摘要:

Quantitative Electrospray Mass Spectrometry of Halides and Halogenic Anions DAVID A. BARNETT AND GARY HORLICK* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T 6G 2G2 A quantitative study of the determination of halides and low to sub-picogram range,16 however no results have been reported for fluorine. halogenic anions by negative ion electrospray mass spectrometry (ES-MS) is presented. Calibration plots were In the present study an electrospray source is used to transfer anions present in solution to the gas phase, which are then found to be linear over a range of 2.5–4 orders of magnitude for F-, Cl-, ClO3-, ClO4-, Br-, I- and IO3- when a sampled into a quadrupole mass spectrometer and subsequently detected.This combination of a low energy source 1.0×10-4 mol l-1 level of external electrolyte was added to the standard solutions. Detection limits ranged from with the very selective detection capabilities of the mass spectrometer oers several advantages over current analytical 0.5 ng ml-1 for the perchlorate ion up to a blank limited 35 ng ml-1 for chloride.An attempt was also made to atomic and chromatographic based techniques. One very important advantage of this technique lies in the fact that determine the level of fluoride present in mouthwash samples. Measurement of the fluoride content utilizing a calibration electrospray can be a very mild source that may be used to sample solution ions directly without loss of speciation curve produced excellent agreement with results obtained by measurement with a fluoride ISE for two synthetic mouthwash information.To date, inorganic electrospray mass spectrometry (ES-MS) samples but poor agreement for two commercial samples. However, excellent agreement between the two methods was has been used primarily for qualitative studies. There have, however, been a few reports on attempts at quantitation with obtained for the commercial samples using the method of standard additions with ES-MS.varying degrees of success in achieving linear calibration plots.21–25 The lack of quantitative studies is not surprising as Keywords: Electrospray ; mass spectrometry ; quantitation; controversy still surrounds the exact mechanism involved in halides ; anions conversion of ions from solution into the gas phase by the electrospray process. Nonetheless, Tang and Kebarle26 have At present the majority of determinations of the halides are presented an excellent study of some of the important features performed by non-spectroscopic techniques including ion- that must be considered in order to undertake quantitation by exchange chromatography1–3 and capillary electrophoresis.4,5 this technique, and Agnes and Horlick25 have shown that The reason for this is two-fold; firstly, halogen atoms are linear calibration plots for simple cationic species can be dicult to observe by atomic absorption because their atomic obtained for up to four orders of magnitude by employing an resonance states lie in the vacuum–UV spectral region and appropriate internal standard.secondly, their excitation and ionization potentials are high resulting in poor sensitivity for both emission and mass EXPERIMENTAL spectrometry unless sources with high excitation temperatures are used, such as helium based plasmas. Despite these limi- All ESMS experiments were carried out on a modified Perkin- Elmer–SCIEX ELAN Model 250 ICP-MS instrument.The tations there have been several reports published on the determination of halides by atomic spectrometry using a electrospray source and interface to the mass spectrometer have been described previously27 with the exception that a variety of dierent sources and detection systems. Emission studies have commonly employed an Ar-ICP6, or an plenum chamber has been added to the front plate to isolate the ES needle from the surrounding laboratory environment.He-MIP.7–9 Mass spectrometric studies include both negative and positive ion Ar-ICP-MS,10–12 He-ICP-MS13,14 and The first and second photon stops have also been removed in order to improve sensitivity by increasing ion transmission MIP-MS using both argon15 and helium based plasmas.16–20 In order to determine halide emission by argon based through the ion optics. A syringe pump (Harvard Apparatus, Cambridge, MA, plasmas a vacuum–UV detection system has been developed by LaFreniere et al.6 to monitor the atomic emission lines of USA) was used to deliver analyte solutions at 2.50 ml min-1 through approximately 30 cm of 254 mm i.d. Teflon tubing Cl, Br and I resulting in limits of detection of 8, 15 and 6 ngml-1, respectively.Vacuum-UV detection has also been connected to a length of stainless steel tubing (200 mm o.d., 100 mm i.d., 5 cm long) which served as the electrospray needle used with a He-MIP source7 but the reported detection limits were in the low mg ml-1 range.Limits of detection determined tip. The electrospray capillary was held at a potential of -2.70 kV, the front plate at -600 V, the sampling plate was by positive ion Ar-ICP-MS10 for Cl, Br and I are lower than for the corresponding emission studies (5, 1 and 0.01 ng ml-1, varied between -15 to -80 V and the skimmer was held constant at -6 V. The sampling plate potential was adjusted respectively), however the only quantitative results reported for fluorine are for determination by negative ion Ar-ICP-MS to both decluster the analyte species of interest eciently and to give optimal sensitivity.The curtain gas used was prepurified with a reported detection limit of 110 ng ml-1.10 Halide determination by He-MIP-MS has focused on the detection of nitrogen at a backing pressure of 10 psi (1 psi=6894.76 Pa) and flow rate of 1.3 l min-1. volatile halo-organic compounds using gas-phase sample introduction.This is a direct result of the tendency for helium Mass spectra were collected by scanning with an integration time of 100 ms per point (10 points per u), while data used for plasmas to become unstable and lose sensitivity upon the introduction of water. Typical absolute detection limits for quantitation were acquired in the peak-hopping mode with a dwell time of 10 ms and total measurement time of 100 ms per halides detected in gas chromatographic eluates are in the Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (497–501) 497point (1 point per u).Average signal intensities were calculated The concentration of each component was approximately 1.0×10-4 mol l-1. The first spectrum, Fig. 1(a), was acquired for ten measurements resulting in typical RSDs of 2–5%. The most abundant isotope was used for each of the species with the sampling plate potential set at -15 V. This results in a relatively low CID energy in the region between the sampling monitored.Typical background levels were 20–40 counts s-1 except for chloride which was observed to be blank limited. plate and the skimmer and hence an abundance of aqua-halide clusters, X(H2O)n-, where X=F, Cl, Br or I and n represents In order to detect the presence of a corona discharge, which can have deleterious eects on analyte intensities, the back- the number of water ligands, is observed in the spectrum. Fluoride exhibits the greatest degree of solvation or coordi- ground level at m/z -32 (O2-) was always monitored to alert the investigator to the presence of a discharge.nation to water ligands with an observed cluster distribution ranging from n=1 to 5 with a maximum signal for F(H2O)3-. Under the same conditions chloride exhibits a maximum signal Reagents for its doubly solvated anion while bromide favours coordination to one water ligand and the predominant peak for Stock solutions were prepared by dissolving the ACS-grade iodide corresponds to the bare singly charged anion at m/z potassium salt in nanopure water to a concentration of -127.This trend towards a smaller degree of solvation with 1.0×10-2 mol l-1. The stock solution was then diluted increasing ionic size follows the Gibb’s free energy of solvation 100-fold in distilled methanol for mass spectrometric analysis. predicted by the Born equation where solvation energy is Serial dilution of the aqueous stock solutions was used to proportional to -Zi2/r, where Zi is the charge on the ion and prepare lower concentration methanolic standards used for the r is its ionic radius.calibration curves in order to maintain the total water content As the collisional energy is increased by increasing the at around 1–3%. sampling plate potential the degree of solvation is observed to decrease. This decrease in solvation in turn serves to decrease RESULTS AND DISCUSSION the complexity of the spectrum, as observed in Fig. 1(b) in which the potential on the sampling plate has been raised to Halide Spectra -20 V. It is important to notice in this figure that there is A sequence of mass spectra for an equimolar mixture of considerable overlap of the 37Cl species at m/z -37 and -55 fluoride, chloride, bromide and iodide is presented in Fig. 1. with solvated 19F species. It will be especially important when selecting operating conditions for quantitation to ensure that this interference is not present.Fortunately this interference can be eliminated by increasing the sampling plate potential to -40 V as shown in Fig. 1(c). At this potential all of the halides are observed to exist predominantly as bare singly charged anions. From these spectra various prominent background species can be identified which are almost always present when operating the electrospray in negative ion mode with a methanol –water solvent system. A list of these background species as observed in Fig. 1(c) is presented in Table 1. Quantitation It has been shown that stable electrospray operation is limited to a range of solution conductivities, determined primarily by the level of non-volatile electrolyte present in the solvent.25,28 When the conductivity of the solution is outside this range the electrospray current will fluctuate rapidly and produce a mass spectrum characterized by species indicative of corona discharge. This conductivity or concentration window varies with electrospray operating parameters but typically results in stable signals for 10-5–10-3 mol l-1 electrolyte in methanolic solvent.In order to extend this concentration range to lower regions suitable for trace analysis it has been found beneficial to add a constant level of ‘supporting’ or stabilizing electrolyte at the Table 1 Some prominent background species observed in negative ion ES-MS -m/z Background species 17 OH- 31 CH3O- 32* O2- 45 HCO2- 46 NO2- 59 CH3CO2- 61 HCO3- 62 NO3- 75 CH3O(CO2)- Fig. 1 Mass spectrum of an equimolar (1.0×10-4 mol l-1) mixture of F-, Cl-, Br- and I- acquired with a sampling plate potential of * Indicative of corona discharge, not present during stable operating conditions. (a) -15 V; (b) -20 V; and (c) -40 V. 498 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 121.0×10-4 mol l-1 level in the presence of the analyte of iodide signal is relatively constant up to a bromide concentration of 3×10-6 mol l-1, above this concentration the iodide interest.In the present study it has been observed that the addition signal begins to decrease sharply while the bromide signal experiences an equally abrupt increase. The cause of the of 1.0×10-4 mol l-1 of potassium iodide to methanolic solutions of potassium bromide results in the observation decrease or ‘suppression’ of the supporting electrolyte signal has been the focus of much attention in positive ion electros- of stable bromide signals for concentrations as low as 1×10-8 mol l-1. The presence of iodide ion carries out a pray studies.Nevertheless it is seen from Fig. 2, and in a previous study by Agnes and Horlick,25 that regardless of the multi-function role by both stabilizing the electrospray process and acting as an internal standard. A log–log plot of the ratio eect of suppression, the ratio of the analyte to electrolyte signal is a linear function of analyte concentration. of the analyte signal (79Br) to the internal standard (127I) shown in Fig. 2(a) results in a calibration curve for bromide linear In addition to the high selectivity aorded by the mass spectrometer to distinguish between two elements, the low over almost four orders of magnitude. Shown in Fig. 2(b) is a similar calibration curve for fluoride linear over 2.5 orders of energy electrospray source also makes it possible to dierentiate between two or more forms of the same element in magnitude. In Fig. 3 the absolute signal intensities detected for iodide and bromide have been plotted as a function of the solution. Several qualitative reports have been published on the capabilities of ES-MS to speciate both cationic and anionic increase in bromide concentration. It is observed that the inorganic species,29–32 however there have been no reported attempts thus far at quantitation of simple anionic species. In Fig. 4 are depicted a series of spectra of an equimolar mixture of chloride, chlorate and perchlorate.In the first spectrum the sampling plate potential is -40 V (the same potential used to desolvate the halides eciently) and the result is a relatively clean spectrum containing the usual background peaks and the characteristic chlorine isotope profile for the three chlorine containing species. The second spectrum was acquired at a sampling plate potential of -60 V and it is observed that the increased collisional energy has resulted in the formation of Fig. 2 Calibration curve for (a) Br- and (b) F- with a 1.0×10-4 mol l-1 level of I- as the electrospray stabilizer and internal standard. Fig. 4 Mass spectra of an equimolar 1.0×10-4 mol l-1 mixture of Fig. 3 Signal intensities of Br- (+) and 1.0×10-4 mol l-1 I- (') as Cl-, ClO3- and ClO4- acquired with sampling plate potentials of (a) -40 V; (b) -60 V; and (c) -80 V. a function of increasing Br- concentration. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 499Table 2 Detection limits and statistical data from calibration curves Detection limit/ Log–log Dynamic range/ Analyte ng ml-1 slope 10-4 mol l-1 19F- 0.8 1.00 0.003–3 35Cl- 35 1.00 0.01–3 35ClO3- 0.7 1.03 0.001–3 35ClO4- 0.5 1.03 0.001–3 79Br- 0.9 1.00 0.001–3 127I- 0.8 1.00 0.001–3 127IO3- 1.0 1.01 0.001–3 based on three times the standard deviation of the blank signal and are listed in Table 2 along with the log–log slopes and the linear dynamic ranges of the calibration curves.In the case of iodide, 1.0×10-4 mol l-1 potassium bromide was used as the stabilizer and internal standard. The high detection limit for chloride was the consequence of a high blank level. Fluoride Determination A log–log calibration curve for fluoride, linear over 2.5 orders Fig. 5 Simultaneous calibration curves for Cl- (+), ClO3- (() and of magnitude with a slope of 1.00, was shown in Fig. 2(b). In ClO4- (2) with a 1.0×10-4 mol l-1 level of I- as the electrospray order to determine fluoride in a more complex matrix another stabilizer and internal standard.calibration curve was established over a much narrower range known to span the concentration of samples to be analysed. the decomposition product ClO2-. The sampling plate poten- This calibration curve is shown in Fig. 6. It is seen to be linear tial in the third spectrum was increased to -80 V resulting in with a near-zero intercept. Volumes of 100 ml of two synthetic a very significant decrease in the observed intensity for the mouthwash samples used in an undergraduate teaching labora- ClO4- ion coupled with the appearance of a ClO- species and an increase in the ClO2- intensity.Similar decomposition tory (M1169 and M1259) containing sodium fluoride, methyl products have been observed when only ClO3- is present in salicylate and 4-chloro-1-butanol as well as two commercial solution and so even though the chlorate signal seems to samples (Cepacol and Oral-B) were diluted in 50 ml of methremain constant in these three spectra, it is likely that while anol, spiked to 1.0×10-4 mol l-1 potassium iodide and some chlorate is being dissociated more is being formed by analysed for fluoride content.The same samples were also the decomposition of the larger perchlorate ions. A detailed analysed using a fluoride ISE as a check on the ES-MS results. study of the decomposition mechanism that is occurring in The results are summarized in Table 3.It is seen that the this example was not undertaken. The point is, that it is fluoride concentration for the two synthetic samples deterimportant to choose CID conditions for quantitation which mined by ES-MS and using the fluoride electrode are in will eectively desolvate the analytes of interest without agreement. The results for the two commercial samples were resulting in their decomposition. Fortunately for these three much lower than expected (0.021% NaF and 0.009% NaF for particular species this CID energy ‘window’ is relatively Cepacol and Oral-B, respectively) when determined from the large so it was possible to choose one set of condi- calibration curve in Fig. 6. However, when determined by the tions to construct calibration curves for all three species method of standard additions the two commercial samples simultaneously. agree with the fluoride ISE results. The results obtained by Log–log calibration curves achieved for the three chlorine species present in the same 1.0×10-4 mol l-1 methanolic potassium iodide solution are depicted in Fig. 5. The curves for chlorate and perchlorate are both linear over three orders of magnitude while the chloride signal levels o just below a concentration of 1.0×10-6 mol l-1. Chloride has a much shorter linear range due to impurity present in the potassium iodide supporting electrolyte. One of the more subtle yet most significant features of this calibration is that despite the fact that the three species were present in the same solution their individual signal ratios to the iodide signal increase linearly with concentration.This implies that the suppression of the ion signal intensities is dependent on the total ion concentration rather than the individual properties of the ions present in solution. This is a particularly important observation in consideration of the number of possible concomitant ions present in real samples. Given that the signals of various analytes are aected equally by changing electrolyte concentrations it should be possible to obtain meaningful analyte concentrations in samples from calibration curves by employing an appropriate internal standard. Fig. 6 Calibration curve for F- with a 1.0×10-4 mol l-1 level of Detection Limits I- as the electrospray stabilizer and internal standard. Error Calibration curves were established for F-, Cl-, ClO3-, bars represent ±s of the signal ratio for ten measurements in peak-hopping mode.ClO4-, Br-, I- and IO3-. Detection limits were estimated 500 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Table 3 Results of determination of fluoride in synthetic and commer- Reliable determination of fluoride present in two real cial mouthwash samples mouthwash samples was not achieved by a determination utilizing a simple calibration curve, however the method of Concentration (% NaF) standard additions did result in a measured fluoride content in agreement with that determined by a fluoride ISE.This Sample ES-MS Fluoride ISE implies that additional matrix eects not accounted for in the M1169 0.050±0.003* 0.051±0.001† simple inorganic systems studied thus far may be present in M1259 0.026±0.002 0.026±0.001 real samples. It is therefore concluded that reliable quantitation Cepacol‡ 0.062±0.003 0.063±0.001 Oral-B‡ 0.054±0.003 0.056±0.001 by ES-MS may require the method of standard additions coupled with the additions of an appropriate internal standard * Error based on least squares analysis of curve depicted in Fig. 6. and electrospray stabilizer. † Error based on least squares analysis of calibration curve for two measurements of sample and standards. Financial support by the Natural Sciences and Engineering ‡ Determined with ES-MS by standard additions. Research Council of Canada (NSERC) and the University of Alberta are gratefully acknowledged. use of standard additions methodology are the ones reported in Table 3.REFERENCES The fact that the fluoride concentrations determined in the 1 Maki, S. A., and Danielson, N. D., Anal. Chem., 1991, 63, 699. commercial mouthwash samples from the calibration curve 2 Umile, C., and Huber, J. F. K., J. Chromatogr., 1993, 640, 27. did not match the results obtained by standard additions 3 Shotyk, W., J. Chromatogr., 1993, 640, 309. indicates that there may be an additional matrix eect occur- 4 Rhemrev-Boom, M.M., J. Chromatogr. A, 1994, 675, 680. ring in electrospray that is not accounted for in the simple 5 Wojtusik, M. J., and Harrold, M. P., J. Chromatogr. A, 1994, electrolytic solutions examined thus far. Some possible sources 671, 411. 6 LaFreniere, B. R., Houk, R. S., and Fassel, V. A., Anal. Chem., of this matrix eect are outlined below. 1987, 59, 2276. Both commercial mouthwash samples analysed contain 7 Alvarado, J., and Carnahan, J. W., Appl. Spectrosc., 1993, 47, 2036. 0.05% cetylpyridinium chloride [C16H33(C5H5N)+Cl-], 8 Jin, Q., Zhang, H., Ye, D., and Zhang, J., Microchem. J., 1993, added as an anti-bacterial agent, in addition to 0.05% NaF. 47, 278. The cetylpyridinium ion is a large solvophobic cation which 9 Camuna, F., Sanchez Uria, J. E., and Sanz Medel, A., Spectrochim. has a relatively high surface activity. The high surface activity Acta, Part B, 1993, 48, 1115. 10 Fulford, J. E., and Quan, E. S. K., Appl. Spectrosc., 1988, 42, 425.of this cation could quite conceivably have a pronounced eect 11 Vickers, G. H., Wilson, D. A., and Hieftje, G. M., Anal. Chem., on both droplet formation and evaporation, which are crucial 1988, 60, 1808. phenomena in the electrospray process. There may also be a 12 Chong, N. S., and Houk, R. S., Appl. Spectrosc., 1987, 41, 66. tendency for this large cation to form ion pairs with fluoride 13 Mohammed, A. H., Creed, J. T., Davidson, T. M., and Caruso, especially in a low dielectric solvent such as methanol. Finally, J.A., Appl. Spectrosc., 1989, 43, 1127. the eect of dierences in pH of the sample and standard, as 14 Montaser, A., Chan, S. K., and Koppenaal, D. W., Anal. Chem., 1987, 59, 1240 well as the eect on pH of the electrospray sampling process 15 Douglas, D. J., and French, J. B., Anal. Chem., 1981, 53, 37. in which droplets are continually decreasing in size as a result 16 Story, W. C., and Caruso, J. A., J. Anal. At. Spectrom., 1993, 8, 571.of solvent evaporation and fission, may also have a profound 17 Creed, J. T., Davidson, T. M., Shen, W.-L., and Caruso, J. A., eect on the fluoride anion which itself acts as a weak base. J. Anal. At. Spectrom., 1990, 5, 109. These three factors may all be responsible in part for the 18 Brown, P. G., Davidson, T. M., and Caruso, J. A., J. Anal. At. decrease in sensitivity observed for fluoride between sample Spectrom., 1988, 3, 763. 19 Creed, J. T., Mohammed, A.H., Davidson, T. M., Ataman, G., and standard. Regardless, the fact that the standard additions and Caruso, J. A., J. Anal. At. Spectrom., 1988, 3, 923. method did produce results in agreement with the fluoride 20 Satzger, R. D., Fricke, F. L., Brown, P. G., and Caruso, J. A., ISE measurement is promising evidence for the quantitative Spectrochim. Acta, Part B, 1987, 42, 705. capabilities of ES-MS. 21 Rafaelli, A., and Bruins, A. P., Rapid Commun. Mass Spectrom., 1991, 5, 269. 22 Mann, M., Org. Mass Spectrom., 1990, 25, 575. CONCLUSIONS 23 Ikonomou, M. G., Blades, A. T., and Kebarle, P., Anal. Chem., 1990, 62, 957. The analytical utility of negative ion ES-MS for quantitation 24 Fenn, J. B., J. Am. Soc. Mass Spectrom., 1993, 4, 524. and speciation of several halogenic anions has been presented. 25 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1994, 48, 649. For these simple inorganic systems it was found that despite 26 Tang, L., and Kebarle, P., Anal. Chem., 1993, 65, 3654. the fact that analyte signal intensities are suppressed when the 27 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1992, 46, 401. 28 Stewart, I. I., and Horlick, G., J. Anal. At. Spectrom., 1996, 11, 1203. total electrolyte concentration exceeds 1.0×10-4 mol l-1 the 29 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1994, 48, 655. relative intensities of these simple inorganic ions present at 30 Agnes, G. R., Stewart, I. I., and Horlick, G., Appl. Spectrosc., consistent levels in solution remain constant. It was also 1992, 48, 1347. observed that analyte signals are independent of the identity 31 Stewart, I. I., and Horlick, G., Anal. Chem., 1994, 66, 3983. of foreign electrolyte present in solution provided they do not 32 Stewart, I. I., Barnett, D. A., and Horlick, G., J. Anal. At. introduce a chemical interference. Detection limits were found Spectrom., 1996, 11, 877. to be of the order of #1 ngml-1, which is comparable to current atomic techniques for bromide and iodide, however Paper 6/03643K the most significant advantage of this method is in its ability ReceivedMay 24, 1996 to speciate dierent forms of the halides directly and especially Accepted September 27, 1996 its sensitivity in the determination of fluoride. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 501

ISSN:0267-9477    

DOI:10.1039/a603643k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Speciation of Chromium(vi) and Chromium(iii) Using Pneumatically Assisted Electrospray Mass Spectrometry ALBIN B. GWIZDALA III†, STEVE K. JOHNSON, SAHANA MOLLAH AND R. S. HOUK* Ames L aboratory—U.S. Department of Energy, Department of Chemistry, Iowa State University, Ames, IA 50011, USA Mass spectra can be obtained from aqueous solutions and oxoanions of the same element under the same spray conditions. containing both Cr3+ and Cr2O72- as negative ions under the same spray conditions.An excess of HCl is used so that Cr3+ sprays as an anionic chloro complex. Moderately energetic EXPERIMENTAL collision conditions produce CrO3- from CrVI and CrOCl2- from CrIII. Detection limits are 100 and 60 ppb for CrIII and Standard solutions were prepared by diluting aliquots from CrVI, respectively. Reasonable calibration curves are provided 1000 ppm aqueous standards (ICP emission standards from by plotting the ratio of the analyte signal to that for 37Cl-. Aldrich Chemicals) with a 50% methanol–50% water solvent. The methanol–water solution was prepared using HPLC grade Keywords: Chromium determination; speciation; electrospray; methanol(Fisher Scientific) and water de-ionized to a resistivity ion spray; mass spectrometry of 18 MV cm( with a Barnsted Nanopure-II system (Newton, MA, USA).ULTREX II ultra pure grade HCl (J. T. Baker) Speciation information is needed as the toxicity and biological was used to bring the final concentration (v/v) of solutions up role of a particular element can vary greatly depending on the to 1% HCl.Approximately 500 ml of solution were drawn into chemical form. At low levels, CrIII is essential for living organ- a 1 ml syringe (Hamilton). Solutions were transported from isms. It is an essential nutrient and is believed to help activate syringe to ion source through a 100 mm id fused silica capillary insulin.1 On the other hand, CrVI can cross cell membranes (Polymicro Technologies) using a syringe pump (74900 Series, and cause skin lesions, lung cancer and other forms of cancer.2 Cole-Parmer Instruments).Chromium has many industrial applications such as dyeing, A Perkin-Elmer SCIEX API 1 mass spectrometer was used tanning and use in the steel industry. Accurate determination (Fig. 1). Typical conditions used for the instrument are summaof each species rather than just the total chromium level is rized in Table 1. Voltages were optimized on a daily basis to important in determining toxicity.maximize the signal for the species of interest. The ‘best’ In the late 1960s, Dole and co-workers3,4 described the voltages varied slightly (±5 V) from day to day; typical values formation of gas-phase ions when a liquid was sprayed out are listed in Table 1. Peak hopping data were collected using of a capillary held at high voltage. Since that time, the eorts a 100 ms dwell time. Spectral scans were collected by adding of Fenn and co-workers5–8 have led to the development of ten consecutive scans together using a 10 ms dwell time.electrospray mass spectrometry (ES-MS). Pneumatically assisted electrospray, also referred to as ion spray, was first RESULTS AND DISCUSSION reported by Bruins et al.9 Ion spray can be considered as a concentric pneumatic nebulizer combined with electrospray. Selection of Collision Conditions These two terms are often used interchangeably. Since the The polarity of the voltage applied to the electrospray needle development of electrospray and ion spray, many applications determines whether positive or negative ions are observed.The have been discovered. Electrospray has found widespread use collision conditions then determine the extent of fragmentation for the determination of the relative molecular mass of large observed. In this experiment, the dierence between the orifice biological molecules.10–12 This technique has also been used plate voltage (VOR, see Fig. 1) and the voltage on the RF-only for the determination of many inorganic species, from ion rods (VRF) has the largest eect on the chromium species clusters to bare metal ions.13–21 Using electrospray mass spectrometry, it is easy to distinguish Cr3+ from Cr2O72- using separate spray conditions.21 A positive voltage is applied to the electrospray needle to spray Cr3+, while a negative voltage produces anions from Cr2O72-. This procedure requires separate runs to observe both cations and anions.The present work describes a possible way to distinguish CrIII and CrVI with the same spray conditions. The cation Cr3+ is sprayed as a negative chloro complex. Horlick and co-workers refer to this as the ‘intermediate’ or ‘counter ion’ mode because collisions during the ion extraction process occur at moderate kinetic energies such that anions remain associated with the metal cation.20 This concept could become a general procedure for distinguishing cations Fig. 1 Schematic diagram of electrospray mass spectrometer including ion source, interface assembly and some ion optics. Representation † Present address: U.S. Silica, Box 187, RT 522 North, Berkeley Springs, WV 25411, USA. is not drawn to scale. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (503–506) 503Table 1 Typical operating conditions 17 ml min-1 Sample flow rate Nebulizer gas Nitrogen Nebulizer gas pressure 280 kPa Curtain gas Nitrogen, ultra pure carrier grade Curtain gas back pressure 550 kPa Curtain gas temperature 60 °C Ionization needle voltage (VISV) -3200 V Interface plate voltage (VIN) -500 V Orifice plate voltage (VOR) -125 V RF only quadrupole voltage -60 V (VRF) Mass analyzer quadrupole -59 V voltage (VR1) CEM detector voltage +2500 V Fig. 3 A plot of signals from the major Cr species, A 52CrO3-, m/z= Operating pressure of 5.333×10-3 Pa 100; B 52CrO335Cl-, m/z=135, observed from a 10 ppm CrVI solution quadrupole chamber while varying the voltage on the orifice plate.Fig. 2 A plot of signals from the major Cr species, A 52CrO235Cl-, m/z=119; B 52CrO35Cl2-, m/z=138; C 52CrO35Cl2- H2O, m/z=156; D 52CrO35Cl2- 2H2O, m/z=174, observed from a 50 ppm CrIII solu- Fig. 4 Mass spectrum of a 50 ppm CrIII solution. The inset is the tion while varying the voltage on the orifice plate. region from m/z=95 to 105. (CrCl4 2H2O)-, or something similar, but such ions could not detected. Ions from the spray flow through the 200 mm diameter be observed even under the ‘softest’ extraction conditions.orifice in the orifice plate, along with nitrogen from the curtain Fig. 3 shows a plot of signal versus orifice voltage (VOR ), gas. As the ions are accelerated through the potential dierence from a 10 ppm CrVI solution. At a VOR-VRF of between 10 (VOR-VRF), they collide with nitrogen molecules in the superand 50 V, the chromium oxochloro species CrO3Cl- (m/z= sonic jet that forms behind the orifice plate.The larger the 135) is observed. At slightly more harsh collision conditions, voltage dierence (VOR-VRF), the more energetic the collision the ion CrO3- (m/z=100) is observed. Using conditions that conditions, which produce more extensive fragmentation. This are favorable for detecting CrOCl2- (m/z=138) from the CrIII voltage dierence is essentially the collision energy for collisionsolution (-125 V on VOR, VOR-VRF#65 V), both CrO3- and induced dissociation in the laboratory frame of reference.CrO3Cl- are observed from CrVI, with CrO3- being the In the present work, the voltage dierence (VOR-VRF) was dominant species. varied by changing VOR only. The ion kinetic energy inside the Thus, a voltage dierence of 65 V between VOR and VRF mass filter is determined by the dierence (VRF-VR1); this produces stable chromium oxo or oxochloro species from both dierence is kept constant so the resolution and peak shapes CrIII and CrVI solutions.These conditions were used to obtain do not change greatly as VOR is altered. all the following results unless otherwise indicated. It should Fig. 2, a plot of signal versus VOR, was generated from a be noted in Fig. 3 that the signal at m/z=100, between -60 V 50 ppm CrIII solution. The plots shown in Fig. 2 are for various and -90 V, is not due to Cr. Scans under these conditions chromium oxochloro species identified in the caption. The ions reveal no Cr isotope pattern in this region.Scans do reveal selected contain 52Cr and 35Cl and are the most abundant species present at both m/z=98 and m/z=100, in a 3 to 1 ones from the various isotope patterns. The main ions under intensity ratio. This would seem to indicate that this species soft conditions (VOR#90 V, VOR-VRF #30 V) still have chrocontains chlorine and is probably due to a background ion. mium as CrIII and intact oxygen ligands. As can be seen in Fig. 2, using a VOR value of -125 V (i.e., VOR-VRF# 65 V) yields a maximum signal for CrOCl2- (m/z=138).Mass Spectra Using -125 V on VOR minimizes the abundance of (CrOCl2 nH2O)- ions. Fig. 4 shows a mass spectrum of a 50 ppm CrIII solution obtained under the optimum conditions (VOR-VRF=65 V) The onset voltage for observation of the hydrated ions is VOR#-80 V. The onset voltages for the analyte species described above. The most abundant chromium oxochloro species are observed at m/z=138, 140 and 142.This set of CrO2Cl- and CrOCl2- are approximately 15 V more negative. This 15 V dierence represents the additional kinetic energy peaks is attributed to CrOCl2- and appears in the appropriate isotope ratios, 95651, for an ion with two chlorine atoms. Less necessary to create CrO2Cl- and CrOCl2- from whatever anion is formed originally. Presumably, this precursor ion is intense peaks can also be observed at m/z=119 and 121; the 504 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12351 isotope pattern shows they have one chlorine atom. These are m/z=100, CrO3- from CrVI, and m/z=138, CrOCl2- from CrIII. In addition to the peaks associated with the isotope peaks are likely to be CrO2Cl-. The region at m/z=100, shown in the inset of Fig. 4, does not contain ions from Cr3+. patterns from the previously mentioned species, CrO2Cl- from CrIII and CrO3Cl- from CrVI are also observed. A mass spectrum of CrVI at 10 ppm is shown in Fig. 5. Chromium species can be observed in the m/z=100 and m/z= The CrO3- ion that is formed from CrVI actually contains Cr in the oxidation state +5. No Cr2O72- or CrO42- were 135 regions. The intensities of the peaks around m/z=100 agree very well with the known isotope ratio of chromium. detected, even under the softest ion extraction conditions. As pointed out by Stewart and Horlick,21 Cr2O72- is readily The dominant peak at m/z=100 is assigned to 52CrO3-. The set of peaks at m/z=135 and 137 are probably CrO3Cl-.The reduced by methanol, which explains why intact ions from chromate or dichromate were not observed. Nevertheless, the peak observed at m/z=113 in Fig. 5 and another fragment peak at m/z=69 (not shown) are due to contamination of the CrO3- ion is characteristic of CrVI in the original sample, i.e., before the methanol was added to enhance the electrospray instrument with trifluoroacetate (TFA-). The TFA contamination can be seen throughout this study and is a result of process.other experiments on electrospray of organic acids. Cleaning procedures have as yet been unable to remove the TFA Calibration Curves and Detection Limits completely from the instrument. TFA does not interfere with the determination of chromium but does appear in these scans. Calibration curves and detection limits were determined for However, this observation does illustrate one possible problem: both CrIII and CrVI solutions. Calibration curves for CrIII and organic ions that remain intact can cause memory or spectral CrVI are shown in Figs. 7 and 8, respectively. These plots were interferences. generated by taking the ratio of the analyte signal, m/z=138 During this work, when an aqueous CrVI solution was for CrIII or m/z=100 for CrVI , to the signal at m/z=37 due to allowed to stand, a small amount of CrVI was reduced to CrIII. 37Cl- from the aqueous HCl solvent. As described by Agnes The characteristic peaks for CrIII can then be seen from the and Horlick, it is often advantageous in ESMS to ratio the CrVI solution. This is a common problem with inorganic analyte signal of interest to another signal to compensate for speciation.When species are sampled, care should be taken to avoid changing the species present in the initial sample. Sampling and sample preparation work can alter the speciation information and lead to inaccurate measurements. Fig. 6 shows a mass spectrum of a mixture containing 50 ppm CrIII and 10 ppm CrVI.The dominant peaks observed Fig. 7 Calibration curves for CrIII solutions: (a) wide range concentration plot, including non-linear region; and (b) a plot of the lower concentration region of (a). Fig. 5 Mass spectrum of a 10 ppm CrVI solution. Fig. 8 Calibration curves for CrVI solutions: (a) wide range concen- Fig. 6 Mass spectrum of a mixture containing 50 ppm CrIII and tration plot, including non-linear region; and (b) a plot of the lower concentration region of (a). 10 ppm CrVI. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 505Table 2 Detection limits REFERENCES 1 da Silva, J., and Williams, R., T he Biological Chemistry of the Species Relative/ppb Absolute/pg Elements, the Inorganic Chemistry of L ife, Clarendon Press, CrIII 100 3 Oxford, NY, 1991, p. 541. CrVI 60 1.5 2 Ottaway, J. M., and Fell, G. S., Pure Appl. Chem., 1986, 58, 1701. 3 Dole, M., Mach, L. L., Hines, R. L., Mobley, R. C., Ferguson, L.P., and Alice, M. B., J. Chem. Phys., 1968, 49, 2240. 4 Mach, L. L., Kralik, P., Rheude, A., and Dole, M., J. Chem. Phys., the variation of electrospray signal with the total ionic com- 1970, 52, 4977. position of the sample.20 Fairly linear calibration curves (cor- 5 Whitehouse, C. M., Dreyer, R. N., Yamashita, M., and Fenn, J. B., Anal. Chem., 1985, 57, 675. relation coecients of 0.989 for CrIII and 0.992 for CrVI, 6 Meng, C. K., Mann, M., and Fenn, J. B., Z.Phys. D, 1988, 10, 361. respectively) were observed at low analyte concentrations, i.e., 7 Wong, S. F., Meng, C. K., and Fenn, J. B., J. Phys. Chem., 1988, up to 1 ppm for CrIII and 10 ppm for CrVI. The calibration 92, 546. curves roll over at higher analyte concentrations. Detection 8 Mann, M., Meng, C. K., and Fenn, J. B., Anal. Chem., 1989, limits are shown in Table 2. These values represent the solution 61, 1702. concentration necessary to produce a net signal equivalent to 9 Bruins, A.P., Covey, T. R., and Henion, J. D., Anal. Chem., 1987, 59, 2642. three times the standard deviation of background during 10 Fenn, J. B., Mann, M., Meng, C. K., Wong, S. F., and Whitehouse, single-ion monitoring for the dwell time used (0.1 s). The C. M., Science, 1989, 246, 64. detection limits are 60–100 ppb (relative) or 1.5–3 pg (absolute). 11 Smith, R. D., Loo, J. A., Edmonds, C. G., Barinaga, C. J., and Udseth, H. R., Anal. Chem., 1990, 62, 882. 12 Smith, R.D., Loo, R. J., Ogorzalek-Loo, R. R., Busman, M., and CONCLUSION Udseth, H. R., Mass Spectrom. Rev., 1991, 10, 359. 13 Siu, K. W. M., Gardner, G. J., and Berman, S. S., Rapid Commun. The concept described in this paper could become a general Mass Spectrom., 1988, 2, 201. procedure for distinguishing cations and oxoanions of a par- 14 Cheng, Z. L., Siu, K. W. M., Guevremont, R., and Berman, S. S., ticular element under the same spray conditions, in cases where Org. Mass Spectrom., 1992, 27, 1370. both forms remain stable in the same solution. Diculties 15 Blades, A. T., Jayaweera, P., Ikonomu, M. G., and Kebarle, P., include high background, mediocre detection limits and prob- Int. J. Mass Spectrom. Ion Processes, 1990, 101, 325. able spectral interferences from either organic or inorganic 16 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1992, 46, 401. 