摘要:

The influence of true simultaneous internal standardization and background correction on repeatability for laser ablation and the slurry technique coupled to ICP emission spectrometry† Joachim No�lte,*a Franziska ScheZer,b Sabine Mann,‡c and Michael Pauld aBodenseewerk Perkin-Elmer GmbH, D-88647 U� berlingen, Germany bMartin-Luther-Universita�t Halle-Wittenberg, Institut fu�r Technische Chemie, D-06108 Halle, Germany cInstitut fu�r Umweltforschung, Universita�t Dortmund, D-44221 Dortmund, Germany dPerkin-Elmer Deutschland, D-88647 U� berlingen, Germany Received 4th November 1998, Accepted 5th February 1999 For the solid sampling techniques laser ablation and the slurry technique, the influence of simultaneous measurement vs.sequential measurement of the signal and background and the internal standard was investigated on an identical data set. The experiments were carried out with an array spectrometer, which generates spectra rather than single points.These spectra, typically measured around the analyte wavelength, are measured simultaneously. The ICP spectrometer used has a ‘profiling’ mode, whereby the entrance slit is moved fourfold and the spectra are recorded with a small spectral shift in a sequential manner. These images are then combined to form a spectrum which is composed of simultaneous and sequential measured data. Hence, by carefully selecting the data points in a measured spectrum, the diVerence between simultaneous and sequential measurements can be estimated from an identical data set.In the example studied, simultaneous background correction and simultaneous internal standardization (typical for array ICP-OES) gave a mean RSD of 3.6%, sequential background correction and simultaneous internal standardization (typical for simultaneous ICP-OES) produced an average RSD of 6.1% and sequential background correction and sequential internal standardization (typical for sequential ICP-OES) resulted in a mean RSD of 6.4%.The RSDs reflect the solid sampling procedure. Some applications of laser ablation and slurry sampling are presented. (ICP-MS) than for LA-ICP-OES. In a literature survey,4 not Introduction taking monographs and review articles into account, 101 In inductively coupled plasma optical emission spectrometry references were found for LA-ICP-MS compared with 35 for (ICP-OES), the analyte signal is always calculated as the net LA-ICP-OES. The use of an array-type spectrometer greatly signal, being the diVerence of the gross signal (analyte plus improves the quality of data for ICP-OES combined with background) minus the background signal (measured oV-line).LA.5,6 The slurry technique is usually associated with AA (228 The analyte signal and the background signal are influenced references) and there are considerably fewer publications on by the excitation temperature, which depends on the mass slurry ICP-OES (58 references).However, a large number of being introduced into the plasma. The eVect on the analyte papers on the slurry technique for ICP-OES have been pubsignal can be compensated with the use of an internal standard. lished for materials which are comparatively easy to digest or The height of the background signal in practice is only have weak chemical bonds (e.g., biological materials or available by measurement. If a steady-state signal can be coal ).5–8 For these materials, slurry sampling represents a generated, the fact that the signal and background are meas- practical technique.ured in a sequential manner with conventional ICP emission In this work, material which is diYcult to digest was used spectrometers does not present a problem. to investigate the potential and limitations of these methods Solid sampling techniques typically produce a far less uni- of solid sampling and how simultaneous measurements can form flow of sample to the plasma,1 which causes a fluctuating help to improve the reproducibility.signal. The only way to approach this situation is to measure all signals at the same time. However, in conventional ICP emission spectrometers, both signals are measured sequentially, Experimental even in most types of simultaneous spectrometers. The advent Instrumentation of array detectors for ICP emission spectrometry allowed the measurement of both signal contributions simultaneously.2,3 The ICP emission spectrometers used were Optima 3000 and Much more information is available for laser ablation (LA) 3300 DV (Perkin-Elmer, Norwalk, CT, USA).The Optima combined with inductively coupled plasma mass spectrometry series of spectrometers use two segmented charge coupled device (CCD) detectors (SCD), one for the UV section and the other for visible light. The detector and optics have been †Presented at the 8th Solid Sampling Spectrometry Colloquium, described in detail elsewhere.9,10 The Optima 3000 has a Budapest, Hungary, September 1–4, 1998.vertical torch with a radial view plasma and the Optima 3300 ‡Present address: Perkin-Elmer Deutschland, D-88647 U� berlingen, Germany. DV has a horizontal torch with a choice of axial11 or radial J. Anal. At. Spectrom., 1999, 14, 597–602 597Table 1 Operating conditions for the ICP emission spectrometers standard should be measured at the same time. The diVerence in repeatability when varying the degree of simultaneity was Power 1300 W studied in a data set generated with SRM BCR 142 ( light Outer gas flow rate 15 L min-1 sandy soil ).For this purpose, a ‘worst case scenario’, a soil Intermediate gas flow rate 0.5 L min-1 with a high degree of heterogeneity, was chosen in order to Carrier gas flow rate 0.7 L min-1 Viewing height (radial viewing) 15 mm have a more pronounced eVect. For this comparison of sequential and simultaneous measurement modes on the identical instrument and data set, view.12 The operating parameters for the plasma are summar- the profiling option of the ICP spectrometer was used.It ized in Table 1. should be emphasized that the described profiling option is The ICP mass spectrometer used was an ELAN 6000 not the recommended mode of recording data. Its only purpose (Perkin-Elmer SCIEX, Concord, ON, Canada). It has a dual in this context is to use a sequential measurement mode in the stage detector for pulse and analog reading.The quadrupole system. Profiling is a fast sequential measurement option while settling time is 200 ms for the pulse mode and 3 ms for the partially retaining simultaneity. When invoking profiling, the analog mode. The integration time was set to 1 s and the dwell entrance slit is moved one quarter of the slit width four times time to 10 ms. The plasma power was set at 1000 W with a consecutively. The spectral image falls on to the detector four outer gas flow rate of 15 L min-1 and a carrier gas flow rate times with the wavelength slightly shifted.By carefully selecting of 0.95 L min-1. the points that are used for peak maximum and background For laser ablation, a Laser Sampler 330 (Bodenseewerk correction, one can either choose a situation where all points Perkin-Elmer GmbH, U� berlingen, Germany) was used. It are gathered simultaneously (the points which are spaced four houses an excimer laser filled with KrF operating at the data points apart) or a situation where the points are gathered fundamental wavelength at 248 nm.The energy pulse was sequentially (all points not four data points apart). Fig. 1 greater than 20 mJ with a pulse duration of 10 ns. The unit illustrates the data generation process for the profiling option. was operated at a pulse frequency of 5 s-1. A Tygon tube The repeatability vs. processing mode was calculated for a (1 m×6 mm id) was used to carry the ablated material directly data set generated by laser ablation of SRM BCR 142 ( light to the injector of the plasma torch.Axial viewing was used sandy soil ). Twelve firings were made on a single spot on the throughout. sample. After a read delay time of 20 s,6 the spectra were For slurry sampling, a magnetic stirrer was used. The slurries recorded with an integration time of 0.1 s using the profiling were aspirated using standard 0.6 mm id tubing materie was set intentionally to such a short time in (PTFE) and pumped at 1 mL min-1 through a standard cross- order to have a time comparable to the laser pulse frequency flow nebulizer with GemTips, connected to a Scott-type spray of 5 s-1 or a time lag between two firings of 0.2 s.When using chamber. It was tested for horizontally and vertically this option, four images are generated, so the total measureoriented plasmas. ment time was 0.4 s. Eight element and wavelength combinations were recorded. The selection is based mainly on Sample pre-treatment freedom from spectral interference and on the amplitude of the signals and included Cu at 327 nm, Fe at 273 and 302 nm, Laser ablation.If block samples (such as metals or glass) Mg at 279 nm, Mn at 260 nm, Si at 221 nm, Ti at 368 nm and were to be analyzed, disks of 32 mm diameter or small pieces Zn at 206 nm; the internal standard was Al at 237 nm. glued to a mounting block were directly mounted in the sample The overall relative standard deviation (RSD) for eight cell of the laser sampler.Powdered samples (soils) were pressed wavelengths and corrected with one internal standard was to pellets after the addition of 20% Spectroblend (Chemplex calculated for these 12 firings. For further data processing, the Industries, Tuckahoe, NY, USA). The mixture was pressed RSDs were again averaged. If background correction and for 3 min at 690 MPa in an IR press.6 internal standard were applied simultaneously, an average RSD of 3.6% was calculated.The opposite, performing all Slurry technique. The slurry technique was used for zeoliths measurements sequentially, produced an RSD of 6.4%, while (faujasite LSX, mordenite, pentasil ZSM-5 and the layer sequential background correction but simultaneous internal silicate hectorite) and boron nitride (BN). Particle size recstandard gave 6.1%, which does not diVer significantly. One ommendations in the literature are generally 2 or 3 mm.13–17 can deduce from this that sequential and simultaneous ICP The zeolith samples were ground to this size with a ball mill emission spectrometers, which typically measure the back- made of stainless steel (Wissenschaftlicher Gera�tebau, ground in a sequential manner, produce a similar quality of Magdeburg, Germany).For zeoliths, it was found that after data, which contrast with those generated with an array type 15 min a median particle size of 3 mm resulted. After prolonged instrument.All subsequent experiments were conducted with- periods of grinding, the particles coagulated to larger particles, out the profiling option so as to ensure simultaneous back- so grinding was stopped after 15 min. ground correction. Also the integration time was set to be Amounts of 50–100 mg of sample were weighed and 100 mL identical for all elements in a given method, so all readings of 2% v/v HNO3 were added to the powder. For the prepwere taken simultaneously in order to allow real-time internal aration of boron nitride slurries, one drop of 0.1% Triton-X standardization. solution was added.The mixture was stirred vigorously with As mentioned above, the measuring time was deliberately a magnetic stirrer, after mixing, prior to sample uptake and set to be very short. For normal applications, longer measuring during the measurement.18 times are used (typically 1–20 s), resulting in a repeatability of around 0.5–2%.6,19 Results and discussion Measuring mode and repeatability Laser ablation applications In a laser ablation application, ICP-MS and ICP-OES were For laser ablation, a repeatability of typically around 10% is achieved.This can be improved to a few per cent by internal compared for a homogeneity test of the main components in glass. For homogeneity tests, the absolute concentration is not standardization.6 However, it is strongly influenced by the homogeneity of the sample and the elements to be determined.of importance but rather how precise a measurement can be made in order to distinguish between data points gathered for For a non-steady-state signal it is important that the internal 598 J. Anal. At. Spectrom., 1999, 14, 597–602A B C D Fig. 1 The profiling option of the Perkin-Elmer Optima 3000 series ICP emission spectrometer produces four simultaneous images of spectra which are generated sequentially by slightly moving the entrance slit four times ( left), so the spectra A, B, C and D are collected one after another. These four images are then overlaid to produce one composite image (right). All spectra are plotted as intensity vs.wavelength. the same material. The major components of this speciality butions of the background have a comparatively larger impact than for higher concentrations. Thus, good repeatability and glass were Ba (53%), Al (2.6%) and Si (16%). Single shots were fired at the glass. Fig. 2 shows the dot raster generated.a good linear calibration function are indications of proper correction of background fluctuations. A calibration with four Si was used as an internal standard. The RSDs of the resulting signals were then calculated. For ICP-MS, being a fast sequen- standards gave correlation coeYcients of 0.995 for all elements. Examples of calibration functions of two critical tial technique, the RSDs were 2.3% for Al and 2.5% for Ba. For ICP-OES, with all simultaneous measurements, the corre- elements, Pd and Pt, are shown in Fig. 3. sponding RSDs were 0.6 and 0.5%, respectively, allowing statements on the homogeneity of the sample. Slurry technique applications In another application, contaminants in silver alloys were For applications of the slurry technique, zeoliths and BN were determined. These included Au (at 208.209, 242.795 and used. To characterize the particle sizes of the materials, scan- 267.595 nm), Bi (at 190.178 and 223.061 nm), Cd (at 214.438, ning electron microscope (SEM) images were recorded as 226.502 and 228.802 nm), Cu (at 224.700, 324.754 and shown in Fig. 4 for zeolith. The chemical composition of the 327.396 nm), Ni (at 221.647, 231.604, 232.003 and zeoliths varied considerably (Table 2). The data were measured 341.476 nm), Pb (at 220.353 nm), Pd (at 324.270, 340.458 and after a microwave digestion with HF, HCl and HNO3. The 363.470 nm), Pt (at 214.423 and 265.945 nm), Sb (at 217.581 water content was determined after heating at 1000 °C for 2 h.and 231.147 nm) and Zn (at 202.548, 206.200 and 213.856 nm). The sum of the components is around 93% for the first five The repeatability was found to be around and below 1% using materials and 100% for the other two. the Ag line at 206 nm for internal standardization. The concen- On introducing particles into the sample introduction trations of these elements were relatively low (in the range up system, namely into the large volume Scott spray chamber, a to 500 mg kg-1).For low concentrations, the noise contriconcern is that there would be a prolonged stabilization time of the signal when the sample is introduced or removed. Fig. 5 and 6 show wash-in and wash-out for hectorite. After about Fig. 2 Microphotograph of glass with equally spaced laser craters 641 Pd 324.270 Pt 265.945 0 0.05 0 0.05 321 Intensity/counts s–1 Concentration (wt%) Correlation coefficient : Correlation coefficient : 0.9996 0.9992 Fig. 3 Calibration diagrams for Pd at 324.270 nm and Pt at 265.945 nm from an excimer laser, operating at 248 nm, across the surface for homogeneity studies with ICP-MS and ICP-OES.With kind per- for purity determinations using LA-ICP-OES of silver alloys with the Ag line at 206.117 nm for internal standardization. mission from R. Sievers and S. Rings, University of Bonn, Germany. J. Anal. At. Spectrom., 1999, 14, 597–602 599Fig. 4 SEM image of zeolith NaY using a magnification of 5000.One can clearly identify clusters of crystals. The individual crystals have a diameter of slightly above 1 mm. Fig. 6 Wash-out behavior for hectorite introduced as a slurry using the same conditions as described earlier. The remaining high signal for Na can be attributed to contamination of the water used for dilution. The stability of the signal of this element in solution is comparable to that of the elements meared as slurries in Fig. 5. Fig. 5 Time diagram to illustrate the stabilization period for introducing slurries.In this case, hectorite was aspirated. At 4 s, a new measurement is made. It takes about 1 min until the signal reaches steady-state conditions. After this period, the signal remains stable. The intensities for Ca and K were multiplied by a factor of 10 for better visibility. 1 min, the signal stabilizes, which corresponds to the stabilization time for liquid analysis. For the remaining time of the Fig. 7 Stability of slurries of boron nitride.Occasional spikes are quasi-steady-state signal, the precision was calculated for a observed for all signals. When calculated for all 50 replicates, the series of five consecutive points and was of the order of 1–2%. RSD is 3.2%. On introducing slurries of BN, occasional spikes were observed, as shown in Fig. 7, that degraded the repeatability to an extent not acceptable when using a set of five consecutive trations. The normal routine in ICP-OES is to use matrixmatching even for aqueous solutions.This becomes more measurements as one would do in a routine application. The RSD calculated for all 50 readings in the given example was important the higher is the concentration of the matrix. It would be expected that for slurry sampling, a high matrix 3.2%, still too high for stoichiometric calculations but acceptable for purity determination. Since all elements follow the eVect would result. In contrast, some workers have suggested calibrating against aqueous standards for ZrO2 powders.20 same trend, the precision can be improved significantly by taking one element as internal standard.When Fe at 259 nm Since no matrix-matched standards were available, we tested whether it would be possible to calibrate against aqueous is used as an internal standard, the RSD for 50 readings is then 0.5% (Fig. 8). Since the concentration ratio in the solid standards. The results are presented in Table 3. The zeolith of faujasite type, LSX, shows acceptable agree- is important for stoichiometric calculations, this procedure is acceptable.ment with the results for the digested material. The results for the slurry are generally higher, which is also reflected by the As a next step, we tried to quantify the element concen- Table 2 Chemical composition (% m/m) of zeoliths determined after digestion Faujasite Pentasil Layer silicate Component LSX Mordenite-I Mordenite-II ZSM-5 hectorite SiO2 32 66 63 81 50.8 Al2O3 25 9 10 2.3 — Na2O 0.15 5 5.9 1.9 2.6 MgO — — — — 24.6 Li2O 7.8 — — — 2.3 H2O 28 14.3 13.8 8.1 21 Sum 93 94 93 93 101 600 J.Anal. At. Spectrom., 1999, 14, 597–602120000 10000 80000 60000 40000 20000 0 1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 4 10 16 22 28 34 40 46 Replicate Intensity/counts s–1 52 58 64 70 76 82 88 94 Fig. 9 Repeat experiment to verify Si signal spikes (arrows) on reinserting the sample tube after the aspiration of air for 4 s.go in order to compensate for the low readings found during the quasi-steady-state measurements. Fig. 8 By referencing the readings of Fig. 7 to an internal standard A repeat experiment to verify this behavior is depicted in (in this case Fe at 259.940 nm), the RSD improves to 0.48% (n=50). Fig. 9 for the Si signal in pentasil ZSM-5. At first, it was assumed that the introduction of air would raise the signal intensity. In order to check this hypothesis, nitrogen and fact that the sum is calculated to be 105% for the slurry technique in contrast to 93% for the digested sample.When oxygen (up to 10% of the carrier gas) were added with the aid of another external mass flow controller. However, the signal calculating the Si/Al ratio, which is an important figure for characterizing a zeolith, there is acceptable agreement between intensity increased by only about 10%. It appears that the first particles entering the plasma still find the plasma in a much slurry (151) and digest (1.1 : 1).For the other materials, the Si and Al values diVer considerably, as does also the Si/Al hotter state. The introduction of more particles then cools the plasma until an equilibrium is reached. During this initial ratio. Si and Al make up the structure with strong bonds via an oxygen atom. The other constituents are loosely bound period, the particles are in an environment hot enough to ‘crack’ them completely. This assumption is also backed by and therefore show much better agreement. It is assumed that the particles are ‘digested’ in the plasma the fact that the Si/Na ratio increases when the temperature in the analyte channel of the plasma, reflected by the ratio of starting from their surfaces.The elements on the surface of the particles or with loose bonds are therefore more easily Ca ionic-to-atomic transitions, is raised by changing the power of the rf generator or changing the carrier gas flow.removed and atomized. This assumption is further backed by the fact that in zeolith NaY, Al is not homogeneously distributed but found preferentially on the surface. For this material, Conclusions the Al concentrations measured in the digest and slurry are in good agreement. Also, mordenite-I is a finer material than Solid sampling for ICP-OES has the potential to present analytical information when all information is collected in real mordenite-II, which is otherwise very similar in chemical composition and identical in structure.As a consequence, the time. A key role is the use of an internal standard. It is important that this is matched as closely as possible to the agreement is much better for the material which has a greater portion of smaller particles. Hence it can be concluded that analytes with respect to excitation conditions. Calibration reacts sensitively to the nature of the calibration standard, so the general rule to matrix-match must also be followed here.This applies to the material type and size distribution. it is imperative that the chemical composition and physical properties resemble each other as much as possible. During the stability runs, it was observed by coincidence that spikes of Si and Al signals appeared immediately after If these basic rules are followed, more information can be extracted when using the lateral information provided by laser re-aspirating the sample during a run when the tube had slipped out of the slurry mixture, similar to those shown in ablation with excellent precision and good accuracy.The slurry technique is very simple to realize. The main Fig. 9. These spikes were not observed for the other constituents. The magnitudes of these spikes were so large that when attraction of this technique lies in the minimum sample pretreatment for materials which are diYcult to digest. The the maximum intensity was used for concentration calculation, the result was very close to the expected concentration.Hence particle size is very important. In our experiments, it was typically around 3 mm, which appears to be too large for this these spikes appear to indicate in which direction one should Table 3 Chemical composition (% m/m) of zeoliths determined using slurry technique Faujasite Pentasil Layer silicate Component LSX Mordenite-I Mordenite-II ZSM-5 hectorite SiO2 39 18 12 35 12 Al2O3 30 3.2 0.7 0.5 — Na2O 0.3 6 6.8 2.8 2.8 MgO — — — — 23 Li2O 8 — — — 1.2 H2O 28 14.3 13.8 8.1 21 Sum 105 42 33 46 60 J.Anal. At. Spectrom., 1999, 14, 597–602 6017 E. Poussel and J.-M. Mermet, Spectrochim. Acta, Part B, 1996, type of material. The wash-in and wash-out behavior is 51, 75. favorable, as are the short- and long-term reproducibilities, 8 J. No� lte, J. Scho�ppenthau, L. Dunemann, T. Schumann and which can be further improved with an internal standard L. Moenke-Blankenburg, J. Anal.At. Spectrom., 1995, 10, 655. measured simultaneously. The slurry technique thus far has 9 T. W. Barnard, M. I. Crockett, J. C. Ivaldi and P. L. Lundberg, yielded acceptable to good results for elements on the surface Anal. Chem., 1993, 65, 1225. 10 T. W. Barnard, M. I. Crockett, J. C. Ivaldi, P. L. Lundberg, D. A. and loosely boto the core material when calibrated against Yates, P. A. Levine and D. J. Sauer, Anal. Chem., 1993, 65, 1231. aqueous standards. In order to obtain good results for elements 11 J. C. Ivaldi and J. F. Tyson, Spectrochim. Acta, Part B, 1995, that have strong bonds in a material, which consists of larger 50, 1207. particles, at present a perfect matrix match seems to be 12 M. DuVy and R. Thomas, At. Spectrosc., 1996, 17, 128. essential. In order to calibrate against aqueous standards for 13 J. G. Williams, A. L. Gray, P. Norman and L. Ebdon, J. Anal. At. Spectrom., 1987, 2, 469. these types of samples, more research is needed. 14 L. Ebdon, M. E. Foulkes and S. J. Hill, J. Anal. At. Spectrom., 1990, 5, 67. 15 I. Halicz and I. B. Brenner, Spectrochim. Acta, Part B, 1987, 42, 207. References 16 D. A. Laird, R. H. Dowdy and R. C. Munter, J. Anal. At. 1 M. Thompson, C. D. Flint, S. Chenery and K. Knight, J. Anal. Spectrom., 1990, 5, 515. At. Spectrom., 1992, 7, 1099. 17 P. Goodall, M. E. Foulkes and L. Ebdon, Spectrochim. Acta, Part 2 J. C. Ivaldi and J. F. Tyson, Spectrochim. Acta, Part B, 1996, B, 1993, 48, 1563. 51, 1443. 18 L. Ebdon and A. R. Collier, J. Anal. At. Spectrom., 1988, 3, 557. 3 J.-M.Mermet and J. C. Ivaldi, J. Anal. At. Spectrom., 1993, 8, 795. 19 M. Ducreux-Zappa and J.-M.Mermet, Spectrochim. Acta, Part B, 1996, 51, 333. 4 M. Sperling, PELIDAS—Personal Literature Data System, 20 R. Lobinski, W. Van Borm, J. A. C. Broekaert, P. Tscho� pel and Bodenseewerk Perkin-Elmer GmbH, U� berlingen, 1998. G. To� lg, Fresenius’ J. Anal. Chem., 1992, 342, 563. 5 J.No� lte, L. Moenke-Blankenburg and T. Schumann, Fresenius’ J. Anal. Chem., 1994, 349, 131. 6 J.No� lte, Fresenius J. Anal. Chem., 1996, 355, 889. Paper 8/08591I 602 J. Anal. At. Spectrom., 1999, 14, 597

