摘要:

Analytical performance of axial inductively coupled plasma time of Øight mass spectrometry (ICP-TOFMS) Xiaodan Tian, Ha kan Emteborg and Freddy C. Adams* Micro and Trace Analysis Center, MiTAC, Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerp, Belgium Received 19th August 1999, Accepted 11th October 1999 Analytical performance of an axial ICP-TOFMS, in terms of accuracy, precision, resolution, signal stability, repeatability, reproducibility, precision of isotope ratios and detection limits is reported. Cool plasma conditions (low power and high central gas Øow) allow the determination of K, Ca and Fe at trace levels.Detection limits for 64 elements are reported with typical values from 0.5±20 pg ml21 (3 s criterion). The long-term stability over 4 h for the raw signal ranges between 1.0% RSD for 7Li to 2.1% RSD for 40Ca operating under cool plasma conditions. Under normal plasma conditions the corresponding values are 6.9% RSD for 208Pb to 12.8% RSD for 59Co. Accuracy for the determination of 16 different elements is assessed by measuring NIST 1643d (trace elements in water), yielding results in agreement with certiÆed values.Results for isotopic ratio determinations demonstrate that a precision of 0.07±0.7% RSD is obtained within a short data acquisition period. Introduction Since inductively coupled plasma mass spectrometry (ICP-MS) was introduced in the early '80s, it has become a widespread multi-elemental technique for elemental analysis in many sample types.1,2 It offers almost complete elemental coverage combined with a high sensitivity for most elements, detection limits being in the 1±10 pg ml21 range.Another important advantage of ICPMSover other analytical techniques, such as inductively coupled plasma atomic emission spectrometry (ICP-AES) and atomic absorption spectrometry (AAS), is the possibility of quantiÆcation by isotope dilution. Most ICP-MS instruments employ quadrupole mass Ælters, resulting in high sensitivity, low operating costs, and modest vacuum requirements.Although quadrupole ICP-MS has matured into a powerful technique, there are still several remaining weaknesses, such as isobaric overlaps, matrix interferences, limited precision (especially for isotope ratios) and limitations in elemental coverage when measuring rapid transient signals.3 The two latter shortcomings are a direct consequence of the fact that quadrupole ICP-MS is a sequential (scanned) mass spectrometer.As an alternative, Myers et al. Ærst described in 1993 an ICP-MS based upon a time-of-Øight (TOF) mass spectrometer placed orthogonal to the ion source.4 In later work, using the same system, results concerning optimization, characteristics and performance for isotope ratios were published.5±7 TOFMS is inherently a simple type of mass spectrometer with high transmission efÆciency. The three most important assets are simultaneous ion extraction, quasi-simultaneous nature of spectral generation and extremely high data acquisition speed over the whole mass spectrum.Ions are generated and extracted from the ion source through a sampler and a skimmer cone. The ions are then guided into an acceleration region where a high voltage pulse is imposed, accelerating the ions to a similar kinetic energy and subsequently directing them into the Æeld-free Øight tube. Lighter ions will consequently travel faster than heavier ions, therefore, ions of different mass (m/z) will separate into their individual mass components as they travel through the Øight tube and generate a complete mass spectrum. Generally, TOFMS was previously only used with pulsed ion sources, such as glow discharge (GD) matrix-assisted laser desorption/ionization, since the ions have to enter the Øight tube in the form of discrete packets (i.e.modulation). Since the repetition rate of a TOF instrument is determined by the Øight time of the heaviest ion through the Æeld-free region, and with a Øight time for atomic species of less than 40 ms, more than 20 000 spectra per s can be generated in ICP-TOF-MS.The high repetition rate and simultaneous detection capabilities of the TOF mass spectrometer not only reduce analysis time and improve precision on isotope ratios, but also make the TOF instrument an ideal choice for the analysis of transient signals. Especially for a better precision on isotope ratios, simultaneous extraction of ions from the plasma improves the performance compared to quadrupole ICP-MS through a better elimination of Øicker noise.ICP-TOFMS thereby overcomes some of the remaining weaknesses of quadrupole ICP-MS. Recently, the application of ICP-TOFMS for transient signal detection generated by ETV and laser ablation have been reported.8,9 Axial ICP-TOFMS was made commercially available by LECO (St. Joseph, MI, USA) in 1998.Compared with orthogonal ICP-TOFMS, an axial arrangement has less divergence of ions, which minimizes mass bias and increases resolution. When comparing time-of-Øight systems with quadrupole systems the duty factor is an important parameter. This is the fraction of ions that are extracted for mass analysis. In the axial system used here approximately 10% of the ions are extracted for analysis. It results in a lower overall sensitivitywhen compared with quadrupole systems used for detection of a single m/z.This is a direct consequence of the ion source modulation. An important feature of the axial ICP-TOFMS is its scheme to avoid detector overload. In quadrupole ICP-MS, the high background obtained at certain m/z from the plasma gas, entrained atmospheric gases and water can be eliminated by skipping over certain dc/rf settings that allow their transmission. In contrast to this an ICP-TOFMS instrument accepts ions of all m/z into the acceleration region regardless of whether they are of analytical interest or not.This may cause detector overload or even destruction by the intense Øux of certain matrix ions. Given this situation, ICP-TOFMS has to be equipped with an additional device to avoid detector overload, called the ``Transverse Rejection Ion Pulse (TRIP)''. This device involves the application of a high voltage pulse imposed at the time when a matrix ion passes by an ion gate in order to remove it from the ion beam.In some cases, analyte ions are also concomitant with matrix ions from the sample. Therefore, J. Anal. At. Spectrom., 1999, 14, 1807±1814 1807 This journal is # The Royal Society of Chemistry 1999additional deØection windows need to be implemented besides certain standard deØections for background from the plasma such as argides and ions derived from water ions. More details may be found in a previous publication.10 The time spent in determining 66 isotopes (the maximum number that can be measured with 30 bins per isotope acquisition window) is only determined by the integration time and number of replicates selected resulting in considerable time-saving. Each bin corresponds to a 2 ns mass window and over the whole mass spectrum 2000 bins are distributed.Note that time and mass can be used interchangeably when describing TOF mass spectrometry. Through the software the analyst can control the width of the isotope acquisition window.By collection of, e.g., a 15 bin mass window for each isotope, 2000/15~133 isotopes can be measured. Naturally a slight sensitivity loss is encountered in such a case. It should also be mentioned that a maximum of 19 bins can be used by the software for calculating the peak area so that peaks at higher mass are not fully integrated. This may inØict problems for the accuracy of isotope ratio measurements. Using quadrupole ICP-MS, the time spent for analysis is directly proportional to the number of isotopes multiplied by the integration time including settling time for the rods.The analysis time using ICP-TOFMS is thus independent of the number of isotopes being measured within the limits described above. In this paper, a set of results obtained by ICP-TOFMS will be presented. The results include basic analytical performance and Ægures of merit like detection limits, signal stability, precision of isotope ratios and accuracy of analytical results using a standard reference material.The instrumentation is competitive with quadrupole ICP-MS in terms of detection limits and precision, but offers the additional advantage of simultaneous ion extraction, which is important for achieving high precision of isotope ratios. The analysis time is reduced and hence sample throughput is increased thanks to the high speed. Rapid spectral generation and quasi-simultaneous detection capabilities also results in complete elemental coverage in micro volumes and rapid transients.Experimental Instrumentation The ICP instrument used in this study is shown in Fig. 1 (axial ICP-TOFMS, Renaissance, LECO). The basic elements include a plasma torch for generation of a continuous ion beam, an ion transmission system including a sampler, a skimmer, an extraction cone (ion lens 1), an ion optical array to position the ion beam for accelerating and repelling the ion beam segments in a modulated fashion, an ion mirror and a detector.Segments of ions enter the Øight tube and are directed to the ion mirror where they are refocused and reØected back towards the detector. Use of an ion mirror results in a higher resolution, lower level of background noise and provides a doubling of the Øight path, thereby allowing a more compact size of the vacuum system. The instrument has a dual frequency rf generator, which generates frequencies of 27.12 MHz and 40.68 MHz; in this work 40.68 MHz was used.For an outline of the performance for the two different frequencies, readers can Ænd useful information in an article by Vickers et al.11 The torch cartridge assembly with a standard torch (Fassel type) is mounted horizontally in the torch box. A Wu±Hieftje FAST spray chamber and a Meinhard nebulizer are used for sample introduction. Sample solutions are delivered at a Øow rate of 1 ml min21 using a peristaltic pump (Gilson, Villiers Le Bel, France, Miniplus 3), where the pump speed can be controlled from the software or directly on the pump.Mass Øow controllers control the gas Øow rates; the plasma (coolant), nebulizer and auxiliary (intermediate) argon Øows are preset at 16, 0.8 and 1.0 l min21, respectively, with a primary argon input pressure of 50 psi. The torch position is adjustable via precise software control in the X, Y and Z directions in relation to the sampling oriÆce for optimization purposes. The torch position together with forward power, coolant, auxiliary and nebulizer Øows are important initial parameters for achieving maximum signal-to-noise ratio.The sampler and skimmer oriÆces are maintained at ground potential. The sampler is made from nickel with an aperture diameter of 0.9 mm. Because of the high temperature from the plasma, the sampler is mounted on a water-cooled plate, which forms the front wall of the vacuum system. An external recirculating chiller (Model CFT-75, Neslab Instruments, Inc, Newington, NH, USA) is used to supply cooling water also for the load coil.The skimmer cone is more sharply angled than the sampler cone, with an aperture diameter of about 0.5 mm. The distance between the sampler and skimmer is 7 mm (standard for this instrument). The vacuum chamber of the mass spectrometer is divided into three separate chambers as shown in Fig. 1. The Ærst stage vacuum chamber is immediately behind the sampler cone. The Ærst chamber's pressure is pumped down with a rotary vane vacuum pump to approximately 2±3 Torr.The expanding gas from the ICP is sub-sampled through skimmer oriÆce into the second-stage vacuum region. The second-stage vacuum is pumped down with a turbo molecular pump, to a pressure of 1023 Torr. Behind the skimmer, there are ion stacks, which control the ion Øow (as shown in Fig. 1). The ions selected by the modulation optic sends all ions into the dc acceleration region of the TOF-MS where all ions reach a similar kinetic energy.The third stagevacuumispumpedby a second turbo molecularpump to 1026 Torr, the actual pressure when the ICP is being operated. At the opposite end of the Øight tube, an ion mirror is placed to double the Øight path of the ions and focus their energy spreads. The detector is located above the ion-optics, opposite the ion mirror. In front of the detector are three grids used to reduce the number of low-energy background ions striking the detector surface.The detector is a discrete dynode multiplier (ETP Model AF831H, Ermington, NSW, Australia). The detector is constructed so that ion counting and analog signals are obtained simultaneously. Ion counting is used for the most sensitive work near the detection limit while analog mode is used to extend the dynamic range at higher signal levels since the signal from the ion counting mode is not linear at concentrations above 10 ng ml21 for the most sensitive isotopes.Optimization of the instrument Three standard plasma conditions, corresponding to ``normal'', ``hot'' and ``cold'' plasma conditions, were adapted for this work. The plasma and instrumental settings used are indicated in Table 1. Slight modiÆcations to these settings are normally necessary on a daily basis. It is obvious from the table that a number of important parameters need be studied and optimized to obtain adequate sensitivity and resolution. Following plasma ignition and prior to optimization, it is important to ensure that deØection settings are valid, so thatOzand Arzare deØected to Fig. 1 Schematic diagram of LECO Renaissance axial ICP-TOFMS system used in this work. The function and operation of the various components are described in the text. The Øight path is 260.5 m. 1808 J. Anal. At. Spectrom., 1999, 14, 1807±1814avoid detector overload. Normally, a 100 ng ml21 mass calibration solution is used to optimize the instrument by maximizing the signal intensity for 115In.The optimization starts with adjustments of the torch position by changingX±Y(vertical position of the plasma relative to the sampler aperture) and Z (horizontal position which also deÆnes the sampling depth) positions for a maximum 115In signal. From that optimum the nebulizer Øow, together with forward power, is changed to obtain a new maximum. A number of parameters of the mass spectrometer are also important for obtaining maximum signal intensity such as the potential on ion lenses 1 and 2, Einzel lenses 1 and 2 and X- and Y-steering.For optimal resolution 208Pb and 209Bi should be observed by adjusting reØectron (ion mirror) settings so as to obtain symmetrical peaks with no tailing or fronting. It is also possible to measure the resolution directly in the optimization menu in the software to reach a value ofw400. Initial optimization of the cool plasma conditions is obtained by Ærst setting the plasma power and nebulizer Øow for minimum background at 40Arz.Under cool plasma conditions, the deØection at m/z 40 should be removed. Mass calibration is mandatory following all optimization events and prior to data collection, especially if parameters in the mass spectrometer have been changed. The Øight time (mass calibration) for each isotope is normally highly stable and does not need to be recalibrated once this has been done properly. Reagents For the performance studies described here, sample solutions were prepared in the laboratory from analytical-reagent grade standard solutions and diluted on a balance (Sartorius LA620S, Go» ttingen, Germany) to 2% HNO3 (by addition of 65% HNO3 Merck, Darmstadt, Germany, Suprapur grade).A 10 mg ml21 mass calibration solution containing Li, Mg, Sc, Co, Y, In, Ce, Ba, Pb and Bi was used. Rare earth elements were separated into two groups to avoid isobaric overlaps, 10 mg ml21 stock standard solutions of Pr, Sm, Tb, Dy, Ho, Tm, and Yb, and a second standard containing La, Nd, Eu, Cd, Er and Lu.For high ionization elements stock solutions containing 10 mg ml21 of Be, Ti, Ge, Y, Mo, Rh, Pd, Sb and Hf and another solution containing Zr, Ru, Cd, Ce, W, Ir, Pt, Tl, Th and U were used. Similarly, 10 mg ml21 stock solutions containing memory prone elements such as B, Nb, Se, Ta, Hg, Au and Te were also measured separately to obtain information on the detection limits. All these standards were obtained from LECO.A 10 mg ml21 mixed standard solution of Al, Sb, As, Ba, Be, Bi, Cd, Ca, Cr, Cs, Co, Cu, Ga, Fe, Rb, Li, Mg, Mn, Na, Ni, K, Se, Ag, Na, Sr, Tl, U, V and Zn (Perkin Elmer, U» berlingen, Germany, PE-mix) was also used throughout. Deionized water was obtained from a Milli-Q system (Millipore, Milford, MA, USA). The isotopic reference materials were obtained from the European Commission's Joint Research Center, Institute for Reference Materials and Measurements, Geel, Belgium. The isotopic reference material for magnesium (IRMM-009) was taken from a Øame-sealed quartz ampoule with aMg content of approximately 1023 mol kg21. The ampoule contained 4 ml of acidiÆed solution (0.2 mol dm21 HNO3).For Pt (IRMM-010), the Pt-wire supplied was dissolved in aqua regia, and then diluted to 1 mg ml21 for stock solution. For Rb another certiÆed reference material was used (NIST-SRM-984). The stock solutions were diluted to a concentration of 100 ng ml21 for Mg and 200 ng ml21 for Pt prior to isotope ratio measurement and all solutions were acidiÆed to 2%HNO3 (Merck, Suprapur).Table 1 Typical operating conditions for ICP-TOFMS.Mass calibration data included illustrating typical values only since they change slightly for different optimizations. Ion deØection settings reported are only valid for normal and hot plasma ICP Source Normal plasma Hot plasma Cool plasma Forward Power/kW 1.27 1.49 0.83 Plasma Flow/l min21 15.4 14.0 16.1 Auxiliary Flow/l min21 1.29 0.701 1.1 Nebulizer Flow/l min21 0.801 0.841 1.0 Mass Spectrometer/V Flight Tube ±1470 21470 21485 ReØectron Low 199 199 193 ReØectron High 1530 1530 1524 X Steering ±1470 21470 21470 Y Steering ±1610 21620 21620 Einzel Lens 1 ±1250 21200 21353 Einzel Lens 2 ±797 2810 2748 Ion Lens 1 ±220 2393 2125 Ion Lens 2 ±19 247 2130 Detector ±2790 22790 22700 Mass calibration/ns Li-7 6746 Mg-24 12138 Co-59 18800 Y-89 22996 In-115 26084 Ba-138 28536 Ce-140 28740 Pb-208 34942 Bi-209 35024 Ion DeØectionsa DeØected Ions Start/ms Width/ms DeØ. 1 (12±19 amu) Cz, Nz, Oz, H2Oz, H3Oz, 0.988 0.12 DeØ. 2 (28±36 amu) N2 z, NOz, O2 z, O2Hz, 36Arz 1.302 0.09 DeØ. 3 (40 amu) 40Arz 1.480 0.1 DeØ. 4 (56 amu) ArOz 1.528 0.03 DeØ. 5 (80 amu) Ar2 z 2.068 0.03 aRemoval of plasma matrix ions. J. Anal. At. Spectrom., 1999, 14, 1807±1814 1809Application–assessment of accuracy using NIST 1643d One milliliter portions of NIST Standard Reference Material 1643d were transferred to eight different 50 ml polypropylene centrifuge tubes (Falcon1, Becton Dickinson & Co, Franklin Lakes, NJ, USA) weighed precisely and then diluted 10 times using Milli-Q water.Small volumes (50±200 ml) of PE solution were added to the centrifuge tubes to increase the initial concentrations 0.5 to 2 times.12 Following adjustments of the Ænal volume with Milli-Q water, so as to dilute the NIST 1643d sample equally for all added concentrations, the following concentrations of the different elements in the PEmix were obtained: 0, 0.158, 0.319, 1.035, 4.913, 8.457, 19.86 and 39.33 ng ml21.The resulting solutions were then analysed by ICP-TOFMS monitoring the following 33 isotopes: 7Li, 9Be, 11B, 23Na, 24Mg, 27Al, 39K, 40Ca, 51V, 50Cr, 52Cr, 53Cr, 55Mn, 54Fe, 56Fe, 58Ni, 59Co, 63Cu, 64Zn, 75As, 76Se, 77Se, 80Se, 82Se, 88Sr, 89Y, 98Mo, 107Ag, 114Cd, 121Sb, 138Ba, 205Tl and 208Pb. A 10 s integration time was used throughout and each sample was analysed with 6 replicates (repeated twice).Certain elements such as Mg, Na, Ca and K were present at mg l21 levels and deØection windows were implemented at those masses. Iron and selenium were severely affected by background from 56ArO and 80ArAr, respectively. The isotopes selected were nevertheless supposed to cover all elements certiÆed in NIST 1643d, regardless of whether instrumental parameters and deØection settings would render some of them non-quantiÆable.The concentrations were calculated in an Excel spreadsheet by dividing the intercept of the y-axis with the slope for the standard addition curve element by element. Measurements of isotope ratios The isotope ratios were measured in the analog mode at high concentrations since these measurement conditions resulted in the best precision. No dead time correction is necessary using the analog mode and obviously the contribution from fundamental noise is smaller at higher concentrations (high signal intensities).Since it was established that close proximity of deØection pulses affect the accuracy of isotope ratios, the TRIP deØections were minimized prior to measurements of isotope ratios. Two deØections for plasma matrix ions with an m/z value close to magnesium were adjusted to have as low impact on magnesium as possible at the same time making sure that the detector was not overloaded. The deØection at m/z 80 was completely removed when measuring 85Rb/87Rb.For platinum, hot plasma conditions were used. In order to obtain backgroundcorrected ratios subtraction of blanks was performed on the basis of the average signals observed at signal-free adjacent masses. For magnesium, the signal at m/z 22 was used for subtraction of the continuum background, and for rubidium and platinum, masses 86 and 189 were used, respectively. In addition, blank solutions were measured to check for presence of possible isobaric overlaps.Results and discussion Full spectral scan spectra A mass spectrum obtained in the analog mode of a solution of 100 ng ml21 mass calibration solution is shown in Fig. 2. The spectrum was obtained under normal plasma operating conditions used for multi-element analysis (Table 1). The background spectrum is similar to those obtained by quadrupole ICP-MS. The Ærst group of background peak results from oxygen and hydrogen (or water) between m/z 16 and 19. The second group is found at m/z 28, 30, 32 and 33 which is due to N2 z, NOz, O2 z, O2Hz.The last major background peaks are found at m/z 40 and 41 and result from 40Arz and 41ArHzinterfering in the determination of 39K and 40Ca. The presence of ions, such as 56ArOz and 80Ar2 z, complicates the determination of trace levels of 56Fe and 80Se. The analog channel spectrum obtained under hot plasma conditions for 100 ng ml21 PE-mix standard is shown in Fig. 3. The observed mass spectra are simple enough for unit mass resolution, which is sufÆcient for most analytical purposes. From Fig. 2, the presence of CeOz at m/z 156, YOz at 105, and UOz at 256 is more evident than signals at the doubly charged mass positions, for example, Ba2z (m/z 69), or Ce2z (m/z 70). The percentage of oxide formation CeOz to Cez is 24.7% at 100 ng ml21 in this example. It is clear that with this instrument, special attention should be given to the likelihood of oxide interferences for certain elements.Oxide formation can be largely eliminated by using an ultrasonic nebulizer with a membrane dryer if desired. Resolving power In the TOF-MS system, the resolving power depends, in part, upon the initial velocity spread of the ions along the Øight-tube axis,13 in part, on the initial spatial distribution of the ion packet along the Øight-tube axis. Here, a two-stage acceleration technique is implemented to partially correct for the initial spatial distribution of the ions and to improve resolving power.14 Moreover, an ion mirror or reØectron is employed to accomplish energy focusing of ions of the same mass with slightly different kinetic energies.An additional advantage of using the ion mirror is that it works as a photon stop that lowers the background count to 1±3 counts per s21 across each m/z. For 100 ng ml21 of Pb and Bi (as shown in Fig. 2 and 3), the resolution is w415 at 10% peak height. This resolving power is sufÆcient for most purposes and a similar performance is possible in quadrupole ICP-MS.For 1 ng ml21 of Bi the resolution increases to 615 at 10% peak height. At low m/z the resolution is signiÆcantly better than for quadrupole ICP-MS. It is noteworthy that peaks become broader, and resolution becomes worse under cool plasma conditions, for reasons that cannot be explained at the present time. Detection limits Detection limits were obtained using the ion counting mode of the detector. No dead time correction was used to improve linearity.Each peak was integrated over 19 successive bins (38 ns of mass window), and the background was determined by integration of the same number of bins from the blank solution at the same m/z position. The integration time was set to 10 s and standards and blanks were determined with 10 replicates. Net signals were taken as the difference between Fig. 2 ICP-TOFMS mass spectrum for 100 ng ml21 mass calibration solution under normal plasma operating conditions.Integration time was 1 s using full spectral scan mode of instrument (2000 bins equally distributed over the whole mass range). 1810 J. Anal. At. Spectrom., 1999, 14, 1807±1814signal and blank levels. Three times the standard deviation of the blank response at the given m/z position divided by the slope of the calibration graph for each isotope was used to calculate detection limits. Typical detection limits of ICPTOFMS are reported in Table 2 together with typical values for quadrupole ICP-MS, ionization energies, number of stable isotopes for each element and abundance of the reported isotope. In general the detection limits are somewhat worse than those reported for quadrupole ICP-MS.The lower sensitivity of approximately 10 times for ICP-TOFMS system originates from the modulation of the ion source. Due to difÆculties in the sensitive measurement of low mass analytes such as 39K, 40Ca and 56Fe under normal plasma conditions, cool plasma conditions were used for these elements. In fact, accurate determinations of 39K, 41Ca and 56Fe are virtually impossible under normal operating conditions, because of serious interferences from argon and argonrelated ions such as 56ArOz.Jiang and Houk et al. reported that operating conditions of the ICP-MS plasma could be optimized to reduce or completely eliminate poly-atomic spectral background associated with certain argide ionic species.15 By employing a lower rf power and higher nebulizer Øow rate, they observed that NOz, O2 z and H3Oz largely dominated the background spectrum, while most Ar-related ions are considerably reduced which make it possible to measure isotopes such as 39K, 40Ca and 56Fe.16 Here, the forward power and nebulizer Øow rate were optimized with a blank solution at m/z 39, 40 and 56, until the background levels were 5±10 cps.The working conditions are listed in detail in Table 1. The three isotopes 39K, 40Ca and 56Fe reach detection limits which are slightly higher than those of other isotopes.It should be pointed out that the instrumentation was not operated in a clean room, hence contamination from the outside atmosphere, acids and Milli-Q water dictates the conditions under which the detection limits were obtained. Moreover, the Milli-Q water apparatus is not placed in the same building as the ICP-TOFMS equipment. Under optimal clean room conditions slightly lower values are attainable. Signal stability Variations of the signals for 7Li, 24Mg, 39K, 40Ca, 56Fe, 133Cs and 85Rb over 4 h under cool plasma conditions are shown in Fig. 4. Long term variations over 4 h for 45Sc, 59Co, 89Y, 115In, 138Ba, 140Ce, 207Pb and 209Bi under normal plasma conditions are shown in Fig. 5. The data collection started 1 h after the plasma was ignited. The working conditions used are as listed in Table 1. The relative standard deviations and correlation coefÆcients following linear regression of the data displayed in Fig. 4 and 5 are given in Tables 3 and 4, respectively. It is clear for the cool plasma that no drift is present over 4 h. On the other hand for 45Sc, 59Co, 89Y, 115In, 138Ba, 140Ce, 207Pb and 209Bi under normal plasma conditions a clear downward drift is present, as shown in Table 4 and Fig. 5. The values for the correlation coefÆcient, r, from 20.895 to 20.948, indicate a time-correlated dependence conÆrming downward drift.It is noteworthy that at the beginning of the measurement when the signals were clearly decreasing, the short term drift over 30 min is only 1.1%RSDfor 208Pb to 4.1% RSD for 59Co, which is better than literature values reported using quadrupole ICP-MS.11,20 Long term drift for ratios of different elements–internal standardization The ions in ICP-TOFMS are sampled from the plasma at the same instant in time, and extracted synchronously from the primary ion beam, so ratioing of the signals from two different isotopes should theoretically overcome transport efÆciency changes from the peristaltic pump, nebulizer Øuctuations and Øuctuations in the plasma tail.Raw signal Øuctuations thus track each other, as can be seen in Fig. 5, and ratioing cancels out Øicker noise sources. Obviously, fundamental noise (shot noise) cannot be overcome.12 The uncertainty, or variation, of the collected signal is nevertheless largely determined by Poisson statistics. By accumulating more signal (higher concentration) the standard deviation can be reduced further in a predictable way. For w30 observations the data should be Fig. 3 ICP-TOFMS mass spectrum for 100 ng ml21 PE-mix standard solution under hot plasma conditions, integration time was 1 s using full spectral scan mode of instrument. Fig. 4 Signal stability over 4 h under cool plasma operating conditions using ion counting mode. Concentration of elements, 4 ng ml21 and a 10 s integration time was used.Fig. 5 Signal stability over 4 h under normal plasma operating conditions using ion counting mode. Concentration of elements, 1 ng ml21 and a 10 s integration time was used. J. Anal. At. Spectrom., 1999, 14, 1807±1814 1811Table 2 Detection limits (3s criterion, pg ml21) using axial ICP-TOFMS and literature values for quadrupole ICP-MS.17 For ICP-TOFMS working conditions see Table 1 Isotopes Detection limits of ICP-TOFMS Detection limits of Quad-ICP-MS17 1st Ionization energy18/eV Number of stable isotopes19 Abundance of major isotope19(%) ICP-TOFMS working conditionsc 7Li 0.5 v1 5.392 2 92.41 C 9Be 23 v1 9.322 1 100 H 11B 165 10±100 8.298 2 80.1 H 23Na 5 10±100 5.139 1 100 C 24Mg 5 10±100 7.646 3 78.99 N 27Al 9 10±100 5.986 1 100 N 28Si 2005 100±1000 8.151 3 92.23 H 39K 6 100±1000a 4.341 3 93.26 C 40Ca 29 100±1000a 6.113 6 96.94 C 45Sc 26 10±100 6.54 1 100 H 48Ti 28 10±100 6.82 5 73.72 H 51V 11 1±10 6.74 2 99.75 H 52Cr 55 1±10 6.766 4 83.79 H 55Mn 2 1±10 7.435 1 100 C 56Fe 36 10±100a 7.870 4 91.75 C 58Ni 15 1±10 7.635 5 68.08 N 59Co 17 v1 7.86 1 100 H 63Cu 4 1±10 7.726 2 69.17 C 64Zn 57 1±10 9.394 5 48.63 H 69Ga 6 v1 5.999 2 60.11 N 74Ge 36 10±100 7.889 5 36.28 H 75As 53 1±10 9.81 1 100 H 82Seb 641 10±100 9.752 6 8.73b H 85Rb 2 v1 4.177 2 72.17 C 88Sr 4 v1 5.695 4 82.58 H 89Y 3 v1 6.38 1 100 H 90Zr 3 1±10 6.84 5 51.45 H 98Mo 19 1±10 7.099 7 24.13 H 103Rh 4 1±10 7.46 1 100 H 102Ru 16 1±10 7.37 7 31.55 H 106Pd 12 1±10 8.34 6 27.33 H 107Ag 7 1±10 7.576 2 51.84 H 114Cd 7 1±10 8.993 8 28.73 H 115In 1 v1 5.786 2 95.71 C 120Sn 21 1±10 7.334 10 32.58 H 121Sb 14 1±10 8.641 2 57.21 H 130Te 31 10±100 9.009 8 34.08 H 133Cs 1 v1 3.894 1 100 C 138Ba 4 v1 5.212 7 71.70 H 139La 3 v1 5.577 2 99.91 H 140Ce 2 v1 5.47 4 88.45 H 141Pr 2 v1 5.42 1 100 H 142Nd 7 v1 5.49 7 27.25 H 152Sm 7 v1 5.63 7 26.75 H 153Eu 4 v1 5.67 2 52.19 H 158Gd 11 v1 6.14 7 24.84 H 159Tb 2 v1 5.85 1 100 H 164Dy 8 v1 5.93 7 28.18 H 165Ho 2 v1 6.02 1 100 H 166Er 5 v1 6.10 6 33.61 H 169Tm 2 v1 6.18 1 100 H 174Yb 4 v1 6.254 7 31.83 H 175Lu 2 v1 5.426 2 97.41 H 180Hf 7 1±10 7.0 6 35.08 H 184W 15 1±10 7.98 5 30.64 H 191Ir 2 1±10 9.1 2 62.7 H 196Pt 16 1±10 9.0 6 33.83 H 197Au 9 1±10 9.225 1 100 H 202Hg 6 v1 10.437 7 29.86 H 205Tl 2 v1 6.108 2 70.48 H 208Pb 4 v1 7.416 4 52.4 N 209Bi 3 v1 7.289 1 100 H 232Th 5 v1 – 1 100 H 238U 5 v1 – 3 99.27 H aIt is not clear from Ref. 17 under which plasma operation conditions the detection limits were obtained for K, Ca and Fe; the values reported suggest that cool plasma was not used.bIt should be noted that 82Se is not the most abundant isotope in contrast to all the other isotopes mentioned above. cN, H, and C are abbreviations of normal-, hot- and cool plasma. 1812 J. Anal. At. Spectrom., 1999, 14, 1807±1814normally distributed around the mean as outlined by Hooton et al.21 The following holds true for data that are normally distributed: 68.3% of the observations are within x°1s, 95.5% within x°2s and 99.7% within 3s. As can be seen from Table 5, the data generally shows a normal distribution demonstrating the high stability of isotope ratio measurements.Here, elements with similar chemical properties, ionization energies or atomic mass were measured as ratios under hot plasma conditions, as to outline the stability of internal standardization. The results can be directly compared with the raw signals in Table 3 and 4.Repeatability and reproducibility Under normal plasma conditions using the working conditions listed in Table 1, a 10 ng ml21 mass calibration solution was used to investigate the repeatability and reproducibility of the system. Employing a 10 s integration time and 5 replicates, the signals for 138Ba, 209Bi, 140Ce, 59Co, 115In, 208Pb, 45Sc and 89Y were monitored, and the repeatability was calculated from 30 discontinuous determinations, nebulizing a blank solution between each measurement for 2 min.The measurements were repeated after 24 h using the working conditions used the previous day and re-optimized monitoring m/z 115. Another 30 discontinuous measurements were performed, and the resulting signals together with the 30 previous measurements were used to calculate the reproducibility, which is shown in Table 6. The measurements were initiated following 2 h of instrumental warm-up. The time spent for 30 discontinuous measurements was about 1.5 h.It can be seen from the Table 6 that for the measured isotopes, the instrument gives very good repeatability and acceptable reproducibility. Analysis of NIST 1643d The results reported for the NIST standard reference material 1643d are generally in agreement with the certiÆed concentrations (Table 7). Out of 26 certiÆed elements only 18 were useful for determinations in the instrumental and analytical conditions used. Since uncertainties both in the slope of the regression line and the intercept of the Y-axis are present, a check of the robustness for the regression data is appropriate.In this example the data obtained for Tl is used. It should be noted that Tl had the worst correlation coefÆcient for all concentrations reported in Table 7, (r2w0.998), the others were better than this. Using the lower 95% conÆdence interval of the slope and intercept obtained using the data PAK-Excel data analysis tool, a concentration of 7.46 ng ml21 is obtained and on the high side 8.03 ng ml21.With a certiÆed concentration of 7.28°0.25 ng ml21 for Tl, the regression data are robust enough to ensure that a result of reasonable quality is maintained. The basic idea of standard additions is thus to make a standard out of the sample and thereby compensate for matrix effects.12 It should be noted that the method of standard addition is less precise than interpolation techniques.22 Measurement of isotope ratios Precisions for seven isotope ratios are reported in Table 8.Even though the deØections were minimized, the accuracy for 25Mg/24Mg, and 26Mg/24Mg is still affected to a rather high degree. Mass bias, given as deviation in % from the true ratio, has been found to be high in the low mass range, 29.2% for 26Mg/24Mg and approximately 1% in the high mass range, 194Pt/195Pt. We will report in detail on this effect in another paper. The precision for isotope ratios of magnesium and rubidium is better than 0.1% under normal working conditions.The slightly deteriorated precision for platinum-ratios possibly results from the relatively poorer response even using a concentration of 200 ng ml21. A higher concentration could not be used since 194Pt, 195Pt and 196Pt were not properly resolved under such conditions. Contrary to observations made with quadrupole ICP-MS,23,24 memory effects were not signiÆcant. Using 192Pt which is the least abundant isotope, a RSD of 0.74% for 192Pt/195Pt could be obtained.In this circumstance the relative contribution from fundamental noise to the 192Pt signal is high due to the low signal intensity, thus resulting in a higher RSD. A more detailed study of accuracy, stability of mass bias and precision for isotope ratios is currently performed in our laboratory. It can be concluded that the precision for isotope ratios using axial ICP-TOFMS is approximately ten times better than ratios measured by quadrupole ICP-MS.25±27 It should also be underlined that this high precision is obtainable for a large number of isotope Table 3 Signal stability for cool plasma, concentration is 4 ng ml21 RSD (%) Isotope 30 min 4 h Correlation coefÆcient r, 4 h 7Li 0.6 1.0 20.295 23Na 0.7 1.0 20.628 24Mg 0.8 1.7 20.186 39K 0.7 2.0 0.071 40Ca 1.8 2.1 0.204 56Fe 2.1 4.3 0.406 85Rb 1.2 1.7 20.427 115In 1.6 2.3 0.640 133Cs 1.2 1.1 0.689 Table 4 Signal stability for normal plasma, concentration is 1 ng ml21 Isotope RSD (%) Correlation coefÆcient r, 4 h 30 min 4 h 45Sc 3.8 11.4 20.947 59Co 4.1 12.8 20.942 89Y 2.5 11.2 20.943 115In 1.9 9.4 20.948 138Ba 1.6 8.7 20.895 140Ce 2.5 10.5 20.933 208Pb 1.1 6.9 20.924 209Bi 1.3 6.7 20.911 Table 5 Signal stability for measurements of intensity ratios over 4 h under normal plasma conditions (concentration is 1 ng ml21) Intensity ratio 138Bi/115In 140Ce/115In 59Co/89Y 24Mg/45Sc 208Pb/209Bi Mean intensity, x 1.0119 1.0383 0.4770 0.3017 0.7233 Standard deviation, s 0.0086 0.0140 0.0091 0.0051 0.0047 % RSD 0.85 1.35 1.91 1.70 0.65 Distribution of measurements Mean°1s 74.1 75.4 75.9 74.1 73.2 Mean°2s 96.4 96.0 94.6 95.1 96.9 Mean°3s 100.0 100.0 100.0 100.0 100.0 J.Anal. At. Spectrom., 1999, 14, 1807±1814 1813ratios in the same measurement. When using quadrupole ICPMS one is limited to one isotope pair at a time if the best possible precision is desired. Conclusions Axial ICP-TOFMS is highly competitive to quadrupole ICPMS despite somewhat lower sensitivity and higher detection limits.Extremely rapid data acquisition makes it possible to obtain qualitative and quantitative information for 66 isotopes in a very short time. Simultaneous ion extraction likewise results in high precision for isotope ratios. Oxide formation is however somewhat more prominent than for quadrupole ICPMS which deserves special attention when measuring certain elements. Accuracy has been demonstrated through the analysis of a standard reference material.Acknowledgements The authors wish to thank Dr. M. Berglund (IRMM, Geel, Belgium) for help with isotope ratio measurements. Dr Janos Fucsko (LECO) is acknowledged for supplying mixed standards of a large number of elements. The Renaissance LECO ICPTOFMS was purchased through funds provided by the Belgian National Lottery, and the work is Ænally supported by FWO, Brussels, Belgium. H.E. also gratefully acknowledges a visiting postdoctoral fellowship from FWO.The LECO Corporation, St. Joseph, MI, USA is acknowledged for supplying Fig. 1. References 1 R. S. Houk, V. A. Fassel, G. D. Flesch, H. J. Svec, A. L. Gray and C. E. Taylor, Anal. Chem., 1980, 52, 2283. 2 A. R. Date and A. L. Gray, Analyst, 1981, 106, 1255. 3 P. P. Mahoney, S. J. Ray and G. M. Hieftje, Appl. Spectrosc., 1997, 51, 16A. 4 D. P. Myers and G. M. Hieftje, Microchem. J., 1993, 48, 259. 5 D. P. Myers, G. Li, P. Yang and G. M. Hieftje, J. Am. Soc.Mass Spectrom., 1995, 6, 400. 6 D. P. Myers, G. Li, P. Yang and G. M. Hieftje, J. Am. Soc. Mass Spectrom., 1995, 6, 411. 7 D. P. Myers, G. Li, P. Yang and G. M. Hieftje, J. Am. Soc. Mass Spectrom., 1995, 6, 920. 8 P. P. Mahoney, S. J. Ray, G. Li and G. M. Hieftje, Anal. Chem., 1999, 71, 1378. 9 P. P. Mahoney, G. Li and G. M. Hieftje, J. Anal. At. Spectrom., 1996, 11, 401. 10 H. Emteborg, X. Tian and F. C. Adams, J. Anal. At. Spectrom., 1999, 14, 1567. 11 G. H. Vickers, D.A. Wilson and G. M. Hieftje, J. Anal. At. Spectrom., 1989, 4, 749. 12 J. D. Ingle and S. R. Crouch, Spectrochemical Analysis, Prentice Hall, Englewood Cliffs, NJ, USA, ISBN 0-13-826876-2. 13 R. J. Cotter, Anal. Chem., 1992, 64, 1027. 14 W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum., 1955, 26, 1150. 15 S. J. Jiang, R. S. Houk and M. A. Stevens, Anal. Chem., 1988, 60, 1217. 16 S. D. Tanner, J. Anal. At. Spectrom., 1995, 10, 905. 17 E. H. Evans, J. J. Giglio, T. M. Castillano and J.A. Caruso, Inductively Coupled and Microwave Induced Plasma Sources for Mass Spectrometry, The Royal Society of Chemistry, Cambridge, UK, 1995, ch. 3, pp. 35. 18 Chemical Rubber Company Handbook of Chemistry and Physics, 64th edition, CRC Inc., Boca Raton, Fl, USA, 1984. 19 K. J. R. Rosman and P. D. P. Taylor, Pure & Appl. Chem., 1998, 70(1), 217. 20 J. J. Thompson and R. S. Houk, Appl. Spectrosc., 1987, 41, 801. 21 K. A. H. Hooton and M. L. Parsons, Anal. Chem., 1973, 45, 2218. 22 J. C. Miller and J. N. Miller, Statistics for Analytical Chemistry, Ellis Horwood Limited, NY, USA, ch. 5, p. 117. 23 F. Vanhaecke, M. van Holderbeke, L. Moens and R. Dams, J. Anal. At. Spectrom., 1996, 11, 543. 24 L. Moens, F. Vanhaecke, J. Riondato and R. Dams, J. Anal. At. Spectrom., 1995, 10, 569. 25 G. P. Russ and J. M. Bazan, Spectrochim. Acta, PartB, 1987, 42, 49. 26 H. P. Longerich, B. J. Fryer and D. F. Strong, Spectrochim,. Acta, Part B, 1987, 42, 39. 27 B. T. G. Ting and M. Janghorbani, J. Anal. At. Spectrom., 1988, 3, 325. Paper 9/06762K Table 6 Repeatability (within run precision) and reproducibility (between run precision), n~30 for repeatability sets and n~60 for reproducibility data 138Ba 209Bi 140Ce 59Co 115In 208Pb 45Sc 89Y Day 1 Average intensity 7415 9775 8522 1492 7090 6596 1231 4481 Repeatability (%) 2.1 1.4 2.2 3.6 2.2 1.6 2.7 2.6 Day 2 Average intensity 8088 9644 9436 1920 7859 6471 1460 5261 Repeatability (%) 1.3 2.4 1.2 1.7 1.1 2.4 1.0 1.1 Day 1 and Day 2 Average intensity 7752 9710 8979 1706 7475 6533 1346 4873 Reproducibility (%) 4.7 2.1 5.4 12.9 5.5 2.2 8.8 8.2 Table 7 Comparison of certiÆed concentrations and concentrations measured using axial ICP-TOFMS of trace elements in NIST1643d. Spread is given as 95% conÆdence intervals for n~6 Concentration/ng ml21 Element Measured CertiÆed Li 16.37°0.34 16.5°0.55 Be 13.08°0.75 12.53°0.28 As 54.1°0.91 56.02°0.73 Zn 70.25°0.88 72.48°0.65 Cu 20.7°0.22 20.5°3.8 Tl 7.77°0.05 7.28°0.25 Cd 6.63°0.18 6.47°0.37 Pb 18.51°0.16 18.15°0.64 Mn 36.61°0.35 37.66°0.83 Co 24.27°0.20 25.00°0.59 Cr 18.46°0.44 18.53°0.20 Sb 53.75°0.62 54.1°1.1 Al 122.1°1.54 127.6°3.5 B 141.9°3.6 144.8°5.2 Mo 106.8°0.78 112.9°1.7 Ni 53.1°0.67 58.1°2.7 Table 8 Mass bias and precision of isotope ratios for isotopic reference materials measured using axial ICP-TOFMS (30 s integration time, 10 replicates). Mass bias is given as deviation in % from certiÆed ratio Isotope ratio CertiÆed ratio Measured ratio Mass bias (%) RSD (%) 25Mg/24Mg 0.1266 0.1213 24.19 0.07 26Mg/24Mg 0.1393 0.1265 29.19 0.09 85Rb/87Rb 2.5932 2.5307 22.41 0.07 192Pt/195Pt 0.02315 0.02389 3.20 0.7 194Pt/195Pt 0.9731 0.9584 21.51 0.1 196Pt/195Pt 0.7465 0.7549 1.13 0.2 198Pt/195Pt 0.2178 0.2230 2.39 0.2 1814 J. Anal. At. Spectrom., 1999, 14, 1807±1814