17 Stewart, I. I., and Horlick, G., T rends Anal. Chem., 1996, 15, 80. anions. These last problems could be alleviated to a large 18 Colton, R., D’Agostino, A., and Traeger, J. C., Mass Spectrom. extent by the use of tandem mass spectrometry, which should Rev., 1995, 14, 79. eliminate much of the background. 19 Corr, J. J., and Anacleto, J. F., Anal. Chem., 1996, 68, 2155. 20 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1994, 48, 649, 655. Ames Laboratory is operated for the U.S. Department of 21 Stewart, I. I., and Horlick, G. J. Anal. Atom. Spectrom., 1997, Energy by Iowa State University under Contract Number 11, 1203. W-7405-Eng-82. This research was supported by the Oce of Technology Development, Environmental Management Paper 6/06413B Program (EM-50). Received September 17, 1996 Accepted January 2, 1997 506 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a606413b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Ion Spray Mass Spectrometry for Elemental Speciation in Aqueous Samples: Preliminary Investigation of Experimental Parameters, Matrix Effects and Metal–Ligand Complexation JOHN W. OLESIK*a , KURT K. THAXTONa,b AND SUSAN V. OLESIKb aL aboratory for Plasma Spectrochemistry, L aser Spectroscopy andMass Spectrometry, Department of Geological Sciences, T he Ohio State University, 275 Mendenhall L aboratory, 125 S. Oval Mall, Columbus, OH 43210, USA bDepartment of Chemistry, T he Ohio State University, Columbus, OH 43210, USA Ion spray mass spectra obtained from aqueous solutions are out prior separation or in combination with a separation technique12 to reduce the complexity of the mass spectra.discussed. Ion clusters with large numbers of water molecules, very few water molecules or bare elemental ions could be There is strong evidence that ions observed by ES-MS are those ions that were present in solution6,20 (as indicated from obtained by adjusting the sampling orifice voltage, the skimmer voltage or the position of the ion spray nebulizer independent measurement by NMR or electrochemistry or from solution models) or are the result of change reduction relative to the sampling orifice.In some cases submillimeter changes in the position of the ion spray nebulizer can reactions in the gas phase.9,13,21 ES-MS has also been used to investigate metal–ligand complexation6,22 and to successfully dramatically aect the nature of the spectra observed.There is evidence of declustering between the skimmer and first ion lens monitor even rapidly exchanging ions.23 Therefore, ES-MS has the potential to be particularly suitable for the investigation as well as between the sampling orifice and skimmer. Interpretation of the mass spectra must be carried out with of metal–ligand complexation in both aqueous and mixed aqueous–organic solvents, where data are scarce and often care as potential spectral overlaps are common.Use of a triple quadrupole mass spectrometer allows assessment of potential dicult to obtain by other methods. Most of the previously reported ES-MS methods for elemen- spectral overlaps. Ni–EDTA complexation was investigated. The Ni+ signal correlated with the expected concentration of tal speciation have involved organic solvents or mixtures of organic and aqueous solvents because the ES is generally more free metal ions in the solution not the total metal concentration even when relatively harsh conditions that stable with organic solvents such as methanol.Formation of discharges when electrospraying water is common. However, produced bare metal ion spectra were used. Matrix eects due to high concentrations of NaCl and HCl were briefly as much more information is available on metal–ligand speciation in water, we chose to begin with aqueous solutions. investigated. Pneumatically-assisted ES (ion spray24) can allow the use of Keywords: Ion spray mass spectrometry; elemental speciation; higher liquid flow rates and higher fractions of aqueous solvent aqueous samples; matrix eects ; metal–ligand complexation; as well as producing aerosols from higher conductivity solu- electrospray mass spectrometry tions than ES.Here, ion spray was used with purely aqueous samples. The importance of elemental speciation in toxicity, nutrient benefit, biological activity, movement through soils and trans- EXPERIMENTAL port in water systems is increasingly recognized.Many of the Quadrupole Mass Spectrometers current methods for elemental speciation are limited by long analysis times, insucient detection limits, a narrow dynamic Two dierent mass spectrometers were used (Fig. 1): a three stage, modified Extranuclear (now Extrel) EMBA (Extra- range or insucient selectivity. The combination of a separation technique (such as ion exchange chromatography, liquid nuclear Modulated Beam Analyzer) II (with the chopper removed) quadrupole with analog detection and a single chromatography or capillary electrophoresis) with an element selective detector, such ICP-MS, is one approach to elemental vacuum stage SCIEX TAGA 6000E triple quadrupole with pulse counting detection.speciation. Great care must be taken to be sure that the speciation is not aected during the separation. Furthermore, The Extrel MS, mounted vertically, was used with a 4 mm thick front plate (with a 3 mm diameter hole) and an orifice while each element can be identified unequivocally, the species must be inferred from the migration or elution time.plate (with a 100 mm diameter hole), separated by a 4 mm thick Teflon spacer. The front plate was maintained at 650 V Electrospray mass spectrometry (ES-MS) provides a means of detecting ions in liquid solution.1–3 Although ES-MS has (Thorn EMI Gencom 3000R power supply). The orifice plate voltage was controlled by a Heath-Zenith model SP-2717 been used mainly for the detection of biological molecules, inorganic ions have been the subject of early investigations.2,4,5 power supply.Nitrogen curtain gas, controlled by a mass flow controller (MKS model 1159B with model 246 control unit), Since then ES-MS has been extensively used in inorganic and organometallic chemistry.6 Kebarle and co-workers first was introduced between the front and sampling orifice plates using an 1/8 id copper tube. The skimmer was 5.6 mm behind observed solvated +2 and +3 metal ions by ES-MS.7,8,9 Application of ES-MS to inorganic analysis and elemental the sampling orifice (unless noted otherwise) and had a 1.0 mm orifice.The skimmer was 34 mm long with a base diameter of speciation has been lead by investigators at SCIEX10,11,12 and by Horlick and co-workers.13–19 36 mm. The skimmer was either grounded or biased using a Hewlett Packard model E3612A power supply. ES-MS shows promise for direct elemental speciation with- Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (507–515) 5071 mm outside the end of the central steel tube. For some experiments, a Meinhard SB-30-A3 nebulizer was used. The fused silica capillary was inserted into the nebulizer so that the tip protruded 1 to 2 mm past the face of the nebulizer. A union (Upchurch Scientific model U402) was used at the inlet end of the nebulizer to hold the capillary in place and to form an air-tight seal.Sample was delivered to the nebulizer at a flow rate of 5 ml min-1 using a 10 ml syringe (Hamilton model 81630) driven by a syringe pump (KD Scientific). Nitrogen was used as the nebulizer gas at a flow rate of 0.4 l min-1 (unless noted otherwise), controlled by a mass flow controller (Brooks model 5850/5876). The ion spray voltage was applied either by connection to the metal SCIEX Ionspray nebulizer or to the metal fitting at the outlet end of the syringe. Formation of discharges was significantly reduced in the latter case.The end of the nebulizer was placed about 1 to 2 cm from the front plate of the Extrel MS at an angle of 23 to 30° relative to the center line of the ion optics. The tip of the Fig. 1. Mass spectrometers used for ion spray MS. Top, Extrel MS. nebulizer was placed on axis (unless noted otherwise). For Bottom, SCIEX TAGA MS: D, detector; Q1, Q3, quadrupole mass experiments with the TAGA instrument, the end of the capillary filters, Q0, rf only quadrupole, Q2; rf only quadrupole and collision was placed about 2 cmfrom the orifice, at an angle of about 30°.cell for MS–MS measurements. Reagents The first stage had a base pressure of 0.2 Torr (using a Sargent-Welch Directorr 8851C mechanical pump). The second Solutions were prepared of nickel (II) chloride, potassium stage had a base pressure of 1×10-4 Torr (using a Varian chloride, sodium chloride and barium chloride (all from Fisher, VHS-6 diusion pump).Three cylindrical lenses were used in A.C.S. certified in 18MV deionized water (from a Millipore the second stage for most of the experiments. All lenses were Milli-Q Plus). Cobalt (II) nitrate, lead (II) nitrate and sodium 1.1 cm in diameter. Lenses one, two and three were 1.5, 5.1 acetate solutions were prepared from A.C.S. certified salts (J.T. and 3.3 cm long. Lens one was 18.2 mm from the tip of the Baker). Other materials (all analytical grade) included yttrium skimmer. For some experiments lens one was removed to allow chloride (Aldrich), strontium chloride (Aldrich), disodium easier swapping between the interface for ion spray MS and EDTA (GFS Chemicals) and trisodium citrate (Jenneile the one for ICP-MS.A 1.9 mm diameter aperture, electrically Chemica). connected to lens three, separated the second and third stages. The entrance ELFS (Extranuclear Labs Field Separator) lens RESULTS AND DISCUSSION (1 cm long, 0.5 cm id, 1 cm od ferromagnetic material inserted in a cylindrical lens) was placed 1.5 cm from lens three.The The ions observed by electrospray or ion spray mass specquadrupole rods were 1 cm back from the ELFS lens. Two trometry depend on the ions present in the sample solution, dierent detectors were used on the Extrel MS during the the extent of solvent molecule declustering as well as charge experiments: either a Galileo Channeltron electron mulitiplier reduction in the gas phase and the species dependent trans- (model 4870E) or an ETP Active Film Multiplier (model mission of ions to the MS detector.The voltages applied to AF553). Current from the multiplier was converted to voltage the needle, the front plate, sampling orifice and skimmer (for by a Keithley 428 current amplifier. The resulting voltage was the Extrel MS) or the conical lens (for the SCIEX TAGA MS) digitized by a Tecmar LabMaster A/D board in a Gateway as well as the curtain gas flow rate can aect declustering and 486–33 PC using a program written in ASYST 4.0.Mass ion transmission.16,18 scanning was controlled by a D/A converter on the LabMaster During our initial studies with aqueous solutions, we found using the same program. it easy to obtain spectra with high degrees of clustering on the The single stage TAGA instrument was cyropumped. The TAGA instrument, but obtained only bare metal ion spectra front plate had a 3 mm diameter hole. The distance between on the Extrel instrument when the skimmer was grounded.the front plate and the 100 mm diameter orifice was 6 mm. A When a small potential (10 V, for example) was applied to the 5 mm long conical electrostatic lens was 6 mm behind the skimmer, it was then possible to obtain spectra of highly orifice. The conical lens had a 4 mm diameter hole in the input clustered ions using the Extrel instrument. Agnes and Horlick end and a 8 mm diameter hole in the output end. There was previously reported15 that signals improved by about a factor an rf-only quadrupole between the conical lens and the first of 20 when the skimmer was biased and the ion extractor was quadrupole mass filter.For some experiments, collisionally changed from a solid disk to a cylinder. We observed about a activated dissociation was carried out in the second quadrupole factor of 3 to 4 improvement in the bare K+ signal when the which is rf only. Data were collected through pulse counting.skimmer was biased but a much larger increase in the intensity All data were taken on an Apple Macintosh microcomputer of solvated ion clusters. using the SCIEX software. Spectra may consist of cluster ions with a large number of solvent molecules (Fig. 2a,b), ions with a small number of solvent molecules (Fig. 2c) or bare elemental ions (Fig. 2d,e). Ion Spray Nebulizer Declustering is thought to occur by collisionally induced dissociation mainly occurring between the sampling orifice The SCIEX IonSpray nebulizer consisted of two concentric steel tubes (21 and 26 gauge) mounted in a T (Valco, model and the skimmer.The data shown in Fig. 2 suggest that declustering is more complete as the voltage dierence between ZT1C). The ends of the inner and outer tubes were flush. Sample was carried in a 23 cm long fused silica capillary the sampling plate and the skimmer increased from 10 to 50 V by either increasing the sampling plate voltage or (Polymicro Technologies, 150 mm od, 75 mm id) that was inserted inside the IonSpray nebulizer and protruded about decreasing the skimmer voltage. This is consistent with 508 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Fig. 2. Eect of potential dierence between sampling orifice and skimmer on ion spray mass spectra. Sampling orifice voltage was varied while skimmer voltage was maintained at 20 V (a,c,d). Skimmer voltage was varied while sampling orifice voltage was maintained at 50 V (b,c,e).The first ion lens voltage was 0 V. Sample contained 5mM KCl and 1 mM NiCl2 in water. Under the conditions of this experiment, the Ni ion species sensitivities were much lower than the K ion species sensitivities. Curtain gas flow rate was 0.076 l min-1. previous reports that used methanol rather than aqueous ion [NiOH+ or NiOH(H2O)n+]. The dominant strontium ions are Sr2+ and Sr(H2O)n2+. Cluster ions of yttrium with solutions.13,16,18 Similar control over the extent of clustering could be water are still predominant.This shows the importance of the element dependent gas phase chemistry in aecting conditions obtained by varying both the sampling plate and skimmer voltage while maintaining a constant 10 V potential dierence necessary to obtain metal–solvent ion clusters, bare ions and charge reduced species, as discussed previously in the electro- between the sampling plate and skimmer (Fig. 3). However, the potential between the skimmer and first ion lens (0 V) spray literature.7,8,9,13,18 As the orifice voltage is increased further to 125 V, bare increased from 20 to 90 V.This suggests that declustering can occur between the sampling orifice and skimmer (and is metal ions are dominant for nickel (although Ni+ is observed and not Ni2+) and strontium (Sr2+) but cluster ions are still therefore a function of the voltage dierence between these two as shown in Fig. 2) or between the skimmer and first ion observed for yttrium.Bare Ni2+ ions were not observed under any of the conditions investigated. When the orifice voltage is optic (and is therefore also a function of the voltage dierence there). increased to 200 V, yttrium is observed predominately as Y2+, YOH2+, Y+ and YO+, all with similar intensities. In contrast Fig. 4 shows mass spectra observed from nickel, strontium and yttrium solutions (1 mM, single element) as a function of to Corr and Anacleto,12 yttrium–water clusters were not observed under these conditions and the dominant strontium the orifice potential with a sampling orifice–skimmer spacing of 1.5 mm (rather than 5.6 mm).In the highly clustered mode ion observed continued to be Sr2+, rather than Sr+, even at orifice voltages up to 300 V. (35 V orifice voltage, 25 V skimmer voltage), the charge state of the metal ion is maintained as in solution, although yttrium Little change in the nature of the mass spectra was observed over a wide range of ion spray voltages applied at the syringe.is observed as charge reduced YOH(H2O)n2+.The mechanisms of charge reduction in the gas phase have been discussed Agnes and Horlick14 reported that the range of electrospray voltages which could be used to obtain observable signals was previously.7,8,9,13,18 As the orifice voltage is increased (to 55 and then 75 V), narrower (+3.8 to +4.1 kV) for aqueous solutions than for methanol solutions (+3 to +6 kV).Furthermore, for any one declustering becomes more extensive, but nickel is observed with a+1 charge (Ni+) or in the+2 state in a charge reduced solution (of a particular conductivity) and liquid flow rate a Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 509in the solution, the spatial distribution of charged drops or the transport of ions from the charged droplets into the gas phase could be aected by the dierence in solution conductivity. Comparison of SCIEX Ion Spray and Meinhard Nebulizers Ikonomou et al.26 reported that electrospray ion signals vanished abruptly below a certain applied voltage because drops were no longer formed. In contrast, ion spray ion signals fell o more slowly and continued to be observed even at low applied voltages because droplets are formed by pneumatic nebulization rather than by formation of a Taylor cone.The applied voltage leads to charging of the pneumatically produced droplets. Light scattering from a 0.95 mW HeNe laser could be observed by eye from the aerosol generated by the Meinhard nebulizer with and without a voltage applied to the solution.In contrast, we were able to observe laser light scattering from the SCIEX IonSpray device only when a voltage was applied. The nature of the aerosol generated by the two devices was also dierent. A narrow line of bright scattering was observed along the SCIEX IonSpray axis, suggesting the presence of larger drops on the axis, as one would expect from electrospray.Light scattering from the aerosol generated by the Meinhard nebulizer (with voltage applied) was more diuse, without a bright narrow line on-axis. We observed a rapid fall o in the Rb+ signal produced using the SCIEX IonSpray at about 2 kV while the signal produced using the Meinhard nebulizer fell rapidly when the voltage was less than 500 V (Fig. 5). When the Meinhard nebulizer was operated without a nebulizer gas flow, the signal rapidly fell at voltages of less than 3.5 kV, which is consistent with the observations of Ikonomou et al.that a higher voltage was necessary to observe ions by electrospray compared to ion spray. We were unable to detect ion signals when the SCIEX IonSpray nebulizer was used without a nebulizer gas flow regardless of the applied voltage. Eect of Nebulizer Position on Ion Signals and Spectra The degree of declustering of the ions observed is even more Fig. 3. Eect of sampling orifice and skimmer voltage (with a fixed critically dependent on the position of the nebulizer relative 10 V dierence) on ion spray mass spectra.The first ion lens voltage was 0 V. Sample contained 5 mM KCl and 1 mM NiCl2 in water. Under to the mass spectrometer (for a fixed set of ion spray, front the conditions of this experiment, the Ni ion species sensitivities were plate, sampler, skimmer and ion optic voltages) than the total much lower than the K ion species sensitivities.Curtain gas flow rate ion signal (Fig. 6). When the tip of the Meinhard nebulizer was 0.076 l min-1. was 14 mm below the front plate and 4.0 mm o axis, the mass spectrum was predominately Sr(H2O)n 2+ ions with values of n from 3 to 9 being the most intense ion signals (Fig. 6a). stable Taylor cone is obtained only over a small range of voltages (e.g. 3.9 to 4.1 kV for a particular sample).25 While When the nebulizer was moved 0.5 mm further o-axis, the spectra were more declustered, with Sr2+, Sr(H2O)2+ and signal intensities varied somewhat for 1 mM strontium chloride solutions, the ratios of observed peaks changed little for Sr2+, Sr(H2O)22+ ions producing the largest signals (Fig. 6b). Charge reduced Sr+, Sr(OH)+ and Sr(OH)(H2O)+ ions that were not Sr(H2O)2+, Sr(H2O)22+ and Sr(H2O)32+ as the ion spray voltage was varied from 3.5 to 7 kV. Sometimes a discharge observed 4.0 mm o-axis, were easily detectable 4.5 mm o- axis. Further declustering was observed as the nebulizer was was formed at ion spray voltages of 8 kV or above.However, at times, ion spray voltages of up to 11 kV could be used moved 5.5 mm o-axis (Fig. 6c). At 7.0 mm o axis no Sr(H2O)n2+ ions were observed, although the Sr2+ signal without severe discharging (indicated by peak broadening, peak splitting and erratic signals). remained strong (Fig. 6d). Sr+ was the dominant charge reduced ion although Sr(OH)+ and Sr(OH)(H2O)+ signals When the set of data shown in Fig. 4 was acquired, a higher voltage (about 6 kV) was necessary to observe signals from remained significant. For each of the species observed, there was an optimum o-axis location for maximum signal. The 1 mM rubidium chloride solution (data not shown) than for 1 mM strontium chloride. Ion spray voltages of 6 to 12 kV location of the optimal locations is also likely to be dependent on the curtain gas flow rate and the sampler–skimmer voltage. could be used without aecting relative rubidium species peak ratios. The conductivity of the rubidium chloride solution was In electrospray, charged drops tend to remain on axis while the small partially evaporated drops and ions are thought to lower (139.8 mS) than that of the strontium chloride solution (223 mS).Because the high voltage was connected to the be driven o-axis by the space charge field.27,28 In ion spray, the situation will be somewhat more complex because drops syringe, there could be some voltage drop between the syringe and the tip of the solution carrying capillary in the ion spray with a range of sizes are produced and dispersed by pneumatic nebulization. The dependence of ion signals on nebulizer nebulizer.Alternatively, the electrophoretic separation of ions 510 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Fig. 4. Eect of sampling orifice voltage on ion spray mass spectra for separate solutions of 1 mM NiCl2, SrCl2 and YCl3 .Skimmer voltage was 25 V in all cases. First ion lens was held at 15 V. Ion spray voltages were 4, 6 and 5 kV, respectively. Nebulizer gas flow rates were 0.4, 0.6 and 0.4 l min-1, respectively. Past a minimum voltage, the nature of the spectra was not aected by increases in ion spray voltage as long as a discharge was not formed. Curtain gas flow rate was 0.20 l min-1. Sampling orifice skimmer distance was 1.5 mm. position may depend on the drop number density and drop set of positions that were between 8 and 9.5 mm o the spray axis and 6 to 14 mm from the nebulizer tip (Fig. 7b). The Sr2+ size distribution as a function of position relative to the nebulizer tip, movement of ions and small drops in the space signal showed a spatial distribution that one might expect at the edge of the aerosol spray (Fig. 7c). The charge reduced charge and applied fields and the kinetics of ion formation (and declustering or clustering) from charged droplets.Sr(OH)+ signal was high under conditions where the Sr2+ existed mainly as declustered ions (Fig. 7d). Undoubtedly, the Sr(H2O)62+, Sr(OH)+ and Sr2+ signal peak heights are shown as a function of sampling position in Fig. 7 for a fixed spatial dependence of the Sr(H2O)62+, Sr(OH)+, Sr2+ signals would be dierent if the sampler–skimmer voltage was changed set of nebulizer (1.0 l min-1) and curtain gas flow rates (0.25 l min-1) and fixed voltages applied to the sample solution to produce more or less clustered ions.However, the spatial dependence of the spectra is also likely to be related to the (4 kV), front plate (650 V), sampling orifice (150 V), skimmer (25 V) and ion optics. The data were obtained by moving the aerosol properties, charge distribution on the drops and subsequent transport of ions produced from drops. nebulizer to a set of horizontal and vertical positions relative to the center of the axis through the ion optics and the front plate.The data were transformed to x, y coordinates relative Comparison of Results on Two Dierent Mass Spectrometers to nebulizer with the y axis along the nebulizer center line and (0,0) at the nebulizer tip (Fig. 7a). Initially we found that bare metal ion spectra were more easily obtained using the three stage MS than the single stage TAGA The Sr(H2O)62+ signal was observed only at a very limited Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 511in the region between the sampling orifice and the skimmer of the Extrel instrument.Although dierent voltages and curtain gas flow rates were required on the two instruments, similar mass spectra can be obtained in a bare metal ion mode, ion cluster mode and intermediate mode. The dierences in location of the nebulizer tip relative to the front plate of the instruments could also produce what would appear to be instrument dependent behavior. Spectral overlaps can create diculties in interpreting mass spectra even with relatively simple samples, particularly because the ions detected can be a strong function of the applied voltages and sampling position relative to the nebulizer tip.A triple quadrupole mass spectrometer can therefore be Fig. 5. Rb+ signal as a function of voltage applied to the sample advantageous during both fundamental studies and for practi- solution observed using a Meinhard nebulizer (with gas flow rates of cal sample analysis.Figs. 8aand 8b show mass spectra obtained 1.0 or 0 l min-1) and a SCIEX IonSpray nebulizer (with a gas flow using the TAGA instrument in the single quadrupole mode rate of 0.6 l min-1). The tip of the SCIEX IonSpray nebulizer was about 1 cm directly below the front plate, oriented at an angle of 30° with dierent orifice voltages. Peaks at m/z of 23, 41, and 59 relative to the vertical axis through the sampler, skimmer and ion were observed when 50 or 100 V orifice potentials were used.optics. The Meinhard nebulizer was about 2 cm directly below the However, product spectra show that the peak observed at m/z front plate, spraying at an angle of 15°. The sampler and skimmer 59 in the single quadrupole mode is due only to Co+ at an voltages were 100 and 25 V, respectively. The curtain gas flow rate orifice voltage of 100 V (Fig. 8c), but is due to a combination was 0.25 l min-1. of Co+ and Na(H2O)+ when an orifice voltage of 50 V was used (Fig. 8d). instrument. Conversely, cluster ion spectra were more easily observed using the TAGA. However, initial experiments using the three stage MS were carried out with the skimmer Metal–ligand complexation grounded. Once a small voltage was applied to the skimmer, highly clustered ions could also be observed using the three Electrospray and ion spray mass spectrometry have the potentialto investigate metal–ligand complexation in aqueous– stage MS. It is not clear why this occurs.In some cases we were able to obtain positive ion spectra even when the skimmer organic and mixed aqueous–organic solvents without prior separation, if the ions in solution can be transferred to the gas was at a more positive potential than the sampling orifice or when the first ion optic was at a more positive potential than phase without changing the speciation. Some workers have proposed the use of electrospray mass spectrometry for total the skimmer. In general, higher orifice voltages and curtain gas flow rates elemental analysis as an alternative to ICP-MS by using harsh conditions that produce bare metal ion spectra.29 were needed to obtain bare metal ion spectra using the single stage TAGA instrument than with the three stage Extrel Fig. 9 shows the ion spray mass spectrum acquired from a mixture of lead nitrate and sodium acetate. Peaks were instrument, perhaps because significant declustering is gained Fig. 6. Ion spray spectra obtained at various Meinhard nebulizer positions.The nebulizer was 14 mm below the front plate. The nebulizer was pointed away from the orifice at an angle of 24°. a, Nebulizer 4.0 mm o the axis through the ion optics; b, nebulizer 4.5 mm o axis; c, nebulizer 5.5 mm o axis; and d, nebulizer 7.0 mm o-axis. Sampler and skimmer voltages were 150 and 25 V, respectively. Curtain gas flow rate was 0.25 l min-1. The first ion lens voltage was -15 V. 512 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Sr (H2O)6 2+ Sr (OH)+ Sr2+ a b d c Sampling ref. pt. X (mm to neb. center line) X (mm from neb. axis) X (mm from neb. axis) Y ( m m f r o m n e b . t i p ) Y (mm downstream of neb.tip) 30 27 24 21 18 15 12 9 6 3 0 22 20 18 16 14 12 10 8 6 22 20 18 16 14 12 10 8 6 22 20 18 16 14 12 10 8 6 100% 0 -18 -15 -12 -9 -6 -3 -18 -16 -14 -12 -10 -8 -6 -18 -16 -14 -12 -10 -8 -6 -18 -16 -14 -12 -10 -8 -6 Fig. 7. Contour plots of Sr2+, Sr(H2O)62+ and Sr(OH)+ signal magnitudes as a function of the sampling position.a, Locations of the center of the front plate orifice relative to the Meinhard nebulizer where data were obtained; b, Sr(H2O)62+ signal magnitude; c, Sr2+ signal magnitude; and d, Sr(OH)+ signal magnitude. Parallelogram indicates region where data were acquired. observed due to Pb+, Pb(OH)+ and Pb(C2H3O2)+. These ively using the Extrel MS. Because these experiments were carried out with a grounded skimmer, a relatively high orifice data suggest that metal–ligand complexation could be investigated by ion spray MS.However, there are some potential voltage (150 V) was used and only bare metal spectra were observed under these conditions. Negative ion spectra were complications. First, sensitivity can be highly species dependent. Therefore, the sensitivity of each species would need to not obtained to look for negatively charged Ni–EDTA complexes. be experimentally calibrated. Secondly, the collision induced dissociation process used to decluster ions might also convert Solutions of fixed total Ni concentration and a fixed, excess K concentration (to maintain a constant conductivity) were metal–ligand complexes into free metal ions and ligand ions, if the metal–ligand complex bond was weak.used while varying the amount of EDTA added. The Ni+ peak magnitude decreased as EDTA was added to the solution Isotope ratio measurements are complicated by poor resolution on our instruments, possibly due to a large spread of while the K+ signal did not.The agreement between the theoretical response, assuming the Ni+ signal is produced only ion kinetic energies, and the propensity for the formation of PbH+ ions. The use of collisional focussing30 following the from free Ni2+ (but not from collisional dissociation of nickel EDTA complexes) in solution, and the experimentally acquired skimmer has been reported to significantly improve both mass spectral resolution and sensitivities.The resolution we obtained Ni+ peak signal data is excellent (Fig. 10). This was true even for harsh ion generation conditions where the only positive Ni was at least a factor of two to four poorer than that reported by Corr and Anacleto.12 containing ion was Ni+. At least for strongly bound metal– ligand complexes, quantitative speciation measurements can The complexation of Ni and EDTA was studied quantitat- Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 513Fig. 8. Ion spray mass spectra from 1 mM CoNO3 and 1.25 mM Na(C2H3O2) in water obtained using the SCIEX TAGA 6000E instrument; a, single quadrupole spectrum with 50 V orifice; b, single quadrupole mode spectrum with 100 V orifice; c, product spectrum, with first quadrupole set to m/z 59, with 50 V orifice; and d, product spectrum, with first quadrupole set to m/z 59, with 100 V orifice. Curtain gas flow rate was 0.86 l min-1. ICP-MS. A minimum sample conductivity is necessary for electrospray31,32 but sensitivity decreases if the solution conductivity is too high.33,34 Agnes and Horlick15 discussed addition of a ‘stabilizer’ and the use of internal standardization to establish linear calibration curves.A set of analytes, each at a concentration of 1 mM in a single aqueous solution were chosen so that the sample contained +1,+2 and +3 ions. Rather harsh conditions were used with a grounded skimmer. Potassium was observed as K+.Nickel, present in solution in the +2 charge state was charge reduced to Ni+ as the predominant ion. Barium, present in solution in the +2 charge state, was observed as Ba2+ and Ba+ in the ion spray mass spectrum. Yttrium, a +3 ion in solution, was Fig. 9. Ion spray mass spectrum of 1 mM PbNO3 and 1.24 mM observed as Y+ and YO+. Na(C2H3O2) obtained using the SCIEX TAGA 6000E instrument. Acid is often added to samples prior to storage to stabilize Orifice voltage was 100V.Curtain gas flow rate was 0.68 l min-1. the sample and to minimize loss of sample ions to the walls of the container. However, the presence of high concentrations of acid, has deleterious eects on ion spray signals. Fig. 11a shows the eect of HCl concentration on ion spray signals. Each of the ion signals observed decreased by more than 35% in the presence of 0.01% v/v (0.0012 M) HCl. Fig. 11b shows the eect of NaCl concentration on the detected ion spray signal for the dominant ions observed for each of the species in the sample.The ion spray signal is depressed by more than 20% by the presence of 0.001 M NaCl. The signals are depressed by more than 90% when 0.1 M NaCl is present in the sample. Because the signal for each of the ions decreased similarly to a first approximation, use of an internal standard could reduce potential analysis errors. Fig. 10. Ni+5K+ signal ratio versus percent equivalent EDTA added. Total Ni concentration was 1 mM, K concentration was 5 mM.Orifice However, detection limits would still be degraded due to the voltage was 150 V. Skimmer was grounded. Curtain gas flow rate was loss of sensitivity. 0.15 l min-1. The eects of NaCl and HCl did not have a strong analyte species dependence, to a first approximation. Considering that K+ is formed by direct transfer of the ion from solution and be made by ion spray MS, in agreement with previous reports.6 solvent declustering while the Ba+ , Y+, YO+ ions would This also shows that the Ni+ ions are not produced from the require further steps involving charge reduction, it is somewhat Ni–EDTA complexes, so the bare metal ion mode cannot be surprising that the matrix eects do not show a stronger viewed as total elemental analysis as would be the case dependence on the ion observed.However,further investigation for ICP-MS. is required to determine if the matrix eects are more species dependent under milder declustering conditions.Matrix eects In general, HCl caused a somewhat more severe matrix eect than NaCl for similar added concentrations. This is not Quantitation in electrospray and ion spray mass spectrometry appears to be more complex and matrix dependent than surprising since the conductivity of HCl solutions is higher 514 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Potential errors in quantitative analysis by electrospray or ion spray mass spectrometry may be severe compared to elemental analysis methods, such as ICP-MS.Further investigation of the origin of the matrix eects and potential means to overcome or minimize their eect on analysis accuracy is needed. This research was supported by the National Science Foundation (CHE-9217170). Donald Kenny (Battelle Memorial Institute, Columbus, OH, USA) is thanked for use of the TAGA instrument and many useful discussions. Jay Corr (SCIEX) is acknowledged for providing the IonSpray nebulizer and useful discussions regarding this work.Ian Stewart is thanked for his comments on this work. REFERENCES 1 Yamashita, M., and Fenn, J. B., J. Phys. Chem., 1984, 88, 4451. 2 Yamashita, M., and Fenn, J. B., J. Phys. Chem., 1984, 88, 4671. 3 Kebarle, P., and Tang, L., Anal. Chem., 1993, 65, 972A. 4 Iribarne, J. V., Dziedzic, P. J., and Thomson, B. A., Int. J. Mass Spectrom. Ion Processes, 1983, 50, 331. 5 Smith, R. D., Barinaga, C. J., and Udseth, H. R., Anal. Chem., 1988, 60, 1948. 6 Colton, R., D’Agostino A., and Traeger, J.C., Mass Spectrom. Rev., 1995, 14, 79. 7 Jayawiera, P., Blades, A. T., Ikonomou, M. G., and Kebarle, P., J. Am. Chem. Soc., 1990, 112, 2452. Fig. 11. a, Eect of HCl on ion spray mass spectrometry signals; b, 8 Blades, A. T., Jayaweera, P., Ikonomou, M. G., and Kebarle, P., eect of NaCl on ion spray mass spectrometry signals. Orifice voltage J. Chem. Phys., 1990, 92, 5900. was 150 V. Skimmer was grounded. Curtain gas flow rate was 9 Blades, A.T., Jayaweera, P., Ikonomou, M. G., and Kebarle, P., 0.15 l min-1. All signals were normalized to values of 100 with no Int. J. Mass Spectrom. Ion Processes, 1990, 102, 251. NaCl or HCl added. Measured solution conductivities are shown in mS. 10 Douglas, D. J., in Inductively Coupled Plasmas in Analytical Atomic Spectrometry, ed. Montaser, A., and Golightly, D. W., VCH (New York), 2nd edn., 1992, pp. 642–646. than for similar concentrations of NaCl.Tang and Kebarle 11 Corr, J. J., and Douglas, D. J., presented at the 41st ASMS have discussed the eect of solution conductivity on electro- Conference on Mass Spectrometry and Applied T opics, San spray current and concluded that the addition of electrolytes Francisco, 1993, p. 202. other than the analyte will always decrease the analyte 12 Corr, J., and Anacleto, J. F., Anal. Chem., 1996, 68, 2155. sensitivity.33 13 Stewart, I. I., and Horlick, G., T rAC, T rends in Anal.Chem. (Pers. Ed.), 1996, 15, 80. 14 Agnes, G. R., and Horlick G., Appl. Spectrosc., 1992, 46, 401. CONCLUSIONS 15 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1994, 48, 649. 16 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1994, 48, 655. Our initial investigation shows that ion spray mass spec- 17 Agnes, G. R., Stewart, I. I., and Horlick, G., Appl. Spectrosc., trometry is a promising technique for elemental speciation in 1994, 48, 1347. aqueous samples. A wide range of ion spray voltages could be 18 Agnes, G.R., and Horlick, G., Appl. Spectrosc., 1995, 49, 324. 19 Stewart, I. I., and Horlick, G., Anal. Chem., 1994, 66, 3983. used to produce observable mass spectra without formation of 20 Cheng, Z. L., Siu, K. W. M., Guevremont, R., and Berman, S. S., severe discharges, in contrast to previous reports using ES-MS J. Am. Soc. Mass Spectrom., 1991, 3, 281. for aqueous solutions. 21 Blades, A. T., Jayaweera, P., Ikonomou, M. G., and Kebarle, P., Control of declustering and charge reduction is qualitatively Int.J. Mass Spectrom. Ion Processes, 1990, 101, 325. similar to that previously discussed for inorganic ions in 22 Katta, V., Chowdhury, S. K., and Chait, B. T., J. Am. Chem. Soc., organic solvents. Similar spectra (highly clustered, intermediate 1990, 112, 5348. 23 Colton, R., James, B. D., Potter, I. D., and Traeger, J. C., Inorg. and bare metal ion) could be obtained using a three stage, Chem., 1993, 32, 2626. dierentially pumped Extrel mass spectrometer and a single 24 Bruins, A. P., Covey, T. R., and Henion, J. D., Anal. Chem., 1987, stage TAGA 6000 E mass spectrometer, although specific 59, 2642. voltages required may be dierent. 25 Stewart, I. I., Ph. D. Thesis, University of Alberta, 1995. The role of sampling position on the spectra obtained 26 Ikonomou, M. G., Blades, A. T. and Kebarle, P., Anal. Chem., warrants further investigation. Experiments to correlateaerosol 1991, 63, 1989. 27 Tang, K., and Gomez, A., Phys. Fluids 1994, 6, 2317. properties with the signal magnitude and extent of declustering 28 Hiraoka, K., Murata, K., and Kudaka, I., Rapid Commun. Mass and charge reduction are needed. Spectrom., 1993, 7, 363. Quantitative metal–ligand complexation measurements in 29 Brown, F. B., Olson, L. K., and Caruso, J. A., J. Anal. At. aqueous solutions can be made using ion spray mass spec- Spectrom., 1996, 11, 633. trometry. The observed bare metal ion signal is related to the 30 Douglas, D. J., and French, J. B., J. Am. Soc. Mass Spectrom., 1992, 3, 398. concentration of uncomplexed ion in solution, at least for a 31 Ikonomou, M. G., Blades, A. T., and Kebarle, P., Anal. Chem., strongly bound metal ligand complex. Further investigation of 1991, 65, 3654. ion spray mass spectrometry for metal–ligand complexation 32 Blades, A. T., Ikonomou, M. G., and Kebarle, P., Anal. Chem., measurements is needed. 1991, 63, 109. Care must be exercised if one attempts to use electrospray 33 Tang. L., and Kebarle, P., Anal. Chem., 1991, 63, 2709. or ion spray mass spectrometry for elemental analysis in a 34 Tang, L., and Kebarle, P., Anal. Chem., 1993, 65, 3654. way analogous to ICP-MS. The electrospray or ion spray mass spectrometry signal will not necessarily be related to the Paper 6/06420E total elemental concentration of the metal, even if a bare metal Received September 17, 1996 AcceptedMarch 25, 1997 ion spectrum is obtained. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 515

ISSN:0267-9477    