ISSN:0267-9477    

DOI:10.1039/a808591i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

INTER-LABORATORY NOTE Direct elemental analysis of lead in micro-samples of human fingernails by rhenium-cup in-torch vaporization-inductively coupled plasma atomic emission spectrometry (ITV-ICP-AES)† Hamid R. Badiei and Vassili Karanassios* Guelph–Waterloo Center for Graduate Work in Chemistry, University of Waterloo, Department of Chemistry, Waterloo, Ontario, Canada N2L 3G1 Received 8th September 1998, Accepted 17th December 1998 Micro-samples of human fingernails were analyzed for lead using Rhenium-cup, in-torch vaporization (ITV) sample introduction and inductively coupled plasma atomic emission spectrometry (ICP-AES).Lead concentrations were found to be in agreement with those reported in the literature. Direct elemental analysis of micro-samples of fingernails, minimum sample pre-treatment and, overall, increased speed of analysis are the main advantages of the approach. ITV, a sample is placed onto or into a probe (e.g., coiled 1 Introduction filament or metal cup), the sample carrying probe is inserted Lead is widely monitored in the environment because of its into a vaporization chamber that clips onto a typical ICP toxicity.For example, chronic lead poisoning is a key concern torch and a seal is formed at the bottom of the chamber.9,12,14 in many parts of the world. In humans, chronic lead poisoning During retraction, the plasma is operated un-interrupted with is manifested by abnormalities such as encephalopathy, ner- the seal open.The sample is vaporized by applying electrical vous irritability, kidney disease and altered heme synthesis power to the sample carrying probe. Due to the use of an and reproductive functions.1–4 Such poisoning is associated with low to intermediate levels of chronic exposure to lead, with primary sources being intake of food, water and air. Suitable indicators are required in order to evaluate the extent of human exposure to Pb. Fingernails are often used as such an indicator.4 From the analytical point of view, fingernail samples do not deteriorate over time and can be obtained easily without traumatizing the donor.The average composition of Pb in fingernail samples from healthy adults cannot be determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) with pneumatic nebulization sample introduction unless a relatively large amount of sample is acid-digested (e.g., multiple fingernail clippings). Further diYculties arise if there is only a small amount of sample available for analysis, as is often the case with fingernails and other micro-samples of biological, clinical or forensic origin.One way of addressing this problem is by using a sample introduction system that permits direct elemental analysis of small amounts of solid samples by ICP-AES, such as, direct sample insertion (DSI),5,6 electrothermal vaporization (ETV)7,8 or in-torch vaporization (ITV).9–14 The object of this work was to bring to ICP-AES the capability of direct elemental analysis of solid micro-samples using Re-cup ITV sample introduction and to use the determination of Pb in fingernails as an example. 2 Instrumentation 2.1 In-torch vaporization A schematic illustration of ITV sample introduction and of the instrumentation used for this work is shown in Fig. 1. In Fig. 1 Schematic illustration of the metal-cup ITV-ICP-AES system †Presented at the 8th Solid Sampling Spectrometry Colloquium, Budapest, Hungary, September 1–4, 1998. used in this work.J. Anal. At. Spectrom., 1999, 14, 603–605 603external power supply, some in-situ (i.e., inside the vaporization ensure that there was no sample carryover from run to run, the cup was washed with 5 ml of 0.1% (v/v) HCl and this was chamber) sample processing becomes possible. For example, samples can be dried and ashed/charred in-situ by applying followed by another vaporization cycle. Using this approach, memory eVects were not observed. progressively higher power levels to the sample carrying probe.Thus far, coiled-filament ITV-ICP-AES has been tested with In coiled-filament ITV-ICP-AES,14 carrier-gas flow rate, coiled-filament insertion position inside the vaporization micro-samples of liquids and slurries but not for direct elemental analysis of solid micro-samples. For the present work, chamber and vaporization power were found to aVect analytical performance characteristics significantly.These parameters several changes were made to previous ITV designs.9,14 For instance, to facilitate solid micro-samples, the coiled filament were briefly optimized for Re-cup ITV-ICP-AES as well, and examples of optimized analyte emission and blank signals are was replaced by a ~5×5 mm Re cup. Rhenium was chosen due to its high melting point (3186 °C), its low thermal shown in Fig. 2. conductivity, its relatively high electrical resistivity and its resistance to chemical attack.13,14 Also, the ceramic which was 4 Results and discussion used to support coiled filaments14 was replaced by an insulated metallic support with a Teflon collar on its upper part (Fig. 1). Instrument calibration is a key problem in the direct elemental These changes kept the cup centered within the insertion tube analysis of solids, in particular, if there are no standard and the vaporization chamber, thus eliminating the possibility reference materials available, as is the case with fingernails.of cup contamination during insertion/retraction and improv- Calibration curves were constructed using the standard ing the reproducibility of insertion. The support was fastened additions method and the Pb content of the fingernail samples onto a drive mechanism which has been used in our laboratory was found to be 0.7±0.15 mg g-1. This result compares favorfor DSI work6 and it was manually driven in or out of the ably with what has been reported in the literature for healthy vaporization chamber. The vaporization chamber was modi- adults.15 Liquid standards were also used for calibration and fied as well and a new chamber (Fig. 1), with a wider diameter the Pb concentration was found to be 0.7±0.2 mg g-1. The in its lower part (to accommodate the Teflon collar), was tested. relatively large standard deviation is most likely due to uneven distribution of Pb within the fingernail samples. The agreement 2.2 Instrumentation between results obtained using the standard additions method and calibration using liquid standards is most likely due to A JY-48 (Jobin–Yvon Instruments SA, Edison, NJ, USA) use of a micro-amount of a sample with a matrix, such as ICP-AES system which was equipped with a 32-channel ceratin, which vaporizes prior to analyte vaporization (e.g., polychromator was used.The current output of the photomulduring charring/ashing). tiplier tube detector was converted to a voltage using a Stanford Research Systems (Sunnyvale, CA, USA) preamplifier/ low-pass filter (SRA, Model SR570). The analog 5 Conclusions voltage was digitized at 100 Hz using a 12-bit analog-to-digital (ADC) board (NB-MIO-16L-25, National Instruments@, Fingernails are important indicators of human exposure to Pb Austin, TX, USA) and an Apple Macintosh personal com- because they literally ‘freeze’ past exposure in time.2–4 In a puter.The data acquisition software was written in LabViewA broad sense, they can be thought of as metal burden indicators (National Instruments@). in the body or as built-in sensors which, when analyzed, can provide a record of the history of personal exposure.Rhenium-cup ITV-ICP-AES enabled rapid and direct 3 Experimental elemental analysis of Pb in micro-samples of human fingernails, 3.1 Reagents and samples thus opening the door to rapid screening for chronic exposure to lead, to homogeneity studies and to determinations of the Standard solutions were prepared from 1000 ppm standard localized (rather than average) composition of fingernails by stock solution (2–5% (v/v) in HNO3 purchased from SCP ICP-AES.Such applications my be useful in toxicology or ScienceA, Quebec, Canada). The solutions were prepared via in high-risk industries (e.g., battery plants or sheet metal repetitive dilutions using distilled, de-ionized water (18MV). production facilities) for risk assessment and management. Fingernail samples were collected from healthy adults using a Ni-coated steel clipper and were cleaned using stainless steel scissors.Subsequently, the samples were washed in an ultrasonic cleaner with 30 ml of 95% ethyl alcohol for 20 min, rinsed repeatedly with de-ionized water and placed under an infrared lamp for 20 min to dry. Dried micro-samples (from 200 to 400 mg) were weighed using a micro-balance (Cahn Electrobalance, Ventron Instruments Corporation, Cerritos, CA, USA) and were placed into the Re cup of the ITV sample introduction system using stainless steel forceps. 3.2 Plasma and ITV operating conditions Plasma operating conditions were 1200 W of applied forward power, outer-tube gas 13 l min-1 (Ar), intermediate-tube gas 0.6 l min-1 (Ar) and central-tube or carrier gas 0.45 l min-1 (Ar–H2 3% v/v). Hydrogen was added to suppress the formation of volatile Re oxides.14 The observation height was 14.5 mm above the load coil. The cup insertion position inside the vaporization chamber was ~1.5 cm above the carrier gas inlet.Drying and charring were carried out in-situ. For example, drying time was 1 min (at 5 W), ashing/charring time was 1 min (at 15W and, at this power level, analyte loss was Fig. 2 Typical signals obtained following a brief optimization (see text for discussion). not observed) and vaporization power was 75W (~5 s). To 604 J. Anal. At. Spectrom., 1999, 14, 603–6057 V. Karanassios, J. M. Ren and E.D. Salin, J. Anal. At. Spectrom., Acknowledgements 1991, 6, 527. 8 J. M. Carey and J. A. Caruso, Crit. Rev. Anal. Chem., 1992, Part of this work was reported in an undergraduate thesis 23, 397. (Chem 492, April 1998) by HRB. Financial assistance from 9 V. Karanassios, K. P. Bateman and G. A. Spiers, Spectrochim. the University of Waterloo and from NSERC is gratefully Acta, 1994, 49, 847. acknowledged. 10 V. Karanassios, K. P. Bateman and G. A. Spiers, Spectrochim. Acta, 1994, 49, 867. 11 V. Karanassios, K. P. Bateman and G. A. Spiers, Spectrochim. Acta, 1994, 49, 989. References 12 V. Karanassios, P. Drouin and G. G. Reynolds, Spectrochim. 1 A. Taylor, S. Branch, D.J. Halls, L.M.W. Owen and M. White, Acta, 1995, 50, 415. J. Anal. At. Spectrom., 1998, 13, 57R. 13 V. Grishko and V. Karanassios, Proceedings, Second Biennial International Conference on Chemical Measurement and 2 Trace Element Analysis in Biological Specimens, ed. R. F. M. Monitoring of the Environment, ed. R. Clement and B. Burk, 1998, Herber and M. Stoeppler, Elsevier, NY, USA, 1994. vol. 2, p. 507. 3 Biological Monitoring of Exposure to Chemicals: Metals, ed. H. K. 14 V. Karanassios, V. Grishko and G. G. Reynolds, J. Anal. At. Dillon and M. H. Ho,Wiley, NY, USA, 1991. Spectrom., 1999, 14, 565. 4 Quantitative Trace Analysis of Biological Materials, ed. H. A. 15 http://www.arup-lab.com/ug/u8–1251.htm (#P30235, Test No. McKenzie and L. E. Smythe, Elsevier, NY, USA, 1988. 0099044). 5 V. Karanassios and G. Horlick, Spectrochim. Acta Rev., 1990, 13, 89. 6 V. Karanassios and T. J. Wood, Appl. Spectrosc., 1999, 53, 197. Paper 8/07033D J. Anal. At. Spectrom., 1999, 14, 603–605 605

ISSN:0267-9477    

DOI:10.1039/a807033d

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

A comparison of automated and traditional methods for the extraction of arsenicals from fish John W. McKiernan,†a John T. Creed,*a Carol A. BrockhoV,a Joseph A. Carusob and Roseanne M. Lorenzanac aUnited States Environmental Protection Agency, National Exposure Research Laboratory, Microbiological and Chemical Exposure Assessment Research Division, 26W. Martin Luther King Drive, Cincinnati, OH 45268, USA. E-mail: creed.jack@epamail.epa.gov bDepartment of Chemistry, University of Cincinnati, Cincinnati, OH 45221–0172 cUnited States Environmental Protection Agency, US EPA Region 10, 1200 6th Avenue, Seattle, WA 98101, USA Received 11th November 1998, Accepted 9th February 1999 An automated extractor employing accelerated solvent extraction (ASE) has been compared with a traditional sonication method of extraction for the extraction of arsenicals from fish tissue.Four diVerent species of fish and a standard reference material, DORM-2, were subjected to both extraction methods.Arsenicals that were extracted with 50% (m/m) methanol–18MV water were speciated with chromatographic separation and inductively coupled plasma mass spectrometric (ICP-MS) detection. Both extraction methods produced extraction eYciencies of greater than 71% with RSDs on replicates of less than 5.5%. The chromatographic separation employed a PRP-X100 anion exchange column. An ammonium nitrate and ammonium carbonate buVer at pH 9.0 was used to resolve five arsenicals.The speciation data indicates that the predominant species were arsenobetaine and arsenocholine. Two unknown arsenic species were present in most of the samples. The two extraction techniques produce similar relative distribution of arsenobetaine–arsenocholine (AsB–AsC) and dimethylarsinic acid (DMA) with relative area distributions of>95% and<2%, respectively. Arsenicals have widely varying ionic characteristics which Introduction are pH dependant. Therefore, finding one separation scheme Speciation of arsenicals is a growing area of interest in both to separate all of the relevant arsenicals that are present within mechanistic and exposure assessment research.This is due in a sample is diYcult. Anionic arsenicals are usually separated part to the species dependent toxicity of arsenic. While the using anion exchange chromatography.14,19–22 Gailer and inorganic species, arsenite (AsIII) and arsenate (AsV), have Irgolic have studied the separation of arsenic anions using the been classified as carcinogenic,1 the methylated forms, mono- Hamilton PRP-X100 anion exchange column.23 Some methylarsonic acid (MMA) and dimethylarsinic acid (DMA), researchers have used a cationic separation in addition to an have recently been identified as cancer promoters.2 It is believed anionic separation in order to resolve all of the ionic species that arsenobetaine (AsB) and arsenocholine (AsC), which present.9,24,25 Others have performed separations of arsenic are highly substituted organoarsenicals, are relatively standards by ion pair chromatography.6,26,27 Finally, inducnontoxic. 3 tively coupled plasma mass spectrometry (ICP-MS) is the The two major sources of arsenic exposure are drinking detection system of choice in the speciation of arsenic due to water and dietary ingestion. According to the 1986–1991 FDA its sub ng g-1 detection capabilities. Total Diet Study,4 seafood is a major source of total arsenic The focus of this manuscript is to compare accelerated exposure.Seafoods are a dietary staple for coastal populations. solvent extraction (ASE) to conventional sonication as a This exposure profile for arsenic in seafood has made the means of extracting arsenicals from dietary fish. The extraction speciation of arsenic in seafoods an active area of research.5–18 process is often the most time consuming and labor intensive The extraction of arsenicals from fish tissue is an area of portion of metal speciation studies.ASE, an automated extracincreasing environmental importance. Both chloroform5,9,17,18 tion method which has been described in detail by Richter and acetone7 have been used to extract the nonpolar fats and et al.,28 has the capability of performing static extractions at lipids from fish tissue prior to a polar extraction, generally elevated temperatures and pressures. Of particular interest is performed using a methanol and water mixture.The majority the investigation of whether ASE is universally applicable to of arsenicals present are extracted into the polar solvent. extraction of arsenicals from fish without the loss of arsenic Studies have been reported in which the fat is not removed species specific integrity. from fish prior to arsenic extraction.15 This was attempted in The direct comparison of conventional sonication with ASE this study using the sonication method, however, the high fat will be made using a mass balance approach to calculate content of some fish (particularly salmon) hindered both the extraction eYciencies of arsenic from fish tissues.18 A second recovery and the filtration of the liquid extracts.comparison is the distribution of arsenic in the two extraction techniques. This distribution comparison is made based on relative area percentages and, therefore, speciation based cali- †Fellow, Oak Ridge Institute for Science and Education. J.Anal. At. Spectrom., 1999, 14, 607–613 607bration curves and associated quantitation QC are not neces- NH4NO3+5.0 mM (NH4)2CO3, pH 9.0; 175–180 s, step; 180– 696 s, 50.0 mM NH4NO3+5.0 mM (NH4)2CO3, pH 9.0; 696– sary. This allows for a direct species by species comparison without the need for species specific quantitation. 702 s step; 702–960 s, re-equilibrate with 5.0 mM NH4NO3+ 5.0 mM (NH4)2CO3, pH 9.0. The chromatograph, from Dionex Corporation (Sunnyvale, CA, USA), was coupled to an Experimental ICP-MS instrument by narrow bore PEEK tubing (Dionex Instrumentation Corporation) inserted into the introduction tube of the glass concentric nebulizer of the ICP-MS unit.The dead volume in A VirTis (Gardiner, NY, USA) lyophilizer was used to freeze the connection was minimized to reduce peak broadening. dry the fish. An 8-ounce Osterizer was used for homogenization. A Zymark TurboVap LV Evaporator (Hopkinton, MA, Sample preparation USA) was used to evaporate the organic solvents and water used in the extraction process.The centrifuge used in this Fish samples were freeze dried and then homogenized. The study was a Model UV from IEC International Centrifuge samples were then freeze dried again until a constant mass Universal. The ASE instrumentation used was an ASE 200 was attained. The DORM-2 sample was used as received. from Dionex Corporation (Sunnyvale, CA, USA). Total arsenic The ICP-MS instrument, used for total arsenic determination and for detection of arsenicals during speciation, was Three 0.1 g samples of each freeze-dried, homogenized fish a Plasma Quad 3 from VG Elemental (Winsford, Cheshire, were digested using concentrated nitric acid and 30% hydrogen UK).For total arsenic determination, a Gilson 222 auto- peroxide, as described in EPA method 200.3.29 The addition sampler was used. A glass concentric nebulizer from Glass of hydrochloric acid was omitted to limit the amount of Expansion (Camberwell, Victoria, Australia) was used in com- chloride present that can form 40Ar35Cl when introduced into bination with a cooled (5 °C), double-pass spray chamber. the argon plasma.Blank, fortified blank, and fortified samples The SAS System for Windows, SAS Institute Inc. (Cary, were treated in the same manner. Total arsenic determinations NC, USA), was programmed to obtain integrated areas from were performed using ICP-MS. the IC-ICP-MS data. Extraction Reagents The sonication extraction procedure described was based on The acetone and methanol, used in the extraction portion of work by Alberti et al.15 The ASE conditions were not optimthis study, were high purity grade reagents purchased from ized, rather, they were chosen to be similar to the sonication Baxter Healthcare Corporation (Muskegon, MI, USA).Ultrex extraction procedure in order to facilitate a direct comparison. II ultra-pure grade concentrated nitric acid from J.T. Baker Parameters such as the number of cycles, pressure, flush (Phillipsburg, NJ, USA) and 30% hydrogen peroxide from volume, and purge time were chosen as they were the default Alfa Aesar (Ward Hill, MA, USA) were used to perform values for the ASE instrument. The temperature during the digestions. Trace metal grade ammonium hydroxide and extraction was ambient to avoid degradation of the arsenicals HPLC grade ammonium carbonate, both from Fischer during extraction.An outline of the extraction procedure can Scientific (Fairlawn, NJ, USA), were used to prepare the be found in Fig. 1. eluents used in the chromatographic separations. Inorganic In this study, a total digestion method has been used to arsenic standards, 1000 mg ml-1 AsIII and 1000 mg ml-1 AsV, determine the total arsenic concentration (Asacid digest in Fig. 1) purchased from Spex Industries Inc. (Edison, NJ, USA), were in dry fish tissue. In order to remove fats and other matrix used to prepare arsenic standards for total arsenic determi- components that might interfere in the extraction of arsenicals nation.AsB was acquired from the University of British or which may confound the chromatographic separation of Columbia, Department of Chemistry, Vancouver, Canada. arsenicals, the fish tissue was subjected to a non-polar solvent Dimethylarsinic acid and disodium methyl arsenate, both 98% extraction using acetone. This was followed by a polar extracpurity (Chem Service, West Chester, PA, USA), were used to tion using 50% methanol in 18MV water.The extraction prepare the DMA and MMA standards, respectively. All stock eYciency was calculated as a percentage based on the concenstandards were made based on arsenic and their concentrations tration of arsenic extracted with the polar solvent (Aspolar in were validated using NIST 1643c. All water used in this study Fig. 1) over the digested concentration (Asacid digest). for dilution and mobile phase preparation was 18 MV Conversely, the ineYciency of the extraction can be represented (Millipore Corporation, Bedford, MA, USA).by the percentage recoveries in the nonpolar extraction Fish samples used in this study were chosen based on (Asnonpolar in Fig. 1), and the arsenic remaining in the residual expected fat levels and arsenic concentrations. Salmon, white- fraction (Asresidual in Fig. 1). Combining these recoveries yields fish, and shark were purchased as fresh fillets from a local a procedural mass balance approach which will be utilized in market.Tuna, also purchased from a local market, was canned evaluating the two extraction procedures. in spring water. The standard reference material, DORM-2, was purchased from the National Research Council of Canada. Sonication procedure. Each sample set contained three replicate samples and a blank. A 0.5 g sample of freeze-dried, Chromatography homogenized fish tissue was extracted in triplicate with 10 ml of acetone.Each extraction required Vortex mixing, 10 min Extracts obtained using 50% (m/m) methanol–18MV water were filtered using 0.45 mm polypropylene filters from Gelman for sonication, and 10 min of centrifugation at 3200 rpm. The acetone portions for each sample were combined, dried, and Sciences (Ann Arbor, MI, USA). For speciation studies, the Hamilton PRP-X100 anion exchange column (Hamilton digested for a total arsenic determination.The arsenic extracted with acetone is referred to as Asnonpolar in Fig. 1. This was Company, Reno, NV, USA), and the matching guard column, were maintained at 50 °C. A 0.05 cm3 sample loop was used. followed by a polar extraction, performed in triplicate using 10 ml of 50% (m/m) HPLC grade methanol in 18MV water. The flow rate was 1.0 ml min-1. It was necessary to use gradient elution to resolve the arsenicals with highly varying The samples were dried to remove the methanol, rewetted with 18MV water and total arsenic was determined. The retention characteristics, while preserving adequate resolution between a large AsB–AsC peak and the adjacent AsIII.arsenic extracted with methanol and 18MV water is referred to as Aspolar in Fig. 1. Speciation was performed on these The chromatographic program was: 0–174 s, 5.0 mM 608 J. Anal. At. Spectrom., 1999, 14, 607–613Fig. 1 Flow diagram of extraction procedure. Ideally, Asacid digest=Assum. polar extract samples.The residual solids were retained for the collection vial and brought to a final mass with 18 MV water. The residual solids were digested in the same manner. digestion followed by determination of total remaining arsenic. The arsenic remaining in the residual solids is referred to as Asresidual in Fig. 1. The total extraction time for sonication Total arsenic determination in digested fish and extracted was 2 h. fractions. Prior to sample analysis, the ICP-MS was tuned at m/z 75 using a solution of 5 ng g-1 As and Y, 15 ng g-1 Ge, ASE procedure.The ASE parameters were set as follows: 3 and 1% HNO3 in 18MV water. Short term precision was cycles at 1500 psi; ambient temperature, flush volume, 60% of measured by performing ten replicate data collections on the the extraction cell volume; 5 min static time (the time sample tuning solution using an integration time of 1 min. The relative is held at specified temperature and pressure); 60 s nitrogen standard deviation (RSD) of the signals at m/z 72, m/z 75 and purge at the end of the extraction.m/z 89 was less than 2%. The samples were prepared by placing 0.5 g freeze-dried Internal standards, 10 ng g-1 Y and 30 ng g-1 Ge, were homogenized fish tissue on top of washed high density glass added to all standards and samples. All of the samples were beads in an 10 ml extraction cell. The sample size of the determined for total arsenic by monitoring m/z 75, while m/z standard reference material, DORM-2 was 0.1 g.Each sample 72, 73, 76, 77, 79, 82, 83, 84, 86 and 89 were monitored to analysis set contained three replicate samples and a blank. determine the presence of interferences, correct for instrument Nonpolar extractions were performed on each of the three drift, and/or determine changes in the plasma due to matrix replicates of each fish using acetone under the ASE parameters eVects. This detection was performed in peak jump mode listed above.The liquid portions for each sample were dis- under normal operating conditions. The signal at m/z 75 was pensed automatically into a 40 ml borosilicate collection vial. corrected for possible interferences due to 40Ar35Cl by use of This was retained for total arsenic determination (Asnonpolar, the signal at m/z 77. Four-point calibration curves were Fig. 1). constructed from AsV standards, and all of the data collected The solid material remaining from the above extraction was fell within the range of the calibration curves.The data were extracted three times using a polar solvent [50% (m/m) internal standard corrected, based on an interpolation between methanol–18MV water]. The liquid portions were collected, 72Ge and 89Y signals. The internal standard correction factor dried, rewetted with 18 MV water and determined for total was between 80 and 120% for all samples. The slope of a arsenic (Aspolar, Fig. 1). Speciation was performed on these typical calibration curve was 4000 cps per ppb while digested samples.The residual solids in the extraction cells were blanks produced 675 cps. The lowest concentration determined reclaimed for digestion. The arsenic determined in the residual in any of the total concentration measurements reported in solids is referred to as Asresidual in Fig. 1. Table 1 was 1.2 ng g-1. Some samples required dilution in order to minimize the internal standard correction required.Treatment of collected material The liquid samples obtained from each extraction were all Arsenic speciation. The samples containing arsenicals extracted using 50% (m/m) methanol–18MV water were fil- taken to dryness in the original collection vials using an automated evaporator set at 60 °C with a constant flow of tered, and chromatographically separated on a Hamilton PRPX100 anion exchange column. Detection was performed at nitrogen. The nonpolar (acetone) extraction was digested in J.Anal. At. Spectrom., 1999, 14, 607–613 609Table 1 Total arsenic concentrations in fish and fish extracts DORM-2 Whitefish Shark Salmon Tuna [As] Percentage [As] Percentage [As] Percentage [As] Percentage [As] Percentage Sample and treatment (ppm±2s)a of digestb (ppm±2s) of digest (ppm±2s) of digest (ppm±2s) of digest (ppm±2s) of digest Acid Digestionc 17.8±0.7 100.0 60.6±1.4 100.0 28.0±1.2 100.0 2.99±0.03 100.0 3.13±0.11 100.0 ASE Nonpolar extractd 0.9±0.7 5.1 0.8±0.4 1.3 0.4±0.2 1.4 0.08±0.01 2.6 0.07±0.01 2.2 Polar extracte 16±1.7 86.8 59±2.5 97.7 22.5±0.7 80.4 2.2±0.06 74.7 2.3±0.2 71.8 Residualf 0.6±0.1 3.6 1.1±0.7 1.9 2.6±0.7 9.3 0.4±0.1 12.3 0.5±0.1 14.6 Totalg 17.0 95.5 61.2 100.9 25.5 91.1 2.7 89.6 2.8 88.6 Sonication Nonpolar extractd 0.6±0.7 3.5 0.2±0.2 0.4 0.4±0.1 1.4 0.08±0.01 2.7 0.02±0.01 0.7 Polar extracte 16±1.0 87.0 52±4.0 84.9 24.9±0.3 89.1 2.7±0.01 88.4 2.7±0.1 84.9 Residualf 2±1.5 13.2 3±1.8 4.7 2.0±0.4 7.0 0.2±0.02 6.0 0.5±0.1 15.3 Totalg 18.5 103.7 54.6 90.0 27.2 97.5 2.9 97.1 3.16 100.9 aAverage concentration of As in parts per million of dry fish±2s for n=3 as determined by ICP-MS.bPercent As extracted in specific phase or residual divided by the total acid digestion concentration multiplied by 100. For example [for DORM-2 ASE nonpolar extract (0.9/17.8)×100=5.1]. Calculated on average concentration prior to rounding. cTotal arsenic in a sample which was not subjected to extraction.dArsenic extracted using acetone. eArsenic extracted using 50% (m/m) MeOH in water after acetone extraction. fArsenic remaining in the solid. gSum, prior to rounding, of arsenic concentrations determined for extracts and residuals. 610 J. Anal. At. Spectrom., 1999, 14, 607–613m/z 75 using ICP-MS in the single ion monitoring mode. Speciation Chromatograms were corrected for instrumental drift by an The chromatographic separation of five arsenic standards and oV-column injection of a reference arsenic standard into the the separation as applied to a polar extract from salmon are mobile phase prior to the chromatographic separation.Peak shown in Fig. 2a and b, respectively. Fig. 2a demonstrates the areas were determined by use of SAS, a software program. excellent sensitivity associated with ICP-MS. The chromatog- The area of the reference peak was used to correct for ram of the salmon (Fig. 2b) extract clearly indicates the need instrument drift during the analysis of a sample set.for excellent resolution between AsB–AsC and a trace peak for AsIII (see Fig. 2a, peak B). The (inset) enhanced region in Fig. 2b illustrates the relative size of the major peak from Results and discussion AsB–AsC to the minor peaks from MMA, DMA, and AsV. Total arsenic The speciation results shown in Table 2 are reported as relative area per cent. This facilitates a direct comparison The total arsenic concentration for the ‘acid digested’ fish species by species across the two extraction techniques to tissue and the ‘total’ arsenic for each of the extraction phases assure the distribution is unaVected by the means of extraction (nonpolar, polar and residual, see Fig. 1) are shown in Table 1.while minimizing the required speciation based quantitation The top half of this table represents the results from a mass balance analysis for ASE while the sonication results are reported in the lower portion of the table.The extracted percentage reported to the right of each total arsenic concentration is calculated by dividing the arsenic concentration in the extracted phase by the arsenic concentration determined in the (unextracted) acid digestion sample. The reproducibility, as RSD, for the ‘acid digestion’ data ranged from 0.5 to 2.6%. For each fish, the nonpolar (acetone) extraction removed 0.4–5.1% of arsenic from the tissue. In three of the fish samples, DORM-2, whitefish, and tuna, 1–4 times as much arsenic was extracted in the nonpolar (acetone) extract using ASE than was extracted using sonication extraction. These diVerences are relatively small in comparison to the large percentage (greater than 90%) of the arsenic which is recovered in the polar (50% methanol ) extract or which remains in the residual solids.The concentration of arsenicals extracted using 50% (m/m) methanol–18 MV water is reported in Table 1 as polar extract and corresponds to Aspolar in Fig. 1. The extraction eYciency for ASE ranged from 71.8–97.7% relative to total arsenic concentration (acid digestion) reported at the top of each column. The 97.7% extraction eYciency obtained in the white- fish sample in conjunction with an RSD of less than 2.1% and in the worst case 5.5% (DORM-2) indicate the quantitative nature and reproducibility of ASE in extracting arsenicals from dietary fish. The relatively low extraction eYciency (71.8%) reported for tuna utilizing the ASE could potentially be increased by using higher pressures and temperatures.The ASE parameters used were chosen in order to make a more direct comparison with sonication: the parameters were not optimized to determine the most eYcient extraction conditions. A comparison of the polar extraction eYciencies for three of the samples are: shark (ASE 80.4%; sonication 89.1%), salmon (ASE 74.7%; sonication 88.4%), and tuna (ASE 71.8%; sonication 84.9%). From these results, the polar extraction using the sonication method is 1.1 to 1.2 times more eYcient than is the ASE polar extraction.The extraction eYciency of sonication for the 5 samples ranged from 84.9–89.1, while the range observed for ASE was 71.8–97.7. This demonstrates less cross-matrix variation for sonication relative to ASE and an increase of approximately 18% in the extraction eYciency of salmon and tuna for sonication relative to ASE. The percentage of arsenic remaining in the residual solids Fig. 2 (a) IC-ICPMS chromatogram of 5 arsenic standards. A, 3 ng after extraction via the ASE for tuna and salmon are 14.6 and AsB; B, 1 ng AsIII; C, 0.5 ng DMA; D, 0.5 ng MMA; E, 1 ng AsV. 12.3, respectively. While these relatively high residual concen- (b) IC-ICPMS chromatogram of salmon extract. Labeled peaks are: trations indicate that ASE is not quantitatively extracting all A, AsB–AsC; C, DMA; D, MMA; E, AsV. Inset shows an enhancement of part of the chromatogram.IC-ICPMS chromatograms the arsenic from the tissue, extraction conditions were chosen obtained using a Hamilton PRP-X100 anion-exchange column and to maximize the comparison with sonication, and therefore it step gradient elution. Detection was performed at m/z 75. Flow rate is likely that the amount of arsenic in the residuals may be 1.0 ml min-1. Mobile phase A: 5.0 mmol l-1 NH4NO3, 5.0 mmol l-1 minimized by using higher temperature or pressure. The (NH4)2CO3, pH 9.0.Mobile phase B: 50.0 mmol l-1 NH4NO3, optimization of the polar extraction, or the minimization of 5.0 mmol l-1 (NH4)2CO3, pH 9.0. 100% A CA 174 S 100% A CA 6 S the Asresidual, is an active area of research within our laboratory. 100% B CA 516 S 100% B CA 6 S 100% A CA 285 S 100% A. J. Anal. At. Spectrom., 1999, 14, 607–613 611Table 2 Relative area percent distribution of chromatographically separated arsenicals Rel area % dista Rel area % dist Rel area % dist Rel area % dist Rel area % dist Rel area % dist Fishb Method AsB–AsC Unknown 1 DMA Unknown 2 MMA AsV Dorm-2 ASE 96.6±0.8 0.04±0.00 1.6±0.2 0.1±0.1 0.10±0.02 1.6±0.9 Dorm-2c Sonication 97.8±0.91d 1.5±16.0d 0.1±86d 0.08±92d 0.6±88d Salmon ASE 97.1±0.1 0.4±0.6 0.17±0.04 2.3±0.6 Salmon Sonication 98.8±0.7 0.6±0.5 0.1±0.1 0.5±0.2 Shark ASE 99.6±0.1 0.06±0.04 0.1±0.02 0.03±0.04 0.3±0.1 Shark Sonication 99.6±0.2 0.1±0.02 0.02±0.01 0.4±0.2 Tuna ASE 96±5 1.6±4.8 0.8±0.3 2.1±0.3 Tuna Sonication 99±1 0.4±0.3 0.8±0.1 0.2±0.9 Whitefish ASE 99.3±0.3 0.4±0.03 0.01±0.02 0.3±0.3 Whitefishc Sonication 99.3±0.06d 0.5±15.5d 0.01±200d 0.2±13d aRelative area per cent.distribution of arsenic species X, RAX=100×Aspecies X/Atotal, where Aspecies X is the average peak area of species X, and Atotal is the average total peak area of all detected species. bn=3 except where noted, uncertainty expressed as 2s. cn=2. dRelative percent diVerence (RPD) is used because n=2. RPD of species X=100×|RAX1-RAX2|/[(RAX1+RAX2)/2], where RAX1 is the relative area percent of species X1, as determined in the first sample, and RAX2 is that determined in the second sample.and analysis time. In addition, this type of comparison is the the distributions of extracted arsenicals utilizing ASE compare favorably with those obtained using sonication. only means of verifying relative arsenic distributions in the presence of unknown chromatographic peaks with unavail- The results of this study show that both sonication and ASE are useful methods for extracting arsenicals from fish able standards.The separation conditions, including the pH of the mobile using acetone followed by 50% MeOH (m/m) in 18MV water. For most of the fish studied, nearly all of the arsenic found in phase, were optimized to give maximum resolution between AsB and AsC (the relatively non-toxic arsenicals which are the total acid digest was accounted for when using mass balance to monitor the extraction processes.Only a small the predominant arsenicals in fish, see Fig. 2a) and AsIII, a carcinogen. The identities of the species (AsB–AsC, AsIII, AsV, percentage (5.1%) of arsenic was removed with acetone in all of the fish studied, while the majority (71.8%) of the MMA, and DMA) were determined by comparison of their retention times with those of standards. The standard injections arsenicals were collected using 50% MeOH (m/m) in 18 MV water.This is shown in Table 1. The extraction methods were of AsB and AsC indicated a co-elution of these two arsenicals and for this reason they are grouped together within Table 2. found to extract similar distributions of arsenicals (Table 2). The majority (96%) of the arsenicals extracted from all of Trimethylarsine oxide may also co-elute near the void volume using this chromatographic separation but the standard mate- the fish studied were unretained on an anion exchange column.This preliminary evaluation of ASE based on total arsenic rial was not available to validate its retention time. The co-elution of these highly substituted arsenicals, which are and relative area percentages relative to sonication indicates the potential of ASE in the extraction of arsenicals from fish. thought to be relatively non-toxic, is not critical to characterizing the risk associated with the ingestion of toxic inorganic The use of an automated extraction method was a definite improvement in terms of time required of the laboratory arsenic from dietary fish.Two unknown species were present in the samples with relative areas of less than 2%. One personnel. It is hoped that optimization of the ASE parameters may facilitate the extraction of arsenicals from fish, especially unknown eluted after AsIII and before DMA, the other eluted after DMA and before MMA. those which yield low extraction eYciencies under sonication extraction conditions.The use of elevated extraction tempera- Table 2 compares the distribution of individual arsenicals across the extraction techniques. Calculations using this data tures, and solvents which are generally avoided in sonication studies, may lead to very eYcient, reproducible extractions. indicate the relative percent diVerences in distribution of arsenicals for salmon, shark, whitefish and DORM-2 are Further studies are required to assure that species integrity is maintained under these extraction conditions.within 2% for AsB–AsC, 28% for DMA, and 124% for AsV. The greatest relative percent diVerence between the techniques for these samples was in extracting AsV in salmon. Tuna Acknowledgments displayed a dramatic relative percent diVerence in DMA The authors would like to thank Ted Martin, US EPA, for (120%) and AsV (165%), which is due to one ASE extraction his expertise in arsenic determinations using ICP-AES, and which had an unusual arsenic distribution.The large diVerence Xinyi Wei, NRC postdoctoral researcher, for her assistance between the two extraction procedures in the extraction of with development of chromatographic separations. Additional DMA and AsV from tuna, and the diVerences seen in extracting acknowledgments are due to Isa Chamberlain, US EPA region AsV in salmon, are significant but it should be noted that these 10, for providing information for total digestion studies. species represent less than 4% of the total arsenic peak area.These diVerences may be due to thermal decomposition of the arsenicals during the evaporation step of the procedure. The References relative abundances of AsV are slightly higher under ASE 1 W. P. Tseng, H. M. Chu, S. W. How, J. M. Fong, C. S. Lin and extraction conditions in most samples than when extracted S. Yen, J. Natl. Cancer Inst., 1968, 40, 453. using sonication methods. This variation may be partially 2 J. Brown and K. Kitchin, Teratog.Carcinog. Mutagen., 1997, 17, explained by the variation induced by the presence of AsV in 71. 3 Toxicological Profile for Arsenic, ATSDR/TP-88/02, prepared by blank determinations. This could also be due to a more Life Systems Inc. for Agency for Toxic Substances and Disease eYcient extraction of AsV under these conditions or could be Registry U.S. Public Health Service, in collaboration with US evidence of some degradation of organic arsenicals. Finally, EPA, March 1989, Oak Ridge National Laboratory, TN, USA, DORM-2 was the only sample for which MMA is recorded. 1989. The relative percent diVerence, in DORM-2, between the two 4 E. L. Gunderson, J. AOAC Int., 1995, 78, 1353. extraction techniques was 20% for MMA. Excluding the above 5 E. H. Larsen, G. Pritzl and S. H. Hansen, J. Anal. At. Spectrom., 1993, 8, 1075. exceptions, this species by species comparison indicates that 612 J. Anal. At. Spectrom., 1999, 14, 607–6136 S. X. C. Le, W.R. Cullen and K. J. Reimer, Environ. Sci. Technol., 19 M. L. Magnuson, J. T. Creed and C. A. BrockhoV, J. Anal. At. Spectrom., 1996, 11, 893. 1994, 28, 1598. 20 B. S. Sheppard, J. A. Caruso, D. T. Heitkemper and K. A.Wolnik, 7 D. Velez, N. Ybanez and R. Montoro, J. Agric. Food Chem., 1995, Analyst, 1992, 117, 971. 43, 1289. 21 S. J. Haswell, P. O’Neill and K. C. C. Bancroft, Talanta, 1985, 8 D. Velez, N. Ybanez and R. Montoro, J. Agric. Food Chem., 1996, 32, 69. 44, 859. 22 G. Rauret, R. Rubio and A. Padro, Fresenius’ Z. Anal. Chem., 9 D. Beauchemin, K. W. M. Siu, J. W. McLaren and S. S. Berman, 1991, 340, 157. J. Anal. At. Spectrom., 1989, 4, 285. 23 J. Gailer and K. J. Irgolic, Appl. Organometall. Chem., 1994, 8, 10 J. P. Buchet, J. Pauwels and R. Lauwerys, Environ. Research, 129. 1994, 66, 44. 24 R. Cornelis and J. De Kimpe, J. Anal. At. Spectrom., 1994, 9, 945. 11 G. Lunde, J. Sci. Food Agric., 1973, 24, 1021. 25 E. H. Larsen, G. Pritzl and S. H. Hansen, J. Anal. At. Spectrom., 12 J. C. Lo� pez, C. Reija, R. Montoro, M. L. Cervera and M. de la 1993, 8, 557. Guardia, J. Anal. At. Spectrom., 1994, 9, 651. 26 P. Thomas and K. Sniatecki, J. Anal. At. Spectrom., 1995, 10, 615. 13 T. Kaise, H. Yamauchi, T. Hirayama and S. Fukui, Appl. 27 W. C. Story, J. A. Caruso, D. T. Heitkemper and L. Perkins, Organometall. Chem., 1988, 2, 339. J. Chromatogr. Sci., 1992, 30, 427. 14 Y. Shibata and M. Morita, Anal. Chem., 1989, 61, 2116. 28 B. E. Richter, B. A. Jones, J. L. Ezzell, N. L. Porter, N. Avdalovic 15 J. Alberti, R. Rubio and G. Rauret, Fresenius’ Z. Anal. Chem., and C. Pohl, Anal. Chem., 1996, 68, 1033.. 15, 351, 415. 29 U. S. Environmental Protection Agency, Sample Preparation 16 K. J. Lamble and S. J. Hill, Anal. Chim. Acta, 1996, 334, 261. Procedure for Spectrochemical Determination of Total 17 S. Branch, L. Ebdon and P. O’Neill, J. Anal. At. Spectrom., 1994, Recoverable Elements in Biological Tissues—Method 200.3, Revision 1.0, April 1991. 9, 33. 18 J. Alberti, R. Rubio and G. Rauret, Fresenius’ Z. Anal. Chem., 1995, 351, 420. Paper 8/08824A J. Anal. At. Spectrom., 1999, 14, 607–613 613