ISSN:0267-9477    

DOI:10.1039/a906762k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

UV-irradiation and MW-digestion pre-treatment of Port wine suitable for the determination of lead isotope ratios by inductively coupled plasma mass spectrometry C. Marisa R. Almeida and M. Teresa S. D. Vasconcelos* LAQUIPAI, Departamento de Quý�mica, Faculdade de CieÃncias, Universidade do Porto, Rua do Campo Alegre, 687, P4169-007 Porto, Portugal. E-mail: mtvascon@fc.up.pt Received 6th July 1999, Accepted 4th October 1999 An UV-irradiation pre-treatment for the determination of Pb isotope ratios (IRs) in Port wine by ICP-MS has been developed. Optimised conditions: mixtures of 20 ml Port wine and 120 ml 30% H2O2 (150 : 1 mixture) are UV-irradiated (mercury high pressure vapour lamp, 1000 W) in PTFE-capped silica tubes for 1.5 h; the sample is then Æltered and 10 times diluted with a 0.5% HNO3 solution.The procedure was compared with two other implemented methods: (1) UV-irradiation for 0.5 h of 10 ml mixture of 1 : 1 (v : v) Port wine and 30% H2O2, and dilution up to 50 ml with 0.5% HNO3 solution; and (2) high pressure MW-digestion of 1.5 ml aliquots of Port wine with 150 ml concentrated HNO3 and 1.5 ml 30% H2O2, and dilution up to 15 ml with de-ionised water.All these procedures were found to be suitable for determination of Pb IRs in Port wines. As the UVirradiation of 150 : 1 mixture of wine ±H2O2 has the advantage of requiring lower amounts of chemicals and providing larger volumes of solution for analysis of Port wine, it was selected for further studies. The method was tested for eight different samples of Port wine.A precision study showed that the standard deviation associated with the overall procedure (sample pre-treatment and subsequent determination) was mainly due to the ICP-MS determinations, the sample pre-treatment giving only a small contribution. Precisions (relative standard deviation) of about 0.3% for 207Pbz/206Pbz and for 208Pbz/206Pbz and about 0.8% for 204Pbz/206Pbz have been routinely obtained and shown to be sufÆcient to differentiate natural variations of the Pb isotope abundances in Port wine samples.Most of the lead (a toxic heavy metal) intake by man comes from food and beverages, while respiration through lungs and skin contributes only to a small extent. A small amount of the lead ingested can pass through the intestinal barrier and be transferred from the blood to the tissues where it is accumulated. The regular absorption of small amounts of lead may result in serious effects on human health, particularly in individuals at risk (people more susceptible to getting sick by reason of lead absorption). Therefore, efforts should be made in order to be able to control the levels of the metal intake.Lead contamination of wine is a problem dating back to ancient times. Along the oenological chain, contamination with lead can have multiple and diversiÆed origins: the soil of the vineyard, atmospheric precipitation, pesticides and fertilisers,1 materials used to produce, transport and store the wine, etc.The role of the different lead sources in the levels of the metal in the Ænal product is unknown but it is important to clarify this issue in order to be able to reduce the lead level in wine. Lead is composed of four stable isotopes, three of which are of radiogenic origin: the radioactive decay of 238U, 235U and 232Th generates, respectively, 206Pb, 207Pb and 208Pb. 204Pb is non-radiogenic.The respective abundances of these lead isotopes, originating from the genesis of the rocks and ore deposits, vary with geological ages and consequently with geographical locations.2 This property may be explored in order to identify the source of lead in a given sample. Until recently, Thermal Ionisation Mass Spectrometry (TIMS) has been considered the most suitable technique for determination of isotope ratios with enough precision to distinguish lead of different origins in a sample.ICP-MS is now also a suitable and convenient technique for that purpose3±9 being less time-consuming than TIMS.10 The application of ICP-MS to the determination of lead isotope ratios in wines is a very unexplored subject. As far as we know, there are only a few results published on table wines.11,12 A dilution with an acid solution is the only pre-treatment required for these type of wines.11,12 ICP-MS has never been applied before to fortiÆed wines in which the genuine Port wine, from Portugal, is included.Port wine is an aged fortiÆed wine, very rich in alcohol, particles in suspension and polymeric organic compounds, particularly sugars. All of these substances strongly interfere with ICP-MS measurements. Alcohol causes suppression of the signal (which drastically reduces the sensitivity) and signal instability.13 Although a signal depression does not necessarily prevent the isotopic ratio determinations, it may affect the measurement precision if the ion intensities of the different isotopes are low.The polymeric organic matter causes blockage of the injector tube and cones of the ICP, preventing the Øux of the ions of the analyte to the MS detector. Such blockage is the result of an incomplete pyrolysis of the sugars in the plasma and formation of residual carbon deposits (carbon build-up). Therefore, a simple dilution of the wine does not solve the problem of interference of the Port wine matrix in ICP-MS measurements, because the maximum dilution allowed on account of sensitivity (ten times) is not sufÆcient to reduce either the alcohol or the sugar to acceptable levels.Such problems could probably be reduced by the addition of a small stream of oxygen to the plasma region, but this procedure requires a speciÆc accessory not available in most of the ICP-MS apparatus. So, a more drastic sample pre-treatment of Port wine is required for ICP-MS measurements. The aim of this study was the development and optimisation of sample pre-treatment procedures suitable for the determination of lead isotope ratios in Port wine using ICP-MS, with precision sufÆcient to warrant the investigation of the provenance of the metal.Pre-treatment which requires less chemical oxidising, to J. Anal. At. Spectrom., 1999, 14, 1815±1821 1815 This journal is # The Royal Society of Chemistry 1999prevent contamination, and provides analytical signal intensity as high as possible in the wine samples, was investigated. Goossens et al.13 used classical acid digestion, in open vessels, for determination of total lead in a liqueur red wine by ICP-MS.As such methods are generally very time consuming and require the addition of high concentrations of reagents to the sample, procedures involving UV-irradiation that required less chemical addition were implemented. An UV-irradiation pre-treatment has been applied recently by Sanllorente et al.14 for determination of nickel in commercial table wines by differential-pulse adsorptive stripping voltammetry. For comparison purposes, a high pressure microwave digestion procedure was also implemented.The high pressure microwave digestion (HPMW-digestion) procedure has been used for determination of total metals concentrations in different types of samples using different analytical techniques, 15 including determination by ICP-MS of several trace metals in biological materials,16 chromium and nickel in human blood,17 rare earth elements in tea18 and arsenic, cadmium and lead in seafood products.19 In this paper the selected UV-irradiation pre-treatment is described in detail and the quality of the results it provides is discussed and compared with other implemented pretreatments requiring greater amounts of chemicals.Experimental Material and reagents Suprapure concentrated HNO3 (65% m/m, d~1.40 g ml21) and solution of 30% H2O2, p.a., from Merck, were used without further puriÆcation.A stock NIST SRM-981 Common Pb Isotopic Standard solution (1000 mg l21 of Pb) was prepared by dissolving a portion of the metal in 1% v/v HNO3. All the other reagents used were p.a. grade or equivalent. Standards solution were prepared daily from the stocks, in polyethylene tubes, by weight, with de-ionised water (resistivity w14 MV cm) or diluted HN, as necessary. All material was soaked in 20% v/v HNO3 for at least 24 h, rinsed several times with de-ionised water and dried in a Class 100 laminar Øow hood.The sample manipulation was carried out in a clean room with Class 100 Æltered air. Wine samples Eight samples of genuine Port wine from the Douro Region, North of Portugal, supplied (by the ``Instituto do Vinho do Porto'', Porto, Portugal) in small glass bottles, were used. They were of three different types: 2 Late Bottled Vintage (LBV) harvest of 1988 (LBV 88a and LBV 88b), 5 Dated wines (DP) harvests of 1935 (DP 35), 1952 (DP 52), 1969 (DP 69), 1974 (DP 74) and 1984 (DP 84), and one wine with an indication of age of 10 years (IA 10). LBV are wines from a single year of harvest (year of excellent quality) that are bottled between the fourth and sixth year after they were made; DP are also wines from a single year that are aged in wooden barrels for several years and can only be sold after they have attained 7 years of age; IA Port wines are similar in style to DP but, unlike the latter, are blended from wines of different years.All the wines studied were bottled in 1992. The UV-irradiation pre-treatment procedures implemented in this work were also applied to two other types of commercial available Portuguese fortiÆed wines: Madeira wine, from the island of Madeira, and Favaios wine, from the Douro Region. The wines were sampled by removing the cork and pouring the required volume into silica tubes of ca. 45 ml of capacity.UV-irradiation pre-treatment UV-irradiation of the samples was carried out with a 1000 W mercury high pressure vapour lamp (with 3 cm diameter and 6 cm length) from Osram, in polytetraØuoroethylene (PTFE) -capped silica tubes (with 2 cm diameter and 15 cm length) positioned concentrically around the lamp (distance between the lamp and the centre of the tubes was 5 cm). This system was inserted in a closed aluminium box, provided with appropriate ventilation, which protects workers against UV-radiation.The optimisation of the pre-treatment of the Port wine included the study of the inØuence on the analytical results of the following parameters: volume of sample (varied between 10 and 40 ml), time of UV-irradiation (between 0.5 and 2.0 h) and addition of different volumes of 30% H2O2 (wine :H2O2 proportions between 150 : 1 and 1 : 1 v/v). When the wine samples were hazed after UV-irradiation, they were Æltered through a 0.45 mm pore size Ælter of mixed cellulose esters, from Schleicher & Schuell, using a syringe with an appropriate Ælter holder. The treated samples were kept at 4 �C and analysed within 48 h, after dilution with 0.5% v/v HNO3 solution.For comparison purpose, dilution with de-ionised water or 1% HNO3 solution was also used. HPMW-digestion Aliquots of 1.5 ml of wine were digested with 150 ml concentrated HNO3 and 1.5 ml of 30% H2O2 in closed PTFE vessels in a high pressure microwave system MLS-1200 Mega, from Millestone, coupled to an exhaust EM-30 of the same brand.The digestion programme and procedure was based on that developed by Alimonti et al.17 for determination of Ni and Cr in blood by ICP-MS. After cooling to room temperature, the vessels were opened and the obtained solutions were quantitatively transferred to polyethylene tubes being diluted to 15 ml. The Ænal solutions were kept at 4 �C and analysed within 48 h. ICP-MS isotope ratios measurements The analytical measurements were carried out in a Perkin- Elmer SCIEX Elan 5000 ICP-MS (Perkin-Elmer, U» berlingen, Germany) apparatus equipped with a crossØow nebulizer, nickel cones and a peristaltic sample delivery pump.The operating conditions for ICP-MS measurements were optimised daily, by using a solution with 10 mg l21 of Mg, Rh and Pb and monitoring the isotopes 24Mg, 103Rh and 208Pb. Since doubly charge ions were not a concern and oxide ions were considered unlikely in the m/z 204-208 regions, the operating conditions that maximised the ion intensity for mass 208 were selected.Operating conditions used: RF power of 1200 W; sample uptake rate of 0.800 l min21; plasma Øow rate of 15.00 l min21, nebulizer Øow rate between 0.750 and 0.810 l min21, and auxiliary Øow rate of 0.800 l min21; the ions lens settings (in arbitrary units) were P~52, S2~24, B~70 and E1~15. For the optimisation of the data acquisition procedure, the inØuence of the instrument parameters: number of replicates (varied between 3 and 10, with one reading per replicate), dwell time (varied between 5 and 10 ms) and number of sweeps per reading (varied between 200 and 1500), on the three Pb isotope ratios (IRs) 204Pbz/206Pbz, 207Pbz/206Pbz and 208Pbz/206Pbz (determine individually or together) and on the respective relative standard deviation (RSD), obtained for a Pb isotopic standard solution (50 mg l21 Pb concentration) was studied.In the optimised procedure, in order to obtain the best precision (lowest RSD), only two Pb isotopes were measured each time, using 1500 sweeps per reading and a dwell time of 10 ms, and three replicates for each measurement were carried out.The Pb isotopes were measured using the peak hopping mode, at 1816 J. Anal. At. Spectrom., 1999, 14, 1815±1821normal resolution. The results of each measurement was the mean of the replicates with the respective standard deviation, after blank subtraction.Since 204Pb is the least abundant stable isotope of Pb, to obtain similar precision for the four isotopes this isotope signal was measured for twice (by choosing the ``time factor'' 2 in the parameter Æle of the ICP-MS software) as long as the other Pb isotopes' signals. Although wines usually have very low contents of Hg,12 the IR with the Pb isotope 204Pb may be altered if this correction is not performed. Therefore, a mathematical correction of 204Hg interference with 204Pb was systematically carried out (by the software of the equipment) for all the determinations: the net signal at m/z 202 (202Hgz) was multiplied by 0.229 (the 204Hg/202Hg natural abundance ratio) and then subtracted from the signal at m/z 204 (204Hgz plus 204Pbz).All the Pb IRs were corrected using the Pb isotopic standard20 (Pb concentration similar to that pre-estimated for the samples). In order to underscore a possible shift with time, every workday the standard was analysed Ærst and then after every two samples (for IR 204Pbz/206Pbz it was measured after every four samples, because it was more time consuming and for this IR the repeatability was higher than for the other two IRs).Due to this correction the standard deviation was calculated for each measurement according to propagation of errors (resulting from the determination of the Pb IR both in the sample and in the isotopic standard). The correction of the Pb IRs with Tl IR (constant natural IR 205Tl/203Tl~2.3871) was also carried out, as discussed elsewhere,21 but no signiÆcant differences were observed between the results obtained using both types of corrections.For signal stabilisation, a sample read delay of 1.5 min was chosen. Between solutions of samples or standard the sampling system was rinsed with 2% HNO3 for 1.5 min. Results and discussion Optimisation of a pre-treatment of Port wine for ICP-MS determinations Development of the UV-irradiation pre-treatment.Volumes of 10 ml of mixtures of Port wine and 30% H2O2, in v:v proportions of 150 : 1, 20 : 1, 5 : 1, 2 : 1 and 1 : 1, were UVirradiated, in parallel, for 0.5 h. The obtained solutions were diluted with a 0.5% HNO3 solution, in order to obtain a 10 times dilution of the wine, and the ion intensities of the different Pb isotopes were measured. That set of experiments was repeated for 1.0, 1.5 and 2.0 h of UV-irradiation. The inØuence of UV-irradiation time and wine: H2O2 proportions on the ion intensities of 207Pb and 206Pb isotopes are illustrated in Table 1.For the other Pb isotopes similar relative intensities and precisions were obtained. The different results were statistically compared through a test for comparison of several means, the least iÆcance difference (LSD) test.22 For 0.5 h of UV-irradiation the highest signal was obtained for the 1 : 1 mixture, indicating that the highest H2O2 concentration originated the most efÆcient elimination of matrix interference.For 1.0 h of irradiation no improvement of sensitivity was observed for the 1 : 1 mixture. In a few other solutions an improvement of the signals was observed but in all cases they were statistically lower than those obtained for 1 : 1 mixture. For 1.5 h of irradiation, statistically identical signals were obtained for 1 : 1, 20 : 1 and 150 : 1. For 5 : 1 and 2 : 1 mixtures lower signals were observed. Higher UV exposure time (2.0 h) improved neither the magnitude nor the precision of the signals, for any of the wine :H2O2 mixtures.Higher signals for 150 : 1 and 20 : 1 than for 5 : 1 and 2 : 1 mixtures were probably a result of evaporation of alcohol (an important interference in ICP-MS) during UV-irradiation which occurred only for the less diluted wine. In fact, after irradiation of 150 : 1 and 20 : 1 mixtures, their volumes were, respectively, 80% and 85% of the initial ones. The treated solutions where evaporation of the alcohol occurred became hazed and were Æltered before analysis.In terms of sensitivity, the 150 : 1 and 20 : 1 mixtures UVirradiated for 1.5 h and the 1 : 1 mixture UV-irradiated for 0.5 h provided statistically identical results (see Table 1). The 150 : 1 mixture has the advantage of enabling the treatment of a larger volume of wine per batch: up to 20 ml (higher volumes cannot be used because solution came out of the silica tube during the UV-irradiation due to the heating) provides about 160 ml of solution for analysis after a 10 times dilution of the treated solution (since about 4 ml evaporate during the UV-irradiation).For 1 : 1 wine :H2O2 mixture more than 10 ml (a quart of the vessel volume) cannot be used because a tumultuous release of gas occurs during UV-irradiation. Therefore, after a 10 times dilution of the wine, only 50 ml of Ænal solution is obtained. Larger solution volumes are useful when different analyses by ICP-MS have to be performed.The 150 : 1 mixture has the additional advantage of requiring much less volume of H2O2. One the other hand, the 1 : 1 mixture has the advantage of being less time consuming since it requires less time of UVTable 1 InØuence of UV-irradiation time and wine :H2O2 proportions on the ion intensities of 206Pb and 207Pb isotopes and on the respective precision, observed for a Port wine sample Wine :H2O2 Timea/h Haze after pre-treatment 206Pb ion intensityb61023/ion s21 207Pb ion intensityb61023/ion s21 150 : 1 0.5 No 8.4 (0.4) 7.34 (0.04) 1.0 Yesc 10.6 (0.1) 9.25 (0.03) 1.5 Yesc 13.08 (0.04) 11.39 (0.07) 20 : 1 0.5 No 11.1 (0.2) 9.74 (0.02) 1.0 No 11.5 (0.1) 10.04 (0.02) 1.5 Yesc 13.13 (0.04) 11.45 (0.02) 5 : 1 0.5 No 12.39 (0.06) 10.85 (0.07) 1.0 No 12.49 (0.02) 10.97 (0.04) 1.5 No 11.94 (0.03) 10.39 (0.04) 2 : 1 0.5 No 11.2 (0.2) 9.79 (0.02) 1.0 No 11.4 (0.1) 9.95 (0.04) 1.5 No 11.5 (0.1) 10.07 (0.02) 1 : 1 0.5 No 13.14 (0.05) 11.54 (0.07) 1.0 No 13.26 (0.02) 11.57 (0.01) 1.5 No 13.05 (0.03) 11.41 (0.07) aFor 2.0 h of UV-irradiation the results were statically identical to those of 1.5 h.bMean and standard deviation (n~3). cThe solutions were Æltered before the analysis. J. Anal. At. Spectrom., 1999, 14, 1815±1821 1817irradiation. The 20 : 1 mixture offers no advantages compared with the other two mixtures. For comparison purposes, UV-irradiation pre-treatments of 150 : 1 and 1 : 1 mixtures were applied, in parallel, to two Port wine samples.The results obtained for the different IRs were statistically identical (see Table 2). The same pre-treatments were also applied to other types of fortiÆed wines (one sample of Madeira wine and one sample of Favaios wine). The obtained results (not shown) suggested that both UV-irradiation procedures are also suitable for ICP-MS analysis of those fortiÆed wines. Since the 150 : 1 mixture requires lower addition of chemicals and can provide larger sample volume for analysis it was selected for subsequent studies.Optimisation of a HPMW-digestion. A more classical pretreatment procedure was also optimised and applied to Port wine. A sample volume of 1.5 ml was chosen, because it was a compromise between the maximum volume suitable for digestion in the closed vessels of the microwave system and the volume required for the ICP-MS determinations (after a 10 times dilution of the wine).The volumes of HNO3 and H2O2 used in the attack were varied between 150 ml and 1.5 ml and 150 ml and 3 ml, respectively. Volumes of 150 ml HNO3 and 1.5 ml H2O2 were selected because they provided clear solutions, and higher concentrations of this two reagents did not improved the analytical signals in ICP-MS. Comparison of MW-digestion and UV-irradiation procedures. The HP-MW-digestion and the UV-irradiation of 150:1 Port wine:H2O2 mixture pre-treatments were applied to Æve wines (DP 35, DP 74, LBV 88a, LBV 88b and IA 10), in parallel, for comparison.For each sample, two replicates were independently pre-treated by each procedure and the mean of the ICP-MS signals (three per replicate) and the respective RSD (n~6, calculated according to the propagation of errors) were obtained. The UV-irradiation procedure provided results with RSD between 0.045 and 0.330%, more precise than those obtained by microwave digestion whose RSD were between 0.100 and 1.82%.In Fig. 1 the results obtained for the various Pb IRs in samples pre-treated by both procedures are compared by linear regression. A linear least-squares adjustment of the global results for the different Pb IRs yield the equation: [Microwave]~( 1.003°0.003) [UV]2(0.0004°0.0037). When each IR was treated separately, the following equations were Table 2 Comparison, through the test of comparing two means,22 of the Pb IRs obtained with two UV-irradiation pre-treatments (see the text) (n~3) 207Pb/206Pba 208Pb/206Pba 204Pb/206Pba UV-irradiation (150 : 1) UV-irradiation (1 : 1) UV-irradiation (150 : 1) UV-irradiation (1 : 1) UV-irradiation (150 : 1) UV-irradiation (1 : 1) DP 52 0.857 (2) 0.859 (3) 2.101 (4) 2.103 (2) 0.0553 (3) 0.0550 (3) 0.858 (2) 0.857 (3) 2.102 (4) 2.100 (7) 0.0552 (1) 0.0552 (2) 0.858 (3) 0.858 (3) 2.100 (3) 2.098 (6) 0.0551 (3) 0.0551 (1) RSD: 0.106% RSD: 0.129% RSD: 0.063% RSD: 0.127% RSD: 0.105% RSD: 0.181% DP 69 0.866 (4) 0.865 (6) 2.100 (5) 2.109 (3) 0.0555 (2) 0.0554 (3) 0.866 (3) 0.864 (7) 2.099 (8) 2.099 (6) 0.0560 (4) 0.0555 (3) 0.866 (3) 0.866 (6) 2.094 (8) 2.103 (4) 0.0562 (2) 0.0554 (5) RSD: 0.027% RSD: 0.072% RSD: 0.165% RSD: 0.248% RSD: 0. 645% RSD: 0.104% aMean and standard deviation (calculated according to the propagation of errors, value affecting last digit) of each measurement. Fig. 1 Comparison, by linear regression, of the Pb IR obtained for Port wine samples pre-treated by 150 : 1 UV-irradiation and by microwave digestion (the °values are the 95% conÆdence limits). 1818 J.Anal. At. Spectrom., 1999, 14, 1815±1821obtained: [Microwave]~(0.98°0.44) [UV]2(0.02°0.37) for 207Pbz/206Pbz; [Microwave]~(1.0°0.8) [UV]2(0.03°1.62) for 208Pbz/206Pbz; and [Microwave]~(1.2°0.4) [UV]2(0.009°0.022) for 204Pbz/206Pbz. This statistical analysis showed no evidence of either relative or Æxed bias in the measured range. Therefore, both the proposed methods were considered to be acceptable for the determination of Pb IRs by ICP-MS.Analysis of blanks. The inØuence of the pre-treatment in the blank signals (ion intensities of the various Pb isotopes) is illustrated in Table 3. For comparison purposes, dilution of the pre-treated samples with de-ionised water or a 1% HNO3 solution were also carried out (see below) and the signals referring to those blanks (not submitted to the pre-treatment) and to a pre-treated wine sample are also shown. Although the ion intensities of Pb isotopes in the blank pre-treated by 150 : 1 UV-irradiation were signiÆcantly higher than those on the blank not UV-irradiated, they were signiÆcantly lower than those obtained for the other pre-treatments blanks, which included larger amounts of chemicals. A comparison of the signals for de-ionised water and the HNO3 solutions indicates that even the suprapure HNO3 contributes signiÆcantly to the blank signal. Nevertheless, Table 3 also shows that, in all cases, the blank signals were two to three orders of magnitude lower than the signals obtained for the Port wine samples.Precision of the Pb IR measurements The data acquisition procedure was optimised with the Pb isotopic standard NIST SRM-981, the Pb IRs of this Table 3 Comparison of the ion intensitiesa obtained for the different Pb isotopes in the blanks of the different pre-treatment procedures. Results for a Port wine solution were included for comparison 204Pb ion intensity/ion s21 206Pb ion intensity /ion s21 207Pb ion intensity /ion s21 208Pb ion intensity /ion s21 De-ionised H2O 3.1 (0.2) 7 (1) 8 (2) 10 (3) 0.5% HNO3 4.4 (0.7) 46 (1) 40 (1) 82 (3) 1% HNO3 6.2 (0.5) 56 (3) 46 (2) 111 (5) UV-irradiation (150 : 1) 17.7 (0.5) 143 (2) 134 (2) 259 (4) UV-irradiation (1 : 1) 24.1 (0.3) 385 (3) 339 (4) 818 (9) HPMW-digestion 30.5 (0.5) 537 (1) 465 (4) 1127 (3) DP 84 wine 7.67 (0.02)6102 141.6 (0.2)6102 123.8 (0.4)6102 301.5 (0.5)6102 aMean and standard deviation (n~3).Different replicates provided statistically identical results. Fig. 2 Variations of the IRs of the Pb isotopic standard NIST SRM- 981, observed over a period of 50 days (s~standard deviation of the mean). CertiÆed values for 207Pbz/206Pbz, 208Pbz/206Pbz and 204Pbz/206Pbz are 0.91464°0.00033, 2.1681°0.0008 and 0.059042°0.000037, respectively. Fig. 3 Pb IRs (mean and standard deviation, n~3), obtained for eight different samples of Port wine. The symbol (*) indicates a sample that displayed IRs signiÆcantly different from all the others, according to the least signiÆcance difference (LSD) test.22 The other symbols (#,+, %) indicate samples with IRs signiÆcantly different between themselves (for example, DP 35 and DP 84 displayed 207Pbz/206Pbz signiÆcantly different, indicated by %).J. Anal. At. Spectrom., 1999, 14, 1815±1821 1819standard (207Pbz/206Pbz, 208Pbz/206Pbz and 204Pbz/206Pbz, certiÆed values: 0.91464°0.00033, 2.1681°0.0008 and 0.059042°0.000037, respectively) being determined every working day.Considering a period of about two months, the means and respective RSDs of all the determinations (n~ 11±13) were calculated (see also Fig. 2): 207Pbz/206Pbz~ 0.920, RSD: 0.08%; 208Pbz/206Pbz~2.208, RSD: 0.17%; 204Pbz/206Pbz~0.0582, RSD: 0.38%. The precisions for long-term measurements were similar to those obtained for short-term measurements (except for 204Pb/206Pb ratio, to which short-term precision was lower) and even for single determinations of the respective Pb IR.The long-term precision obtained for the 207Pbz/206Pbz ratio was better than that obtained for the same Pb isotopic standard by Dean et al.11 and Campbell et al.,23 by using VG PlasmaQuad ICPMS apparatus, but was similar to that obtain by Halicz et al.10 using a Perkin-Elmer SCIEX Elan 6000 ICP-MS. For the 204Pbz/206Pbz ratio, a worse precision was obtained, compared to that obtained for the other two Pb IRs, probably due to poor counting statistic on the 204Pb isotope relative to the other Pb isotopes, since this is the least abundant isotope.Stroh et al.24 reported that the addition of acid to table wines increases the stability of the signals for Pb IRs determined by ICP-MS. In order to test the inØuence of acid in the signals, measurements were carried out, in parallel, in samples pretreated by the chosen procedure either not acidiÆed or acidiÆed with 0.5% or 1% HNO3.Three different Port wine samples and the Pb isotopic standard solution were analysed. It was observed that the acidiÆcation improved markedly the intensity of the ion intensities but not the precision of the Pb IRs (results not shown). However, no signiÆcant differences were observed, by the statistical test of comparison of two means,22 between the results obtained in solutions with 0.5% and 1% HNO3. From these results it was decided to acidify to 0.5% HNO3 all the subsequent solutions (samples, blanks and standards) before the ICP-MS measurements. In order to test the repeatability of the results obtained with the chosen pre-treatment procedure, three replicates of each wine sample were independently pre-treated and analysed.The respective mean and variance were calculated and compared with the mean and variance obtained for a single pre-treated sample which was analysed three times. All the experimental work was carried out in a single working day.Since the measurement of each Pb IR is affected by an error resulting from its own determination and that of the Pb isotopic standard, and since three analyses were performed for each wine, the variance associated with the respective mean was calculated according to propagation of errors. The obtained results, presented in Table 4, show that the variance obtained for the three independently pre-treated samples were of the same order of magnitude as that obtained for a single pretreatment. 204Pbz/206Pbz data referring to the wine samples DP 69 and LBV 88a are not presented due to insufÆcient sample volume but similar results are expected. These results indicated that the implemented pre-treatment did not contribute markedly to the variance of the overall method (pre-treatment plus determination). The decisive factor for the precision of the obtained Pb IRs being the ICP-MS determinations. Pb IRs in Port wines The three different Pb IRs (207Pbz/206Pbz, 208Pbz/206Pbz and 204Pbz/206Pbz) were determined in eight Port wine samples of different ages and types (Fig. 3). The RSDs associated with the mean values of the 207Pbz/206Pbz and 208Pbz/206Pbz ratios in the wine samples were between 0.15 and 0.50%, being lower than 0.3% in most of the cases. Similar precisions were obtained by Dean et al.11 and Table 4 Precision of the Pb isotope ratios observed in different Port wine samples 207Pb/206Pb Analysis of three aliquots of a single UV-irradiated solution Analysis of three independently UV-irradiated solutions s2 (UV-irradiation)a DP 52 Mean 0.858 0.858 s2 1.261025 1.8610-5 5.6610-6 DP 69 Mean 0.866 0.866 s2 2.461025 3.461025 1.061025 DP 74 Mean 0.858 0.860 s2 3.961025 3.661025 22.861026 DP 84 Mean 0.870 0.871 s2 3.761025 4.361025 6.361026 LBV 88a Mean 0.862 0.862 s2 1.261025 2.461025 1.261025 208Pb/206Pb DP 52 Mean 2.100 2.101 s2 7.861025 9.761025 1.961025 DP 69 Mean 2.099 2.097 s2 1.961024 1.661024 23.661025 DP 74 Mean 2.088 2.086 s2 1.861024 1.761024 21.261025 DP 84 Mean 2.092 2.093 s2 6.461025 1.261024 5.761025 LBV 88a Mean 2.084 2.080 s2 9.461025 1.361024 3.861025 204Pb/206Pbb DP 52 Mean 0.0552 0.0552 s2 1.161027 2.761027 1.361027 DP 74 Mean 0.0557 0.0556 s2 2.261027 2.561027 3.261028 DP 84 Mean 0.0557 0.0557 s2 2.461028 2.861027 2.661027 aDifference between the total variance (analysis of three independently UV-irradiated solution) and the determined variance (analysis of three aliquots of a single UV-irradiated solution).bNot measured in the wines DP 69 and LVB 88a, due to insufÆcient sample volume. 1820 J. Anal. At. Spectrom., 1999, 14, 1815±1821Augagneur et al.12 in table wines and were considered to be sufÆcient to differentiate natural variation of the Pb isotope abundances.12 For the 204Pbz/206Pbz ratio the RSDs were between 0.2 and 1%, seven of the eight samples displaying RSD values lower than 0.8%. Therefore, the precision of this IR was worse than that obtained for the remainders, presumably entirely due to poor counting statistic on the 204Pb isotope, relative to the other Pb isotopes, since this is the least abundant isotope. A similar result was observed by Goossens et al.25 in table wines and is typical of IRs involving an isotope of low abundance, such as 204Pb.26 With the purpose of testing if among the analysed Port wines signiÆcant differences in the values of the Pb IRs occurred, a test for comparison of several means, the LSD,22 was applied.As shown in Fig. 3, some signiÆcant differences were found (values of Pb IR indicated with the same symbol). For example, the LBV 88a wine displayed both 207Pbz/206Pbz and 208Pbz/206Pbz ratios signiÆcantly different from those of all the other wines and a 204Pbz/206Pbz ratio signiÆcantly different from that of the DP 74, DP 84 and LBV 88b wines. These results indicate that the precision of the Pb IRs obtained with the proposed method was sufÆcient to distinguish Pb isotopic composition in some of the Port wines analysed. SigniÆcant correlation (Pv0.05)22 between the age of DP wines and the 207Pbz/206Pbz ratio or the 204Pbz/206Pbz ratio were found, but at this stage no explanation can be provided for this observation.SigniÆcant correlation between the age of the wines and the 208Pbz/206Pbz ratio was not observed. The two LBV wines, although with the same age and designation, displayed signiÆcantly different Pb IRs.This indicates that the Pb present in the LBV 88a wine was from a source different of that of the LBV 88b. As the IA 10 Port wine is a mixture of wines of different years the comparison between age and Pb IRs is not signiÆcant. Acknowledgements To ``FundacÀaƒo para a CieÃncia & Tecnologia'', Lisbon, Portugal, a PhD scholarship to C. M. R. A. (PRAXIS XXI/ BD/16028/98) and the equipment (Proj. 27/M/90) and to ``Instituto do Vinho do Porto'', Porto, Portugal, the Port wine samples and related technical information.References 1 B. L. Gulson, T. H. Lee, K. J. Mizon, M. J. Korsch and H. R. Eschnauer, Am. J. Enol. Vitic., 1992, 43, 180 and references therein. 2 T. D. Bullen and C. Kendall, in Isotope Tracers in Catchment Hydrology, ed. C. Kendall and J. J. McDonnels, Elsevier Science B. V., Amsterdam, 1998, ch. 18. 3 M. E. Ketterer, M. J. Peters and P. J. Tisdale, J. Anal. At. Spectrom., 1991, 6, 439. 4 T. Catterick, H. Handley and S. Merson, At. Spectrosc., 1995, 16, 229. 5 M. Viczian, A. Lasztity and R. M. Barnes, Acta Chim. Hung., 1991, 128, 639. 6 A. J. Walder and N. Furuta, Anal. Sci., 1993, 9, 675. 7 A. J. Walder, D. Koller, N. M. Reed, R. C. Hutton and P. A. Freedman, J. Anal. At. Spectrom., 1993, 8, 1037. 8 A. J. Walder, I. Platzner and P. A. Freedman, J. Anal. At. Spectrom., 1993, 8, 19. 9 J. R. Dean, L. Ebdon and R. Massey, J. Anal. At. Spectrom., 1987, 2, 369. 10 L. Halicz, Y. Erel and A. Veron, At. Spectrosc., 1996, 17, 186. 11 J. R. Dean, L. Elbon and C. Massey, Food Addit. Contam., 1990, 7, 109. 12 S. Augagneur, B. Medina and F. Grousset, Fresenius' J. Anal. Chem., 1997, 357, 1149. 13 J. Goossens, T. De Smaele, L. Moens and R. Dams, Fresenius' J. Anal. Chem., 1993, 347, 119. 14 S. Sanllorente, M. C. Ortý�z and M. J. Arcos, Analyst, 1998, 123, 513. 15 F. E. Smith and E. A. Arsenault, Talanta, 1996, 43, 1207. 16 D. Beauchemin, J. McLaren W. and S. S. Berman, J. Anal. At. Spectrom., 1988, 3, 775. 17 A. Alimonti, F. Petrucci, B. Santucci, A. Cristaudo and S. Caroli, Anal. Chim. Acta, 1995, 306, 35. 18 X. Cao, G. Zhao, M. Yin and J. Li, Analyst, 1998, 123, 1115. 19 B. S. Sheppard, D. T. Heitkemper and C. M. Gaston, Analyst, 1994, 119, 1683. 20 Perkin Elmer Users Manual Elan 5000 Inductively Coupled Plasma Mass Spectrometry, Norwalk, Connecticut, USA, 1992. 21 C. M. R. Almeida and M. T. S. D. Vasconcelos, Anal. Chim. Acta, 1999, 396, 45. 22 J. C. Miller and J. N. Miller, in Statistics for Analytical Chemistry, Wiley, New York, 1984, ch. 3 and 4. 23 M. J. Campbell and H. T. Delves, J. Anal. At. Spectrom., 1989, 4, 235. 24 A. Stroh, P. Bru»ckner and U. Vo» llkopf, At. Spectrosc., 1994, 2, 100. 25 J. Goossens, L. Moens and R. Dams, Anal. Chim. Acta, 1994, 293, 171. 26 M. E. Ketterer, J. Anal. At. Spectrom., 1992, 7, 1125. Paper 9/05426J J. Anal. At. Spectrom., 1999, 14, 1815±1821 18