DOI:10.1039/a606420e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Electrospray Mass Spectrometry as a Technique for the Elemental Analysis of Metals and Organometals GRACE ZOOROB†, FRANCINE BYRDY BROWN AND JOSEPH CARUSO* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, USA. Email: joseph.caruso@uc.edu An investigation of an in-house constructed electrospray (ES) AES allows multi-elemental analysis, and the detection limits are improved when ICP-MS is employed. Unfortunately, none ion source and interface has been carried out with the focus on elemental analysis. An evaluation of the 29, 30 and 31 gauge of these techniques has the capability to analyze the metal and the organic species at the same time in its native form.capillary sizes for the ES source is presented. For multielement solutions of alkali metals, alkaline earth metals and Theoretically, this can be achieved by ES-MS, where one can detect both the metal and the organic complex. transition metals studied in the bare metal ion mode, the smaller 30 gauge capillary improved the detection limits by an Porphyrins are a set of moderate molecular weight macrocyclic tetrapyrroles amenable to analysis by the ES-MS order of magnitude over those of the larger 29 gauge.The 31 gauge was too fragile for easy handling. The ion-cluster modes technique. Qualitative and quantitative identification and characterization of porphyrins, whether they be new synthetic of CrII, CrIII and CoII were investigated and the resulting spectra indicate the possibility of attaining speciation compounds (i.e., those used in tumor localization and treatment), 28 or those that occur naturally in biological systems29,30 information with this set-up.Matrix eects on the analyte signal of a multielement solution of transition metals using the and fossil fuels (i.e., geoporphyrins)31 is desirable. Van Berkel and co-workers used ES with ion trap mass spectrometry for 29 and 30 gauge capillaries are discussed.The same trend is observed with both capillaries where the signal intensity the analysis of a wide range of porphyrins.32 In their study, a single molecular ionic species was observed. Also, the free improves with the addition of 0.15% and 0.30% total dissolved solids. At higher concentrations of total dissolved solids signal bases of porphyrins were detected as the protonated molecules, (M+H)+, the trivalent metalloporphyrins as (M-L)+, where suppression is seen. Additionally, ES-MS was demonstrated as an elemental analysis tool for the study of the bare metal ion L- is the counter ion, and the divalent metalloporphyrins as MV+, (M+H)+ and (M+Na)+, depending on the metal mode of larger organometallics, mainly organolead compounds and metalloporphyrins.complex and the composition of the electrospray solvent. However, they were unable to induce sucient fragmentation Keywords: Electrospray mass spectrometry; elemental in the collision-induced dissociation to detect the elemental analysis; organometallics form of the metal.This study investigates some quantitative aspects of ES-MS for the elemental analysis of organometallics. Initial work Electrospray mass spectrometry (ES-MS) is a well-established, powerful technique for the analysis of organic compounds. included instrumental modification to the electrospray capillary in order to improve signal intensity as well as day-to-day The focus has mainly been on the analysis of proteins and amino acids.However, the technique is not limited to this field signal reproducibility of a multi-element solution. In addition, a study of matrix eects on a multi-element solution is dis- of study and has been applied successfully to a variety of metal ion species. Recently, it has been shown that ES-MS can be cussed, followed by a brief study of metalloporphyrins and organometallics. used to quantitatively determine trace levels of both elemental and molecular inorganic cations and anions.ES-MS has been applied to the generation and study of metal–ion molecule EXPERIMENTAL clusters,1–3 to the direct determination of solution components4,5 and for elemental analysis.6–9 Blades, Kebarle, et al. Electrospray Source pioneered the area of ES-MS for elemental analysis with their A schematic diagram for the electrospray source and interface investigations of M2+ and M3+ ions.1,2 Earlier attempts by that were previously designed and constructed in our labora- other investigators in this area were unsuccessful10,11 or went tory by Byrdy et al.33 is shown in Fig. 1. Sample solution was unreported.9 Presently, there are a number of reports on the pumped through a stainless steel capillary at a flow rate of determination of uncomplexed metal ions in solution in dier- 1–10 ml min-1 with a syringe pump (Sage Instruments, Model ent solvents.1–5,8,9 Numerous reports utilizing ES-MS for speci- 341 B, Fisher Scientific, Fairlawn, NJ, USA).A glass sample ation-related investigations have appeared in the literature. syringe (1 ml, Hamilton Gastight 1001 with a removable 22 They include analysis by ES-MS of inorganic species,6,7,9,12–14 gauge needle) or a disposable plastic sample syringe (5 ml, inorganic complexes,1–3,15 metal ion coordination in bio- Hamilton, with a 20 gauge needle) were connected with peristal- organic molecules,16–21 anticancer drugs,22 tagging of neutral tic pump tubing (Fisher Scientific, 0.025 mm od and 0.020 mm organic molecules with metal ions,23 phosphopeptides,24 pesticid) to the electrospray capillary. ides and parasiticides,25,26 and the lanthanides.27 The original capillary was made using hypodermic stainless The metal content of organometallic compounds is usually steel tubing where a piece of smaller bore tubing (HTX-29-24 analyzed by atomic absorption spectrophotometry (AAS) or Type 304 s/s 29 gauge from Small Parts Inc., Miami Lakes, inductively coupled plasma atomic emission spectrometry FL, USA, 0.330 mm od and 0.178 mm id) was soldered inside (ICP-AES). Each element of interest has to be analyzed a piece of larger bore tubing (HTX-17 Type 316 s/s 17 gauge individually in AAS, and it is usually time-consuming. ICPwith 1.47 mm od, 1.07 id and a length of 6 in).The larger bore tube was used to obtain a more rigid structure upon which to † Present Address: A274 ASTeCC Building, College of Pharmacy, University of Kentucky, Lexington, KY 40506-0286, USA.make an electrical connection. For this study, two other smaller Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (517–524) 517Reagents A variety of inorganic ions were investigated. Stock standards (1×10-1 M) were prepared by dissolving the following metal salts in distilled, de-ionized 18 Megaohm water (Barnstead, Boston, MA, USA): KCl (MC/B, Norwood, OH, USA), RbCl (99+%, Aldrich, Milwaukee, WI), CsCl (MC/B), MgCl2·6H2O (ACS reagent, Fisher, Fairlawn, NJ, USA), CaCl2·2H2O (99.999%, Aldrich), VO(SO4·3H2O (Aldrich), CdCl2 ·2c H2O (Aldrich), Cr(NO3 )3 ·9H2O (MC/B), NiCl2·6H2O (J.T. Baker, Phillipsburg, NJ, USA), Co(NO3)3·6H2O (Fisher), Cu(NO3 )2 ·3H2O (Fisher), Zn(NO3)2·6H2O (Fisher), BaCl2 (Fisher) and UO2(NO3)2·6H2O (Fluka, Ronkonkoma, NY, USA), CrIII acetylacetonate (Aldrich) and CrII acetylacetonate (Aldrich). In addition, Pb(NO3 )2 (Fisher), (butyl)3PbOAc (Alfa Inorganic Venton, Beverly, MA, USA), (Ph)3PbCl (Alfa Inorganic Venton), (propyl)3PbOAc, (Alfa Inorganic Venton) and (isobutyl)3PbOAc (Alfa Inorganic Venton) were used.The following metalloporphyrins were obtained from Porphyrin Products (Logan, UT, USA): cobalt protoporphyrin IX chloride (CoPP), ferriprotoporphyrin IX chloride (commonly known as hemin), and manganese protoporphyrin chloride (MnPP). Further dilutions were made using HPLC-grade Fig. 1 Schematic diagram of the ES capillary and interface.methanol (Fisher). Standard solutions were prepared in the concentration range from 10-7 to 10-3 M. A lead standard reference material in fuel was obtained from the National Institute of Standards and Technology (NIST) (Gaithersburg, capillaries were used where the smaller bore tubings were MD, USA), Standard Reference Material (SRM 2715). HTX-30-24 Type 304 s/s 30 gauge, 0.305 mm od and 0.152 mm id and HTX-31-24 Type 304 s/s 31 gauge, 0.254 mm od and Synthetic Ocean Water Matrix Study 0.127 mm id from Small Parts Inc.The larger bore supporting capillary was the same in all cases. The capillary was positioned The eects of synthetic ocean water on signal intensities were 8 mm away from the MS front plate and extended 5 cm out investigated by ES-MS. The eect of ocean water was calcu- from the capillary holder. The diameters and spacings of the lated as a suppression/enhancement factor (eqn. 1): interface components are given in the paper by Byrdy et al.33 Suppression/enhancement factor The nitrogen curtain gas was used at a flow rate of 0.1–0.5 l min-1 and was controlled using a mass flow controller =net signal in matrix solution net signal in methanol only (1) (Tylan RO-28, San Diego, CA).The curtain gas influences the rate of evaporation of the charged droplets and aids in declustering and desolvation of the ion–solvent clusters.34,35 Multi-element solutions that were 10-5 M in several elements (Ba, Zn, Cu, U, Cd, Cr, Co, V and Ni) were evaluated in The sampling plate was used to control the extent of solvent declustering in the collision induced dissociation region methanol as well as in synthetic ocean water (SOW).The SOW standards were made up to produce final percentage (between the sampling plate and the skimmer), and the potential dierence between the sampling plate and the skimmer total dissolved solids of 0, 0.15, 0.30, 0.45 and 0.60%. The composition of SOW stock solution is shown in Table 1.controls the collisional energy of the ions.34 Voltages were applied to the capillary, front plate, curtain gas plate and skimmer using 4 dierent supplies. A Varian Data Collection Model 921-0067 (6 kV maximum) supply was used for the capillary tip. To monitor the current at the capillary tip, an The mass range investigated was 8–240 amu. No skip regions in-house constructed ammeter was used (2 mA maximum) and were defined.The dwell time was 320 ms, with 100 sweeps and this was useful for detecting the onset of a corona discharge 2048 channels being used. The acquisition time was 65.5 s. at the capillary tip. The front plate utilized a Keithley Experiments were run in the positive ion mode, where the (Cleveland, OH, USA) 247 HV supply (2999 V maximum). For potential dierence was increasingly more negative from the the sampling plate, a Palo Alto, California Model 80–375 HV capillary tip to the skimmer.It was not possible to monitor supply was employed (2012 V maximum).The skimmer utilized negative ions with the present mass spectrometer and the a dc power supply, BK Precision (Chicago, IL, USA), Model highest detectable mass was 240 units. 1635 (±30 V maximum). All calculations were based on three runs except for the standards used to obtain reproducibility information. Those standards were each run eight times. The mass spectral data Mass Spectrometric Parameters The mass spectrometer used was a VG PlasmaQuad I, Table 1 The composition of synthetic ocean water stock solution ICP-MS, which is a quadrupole instrument with unit mass Compound % (m/m) resolution.Since this instrument was designed for ICP-MS analysis the mass range was limited to 240 units. Because the NaCl 2.11 MgCl2 0.41 instrument used for this work was originally an ICP instru- CaCl2 0.12 ment, the photon stop, which was used to prevent light from KCl 0.08 passing to the electron multiplier, was removed for the ES HCl 0.40 work.This has significantly improved the signal intensity for H2SO4 0.20 the instrument with the ES interface. 518 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12were transferred to an in-house computer program so that that obtainable if the lenses were reset to maximize the signal for uranium alone. Thus, the mass bias eect should be results could be easily imported into Lotus 123. recognized. Additionally, the optimum solution flow rate is lower than that of the transition metals, giving smaller droplets and an improvement in ionization and desolvation eciences, RESULTS AND DISCUSSION which achieves an overall increase in the eciency of the Optimization of Electrospray Operating Parameters electrospray process.Improvement of the electrospray signal by lower flow rate has been reported in the literature.37,38 There are three major processes in ES-MS: the production of charged droplets from electrolyte dissolved in solvent, the production of gas phase ions from charged droplets, and ion Investigation of the Ion Cluster Mode sampling and mass analysis.These processes have been discussed in detail elsewhere.5,6,9 Agnes and Horlick have investigated the qualitative and quantitative aspects of ES-MS for elemental analysis, including The ES source and interface conditions exert a strong influence on the nature of the resulting mass spectra.The the typical ion species which could be observed in the three positive ion modes: the ion cluster mode; the intermediate conditions used for these experiments allowed the detection of all ions as singly charged atomic ions (M+) regardless of their mode; and the bare metal ion mode.6 They studied the ion cluster mode and used an electric field gradient of 16000 V m-1 charge state in solution. The electrospray conditions for the bare metal ion mode to reduce the energy deposited per collision, so that the level of fragmentation is reduced in the collision-induced dis- were optimized for 3 sets of 10-4 M standards of alkali and alkaline earth metals, transition metals, and uranium, respect- sociation region.Although the bare metal ion mode was the most important for elemental analysis in this work, it was ively, using the 29 gauge capillary. In the bare metal ion mode, higher voltages for the capillary, sampling plate and skimmer, interesting to investigate other modes of the in-house constructed ES interface.Furthermore, the conditions used for as well as higher curtain gas flow rates, are used. These conditions will allow for adequate declustering of the solvent these experiments allowed the detection of all ions as singly charged atomic ions (M+) regardless of their charge state ions so that the bare-metal ion can be detected. The following were the optimal conditions for the first set of elements studied, in solution.In ES-MS, the source and interface conditions employed by i.e., the alkali and alkaline earth metals: ES capillary (+5.0 kV), front plate (+842 V), sampling plate (+330 V), and skimmer ES-MS are very important as they influence the nature of the resulting mass spectrum.6,7,33 A solution of 10-4 M of CrIII (-23 V). The curtain gas flow rate was maintained at 350 ml min-1, and a solution flow rate of 5 ml min-1 was used. acylacetonate was studied to investigate the ion cluster mode.‘Softer’ conditions are necessary to attain the clusters in the The signal was optimized at Rb (m/z 85). These conditions were similar to those employed by Byrdy et al.33 ion cluster mode. The most important parameters that promote detection of clusters for this mode are the capillary voltage, The electrospray process and the process of generating singly charged metal ions as solvated electrosprayed ion clusters are sampling plate voltage, skimmer voltage and curtain gas flow rate.The ES capillary was held at 4.5 kV. The front plate, not the same for singly and doubly charged solution ions.2,5,6 A divalent solution ion will retain its doubly charged state sampling plate and skimmer voltages were 950, 210 and -4 V, respectively. The electric field gradient necessary for the ion when the solution is sprayed. However, as a result of solvent evaporation and insucient solvation energy, charge reduction cluster mode for this ES interface was 53500 V m-1, which was decreased from 102 000 V m-1 for the bare metal ion of the metal occurs.2,6 Therefore, to detect de-clustered singly charged metal ions in the bare metal ion mode slightly harsher mode.The optimum solution flow rate was 0.8 ml min-1, which improved the electrospray process, as discussed previously. conditions were employed for the transition metals. For adequate de-clustering in the collision-induced dissociation The curtain gas flow rate was 350 ml min-1.The bare metal ion and the ion cluster modes for chromium are presented in (CID) region located between the sampling plate and the skimmer, an electric field gradient in the range of 38000 to Fig. 2. In Fig. 2(a) the bare metal ion mode is seen. In addition, a peak at m/z 64 corresponding to (CH3OH)2+ is shown. In 95000 V m-1 can be employed.33,34,36 The optimum operating conditions for the transition metals were as follows: ES capil- this mode, parameters that induce declustering are favored and thus higher capillary tip, sampling plate and skimmer lary (+5.7 kV), front plate (+950 V), sampling plate (+384 V), skimmer (-24 V).The solution flow rate was 5 ml min-1 and voltages, as well as higher curtain gas flow rate, are used. Fig. 2(b) presents an ion cluster mode where the bare metal, the curtain gas flow rate used was 470 ml min-1. The signal was optimized at Cr (m/z 52). The electric field gradient was and the metal complexed to a (COH) group, possibly a portion of the original acetate molecule, are observed. Another set of 102 000 V m-1.These parameters were similar to those employed by Byrdy et al.33 ion clusters was observed as CrMeOH(H2O)2+ when the curtain gas flow rate was decreased to 200 ml min-1 [Fig. 2(c)], Uranium, m/z 238, lies on the higher mass range of the mass spectrometer employed for this study. It was of interest to which shows that when the curtain gas flow rate decreases, more solvent clusters are observed in the spectra.The flow improve its signal in the third set of experiments. When a uranium solution (10-4 M) was studied in a multi-element rate of the curtain gas influences the rate of evaporation of the charged droplets and helps in declustering and desolvation of solution under the same conditions as the transition metals, the uranium signal was 230 counts (peak height) in 30 ms. the ion clusters. The ES-MS spectra of a 10-4 M solution of CrII is shown in When a 10-4 M solution of uranium only was run, the optimum conditions were similar to the transition metals, but the signal Fig. 3 and was run under the same conditions as those of a 10-4 M solution of CrIII [Fig. 2(b)]. Under the same conditions, improved to 1400 counts (peak height) in 30 ms at m/z 238. The dierences between the optimum conditions for the trans- the ion clusters formed from both solutions were not the same. Although the bare metal ion was still the most intense in the ition metals and uranium are the ion optics settings and the solution flow rate of 0.8 ml min-1. A possible explanation is spectra, the ion clusters that were detected for CrII and CrIII were dierent.Fig. 3 shows the clusters of CrII as CrO+, that in ICP-MS instruments, the ion lens voltages necessary to yield a maximum signal for lighter elements may be dierent Cr(MeOH)+, Cr(MeOH)2+ and Cr(MeOH)3+. This result was interesting since speciation of solution components was from the optimum voltages for heavier ones.In these experiments, the signal is optimized on Rb+. The transmission for observed in the spectra. However, considerable identification, study and development of spectra resulting from various ES U+ is sacrificed in a multi-element solution compared with Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 519Fig. 2 Two modes for chromium(III) (10-4 M): (a), bare metal ion mode using ES capillary (+5.7 kV), front plate (+950 V), sampling plate (+384 V), skimmer (-24 V), curtain gas flow rate (470 ml min-1); (b), ion cluster mode using ES capillary (+4.5 kV), front plate (+950 V), sampling plate (210 V), skimmer (-4 V), curtain gas flow rate (350 ml min-1); (c), ion cluster mode at curtain gas flow rate of 200 ml min-1 ; other conditions as in (b).Fig. 3 Ion cluster mode for CrII at 10-4 M. ES conditions are similar to those of Figure 2(b). conditions and development of ES methodology will be required to ensure further reliable and accurate results. On the one hand, an ES ionization source that yields bare singly charged metal ions would be favorable, and it remains to be seen whether ES conditions can be adjusted to achieve this goal.On the other hand, it is desirable to adjust the ES ionization conditions to yield speciation as well as elemental analysis information. At this point, ES ionization for elemental Fig. 4 Cluster ion mode for 10-4 M solution of Co.(a), ES capillary speciation studies is still in its infancy and it is dicult to use 4.5 kV, front plate 950 V, sampling plate 210 V, skimmer-4 V, curtain it for speciation and elemental analysis studies of real samples. gas flow rate 350 ml min-1. (b), same conditions as (a), except curtain gas flow rate is decreased to 200 ml min-1. Complex samples will have other constituents present, which will significantly complicate obtaining and interpreting the spectra, not to mention the complexities introduced with additional competing equilibria.stable current, which produces a ‘better’ spray, and improves The ion cluster mode of cobalt was investigated. Cobalt signal intensity and sensitivity, two smaller capillary sizes (30 clusters were seen at the same conditions as those for chromium gauge and 31 gauge) were investigated to examine their eect in Fig. 3: ES capillary 4.5 kV, front plate 950 V, sampling plate on current stability. The larger bore tubing was the same 210 V, skimmer -4 V, curtain gas flow rate 350 ml min-1 as before.(Fig. 4). Clusters of Co were detected as CoH+, CoO+, and A smaller capillary, 30 gauge, was investigated. Multi- CoOH+ in Fig. 4(a). As mentioned earlier, the curtain gas flow element standard solutions were run with concentrations rangrate played an important role in the declustering process. At ing from 1×10-7 to 5×10-4 M. The monitored current was softer declustering conditions, i.e., lower curtain gas flow rates, much more stable than that previously observed with the more clusters were apparent in the spectra.When the curtain larger bore capillary. The current fluctuated by ±0.05 mA gas flow rate was decreased from 350 to 200 ml min-1 and instead of ±0.5 mA. The background signal was identical with keeping other parameters identical with those in Fig. 4(a), an that of the 29 gauge capillary. The sample flow rate was similar additional peak at m/z 91 appeared, representing the detection in both capillaries.The optimized conditions used for the of a CoMeOH+ cluster [Fig. 4(b)]. alkali, alkaline earth and transition metals with the 30 gauge capillary are presented in Table 2. Figures of merit of the same elements with the 30 gauge capillary are presented in Table 3. Investigation of Capillary Sizes The calibration curves were linear. The linear dynamic range extended over 3 orders of magnitude for V, Zn, and U, and The monitored current was unstable with the 29gauge capillary and was fluctuating about ±0.5 mA from the desired setting over 4 orders of magnitude for all other metals in this study.The detection limits were calculated as 3s in all cases. The during the experiment. Since it was desirable to attain a more 520 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Table 2 Optimum conditions for the alkali, alkaline earth and trans- where H is a constant (that can be evaluated and depends on ition metals with the 30 gauge capillary the dielectric constant and surface tension of the solvent, the flow rate, and the electric field at the capillary tip) and n= Alkali and alkali 0.2–0.4. This shows that the ES current is a function of the earth metals Transition metals physical properties of the solution as well as its conductivity.ES capillary +5.4 kV +5.5 kV The eects caused by high total dissolved solids on the Front plate +874 V +1056 V analyte signal in ES-MS, which are probably the more complex and less well understood matrix eects, were investigated.Two Sampling plate +384 V +386 V capillary sizes (29 and 30 gauge) were used and a similar trend Skimmer +14.7 V +31.5 V was observed with both capillaries. The results are given in Curtain gas flow rate 380 ml min-1 510 ml min-1 Table 4 and Table 5 for the 29 gauge and 30 gauge capillaries, respectively. The background signal was the same with both capillaries.When 0.15–0.30% of SOW was added to the multi- Table 3 Figures of merit for a multi-element solution with the 30 element solution, signal enhancement for most elements was gauge capillary seen. Signal enhancement may be attributed to increased solution conductivity and more ‘favorable’ physical character- Rb Cs V Cr Ni istics of the solution that allow the formation of a better spray. R2 0.9989 0.9993 0.9757 0.9835 0.9865 Conversely, when SOW was added in higher concentrations Slope log-log 1.014 0.9981 0.8204 0.9975 1.012 than 0.30% total dissolved solids, it was more dicult to form LDR 4 4 3 4 4 a spray and thus a suppression of the signal was observed.Sr (%) 3 4 4 3 5 DL/ng ml-1 1 3 53 1 2 This has also been previously reported in the literature.5,6,41 Signal suppression may be due to dramatic changes in the Co Cu Zn Ba U physical properties of the solution (surface tension, dielectric R2 0.9732 0.9765 0.9842 0.9297 0.9635 constant, etc.) which suppress spray formation.Additionally, Slope log-log 0.9389 0.9761 0.9836 1.048 1.132 under those conditions the solution may have reached maxi- LDR 4 4 3 4 3 mum conductivity and signal suppression is inevitable. Matrix Sr (%) 5 7 6 3 6 eects like those noted here can be dicult to measure and DL/ng ml-1 2 12 28 4 9 quantify. High concentrations of matrix elements lead to blocking of the capillary and the orifices of the ES interface, as well as memory eects. To eliminate these eects, dilution relative detection limits (Table 3) with the 30 gauge capillary of the samples and internal standardization may be necessary. were improved by an order of magnitude over the equivalent Although the trend was similar using both capillaries, a elements using the 29 gauge capillary.33 The detection limit dramatic increase in signal sensitivity was observed when the for V was higher than the rest of the elements studied due to 30 gauge was compared to the 29 gauge capillary at total the formation of V+ and VO+ because (VO)SO4·3H2O was dissolved solids of up to 0.3%.Chromium signal was not used to prepare the sample. As for Zn (m/z 64), the higher suppressed at 0.6% total dissolved solids. No explanation for detection limit was caused by the higher background at m/z the trend with chromium is available at this point. As was 64 due to the detection of an interferent (CH3OH)2+ peak. discussed previously, higher sensitivity was attributed to the The interfering peak is from the blank methanol solution.The more stable current that was attainable by the 30 gauge blank spectrum under the operating conditions used showed capillary and the physical characteristics of the solutions which peaks at m/z values of less than 38 and at 64. For all elements favors formation of the spray. Signal suppression was also studied, the improved results may be attributed to the more observed at 0.45% and higher percentages of total dissolved stable current generated by the 30 gauge capillary, which leads solids.The trend was explained in the previous section. to signal stability and improved signal sensitivity. In these experiments, the capillary current was the major contributor to signal stability and magnitude. Quantitative Aspects of Lead Organometals Another smaller capillary, 31 gauge, was investigated. Its A series of organolead compounds were run in the bare metal performance was similar to that of the 30 gauge.However, it ion mode. Mass spectra were monitored up to m/z 240. clogged more easily and was very fragile, so the 30 gauge Solutions of 10-5 M each of Pb(NO3)2, (butyl)3PbOAc, capillary was the capillary of choice for later studies. Table 4 Eect of SOW on a multi-element solution with the 29 Matrix Eects on the ES Signal gauge capillary The formation of charged droplets in the ES process has been SOW Cr Ni Co Cu Zn Rb Cd Cs Ba the subject of intensive studies; however, a full understanding 0% 1.2 2.4 4.0 5.5 6.0 1.0 1.0 0.9 1.5 has not been achieved and complete equations relating the 0.15% 1.5 2.8 7.4 5.8 6.7 0.9 1.1 1.2 1.6 pertinent variables are not yet available.38 It has been found 0.30% 1.6 2.3 7.8 2.1 7.9 0.7 1.0 1.5 1.1 that the charged droplet emission current is dependent on the 0.45% 1.0 0.9 1.0 0.8 0.5 0.7 0.9 0.9 0.8 0.60% 0.9 0.5 0.5 0.4 0.3 0.6 0.8 0.9 0.7 presence of ionized electrolytes in the solvent to be sprayed.No spray is observed in totally de-ionized solutions.39 The threshold conductivity, s, where the spray is still intermittent, Table 5 Eect of SOW on a multi-element solution with the 30 occurs for methanol containing dissolved electrolytes at s= gauge capillary 10-7 V-1 cm-1. For electrolytes such as the alkali metal halides present in sea-water, the above s corresponds to a SOW Cr Ni Co Cu Zn Rb Cs Ba U total electrolyte concentration of 10-6 M.40 As the conductivity 0% 5.2 5.0 12.0 2.0 15.3 3.1 2.5 1.9 0.9 is increased above the threshold, the ES current, I, increases 0.15% 6.0 5.0 12.5 2.1 15.6 3.4 2.3 2.0 1.0 and becomes stable.I is found to approximately follow a 0.30% 5.8 3.5 5.7 1.6 5.4 2.6 1.5 1.7 1.0 power law with the conductivity as shown in eqn. 2.40 0.45% 1.2 0.8 0.9 0.8 1.1 1.0 0.9 0.9 0.9 0.60% 1.0 0.6 0.9 0.5 0.9 0.9 0.9 0.9 0.7 I=Hsn (2) Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 521(Ph)3PbCl, (propyl)3PbOAc, and (isobutyl)3PbOAc were investigated with the 30 gauge capillary.The capillary was held at +5.3 kV, the front plate at +1101 V, the sampling plate at +670 V, the skimmer at -16.6 V and the solution flow rate was 5 ml min-1. The optimum flow rate for nitrogen gas was 230 ml min-1. Many conditions were investigated, and it was necessary to hold the sampling plate at this voltage to determine the bare metal ion for these larger molecules. In general, the degree of declustering is dependent on the voltage between the sampling plate and the skimmer (687 V), as well as high voltages between the front plate and the sampling plate (431 V).Under these conditions, the bare metal ion was the most intense in the ES spectrum. The electric field gradient in the CID region was higher than that used for the transition metals (about 171 750 V m-1). At higher electric field gradients (>171750 V m-1) some discharge peaks were observed at m/z 19, 33 and 65. The discharge peaks were identified by running a methanol blank solution at the same conditions as those of the sample.However, at the conditions reported above, minimal discharge peaks were seen and the bare metal ion peak was the most intense. Scans of 10-5 M Pb(NO3 )2 and (butyl)3PbOAc are shown in Figs. 5 and 6, respectively. The correct lead isotope ratios were determined in the bare metal ion mode. For tributyllead acetate, decreasing the sampling Fig. 6 ES-MS spectrum of (butyl)3PbOAc (10–5 M) in: (a), the bare metal ion mode (ES capillary 5.3 kV, front plate 1101 V, sampling plate voltage and the curtain gas flow rate resulted in a peak plate 670 V, skimmer -16.6 V, curtain gas flow rate 230 ml min-1); at m/z 57, which may be due to a butyl fragment of the original (b), the same conditions except sampling plate 600 V, and curtain gas molecule [see Fig. 6(b)]. Figures of merit for lead in a flow rate 150 ml min-1. Pb(NO3)2 standard are shown in Table 6.To assess the reliability of this method, NIST SRM 2715 (a Table 6 Figures of merit for Pb in Pb(NO3)2 by ES-MS lead standard reference material in fuel) was run to determine the lead content by ES-MS. The optimum conditions for the Pb lead signal were as follows: ES capillary (+5.4 kV), front plate R2 0.9987 (+1101 V), sampling plate (+684 V), skimmer (-16.6 V). The Log-log slope 1.092 flow rate used was 8 ml min-1 and the curtain gas flow was at LDR 3 220 ml min-1.The NIST SRM 2715, certified to contain Sr (%) 3 7.8±1.4 mg ml-1, was diluted 1+99 with methanol. The exper- DL/ng ml-1 3 imentally determined lead concentration was determined using a Pb(NO3)2 standard calibration curve. Since no speciation Metalloporphyrins information was given on the lead present in the NIST SRM, it was assumed to be Pb2+ and a standard containing Pb2+ Since the ES interface was successfully used for the detection was used to generate the calibration curve.The experimental of the lead ion in organolead compounds, it was interesting value for lead (8.2±3 mg ml-1) falls within the certified range. to evaluate the bare metal ion mode of organometals with larger organic molecules. Metalloporphyrins of the type [(metal3+)(porphyrin2-)]+L- are associated with a negatively charged counter ion L-, are positively charged in solution and thus are amenable to analysis by ES-MS in the positive ion mode. At the onset of this project it was realized that the mass spectrometer used in this study had several shortcomings with respect to the analysis of large molecules.The scan range of the mass spectrometer was limited to 240 units, which made the detection of ions with m/z larger than 240 impossible. Also, since the mass spectrometer of our instrument was mainly used for ICP-MS work, it could be operated in the positive mode only and did not have the option to use the negative ion mode.This added an extra limitation to the types of solvents and metalloporphyrins that could be used, since the analysis had to be performed in the positive mode and the ES solvent needed to be slightly acidic to prevent the dissociation of carboxylic acid substituents on the macrocycle. Many conditions were investigated for the determination of the bare metal ion of metalloporphyrins. The best conditions were similar to those utilized for the organolead compounds. It was also found that the addition of acetic acid (less than 1%by volume)to the methanol solvent increased and stabilized the signal of the metalloporphyrins.However, it has been reported in the literature that solvent systems containing Fig. 5 ES-MS spectrum of Pb(NO3 )2 (10–5 M solution). The following trifluoroacetic acid resulted in the demetallization of zinc from conditions were used: ES capillary 5.3 kV, front plate 1101 V, sampling plate 670 V, skimmer -16.6 V, curtain gas flow rate 230 ml min-1.zinc porphyrin.42 As a result, acetic acid was used in this study 522 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12instead of trifluoroacetic acid. Additionally, spectra were tation. However, when the 30 gauge capillary was employed, slightly milder conditions were used (capillary tip 5.3 kV, recorded at 3 and 45 min, respectively, after the porphyrin was first dissolved in the ES solvent and they did not show any sampling plate 674 V and curtain gas flow rate 310 ml min-1, the other parameters being similar to those used with the 29 indication of demetallization.Initial work was performed using the 29 gauge capillary and gauge capillary). Solutions of 10-4 M ferriprotoporphyrin IX chloride, manganese protoporphyrin chloride, and cobalt pro- the spectra obtained contained significant fragmentation. A spectrum of 10-4 M solution of ferriprotoporphyrin IX chloride toporphyrin IX chloride gave the spectra presented in Figs. 10–12, respectively. These spectra do not show significant (Mr 652) or hemin is shown in Fig. 7. Similar spectra were obtained for manganese protoporphyrin chloride (Mr 651), fragmentation and the bare metal ion is the most intense signal in the range monitored. The sampling plate voltage was similar and cobalt protoporphyrin IX chloride (Mr 655) and are presented in Figs. 8 and 9, respectively. Although several in both cases. A possible explanation for the dramatic dierence in the spectra is that the milder conditions used with the conditions were investigated, the optimum ES conditions used were as follows: capillary tip +6.0 kV, front plate +368 V, 30 gauge capillary result primarily in unfragmented molecular ions.While this may be possible, it is not highly probable. sampling plate +690 V, skimmer -5 V, curtain gas flow rate 210 ml min-1. The flow rate of the solution was 5 ml min-1. Perhaps with the harsher conditions exhibited for the 29 gauge capillary (Figs. 7–9), a discharge was present, causing extensive Under these conditions significant fragmentation of the metalloporphyrins is observed. It is dicult to attempt to identify fragmentation, whereas under the milder conditions for the 30 gauge capillary, the metal ions resulting from solution equilib- the peaks due to the lack of spectra that includes information on the molecular metalloporphyrin ion or larger fragments. rium were detected (Figs. 10–12). Further studies will be necessary to provide an unambiguous explanation.Such spectra would help to elucidate the nature of the fragmen- Fig. 10 ES-MS spectrum of a 10-4 M solution of hemin in the bare Fig. 7 ES-MS spectrum in the bare-metal ion mode of a 10-4 M metal ion mode using the 30 gauge capillary. solution of hemin using the 29 gauge capillary. Fig. 11 ES-MS spectrum of a 10-4 M solution of MnPP in the bare Fig. 8 ES-MS spectrum in the bare-metal ion mode of a 10-4 M metal ion mode using the 30 gauge capillary.solution of MnPP using the 29 gauge capillary. Fig. 12 ES-MS spectrum of a 10-4 M solution of CoPP in the bare Fig. 9 ES-MS spectrum in the bare-metal ion mode of a 10-4 M solution of CoPP using the 29 gauge capillary. metal ion mode using the 30 gauge capillary. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 523American Society for Mass Spectrometry, New York, USA, CONCLUSION 1993, p. 202. The in-house constructed ES interface was used in both the 10 T homson, B. A., and Irbarne, J.V., J. Chem. Phys., 1979, 71, 4451. 11 Yamashita, M., and Fenn, J. B., J. Phys. Chem., 1984, 88, 4451. bare metal ion mode and the ion-cluster mode. The former 12 Siu, K. W. M., Gardner, G. J., and Berman, S. S., Anal. Chem., mode is important for elemental analysis purposes and was 1989, 61, 2320. applied for the analysis of multielement solutions containing 13 Siu, K. W. M., Guevremont, R., LeBlanc, J. C.Y., Gardner, G. J., alkali metals, alkaline earth metals and transition metals. In and Berman, S. S., J. Chromatogr., 1991, 554, 27. addition, a study of dierent capillary sizes (29, 30, and 31 14 Curtis, J. M., and Boyd, R. K., Rapid Commun. Mass Spectrom., gauge capillaries) showed that the 30 gauge capillary improved 1993, 7, 409. 15 Katta, V., Chowdhury, S. K., and Chait, B. Y., J. Am. Chem. Soc., the detection limits by an order of magnitude over those 1990, 112, 5348.obtained using the 29 gauge one. The 31 gauge capillary 16 Fura, A., and Leary, J. A., Anal. Chem., 1993, 65, 2805. worked as well as the 30 gauge, but was fragile. The ES current 17 Schneider, R. P., Lynch, M. J., Ericson, J. F., and Fouda, H. G., in the smaller capillaries was more stable and led to higher Anal. Chem., 1991, 63, 1789. signal intensity and improved sensitivity. Matrix eects on the 18 Li, Y. T., Hsieh, Y. L., Henion, J. D., and Ganrm, B., J. Am. Soc. analyte signal for multi-element solutions with the 29 and 30 Mass Spectrom., 1993, 4, 631. 19 Yu, X., Wojciechowski, M., and Fenselau, C., Anal. Chem., 1993, gauge capillaries showed signal enhancement when 0.15 and 65, 1355. 0.3% of total dissolved solids were added and signal suppres- 20 Jaquinod, M., Leize, E., Potier, N., Albecht, A. M., Shanzer, A., sion at higher percentages of total dissolved solids. The scans and Van Dorrsselear, A., T etrahedron L ett., 1993, 34, 2771. of two chromium species (CrII and CrIII in the ion cluster mode 21 Leize, E., Kramer, R., Lehn, J.M., and Van Dorsselear, A., showed dierent clusters for each species, indicating the pos- Proceedings of the 41st ASMS Conference on Mass Spectrometry sibility of attainingspeciation information with the electrospray and Allied T opics, San Francisco, CA, USA, American Society for Mass Spectrometry, New York, USA, 1993, p. 878a. set-up. Other organometallic molecules were studied in the 22 Poon, G.K., Bisset, M. F., and Mistry, P., J. Am. Soc. Mass bare metal ion mode, mainly organolead compounds and Spectrom., 1993, 4, 588. metalloporphyrins. 23 Wilson, S. R., and Wu, Y., J. Am. Soc.Mass Spectrom., 1993, 4, 596. Although the detection limits and sensitivities obtained in 24 Huddleston, M. J., Annan, R. S., Bean, M. F., and Carr, S. A., this work are not yet competitive with the more standard J. Am. Soc. Mass Spectrom., 1993, 4, 710. 25 Hughes, B. M., McKinzie, D.E., and Dun, K. L., J. Am. Soc. elemental analysis techniques, a possible advantage of ES-MS Mass Spectrom., 1993, 4, 604. is the speciation capability demonstrated in the chromium 26 Lin, H. Y., and Voyksner, R. D., Anal. Chem., 1993, 65, 451. spectra. Alternative analyzers may restore and enhance detec- 27 Stewart, I., and Horlick, G., Anal. Chem., 1994, 66, 3983. tion levels with elemental speciation. 28 Doiron, D. R., and Gomer, C. J., ed. Porphyrin L ocalization and T reatment of T umors, Alan R. Liss Inc., New York, 1984.The authors thank the Chemistry Machine Shop as well as 29 Porphyrins and Metalloporphyrins, ed. Smith K.M., Elsevier, Amsterdam, The Netherlands, 1975. the Electronics Shop at the University of Cincinnati for their 30 T he Porphyrins, ed. Dolphin, D., Wiley, New York, USA, 1978, help. We also acknowledge the National Institute of vols. I–VI. Environmental Health Sciences for support through grant 31 Baker, E. W., and Louda, J. W., in Biological Markers in the ESO4908, Project 5. Sedimentary Environment, ed. Johns, R. B., Elsevier, Amsterdam, The Netherlands, 1986. 32 Van Berkel, G., McLuckey, S. A., and Glish, G. L., Anal. Chem., REFERENCES 1991, 63, 1098. 