ISSN:0267-9477    

DOI:10.1039/a808824a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

The eVects of a torch shield on performance of the vacuum interface of an inductively coupled plasma mass spectrometer Brett S. Duersch, James E. Patterson and Paul B. Farnsworth* Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, USA Received 19th November 1998, Accepted 3rd February 1999 The eVects of a torch shield on the performance of a vacuum interface for an inductively coupled plasma mass spectrometer (ICP-MS) have been studied by laser excited atomic and ionic fluorescence.A metallic shield was inserted between the torch and load coil of the ICP, which was alternately grounded and floated. Relative ion and atom densities upstream from the sampling cone and downstream from the tip of the skimmer cone of the interface were determined by laser excited fluorescence. Doppler shifts in the excitation spectra were used to determine ion and atom velocities inside the skimmer cone. Pb atoms, Ba and Sc ions, and Ar metastable atoms were used as the probe species. The shield had no measurable eVect on the densities of the probe species upstream from the sampling cone.Inside the skimmer cone, the densities of all probe species dropped significantly when the shield was floated. No changes were observed in the velocities of the probe species as the shield was floated and grounded. solutions were introduced into the plasma with an ultrasonic Introduction nebulizer (U 5000, Cetac, Omaha, NE, USA).The aerosol When ICP sources that had been developed for optical spec- was desolvated by a heater and condenser that were integral troscopy were adapted for use as ion sources for plasma mass parts of the nebulizer. Pertinent operating parameters are spectrometry, many of them produced secondary discharges listed in Table 1. The three-turn load coil was grounded by a between the plasma and the grounded first stage of the vacuum piece of copper braid that was soldered at one end to the interface. Such discharges caused numerous deleterious eVects, downstream turn of the coil and bolted at the other end to including rapid deterioration of the sampling and skimming the first stage sampling plate.During all experiments a comcones, contamination of the plasma with sampler and skimmer mercial torch shield (Hewlett-Packard, Palo Alto, CA, USA) material, high photon noise and high ion energies with large was inserted between the load coil and the torch.A highenergy spreads.1–4 Secondary discharges arose because the voltage switch connected the shield to ground. The plasma grounding configurations used for emission source ICPs give was ignited with the shield retracted toward the base of the rise to a net positive potential in the plasma with respect to torch. With the plasma running, the shield was pushed into the grounded sampling interface.4,5 The positive potentials are place, and if desired, grounded by closure of the high-voltage the combined result of capacitive coupling between the load switch.coil and the plasma and diVerent mobilities of the positive The decision to float the shield rather than retract it was and negative charge carriers. based on early observations that there was little diVerence in A number of diVerent load coil configurations have been instrument performance between the two modes of disabling used to minimize the plasma potential and to eliminate the the shield.Because our home-built instrument provided no secondary discharge. They include ‘reversed’ load coils,5,6 automatic mechanism for shield retraction, the option of center tapped load coils,4 interleaved load coils,7 balanced breaking the ground connection was chosen for convenience load coils and shielded load coils.8 The performance character- and safety. istics of the various load coil geometries have been widely A viewport in the first vacuum stage allowed visual obserdiscussed, both in the scientific literature and in promotional vation of the region between the sampling and skimming literature by instrument manufacturers.The scientific charac- cones. Under the conditions given in Table 1, there was no terizations have focused on the ion kinetic energy distributions visible change in the first vacuum stage as the shield grounding produced by diVerent geometries or on the eVect of geometry switch was opened and closed. An obvious secondary discharge and plasma potential on overall instrument performance. 5,6,8–11 The development of optical probes that can meas- Table 1 Instrument operating conditions ure ion and atom densities and velocities in the second vacuum Incident power 1.25 kW stage of an ICP-MS12–14 has given us a unique tool with which Outer gas flow 14 l min-1 to study the eVect of load coil configuration on the perform- Intermediate gas flow 0.4 l min-1 ance of a critical component of an ICP-MS, namely, the Nebulizer gas flow 1.2 l min-1 a vacuum interface.In this paper we report the characterization Sampling depth 10 mm of a single load coil geometry, a reversed load coil, combined Sampler orifice diameter 1.0 mm with a torch shield that was either grounded or allowed to float. Skimmer orifice diameter 0.70 mm Sampler–skimmer separation 10 mm First stage pressure 1.1 Torr Second stage pressure 13×10-3 Torr Experimental aThe central gas flow was turned oV for the argon fluorescence The ICP-MS that was used in these experiments has been measurements.described previously,15 as have the optical probes.12,13 Sample J. Anal. At. Spectrom., 1999, 14, 615–619 615was present only when the switch was opened at extremely cence signals from the two atom reservoirs were recorded simultaneously on two boxcar averagers. The hollow cathode high central channel flow rates. The evidence for a secondary discharge was a marked brightening between the sampler and lamps were operated at their maximum rated dc current level, then switched oV for 100 ms 20–30ms before the laser was skimmer cones.There were no obvious changes in the plasma outside the vacuum system. It is also important to note that fired. When the lamps were switched oV the background emission dropped much more rapidly than the atom density, in the vast majority of the experiments there were no measurable changes in the pressures of the first and second vacuum and clean excitation spectra were obtained for all three elements. stages coincident with switching of the ground connection.On two occasions, a slight increase (0.02 Torr) was noted in the The apparatus was modified for the measurement of Ar metastable atom fluorescence, as shown in Fig. 2. In this case first stage pressure as the grounding switch was opened, but in both of these cases, irregularities in other parts of the the diode laser was CWinstead of pulsed. The boxcar averagers were replaced by lock-in amplifiers, and an optogalvanic apparatus were noted and the data were not used.Sc, Ba and Pb were used as test analytes at concentrations (OGE) signal from the argon-filled hollow cathode lamp served as a stationary reference. of 200, 50 and 100 mg l-1, respectively. Pb and Ba solutions were prepared from the nitrate salts, and the Sc solution from Reference density measurements made in the ICP upstream from the sampling orifice were also essential to confirm that Sc2O3 dissolved in acid.Sc and Ba were detected as singly charged ions, lead as a neutral atom. Argon metastable atoms changes measured in the second vacuum stage did not simply reflect changes in the plasma itself. Such reference measure- (4s[3/2] J=2) were detected as representative of the support gas. All four species were chosen because they have eYcient ments were made with an external fluorescence probe that has been described in ref. 14. It recorded relative densities in the non-resonant fluorescence schemes with large diVerences between the excitation and fluorescence wavelengths.The plasma from a volume of approximately 1 mm3 located just upstream of the sampling orifice. excitation and detection wavelengths are listed in Table 2. It was essential in these experiments that we had the ability to record both relative density and velocity data for the probe Results and discussion species. Velocity information was extracted from Doppler shifts in the excitation wavelengths listed in Table 2.To With the plasma operating conditions set as indicated in Table 1, we recorded fluorescence intensities upstream from eliminate the eVects of drift in the wavelength readouts of the lasers used in the experiments, we simultaneously recorded the the tip of the sampling cone and inside the tip of the skimmer cone with the shield switch open and closed. The results of excitation spectra of the selected species in a stationary atom reservoir (a hollow cathode lamp) and in the 2 mm downstream these experiments are summarized in Table 3.The ratios in Table 3 were recorded at the peak excitation wavelengths for from the tip of the skimmer cone, a region defined by the overlap of the excitation and emission optical paths.14 Two each probe species, which were diVerent inside and outside the vacuum chamber. The drops in the intensities of the ionic diVerent experimental configurations were used: one for the analytes and the other for the argon metastable atoms.The fluorescence inside the vacuum chamber were expected. Based on reported diVerences in ion energies between shielded and configuration used for the analytes is shown in Fig. 1. The pulsed dye laser was scanned in 1 pm increments and fluores- unshielded coils, an increase in ion velocities in the ungrounded configuration that would shift the absorption resonance out- Table 2 Laser fluorescence wavelengths Excitation Fluorescence Species wavelength/nm wavelength/nm Sc II 358.094 432.501 Ba II 455.404 614.172 Pb I 283.306 405.783 Ar I 801.479 842.465 Fig. 2 Experimental block diagram for measurements of Ar metastable fluorescence. Table 3 Ratio of fluorescence intensities (shield floating/shield grounded). The uncertainties are short-term standard deviations of the ratios In plasma, Downstream upstream from tip from tip of Species of sampling cone skimmer cone Sc II 0.99±0.09 0.53±0.03 Ba II 1.0±0.1 0.45±0.03 Pb I 1.0±0.1 0.76±0.05 Fig. 1 Experimental block diagram for measurements of Sc, Ba, and Ar I 0.480±0.003 Pb fluorescence. 616 J. Anal. At. Spectrom., 1999, 14, 615–619side the laser bandwidth was expected. The drops in the argon and lead fluorescence intensities were surprising. Neutral species should be unaVected by plasma potential. Two additional sets of experiments were conducted in an eVort to understand the nature of the changes induced by grounding or floating the torch shield.In the first set, fluorescence excitation scans of ions inside the tip of the skimmer cone and in hollow cathode lamps were recorded simultaneously. The results of these experiments are presented in Fig. 3–6. Perhaps the most important feature of these four figures is that grounding or floating the torch shield only aVects the magnitude of the fluorescence signals. The normalized wavelength profiles produced by the excitation scans Fig. 5 Excitation scans for Pb atom. Top, raw data; bottom, normalized scans.&, Hollow cathode lamp; +, shield grounded; *, shield floating. Fig. 3 Excitation scans for Sc ion. Top, raw data; bottom, normalized scans.&, Hollow cathode lamp; +, shield grounded;*, shield floating. Fig. 6 Excitation scans for Ar metastable atoms. Top, raw data; bottom, normalized scans. ··-··, Hollow cathode OGE signal; ——, shield grounded; - - - -, shield floating. under the two conditions are identical within experimental error.A second set of experiments was performed to ensure that the results of the excitation scans were not unique to a single set of plasma conditions. For the three probe metals, the shield was alternately grounded and ungrounded as the central flow rate was raised in small increments. The results are shown in Fig. 7. Clearly, the behavior observed at a central flow rate of 1.2 l min-1 is not unique. Consistent with reports by Ross et al.,6 the optimum flow rates for the grounded and ungrounded conditions are diVerent.However, the sensitivities measured by fluorescence are worse under all flow conditions for the ungrounded shield than they are for the grounded Fig. 4 Excitation scans for Ba ion. Top, raw data; bottom, normalized shield. The diVerences between the Pb atom traces and those scans.&, Hollow cathode lamp; +, shield grounded;*, shield floating. for the Sc and Ba ions are expected. The ion curves are typical J.Anal. At. Spectrom., 1999, 14, 615–619 617excitation spectrum is almost certainly produced by atoms that are thermalized in a shock structure or other flow disturbance at the tip of the skimmer cone. A detailed study of the argon spectra is reported in ref. 13. The optically-measured velocities reported here are consistent with those reported by Fulford and Douglas,10 but diYcult to reconcile with those measured by other workers.8,9,11 The most direct comparison can be made with the study by Gray,8 in which shielded and unshielded coils were compared.Although in our study the comparison is between a grounded shield and a floating shield, the qualitative diVerences we observed between the two conditions should be similar to those reported by Gray. The floating shield changes the capacitance between the load coil and the plasma but does not eliminate it. In Gray’s study, significant decreases in the energy of cobalt ion were reported as an unshielded coil was replaced by a shielded coil.The changes were measured over a broad range of central channel flow rates. We see no such changes. The interpretation of Doppler shifts in a fluorescence excitation spectrum is very straightforward. This suggests that the discrepancies between the two studies must be due to subtle diVerences between the interfaces, residual shielding by the floating shield, changes in velocities arising downstream from the skimmer cone, or artifacts in the ion stopping Fig. 7 EVect of torch shield as a function of central gas flow rate. +, measurements. Shield grounded;*, shield floating. Top graph, Sc ion; middle Interpretation of the decreases in the magnitudes of the graph, Ba ion; bottom graph, Pb atom. fluorescence signals is not straightforward. If the fluorescence signals were representative of the total flux of material through of those recorded for mass spectrometric detection.16 The the interface, then the opening of the grounding switch would atom curves are typical of a species favored by low tempera- of necessity be accompanied by a large drop in vacuum tures.At intermediate flow rates the lead ion dominates and pressure in the first and second vacuum stages. Clearly, the the atom signal is low. At high flow rates, cooler gas is pushed drops in fluorescence signal do not signify a proportional drop toward the sampling cone, and the fraction of the lead that is in total flux through the interface.Two alternative explanations ionized decreases. At low flow rates, some lead atoms appear could account for the lack of a pressure change accompanying as a result of ion–electron recombination. the large drops in fluorescence intensity. One is that the species In Fig. 3–6 the magnitudes of the Doppler shifts are prob- that accounts for the bulk of the flow through the interface, ably the most significant feature. Those shifts, with the corre- ground state neutral argon, is not aVected by the shield sponding velocities of the atoms or ions, are summarized in changes in the same manner as the minority species measured Table 4.In the Doppler shift data there is no evidence of any by the fluorescence experiments. The second is that the changes acceleration of the ions by electric fields inside or outside the in the fluorescence signals reflect not a change in the total interface. Rather the data are consistent with a free expansion density of the respective species, but rather a shift to other of the plasma with no other accelerating forces.The most states. Of the two possibilities, the former seems the most probable velocities of neutral lead and barium and scandium plausible. In either case, our experiments do not reveal the ions are the same within experimental error, with no clear source of the changes in fluorescence intensity. We plan further dependence on mass. The most probable velocity for Ar, which experiments in the first vacuum stage that we expect to provide is the most precise of the measured values, corresponds to a more insight into the changes in interface performance that gas kinetic temperature in the plasma of 7500 K.10 This is occur with the grounding and floating of the torch shield. The within the range of values expected for a 27 MHz ICP at experiments reported here certainly do not answer all remain- 1.25 kW.17 Because the argon measurements were made with ing questions about the eVects of load coil configuration and the central gas flow turned oV, the temperature and hence the plasma potential on vacuum interface performance, but rather terminal expansion velocity should be slightly higher than for should serve to provide impetus and direction for further study.the other probe species. In addition to the argon population with a mean velocity Acknowledgement of 2800 m s-1, there is a significant population with a mean velocity near zero.The low velocity peak in the argon This work was supported by the National Science Foundation, Grant no. CHE-9415384. Table 4 Doppler shifts and ion/atom velocities. Uncertainties for Sc, Ba, and Pb are based on a wavelength uncertainty of ±0.5 steps of References the laser wavelength drive. The uncertainty for Ar is based on an estimated uncertainty in peak location of ±5 steps in the diode 1 H. Niu and R. S. Houk, Spectrochim. Acta, Part B, 1996, 51, 779.laser scan 2 R. S. Houk, V. A. Fassel, G. D. Flesh, H. J. Svec, A. L. Gray and C. E. Taylor, Anal. Chem., 1980, 52, 2283. Stationary 3 D. J. Douglas, Can. J. Spectrosc., 1988, 34, 38. Species wavelength/nm Doppler shift/pm Velocity/m s-1 4 D. J. Douglas and J. B. French, Spectrochim. Acta, Part B, 1986, 41, 197. Sc 358.094 3 2600±400 5 A. L. Gray, R. S. Houk and J. G. Williams, J. Anal. At. Spectrom., Ba 455.404 4 2700±300 1987, 2, 13. Pb 283.305 2 2200±500 6 B. S. Ross, P. Yang, D. M. Chambers and G. M. Hieftje, Ar 801.479 7 2800±40 Spectrochim. Acta, Part B, 1991, 46, 1667. 618 J. Anal. At. Spectrom., 1999, 14, 615–6197 I. L. Turner, U. S. Patent 5,194,731, March 16, 1993. 14 B. S. Duersch and P. B. Farnsworth, Spectrochim. Acta, in the press. 8 A. L. Gray, J. Anal. At. Spectrom., 1986, 1, 247. 15 Y. Chen and P. B. Farnsworth, Spectrochim. Acta, Part B, 1997, 9 J. A. Olivares and R. S. Houk, Appl. Spectrosc., 1985, 39, 1070. 52, 231. 10 J. E. Fulford and D. J. Douglas, Appl. Spectrosc., 1986, 40, 971. 16 G. Horlick, C. A. Rose, S. H. Tan and M. A. Vaughan, 11 N. Jakubowski, B. J. Raeymaekers, J. A. C. Broekaert and D. Spectrochim. Acta, Part B, 1985, 40, 1555. Stuewer, Spectrochim. Acta, Part B, 1989, 44, 219. 17 M. Huang, S. A. Lehn, E. J. Andrews and G. M. Hieftje, 12 B. S. Duersch, Y. Chen, A. Ciocan and P. B. Farnsworth, Spectrochim. Acta, Part B, 1997, 52, 1173. Spectrochim. Acta, Part B, 1998, 53, 569. 13 J. E. Patterson, B. S. Duersch and P. B. Farnsworth, Spectrochim. Acta, in the press. Paper 8/09063G J. Anal. At. Spectrom., 1999, 14, 615–619 619