ISSN:0267-9477    

DOI:10.1039/a905426j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Inter-element fractionation and correction in laser ablation inductively coupled plasma mass spectrometry Zhongxing Chen{ School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC, Canada V8W 3P6 Received 26th April 1999, Accepted 6th September 1999 Inter-element fractionation in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis is one of the major challenges for using the technique for in situ trace element determination and isotopic ratio measurement of geological, environmental and biological solid samples.Attempts have been made to reduce inter-element fractionation in LA-ICP-MS analysis. However, this fractionation cannot be eliminated. The mechanism of the fractionation in LA-ICP-MS analysis is not very well understood. This study investigated the inter-element fractionation of seven elements (Ca, V, Zn, Ga, Sr, La and Nd) in three different sample matrices (NIST 613, BCR-2 and SY-4) using a UV 266 nm laser.The study showed that the inter-element fractionation depends on the sample matrices and varies with time. The inter-element fractionation behaviour of V, Zn and Ga in the synthetic silicate glass NIST 613 is different from that in the quenched glass of fused silicate rocks (BCR-2 and SY-4). Relative to Ca, V, Zn and Ga show less fractionation in NIST 613 but larger fractionation in BCR-2 and SY-4. The relative internal standard normalized element intensity (RISNEI) is not linear with time for a laser ablation period of 210 s.Therefore, data acquisition using prolonged laser ablation without a matrix match will not improve the precision and accuracy for elements whose fractionation behavior is different from that of the internal standard element. The RISNEI versus time relationship for the Ærst 100 s laser ablation can be treated as linear to simplify the data calculation. In this paper, the internal standard normalized fractionation factor (ISNFF) is deÆned as the sum of the second half average RISNEI and the difference between the second and Ærst half average RISNEI, divided by the second half RISNEI of data acquisition, for the analyte concentration calculation.The ISNFF was applied for the correction of the data reduction in LA-ICP-MS analysis. The data accuracy for these seven elements is generally improved, particularly for an element whose calibration standard normalized ISNFF is signiÆcantly greater or less than 1 (e.g., Zn and Ga in this study).Good accuracy can be obtained for elements without ISNFF correction and matrix matches only if the calibration standard normalized ISNFF of the elements is close to 1. Since laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was Ærst introduced by Gray1 in 1985, it has been widely used as a powerful analytical technique in various disciplines2±7 for in situ solid micro-sampling analyses. For LA-ICP-MS analysis, a combination of external and internal standardization is most commonly used for calibration and quantiÆcation.Good precision and accuracy have been reported if an element whose fractionation is similar to those of the analytes was chosen as an internal standard.2,3,8±10 However, if the fractionation of the analyte is signiÆcantly different from that of the internal standard element, poor accuracy is obtained. The analytically suitable and naturally occurring major elements with known concentrations (e.g., Ca and Si in silicate rocks and minerals) are commonly used as an internal standard for in situ multi-element LA-ICP-MS analysis.The selection of these elements is very limited. It is difÆcult to choose an internal standard which is capable of standardizing elements with different chemical characteristics (e.g., chalcophile, alkali and lithophile elements). Inter-element fractionation in LA-ICP-MS analysis has been studied by several investigators recently.9±13 However, the mechanism of inter-elemental fractionation in LA-ICP-MS analysis is still not very well understood.Every effort has been made to minimize inter-element fractionation. The local temperature has been reported to be an important factor in the fractionation and methods were adopted to reduce it.9,11 Active focusing of the laser beam also reduced the interelement fractionation in LA-ICP-MS analysis.9,11 Laser ablation using a 266 nm UV laser produced less inter-elemental fractionation than using 1064 and 532 nm lasers.9±11 More recently, Jeffries et al.13 reported that laser ablation using a frequency quintupled Nd:YAG laser (213 nm) signiÆcantly decreased the inter-element fractionation.Despite all these efforts, inter-element fractionation in LAICP- MS cannot be eliminated. In this work, inter-element fractionation in three different matrices (NIST 613, BCR-2 and G-2) during laser ablation at 266 nm was investigated.Interelement fractionation in LA-ICP-MS analysis depends on the sample matrix and varies with time during the laser ablation and transport. A fractionation factor was deÆned and applied to the correction of the LA-ICP-MS data reduction. Data accuracy for elements whose fractionation is different from that of an internal standard element was signiÆcantly improved after inter-element fractionation was corrected. Experimental Instrumentation A VG Elemental (Winsford, Cheshire, UK) PQ II S high sensitivity ICP-MS instrument was used.The software was upgraded to the OS/2 based PQ version 4.36. A two-directional communication is interfaced between the ICP-MS and the LA systems via a serial RS-232 (i.e., COM port). With the new PQ version software, the execution of each deÆned time setting by {Present address: Department of Marine Science, University of Southern Mississippi, Stennis Space Centre, MS 39529, USA. E-mail: zhongxing.chen@usm.edu; Tel: z1 228 688 1180; Fax: z1 228 688 1121.J. Anal. At. Spectrom., 1999, 14, 1823±1828 1823 This journal is # The Royal Society of Chemistry 1999LA corresponds to the ICP-MS data acquisition, allowing samples to be run in automated cycle sequences. Optimization of the plasma and mass spectrometer conditions was accomplished using NIST 613 glass containing about 50 mg g21 of trace elements. For each analytical run, the nebulizer gas Øow rate, torch position and lens setting were adjusted to optimize the signal intensity while ablating NIST 613 with a spot size of approximately 50 mm and a laser beam energy of less than 3 mJ, so that (1) sensitivity of 42Caw7.0 counts s21 mg21, (2) sensitivity of 139Law4200 counts s21 mg21 and (3) sensitivity of 232Thw3250 counts s21 mg21 and 232Th16O/232Thv0.3%.Typical ICP-MS operating conditions are given in Table 1. The LA used in this work was a Merchantek (Carlsbad, CA, USA) EO UV laser system. It is a compact computercontrolled solid state Nd:YAG laser whose output is frequency quadrupled to 266 nm with a maximum energy of 4 mJ.The LA software was upgraded to 2.12.0 UV. The laser was operated in a gated Q-switch mode for optimum stability, i.e., the Øash lamp was Æred at a frequency of 20 Hz and the laser output at computer-controlled pulse rates between 1 and 20 Hz. The UV laser beam is split by a beamsplitter that allows approximately 2% of the laser beam to go to a built-in energy meter and the other 98% of the beam to the objective lens, allowing an operator to monitor continuously the laser power before the Ænal objective lens during the ablation.Typical LA operating conditions are given in Table 2. Standards and samples NIST 613 synthetic silicate reference material was used as a test and calibration standard. The NIST 613 glass button was analysed as received. Major and trace element concentrations of the reference material, determined by solution nebulization ICP-OES and ICP-MS, were given by Chen et al.3 Two geological reference materials, Canada Centre for Energy and Mineral Technology (CANMET) SY-4 (diorite gneiss rock) and United State Geological Survey (USGS) BCR-2 (basalt) were analysed by LA-ICP-MS.Approximately 4±5 g of rock powder were weighed into a 20 cm3 Pt crucible and heated in a mufØe furnace (Lindberg Blue M) at 1550 �C in an air atmosphere for 12±15 h for melting and sample homogeneity.No Fe loss occurred from the sample to the Pt crucible in an air atmosphere. The crucible was removed from the furnace and the rock melt was quickly quenched to a glass in water. Several centimetre-sized chips of the glass were removed from the crucible, mounted in epoxy and polished with SiC powder under distilled water. The resultant glass chips were examined using a petrographic microscope and were found to be free of any crystal phases. Data acquisition and calibration After the instruments had been optimised and a procedure set up, both LA and ICP-MS were run automatically following the analytical procedure from the Ærst to last analysis.Experiments were carried out in the following cycle sequences: NIST 613, BCR-2, SY-4, NIST 613, º, NIST 613, BCR-2, SY-4, NIST 613. Each analysis consists of a 100 s pre-ablation delay for the background (data collected), 210 s lasering for signal (data collected) and a 90 s sample delay for Øush (no data collected).Seven elements (42Ca, 51V, 66Zn, 69Ga, 88Sr, 139La and 146Nd) which range from the highest (Zn) to least (Nd) fractionation relative to Ca were investigated for their fractionation behaviour and correction procedure. Data were acquired in the peak-jumping mode with a dwell time of 10.24 ms. Background levels for each element were obtained by acquiring data for a gas blank for 100 s prior to laser sampling. Sample data were corrected from 210 s ablation and data acquisition.Count rates were collected and exported as CSV (comma delimited values) Æles by PQ Version 4.36 Time Resolved Analysis (TRA) software. All subsequent data manipulations were later accomplished off-line using a commercial spreadsheet program by manual operation and by in-house written software. All sample data reported were background subtracted. The Ærst few data after initial laser ablation were not included in the data reduction to avoid potential surface contamination.Calibration and quantiÆcation of the analysis utilized both external and internal calibrations. NIST 613 was used as an external calibration standard and Ca as an internal standard. Results and discussion Matrix effect The relative internal standard (e.g., Ca in this study) normalized element intensity (RISNEI) is deÆned here as RISNEI~ÖIElement=IInternal standardÜt =ÖIElement=IInternal standardÜInitial ablation Fig. 1. shows the RISNEI of Zn, Ga, V and La in three different matrices (NIST 613, BCR-2 and SY-4).The data shown in the Ægures are averages of every nine readings from TRA data acquisition to smooth the curve. The RISNEI versus time relationship of Sr and Nd in the three matrices is similar to that of La, and is not shown in the Ægure. Inter-element fractionation of the elements depends on the sample matrices. The interelement fractionation behaviour of the elements in the synthetic silicate glass NIST 613 is different from that in the the fused silicate rocks (BCR-2 and SY-4).Relative to Ca, V, Zn and Ga show less fractionation in NIST 613 but greater fractionation in BCR-2 and SY-4. The fractionation of an element in SY-4 is higher than that of the same element in BCR-2. Although the fractionation of the same element is different in three sample matrices (Fig. 1), the RISNEI of all elements in the same sample matrix displays a similar trend, e.g., the RISNEI of all the Table 1 ICP-MS operating conditions Inductively coupled plasma– Plasma gas Argon Forward power 1350W ReØected power v5W Gas Øow rates– Plasma gas Øow rate 14 dm3 min21 Auxiliary gas Øow rate 0.94 dm3 min21 Inner gas Øow rate y1.20 dm3 min21 (see text) Interface– Sampling distance (load coil to sample aperture) 16 mm Sampling aperture Nickel, 1.0 mm diameter Skimmer aperture Nickel, 0.7 mm diameter Ion lens settings– Extraction lens 2365 V Collector lens 277.9 V L1 lens z2.8 V L2 lens 210.7 V L3 lens z4.8 V L4 lens 229.9 V Pole bias 0 V Data acquisition parameters– Measurement mode Peak jumping Dwell time 10.24 ms Data acquisition time 310 s Points per peak 1 Table 2 LA operating condition Laser mode Q-switch Wavelength 266 nm Flash lamp frequency 20 Hz Laser output frequency 10 Hz Laser energy before objective lens v3 mJ Spot size 3 (y50 mm) 1824 J.Anal. At. Spectrom., 1999, 14, 1823±1828elements in SY-4 increases with time after the initial ablation and transport, reaches its maximum at approximately 275 s, and then decreases with time.The RISNEI is not linear with time for a period of 210 s during laser ablation and transport. Therefore, data acquisition using prolonged laser ablation without a matrix match will not improve the precision and accuracy for elements whose fractionation behaviour is different from that of the internal standard. Variations with time Fig. 2 shows the RISNEI of Zn and Ga in SY-4 and BCR-2 of the Ærst and last analyses.The last analysis was repeated using the same LA-ICP-MS operating conditions approximately 2 h after the Ærst analysis. Fig. 2 indicates that the inter-element fractionation varies with time. Physical conditions during the laser ablation and transport may cause this variation. The oxygen content in the sample chamber signiÆcantly affects the inter-element fractionation during the laser ablation and transport and the mechanism is still under investigation. It was observed that different laser beam focusing may also affect the fractionation.9,11 The local temperature of the sample is an important factor in fractionation.11 Despite the RISNEI changes for the two different analyses, interestingly, all the elements in the same sample matrix displayed the same RISNEI versus time trend described above (Fig. 2).Internal standard normalized fractionation factor (ISNFF) Fig. 3 shows the relative Ca normalized Ga and Zn intensity in SY-4 versus time for the Ærst 100 s laser ablation.A very good linear relationship was observed. To simplify the calculation here, it is assumed that the RISNEI is linear with time for the Ærst 100 s during the laser ablation and transport. This is reasonable for our routine LA-ICP-MS analysis which uses 60 s for data acquisition. Fig. 4 is a schematic diagram shown the RISNEI versus time. AB is the RISNEI when the laser is initially Æred, CD the RISNEI when the laser is off, EF the average RISNEI of the data acquisition, GH the average RISNEI of the Ærst half of the data acquisition and IJ an average RISNEI of the second half of the data acquisition. The internal standard normalized fractionation factor (ISNFF) is deÆned here as the sum of the second half average RISNEI and the difference between the second and Ærst half average RISNEI, divided by the second half RISNEI: ISNFF~âIJzÖIJ{GHÜä=IJ~Ö2IJ{GHÜ=IJ~2{GH=IJ The ISNFF calculated varies with the time interval selected for data reduction.AnaverageISNFFof individual elements for the Ærst 100 s laser ablation in three different matrices is presented in Table 3.Relative to Ca, six elementsshow different fractionation behaviour in three sample matrices during the LA-ICP-MS analysis. ISNFF ranges from 0.965 for La to 1.259 for Zn in NIST 613, from 0.991 for La to 1.332 for Zn in BCR-2 and from 0.992 forLato 1.432 forZnin SY-4. The fractionation behaviour of the elements in fused glass chips of two geological silicate reference materials (BCR-2 and SY-4) is different from those in synthetic silicate glass standard. The relationship between ISNFF and the major components (SiO2 and Al2O3) in three sample matrices is shown in Fig. 5. The ISNFF increases and decreases with Al2O3 and SiO2 contents in the sample matrices, respectively. The relationship between the fractionation and elemental properties such as Æeld strength and melting temperature has been investigated previously,9 but no signiÆcant correlation was observed.A linear relationship between the ISNFF and the sum of Ærst and second ionizati enthalpies is shown in Fig. 6, indicating that inter-element fractionation during laser ablation dependsonionization energy. This suggests that a low ionization energy element was ablated easier than that of high ionization energy, which lead to the progressive enrichment of high ionization-energy elements in the ablation Fig. 1 Relative Ca normalized element intensity of (a) Zn, (b) Ga, (c) V and (d) La in NIST 613, BCR-2 and SY-4 in LA-ICP-MS analysis.J. Anal. At. Spectrom., 1999, 14, 1823±1828 1825pit. The mechanism of inter-element fractionation in LA-ICPMSis not very well understood, and will be investigated further. Inter-element fractionation correction The ISNFF for individual elements in NIST 613 standard and samples was calculated using the equation described above. Fig. 2 Relative Ca normalized element intensity of (a) Zn and (b) Ga in SY-4 and of (c) Zn and (d) Ga in BCR-2 for the Ærst and last analyses.The last analysis was repeated using the same LA-ICP-MS operating conditions approximately 2 h after the Ærst analysis. Fig. 3 Relative Ca normalized element intensity of (a) Zn and (b) Ga versus time, showing linearity for the Ærst 100 s laser ablation and transport of LA-ICP-MS. Fig. 4 Simplied diagram of relative internal standard normalized element intensity (RISNEI) versus time.AB is the RISNEI when the laser is initially Æred, CD the RISNEI when the laser is off, EF the average RISNEI of the data acquisition, GH the average RISNEI of the Ærst half of the data acquisition and IJ the average RISNEI of the second half of the data acquisition. The internal standard normalized fractionation factor (ISNFF) is deÆned here as the sum of the second half average RISNEI and the difference between the second and Ærst half average RISNEI, divided by second half RISNEI of data acquisition. 1826 J. Anal. At. Spectrom., 1999, 14, 1823±1828The ISNFF of the element in the samples was then normalized to that of the same element in standard, and applied to the Ænal elemental concentration calculation described previously:15 CÖSample analyte; correctedÜ~CÖSample analyteÜ |ISNFFÖStandard analyteÜ=ISNFFÖSample analyteÜ The results with and without inter-element fractionation correction are present in Table 4. The accuracy expressed as the relative difference between this work and literature value is shown in Fig. 7. The data accuracy by LA-ICP-MS with ISNFF correction developed in this study (using the equation described above) is signiÆcantly improved, particularly for Zn and Ga. The relative difference for Zn is reduced from 53.5 to 2.6% for BCR-2 and from 81.5 to 7.2% for SY-4, and that for Ga from 16.3 to 23.0% for BCR-2 and from 30.9 to 25.4% for SY-4. Since Gray1 performed the Ærst LA-ICP-MS analysis in 1985, good precision and accuracy by LA-ICP-MS have been obtained without inter-element fractionation correction and matrix matches if an element whose fractionation is similar to those of the analytes was chosen as an internal standard.2,3,9,10 This can be very clearly explained in Table 3.Relative to Ca, the elements Sr, La and Nd have ISNFFs very close to 1 in both the external calibration standard (NIST 613) and samples (BCR-2 and SY-4). The results calculated using NIST 613 as an external standard and Ca as an internal standard are not very different between the data with and without ISNFF correction (Table 4).However, if the calibration standard normalized ISNFF of the elements (e.g., Zn and Ga in this study) is signiÆcantly greater or less than 1, an error is introduced into the calculated results of LA-ICP-MS analysis if the procedure used here is not adopted. Conclusions Inter-element fractionation of seven elements (Ca, V, Zn, Ga, Sr, La and Nd) in three different sample matrices (NIST 613, BCR-2 and SY-4) during LA-ICP-MS analysis using a UV 266 nm laser was investigated.The fractionation depends on the sample matrices and varies with time. A linear relationship between the ISNFF and the ionization enthalpy indicates that Table 3 ISSF of elements for the Ærst 100 s laser ablation in three different matrices Element 51V 66Zn 69Ga 88Sr 139La 146Nd NIST 613 (average, n~5) 1.057 1.259 1.142 1.012 0.965 0.974 RSD (%) 1.8 3.0 2.3 0.3 1.6 1.3 BCR-2 (average, n~5) 1.109 1.332 1.199 0.991 0.983 0.985 RSD (%) 1.4 1.6 2.1 2.0 2.5 3.0 SY-4 (average, n~5) 1.131 1.432 1.286 0.992 0.998 0.980 RSD (%) 2.1 3.0 3.0 1.3 1.1 1.7 Fig. 5 Relationship between the ISNFF of V, Zn and Ga and major components [(a)SiO2 and (b) Al2O3] in three different sample matrices. Fig. 6 Relationship between the ISNFF of V, Zn and Ga and the sum of Ærst and second ionization enthalpies in three sample matrices: (a) NIST 613, (b) BCR-2 and (c) SY-4.Ionization enthalpy values from ref. 14. J. Anal. At. Spectrom., 1999, 14, 1823±1828 1827inter-element fractionation during laser ablation depends on ionization energy. In this paper, the internal standard normalized fractionation factor (ISNFF) is deÆned as the sum of the second half average relative internal standard (e.g., Ca in this study) normalized element intensity (RISNEI) and the difference between the second and Ærst half average RISNEI, divided by second half RISNEI of data acquisition.A procedure was developed to correct inter-element fractionation. The ISNFF was applied to the LA-ICP-MS data reduction to correct the inter-element fractionation during laser ablation and transport. Data accuracy is generally improved, particularly for an element whose ISNFF normalized to the calibration standard is signiÆcantly greater or less than 1. Good accuracy can be obtained without ISNFF correction and matrix matches only if the calibration standard normalized ISNFF is close to 1.Further work is needed to investigate the inter-element fractionation of more elements in various matrices, e.g., Pb, Th and U in zircons. The isotopic ratio measurement by LA-ICP-MS in zircons has been used for geochronology.18,19 However, owing to the inter-element fractionation, this measurement has been limited to the determination of only Pb207/Pb206 ratios. With the interelement fractionation correction, it will be possible to measure more useful U/Pb and Th/Pb ratios.The PQ II S ICP-MS and Merchantek EO LA system were purchased through an NSERC major installation grant (Grant No. EQM0184535) to J. K. B. Bishop, D. Canil and K. M. Gillis. The author thanks H. Longerich and D. Canil for their useful comments and suggestions in improving the manuscript. References 1 A. L. Gray, Analyst, 1985, 110, 551. 2 S. E. Jackson, H. P. Longerich, G. R. Dunning and B. J.Fryer, Can. Mineral., 1992, 30, 1049. 3 Z. Chen, W. Doherty and D. C. Gregoire, J. Anal. At. Spectrom., 1997, 12, 653. 4 R. D. Evans, P. M. Outridge and P. Richner, J. Anal. At. Spectrom., 1994, 9, 985. 5 S. Tanaka, N. Yasushi, N. Sato, T. Fukasawa, S. J. Santosa, K. Yamanaka and T. Ootoshi, J. Anal. At. Spectrom., 1998, 13, 135. 6 C. Leloup, P. Marty, D. Dallava and M. Perdereau, J. Anal. At. Spectrom., 1997, 12, 945. 7 A. Raith, R. C. Hutton, I. D. Abell and J. Crighton, J.Anal. At. Spectrom., 1995, 10, 591. 8 B. J. Fryer, S. E. Jackson and H. P. Longerich, Can. Mineral., 1995, 33, 303. 9 T. E. Jeffries, N. J. Pearce, W. T. Perkins and A. Raith, Anal. Commun., 1996, 33, 35. 10 H. P. Longerich, D. Gunther and S. E. Jackson, Fresenius' J. Anal. Chem., 1996, 355, 538. 11 D. Figg and M. S. Kahr, Appl. Spectrosc., 1997, 51, 1185. 12 P. M. Outridge, W. Doherty and D. C. Gregoire, Spectrochim. Acta, Part B, 1997, 52, 2093. 13 T. Jeffries, S. E. Jackson and H. P. Longerich, J. Anal. At. Spectrom., 1998, 13, 935. 14 A. M. James and M. P. Lord, in Macmillan's Chemical and Physical Data, Macmillan, London, 1992. 15 H. P. Longerich, S. E. Jackson and D. Gunther, J. Anal. At. Spectrom., 1996, 11, 899. 16 S. A. Wilson, United States Geological Survey, Open File Report, in the press. 17 W. S. Bowman, Geostand. Newsl., 1995, 19, 101. 18 B. J. Fryer, S. E. Jackson and H. P. Longerich, Chem. Geol., 1993, 109, 1. 19 R. Feng, N. Machado and J. Ludden, Geochim. Cosmochim. Acta, 1993, 57, 3479. Paper 9/03272J Table 4 Concentrations determined by LA-ICP-MS before and after the ISNFF correction, and literature values for reference materials BCR-2 and SY-4 Element 51V 66Zn 69Ga 88Sr 139La 146Nd Before ISNFF correction– BCR-2 (average, n~5)/mg g21 431 195 27 312 24.2 27.2 RSD (%) 1.1 8.8 3.8 2.2 3.9 4.8 SY-4 (average, n~5)/mg g21 6.18 169 45.8 1099 54.9 53.8 RSD (%) 2.0 7.0 3.4 1.6 1.6 1.7 After ISNFF correction– BCR-2 (average, n~5)/mg g21 399 130 22.3 322 23.6 26.7 RSD (%) 2.8 12.7 5.0 1.7 7.6 8.4 SY-4 (average, n~5)/mg g21 5.67 100 33.1 1130 52.6 52.7 RSD (%) 0.1 11.1 13.6 9.1 3.4 3.4 Literature value– BCR-216/mg g21 416 127 23 346 25 28 SY-417/mg g21 8.0 93 35 1191 58 57 Fig. 7 Accuracy expressed as the relative difference between results obtained in this work (A) before and (B) after ISNFF correction, and literature values for (a) BCR-2 (ref. 16) and (b) SY-4 (ref. 17). 1828 J. Anal. At. Spectrom., 1999, 14, 1823±1828