33 Byrdy, F., Doctoral Dissertation, University of Cincinnati, 1 Blades, A. T., Jayaweera, M. G., Ikomonou, M. G., and Department of Chemistry, 1995; Byrdy Brown, F., J. Anal. At. Kebarle, P., Int. J. Mass Spectrom. Ion Processes 1990, 102, 251. Spectrom., 1996, 11, 633. 2 Blades, A. T., Jayaweera, M. G., Ikomonou, M. G., and 34 Agnes, G. R., Stewart, I. I., and Horlick, G., Appl. Spectrosc., Kebarle, P., Int. J. Mass Spectrom. and Ion Processes, 1990, 1994, 48, 1347. 101, 325. 35 Bruins, A. P., Mass Spectrom. Rev., 1991, 10, 53. 3 Blades, A. T., Jayaweera, M. G., Ikomonou, M. G., and 36 Agnes, G., Doctoral Dissertation, University of Alberta, Canada, Kebarle, P., J. Chem. Phys., 1990, 92, 5900. Department of Chemistry, 1993. 4 Cheng, Z. L., Siu, K. W. M., Guevremont, R., Berman, S. S., 37 Wilm, M. S., and Mann, M., Int. J. Mass Spectrom. Ion Processes, J. Am. Soc. Mass Spectrom., 1992, 3, 281. 1994, 136, 167. 5 Cheng, Z. L., Siu, K. W. M., Guevremont, R., Berman, S. S., Org. 38 Wilm, M., and Mann, M., Anal. Chem., 1996, 68, 1. Mass Spectrom., 1992, 27, 1370. 39 Tang, L., and Kebarle, P., Anal. Chem., 1991, 63, 2709. 6 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1992, 46, 401. 40 Ikonomou, M. G., Blades, A. T., and Kebarle, P., Anal. Chem., 7 Agnes, G. R., Horlick, G., Appl. Spectrosc., 1994, 48, 649. 1991, 63, 1989. 8 Douglas, D. J., in Fundamental Aspects of Inductively Coupled 41 Pfeifer, R. J., and Hendricks, C. D. AIAA J., 1968, 6, 496. Mass Spectrometry, ed. Montaser, A., and Golightly, D.W., VCH 42 Smith, D. P., IEEE T rans. Ind. Appl., 1986, IA-22, 527. Publishers, New York, 1992, 2nd edn., p 642. 9 Corr, J. J., and Douglas, D. J., ‘Elemental Ion Spray-Mass Paper 6/02682F Spectrometry: Analysis of Solutions of Metal Salts’, in Abstracts Received April 17, 1996 and Index of Authors of the 41st ASMS Conference on Mass Spectrometry and Applied T opics, San Francisco, CA, USA, Accepted February 26, 1997 524 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a602682f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Use of Ionspray Mass Spectrometry in the Speciation and Elemental Characterization of Metallothioneins J. C. YVES LE BLANC Sciex, 71 Four Valley Drive, Concord, Ontario, Canada L 4K 4V 8 Results are presented to illustrate the potential and limitations chemical speciation and elemental analysis. By carefully controlling the collision induced dissociation (CID) processes of ionspray mass spectrometry in the characterization of metal containing proteins, metallothioneins (MTs). Through collision in the interface sampling region, they could either observe intact molecular ions of arsenic species (e.g., arsenobetaine) or induced dissociation in the interface sampling region of a mass spectrometer, ionspray can be used to obtain species and elemental arsenic ions.It was shown that the determination of arsenic species in the elemental mode would improve detection elemental information on small molecules. When larger ions, such as MTs, are analyzed after chromatographic separation, by a factor of three over the determination of the same species in the molecular ion mode (based on signal-to-noise ratio).15 similar information can only be obtained indirectly. Postcolumn acidification of the solution was used to release protein A single quadrupole mass spectrometer was also modified in the same way and used as a specific detector in the analysis of bound metal ions into solution prior to their determination by ionspray mass spectrometry.This approach allows MT species inorganic species by means of capillary electrophoresis (CE–MS), where most elements were detected in their elemental and metal ions freed from MTs to be determined using a single experimental set-up.Extensive on-column metal form.16 Therefore, an instrument equipped with such a modifi- cation was evaluated for the characterization of a class of displacement was observed under the chromatographic conditions used. metal containing proteins, metallothioneins.Metallothioneins (MTs) are small proteins (6–7 kDa) which Keywords: Ionspray mass spectrometry; metallothionein are rich in cysteine residues (20 Cys) and with none involved characterization in disulfide bridges.17 In mammals, MTs are believed to be induced by exposure to metal ions such as Cd2+, Zn2+, Hg2+ Ionspray (IS) is a soft desorption technique that can be used and Cu2+. Metals are bound through metal–thiolate bonds to desorb intact molecular ions from aqueous solutions into with the Cys residues and their binding properties have been the gas phase for MS analysis.It is considered the method of widely studied,18 especially for rabbit MTs. Studies have shown choice for the introduction of thermally labile and/or non- that within the protein, there exist two well defined binding volatile compounds into a mass spectrometer. Ionspray is also domains (a and b) where a total of seven metal atoms may be the ideal desorption technique to produce gas-phase multiply bound.19,20 The arrangement of cadmium and zinc for each charged ions of high molecular mass compounds, such as domain is shown in Fig. 1. The b domain, which carries three proteins, from aqueous solutions. These multiply charged ions metal ions, is found on the amino terminal end of the sequence, may then be determined using a quadrupole mass spectrometer whereas the a domain is found at the carboxylic end of the whose upper m/z limits are comparatively low (<3000), thus sequence and binds four metal ions.The binding of metal atoms permitting an extension of the mass limit of the sample in both the a and b domains gives a strong three-dimensional analyzed. Mann et al.1 and Covey et al.2 were among the first structure to metallothionein. The complexing capabilities of to demonstrate the analytical capabilities of IS-MS in the MTs are known to be aected by the acidity of the solution bioanalytical field; since then the number of applications using and/or the presence of other chelating agents in solution.21 Yu this technique has increased significantly.IS-MS allows one to et al.22 have shown that IS-MS could be used to identify the determine protein molecular masses accurately and rapidly.1,2 level of metal coordination as a function of the solution pH. Furthermore, adducts of proteins that are covalent3 and non- In their work, reconstituted Cd7MT samples were infused at covalent4–6 in nature may also be characterized with this various solution pH values.In agreement with previously technique. published data,23 they observed intact multiply charged In addition to proteins, IS may also be applied to lower Cd7MT-2a ions at pH 4, whereas when the solution pH was molecular mass species such as inorganic cations. When metal lowered to 3.5 they observed Cd4MT-2a ions as the most species are determined by means of IS, the mass spectrum abundant species. The resulting loss of three Cd ions was generally shows high degrees of ion–solvent clustering, as attributed to opening of the b domain and it was concluded demonstrated by Kebarle and co-workers7–10 and Siu and co-workers.11,12 A number of studies have examined the relationship between species observed in the mass spectrum and the inorganic ionic species present in solution.9–12 Agnes and Horlick13,14 have shown that relatively clean mass spectra of declustered singly charged transition and alkali metal ions could be observed in electrospray MS.In order to observe these elemental mass spectra, they used elevated declustering potentials when sampling ions into the mass spectrometer, thus producing more energetic collisions between ion adducts and the nitrogen gas molecules in the dierentially pumped interface region. More recently, Corr and Larsen15 showed that a commercially available triple quadrupole mass spec- Fig. 1 Arrangement of metal ions in the a and b domains of metallothioneins.trometer, after minimal modification, could be used for both Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (525–530) 525that the four remaining Cd ions were most likely in the a domain. Species with an intermediate number of cadmium atoms bonded, such as Cd5MT-2a and Cd6MT-2a, were also observed in smaller amounts and their presence was attributed to the ability of the b domain to bind cadmium in a less cooperative way.These observations were made from fractions collected after gel filtration or from lyophilized fractions that were diluted and infused at 3 ml min-1. The metal content results obtained by means of IS-MS were in agreement with those obtained by ETAAS.22 Conventionally, the metal content in proteins is determined using destructive, element-specific techniques such as ICP, FAAS or ETAAS.22,24,25 Even though these techniques have proved to be extremely sensitive for elemental work; they do not provide any information about the protein in solution.The use of IS in a mass spectrometer equipped with a dieren- Fig. 2 Schematic diagram of the dierentially pumped interface and tially pumped interface would provide information on the ion optics of the API 100 mass spectrometer. The voltage dierence intact protein in addition to its metal content. Further, coupling between the orifice and the skimmer is referred to as the declustering the above system on-line to LC separation would reduce potential.sample handling and minimize the risk of sample contamination. The mass spectrometer was operated with a resolution such that the valley between peaks diering by 1 m/z unit was EXPERIMENTAL <20% of the peak maximum and the full width at half-height Materials was <0.7 m/z. Mass spectra were acquired using a 0.3 u step size and a dwell time of 0.5 ms, resulting in a typical scan time Formic acid, ammonium formate and rabbit liver of about 3 s per scan.Limited range mass spectra of ions of metallothionein-II (Cd4Zn3MT-2) were purchased from Sigma m/z 60–130 were acquired using a 0.042 u step size and a dwell (St. Louis, MO, USA) and used without further purification. time of 2 ms, resulting again in a typical scan time of about Zinc metallothioneins (Zn7MT) was induced in rabbits and 3 s per scan. the isoforms were isolated and purified as described by Zelazowski et al.26 Stock solutions containing 2 mg ml-1 of MT were prepared in distilled, de-ionized water, which was LC–MS Conditions purified in-house with a Waters Milli-Q purification system Metallothioneins were separated on a reversed-phase C4 (Millipore, Bedford, MA, USA). For the determination of MTs column (100×1 mm id, 5 mm) (Vydac, Hesperia, CA, USA), by infusion, the stock solutions were diluted to 50 mg ml-1 using a mobile phase of a mixture of 10 mM ammonium with aqueous ammonium formate (10 mM) buered to the formate buered to pH 5.0 with formic acid in water (solvent desired pH with formic acid.For the determination of MTs A) and methanol (solvent B). Solvent A and B were delivered by LC–MS, the stock solution was diluted to 50 mg ml-1 with with two LC-10AD pumps (Shimadzu, Kyoto, Japan) at a water. Methanol used in LC–MS analysis was of HPLC grade combined flow rate of 50 ml min-1 using the following gradient (Fisher, Nepean, ON, Canada). conditions: 10% of solvent B for 5 min and from 10 to 70% of solvent B in 75 min.Protein solutions were injected using a Mass Spectrometry 50 ml loop injector (Model 8125, Rheodyne, Cotati, CA, USA). Post-column acidification of the LC efluent was achieved via Experiments were performed on a modified API 100 single a mixing T located between the column and the IS probe quadrupole mass spectrometer (PE-Sciex, Concord, ON, (Fig. 3). The formic acid solution used for this purpose was Canada) equiped with IS.The mass spectrometer was fitted with a dierentially pumped interface behind the orifice plate (Fig. 2). Analyte solution and LC eluent were introduced via a fused silica capillary tube that was inserted into the IS needle held at 5.2 kV. Ions desorbed from the solution were sampled into the mass spectrometer system through a dry nitrogen curtain gas, which assisted in declustering of ion–solvent adducts and prevented the entry of vapors and contaminants. The ions underwent a free-jet expansion through the orifice into the first dierentially pumped region, which was maintained at 1–2 Torr, before passing through the skimmer and into the rf-only quadrupole maintained at a pressure of 5–8 mTorr.An external power supply (Model 6384, Hewlett- Packard, Avondale, PA, USA) was used for biasing of the orifice from 0 to 400 V. The dierence between the orifice potential and the skimmer potential (at ground) is typically referred to as the declustering potential.In the speciation mode, the orifice potential was typically adjusted to 40–50 V for the determination of intact molecular ions. In the elemental mode, the orifice potential was typically adjusted to 340–350 Fig. 3 Experimental set-up used for post-column acidification of the V to fragment metal containing ions to bare metal ions. In the solution. The chromatography was performed on a Vydac C4 column rf-only region, energetic cooling of the ions focused them onto (100×1 mm id) at a flow rate of 50 ml min-1.The post-column the axis of the quadrupole, as described by Douglas and modifier solution was delivered via a syringe pump at a flow rate of 5 ml min-1. French.27 526 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12infused with a Model 22 syringe pump (Harvard Apparatus, lowered to 3.0, only the fully demetallated metallothionein was observed, as shown in Fig. 4(C). The measured molecular mass South Natcik, MA, USA) at a flow rate of 5 ml min-1.of 6125 is identical with the calculated molecular mass of 6125 Da based on the published sequence information. [The RESULTS AND DISCUSSION terminology used to identify MTs is based on the IUPAC–IUB Commission on Biochemical Nomenclature (CBN).28 The MS of Metallothioneins numerical identifier refers to the order of elution of the In order to obtain a better appreciation of the importance of MT variants from anion-exchange chromatography. The the solution pH on metal bindings of MTs, a solution of letter identifiers are used to denote homology of the decoded Zn7MT-2a in 10 mM ammonium formate and formic acid was amino acid sequence (i.e., dierent amino acid sequence for first infused and analyzed by IS-MS.Fig. 4(A) shows the same variant). All sequences were obtained from the deconvoluted IS mass spectrum obtained from a solution of EMBL Sequence database (European Molecular Biology Zn7MT-2a at pH 5.1. The calculated molecular mass of the Laboratory)].At this pH, species with one, two or three zinc most intense protein peak was 6569 Da, which corresponds to atoms attached were not observed, in agreement with obser- (apo-MT-2a+7Zn2+-14H+). Lowering of the solution pH to vations made for Cd7MT-2a under similar pH conditions.22 approximately 4.1 resulted in two MT species that correspond to Zn4MT-2a and Zn0MT-2a [Fig. 4(B)]. The loss of three zinc ions from the MTs was probably due to ‘unfolding’ of the LC–CID–MS of Metallothioneins b domain, while the remaining zinc ions were held in the As mentioned earlier, the potential dierence between the a domain, where cooperative binding of the metal ions is orifice (OR) and the skimmer cone of the API 100 interface believed to be stronger.23 The presence of a small amount may be raised to induce fragmentation of ions sampled into of demetallated-MT (apo-MT-2a) suggests that the analytical the mass spectrometer (see Fig. 2). This process is often referred conditions used were approaching the solution pH where the to as source induced fragmentation.When Cd4Zn3MT-2a was a domain started to unfold, thus releasing all four ions of zinc infused at 5 ml min-1 in a solution of pH 6, no Cd+ or Zn+ into solution. At pH 4.1 and 5.11, no intermediate complexes ions were evident in the mass spectrum even when the orifice of the form ZnxMT-2a, where x=1, 2, 3, 5 or 6, were observed. voltage was set to 400 V. However, when the Cd4Zn3MT-2a This observation agrees partially with results obtained by Yu solution was acidified to pH 3.5, Zn+ could be observed; when et al.,22 where the only reconstituted CdxMT-2a species not it was acidified to pH 2.5, Zn+ and Cd+ could be observed observed at pH 3.5 and pH 4 were with x=1, 2 and 3.The (data not shown). These results suggest that the CID processes inability to observe intermediates of Zn5MT-2a and Zn6MT-2a that occur in the interface region were not energetic enough as was observed for Cd-MT solutions could be inherent to the to induce the fragmentation of MTs from their native form to binding stability of the cadmium complexes in comparison bare singly charged metal ions (Zn+ and Cd+).Based on these with the zinc complexes. When the solution pH was further observations, the LC–MS system was modified so that postcolumn adjustment of the solution pH would be feasible, as shown in Fig. 3. Such a configuration allowed the use of neutral solution conditions to separate metallatedMT isoforms and release the metal ions into solution via an on-line step of pH adjustment.Methanol as a Post-column Modifier Methanol was first used as a post-column modifier in order to obtain information on the metallated MT isoforms that are present in the commercially available sample of MT-2. Fig. 5 shows the LC–MS total ion chromatogram (TIC) trace of a Cd4Zn3-MT-2 sample, where as many as nine proteins could be detected.Reversed phase LC has previously been used to separate MT isoforms. For rabbit metallothioneins, as many as seven isoforms were separated with a C8 column.29 Table 1 lists the molecular masses obtained for most of the peaks labelled in Fig. 5. In order to simplify the discussion, the remainder of this paper will focus only on the most predominant species separated, peaks C and I. The mass spectra for peaks C and I are shown Fig. 6(A) and (B), respectively.After Fig. 4 Deconvoluted mass spectra of Zn7MT-2a with the solution Fig. 5 Total ion chromatogram (TIC) for the separation of Cd4Zn3MT-2a with methanol as a post-column modifier. The mass pH adjusted to A, 5.1, B, 4.1 and C, 3.0. Solutions were prepared using water–10 mM ammonium formate and the pH was adjusted with spectrometer was operated in the speciation mode (i.e., low declustering potential) with the separation conditions given under Experimental. formic acid. Analyses were performed by continuous infusion of the solution at 5 ml min-1.The masses obtained for each of the peaks labeled are listed in Table 1. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 527Table 1 Experimental molecular mass for each peak observed in Fig. 4(A). Peaks E and F had poor signals and no molecular mass could be assigned to them. See ref. 28 for the terminology used Peak label Expt. molecular mass/Da Potential species A 6915.7±3.4 Cd7MT-1a B 6924.0±1.9 Cd7MT-2b C 6901.3±1.7 Cd7MT-2a D 6918.7±0.8 Cd7MT-1a or Cd7MT-2a G 6566.1±0.6 Cd4MT-2a H 6568.2±0.6 Cd4MT-2a I 6569.5±0.8 Zn7MT-2a or Cd4MT-2a Fig. 7 A, TIC obtained for the separation of Cd4Zn3MT-2a with methanol as a post-column modifier and with the mass spectrometer operated in the elemental mode (i.e., high declustering potential).B, Mass spectrum of peak 1 where Zn ions are observed. C, Extracted ion chromatogram for all Zn isotopes (i.e., m/z=64, 66, 67 and 68).Fig. 6 Mass spectrum obtained for peaks C (A) and I (B) of the TIC shown in Fig. 5. Methanol was used as a post-column modifier and the mass spectrometer was operated in the speciation mode (i.e., low declustering potential). Two distinct species of MTs are observed; Cd7MT-2a (A) and Cd4MT-2a (B). charge state assignment, species C was found to have a Fig. 8 Mass spectrum obtained for peaks C (A) and I (B) of the TIC molecular mass of 6901.3±1.7 Da and was assigned to shown in Fig. 7. Methanol was used as a post-column modifier and Cd7MT-2a (apo-MT-2+7Cd2+-14H+). The molecular mass the mass spectrometer was operated in the elemental mode (i.e., high declustering potential). No metal ions were detected under these of species I was found to be 6569.7±0.7 Da, which could conditions. assigned to either (apo-MT-2+7Zn2+-14H+) or (apo- MT-2+4Cd2+-8H+), two species that are only 2 Da apart. with these MT species and that no metal ions were produced The observation of Cd7MT-2a and potentially Zn7MT-2a was in the source fragmentation reaction of metallothioneins.Even puzzling since a solution of Cd4Zn3MT-2a was injected more surprisingly, injection of a Zn7MT-2a solution resulted on-column. in an LC–MS trace that was identical with that shown in For further characterization, a second injection of the same Fig. 5, and species of identical molecular mass were identified solution was made under identical chromatographic con- (data not shown).Therefore, it appears that there was an ditions, except in this run the orifice potential was raised to ecient displacement process on-column that resulted in elu- 350 V in order to obtain metal composition information. tion of cadmium bound MTs and free Zn+. The source of Fig. 7(A) shows the LC–MS trace obtained when the mass cadmium is unknown; it was speculated that the acquired Cd spectrometer was scanned from mass 60 to 130. The appearance ions may come from previous injection of MTs.of an additional peak at 3.02 min is observed when Fig. 7(A) is compared with Fig. 5. Fig. 7(B) shows the mass spectrum of this new chromatographic peak and clearly displays the zinc 1.0% Formic Acid as a Post-column Modifier isotope pattern (m/z 64 and 66). When the ion chromatogram for the zinc isotopes is extracted from the LC–MS trace, a The same Cd4Zn3MT-2a sample solution was injected when the post-column modifier consisted of methanol and formic single chromatographic peak showing up after the void volume is obtained [Fig. 7(C)]. These observations suggest that a acid (1% v/v) and analyzed in the speciation mode. Fig. 9(A) shows that the LC–MS trace obtained was very significant amount of free zinc ions was present in the injected solution or that the injected Cd4Zn3MT-2 lost its Zn metal similar to that observed when the post-column modifier was pure methanol (Fig. 5). The mass spectra corresponding to during the chromatographic run.Fig. 8(A) and (B) show the elemental mode mass spectra corresponding to peak C and I peaks C and I are shown in Fig. 9(B) and (C), respectively. This time, peaks C and I were found to have the same from Fig. 7(A), respectively. No Cd+ and Zn+ ions are evident in these mass spectra. This means that no metal ions eluted molecular mass of 6569.2±1.2 Da, which would most likely 528 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Fig. 9 A, TIC obtained for the separation of Cd4Zn3MT-2a with 1% Fig. 11 A, Extracted ion chromatograms of A, the Zn isotopes and formic acid as a post-column modifier and with the mass spectrometer B, the Cd isotopes (i.e., m/z=110, 111, 112, 113, 114 and 116) for the operated in the speciation mode (i.e., low declustering potential). The separation of Cd4Zn3MT-2a with 1% formic acid as a post-column mass spectra corresponding to peaks C (B) and I (C) indicate that modifier and with the mass spectrometer operated in the elemental post-column acidification yielded the same MT species, Cd4MT-2a.mode (i.e., high declustering potential). correspond to Cd4MT-2a. This is the same molecular mass as was observed for peak I when methanol was used as a post- [Fig. 11(A)]. This observation further confirms that the MT column modifier [see Fig. 6(B)]. With the modifier solution species eluting at peak I contained Cd and not Zn. Once again, delivered at 5 ml min-1 into the column eluent of 50 ml min-1, the mass dierence observed between peak I when methanol the final concentration of formic acid was about 0.1% v/v, was used as the post-column modifier and when methanol– yielding a pH of about 3.This means that irrespective of the formic acid was used as the post-column modifier corresponds MT species eluting from the column, the decrease in solution to the loss of three Cd ions. pH would result in the loss of three metal ions into solution from the b domain, while the a domain would retain its four metal ions.This is consistent with the results portrayed in 10.0% Formic Acid as a Post-column Modifier Figs. 6(A) and 9(B), where the molecular mass dierence observed for peak C was a decrease of 332 Da from methanol The same Cd4Zn3MT-2a sample solution was injected when to methanol–0.1% formic acid, corresponding to the loss of the post-column modifier consisted of methanol and formic three Cd and the gain of six protons.acid (10% v/v) and analyzed under speciation conditions. With The same solution was re-injected and analyzed under the conditions used, the expected concentration of formic acid elemental mode conditions. The TIC from the LC-MS run would be about 1.0%, which gives a solution pH of about 2. under elemental mode conditions and the mass spectrum Because of this pH, apo-MT-2a (6127.3±1.8 Da) and corresponding to peak C are shown in Fig. 10(A) and (B), Cd4MT-2a (6570.3±1.9) could be produced in solution and respectively.The cadmium isotopic pattern of peak C is unmis- these were identified in the mass spectrum of peak I (Fig. 12). takable, suggesting that cadmium was released into solution The same species were observed in the mass spectrum of peak upon post-column acidification [Fig. 10(B)]. Fig. 11(A) and C (data not shown). When the sample was re-injected and (B) show the extracted ion traces for the Zn and Cd isotopes, analyzed under elemental mode conditions, the Cd signal respectively, when methanol–formic acid was used as the post- intensity increased but no other metal ion could be observed column modifier.It can be seen that zinc was present only in in either peak C or peak I. Fig. 13(A) and (B) show the a peak eluting soon after the void volume, suggesting that extracted ion chromatograms of the Cd isotopes when the it was present either as unbound Zn in solution or released post-column solutions were 1% and 10% formic acid, into solution because of an ecient on-column reaction respectively.Fig. 12 Mass spectrum observed for the separation of Cd4Zn3MT-2a Fig. 10 A, TIC obtained for the separation of Cd4Zn3MT-2a with with 10% formic acid as a post-column modifier and with the mass spectrometer operated in the speciation mode (i.e., low declustering 1% formic acid as a post-column modifier and with the mass spectrometer operated in the elemental mode (i.e., high declustering potential).The mass spectrum was taken at the retention time of peak I. Two distinct species are observed; apo-MT-2a (+) and Cd4MT-2a potential). B, Mass spectrum corresponding to peak C shows that Cd ions have been released in solution. (#). Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 529The author thanks Dr. Martin Stillman for supplying Zn7MT-2a samples and Dr. Jean-Simon Blais for the helpful discussions and instructions on the separation of MTs. The author also thanks Dr.K. W. M. Siu for reviewing this paper before its submission and Dr. Jay Corr for instructions on the operation of the modified API 100 instrument. REFERENCES 1 Mann, M., Meng, C. K., and Fenn, J. B., Anal. Chem., 1989, 61, 1702. 2 Covey, T. R., Bonner, R. F., Shushan, B. I., and Henion, J. D., Rapid Commun. Mass Spectrom. 1988, 2, 249. 3 Bolton, J. L., Le Blanc, J. C. Y., and Siu, K. W. M., Biol. Mass Spectrom., 1993, 22, 666–668. 4 Katta, V., and Chait, B. T., J.Am. Chem. Soc., 1991, 113, 8534. 5 Ganem, B., Li, Y.-T., and Henion, J. D., J. Am. Chem. Soc., 1991, 113, 6294. Fig. 13 Extracted ion chromatogram for Cd isotopes when the post- 6 Light-Wahl, K. J., Springer, D. L., Winger, B. E., Edmonds, C. G., column modifier is A, 1% formic acid and B, 10% formic acid. The Camp, D. G., II, Thrall, B. D., and Smith, R. D., J. Am. Chem. mass spectrometer was operated in the elemental mode (i.e., high Soc., 1993, 115, 803. declustering potential). 7 Blades, A. T., Jayaweera, P., Ikonomou, M. G., and Kebarle, P., Int. J. Mass Spectrom. Ion Processes, 1990, 101, 325. 8 Blades, A. T., Jayaweera, P., Ikonomou, M. G., and Kebarle, P., CID of Metallothionein Ions Int. J. Mass Spectrom. Ion Processes, 1990, 102, 251. 9 Blades, A. T., Jayaweera, P., Ikonomou, M. G., and Kebarle, P., For small organometallic and inorganic ions, source CID is Int. J. Chem. Phys., 1990, 92, 5900. suciently energetic to yield elemental ions in most cases. 10 Jayaweera, P., Blades, A. T., Ikonomou, M. G., and Kebarle, P., Agnes and Horlick30 have suggested that the formation of J. Am. Chem. Soc., 1990, 112, 2452. elemental ions is a multiple step process that originates with 11 Cheng, Z. L., Siu, K. W. M., Guevremont, R., and Berman, S. S., sampling of the ion–solvent cluster. With increasing declus- J. Am. Soc. Mass Spectrom., 1992, 3, 281. 12 Cheng, Z. L., Siu, K. W. M., Guevremont, R., and Berman, S. S., tering potential, more of the solvent molecules are eliminated Org.Mass Spectrom., 1992, 27, 1370. to a point where an elemental ion is produced. For a protein 13 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1992, 46, 401. ion, a similar process occurs in terms of desolvation. Once 14 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1994, 48, 655. desolvation is complete, further deposition of energy via CID 15 Corr, J. J., and Larsen, E. H., J. Anal. At. Spectrom., 1996, 11, 1215. would result in fragmentation of the bare protein ion.From 16 Corr, J. J., and Anacleto, J. F., Anal. Chem., 1995, 68, 2155. the experimental results presented here, it appears that the 17 Bremner, I., Methods Enzymol., 1991, 205, 25. 18 Hunziker, P. E., Methods Enzymol., 1991, 205, 421. remaining energy is insucient to break the protein ion into 19 Messerle, B. A., Schaer, A., Vasak, M., Kagi, J. H. R., and its elemental constituents. Because of its multitude of Wuthrich, K., J.Mol. Biol., 1992, 225, 433. vibrational freedom, the energy acquired by the protein ion is 20 Worgotter, E., Wagner, G., Vasak, M., Kagi, J. H. R., and probably distributed among a large number of bonds within Wuthrich, K., Eur. J. Biochem., 1987, 167, 457. the protein and only a few of these bonds will acquire sucient 21 Kagi, J. H. R., and Valle, B. L., J. Biol. Chem., 1961, 236, 2435. energy to fragment. Therefore, for a relatively large ion, such 22 Yu, X., Wojciechowski, M., and Fenselau, C., Anal. Chem., 1993, 65, 1355. as the MT ions, it was necessary to release the metal ions in 23 Neilson, K. B., and Winge, D. R., J. Biol. Chem., 1983, 258, 13063. solution prior to MS analysis. Although an extra chemical 24 Suzuki, K. T., Sunaga, H., Aoki, Y., and Yamamura, M., step was needed, it was possible to mimic the results of a J. Chromatogr., 1983, 281, 159. previous study in obtaining both species and elemental infor- 25 Mason, A. Z., Storms, S. D., and Jenkins, K. D., Anal. Biochem., mation from a single experimental set-up.22 Other combined 1990, 186, 187. approaches, such as post-column addition of a chelating agent 26 Zelazowski, A. J., Gasyna, Z., amd Stillman, M. J., J. Biol. Chem., 1989, 264, 17091. from which protein elemental composition may be obtained 27 Douglas, D. J., and French, J. B., J. Am. Soc. Mass Spectrom., along with protein molecular information, are currently under 1992, 3, 398. investigation. 28 IUPAC–IUB Commission on Biological Nomenclature, J. Biol Chem., 1977, 252, 5939. 29 Richards, M. P., Methods Enzymol., 1991, 205, 217. CONCLUSION 30 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1995, 49, 324. Although IS-MS could be used to obtain species of and elemental information on small molecules, similar information Paper 6/06422A on metallothioneins species could not be obtained directly. Received September 9, 1996 Post-column acidification was employed to release the metal Accepted February 10, 1997 ions from the protein. On-column metal displacement occurred to a significant extent. Other chromatographic solid supports and capillary zone electrophoresis are currently being investigated to minimize this artifact. 530 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a606422a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Ten Organoarsenic Compounds Using Microbore Highperformance Liquid Chromatography Coupled With Electrospray Mass Spectrometry–Mass Spectrometry SPIROS A. PERGANTIS†, WITOLD WINNIK‡ AND DON BETOWSKI* U. S. Environmental Protection Agency, National Exposure Research L aboratory, Characterization Research Division, P. O. Box 93478, L as Vegas, NV 89193-3478, USA. E-mail: betowski.don@epamail.epa.gov An analytical method based on reversed-phase microbore sensitive and highly selective element specific detector.HPLC HPLC coupled on-line with electrospray mass spectrometry coupled on-line with ICP-MS has greatly enhanced our (ES-MS) is described. The method allows for the capabilities of speciating metals present in a variety of matedetermination of up to ten organoarsenicals in a single rials.1,8–11 This technique provides excellent sensitivity and chromatographic run. Excellent sensitivity and selectivity is detection limits, good selectivity, simplicity and short analysis achieved by operating the triple quadrupole mass spectrometer times.However, the technique has a fundamental limitation, in the selected-reaction monitoring mode. This is possible originating from the fact that identification of metal combecause the ions produced by collision-induced dissociation of pounds is based solely on the matching of retention times. As the protonated molecules or molecular ions of the ten a consequence, errors can result from using this approach, organoarsenicals are characteristic of each compound.The especially in cases of co-eluting metal species. Furthermore, best LODs, achieved in the positive-ion mode, were between 2 lack of appropriate standards could prevent identification and 21 pg of arsenic and corresponded to arsenicals which altogether.12 exist as cations in acidic solutions. The selectivity achieved by Electrospray mass spectrometry (ES-MS) is a relatively new using this method allows for successful determination of technique used with great success for the structural characterizarsenicals co-eluting during HPLC.This is a major ation of numerous compounds of biological and environmental improvement over other hyphenated techniques already used interest.13–15 The fact that ES is an extremely eective sample for arsenic speciation, e.g., HPLC–ICP-MS. Furthermore, the introduction technique for mass spectrometry, especially for method was used for the analysis of an undiluted urine SRM non-volatile and/or thermally labile species, has convinced in which arsenobetaine was determined to be present at the researchers to evaluate its potential use for the determination low mg l-1 level. of organometallic compounds, particularly those of tin and arsenic.16–21 For arsenic determinations only three studies have, Keywords: Arsenic; speciation; microbore high-performance so far, evaluated the usefulness of ES-MS.16, 17, 21 Initially, Siu liquid chromatography; electrospray mass spectrometry ; et al.16 reported on the atmospheric pressure chemical ioniz- triple quadrupole; mass spectrometry–mass spectrometry; ation MS and ES-MS of three arsenic compounds [dimethyl- collision-induced dissociation; selected-reaction monitoring arsinic acid (DMA), arsenobetaine (AsB) and arsenocholine (AsC)] and presented their collisionally-induced dissociation Elemental speciation has become increasingly important in spectra.In later work, Siu et al.17 reported on the development making risk assessments of toxic elements.1,2 A prime example of an HPLC–ES-MS–MS procedure for the determination of of this requirement is the need for arsenic speciation.To date three arsenic species [AsB, AsC and the tetramethylarsonium numerous arsenic compounds have been identified in biological ion (TMAs+)], which exist as cations in acidic solutions. The and environmental materials.3–5 These compounds exhibit procedure that was developed involved the separation of significant variations in toxicity towards living organisms, and the three arsenicals using cation exchange chromatography.thus must be identified and quantified if accurate risk assess- Detection was accomplished using ES-MS–MS operated in ments are to be made. Currently, no U.S. Environmental the selected-reaction monitoring mode. This was the first study Protection Agency methods exist for their speciation. to demonstrate the potential use of HPLC–ES-MS–MS for The initial approach for the identification of metal arsenic speciation in environmental samples.However, it only compounds present in environmental and biological samples included the determination of three organoarsenicals and the has involved compound isolation, purification and finally analysis of a single RM of marine origin. It is therefore structural characterization using various spectrometric tech- necessaryto investigatethe applicability of HPLC–ES-MS–MS niques.4–7 This approach requires the use of numerous, often for the speciation of arsenic in other materials as well.tedious, purification steps which result in increased risk of In the present study, the separation and detection of ten sample contamination and loss of analyte. Current state-of- arsenic compounds obtained using reversed-phase (RP) micro- the-art procedures for metal speciation involve the use of bore HPLC (mHPLC) coupled on-line with an ES triple hyphenated techniques.These techniques consist of a high- quadrupole mass spectrometer are reported. The method was resolution chromatographic method coupled on-line with a developed, optimized and evaluated for sensitivity, detection limits and capability to provide structural information on each † Present address: Institute for Environmental Studies, Free of the ten organoarsenicals examined. The analytical technique University, Amsterdam, The Netherlands.which resulted from this investigation was further evaluated E-mail: spiros.pergantis@ivm.vu.nl for its capability to identify arsenic species present in a ‡ Present address: New York University Medical Center, Nelson Institute of Environmental Medicine, Tuxedo Park, NY 10987, USA. urine RM. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (531–536) 531EXPERIMENTAL 3-nitro-4-hydroxyphenylarsonic acid (Roxarsone, ICN Biochemicals, Cleveland, OH, USA), p-arsanilic acid (p-ASA, Instrumentation Eastman Organic Chemicals, Rochester, NY, USA), A Finnigan MAT TSQ 700 triple-stage quadrupole (Q1, Q2, 4-nitrophenylarsonic acid (4-NPAA, Aldrich Chemicals, Q3) mass spectrometer (San Jose, CA, USA) equipped with Milwaukee, WI, USA), dimethylarsinic acid (DMA, Sigma an electrospray source was used throughout this study.A Chemicals, St. Louis, MO, USA),and disodium methylarsonate spray voltage of 4 kV and a capillary temperature of 225 °C (MMA, Chemical Service, West Chester, PA, USA).were applied. The mass spectrometer was operated in the Arsenobetaine (AsB),22 arsenocholine (AsC),23 trimethyl- positive ion mode. Argon was used as the collision gas in the arsine oxide (TMAsO)24 and tetramethylarsonium (TMAs+) second quadrupole. The third quadrupole was set to monitor iodide25 were synthesized using literature methods. Dr. the product ion of each arsenical in the selected-reaction W. R. Cullen (Department of Chemistry, University of monitoring mode using a 1.6 u window.British Columbia, Vancouver, Canada) kindly donated the An analytical microbore HPLC 150×1.0 mm i.d. stainless 4-hydroxyphenylarsonic acid (4-OH) compound. steel column (Isco Lincoln, NE, USA), packed with Spherisorb The NIST SRM 2670 Toxic Metals in Urine (labelled by 3 mm C18 material, was used throughout this study. HPLC NIST as containing normal levels of arsenic) was analyzed mobile phases were filtered and de-gassed using a 35 mm, all- during this study.The material was prepared as recommended glass filter holder (Millipore, Bedford, MA, USA), fitted with by NIST. The dried powder was dissolved in 20 ml of a 0.45 mm hydrophilic nylon filter (Cuno, Meriden, CT, USA) de-ionized water (18 MV cm resistivity). for aqueous solvents and a 0.5 mm poly(tetrafluoroethylene) filter (Cole-Parmer Instruments, Chicago, IL, USA) for organic solvents. RESULTS AND DISCUSSION A 1 ml internal loop injector (Valco Instruments, Houston, TX, USA) was used for mHPLC sample loading. A syringe Electrospray Ionization of Organoarsenicals pump (Model 100DM, Isco) was used to deliver pulse-free As part of this study the use of electrospray (ES) ionization microflows of 20 ml min-1.was evaluated for the structural characterization of ten organoarseniccompounds having a wide range of chemical structures Reagents and Samples and polarities (Fig. 1). Chromatographic standards were prepared from 1000 mg l-1 Arsenic animal feed additives have been used for their of As aqueous solutions of the following arsenic compounds: growth-promoting and disease-controlling properties in both Fig. 1 Structures of the ten organoarsenic compounds studied. 