ISSN:0267-9477    

DOI:10.1039/a809063g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of impurities in antique silver objects for authentication by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) W. Devos, Ch. Moor* and P. Lienemann Swiss Federal Laboratories for Materials Testing and Research (EMPA), U� berlandstrasse 129, CH-8600 Du�bendorf, Switzerland Received 4th January 1999, Accepted 25th February 1999 In addition to visual characteristics, a less manipulable criterion for authenticity verification of silver antiques is given by trace and minor element patterns in the silver alloy. The analytical method used to analyse precious silver antiques should not visibly damage the object and should enable the determination of impurities in the ppm–0.5% range.Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), visible damage can be restricted to an acceptable minimum. Because most antique silver objects are too large to fit into a normal laser ablation cell, an alternative cell design was used that allows a direct, virtually non-destructive analysis of entire antique silver objects.This cell is placed upon the object to be analysed. A micro-amount of the object is then ablated through an aperture in the bottom of the cell. The 100 mm wide craters are almost invisible on an antique silver object. The analytes Zn, Cd, Sn, Sb, Au, Pb and Bi were measured. Signals were normalized to the Ag signal and silver standard materials were used for external calibration.The crater-to-crater repeatability of the normalized signals in a homogeneous silver sample was below 10% RSD (n=3) for most elements. Detection limits lie within the sub-ppm to 2 ppm range. The accuracy was validated with comparative ICP-MS measurements after digestion and with X-ray fluorescence (XRF) measurements. The analysis of eight antique silver objects, including one forgery, illustrates the application of the method. Classical methods for direct solid sample analysis, such as Introduction X-ray fluorescence (XRF) analysis, scanning electron Various criteria are available to verify if an antique silver microscopy with an energy-dispersive X-ray detection system object is authentic or a forgery.A trained expert in silver (SEM-EDXRF) or electron probe microanalysis (EPMA), antiques may be able to identify forgeries by a careful visual are fully non-destructive, but lack sensitivity in the study of stylistic characteristics, hallmarks, fabrication tech- lower mg g-1 range.Furthermore, most antique silver objects nique and signs of wear. However, a malevolent silversmith are too large to be fitted into the sample chambers commonly with suYcient expertise can manipulate these criteria so as to used with these techniques. make a forgery look real, even to a trained eye. An additional Wet analytical techniques, e.g., atomic absorption speccriterion much more diYcult to manipulate is given by the trometry (AAS) and inductively coupled plasma mass specchemical composition of the silver alloy.In addition to the trometry (ICP-MS) of solutions, require acid digestion of the main constituents of silver (75–95%) and copper (5–25%), a Ag–Cu alloy. This is particularly troublesome if Au also has silver alloy contains minor or trace impurities typical of the to be determined, as traces of Au are only partially soluble in metallurgical manufacturing and refinement process of the nitric acid, whereas Ag will precipitate as silver chloride in silver, such as Zn, Cd, Sn, Sb, Au, Pb and Bi.Silver refinement aqua regia, giving rise to losses of trace elements due to has undergone fundamental changes during the past centuries, coprecipitation. It is possible to keep Ag in solution as chloride especially in the 19th century, as the introduction of the Parkes complexes with a carefully balanced mixture of nitric and process around 1850, followed by electrolytic refinement in hydrochloric acid, but this demands quite a lot of experience 1884, led to much lower contents of Zn, Sn, Sb, Au, Pb and and time.To overcome this problem, Hinds3 used solid sam- Bi. Cd, on the contrary, only discovered in 1817, has tempor- pling graphite furnace AAS (GFAAS) for the determination arily been used as an additive in 20th century silver solders of Au, Pd and Pt in high purity silver, and Moor et al.4 and special silver alloys and is a clear indication of a modern reported the application of solid sampling electrothermal silver alloy.1,2 As it was not until the latter part of the 19th vaporization ICP-MS (ETV-ICP-MS) to determine Au and century that collecting antique silver came into vogue, most other impurities in silver alloys.In order to allow practical fake silver objects stem from that period on. Therefore 1850 sample handling, both techniques require in practice at least is a significant date to distinguish forgeries from earlier genuine 50–100 mg of sample.Coupling solid sample introduction silver antiques. techniques to ICP-MS has a powerful advantage over solid It is evident that any analytical method used for authenticity sampling GFAAS in that it oVers a quasi-simultaneous multiverification of often precious antique silver objects should be element analysis in one signal acquisition run. either non-destructive or require an extremely small sample In recent years, laser ablation (LA) has gained increased amount.Furthermore, it should allow the determination, popularity as a solid sample introduction technique to preferably simultaneously, of the relevant elements in a concen- ICP-MS, thanks to its low detection limits, its wide linear dynamic range, its spatial resolution and its low consumption tration range from a few mg g-1 to about 0.5%. J. Anal. At. Spectrom., 1999, 14, 621–626 621of sample material.5–9 The benefits of LA-ICP-MS in trace objects, a flat rubber sealing-ring can be used instead.For the analyses in this study, the bottom disc with a 5 mm aperture analysis of silver and gold materials for assessing the provenance of stolen gold,10 for prospecting purposes11 and for was used. A supporting platform, adjustable in three dimensions and the determination of impurities in pure coinage silver and gold12 have been reported. mounted on the Laser Sampler Model 320 supporting block with its three stepping motors, allows the aperture of the cell In LA-ICP-MS, a sample is usually mounted into a relatively small, closed laser ablation cell, in which the ablation process and, consequently, the area of interest on the object to be exactly positioned under the focusing lens.The autofocus takes place and through which a carrier gas stream flows to transport the ablated material to the ICP-MS. Objects larger system then automatically adjusts the surface of the object within the focal plane of the laser beam.than a few centimetres, however, e.g., the silver antiques under study here, require the removal of a fragment of the object in order to fit into a normal laser ablation cell and can thus be Laser analysis and signal processing analysed only indirectly. For a high quality piece of antique The carrier gas flow rate was optimized for maximum inte- this is unacceptable. Special cell designs have been proposed grated signals with a silver standard material.A short bivariate by Arrowsmith and Hughes13 and Gu� nther14 which consist of optimization study was carried out for the number of laser a cell that encloses the plume of ablated material but not the pulses per analysis point and the excitation lamp energy in sample. In this work, an alternative cell design was used in order to obtain craters of a suitable size. By increasing the combination with a home-built autofocus system, allowing the excitation lamp energy, which directly influences the laser direct analysis of entire antique silver objects by LA-ICP-MS.beam energy, wider and deeper craters are produced, whereas Quantitative information was obtained using matrix matched increasing the number of shots only produces deeper craters. standards. The operating conditions of the ICP-MS and the laser sampler are listed in Table 1. The laser was focused on the sample Experimental surface. Analysis points were chosen a few millimetres apo correct for diVerences in ablation eYciency between Instrumentation analysis points, 107Ag was used as an internal standard.This requires the silver content of the samples and external stan- An Elan 6000 ICP-MS (Perkin-Elmer SCIEX, Thornhill, Ontario, Canada) was used in combination with a Laser dards to be approximately known. The external standards used in this work (see below) contained between 99.4 and Sampler Model 320 (Perkin-Elmer SCIEX, Thornhill, Ontario, Canada). Measurements were performed in the dual detection 100% silver.Unless given by other sources, a silver content of 84% was used as an approximation of the silver content in the mode of the Elan 6000, which oVers the measurement of pulsecounting and analogue signals simultaneously.15 The Laser alloy of antique silver objects. Some commonly used antique alloys were 12/16 (i.e., m/m Ag/alloy ratio), 13/16, 14/16 and Sampler Model 320 was modified by adding a home-built autofocus system.16 The laser used in this work is an Nd5YAG 15/16 silver (1/16=1 ‘Lot’), Sterling silver (92.5% fine silver) and 9/12, 10/12 and 11/12 silver (1/12=1 ‘denier’ or laser (Spectra-Physics, Darmstadt, Germany), operated at 532 nm.The area of ablation can be observed by a video ‘dinero’).17–19 Generally, the silver content of antique and modern alloys for silverware lies between about 75% and 95%. camera. An alternative laser ablation cell was used to analyse entire Thus, using an approximation of 84% silver for internal standardization introduces a supplementary relative uncer- antique silver objects directly, i.e., without the need to remove a fragment from the object.This cylindrical borosilicate glass tainty of, at most,±12%, which is still acceptable for authentication purposes. cell (r=2.1 cm; h=1.7–2.4 cm) has a flat exchangeable Plexiglas bottom with a central aperture of 5 mm, 10 mm or Based on the literature1 and preliminary experiments with solid sampling ETV-ICP-MS and conventional ICP-MS,4 the 20 mm, and is placed upon the object to be analysed (Fig. 1).Part of the object is then ablated by the laser beam through following isotopes were chosen for analysis: 66Zn, 107Ag, 111Cd, 117Sn, 121Sb, 197Au, 208Pb and 209Bi. The transient signals were the aperture. The cell is continuously flushed with a carrier gas (argon) via a lateral inlet and outlet tube to transport the ablated material to the ICP-MS. To prevent the Ar carrier Table 1 Operating conditions for the ICP-MS and the laser sampler gas, and therewith the ablated material, from escaping out of ICP-MS the cell through the aperture, modelling plasticine is applied Rf power/W 1050 between the cell and the object, around the aperture.For flat Gas flow rates/l min-1 Plasma gas 15 Auxiliary gas 0.8 Carrier gas 1.0 Lens setting Autolens mode (variable) Points per peak 1 (at peak maximum) Measuring mode Peak hopping Isotopes measured 66Zn, 107Ag, 110Cd (dummy), 111Cd, 117Sn, 121Sb, 197Au, 208Pb, 209Bi Dwell time/ms 10; 111Cd and 121Sb: 50 Sweeps per reading 1 Readings per replicate 91 Detector mode Dual (pulse counting and analogue) Laser sampler Mode Q-switch Q-switch time/ms 240 Excitation lamp energy/J 45 Pulse frequency/Hz 10 Fig. 1 The alternative laser ablation cell used in this work, placed Number of pulses/analysis point 60 upon a silver object: 1, laser ablation cell; 2, focusing lens; 3, autofocus; Focus On sample surface (autofocus) 4, supporting platform. 622 J. Anal. At. Spectrom., 1999, 14, 621–626integrated over a period of 20 s after the laser had started firing, thus covering the entire analyte signals, while allowing the silver signal to decrease to about 1% of its maximum intensity. A blank correction was carried out by subtracting the Ar gas blank signal, integrated over the same period of time and measured with the laser running but the laser beam path blocked.Calibration To obtain similar ablation conditions for external standards and samples, matrix-matched standards or standard materials with chemical and physical properties very similar to the samples are preferred in laser ablation analysis. Three silver calibration standards were purchased from VEB Bergbau- und Hu�ttenkombinat (Freiberg, Germany), containing the elements of interest at known concentration levels of around 500 mg g-1 (‘Ag500’), 100 mg g-1 (‘Ag100’) and 10 mg g-1 (‘Ag10’) respectively.As these standards were in the form of 10 cm rods with a diameter of only 7 mm, and were therefore too small to place the cell on, a cross-section was made of each standard after embedding in a Plexiglas resin (RESINAR F, Wirtz Buehler, Du� sseldorf, Germany). Analysed objects and cleaning procedure Eight silver objects, provided by a private antique silverware dealer, were analysed in order to investigate their authenticity. Before analysis, the objects were consecutively cleaned with silver polish to remove any AgS patina, pro analysi grade ethanol (Merck, Darmstadt, Germany) and ultrapure water (Milli-Q, Millipore, Bedford, MA, USA).Comparative analyses by solution ICP-MS and WD-XRF For comparative analysis by ICP-MS of solutions, four samples of 3–5 mg each were taken from an antique silver alloy and submitted to the following open acid digestion procedure. Fig. 2 Details of the silver ‘deer’ from Fig. 1 after laser ablation After dissolving the silver alloy with 1 ml concentrated HNO3 analysis.(a) Optical microscopy view. The arrows indicate the ablated for ca. 10 min in a 100 °C hot-water bath, 3 ml concentrated craters. (b) Scanning electron microscope (SEM) view of one crater. HCl was added (causing precipitation of Ag as AgCl) to The diameter of the crater is ca. 100 mm. The not quite circular crater dissolve the remaining undissolved Au, and the solution was shape is possibly due to imperfect optical alignment of the home-built left to boil for another 30–60 min.After cooling, the solution frequency doubling upgrade. was made up to 10 ml with deionized water. Comparative analyses by wavelength dispersive XRF Calibration and detection limits (WD-XRF) were carried out on a Philips PW1404 (Philips, The three calibration standards ‘Ag10’, ‘Ag100’ and ‘Ag500’ Almelo, The Netherlands) with a Cr tube for the determination were analysed to investigate whether a linear calibration could of Sn and Sb and a Philips PW2400 with an Rh tube for the be obtained with the alternative cell design.The linearity of other elements of interest. A sample with a flat geometry was the calibration graphs is excellent, with correlation coeYcients prepared from a piece of an antique object which could be of 0.996 or better (Fig. 3). Furthermore, the fitted calibration destroyed for analysis. The measurements were calibrated with lines go almost perfectly through zero, which confirms that standards ‘Ag100’ and ‘Ag500’.the subtraction of an Ar gas blank is an adequate method for blank correction for the given experimental set-up and this Results and discussion type of sample. The 111Cd signal initially gave rise to a problem. When Quasi-non-destructiveness 111Cd was measured immediately after 107Ag in the peak hopping sequence, a steady state signal instead of the normal The laser parameters shown in Table 1 resulted in ablation craters of about 100 mm across (Fig. 2b).Three craters per transient signal could be observed at mass 111, which completely masked the transient 111Cd signal for the standard with sample were shot. The craters are small enough to be almost invisible with the naked eye (Fig. 2a) on antique silver, and the lowest Cd concentration (‘Ag10’: 8.3 mg g-1). This resulted in far too high a signal for Cd in the Ag10 standard. The thus are virtually non-destructive. Although the crater size could easily be reduced further, smaller craters would decrease constant 111Cd signal abruptly dropped to background level at the point where the 107Ag signal had decreased to about the relative sensitivity and would be less representative of the silver alloy.From the crater width and d, the ablated 107 counts per second (Fig. 4a). This phenomenon was reproducible and was probably caused by a memory eVect in the mass per crater was estimated to be roughly about 3 mg.After ablation, sometimes a black discolouration of the silver could discrete dynode electron multiplier or the electronics of the detection system, due to the high count rates for 107Ag. be observed around the craters. This discolouration could easily be wiped oV with a silver polishing cloth. Introducing a ‘dummy’ mass between 107Ag and 111Cd, e.g., J. Anal. At. Spectrom., 1999, 14, 621–626 623110Cd, solved the problem and resulted in a normal transient signal (Fig 4b) and a linear calibration curve for 111Cd (Fig. 3). The detection limits for the elements measured, calculated as 3s from three Ar blank replicates, lie between 0.06 ppm (Pb and Bi) and 1.6 ppm (Zn). Accuracy and repeatability The linearity of the calibration lines based on the solid silver calibration standards suggests a good accuracy, limited by the quality of the standards. Furthermore, an antique silver sample, indicated here as ‘Alloy A’, was analysed both by LA-ICP-MS and by ICP-MS using a cross-flow nebulizer.As shown in Table 2, the results of the two approaches were in fairly good agreement, considering that only three craters were shot. The diVerences between the corresponding mean values obtained by the two methods are statistically insignificant (95% confidence level t-test) for all elements except Au and Bi. The somewhat lower values for these elements with the solution method may be due to incomplete dissolution. Additionally, the results of laser ablation analysis of another antique silver sample (’Alloy B’) were compared with XRF measurements (Table 2). The concentrations of Sb and Cd were below the detection limits of XRF for these elements in this type of matrix.A statistically significant diVerence between the two methods was observed for the Pb values. The reason for this diVerence is not clear, but it might be attributed to surface contamination eVects during sample preparation for XRF, as the comparison of LA-ICP-MS with solution ICP-MS did not reveal any systematic deviation of LA-ICP-MS for lead.The agreement between LA-ICP-MS and XRF for the other elements was satisfactory in view of the objectives of this study. The crater-to-crater repeatability (n=3) of the analyte signals (ratioed to the Ag signal ) is summarized in Table 3 for the measurements of the standards and the two aforementioned antique silver alloys. The uncertainties shown in Table 2 are larger than the RSDs in Table 3 as they were calculated on a Fig. 3 Calibration graphs of the integrated signal intensity of the 95% confidence level and include the propagated uncertainty isotope monitored ratioed to the integrated Ag signal intensity for the of the calibration. For a homogeneous silver alloy, like ‘Alloy three solid silver calibration standards (VEB Bergbau- und Hu� ttenkombinat, Freiberg, Germany). B’, RSDs (n=3) well below 10% could be observed. A larger relative uncertainty was found for Cd in this alloy because its signal was only just above the background noise.In other samples, sometimes a spreading up to 30% RSD was seen for some elements, possibly due to inhomogeneity. Analysis of the antique silverware objects Finally, eight antique objects dating from the 16th to the 20th century were directly analysed by LA-ICP-MS using the alternative cell design (Table 4). Their provenance is mainly West European, except for one small spoon from the USA.For gilded objects, an area was analysed where the gilding was not applied or where it had worn oV. It can be observed from Fig. 5 that the small spoon from around 1900 and the 20th century meat fork are characterized by relatively low impurity levels, except for Cd in the meat fork. The relatively high Cd concentration in the meat fork is a clear indication (but not a prerequisite) of a 20th century silver alloy (see Introduction). With the exception of the ‘deer’ and its matching socle, all objects from before 1850 show generally much higher concentrations of the measured impurities.For the ‘deer’ and socle, however, the impurity levels are much too low to be consistent with a silver alloy from before 1850. In view of these results, this object is to be regarded as a forgery, probably from the late 19th century. The high value for Au in the ‘deer’ is most Fig. 4 Transient 111Cd signal intensity for standard ‘Ag10’ measured probably due to remaining traces of the gilding.A fine silver without 110Cd (a) and with 110Cd (b) as dummy isotope. The 107Ag and the 117Sn signal profiles are given for comparison. determination by ICP-OES after digestion of a 3 mg sample 624 J. Anal. At. Spectrom., 1999, 14, 621–626Table 2 Comparison of LA-ICP-MS (n=3) with ICP-MS from solutions after digestion (n=4) for silver Alloy A and with WD-XRF (n=1) for Alloy B. All values are in mg g-1. The uncertainties, given as 95% confidence intervals, are propagated values, including the uncertainty of the calibration Alloy A Alloy B Analyte LA-ICP-MS Solution ICP-MS LA-ICP-MS WD-XRF Zn 7100±2300 7500±2400 1700±500 1400±200 Cd 34±10 26±10 <2 <200 Sn 290±90 290±60 400±70 580±200 Sb 300±100 300±70 40±20 <200 Au 2800±500 2200±500 1400±100 1600±200 Pb 2200±1200 1900±300 1900±500 2900±300 Bi 200±100 110±50 120±20 200±100 from the bottom of the socle additionally confirmed a younger Table 3 RSDs (%) for the normalized signal ratios (n=3) measured age, as a silver content of almost exactly 80% was found.This in the external standards and in two antique silver alloys. The corresponds to ‘silver 800’, an alloy that has been used only concentrations (mg g-1) of the analytes are given in parentheses since the 19th century. RSD (%) (and concentration/mg g-1) Analyte ‘Ag10’ ‘Ag100’ ‘Ag500’ ‘Alloy A’ ‘Alloy B’ Conclusion Zn 17 (9.8) 4 (120) 3 (424) 13 (7100) 6 (1700) The presented method has been shown to be suYciently precise Cd 8 (8.3) 3 (95) 5 (349) 12 (34) 33 (<2) and accurate to allow an ante quem/post quem dating (before Sn 11 (11.5) 13 (101) 2 (497) 12 (290) 4 (400) or after 1850) of antique silverware. The main advantage of Sb 4 (11) 5 (89.2) 3 (463) 18 (300) 5 (40) this method over wet techniques or electron probe microanal- Au 31 (13.6) 4 (101) 11 (506) 7 (2800) 2 (1400) ysis is that relatively large silver objects can be analysed Pb 5 (15.8) 11 (97.6) 1 (489) 22 (2200) 6 (1900) Bi 5 (14.7) 5 (107) 3 (566) 21 (200) 3 (120) directly without having to take a visible fragment of the object for digestion or in order to fit into a vacuum sample chamber.Table 4 Description of the antique silverware objects analysed by LA-ICP-MS Object Provenance Date Bern (Switzerland) 2nd half 18th century Fork Fork Paris (France) ca. 1780 Meat fork SchaVhausen (Switzerland) 20th century Small spoon USA ca. 1900 Spoon Amsterdam (The Netherlands) 2nd half 18th century Deer with matching socle (gilded) Southern Germany Mid-17th century (?) Pomegranate beaker (partially gilded) Southern Germany 1580 Polychromed wooden sculpture of a man (with gilded silver Schwa�bisch-Gmu�nd (Germany) 1620 decoration) Fig. 5 Impurity patterns in the antique silver objects analysed by LA-ICP-MS. For security reasons, only relative values are shown. The concentration of each element has been normalized to its maximum concentration measured in these objects.The low impurity levels in the ‘deer’ and the matching socle suggest a post-1850 origin for the objects. J. Anal. At. Spectrom., 1999, 14, 621–626 6258 E. F. Cromwell and P. Arrowsmith, Anal. Chem., 1995, 67, 131. As the craters obtained after firing the laser are almost invisible 9 W. T. Perkins, N. J. G. Pearce and J. A. Westgate, Geostandards with the naked eye, the proposed method can be re as Newsletter.The Journal of Geostandards and Geoanalysis, 1997, being virtually non-destructive. 21, 175. 10 R. J. Watling, H. K. Herbert, D. Delev and I. D. Abell, Spectrochim. Acta, 1994, 49B, 205. Acknowledgements 11 P. M. Outridge, W. Doherty and D. C. Gregoire, J. Geochemical Exploration, 1998, 60, 229. The authors wish to thank Mr. Martin Kiener for providing 12 V. V. Kogan, M. W. Hinds and G. I. Ramendik, Spectrochim. the antique silverware and the very helpful background Acta, 1994, 49B, 333. information. 13 P. Arrowsmith and S. K. Hughes, Appl. Spectrosc., 1988, 42, 1231. 14 D. Gu¡§ nther, Doctoral Thesis, Martin-Luther-University Halle Wittenberg, Halle, Germany, 1990. References 15 U. Vo¡§ llkopf and K. Barnes, At. Spectrosc., 1995, 16, 19. 16 B. Wanner, Ch. Moor, P. Richner, R. Bro¡§nnimann and B. 1 E.-L. Richter, Altes Silber, Imitiert-Kopiert-Gefa¡§lscht, Keyser, Magyar, Spectrochim. Acta, 1999, 54B, 287. Munich, 1983, ch. 15, pp. 245.249. 17 E.-L. Richter, Altes Silber, Imitiert-Kopiert-Gefa¡§lscht, Keyser, 2 R. D. Mushlitz, in McGraw-Hill Multimedia Encyclopedia of Munich, 1983, ch. 11, p. 166. Science and Technology.¡®Silver Metallurgy¡�, McGraw-Hill, CD- 18 J. Bly, Discovering Hallmarks on English Silver, Shire Publications, ROM Version 1.0, 1994. Buckinghamshire, 8th edn., 1997, pp. 3.13. 3 M.W. Hinds, Spectrochim. Acta, 1993, 48B, 435. 19 J. Divis¢§, Silber-Stempel aus aller Welt, Battenberg Verlag, 4 C. Moor, P. Boll and S. Wiget, Fresenius¡� J. Anal. Chem., 1997, Augsburg, 1995, p. 39 (original title: Znac¢§ky Str¢§©¥¢¥ bra, Aventinum 359, 404. Verlag, Prague, 1976). 5 P. Arrowsmith, Anal. Chem., 1987, 59, 1437. 6 E. R. Denoyer and K. J. Fredeen, Anal. Chem., 1991, 63, 445A. 7 L. Moenke-Blankenburg, Spectrochim. Acta, 1993, 15, 1. Paper 9/00073I 626 J. Anal. At. Spectrom., 1999, 14, 621