ISSN:0267-9477    

DOI:10.1039/a903272j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Detection of arsenosugars from kelp extracts via IC-electrospray ionization-MS-MS and IC membrane hydride generation ICP-MS{ Patricia A. Gallagher, Xinyi Wei,{ Jody A. Shoemaker, Carol A. Brockhoff and John T. Creed* US EPA NERL Microbiological and Chemical Exposure Assessment Research Division, Cincinnati, OH 45268, USA Received 2nd August 1999, Accepted 21st September 1999 The selectivity and the ability to obtain structural information from detection schemes used in arsenic speciation research are growing analytical requirements driven by the growing number of arsenicals extracted from natural products and the need to minimize misidentiÆcation in exposure assessments.Three arsenosugars were extracted from ribbon kelp utilizing accelerated solvent extraction. The three arsenosugars were separated from other arsenicals with near baseline resolution using a PRP-X100 column and a 20 mM (NH4)2CO3 mobile phase at a pH of 9 with IC-ICP-MS detection. Utilizing these chromatographic conditions, the molecular weight was determined for each arsenosugar utilizing ion chromatography-electrospray ionization-mass spectrometry (IC-ESI-MS) in the positive ion mode. The molecular weight and retention times for the three arsenicals are 328 u (4.6 min), 482 u (8.2 min) and 392 u (14.2 min).The IC-ESI-MS-MS spectra from each of the arsenosugars were compared to the spectra reported in the literature, which were obtained via direct infusion of standard materials.All three MS-MS spectra contain m/z 237, 195 and 97, which are fragments of the base dimethylarsinylriboside common to all the arsenosugars. Adequate sensitivity for each arsenical was achieved using a 6.1 ng and a 22 ng injection for IC-ESI-MS and IC-ESI-MS-MS, respectively. Given the unavailability of standards, the arsenosugar distribution was determined via relative chromatographic areas using IC-ICP-MS. The IC-ICP-MS indicated the presence of an arsenic heteroatom within the same retention windows in which the arsenosugars were detected via IC-ESI-MS. The IC-ESI-MS and IC-ESI-MS-MS detection scheme provided structural information but at reduced sensitivity.In an attempt to preserve sensitivity and improve selectivity of the IC-ICP-MS, an on-line membrane hydride generation detection scheme was evaluated. The hydride system indicated that the three unknown peaks (arsenosugars) were not hydride active, thereby simplifying the chromatographic resolution needed to quantitate the more toxicologically important arsenicals, such as MMA, DMA, As(III) and As(V), while minimizing the potential for misidentiÆcation.Introduction The two major pathways for arsenic exposure are drinking water and dietary intake. The geographical distribution of arsenic in surface water and ground water in the US has been estimated by Frey and Edwards.1 Inorganic arsenic is the predominant species present in drinking water and it is estimated that approximately 15% of the US population is exposed to arsenic in drinking water at concentrations greater than 2 mg L21.Dietary exposure to total arsenic (arsenic determined after acid digestion) has been estimated.2±4 The naturally occurring and anthropogenic arsenicals are assimilated into many foods with the highest concentrations being found in Æsh and shellÆsh. Unlike water, the dietary sources of arsenic contain a variety of arsenicals.5±14 Some of these arsenicals are thought to be carcinogenic, others are thought to be cancer promoters, while some are considered non-toxic.This species-dependent toxicity of arsenicals requires analytical techniques capable of distinguishing the toxic from the nontoxic chemical forms. Therefore, an accurate risk assessment which incorporates the two major sources of arsenic exposure must estimate the toxic and non-toxic species found in the dietary components. One essential aspect of an accurate risk assessment for arsenic is sensitive and selective analytical methods capable of speciating arsenic in a variety of exposure assessment matrices.The analytical capability to speciate arsenic in a wide variety of sample matrices is a rapidly growing research area. Analytical capability to speciate arsenic has been demonstrated in environmental matrices,15±19 biologicals20±32 and dietary samples5±14,33±36. Much of this research has focused on the use of atomic spectroscopy and more recently ICP-MS as the detector of choice because of the inherent sensitivity and elemental selectivity.The ability to obtain structural information from detection schemes used in arsenic speciation research is a growing analytical requirement driven by the growing number of arsenicals extracted from natural products and the need to minimize misidentiÆcation in exposure assessments. This growing list produces the potential for false positives based on insufÆcient chromatographic resolution or detector selectivity.Improved chromatographic resolution via capillary electrophoresis is an active area of research within arsenic speciation.37,38 Alternatively, the use of electrospray ionization (ESI) mass spectrometry as a detector for LC can provide structural information39,40 on eluting arsenicals and, in turn, improve detector selectivity.41±43 This structural information is obtained at the cost of reduced sensitivity relative to ICP-MS. A third approach to minimizing misidentiÆcation is to capitalize on the inability of highly derivatized arsenicals to produce a volatile hydride.44,45 This in effect improves the selectivity of atomic spectroscopy based detection systems while preserving the required sensitivity.This paper will report on the use of {US Government Copyright. {National Research Council Postdoctoral Fellow. J. Anal. At. Spectrom., 1999, 14, 1829±1834 1829 This journal is # The Royal Society of Chemistry 1999IC-ESI-MS-MS and IC hydride generation ICP-MS as a means of improving detector selectivity relative to conventional atomic spectroscopy.The IC-ESI-MS-MS system will be used to characterize three arsenosugars as they elute from a conventional anion exchange column while IC hydride generation ICP-MS will be used to preserve sensitivity and demonstrate an added degree of selectivity. Experimental Reagents The ribbon kelp (Alaria marginata) was received from Puget Sound, WA, USA.The HPLC grade methanol (MeOH) and trace metal grade ammonium hydroxide (NH4OH) originated from Fisher ScientiÆc (Pittsburgh, PA, USA). The ACS reagent grade ammonium carbonate [(NH4)2CO3] was purchased from Aldrich (Milwaukee, WI, USA) while the ammonium phosphate [(NH4)2HPO4] and the ultrapure reagent grade nitric acid (HNO3) were from J. T. Baker (Phillipsburg, NJ, USA). For hydride generation, trace metal grade HCl (Fisher, Pittsburgh, PA, USA) was used, and NaBH4 (97z%, Alfa AESAR, Johnson Matthey, Ward Hill, MA, USA) was prepared in 0.1 M NaOH (97z%, certiÆed ACS; Fisher, Fair Lawn, NJ, USA) solution for stabilization.K2S2O8 (99z%, ACS reagent) was purchased from Aldrich. The water used was always 18MV by Millipore (Bedford, MA, USA). The arsenite [As(III)] and arsenate [As(V)] were purchased from Spex CertiPrep (Metuchen, NJ, USA). Dimethylarsinic acid (DMA) and disodium methylarsenate (MMA) were from Chem Services (West Chester, PA, USA).Arsenobetaine (AsB) was from University of British Columbia, Department of Chemistry (Vancouver, Canada). All standard materials were prepared based on arsenic and standardized against NIST 1643c obtained from the US Department of Commerce National Institute of Standards and Technology (Gaithersburg, MD, USA). Sample preparation±accelerated solvent extraction The ribbon kelp was freeze dried utilizing a VirTis lyophilizer (Gardiner, NY, USA) followed by homogenization in an Osterizer blender (Milwaukee, WI, USA).The ribbon kelp samples were extracted using an ASE 200 accelerated solvent extractor system (Dionex, Sunnyvale, CA, USA). A specialized 3 mL ASE cell was utilized for this study. The ribbon kelp was mixed with Empore Filter Aid 400 high density glass beads from Varian (Harbor City, CA, USA). The glass beads were used as a support medium and prevented the ASE cell from clogging due to the ribbon kelp expanding.The ASE parameters were: 30z70 MeOH±H2O, 500 psi, ambient temperature, 1 min heat step, 1 min static step, 30% Øush step, 1 cycle and a 120 s purge. The Zymark TurboVap LV evaporator was set to 50 �C with a nitrogen purge. The extract was next brought to dryness. The residue was then redissolved in water and treated with a maxi clean C18 cartridge (900 mg) from Alltech Associates, Inc. (DeerÆeld, IL, USA). The C18 puriÆed sample was used in all further testing. A summary of the ASE instrumental conditions can be found in Table 1.Chromatography ICP-MS and ESI-MS-MS The ion chromatography was completed utilizing a Dionex Gradient pump (Model GPM2), which utilizes a pre- and postcolumn six-way valve injector. The post-column injection introduces the marker peak and the pre-column injection is used for chromatographic separation. Three different chromatographic separations were utilized. The Ærst chromatographic separation conditions consist of an ION 120 column from Interaction Chromatography (San Jose, CA, USA) with a 40 mM (NH4)2CO3, pH 10.5 mobile phase.The Øow rate was 1 mL min21 with a 100 mL injection loop. This chromatographic separation was used with direct nebulization ICP-MS detection. The second chromatographic separation conditions involve a PRP-X100 column from Hamilton (Reno, NV, USA) and a 20 mM (NH4)2CO3, pH 9.0 mobile phase. The Øow rate was 1 mL min21 with a 100 mL (ICP-MS and ESI-MS) or a 360 mL (ESI-MS-MS) injection loop.This chromatographic separation was used with direct nebulization ICP-MS and ESIMS- MS detection. The third chromatographic separation used in the hydride research incorporates the Hamilton PRP-X100 column with a 14 mM (NH4)2HPO4±14 mM HNO3, pH 6.0, mobile phase. The Øow rate was 1 mL min21 with a 50 mL injection loop. A summary of the chromatographic conditions can be found in Table 1. Direct nebulization and membrane hydride ICP-MS detection The ICP-MS used for direct nebulization was a Plasma Quad 3 from VG Elemental (Franklin, MA, USA).The Øow rates for the plasma, auxiliary and nebulizer were 13.0 L min21, 0.80 L min21 and 0.68 L min21, respectively, with a forward power of 1350 W. Data collection was completed using single ion monitoring of m/z 75. The hydride generation ICP-MS instrument was a Hewlett- Packard (Avondale, PA, USA) 4500 series. The membrane hydride generation system has been described previously.18 This system has been modiÆed to include the on-line postcolumn photo-oxidation process to allow for the detection of highly derivatized arsenicals.The on-line photo-reactor PHRED was purchased from Aura Industries, Inc. (State Island, NY, USA). The reactor compartment was equipped with an 8 W UV lamp (254 nm) and a polished stainless steel support plate with holes at the bottom, which allowed the TeØon tubing reaction coil to extend in or out of the reactor. For better irradiation efÆciency, the original Øat-shaped PHRED knitted reaction coil (designed to Æt on the bottom support plate under the UV lamp) was replaced with a 5 m thin-wall microbore PTFE tubing (id 0.56 mm, wall 0.02 mm, Cole-Parmer Instrument Company, Vernon Hills, IL, USA) reactor, which was braided around the UV lamp.The IC efØuent was mixed with 2% K2S2O8 solution at a three-way PTFE manifold mixer and passed through the reaction coil under UV irradiation. A cooling device for the UV lamp was not provided with PHRED, and for this reason a warm up period of 15±20 min was used to ensure that thermal equilibrium was achieved.ESI-MS-MS The Finnigan MAT TSQ 700 (San Jose, CA, USA) triple quadrupole mass spectrometer, equipped with an API I interface, was utilized in the ESI-MS and ESI-MS-MS mode. The instrument was initially tuned for ESI using a myoglobin± MRFA mixture. The electrospray needle was held at 5 kV and was operated in the positive ion mode.The heated capillary (275 �C), the nitrogen sheath gas (80 psi) and the nitrogen auxiliary gas (35) were optimized using arsenobetaine infused at 1 mL min21 in 20 mM (NH4)2CO3. The heated capillary and manifold temperatures were 275 and 100 �C, respectively. In MS experiments, the instrument was scanned from 130 to 550 u s21. The MS-MS experiments were conducted using a collision energy of 30 eV and an argon pressure of 1 mTorr. The IC-ESI-MS-MS experiment was performed as follows: during retention time windows of 0±6 min, 6±10 min and 10± 16 min, Q1 was set to pass m/z 329, 483 and 393, respectively, and Q3 was scanned at 75±331, 75±485 and 75±395 u s21, respectively.A summary of these instrumental conditions can be found in Table 1. 1830 J. Anal. At. Spectrom., 1999, 14, 1829±1834Results and discussion Edmonds and Francesconi46 characterized 2 arsenosugars extracted from brown kelp using IR and NMR in 1981. Corr and Larsen41 characterized three arsenosugars via ESI-MS-MS using direct infusion for sample introduction.These three arsenosugars are retained on the ION 12047 and PRP-X10047 anion exchange columns using typical mobile phases [(NH4)2CO3] and pHs (wpH 9). Given these chromatographic conditions, the arsenosugars can potentially co-elute with As(III), DMA and AsB producing a false positive or misidentiÆcation. Fig. 1 demonstrates this co-elution problem utilizing the ION 120 column, 40 mM (NH4)2CO3 at pH 10.5.These conditions are standard conditions used in our laboratory for arsenic speciation in seafood extracts. Fig. 1 contains two chromatograms collected using these separation conditions and direct nebulization ICP-MS detection. The Ærst chromatogram (top, broken line, y axis on right of graph) is an injection of a kelp extract which contained three arsenosugars (combined 6.1 ng injected, based on a total arsenic measurement). See Table 2 for arsenical structures.The second chromatogram (bottom, solid line, y axis on left of graph) is a standard injection of 1.0 ng of each of AsB, DMA and As(III). These two chromatograms clearly indicate the potential for misidentiÆcation of arsenosugar 482 (482 is based on molecular mass, see IC-ESI-MS section and Table 2) as DMA and arsenosugar 328 as AsB. Similar chromatograms are obtained if a PRP-X100 column is used.47 Le et al.48 have reported similar co-elution and potential misidentiÆcation effects (on anion exchange columns) in human urine produced by the ingestion of arsenosugars from seafoods.Le et al.44 utilized ICP-MS and atomic Øuorescence as detectors and relied on elution times for identiÆcation. In order to minimize the potential for misidentiÆcation, a separation using a PRP-X100 column and 20 mM (NH4)2CO3 mobile phase at pH 9.0 was developed. This produces a near baseline separation of all known anionic arsenicals in 30 min. This separation allowed the retention times obtained for the three arsenosugars using IC-ICP-MS to be used to verify the retention times of the [MzH]z species found utilizing IC-ESI-MS.The relative abundance (based on chromatographic peak areas) of the three arsenosugars in the kelp extract determined by IC-ICP-MS was 5.0% (arsenosugar 328, m/z 329), 11.0% (arsenosugar 482, m/z 483) and 84.0% (arsenosugar 392, m/z 393). This distribution was calculated using the extraction conditions in Table 1 and an extraction efÆciency of 55% (i.e., the kelp after extraction contained 45% of the available arsenic based on a total arsenic digestion). IC-ESI-MS and MS-MS of arsenosugars from ribbon kelp Electrospray ionization coupled with IC-MS and IC-MS-MS was utilized to determine the identity of the three unknown arsenic peaks (from a ribbon kelp extract) observed in Fig. 1.The chromatographic conditions were optimized (as mentioned above) to produce near baseline resolution of all arsenicals.The separation and detection conditions used in Fig. 2 and 3(a±c) are listed in Table 1. Fig. 2 is a chromatogram of the ribbon kelp extract obtained by positive ion IC-ESI-MS. The Ægure has threlapping traces of m/z 329 ([MzH]z, arsenosugar 328), m/z 483 ([MzH]z, arsenosugar 482) and m/z 393 ([MzH]z, arsenosugar 392). Except for arsenosugar 392, the signal at these masses is a minor component of the total ion current. The retention times for the three unknown peaks via IC-ESI-MS are within 0, 4 and 7% (respectively) of the retention times obtained via IC-ICP-MS.This slight shift in retention times for late eluting peaks may be produced by the relatively weak mobile phase in combination with the 1 : 10 dilution factor used in the IC-ICP-MS analysis. The net effect is that the matrix anions in the IC-ESI-MS analysis are depleting the available sites (relative to IC-ICP-MS analysis) on the anion PRP-X100, thereby producing shorter retention times. From the ESI mass spectra obtained at these retention times, it was determined that the unknown arsenic peaks had molecular weights of 328 (arsenosugar 328), 482 (arsenosugar 482), and 392 (arsenosugar 392).These molecular weights matched molecular weights published by Corr and Larsen41 for the arsenosugars. This molecular weight match in combination with the conÆrmation of the presence of an arsenic heteroatom (via IC-ICP-MS) made the three arsenosugars a logical choice for tentative identiÆcation.Collisionally activated decomposition (CAD) was performed on the [MzH]z of each unknown arsenic peak to further Table 1 Summary of experimental conditions Chromatography– PRP-X100 column Direct analysis: 20 mM (NH4)2CO3, pH 9.0 Hydride analysis: 14 mM (NH4)2HPO4±14 mM HNO3, pH 6.0 ION 120 column Direct analysis: 40 mM (NH4)2CO3, pH 10.5 Flow rate and injection volume ICP-MS and ESI-MS: 1 mL min21 and 100 mL Hydride analysis ICP-MS: 1 mL min21 and 50 mL ESI-MS/MS analysis: 1 mL min21 and 360 mL Accelerated solvent extraction– Solvent: 30z70 (w/w) MeOH±H2O Pressure: 500 psi Temperature: ambient Flush percentage: 30% Cell size: 3 mL Static time: 1 min Purge time: 120 s Hydride generation– NaBH4: 1.5% (w/w) in 0.1 M NaOH HCl: 35% (w/w) Potassium persulfate: 2% (w/w) Membrane: expanded polytetraØuoroethylene microporous tubing Electrospray MS and electrospray MS-MS– Finnigan MAT TSQ 700 Auxiliary Øow: 35 nitrogen (arbitrary units) Sheath Øow: 80 psi nitrogen Capillary temperature: 275 �C Electrospray needle potential: 5 kV Fig. 1 Potential for misidentiÆcation of arsenicals using IC-ICP-MS detection. Chromatographic conditions: ION 120 column; isocratic 40 mM (NH4)2CO3, pH 10.5, 1 mL min 21. Solid line, 1 ng of each arsenical; broken line, 6.1 ng arsenosugars based on total As measurement. Numbers in parentheses refer to molecular weights established by ESI-MS. J. Anal. At. Spectrom., 1999, 14, 1829±1834 1831conÆrm that the unknown peaks were arsenosugars.To obtain adequate sensitivity in the MS-MS mode it was necessary to inject 360 mL of an undiluted extract (22.1 ng). The MS-MS mass spectra obtained on the eluting peaks are shown in Fig. 3(a±c). The MS-MS spectra of the three unknown peaks (arsenosugar 328; arsenosugar 482; arsenosugar 392) are similar to the MS-MS spectra of the arsenosugars published by Corr and Larsen41, given the differences in the instrumentation used in the experiments.All three MS-MS spectra contain m/z 237, 195 and 97, which are fragments of the base dimethylarsinylriboside, common to all the arsenosugars. While m/z 97 could be OSO3 z in arsenosugar 392, this would not explain its presence in arsenosugar 328 or arsenosugar 482. Thus, the structure shown in Fig. 3(a±c) involving the pentose moiety common to all the sugars is one possible assignment for m/z 97 in all three arsenosugars. This assignment is in agreement with Pergantis et al.39 In the case of arsenosugar 392, some of the m/z 97 signal in the MS-MS spectrum may be due to OSO3 z.The MS-MS spectrum of arsenosugar 482 shows additional fragments at m/z 465 (loss of water), 391 and 329, which are fragments of the phosphate functional group. The MS-MS spectrum of arsenosugar 392 does not contain the daughter ions at m/z 149, 167 and 279 reported by Corr and Larsen.41 One possibility for this difference is that Corr and Larsen41 noted large phthalate contamination in their arsenosugars, which dominated the MS spectra.If this contamination was extremely severe, it is Table 2 Arsenical chemical structures Fig. 2 Electrospray IC-MS mass chromatogram of arsenicals extracted from ribbon kelp. Chromatographic conditions: PRP-X100 column, isocratic 20 mM (NH4)2CO3, pH~9.0, 1 mL min21. Broken line, m/z 329; dotted line, m/z 483; solid line, m/z 393. Fig. 3 MS-MS spectra of three arsenosugars. (a) MS-MS of As (328); (b) MS-MS of As (482); (c) MS-MS of As (392). 1832 J. Anal. At. Spectrom., 1999, 14, 1829±1834possible that the daughter ions at m/z 149, 167 and 279 observed during direct infusion were produced by dioctylphthalate (nominal MW 390) via carry over. The MS-MS spectra shown in Fig. 3(a±c) provide additional data which indicate that the unknown chromatographic peaks are arsenosugars rather than the typical arsenicals found in the environment. These IC-ESI-MS and IC-ESI-MS-MS spectra, coupled with the retention time veriÆcation with ICP-MS, give credence to the presence of the three arsenicals but without arsenosugar standards this conÆrmation is tentative. Ion chromatography membrane hydride generation ICP-MS The arsenicals commonly used to assess exposure are inorganic arsenic [As(III) and As(V)], MMA and DMA.Therefore, an arsenic exposure assessment methodology should provide interference free detection of these species and minimize the potential for false positives from other arsenicals.This added degree of detector selectivity can be achieved by using a membrane hydride ICP-MS methodology without a postcolumn photo-oxidation step. A summary of these chromatographic and hydride generation conditions can be found in Table 1. The highly substituted arsenicals (arsenobetaine, etc.) are not hydride active without photo-oxidation.44,45 This allows the highly substituted arsenicals to chromatographically co-elute with As(III), As(V), MMA or DMA without producing a false positive.This selectivity is demonstrated in Fig. 4(a±b). Fig. 4(a) contains two chromatograms collected with the hydride generation system operating with a post-column online photo-oxidation step prior to detection. The separation conditions are dramatically different from those used in Fig. 1± 3. The column is a PRP-X100 with a 14 mM (NH4)2HPO4± 14 mM HNO3, pH 6.0, mobile phase. This mobile phase is used because it does not produce a gaseous species (i.e., CO2 from an ammonium carbonate mobile phase) within the hydride generation reaction and for this reason produces a more reproducible/steady baseline.The Ærst chromatogram in Fig. 4(a) (indicated by the solid line) is an injection of 50 pg of AsB, As(III), MMA and 100 pg of DMA and As(V). The arsenobetaine peak marked AsB/STD is a post-column injection. This post-column injection of AsB indicates the on-line photo-oxidation step is functioning.Similar selectivity has been demonstrated by Le et al.44,48 using on-line microwave HGAAS. This chromatogram indicates a clean separation of the four arsenicals in approximately 9 min. The second chromatogram (indicated by a broken line) in Fig. 4(a) is the separation of the ribbon kelp extract (0.6 ng arsenosugar injected, based on a total measurement) achieved using the exact same chromatographic/hydride condition. All three arsenosugars are hydride active after being subjected to the on-line photo-oxidation process.In addition, the elution order is the same as in Fig. 1 and 2 and all three species still maintain a retention characteristic on the PRP-X100 column at a pH of 6.0. The combination of both chromatograms clearly indicates that the three arsenosugars do not produce a direct chromatographic overlap but do complicate the identiÆcation process based on retention time. Fig. 4(b) contains two chromatograms in which the on-line photo-oxidation step is not utilized. The Ærst chromatogram ndicated by a solid line) is a standard mix [100 pg As(V), DMA and 50 pg of AsB, As(III) and MMA].This clearly indicates the excellent sensitivity achievable using a hydride ICP-MS system. Note: The post-column injection of AsB does not produce a response. This indicates the photo-oxidation step is inoperable. The second chromatogram (indicated by a broken line) is a 50 mL injection of the ribbon kelp extract (12.3 ng g21 arsenosugar based on a total measurement) without photo-oxidation.The Øat baseline indicates that the arsenosugars in the ribbon kelp extract shown in Fig. 4(a) are not hydride active without some type of oxidative step prior to detection. Le et al.44 have demonstrated this via microwave digestion of arsenosugars in urine samples. The highly substituted arsenicals, e.g., AsB, arsenocholine (AsC), etc., are normally not hydride active without photo-oxidation. Arsenosugars produce non-volatile hydrides which are not detectable.Therefore, by carefully monitoring experimental conditions, highly substituted arsenicals could be co-eluted with As(III), As(V), MMA or DMA and detected without producing a false positive. These data provide additional evidence to indicate the presence of arsenosugars and that these arsenosugars [characterized by IC-ESI-MS-MS in Fig. 3(a±c)] are the only detected arsenicals in the ribbon kelp. In addition, the hydride generation mode of detection without the photooxidation step signiÆcantly simpliÆes the chromatograms while photo-oxidation provides the analyst with the Øexibility of detecting the arsenosugars for exposure source information. The above data clearly indicate the selectivity differences between IC-ESI-MS, IC-ICP-MS and IC-hydride-ICPMS.Selectivity is one aspect which is a growing requirement in arsenic speciation methodologies while instrumental sensitivity is challenged, especially in solid sample extracts.In an attempt to make a sensitivity comparison between IC-ICP-MS, IC-ESI-MS and IC-hydride-ICP-MS, a concentration for the m/z 483 peak was calculated from the relative area distribution and the total arsenic concentration in the extract. The relative area per cent of the m/z 483 peak is 11% and the total arsenic concentration in the extract is 613 ng g21, therefore the m/z 483 peak is 67 ng g21. Using 67 ng g21 of arsenic as the concentration for the m/z 482 peak and calculating a 3 sigma Fig. 4 Chromatographic separation of arsenicals using hydride generation with (a) on-line photo-oxidation. Chromatographic conditions: PRP-X100 column, isocratic 14 mM (NH4)2HPO4±14 mM HNO3, pH 6.0, 1 mL min21. Solid line, 100 pgDMAand AsV, 50 pg AsB, AsIII and MMA; broken line, 615 pg of total arsenosugars injected. Numbers in parentheses refer to molecular weights established by ESI-MS. (b) Without on-line photo-oxidation.*Dwell time is 2 s. J. Anal. At. Spectrom., 1999, 14, 1829±1834 1833detection limit based on peak height, the following detection limits were calculated. The 3 sigma detection limits were 4.5, 150 and 1.5 pg for the IC-ICP-MS, IC-ESI-MS, and IChydride- ICP-MS, respectively. It should be noted that the ICESI- MS data collection and ESI experimental parameters were not optimized for detection limits but rather were compromised to facilitate the IC separation developed for IC-ICP-MS.Conclusions The elemental selectivity of atomic spectrometric detection provides excellent sensitivities relative to IC-ESI-MS but does not provide structural information available with IC-ESI-MS and IC-ESI-MS-MS. Hydride IC-ICP-MS provides unparalleled sensitivity and increased selectivity but still does not provide a structural identiÆcation capability for unknowns. Unlike ICP-MS the total ion current chromatograms obtained from an IC-ESI-MS-MS indicate a wide variety of co-extracted organic constituents eluting from the IC.This makes the identiÆcation of species which may contain arsenic very difÆcult without the use of the elemental information obtained via IC-ICP-MS. The arsenicals are, in effect, minor constituents via IC-ESI-MS while the ICP-MS can provide the elemental information necessary to identify IC retention windows of interest for the identiÆcation of arsenosugars from the ribbon kelp extract. The IC-ESI-MS spectra clearly indicate that the eluting species have the same molecular ion as those reported in the literature while the IC-ESI-MS-MS spectra conÆrm structural similarities to those reported in the literature.Finally, the ICP-MS chromatograms indicate the presence of an arsenic heteroatom during each of the three corresponding retention windows. From this perspective and a methods development standpoint the techniques are complementary and each provide supporting structural or elemental conÆrmation.Clearly, ICP-MS elemental information is inadequate in providing the structural information needed to identify these arsenosugars; on the other hand, IC-ESI-MS and IC-ESI-MSMS as applied in this paper lack the sensitivity necessary for arsenic exposure assessment. Hydride ICP-MS does provide an added degree of selectivity, which minimizes the required chromatographic resolution while providing an analytical response for the most toxicologically relevant arsenicals.From this perspective, the IC-ESI-MS-MS structural information has allowed the arsenosugars to be identiÆed and this identiÆcation has allowed a hydride IC-ICP-MS analytical approach to be tested for false positives produced from arsenosugars. Acknowledgements The authors would like to thank Roseanne M. Lorenzana, US EPA Region 10, for her generosity in supplying us with the ribbon kelp sample utilized in this study. We would also like to recognize Douglas T. Heitkemper, US FDA, whose help through a cooperative study has been invaluable.References 1 M. M. Frey and M. A. Edwards, J. Am. Water Works Assoc., 1997, 89, 105. 2 R. W. Dabeka, A. D. McKenzie, G. M. A. Lacroix, C. Cleroux, S. Bowe, R. A. Graham, H. B. S. Conacher and P. Verdier, J. AOAC Int., 1993, 76, 14. 3 E. L. 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A. Shoemaker, J. T. Creed and J. A. Caruso, European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999, C7. 48 X. C. Le, W. R. Cullen and K. J. Reimer, Talanta, 1994, 41, 495ss. Paper 9/06249A 1834 J. Anal. At. Spectrom., 1999, 14, 1829±1834

ISSN:0267-9477    

DOI:10.1039/a906249a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

High-precision measurement of calcium isotopes in carbonates and related materials by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) Ludwik Halicz,a,b Albert Galy,a Nick S. Belshawa and R. Keith O'Nionsa aDepartment of Earth Sciences, University of Oxford, UK OX1 3PR bGeological Survey of Israel, 30 Malkhey Israel St., 95501 Jerusalem, Israel. E-mail: Ludwik@mail.gsi.gov.il Received 6th August 1999, Accepted 21st September 1999 Multi-collector ICP-MS has been used for the precise measurement of natural variations in the isotopic composition of Ca.The interference of Ar in the Ca mass region is assessed and the repeatability of the 44Ca/42Ca ratio of a sample calcium solution relative to the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 915a Calcium Carbonate (Clinical) standard is better than 0.1ù at 95% conÆdence. Variations in sample 44Ca/42Ca ratio are expressed as d44Ca units, which are deviations in parts per 103 from the same ratio in the NIST SRM 915a Ca standard.Measurements of d44Ca are presented for terrestrial and marine carbonates, which show a variation of up to 0.7ù, in agreement with previous studies by thermal ionisation mass spectrometry (TIMS). Introduction Calcium is the Æfth most abundant element in the silicate earth, with an abundance of y3%. It has six stable isotopes, 40Ca, 42Ca, 43Ca, 44Ca, 46Ca and 48Ca and is an essential element in both animal and plant tissues. Isotopic studies of Ca are relatively few and include investigation of environmental samples as well as the dietary inØuence on calcium intake by the human body.1 A study of Ca isotopes in natural samples2±5 has revealed a variation up to y4ù in the 44Ca/40Ca ratio,4 suggesting that biological fractionation of Ca isotopes takes place in the food chain.4,5 Additionally, Ca in oceanic carbonate sediments has been observed to be lighter than the seawater from which it precipitated.5 Recent studies have concluded that the Ca isotope composition of the oceans are maintained at their present value through biological fractionation.4,5 Conventional ICP-MS is a relatively insensitive technique for the measurement of calcium isotopes with a typical precision of y5ù for isotope ratios.This is a consequence of the relatively poor peak shapes achievable with quadrupole mass spectrometers and the requirement for sequential rather than simultaneous measurement of the isotopes of interest.More precise measurement of Ca isotope ratios (y0.1ù) have been made using thermal ionisation mass spectrometry (TIMS) using a double-spike to allow corrections to be made for instrumental mass fractionation.2±6 This technique, although accurate, has not been widely exploited, in part, because of the necessity to accurately calibrate spikes and to chemically purify Ca prior to isotopic analysis. The technique reported here employs a multiple collector (MC)-ICP-MS to produce highprecision measurements of Ca isotopes even in the presence of 40Arz.This technique is relatively rapid and has the additional advantage of requiring minimal sample puriÆcation. Experimental Calcium isotopes The measurement of Ca isotopes using Ar-ICP source mass spectrometers is potentially hindered by the presence of 40Arz, preventing accurate measurement of 40Caz. However, Ca has Æve other isotopes, of which three are both sufÆciently abundant and convenient for measurements to be made at high precision.These are 42Ca, 43Ca and 44Ca with abundances of y0.65%, 0.13% and 2.1%, respectively. Interference at these masses may include molecular species and doubly charged ions.7,8 In addition, 46Ca has a very low abundance, of only 0.003%, and 48Ca, although of adequate abundance (0.18%), is too far removed from 42Ca on the mass focal plane of the spectrometer for simultaneous collection of all Ca isotopes to be possible.Furthermore, possible interference from 46Ti and 48Ti also serve to make 46Ca and 48Ca less suitable than the lighter isotopes of Ca. For 42Ca, 43Ca and 44Ca to be useful any interference at these masses, which may include molecular species and doubly charged ions,7,8 must be either absent or sufÆciently small that an accurate correction can be made. In addition to the spectral interference mentioned above, there are possible scattered background contributions from the large ( y5 nA) 40Arz and 40Caz ion beams, which enter the collector array along with the masses of interest.This study describes a technique, which both minimises and corrects for, potential isobaric interference on 42Ca, 43Ca and 44Ca and other non-spectral contributions to the background. Sample preparation All samples and standards were prepared in dilute 0.1 M nitric acid solution and introduced into the Ar-plasma source through a modiÆed Cetac (Omaha, NE, USA) MCN 6000 desolvating nebuliser.This device minimises the introduction of H2O, CO2, O2 and N2 into the plasma thus reducing the abundance of interfering molecular species. Natural samples of carbonate mineral were dissolved in 3 M HCl at room temperature. The residue after dissolution was separated by centrifugation and the supernate evaporated to dryness and then re-dissolved in 2 M HNO3. Samples and standards were diluted with 0.1 M HNO3 to 20±30 ppm Ca to obtain the best counting statistics for mass spectrometric analysis.J. Anal. At. Spectrom., 1999, 14, 1835±1838 1835 This journal is # The Royal Society of Chemistry 1999Mass spectrometry Ca isotope ratios were measured using a Nu Instruments MCICP- MS (Nu Instrument Ltd, Wrexham, Wales). This instrument is a double focusing magnetic sector instrument with variable dispersion ion optics and a Æxed array of 12 Faraday collectors.9 The normal operating conditions adopted for the mass spectrometer are summarised in Table 1.The isotopes of interest, 42Ca, 43Ca and 44Ca, were positioned as indicated in Fig. 1. Since the Faraday collectors are Æxed and the instrumental mass dispersion can be varied by a factor of y2, these isotopes were collected by Faradays 4, 8 and 10, respectively, within the overall array. In this conÆguration the 40Arz and 40Caz ion beams also enter the collector and are a potential source of scattered ions and secondary electrons.It is important, in this case, to assess the contribution of scattered ions to the measured signals. Faraday collectors corresponding to m/z 41.5, 42.5, 43.25, 43.75 and 44.25 are used for this purpose, allowing corrections to be applied for elevated background levels. Mass discrimination for Ca isotopes in the MC-ICP-MS is y5% u21 and is monitored with reference to an external Ca isotope standard, using the standard±sample±standard bracketing technique. Samples and standards of similar Ca concentration (20±30 ppm) are introduced into the instrument in 0.1 M HNO3.Results and discussion The measured 44Ca/42Ca isotope ratios are expressed relative to the same ratio in the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 915a Calcium Carbonate (Clinical) Ca isotope reference standard as follows: d44CaÖùÜ~fÖ44Ca=42CaÜsample=Ö44Ca=42CaÜSRM 915a{1g |1000 Previous studies2,4,5 reported variations in 44Ca/40Ca or 40Ca/44Ca isotope ratios as d44Ca.However, the variation in 40Ca abundance by decay of 40K implies that 44Ca/40Ca isotope ratios are not only dependent on mass-dependant fractionation. For instance, the 44Ca/40Ca isotope ratios differ by up to 0.3ù between crustal rocks and mantle-derived material only because of differences in age and K/Ca ratios.10 By analogy with the notation used for all other stable isotopes we prefer to normalise to 42Ca, the lighter isotope not affected by radiogenic decay of other elements.Interferences The mass spectrometric measurement of Ca isotopes is potentially complicated by the presence of either spectral or non-spectral interference. Accurate analysis depends upon an understanding of, and allowance for, these as follows. Non-spectral interferences. The optimum situation in magnetic sector mass spectrometry is where the background signal levels are indistinguishable from the intrinsic background noise of the detector system.The region of the mass spectrum around m/z 40 in MC-ICP-MS instruments may present both spectral problems from the 40Ar isobar on 40Ca as well as non-spectral interference arising from scattering of the 40Arz and 40Caz ion beams entering the collector housing. As a result elevated levels of background may occur on all detectors, which in this case is approximately 1 mV (Fig. 2). Correction for this non-spectral contribution was made by the simultaneous monitoring of signal levels in the relevant Faraday collectors at positions either 0.25 or 0.5 u removed from the Ca masses.Thus for 44Caz the scattered background was monitored at 44.5 u, whilst for 43Caz and 42Caz the backgrounds at 43.25 and 42.5 were monitored. Interferences arising from multiply charged species at these monitor positions are not present, and similarly low signal levels are observed at 41.5, 43.25, 43.5, 43.75, 44.25 and 45 u (Fig. 1). Doubly charged ion interferences.The most likely source of doubly charged ion interference is from 88Sr2z, 86Sr2z and Table 1 Instrumental operating conditions and signal measurement parameters RF power 1400W Plasma gas Øow rate 12 l min21 Interface cones Nickel Acceleration voltage 4 kV Ion lens setting Optimised for max. intensity Instrument resolution y300 Mass analyser pressure 361029 mbar Detector 12 Faraday collectors Nebuliser Microconcentric Spray chamber temp. 75 �C Desolvator temp. 160 �C Sweep gas (argon) 3.65 l min21 (optimised daily) Sample uptake rate 70 ml min21 Typical Ca44 sensitive 0.3 V (ppm)21 Sampling time 3 repetitions of 20610 s Fig. 1 Typical background levels from spectral interferences and scattered contribution from 40Arz observed during nebulisation of 0.1M HNO3. The collector conÆguration is that used for static measurement of 42Ca, 43Ca and 44Ca described in the text. The precision of background measurements is 3±4% RSD. Fig. 2 Spider diagram of background signal in the mass range 41.5± 45.5 u during nebulisation of 0.1 M HNO3 (solid line) and a 30 ppm solution of calcium in 0.1M HNO3 (dashed line).The scattering of 40Arz and 40Caz are observed to be very similar. 1836 J. Anal. At. Spectrom., 1999, 14, 1835±183884Sr2z on 44Caz, 43Caz and 42Caz, respectively. These interferences cannot be mass resolved whilst maintaining high ion-transmission using the current generation of small geometry MC-ICP-MS instruments. 87Sr2z, however, may be monitored at 43.5 u. After background correction at 43.5 u, based upon interpolation of the backgrounds at 43.75 and 43.25 u, a correction for Sr2z is applied to the measured 44Caz, 43Caz and 42Caz, if necessary. Under the conditions adopted for Ca isotope analysis, Sr2z/Srz varies between 0.02 and 0.05 and for natural carbonates the 87Sr/86Sr ratio will be in the range 0.706±0.720.11 For a maximum Sr2z/Caz ratio of 0.05, this range of 87Sr/86Sr ratios would introduce an uncertainty in d44Ca of less than 0.5ù.In natural carbonates with less than 2000 ppm of Sr, the uncertainty arising from the Sr correction is below the precision of the d44Ca measurement. If required, separation of Ca from Sr is easily achieved by liquid chromatography. In some samples, particularly silicates, 87Rb2z may also be present at 43.5 u, but this is not usually a problem with carbonates. Molecular interferences. Potential molecular interferences in the Ca mass region include 14N2 16Oz and 12C16O2 z on 44Caz and 40ArH2 z on 42Caz.7,8 The use of a desolvating nebuliser acts to reduce O, N, C and H interference to an insigniÆcant level (Fig. 1). The measured interferences on Ca isotopes, after correcting for non-spectral contributions are less than 0.2 mV at 42, 43 and 44 u. Repeatability of standard Ca isotope ratios The Nu Instruments MC-ICP-MS produces Ca peaks with Øat tops at a working mass resolution of y300, as required for high-precision Ca isotope ratio measurement. The Ca isotopes of interest are positioned in the multiple collector for simultaneous measurement as shown in Fig. 3. The optimum Ca concentration for the standard solution is 15±30 ppm, given the sensitivity for 44Ca of y0.3 V (ppm)21. Higher concentrations of Ca were found to cause a reduction in instrument sensitivity due to material deposition on the sample and skimmer cones. Ca isotopes of samples or standards analysed by MC-ICPMS are not corrected for instrumental mass discrimination, either with internal or external standards.The standard± sample±standard bracketing technique has been adopted here to examine repeatability of measurement, as this permits a correction to be made for instrumental drift. Individual measurements of sample and standard isotope ratios comprised 20 measurements of 10 s integration with simultaneous measurement of 42Ca, 43Ca and 44Ca and 87Sr2z at 43.5 u as described above.The stability of uncorrected of 44Ca/42Ca and Fig. 3 Peak shapes and coincidences of calcium isotopes 44Ca, 43Ca, 42Ca and strontium double charge 87Sr (m/z~43.5) with the variable mass dispersion optics arranged for measurement of isotopes in the mass region of the mass spectrum. Fig. 4 Evolution of the measured, uncorrected calcium isotope ratio (44Ca/42Ca) of standard and samples through time. $, NIST SRM 915a; , CaCO3 from Aldrich; %, CaCO3 from Alfa; , specpure CaCO3 from Merck; © speleotherm 2-8-E3.Table 2 Ca isotopic composition for selected carbonate and commercial reagents (N~number of replicates during a 2 month period) Sample d44Ca °1s N Commercial Ca reagent– SRM 915aa CaCO3 0.01 0.08 9 Specpureb CaCO3 0.34 0.10 3 Alfac CaCO3 0.54 0.05 4 Aldrichd Ca solution 0.54 0.11 2 Merckb CaCO3 0.61 0.14 4 Continental environment– 2-8-E3 Speleotherm Israel 0.25 0.16 3 2-8-G Speleotherm Israel 0.17 0.19 4 2-8-J Speleotherm Israel 0.04 0.08 3 SA 310 Calcrete Israel 0.44 0.11 4 SA 495 Calcrete Israel 0.49 0.20 5 Marine environment– Chalk Jurassic France 0.65 0.14 2 Pocillopora Coral Red Sea 0.60 0.02 3 Acropora Coral Red Sea 0.58 0.08 2 Metamorphic rocks– Carrara Marble Italy 0.75 0.05 3 Spar Calcite Israel 0.58 0.03 3 aNIST, Gaithersburg, MD, USA.bMerck, Darmstadt, Germany. cJohnson Matthey, Karlsruhe, Germany. dMilwaukee, WI, USA. J. Anal. At. Spectrom., 1999, 14, 1835±1838 183743Ca/42Ca ratios during extended runs of up to 5 h was 0.1 and 0.15ù h21, respectively (Fig. 4). The corresponding typical 2s precision after correction for background and interference was 0.1ù. The precision of a measured sample using the sample± standard bracketing technique was observed to be 0.1ù, assessed from the external repeatability measured over a period of 2 months (Table 2). The measured isotope ratios were found to be unaffected by the presence of magnesium in concentrations up to twice that of calcium.Several carbonate samples from continental, marine and metamorphic sources were analysed (Table 2). These show a range of 0.7ù on d44Ca. Conclusion The performance of the MC-ICP-MS for Ca isotope ratio measurement is evaluated and shown to equal the precision and accuracy of the best results obtained by TIMS. Elevated background levels resulting from ion scattering of 40Arz and 40Caz are corrected for during the analysis and 87Sr2z is monitored to allow corrections for Sr interference.The throughput of this proposed method is about 12±15 samples per day compared to 2±3 for TIMS. The range in isotopic composition of pure calcium obtained commercially (y0.7ù) conÆrms previous studies2 and suggests the use of a homogenised and well-distributed standard, such as NIST SRM 915a, for normalisation of stable isotopic composition of Ca. The preliminary results for several carbonate samples indicate a different isotopic composition of Ca in marine and terrestrial environments.It is necessary to investigate the systematics of the biological and geochemical control of calcium isotope rs in the global calcium cycle. Acknowledgements This research has been supported by a grant from the Natural Environment Research Council. AG has been supported by the EC through the TMR ``Marine Record of Continental Tectonics and Erosion'' No. ERBFMXCT 960046. References 1 R. I. Price, G. N. Kent, K. J. B. Rosman, D. H. Gutteridge, J. Reeve, J. P. Allen, B. G. A. Stuckey, M. Smith, G. GuelÆ, C. J. Hickling and S. L. Blakeman, Biomed. Environ. Mass Spectrom., 1990, 19, 353. 2 W. A. Russell, D. A. Papanastassiou and T. A. Tombrello, Geochim. Cosmochim. Acta, 1978, 42, 1075. 3 I. Platzner and N. Degani, Biomed. Environ. Mass Spectrom., 1990, 19, 822. 4 J. Skulan, D. J. DePaolo and T. L. Owens, Geochim. Cosmochim. Acta, 1997, 61, 2505. 5 P. Zhu and J. D. Macdougall, Geochim. Cosmochim. Acta, 1998, 62, 1691. 6 M. L. Coleman, Earth Planet. Sci. Lett., 1971, 12, 399. 7 S. Sturup, M. Hansen and C. Molgaard, J. Anal. At. Spectrom., 1997, 12, 919. 8 N. M. Reed, R. O. Cairns, R. C. Hutton and Y. Takaku, J. Anal. At. Spectrom., 1994, 9, 881. 9 N. S. Belshaw, P. A. Freedman, R. K. O'Nions, M. Frank and Y. Guo, Int. J. Mass Spectrom., 1998, 181, 51. 10 B. D. Marshall and D. J. DePaulo, Geochim. Cosmochim. Acta, 1989, 53, 917. 11 W. H. Burke, R. E. Denison, E. A. Hetherington, R. B. Keopnick, H. F. Nelson and J. B. Otto, Geology, 1982, 10, 516. Paper 9/06422B 1838 J. Anal. At. Spectrom., 1999, 14, 1835±1838