532 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12poultry and swine.26 Because of their non-volatile nature it is Dierences would only be expected when source and interface conditions are not adjusted to maximize declustering and not possible to analyze them without prior derivatization using GC, or using electron impact or desorption chemical ionization minimize in-source CID fragmentation.For compounds (4) and (7) the only ions observed were at MS. Other mass spectrometric techniques used with relative success for the analysis of this group of arsenicals include: m/z 139 and 179, respectively, corresponding to the protonated molecules of each compound. For compounds (9) and (10), thermospray (TS),27 liquid secondary ion mass spectrometry (LSIMS)27 and HPLC–ICP-MS.28,29 For the first time ES which exist as cations in solution, intense (100%) molecular ions were observed at m/z 165 and 135, respectively.Again, no mass spectra of arsenic animal feed additives [compounds (2), (5) and (6)] are presented (Fig. 2). In the ES mass spectra of fragmentation was detected. In a previous ES-MS study it was reported that no response all these compounds the protonated molecule was the most intense ion.In addition, very little fragmentation, if any at all, was observed from solutions of compound (1), whether acidic or basic, in either positive or negative modes.16 However, the was observed. This allows for the selection of a very intense precursor ion for the collision-induced dissociation (CID) of observation of an ES mass spectrum for compound (1), containing an intense ion at m/z 141 (100%), corresponding each of the arsenic animal feed additives.Of considerable analytical interest is the examination to its protonated molecule can be reported here. Furthermore, the ES mass spectrum obtained for compound (8) is also of 4-hydroxyphenylarsonic acid [(3), 4-OH] by means of ES-MS. This arsenical is not an animal feed additive and has reported. Once again, the only ion observed was its protonated molecule at m/z 137 (100%). not been found to exist in the environment. However, it has been successfully used as an internal standard for mHPLC– ICP-MS.10,28,30 Thus, it is of interestto investigate its usefulness as an internal standard for mHPLC–ES-MS.The ES mass Collision-induced Dissociation of Organoarsenicals spectrum of 4-OH is presented in Fig. 2. As was the case with Adequate sensitivity and selectivity are the primary require- the other three phenylarsonic acids (Fig. 2), the spectrum of ments for the successful development and application of a 4-OH exhibits an intense ion (100%) corresponding to its mass spectrometric technique for the determination of organ- protonated molecule and very little fragmentation. The mass oarsenicals in complex sample matrices.One way of assuring spectral features of all four phenylarsonic acids obtained under this is by taking advantage of the MS–MS capabilities of the ES conditions are similar to those observed using continuous triple quadrupole analyzer. In order to study the behavior of flow LSIMS.27 the organoarsenicals under CID conditions, each compound The ES mass spectra obtained for organoarsenicals (4) was introduced into the ES mass spectrometer in the continu- (DMA), (7) (AsB), (9) (AsC) and (10) (TMAs+) are similar to ous flow or infusion mode, using a carrier solution containing those reported by Siu et al.16 This is to be expected since the 1% acetic acid in water–methanol (80+20% v/v) with a flow ions generated by ES reflect pre-formed ions in solution.rate of 20 ml min-1.Because ES is a ‘soft’ ionization technique (produces abundant molecular ion and protonated molecule species, and limited fragmentation), the most intense ion for the arsenicals examined was either their protonated molecule [(M+H)+] or in the case of pre-formed cationic species their molecular ion (M+). In the CID experiments the precursor (parent) ion selected for each compound was its protonated molecule, except for AsC and TMAs+, in which case their molecular ions were selected.The product (daughter) ion mass spectra of all ten arsenicals were recorded under these conditions. The relative intensities of the fragment ions resulting from CID are listed in Table 1. The purpose of obtaining this information was to enable selection of the appropriate pair of precursor and product ions, that would ensure maximum sensitivity, selectivity and stability during selected-reaction monitoring experiments. Acquisition in the selected-reaction monitoring mode substantially improves overall sensitivity.This is accomplished because each of the quadrupoles (Q1 and Q3) is operated in the single-ion monitoring mode. This allows for Fig. 2 Electrospray mass spectra of four phenylarsonic acids, maximization of the duty cycle of each analyzer. A list contain- obtained in the flow-injection mode. Solutions contained 1 mg ml-1 arsenic in water–methanol (80+20% v/v) with 1% acetic acid. ing the pairs of precursor–product ions used for monitoring Table 1 CID fragment ions and selected-reactions monitored for ten organoarsenic compounds.Fragment ions were obtained using 30 eV collision energy and 1 mTorr (0.1333 Pa) collision cell pressure Compound Mn* Precursor ion CID fragment ions† CID reaction monitored Methylarsonic acid (1) 140 141 141 (100); 123 (15); 109 (7); 91 (5) 141�123 p-Arsanilic acid (2) 217 218 218 (100); 109 (37); 108 (13); 92 (13) 218�109 4-Hydroxyphenylarsonic acid (3) 218 219 219 (100); 201 (16); 110 (31); 93 (10) 219�110 Dimethylarsinic acid (4) 138 139 139 (100); 121 (27); 108 (8); 91(8) 139�121 3-Nitro-4-hydroxyphenylarsonic acid (5) 263 264 264 (100); 246 (12) 264�246 4-Nitrophenylarsonic acid (6) 247 248 248 (100); 230 (12); 202 (16); 122 (5) 248�202 Arsenobetaine (7 ) 178 179 179 (100); 120 (18) 179�120 Trimethylarsine oxide (8) 136 137 137 (100); 122 (17); 107 (23) 137�107 Arsenocholine (9 ) 165 165 165 (100); 121 (5); 45 (9) 165�45 Tetramethylarsonium ion (10) 135 135 135 (100); 120 (44); 105 (15) 135�120 * Nominal relative molecular mass; the monoisotopic molecular mass rounded to the nearest whole number.† Numbers in parentheses correspond to % relative intensity. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 533each arsenical during FI and mHPLC analyses is also included in Table 1. It should be noted that the selected-reaction monitoring approach used for the determination of three organoarsenicals was first demonstrated by Siu et al.17 Once again, the data obtained in the present study for the three compounds (AsB, AsC and TMAs+) show agreement with the pairs of precursor–product ions that were reported in the study by Siu et al.However, it should be noted that the daughter ions obtained in the present study were of much lower relative intensity than those reported by Siu et al. This may result in sensitivity loss and thus should be further optimized.In the present study the collision energy has been found to have an optimum at around 30 eV in the laboratory frame of reference, for the formation of product ions from all arsenicals examined. Optimization was carried out by varying the collision energy in 5 eV increments. It should be noted that dierences in the relative inteter ions observed in the two independent studies are most likely since even if collision energies were the same, the target thicknesses were probably not identical.Reversed-phase mHPLC Coupled with ES-MS–MS Analytical methods involving ion-pair (IP) reversed-phase (RP) mHPLC with ICP-MS have been used with success for arsenic speciation.8–10 In the present study the development of a RP-mHPLC–ES-MS–MS procedure for the separation and detection of ten organoarsenicals is described. Use of IP reagents was avoided because of the analyte signal suppression they cause during ES ionization. Siu et al.17 have shown that in some cases the use of IP reagents causes considerable analyte signal loss. Also, in the present procedure, in contrast to that of Siu et al.,17 there was no need to split the mobile Fig. 3 Time-scheduled selected-reaction monitoring chromatogram phase flow rate. This was because a microbore (1 mm i.d.) obtained using mHPLC–ES-MS–MS for a solution containing ten column together with syringe pumps for solvent delivery were organoarsenic compounds. Mobile phase consisted of 1% acetic acid used.Both of these features provide improved stability and in water–methanol (80+20% v/v) and had a flow rate of 20 ml min-1. add to the overall ruggedness of the system. The mHPLC– Two ng of arsenic were injected for each compound, for AsB 0.5 ng of ES-MS–MS chromatogram obtained for ten organoarsenicals arsenic was injected. is presented in Fig. 3. As can be seen from the total-ion current chromatograms, complete chromatographic separation was not achieved.However, for the purpose of quantification, complete resolution of the analyte signals for the overlapping arsenicals is achieved using unique pairs of precursor–product ions during the selected-reaction monitoring. An additional feature used in this study is that of time-scheduled acquisition. This allows specification of a time period corresponding to the elution time of each arsenical, during which the mass spectrometer monitors a specific pair of precursor–product ions unique to each analyte.When two or more compounds co-elute, then the mass spectrometer is programmed to scan rapidly through the pairs of ions in sequential order. It should be noted that when using HPLC–ICP-MS all ten arsenicals were not resolved and thus could not be quantified in a single run (Fig. 4), as was accomplished using ES detection. Instead, at least two dierent chromatographic procedures are Fig. 4 mHPLC–ICP-MS chromatogram of ten organoarsenic com- required for complete separation of all ten arsenic compounds.pounds. Mobile phase of 1% acetic acid in water–methanol (80+20% This translates into requirements for additional HPLC columns v/v) was delivered at a flow rate of 20 ml min-1. Details of instrumen- and mobile phases, and ultimately results in a tremendous tation are given in ref. 10. increase in analysis time. The LODs obtained for all ten organoarsenicals using mHPLC–ES-MS–MS are presented in Table 2. It should be elute from the mHPLC system in relatively broad bands, however, they still exhibit the best LODs.The best LOD was noted that a number of factors significantly influence the LOD of each compound. Most significant of these factors are the achieved for AsB. Even though AsB is a zwitterion, it exists as a cationic species in acidic environments. Analysis of AsB by eciency with which each compound ionizes during the ES process, the intensity of the CID fragment ion monitored in using FI in the selected-reaction monitoring mode is presented in Fig. 5. Overall, the LODs achieved allow for the use of the the selected-reaction mode and also the chromatographic peak width of each arsenical. As expected, all the cationic arsenic mHPLC–ES-MS–MS method for ultratrace detection of organoarsenicals in environmental and biological samples. species provide a much higher ion signal compared with those of the remaining arsenicals. The preformed cationic species Compared with the LODs obtained using other mass 534 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Table 2 LODs for organoarsenicals, obtained by using mHPLC–ES-MS–MS in the selected-reaction monitoring mode Selected-reaction monitoring* LOD† pg of As per ml of Compound solution Methylarsonic acid (1) 305 4-Hydroxyphenylarsonic acid (3) 250 p-Arsanilic acid (2) 107 Dimethylarsinic acid (4) 72 3-Nitro-4-hydroxyphenylarsonic acid (5) 150 4-Nitrophenylarsonic acid (6) 145 Arsenobetaine (7) 2 Trimethylarsine oxide (8) 40 Arsenocholine (9) 21 Tetramethylarsonium ion (10) 15 * The corresponding CID reactions monitored are listed in Table 1.† Calculations based on SNR=3; 1 ml sample injections. Fig. 6 Selected-reaction monitoring chromatograms of a urine SRM (NIST SRM 2670) sample: A, signal obtained from monitoring reaction m/z 179�120, undiluted urine sample; B, signal obtained from monitoring reaction m/z 179�120, injected urine sample contained 200 pg of spiked arsenic as AsB; and C, signal obtained from monitoring reaction m/z 165�45, injected urine sample contained 500 pg of spiked arsenic as AsC.Fig. 5 FI-ES-MS–MS of AsB; acquisition in the selected-reaction was analyzed for its content of arsenic species. The only monitoring mode (m/z 179�120); 1 ml sample injections. arsenical detected in the urine sample was AsB (Fig. 6A). Quantitation by means of standard additions was required spectrometric techniques (fast atom bombardment,31 LSIMS,27 because of the severe matrix eects that cause suppression of DCI,32 FD–FI,33 matrix-assisted laser desorption/ionization34) the AsB signal by approximately an order of magnitude.The the LODs obtained using ES-MS–MS demonstrate that it is AsB was determined to be present in the NIST urine material the most sensitive mass spectrometric technique for arsenic at 11±3 mg l-1. It should also be noted that matrix eects speciation described to date. ICP-MS is an atomic spectro- cause a decrease in the retention time of AsB present in the metric technique and is thus not considered in the comparison.urine sample by approximately 30 s, compared with that of Further improvement of the ES sensitivity for some of the AsB in de-ionized water. Thus, even for identification purposes arsenicals may be achieved by acquiring data in the negative- sample spiking with AsB was necessary (Fig. 6B). The concenion mode. This can be accomplished by appropriately adjusting tration of arsenic as AsB in NIST SRM 2670 has been the mobile phase pH so that the arsenicals are present as determined to be 11.3±2.1 mg l-1 using HPLC–ICP-MS.29 anions in solution.It is expected that the sensitivity for This value is in very good agreement with the concentration compounds (1)–(6) could readily be improved by employing obtained in the present work. A urine sample spiked with AsC this approach. The inorganic forms of arsenic [arsenite was analyzed in order to evaluate this method for the detection (AsO33-) and arsenate (AsO43-)] may also be detected in this of other arsenic species.The resulting selected-reaction monifashion. Attempts to determine these two compounds using toring chromatogram of this sample (Fig. 6C) shows that ES-MS in the positive-ion mode were not successful. As stated 500 pg of arsenic as AsC can easily be detected. under Experimental all data reported were acquired in the The analysis of the urine NIST SRM 2670 using HPLC– positive-ion mode.ICP-MS has shown that anumber of arsenicals are present,30,35 mainly DMA, MMAand some inorganic arsenic species. These arsenicals were not detected when using mHPLC–ES-MS–MS, Determination of Arsenobetaine in Urine even though they are present at higher concentrations than AsB, this is because their LODs (Table 2) are significantly Arsenobetaine (AsB) is a major arsenic compound present in large amounts (usually mg g-1) in seafood.Upon consumption poorer than the LOD obtained for AsB. Further development of the ES technique is required in order to achieve their of crab, lobster, shrimph it is excreted rapidly into urine unchanged. Because urinary excretion is the major detection. It is expected that the use of negative-ion monitoring will substantially improve the overall sensitivity for these pathway for the elimination of arsenic from the human body, the analysis of urine samples is a convenient approach to the particular arsenic species.It should also be pointed out that when using anion-pairing HPLC–ICP-MS for urine analysis, study of arsenic metabolism. Therefore, it is important to develop an analytical technique capable of determining all the AsB may not be detected as it co-elutes with arsenite. This could lead to the incorrect identification of AsB as arsenite or major and minor arsenic compounds present in urine. In order to evaluate the mHPLC–ES-MS–MS method, a urine SRM vice versa.Recent mHPLC–ICP-MS studies involving two Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 5356 Lawrence, J. F., Michalik, P., Tam, G., and Conacher, H. B. S., dierent modes of chromatography have allowed the resolution J. Agric. Food Chem., 1986, 34, 315. of AsB and arsenite and thus have confirmed the presence of 7 Norin, H., Christakopoulos, A., Sandstro�m, M., and Ryhage, R., the former in NIST SRM 2670.30 Chemosphere, 1985, 14, 313. 8 Beauchemin, D., Bednas, M. E., Berman, S. S., McLaren, J. W., Siu, K. W. M., and Sturgeon, R. E., Anal. Chem., 1988, 60, 2209. CONCLUSIONS 9 Edmonds, J. S., Shibata, Y., Francesconi, K. A., Yoshinaga, J., and Morita, M., Sci. T otal Environ., 1992, 122, 321. It has been demonstrated that ES-MS–MS can be used for the 10 Pergantis, S. A., Heithmar, E. M., and Hinners, T. A., Anal. ecient speciation of arsenic compounds. The sensitivities, Chem., 1995, 67, 4530.LODs and selectivities achieved potentially allow for the 11 Morita, M., and Shibata, Y., Anal. Sci., 1987, 3, 575. analysis of environmental and biological samples. The determi- 12 Le, X.-C., Cullen, W. R., and Reimer, K. J., Clin. Chem., 1994, 40, 617. nation of AsB in an undiluted urine sample demonstrates the 13 Winger, B. E., Hofstadler, S. A., Bruce, J. E., Udseth, H. R., and capability of the technique for the analysis of extremely dicult Smith, R. D., J. Am. Soc.Mass Spectrom., 1993, 4, 566. matrix types, e.g., high salt content. However, this method of 14 Poon, G. K., Bisset, G. M. F., and Mistry, P., J. Am. Soc. Mass detection can further benefit from the development of chroma- Spectrom., 1993, 4, 588. tographic procedures that will enable the separation of the 15 Hughes, B. M., McKenzie, D. E., and Dun, K. L., J. Am. Soc. Mass Spectrom., 1993, 4, 604. analytes from the matrix components to be achieved. This is 16 Siu, K. W. M., Gardner, G.J., and Berman, S. S., Rapid Commun. of importance, especially for urine analysis, because of the Mass Spectrom., 1988, 2, 69. severe matrix eects causing analyte signal suppression. 17 Siu, K. W. M., Guevremont, R., Le Blanc, J. C. Y., Gardner, Also of interest for future research is the analysis of other G. J., and Berman, S. S., J. Chromatogr., 1991, 554, 27. arsenic-containing materials. 18 Siu, K. W. M., Gardner, G. J., and Berman, S. S., Anal. Chem., The combination of mHPLC–ICP-MS and mHPLC– 1989, 61, 2320. 19 Siu, K. W. M., Gardner, G. J., and Berman, S. S., Rapid Commun. ES-MS–MS will potentially provide the appropriate tools to Mass Spectrom., 1988, 2, 201. conduct successfully elemental speciation research at a more 20 Jones, T. L., and Betowski, L. D., Rapid Commun.Mass Spectrom., advanced level. Emphasis should be placed on using the 1993, 7, 1003. ES-CID technique when HPLC co-elution occurs or when 21 Corr, J. J., and Larsen, E.H., J. Anal. At. Spectrom. 1996, 11, 1215. standard compounds, corresponding to chromatographic 22 Edmonds, J. S., Francesconi, K. A., Cannon, J. R., Raston, C. L., peaks detected using ICP-MS, are not available. The latter Skelton, B. W., and White, A., T etrahedron L ett., 1977, 18, 1543. 23 Irgolic, K. J., Junk, T., Kos, C., McShane, W. S., and Parppalardo, may be an insurmountable problem for HPLC–ICP-MS, C. C., Appl. Organomet. Chem., 1987, 1, 403. especially when the structures of the compounds are not 24 Kaise, K., Hanaoka, K., and Tagawa, S., Chemosphere, 1987, known. 16, 2551. 25 Cullen, W. R., and Dodd, M., Appl. Organomet. Chem., 1989, 3, 401. 26 Gilbert, F. R., Wells, G. A. H., and Gunning, R. F., Vet. Rec., The U.S. Environmental Protection Agency (EPA), through 1981, 109, 158. its Oce of Research and Development (ORD), funded and 27 Pergantis, S. A., Cullen, W. R., Chow, D. T., and Eigendorf, collaborated in the research described here. It has been sub- G. K., J. Chromatogr., A, in the press. jected to the Agency’s peer review and has been approved as 28 Pergantis, S. A., Heithmar, E. M., and Hinners, T. A., in an EPA publication. The U.S. Government has a non-exclusive, preparation. royalty-free license in and to any copyright covering this 29 Dean, J. R., Ebdon, L., Foulkes, M. E., Crews, H. M., and Massey, R. C., J. Anal. At. Spectrom., 1994, 9, 615. article. Mention of trade names or commercial products does 30 Pergantis, S. A., Momplaisir, G.-M., Heithmar, E. M., and not constitute endorsement or recommendation of use. This Hinners, T. A., Proceedings of the 44th ASMS Conference on Mass work was carried out while S. A. P. and W. W. held National Spectrometry and Allied T opics, Portland, Oregon, 1996, p. 21. Research Council/CRD-LV Research Associateships. 31 Lau, B. Y., Michalik, P., Porter, C. J., and Krolik, S., Biomed. Environ. Mass Spectrom., 1987, 14, 723. 32 Cullen, W. R., Eigendorf, G. K., and Pergantis, S. A., Rapid REFERENCES Commun. Mass Spectrom., 1993, 7, 33. 33 Norin, H., and Christakopoulos, A., Chemosphere, 1982, 11, 287. 1 Vela, N. P., Olson, L. K., and Caruso, J. A., Anal. Chem., 1993, 34 Pergantis, S. A., Cullen, W. R., and Eigendorf, G. K., Biol. Mass 65, 585A. Spectrom., 1994, 23, 749. 2 Cullen, W. R., and Reimer, K. J., Chem. Rev., 1989, 89, 713. 35 Heitkemper, D., Creed, J., Caruso, J., and Fricke, F. L., J. Anal. 3 Edmonds, J. S., and Francesconi, K. A., Nature (L ondon), 1977, At. Spectrom., 1989, 4, 279. 265, 436. 4 Shiomi, K., Kakehasi, Y., Yamanaka, H., and Kikuchi, T., Appl. Paper 6/06416G Organomet. Chem., 1987, 1, 177. Received September 17, 1996 5 Edmonds, J. S., and Francesconi, K. A., Nature (L ondon), 1981, 289, 602. Accepted December 12, 1996 536 Journal of Analytical Atomic Spectrometry, May 1997, Vol.

ISSN:0267-9477    

DOI:10.1039/a606416g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Measurement of Molecular Species of Arsenic and Tin Using Elemental and Molecular Dual Mode Analysis by Ionspray Mass Spectrometry JAY J. CORR MDS SCIEX, 71 Four Valley Drive, Concord, Ontario, Canada L 4K 4V 8 Ionspray mass spectrometry has been employed for dual mode, unique opportunities for analysis of molecular forms of elemenmolecular and elemental, detection and quantification of tal species that standard inorganic detection techniques do not tributyltin and arsenobetaine. A selectable degree of ion– provide.ES-MS(–MS) and IS-MS(–MS) are capable of persolvent declustering and molecular fragmentation in the ion forming as dual mode detectors: as elemental analyzers, as well source–mass spectrometer interface region permitted detection as in their more typically utilized molecular modes. Such of fully intact molecular species, or fragments of the molceular techniques find natural application in elemental speciation species to any selected degree.Complete dissociation of the analysis. Since the toxicity, bioavailability and transport of molecular species in the interface region enabled detection of various elements are dependent on the exact molecular chemithe underlying element of interest. Tandem mass spectrometry cal form of the element present in a sample or system, it is (MS–MS) was used in the molecular detection mode to essential to determine the particular individual species present increase selectivity of the analysis.Instrumental parameter in the sample or system selectively. adjustment permitted dual mode detection for the same sample This requirement of individual species determination has on subsequent injections. Dual mode detection using FI was resulted in the coupling of element specific detectors with a performed to determine the tributyltin concentration in the variety of separation techniques, of which HPLC has been the National Research Council of Canada (NRCC) harbour most frequently employed. Separation techniques such as sediment reference material PACS-1.Measured tributyltin HPLC serve the purposes of separating the analytes of interest concentrations were 1.24±0.04 mg g-1 of Sn in the sediment from the sample matrix, as well as separating the individual (MS–MS mode) and 1.29±0.04 mg g-1 of Sn in sediment elemental species to be detected or measured. The most (elemental mode), compared with the certified value of frequently utilized element specific detectors for HPLC separa- 1.27±0.22 mg g-1 of Sn in sediment.The dual mode detection tions have been AAS, ICP, AES and ICP-MS.17 Although system was coupled to two forms of liquid chromatography, techniques such as HPLC–ICP-MS oer the advantages of cation exchange and ion-pairing, to determine arsenobetaine high selectivity and sensitivity as well as low detection limits, concentration in the NRCC dogfish muscle reference material species identification is based on chromatographic retention DORM-2, as part of the NRCC certification process.Results times compared with those of available standards. Since no of the two detection modes with both chromatographic molecular information is available from a standard elemental methods were consistent, and yielded a mean arsenobetaine detector, identification of dierent species of the same element concentration of 16.6±0.6 mg g-1 of As in the material. depends entirely on the separation technique.Species which Although the certification process is not complete, this result co-elute require modification of the separation conditions and, was consistent with an expected arsenobetaine level slightly again, comparison with separations of standards, often a higher than previously measured values of 15.7±0.4 mg g-1 of tedious or impossible proposition for certain species. For real As in for DORM-1, the NRCC predecessor reference material samples and the presence of complicated sample matrices, to DORM-2.Additionally, an intermediate mode of interface retention times of chromatographic peaks may shift compared fragmentation coupled with MS–MS enabled the detailed with those of a mixture of standard substances, requiring mapping of possible collisionally induced dissociation channels determination of the proper chromatography and retention for arsenobetaine. time matching for the particular matrix. Chromatographic Keywords: Organotin; organoarsenic; tributyltin; peaks which are not identified by retention time matching arsenobetaine; speciation; ionspray mass spectrometry against standard substances resist identification, and will continue to do so until the appropriate standard substances become available. For these reasons, and particularly since Recently in the literature there has been an increasing number speciation analysis is becoming increasingly involved with of reports on the use of two atmospheric pressure ionization larger relative molecular mass species of the elements of (API) techniques, electrospray (ES) and ionspray (IS), as ion interest, a technique that combines molecular as well as sources for mass spectrometric analyses of monatomic and elemental detection capabilities is required.molecular elemental species.1–16 Both ES and IS, the pneumati- Triorganotins are utilized as fungicides, herbicides and insec- cally assisted version of ES, are ‘soft’ ionization techniques ticides owing to their toxicity to a variety of living organisms.which produce gas-phase analyte ions directly from pre-existing The trialkyltin compounds are extremely toxic to a number of ions in solution. They have been employed extensively, often aquatic organisms and have been found to accumulate in coupled with LC, for analyses of large organic moleclues such sediments. Tributyltin {TBT, [CH3(CH2)3]3Sn} or Bu3Sn as proteins, peptides and other biomolecules by MS and leaches into the marine environment mainly through its eec- tandem mass spectrometry (MS–MS).MS-MS, a molecular tive use in antifoulant paints for ocean-going vessels. The risks detection technique, facilitates elucidation of molecular strucassociatedwith TBT have been formally recognized, but despite ture through characteristic fragmentation patterns of a precurits ban or restriction as a component of antifoulants in a sor molecule as a result of collision induced dissociation (CID).The ES and IS sources for mass spectrometric detection oer number of countries, TBT continues to enter the marine Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (537–546) 537environment in large amounts. Gas chromatography has been elemental mode IS-MS, or ES-MS, quantification results involving coupling with HPLC. the most frequently used separation technique for speciation of organotin compounds, and is often coupled with one of the element specific detection methods discussed above, though these are not the most common GC detectors for these EXPERIMENTAL analyses. Derivatization procedures are used in conjuction Chromatographic Systems with GC, but the derivatization process and resulting cleanup procedures involved are problematic and result in long For arsenic speciation two chromatographic methods, ion- analysis times.18,19 HPLC has been coupled with AAS,20 ICP- pairing and cation exchange, were employed.Ion-pairing AES21 and ICP-MS22 for organotin speciation.However, these chromatography was performed using a diisobutyl techniques involving HPLC depend entirely on the chromato- n-octadecylsilane based Zorbax SB-C18 analytical column graphic separation and retention time matching against stan- (150×2.1 mm, 5 mm particle size). For arsenic standard sub- dards for species identification and oer no molecular or stances the mobile phase consisted of 10 mmol l-1 octanesul- structural information for confirmation of detection of tin fonate in a water–methanol (60+40) mixture adjusted at species.Hence, it would be advantageous to develop new pH 2.7 with acetic acid, delivered isocratically at 200 ml min-1. speciation techniques for determination of organotin com- For analysis of DORM-2 dogfish muscle reference material pounds. Organotin standard substances have been studied by the mobile phase was adjusted to a water–methanol (80+20) IS-MS(–MS) and ES-MS4,7,8,10,15 and quantification of mixture and the flow rate was increased to 250 ml min-1. For TBT in the National Research Council of Canada (NRCC) DORM-2 analyses an Applied Biosystems Aquapore C18 guard sediment reference material PACS-1 was accomplished by column (30×2.1 mm, 7 mm particle size) was also installed. IS-MS(–MS).10 The analytical power of dual mode detection Cation exchange chromatography was performed using a silica by IS-MS(–MS) is demonstrated in the present study by based Chrompack Ionospher Canalytical column (100×3 mm, performance of quantification of TBT in this same reference 5 mm particle size) with sulfonic acid functional groups.The material in both molecular and elemental modes of detection. mobile phase consisted of 15 mmol l-1 pyridinium formate in This includes presentation of the first quantification of a tin a water–methanol (80+20) mixture adjusted at pH 2.7 with species by elemental mode IS-MS, or ES-MS. formic acid,14,33 delivered isocratically at 1 ml min-1.For Arsenic occurs in the environment and in biological systems arsenic speciation studies the chromatographic mobile phases in a number of dierent organic and inorganic molecular forms were delivered by a Perkin-Elmer Series 200 LC pump, while or species. The quaternary arsonium compounds, arsenob- for tributyltin FI quantification experiments a Shimadzu etaine (AsB), arsenocholine (AsC) and the tetramethylarsonium LC-10AD HPLC pump was employed.The post-column ion (CH3 )4AS+ are essentially non-toxic. The formulae of the euent was split such that only 20 ml min-1 were pumped arsenic species of interest to this study are shown in Table 1. through the ionspray capillary. For arsenic speciation studies AsB has been the most extensively studied organoarsenic a Rheodyne 8125 low dead volume injector with a 5 ml (ion- species owing to the high levels of this compound found in pairing chromatography) or 20 ml (cation exchange chromatog- many edible fish, shellfish and seafood products.For arsenic raphy) stainless steel sample loop was used. For tributyltin speciation studies a variety of hyphenated analytical tech- quantification a Rheodyne 7520 fixed loop injector with a niques, involving the coupling of separation techniques with 0.5 ml sample loop was employed with a carrier solution flow element selective detectors, have been reported in the litera- rate of 20 ml min-1.For infusion experiments analyte solution ture.23,24 Soft ionization techniques, such as field desorp- was delivered by a Harvard Apparatus Model 22 syringe pump tion,25–27 thermospray28 and electron impact,29 have been used at 5 ml min-1. for introduction of organoarsenicals for MS detection. Fast atom bombardment27,30,31 and desorption chemical ionization32 MS permitted probing of organoarsenic molecular Ionspray Source structure by characteristic fragmentation induced in the ion source.Atmospheric pressure chemical ionization and Previous in-house investigations of elemental cations and anions, as well as oxo-anions, indicated the possibility of ES-MS(–MS) have facilitated deeper elucidation of CID characteristic fragmentation patterns.9 Dual mode, elemental memory eects in the ionspray source. Hence, for the present study and other concurrent elemental analysis studies, the and molecular, HPLC–IS-MS(–MS) has been used to study a number of organoarsenic species, including dimethylarsinyl- ionspray source was modified to reduce possible memory eects owing to adsorption of analytes on the fused silica riboside derivatives (arsenosugars), as standards and in real samples.14 That study presented the first coupling of HPLC capillaries normally used to carry solution through the ion spray device.The use of Teflon and stainless steel solution with elemental mode IS-MS, or ES-MS, for analysis of standards and real samples.Quantification of AsB in the dogfish transfer tubing in the ionspray source oerred no indication of adsorption of the analytes of interest in this study. A 0.1 cm muscle reference material DORM-1 has been performed by HPLC–ES-MS(–MS).11 The present study details the power i.d. stainless steel capillary carried solution from the splitter T and served as the needle probe. Coaxial to the solution carrying of dual mode detection by demonstration of the coupling of IS-MS(–MS) to cation exchange and ion-pairing capillary was a 0.4 cm i.d.stainless steel tube that delivered the nebulizing gas at 0.95 l min-1 for molecular mode oper- chromatography for molecular mode and elemental mode quantification of AsB in the NRCC reference material ation, and at 1.44 l min-1 for elemental mode analyses. The solution carrying capillary protruded approximately 2 mm DORM-2, the successor to DORM-1. Presented are the first from the nebulizer tube. The ionspray probe was oriented at Table 1 Formulae of organoarsenic species a severely oblique angle to the curtain plate (or sampling plate) of the mass spectrometer enabling sampling of only the edge Compound Formula of the ionspray aerosol cone into the mass spectrometer, Dimethylarsinic acid (DMA) (CH3)2AsO(OH) resulting in enhanced ionspray signal stability and sampling Trimethylarsine oxide (TMAO) (CH3)3AsO of less highly clustered analyte molecules.The two modes of Tetramethylarsonium ion (TMAs) (CH3)4As+ analysis required dierent ionspray needle probe potentials: Arsenobetaine (AsB) (CH3)3As+CH2COO- approximately 4.5 kV for molecular mode, and an increase of Arsenocholine (AsC) (CH3)3As+CH2CH2OH approximately 0.5 kV for elemental mode. 538 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Mass Spectrometer respectively)], or to scan the full mass range. In elemental mode, source and interface parameters were adjusted such that A triple quadrupole PE-SCIEX API 300 mass spectrometer not only were solvent clusters stripped from the molecules of (Fig. 1) was modified to permit elemental analysis as well as interest, but the molecules were fragmented or dissociated to the standard molecular analysis performed with such an instru- their underlying elements of interest. As in fully intact molecu- ment. This system utilized a dierentially pumped atmosphere- lar mode, the instrument was operated as a single quadrupole to-vacuum interface.Ions from the ionspray source were device with only the first quadrupole (Q1) performing mass sampled into the mass spectrometer system through a dry analysis for the purpose of monitoring particular ion masses nitrogen curtain gas between the curtain plate (or sampling as a function of time, or to scan the full mass range. plate) and the orifice. The pressure due to curtain gas flow in The mass spectrometer was usually operated such that in this region was minimally higher than atmospheric pressure.full scan mode the valleys between peaks diering by 1 Da The curtain gas assisted in the declustering of ion–solvent were less than 10% of the more intense peak. Full widths at clusters and the desolvation of ionspray droplets, as well as in half height were 0.6 Da. Full scan spectra were obtained using preventing contaminants from entering the mass spectrometer. 