ISSN:0267-9477    

DOI:10.1039/a900073i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of natural uranium and thorium in environmental samples by ETV–ICP-MS after matrix removal by on-line solid phase extraction Jason B. Truscott,a Lee Bromley,a Phil Jones,a E. Hywel Evans,*a Justine Turnerb and Ben Fairmanb aUniversity of Plymouth, Department of Environmental Sciences, Drake Circus, Plymouth, UK PL4 8AA bLGC, Queens Road, Teddington, Middlesex, UK TW110LY Received 30th September 1998, Accepted 17th February 1999 An on-line solid phase extraction method has been developed for the determination of 238U and 232Th biological certified reference material using inductively coupled plasma mass spectrometry (ICP-MS).Absolute detection limits were 2.7 pg and 3.1 pg for the determination of 238U and 232Th respectively, both being blank limited. The result for the determination of 238U in NASS-4 Open Ocean Sea Water was 2.13±0.28 ng ml-1 compared with a certified value of 2.68±0.12 ng ml-1. The results for the determination of 238U in SLRS-3 River Water was 0.043±0.002 ng ml-1 compared with an indicative value of 0.045 ng ml-1.Results for the determination of 238U and 232Th in NIST 1575 Pine Needles were 14.6±3.4 ng g-1 and 28.3±4.5 ng g-1 respectively compared with certified values of 20±4 ng g-1 and 37±3 ng g-1, using a dry and wet ashing sample preparation method. Results for the determination of 238U and 232Th in NIST 1566a oyster tissue were 121±21 ng g-1 and 29±8 ng g-1 for 238U and 232Th compared to certified and indicative values of 132±12 ng g-1 and 40 ng g-1, using the same method.When a lithium metaborate fusion method was used, results for 238U and 232Th were 23.3±2.0 ng g-1 and 36.2±5.6 ng g-1 respectively in NIST 1575 Pine Needles. The application of electrothermal vaporisation ICP-MS (ETV–ICP-MS) to NASS-4 Open Ocean SeaWater gave 2.81±0.54 ng ml-1 and SLRS-3 River Water 0.045±0.004 ng ml-1 for 238U. When the fused NIST 1575 samples were analysed using ETV–ICP-MS, results for 238U and 232Th were 19.5±1.7 ng g-1 and 38.8±2.2 ng g-1 respectively.Absolute detection limits for ETV– ICP-MS were 30 fg and 9 fg for 238U and 232Th respectively, both being blank limited. cluded that results for the determination of 239Pu in urine were Introduction comparable to those obtained using a-spectrometry. Similarly, Inductively coupled plasma mass spectrometry (ICP-MS) is a 230Th and 234U have been determined in the soil reference technique ideally suited to the determination of the concen- material TRM-4 (ref. 8) using hydrofluoric acid for sample tration and isotopic composition of the actinide elements. The digestion. Chiappini et al.9 have quoted values close to 1.2 fg principal advantages of ICP-MS are speed and sensitivity, detection limits for uranium, using a new high sensitivity ICPwith the capability of determining all the actinide elements MS10 and a high-eYciency desolvating nebulizer. Aldstadt within a minute, at concentrations as low as 1 pg ml-1 in et al.11 have also reported good results for the determination liquid samples.In addition, there is no need to separate the of 238U by FI-ICP-MS using TRU-SpecTM Resin. The use of elements one from another, as there is in a-spectrometry, 209Bi or 205Tl as internal standards has been quoted to be because this is achieved by the mass spectrometer, hence, the applicable for use in thorium and uranium determination in number of sample pre-treatment stages can be greatly reduced.biological samples.12 In this work the application of an actin- However, it is still necessary to separate the radionuclides ide-specific resin for pre-concentration and matrix removal from the matrix, a procedure for which column pre- prior to analysis by ICP-MS, with and without ETV sample concentration methods are ideal. A number of resins have introduction, has been addressed. been used for the pre-concentration and separation of the actinides.Recently a number of very specific chelating resins have become available which are particularly suited to this task. Some extraction procedures and application of these Experimental resins have been addressed by Horwitz and co-workers,1–4 and Pneumatic nebulization ICP-MS detection Crain et al.5 have quoted 20 fg mL-1 detection limits for 239Pu and 235U using TRU-SpecTM resin as a pre-concentration step An inductively coupled plasma mass spectrometer prior to analysis by ICP-MS.Alvardo and Erickson6 obtained (PlasmaQuad 2+, VG Elemental, Winsford, Cheshire, UK) 5 fg and 2 fg detection limits for 238U and 232Th respectively was used. Data was acquired using the time resolved analysis when using electrothermal vaporisation (ETV) coupled with software, which allows time resolved monitoring of multiple ICP-MS and trifluoromethane as a modifier gas, compared to isotopes, and manipulated oV-line using MassLynx software 180 fg and 1600 fg for an unmodified ETV.Wyse and Fisher7 (Micromass Ltd., Manchester, UK). Operating conditions are have reported a potential 3 fg absolute detection limit for shown in Table 1. The flow injection manifold comprising a 500 ml injection loop on a 6 port valve (Model 5020, Rheodyne, plutonium using ICP-MS and TRU-SpecTM resin, and con- J. Anal. At. Spectrom., 1999, 14, 627–631 627Table 1 Operating conditions for ICP-MS VG PQ2+ PE ELAN 5000A ICP— Forward power/W 1350 1080 Plasma gas/l min-1 16.5 15 Auxiliary gas/l min-1 0.7 1.0 Nebulizer gas/l min-1 0.8 0.8 Sampling depth/mm 10 15 Sample flow/ml min-1 0.5 1.0 Torch Fassel (quartz) Fassel (quartz) Nebulizer Concentric (quartz) Cross-flow (Gem-tip) Spray Chamber Scott type (quartz) Interface— Sampler Ni Pt Skimmer Ni Pt Mass spectrometer— Ion masses (m/z) 232Th, 238U, 209Bi 232Th, 238U, 235U Data acquisition Time resolved mode Transient, peak hopping Points per peak 3 1 DAC step 3 n/a Dwell time/ms 20 40 Time-slice duration/s 1 Cotati, CA, USA) was interfaced with the ICP-MS instrument France) in commercially available glass chromatography columns of 3 mm id and 50 mm length (Omnifit microbore as shown in Fig. 1. columns, Omnifit, Cambridge, Cambs., UK). When not in use the columns were filled with 2 M HNO3, and prior to use ETV–ICP-MS detection they were washed with successive portions of 0.1 Mammonium An inductively coupled plasma mass spectrometer (Elan bioxalate and 2M HNO3 at a flow rate of 0.5 ml min-1 for 5000A, Perkin Elmer, Beaconsfield, Bucks., UK) interfaced 6 min, and finally 1 ml of column feed solution.with an electrothermal vaporisation (ETV) sample introduction system (HGA 600MS, Perkin Elmer) was used. Data Reagents were acquired in transient peak hopping mode, which allows time resolved monitoring of multiple isotopes. Operating con- All solutions were prepared using analytical grade reagents ditions for the ICP are shown in Table 1, with the associated and deionised water (Ultra Pure Water, Elgastat Maxima, temperature program for the ETV shown in Table 2.Elga Ltd, High Wycombe, Bucks., UK). Analytical reagents Samples were applied to the column and eluted with 5 ml were: nitric acid, 2 M (Aristar, BDH, Poole, Dorset, UK); of 0.1M ammonium bioxalate into ETV autosampler vials. eluting solution, (0.1M NH4HC2O4 (Fisons Scientific Portions (30 ml ) were pipetted into the ETV furnace tube and Equipment, Loughborough, UK) filtered through a 47 mm the temperature program initiated.diameter 0.45 mm sterile membrane filter paper (Whatman Laboratory Division, Maidstone, Kent, UK); internal stan- Analytical columns dard solution (15 ng ml-1 Bi) to allow correction for instrumental drift; column feed solution, 1 M Al(NO3)3 (Analytical Columns were prepared with a dry powder of resin Grade, Fisher Scientific UK, Loughborough, Leics., UK) (50–100 mm, TRU-SpecTM, EiChrom Europe, 75010 Paris, purified by passing through a 1.2 cm3 bed of Dowex 1-X8 anion exchange resin (BDH, Poole, Dorset, UK) then a 0.6 cm3 bed of Tru-Spec resin; column feed solution, 0.5M Al(NO3)3+2 M HNO3.Standard solution preparation A mixed standard solution of 10 mg ml-1 232Th and 238U, was prepared in 5% HNO3 from 1000 mg ml-1 stock solutions of the individual elements (Johnson Matthey Ltd., Reading, Berks., UK). In order to ensure that the analytes were in the correct oxidation states to be retained on the column [i.e.U (VI) and Th (IV)], 10 ml of the 10 mg ml-1 standard solution was boiled to dryness in two successive 10 ml portions of Fig. 1 Schematic of the flow injection manifold interface with ICP-MS. conc. HNO3. Table 2 Operating conditions and gas flows for the ETV system Internal furnace gas Program step Temp/°C Ramp time/s Hold time/s flow/ml min-1 1 100 10 15 300 (Ar) 2 120 10 60 300 (Ar) 3 800 5 30 10 (CHF3) 4 2500 0.2 2 0 (to ICP) 5 2700 0 1 0 (to ICP) 6 20 15 1 0 (to ICP) 628 J.Anal. At. Spectrom., 1999, 14, 627–631Sample preparation Water samples. Two certified reference materials were studied, namely NASS-4 Open Ocean Sea Water and SLRS-3 River Water (National Research Council, Ottawa, Ontario, Canada). Samples, 10 ml of NASS-4 and 25 ml of SLRS-3 were treated in the same way as the mixed standard solution, except that they were made up to final volumes of 25 ml and 50 ml respectively with column feed solution.Biological samples. Initially the sample preparation procedure was based on a method by Nelson and Fairman.13 Two certified reference materials (CRMs) were studied, namely Fig. 2 Elution profiles for 238U and 232Th. NIST 1566a Oyster Tissue and NIST 1575 Pine Needles (National institute of Science and Technology, Gaithersburg, concentration step was required. The solution was deposited MD, USA). Samples (0.5 g) were weighed into porcelain onto the column by pumping through the manifold using the crucibles, placed in a muZe furnace and dry-ashed at 200 °C tubing normally immersed in the carrier stream.During depos- for 2 hours, 400 °C for 2 h, 600 °C for 2 h, and 800 °C for 2 h. ition the column was diverted to waste. The centrifuge tube This step was omitted for the oyster tissue. Nitric acid (10 ml ) was rinsed with 1.5 ml of 2 M HNO3, to remove any residual was added to each crucible, followed by gentle warming on a sample from the tube, and subsequently with 1 ml of 2 M hot-plate to digest the samples, boiling to dryness and heating HNO3 to flush through any residual column feed solution on a hotplate.This procedure was repeated until a white ash prior to diverting the column to the ICP-MS. The column was was left. On the last iteration the samples were boiled down diverted to the ICP-MS instrument, the analytes eluted with until almost dry, then 10 ml of the column feed solution was 0.1 M ammonium bioxalate, and the analyte masses moni- added to each beaker to dissolve the ash.Samples were made tored. After elution the column was again diverted away from up to final volumes of 50 ml and 25 ml, for oyster tissues and the ICP-MS and flushed with 1 ml of column feed solution to pine needles respectively, with column feed solution. Three remove residual ammonium bioxalate solution prior to further sample blanks were also prepared. deposition. Each injection was repeated at least three times.Fusion of biological samples. Subsequently sample preparations have been performed by lithium metaborate fusion. Results and discussion Similar procedures of lithium metaborate fusions for soil ICP-MS detection samples have recently been used for uranium and plutonium determinations.14 One certified reference material was studied, Elution profiles and detection limits. The elution peaks for namely NIST 1575 Pine Needles. Samples (0.5 g) were weighed 238U and 232Th are shown in Fig. 2, with uranium being eluted into platinum crucibles and 0.8 g of lithium metaborate completely in approximately 50 s and thorium in approxi- (Spectroflux, Johnson Matthey) was added to each, then mately 30 s. The elution times corresponded to volumes of heated over a Meeker burner. A platinum lid for the crucible approximately 0.4 and 0.25 ml for 238U and 232Th respectively. was used to improve heat retention and thus encourage fusion, The standards were deposited from a 500 ml loop so under some flaming was initially observed from the pine needles, these circumstances both 238U and 232Th were eluted in a while the organic matter was burnt oV.The molten fused smaller volume than the sample loop. sample was then quickly poured into a beaker containing Instrumental and method detection limits for 238U and 232Th approximately 30 ml of column feed solution. Any undissolved are shown in Table 3. Instrumental detection limits were fused matter was allowed to dissolve in the solution, mixing determined using solutions prepared in the column-eluting was aided by use of a magnetic stirrer.Samples were made up solution (0.1 M ammonium bioxalate) but had not been eluted to final volumes of 50 ml in column feed solution. Three from the column, thus reflecting the level of the blank in the sample blanks were also prepared. column-eluting solution. Method detection limits were determined by pre-concentrating a 0.5 ml aliquot of column feed Calibration solution onto the column and eluting with 0.1 M ammonium A series of calibration standards containing both 232Th and bioxalate solution. The method detection limits were blank 238U were prepared and deposited onto the column by flow limited and can be improved by a factor of at least 100 if the injection, into a carrier stream of column feed solution at a reagents are purified more eVectively.This will also allow flow rate of approximately 0.5 ml min-1 for 1 min.During greater pre-concentration factors to be realised, thereby deposition the outlet from the column was diverted to waste improving detection limits further. to prevent the column feed solution entering the ICP-MS instrument. After a deposition, the column was rinsed with Analysis of reference materials. The certified reference mate- 1 ml of 2 M HNO3 to remove any residual column feed rials NASS-4 (Open Ocean Sea Water) and SLRS-3 (River solution before the column was diverted back to the ICP-MS, the analytes were eluted with 0.1 M ammonium bioxalate, and Table 3 Instrumental and method detection limits for uranium and the analyte masses were monitored.After elution the column thorium using pneumatic nebulization PN-ICP-MS and ETV-ICP-MS was again diverted away from the ICP-MS and flushed with U Th 1 ml of column feed solution to remove residual ammonium bioxalate solution prior to further deposition. Each injection Absolute/ Relative/ Absolute/ Relative/ was repeated three times.pg pg ml-1 pg pg ml-1 Analysis of samples Instrumental (PN) 2.7 5.4 3.1 6.2 Method (PN) 24 48 60 120 An accurate volume of the prepared sample was either meas- Instrumental (ETV) 0.03 0.9 0.009 0.3 ured into a clean polypropylene centrifuge tube or injected Method (ETV) 0.6 21 0.3 9 into the 500 ml sample loop, depending on whether a pre- J. Anal. At. Spectrom., 1999, 14, 627–631 629Table 4 Results for the determination of uranium in certified reference materials NASS-4, SLRS-3 by PN-ICP-MS and ETV-ICP-MS 238U found/ng ml-1 Analysed without Certified reference Certified value/ column Analysed with Detection material ng ml-1 (10×dilutiona) columna PN NASS-4 2.68±0.12 2.13±0.28 ETV NASS-4 2.68±0.12 1.98±0.11 2.81±0.54b PN SLRS-3 (0.045)c 0.043±0.002 ETV SLRS-3 (0.045)c 0.042±0.002 0.045±0.004d amean±s; b0.5 ml sample; cuncertified indicative value; d2.5 ml sample.Table 5 Results of the determination of uranium and thorium in certified reference materials by PN-ICP-MS after dry/wet ashing U Th Certified reference Certified value/ Founda/ Certified value/ Founda/ Material ng g-1 ng g-1 ng g-1 ng g-1 1566a Oyster Tissue 132±12 121±21b (40)c 29±8d 1575 Pine Needles 20±4 14.6±3.4d 37±3 28.3±4.5d amean±s; bn=11; cindicative value; dn=5.Table 6 Results of the determination of uranium and thorium in pine needles by PN-ICP-MS and ETV-ICP-MS after lithium metaborate fusion, by calibration with and without the column in place U Th Certified value/ Founda/ Certified value/ Founda/ Detection Calibration method ng g-1 ng g-1 ng g-1 ng g-1 PN Calibration with 20±4 23.3±2.0 37±3 36.2±5.6 columnb PN Calibration without 20±4 18.1±1.4 37±3 33.6±6.8 columnb PN Calibration without 20±4 16.6±1.5 37±3 38.1±0.8 column, 5 ml preconc.c ETV Calibration without 20±4 19.5±1.7 37±3 38.8±2.2 columnd amean±s; bn=3; cn=1, 3 injections; dn=6.Water) were analysed by pre-concentrating known volumes of Table 5.For oyster tissue no significant diVerence was found between the found value and the certified mean for uranium the prepared material, eluting and comparing the peaks to the calibration curve after normalising using the Bi internal stan- at the P=0.05 level. For the pine needles, low recoveries for both thorium and uranium were observed in comparison with dard. Results are shown in Table 4, though it was only possible to compare uranium as the reference materials were only the certified mean, though there was no significant diVerence between the found value and the bottom of the certified range certified for this element.One particular problem that was encountered was that the reproducibility for thorium was for both uranium and thorium (i.e. 16 ng g-1 and 34 ng g-1 respectively) at the P=0.05 level. Other workers have reported unpredictable, and this element was prone to carry-over and high blank values.Low recoveries were obtained for uranium losses of uranium through the use of porcelain crucibles,15,16 by adsorption of 238U onto the surface. However, low recover- in NASS-4 samples using pneumatic nebulization (PN)- ICP-MS. However full recoveries were found for uranium in ies could also be the result of analyte losses by volatilisation in the muZe furnace, as by incomplete sample digestion of NASS-4 when using ETV-ICP-MS, with no significant diVerence between the found value and the mean of the certified silicate material.When the lithium metaborate fusion method was used (Table 6) recoveries were within the certified range, value at the P=0.05 level. The analyses were repeated on two separate days and the results were very similar. Good agree- probably due to complete digestion of silicates within the pine needle matrix, with no significant diVerence between the found ment was obtained between the analytical result and the indicative value for SLRS-3, though no firm conclusions can value and the certified mean for both uranium and thorium at the P=0.05 level.In order to try and speed up the analysis, be drawn because this material was not certified. This clearly shows the value of pre-concentration since the indicative value the eVect of calibrating the analysis by simply flow injecting the standards, rather than depositing them on the column, of 0.045 ng ml-1 was close to the detection limit for the ICP-MS instrument used, and was twice the absolute detection was investigated.Results are shown in Table 6 and indicate that full recoveries were obtained for both 238U and 232Th. limit for the method detailed here. However, a preconcentration factor of 5 eVectively raised the level of uran- When the pre-concentration factor was increased by a factor of 10 (i.e. 5 ml were deposited instead of 0.5 ml ) recoveries ium to 10 times the detection limit, making analysis feasible. were still within the certified range, again with no significant diVerence between the found value and the certified mean for Results for the analysis of oyster tissue and pine needles after sample preparation by dry/wet ashing are shown in both uranium and thorium at the P=0.05 level. 630 J. Anal. At. Spectrom., 1999, 14, 627–631Table 6. Results were within the certified range of the reference material. Conclusions The determination of 238U and 232Th in certified reference materials was successfully performed in most instances.Low recoveries were observed for the determination of 238U in NASS-4 Open Ocean Sea Water without matrix removal using the column pre-treatment for ETV-ICP-MS, however, with column pre-treatment full recoveries were obtained. Results Fig. 3 Vaporisation profiles for 238U (3 pg) and 232Th (30 pg) for for the freshwater (SLRS-3) were in good agreement with the ETV-ICP-MS using only argon gas. indicative value. Agreement with certified values was observed for the determination of 238U and 232Th in NIST 1575 Pine Needles after pre-concentration and matrix elimination after lithium metaborate fusion, and detection by ICP-MS and ETV-ICP-MS. However, losses were apparent when using a dry/wet ashing method.The addition of freon gas to the ETV improved sensitivity for 238U and 232Th 10-fold and 50-fold respectively. Acknowledgements The work described in this paper was supported in part by the Department of Trade and Industry, UK, as part of the Government Chemist Programme.References Fig. 4 Vaporisation profiles for 238U (3 pg) and 232Th (3 pg) for ETVICP- MS using CHF3 modifier gas. 1 E. P. Horwitz, New Chromatographic Materials for Determination of Actinides, Strontium, and Technetium in Environmental, ETV–ICP-MS detection Bioassay, and Nuclear Waste Samples, Chemistry Division, Argonne National Laboratory, Argonne, IL, May 1992. EVect of freon gas, elution profiles and detection limits. 2 E. P. Horwitz, M. L. Dietz, R. Chriarizia, H. Diamond, S. L. Vaporisation profiles for 238U and 232Th with and without Maxwel and M. R. Nelson, Anal. Chim. Acta, 1995, 310, 63. 3 E. P. Horwitz, M. L. Dietz, R. Chriarizia, H. Diamond, A. M. freon added during the ashing stage are shown in Figs. 3 and Essling and D. Graczyk, Anal. Chim. Acta, 1992, 266, 25. 4. In the absence of freon (Fig. 3) the peaks were approxi- 4 E. P. Horwitz, R. Chiarizia, M. L. Dietz and H.Diamond, Anal. mately 2.5 s wide, and 232Th vaporised slightly later than U. Chim. Acta, 1993, 281, 361. However, when freon was added (Fig. 4), peak height and 5 J. S. Crain, L. L. Smith, J. S. Yaeger and J. A. Alvarado, peak area signals increased by approximately 10 times and 50 J. Radioanal. Nucl. Chem., 1995, 1, 133. times for 238U and 232Th respectively, resulting in much 6 J. S. Alvardo and M. D. Erickson, J. Anal. At. Spectrom., 1996, 11, 923. improved detection limits.Other workers have also noted the 7 E. J.Wyse and D. R. Fisher, Radiat. Prot. Dosim., 1994, 55, 199. beneficial eVect of freon gas in ETV,6,17,18 which prevents the 8 M. Hollenbach, J. Grohs, S. Mamich and K. Marilyn, J. Anal. At. formation of refractory carbides on the surface of the graphite Spectrom., 1994, 9, 927. tube, however, it is advisable to only introduce the gas during 9 R. Chiappini, J.-M. Taillade and S. Bre�bion, J. Anal. At. the ashing stage. If freon is introduced during the vaporisation Spectrom., 1996, 11, 497. stage, tube lifetimes are reduced substantially. 10 Practical Benefits of an Ultra Sensitive ICP-MS System—Actinide Determination at PPQ Level, Hewlett-Packard Technical Note Instrumental and method detection limits are shown in Pub. No. (43) 5965–5181E, 1996. Table 3 and were determined as before. As for pneumatic 11 J. H. Aldstadt, J. M. Kuo, L. L. Smith and M. D. Erickson, Anal. nebulization, detection limits were blank limited, so improve- Chim. Acta, 1996, 319, 135. ments might be expected if the purity of reagents is improved. 12 Y. Igarahi, H. Kawamura, K. Shiraishi and Y. Takaku, J. Anal. At. Spectrom., 1989, 4, 571. Analysis of reference materials. Results for the analysis of 13 D. M. Nelson and W. D. Fairman, presented at the 36th Annual Conference on Bioassay, Analytical and Environmental water reference materials are shown in Table 4. The samples Radiochemistry, Oak Ridge, Tennessee, USA, 1990. were analysed after straightforward 10-fold dilution, and after 14 I. Croudace, P. Warwick, R. Taylor and S. Dee, Anal. Chim. Acta, pre-treatment on the column. Low recoveries were obtained 1998, 371, 217. for the diluted NASS-4 Open Ocean Sea Water samples 15 W. F. Neumann, R. W. Fleming, A. B. Carlson and N. Glover, without matrix removal on the column, but agreement with J. Biol. Chem., 1948, 173, 41. the certified value was obtained when the matrix was removed 16 A. K. Babko and V. N. Danilova, Zh. Anal. Khim., 1963, 18, 1036. 17 D. Goltz, D. C. Gre�goire, J. P. Bryne and C. L. Chakrabarti, using the column pre-treatment. Similar results were obtained Spectrochim. Acta, Part B, 1995, 50, 803. for the SLRS-3 River Water samples regardless of which 18 B.Wanner, P. Richner and B. Magyar, Spectrochim. Acta, Part B, method was used, reflecting the relative simplicity of this 1996, 51, 817. matrix compared to sea water. Results for the analysis of pine needles reference materials Paper 8/08430K using the lithium metaborate fusion method are shown in J. Anal. At. Spectrom., 1999, 14, 627–631 6