ISSN:0267-9477    

DOI:10.1039/a906422b

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Miniaturisation of a matrix separation/preconcentration procedure for inductively coupled plasma mass spectrometry using 8- hydroxyquinoline immobilised on a microporous silica frit Simon D. Lofthouse,a Gillian M. Greenwaya and Sharon C. Stephenb aUniversity of Hull, Cottingham Road, Hull, N. Humberside, UK HU6 7RX bAvecia Ltd, Blackley, Manchester, UK M9 8ZS Received 9th September 1999, Accepted 11th October 1999 A comparison has been performed on miniaturised matrix separation/preconcentration procedures using packed micro-columns of imminodiacetate (IDA) chelating reagents and an 8-hydroxyquinoline (8-HQ) micro-column. Commercially available IDA reagents, Prosep and Muromac, were packed into micro-columns and 8-HQ has been immobilised on a novel microporous silica structure. These have been successfully utilised for the determination of several trace elements in complex matrix samples.The miniaturised matrix separation/ preconcentration procedures have been developed to reduce sample analysis time. A microconcentric nebuliser in the ICP-MS permits the multielement analysis on a smaller volume of solution leading to a reduction in reagent consumption and a more efÆcient procedure.Preparation of the micro-columns is described along with optimisation of the procedures with respect to the variables buffer concentration, buffer pH, eluent acid concentration and reagent Øow rates. Sample analysis times are compared for both miniaturised systems.Analysis times of 3.0 min for the IDA column and 2.3 min for the 8-HQ column are reported. Calibrations showed good linearity with correlation coefÆcients of 0.999±0.9998 for IDA columns and 0.999±0.9997 for 8-HQ column for a range of analytes. Recoveries ranging from 91±102% for IDA columns and 96±105% for the 8-HQ column are reported for a range of elements. The method was validated by the analysis of estuarine (SLEW-1) and coastal (CASS-2) certiÆed reference materials.Good agreement between the certiÆed and reference values was obtained for the materials. Introduction ICP-MS has evolved into one of the most accurate, sensitive and reliable trace element measurement techniques. The determination of trace and ultratrace elements in a wide variety of matrices is now common practice. Nevertheless, early work with ICP-MS identiÆed major disadvantages including a low tolerance to dissolved solids (v0.2% m/v) and the formation of polyatomic interferences.1,2 High levels of dissolved solids lead to blockage of the interface sampling oriÆce and/or injector tube of the torch.Matrix elements can also combine with the plasma gas or solvent components leading to interferences degrading quality of analysis. Alongside these, easily ionisable matrix elements can cause suppression effects in the plasma because of an increase in electron density. Considerable effort has been invested in the alleviation of these problems using on-line matrix separation and preconcentration techniques.3,4 On-line methods can themselves introduce other problems, mainly that the matrix separation/ preconcentration step takes between Æve and ten minutes.During this stage, the ICP-MS is idle and waiting for the sample to reach the plasma, thus wasting expensive resources. Although in general automated processes are preferred, often an off-line batch preparation procedure may be more cost effective. If the matrix separation/preconcentration can be shortened then the on-line technique could become more efÆcient.Miniaturisation can help speed up the system, but problems of poor recoveries and low volumes of eluent need to be overcome. A microconcentric nebuliser has been reported5,6 which provides stable sample introduction into an ICP at Øow rates °30 ml min21. This allows multielement determinations on a ml fraction of a sample. If such a nebuliser is employed in the online ICP-MS system, the sample size can be reduced and consequently the matrix separation/preconcentration step can be miniaturised leading to a reduction in analysis time.Nelms et al.7 successfully utilised a preconcentration column containing 0.04 g of a commercially available reagent in which imminodiacetate (IDA) is immobilised on controlled pore glass (Prosep) in a Øow injection manifold. Our communication8 recently described the preparation and evaluation of four columns prepared containing 0.0045±0.025 g of chelating agent.The small matrix separation/preconcentration column enabled the reduction in time of the sample preparation procedure from approximately 5 to 3 min with no loss of accuracy or precision. This was possible because the microconcentric nebuliser in the ICP-MS system permits the analysis of a smaller volume of solution. However, problems remain with the efÆcient packing of the mini-columns for the miniaturised Øow injection systems.It is well documented that 8-hydroxyquinoline (8-HQ) when covalently bonded on a support can be used to chelate a large number of metals under deÆned pH conditions. Supports used for this process include controlled pore glass,9 polymers,10 and silica.11 These materials were chosen such that there is no change in volume with changing pH or sample composition. The chelating surface is very reactive and is conditioned rapidly between samples, allowing a faster sample throughput.Sturgeon et al.12 immobilised 8-HQ onto silica gel for the preconcentration of a range of metals from seawater prior to determination by GF-AAS. The procedure provided a simple, rapid and reliable technique for the separation of the elements from sea-water. However, the batch capacity of the material was low but considered adequate for trace element determination. The capacity value reported for Cu was 0.061 mmol g21. Shan and co-workers13 described a novel support for immobilisation of 8-HQ to preconcentrate rare earth elements (REE) in sea-water prior to ICP-MS detection.The poly- J. Anal. At. Spectrom., 1999, 14, 1839±1842 1839 This journal is # The Royal Society of Chemistry 1999acrylonitrile (PAN) hollow Æber membrane immobilised with 8-HQ offers a relatively short preparation time and preconcentration of REE over a wide pH range. The Æbre was packed into a 6066 mm column for the preconcentration procedure. Samples are not buffered before analysis, reducing contamination, though extreme sample pHs could cause recovery problems from the column.The capacity of the 8-HQ PAN Æber membrane was determined to be 0.0715 mmol g21. Excellent recoveries were presented and agreement with previously analysed water samples. However, there is no mention of the time taken for the preconcentration or analysis of a sample. Recently a new method for the production of microporous silica structures has been reported14 for use in micro-reactor technology and it is suggested that this would provide an ideal support for 8-hydroxyquinoline in micro columns. This paper compares two rapid miniaturised matrix separation systems for ICP-MS with a microconcentric nebuliser, using packed IDA micro-columns and a novel micro-column immobilised with 8-HQ, for the analysis of trace elements in high salt matrices.Results regarding analysis time and effectiveness of the matrix separation procedures are reported, together with validation of the processes using certiÆed reference materials.Experimental Instrumentation The ICP-MS instrument used was a VG Elemental Plasma- Quad IIz (VG Elemental, Winsford, Cheshire, UK). Table 1 shows the operating conditions and measurement parameters for the ICP-MS instrument. The microconcentric nebuliser was an MCN-100 M2 (Cetac Technologies, Omaha, NE, USA) which was supplied with an end cap that Æts directly onto the Scott double pass glass spray chamber.The sample was fed using special low Øow capillary tubing supplied with the nebuliser. The polyamide nebuliser capillary was 150 mm in diameter with a dead volume of 2 ml. IDA packed micro-columns8 were made from glass capillaries, 0.2 mm in diameter, of varying lengths depending on the amount of packing material required. This compares to previous work13 where IDA columns were at least 3 mm in diameter and 2.5 cm in length. Microporous silica structures (frits) were prepared in glass capillaries 0.5 mm in diameter and 5 mm in length.Reagents Column packing materials were imminodiacetate immobilised on controlled pore glass (PROSEP Chelating-1 Bioprocessing, Consett, Co Durham, UK) and imminodiacetate immobilised on a cross linked polymer resin, (Muromac A-1 50±100 mm mesh, Muromachi Chemicals, Tokyo, Japan). Varying amounts of chelating material were slurry packed into glass capillary columns. Cleaned quartz wool was used for the end frits.The reagents 8-hydroxyquinoline, 3-aminopropyltriethoxysilane and p-nitrobenzoyl chloride (Sigma, Poole, Dorset, UK), sodium dithionite, sodium nitrite (Fisher ScientiÆc, Loughborough, Leicestershire, UK) and concentrated hydrochloric acid (Aristar, Merck Ltd., Poole, Dorset, UK) were used in the immobilisation procedure. Ammonium acetate buffer was prepared from the solid and puriÆed by passing through a column of Chelex-100. Microporous frits were fabricated from 10% formamide (Avocado Research Chemicals Ltd., Heysham, Lancashire, UK) and potassium silicate (21% SiO2, 9% K2O, Prolabo, Manchester, UK).National Research Council of Canada certiÆed reference samples (Ottawa, Canada) CASS-2 and SLEW-1 were used as received. High purity deionised water (18 MV cm resistivity, Elgastat UHQ PS, Elga, High Wycombe, UK) and super purity nitric acid (Romil, Cambridge, UK) were used throughout. Elemental stock solutions (1000 mg ml21, SpectrosoL, Merck) were used in the preparation of calibration and spiked solutions.Microporous frit preparation The fabrication of the frit for immobilisation was taken from a recent communication by Christensen et al.14 Formamide (18 ml) was mixed for 30 s with 140 ml of potassium silicate. The resulting solution was positioned in a glass capillary using a slow speed peristaltic pump (Minipuls 3, Gilson, Villiers-le-Bel, France). The frit was then placed in an oven for 1 h at 60 �C. Finally, an extra washing step, with dilute sodium hydroxide, was included to remove excess reagents.Immobilisation procedure The procedure for immobilisation of 8-HQ was taken from Nelms et al.15 This procedure for the immobilisation of 8-HQ onto controlled pore glass needed to be modiÆed because our micro-column frit was already prepared so the reagents were pumped through the frit using a peristaltic pump. Firstly, hot 10% v/v nitric acid was pumped through the frit to prepare the microporous silica structure. The frit was then dried in an oven at 80 �C and subsequently silanised by reaction with 20% v/v 3- aminopropyltriethoxysilane in anhydrous toluene for 30 min at room temperature.Again the frit was oven dried and reacted with 20% m/v p-nitrobenzoylchloride in chloroform for 24 h at room temperature. The frit was oven dried at 50 �C and further treated with a 20% boiling solution of sodium dithionite for 60 min, reducing the nitro group to the amine.After being oven dried at 50 �C, hydrochloric acid (2 mol l21) was pumped into the frit and reacted with sodium nitrite (2% m/v in water, slow pumping) at 0 �C to yield the diazonium salt. The Ænal stage was to pump 8-hydroxyquinoline (4% m/v in absolute ethanol) through the frit. A deep red colour in the frit indicated that immobilisation was successful and the diazo compound had been formed. The frit was Ænally washed with hydrochloric acid (2 mol l21) and water and stored in a desiccator until use.Determination of exchange capacity The capacity of the immobilised 8-HQ frit was determined for Mn by both a batch and a dynamic method using ICP-OES detection. Batch determination. The 8-HQ frit was removed from the glass capillary that it was formed in, powdered and weighed. This step was to ensure maximum interaction with the solution to obtain an absolute capacity value. A 0.05 g portion of immobilised 8-HQ frit was added to a solution of Mn (20 mg ml21) prepared in ammonium acetate buffer (0.05 mol l21). The solution was left to equilibrate overnight Table 1 Instrumental conditions and measurement parameters for the VG PlasmaQuad IIz ICP mass spectrometer Rf forward power/W 1350 ReØected power/W 0 Coolant gas Øow rate/l min21 14 Auxiliary gas Øow rate/l min21 1.2 Nebuliser gas Øow rate/l min21 0.880 Spray chamber Glass, water cooled at 4 �C Data acquisition mode Peak jumping Points per peak 3 Dwell time/ms 10.24 Detector mode Pulse counting 1840 J.Anal. At. Spectrom., 1999, 14, 1839±1842with stirring and the concentration of the resulting Mn solution was compared with the initial concentration. The decrease in the concentration of the solution was then used to calculate the capacity.15 Dynamic determination. Aliquots (1000 ml) of increasing concentration of Mn standard solution were injected onto the 8-HQ frit and eluted with 250 ml of 2 mol l21 HNO3 into the ICP-OES instrument.Three repeat analyses were made at each concentration and the results were evaluated in terms of peak height. The concentration beyond which no further increase in the emission intensity of the eluted peak was obtained was deemed to represent the dynamic capacity. This value was then used to calculate the dynamic capacity for the 8-HQ frit.15 Matrix separation procedure Aliquots of 100 ml of sample were injected into the buffer stream (1.5 mol l21 ammonium acetate) and loaded onto the column.The column was then Øushed with 2 mol l21 nitric acid. The Ærst 50 ml of acid stream was collected and measured by ICP-MS. The column was washed with nitric acid then with buffer before the next sample was loaded. Results and discussion Micro-columns were prepared containing IDA chelating reagents. These columns were slurry packed using a very slow pump speed to enable more efÆcient packing. The Ærst column contained 0.005 g Prosep and the second 0.0045 g Muromac.Although the potential of micro-columns packed with IDA chelating material has been shown in our previous communication,8 it is often the case that the reduction in analysis time created by the miniaturisation is outweighed by the difÆculty in efÆciently packing these columns. Frits were Ærst assigned for end stops for columns and to hold materials in place in micro technologies.12 Having looked at its structure it was considered an ideal support for a chelating reagent that could easily be used in a miniaturised system.The immobilisation procedure for immobilising 8-HQ onto controlled pore glass (cpg) was taken and applied to the immobilisation onto the frit structure. Initially no immobilisation was observed onto the frit and it was concluded that the porosity of the frit was too low. Ways of improving the porosity of the frit were looked at and it was found that by making the frits with 50% potassium silicate solution (diluted 1z1 with water), the porosity of the frits was increased to around 80%. The immobilisation procedure was then reapplied to the new frit.After the Ænal step it was clear that the immobilisation was more effective though not along the whole length of the frit. The deep red colour indicative of successful 8-HQ immobilisation appeared only at one end of the frit and a few tracks along its length. Due to the difference between cpg and the frit structure it was decided to change the immobilisation procedure.Owing to the fact that the reagents were being pumped through the frit, the time dependent stages of the procedure were increased. By increasing these time intervals, the interaction time between reagents and frit would also increase. Alongside this we decided to increase the concentration of the reagents as we thought that there may be more immobilisation sites available in the frit than the cpg. By increasing the time and concentration of the reagents immobilisation would be more successful. Once the procedure had been optimised successful immobilisation of 8- HQ along the whole frit was achieved.The immobilisation procedure is described in the experimental section of this paper. Capacity valuDA reagents has been well documented over the years.7,16 Capacity values in the order of 0.14 mmol g21 for Prosep and 0.22 mmol g21 for Muromac have been reported for Mn. Capacity of the immobilised 8-HQ frit was evaluated for Mn.A batch capacity value of 0.136°0.010 mmol g21 and a dynamic capacity value of 0.102°0.004 mmol g21 were measured for Mn. These capacity values are consistent with earlier reported values for similar 8- HQ immobilised materials. Nelms et al.15 reported a batch capacity value of 0.086 mmol g21 for 8-HQ immobilised on cpg. Sturgeon et al.12 and Marshall and Mottola's work17 showed values of 0.061 mmol g21 and 0.185 mmol g21 respectively for Cu on silica gel immobilised 8-HQ.Finally, Shan and co-workers13 presented a capacity value of 0.0715 mmol g21 for Cu PAN Æber membranes. The capacities with respect to other elements for the 8-HQ frit have not been investigated because the recovery results show that the capacity of the material is sufÆcient for a wide range of elements. The matrix separation procedures were optimised with respect to ammonium acetate buffer concentration, pH and acid eluent concentration for both the IDA micro-columns and the 8-HQ frit.The proÆles of the optimisation curves did not differ greatly from previous work15 where 8-HQ is immobilised on cpg for preconcentration prior to analysis by ICP-MS. Fig. 1 shows the variation in cps for Mn on changing the pH of the ammonium acetate buffer for the three miniaturised columns.A pH of 6 was chosen for the analysis. Decreased pH values gave reduced elemental recoveries and higher pH values could lead to the retention of matrix elements. A compromise set of conditions were selected for the miniaturised procedures: 1.5 mol l21 ammonium acetate buffer at a pH of 6.0 and an acid eluent concentration of 2 mol l21.The matrix separation Øow rate was investigated for each micro-column in order to reduce the analysis time. An optimum of 1 ml min21 was possible for both the IDA micro-columns resulting in a sample analysis time of 3 min including column reconditioning. The optimum Øow rate for the frit immobilised 8-HQ was 1.5 ml min21 resulting in an analysis time of 2.3 min.Increasing the Øow rate, hence reducing the analysis time further, led to decreased recoveries due to incomplete chelation and elution. Column washing and reconditioning periods contribute signiÆcantly to the total sample analysis time for the procedure and experiments were carried out to reduce this. It was found that by reducing this time to less than 60 s for the micro IDA columns produced unacceptable recoveries from spiked solutions.This was probably due to a sample to sample carry over effect and improper reconditioning of the column. However, a reconditioning time of 45 s for the 8-HQ frit still gave acceptable recoveries from a spiked solution. This is probably due to the quicker chelating recovery time when using immobilised 8-HQ. The total sample analysis time for the 8- HQ frit was 2.3 min, fractionally less than the micro IDA columns because of the more efÆciently packed column and the reduced reconditioning time. Once the procedures were optimised and the sample analysis times reduced signiÆcantly to 3 and 2.3 min for the different micro-columns, recovery experiments, matrix separation effectiveness and the analysis of certiÆed reference materials were studied.Recovery experiments were performed on a 5 ng ml21 solution containing the elements V, Cr, Mn, Co, Ni, Cu, Zn, Fig. 1 Effect of ammonium acetate buffer pH on Mn signal for miniaturised procedures.J. Anal. At. Spectrom., 1999, 14, 1839±1842 1841Cd, and Pb. Recoveries in the range 96 to 105% were observed for the 8-HQ frit. Recoveries between 71±101% and 91±102% were obtained for the Prosep and the Muromac micro-columns respectively. To determine the effect of repeated use on the columns, fresh columns were prepared and compared against the aged columns. There were no signiÆcant differences in the recoveries from a 5 ng ml21 solution after the columns had been in use for at least 100 h.To establish the efÆciency of the matrix separation procedure the 63Cu : 65Cu isotope ratios were determined. Procedures have been described7,13 for evaluating the inØuence of residual matrix on the ICP-MS results, using the 63Cu : 65Cu isotope ratio. If Na is present in the plasma the 63Cu : 65Cu is anomalously high due to the polyatomic overlap of 40Ar23Na on 63Cu or low due to the possible interference of 33S16O2 z on 65Cu. Table 2 shows that the matrix separation procedure is successful for all three micro-columns as the measured isotope ratio for 63Cu to 65Cu are close to the expected natural ratio.This can be difÆcult to achieve with the direct analysis of salt waters by ICP-MS. The IDA micro-columns and the immobilised 8-HQ frit column were used for the analysis of two reference materials, CASS-2 and SLEW-1. Six standards across the concentration range 0±15 ng ml21 (Mn), 0±1 ng ml21 (Co, Ni, and Cd) and 0±5 ng ml21 (Cu and Zn) were used.Table 3 details the correlation coefÆcient of the standards and the limit of detection calculated as three times the standard deviation of the blank plus the blank for the 8-HQ frit. Similar data were obtained for the other two IDA micro-columns. The elemental concentrations for the reference materials CASS-2 and SLEW- 1 are displayed in Table 4 for the 8-HQ frit and Table 5 for the Muromac micro-column. There is good agreement between the measured and certiÆed values.Conclusion A new miniaturised method incorporating the chelating agent 8-HQ has successfully been utilised to preconcentrate a suite of metals and separation of matrix components prior to analysis by ICP-MS. The frit structure provided an ideal support for 8-HQ avoiding the problems of efÆciently packing microcolumns. The 8-HQ frit column has been compared to miniaturised IDA based columns for the preconcentration of a range of metals in sea-water. The IDA micro-columns enabled the reduction in time of the sample preparation procedure from approximately 5 to 3 min with no loss of accuracy or precision.With the 8-HQ frit incorporated in the miniaturised system the sample analysis time is reduced further to 2.3 min. These results are possible because the microconcentric nebuliser in the ICPMS system permits the analysis of a smaller volume of solution. Further work will use the miniaturised 8-HQ frit system with a commercial automated sample preparation system so that when one sample is being analysed, the column is being washed and the next sample is loading.This would further reduce the sample preparation time giving a sample analysis rate of at least 30 samples h21. Such an automated system will reduce the time of sample preparation enabling efÆcient and cost effective analysis of multiple samples. Acknowledgements SDL thanks Avecia Ltd and EPSRC for funding this research. References 1 D. Beachemin, TrAC, Trends Anal.Chem. (Pers. Ed.), 1991, 10, 71. 2 J. W. McLaren, At. Spectrosc., 1993, 14, 191. 3 M. J. Bloxham, S. J. Hill and P. J. Worsfold, J. Anal. At. Spectrom., 1994, 9, 935. 4 D. Beauchemin and S. S. Berman, Anal. Chem., 1989, 61, 1857. 5 F. Vanhaecke, M. Van Holderbeke, L. Moens and R. Dams, J. Anal. At. Spectrom., 1996, 11, 543. 6 S. D. Lofthouse, G. M. Greenway and S. C. Stephen, J. Anal. At. Spectrom., 1997, 12, 1373. 7 S. M. Nelms, G. M. Greenway and D. Koller, J.Anal. At. Spectrom., 1996, 11, 907. 8 S. D. Lofthouse, G. M. Greenway and S. C. Stephen, Anal. Commun., 1998, 35, 177. 9 D. Beauchemin and S. S. Berman, Anal. Chem., 1989, 61, 1857. 10 J. A. Resing and M. Mottl, J. Anal. Chem., 1992, 64, 2682. 11 B. K. Daih and H. Huang, Anal. Chim. Acta, 1992, 258, 245. 12 R. E. Sturgeon, S. S. Berman, S. N. Willie and J. A. H. Desaulniers, Anal. Chem., 1981, 53, 2337. 13 B. Wen, X. Shan and S. Xu, Analyst, 1999, 124, 621. 14 P. D. Christensen, S.W.P. Johnson, T. McCreedy, V. Skelton and N. G. Wilson, Anal. Commun., 1998, 35, 341. 15 S. M. Nelms, G. M. Greenway and R. C. Hutton, J. Anal. At. Spectrom., 1995, 10, 929. 16 Y. Sung, Z. Liu and S. Huang, Spectrochim. Acta, Part B, 1997, 52, 755. 17 M. A. Marshall and H. A. Mottola, Anal. Chem., 1985, 57, 729. Paper 9/07308F Table 2 Isotope ratio measurements from the three micro-columns. Values quoted with range °2s (n~5), except natural ratio Isotope ratio 63Cu : 65Cu Prosep mico-column Muromac micro-column 8-HQ frit Natural ratio 2.24 2.24 2.24 Spiked pure water 2.28°0.05 2.20°0.05 2.26°0.05 Spiked sea water 2.26°0.05 2.29°0.05 2.28°0.05 Table 3 Calibration results for the 8-HQ frit Mn Co Cu Zn Cd RSD at 5 ng ml21 (%, n~5) 2.8 3.2 2.5 4.1 3.8 Correlation coefÆcient, r 0.9990 0.9992 0.9995 0.9990 0.9997 LOD/ng ml21 0.15 0.01 0.04 0.20 0.008 Table 4 Analysis results for reference material CASS-2 and SLEW-1 for 8-HQ frit. Concentrations in ng ml21. Uncertainty expressed as 2s of the instrument response to each analyte (95% conÆdence limit, n~3) CASS-2 SLEW-1 Element Measured CertiÆed Measured CertiÆed Mn 1.95°0.10 1.99°0.15 12.0°0.9 13.1°0.8 Co 0.024°0.004 0.025°0.006 0.044°0.005 0.046°0.007 Ni 0.301°0.009 0.298°0.036 0.752°0.070 0.743°0.078 Cu 0.670°0.090 0.675°0.039 1.79°0.15 1.76°0.09 Zn 2.00°0.10 1.97°0.12 0.90°0.10 0.86°0.15 Cd 0.022°0.004 0.019°0.004 0.019°0.002 0.018°0.003 Table 5 Analysis results for reference material CASS-2 and SLEW-1 for Muromac micro-column. Concentrations in ng ml21. Uncertainty expressed as 2s of the instrument response to each analyte (95% conÆdence limit, n~3) CASS-2 SLEW-1 Element Measured CertiÆed Measured CertiÆed Mn 1.90°0.10 1.99°0.15 10.5°0.9 13.1°0.8 Co 0.028°0.004 0.025°0.006 0.050°0.005 0.046°0.007 Ni 0.277°0.009 0.298°0.036 0.740°0.070 0.743°0.078 Cu 0.699°0.090 0.675°0.039 1.81°0.15 1.76°0.09 Zn 1.90°0.10 1.97°0.12 0.79°0.10 0.86°0.15 Cd 0.020°0.004 0.019°0.004 0.020°0.002 0.018°0.003 1842 J. Anal. At. Spectrom., 1999, 14, 1839±1842