0.1 Da steps with 5 ms dwell times. SIM and MRMchromatog- Ions underwent a free jet expansion through the orifice into a rams were acquired with 250 ms dwell times.For TBT determi- dierentially pumped region maintained at a pressure of nation, SIM and MRM experiments involved setting Q1 for approximately 1 Torr (1 Torr=133.3 Pa). The potential dier- selective transmission of m/z 120 (elemental mode) or m/z 291 ence between the orifice and the electrically grounded skimmer (molecular modes), corresponding to the most abundant iso- is referred to as the declustering potential. Although a number tope of tin.of source and interface parameters were adjusted to select a particular mode of operation, the curtain gas flow and the declustering potential were the most critical. Upon passing Chemicals through the skimmer, ions entered an rf-only quadrupole (Q0) Tributyltin chloride was obtained from Aldrich Chemicals maintained at #5 mT. In the Q0 region, collisional focusing (96% purity, Milwaukee, WI, USA). This substance was dis- caused energetic cooling of the ions and forced them onto the solved in distilled, de-ionized water to form a 10-2 mol l-1 axis of the mass spectrometer system,34 resulting in increased stock solution of TBT (as the molecule).Distilled, de-ionized sensitivity. water was purified in-house with a Waters Milli-Q purification There were two forms of molecular mode operation: detec- system (Millipore, Bedford, MD, USA). The aqueous standard tion of fully intact molecular species and detection of collision- substances used to prepare arsenic standard solutions have ally induced fragments via MS–MS.In fully intact molecular been described previously.14,33 The species represented were mode, source and interface parameters were adjusted such that AsV, MMA (monomethylarsonic acid), DMA, TMAs, AsB and solvent clusters were stripped from the molecules of interest, AsC. Aqueous mixtures of these standards were prepared at yet the molecules were not fragmented.In this mode, the levels of 1 mg ml-1 or 200 ng ml-1 each (as species concen- instrument was operated as a single quadrupole device with tration) for HPLC–IS-MS(–MS) experiments. For infusion only the first quadrupole (Q1) performing mass analysis for experiments the individual arsenic and tin standard substances the purpose of monitoring particular ion masses as a function were prepared at the 1 mg ml-1 level in water–methanol of time [single ion monitoring (SIM)], or to scan the full mass (50+50) with 20 ml of 0.1 mol l-1 HCl added for each 5 ml of range.The second and third quadrupoles (Q2 and Q3, respect- solution (to aid signal stability). HPLC grade chemicals were ively) were operated in rf-only mode, resulting in transmission, purchased from Aldrich or Fisher Scientific (Nepean, Ontario, but not mass analysis, of ions. For MS–MS analyses the same Canada). The harbour sediment reference material, PACS-1, source and interface parameters were employed as for fully and the dogfish muscle reference material, DORM-2, were intact molecular mode.In this mode, mass analysis was obtained from the NRCC (Ottawa, Ontario, Canada). performed in both Q1 and Q3, while the enclosed rf-only Q2 quadrupole was filled with nitrogen collision gas to create a high pressure collision cell.35 Q1 was operated to allow trans- Sample Preparation mission of only masses corresponding to particular molecules Extraction of TBT from the sediment material followed the of interest, the precursor ions.These precursor ions impinged butan-1-ol method given by Siu et al.10 Briefly, 4 g of PACS-1 upon the collision gas in Q2 with a collision energy given by were placed in each of three 50 ml glass centrifuge tubes. the dc oset potential dierence between Q1 and Q2, multiplied Employing the method of standard additions, appropriate by the charge of the precursor ion exiting Q1. CID of the spikes were added to each tube as well as 8 ml of butan-1-ol.precusor ion occurred, resulting in characteristic fragment or The mixtures were placed in an ultrasonic bath for 1 h, and product ions for that specific collision energy. The third then centrifuged at 2000 rev min-1 for 10 min. The butan-1-ol quadrupole (Q3) was operated as a mass analyzer for the phase was removed from each centrifuge tube and diluted purpose of monitoring specific fragment ions as a function of to 25 ml with methanol containing 1 mmol l-1 ammonium time [single or multiple reaction monitoring (SRM or MRM, acetate, the FI carrier solution.Extraction of arsenic species from the DORM-2 reference material proceeded via the method employed by Beauchemin et al.29 for DORM-1. Briefly, 2 g of DORM-2 were placed in each of three 50 ml glass centrifuge tubes. Employing the method of standard additions, appropriate spikes were added to each tube as well as methanol (20 ml) and chloroform (10 ml) .The mixtures were placed in an ultrasonic bath for 30 min, and then centrifuged at 2000 rev min-1 for 10 min. The orange methanol–chloroform liquid containing the extract was removed from each tube and placed in 125 ml separatory funnels. The entire process was repeated, and the two extracts for each spike level combined in separatory funnels. Water (20 ml) and chloroform (20 ml) were added to the separatory Fig. 1 Schematic diagram of the ionspray tandem mass spectrometer system.funnels, which were then shaken vigorously and allowed to Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 539stand while phase separation occurred. The lower chloroform phases contained little arsenic and hence were drained and set aside. The upper water–methanol phases were drained and allowed to evaporate overnight with the assistance of a dry nitrogen gas flow. The yellow–brown residues of extraction were dissolved in 1 ml of water and sonicated for 10 min.Sample clean-up was performed to reduce the level of hydrophobic biogenic substances in the samples and therefore reduce matrix eects for ionspray sample introduction. This was accomplished through use of a Varian Mega Bond Elut C18 solid phase extraction (SPE) cartridge. Through this process the redissolved 1 ml of extract was diluted to 10 ml in water. RESULTS AND DISCUSSION Dual Mode Detection of TBT The ionspray or electrospray mass spectra of inorganic and low relative molecular mass organometallic cations generally contain high degrees of ion–solvent clustering which may dominate the spectra.These clusters and their relationship to the ionic species in solution have been studied in detail Fig. 2 Detection of TBT in three modes: (a) molecular, with 25 V by a number of groups.36–41 Results have been presented declustering potential; (b) MS–MS, product ions of m/z 291 with 10 eV which detail source–interface fragmentation of inorganic cat- collision energy; and (c) elemental, with 250 V declustering potential.TBT concentration was 1 mg ml-1 (as Sn). ions1–7,12,13,16 or low relative molecular mass organometallic cations4,7,8,10,14–16 to reveal the underlying element of interest in a bare or slightly clustered elemental form. A number of these studies presented relatively clean full scan spectra which molecular structure information in this mode are demonstrated in Fig. 2(b). All instrumental parameters up to and including were dominated by singly charged, completely declustered elemental ions, regardless of the charge of the analyte in Q1 were identical with those used in Fig. 2(a), including the declustering potential and the curtain gas flow. In this mode solution.1,3,7,12–16 One of the most important factors in obtaining such spectra was the use of an elevated declustering Q1 was not scanned, but was operated to allow selective transmission of only the TBT+ molecule, while Q3, the second potential, which produced more energetic collisions between ion–adduct molecules and the curtain gas in the atmosphere- mass analyzing quadrupole, was scanned across the mass range.This resulted in a product ion spectrum for TBT+. The to-vacuum interface of the mass spectrometer system. Kebarle and co-workers36–39 have explained the observation of singly dc quadrupole oset potential dierence between Q1 and Q2 was 10 V, providing a collision energy of 10 eV for the singly charged metals as being due to gas phase charge reduction.Another factor critical to obtaining complete dissociation to charged TBT+ molecule with the nitrogen collision gas. This collision energy was chosen since it resulted in the highest the underlying element of interest is an elevated curtain gas flow. Such a curtain gas flow assists in the declustering of ion– intensity for any fragment or product ion (m/z 235), an important consideration for analytical purposes, particularly adduct molecules and the desolvation of ionspray droplets, permitting optimized utilization of the declustering potential for quantification at low levels.The product ion at m/z 235 is [CH3(CH2)3]2SnH+ (or Bu2SnH+), corresponding to loss of for fragmentation or dissociation of molecules. This mode of ionization was applied for elemental mode analysis in the a butene group, CH2CHCH2CH3, as suggested by Siu et al.8,10 Ion path parameters downstream of the collision cell were present study. Three modes of detection (fully intact molecular, MS–MS optimized for maximum sensitivity for this product ion, while retaining mass resolution in Q3.Maximum sensitivity for the and elemental) are demonstrated in Fig. 2 for infusion of a 1 mg ml-1 (as Sn) standard solution of tributyltin (TBT). The product ion at m/z 235 was achieved at a collision energy for which complete dissociation of the precusor ion, TBT+, did full scan, fully intact molecular mode detection of the TBT+ ion, [CH3(CH2)3]3Sn+ or Bu3Sn+, at m/z 291 is shown in not occur. This is a common phenomenon in MS–MS.Small increments in collision energy permit access to further CID Fig. 2(a). A declustering potential of 25 V provided suciently energetic collisions in the dierentially pumped region to channels, distributing the total ion signal over an extended range of dissociation products. The other major product eliminate TBT–adduct clusters, yet resulted in no fragmentation of the TBT molecular ion.For most organometallic ion in the MS–MS spectrum is [CH3(CH2)3]SnH2+ (or BuSnH2+), at m/z 179, corresponding to further loss of a species, observation of the uncomplexed, yet unfragmented, molecule occurs only for a very narrow range of declustering butene group. Origin of the minor peaks at m/z 197 and m/z 253 is uncertain, but may be due to water adducts of the potentials and interface parameters. Attaining detection in this manner is desirable for analytical purposes since the TBT product ions as a result of water impurity in the N2 collision gas.Increasing the collision energy by 10 eV resulted in signal is not diluted over a range of clusters or fragments. Optimization of the fully intact molecular signal included complete dissociation of TBT+ and the appearance of a product ion at m/z 123 (SnH3+), corresponding to subsequent adjustment of the declustering potential, the curtain gas flow, the ionspray potential and nebulizer gas flow, and the ion loss of another butene group.Under conditions corresponding to the fully intact molecular mode, increasing the declustering optics up to Q1 . The background signal intensity across the full scan mass range is significant owing to the soft mode of potential to #100 V resulted in detection of this same set of fragment ions under single-MS conditions. The background detection used. While it is possible to optimize the system for maximum TBT+ signal, it is not possible to obtain selective level in the MS–MS mode is extemely low, particularly compared with the fully intact molecular mode [Table 2 lists signal- fragmentation of other molecules simultaneously, which therefore contribute to the background mass spectrum.to-background (S/B) ratios for the three modes of detection]. This is due to the high selectivity of the MS–MS process, and The MS–MS mode of detection and the availability of 540 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Table 2 S/B and DL for TBT and AsB for three modes of detection. concentrations in this same reference material, but their deter- S/B values are for infusion of standards. DLs are for FI minations were limited to the MS–MS mode only. of PACS-1 (TBT) and cation exchange chromatographic analysis of Siu et al.10 indicated that dilution of the PACS-1 extract DORM-2 (AsB) into a polar methanol–acetonitrile FI carrier solution has two major advantages.Firstly, TBT ionization was promoted since TBT AsB TBT compounds, such as acetates, are polar and are expected Mode S/B DL/fg S/B DL/fg to dissociate readily, by heterolytic cleavage of the TBT– acetate bond, into TBT+ and a counter ion. This is critical, Molecular 350 — 200 — MS–MS 90550 64 92650 415 since for a particular compound to be observed from an Elemental 13550 64 7550 415 electrospray or ionspray source, the compound must be ionized in the solvent being sprayed.Secondly, this carrier solution is very favourable to the ionspray process. The eciency of oers decreased detection limits (DL), particularly for MRM transfer of a particular ion from the liquid to the gas phase experiments in which precusor and product ion masses are depends, among other factors, upon ion evaporation at atmosspecifically selected. Hence the MS–MS mode and MRM pheric pressure. The lower surface tension of methanolexperiments are the methods used most often for organic containing droplets facilitates disruption of the bulk liquid analyses.surface of a droplet, allowing for more rapid evaporation of Elemental mode detection is demonstrated in Fig. 2(c), a full solvent. Hence the sprayed droplets more readily achieve the scan single-MS spectrum. The only feature in the spectrum is critical electric field on their surfaces necessary for ion transfer the tin isotopic distribution pattern near m/z 120, resulting in to the gas phase.The result is improved stability and sensitivity a spectrum similar to that obtained by ICP-MS. A declustering of signal. potential of 250 V was used and the curtain gas flow was The triplicate injection, dual mode quantification of TBT in increased 52% to 1.44 l min-1. The increased curtain gas flow PACS-1 sediment reference material is shown in Fig. 3. In each assisted in solvent cluster removal and desolvation of ionspray of the figures, the first three peaks correspond to triplicate droplets, leaving more of the collision energy in the orifice– injections of unspiked extracts, while the second and third skimmer dierentially pumped region available for molecular sets of triplicate injections correspond to extracts including fragmentation.It also shortened the mean free path of the spikes of 1.48 and 2.96 mg g-1 of Sn in sediment, respectively. molecules and their fragments in the interface region, resulting Elemental mode detection is shown in Fig. 3(a), and MS–MS in enhanced dissociation. The degree of fragmentation attain- mode detection is displayed in Fig. 3(b). In the elemental mode, able is equally a function of the number of collisions the all operating conditions were the same as for Fig. 2(c), except molecules experience and the energies of the collisions. In that the declustering potential was increased to 350 V to general, elemental fragments or significantly dissociated mol- prevent detection of the tin hydride.In this mode of operation ecules passing through the skimmer and into Q1 have experi- the declustering and fragmentation conditions were sucient enced a substantial electric field between the orifice and the to dissociate most compounds present in the PACS-1 extract. skimmer. This results in fragment kinetic energies too large This mode of analysis is analogous to ICP-MS detection in for ecient quadrupole mass analysis with good mass reso- that the harsh ionization conditions are expected to dissociate lution.In the literature, there are several examples of mass any concomitant compounds which may be present at the spectral resolution being sacrificed in favour of a higher degree detected mass of interest. As in ICP-MS analyses, sample of fragmentation and increased sensitivity. Collisional focusing34 in Q0 of this mass spectrometer energetically cooled the fragment or elemental ions such that they trickled out of Q0 and into the mass analyzing quadrupole, Q1, with approximately 1 eV of kinetic energy, enabling improved mass resolution.This same collisional focusing forced the elemental or fragment ions onto the axis of the mass spectrometer as they were energetically cooled, permitting increased transmission and hence higher sensitivities than in other studies. Expansion of the tin isotope distribution pattern in Fig. 2(c) reveals a small peak at m/z 121, where no tin isotope exists.This is due to the fact that TBT+ readily dissociates to SnH3+, but in this configuration the hydrogen is bound substantially tighter to the tin atom. Under the fragmentation conditions employed for this spectrum there is a low level of SnH+ detected since the declustering potential is sucient to dissociate two of these hydrogens, but not sucient to fragment the SnH+ molecule completely. Operation at a higher declustering potential (#350 V) permitted elimination of the final hydrogen, but resulted in a sensitivity compromise of approximately 25%.This is the first report of the use of IS-MS (or ES-MS) for predominant detection of Sn+ at m/z 120 compared with detection of SnH3+ at m/z 123. Dual Mode Quantification of TBT To investigate the dual mode quantification possibilities of IS-MS(–MS), TBT concentrations were determined in the Fig. 3 Dual mode FI quantification of TBT in sediment reference NRCC harbour sediment reference material PACS-1, using material PACS-1: (a) elemental, single ion monitoring of m/z 120; and both MS–MS and elemental modes of detection for FI.Siu (b) MS–MS, selected reaction monitoring of Q1 at m/z 291, Q3 at m/z 235 with 10 eV collision energy. et al.10 have previously used IS-MS(–MS) to measure TBT Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 541preparation is critical for elemental mode IS-MS since any other tin species extracted from the sediment, and which the interface region was capable of dissociating to elemental constitients, would be detected in the elemental mode.Fortunately, the extraction method utilized was exclusive for TBT and there were no other tin species present in the extract. Otherwise, using FI with the elemental mode would not be sucient for unambiguous determination and a separation technique would become neecessary. For five replicate analyses the TBT concentration in PACS-1 was determined to be 1.29±0.04 mg g-1 of Sn in sediment by elemental mode IS-MS, compared with the certified value of 1.27±0.22 mg g-1.Background subtracted FI peak areas were calculated to determine the measured TBT concentrations. In the MS–MS mode detection displayed in Fig. 3(b), single reaction monitoring (SRM) was utilized to monitor a specific precursor ion–product ion transition as a function of time. Q1 was operated to transmit selectively only ions of m/z 291, corresponding to TBT+, while Q3 analyzed only product ions of m/z 235, corresponding to CID loss of a butene group.A Q2 collision energy of 10 eV was selected for optimum product ion production at m/z 235. Scans were performed for many Fig. 4 Detection of AsB in three modes: (a) molecular, with 30 V other tin species, but none was detected. The high specificity declustering potential; (b) MS–MS, product ions of m/z 179 with 27 eV of this mode of detection allows for a less ambiguous analysis, collision energy; and (c) elemental, with 300 V declustering potential. which is often necessary for complex samples.Although the AsB concentration was 1 mg ml-1 (as the molecule). TBT extraction used for this analysis was specific for TBT, such extractions are not available or practical for all analytes of interest. For five replicate analyses the TBT concentration a collision energy of 27 eV. This collision energy was chosen since it yielded the highest sensitivity for any product ion, the in PACS-1 was determined to be 1.24±0.04 mg g-1 of Sn in sediment by MS–MS mode ionspray mass spectrometry, com- fragment at m/z 120.All instrumental parameters up to and including Q1 were identical with those used in Fig. 4(a). Q1 pared with the certified value of 1.27±0.22 mg g-1. Experiments in which the spike was added after the extrac- was operated to allow selective transmission of only the AsB H+ precursor molecule, and Q3 was set to perform full tion yielded identical TBT concentrations.The above experiments corresponded to the actual injection of 12.7 pg of TBT mass scans. The resultant product ion spectrum for AsB H+ is significantly more complicated than that oerred by TBT in (as Sn) and detected concentrations of 25.4 ng ml-1. Further experiments were conducted to determine the levels of detection Fig. 2(b). For TBT, the CID characteristic fragmentation followed one route only, loss of butene groups. For AsB a more possible, for this sample, using both modes of detection.For both elemental mode and MS–MS mode it was possible to diverse set of CID channels is apparent. The most intense product ion is the trimethylarsine ion, (CH3)3As, at m/z 120, dilute the PACS-1 extract by a factor of 200 and obtain an S/B of 251, corresponding to injection of 63.5 fg of TBT (as representing loss of CH3CO2 or loss of CH2CO followed by loss of OH. The product ion at m/z 105, (CH3)2As, corresponds Sn) and a detected concentration of 127 pg ml-1 (DLs are listed in Table 2).The agreement between the two modes of to the further loss of a methyl group and appeared only as a result of dissociation of the trimethylarsine ion. Another major, detection for quantification of TBT in PACS-1, their agreement with the certified value and these levels of detection indicate though less intense, dissociation channel was loss of water to yield the product ion (CH3)3AsCHCO at m/z 161. The other that dual mode ionspray mass spectrometry is a viable technique for elemental speciation studies.major CID characteristic fragment ion, at a collision energy of 27 eV, was (CH3 )3AsOH at m/z 137, representing loss of CH2CO. Further dissociation of these product ions and the Dual Mode Detection of Arsenobetaine study of minor, less intense, product ions will be presented in the next section. Three modes of detection (fully intact molecular, MS–MS and elemental) are demonstrated in Fig. 4 for infusion of a Elemental mode detection of AsB is demonstrated in Fig. 4(c), which is a Q1 single-MS, full scan spectrum. The 1 mg ml-1 (as the molecule) standard of arsenobetaine (AsB). The corresponding arsenic concentration was 421 mg ml-1. spectrum was acquired with a declustering potential of 300 V and the curtain gas flow was increased in the same manner as The previously described acid addition to the sample matrix assisted in protonation of the AsB molecule. Although AsB is the TBT case.The dominant features in the spectrum are a peak for As+ at m/z 75, and a peak for AsO+ at m/z 91. a zwitterion it exists as a cation in acidic solutions. The full scan spectrum of Fig. 4(a) shows fully intact molecular mode Otherwise the spectrum is extremely clean, though at lower masses the background is increased owing to fragmentation of detection of the protonated AsB molecule at m/z 179. A declustering potential of 30 V was employed and Q1 single-MS other substances in the solution. In ICP-MS analyses, detection of the bare arsenic ion presents a problem owing to the ArCl+ scanning parameters were the same as for Fig. 2(a). As in Fig. 2(a), the declustering and interface parameters were interference at m/z 75. No such interference exists for IS-MS detection in the elemental mode, suggesting this technique as adjusted such that the solvent adducts were stripped from the fully intact molecule, yet little molecular fragmentation complementary to ICP-MS, specifically for m/z values at which ICP-MS experiences interferences. The oxide at m/z 91 is occurred.The only other significant feature in this spectrum is the protonated methanol dimer at m/z 65, originating from the dicult to eliminate, and is extremely dependent on the position of the ionspray probe, as well as on the interface and sample matrix. Use of a dierent solvent in the standard solution generally yields analogous detection of a correspond- ionspray parameters.Small changes in ionspray probe position, nebulizer gas flow, ionspray potential, curtain gas flow rate ing solvent molecule. In Fig. 4(b) the MS–MS mode of detection is displayed for and interface potentials may have profound eects on oxide 542 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12formation and detection. In general the same situation exists ceeded to yield a fragment at m/z 103, which could correspond to CH3AsCH (loss of CO) or AsCO (loss of CH3CH).for any element which has a high anity for oxide formation. This is in contrast to the situation for TBT, where detection Observation of the next fragment at m/z 91 could represent CH3AsH or AsO, but the technique is not capable of oerring of the bare metal ion, Sn+, was accomplished with little eort. Positioning the ionspray needle probe at very oblique angles further elucidation of the dissociation pathway. As the collision energy was increased from 0 to 10 eV the first product of to the curtain plate, and spraying past the aperture in the curtain plate, such that only the edge of the aerosol cone was AsB H+ to appear was (CH3 )3As at m/z 120, indicating that loss of CH3CO2 was the preferred mechanism.At 15 eV sampled into the mass spectrometer, was empirically found to facilitate reduction of oxides. The S/B values for the three collision energy, the previously discussed product ion at m/z 137 appeared, which could loseOH to yield this same fragment modes of detection are listed in Table 2.at m/z 120. Another observed path of dissociation for m/z 137 was sequential loss of methyl groups to yield (CH3)2AsOH at Moderate Dissociation of Arsenobetaine in the Interface m/z 122 and CH3AsOH at m/z 107. All dissociation pathways were verified by precursor ion scans. In such a scan, Q3 is The fully intact molecular and elemental modes shown in operated to permit only transmission of product ions of a Fig. 4 demonstrate dissociation and fragmentation in the particular m/z and Q1 is scanned over the selected full m/z source–interface region at two extremes. Moderate declustering range to indicate which precursor ions resulted in the selected potentials may be selected which produce substantial, but not fragment. The dissociation mechanisms discussed above are total, dissociation of the molecular species. Operation in such only tentatively suggested and have only involved fragment a mode provides another level of information on molecular ions previously proposed by other groups.9,26,27,30–32 Such structure by, in eect, oerring another quasi-degree of MS.A operation of the IS-MS(–MS) system clearly permitted more Q1 single-MS full scan spectrum acquired for a 1 mg ml-1 (as extensive probing into the structure and dissociation of the the molecule) standard of AsB with a declustering potential of AsB molecule than previous studies were able to achieve. 150 V, is shown in Fig. 5. Mass spectral resolution was compro- Similar dissociation pathways have been mapped out for the mised somewhat to enhance sensitivity owing to the fact that less complicated TMAs, DMA and AsC molecules, but are not the total ion signal was spread over a number of fragment ion included in this paper. masses under these dissociation conditions. The m/z values detected range from the fully intact protonated molecule at m/z 179 to the bare elemental arsenic ion at m/z 75.The LC–IS-MS(–MS) of Arsenic Standard Substances MS–MS product ions of Fig. 4(b) reside in the moderate declustering potential spectrum of Fig. 5. The cation exchange chromatography performed in this work has been described in detail elsewhere,14 hence only the ion- To probe more extensively into AsB molecular structure and dissociation, source and interface conditions were adjusted to pairing chromatography will be described in this section. Three modes of detection (fully intact molecular, MS–MS and optimize production of a particular fragment, upon which MS–MS was then performed at a number of dierent collision elemental) for ion-pairing separation of five co-injected arsenic species are demonstrated in Fig. 6. The arsenic species were at energies. Every fragment produced in the source–interface region and then studied by MS–MS was subjected to extreme concentrations of 200 ng ml-1 each (as the molecule), corresponding to injections of 4 ng of each molecule, or approxi- MS–MS conditions to verify that it contained arsenic. Under such extreme CID conditions bare As+ was observed at m/z mately 2 ng of As for each species.Euent splitting prior to the ionspray probe resulted in approximately 0.4 ng of each 75, but the conditions necessary to achieve such extensive fragmentation were detrimental to signal intensity and resolution, rendering the CID spectra of little use other than to verify the presence of As+.For example, source and interface conditions were adjusted to provide optimum sensitivity for the previously discussed major fragment at m/z 161. MS–MS was then performed with Q1 operating such as to permit only transmission of ions of m/z 161. With Q3 operating in full scan mode, product ion spectra for m/z 161 were produced. It was found that m/z 161 dissociated preferentially, and exclusively, by loss of a methyl group to yield (CH3 )2AsCHCO at m/z 146. This CID product ion, in turn, lost another methyl group to yield CH3AsCHCO at m/z 131.The CID reaction then pro- Fig. 6 Ion-pairing LC–IS–MS(–MS) detection of five co-injected arsenic species in three modes: (a) molecular, with 35 V declustering potential; (b) MS–MS, with 25 eV collision energy; and (c) elemental, single ion monitoring of m/z 75 with 350 V declustering potential. Fig. 5. Moderate fragmentation of AsB in the interface region, with Figs. (a) and (b) are the summed, or TIC chromatograms, for the separate chromatograms corresponding to the arsenic species. 150 V declustering potential. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 543molecule, or#0.2 ng As for each species, reaching the detector. which target a single particular species often significantly improve detection for that particular analyte. Seven arsenic species were included in the samples injected, The fully intact molecular mode single ion chromatograms but AsV and MMA were not detectable in the positive ion for the five arsenic species are shown in Fig. 7. The TIC mode under the experimental conditions used. Using continuchromatogram presented in Fig. 7(a) is the same as the chroma- ous infusion of AsB in the mobile phase, the IS-MS(–MS) togram in Fig. 6(a). In fully intact molecular mode the five system was optimized for detection of AsB in each of the three single ion chromatograms provide selective detection of the modes of detection. Sprayer position was optimized for AsB five arsenic species, and in particular, the ability to distinguish detection in the elemental mode, since of the five species chromatographic peaks corresponding to TMAs and TMAO.detected AsB was the most dicult to dissociate to bare The single ion chromatograms for m/z 135 (TMAs) and m/z elemental As+. The sprayer position was not changed when 137 (TMAO) indicate that the chromatographic peaks corre- switching between detection modes. In each of the three modes, sponding to these species contribute peak heights of 12000 the 10 mmole l-1 octansulfonate mobile phase was responsible and 16000 cps (counts s-1), respectively, towards the total for suppression of the sensitivity, owing to matrix eects, by peak height of 48000 cps in Fig. 6(a). However, fully intact approximately a factor of five as compared with infusion of molecular mode detection oers the highest DLs of the three AsB in the 50% methanol solution previously described.detection modes owing to elevated background levels, and However, the low pH of the mobile phase promoted cation hence is very limited for analytical and quantification purposes. formation by protonation of the DMA, TMAO and AsB In elemental mode, Fig. 6(c), the elevated declustering potential molecules which were detected as protonated molecules in the resulted in a minimization of the background level, therefore molecular modes. Protonation was not necessary for the increasing S/B values.The high selectivity of MS–MS also permanent cations TMAs and AsC. results in high S/B values. The total ion current (TIC) chromatogram for fully intact The extracted ion chromatograms for the individual MS–MS molecular detection is displayed in Fig. 6(a). The TIC chroma- transitions are shown in Fig. 8. The TIC chromatogram pre- togram is the sum of the individual chromatograms corre- sented in Fig. 8(a) is the same as in Fig. 6(b). Each of the SRM sponding to the single ion monitoring of TMAs (m/z 135), chromatograms is the result of the monitoring of the most TMAO H+ (m/z 137), DMA H+ (m/z 139), AsC+ (m/z 165), intense CID fragment of a particular precursor molecule, after and AsB H+ (m/z 179).Only Q1 was utilized as a mass only that particular precursor molecule was transmitted analyzer, and the declustering potential was 35 V for optimized through the first analyzing quadrupole, Q1. For example, the AsB sensitivity.The TIC chromatogram for MS–MS detection chromatogram in Fig. 8(c) corresponds to Q1 operating such of five individual CID transitions, with a collision energy of as to permit only transmission of ions of m/z 179 (AsB H+), 25 eV, is shown in Fig. 6(b). For DMA, TMAO and AsB and Q3 detecting only the m/z 120 fragment ions. For DMA, detection the precusor ion was the protonated molecule. For TMAO and AsB detection the precursor ions were the pro- the individual SRM chromatograms the fragment with the tonated molecules.Owing to the co-elution of TMAs and highest sensitivity under these CID conditions was monitored TMAO it is not possible to claim that these species do not as a function of time. All instrumental parameters up to and interfere with one another in some manner, but it may be including Q1 were identical with those employed in Fig. 6(a). stated that there is no interference between DMA and AsB The chromatogram for single-MS elemental mode detection of and either of the other two species.These individual chromato- As+ at m/z 75, using a declustering potential of 350 V is given grams represent extremely selective detection of specific arsenic in Fig. 6(c). The chromatograms in Fig. 6 each display less species, a fact that presents several advantages for chromato- than five chromatographic peaks since TMAs and TMAO graphic detection. The specificity oerred by IS-MS(–MS) co-eluted at a retention time of 7.2 min.Additionally, column detection for chromatographic separations may be used to problems resulted in inconsistent retention of AsC. AsC eluted reduce the necessity of complete chromatographic resolution in the void volume or at retention times greater than 20 min. of all species. This results in shorter analysis times and less The reason for this behaviour is unknown, though the column expired shortly thereafter (these chromatograms were obtained after the AsB quantification work).Hence, AsC was detected in fully intact molecular mode and as a shoulder on the leading edge of the peak for DMA in the elemental mode data, while in the MS–MS mode AsC was not detected in the 9 min data acquisition. The elemental mode chromatogram in Fig. 6(c) is analogous to LC detection by ICP-MS in that no further selectivity is available. In order to detect TMAs and TMAO unambiguously the chromatography would require modifi- cation. However, molecular detection modes provide the speci- ficity necessary for unambiguous detection of the various species.Although the gross features of the three chromatograms presented in Fig. 6 are similar, relative intensities for chromatographic peaks dier depending on the detection mode. These dierences in sensitivities for the various species indicate factors which must be considered when employing molecular ionization techniques for analysis of more than one species in a single sample. Several of these factors have been presented in a recent publication,14 and will not be repeated in the present discussion.It is sucient to summarize that for any given solution or mobile phase, the ionspray (or electrospray) and Fig. 7 Fully intact molecular mode ion-pairing LC–IS–MS detection mass spectrometer interface parameters selected will promote of five co-injected arsenic species: (a) TIC chromatogram; (b) AsC, m/z more intense detection of some species than others. Since many 165; (c) DMA, m/z 139; (d ) AsB, m/z 179; (e) TMAO, m/z 137; and ( f ) TMAs, m/z 135. The declustering potential was 35 V.compounds react dierently to the ionspray process, analyses 544 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12method, five replicate measurements were made. In all cases, the standard deviation of the variance of the background subtracted peak areas (for the same spike level) was less than 5%, and in two cases was under 3%. Results of dual mode determination of AsB in DORM-2 are shown in Table 3.Ionpairing results were identical for MS–MS and elemental modes of detection. For cation exchange chromatography the dual mode results were within statistical error of each other. The results of the two chromatographic methods, in either detection mode, were also consistent. These results were included in the NRCC certification procedure for AsB in DORM-2. Since the process is not yet complete, a final certified value for the AsB concentration in DORM-2 is not yet available. However, it is expected that the AsB concentration will be slightly higher than previously measured value11,29 of 15.7±0.4 mg g-1 of As in material for DORM-1, the NRCC predecessor reference material to DORM-2.The results presented above are consistent with this expectation. For cation exchange chromatography the above quantifi- cation measurements represented detection of AsB at a concentration of 133 ng ml-1 (as As), corresponding to an injected amount of 2.7 ng, and 66 pg of As reaching the detector.For ion-pairing chromatography all levels were a factor of ten higher. Further experiments were conducted to determine the Fig. 8 Molecular mode ion-pairing LC–IS–MS(–MS) detection of levels of detection attainable employing these chromatographic four co-injected arsenic species: (a) TIC chromatogram; (b) DMA, Q1 methods and ionspray mass spectrometry detection. For both at m/z 139, Q3 at m/z 121; (c) AsB, Q1 at m/z 179, Q3 at m/z 120; (d) MS–MS and elemental modes of detection for cation exchange TMAO, Q1 at m/z 137, Q3 at m/z 122; and (e) TMAs, Q1 at m/z 135, chromatography it was possible to detect, at a 251 S/B, Q3 at m/z 120.The collision energy was 25 eV. concentrations of 830 pg ml-1 (as As), which corresponded to 16.6 pg of AsB (as As) injected, and 415 fg reaching the detector complex chromatography. For ionspray or electrospray, further (detection limits are listed in Table 2).These levels of detection advantages may be realized with the possibility of reducing were a significant improvement over the values presented by the ionic strengths of the mobile phases, thus reducing matrix Corr and Larsen14 using the same cation exchange chromatoge ects, and therfore promoting increased sensitivity and raphy. In that work crude extracts were injected with no prior reduced detection limits. sample clean-up procedure occurring.This indicates the significance of presenting samples in a matrix as amenable as possible to the ionspray process. Dual Mode Quantification of Arsenobetaine To test the dual mode quantification capabilities of IS-MS(–MS) further, AsB concentration was determined in CONCLUSIONS the NRCC dogfish muscle reference material DORM-2, using The agreement between the two modes of detection (elemental both MS–MS and elemental modes of detection coupled with and MS–MS) for quantification of TBT and AsB in certified LC.Again, Siu et al.11 have previously used IS-MS(–MS) to reference materials, and the detection levels attainable for these measure the AsB concentration in DORM-1, the predecessor samples, indicates that ionspray mass spectrometry is a viable to DORM-2, but their determinations were limited to the technique for elemental speciation studies. The molecular MS–MS mode only. structure and dissociation information provided by the molecu- Quantification of AsB in DORM-2 was performed using lar modes of this technique, as illustrated by the AsB example, both cation exchange and ion-pairing chromatography.indicate that ionspray mass spectrometry has significant poten- MS–MS and elemental modes of detection were employed for tial for the development of future speciation methodologies. each chromatography. Infusion of AsB in the appropriate The dual mode capability of the technique oers possibilities mobile phase was used to optimize the IS-MS(–MS) system that other methods of detection do not provide and hence this for AsB detection for each method.Owing to the expectation technique may prove important in furthering the state-of-the- that AsB would be present at significant levels in the DORM-2 art in speciation analysis. reference material, sample dilution prior to injection was considered. In fact, sample dilution was necessary owing to severe column overload in the void volume, particularly since The author thanks Michael Siu of NRCC for many helpful AsB eluted close to the void volume.This was especially true discussions and for the DORM-2 reference material. for ion-pairing chromatography, in which the myriad other substances in the sample caused the AsB retention time to be reduced (compared with the retention time of the standard substance) to the point where AsB detection was interfered Table 3 Quantification results for determination of AsB in DORM-2.Concentration units mg g-1 of As in the material; n=5 replicates for with by matrix eects from the void volume. As a result, a each detection mode for each chromatographic method mobile phase with increased aqueous content, compared with the mobile phase used for standards separations, was employed Both chromatographic for ion-pairing chromatography. For cation exchange chroma- Mode Cation exchange Ion-pairing methods tography the sample was diluted a further factor of 1+49 in Elemental 16.6±0.5 16.4±0.7 16.5±0.6 water while for ion-pairing chromatography the dilution factor MS–MS 17.1±0.5 16.4±0.6 16.7±0.6 was 1+4.Overall 16.8±0.6 16.4±0.6 16.6±0.6 For each mode of detection, and for each chromatographic Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 545Instrumentation in Analytical Chemistry, Volume 13, Environmental REFERENCES Analysis: T echniques, Applications and Quality Assurance, ed. 1 Agnes, G. R., and Horlick, G., Appl.Spectrosc., 1992, 46, 401. Barcelo, D., Elsevier, Amsterdam, 1993, p. 549. 2 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1994, 48, 649. 25 Norin, H., and Christakopoulos, A., Chemosphere, 1982, 11, 287. 3 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1994, 48, 655. 26 Luten, J. B., Riekwel-Booy, G., and Rauchbaar, A., Environ. 4 Agnes, G. R., Stewart, I. I., and Horlick, G., Appl. Spectrosc., Health Perspectives, 1982, 45, 165. 1994, 48, 1347. 27 Luten, J. B., Riekwel-Booy, G., Greef, J.v. d., and ten Noever de 5 Stewart, I. I., and Horlick, G., Anal. Chem., 1994, 66, 3983. Brauw, M. C., Chemosphere, 1983, 12, 131. 28 Cullen, W. R., and Dodd, M., Appl. Organomet. Chem., 1989, 3, 401. 6 Agnes, G. R., and Horlick, G., Appl. Spectrosc., 1995, 49, 324. 29 Beauchemin, D., Bednas, M. E., Berman, S. S., McLaren, J. W., 7 Stewart, I. I., and Horlick, G., T rends Anal. Chem., 1996, 2, 80. Siu, K. W. M., and Sturgeon, R. E., Anal. Chem., 1988, 60, 2209. 8 Siu, K. W. M., Gardner, G. J., and Berman, S. S., Rapid Commun. 30 Lawrence, J. F., Michalik, P., Tam, G., and Conacher, H. B. S., Mass Spectrom., 1988, 2, 201. J. Agric. Food Chem., 1986, 34, 315. 9 Siu, K. W. M., Gardner, G. J., and Berman, S. S., Rapid Commun. 31 Lau, B. P.-Y., Michalik, P., Porter, C. J., and Krolik, S., Biomed. Mass Spectrom., 1988, 2, 69. Environ. Mass Spectrom., 1987, 14, 723. 10 Siu, K. W. M., Gardner, G. J., and Berman, S. S., Anal. Chem., 32 Cullen, W. R., Eigendorf, G. K., and Pergantis, S. A., Rapid 1989, 61, 2320. Commun. Mass Spectrom., 1993, 7, 33. 11 Siu, K. W. M., Guevremont, R., Le Blanc, J. C. Y., Gardner, 33 Larsen, E. H., Pritzl, G., and Hansen, S. H., J. Anal. At. Spectrom., G. J., and Berman, S. S., J. Chromatogr. A, 1991, 554, 27. 1993, 8, 1075. 12 Corr, J. J., and Douglas, D. J., in Proceedings of the 41st ASMS 34 Douglas, D. J., and French, J. B., J. Am. Soc. Mass Spectrom., Conference on Mass Spectrometry and Allied T opics, San Francisco, 1992, 3, 398. CA, American Society for Mass Spectrometry, Santa Fe, NM, 35 Thomson, B. A., Douglas, D. J., Corr, J. J., Hager, J. W., and USA, 1993, p. 732a. Jollie, C. L., Anal. Chem., 1995, 67, 1696. 13 Corr, J. J., and Anacleto, J., F., Anal. Chem., 1996, 68, 2155. 36 Blades, A. T., Jayaweera, P., Ikonomou, M. G., and Kebarle, P., 14 Corr, J. J., and Larsen, E. H., J. Anal. At. Spectrom., 1996, 11, 1215. Int. J. Mass Spectrom. Ion Process., 1990, 101, 325. 15 Jones, T. L., and Betowski, L. D., Rapid Commun. Mass Spectrom., 37 Blades, A. T., Jayaweera, P., Ikonomou, M. G., and Kebarle, P., 1993, 7, 1003. Int. J. Mass Spectrom. Ion Process., 1990, 102, 251. 16 Ketterer, M. E., and Guzowski, J. P., Anal. Chem., 1996, 68, 883. 