ISSN:0267-9477    

DOI:10.1039/a808430k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Simultaneous determination of Pt and I by ICP-MS for studies of the mechanism of reaction of diiodoplatinum anticancer complexes Marina Patriarca,† Nicole A. Kratochwil and Peter J. Sadler* Department of Chemistry, University of Edinburgh, Joseph Black Building, King’s Buildings, West Mains Rd., Edinburgh, UK EH9 3JJ. E-mail: P.J.Sadler@ed.ac.uk Received 14th October 1998, Accepted 19th January 1999 The determination of Pt by ICP-MS in environmental and biological samples is well documented and generally performed after dissolution in dilute HNO3.On the other hand, I is poorly ionised in the plasma and, at low pH, memory eVects and instability arise from the formation of potentially volatile species, such as I2 and HI, depending on the oxidation state of I. In order to investigate the role of iodo ligands in the design of Pt anticancer complexes, we have optimised conditions for the simultaneous determination of Pt and I. Standards and samples were diluted in 10 mM KOH and improved ion extraction into the quadrupole was achieved by means of an additional pump (S-Option@), leading to about a 2-fold increased sensitivity.The limits of detection in water were 10 and 0.6 ng l-1 for I and Pt, respectively, but increased to 23 ng l-1 for I and 2.2 ng l-1for Pt in KOH (10 mM). The analysis of certified reference materials yielded the following results: 0.84±0.05 mg g-1 I (certified value: 0.81±0.05 mg g-1) in BCR CRM 063R ‘Skimmed Milk Powder’, and 0.121±0.006 mg ml-1 Pt (indicative value: 0.12 mg ml-1) in NIST SRM 2670 ‘Toxic Metals in Urine’. Pt5I ratios ranging from 0.240 to 1.035 were measured with an accuracy of 101.3±2.4%.The determination of the Pt5I ratio in the low Mr fraction of reaction mixtures of diiodo Pt complexes and human albumin provided evidence for the release of iodide and for diVerent kinetics for the reactions of diiodo Pt(IV) and Pt(II) complexes. Introduction The capability of inductively coupled plasma mass spectrometry (ICP-MS) for multi-elemental detection with almost unrivalled sensitivity is of particular interest in biomedical research for the investigation of the functions of biological molecules with aYnity for more than one element (e.g.the metal transport proteins metallothionein and transferrin) and studies of the mechanism of action of newly designed drugs involving metal centres. The full exploitation of this technique, however, requires careful optimisation of analytical param- Fig. 1 Structures of the diiodo Pt complexes used in this work: (1) eters, especially when elements with diVerent chemistry are to trans,cis-[Pt(en)(OH)2I2] and (2) [Pt(en)I2]. be determined. As part of current work on the role of iodo ligands in the design of Pt anticancer complexes,1–3 we have investigated The determination of Pt by ICP-MS is relatively straightoptimum conditions for the simultaneous determination of Pt forward,5 the only reported spectral interferences being due to and I by ICP-MS. Release of iodide may be an important step oxides of less common elements, such as hafnium, ytterbium in the mechanism of reaction of diiodo Pt complexes (Fig. 1) and tungsten.6 In contrast, I is poorly ionised in the plasma with human albumin, the major protein in blood plasma.3 The and the analytical performance is aVected by the oxidation determination of the Pt5I ratio in the protein and low Mr state of I.Poor stability and memory eVects occur at low pH fractions of the reaction mixture can provide evidence for the in the presence of iodide and have been attributed to the release of I and clarify the mechanism of reaction.formation of volatile species such as HI and I2.7,8 Therefore, Previous work has been reported on the determination of the dilution of standards and samples in alkaline media is either I or Pt in biological matrices by diVerent techniques, recommended.7,9–11 Both inorganic and organic bases, such as including spectrophotometry, voltammetry, gas chromatogra- NH3,7–9 KOH or NaOH, at concentrations between 50 and phy, neutron activation analysis, atomic absorption spec- 70 mM,11,12 tetramethylammonium hydroxide11 and a mixture trometry, inductively coupled plasma optical emission of water-soluble tertiary amines10 have been used to eliminate spectrometry and ICP-MS.4,5 However, few of these allow the memory eVects.simultaneous determination of both elements with comparable In this paper, we focused on the validation of a method for sensitivity. The use of diVerent techniques for the determi- the simultaneous determination of I and Pt, which could nation of the two elements would be prone to the introduction provide a comparable degree of reliability for both elements of a wider range of uncertainty and would require larger and allow the accurate determination of their ratio.Previous volumes of samples.reports have indicated influences of the matrix on the analytical behaviour of individual elements, according to specific characteristics. For example, the addition of organics, such as †On leave from Istituto Superiore di Sanita`, Rome, Italy. J. Anal. At. Spectrom., 1999, 14, 633–637 633glycerol,13 Triton X-100,14 methanol15,16 and organic amines,10 exchange–reverse osmosis system (Elga, Bucks., UK) and used in all experiments. enhances selectively the sensitivity of elements with ionisation potentials>9 eV, including I, by increasing the torch tempera- Diiodo Pt complexes, [Pt(en)I2] and trans,cis- [Pt(en)(OH)2I2], were synthesised according to previously ture.Therefore, since Pt and I diVer in ionisation potential (8.70 eV; 10.08 eV) and degree of ionisation (62%; 29%),17 described procedures.1 Recombinant human albumin (rHA, batch R970103) was supplied by Delta Biotechnology particular attention was paid to the influence of increasing concentrations of KOH on signal intensity and to the eYcacy (Nottingham, UK).of internal standardisation. Procedures Experimental ICP-MS analysis. Working standard solutions containing both I and Pt in a molar ratio of 251 were prepared freshly Instrumentation each day as follows. A stock standard solution at a concentration of 100 mg l-1 was made up from KI in ultrapure water, A PlasmaQuad 3 inductively coupled plasma mass stored frozen at -20 °C and replaced every 30 d.A solution spectrometer from VG Elemental (Winsford, UK) was used containing I (1.3 mg l-1) and Pt (1.00 mg l-1) was prepared for all measurements. The instrumental apparatus included a by dilution of the stock solutions (I 100 mg l-1, Pt high eYciency interface (S-Option@), which improves sensi- 1000 mg l-1) in water, from which working standard solutions tivity by increasing the eYciency of the extraction of ions in at concentrations of 0, 1.3, 3.2, 6.4 and 12.7 mg l-1 I and 0, the plasma through to the mass spectrometer itself.This 2.5, 5.0 and 10.0 mg l-1 Pt, were prepared by dilution with feature is achieved by means of improved pumping of the 10 mM KOH containing 20 mg l-1 Te as internal standard expansion chamber via a high capacity rotary pump, which (diluent). The certified reference materials were prepared for reduces the pressure within the expansion region to less analysis as follows. Portions of about 200 mg of CRM 063R than 1 mbar.‘Skimmed Milk Powder’ were weighed and dissolved overnight Instrumental settings, operating conditions and acquisition in 40 g of the diluent to obtain clear solutions; the moisture parameters are given in Table 1. Lens settings were optimised content of CRM 063R was determined according to the every day at mass 127 with a solution containing 12.7 mg l-1 manufacturer’s instructions and used to correct results to dry I and 9.75 mg l-1 Pt in KOH (10 mM). mass. After reconstitution, the SRM 2670 ‘Toxic Metals in Urine’ (Elevated Level ) was diluted 1+24 with 10 mM KOH Reagents containing 20 mg l-1 Te.Potassium iodide (99.5%) and single element standards for ICP at 1000 mg l-1 of Pt (Pt in 20% HCl) and Te (Te in Studies of the reactions of diiodo Pt complexes with human HCl), all ‘Aristar’ grade, from Merck (Leics., UK), were used albumin. The diiodo Pt complexes were separately incubated for calibration. KOH was of analytical-reagent grade from with rHA at a molar ratio of 151 (100 mM) in 100 mM Fisher Scientific (Loughborough, UK). The reference mate- NaCl–10 mM NaH2PO4 at 37 °C, pH 7.4.Aliquots (900 ml ) rials NIST SRM 2670 ‘Toxic Metals in Urine’ (National were taken from the reaction mixture (10 ml ) at 0, 0.5, 1, 1.5, Institute of Standards and Technology, Gaithersburg, MD, 2.5, 6.0, 20.5 and 24 h. To evaluate the distribution of Pt and USA), and BCR CRM 063R ‘Skimmed Milk Powder’ (EU I among the high and low Mr fractions at diVerent stages of Institute for Reference Materials and Measurements, Geel, the reaction with rHA, the two fractions were separated by Belgium) were used to assess the accuracy of Pt and I ultrafiltration using a Centricon-30 concentrator (10 min at determination, respectively.Ultrapure water (resistivity 2516 g, 4 °C). Since significant binding of iodide to human 18 MV, TOC <5 ppb) was obtained by a combined ion serum albumin has been shown to occur (Ka1=6.15×103),18 reversibly bound I was removed from the protein fraction by repeated washing with the reaction buVer, i.e.after the first Table 1 VG PlasmaQuad 3 instrumental settings, operating conditions centrifugation, 900 ml of buVer were added to the protein and acquisition parameters fraction and the centrifugation was repeated. This step was repeated three times for each sample, the last centrifugation Operating conditions— Plasma: Rf power Forward: 1350 W carried out for 15 min and all ultrafiltrates were combined.Reflected: <2 W The final volumes of both fractions were recorded. Ar gas flow rates Plasma: 13.1 l min-1 Determinations of Pt and I were carried out by ICP-MS on Auxiliary: 0.80 l min-1 both fractions of all samples, after dilution with 10 mM KOH Nebuliser: 0.80 l min-1 containing 20 mg l-1 Te. Sample Peristaltic pump Minipuls 3, (Gilson) introduction: Nebuliser Meinhard concentric glass nebuliser Results Spray chamber type Scott double-pass type, Choice of experimental conditions water cooled at 5 °C Sample uptake rate 0.6 ml min-1 Initial studies were carried out to assess the feasibility of the Vacuum: Expansion stage 0.8 mbar (S-Option@ on) simultaneous determination of both I and Pt in KOH and to Intermediate stage 10-5 mbar determine the minimum concentration of KOH needed.The Analyser stage 3.4×10-6 mbar signal intensity profile obtained for a 10 mg l-1 solution of KI Sampling depth 5 mm from load coil in 50 mM KOH (Fig. 2) showed a flat top peak with little Acquisition Isotopes 127I, 195Pt, 126Te fronting and a shorter tail, as compared with 10 mM KOH, parameters: Time per sweep 0.2 s thus indicating a more eVective stabilisation of I species, but Acquisition mode Peak jumping poorer sensitivity. The estimated time for complete wash-out Dwell time 10.24 ms Points per peak 3 was 3 min in 50 mM KOH and 8 min in 10 mM KOH. The Uptake 120 s analysis of a set of standard solutions, prepared in either 10, Acquisition time 10 s 20 or 50 mM KOH, indicated a drop in signal intensity with Rinsing time 420 s increasing concentration of KOH, aVecting Pt and I to diVerent Replicates 5 extents (Table 2).Total dissolved solids increased from 0.06% 634 J. Anal. At. Spectrom., 1999, 14, 633–637Fig. 2 Signal intensity profiles for 10 mg l-1 I in 10 and 50 mM KOH. Table 2 Changes of signal intensity (%) for I and Pt in increasing concentrations of KOH relative to 10 mM KOH (mean±s, n=5) KOH concentration Fig. 4 Long-term stability profile for 127I, 195Pt and 126Te. 10 mM 20 mM 50 mM Te (internal standard) in 10 mM KOH, studied over a period I 100 89.1±1.1 68.9±1.7 of 10 h, by repeated analysis of a solution containing all three Pt 100 84.8±3.2 61.5±0.8 elements, was satisfactory for a period of about 5 h, after which diVerences between the analytical behaviour of Pt and Te became evident (Fig. 4). in 10 mM KOH to 0.28% in 50 mM KOH.In addition, poorer long-term stability, higher imprecision, and higher blank levels Analytical performance were observed when 20 or 50 mM KOH was used for the dilution of standards and samples. Therefore, 10 mM KOH In standard sensitivity mode, the limits of detection (LODs, was used for all subsequent work. To prevent memory eVects, 3s of ten measurements of the blank) were 40 ng l-1 for I and the rinsing time between samples was increased to 7 min, using 0.5 ng l-1 for Pt in water, but increased to 58 ng l-1 for I and water rather than KOH to limit the build-up of salts and 4.9 ng l-1 for Pt in KOH (10 mM).When the S-Option@ was damage to the torch. This approach proved to be eVective in used, the LODs for I improved to 10 ng l-1 in water and the range 0–12.7 mg l-1 I. 23 ng l-1 in KOH (10 mM). For Pt, the LODs were 0.6 ng l-1 The variations in signal intensity with the plasma in water and 2.2 ng l-1 in KOH (10 mM). The average concentemperature are shown in Fig. 3 for both Pt and I, in terms trations of I and Pt in the blank (10 mM KOH, 20 mg l-1 Te) of absolute intensity, normalised for concentration and isotope were 0.35 mg l-1 and 10.3 ng l-1, respectively.abundance, for rf power varying from 1050 to 1500 W. Both The analysis of certified reference materials yielded the elements show a maximum at 1450 W, but with little variation following results. For the CRM 063R ‘SkimmedMilk Powder’, within the range 1350–1450 W (+12 and +5%, respectively).we found an I value of 0.84±0.05 mg g-1 (6.0% RSD, n=5) In addition, the signal-to-noise ratio reached maximum values as compared with the certified value of 0.81±0.05 mg g-1. We for both I and Pt between 1350 and 1400 W. With improved found no detectable Pt in this CRM for which no Pt value is ion extraction (S-Option@), we observed an increase in signal reported. The Pt value measured in the SRM 2670 ‘Toxic intensity of 2.6-fold for I and 1.2-fold for Pt.Optimisation of Metals in Urine’ was 0.121±0.006 mg ml-1 (4.7% RSD, n= lens settings at mass 127 proved to be critical to achieve 9) as compared with an indicative value of 0.12 mg ml-1. No maximum sensitivity and good short-term stability for I, I value is reported for this SRM, but we measured an I content whereas Pt was less sensitive to optimisation of lens settings of 0.244±0.014 mg ml-1 (5.7% RSD, n=9). The precision of at either mass 127 or 195. The long-term stability of I, Pt and the Pt5I ratio measured in the SRM 2670 was 0.496±0.013 (RSD 2.7%, n=9).The accuracy of the determination of the Pt5I ratio was tested by analysing a solution of the diiodo Pt(IV) complex, trans,cis-[Pt(en)(OH)2I2], which has a stoichiometric Pt5I ratio of 0.5, and in solutions of the same complex spiked with diVerent amounts of either I or Pt standards, to obtain Pt5I ratios ranging from 0.240 to 1.035 (Table 3). The regression analysis between measured ( y) and expected (x) values gave y=-0.018+1.050 x, r2=0.998.The within-day precisions for I, Pt and Pt5I ratio Table 3 Accuracy of the determination of the Pt5I ratio Measured Pt5I ratio Expected Pt5I ratio Mean s RSD (%) n 0.240 0.245 0.001 0.4 3 0.324 0.329 0.005 1.6 3 0.500 0.492 0.006 1.2 13 0.767 0.762 0.008 1.1 3 Fig. 3 Molar response for 127I, 194Pt and 195Pt as a function of 1.035 1.089 0.009 0.9 3 rf power. J. Anal. At. Spectrom., 1999, 14, 633–637 635tivity for both elements, lower blank values for Pt, and reduced content of total dissolved solids, which in turn ensures better stability.Tellurium has been used as an internal standard for I determination7,9 and is among the group of elements with higher ionisation potential; a disadvantage is that it is a multiisotopic element and the most abundant isotopes (130Te, 128Te) are both prone to isobaric interferences with Xe. The suitability of Te as an internal standard was verified by studying the long-term stability of the signal for a period of 10 h.This showed that 126Te closely followed the analytical behaviour of I, but diVerences from Pt became increasingly evident after 5 h. In addition, we used frequent recalibration (every 20 samples) and the analysis of control samples to monitor drift and variation of performance. For analysis over a long period of time, an additional internal standard should be considered. In this case, 197Au, 193Ir or isotope dilution with a minor Pt isotope could be suitable choices.The quest for higher sensitivity and lower detection limits has stimulated the development of alternative methods to improve these analytical performances, in particular novel methods for sample introduction, including direct injection, Fig. 5 Pt5I ratio in the protein (a) and low Mr (b) fractions of the ultrasonic and microconcentric nebulisers. In this study, we reaction mixture containing trans,cis-[Pt(en)(OH)2I2] (6) or applied a relatively new instrumental option, which provided [Pt(en)I2] (&) and rHA as a function of time.a substantial improvement in sensitivity, especially for the most diYcult ion (I ). Reported LODs for I range from 100 to 4000 ng l-1,7–9,11,16,19 but values of 15 and 18 ng l-1, in determinations, measured as RSD (n=13), were 0.9, 0.9 and NaOH and a mixture of tertiary amines, respectively, have 1.2%, respectively. Between-day precisions were determined by recently been reported.10 The LOD of our method is 23 ng l-1, repeated analysis of trans,cis-[Pt(en)(OH)2I2] on diVerent days which compares well with this last report.Literature data for (n=10) and were 2.3, 2.2 and 3.7%, respectively, for I, Pt and LODs of Pt range from 14 to 50 ng l-1,20–23 using conventional the Pt5I ratio. Attempts to reduce the KOH concentration to instrumentation, although LODs of 0.02 ng l-1 can be 1 or 0.1 mM yielded lower values for the Pt5I ratio obtained by double-focusing magnetic sector field ICP-MS.24 (0.484±0.004 and 0.442±0.004, respectively) and were Our LOD for Pt (2.2 ng l-1) is, therefore, much lower than therefore abandoned.previously reported values for quadrupole ICP-MS, and even lower LODs can be measured in water (0.5 ng l-1). Kinetic studies The method allows the reliable determination of the Pt5I ratio with an overall precision of 3.7% and an average accuracy Initial experiments were carried out to assess the extent of non-specific binding of iodide to albumin.After 24 h incu- of 101.3±2.4%. We studied the kinetics of the interactions between both diiodo Pt(II) and Pt(IV) complexes and rHA, by bation of trans,cis-[Pt(en)(OH)2I2] with rHA, the Pt5I ratio in the highMr fraction was measured before and after repeated determining the ratio of the two elements in the protein and low Mr fractions of samples taken from the reaction mixture (×3) washing with the reaction buVer.The Pt5I ratio was 0.645 initially, but increased to 4.08 when reversibly bound over a period of 24 h. The determination of the ratio between the two elements rather than their absolute amount or concen- iodide was removed by repeated washing. Fig. 5 shows the variations of the Pt5I ratio in the protein tration overcomes problems associated with uncertainties such as recovery, dilution and volume measurements. and low Mr fractions of the reaction mixture between the diiodo Pt(IV) and Pt(II) complexes with rHA over 24 h.In the The rapid decrease in the Pt5I ratio in the low Mr fraction of the reaction mixture indicates the presence of increasing two fractions, the Pt5I ratio varies rapidly in opposite directions within the first 2.5 h, indicating the release of iodide at amounts of free iodide and provides evidence for the release of I at an early stage of the reaction of both complexes with an early stage of the reaction. The pattern of variation of the Pt5I ratio in the protein fraction (from 1 to 4) also suggests albumin.The Pt5I ratios for the protein fraction suggest that the release of the two I ligands occurs at diVerent stages. The that the two iodide ligands are released from Pt in separate stages. study of Pt5I ratios has highlighted diVerences in the kinetics of the reactions of the diiodo Pt(IV) and Pt(II) complexes with albumin. A more detailed discussion of the reaction Discussion mechanisms will be reported elsewhere.3 Previous work has shown that I is best determined at alkaline pH, to prevent severe memory eVects and signal instability.7,9,10 Acknowledgements The drawbacks of this approach are mainly the deterioration of analytical performances, because of the wearing of glassware We thank the EU (Marie Curie Research Training Grant, and build-up of deposits on cones, and higher blank values, ERB4001GT963865 to NAK, and COST Action D8), BBSRC due to impurities in the reagents.In this work, we used KOH, and EPSRC, for their support to this work, and Delta which could be obtained with a higher purity than NaOH. Biotechnology for the gift of recombinant human albumin. Ammonia was avoided, because it forms stable complexes with Pt. The comparison of diVerent concentration of KOH has References shown that at least 10 mM KOH is necessary for the stabilisation of the I signal. Under these conditions, the complete 1 N. A. Kratochwil, M. Zabel, J.J. Range and P. J. Bednarski, elimination of residual memory eVects requires a rather long J. Med. Chem., 1996, 39, 2499. 2 N. A. Kratochwil, Z. Guo, P. S. Murdoch, J. A. Parkinson, washing time (7 min) but has the advantages of higher sensi- 636 J. Anal. At. Spectrom., 1999, 14, 633–637P. J. Bednarski and P. J. Sadler, J. Am. Chem. Soc., 1998, 120, 14 M. J. Campbell, C. Demesmay and M. Olle, J. Anal. At. Spectrom., 1994, 9, 1379. 8253. 15 E. H. Larsen and S. Stu�rup, J.Anal. At. Spectrom., 1994, 9, 1099. 3 N. A. Kratochwil, A. I. Ivanov, M. Patriarca, J. A. Parkinson and 16 E. H. Larsen and M. B. Ludwigsen, J. Anal. At. Spectrom., 1997, P. J. Sadler, unpublished work. 12, 435. 4 Encyclopedia of Analytical Science, ed. A. Townshend, 17 R. S. Houk, Anal. Chem., 1986, 58, 97A. P. J.Worsfold and S. J. Haswell, Academic Press, London, 1995. 18 D. C. Carter and J. X. Ho, Adv. Protein Chem., 1994, 45, 153. 5 M. Balcerzak, Analyst, 1997, 122, 67R. 19 G. Ra�dlinger and K. G. Heumann, Anal. Chem., 1998, 70, 2221. 6 H. Mukai, Y. Ambe and M. Morita, J. Anal. At. Spectrom., 1990, 20 T. Minami, K. Hashii, I. Tateyama, E. Kadota, Y. Tohno, 5, 75. S. Tohno, M. Utsumi, M. Yamada, M. Ichii, K. Namikawa and 7 H. Vanhoe, F. Van Allemeersch, J. Versieck and R. Dams, Y. Okazaki, Biol. Trace Elem. Res., 1994, 42, 253. Analyst, 1993, 118, 1015. 21 T. Minami, M. Ichii and Y. Okazaki, Biol. Trace Elem. Res., 1995, 8 M. Haldimann, B. Zimmerli, C. Als and H. Gerber, Clin. Chem., 48, 37. 1998, 44, 817. 22 J. Christodoulou, M. Kashani, B. M. Keohane and P. J. Sadler, 9 H. Baumann, Fresenius’ J. Anal. Chem., 1990, 338, 809. J. Anal. At. Spectrom., 1996, 11, 1031. 10 Y. Gelinas, A. Krushevska and R. M. Barnes, Anal. Chem., 1998, 23 W. R. L. Cairns, C. W. McLeod and B. Hanck, Spectroscopy, 70, 1021. 1997, 12, 16. 11 S. Stu�rup and A. Buchert, Fresenius’ J. Anal. Chem., 1996, 354, 24 J. Bergerow, M. Turfeld and L. Dunemann, J. Anal. At. 323. Spectrom., 1997, 12, 1095. 12 P. Schramel and S. Hasse, Mikrochim. Acta, 1994, 116, 205. 13 P. Allain, L. Jaunault, Y. Mauras, J. M. Mermet and T. Delaporte, Anal. Chem., 1991, 63, 1497. Paper 8/07981A J. Anal. At. Spectrom., 1999, 14, 633–6