ISSN:0267-9477    

DOI:10.1039/a907308f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of cerium, neodymium and samarium in biological materials at low levels by isotope dilution inductively coupled plasma mass spectrometry Bing Li,* Yali Sun and Ming Yin Institute of Rock and Mineral Analysis, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China. E-mail: irma@mail.sparkice.cn Received 2nd July 1999, Accepted 24th September 1999 A method was developed for the simultaneous determination of Ce, Nd and Sm in biological materials at subor low ng g21 levels by isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS).The monitored isotopic pairs for sample analysis were 140Ce/142Ce, 143Nd/146Nd and 147Sm/149Sm. The isobaric interference of 142Nd on 142Ce was corrected by measuring 143Nd and calculating the percentage contribution of 142Nd to the 142Ce analyte peak. The total mass bias was determined experimentally and corrected by use of a standard solution of natural abundances. The overall performance of the procedure was checked by analysing standard solutions of natural Ce, Nd and Sm.The recoveries of Ce, Nd and Sm at the 10 ng level were 99.7, 103 and 102% with precisions of 1.4, 1.5 and 2.6% RSD, respectively. The validity of the analytical procedure was further examined by analysing a certiÆed reference material (CRM) (Bush Leaves, GBW 07603, China). The results were in good agreement with the certiÆed values of the CRMs (all results fell within the speciÆed uncertainties), with RSDs of 6.3±8.5%.The method detection limits for Ce, Nd and Sm were 0.55, 0.17 and 0.10 ng g21, respectively. The method was used to determine Ce, Nd and Sm in two Chinese CRMs of Human Hair (GBW 09101) and Wheat Flour (GBW 08503). The results obtained by ID-ICP-MS agreed well with those obtained by external calibration ICP-MS from four laboratories; the deviations of Ce, Nd and Sm for Human Hair were 0.5, 3.4 and 1.9%. and for Wheat Flour 1.4, 4.7 and 10.1%, respectively. A t-test indicated that the results determined by ID-ICP-MS showed no signiÆcant difference from those obtained by external calibration ICP-MS (Pw0.05), except for Sm in Wheat Flour (Pv0.05).The results were also compared with those of an NAA method. The deviations of Ce, Nd and Sm for Human Hair were 2.1, 19.7 and 4.1%, and for Wheat Flour 3.7, 14.0 and 18.0%, respectively. A t-test indicated that the results for Ce and Sm in Human Hair and Ce in Wheat Flour showed no signiÆcant difference between ID-ICP-MS and NAA (Pw0.05); however, for Nd in both samples and Sm in Wheat Flour there were signiÆcant differences between ID-ICP-MS and NAA (Pv0.05).A z-score assessment program was carried out following similar procedures to those used in the International ProÆciency Test for Analytical Geochemistry Laboratories. The results indicated that all zscore results for Ce, Nd and Sm for both Human Hair and Wheat Flour samples were in the range 22vzv2.Hence, the analytical results of the present work were considered to be satisfactory. In recent years the monitoring and evaluation of rare earth elements (REEs) in some biological materials has received increasing attention, from both a nutritional and a toxicological point of view, owing to the use of REEs in agrotechniques in China. The study of REE application in the agricultural and biological Æelds has achieved signiÆcant progress in recent years.1 The optimum concentration of some light REEs can promote the growth of plants signiÆcantly.2 Information about REE distribution in biological samples is basic to an understanding of their physiology and is useful in agronomy, as for instance in the design of fertilizer operations, as well as environmental pollution assessments or in studies of biochemical processes. The quality of analytical data is of course a prerequisite for such investigations. CertiÆed reference materials (CRMs) play an important role in quality assurance.At present, however, few certiÆed values for REEs in biological reference materials are available.3 The concentration of REEs in some biological materials remains very poorly characterised. For example, Human Hair (NIES CRM 5 and GBW 09101) and Wheat Flour (NIST SRM 1567 and GBW 08503) are certiÆed for only a few trace elements and values for the REEs are not available. This is mainly due to the limitations of available instrumentation historically and the very low levels of REEs present in these materials.This work is a continuation of our previous work,4 which is part of a project intended to study the risk of application of REEs in agrotechniques under the auspices of the Chinese National Natural Science Foundation. We were assigned to provide the concentrations of Ce, Nd and Sm by ID-ICP-MS for two Chinese reference materials of Human Hair and Wheat Flour. Isotope dilution is a powerful strategy for elemental analysis.The combination of ID with ICP-MS offers further suitability because it permits accurate and precise determination of elements, particularly in the certiÆcation of environmental and biological CRMs where analytical values from several analytical methods with different analytical principles are required. ID-ICP-MS has a number of distinct advantages over ICP-MS with external calibration, as with ID-ICP-MS the results are hardly affected by, e.g., signal drift or matrix effects, or partial loss of the analyte during sample preparation; additionally, it is also a more `traceable' technique.Although ID-ICP-MS has frequently been applied to the determination of a number of elements in various matrices,5±17 only a few applications have been reported for the REEs at ultratrace levels in biological materials, such as human hair and wheat Øour samples. Field and Sherrell18 and Esser et al.19 have successfully determined trace levels of REEs in natural samples and waters by using ID-ICP-MS.The purpose of the present study was to develop an ID-ICPJ. Anal. At. Spectrom., 1999, 14, 1843±1848 1843 This journal is # The Royal Society of Chemistry 1999MS method for the determination of Ce, Nd and Sm in biological materials and provide the ID-ICP-MS results for Ce, Nd and Sm in Human Hair and Wheat Flour reference materials. The validity of the analytical procedure was examined by analysing a Bush Leaves CRM (GBW 07603, China).The analytical results obtained by ID-ICP-MS were compared with those obtained by neutron active analysis (NAA) and external calibration ICP-MS methods. The problems of optimum dilution ratio and precision of IDICP- MS are discussed. The effectiveness of the correction for both mass bias and isobaric interferences is also discussed. Experimental Instrumentation The ICP-MS instrument used was a POEMS (Thermo Jarrell Ash, Franklin, MA, USA).In order to obtain the optimum instrumental conditions, the parameters inØuencing isotope ratio determinations were carefully optimized. The ion lens voltage settings and other parameters of the instrument were tuned to obtain a compromise between maximum sensitivity and minimum mass bias. Under the compromise conditions, nearly uniform count rates (about 100 000 counts s21) for Rh, In and Tb (10 ng ml21) were obtained. The accuracy and precision of isotope ratios were tuned, generally being 0.2±0.5% RSD, by tuning the instrumental parameters (e.g., ion lens voltages, number of sweeps and examination points per peak) for the analysis of NIST SRM 981 Natural Lead (about 0.5 mg ml21).The optimized instrumental parameters established for all further experiments are summarized in Table 1. Reagents and spike isotopes Standard solutions of REEs were prepared by diluting the stock standard solutions available from the National Research Center for CertiÆed Reference Materials (Beijing, China).All acids used were of ultrapure grade (Beijing Institute of Chemical Reagent Research, China). Nitric acid and water were further puriÆed in a clean room by quartz sub-boiling distillation. The enriched spike isotope 142Ce was purchased from the China Institute of Atomic Energy. The 142Ce spike stock solution was prepared by dissolving 142CeO2 powder in 1M nitric acid (concentration of Ce, 320.03 mg g21). The spike isotope solutions for 146Nd and 149Sm were obtained from the Laboratory of Isotope Geology, Chinese Academy of Geological Sciences (concentrations of Nd and Sm, 1.6812 and 0.8373 mg g21, respectively).The stock solutions were stored in polyethylene containers at 4 �C in a refrigerator. A mixed working spike isotope solution for 142Ce, 146Nd and 149Sm was prepared gravimetrically by gradually diluting the stock spike solutions to the target concentrations (142Ce about 75 ng, 146Nd about 15 ng, 149Sm about 8 ng).The accurate concentrations of the spike solutions were determined by the reverse ID technique. The isotopic compositions of the spikes were accurately checked by using thermal ionization mass spectrometry (TIMS). These data were supplied by the Laboratory of Isotope Geology, Chinese Academy of Geological Sciences. A listing of the enriched isotopic abundances for these materials is given in Table 2. Samples Three Chinese certiÆed reference materials were analysed: Human Hair GBW 09101, Wheat Flour GBW 08503 and Bush Leaves GBW 07603.All the CRMs were dried in an oven at 80 �C for 6 h and then stored in a desiccator. Sample dissolution A sample portion of about 2°0.1 g (for Human Hair and Wheat Flour) or 0.1°0.01 g (for Bush Leaves) was accurately weighed and placed in a glass beaker. About 1 g of mixed working spike solution was then added gravimetrically to the sample, after which 10 ml of HNO3 and 2 ml of HClO4 were added in sequence.After standing overnight at room temperature to ensure isotope equilibrium, the sample was evaporated to incipient dryness on a hot-plate. The residue was treated with about 8 ml of 5% HNO3 solution, then heated gently until the solution became clear. The Ænal dilution factor is about 4 for Human Hair and Wheat Flour samples, and about 80 for Bush Leaves. The solution was ready for analysis by ID-ICP-MS. The blank was prepared in exactly the same way as the samples.Isotope dilution The ID-ICP-MS calculation is based on the following conventional equation:20 Cs~ MspKÖBsR{AsÜ WsÖAx{BxRÜ (1) where Cs is the concentration of the determined element in the sample, Msp the mass of the spike, K the ratio of the natural atomic weight to the atomic weight of the enriched material,Ws the weight of the sample, Ax the natural abundance of the `reference' isotope, Bx the natural abundance of the `spike' isotope, As the abundance of the `reference' isotope in the enriched spike, Bs the abundance of the `spike' isotope in the enriched spike, andRthe measured reference/spike isotope ratio.It should be noted that all counts of `spike' and `reference' were corrected by a total mass bias factor and isobaric interference factor (see below) prior to concentration calculation. Results and discussion Optimum dilution ratio and precision The ID method is based on addition of a known amount of enriched isotope to a sample. After equilibration of the spike Table 1 Operating conditions for ID-ICP-MS Instrument POEMS (Thermo Jarrell Ash) Forward power 1350 W ReØected power v5W Coolant Øow rate (Ar) 15 l min21 Auxiliary Øow rate (Ar) 1.5 l min21 Carrier gas Øow rate (Ar) 0.8 l min21 Sampling cone oriÆce (Ni) 1.2 mm Skimmer cone oriÆce (Ni) 1.0 mm Resolution 0.8 u Acquisition mode Pulse Number of sweeps 100 Examination points per peak 5 Scan time per u 2 s No.of replicates 6 Table 2 Enriched isotope spike information for the elements determined in this study Abundance (%) Element Isotope Spike Natural Ce 140 7.525 88.48 142 92.202 11.07 Nd 143 0.392 12.17 146 96.837 17.22 Nd 144 0.692 23.85 146 96.837 17.22 Sm 147 0.273 14.97 149 96.498 13.83 152 0.517 26.72 149 96.498 13.83 1844 J.Anal. At. Spectrom., 1999, 14, 1843±1848isotope with the analyte in the sample, the altered isotope ratio is measured to calculate the analyte concentration. As a general rule in ID-ICP-MS,20 the amount of the spike is usually selected so that the measured ratio is near unity, to maximize the mass spectrometry analytical precision.Also, it is normal to use the most abundant isotope as the reference in order to obtain the best possible sensitivity. The existence of isobaric interferences can seriously degrade the accuracy of isotope dilution analysis, and the ideal is always to Ænd two isotopes, which are completely free from such interferences. However, in practice, it is sometimes hard to meet all these requirements because of the ultratrace levels of the elements to be determined and/or the limited number of available spike isotopes.Hence, the isotope dilution target ratio for spiked to reference isotope is usually determined as a compromise between minimizing the error magniÆcation factor and minimizing measurement uncertainties which increase as ratios deviate from 1 : 1. The problem of precision in the ID method has been described in the literature.12,18±21 The isotope dilution target ratio can be different when applying different pairs of isotopes. For example, the optimum dilution ratios for 187Re/185Re and 192Os/190Os are 0.21 and 0.16, respectively, in the study of the Re±Os geochronometry.21 The geometric mean of spiked to reference isotope ratio was 14 : 1 (R~0.07) for Nd and 11 : 1 for Yb (R~0.09).18 An expression for the relative error in the determination of the concentration can be derived from eqn.(1): dC dR ~f 0ÖRÜ (2) dC~C Bs BsR{As { Bx BxR{Ax dR (3) dC C 2 ~ R Bs BsR{As { Bx BxR{Ax 2 dR R 2 (4) dC/C is the relative error in the concentration. Thus, it is apparent that the relative error in the concentration depends on the error in measuring the ratio, dR, and on the magnitude of the ratio, R. Let R Bs BsR{As { Bx BxR{Ax 2 ~P (5) where P is the error magniÆcation factor. By plotting P against R, it is possible to examine the dependence of precision on the magnitude of the R-value.Fig. 1 shows the relationships between P and R for Ce, Nd and Sm. It is clear that for the pair of 140Ce/142Ce, it provides a wide spiking ratio range (from 0.5 to 1.3) without signiÆcantly sacriÆcing the precision of the calculated concentrations. However, for the pairs of Nd and Sm, the optimum ratio range is very limited. For example, the error magniÆcation factors increase rapidly at ratios of 143Nd/146Nd below 0.05 or above 0.2. In practice, the optimum isotope dilution target ratio can be derived from eqn. (5): R~ ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ Rs|Rx p (6) where R is the optimum ratio for the lowest error magniÆcation factor, determined as the geometric mean of the reference to spiked isotope ratio.In this work, the optimum ratio for the lowest error magniÆcation factor is 0.808 for 140Ce/142Ce, 0.054 for 143Nd/146Nd, 0.099 for 144Nd/146Nd, 0.055 for 147Sm/149Sm and 0.102 for 152Sm/149Sm. According to these calculations, a mixed spike working solution of enriched 142Ce (about 75 ng), 146Nd (about 15 ng) and 149Sm (about 8 ng) was added gravimetrically, at which the ratio values determined for Human Hair and Wheat Flour were close to the calculated optimum ratios for the lowest error magniÆcation factor. In quadrupole ICP-MS, isotope ratio measurements are preferably carried out at very high count rates (about 105±106) to improve the precision of the results. At high count rates, a typical precision of 0.2±0.5% RSD can be obtained.In this work, because of the low concentrations in both samples, the counts of the `reference' 147Sm and 143Nd ( about 103) did not provide the best sensitivity even with a large amount of sample (2 g). Table 3 shows the typical precision of isotope ratios determined for the Human Hair and Wheat Flour CRM samples. The precision of the isotope ratios was generally less than 3% RSD. However, the precision is acceptable for the present work.An accurate concentration can also be obtained, e.g., the results for the Bush LeaCRM were in good agreement with the certiÆed values, provided that the isotope dilution ratio was suitable. However, it should be noted that for both the `spiked' and `reference' isotope, the signal intensity of Ce, Nd and Sm for both the Human Hair and Wheat Flour samples signiÆcantly exceeded the blank levels (at least 100 times) in the present work. Proposed pairs of isotopes and correction of isobaric overlap For determination of Nd and Sm, 143Nd/146Nd and 147Sm/149Sm were selected. Both isotope pairs are free from isobaric interferences.For determination of Ce, 140Ce/142Ce was selected. For the purpose of comparison, the isotope pairs 144Nd/146Nd and 152Sm/149Sm, in which 144Nd and 152Sm are overlapped by 144Sm and 152Gd, respectively, were also monitored. The reference 140Ce is the most abundant isotope; however, a more serious isobaric overlap problem exists from 142Nd (11.1% relative natural abundance) on the spike isotope 142Ce.Fig. 1 Error magniÆcation factor (P) against the ratio for reference to spiked isotope (R). J. Anal. At. Spectrom., 1999, 14, 1843±1848 1845Thus, a correction is necessary to m/z 142 to account for the contribution of 142Nd. This was performed by measuring 143Nd (12.7% relative natural abundance) and calculating the percentage contribution of 142Nd to the 142Ce analyte peak. It is noteworthy that the 142Ce spike was found not to contain Nd in sufÆcient amount to necessitate correction of 142Nd interference on 142Ce.Since a certain degree of error is always introduced into the measurement when corrections are applied, the effectiveness of the correction for the isobaric overlap, and also for mass bias correction in ID-ICP-MS, was assessed. The isobaric interference corrections are based on the following equations: 142Ce~142integral{R142Nd=143Nd|143Nd ~142integral{Ö2:227|143NdÜ (7) 144Nd~144integral{R144Sm=147Sm|147Sm ~144integral{Ö0:2064|147SmÜ (8) 152Sm~152integral{R155Gd=152Gd|155Gd ~152integral{Ö0:0136|155GdÜ (9) Isotope ratio measurements and mass bias correction It is well known that, in ICP-MS, the space charge effect and a nozzle separation effect always cause mass discrimination problems which inØuence the accuracy of isotope ratio determinations.From the equation of ID-ICP-MS, the only parameter that requires measurement is the isotope ratio, R; thus, precise and accurate isotope ratio measurements are very important.The value of R is inØuenced by a number of factors, such as the sensitivity of the instrument, the mass bias of the ICP-MS and the dead time of the detector.22,23 However, the total mass bias in a mass spectrometer can be determined experimentally by the mass discrimination factor. It is common practice to compensate for the mass bias effect by bracketing the analysis of the spiked sample with a pure reference standard of known isotopic composition.In this work, the total mass bias was determined experimentally and corrected daily by use of a standard solution of natural abundances for all counts determined prior to calculation of concentration. The mass bias factor was obtained as follows: ÖA=BÜa~ÖA=BÜtÖ1zanÜ (10) where (A/B)a is the observed ratio of isotope A to isotope B, (A/B)t the known isotope ratio of the reference standard, a the bias per mass unit and n the mass difference between the measured isotopes.Table 3 shows the typical isotope ratios and precisions for the samples. Table 4 shows the typical total mass bias determined on different days. It is clear that the mass bias is not constant across the mass range, but in contrast is strongly dependent on the analyte mass number. In addition, the mass bias factor is also not constant on different days of operation. Hence, in this work the mass bias factor used is the actual isotope ratio of interest itself and is corrected daily.It can also be seen that the mass bias for the 140Ce/142Ce isotope ratio was sometimes higher than the corresponding theoretical value. This is contrary to what is expected. There should be no Nd contamination problem, because the counts were corrected by means of eqn. (7) prior to calculation of the mass bias factor. This might be due to measurement error and isobaric interference correction error.Further investigation is required to clarify the reason. However, this is not likely to be a problem for the concentration of Ce in this work, as can be demonstrated from Table 5, in which the results for Ce with and without mass bias correction show no statistical difference. Heumann et al.22 have reported that, for ID-ICP-MS analysis, mass discrimination of the different isotopic terms compensates for each of them so that an excellent approximation of the accurate result can usually also be obtained without correcting Table 3 Precision of isotope ratios determined for Human Hair and Wheat Flour Isotope ratio determined (R) 140Ce/142Ce 143Nd/146Nd 147Sm/149Sm Replicate Human Hair Wheat Flour Human Hair Wheat Flour Human Hair Wheat Flour 1 0.5882 0.8190 0.1110 0.1800 0.0536 0.0764 2 0.5742 0.7979 0.1056 0.1760 0.0531 0.0831 3 0.5753 0.8004 0.1057 0.1776 0.0531 0.0791 4 0.5853 0.7994 0.1060 0.1793 0.0548 0.0787 5 0.5855 0.8325 0.1058 0.1710 0.0539 0.0820 6 0.5871 0.7937 0.1056 0.1772 0.0513 0.0804 Mean (n~6) 0.5826 0.8073 0.1066 0.1769 0.0533 0.080 s 0.0062 0.0153 0.0022 0.0032 0.0012 0.0024 RSD (%) 1.1 1.9 2.0 1.8 2.3 3.0 Table 4 Mass bias of isotope ratios and precision for a 50 ng ml21 standard solution Standard solution 140Ce/142Ce 143Nd/146Nd 147Sm/149Sm Date Theoretical ratio 7.9730 0.7093 1.0870 26/11/98 Determined ratio 7.9412 0.6949 1.0805 Mass bias factor 20.0020 20.0068 20.0030 30/11/98 Determined ratio 7.9134 0.6914 1.0824 Mass bias factor 20.0037 20.0084 20.0021 2/12/98 Determined ratio 7.9575 0.6935 0.0777 Mass bias factor 20.0010 20.0074 20.0043 8/12/98 Determined ratio 8.0480 0.6933 1.0745 Mass bias factor 0.0047 20.0075 20.0057 10/12/98 Determined ratio 8.0547 0.6983 1.0807 Mass bias factor 0.0051 20.0052 20.0029 1846 J.Anal. At. Spectrom., 1999, 14, 1843±1848any data. A comparison was made between the results with and without mass bias correction in this work. As can be seen from the results listed in Table 5, the results for Ce with and without mass bias correction are indeed identical.However, the fact that all data for Nd and Sm were systematically biased suggests mass-dependent variations in the mass bias correction. Of course, for accurate isotope ratio determination, mass discrimination has to be appropriately corrected for. Validation of the proposed method and sample analysis To examine the reliability of the proposed method, a series of solutions containing Ce, Nd and Sm at 10, 20 and 30 ng were prepared and analysed.The recoveries are listed in Table 6. The recoveries of Ce, Nd and Sm at the 10 ng level were 99.7, 103 and 102% with precisions of 1.4, 1.5 and 2.6%, respectively. The validity of the analytical procedure was further examined by analysing a Bush Leaves CRM (GBW 07603). In order to match the concentration levels with those of the Human Hair and Wheat Flour samples, about 0.1 g of Bush Leaves was weighed for analysis.As can be seen from Table 7, all results for the three elements were in good agreement with the certiÆed values (all results fell within the speciÆed uncertainties). Hence, the present method is satisfactory for the determination of Ce, Nd and Sm in biological materials at such low concentration levels. The means of the analytical results for Human Hair and Wheat Flour reference materials prepared and analysed independently (n~7) were compared with those obtained by NAA and external calibration ICP-MS methods.4 As shown in Table 7, the values found by ID-ICPMS agree well with those obtained by external calibration ICPMS from four laboratories; the deviations of Ce, Nd and Sm for Human Hair were 0.5, 3.4 and 1.9%, and for Wheat Flour 1.4, 4.7 and 10.1%, respectively.A t-test indicated that the results determined by ID-ICP-MS showed no signiÆcant difference from those obtained by ICP-MS (Pw0.05), except for Sm in Wheat Flour (Pv0.05).The results were also compared with those of the NAA method. The deviations of Ce, Nd and Sm for Human Hair were 2.1, 19.7 and 4.1%, and for Wheat Flour 3.7, 14.0 and 18.0%, respectively. A t-test indicated that the results for Ce and Sm in Human Hair and Ce in Wheat Flour showed no signiÆcant difference between IDICP- MS and NAA (Pw0.05); however, for Nd in both samples and Sm in Wheat Flour, there were signiÆcant differences between ID-ICP-MS and NAA (Pv0.05).In order to assess further the reliability of the results obtained by the present IDICP- MS method, a z-score assessment was carried out following similar procedures to those used in the International Table 5 Comparison of results for Human Hair (ng g21) with and without mass bias correction Ce Nd Sm Date Uncorrected Corrected Uncorrected Corrected Uncorrected Corrected 26/11/98 19.4 19.2 7.97 8.15 1.33 1.43 30/11/98 19.2 19.1 8.15 8.31 1.34 1.43 2/12/98 20.1 19.8 8.51 8.68 1.43 1.52 8/12/98 20.9 20.6 8.46 8.63 1.46 1.56 Table 6 Comparison of recoveries and precisions for standard additions at different concentrations.n~6 in all instances 140Ce/142Ce 143Nd/146Nd 144Nd/146Nd 147Sm/149Sm 152Sm/149Sm Added/ng Found/ng Recovery (%) Found/ng Recovery (%) Found/ng Recovery (%) Found/ng Recovery (%) Found/ng Recovery (%) 10 9.97 99.7 10.3 103 10.4 104 10.2 102 10.4 104 Ra 0.2302 0.0741 0.1443 0.1585 0.2888 RSD (%) 1.41 1.50 1.97 2.62 0.93 20 23.4 117 20.8 104 21.2 106 20.5 103 21.1 106 R 0.3991 0.1311 0.2588 0.2767 0.5032 RSD (%) 0.82 1.63 1.19 1.04 1.52 30 29.9 99.7 31.4 105 31.9 106 31.1 104 32.0 107 R 0.4783 0.1782 0.3523 0.3690 0.6692 RSD (%) 0.84 0.74 1.41 1.38 1.04 aR: Measured isotope ratio. Table 7 Comparison of results (mean°standard deviation, n~7) for CRMs by different methods Sample Method Ce Nd Sm Human Hair, GBW 09101/ng g21 ID-ICP-MS 19.9°0.65 8.59°0.29 1.53°0.08 ICP-MSa 20.0°0.80 8.31°0.56 1.56°0.04b t-test Pw0.05 Pw0.05 Pw0.05 NAAc 19.5°0.9 10.7°0.3 1.47°0.08 t-test Pw0.05 Pv0.05 Pw0.05 Wheat Flour, GBW 08503/ng g21 ID-ICP-MS 27.8°1.67 15.5°0.92 2.22°0.16 ID-ICP-MSa 28.2°1.2 14.8°0.80 2.47°0.11b t-test Pw0.05 Pw0.05 Pv0.05 NAAc 25.8°0.6 13.6°0.6 1.88°0.06 t-test Pw0.05 Pv0.05 Pv0.05 Bush leaves, GBW 07603/mg g21 ID-ICP-MS 2.27°0.19 1.03°0.09 0.19°0.01 ICP-MSa 2.13°0.11 0.99°0.08 0.20°0.01b CertiÆed 2.2°0.1 1.0°0.1 0.19°0.02 aExternal calibration ICP-MS data calculated as mean and standard deviation of 28 replicates from four laboratories.4 bData calculated as mean and standard deviation of seven replicates by external calibration with internal standardization ICP-MS from present work.4 cData provided by Dr.Ni Bangfa of China Institute of Atomic Energy. J. Anal. At. Spectrom., 1999, 14, 1843±1848 1847ProÆciency Test for Analytical Geochemistry Laboratories.24 The proÆciency testing program has now become well established as the standard procedure for contributing to the quality control assessment of data from analytical geochemistry laboratories.By evaluating the magnitude of the z-score, participating laboratories can decide whether the quality of their analytical data is satisfactory. A similar procedure24 was followed here. `Method consensus values', being robust estimates of the mean composition of the sample, were derived from the contributed data by different laboratories and methods, using a statistical procedure that accommodates outliers.The resulting method consensus values were used as the assigned value for elemental compositions [X(a)]. The target precision was calculated for each assigned value using a modiÆed form of the Horwitz equation: h(a)~kX(a)0.8495, where X(a) is the assigned concentration and k is a constant equal to 0.02 for applied geochemistry laboratories. A z-score was calculated from z~[x2X(a)]/h(a). z-Scores in the range 22vzv2 were considered to be satisfactory.The results of zscore calculations are listed in Table 8. It is clear that all z-score results for Ce, Nd and Sm for both Human Hair and Wheat Flour samples are in the range 22vzv2. Hence, the analytical results of the present work are considered to be satisfactory. Detection limits The detection limits and the corresponding procedural blank concentrations are listed in Table 9. The blank concentrations were measured by ID analysis of blank solutions, which went through the same acid digestion procedure as the sample.The Ce blank value shown in Table 9 is signiÆcantly higher than those of the other analytes. This is mainly because of the reagent blank from the HClO4 used in the sample digestion procedure, which was not puriÆed further. However, this is not a serious problem in the determination of Ce, because of the higher concentration of Ce in both the samples analysed compared with Nd and Sm. The detection limits were determined by ID analysis of six procedural blank solutions and calculation of the analyte concentration that yielded a signal three times the standard deviation of the blank signal. Conclusion In this study, a method was developed to determine Ce, Nd and Sm in human hair and wheat Øour samples simultaneously with ID-ICP-MS.The isotope dilution target ratio for spiked to reference isotope was determined as a compromise between minimizing the error magniÆcation factor and minimizing measurement uncertainties.The isobaric interference of 142Nd on 142Ce was corrected by measuring 143Nd and calculating the percentage contribution of 142Nd to the 142Ce analyte peak. The method precision is in the range 1.1±3.0% RSD. For accurate isotope ratio determination, mass discrimination has to be appropriately corrected for. The validity of the analytical procedure was examined by analysing a Bush Leaves CRM. All results for the three elements were in good agreement with the certiÆed values, with RSDs of 6.3±8.5% (all results fell within the speciÆed uncertainties).Comparison of the results of IDICP- MS for human hair and wheat Øour with those of external calibration ICP-MS and a z-score proÆciency test indicated that the present method is satisfactory for the determination of Ce, Nd and Sm in biological materials at low or sub-ng g21 concentration levels. Financial Support of the Chinese National Natural Science Foundation and Thermo Jarrell Ash Corporation is gratefully acknowledged.The comments and suggestions made by the referees and the helpful discussion of the precision problem with Professor Du Andao, He Hongliao and Luo Daihong are also greatly appreciated. References 1 B. Guo, Chin. Rare Earths, (in Chinese), 1999, 20(1), 64. 2 W. Chen, Y. Gu and G. Zhao, Chin. Rare Earths, (in Chinese), 1999, 20(1), 58. 3 Laboratory of the Government Chemist (LGC), CertiÆed Reference Materials Catalogue, 1996, Issue No. 3, pp. 15±85. 4 M. Yin and B. Li, Spectrochim. Acta, Part B, 1998, 53, 1447. 5 J. W. McLaren, D. Beauchemin and S. S. Berman, Anal. Chem., 1987, 59, 610. 6 J. R. Garbarino and H. E. Taylor, Anal. Chem., 1987, 59, 1568. 7 T. G. B. Ting and M. Janghorbani, Anal. Chem., 1986, 58, 1334. 8 J. R. Dean and L. Ebdon, J. Anal. At. Spectrom., 1987, 2, 369. 9 D. Beauchemin, J. W. McLaren, A. P. Mykytiuk and S. S. Berman, Anal. Chem., 1987, 59, 778. 10 G. E. M. Hall, C. J. Park and J. C. Pelchat, J. Anal. At. Spectrom., 1987, 2, 189. 11 J. D. Fassett and P. J. Paulsen, Anal. Chem., 1989, 61, 643A. 12 Y. Sun, N. Yin and X. Yuan, Rock Miner. Anal., (in Chinese), 1995, 14(1), 15. 13 J. P. Chang and K. S. Jung, J. Anal. At. Spectrom., 1997, 12, 573. 14 J. Yoshinaga and M. Morita, J. Anal. At. Spectrom., 1997, 12, 417. 15 U. Ornemark, P. D. P. Taylor and P. de Bievre, J. Anal. At. Spectrom., 1997, 12, 567. 16 F. Vanhaecke, S. Boonen, L. Moens and R. Dams, J. Anal. At. Spectrom., 1997, 12, 125. 17 M. Nonose and M. Kubota, J. Anal. At. Spectrom., 1998, 13, 151. 18 M. P. Field and R. M. Sherrell, Anal. Chem., 1998, 70, 4480. 19 B. K. Esser, A. Volpe, J. M. Kenneally and D. K. Smith, Anal. Chem., 1994, 66, 1736. 20 K. E. Jarvis, A.L. Gray and R. S. Houk, Handbook of ICP-MS, Blackie, Glasgow, 1992. 21 A. Du, H. He, N. Yin, X. Zou, Y. Sun, D. Sun, S. Chen and W. Qu, Acta Geol. Sin. (in Chinese), 1994, 68(4), 339. 22 K. G. Heumann, S. M. Gallus, G. Radlinger and J. Vogl, J. Anal. At. Spectrom., 1998, 13, 1001. 23 I. S. Begley and B. L. Sharp, J. Anal. At. Spectrom., 1997, 12, 395. 24 M. Thompson, P. J. Potts, J. S. Kane, P. C. Webb and J. S. Watson, Geostand. Newsl., 1998, 22, 127. Paper 9/05346H Table 8 z-Score proÆciency test Sample Element Consensus value X(a) Uncertainty (2 s) Target precision h(a) z-score (ID-ICP-MS) Human Hair (GBW 09101) Ce 19.8 2.0 0.455 0.22 Nd 8.5 1.40 0.222 0.41 Sm 1.48 0.36 0.050 1.0 Wheat Flour (GBW 08503) Ce 27.9 3.6 0.609 20.16 Nd 14.8 2.0 0.356 1.97 Sm 2.34 0.64 0.074 21.62 Table 9 Detection limits for Ce, Nd and Sm by ID-ICP-MS Element Concentration of procedural blank/ng g21 Detection limit (3s)/ng g21 Ce 1.08 0.55 Nd 0.35 0.17 Sm 0.18 0.10 1848 J. Anal. At. Spectrom., 1999, 14, 1843±1848

ISSN:0267-9477    

DOI:10.1039/a905346h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Development of a novel method for the determination of 99Tc in environmental samples by ICP-MS Martin McCartney,*a Kaliaperumal Rajendran,a Valerie Olive,a Richard G. Busbyb and Paul McDonaldb aScottish Universities Research and Reactor Centre, East Kilbride, UK G75 0QF bWestlakes ScientiÆc Consulting, Moor Row, Cumbria, UK CA24 3LN Received 30th June 1999, Accepted 7th September 1999 A relatively rapid and efÆcient method for the determination of 99Tc in a range of marine samples by ICP-MS was developed.Ruthenium decontamination is achieved by the use of TEVA.Spec resin following the removal of the bulk of the matrix elements by a simple evaporation±recrystallisation step. The resulting solution is devoid of ruthenium and subsequent analysis by ICP-MS is comparatively free from interfering matrix effects. The validity of the method was demonstrated by participation in an international intercomparison exercise. The chemical yield for marine biota averages 80±90% and for sediment is around 50±70%. The limit of detection for a 10 g sample is 1 Bq kg21.Introduction 99Tc is a long-lived radionuclide (half-life, 2.136105 years) produced with a relatively high yield (6%) from the Æssion of 235U and 239Pu. Releases of 99Tc into the environment result mainly from the reprocessing of spent nuclear fuel. Of particular signiÆcance in this respect is the British Nuclear Fuels reprocessing plant at SellaÆeld, Cumbria, in the UK, which discharges liquid radioactive waste into the Irish Sea.Annual discharges of 99Tc from this site have increased from 4± 6 TBq in the 1980s and early 1990s to 70±200 TBq from 1994 onwards. Unfortunately, relatively little is known about the behaviour of 99Tc in the environment. There are no stable isotopes of technetium to study and the half-life of the longestlived isotope, 98Tc (4.26106 years), is such that no primordial technetium will remain on earth. Trace amounts of natural 99Tc, formed by spontaneous Æssion of 238U and slow neutron induced Æssion of 235U, have been identiÆed in pitchblende ores,1 but the quantity formed in this way is very small compared with present-day arisings from the nuclear fuel cycle.The comparatively high levels of 99Tc now present in the marine environment provide an ideal opportunity for the more detailed study of the behaviour of technetium. Such work is necessary to determine the radiological signiÆcance of past, present and future discharges of 99Tc from the nuclear fuel cycle.To this end, the development of a suitable analytical technique is required. Most published techniques are based on the radiometric counting of the b-particle (Emax~293 keV) emitted in the decay of 99Tc.2±6 These methods are all characterised by extensive sample preparation (to remove interfering radionuclides and convert the 99Tc into a form suitable for radiometric counting) and long counting times. SigniÆcant improvements in counting times can be achieved by using ICP-MS.The advantages of ICP-MS for the measurement of long-lived radionuclides have been well documented7,8 and, in recent years, several laboratories have developed this technique for the determination of 99Tc in environmental samples.9±11 Extensive sample preparation is still required, however, in order to (i) extract the 99Tc from the matrix into a form suitable for analysis by ICPMS and (ii) remove the isobaric interference caused by the presence of 99Ru.The aim of the present study, therefore, was to develop a rapid and efÆcient method for the determination of 99Tc in environmental samples by ICP-MS. Experimental Reagents and materials High purity de-ionised water (15 MV cm) was obtained from a Millipore Milli-U 10 unit (Millipore, Bedford, MA, USA). Aristar grade nitric acid (relative density 1.42), hydrogen peroxide (30% m/v) and AnalaR grade ammonia solution (relative density 0.88) were obtained from BDH (Poole, Dorset, UK). Working standard solutions of Be, Co, Ni, Zn, Mo, In, Pt, Hg, Bi and U were obtained from monoelemental 1 mgml21 certiÆed stock standard solutions (Johnson Matthey, Royston, Hertfordshire, UK).TEVA.Spec resin (particle size 50±100 mm) was obtained from Eichrom Europe (Paris, France). 99Tc calibration standards were derived from a stock standard solution (34 mg g21) obtained from Amersham International (Amersham, Buckinghamshire, UK).The yield tracer, 95mTc, was obtained from the National Physical Laboratory (NPL, Teddington, Middlesex, UK). Instrumentation 95mTc was determined by measuring the intensity of its 204 keV c-ray using a 363 in well-type NaI detector. Samples were measured until over 10 000 counts were observed. 99Tc was measured using a VG Elemental PQ2 Plus quadrupole-based ICP-MS instrument (VG Elemental, Winsford, Cheshire, UK) Ætted with a Meinhard nebuliser and a water cooled glass Scott double pass spray chamber.Instrument parameters were optimised using an In solution (10 ng g21) and a typical response for 99Tc was 36 104 counts s21 (ppb)21. The acquisition parameters are listed Table 1 ICP-MS acquisition parameters Sample uptake rate 0.8 ml min21 Washout time 180 s Uptake time 90 s Acquisition time 30 s Data acquisition mode Peak jumping Masses monitored m/z 99, 101, 115 Dwell time 10.24 ms Points per peak 3 No. of replicates 3 J. Anal. At. Spectrom., 1999, 14, 1849±1852 1849 This journal is # The Royal Society of Chemistry 1999in Table 1.Ruthenium contamination was monitored by measuring the 101Ru count rate and 115In was used as an internal standard. Memory effects were eliminated by employing a 180 s washout with 0.8 M HNO3 between samples. Method summary A summary of the analytical method is presented in Fig. 1 and important aspects of the procedure are discussed in the following sections. Results and discussion Ashing and leaching Ashing is carried out to remove organic carbon, which would otherwise interfere with the subsequent chemical processing and lead to signiÆcant matrix effects in the ICP-MS analysis.At high temperatures, however, technetium may be lost from the sample through the formation of the volatile acid HTcO4, although it has been reported that this mechanism can be suppressed by the addition of ammonia.12 Some studies have suggested that technetium losses can occur at temperatures around 500 �C,5,6 whereas in other studies signiÆcant losses were not observed below 800 �C.9,12 Therefore, an experiment was Ærst carried out to investigate the effects of (i) different ashing temperatures and (ii) the addition of ammonia to the sample prior to ashing.For each ashing temperature, two 10 g aliquots of dried seaweed (Fucus vesiculosus) powder were spiked with 95mTc. One aliquot was wetted with 10±20 ml of ammonia solution (relative density 0.88).The samples were gently dried on a hotplate and then placed in a mufØe furnace. The temperature was ramped at a rate of 100 �C h21 and left at the speciÆed value for 6 h. The results are presented in Fig. 2(a). No signiÆcant losses of technetium were observed at temperatures below 750 �C and the samples treated with ammonia did not appear to differ markedly from the untreated samples. These samples were all subsequently processed and the 99Tc concentrations determined by ICP-MS.The results are presented in Fig. 2(b) (each data point representing the average of two aliquots ashed at a particular temperature). The data obtained at ashing temperatures above 750 �C are unsatisfactory as the poor yields result in very large errors associated with the measurement of both the 99Tc concentration and the 95mTc recovery. The 99Tc concentration determined between 450 and 750 �C gradually increases from 5 to 5.6 Bq g21. This effect was further investigated by repeated analysis of another seaweed sample which had been used in a large intercomparison exercise and thus had a relatively well deÆned 99Tc concentration (17.9°0.9 Bq g21).13 Six aliquots of this sample were ashed at 550 �C (treated with ammonia) and six at 750 �C (treated with ammonia).The 9ns derived from the samples ashed at 550 �C (16.3°0.6 Bq g21) were consistently lower and in poorer agreement with the consensus value than those ashed at 750 �C (18.1°0.8 Bq g21).Wigley et al.6 observed a similar effect although at a lower ashing temperature and concluded that low 99Tc concentrations in samples ashed below 500 �C resulted from reduced efÆciency of the subsequent leaching process for the matrix bound 99Tc. Therefore, an ashing temperature of 750 �C was chosen for routine use and, as a precaution against possible volatilisation, samples were also treated with ammonia. After ashing, sample dissolution (or, in the case of marine sediments, leaching) was achieved by gently heating (v75 �C) on a hot-plate for 4 h following the addition of 50 ml of 8 M HNO3 and 5 ml of 30% m/v H2O2.The hydrogen peroxide ensures that all the technetium present will be in the z7 state. Any residual particulate matter is removed by Æltration. Recrystallisation and ruthenium decontamination Prior to ICP-MS analysis, it is necessary to remove the isobaric interference 99Ru (natural abundance 12.7%). The removal of ruthenium is also required when using radiometric techniques as the radionuclide 106Ru interferes with the b-counting of 99Tc.Ruthenium decontamination has normally been achieved through the use of solvent extraction2±4,14±16 although more recently the use of TEVA.Spec resin, an extraction chromatographic material, has been proposed.6,10,17 Technetium, in the pertechnetate form, is strongly adsorbed by the resin at low concentrations of nitric acid whereas ruthenium is not effectively retained.The technetium can subsequently be eluted with higher concentrations of nitric acid. When applied to environmental samples with complicated matrices and, in some cases, high ruthenium levels, it has been found that complete Tc±Ru separation cannot be achieved by use of TEVA.Spec resin alone. Most workers have found it necessary to include additional clean-up steps to improve the Tc±Ru separation. Butterworth et al.17 used a combination of iron hydroxide precipitation and anion exchange to clean up the samples prior to the use of the TEVA.Spec resin.Beals10 recommended the use of solvent extraction for samples with high ruthenium levels whereas Wigley et al.6 preceded the TEVA.Spec stage with an iron hydroxide precipitation and succeeded it with a solvent extraction step. Although all these solutions to the problem are effective, they add to both the cost and time necessary to complete the analysis. In this study, we made use of a simple yet effective recrystallisation stage prior to the use of the TEVA.Spec Fig. 1 Flow diagram of analytical method. 1850 J. Anal. At. Spectrom., 1999, 14, 1849±1852resin. The Æltrate from the leaching stage is evaporated gently (v75 �C) to incipient dryness. As the volume decreases, recrystallisation of dissolved salts takes place. A 30 ml volume of water is then added in 10 ml aliquots, the slurry Æltered and the Æltrate retained. Analysis of the redissolved solids by ICP-MS indicates that these mainly consist of salts of Group II elements (Mg, Ca, Sr and Ba), transition metals (Fe, Mn, Cu, Ni and Zn), lead and uranium.On average, approximately 20% of the ruthenium in the original solution is retained within these salts whilst losses of technetium are minimal (v2%). Although the extent of Tc±Ru separation achieved by this step is small, it does provide a relatively matrix free solution from which it is possible to separate these two elements completely.The Æltrate from the recrystallisation step is then added to a preconditioned TEVA.Spec column (0.3 g of resin retained in a Pasteur pipette with a glass-wool plug, washed with 30 ml of 4M HNO3 followed by 30 ml of 0.1 M HNO3). At the low nitric acid concentration of the Æltrate, technetium, unlike ruthenium, is retained by the resin. Ruthenium decontamination is then completed by washing the resin with 30 ml of 0.1 M HNO3 and the technetium eluted with 30 ml of 4 M HNO3.The reliability of the method was checked by the multiple analysis of a seaweed sample containing approximately 140 ng g21 (87.5 Bq g21) of 99Tc and 20 ng g21 of Ru. Analysis of the eluates was carried out by ICP-MS. The results are presented in Table 2. It may be that the importance of the recrystallisation step lies in the removal of a large fraction of the matrix rather than in the amount of ruthenium removed. Certainly, the efÆciency of the Tc±Ru separation achievable using the TEVA.Spec resin is enhanced by this pretreatment.This is further demonstrated in Fig. 3(a) and (b), which show the mass spectra (m/z 95±103) of the TEVA.Spec eluate with and without the recrystallisation step. The eluate obtained without use of the recrystallisation step shows a small but signiÆcant ruthenium peak at m/z 101. The efÆciency of the ruthenium decontamination is routinely monitored by measuring the count rate at m/z 101 (the natural abundance of 101Ru is 17%).The ruthenium contribution to the count rate at m/z 99 is directly proportional to that at m/z 101, hence it is possible to correct for small amounts of ruthenium remaining in the Ænal solution. In practice, in the analysis of over 200 environmental samples, no signiÆcant contribution from ruthenium has been observed. From the repeat analysis of a seaweed sample with a Ru concentration of 20 ng g21, a decontamination factor in excess of 103 was determined.It was only possible to determine the lower limit of the decontamination factor as the Ænal solution contained no detectable levels of ruthenium. Hence the lower limit of the decontamination factor was determined using the instrumental detection limit of 2 pg ml21 for 101Ru. Chemical recovery This method was developed primarily for the determination of 99Tc in marine biota. Chemical yields for seaweed and a wide variety of other types of marine biota (lobster, mussels, winkles, crab and Nephrops) average between 80 and 90%.The method has also been applied to sediment samples, which display a lower recovery of around 50±70%. ICP-MS Interferences. In addition to the major isobaric interference presented by ruthenium, there are a variety of other elements which, through the formation of polyatomic (59Co40Arz, 62Ni 37Clz, 64Ni35Clz, 64Zn35Clz and 98Mo1Hz) and doubly charged (198Hg2z and 198Pt2z) species, may contribute to the count rate at m/z 99. Although the concentration of some of these elements may be relatively high in environmental materials, the chemical procedure used for the Tc±Ru separation has the added beneÆt of excluding most of these elements from the Ænal solution. ICP-MS analysis of a variety of sample types (sediment, lobster, mussels and seaweed) suggests that the concentrations of these elements in the Ænal solution are not likely to exceed 10 ng ml21.Solutions containing 100 ng ml21 of these elements were analysed by ICP-MS in order to test the extent of the potential interference.In each case, the contribution to the count rate at m/z 99 was found to be negligible. Internal standard. The suitability of a variety of nuclides (9Be, 59Co, 115In, 209Bi and 238U) was tested for use as an internal standard. The behaviour of 115In was found to be the Fig. 2 Effect of ashing temperature on (a) technetium recovery and (b) Ænal result. Error bars (1s) are shown where they exceed the symbol size.Table 2 Average recoveries of technetium and ruthenium in the eluate from the three stages of the TEVA.Spec resin decontamination process (as a percentage of the concentration of these elements in the loading solution), based on 10 repeats Process Technetium (%) Ruthenium (%) Loading 0 80 Washing 0 20 Elution 95 0 Fig. 3 ICP-MS spectra of Ænal solution (a) with and (b) without recrystallisation step. J. Anal. At. Spectrom., 1999, 14, 1849±1852 1851most similar to that of 99Tc, as would be expected given the proximity of the masses.One potential problem in using 115In as internal standard is caused by the fo9Tc16Oz (m/z 115).18 Formation of the oxide, however, was found to be insigniÆcant (v0.02%) and it was therefore concluded that 115In could be used to correct for changes in sensitivity resulting from changing instrumental conditions throughout the run. Matrix effects. The concentrations of 99Tc in the sample solutions are obtained by reference to a calibration curve produced by the analysis of standard solutions containing known concentrations of 99Tc.It is necessary, however, to check that the slope of the calibration curve derived from the use of the standard solutions does not vary signiÆcantly from that derived from the use of `real' solutions. Seaweed samples, containing low levels of 99Tc, were processed and the Ænal solution spiked with 99Tc to produce concentrations of 0.1, 1, 2, 10 and 20 ng ml21.The slope of the calibration curve obtained [35 100°1200 cps (ppb)21] was then compared with that obtained for the standard solution [34 500°1000 cps (ppb)21]. The results show that there is no signiÆcant difference in the slopes of the two calibration curves and suggest that there are no unforeseen matrix effects which cannot be corrected for by the use of the 115In internal standard. Instrumental limit of detection. The instrumental limit of detection, based on three times the standard deviation of 11 repeated analyses of a reagent blank, is 2.0 pg ml21 (1.25 mBq ml21).Accuracy and precision There are no readily available standard reference materials for 99Tc with which to validate the method. The validity, in this case, was assessed through participation in a relatively large international intercomparison exercise involving 14 participating laboratories from eight countries.13 The exercise involved the analysis of Æve seaweed samples and the initial results obtained by this laboratory are presented in Table 3.The results for samples C, D and E were satisfactory but those for samples A and B appeared to be too high. Subsequent investigations revealed that the cause of the problem was crosscontamination at the ashing stage. It had been our practice to re-use a set of silica crucibles. Tests revealed, however, that despite a rigorous cleaning procedure (soaking overnight in Decon, followed by a 6 h reØux in 8MHNO3), a small amount of carry-over (approximately 0.1%) could still occur.This problem was exacerbated by the large variations in 99Tc levels present in the samples used in the intercalibration study (over four orders of magnitude). The method was subsequently revised and disposable porcelain crucibles are now used. Repeated analysis of sample B, using the revised procedure, produced satisfactory results (61.5 Bq kg21). The 1s standard deviation on 20 repeated analyses of two large seaweed samples averaged 8%.This value for the precision of the method can be taken to be the upper limit of the uncertainty since it is likely that a signiÆcant fraction of the error is attributable to sample heterogeneity. Detection limit of method The detection limit of the method, based on the average yield for a 10 g biota sample taken up in 5 ml of nitric acid, is 1 Bq kg21. Conclusions A rapid and efÆcient method for the determination of 99Tc in a variety of marine samples by ICP-MS has been described.Important parameters, relating to the performance of the technique, are summarised in Table 4. Other important Ændings resulting from the study are detailed below. The choice of ashing temperature appears to be critical to the performance of the technique. At too low a temperature, the subsequent leaching process does not liberate 99Tc quantitatively from the matrix. At too high a temperature, losses of Tc become unacceptably high.For this study, an ashing temperature of 750 �C was found to be ideal. Despite a rigorous cleaning protocol, cross-contamination from the re-use of ashing crucibles was observed. The use of disposable crucibles is recommended. The incorporation of a simple evaporation±recrystallisation step greatly improves the efÆciency of the Tc±Ru separation achieved by the subsequent use of the TEVA.Spec resin. References 1 B. T. Kenne and P. K. Kuroda, J. Inorg. Nucl. Chem., 1964, 26, 493. 2 E. Holm, J. Rioseco and M. Garcia-Leon, Nucl. Instrum. Methods Phys. Res., 1984, 223, 204. 3 Q. Chen, H. Dahlgaard, H. J. M. Hansen and A. Aarkrog, Anal. Chim. Acta, 1990, 228, 163. 4 M. Garcia-Leon, J. Radioanal. Nucl. Chem., 1990, 138, 171. 5 B. R. Harvey, R. D. Ibbett, K. J. Williams and M. B. Lovett, The Determination of Technetium-99 in Environmental Materials, Ministry of Agriculture, Fisheries and Food, Lowestoft, 1991. 6 F. Wigley, P. E. Warwick, I.W. Croudace, J. Caborn and A. L. Sanchez, Anal. Chim. Acta, 1999, 380, 73. 7 M. R. Smith, E. J. Wyse and D. W. Koppenaal, J. Radioanal. Nucl. Chem., 1992, 160, 341. 8 J. I. Garcia Alonso, D. Thoby-Schultzendorff, B. Giovannone and L. Koch, J. Radioanal. Nucl. Chem., 1996, 203, 19. 9 K. Tagami and S. Uchida, Radiochim. Acta, 1993, 63, 69. 10 D. M. Beals, J. Radioanal. Nucl. Chem., 1996, 204, 253. 11 A. E. Eroglu, C. W. McLeod, K. S. Leonard and D. McCubbin, J. Anal. At. Spectrom., 1998, 13, 875. 12 S. Foti, E. Delucchi and V. Akamian, Anal. Chim. Acta, 1972, 60, 269. 13 M. McCartney, V. Olive and E. M. Scott, J. Radioanal. Nucl. Chem., in the press. 14 N. Matsuoka, T. Umata, M. Okamura, N. Shiraishi, N. Momoshima and Y. Takashima, J. Radioanal. Nucl. Chem., 1990, 140, 57. 15 S. Morita, C. K. Kim, Y. Takaku, R. Seki and N. Ikeda, Appl. Radiat. Isot., 1991, 42, 531. 16 S. Nicholson, T. W. Sanders and L. M. Blaine, Sci. Total Environ., 1993, 130(131), 275. 17 J. C. Butterworth, F. R. Livens and P. R. Makinson, Sci. Total Environ., 1995, 173(174), 293. 18 J. I. Garcia Alonso, F. Sena and L. Koch, J. Anal. At. Spectrom., 1994, 9, 1217. Paper 9/05274G Table 3 99Tc intercalibration results (Bq g21 dry°1s) Sample This laboratory Consensus value A 0.0140°0.0008 0.0059°0.0011 B 0.0885°0.0053 0.0583°0.0046 B (0.0615°0.0048)a 0.0583°0.0046 C 3.15°0.20 3.91°0.13 D 18.10°0.92 17.91°0.78 E 138°10 133.1°5.4 aRepeat analysis using disposable crucible (see text). Table 4 Performance of method Parameter Performance Chemical recovery for marine biota 80±90% Chemical recovery for sediment 50±70% Ru decontamination factor w1023 Detection limit of method 1 Bq kg21 1852 J. Anal. At. Spectrom., 1999, 14, 1849±18