38 Blades, A. T., Jayaweera, P., Ikonomou, M. G., and Kebarle, P., 17 Szpunar-Lobinska, J., Witte, C., Lobinski, R., and Adams, F. C., J. Chem. Phys., 1990, 92, 5900. Fresenius J. Anal. Chem., 1995, 351, 351. 39 Jayaweera, P., Blades, A. T., Ikonomou, M. G., and Kebarle, P., 18 Quevauviller, P., Donard, O. F. X., Maier, E. A., and Griepink, J. Am. Chem. Soc., 1990, 112, 2452. B., Mikrochim. Acta, 1992, 109, 169. 40 Cheng, Z. L., Siu, K. W. M., Guevremont, R., and Berman, S. S., 19 Quevauviller, P., Appl. Organomet. Chem., 1994, 8, 715. J. Am. Soc. Mass Spectrom., 1992, 3, 281. 20 Schulze, G., and Lehmann, C., Anal. Chim. Acta, 1994, 288, 215. 41 Cheng, Z. L., Siu, K. W. M., Guevremont, R., and Berman, S. S., 21 Rivaro, P., Zaratin, L., Frache, R., and Mazzucotelli, A., Analyst, Org. Mass Spectrom., 1992, 27, 1370. 1995, 120, 1937. 22 Dauchy, X., Cottier, R., Batel, A., Jeannot, R., Borsier, M., Astruc, Paper 6/06421C A., and Astruc, M., J. Chromatogr. Sci., 1993, 31, 416. Received September 17, 1996 23 Cullen, W. R., and Reimer, K. J., Chem. Rev., 1989, 89, 713. 24 Donard, O. F. X., and Ritsema, R., in T echniques and Accepted January 27, 1997 546 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a606421c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Vaporization of Radium and Other Alkaline Earth Elements in Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry ROY ST. C. MCINTYREa , D. CONRAD GRE� GOIREb and CHUNI L. CHAKRABARTIa aOttawa-Carleton Chemistry Institute, Department of Chemistry, Carleton University, Ottawa, Ontario, Canada K1S 5B6 bGeological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8 Reported are the mechanism of vaporization and optimum have shown promise, no in-depth study on the vaporization of Ra or study of optimum experimental conditions for ETV experimental conditions for the determination of Ra and other alkaline earth elements (Be, Mg, Ca, Sr and Ba) by determinations has been reported.Alpha spectrometry1,2 is the most commonly used technique electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). Calculated and published data for the determination of 226Ra. Sample sizes range from 1–50 ml of solution or 1–5 g of solid.The limit of detection along with new experimental results suggest that these elements are vaporized from the surface of the graphite tube obtained reported for alpha spectrometry1 was 1.8×10-4 Bq (0.1 fg ml-1) for a 50 ml sample, pre-concentrated and counted as oxides. These oxides are then transported to the argon plasma where dissociation and ionization take place. for 1000 min. Using TIMS,3,4 sample sizes can be as small as 1 g and yield detection limits of about 10 fg ml-1.Techniques Appearance temperatures and maximum pyrolysis temperatures obtained experimentally generally agree with based on radon emanation5 involve the collection of 222Rn, a decay product of 226Ra. Large sample sizes (litres) are required values obtained using graphite furnace atomic absorption spectrometry (GFAAS). For Ra, the optimum pyrolysis and and ingrowth of radon takes from several days to weeks before sucient quantities of Rn are produced for an accurate analy- vaporization temperatures were 1400 and 2500 °C, respectively.Diluted (15500) seawater, used as a physical sis. Cerenkov counting,6 based on b-particle detection using a liquid scintillator, also requires a relatively large sample and carrier, was eective in improving sensitivity when used in small quantities, but caused significant suppression of the Ra involves a relatively long sample preparation time giving a limit of detection of 0.035 Bq l-1 (0.95 fg ml-1). Although all signal when the analyte was co-vaporized with quantities of salt in excess of 40 mg.An absolute limit of detection of 1.7 fg of the techniques discussed above generally oer low limits of detection for Ra, this advantage is somewhat oset by long was obtained corresponding to 34 fg ml-1 in a 50 ml sample. preparation and analysis times. Preparation of liquid samples Keywords: Alkaline earth elements; electrothermal usually involves an ion-exchange separation of Ra followed by vaporization ; inductively coupled plasma mass spectrometry; an electrodeposition preconcentration step.Each step necessar- radium ily leads to a greater possibility of contamination and/or analyte loss. Studies on the determination of 226Ra by ICP-MS using Of the four naturally occurring isotopes of Ra, 226Ra is the most abundant having a half-life of approximately 1600 years. solution nebulization7 and ETV8,9 sample introduction gave The other isotopes are of lesser importance since they are not limits of detection of 0.2 pg ml-1 for solution nebulization7 as persistent with half-lives of 6.7 years for 228Ra, 11.7 days and 0.27 fg ml-1 for ETV when 50 ml of sample solution were for 223Ra and 3.64 days for 224Ra.No stable isotope of Ra used. These methods used either or both preconcentration by exists. Radium found in nature is derived from both natural ion exchange and evaporation techniques to improve sensiand anthropogenic sources and is highly toxic.The element tivity. Analysis by ETV-ICP-MS8 was done by drying successreplaces Ca in bone structure and can result in bone degra- ive samples without vaporization (multiple deposition) to dation and cancer. Radium (226Ra) is also the natural precursor increase sensitivity. This approach, however, is not suitable to 222Rn, which is retained in the lungs in the form of 210Pb when sample solutions contain large quantities of dissolved and 210Po.salts. Alvarado and Mitchell9 used Freon-23 as a chemical There is a substantial body of literature reporting on GFAAS modifier and obtained a limit of detection of 0.6 fg ml-1 for a studies on the mechanism of atomization of all of the alkaline 25 ml sample aliquot. A limit of detection of 1 fg ml-1 was earth elements with the exception of Ra. As will be shown obtained without the use of Freon-23. below, much of this information is useful when applied to The use of ETV sample introduction provides an alternative ETV-ICP-MS studies.Since little published information exists means of sample introduction for ICP-MS, which serves to on Ra, the vaporization of 226Ra compounds and its extend the range of application of the technique. The main determination by ETV-ICP-MS is the focus of this paper. advantages provided by ETV sample introduction include the Radium, as 226Ra, is currently determined largely by such use of microlitre or microgram sample sizes, the removal of techniques as alpha spectrometry,1,2 thermal ionization mass matrix interferences by thermal pretreatment of the sample spectrometry (TIMS),3,4 radon emanation5 and Cerenkov prior to vaporization and the ability to analyze solids, slurries counting.6 Recently, 226Ra has been successfully determined by and organic materials directly.Electrothermal vaporization inductively coupled plasma mass spectrometry (ICP-MS) using sample introduction provides for rapid analysis in minutes solution nebulization sample introduction7 and electrothermal compared with days or even longer for counting techniques.5,6 vaporization (ETV) sample introduction.8,9 While these studies As an additional advantage, many samples do not require preconcentration owing to the inherent high sensitivity of ICP-MS.In fact, in a recent paper by Smith et al.,10 it was GSC Publication No. 1996264. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (547–551) 547shown that it was advantageous to use ETV-ICP-MS rather modifier solutions were 10 ml in volume.The 226Ra solution (provided by the Atomic Energy Commission of Canada, than counting techniques for the determination of all radionuclides whose half-lives were greater than about 570 years. Pinawa, Manitoba) contained 5.2 Bq g-1 of the element. The concentrated Ra stock solution was calibrated against NIST The purpose of this study is to elucidate the mechanism of vaporization of Ra and the other alkaline earth elements in SRM 4966 (Radium-226) which was certified to contain 300 Bq g-1 of 226Ra. NASS-3 Open Ocean Seawater reference ETV-ICP-MS and to determine the optimum measurement conditions for these elements.This work is to serve as the material was obtained from the National Research Council of Canada and diluted 500-fold with deionized water prior to use basis for the development of methodology for the direct determination of Ra in solids, sampled as slurries, using as a chemical modifier or simulated sample matrix.ETV-ICP-MS. RESULTS AND DISCUSSION EXPERIMENTAL Mechanism of Vaporization of Be, Mg, Ca, Sr and Ba A Perkin-Elmer SCIEX Elan 5000a ICP mass spectrometer The mechanisms of atomization for a number of elements have equipped with an HGA-600 MS electrothermal vaporizer and been studied using GFAAS.11–13 Sturgeon et al.11 showed that a Model AS-60 autosampler was used. Pyrolytic graphite three processes occurred leading to the production of free coated tubes were used throughout.The experimental conatoms: (1) carbon reduction of the analyte oxide followed by ditions for both the Elan 5000a and the HGA-600MS are sublimation of the metal; (2) thermal dissociation of the oxide given in Table 1. on the graphite surface or in the gas phase; and (3) thermal A PTFE tube of 80 cm and 6 mm id was used to connect dissociation of the metal chloride. Thermogravimetric14 and the HG600MS to the plasma torch.Optimization of the atomic absorptiondata11–13 show that the oxides of the alkaline plasma and mass spectrometer was accomplished using soluearth elements (Mg, Ca, Sr and Ba) are common intermediates, tion nebulization, prior to switching to ETV mode. No further resulting from the heating (prior to vaporization) of either the optimization of the ICP mass spectrometer was required with chloride or nitrate form of the element. the exception of small (±50 ml min-1) variations in the carrier In GFAAS, Mg oxide (MgO) and Ca oxide (CaO) vaporize Ar flow rate.The operation of the HGA-600 MS was comfrom the graphite surface as oxides and in the vapour phase pletely computer controlled. During the drying and pyrolysis dissociate giving atoms.15 Hutton et al.16 showed by molecular steps of the temperature program, opposing flows of argon emission measurements in a carbon furnace that the gaseous (300 ml min-1) originating from both ends of the graphite tube oxide species for Mg, Ca and Sr exist, suggesting dissociation removed water and other vapours through the dosing hole of into the elements in the vapour phase.Sturgeon et al.11 the graphite tube. During the high-temperature or vaporization suggested that the vapour phase composition was dependent step, the dosing hole was sealed by a pneumatically activated upon the sample size; large samples produced gaseous elements graphite probe. Once the graphite tube was sealed, a valve directly, with thermal dissociation occurring on the surface, located at one end of the HGA workhead directed the carrier and small samples produced gaseous oxides before dissociating argon gas flow, originating from the far end of the graphite into gaseous atoms.Kantor et al.17 supported this interpret- tube, directly to the argon plasma at a flow rate of ation and Prell et al.13 used mass spectrometry coupled with 800 ml min-1. a graphite furnace to show that the oxide directly precedes the production of the analyte atoms.Standards and Reagents There is some uncertainty surrounding the vaporization of Sr oxide. It was suggested by Moore et al.18 that SrO(s) High purity argon gas (99.995%, Matheson Gas Products, vaporizes to SrO(g). However, mass spectrometric studies by Ottawa, Ontario, Canada) was used. A solution of mixed Porter et al.19 suggested direct formation of the gaseous alkaline earth elements was prepared by dilution of SPEX elements from the solid oxide.Nagdaev and Bukreev20 (using standard 1000 or 10000 mg ml-1 stock solutions (SPEX a graphite rod atomizer) proposed that the most probable Industries, Edison, NJ, USA) using de-ionized water (Millipore, mechanism for free Sr and Ba atom formation is sublimation Mississauga, Ontario, Canada). Aliquots of all samples and of the oxide with dissociation in the gas phase. Hutton et al.16 obtained similar results using molecular emission measure- Table 1 Instrumental operating conditions and data acquisition ments of SrO and Prell et al.13 also showed that the oxide parameters directly preceded the appearance of free Sr atoms.For Ba, the oxide is thought to exist predominately in the ICP mass spectrometer— vapour phase as a gas.15 Jasim and Barbooti21 suggested that Rf power/W 1000 Coolant argon gas flow/l min-1 15.0 free Ba is formed from the gas phase thermal dissociation of Auxiliary argon gas flow/ml min-1 900 the oxide as did Nagdaev and Bukreev.20 Frech et al.22 used Carrier argon gas flow/ml min-1 800 thermodynamic calculations to show that the hydroxide (gaseous and liquid) and the gaseous oxide were the precursors to HGA-600MS electrothermal vaporizer— gaseous Ba atom formation.Byrne et al.23 added oxygen to Sample volume/ml 10–20 the purge gas to produce signal shifts giving evidence in Dry step 10 s ramp 110 °C for 30 s support of the gaseous oxide dissociation hypothesis.Prell Pyrolysis step 1 s ramp et al.13 showed BaO as the precursor to Ba atom formation. 400–2650 °C for 30 s Sucient vapour pressure (a few Torr) for all oxides15 exists at Vaporization step 1 s ramp their appearance temperatures to produce a signal in ETV- 900–2700 °C for 6 s ICP-MS. Beryllium has an atomization mechanism similar to the Data Acquisition— Dwell time/ms 20 other alkaline earth elements12 with one significant dierence. Scan mode peak hopping For the alkaline earth elements (Mg, Ca, Sr and Ba), the Points/spectral peak (m/z) 1 species preceding the appearance of the free element has been Signal measurement Integrated counts the simple oxide, MO.Beryllium, on the other hand, forms a Resolution 0.7 u at 10% peak-height polymeric oxide, (BeO)n and releaseof the free element proceeds 548 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12as follows: and as high as 1230 °C. Thermochemical properties of the alkaline earth elements follow well defined trends26 and much (BeO)n(ad)�(BeO)n-1(g)+.. .+BeO(g) of the information available on Ra is currently derived from these trends. Few experimental measurements of the thermo- BeO(ad)�Be(g)+O(g) dynamic properties of RaO have been completed owing to its There are, however, some dierences in the mode of oper- high reactivity.25 Radium oxide (RaO) is a highly aggressive ation of ETV-ICP-MS that may aect the usefulness of data substance that readily attacks crucible and calorimeter mate- obtained by GFAAS studies.For example, in GFAAS, during rials and, to date, no definite material has been conclusively the signal measurement step (high-temperature step) no argon identified for which the formula RaO can be assigned.25 is flowing through the graphite tube. Production of atoms for In ETV-ICP-MS, it is often observed that for many sub- atomic absorption can proceed by the vaporization of metal stances there is sucient vapour pressure (several Torr) at the from the graphite surface to the gas phase and/or by the melting point to give rise to a signal.Radium oxide was thermal dissociation of molecular species in the gas phase. calculated to melt at 1615 °C.27 This temperature is (within For ETV-ICP-MS, there is a constant Ar flow of about error) the same as the appearance temperature (1600 °C) for 0.8–1 l min-1 through the graphite tube, which serves to Ra obtained from the vaporization of both the nitrate and remove any vaporized material (atomic or molecular) and chloride form.This supports the possibility that the oxide is carry it to the argon plasma where atomization and ionization the most likely form of Ra vaporized at the appearance takes place. For the alkaline earth elements discussed above temperature. and for both GFAAS and ETV-ICP-MS, analyte oxide is As shown above, all the alkaline earth elements most prob- vaporized into the gas phase.This does not mean, however, ably volatilize as oxides. The chemical and thermodynamic that analyte signal will be observed at the same vaporization similarities between Ra and the other alkaline earth elements, temperature for both techniques. As will be shown below, this the appearance temperature for Ra in ETV-ICP-MS and its is because, for GFAAS, the production of the analyte signal is similarity to the melting point of Ra oxide suggest a likely dependent on a high enough gas phase temperature to eect mechanism of vaporization.The evidence provided above is thermal dissociation of the oxide, whereas, in ETV-ICP-MS, incomplete and perhaps circumstantial, but, in the absence of the analyte signal will result at whatever temperature the oxide more quantitative data, a reasonable mechanism for the pro- is vaporized. This means that, in general, appearance tempera- duction of the Ra signal at the appearance temperature in the tures (defined as the lowest temperature at which the analyte ETV is vaporization of RaO(s) to RaO(g) which is then signal can be detected above baseline noise) for ETV-ICP-MS transported to the argon plasma where atomization and will always be lower or equal to appearance temperatures ionization take place.measured using GFAAS. Appearance temperatures for the alkaline earth elements measured using ETV-ICP-MS are compared in Table 2 to literature values for appearance Optimization of Experimental Conditions temperatures determined using GFAAS.Within experimental A typical ETV heating program contains at least three steps. error (±100 °C), appearance temperatures agree except for Ba A low temperature step ranging from 80 to 110 °C is used to for which the ETV-ICP-MS temperature is significantly lower. remove solvent and volatiles not containing the analyte. A This lower temperature may be attributable to the higher second step, generally called the pyrolysis step, is used to sensitivity of ICP-MS compared with GFAAS.There is no removeselected matrix components and/or to activate chemical reported value for the appearance temperature for Ra using modifiers. The third step is the high temperature or vaporiz- GFAAS. ation step which is used to vaporize the analyte. It is during this step that ICP-MS data are collected. The temperature and the rate of heating of the drying step are normally determined Mechanism of Vaporization of Ra experimentally and are usually dependent on the nature of the Radium is the heaviest of the alkaline earth elements and sample matrix.The maximum pyrolysis temperature that can shares many chemical properties with the rest of the group, be used is the temperature at which the analyte is lost in particularly Ba. When the alkaline earth elements are discussed significant quantities. In order to measure the maximum as a group, Be and Mg are usually kept separate because they allowable pyrolysis temperature for the alkaline earth elements, exhibit dierent chemistries than the rest of the group.24 a curve was constructed of the analyte signal obtained at Calcium, Sr and Ba are usually classed together, with Ra often dierent temperatures for a 30 s (1 s ramp time) pyrolysis step.paired to Ba. In each case, the same vaporization temperature (2500 °C) Radium compounds follow the general solubility trends for was used throughout. Increasing the hold time for the pyrolysis the alkaline earths (sulfate solubility decreases and hydroxide step generally decreased the temperature at which analyte solubility increases as atomic number increases, etc.) except for losses were detected.The 30 s hold time selected for this study the nitrate, which is slightly more soluble than the Ba com- is reasonable based on typical heating programs used for the pound. Thermogravimetric analysis of radium nitrate and analysis of real samples.The pyrolysis and vaporization curves carbonate25 have revealed the existence of several phases of an for Ra are shown in Fig. 1. These data show that losses begin oxide, believed to be RaO, at temperatures as low as 300 °C to occur at a pyrolysis temperature of around 1400 °C for Ra. Fig. 2 shows pyrolysis curves for the other alkaline earth elements. The pyrolysis temperatures obtained for these Table 2 Appearance temperatures for the alkaline earth elements elements (Table 3) are generally lower by several hundred degrees than those reported in GFAAS studies.This may be Appearance temperature/°C due to the much smaller quantities of analyte vaporized in Element GFAAS ETV-ICP-MS Reference ETV-ICP-MS (fg–pg) compared with GFAAS (ng–mg).32 As shown in Fig. 2, Be losses occurred at relatively low tempera- Be 1227 1200 11 Mg 1237 1200 10 tures. This may result from the low temperature vaporization Ca 1577 1500 10 of Be compounds such as Be(NO3)2 (bp 142 °C) or BeCl2 (bp Sr 1367 1300 13 520 °C) depending upon the acid used in solution.Maessen Ba 1727 1500 21 et al.33 reported that approximately 10% of the Be is lost from Ra — 1600 this work the furnace due to diusional losses before the free atom signal Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 549Analytical Figures of Merit Analytical figures of merit for the determination of 226Ra by ETV-ICP-MS are given in Table 4. The limit of detection for Ra was calculated as the mass of Ra equivalent to a signal equal to three times the standard deviation of the blank.An absolute limit of detection of 1.7 fg was obtained, which corresponds to a relative limit of detection of 34 fg ml-1 in a 50 ml sample. The analyte signal could be measured with a precision of 3.9% and 5.3% for integrated and peak-height signal measurements, respectively. Eect of Physical Carrier and Signal Suppression Due to Matrix Components The eect of using physical carriers to improve analyte transport from the graphite tube to the argon plasma was shown Fig. 1 Pyrolysis (&) and vaporization (%) curves for 0.25 pg Ra. by Hughes et al.35 In this study, the use of NASS-3 (reference seawater diluted 500 times) was shown to be an eective physical carrier for many elements resulting in signal enhancements of up to a factor of ten. The eects of NASS-3 seawater on the Ra signal (Table 5) was studied by adding 10 ml of solutions containing increasing concentrations of NASS-3 to a 10 ml aliquot of sample solution containing 0.1 ng ml-1 Ra.Pyrolysis and vaporization temperatures of 700 and 2500 °C, respectively, were used. A seawater matrix can also be used to determine the eect of added matrix components on possible signal suppression. When small quantities of salt were added, the Ra signal was enhanced by up to 30%. Upon adding greater quantities of salt, the signal was suppressed by as much as 75% when 360 mg of salt were added.Radium is present in seawater at 8.9×10-11 mg ml-1 (ref. 36) corresponding to 8.9×10-4 fg in a standard 10 ml sample. This quantity of 226Ra is well below the observed ETV-ICP-MS limit of detection for Ra (1.7 fg) and will not aect the observed signal in experiments using Fig. 2 Pyrolysis curves of the alkalineearth elements in ETV-ICP-MS (Vaporization 2600°C): Be (&), Mg (1), Ca (+), Sr (%), and Ba ($). seawater as a modifier. As was reported by Hughes et al.35 the initial enhancement eect is probably due to an increased transport eciency, Table 3 Pyrolysis temperatures for the alkaline earth elements while, at higher added salt masses, signal suppression occurs.The modest enhancement observed for Ra when small quantit- Pyrolysis temperature/°C Element GFAAS ETV-ICP-MS Reference Table 4 Analytical figures of merit for Ra Be 900 500 28 Blank— Integrated Signal Peak height Mg 900 900 29 Ca 1200 700 30 226Ra 53 94 Sr 1500 800 31 s, n=10 8 16 Ba 1500 1500 30 RSD (%) 15 17 Ra — 1400 this work 3s 24 49 Radium— 226Ra (1.4 pg) 19 600 36 737 s, n=5 760 1959 is observed in GFAAS. Vanhoe et al.28 reported loss of Be at RSD (%) 3.9 5.3 900 °C in GFAAS when no modifier was used.Meah34 also LOD (abs) fg 1.7 1.7 recorded Be losses at very low temperatures when vaporized LOD (rel) (50 ml)/fgml-1 34 34 in the presence of acids and salt matrices. Thus if all alkaline LOD/Bq g-1 1.3×10-3 1.3×10-3 earth elements were to be determined as a group by ETVICP- MS, a pyrolysis temperature of 700 °C or less should be used to prevent losses of the more volatile Be.However, with Table 5 Eect of added salt on Ra signal the use of magnesium nitrate as a chemical modifier, Be can Amount of salt/mg Enhancement* be stabilized to a temperature of 1500 °C. A second set of experiments exploring the relationship 0 1.00 between the vaporization temperature and the integrated signal 0.72 1.32 3.6 1.29 for the alkaline earth elements showed that the maximum 7.2 1.12 signal was obtained at a vaporization temperature of 2500 °C 36 0.78 or greater when a 500 °C pyrolysis step was used.As an 72 0.61 example, the vaporization curve for Ra is given in Fig. 1. Using 360 0.25 these vaporization conditions completely removes the analyte from the surface of the graphite tube with no memory eects * Defined as the ratio of the signal obtained when Ra is vaporized with salt divided by the Ra signal obtained when vaporized alone.observed. 550 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 1211 Sturgeon, R. E., Chakrabarti, C. L., and Langford, C. H., Anal. Chem., 1976, 48, 1792. 12 Styris, D. L., and Redfield, D. A., Anal. Chem., 1987, 59, 2897. 13 Prell, L. J., Styris, D. L., and Redfield, D. A., J. Anal. At. Spectrom., 1991, 6, 25. 14 Duval, C., Inorganic T hermogravimetric Analysis, 2nd edn., Elsevier Publishing Co., New York, 1963. 15 Margrave, J. L., T he Characterization of High-T emperature Vapors, John Wiley & Sons, USA, 1967, pp. 555. 16 Hutton, R. C., Ottaway, J. M., Epstein, M. S., and Rains, T. C., Analyst, 1977, 102, 658. 17 Ka�ntor, T., Bezu�r, L., Pungor, E., and Winefordner, J. D., Spectrochim. Acta, Part B, 1983, 38, 581. 18 Moore, G. E., Allison, H. W., and Struthers, J. D., J. Chem. Phys., 1950, 18, 1572. 19 Porter, R. F., Chupka, W. A., and Inghram, M. G., J. Chem. Phys., 1955, 23, 1347. 20 Nagdaev, V. K., and Bukreev, Y. F., Zhurnal Prikladnoi Spektroskopii, 1980, 33, 618. Fig. 3 ETV-ICP-MS signals for Ra (A) and Ba (—). 21 Jasim, F., and Barbooti, M. M., Talanta, 1981, 28, 353. 22 Frech, W., Lundberg, E., and Cedergren, A., Prog. Anal. At. Spectrosc., 1985, 8, 257. ies of salt were added may be due to the presence of Ba in 23 Byrne, J. P., Chakrabarti, C. L., Chang, S. B., Tan, C. K., and solution with Ra which itself acts as a physical carrier. The Delgado, A.H., Fresenius’ Z. Anal. Chem., 1986, 324, 448. Ra standard used, as well as the NIST material, contained Ba 24 Cotton, F. A., and Wilkinson, G., Advanced Inorganic Chemistry: which served as a carrier during the Ra purification step. Both A Comprehensive T ext, 4th edn., John Wiley & Sons, US, 1980, pp. 1396. Ra and Ba are co-volatilized during the vaporization step as 25 Gmelin, L., Gmelin Handbuch der Inorganischen Chemie, 8. shown in Fig. 3. Auflage, Radium, Springer-Verlag, Berlin, Germany, 1977. 26 Lowson, R. T., T hermochim. Acta, 1985, 91, 185. The authors are grateful to John Byrne, University of 27 Chekhovskoi, V. Ya., and Irgashov, Kh., Russian J. Phys. Chem., Technology, Sydney, Australia for critically reading the manu- 1990, 64, 2. script and to Rich Hamon, Atomic Energy of Canada Ltd., 28 Vanhoe, H., Vandecasteele, C., Desmet, B., and Dams, R., J. Anal. At. Spectrom., 1988, 3, 703. Pinawa, Manitoba, for providing Ra standards and technical 29 Slavin, W., Graphite Furnace AAS: A Source Book, Perkin-Elmer information. Corp., USA, 1984, pp. 229. 30 Welz, B., Atomic Absorption Spectrometry, 2nd edn., VCH, 1985, pp. 506. REFERENCES 31 Helsbey, C. A., T alanta, 1977, 24, 46. 1 Alvarado, J. S., Orlandini, K. A., and Erickson, M. D., 32 Gre�goire, D. C., Lamoureux, M., Chakrabarti, C. L., J. Radioanal. Nucl. Chem., 1995, 194, 163. Al-Maawali, S., and Byrne, J. P., J. Anal. At. Spectrom., 1992, 2 Ditchburn, R. G., and Whitehead, N. E., J. Radioanal. Nucl. 7, 579. Chem., 1995, 189, 115. 33 Maessen, F. J. M. J., Balke, J., and Massee, R., Spectrochim. Acta, 3 Cohen, A. S., and O’Nions, R. K., Anal. Chem., 1991, 63, 2705. Part B, 1978, 33, 311. 4 Volpe, A. M., Olivares, J. A., and Murrell, M. T., Anal. Chem., 34 Meah, M. D. Y., M.Sc. Thesis, Carleton University, Ottawa, 1991, 63, 913. Ontario, 1981. 5 Chieco, N. A., Bogen, D. C., and Knutson, E. D., Environmental 35 Hughes, D. M., Chakrabarti, C. L., Goltz, D. M., Gre�goire, D. C., Measurements L aboratory Procedures Manual, 27th edn., Sturgeon, R. E., and Byrne, J. P., Spectrochim. Acta, Part B, 1995, HASL-300, US Department of Energy, New York, 1990. 50, 425. 6 Blackburn, R., and Al-Masri, M. S., Analyst, 1993, 118, 873. 36 CRC Handbook of Chemistry and Physics 76th edn., CRC Press, 7 Hodge, V. F., and Laing, G. A., Radiochim. Acta, 1994, 64, 211. USA, 1995–1996. 8 Gray, D. J., Wang, S., and Brown, R., Appl. Spectrosc., 1994, 48, 1316. Paper 6/07270D 9 Alvarado, J. S., and Erickson, M. D., J. Anal. At. Spectrom., 1996, Received October 24, 1996 11, 923. Accepted January 15, 1997 10 Smith, M., Wyse, E., and Koppenaal, D., J. Radioanal. Nucl. Chem., 1992, 160, 341. Journal of Analytical Atomic Spectrometry, May 1997, Vol

ISSN:0267-9477    

DOI:10.1039/a607270d

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Investigation of the Source of Blank Problems in the Measurement of Lead in Sub-micrometre Airborne Particulates by Inductively Coupled Plasma Mass Spectrometry STEPHEN THOMASa, b , LIDIA MORAWSKA*b, NEVILLE BOFINGERa, b AND MARK SELBYa aCentre for Instrumental and Developmental Chemistry, School of Chemistry, Queensland University of T echnology, 2 George Street, Queensland, Australia bCentre forMedical and Health Physics, School of Physics, Queensland University of T echnology, 2 George Street, Queensland, Australia The development of a method for the determination of trace use of such standards for the validation of the extraction of fine particulates less than 1 mm in diameter is not considered amounts of lead in airborne fine particulates using ICP-MS for sample analysis is described.A systematic approach to the to be appropriate. Sub-micrometre airborne particulates are likely to contain less crustal rock material than that in the reduction of blank readings and to the identification of sources of contamination was applied, using the techniques of factorial standards and the elements may be present in forms that are much more easily solubilized. Furthermore, the total masses experimental design and multi-way analysis of variance (ANOVA).The former provides an economical method for of elements present in the reference materials are often much higher than those obtained from usual sampling conditions indicating significant sources of contamination to the blank readings.The latter is a statistical method for the study of the and as a result may require much more rigorous digestion procedures which have been known to compromise sensi- eects of these sources and their interactions. It was found that a significant contribution of the high blank readings is tivity levels.4,15 Sampling over time periods appropriate to assess temporal attributable to the ‘memory eect’ exhibited by the laboratory PTFE-ware.As a result of this study, the detection limit of trends in atmospheric particulates and to determine the concentration levels of elements in the sub-micrometre size region the method, in terms of the mass of lead on the filter, was reduced from 20 to 1 ng. results in a dramatic reduction of the total mass of sample available for analysis. At the trace levels encountered it is Keywords: Inductively coupled plasma mass spectrometry; imperative that sources of contamination are identified and airborne particulate analysis; chemometrics; lead; blanks minimized.Contamination sources inherent in atmospheric analyses that require consideration may include: the walls of the sampling vessels (PTFE) used for both sampling and Typically, lead levels can be expected to be up to 2 mg m-3 in storage; the filter materials; the instrument laboratory ware; rural areas and up to 10 mg m-3 in close proximity to heavy the reagents and water added; and the laboratory air in which trac.1 With the increased concern over the presence of lead any chemical manipulations may be performed.pollution in the environment, there has been a greater need Previous studies on the contribution of dierent processes for the development of sensitive analytical techniques capable to lead blanks have focused on individual procedures.4,15–17 of measuring at the trace level concentrations consistent with This paper describes a systematic, chemometric approach using fine airborne particulate matter.It is accepted2 that particles factorial experimental design and analysis of variance with sizes <1 mm have a higher chance of deposition in the (ANOVA), successfully adopted to reduce the blanks and pulmonary and tracheobronchial regions of the body where sources of contamination experienced in atmospheric analyses. subsequent absorption into body tissues can occur. Ambient air containing airborne particulates in this size range was sampled for this study.Particulate studies in ambient EXPERIMENTAL air have centred on pollutant concentrations averaged over periods of 24 h or greater where samples are collected by low Instrumentation or high volume air samplers onto filters.3–5 The samples Airborne particulate samples were collected using a TSI Model represent total suspended particulate (TSP) or particulates less 3071A Electrostatic Classifier (TSI, St. Paul, MN, USA). The than 10 mm in diameter (PM10).Such studies do not provide instrument setting was such that particles in the size range information on temporal changes of the lead concentration or from 0.02 to 0.63 mm were collected for 15 min at an air flow the potential for lead deposition within the respiratory tract. rate of 0.3 l min-1. Collection of size-fractionated particles over time periods of Lead determinations were made using a VG Elemental approximately 10 min is now achievable with sampling rates PlasmaQuad ICP-MS PQ2 instrument (VG Elemental of the order of 1 l min-1.Under these sampling conditions in Winsford, Cheshire, UK). The detailed optimized instrumental urban atmospheres, the mass of lead present on the filter will parameters are listed in Table 1. be of the order of 1 ng. In previous studies3–16 samples contained much larger particles owing to the sampling technique and in some cases the Reagents extraction and analysis procedures have been validated using non-size segregated certified particulate reference materials15,16 All of the water used to wash laboratory ware and for the preparationof solutionsand standards was distilled and doubly (e.g., NIST SRM 1648 Urban Particulate) which are extracted and analysed under the same conditions as the sample.The de-ionized (resistance >18.2MV) using a Maxima Ultra-pure Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (553–556) 553Table 1 ICP-MS operating conditions conditions, ‘medium’ to 50% power and 3.5×105 Pa pressure and ‘high’ to 100% power and 5.5×105 Pa pressure.Inductively coupled plasma— 3. The digestion number (the number of prior blank diges- Plasma Ar flow rate 16 l min-1 tions performed on the particular bomb), measuring the poten- Auxiliary Ar flow rate 1.5 l min-1 tial ‘memory eect’. It is in essence a parameter representing Nebulizer Ar flow rate 0.800 l min-1 Rf power 1.3 kW the cleanliness of the vessel. Reflected power <5 W 4.The nitric acid concentration used in the digestion (0, 2, Long torch (Fassel configuration) 5, 10 and 20% v/v). Analysis of the eect of acid concentration Meinhard c-3 low-flow glass nebulizer provided information on the presence of contaminants in the Scott-type spray chamber reagent and also, as with the digestion parameters, on the Coil-to-orifice distance 10 mm retained metals on the vessel walls. The low acid concentration Torch-to-orifice distance 5 mm levels were chosen to allow for high sensitivity while at the same time being suciently concentrated to allow the quanti- Water System (Selby Scientific, Acacia Ridge, Queensland, tative extraction of lead from fine airborne particulates (diam- Australia).eters less than 1 mm). Previous studies on lead in airborne The reagents were of Fisons PrimaR high-purity grade particulates employed 0.117 and 0.216 M nitric acid for AAS (Fisons Pty., Homebush, New South Wales, Australia) and the and ETAAS analysis.standards were prepared from certified reference material ICP Multi-element Standard Solution (Art 15474) (BDH Chemicals Calibration Australia Pty., Kilsyth, Victoria, Australia). Aqueous multi-element standards of 0, 2 and 10 ng ml-1 were prepared in nitric acid (at the concentration employed in each Sample Digestion digestion) by serial dilution of the standard stock solution. At the trace levels under consideration, a number of factors Indium was used as an internal standard.may be identified as possible sources of high blanks. The following steps were undertaken where possible to minimize Data Analysis the eects of these factors: washing any laboratory ware with nitric acid (20% v/v); performing all chemical manipulations Sample solutions were introduced into the ICP-MS instrument in a laminar flow hood; and using only Teflon vessels. However, by conventional pneumatic nebulization. The concentrations in order to reduce the high blank levels, it is important to of lead in the samples were determined using the conditions identify the specific contributions of each factor to the blanks.shown in Table 2. The work requires that atmospheric samples are collected All statistical analyses including graphs were performed on HV Millipore membrane filters (0.45 mm maximum pore using the Statgraphics Statistical Graphics System (Version: size, 13 mm diameter) using the TSI Model 3934 SMPS 6.0) (Manugistics and Statistical Graphics, Rockville, MD, system (TSI).USA). Initial investigations centred on the filter membranes and their contribution to the high blanks. The membrane filters RESULTS AND DISCUSSION were washed in nitric acid (20% v/v) and transferred with Teflon tweezers to closed PTFE decomposition vessels (bombs) Preliminary tests were performed on digested filters and where they were digested with 2 ml of nitric acid (10% v/v) at digested blanks to determine, in the first instance, if the lead 5.5×105 Pa and 100% power for 10 min in an MDS-2000 levels were appropriate for trace analysis, that is, to determine microwave digestion unit (maximum power 650 W) (CEM, if the levels were lower than the expected levels of the atmos- Matthews, NC, USA). However, it became apparent that the pheric samples.The critical sample load was taken as 1 ng per lead contamination arose from sources other than the filter filter, as described above.Secondly, the tests were designed to membranes and as a result all further tests were performed indicate the eect of the membrane filters on the blank levels. under similar conditions without involving the filter A summary of the statistics for those blank filters digested in membranes. 2 ml of nitric acid (10% v/v) and blanks digested in the same Instead, a multi-level fractional factorial experimental design acid concentration is provided in Table 3. was devised as an economical method for obtaining useful It can be seen that the averages are the same for each of the information indicating possible eects and interaction eects sample types (6.3 ng Pb for blank and blank filters).At the of a number of other factors. In this technique, all factors of simplest level of interpretation, there being no dierence interest are varied simultaneously, and the individual eects between the averages for the two techniques, the filters were and their interactions can be extracted by mathematical considered not to be responsible for any blank contamination. treatment.The factors assessed included: However, these levels were much higher than the desired levels 1. The vessel (bomb) number. PTFE containers are con- and further investigation into the eects of a number of factors sidered cleaner than glass. However, the material may still on the blank levels is warranted. exhibit a ‘memory eect’, whereby metals are retained on the A factorial experimental design was devised incorporating walls, which can cause cross-contamination of samples.It was the factors: bomb number; digestion parameters; number of intended that, in using the bomb number of each digestion as previous blank digestions; and nitric acid concentration. A a parameter in the analysis, variations in concentration levels multi-way ANOVA test (Table 4) was performed on the data associated with each bomb would be identified. 2. The microwave digestion parameters (none, low, medium Table 2 Conditions for determination of lead by ICP-MS and high).It was proposed that any metals retained on the walls of the vessels may be more likely to be digested under Peak-jumping Mode Element Pb the higher temperature and pressure conditions in the micro- Default dwell 9174; 249; 534; 249 ms for 204Pb; 206Pb; 207Pb; wave. Accordingly, this parameter was used to assess any 208Pb, respectively significant dierences that occurred as a result of the digestion Points per peak 3 stage of the process.