ISSN:0267-9477    

DOI:10.1039/a807981a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Speciation of metal–carbohydrate complexes in fruit and vegetable samples by size-exclusion HPLC-ICP-MS Joanna Szpunar,a Patrice Pellerin,b Alexei Makarov,a Thierry Doco,b Pascale Williamsb and Ryszard £obin�ski*a aCNRS EP132, He�lioparc, 2, av. Pr. Angot, 64 000 Pau, France. E-mail: Ryszard.Lobinski@univpau. fr bInstitut National de la Recherche Agronomique, Institut des Produits de la Vigne, Unite� des Recherches des Polyme`res et des Techniques Physico-Chimiques, 2, pl. Viala, 34 060 Montpellier, France Received 23rd October 1998, Accepted 25th January 1999 Kinetically inert and thermodynamically stable metal complexes with polysaccharides were detected in aqueous leachates and enzymatic digests of apple and carrot samples by size-exclusion chromatography with parallel refractometric and ICP-MS detection.The method developed allowed detection in the water-soluble fraction and the identification of a high molar mass polysaccharide fraction (>50 kDa) containing Pb, Ba, Sr, Ce and B, whereas other metals (Zn, Cu, Mg) eluted as complexes with low molar mass non-carbohydrate compounds.The majority of the metal–carbohydrate complexes were located in the solid water-insoluble fraction of the analysed samples. An extraction procedure with a mixture of pectinolytic enzymes was developed to release these species into the aqueous phase. The metal-binding carbohydrate component was identified as the dimer of rhamnogalacturonan-II, a pectic polysaccharide present in plant cell walls.The unidentified residual metal species contained less than 5% of the metals present. and the quantitative recovery of intact metal species from Introduction solid matrices. Indeed, even for relatively simple organoarsenic Fruits and vegetables are important (and sometimes major) and -selenium compounds, the scarcity of standards has been sources of trace elements in the human diet. In order to be of a considerable shortcoming.Bioligands often exhibit polymorconcern in terms of essentiality and/or toxicity these elements phism and, additionally, mixed metal complexes can be must be bioavailable, i.e. readily absorbable by the gut, and formed. Purification of particular metallocompounds is thus further utilizable in the body. Since the bioavailability depends diYcult and the consequence is again the poor availability of critically on the actual species of an element present, infor- calibration standards. Another critical issue is the sample mation on the total concentration of an element in a foodstuV preparation procedure that should enable the metal species to may be useless with respect to its actual assimilation by be recovered unmodified and to be presented for HPLC in an consumers.1 Precise information regarding the identity, nature aqueous 0.2 mm filtrable solution.To date, most of the studies and concentrations of individual metal compounds present in of the speciation of metal complexes have focused on the a sample (speciation) is therefore required.water-soluble (aqueous buVer) fraction that contained 10–20% Because of the capability of ICP-MS to detect metals and of the elements of concern present in a sample, leaving the metalloids at picogram levels in liquid chromatographic solid residue unexplored. eZuents,2–4 the coupling of HPLC to ICP-MS has been widely FoodstuVs of plant origin contain significant concentrations applied to elemental speciation analysis in foodstuVs of animal of polysaccharides of which the potentially negatively charged and plant origin. The research objectives included the detection oxygen functions can bind cations electrostatically and even and identification of As species in marine edible resources,5 chelate them via polyhydroxy groups.28 In comparison with monitoring of residual As-containing growth additives,6,7 and proteins, however, little is known about the relevance of metal the characterization of Cd– and Zn–metallothionein complexes coordination to carbohydrates that are the most abundant (by in meat and their behaviour during simulated gastrointestinal weight) class of compounds in the biosphere.29 Recently, digestion procedures8–11 and cooking.10 Studies of edible plant attention has been attracted by a structurally complex pectic materials focused on the speciation of Se in nutritional sup- polysaccharide rhamnogalacturonan-II (RG-II).18,20,30–32 This plements12–14 and Se-enriched vegetables,15 B in radish ubiquitous component of primary plant cell walls forms dimers roots,16,17 apple fruit17 and sugar beet,18 metals in tea19 and cross-linked by 152 borate diol esters.30–32 The dimers of Pb in wine.20–22 Speciation of Zn and Cd in vegetables was RG-II borate ester (dRG-II-B) were found to complex in vitro approached by ultra- and dia-filtration techniques23,24 and specific divalent cations31 and the majority of Pb in wine.20–22 SEC25,26 with oV-line detection by AAS23,24,26 or total reflec- The purpose of this paper was to investigate the speciation tion XRF.25 Speciation of trace metals and metalloids in plants of metals in fresh fruits and vegetables by HPLC with both was recently reviewed.27 refractometric and ICP-MS detection. A sample preparation The key challenges to elemental speciation in foodstuVs procedure based on an enzymatic digestion/liquefaction was include the identification of the eluted compounds of which used to gain an insight into metal speciation in the waterinsoluble sample fraction.only the metal component is actually detected by ICP-MS, J. Anal. At. Spectrom., 1999, 14, 639–644 639molecular mass standards (P-5, Mw=5800; P-10, Mw=12 200; Experimental P-20, Mw=23 700; P-50, Mw=48 000, Showa Denko, Tokyo, Instrumentation Japan) using refractometric detection.33,34 P-50 elutes at 8.55 ml, close to the exclusion volume of the column.The Chromatographic separations and flow-injection experiments glucose monomer was injected to obtain the total column were performed using a Series 410 BIO HPLC pump (Perkinvolume of 19.32 ml. The elution volumes for the standards in Elmer, Palo Alto, CA, USA) as the sample delivery system. the fractionation range of the column were 10.05, 11.74 and Injections were performed using a Model 7725 injection valve 14.04 ml for P-20, P-10 and P-5, respectively.It should be with a 50 ml injection loop (Rheodyne, Cotati, CA, USA). noted that pullulans are linear polysaccharides and the use of All the connections were made of PEEK tubing (0.17 mm id). this calibration may lead to some underestimation of the Analyte species were separated on a 13 mm SuperdexTM-75 HR molecular masses of highly ramified polysaccharides. 10/30 SEC column (300×10 mm id) (Pharmacia Biotech, Uppsala, Sweden) with an exclusion limit of 100 kDa and an Speciation analysis of the water-soluble fraction.Apples and eVective separation range between 0.5 and 50 kDa (pollulans). carrots were washed, peeled and sliced. Samples (160 g of apple The presence of polysaccharides was monitored using a and 152 g of carrots) were crushed in aWaring blender for 1 min Model ERC-7512 refractometer (Erma, Tokyo, Japan). after addition of 160 ml (apples) or 152 ml (carrots) of a solution Element-specific detection was realized with an Elan 6000 ICP containing 6 mM of ascorbic acid (to prevent oxidation) and mass spectrometer (Perkin-Elmer SCIEX, Concord, Canada). 0.04% m/v of sodium azide (antibacterial agent).The homogen- The sample introduction system used included a RytonTM ates were centrifuged at 13 000 rpm for 10 min to produce a spray chamber fitted with a cross-flow nebulizer. For total supernatant (180 ml for apples and 117 ml for carrots) and a analyses the samples were fed by means of a Minipuls 3 solid residue (120 g for apples and 187 g for carrots).Aliquots peristaltic pump (Gilson, Villiers-le-Bel, France) that also of the supernatant were analysed by ICP-MS for total metals served for draining the spray chamber. Chromatographic data and by HPLC-ICP-MS for metal species. were processed using the Turbochrom4TM software (Perkin- Elmer). All signal quantifications were performed in the peak Speciation analysis of thr-insoluble fraction. The solid area mode. residues after centrifugation were blended with a solution containing 6 mM of ascorbic acid and 0.04% m/v of sodium Reagents, standards and samples azide (180 and 117 ml, for apple and carrot samples, respect- Analytical-reagent grade reagents purchased from Sigma- ively).Aliquots of 320 and 300 ml, respectively, of the commer- Aldrich (St. Quentin Fallavier, France) were used throughout cial enzyme preparation were added. Samples were incubated unless specified otherwise. Milli-Q water (18 MV) (Millipore, at 30 °C for 24 h.The mixtures were centrifuged as described Bedford, MA, USA) was used throughout. The formate buVer above. Aliquots of the supernatant (301 ml for apples and was prepared by dissolving 30 mM of ammonium formate in 275 ml for carrots) were analysed by ICP-MS and by HPLCwater and adjusting the pH to 5.2 by the addition of formic ICP-MS. The residues (18 g for apples and 30 g for carrots, acid. mainly cellulose) were slurried in an ultrasonic bath and Rapidase Liq+TM (Gist Brocades, Seclin, France) analysed by ICP-MS.and Pectinex Ultra-SPLTM (Novo Nordisk, Copenhagen, Denmark) commercial enzymatic preparations containing pec- Results and discussion tinases, hemicellulases and cellulases were used. These prep- Chromatographic conditions (choice of the column, mobile arations were added to the homogenate samples to give a final phase composition and flow rate) optimized and found success- concentration of 0.1% m/v of each enzyme.ful elsewhere20 for the speciation of biomolecular Pb complexes Two rhamnogalacturonan-II preparations, viz., monomer in wine were adopted in this work. For all peaks shown in the (mRG-II) and dimer (dRG-II ), purified according to Pellerin chromatograms below, the isotopic pattern of the determined et al.,30 were used. The fraction reported earlier30 as RG-II2 element was checked in an independent experiment in order [predominantly (>95%) as monomer] was used as the mRG-II to exclude the possibility of artefacts due to isobaric inter- standard whereas the fraction reported as RG-II3 [predomiferences.Isotopes (one for each element) giving the most nantly (87%) as dimer] was used as the dRG-II standard. intense signals were chosen for the multi-element chromatogra- Apples (Malus domestica; Golden delicious) and carrots phy. The set of eight elements which were monitored included (Daucus carota) were purchased at a local supermarket.Pb as a common toxic element (the signal from Cd was too Procedures small to be monitored), Ba, Sr, and Ce as representative elements known to be complexed by carbohydrates,31 B as an ICP-MS conditions. ICP-MS measurement conditions (nebu- essential nutrient and a component of a diester ligand potenlizer gas flow, rf power and lens voltage) were optimized daily tially binding metals,16–18,20,21,31 and Cu, Zn and Mg (the using a standard built-in software procedure. The isotopes most common essential elements).The fruit and vegetable 138Ba, 139La, 140Ce, 141Pr, 88Sr, 63Cu, 64Zn, 208Pb and 11B were sample homogenates were fractionated by centrifugation into monitored. The dwell time for each isotope was 200 ms and a water-soluble fraction (supernatant) and a water-insoluble the number of replicates allowing for continuous scanning for (solid residue) fraction. The supernatant could be analysed by the duration of the chromatogram was applied. For total size-exclusion HPLC-ICP-MS directly.For the solid fraction, element ICP-MS analyses, a sample was slurried or diluted a procedure to release metal species needed to be developed with HNO3 to reach an acid concentration of 1% m/v in a to allow speciation analysis by HPLC-ICP-MS. 10 ml calibrated flask and placed in an ultrasonic bath for 5 min. Quantification was performed using an external cali- Speciation of metals in fruit and vegetables (water-soluble bration graph.fraction) Molar mass distribution profiles for the diVerent elements. Chromatographic conditions. For size-exclusion HPLC, aliquots of 20–100 ml were injected. The mobile phase was 30 mM Fig. 1 shows a multi-element size-exclusion chromatogram with ICP-MS detection of the water-soluble fraction for apple ammonium formate buVer, pH 5.2, at a flow rate of 0.6 ml min-1. The eluate from the column was fed directly [Fig. 1(a)] and carrot [Fig. 1(b)] samples. The chromatograms (one run for each sample) are each shown in two panels for into the ICP.The column was calibrated with narrow pullulan 640 J. Anal. At. Spectrom., 1999, 14, 639–644column). This fraction represents all the B present in the water-soluble fraction of carrots, but for the apple sample, a small percentage of B is present in the fraction excluded from the column (elution volume 8.0 ml ). The major diVerence between the samples studied is the presence of a strong signal containing Ba, Ce, Pb, Sr and B at an elution volume of 8.0 ml in the chromatogram of the apple water-soluble fraction.This signal corresponds to a compound(s) with an apparent molar mass equal to or above 50 kDa and is absent in the water-soluble fraction of the carrot sample. This compound(s) contains the totality of the Ba, Ce and Sr and the majority (>95%) of Pb present in the sample. For the carrot sample, some Pb co-elutes with the fraction of Zn whereas Ba and Sr elute as a single peak at an elution volume of 11.7 ml corresponding to a compound with a molecular mass of 10–15 kDa. The signals of Ba, Ce, Sr and B for the carrot sample are very low in comparison with those measured for the apple sample, despite similar concentrations of these elements in the initial homogenate.Molar mass distribution profiles for carbohydrates in the water-soluble fraction. Refractometric detection was employed to detect carbohydrates present in the size-exclusion chromatographic eluates of the water-soluble fractions of the apple and carrot samples.The chromatograms are shown in Fig. 2. The presence of polysaccharides excluded from the column (Mw >50 kDa) is detected; the exact elution volume matches, for the apple sample, the elution volume of Ce, Sr, Ba and Pb. No other refractive index signal, except for one from the salts present in the sample (not shown), was observed in any of the analysed samples. It should be noted that the peaks in the chromatograms acquired with refractometric detection are much broader than in those acquired with ICP-MS detection.This suggests that only particular carbohydrates from the polysaccharide fraction bind metals. Identification of metal species in the fruit and vegetable watersoluble fraction. Identification of the metal species detected is hampered by the lack of standards. Nevertheless, in SEC the identity of an element species can be hypothesized on the basis Fig. 1 Size-exclusion HPLC-ICP-MS traces for the supernatants on the centrifugation of (a) an apple sample and (b) a carrot sample. One run for each sample. Two panels are shown for each sample for better clarity of presentation. better clarity of presentation. ForMg, Cu and Zn the molecular size distribution patterns for apple and carrot samples are identical. Magnesium shows a single peak (>99%) at an elution volume exceeding the total volume of the column. The totality of Cu and Zn elute in a single (each element in a diVerent one) fraction.The average molar mass of the Zn-containing fraction is slightly higher than that of the Cu-containing fraction. Boron elutes principally as a low Fig. 2 Size-exclusion HPLC traces with refractometric detection for molar mass fraction interacting strongly with the stationary total soluble polysaccharides obtained for (a) an apple sample, (b) a carrot sample. phase (the elution volume exceeds the total volume of the J.Anal. At. Spectrom., 1999, 14, 639–644 641of its elution volume. Although the precision of the identification complex is released. This was attempted with enzyme preparations that contain high levels of pectinolytic activity. on the basis of the apparent molar mass is relatively poor, it serves well for a preliminary approach. The peak containing the Homogalacturanans are extensively degraded by pectinases whereas dRG-II-B is resistant to such enzymes. majority of the dissolved B is likely to correspond to a mixture of the borate mono- and diesters as identified by Matsunaga and Nagata17 using 10B NMR.Cu and Zn elute in an area of Enzymatic degradation of macromolecular metal–polysaccharide complexes. Enzyme preparations with high levels of pec- molar masses of phytochelatin. The elution volume of the Mg signal suggests that this element may be present as the Mg2+ ion tinolytic activity (hydrolases, lyases, esterases) are widely used in the fruit processing industry to increase yields, and improve which can be further unspecifically retained by the cationexchange sites of the stationary phase.liquefaction, clarification and filtrability of juices.30 A chromatogram [Fig. 4(a)] with the refractometric detection of the super- The similar ionic radius of metals complexed by the excluded (50 kDa) fraction [Fig. 1(a)], which is a polysaccharide natant of an apple homogenate sample subjected to a 4 h treatment at 35 °C with a mixture of such enzymes shows the fraction (cf.Fig. 2), suggests that the species is a complex of these metals (Ba, Ce, Sr, Pb) with a dimer of RG-II as a absence of the high molar mass polysaccharide species seen in Fig. 2(a). Instead, two peakswith elution volumes corresponding ligand, similarly to Pb in wine.20,21 This hypothesis is further corroborated by the co-elution of B which is necessary to form to the RG-II monomer and dimer (cf. Fig. 3) can be seen.Note that only the dimer is able to complex metals.31 This observation a dimer of RG-II,30–32 and hence to form the complex with the metals. The ultimate verification of this hypothesis can be is confirmed by a size-exclusionHPLC-ICP-MS trace [Fig. 4(b)] obtained after enzymatic treatment of the water-soluble apple obtained by comparing the elution volume of this species with that of a dRG-II standard. homogenate fraction. The elution pattern of Cu, Zn and Mg is identical with that Fig. 3(a) shows the elution profiles of the dimer and monomer of RG-II obtained with refractometric detection. The shown in Fig. 1(a) but the excluded peak (50 kDa) containstandard of the dimer used contained an admixture of the monomer. The chromatogram with ICP-MS detection [Fig. 3(b)] shows that only the dimer contains B and Pb. No peak of any of the elements monitored by ICP-MS matches the elution volume of the mRG-II. The chromatographic conditions were the same as in Fig. 1 and 2. The elution volume of the dRG-II standard indicates that the molar mass of the metal species detected in the apple juice exceeds that of dRG-II by a factor of >5. For the carrot sample, however, some Ba and Sr [Fig. 1(b)] elutes apparently as a complex with dRG-II. Taking into account the co-elution of the metals with a particular ionic radius with B and the ubiquitous presence of dRG-II in fruit and vegetables,18,20,30–32 it was considered possible that the metals in the apple water-soluble fraction may indeed be complexed by dRG-II-B but that the complex may be part of an even more complex molecule (high molecular mass polysaccharide). Experiments were therefore designed to disrupt this structure in such a way that the intact dRG-II-B Fig. 4 Size-exclusion HPLC traces for the apple sample water-soluble Fig. 3 Size-exclusion HPLC traces for the RG-II standards. (a) fraction degraded by enzymatic hydrolysis. (a) Refractometric detection; (b) ICP-MS detection (data are shown in two panels for clarity Refractometric detection; (b) ICP-MS detection. 1, RG-II dimer standard; 2, RG-II monomer standard. of presentation). 642 J. Anal. At. Spectrom., 1999, 14, 639–644Fig. 6 Size-exclusion HPLC traces with refractometric detection for an enzymatic digest of the residue of an apple homogenate after centrifugation. methylmercury, alkyllead ) the stability of which could readily be assured during the hydrolysis of the proteinaceous matrix by tetramethylammonium hydroxide35 or proteolytic enzymes.36 For metal complexes, alkaline or acid hydrolysis cannot be accepted since it would aVect the complexation equilibria.Selective degradation of the matrix at the natural pH of the sample studied appeared therefore to be the only valid solution. Fig. 5 Size-exclusion HPLC-ICP-MS traces for an enzymatic digest The polysaccharide matrix of the water-insoluble residue of the residue of an apple homogenate after centrifugation.Data are after centrifugation of fruit and vegetable homogenates is shown in two panels for clarity of presentation. The peak at 12.2 kDa composed of pectic polysaccharides, hemicelluloses and cellu- corresponds to the metal complex with dRG-II. loses. Therefore, the use of enzymatic preparations containing pectinolytic, hemicellulolytic and cellulolytic activities is neces- Table 1 Quantitative distribution of elements in soluble and insoluble sary to solubilize the solid residues. RG-II was reported to be fractions of the fruit and vegetables studied (in mass percent of the initial homogenate) resistant to such treatments.34 This resistance was attributed to the high degree of ramification and diversity of glycosyl Insoluble fraction, linkages of RG-II.Water-soluble liberated by Residue after It was also expected that the complex of dRG-II-B with Element fraction enzymolysis enzymolysis metals, if present in the solid residue, would be resistant to pectinolysis and would be released into the aqueous phase Carrots (Daucus carota)— B 33.0 9.3 56.8 while other polysaccharides would be degraded. Indeed, the Ce 1.0 5.5 93.5 chromatograms in Fig. 5 show that the enzymatic hydrolysis Cu 65.0 31.4 3.6 releases the complex of dRG-II (bridged by B) with Ce, Ba, Pb 3.5 97.0 0.5 Sr and Pb. The enzyme also releases other metals from the Sr 12.2 82.0 5.8 solid fruit and vegetable fraction in the forms in which they Zn 48.6 49.6 1.8 were present in the water-soluble fraction (cf.Fig. 1). The Apples (Malus domestica)— B 19.8 33.0 47.1 chromatogram with refractometric detection (Fig. 6) confirms Ce 73.9 26.1 Below 0.5% that carbohydrates co-elute with the metals and B. Cu 76.6 19.3 4.1 Unlike the water-soluble fraction, the chromatogram of the Pb 42.5 47.5 10.0 enzymatic digests of the carrot sample solid residue is very Sr 28.6 47.4 14.0 similar to that of the apple one. This suggests that the metals Zn 57.2 30.0 12.8 in the solid fraction are bound to a pectic matrix, the metal complexing component of which is dRG-II-B. ing Ba, Ce, B, Pb and Sr has disappeared.Instead, an intense peak can be seen for these elements at an elution volume of Distribution of metal species between the water-soluble and water-insoluble fractions 11.7 ml which matches exactly that of dRG-II-B (cf. Fig. 3). The data lead to the conclusion that Pb, Sr, Ba and Ce in the The quantitative distribution of metals between the water- apple water-soluble fraction are present in the form of a soluble and water-insoluble fractions of the fruit and vegetable complex with dRG-II but this species is, unlike in wine,20–22 samples was investigated.For the water-insoluble fraction, part of high molecular mass polysaccharides, probably due to further distinction was made between the metal fraction that the covalent linkage with homogalacturonans. can be solubilized by enzymatic hydrolysis and the fraction that is retained in the solid residue after pectinolysis.Results, Speciation of metals in fruit and vegetables (water-insoluble expressed as a percentage of the total element concentration fraction) present in the initial sample, are given in Table 1. Because of the slurry sampling technique employed for the analysis of the Approaches to speciation analyses of water-insoluble samples have been scarce because of the diYculties with selective solid residue the error of this analysis can reach 10–15%.Elements in the supernatants could be determined with a destruction of the solid matrix in such a way that the metal complex of interest is preserved intact. To date, the studies precision of 1–3%. The results show that most of the metals are either already have been limited to some organometallic species (organotin, J. Anal. At. Spectrom., 1999, 14, 639–644 6437 S. A. Pergantis, E. M. Heithmar and T. A. Hinners, Analyst, 1997, present in water-soluble forms or can be released as water- 122, 1063. soluble species by enzymatic leaching with pectinolytic 8 L.M. W. Owen, H. M. Crews and R. C. Massey, Chem. Speciat. enzymes. The highest elemental concentrations in the water- Bioavail., 1992, 4, 89. soluble fraction (49–75%) are obtained for Zn and Cu which 9 H. M. Crews, J. R. Dean, L. Ebdon and R. C. Massey, Analyst, are known to be present as complexes with polypeptide ligands 1989, 114, 895. 10 J. R. Dean, S. Munro, L. Ebdon, H. M. Crews and R. C. Massey, (phytochelatins and metallothioneins). A strong signal from J. Anal. At. Spectrom., 1987, 2, 607. Mg in the water-soluble fraction is also observed. The only 11 L. M. Owen, H. M. Crews, R. C. Hutton and A. Walsh, Analyst, elements largely retained in the residue in a form that is 1992, 117, 649. resistant to pectinolytic enzymes are B and Ce. 12 S. M. Bird, H. Ge, P. C. Uden, J. F. Tyson, E. Block and E. The distribution of Pb, Sr, Ba and Ce that are complexed Denoyer, J.Chromatogr., 1997, 789, 349. by dRG-II-B and can be incorporated in larger polysaccharide 13 S. M. Bird, P. C. Uden, J. F. Tyson, E. Block and E. Denoyer, J. Anal. At. Spectrom., 1997, 12, 785. structures between the water-soluble and water-insoluble frac- 14 J. Zheng, W. Go� ssler and W. Kosmus, Trace Elem. Electrol., 1998, tions is diVerent in carrot and apple samples. Whereas in 15, 70. carrots little of these elements is present in the water-soluble 15 H.Ge, X. J. Cai, J. F. Tyson, P. C. Uden, E. R. Denoyer and E. fraction, the majority of Ce and significant proportions of Sr, Block, Anal. Commun., 1996, 33, 279. Pb and Ba in apples are present as water-soluble species. The 16 T. Matsunaga, T. Ishii and H.Watanabe, Anal. Sci., 1996, 12, 673. relative water-soluble fraction is the highest for Ce and the 17 T. Matsunaga and T. Nagata, Anal. Sci., 1995, 11, 889. 18 T. Ishii and T.Matsunaga, Carbohydr. Res., 1996, 284, 1. lowest for Sr. In the carrot samples the majority of Pb, Sr and 19 K. E. Oedegard and W. Lund, J. Anal. At. Spectrom., 1997, 12, Ba can be released (as water-soluble dRG-II complexes) by 403. the enzymatic hydrolysis procedure developed. This is not the 20 J. Szpunar, P. Pellerin, A. Makarov, T. Doco, P. Williams, B. case with Ce, however. The majority of this element in carrots Medina and R. £obin� ski, J. Anal. At. Spectrom., 1998, 13, 749.is retained in the final insoluble residue, either as a complex 21 P. Pellerin and M. A. O’Neill, Analusis, 1998, 26, M32. or more likely as hydrolysed oxide/hydroxide species. 22 P. Pellerin, M. A. O’Neill, C. Pierre, M. T. Cabanis, A. G. Darvill, P. Albersheim and M. Moutounet, J. Int. Sci. Vigne Vin, 1997, 31, 33. Conclusions 23 K. Lange-Hesse, L. Dunemann and G. Schwedt, Fresenius’ J. Anal. Chem., 1991, 339, 240. Size-exclusion HPLC-ICP-MS combined with sample prep- 24 K.Lange-Hesse, L. Dunemann and G. Schwedt, Fresenius’ aration procedures based on hydrolysis using glycosyl hydro- J. Anal. Chem., 1994, 349, 460. lases oVers an attractive tool to study the speciation of trace 25 K. Guenther and A. Von Bohlen, Spectrochim. Acta, Part B, 1991, metals in fruit and vegetables and can be used for monitoring 46, 1413. the fate of metal species during industrial food processes. 26 K. Gu�nther and H. Waldner, Anal. Chim. Acta, 1992, 259, 165. 27 J. Szpunar and R. Lobinski, in Heavy Metal Stress in Plants— Polysaccharides present in samples of plant origin are import- From Molecules to Ecosystem, ed. M. N. V. Prasad and ant ligands able to bind the majority of some metals in stable J. Hagemeyer, Springer, Heidelberg, 1999, ch. 16. water-soluble complexes. The basic ligand seems to be the 28 D. M. Whitfield, S. Stoijkovski and B. Sarkar, Coord. Chem. Rev., dimer of rhamnogalacturonan-II that may be part of larger 1993, 122, 171. polysaccharide structures. The method developed oVers an 29 J. Monreuil, Pure Appl. Chem., 1984, 56, 859. analytical tool to study the stability of the Pb–dRG-II complex 30 P. Pellerin, T. Doco, S. Vidal, P. Williams, J. M. Brillouet and M. A. O’Neill, Carbohydr. Res., 1996, 290, 183. in the gastrointestinal system which is required to evaluate the 31 M. A. O’Neill, D. Warrenfeltz, K. Kates, P. Pellerin, T. Doco, bioavailability of Pb taken in with foodstuVs. A. G. Darvill and P. Albersheim, J. Biol. Chem., 1996, 271, 22923. 32 M. Kobayashi, T. Matoh and J. L. Azuma, Plant Physiol., 1996, References 110, 1017. 33 T. Doco and J. M. Brillouet, Carbohydr. Res., 1993, 243, 333. 1 H. Crews, Anal. Eur., 1995, August, 28. 34 T. Doco, P. Williams, S. Vidal and P. Pellerin, Carbohydr. Res., 2 K. Sutton, R. M. C. Sutton and J. A. Caruso, J. Chromatogr., 1997, 297, 181. 1997, 789, 85. 35 M. Ceulemans, C. Witte, R. Lobinski F. C. Adams, Appl. 3 G. K. Zoorob, J. W. McKiernan and J. A. Caruso, Mikrochim. Organomet. Chem., 1994, 8, 451. Acta, 1998, 128, 145. 36 D. S. Forsyth and J. R. Iyengar, J. Assoc. OV. Anal. Chem., 1989, 4 A. Makarov and J. Szpunar, Analusis, 1998, 26, 44. 72, 997. 5 M.Morita and J. S. Edmonds, Pure Appl. Chem., 1992, 64, 575. 6 J. R. Dean, L. Ebdon, M. E. Foulkes, H. M. Crews and R. C. Massey, J. Anal. At. Spectrom., 1994, 9, 615. Paper 8/08231F 644 J. Anal. At. Spectrom., 1999, 14, 639&nda