ISSN:0267-9477    

DOI:10.1039/a905274g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Application of a nitrogen microwave-induced plasma mass spectrometer as an element-speciÆc detector for arsenic speciation analysis Amit Chatterjee,* Yasuyuki Shibata, Jun Yoshinaga and Masatoshi Morita National Institute for Environmental Studies, Environmental Chemistry Division, Environmental Chemodynamics Section, 16-2 Onogawa, Ibaraki 305 0053, Tsukuba Science City, Japan Received 24th June 1999, Accepted 14th October 1999 A high power nitrogen microwave-induced plasma (1.3 kW) mass spectrometer (N2-MIP-MS) was successfully coupled with an HPLC system using a silica-based cation-exchange column.It was examined as an elementspeciÆc detector for its applicability to the optimization and determination of seven arsenic compounds [As(V), methylarsonic acid (MA), dimethylarsinic acid (DMA), arsenobetaine (AB), arsenocholine (AC), trimethylarsine oxide (TMAO) and tetramethylarsonium ion (TMI)]. The system is a promising alternative ion source for mass spectrometry for elemental speciation analysis. The MIP was stable with a pyridine mobile phase for up to 6 h.Replacing the MIP-MS fabricated nebulizer (concentric) and sample input tubing (PTFE) with an ICP-MS (PMS-2000) nebulizer (concentric) and PEEK tubing increased the ion signals for anionic and cationic arsenic compounds by 17±30 and 21±25%, respectively. PEEK tubing additionally increased the separation efÆciency for the arsenic compounds. The detection limits of As(V), MA, DMA, AB, TMAO, AC and TMI obtained with the optimized HPLC-N2-MIP-MS system were 0.68, 0.95, 2.01, 0.92, 22.1, 1.31 and 1.75 mg l21, respectively.The repeatability (RSD for three successive analyses) and reproducibility (RSD for three successive analyses performed on three different days) achieved were 0.7±9.22 and 6.5±11.4%, respectively, for the seven different arsenic compounds. No detectable spectroscopic interference of 40Ar35Clz was observed with a high chloride matrix (10 000 mg l21). The developed HPLC-N2-MIP-MS method was successfully applied to the determination of arsenic compounds, principally AB, in NIES Candidate CRM-18 Freeze-Dried Human Urine (134°6 mg l21).The results agreed reasonably well with the HPLC-ICP-MS values. Introduction Trace element speciation can generally be realized by complex analytical systems, in which the species information is generated by some kind of separation method. The detection of the separated species is performed by a spectroscopic analytical method, resulting in atomic or molecular information.Chromatographic separation with element-speciÆc detection is clearly a favorable method for the determination of different forms of an element.1 Chemical speciation of arsenic compounds is a topic under extensive study, because of the very rich chemistry, the diverse toxicity of its compounds, and the dramatic differences in metabolism of the various arsenic species. The powerful speciation techniques developed during the past few years have played an essential role in providing information for understanding the distribution and fate of arsenic in biological and environmental systems.2,3 The combination of chromatographic separation with elementspeciÆc spectrometric detection has proved to be very useful for the speciation of trace levels of arsenic compounds. In particular, liquid chromatographic separation with inductively coupled plasma mass spectrometry (ICP-MS),2±6 electrospray mass spectrometry (ESMS),7 capillary electrophoresis,8 and atomic spectrometry1,9±11 has played an important role in chemical speciation studies.In the development of such hyphenated techniques the matching of separately optimized analytical procedures in different instruments has to be resolved. FAAS, ICP-AES, HGAAS and ICP-MS instruments can easily be matched to the Øow system of HPLC and such methods are readily applicable to metallic speciation analysis; 12,13 however, the organic eluents can disturb the stability of the detector.12 The detection limits obtained with FAAS and ICP-AES are too high.ETAAS provides low detection limits, but suffers from pre-atomization losses. HGAAS also generates low detection limits for arsenic species; however, for arsenobetaine (AB), tetramethylarsonium ion (TMI) and arsenochlorine (AC), prior decomposition to a hydrideforming form is essential prior to HG.4,14 The Ar-ICP-MS has been widely used as an ion source in speciation elemental mass spectrometry, since ICP-MS provides spectral simplicity, multi-elemental analysis and low detection limits for the determined elements.15,16 However, spectral and non-spectral interferences are still a serious problem.The mono/polyatomic ions caused by the Ar plasma sustaining gas, such as 40Ar1Hz, 40Arz, 40Ar12Cz, 40Ar15Nz, 40Ar16Oz, 40Ar18Oz, 40Ar35Clz, 40Ar37Clz, 40Ar38Arz, 40Ar2 z, interfere with the determination of 39Kz, 40Caz, 52Crz, 55Mnz, 56Fez, 58Fez, 75Asz, 78Sez and 80Sez, respectively.16,17 To reduce the Ar-associated polyatomic ions for Ar-ICP, sources other than Ar or mixed gases with Ar and several types of MIP sources have been developed and studied.16±20 The gas-Øow system of a microwave- induced plasma (MIP) can be coupled directly to GC systems; therefore, MIP-AES has long been applied as an element-speciÆc GC detector.12 The introduction of liquid samples into the MIP discharge is, however, not free from difÆculties.The limited thermal energy and the relatively small volume of the MIP restrict the analyte Øow to be introduced.20 A high power nitrogen MIP-MS having a rectangular waveguide and an Okamoto cavity (2.45 GHz, maximum 1.5 kW; constructed by Hitachi, Ibaraki, Japan) is one of these sources which is sustained by nitrogen gas.18,19 The plasma is doughnut-shaped just like the Ar-ICP. It enables the introduction of sample aerosols directly into the center of the J.Anal. At. Spectrom., 1999, 14, 1853±1859 1853 This journal is # The Royal Society of Chemistry 1999plasma.14±19 Because the interference of Ar-associated polyatomic ions is replaced by that of nitrogen-related polyatomic ions, MIP-MS could determine arsenic without spectral interference.15±19 Surprisingly, relatively little work with a high power N2-MIP-MS has been reported for total elemental analysis.15,17,21,22 In previous papers, we reported the use of high power N2-MIP-MS for the isotope dilution analysis of selenium in clinical and marine samples.21,22 The methods are highly sensitive with a low detection limit, and are comparable to those of ICP-MS.23 Very few papers concerning the coupling of HPLC with N2-MIP-MS for elemental speciation analysis, especially for arsenic, have been published. This paper describes the feasibility of high power N2-MIPMS as an arsenic-speciÆc detector, for the determination of seven arsenic species by coupling with an HPLC system.Two types of sample introduction mode, i.e. combination of a Hitachi supplied conventional MIP-MS nebulizer with TeØon tubing and an ICP-MS (PMS-2000) Meinhard-type nebulizer with PEEK pressure tubing, were used. The results obtained by both sample introduction methods are compared in terms of analytical Ægures of merit for the seven arsenic species. Finally, the present HPLC-N2-MIP-MS (PMS-2000 nebulizer along with PEEK tubing) system was applied successfully to the determination of AB in NIES Candidate CRM-18 Freeze- Dried Human Urine.Attention was also paid to the detection limits and sensitivities of the arsenic compounds. Experimental Reagents and solutions NIES Candidate CRM-18 Freeze-Dried Human Urine was used as a reference material (National Institute for Environmental Studies, Environmental Chemistry Division, Tsukuba, Ibaraki, Japan). All solutions were prepared with Milli-Q (18.3 MV cm; Milli-QSP.TOC Reagent Water System, Nihon Millipore, Yonezawa, Japan) water.The mobile phase for cation-exchange chromatography was prepared by dissolving 1.58 g of pyridine (Merck, pro analysi; Merck, Darmstadt, Germany) in Milli-Q water and adjusting the pH of this solution to 2.5 by adding formic acid (#98%, Fluka, puriss, p.a., Fluka, Buchs, Switzerland); the solution was then made up to 1000 ml (20.0 mM) with Milli-Q water. Standard solutions (1000 mg l21 As) for the identiÆcation and quantiÆcation of arsenic compounds were prepared by dissolving 433.0 mg of NaAsO2 (Merck, pro analysi) [As(III)], 1.041 mg of Na2HAsO4?7H2O (Merck, pro analysi) [As(V)], 460.5 mg of dimethylarsinic acid (DMA), 466.6 mg of methylarsonic acid (MA), 594.2 mg of arsenobetaine bromide (AB), 554.2 mg of arsenocholine (AC), 453.9 mg of trimethylarsine oxide (TMAO) and 874.2 mg of tetramethylarsonium iodide (TMI) (Tri Chemical Laboratory, Yamanashi, Japan) in 250 ml of water.Calibration graphs for the HPLC-MIP-MS measurements were obtained by injecting aliquots (0.100 ml) of solutions containing 5.00, 10.0, 25.0, 50.0 or 100.0 mg l21 As and of As(III), As(V), MA, DMA, AB, AC, TMAO and TMI for the separations on the Supelcosil LC-SCX cation-exchange column. Nitrogen microwave-induced plasma mass spectrometry (N2-MIP-MS) A Hitachi P-6000 N2-MIP-MS, which is commercially available from Hitachi (Ibaraki, Japan) was used for this study.In Table 1 the operating conditions for N2-MIP-MS are displayed. The microwave power (2.45 GHz, maximum 1.5 kW) was produced by a magnetron (H3862: Hitachi) operated by a dc power supply and fed to a cavity known as the Okamoto cavity18,19 through a rectangular waveguide (WRJ-2). The cavity was cooled by circulating cold water (20 �C) from a refrigerator. To establish stable impedance matching between the plasma and the microwave power source, the height of the rectangular waveguide was reduced from its regular size (WJR- 2) of 54.2 to 8.0 mm by using a tapered waveguide.An annularshaped 15 electric Æeld, where the microwave discharge is maintained, was produced between an inner conductor and an outer cylindrical conductor terminated by a front plate. A quartz discharge tube of 10 mm id (1 mm thickness) was inserted into the inner conductor. The tube consisted of two concentric tubes, an inner and an outer. The inner tube was tulip-shaped, with a large outer diameter of 9 mm and a small inner diameter of 1 mm.The carrier gas (N2; 1.1 l min21) with the sample aerosol was fed through the central oriÆce (id 1.0 mm) and the plasma support gas (N2; 15 l min21) was fed into the cylindrical gap between the two quartz tubes, respectively. The plasma was ignited by a Tesla coil. A stable, annular (doughnut)-shaped, pink nitrogen plasma was produced. A Meinhard concentric-type nebulizer (Hitachi Electric, Ibaraki, Japan, Part No.P97M170, 300-8350) connected with TeØon sampling tubing (PTFE, Hitachi, Part No. 300-8868) and an ICP-MS concentric-type nebulizer (PMS-2000, Yokogawa Electric, Tokyo, Japan; Part No. K9250YW) with 30060.25 mm id PEEK (polyether ether ketone) capillary tubing without a desolvation system were used for the sample introduction systems. The exit of the HPLC column was connected directly to the nebulizer via the PEEK capillary or PTFE tubing. A Neslab refrigerating circulator was used to maintain the temperature of the glass spray chamber at 5 �C.The normal aqueous sample solution uptake rate was about 0.3 ml min21 when the Øow rate of the nitrogen gas (carrier) was 1.1±1.4 l min21. However, with HPLC connection Table 1 Operating conditions of nitrogen-MIP-MS N2-MIP-MS: Frequency 2.45 GHz Microwave power (a) Forward 1.3 kW (b) ReØected v20 W Plasma gas Øow/l min21 15 Carrier gas Øow/l min21 1.1 Peak point /mass 10 Dwell time 2.0 ms Number of sweeps 1500 Nebulizer (Meinhard) Concentric Temperature of spray chamber v5 �C Sampling cone (Pt) 0.8 mm oriÆce Skimmer cone (Pt) 0.4 mm oriÆce Sample uptake rate/l min21 1.5 Measuring parameters: Monitored signals: 75Asz, 77Sez, 40Ar37Clz m/z 75, 77 Total analysis time 600 s Vacuum pump: Stage 1: Mechanical pump DPF-6ZS, 750 l min21, Daiashinku, Japan Stage 2: Vapour pump DPF-6ZS, 1200 l s21, Kashiyama, Japan Stage 3: Vapour pump DPF-4ZS, 570 l s21, Kashiyama, Japan Both vapour pumps are Ætted with water-cooled bafØe valves Mass analyzer: Quadrupole analyzer: QMG420-4 Balzers, Liechtenstein; Mo rods, 200 mm length, 8 mm diameter, radiofrequency of 2.25 MHz Ion detector: Channeltron electron multiplier 4831 G, Dalileo, USA, mounted on a quadrupole analyzer with 90� ion deØection and off-axis Pulse counting: Pulse ampliÆer C3866, Hamamatsu Photonics, resolution 10 ns, maximum counting rate 107 counts s21. 1854 J. Anal. At. Spectrom., 1999, 14, 1853±1859using 20 mM pyridine as mobile phase at a Øow rate of 1.0± 1.5 ml min,21 the plasma was stable for 6 h.A more detailed description of the microwave system, plasma ignition system, sample introduction system, MS instrument and interface system has been published previously.15,18,19 Chromatography The HPLC system consisted of a Perkin-Elmer Model Series 410 B10 solvent delivery unit (Perkin-Elmer, Norwalk, CT, USA) and a Rheodyne 9725 six-port injection valve (Cotati, CA, USA) with a 100 ml injection loop.The separations were performed on a Supelcosil LC-SCX (Supelco, Bellefonte, PA, USA) cation-exchange column (25 cm64.6 mm id, 5 mm silicabased particles with propylsulfonic acid exchange sites). The column was equilibrated by passing at least 100 ml (Øow rate 1.0 ml min21) of the mobile phase through the column before injection of the arsenic compounds. The exit of the column was connected directly to the nebulizers with PEEK capillary or PTFE tubing. The ion signals at m/z 75 (75As) and m/z 77 (40Ar37Cl) were recorded with the time-resolved analysis software# Version of Hitachi.For quantiÆcation, the chromatograms were exported, peak areas and peak heights determined, and the concentrations calculated with external calibration graphs and with the standard additions technique. Urine preparation Freeze-Dried Human Urine (NIES Candidate CRM-18) served as a reference material for the application study. The reference value for the total arsenic was 134°6 mg l21 when it was dissolved in 9.57 ml of water, but this is not a certiÆed value.The urine was prepared from 10.0 l of urine collected from nonexposed male clerical workers of the NIES in late July 1996. The collected urine was immediately Æltered through a 5 mm membrane Ælter and stored in a freezer in one lot until further treatment. Frozen urine was thawed 2 weeks after collection and Æltered again through a 0.45 mm membrane Ælter.Ten grams of the Æltered urine (10.0°0.05 g) were dispensed into 930 borosilicate bottles, which had been pre-cleaned with nitric acid and numbered. The weight of the dispensed urine was recorded for every 50 bottles for calculation of weight loss due to subsequent freeze-drying. All of the bottles except for 50 were freeze-dried in one batch at Wako Pure Chemicals, Mie, Japan. Freeze-dried urine was stored at 4 �C at NIES. The weight loss of the urine due to freeze-drying was calculated to be 9.57°0.02 g (n~19).A more detailed description of the preparation of the urine samples was given previously.24 A 9.57 ml volume of pure water was added to each bottle for reconstitution. The bottles were swirled gently to dissolve the material completely. The reconstituted urine was stored at 4 �C prior to analysis. Results and discussion Chromatographic separation of arsenic compounds Among the many methods which have been applied to the chromatographic separation of arsenic compounds, a common and reliable method is ion-exchange chromatography.The polarity of cationic and anionic arsenic compounds makes them amenable to ion-exchange HPLC. A cation-exchange chromatographic system was used (LC-SCX), because this provides the possibility of applying a low organic content (20 mM pyridine) of the eluent and is compatible with the limited loading of the MIP source. The normal aqueous sample solution uptake rate in this N2-MIP is very low and about 0.3 ml min21 when the Øow rates of the nitrogen gas (carrier) are 1.1±1.4 l min21.15 However, with the HPLC connection using 20 mM pyridine as mobileØow rate of 1.5 ml min21, the plasma is stable and the count variation is about 5±8% during 6 h of continuous operation.The high power N2-MIP produces an annular (doughnut)-shaped plasma15±19 that has superior operational stability and a higher tolerance for the injection of highly aqueous aerosol samples.18 This plasma is more robust than the Ar-ICP-MS and is not extinguished even if an air sample is injected.Hence, a high mobile phase Øow rate (1.0±1.5 ml min21) is compatible with the MIP system. Fig. 1 and 2 show the retention times of anionic and cationic arsenic compounds in LC-SCX using 20 mM pyridine at pH 2.50. The AB, AC, TMAO, TMI, MA, DMA and As(V) are well separated. However, the signals for arsenite and MA overlap, making their simultaneous determination impossible with the present chromatographic system.Further details of the separation methods have been described previously.1,4,25 Optimization of experimental parameters In an attempt to obtain a maximum line to background intensity ratio and stability for the seven arsenic compounds, various operating parameters were studied and optimized individually with the on-line chromatographic system using a mixture of the seven arsenic compounds, while the other parameters were kept at their optimum values.The parameters investigated were microwave power to the plasma, and nitrogen Øow rates of the plasma and carrier gas for both sample introduction systems. It was observed that on increasing the microwave power the ion signals of all the arsenic compounds (seven) increased up to 1.3 kW (Fig. 3). A further increase of power was not possible. Hence, a 1.3 kW microwave power was used during the measurements. The effect of microwave power on the elemental ion signals is correlated with the ionization potential.18,19,26 A higher Fig. 1 Chromatogram obtained with a solution of dimethylarsinic acid (DMA), methylarsonic acid (MA) and arsenate [As(V)] (10 ng As from each species) in distilled water on a Supelcosil LC-SCX cationexchange column with N2-MIP-MS (for optimized conditions see Table 1; mobile phase, 20 mM pyridine at pH 2.50; injection volume, 0.100 ml; Øow rate, 1.5 ml min21). Fig. 2 Chromatogram obtained with a solution of arsenobetaine (AB), trimethylarsine oxide (TMAO), arsenocholine (AC) and tetramethylarsonium iodide (TMI) (10 ng As from each species) in distilled water on a Supelcosil LC-SCX cation-exchange column with N2-MIP-MS detection (conditions as in Fig. 1). J. Anal. At. Spectrom., 1999, 14, 1853±1859 1855microwave power is necessary for elements with high ionization potentials.26,27 For elements with high ionization potentials such as As (9.8 eV), the intensity is enhanced on increasing the forward microwave power,26 which agrees with our experimental Ændings.The annular-shaped plasma is inØuenced by the microwave power.18 The stable region of the plasma increases with increasing microwave power.18 This is because the plasma pressure and the plasma diameter increase with increasing microwave power. The tail Øame of the plasma also increases with increasing power.18 A long plasma is expected to lead to longer analytical residence times of the analytes in the plasma.18 Consequently, greater decomposition and higher ionization of the arsenic compounds occur, which ultimately increases the central analyte-rich region. Hence, the ion signals increase on increasing the microwave power.As the counts increase with increasing power, further increase in power may increase the concentration of 75Asz in the plasma.18,19,26 It is desirable for the power to be as high as possible to increase the sensitivity. Hence, further work will be directed towards lowering the detection limits of arsenic compounds by increasing the microwave power.The plasma and carrier gas Øow rates were altered to optimize the ion signals of the arsenic compounds, keeping the microwave power at 1.3 kW. On increasing the plasma Øow from 13 to 19 l min21, the ion signals increase up to 15 l min21 and reach a maximum at this value (Fig. 4). A further increase of plasma Øow rate does not cause the signals to vary signiÆcantly. This is probably as a result of the high viscosity of the plasma, which is not able to accommodate the increased gas Øow without some spatial distribution to provide an unimpeded path for the plasma gas.18 The visual hole of the annularshaped plasma produced by the carrier gas Øow decreases with an increase in the plasma Øow rate, as has been observed by Okamoto.18 The increase in ion signals with plasma gas Øow rate up to 15 l min21 is due to an increase in the stable plasma region, which Ænally increases the production of ions. A 1.1 l min21 carrier Øow rate (0.6±1.3 l min21) provides the maximum optimized signals for the seven arsenic species when peak area measurement is considered. A further increase/ decrease results in a decrease in total counts, as has been observed previously for total arsenic determination.15,26 It should be noted that the maximum ion signal value may be obtained as a compromise between the increment of aerosol into the plasma and the decrease of temperature in the central part of the plasma.This is recognized by the observation of a change in the color of the central part of the plasma with an increase in the nebulizer gas Øow rate. The color of the central part of the nitrogen plasma changes to a pale and whitish pink on increasing the nebulizer gas Øow rate above 1.1 l min21. Hence, the ion signals decrease above a 1.1 l min21 carrier Øow. Below 1.1 l min21, the carrier Øow decreases the aerosol production, transportation and injection velocity necessary to produce a central analyte-rich region, which ultimately leads to a decrease of the ion signals.Details of the optimization of the carrier Øow have been given by several workers.15,19,26 Effect of nebulizer and tubing The ICP-MS nebulizer with PEEK tubing increases the ion signals by at least 17±30% for anionic species and 21±25% for cationic species (Table 2) and is more efÆcient than the in-built instrumental nebulizer with PTFE tubing.The PEEK capillary tubing used additionally increases the counts (ion signals) and gives a better baseline separation of the arsenic compounds compared with the PTFE tubing (Table 2). By using the PEEK tubing, the peak heights are also enlarged (35±78%; Table 2) and the peaks are very sharp. In particular, the PTFE tubing has a larger diameter than the PEEK tubing. Hence, after separation, the analytes are diluted and diffused during transportation. Further, the PTFE tubing increases the dead volume more than the PEEK tubing when both are connected individually to the HPLC column and the nebulizer.As a result, the PTFE tubing reduces the separation efÆciencies and lowers the ion signals of the arsenic compounds. The ICP-MS nebulizer with PEEK tubing enhances aerosol production and its transmission efÆciency to the MIP, thereby increasing the sensitivity. Thus, it was used for all further measurements. Figures of merit The performance of the hyphenated technique was characterized by the linearity of the calibration graphs, repeatability, reproducibility and limits of detection (based on 2s of the blanks) within a series, which were calculated for three replicate measurements and are listed in Table 3.The intensities for the HPLC-N2-MIP-MS signals (on-line injection of a mixture of the seven arsenic compounds) given, indicate that a dynamic range of at least two orders of magnitude (Fig. 5) is necessary to cover the response range caused by the arsenic compounds present in the urine.Calibration graphs, obtained from the areas of the signals (three replicates) in chromatograms with standard solutions [As(V), MA, AB, AC, DMA and TMI at concentrations of 5.00, 10.0, 25.0, 50.0 and 100 mg l21 As; TMAO at concentrations of 10.0, 25.0, 50.0 and 100 mg l21 As)], are linear (Fig. 5). Repeatability is good for all the species; with respect to peak areas better than 9.2% and to peak heights 3.97% (Table 3; Fig. 6).Day-to-day reproducibility for the seven arsenic compounds is better than 11.4% for peak areas. We also analyzed and calculated the total counts and the detection limits of the investigated arsenic compounds using on-line injection of a mixture of the seven arsenic compounds with HPLC-ICP-MS. The total counts of the seven (individual) arsenic compounds found by using the HPLC-N2-MIP-MS and HPLC-ICP-MS systems are presented in Table 2. The counts of the seven arsenic compounds obtained with the HPLC-N2-MIP-MS system are lower than those with Fig. 3 Dependence of the ion signals of the seven arsenic compounds [As(V), MA, DMA, AB, AC, TMAO and TMI] on the microwave power using HPLC-N2-MIP-MS (conditions as in Fig. 1). Fig. 4 Dependence of the ion signals of the seven arsenic compounds [As(V), MA, DMA, AB, AC, TMAO and TMI] on the plasma gas Øow rate using HPLC-N2-MIP-MS (conditions as in Fig. 1). 1856 J. Anal. At. Spectrom., 1999, 14, 1853±1859the HPLC-ICP-MS system (Table 2).The detection limits for As(V), MA, DMA, AB, TMAO, AC and TMI are 0.69, 0.32, 0.86, 0.18, 6.33, 0.41 and 0.37 mg l21 As, respectively. Hence, the detection limit obtained for As(V) by using HPLC-N2-MIPMS is the same as that obtained with HPLC-Ar-ICPMS. However, for DMA (2), MA (3), TMAO (3), AC (3), AB (4) and TMI (5 times), the detection limits are about 2±5 times higher than those obtained with HPLC-Ar-ICP-MS (Table 2).The repeatability, reproducibility and correlation coefÆcient obtained with HPLC-high power-N2-MIP-MS are comparable to those obtained with HPLC-ICP-MS.28 The detection limit of arsenic is about 1±2 orders of magnitude lower than those reported for other N2-MIP systems.15,18,19 The main reasons for the lower detection limit may be the difference in the microwave power (500 versus 1300 W) used, and the difference in the shape (annular) of the plasma formed. However, owing to the high ionization potential of arsenic the detection limit is higher than that for Ar-ICP-MS.18,19 The higher detection limit of arsenic in N2-MIP-MS is because of the interference of the high concentration of 30NOz in the nitrogen plasma.18,19 It has been reported that the 30NOz content decreases on increasing the microwave power.18,19 Hence, further investigation will be aimed at lowering the detection limit of arsenic by increasing the microwave power or by using helium as the plasma support gas.Spectroscopic interference An interference study with chloride ion (500±10 000 mg l21) was carried out to observe whether there was any enhancement of the arsenic signal (m/z 75) by 40Ar35Cl.z The typical background signals at m/z 5±100 for Milli-Q water, 20 mM pyridine and 10 000 mg ml21 chloride are displayed in Fig. 7. Background counts of 290±300 were found with Milli-Q water at m/z 75. With the pyridine mobile phase the background signal was increased to 500±520 counts. No detectable arsenic signal (counts) at m/z 75 was observed when 500, 1000, 5000 and 10 000 mg l21 chloride were injected onto the column (Fig. 7). Hence, 40Ar35Clz interference due to chloride is absent, which agrees well with previous Ændings.26,27 This may be because the polyatomic species which are generated from the chloride matrix are about two orders of magnitude less intense than those of the Ar-ICP ion source.27 The increase of background counts in the pyridine mobile phase is owing to arsenic contamination, either from the pyridine or from the formic acid, used for mobile phase preparation.Analysis of real samples To validate the developed HPLC-MIP-MS method, NIES CRM-18 Freeze-Dried Human Urine was analyzed for the certiÆcation of arsenic compounds, particularly AB (Fig. 8). The peaks were identiÆed by comparison of the retention times (Fig. 1 and 2) with those of authentic standards and were conÆrmed by spiking with the standard arsenic compounds.Arsenate, MA, DMA and AB were detected, and conÆrmed by HPLC-ICP-MS using anion,1 cation,25 and reversedphase 6,24,29 columns with different mobile phases described previously.1,6,24,25,29 The AB concentration, estimated using the standard additions technique, was 78.1°1 mg l21, in agreement with the HPLC-ICP-MS value (72.6°8 mg l21). The AB is present as a major arsenic species (58% of the total arsenic) in urine followed by DMA, As(V) and MA.The AB, As(V), MA and DMA concentrations were also calculated using external calibration graphs, but the concentration of AB found was higher (90.8°7 mg l21) with respect to the standard additions method, indicating signal enhancement due to the urine matrix. The sum of all four arsenic species (arsenate, 8.92°1.1 mg l21; MA, 11.2°0.5 mg l21; and DMA, 41.5°0.7 mg l21) is consistent with the total arsenic content (134°6 mg l21). Moreover, Table 2 Total countsa of different arsenic species with two different nebulizing systems and with HPLC-ICP-MS MIP-MS nebulizerzPTFE tubing connecting the nebulizer and the HPLC column ICP-MS nebulizerzPEEK tubing connecting the nebulizer and the HPLC column HPLC-ICP-MS with PEEK tubing Arsenic species (100 mg l21) Area Height Area Height Area Height AB 8500°590 2210°131 10 320°73.5 (25)b 3950°89 (78) 50 600°136 8870°56 TMAO 1970°175 132°20.2 2390°60 (21) 179°2.83 (35) 7460° 499 230°34 AC 9910°850 1920°180 11 960°160 (20) 2950°114 (50) 39 900°3060 3650°240 TMI 9420°580 1640°118 11 560°468 (23) 2270°3.54 (38) 62 700°4410 4330°340 As(V) 12 000°120 2470°35.4 14 660°813 (23) 5860°24.7 (38) 18 900°76 2510°254 MA 15 610°570 2780°169 18 300°687 (17) 4270°76 (53) 45 200°2060 4700°223 DMA 14 140°610 1310°83 18 370°574 (30) 1870°60 (42) 32 600°1280 1740°55 aAverage of three determinations.bPercentage increased value is given in parentheses. Table 3 Detection limits,a repeatability,b reproducibilityc and correlation coefÆcients (R2)d after chromatographic separation of the arsenic compounds Repeatability (%) Reproducibility(%) R2 Analyte Detection limit/mg l21 (% RSD) Area Height Area Height Area Height As(V) 0.68 (5.2) 5.6 2.4 11 12 0.9978 0.9982 MA 0.95 (1.9) 9.2 1.8 8.3 5.2 0.9997 1.000 DMA 2.01 (2.8) 5.8 3.2 9.2 6.1 0.9992 0.9999 AB 0.92 (5.6) 0.7 2.3 6.5 6.2 0.9996 0.9964 TMAO 22.1 (1.6) 2.6 1.6 11 5.3 0.9982 0.9984 AC 1.31 (4.2) 3.8 6.0 7.8 5.2 0.9998 1.000 TMI 1.75 (1.4) 4.1 2.6 11 6.3 0.9999 0.9990 aDetection limits were determined as the elemental concentrations giving a signal twice the standard deviation (n~5) of the blank (Milli-Q water injected with 20 mM pyridine mobile phase).bRepeatability was determined for peak areas and peak heights by calculating the relative standard deviation (RSD of three successive analyses; concentration of each analyte was 25.0 mg l21). cReproducibility was determined for peak areas and peak heights by calculating the RSD of three analyses performed on three different days; the concentration of each analyte was 100 mg l21.dThe concentration range was 5±100 mg l21 for As(V), MA, AB, AC, TMI and DMA, and 10±100 mg l21 for TMAO. J. Anal. At. Spectrom., 1999, 14, 1853±1859 1857the veriÆcation of the anionic arsenic compounds requires the use of other methods and different chromatographic systems with the standard additions technique, before Ænal certiÆcation.Conclusions High power nitrogen-MIP-MS coupled with HPLC was examined as an element-speciÆc detector for its applicability to the determination of cationic and anionic arsenic compounds. The MIP is a promising alternative ion source for mass spectrometry for elemental speciation analysis. The detection limits, repeatability and reproducibility of the system are adequate for the determination of arsenic compounds. The background species found in the nitrogen plasma during the nebulization of aqueous solutions are not as complex as those found with the argon-ICP27 and are absent above m/z 45.18,19 Additionally, a high-purity nitrogen gas supply minimizes argon contamination and is easily obtained from liquid nitrogen boil-off.Moreover, the gas running cost of the N2- MIP-MS is lower than that of the Ar-ICP-MS. On the other hand, the high power nitrogen-MIP is more robust than the Ar- ICP. Hence, the high power N2-MIP-MS coupled with HPLC can be used as an element-speciÆc detector for elemental speciation and successfully used for arsenic species analysis. Limited information is available on elemental speciation using HPLC-N2-MIP-MS. To our knowledge, this is the Ærst report of the coupling of HPLC with MIP-MS for the determination of arsenic compounds.This coupling system may also be applicable to other elemental speciation analysis. Determination of arsenic in urine by ICP-MS is complicated by the formation of the argon chloride (40Ar35Clz) molecular ion, which overlaps with monoisotopic As at m/z 75.Several researchers have endeavored to eliminate or correct 40Ar35Clz formation.30±32 With N2-MIP-MS, the argon-related polyatomic interference is eliminated. No detectable interference in the m/z 70±80 region is observed. The chloride interference as 40Ar35Clz, arising from the urine matrix, in ICP-MS has been overcome. Hence, N2-MIP-MS should be a very useful detector for the determination of arsenic compounds in samples with high chloride concentrations (10 000 mg l21). The procedure is very promising and reliable and was successfully applied to the determination of AB in urine.The sample used here has been distributed to many organizations and will be issued as a certiÆed reference material by our institute, for a collaborative study to certify the arsenic species, especially AB. Although the accuracy of the AB value obtained was conÆrmed by HPLC-ICP-MS, the accuracy of the values for the anionic arsenic species in urine has not yet been established as collaborative work is still in progress.Acknowledgements The authors gratefully acknowledge Dr. Sukti Hazra for helpful comments during the preparation of the manuscript, Mr. Minoru Yoneda for data transformation and JISTEC and STA, Japan, for Ænancial support. Amit Chatterjee also acknowledges Mrs. Shizuko Kinoshita for her technical support and help. Fig. 5 Calibration graphs and variation of the ion signals with concentration for the seven arsenic compounds [As(V), MA, DMA, AB, AC, TMAO and TMI] using HPLC-N2-MIP-MS (conditions as in Fig. 1). Fig. 6 Dependence of the RSD (%) on the concentration of the different arsenic compounds [As(V), MA, DMA, AB, AC, TMAO and TMI] in HPLC-N2-MIP-MS (conditions as in Fig. 1). Fig. 7 Background mass spectra in the range m/z 5±100 when Milli-Q water, 20 mM pyridine and 10 000 mg l21 chloride solutions are introduced into the optimized plasma directly (conditions as in Table 1).Fig. 8 Chromatogram obtained for the reconstituted NIES Candidate CRM-18 Freeze-Dried Human Urine on a Supelcosil LC-SCX cationexchange column with N2-MIP-MS; 50 ml urine injected (conditions as in Fig. 1). 1858 J. Anal. At. Spectrom., 1999, 14, 1853±1859References 1 A. Chatterjee, Sci. Total Environ., 1999, 228, 25 and references cited therein. 2 X. C. Le and M. Ma, Anal. Chem., 1998, 70, 1926. 3 J. Mattusch and R.Wennrich, Anal. Chem., 1998, 70, 3649. 4 E. H. Larsen, G. Pritzl and S. H. Hansen, J. Anal. At. Spectrom., 1993, 8, 1075. 5 X. C. Le and M. Ma, J. Chromatogr. A, 1997, 764, 55. 6 Y. Shibata and M. Morita, Anal. Sci., 1989, 5, 107. 7 S. A. Pergantis, W. Winnik and D. Betowski, J. Anal. At. Spectrom., 1997, 12, 531. 8 H. Greschonig, M. G. Schmid and G. Gubitz, Fresenius' J. Anal. Chem., 1998, 362, 218. 9 A. Chatterjee and A. L. Mukherjee, Sci. Total Environ., 1999, 225, 249. 10 A. Chatterjee, D. Das, B. K. Mandal, T. R. Chowdhury, G. Samanta and D. Chakraborti, Analyst, 1995, 120, 643. 11 A. Chatterjee, D. Das and D. Chakraborti, Environ. Pollut., 1993, 80, 57. 12 Gy. Heltai, T. Jozsa, K. Poresich, I. Fekete and Zs. Jarr, Fresenius' J. Anal. Chem., 1999, 365, 487. 13 R. I. Botto, J. Anal. At. Spectrom., 1993, 8, 51. 14 X.-C. Le, W. R. Cullen and K. Reimer, Appl. Organomet. Chem., 1992, 6, 161. 15 K. Oishi, T. Okumoto, T. Iino and M. Koga, Spectrochim. Acta, Part B, 1994, 49, 901. 16 M. Ohata, T. Ichinose and N. Furuta, Anal. Chem., 1998, 70, 2726, and references cited therein. 17 M. Ohata and N. Furuta, J. Anal. At. Spectrom., 1997, 12, 341. 18 Y. Okamoto, Anal. Sci., 1991, 7, 283. 19 Y. Okamoto, J. Anal. At. Spectrom., 1994, 9, 745. 20 F. Leis and J. A. C. Broekaert, Spectrochim. Acta, Part B, 1984, 39, 1459. 21 J. Yoshinaga, T. Shirasaki, K. Oishi and M. Morita, Anal. Chem., 1995, 67, 568. 22 T. Nakahara and N. Takeuchi, Anal. Sci., 1997, 13, 13. 23 T. Shirasaki, J. Yoshinaga, M. Morita, T. Okumoto and K. Oishi, Tohoku J. Exp. Med., 1996, 81. 24 J. Yoshinaga, Y. Shibata, T. Horiguchi and M. Morita, Accredit. Qual. Assur., 1997, 2, 154. 25 W. Goessler, A. Rudorfer, E. A. Mackey, P. R. Becker and K. J. Irgolic, Appl. Organomet. Chem., 1998, 12, 491. 26 W. L. Shen, T. M. Davidson, J. T. Creed and J. A. Caruso, Appl. Spectrosc., 1990, 44, 1003. 27 W. L. Shen, T. M. Davidson, J. T. Creed and J. A. Caruso, Appl. Spectrosc., 1990, 44, 1011. 28 A. Chatterjee, Y. Shibata, J. Yoshinaga and M. Morita, Anal. Chem., submitted for publication. 29 Y. Shibata and M. Morita, Anal. Chem., 1989, 61, 2118. 30 C. J. Amarasiriwardena, N. Lupoli, V. Potula, S. Korrick and W. Hu, Analyst, 1998, 123, 441. 31 F. Laborda, M. J. Baxter, H. M. Crews and J. Dennis, J. Anal. At. Spectrom., 1994, 9, 727. 32 M. J. Campbell, C. Demesmay and M. Olle�, J. Anal. At. Spectrom., 1994, 9, 1379. Paper 9/05078G J. Anal. At. Spectrom., 1999, 14, 1853±1859 18