‘None’ refers to no digestion performed DAC step 5 in the microwave, ‘low’ to 20% power and 1.4×105 Pa pressure 554 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Table 3 Summary of statistics for filter digestion tests Variable Blank Blank filter Sample size 30 30 Average/ng 6.3 6.3 Median/ng 5.2 5.7 Mode/ng 3.8 10 Variance/ng 32 21 Standard deviation/ng 5.7 4.7 Standard error/ng 1 0.9 Minimum/ng 0 0 Maximum/ng 22 17 Range/ng 22 17 Lower quartile/ng 2.4 2.3 Upper quartile/ng 7.7 10 Interquartile range/ng 5.4 7.7 RSD (%) 89.3 74 Fig. 2 Box-and-Whisker plots foracid concentration factor level data. Table 4 Summary of ANOVA analysis of blanks by factors Factor F statistic Significance level Bomb number 0.789 0.63 Microwave conditions 2.238 0.09 Acid concentration 5.553 0.0007 Digestion number 4.937 0.0001 from this design to separate and estimate the dierent causes of variation.ANOVA tests whether the dierence between sample means is too great to be explained by the random error. The null hypothesis (H0) that the factors had similar means at the 95% confidence level was tested, H0 to be rejected in favour of an alternative hypothesis if p<0.05. Both the acid concentration and pre-blank digestion number factors are significant at the 95% confidence level with p=0.0007 and 0.0001, respectively. Fig. 3 Box-and-Whisker plots for pre-blank digestion factor level This was further illustrated by the Box-and-Whisker plots data. for the variables (Figs. 1–4). The box-plot depicts the upper and lower quantiles of the data by the top and bottom of a rectangle, and the median is depicted by a horizontal line partition within the rectangle. Lines extend from the ends of the box to the upper and lower adjacent values. The upper adjacent value is the largest observation that is less than or equal to the upper quartile plus 1.5×IQR[where the interquartile range IQR=Q(0.75)-Q(0.25)].The lower adjacent value is the smallest observation that is greater than or equal to the lower quartile minus 1.5×IQR. Outside values are those points that fall outside the range of the two adjacent values, and are plotted as individual points using the symbol ‘+’. Characteristic features of the data are exhibited in the boxplot. The median shows the centre of the distribution. The spread of the bulk of the data (the central 50%) is represented Fig. 4 Box-and-Whisker plots for bomb number factor level data. by the length of the box. Comparison of the lengths of the lines with the length of the box reveals how stretched the tails of the distribution are. The individual outside values on the graphs allow the viewer an opportunity to consider the question of outliers, that is, observations that seem unusually large or small. Outside values are not necessarily outliers but any outliers will almost certainly be outside values.It is evident from the plots that there are two main features in the data. Firstly, the lead levels per vessel decreased with each blank digestion performed (Fig. 3). Also, the lead levels tend to increase with acid concentration (Fig. 2). Both features are not unexpected. The significance of the number of preblank digestions can be reduced by performing an acceptable Fig. 1 Box-and-Whisker plots for microwave conditions factor level data.number of blank digestions prior to each analysis. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 555Table 5 Summary of statistics for blank digestion tests performed Although not significant at the 95% confidence level the after nine pre-blank digestions trend observed for the acid concentration still creates a problem. The nitric acid reagent may be rejected as a possible Variable Blank source of lead contamination. The lead contained in this Sample size 34 reagent would make no net contribution to contamination Average/ng 0.34 levels as nitric acid makes the same contribution to the samples Median/ng 0.32 and standards.Mode/ng 0.52 Both trends can be interpreted as retention of lead on the Variance/ng 0.04 Standard deviation/ng 0.19 vessel walls. For the pre-blank digestion number it is evident Standard error/ng 0.03 that as the number increases the contamination levels decrease, Minimum/ng 0 eventually levelling out after approximately nine digestions Maximum/ng 0.85 (Fig. 3). A similar explanation can be made for the acid Range/ng 0.85 concentration factor as lead levels in blanks increase with Lower quartile/ng 0.19 increasing concentration of nitric acid. The levels can therefore Upper quartile/ng 0.49 Interquartile range/ng 0.31 be considered to be a result of retention on the walls from the RSD (%) 56.9 vessels with the higher acid concentrations able to digest a greater amount of the lead from the vessel walls.Given that the blank contamination may be attributed to trend visible in the box-plots for the nitric acid concentrations the retention of lead on the walls of the laboratory ware, a used in the digests. policy of performing a minimum of nine pre-blank digestions prior to analysis was adopted. A summary of the statistics for This research was supported in part by a QUT Meritorious 34 blank samples digested under this criterion in 2 ml of nitric Grant and by the ARC Collaborative Grant C295301023.acid (10% v/v) is provided in Table 5. Allowing for approximately 99.7% of the blank values to fall within three standard deviations of the mean, the detection limit of the method for REFERENCES lead deposited on the filter may be calculated as 1 ng per filter. 1 Boeckx, R. L., Anal. Chem., 1986, 58, 275A. Making use of the multi-element capabilities of ICP-MS, 2 Sub-committee on Airborne Particles, Airborne Particles, similar studies on a number of other trace elements were University Park Press, Baltimore, MD, 1978, p. 107. performed on the same samples with no additional work. The 3 Banerjee, D., and Pandey, G. S., Int. J. Environ. Anal. Chem., results were consistent with those shown for lead. 1989, 35, 169. 4 Janssens, M., and Dams, R., Anal. Chim. Acta, 1973, 65, 41. 5 Usero, J., and Gracia, I., Int. J. Environ. Anal. Chem., 1987, 30, 69. CONCLUSIONS 6 Ward, N. L., Brooks, R. R., Roberts, E., and Boswell, C.R., Environ. Sci. T echnol., 1977, 11, 917. A procedure for the reduction in blank contamination was 7 Valerio, F., Brescianini, C., and Lastraioli, S., Int. J. Environ. established by using a systematic study by ANOVA. The Anal. Chem., 1989, 35, 10. digestion procedure incorporated meticulous washing of appar- 8 Hunt, A., Johnson, D. L., Watt, J. M., and Thornton, I., Environ. atus and the performance of a number of pre-blank digestions, Sci. T echnol., 1992, 26, 1513. suitable for measurement at the trace levels consistent with 9 Galli, B.C., Burki, P. R., Nyeler, U. P., and Schindler, P. W., Int. J. Environ. Anal. Chem., 1989, 35, 111. atmospheric particulate lead. Detection limits of the method 10 Mukai, H., Furuta, N., Fujii, T., Ambe, Y., Sakamoto, K., and are not governed by those of the instrumentation but more by Hashimoto, Y., Environ. Sci. T echnol., 1993, 27, 1347. contamination sources inherent in the type of samples under 11 Hinners, T. A., Heithmar, E. M., Spittler, T. M., and Henshaw, analysis. J. M., Anal. Chem., 1987, 59, 2658. The detection limit for the technique improved from 20 to 12 Paudyn, A. M., and Smith, R. G., J. Anal. At. Spectrom., 1990, 1 ng for lead deposited on the filter. The method has not been 5, 523. 13 Sturges, W. T., and Harrison, R. M., Atmos. Environ., 1985, applied to certified reference materials. However, it is believed 19, 1495. that these materials do not adequately represent sub- 14 Schneider, B., Spectrochim. Acta, Part B, 1989, 44, 519. micrometre airborne particulate matter collected under normal 15 Pakkanen, T. A., Hillamo, R. E., and Maenhaut, W., J. Anal. At. sampling conditions and that the method is viable for the Spectrom., 1993, 8, 79. detection of lead present in these types of samples. 16 Jannssens, M., and Dams, R., Anal. Chim. Acta, 1973, 65, 41. Despite PTFE having a reputation as being generally cleaner 17 Liang, Z., Wei, G. T., Irwin, R. I., Walton, A. P., and Michel, R. G., Anal. Chem., 1990, 62, 1452. than glass, there is still evidence of a ‘memory eect’ when dealing with lead. This is most visible in the box plots for the pre-blank digests in which a gradual decrease in the amount Paper 6/06446I Received September 18, 1996 of lead present in the vessel occurs with each blank digest. The conclusion is also supported by the presence of an upward Accepted January 17, 1997 556 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a606446i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Technetium-99 in Aqueous Solutions by Inductively Coupled Plasma Mass Spectrometry: Effects of Chemical Form and Memory† ROBERT C. RICHTER‡, S. ROY KOIRTYOHANN AND SILVIA S. JURISSON* Department of Chemistry, University ofMissouri, Columbia, MO 65211, USA The eects of chemical form and instrumental memory on the iently low for direct analysis of environmental samples, determination of technetium-99 (99Tc) in aqueous requiring preconcentration of the 99Tc.5 NAA methods, using environmental samples by ICP-MS were investigated. Using either thermal or fast neutron capture reactions, have also an assortment of cationic, anionic and neutral Tc and Re been found inadequate for the low 99Tc levels found in complexes, a comparison of the ICP-MS method with the environmental samples.8,9 established methods of liquid scintillation counting (LSC) for ICP-MS has been used successfully in environmental trace Tc and neutron activation analysis (NAA) for Re gave lower analysis and recently has been applied to 99Tc determithan expected Tc and Re values by ICP-MS owing to loss of nation.6,10–17 The published methods use preconcentration sample in the delivery system.Oxidation of the complexes techniques to increase 99Tc levels above the blank limited prior to analysis and the addition of Triton X-100 to the detection limit.12,17 High blanks result primarily from instrusample solution eliminated this problem. Instrumental mental memory, an increase in the background count rate memory, resulting from interactions of 99Tc with the peristaltic following the analysis of a solution containing 99Tc.pump tubing and the alumina injection port tube, caused Although the pertechnetate ion dominates in aqueous significant increases in the background count rate during environmental samples, the presence of reducing agents and/or analysis. Aspiration with a nitric acid solution between sample complexing agents may alter the chemical form of 99Tc.If 99Tc runs to clean the system eectively eliminated this problem. is reduced in the absence of complexing ligands, it forms TcO2 These techniques were applied to simulated tap water and which can adsorb on any surfaces in contact with the sample. actual river water samples, and the accuracy was assessed 99Tc has a high anity for sulfhydryl ligands, which results in through LSC and spike recovery experiments. The detection absorption by microorganisms (approximately 10–20% of the limit of the ICP-MS method was found to be 0.6 ppt with an 99Tc that enters the river or ground water may be found in RSD of less than 10%, and these results were within 4% of the microorganisms).4,5 99Tc also complexes with naturally occur- LSC results.The sensitivity of the ICP-MS method for the ring ligands such as humic acid.18 Title 10, Chapter 1, Part 20 determination of 99Tc is much superior to that of the of the US Nuclear Regulatory Commission Rules and alternative radioanalytical methods when accounting for the Regulations allows the release of 99Tc resulting from medical data acquisition time for identical, low-concentration samples applications into the sanitary sewer system.This 99Tc is usually such as are often found in the environment. bound to a ligand or protein. This study was undertaken to develop a procedure that Keywords: T echnetium-99 ; inductively coupled plasma mass spectrometry ; radioenvironmental analysis; technetium-99 eliminates instrumental memory and allows the quantification speciation eects; technetium-99 memory eects of the 99Tc in aqueous environmental samples independent of chemical form and which requires minimal sample pretreatment.To minimize instrument contamination, rhenium (a non- Technetium-99 (99Tc) is a low-energy beta emitter (0.292 MeV) radioactive chemical analog of 99Tc) was used in the initial with a half-life of 2.1×105 yr which has entered the environ- studies and method development.Rhenium is often used as a ment through nuclear weapons testing, nuclear power pro- non-radioactive chemical analog for technetium because their duction, medical applications and negligence. The release of chemistries are similar.19,20 The lanthanide contraction makes 99Tc into the environment is of concern because, in addition analogous Tc and Re compounds virtually identical from a to its long half-life, it is highly mobile in aerobic environments structural standpoint.19 The main dierence is in their redox as the pertechnetate ion (TcO4-), its predominant form.2–5 potentials; Re is much more dicult to reduce than Tc and This element does not occur naturally and has no non- thus much easier to oxidize.19,20 The dierence in the redox radioactive isotopes.The current concentration guideline set potentials suciently aects the chemistries in some cases such by the US Department of Energy (DOE) for public exposure that any method developed based on Re must be validated to 99Tc in drinking water in the USA is 4 nCi l-1 (148 Bq l-1 with Tc.or 0.24 mg l-1).6,7 The extent and causes of instrumental memory were deter- Radiometric techniques are the most common methods used mined using standard perrhenate solutions, and a cleaning for the determination of 99Tc. Methods based on beta counting method was developed which eectively eliminates the reten- require the separation of 99Tc from other radionuclides before tion of Re and Tc associated with analyses.The chemical form analysis and long counting times (often 12 h or more) for low- of Tc may not be pertechnetate in all waste streams, so several concentration environmental samples. In addition, the detec- coordination complexes of Re and Tc were analyzed by tion limits for beta counting methods usually are not suc- ICP-MS to assess the impact of metal oxidation state, the complex charge and the coordinated ligand.A pretreatment † See ref. 1. method which eectively oxidizes all complexes to pertechnet- ‡ Present address: New York State Department of Health, ate was developed such that there was good agreement between Wadsworth Center, Room D-349, P.O. Box 509, Albany, NY 12201-0509, USA. the ICP-MS results and either NAA (for Re) or liquid scintil- Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (557–562) 557lation counting (LSC) (for 99Tc) for the various samples.The final test for any method developed is the analysis of real samples. Two types of samples were tested using the methods developed: (1) the various 99Tc complexes were prepared in tap water (to simulate sanitary sewer water) to insure that there was no interference from the matrix itself; and (2) actual river water samples known to contain 99Tc were obtained from the Savannah River site in South Carolina. All samples were validated using LSC.EXPERIMENTAL Materials Radiation safety Caution. The samples used for this work present radiological hazards. Proper radiation safety measures for handling, shielding, storage and disposal of these materials were strictly observed. Reagents Trace metal grade nitric acid (Fisher Scientific, Rochester, NY, USA) was used to prepare ICP-MS calibration and sample solutions. Analytical reagent grade acids (Fisher Scientific) were used to prepare cleaning solutions. Triton X-100 was purchased from Aldrich (Milwaukee, WI, USA).Rhenium Fig. 1 Structures of the various Tc and Re complexes. (1000 mg ml-1 ReO4-), tungsten (1000 mg ml-1 WO42-), and chromium (1000 mg ml-1) ICP-MS standards were purchased from Fisher Scientific. NH499TcO4 was purchased from Oak netium complexes were prepared in either water or acetonitrile. Ridge National Laboratory (Oak Ridge, TN, USA) and puri- The rhenium concentrations were determined by accurate fied prior to use (either by filtration to remove 99TcO2 or by weighing and the technetium concentrations by LSC.All H2O2 oxidation to convert 99TcO2 into 99TcO4-). A 99Tc samples analyzed by ICP-MS were 0.16 M in HNO3 and standard (0.289 mg ml-1 99TcO4-) was obtained from the contained 10.0 ppb of tungsten as an internal standard. Missouri University Research Reactor (MURR). Optifluor scintillation cocktail was purchased from Packard (Meriden, Instrumentation CT, USA). The reagents used for the syntheses of the rhenium and technetium complexes were obtained from Sigma (St.ICP-MS Louis, MO, USA), Aldrich and Fisher Scientific. Distilled, A Perkin-Elmer SCIEX (Thornhill, ON, Canada) ELAN 5000 deionized water was used for sample preparation. River water ICP-MS instrument equipped with either a cross-flow pneu- samples were obtained from Westinghouse Savannah River matic nebulizer with a Scott-type spray chamber or an ultra- Company (Savannah River, SC, USA). sonic nebulizer (CETAC Technologies, Omaha, NE, USA) for sample introduction was utilized.The ICP-MS operating con- T echnetium and rhenium complexes ditions and sample acquisition parameters are given in Table 1. Potassium hexachlororhenate(IV) (K2ReCl6),21 bis(ethylenediamine) dioxorhenium(V) chloride {[ReO2(en)2 ]Cl},22 dioxote- NAA tra(pyridine)rhenium(V) chloride {[ReO2(py)4]Cl},23 tetra- Samples were irradiated at MURR using the parameters given butylammonium (mercaptoacetyltriglycine)oxorhenate(V) {Bu4N in Table 2.Known amounts of sample and chromium (used as [ReO(MAG3)]},24 bis[cyclohexane-1,2-dione dioximato an internal standard) were placed in 2 ml polyethylene vials, (-1)-O][(cyclohexane-1,2-dione dioximato)(-2)-O]methyl- sealed and transported to reflector positions in the reactor borato(-2)-N,N¾,N,N+,N¾¾,N¾¾¾chlororhenium(III ) [ReCl core with a pneumatic tube system. Irradiated samples were (CDO)3BMe],25 bis[cyclohexane-1,2-dione dioximato(-1)- decayed for 3 d and then counted for 10 min at the appropriate O][(cyclohexane-1,2-dione dioximato)(-2)-O]methylborato energies (Table 2) using a high-purity germanium detector (-2)-N,N¾,N,N+,N¾¾,N¾¾¾chlorotechnetium(III) [99TcCl (CDO)3BMe],26 oxo[(3,3,9,9-tetramethyl-4,8-diazaundecane- Table 1 ICP-MS operating conditions 2,10-dione dioximato)(-3)]technetium(V) (99TcOPnAO)27 and tetrabutylammonium tetrachlorooxotechnetate(V) [Bu4N ICP conditions— (99TcOCl4)]28 were synthesized using literature methods. Fig. 1 Rf power 1000 W illustrates the structures of the complexes. Plasma Ar flow rate 15.00 l min-1 Auxiliary Ar flow rate 0.770 l min-1 Nebulizer Ar flow rate 0.900 l min-1 Solutions Solution uptake rate 1.00 ml min-1 A 4.00 ppb rhenium solution, 1.45 ppb 99Tc solution, internal Data acquisition parameters— Dwell time 150 ms standard solution and ICP-MS calibration solutions were Sweeps per reading 250 prepared by diluting the standards with 0.16 M HNO3.HNO3 Readings per replicate 1 (0.8 M), H2SO4 (0.9 M), HCl (0.8 M), H3PO2 (0.9 M) and Number of replicates 3 HNO3–HCl (0.8 M in each) were prepared as potential cleaning Points per peak 1 solutions. Stock standard solutions of the rhenium and tech- 558 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Table 2 NAA parameters for irradiation and counting Sample Analysis Eect of chemical form Thermal flux 4.0×1013 n cm-2 s-1 Epithermal flux 1.0×1012 n cm-2 s-1 The eect of chemical form on the quantification of 99Tc was Fast flux 4.0×1012 n cm-2 s-1 investigated by analyzing a variety of rhenium and technetium Irradiation time 1 h Decay time 3 h complexes (see below) for the content of the given metal.The Counting time 10 min Re samples were prepared as described above and analyzed by ICP-MS and NAA for validation of the results (Table 3). Nuclear reactions Gamma energy/keV Half-life The agreements between the results obtained from ICP-MS 185Re (n,c) 186Re 137 3.7 d and NAA were optimized by the addition of detergents and/or 187Re (n,c) 188Re 155 16.9 h oxidants to the samples and subsequent heating of the samples 50Cr (n,c) 51Cr 320 27.0 d prior to ICP-MS analysis(Table 4).The optimumpretreatment procedures developed for the Re samples were then used to analyze the Tc samples by ICP-MS. LSC was used for validation (Table 5). Additional optimization was necessary for the Tc samples because of the dierences in the redox potentials equipped with a multi-channel analyzer (EG&G Technologies, between Tc and Re.The optimization was achieved by heating Omaha, NE, USA). the 99Tc sample solutions (to ensure oxidation to pertechnetate) prior to ICP-MS analysis. L SC Simulated sanitary sewer samples A Tracor Analytic (Elk Grove Village, IL, USA) Delta 300 liquid scintillation counter was used to determine 99Tc concen- Tap water was used as the diluent to simulate samples from trations in samples consisting of 9 ml of Optifluor liquid sanitary sewers.Aliquots were taken from each of the 99Tc scintillation cocktail and 1 ml of sample solution. The samples complex stock solutions, placed in two 100 ml polymethylpen- were equilibrated in the absence of fluorescent light for 1 h tene flasks, filled to the mark with 0.16 M HNO3, and heated before counting. at 35 °C for 45 min. These samples were analyzed by LSC to determine their 99Tc concentrations and then diluted by a factor of 10000 with tap water and analyzed by ICP-MS Instrumental Memory (Table 6).To minimize instrument contamination from radioactivity, the extent and sources of Tc retention in the ICP-MS system were Environmental Samples determined using Re, the non-radioactive chemical analog for The application of the method developed for the quantification Tc, and Tc was then evaluated beginning with the optimized of 99Tc by ICP-MS was extended to an environmental river conditions. water sample obtained from the Savannah River site in South The extent of Re retention was evaluated by measuring the Carolina.LSC and standard addition experiments were used initial background count rate for Re at m/z 187 while aspirating to validate the ICP-MS results. a 0.16 M HNO3 blank solution, then aspirating the 4.00 ppb Re solution for 2 min, rinsing with water for 30 s and remeasuring the 0.16 M HNO3 blank solution to determine the apparent River water analysis by L SC background count rate.This procedure was repeated nine Duplicate samples (200 ml) of river water were evaporated to consecutive times (Fig. 2). dryness by heating at 60 °C, the residue was dissolved in 10 ml The sources of Re retention were determined by measuring of 2.0 M HNO3 and the solution was transferred into 20 ml the background count rate of the 0.16 M HNO3 blank solution LSC vials and again evaporated to dryness. The residues at m/z 187 before and after individually cleaning each compo- remaining were dissolved in 1 ml of 0.16 M HNO3 and scintil- nent of the ICP-MS system following a 30 min aspiration with lation cocktail (9 ml) was then added.Each sample was the 4.00 ppb rhenium solution and a 30 s rinse with water. The counted for 12 h. The elapsed time for 99Tc quantification sampler and skimmer cones were removed, rinsed with dilute was 72 h. HNO3 and sonicated in an ultrasonic bath to remove any Re deposits.The injection port tube (quartz or alumina) and spray chamber were cleaned by soaking in 2 M HNO3 for 1 h. The River water analysis by ICP-MS peristaltic pump tubing (standard or SolventFlex from Perkin- Direct sample analysis. A sample (1.5 l) of river water was Elmer, Norwalk, CT, USA) was replaced rather than cleaned. acidified with 15 ml of concentrated HNO3, heated at 75 °C The optimum cleaning solution composition was determined for 75 min and filtered to remove undissolved sediment.Twelve by comparing the initial background count rate of the 100 ml aliquots were then analyzed by ICP-MS. Three hours 0.16 M HNO3 blank solution at m/z 187 with that measured were required for quantification of 99Tc using this method, after a 2 min aspiration with the 4.00 ppb rhenium solution with 2 min for sample acquisition and 4 min rinsing between followed by 8 min of rinsing with one of the cleaning solutions samples. (see below). The count rate at m/z 187 was measured every 2 min during the continuous aspiration with one of the rinse Standard addition method. A sample (300 ml) of river water solutions to determine the time course of the cleaning (Fig. 3). was treated as above and used to prepare two 100 ml samples The most eective cleaning solution, as determined for Re, was for standard addition analysis. Dierent aliquots (75 and validated for 99Tc by comparing the initial background count 100 ml) of a 289 pg ml-1 99Tc standard solution were added rate for the 0.16 M HNO3 blank solution at m/z 99 with that to these samples and they were then analyzed by ICP-MS.measured after a 3 min aspiration with the 1.45 ppb 99Tc solution followed by a 4 min rinse with 0.8 M HNO3. This Statistical Analysis procedure was repeated six consecutive times. SolventFlex tubing and an alumina injection tube were used in these The values reported for the Re complexes by NAA are the average of four sample irradiations. With the exception of the experiments.Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 559river water, for which 12 samples were analyzed, all ICP-MS oxoanions are known to have an anity for alumina).20 The use of a quartz injection tube eliminated this memory eect. results are the average of six sample analyses. LSC samples were analyzed in triplicate for the routine analyses and in duplicate for the lengthy river water samples. All errors are Eectiveness of Cleaning Solutions reported as plus or minus one standard deviation from the mean.Dismantling the system to clean the components between sample runs is an unrealistic approach to minimizing the background count rate. A more practical approach is aspir- RESULTS AND DISCUSSION ation of a cleaning solution through the system between samples to remove any residual analyte. A variety of acids Instrumental Memory were tested as cleaning agents and their eectiveness was Instrumental memory eects have been reported for the determined by monitoring the background count rate while ICP-MS determination of 99Tc by a number of groups, but aspirating a cleaning solution following a simulated analysis the solution to the high backgrounds observed for these (Fig. 3).The background count rate decreased rapidly for all analyses has generally been the use of tedious and time- the cleaning solutions except H3PO2. The formation of col- consuming preconcentration procedures.11,12,14–16 As precon- loidal ReO2 and its adsorption to the spray chamber walls, centration increased the analyte concentration, the 99Tc signal accumulation in the dead volume of the spray chamber or was increased and the accompanying increase in background precipitation on to the pump tubing may be responsible.HCl signal was apparently tolerable. Instrumental memory eects more eectively reduced the background count rate, perhaps arise from the retention of 99Tc in the ICP-MS system, perhaps through the formation of mobile Re–Cl complexes or by the from analyte build-up on the cones, adsorption to a surface displacement of perrhenate by chloride in the pump tubing en route to the plasma or retention in the dead volume of the and injection port tube.The most eective cleaning solution spray chamber. The background count rate between sample was found to be 0.8 M HNO3. runs was measured during a simulated analysis to study the The optimum rinse time was determined from Fig. 3. Using eects of instrumental memory. Fig. 2 shows the increase in 0.8 M HNO3 as the rinse solution, aspiration for 4 min reduced the apparent background count rate for 187Re (a non- the count rate to its minimum value and continued rinsing did radioactive chemical analog for 99Tc). After just one run the not further improve the background count rate. apparent background count rate had nearly doubled and after The same cleaning procedure was found suitable for minimiz- nine runs it was 20 times the original value.This sharp increase ing the 99Tc background count rate during analysis. The in the background could result in significant error when background count rate for the 0.1 M HNO3 blank solution analyzing solutions with concentrations near the limit of was found to be 16.2±0.6 counts s-1 prior to and detection. 15.9±0.7 counts s-1 following the analysis of a 1.45 ppb The ICP-MS system was systematically dismantled, the 99TcO4- solution (7439±86 counts s-1 count rate).The individual components were cleaned and the background count absence of any significant change in the background count rates before and after cleaning were compared to determine rate during the simulated analysis indicated that the cleaning the sources of 99Tc and Re retention. The spray chamber and procedure eectively removed any residual 99Tc from the sampling cones had no significant eect on system memory, system.since the apparent background count rate did not change after cleaning. The peristaltic pump tubing, on the other hand, played a major role. When the used piece of standard tubing Eect of Chemical Form on 99Tc Determination was replaced with a new piece, a 10-fold decrease in the The eect of chemical form on 99Tc analysis was probed using apparent background count rate was observed. This suggested a number of dierent Re and 99Tc complexes (Fig. 1). Rhenium that the tubing was permeable to Re or that surface sites was again used as the non-radioactive chemical analog of existed to which the Re bound.The use of SolventFlex tubing 99Tc during method development. The complexes used for this minimized the increases in the apparent background count rate. study exhibited a range of metal oxidation states and overall The injection tube was also involved in the retention of complex charge. Additionally, the 99mTc analogs of rhenium. Cleaning the alumina injection tube resulted in a (Bu4N)[ReO(MAG3)] and MCl(CDO)3BMe (M=Re or 99Tc) fourfold decrease in the background count rate, indicating that are used in nuclear medicine departments for renal and myocar- the Re had adsorbed on the alumina (perrhenate and other Fig. 3 Eectiveness of cleaning solutions. The background count rate at m/z 187 is shown while aspirating each of the cleaning solutions Fig. 2 Eect of instrumental memory on background count rate. The ($, 0.9 M H3PO2; (, 0.9 M H2SO4; #, 0.8 M HCl; ', 0.8 M HNO3; 2, 0.8 M HCl+0.8 M HNO3) immediately following analysis of a 4.00 ppb background count rate for the blank solution at m/z 187 is shown following consecutive analyses of a 4.00 ppb ReO4- standard solution.ReO4- standard solution. 560 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12Table 3 Comparison of NAA and ICP-MS results for rhenium com- Table 5 Comparison of LSC and ICP-MS results for 99Tc complexes.All values in mg ml-1 (ppm) plexes. All values are percentage of rhenium in the complex Complex Calculated NAA ICP-MS Complex LSC ICP-MS K2ReCl6 39.0 39.7±0.2 38.9±0.5 Bu4N99TcOCl4 1.29±0.06 1.14±0.02 99TcOPnAO 1.99±0.01 1.98±0.05 ReO2(en)2Cl 49.8 48.0±0.8 48.1±1.4 ReO2(py)4Cl 32.6 33.0±0.7 32.4±0.8 99TcCl(CDO)3BMe 0.531±0.005 0.453±0.011 ReCl(CDO)3BMe 27.8 26.9±0.4 5.12±0.09 Bu4N[ReO(MAG3)] 26.4 1.07±0.02 1.08±0.03 Table 6 Comparison of LSC and ICP-MS results for 99Tc complexes Table 4 Optimization of solution conditions for ReCl(CDO)3BMe in a tap water matrix.All values in pg ml-1 (ppt) Solution conditions Re (%) ICP-MS 0.16 M HNO3 5.12±0.09 Sample LSC PN* USN* 0.01% (v/v) Triton X-100 Complex Mix 1 26.1±0.1 25.4±0.8 — 0.16 M HNO3 23.3±0.3 Complex Mix 2 57.0±0.1 56.1±0.6 — 0.08% (v/v) Triton X-100 Bu4N99TcOCl4 5170±20 — 5150±50 0.16 M HNO3 24.8±0.5 99TcOPnAO 550±2.0 — 549±7.0 99TcCl(CDO)3BMe 56.1±0.7 — 55.9±1.0 0.16 M HNO3 30 min oxidation 26.2±0.2 * PN=pneumatic nebulization; USN=ultrasonic nebulization. 5.0% (v/v) H2O2 0.16 M HNO3 23.1±0.2 0.01% (v/v) Triton X-100 0.16 M HNO3 26.7±0.3 which already contained an oxidant (HNO3), were allowed to 30 min oxidation stand for 30 min prior to analysis. This led to an improvement NAA result 26.9±0.4 over the Triton X-100 method (Table 4). The use of hydrogen peroxide as oxidizing agent failed to improve the correlation. Since the ICP-MS results for the HNO3 oxidation of ReCl(CDO)3BMe were found to lie just outside one standard dial imaging, respectively, and an isomer of TcOPnAO is used deviation of the NAA values, a small amount (0.01%) of Triton for cerebral blood flow imaging.29 X-100 was added to improve the correlation. This combination The values obtained from ICP-MS analysis were compared resulted in excellent agreement with the NAA data (Table 4) with the results from NAA and LSC to probe the eects of and was used to analyze the 99Tc complexes.chemical form.The Re analyses are reported as percentage of The LSC and ICP-MS results for the 99Tc complexes are Re since accurate masses were obtained. To minimize sample given in Table 5. Good correlation between LSC and ICP-MS handling and potential contamination, the concentrations of was observed only for 99TcOPnAO. The low values for the the 99Tc samples were determined by LSC and the results are other complexes resulted from insucient oxidation of the reported as ppm of 99Tc in solution. 99Tc (Re is easier to oxidize than Tc).20 Heating the 99Tc- The NAA and ICP-MS analyses for the Re complexes are containing solutions for 45 min at 35 °C prior to ICP-MS shown in Table 3. The results from the two methods compare analysis resulted in good agreement between the two methods. favorably with each other and with the known percentage of Re, except for (Bu4N)[ReO(MAG3)] and ReCl(CDO)3BMe. The low value observed for the analysis of (Bu4N) Analysis of Water Samples [ReO(MAG3 )] by both NAA and ICP-MS compared with the theoretical value resulted from the presence of a Bu4NBr The above techniques were applied to the determination of 99Tc in simulated sewer water and river water.Tap water was impurity. The good agreement between the NAA and ICP-MS results for this complex indicated that the chemical form had used as a model for sanitary sewer water. Reliability was validated by LSC and direct and standard addition ICP-MS not aected the analysis.The low value observed for the ICP-MS analysis of ReCl(CDO)3BMe was accompanied by analysis. The results for tap water analysis using pneumatic and ultrasonic nebulization are given in Table 6. There is good an increase in the background count rate, suggesting adsorption to the walls of the pump tubing, spray chamber or sample agreement with LSC for both nebulization methods. Since satisfactory nebulization conditions could not be found when flask, probably resulting from the high lipophilicity of this compound.Triton X-100 was present, it was omitted from the ultrasonic nebulization method. Additional heating (75 min) successfully Several procedural modifications were examined to minimize the adsorption of this neutral, lipophilic analyte. A detergent, compensated for this change. There is excellent agreement between all methods of analysis. Direct ICP-MS analysis Triton X-100, was added to improve the dispersion of the complex in aqueous solutions.Addition of 0.01% (v/v) Triton of the river water sample gave a concentration of 1.72±0.14 pg ml-1 of 99Tc, ICP-MS standard addition analysis X-100 improved the correlation with the NAA data signifi- cantly (Table 4). Even though the correlation was further gave 1.76±0.36 pg ml-1 and LSC gave 1.78±0.09 pg ml-1. These results suggest that LSC is comparable to ICP-MS, but improved by increasing Triton X-100 to 0.08% (v/v), a 50% reduction in the Re count rate was observed along with carbon the dierences in the times for data acquisition per sample (2 min for ICP-MS and 12 h for LSC) and the fact that the deposits on the sampling cones.The reduction in the Re count rate aected the precision, accuracy and detection limit of the LSC sample was concentrated by a factor of 20 illustrate the much superior sensitivity of the ICP-MS method for this low method and the carbon deposits could eventually obstruct the sampler orifice.Hence higher concentrations of Triton X-100 specific activity radionuclide. The large standard deviations for the ICP-MS analysis of the river water sample result were not used to further improve the correlation. Oxidation of ReCl(CDO)3BMe to ReO4-, a more soluble because the analyte concentration is close to the detection limit of 0.6 ppt (3s of the blank), which is equivalent to chemical form of Re, was evaluated as an alternative method. The ReCl(CDO)3BMe solutions used for ICP-MS analysis, 0.6 ng l-1 (0.37 Bq l-1 or 0.01 nCi l-1).Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 5619 Ikeda, N., Seki, R., Kamemoto, M., and Otsuji, M., J. Radioanal. CONCLUSION Nucl. Chem., 1989, 131, 65. The use of these simple preoxidation and cleaning procedures 10 Igarashi, Y., Kim, C. K., Takaku, Y., Shiraishi, K., Yamamoto, M., and Ikeda, N., Anal. Sci., 1990, 6, 157. to analyze successfully river water samples for 99Tc demon- 11 Kim, C.K., Otsuji, M., Takaku, Y., Kawamura, H., Shiraishi, K., strates that in the absence of a significant isobaric 99Ru Igarashi, Y., and Ikeda, N., Radioisotopes, 1989, 38, 151. interference or high concentrations of dissolved solids, the 12 Morita, S., Kim, C. K., Takaku, Y., Seki, R., and Ikeda, N., Appl. tedious preconcentration and separation procedures previously Radiat. Isot., 1991, 42, 531. reported are unnecessary.13,15–17 If 99Ru is present, a correction 13 Morita, S., Tobita, K., and Kurabayashi, M., Radiochim.Acta, 1993, 63, 63. based on the amount of 101Ru present can be applied.6 When 14 Momoshima, N., Sayad, M., and Takashima, Y., Radiochim. Acta, the 99Ru concentration is orders of magnitude higher than that 1993, 63, 73. of 99Tc or the sample is taken from an area where the natural 15 Nicholson, S., Sanders, T. W., and Blaine, L. M., Sci. T otal abundance ratios may have been altered (the fission yields of Environ., 1993, 130, 275.the ruthenium isotopes do not correspond to the natural 16 Sumiya, S., Morita, S., Tobita, K., and Kurabayshi, M., J. Radioanal. Nucl. Chem., 1994, 177, 149. abundances), the 99Ru must be separated from the 99Tc prior 17 Garcia Alonso, J. I., Sena, F., and Koch, L., J. Anal. At. Spectrom., to analysis. If the sample contains only a high concentration 1994, 9, 1217. of dissolved solids, on-line separation can be used to remove 18 Sekine, T., Watanabe, A., Yoshihara, K., and Kim, J.I., Radiochim. the matrix prior to 99Tc determination.30,31 Acta, 1993, 63, 87. 19 Deutsch, E., Libson, K., Vanderheyden, J.-L., Ketring, A. R., and Maxon, H. R., Nucl. Med. Biol., 1986, 13, 465. We acknowledge Perkin-Elmer SCIEX for providing the 20 Cotton, F. A., and Wilkinson, G., Advanced Inorganic Chemistry, ICP-MS system and the Missouri University Research Reactor Wiley, New York, 5th edn., 1988, pp. 850–867. for financial support and the use of their facilities. We thank 21 Watt, W., and Thompson, R. J., Inorg. Synth., 1963, 7, 189. 22 Murmann, R. K., Inorg. Synth., 1966, 8, 173. Dr. Donna Beals (Westinghouse Savannah River Co.) for 23 Ram, M. S., and Hupp, J. T., Inorg. Chem., 1991, 30, 130. providing the river water sample, Dr. Alan Ketring for assist- 24 Rao, T. N., Adhikesavalu, D., Camerman, A., and Fritzberg, ance with the NAA analysis and Dr. Jack Lydon, Teri Parsons A. R., Inorg. Chim. Acta, 1991, 180, 63. and Martha Halihan for help with the syntheses of the 25 Jurisson, S., Treher, E. H., Francesconi, L. C., Malley, M. F., complexes. Gougoutas, J. Z., and Nunn, A. D., Inorg. Chem., 1991, 30, 1820. 26 Treher, E. H., Francesconi, L. C., Malley, M. F., Gougoutas, J. Z., and Nunn, A. D., Inorg. Chem., 1989, 28, 3411. 27 Jurisson, S., Schlemper, E. O., Troutner, D. E., Canning, L. R., REFERENCES Nowotnik, D. P., and Neirinckx, R. D., Inorg. Chem., 1986, 25, 543. 28 Davison, A., Trop, H. S., Depamphilis, B. V., and Jones, A. G., 1 Abstracted in part: Richter, R. C., PhD Thesis, University of Inorg. Synth., 1982, 26, 160. Missouri–Columbia, 1995. 29 Jurisson, S., Berning, D., Jia, W., and Ma, D., Chem. Rev., 1993, 2 Holm, E., Radiochim. Acta, 1993, 63, 57. 93, 1137. 3 Yanagisawa, K., Muramatsu, Y., and Kamada, H., Radioisotopes, 30 Hollenbach, M., Grohs, J., Mamich, S., Kroft, M., and Denoyer, 1992, 41, 397. E. R., J. Anal. At. Spectrom., 1994, 9, 927. 4 Lieser, K. H., Radiochim. Acta, 1993, 63, 5. 31 Shabani, M. B., and Masuda, A., Anal. Chim. Acta, 1992, 261, 315. 5 Lieser, K. H., and Bauscher, C. H., Radiochim. Acta, 1987, 42, 205. 6 Crain, J. S., and Gallimore, D. L., Appl. Spectrosc., 1992, 46, 547. Paper 6/06483C 7 Order Number 5400.5, 8 February 1990, Chapters II and III, Received September 20, 1996 United States Department of Energy, Washington, DC. 8 Trautmann, N., Radiochim. Acta, 1993, 63, 37. Accepted December 12, 1996 562 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a606483c

出版商:RSC    

年代:1997

数据来源: RSC