ISSN:0267-9477    

DOI:10.1039/a905078g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Quantitation of perchlorate ion by electrospray ionization mass spectrometry (ESI-MS) using stable association complexes with organic cations and bases to enhance selectivity{ Edward T. Urbansky,* Matthew L. Magnuson, David Freeman and Christopher Jelks United States Environmental Protection Agency (EPA), OfÆce of Research and Development, National Risk Management Research Laboratory, Water Supply and Water Resources Division, Treatment Technology Evaluation Branch, Cincinnati, OH 45268, USA.E-mail: urbansky.edward@epamail.epa.gov; magnuson.matthew@epamail.epa.gov Received 14th July 1999, Accepted 28th September 1999 Quantitation of trace levels of perchlorate ion in water has become a key issue since this species was discovered in water supplies around the United States. Although ion chromatographic methods presently offer the lowest limit of detection, #40 nM (4 ng ml21), chromatographic retention times are not considered to be unique identiÆers and often cannot be used in legal proceedings without conÆrmatory testing.Mass spectrometry can provide such conÆrmation; however, detection capabilities can impose a practical limitation on its use. Moreover, quadrupole mass spectrometers cannot provide sufÆcient accuracy and precision in m/z to identify conclusively an ion as perchlorate when samples are run directly without prior chromatographic or electrophoretic separation. We report on the abilities of (1) tetralkylammonium cations and (2) minimally nucleophilic, sterically hindered organic bases to increase selectivity in the electrospray ionization mass spectrometric (ESI-MS) determination of perchlorate ion without concomitant loss of sensitivity.Selectivity arises from the formation of a stable association complex between a base molecule and a perchlorate anion. The best results were obtained using 10 mM chlorhexidine in methanolic solution; the lower limit of detection (LLOD) for S/N¢2 was less than or equal to 0.10 mM (10 ng ml21).This compares favorably with the LLOD determined for perchlorate in the absence of any complexing agents (#0.05 mM~5 ng ml21). For the other bases, which were diazabicyclo compounds (DBN, DBU, DBO), sensitivity was lower by 90% or more. The chlorhexidine±perchlorate complex (m/z~605) can be observed even in the presence of equiformal nitrate, nitrite, hydrogensulfate, chloride, bromide, bromate, and chlorate (all together) down to approximately 1 mM; thus, the method is rugged enough to Ænd application to systems containing multiple inorganic anions.Introduction Perchlorate ion was identiÆed in ground and surface waters of the western United States in 1997, and has since been found at sites around the country. It may be found in the ground or surface waters near wherever perchlorate salts have been manufactured, stored, or used. Such sites are often associated with defense or aerospace programs (or supporting industries) since perchlorate salts Ænd use as solid oxidants or energetics boosters in rockets and missiles.The analytical chemistry of perchlorate and the signiÆcance of quantifying this contaminant have been reviewed and described in detail elsewhere.1,2 Because of the low concentrations of perchlorate found at most of the contaminated sites (5±50 ng ml21), the primary technique used for water analysis has been ion chromatography (IC). However, chromatographic retention time is not considered to be a unique identiÆer, and additional conÆrmation is required to initiate legal action.Although IC may be used for routine monitoring, periodic conÆrmatory testing must be carried out. As a consequence, techniques such as mass spectrometry can be expected to Ænd a role in secondary conÆrmation even if the instrumentation is not generally available on-site to potable water utilities, responsible parties, or regulatory governmental agencies for primary environmental monitoring purposes.There are essentially six different ways of assaying perchlorate. Gravimetry and titrimetry only apply to standardization of fairly concentrated laboratory (¢0.01 M) solutions. 1 Ion-selective electrodes suffer from a number of anionic interferences as well as lower limits of detection (LLODs) of #7 mM~70 ng ml21,1 which is at least twice what the no observable adverse effects level is predicted to be based on current research.3 Although Hauser et al.report a detection limit of 10 ng ml21 for an ion-selective electrode, this was in conjunction with a capillary electrophoresis (CE) separation.4 Barnett and Horlick used electrospray ionization mass spectrometry (ESI-MS) to obtain an LLOD of 0.050 mM (5 ng ml21).5 While spectrophotometric determinations are reported to have similar LLODs to the ESI-MS method,6 they are not sufÆciently selective, and dye purity can be an issue. Without prior analyte separation, quadrupole mass spectrometers cannot measure m/z ratios accurately and precisely enough to identify conclusively an ion with a mass of 99 u as perchlorate.Despite the selectivity generally regarded as associated with mass spectrometric identiÆcation, a large number of small mass (v300 u) hydrophilic inorganic or organic anions can be found in natural water sources as well as disinfected potable water supplies. Mass spectrometry offers less ambiguity than IC, but with a reduction in sensitivity.Introduction/ionization techniques for the involatile perchlorate ion are most likely constrained to either electrospray, which was used by Barnett and Horlick,5 or thermospray, which has not been reported for perchlorate. Extraction of perchlorate salts from aqueous solution is possible, but can be difÆcult to take advantage of analytically. Use of large cationic organic dyes, e.g., Brilliant Cresyl Blue or Brilliant Green, is reported for spectrophoto- {US Government copyright.J. Anal. At. Spectrom., 1999, 14, 1861±1866 1861 This journal is # The Royal Society of Chemistry 1999metric determinations of perchlorate.3 With a mass of 99 u, perchlorate anion has an m/z ratio in the region of many other ions commonly found in US waterways, from either natural or anthropogenic sources. Chlorinated potable water supplies contain other anions, e.g., chlorate and chloride, at substantially higher levels than perchlorate can be expected to be found.Additionally, some ions can be created in the electrospray process by direct electrochemical reduction. Consequently, it is highly desirable to improve not only sensitivity, but also selectivity, for perchlorate ion determination by electrospray mass spectrometry. To this end, we have explored the ability of a number of compounds to increase the selectivity of ESI-MS as a technique for determining perchlorate ion concentration in water through the formation of stable association complexes amenable to electrospray ionization.Experimental{ Instrumentation A Finnigan (San Jose, CA, USA) electrospray apparatus and a Finnigan TSQ 700 quadrupole mass spectrometer were used throughout. Samples were introduced via a Rheodyne (Rohnert Park, CA, USA) 7725 injector with a 200 ml sample loop. The liquid carrier (Fisher Optima1 (Pittsburgh, PA, USA) methanol) was supplied by a Waters (Milford, MA, USA) 600- MS pump. Mass spectra were acquired in both positive and negative ion modes.ESI is a sufÆciently soft ionization technique that fragmentation is generally not observed for these analytes. Table 1 gives additional parameters for operation and data acquisition. Reagents Solvents. All organic solvents were pesticide residue analysis grade, such as Fisher Optima1, Aldrich (Milwaukee, WI, USA), or Mallinckrodt Nanograde1 (Phillipsburg, NJ, USA). Inorganic salts. A stock solution of 1000 mg ml21 ClO4 2 was prepared by dissolving ammonium perchlorate, NH4ClO4 [7790-98-9] (Aldrich), in doubly de-ionized water (Barnstead Nanopure, Dubuque, IA, USA).Additional solutions were prepared by quantitative dilution of this stock solution. Stock aqueous solutions of NH4ClO4 (Aldrich), NaClO3 (Fisher), NaCl (J. T. Baker, Phillipsburg, NJ, USA), NaBrO3 (Fisher), NaBr (GFS, Columbus, OH, USA), NaNO2 (GFS), NH4NO3 (GFS), and NaHSO4 (Fluka, Buchs, Switzerland) were prepared at 0.10 M. Potassium salts were not used so as to avoid precipitation.Cationic organic dyes. Stock solutions of Brilliant Cresyl Blue [81029-05-2], Brilliant Green [633-03-4], and Crystal Violet [548-62-9] were prepared at 0.01 M by dissolving the solids in doubly de-ionized water containing 25% v/v methanol. Dyes were obtained from Spectrum (New Brunswick, NJ, USA). Organic bases. Stock solutions of Æve minimally nucleophilic (2� or 3�) sterically hindered organic bases were prepared: 1,4-diazabicyclo[2.2.2]octane (DBO, Dabco2, triethylenediamine) [280-57-9] 1, 1,5-diazabicyclo[4.3.0]nonene (DBN) [3001-72-7] 2, 1,8-diazabicyclo[5.4.0]undec-7-ene, (DBU, 2,3,4,6,7,8,9,10-octahydropyrimido[1,2-a]azepine) [6674-22-2] 3, 1,1'-hexamethylenebis[5-(4-chlorophenyl)-biguanide] (chlorhexidine) [55-56-1] 4, 4,5-dihydro-2,4-diphenyl-5-(phenylimino)- 1H-1,2,4-triazolium hydroxide, inner salt (nitron) [2218-94-2] (structure not shown), and triethanolamine (all from Aldrich). Concentrations of 0.010 or 0.10 M were obtained by dissolving known masses of the commercial reagents in doubly de-ionized water.Ethanoic (acetic) acid (HOAc, Mallinckrodt) at several concentrations was used to solubilize the chlorhexidine and the nitron. Quaternary cations. NMe4OAc [10581-12-1], NEt4OA-c? 4H2O [67533-12-4], NPr4OH [4499-86-9], NBu4Br [1643-19- 2], NHex4Br [4328-16-6], NOct4Br [14866-33-2], carbamyl-b -methylcholine (bethanechol) chloride [590-63-6] and As(C6H5)4Cl?H2O [507-28-8], were obtained from Aldrich or Sigma (bethanechol chloride).Stock solutions (0.10 M) were prepared by dissolving the solids in doubly de-ionized water and/or methanol. NPr4OAc was prepared by reacting NPr4OH with HOAc in situ. Sample preparation and treatment Test solutions. Methanolic (high-purity) solutions (containing 10±16% v/v water from dilution of stock reagents) were prepared containing perchlorate ranging from 0 to 50 mg ml21 and organic complexing agents at concentrations from 1 mM to 10 mM (depending on the complexing agent).Organic bases (except chlorhexidine) were run with and without acetic acid. Organic dye concentration was 1 mM. For comparison, propan-2-ol solutions of several agents were also prepared. Methanolic solutions of ammonium perchlorate with no other reagents were used as a control group. To evaluate ruggedness (resistance to matrix effects), methanolic solutions were prepared as described below (v5% v/v water).Each solution contained the following mix of electrolytes at equiformal conditions: NH4ClO4, NaClO3, NaCl, NaBrO3, NaBr, NaNO2, NaNO3, and NaHSO4. In addition, each solution contained 1.0 mM DBN, DBU, or DBO or 10 mM chlorhexidine. Test solutions were prepared containing 1.0, 2.0, 5.0, 7.5, 10, 20, or 50 mM of each salt with all salts in every test solution. Solvent extraction of cationic organic dye±perchlorate complexes. Aliquots of 100 ml of 1 mM dye and 50 mg ml21 perchlorate were extracted with equal volumes of ethyl acetate (EA) or tert-butyl methyl ether (MTBE).Solvent was drawn off by rotary evaporation under house vacuum at 60 �C (MTBE) {Mention of speciÆc brand names and trademarks should not be construed to reØect an endorsement of products or manufacturers by the United States Government. Table 1 ESI-MS operating conditions and data acquisition parameters Applied ESI spray potential/kV 4.0 Baseline ESI spray current/mA 0.342 Capillary temperature/�C 200 (optimal) Liquid carrier Methanol Carrier Øow rate/ml min21 0.30 Scan time/s 0.50, for D(m/z)~1.0 u Nebulizer gas pressure/kPa 482~70 psi Number of injections 3 Injection volume/ml 50.0 Relative standard deviation of replicate injections (%) v15 Scheme 1 Structures of organic bases. 1862 J.Anal. At. Spectrom., 1999, 14, 1861±1866or 70 �C (EA). The residue was dissolved in a minimum of methanol, transferred into a 50 ml beaker, and evaporated to dryness on a warm hot-plate.The residue was redissolved in 10 ml of methanol and subjected to ESI-MS analysis. Results and discussion Perchlorate complexes with organic bases Complexation. Table 2 gives the masses of molecular ions that were found by ESI-MS or for which we monitored the corresponding m/z ratio (assuming z of unity); masses of signiÆcant neutral species are given for reference. As can be seen from Table 2, the protonated organic bases, DBO, DBN, and DBU, form complexes with the empirical formula [(HBz)(ClO4 2)2].The mass spectrum of the molecular ion for the DBU±perchlorate complex of this form is shown in Fig. 1. As Fig. 2 shows, chlorhexidine is capable of multiple protonations and up to three perchlorate complexations with molecular ions of the form [chlorhexidine?ClO4(HClO4)n]2; n~1 or 2. Although we expected that a source of acid would be needed, we found that there was no effect on the peak areas when the acetic acid concentration was varied from 0 to 0.010 M with DBO, DBN, and DBU; at 0.10 M HOAc, the signal fell off dramatically, presumably due to a loss of electrospray efÆciency induced by ionic strength.Apparently, the water present from the stock perchlorate solution (or the methanol itself) is sufÆcient to provide a source of protons. For chlorhexidine, varying [HOAc] from 0.01 to 106(molar basis) the chlorhexidine concentration had no measurable effect on the peak areas. Selectivity and sensitivity.Of all the bases, chlorhexidine gave the greatest enhancement of selectivity as shown by Fig. 3, which includes perchlorate concentrations in the ppm range (1 ppm~100 mM). Sensitivity must be gauged in terms of instrument response relative to a total concentration of complexing agent. It should be noted that the highest total chlorhexidine concentration in Fig. 3(a) is 10 mM while the DBN concentration in Fig 3(b) is 10 mM, 1000 times greater. Although the DBU and DBO calibration lines in Fig. 3(b) have slopes about 1.5 times that of the chlorhexidine line with highest slope, this is observed where the chlorhexidine concentration is 10 mol% that of the other two bases. Accordingly, the relative sensitivity for chlorhexidine is computed to be about 7 times greater. The chlorhexidine± perchlorate complex gave a signal equal to approximately 10% of the signal found for perchlorate alone at low concentrations. Barnett and Horlick reported a LLOD of 5 ng ml21 for perchlorate ion alone.5 Based on the data in Fig. 4, we determined LLOD°0.10 mM (10 ng ml21) for the perchlorate± chlorhexidine complex.This concentration gave a measurable signal distinct from the average noise by a factor of at least 2. With regard to capillary temperature, we eventually opted for 200 �C, which seemed consistently to give the greatest sensitivity and injection-to-injection reproducibility. In terms of absolute sensitivity, comparison of Fig. 3(a) and (b) suggests that 1 mM DBU would be preferable to 10 mM chlorhexidine. However, at concentrations of perchlorate below 10 mM, the instrumental response for perchlorate was indistinguishable from the blank in 1 mM DBU. Meanwhile, as noted above, Fig. 4 indicates an acceptable response for chlorhexidine complexes in this region of perchlorate concentration. Therefore, chlorhexidine outperforms DBU in terms of absolute sensitivity at the lower perchlorate concentrations and is an overall better choice.In the presence of equiformal anions, molecular ions of the form [(HBz)(ClO4 2)2] were not observed for DBO, DBN, or DBU. Given that the sum of the anion concentrations is more than 8 times the chlorhexidine concentration, we had expected Table 2 Masses of investigated and/or identiÆed molecular ions and reference neutral moleculesa Species Mass/u Species Mass/u DBOz 112 f N(EtOH)3 z 149 f DBO(ClO4)2 211 f N(EtOH)3(ClO4)2 248 f DBO(ClO4)2H2 311 f N(EtOH)3(ClO4)2H2 348 f DBNz 124 f Nitz 312 f DBN(ClO4)2 223 f Nit(ClO4)2 411 n DBN(ClO4)2H2 323 f NMe4(ClO4)2 2 271 n DBUz 152 f NEt4(ClO4)2 2 328 f DBU(ClO4)2 251 f NPr4(ClO4)2 2 384 f DBU(ClO4)2H2 351 f NBu4(ClO4)2 2 440 f CHDz 505 f NHex4(ClO4)2 2 552 f CHD(ClO4)2 604 f NOct4(ClO4HD(ClO4)2H2 704 f Beth(ClO4)2 2 359 f CHD(ClO4)3H2 2 804 f HClO4 0 100 n ClO4 2 99 f aDBO~1,4-Diazabicyclo[2.2.2]octane. DBN~1,5-Diazabicyclo[4.3.0]- non-5-ene.DBU~1,8-Diazabicyclo[5.4.0]undec-7-ene. CHD~Chlorhexidine.N(EtOH)3~Triethanolamine. Nit~Nitron. Bethz~ C7H17N2O2 z, from bethanechol chloride. IdentiÆcation: f~found, n~not found, based on peaks in ESI mass spectra. Fig. 1 Negative ion ESI mass spectrum obtained for complex of 1,8- diazabicyclo[5.4.0]undec-7-ene (DBU) with perchlorate. Similar results were obtained for DBO and DBN. See Table 2 for molecular ion identiÆcation. Fig. 2 Negative ion ESI mass spectrum obtained for complexes of chlorhexidine with perchlorate.Note that multiple perchlorate ions can associate with a single chlorhexidine molecule. Complexes have the following empirical formula: [chlorhexidine?ClO4(HClO4)n]2; n~1 or 2. See Table 2 for molecular ion identiÆcation. J. Anal. At. Spectrom., 1999, 14, 1861±1866 1863to see an unresolvable system of mixed anion complexes in the mass spectrum or perhaps nothing at all, but this was not the case. Instead, we found molecular ions of the form [chlorhexidine? X2] for perchlorate, chloride, nitrate, and bromide. However, there was no evidence for chlorhexidine complexes with nitrite, hydrogensulfate, acetate, bromate, or chlorate.It is worth pointing out that we have never seen a peak that corresponds to an association complex of acetate with any of the reagents. We assume that a peak at m/z~586 corresponds to a chlorhexidine±hydrogensulÆte complex that formed from sulfate reduced by the electrospray process, and we suspect that chlorate, bromate, and nitrite are reduced as well.Further exploration of these peaks and the fate of the other anions was beyond the scope of this work and was not pursued. The peaks for the chlorhexidine complexes of chloride, nitrate, and bromide are larger than the peak for the perchlorate complex; nevertheless, Fig. 5 does show that the perchlorate response remains visible even under competition for the chlorhexidine. Although the sensitivity is reduced relative to perchlorate in the absence of the other inorganic anions, a non-linear response curve can be generated, as shown in Fig. 6. Attenuation of the signal at high concentration is probably due to reduced electrospray efÆciency resulting from ionic strength effects. Based on Fig. 3 and 6, we believe that this method could be applied under a variety of conditions to both analytical solutions and real water samples. Some variation may be needed for optimization, such as changing pH or chlorhexidine concentration. Preconcentration may be advised for some samples, and we expect that standard approaches would work, e.g., simple evaporation, lyophilization, or on-column retention (using a strong anion-exchange resin).The suitability of these would depend on both analyte concentration and matrix constituents. Given the ionic content of the test solutions, we expect that matrix effects could readily be accounted for in drinking water by using the method of standard additions, but this would require validation and optimization on a per case basis. Fig. 5 suggests that chlorhexidine might even prove useful for selectively enhancing a number of different anions, but further exploration of the phenomenon was beyond the scope of this work. Applicability to potable water analysis. Because of the high ionic strength of potable water, which includes sodium chloride at considerably higher concentration than the analyte, suppression of the electrospray signal is to be expected.Two different potable waters were tested: Cincinnati tap water and Tri-Township Water (TTW). The source of Cincinnati tap water is the Ohio River. TTW is supplied by a well in Dearborn County, Indiana, and probably experiences some inÆltration from the Whitewater River. The TTW sample was collected from a residential faucet in Logan Township. A 20 ml sample of TTW water was evaporated to dryness at 60 �C and reconstituted in an equal volume of 10% v/v H2O± MeOH.Aqueous chlorhexidine was added to give a Ænal chlorhexidine concentration of 10 mM in the reconstituted Fig. 3 Calibration graphs (peak area versus perchlorate concentration) for ESI-MS determination of perchlorate: (a) top to bottom, in (#) 10 mM, (%) 1 mM, (©) 0.1 mM chlorhexidine,MeOH solution; (b) (%, ±±) 1 mM DBU, (‡, –) 10mM DBN, ((, -?-) 1 mM DBO. All lines are based on least-squares regression. Fig. 4 Calibration graph (peak area versus perchlorate concentration) for ESI-MS determination of perchlorate in 10 mM chlorhexidine, MeOH solution.First six points used for least-squares regression line. Presumably, points 7 and 8 suffer from loss of electrospray efÆciency due to ionic strength. Inset: Calibration graph for perchlorate anion without any complexing agents, MeOH solution, provided for comparison. Non-linear response at higher perchlorate concentration is observed with and without complexing agents. Fig. 5 Negative ion ESI mass spectrum for 1 mM (equiformal) NH4ClO4, NaClO3, NaCl, NaBrO3, NaBr, NaNO2, NaNO3, and NaHSO4 in 10 mM chlorhexidine, MeOH solution.In this case, the only perchlorate±chlorhexidine association complex observed is that containing one chlorhexidine molecule and one perchlorate ion. See text for additional discussion. 1864 J. Anal. At. Spectrom., 1999, 14, 1861±1866sample. It is important to note that the mineralized residue resulting from evaporation does not redissolve in the Ænal solution.In order to promote as much dissolution of perchlorate salts as possible, the reconstituted samples were permitted to sit for 20±40 min before decanting the solvent. A signal distinguishable from that of unspiked samples was obtained at 20 and 40 mM perchlorate with a response factor of 16 000 area units per 20 mM spiked into the sample. Much of the chlorhexidine peak (m/z~505 u) is lost from the supernatant, presumably due to adsorption onto the mineralized residue.For this reason, spiking the supernatant with additional perchlorate after decanting from the evaporation vessel gives no increase in signal. Nonetheless, if the chlorhexidine concentration is returned to 10 mM (as indicated by the peak at m/z~505 u), further perchlorate spikes give appropriate response. Because of the electrospray suppression, large dilution factors are required if the waters are to be run without preconcentration. We found that a 5% v/v dilution of tap water samples gave the best results.Of course, this would mean that the perchlorate concentration in the water sample would have to be 20 times greater than the post-dilution concentration. In both TTW and Cincinnati samples, a post-dilution concentration of 250 nM perchlorate (1.25 mM pre-dilution) gave a response of about 9200±9500 area units above the unspiked blank. We must point out that the response is not linear in these matrices, and quantiÆcation of perchlorate under such conditions would require careful construction of a calibration graph in the matrix under study.Simple dilution is adjudged not to be a generally suitable approach for quantifying perchlorate in the presence of multiple other inorganic anions. Ideally, a preconcentration technique would select for perchlorate over other anions. We speculate that preconcentration using certain highly selective resins, such as those recently developed by the Oak Ridge National Laboratory,7 would dramatically improve the performance of this method and could reduce the LLOD by a factor of 100 or more.Methanol versus propan-2-ol. We compared the signal of 50 mg ml21 ClO4 2 standards with DBU and chlorhexidine in propan-2-ol versus methanol. No measurable difference was found. In addition, we were concerned that clusters of (MeOH)2(H2O)2 might be responsible for units of D(m/ z)~z100 rather than HClO4. Since these peaks are also observed in the propan-2-ol solutions but not in perchloratefree methanolic blanks, we conclude that the perchlorate is in fact responsible, rather than some cluster of solvent or spectator species.Quaterny cations and other species Performance of the tetraalkylammonium ions as complexing agents was so much poorer than the organic bases that it sufÆces to say that these will be of little analytical utility. No signal was detected for the molecular ions of the tetramethylammonium, tetraethylammonium, or tetrapropylammonium cations, [(NR4 z)(ClO4 2)2].Triethanolammonium and bethanechol gave observable signals, but these did not compare favorably with chlorhexidine or the diazabicyclo bases. Tetrabutylammonium, tetrahexylammonium, and tetraoctylammonium gave signals whose magnitude increased with the length of the carbon chain for equiformal [NR4 z]. However, these salts are limited by their solubilities in water and methanol (or combinations thereof), and did not compare favorably with the results obtained for the organic bases above.No signal was found at the m/z ratio predicted for the molecular ion of tetraphenylarsonium cation associated with a perchlorate anion and either another perchlorate anion or a chloride anion. No signal was detected for a protonated nitron associated with perchlorate anion; neutral nitron is zwitterionic, and it is usually protonated with acetic acid to solubilize it and to form the precipitant cation.In positive ion mode ESIMS, however, all of the expected cations could be identiÆed. Consequently, we feel conÆdent in saying that there is a lack of the complexation behavior characteristic of the other bases rather than an inability of the cations to be carried into the vapor phase. Cationic organic dyes We did not observe any evidence for the presence of complexes of perchlorate with cationic dyes in the ESI mass spectra when directly injecting the methanolic dye±perchlorate solutions or the reconstituted extracts.While we are uncertain whether the extractions were successful, we suspect that the failure of the direct injection is probably due to the inability of the electrospray apparatus to nebulize and ionize these species effectively. Regardless, this particular strategy does not appear to be worth pursuing further, even though it may be suitable for the reported spectrophotometric determinations.6 Conclusions Chlorhexidine gave the greatest improvement in selectivity without loss of sensitivity for perchlorate detection and quantiÆcation, and it was the only base to show resistance to matrix effects, even at much lower base concentration. Although the non-nucleophilic organic bases (i.e., DBO, DBN, DBU) gave acceptable increases in selectivity, losses of sensitivity were rather high.Surprisingly, cationic precipitants used in perchlorate gravimetry, i.e., tetraphenylarsonium and nitron, gave no signal at all.Because the organic bases substantially elevate the mass–by a factor of 3±8–the peak is completely separated from the low mass area where other anions are commonly found. In addition, complexes with water or methanol molecules are not seen in the mass spectrum; thus, peaks of species such as H2O?81Br2 (m/z~99), which was seen by Barnett and Horlick,5 are conveniently absent. The use of complexing reagents therefore permits ESI-MS analysis without separation on account of its increased selectivity. Nonetheless, use of IC or CE is not precluded, and hyphenated techniques (i.e., LC-MS or CE-MS) may Ænd a use for such reagents.The thoughtful use of complexing agents such as chlorhexidine will perhaps provide a valuable tool in terms of Fig. 6 Perchlorate response (peak area versus concentration) for ESIMS determination in 10 mM chlorhexidine in the presence of seven other equiformal anions, MeOH solution; see Fig. 5 caption. This is not a true calibration graph, because the sample matrix (but not the chlorhexidine) varies in concentration, too. Inset: Calibration graph for perchlorate anion without any complexing agents, MeOH solution, provided for comparison. Non-linear response at higher anion concentration is observed, presumably due to ionic strength effects on electrospray efÆciency. J. Anal. At. Spectrom., 1999, 14, 1861±1866 1865increased selectivity without diminished sensitivity in the analytical determination of the perchlorate anion. Acknowledgements The Ænancial support of EPA's summer mentoring program is recognized by D.F. and C.J. References 1 E. T. Urbansky, Biorem. J., 1998, 2, 81, and references cited therein. 2 E. T. Urbansky and M. R. Schock, J. Environ. Manag., 1999, 56, 79. 3 US Environmental Protection Agency, Perchlorate Environmental Contamination: Toxicological Review and Risk Characterization Based on Emerging Information, Review Draft, December 31, 1998, Document No. NCEA-1-503. 4 P. C. Hauser, N. D. Renner and A. P. C. Hong, Anal. Chim. Acta, 1994, 295, 181. 5 D. A. Barnett and G. Horlick, J. Anal. At. Spectrom., 1997, 12, 497. 6 A. A. EnsaÆ and B. Rezaei, Anal. Lett., 1998, 31, 167. 7 B. Gu, G. M. Brown, S. D. Alexandratos, R. Ober and V. Patel, Selective Anion Exchange Resins for the Removal of Perchlorate ClO4 2 from Groundwater, Oak Ridge National Laboratory, Oak Ridge, TN, February, 1999, Doc. No. ORNL/TM-13753, Environmental Sciences Division Publication No. 4863. Paper 9/05721H 1866 J. Anal. At. Spectrom., 1999, 14, 1861±1866

ISSN:0267-9477    

DOI:10.1039/a905721h

出版商:RSC    

年代:1999

数据来源: RSC