ISSN:0267-9477    

DOI:10.1039/a808231f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Sample preparation and HPLC separation approaches to speciation analysis of selenium in yeast by ICP-MS Corinne Casiot, Joanna Szpunar, Ryszard £obin�ski and Martine Potin-Gautier* Laboratoire de Chimie Analytique, Bio-inorganique et Environnement, CNRS EP132, Universite� de Pau et de Pays de l’Adour, He�lioparc, 2, av. Pr. Angot, 64 053 Pau Ce� de�x 9, France Received 18th November 1998, Accepted 8th February 1999 Eight solid–liquid extraction procedures were evaluated for the recovery of selenium species from yeast.Speciation of Se in the extracts was characterized by diVerent types of HPLC, including size-exclusion, anion-exchange and reversed-phase chromatography with ICP-MS detection. The results obtained depended critically on the sample preparation procedure used. Leaching with water and with methanol led only to 10–20% recoveries of Se, split into eight compounds, among which Se(IV) and selenomethionine could be identified. Leaching with pectinolytic enzymes released an additional 20% of selenomethionine. Leaching with sodium dodecyl sulfate solution allowed the solubilization of a selenoprotein that accounted for ca. 30% of the total Se present. Leaching with proteolytic enzymes led to recoveries of Se above 85%, the majority as selenomethionine. Hydrolysis of the yeast with tetramethylammonium hydroxide solubilized the sample completely but the Se species present were entirely degraded to selenomethionine and inorganic selenium.A sequential leaching procedure is proposed for the evaluation of selenium speciation in yeast without the need for a coupled technique. selenium compounds, including selenocysteine, selenomethion- Introduction ine, methylselenocysteine and inorganic forms. The use of Selenium is an essential micronutrient for living organisms but enzymic hydrolysis led Gilon et al.5 to the conclusion that may be toxic in specific forms [e.g., Se(VI)] and at higher more than 80% of Se in the yeast was present in three forms: concentrations.1 It is a component of the human enzyme inorganic selenium, selenocysteine and selenomethionine.glutathione peroxidase and has been reported to show a The objective of this work was to develop and to evaluate protective eVect against cancer.2,3 The observation of statisti- critically diVerent leaching/extraction procedures for the recovcally significant reductions in cancer mortality and cancer ery of selenium species from yeast.Size-exclusion chromatograincidence in a population consuming yeast-derived nutritional phy with ICP-MS was developed to monitor the speciation supplements by Clark et al.4 induced a surge of interest in Se- of selenium in the diVerent extracts and was compared enriched yeast. Since the bioavailability and the toxicity of Se with anion-exchange and reversed-phase chromatography. are closely correlated with its chemical form, this interest has Attempts were made to optimize a sequential extraction protobeen translated into the quest for information on selenium col in such a way that particular classes of selenium species speciation in yeast.5–9 are selectively recovered with the consecutive extraction In terms of analytical approaches, the direct coupling of solutions. high-performance liquid chromatography (HPLC) with ICPMS is now an established approach to the determination of Experimental Se species.10–13 Separation modes used included ion-exchange (anion and cation) and anion-pairing reversed-phase chroma- Apparatus tography (for an exhaustive list of references, see refs. 7 and The ICP-MS instrument used was an HP4500 (Yokogawa 14. The choice of species studied [Se(IV), Se(VI), selenome- Analytical Systems, Tokyo, Japan). The sample introduction thionine and selenocysteine] has been mainly dictated by the system used included a Scott double-pass spray chamber fitted commercial availability of standards, with the exception of with a Babington nebulizer. For chromatographic experiments some recent studies in which a large number of synthetic aModel 9012 HPLC pump (Varian Chromatography Systems, organoselenium compounds were used.6,7 The majority of Walnut Creek, CA, USA) was used as the sample delivery studies have focused on the development of coupled techsystem.All the connections were made of PEEK tubing niques; applications to real samples have been scarce. (0.17 mm id). Injections were made using a Model 7725 Speciation analysis of yeast by HPLC-ICP-MS requires that injection valve (Rheodyne, Cotati, CA, USA) with a 100 ml the endogenous Se species present in the yeast are extracted injection loop.Chromatographic results were processed using without modification of their chemical form or disturbance to Chromatosoftware (Hewlett-Packard, Avondale, PA, USA). the equilibrium between the various species present. Previous Quantifications were performed in the peak area mode.studies of selenium speciation in yeast either concerned the water soluble fraction only (containing ca. 10% of the total Chromatographic columns selenium)6–8 or were aimed at the maximization of the Se recovery by degrading the species originally present with a Size-exclusion chromatographic separations were performed mixture of proteolytic enzymes.5 The results of these pro- on a 300×10 mm id, 13 mm Superdex-200 HR 10/30 SEC cedures have been contradictory. By leaching with an aqueous column (Pharmacia Biotech, Uppsala, Sweden) with an exclusion limit of 1300 kDa and an eVective separation range solution, Bird et al.6,7 reported the presence of more than 20 J.Anal. At. Spectrom., 1999, 14, 645–650 645between 10 and 600 kDa. Anion-exchange and reversed-phase HPLC-ICP-MS conditions chromatographic separations were performed on a The chromatographic separation conditions are summarized 250×4.1 mm id, 10 mm Hamilton PRP-X100 column in Table 1.The ICP-MS measurement conditions, given in (Hamilton, Reno, NV, USA) and a 150×4.6 mm id, 5 mm Table 2, were optimized daily using a standard built-in software Inertsil ODS-2 column (Interchim, Montluc�on, France), procedure for injection of a 10 mg l-1 solution containing Li, respectively. Y, Ce and Tl into a flow of the chromatographic mobile phase at the flow rate given in Table 1. Three major selenium Standards and samples isotopes, 77Se, 78Se and 82Se (with relative abundances of 7.63, 23.78 and 8.73%, respectively), were monitored.DL-Selenocysteine, DL-selenomethionine and selenoethionine were purchased from Sigma (St. Quentin Fallavier, France) Results and discussion and were used without further purification (90% purity for selenocystine). Stock standard solutions containing 1 mg ml-1 Experimental conditions reported in the literature for anionof each compound in de-ionized water (Millipore, Bedford, exchange (AEC)5 and reversed-phase (RPC)6,7 chromatogra- MA, USA; 18 MV) were stored in the dark at 4 °C.phy of selenium species were used. In order to detect the Hydrochloric acid (3%) was used to dissolve selenocysteine. presence of high molecular mass compounds within the same Working standard solutions were prepared daily by dilution run, size-exclusion chromatography (SEC) was optimized. The in de-ionized water. SEC-ICP-MS coupling has been a method of choice for the A sample of industrially produced selenium enriched yeast characterization of high and medium molecular mass metal was used.Saccharomyces cerevisiae was grown in the presence (metalloid) compounds in biological samples.15 Despite the of sodium selenite, from which it naturally synthesizes organic fact that, in theory, the separation should be based on the seleno compounds. It was then pasteurized and dried (to avoid analyte molecular mass, secondary adsorption and ionfurther growth and facilitate handling). exchange eVects apparently play an important role in SEC and the separation of small compounds with similar molecular Reagents masses can be achieved in addition to the possibility of the unambiguous identification of the presence of selenoproteins.Analytical reagent grade sodium dodecyl sulfate (SDS), tetra- The signal intensity ratios measured were: 78Se577Se, methylammonium hydroxide (TMAH) (2in 3.0±0.2, 82Se577Se 0.98±0.07 and 78Se582Se 3.0±0.2 in the water), phenylmethylsulfonyl fluoride (PMSF), methanol, range 1040–1400 s in SEC-ICP-MS.Comparison with the ammonium mono- and dihydrogenphosphate and tris(hydtheoretical values of 3.1, 1.1 and 2.7, respectively, indicates roxymethyl )aminomethane (TRIS) were purchased from that no significant interferences occurred. Aldrich (St. Quentin Fallavier, France). The enzymes Driselase Fig. 1(a), (b) and (c) show the chromatographic profiles and pronase E (protease XIV type) were obtained from obtained by SEC-ICP-MS, AEC-ICP-MS and RPC-ICP-MS, Aldrich.Milli-Q water (18 MV) (Millipore) was used through- respectively, for supernatants obtained by each of the sample out. BuVers were prepared by dissolving the appropriate preparation procedures. The results of procedures A–C ( leach- amount of salts in de-ionized water and the pH was adjusted ing with diVerent aqueous solutions) leading to similar chroma- by dropwise addition of 30% ammonia solution or HCl.The tographic profiles are presented in one panel. The same eluents were filtered (0.45 mm) and de-gassed before use. concerns leaching with the SDS solution with and without the addition of PMSF (procedures E and F). Procedures The recovery of Se in the sample preparation procedures was determined by SEC-HPLC using the peak area quantifi- Eight sample preparation (solid–liquid extraction) procedures cation mode. In all cases except the TMAH digestion (prowere used. For each of them a sample of 200 mg of yeast was cedure H), solid residues were observed after extraction.The placed in a centrifuge tube followed by the addition of the SEC-ICP-MS results obtained for this procedure matched the following reagents: value obtained by flow injection analyses and was taken as A: 5 ml of hot water (85–90 °C); the sample was stirred 100%. Table 2 summarizes the recoveries and relative abun- for 1 h. dances of particular chromatographic signals in the B: 5 ml of 10% MeOH in 0.2 M HCl; the sample was chromatograms.sonicated for 1 h. C: 5 ml of 30 mM TRIS-HCl buVer (pH 7) and 0.1 mM Leaching with aqueous solutions PMSF; the sample was sonicated for 1 h. D: 5 ml of 4% Driselase in 30 mM TRIS-HCl buVer (pH 7) The results obtained by extraction with hot water are almost identical with those obtained with methanol–HCl and with a in the presence of 1 mM PMSF; the sample was incubated for 1 h at room temperature. neutral buVer (TRIS–HCl, pH 7.0) [Fig. 1(a)–(c) top panel ], which is in agreement with an earlier report.7 Only ca. 10% of E: 5 ml of 30 mM TRIS-HCl buVer (pH 7) containing 4% SDS. The sample was homogenized and incubated for 1 h. the selenium present in the sample is extracted. The compounds recovered include the weakly bound and water-soluble selenite, F: 5 ml of 20 mM ammonium phosphate buVer (pH 7.4), 0.15 M NaCl, 0.1 mM PMSF, 1 mM EDTA, 5% SDS; the sample selenoamino acids and possibly trimethylselenonium and selenoglutathione.Injection of standards and spiking experiments was stirred for 1 h. G: 5 ml of phosphate buVer (pH 7.5) containing 20 mg of indicated the presence of selenomethionine and Se(IV) in the low molecular mass elution volume range of the column, pronase and 10 mg of lipase; the sample was incubated for 16 h at 37 °C. which was further confirmed by orthogonal (AEC and RPC) separation chromatographic techniques. The best resolution H: 5 ml of 25% TMAH solution in hot (60 °C) water; the sample was incubated for 4 h.was achieved by anion-exchange chromatography, which allows the discrimination of seven well resolved signals. The After the sample preparation procedure the sample was centrifuged using a Hettich Universal 16 centrifuge at 4000 retention times of two of them match those of Se(IV) and selenomethionine. Selenocysteine would elute in (or close to) min-1 for 30 min. An aliquot of the supernatant was filtered through a 0.45 mm filter and injected on to the chromato- the void in anion-exchange and reversed-phase chromatography and its positive identification in a real sample by these graphic column. 646 J. Anal. At. Spectrom., 1999, 14, 645–650Table 1 Experimental chromatographic conditions Separation mode Parameter Size-exclusion Anion-exchange Ion-pair reversed phase Superdex-200 Hamilton PRP-X100 Inertsil ODS-2 Analytical column Eluent 30 mM TRIS–HCl buVer (pH 7.0) Ammonium phosphate buVer; 0.1% TFA in 2% MeOH (A) 6.25 mM (pH 5.5); (B) 25 mM (pH 11) Eluent flow rate/ml min-1 1.0 1.4 1.0 Elution programme Isocratic Gradient: 0–9 min, 100% A; Isocratic 9–10 min, 100% A to 100% B; 10–20 min, 100% B Injection volume/ml 100 100 100 Run time/min 40 20 40 Table 2 Experimental ICP-MS conditions Leaching with non-proteolytic (pectinolytic) enzymes The first attempt was to release selectively any selenium present Nebulizer Babington Rf power 1350 W in the sample that is not protein bound.The mixture of Nebulizer gas (argon) flow rate 1.1 l min-1 enzymes of choice should therefore be devoid of proteolytic Lens voltages— activity. Driselase, a commercial enzyme preparation contain- Extraction lenses Extract 1: -120 V ing laminarinase, xylanase and cellulase that is used to destroy Extract 2: -115 V the cell walls of fungi,16 was chosen. An attack with this Einzel lenses Einzel 1, 3: -100 V enzyme would therefore release the selenium compounds Einzel 2: -6 V Omega lenses Bias: -40 V trapped in the cell walls, either physico-mechanically or chemi- (+): 3 V cally as coordination complexes with plant cell components.(-): -26 V The chromatograms shown in the panel next to the top in Acquisition parameters— Fig. 1(a)–(c) vary distinctly from those obtained after leaching Data acquisition mode Time resolved analysis with hot water. The solid residue after leaching and centrifu- Intergration time 100 ms gation is smaller that in the case of water and methanol Number of replicates 1 Isotopes monitored 77Se, 78Se, 82Se extractions.The amount of selenium released with Driselase is more than twice that recovered without the enzyme. The retention time of the dominant species matches that of selenomethionine in the size-exclusion and anion-exchange chromatograms but not in the reversed-phase chromatogram. The Se techniques is impossible. The diVerence between the elution volumes of the selenoamino acids studied by SEC (chromato- eluting at the exclusion volume of the column is accompanied by a small signal corresponding to a molecular mass of grams not shown) is too small to allow their positive identification. 20–30 kDa, which is likely to be a polysaccharide liberated from the yeast cell walls. Another important peculiarity of this The results obtained are in agreement with the two literature reports published so far. An intense unidentified compound chromatogram is the absence of the peak that elutes last (1320 s) in the case of water extraction.This suggest that accounting for ca. 35% of extracted selenium (for the hot water and MeOH–HCl extractions, procedures A and B) or Driselase is responsible for the degradation of this compound. 45% (for the extraction with neutral buVer, procedure C) of the water-soluble Se has also been observed elsewhere.8 Leaching with SDS The amount of selenium recovered in a second extraction was below 5% of that extracted with the first portion of the Yeast is known for its high protein content, which is approximately 40% dry mass.17 Because of the similarity of selenium leaching solution.In the analysed sample 90% of Se is present in the form of species that are insoluble in water. EVorts were to sulfur, selenoamino acids can also be metabolized into proteins. Most studies on selenium-containing proteins have therefore made to study the speciation of selenium in that fraction. dealt with human and animal tissues, whereas information on Table 3 Extraction yields and distribution of selenium species in size-exclusion chromatography (total Se concentration: 1183±41 mg g-1) Concentration of major species (as Se) /mg g-1 (% of the extracted Se) Unknown low Procedure Selenium Extraction yield High molecular Selenoamino molecular mass used extracted/mg g-1 (% of total Se) mass fraction Inorganic Se acids compound A 118.3 10 7.1 (6) 41.4 (35) 9.5 (8) 41.4 (35) B 153.8 13 6.2 (4) 52.3 (34) 69.2 (45) C 118.3 10 7.1 (6) 40.2 (34) 10.6 (9) 41.4 (35) D 319.4 27 22.4 (7) 89.4 (28) 143.7 (45) 54.3 (17) E 496.9 42 362.7 (73) 19.9 (4) 9.9 (2) 49.7 (10) F 437.7 37 319.5 (73) 21.9 (5) 13.1 (3) 30.6 (7) G 1041 88 10.4 (1) 187.4 (18) 780.8 (75) 41.6 (4) H 1180 100 — 1147 (97) J.Anal. At. Spectrom., 1999, 14, 645–650 647Fig. 1 HPLC with ICP-MS detection of leachates containing selenium compounds using diVerent sample preparation procedures: (a) sizeexclusion chromatography; (b) anion-exchange chromatography; and (c) reversed-phase chromatography.Capital letters refer to the procedures described in the Procedures section. Data are shown for 82Se. fungi and plants is scarce.18–21 About 80% of Se incorporated The size-exclusion ICP-MS trace [Fig. 1(a), middle panel ] shows an intense signal corresponding to a high molecular in yeast was found associated with high molecular mass compounds (cell walls, mitochondria, microsomes, proteins mass compound (molecular mass >100 kDa) that accounts for ca. 30% of the initial Se present in the sample. The and nucleic acids) or present in water-soluble proteins.17 SDS polyacrylamide gel electrophoresis (PAGE) combined with chromatographic pattern of the low molecular mass fraction (passing through a 10 kDa cut-oV filter) is undistorted by the neutron activation analysis allowed the conclusion that Se in the seeds of coco de mono (Lecythis ollaria) was present in SDS leaching.The amounts of selenium recovered in a second and third extraction were ca. 22% and 6%, respectively, in extremely selenium-rich proteins with molecular masses below 20 kDa.18 A procedure aimed at the recovery of the Se- comparison with that extracted with the first portion of the leaching solution. containing protein fraction was therefore attempted. SDS is widely used to denature proteins and, by forming The data in Table 2 indicate that even after repeated leaching with the SDS solution, 60% of selenium present in the sample ion pairs, to render them water soluble.This reagent has been employed for the recovery of selenoproteins from mammalian still remains behind in the solid residue.We therefore attempted to recover it by partial degradation of the insoluble compound, tissues.22,23 The extraction is typically carried out at 4 °C in the presence of PMSF to retard the naturally present proteo- probably protein. An enzymatic attack with a mixture of proteolytic enzymes as described by Gilon et al.5 was the lytic activity of enzymes liberated during the treatment. Fig. 1(a) (middle panel ) shows no diVerence in the mor- method of choice. phology of a yeast extract chromatogram, regardless of whether the leaching with the 4% SDS aqueous solution was Release of Se from yeast by proteolysis of selenoproteins done in the ice-bath with PMSF (protease inhibitor) or at room temperature without the reagent.This is probably The recovery of selenium compounds with a mixture of proteolytic enzymes has often been used to ensure the quanti- because the enzymes in the dry sample analyzed had already lost their activity or that the selenium compound recovered is tative recovery of Se from biological samples.5,8,9,24 Protease is able to break the peptic bonds of proteins present in the not a protein so it is resistant to the degradation by proteolytic enzymes. sample, so information concerning the original selenium- 648 J.Anal. At. Spectrom., 1999, 14, 645–650Sequential extraction of Se species from yeast The results obtained above suggest the possibility of developing a sequential leaching procedure that is able to provide information on the speciation of selenium in nutritional yeast supplements without the need for a chromatographic separation. Fig. 2 shows chromatograms of sequential extracts of a selenized yeast sample using diVerent leaching agents (in contrast to Fig. 1, now it was the residue after one leaching procedure that was subjected to leaching with another reagent). The three characteristic leaching reagents to be applied sequentially included (1) hot water [Fig. 2(a)] to extract water-soluble fraction, (2) Driselase [Fig. 2(b)] to release selenium present in cell and (3) SDS [Fig. 2(c)] to give the protein-soluble fraction. Considering the limited information available on the identity of chromatographic signals, the determination of total selenium in the sequential leachates gives as much information as HPLC-ICP-MS.Conclusions Size-exclusion chromatography with ICP-MS detection is a useful technique for screening selenium species in yeast extracts since it combines the satisfactory resolution of the small watersoluble Se species with the possibility of monitoring the high molecular mass fraction. The most critical step in speciation analysis for selenium compounds in yeast is the extraction of the intact species from a sample.Basic information on speciation of Se in yeast can apparently be obtained without the need for a coupled technique by sequential leaching with carefully designed reagents. The study reveals the existence of various Se species in yeast, which should be isolated and purified for identification by, e.g., electrospray MS.27 Fig. 2 Size-exclusion chromatograms with ICP-MS detection obtained References for leachates using the sequential extraction approach: (a) leaching 1 M.S. Alaejos and C. D. Romero, Chem. Rev., 1995, 95, 227. with hot water; (b) leaching of the solid residue after (a) with a 2 T. C. Stadtman, J. Biol. Chem., 1991, 266, 257. solution of Driselase; (c) leaching of the solid residue after (b) with a 3 F. Dubois and F. Belleville, Pathol. Biol., 1988, 36, 1017. solution of SDS. Data are shown for 82Se. 4 L. C. Clark, G. F. Combs, Jr., B. W. Turnbull, E. H. Slate, D. K. Chalker, J.Chow, L. S. Davis, R. A. Glover, G. F. Graham, E. G. Gross, A. Krongrad, J. L. Lesher, Jr., H. K. Park, B. B. Sanders, Jr., C. L. Smith and J. R. Taylor, J. Am. Med. species proteins is lost. Indeed, the chromatograms obtained Assoc., 1996, 276, 1957. 5 N. Gilon, A. Astruc, M. Astruc and M. Potin-Gautier, Appl. for a leachate of the yeast sample with proteolytic enzymes Organomet. Chem., 1995, 9, 623. [Fig. 1(a)–(c), panel next to the bottom] indicate the presence 6 S. M. Bird, P.C. Uden, J. F. Tyson, E. Block and E. Denoyer, J. of a single intense peak. This peak was identified also by Anal. At. Spectrom., 1997, 12, 785. anion-exchange chromatography as selenomethionine. 7 S. M. Bird, H.-H. Ge, P. C. Uden, J. F. Tyson, E. Block and E. The proteolytic hydrolysis leads to the solubilization of Denoyer, J. Chromatogr., 1997, 789, 349. almost 90% of the selenium initially present in yeast. 8 J. Zheng, W. Go� ssler and W. Kosmus, Trace Elem. Electrol., 1998, 15, 70. 9 R.Mun� oz Olivas, O. F. X. Donard, N. Gilon and M. Potin- Quantitative recovery of Se from yeast Gautier, J. Anal. At. Spectrom., 1996, 11, 1171. 10 R. Mun�oz Olivas, O. F. X. Donard, C. Camara and Ph. All the procedures investigated above left a small solid residue Quevauvillier, Anal. Chim. Acta, 1994, 286, 357. after centrifugation of the supernatant. This residue always 11 K. Pyrzyn� ska, Chem. Anal. (Warsaw), 1995, 40, 677. contained some selenium. We therefore attempted to solubilize 12 G.Ko� lbl, K. Kalcher, K. J. Irgolic and R. J. Magee, Appl. the yeast sample completely to recover all of the Se. Two Organomet. Chem., 1992, 7, 443. approaches were considered: acid protein hydrolysis with HCl 13 X. Dauchy, M. Potin-Gautier and M. Astruc, Fresenius’ J. Anal. Chem., 1994, 348, 792. at 110 °C, which was reported earlier to degrade the Se 14 W. Go� ssler, C. Ku� hnelt, C. Schlagenhaufen, K. Kalcher, M. compounds in yeast,25 and alkaline hydrolysis using TMAH, Abegaz and K.Irgolic, J. Chromatogr., 1997, 789, 233. which has been used to solubilize biological tissues allowing 15 A. Makarov and J. Szpunar, Analusis, 1998, 26, M44. the quantitative recovery of intact organometallic species.rin and Z. Gunata, Ele�ments d’Oenologie, ed. C. Flanzy, The latter approach was investigated in more detail. Lavoisier, Paris, 1998. Chromatograms obtained after the hydrolysis of a yeast 17 M. Korhola, A. Vainio and K. Edelmann, Ann. Clin. Res., 1986, 18, 65. sample with TMAH are shown in Fig. 1(a)–(c) (bottom 18 C. Hammel, A. Kyriakopoulos, D. Behne, D. Gawlik and P. panel ). No solid residue is left after the solubilization. The Bratter, J. Trace Elem. Med. Biol., 1996, 10, 96. total peak area accounts for 97% of the total Se present. The 19 M. A. Beilstein, P. D. Whanger and G. Q. Yang, Biomed. Environ. morphology of the chromatograms obtained using the diVerent Sci., 1991, 4, 392. separation mechanisms indicates the degradation of the Se 20 T. W.-M. Fan, A. N. Lane and R. M. Higashi, Environ. Sci. species initially present to inorganic selenium [probably Technol., 1997, 31, 569. 21 Z. Ouyang, J. A.Wu and L. Q. Xie, Anal. Biochem., 1989, 178, 77. Se(VI)]. Some traces of selenoamino acids are still left. J. Anal. At. Spectrom., 1999, 14, 645–650 64922 M. A. Beilstein, M. J. Tripp and P. D. Whanger, J. Inorg. 26 R. Van Cleuvenbergen, D. Chakraborti and F. Adams, Anal. Biochem., 1981, 15, 339. Chim. Acta, 1990, 228, 77. 23 J. K. Evenson and R. A. Sunde, Proc. Soc. Exp. Biol. Med., 1988, 27 C. Casiot, V. Vacchina, H. Chassaigne, J. Szpunar, M. Potin- 187, 169. Gautier and R. £obin� ski, Anal. Commun., 1999, 36, 77. 24 M. Potin-Gautier, N. Gilon, M. Astruc, I. De Gregori and H. Pinochet, Int. J. Environ. Anal. Chem., 1996, 67, 15. 25 L. N. Mackey and T. A. Beck, J. Chromatogr., 1982, 240, 455. Paper 8/09027K 650 J. Anal. At. Spectrom., 1999, 14, 645&ndash

ISSN:0267-9477    

DOI:10.1039/a809027k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Sensitive determination of three arsenic species in water by ion exclusion chromatography-hydride generation-inductively coupled plasma mass spectrometry Tadashi Taniguchi,a Hiroaki Tao,*b Mamoru Tominagab and Akira Miyazakib aShimadzu Co. Ltd. 380–1 Horiyamashita Hadano Kanagawa 259–1304 Japan bNational Institute for Resources and Environment 16–3 Onogawa Tsukuba Ibaraki 305–8569 Japan. E-mail hiroaki@nire.go.jp Received 8th December 1998 Accepted 15th February 1999 A sensitive and robust speciation method for arsenic in water is described. The separation of arsenate (AsV) arsenite (AsIII) and monomethylarsonic acid (MMA) was performed using an ion exclusion column packed with a sulfonated polystyrene resin and using dilute trifluoroacetic acid at pH 2.1 as the mobile phase.The hydride generation method was used to improve sensitivity and to eliminate interference from chloride ions. The analysis time per sample was 18 min but could be shortened to 9 min by using a column switching method. The detection limits for the arsenic species were 1.1 pg ml-1 for AsV 0.5 pg ml-1 for AsIII and 0.5 pg ml-1 for MMA with an injection volume of 50 ml. The relative standard deviations of five replicates of a standard containing 1 ng ml-1 As of each species ranged from 0.8 to 2.8%. The method was validated by analyzing reference water samples. Introduction Since the toxicity of an element depends on its chemical form it is important to determine the concentration of individual chemical species in order to evaluate environmental risk.For arsenic and derivatives thereof the toxicity decreases in the order arsenite (AsIII )>arsenate (AsV )>[dimethylarsinic acid (DMA) monomethylarsonic acid (MMA)]>[arsenobetaine (AsB) arsenocholine (AsC) tetramethylarsonium ion (TMA)]. As a result the biological methylation of arsenic is generally thought to represent a detoxification process. However reports have appeared concerning the toxicity of DMA which includes DNA modification and mutagenicity.1,2 Arsenic in water occurs mainly in inorganic forms such as AsV and AsIII but also occurs in methylated forms such as MMA and DMA at very low concentration levels.3,4 The levels of other organoarsenic species found in biological tissues such as AsB AsC TMA and arsenosugars are negligible in water.A variety of methods for the speciation of arsenic have been developed thus far,5–8 and hyphenated methods such as liquid chromatography-inductively coupled plasma mass spectrometry (LC-ICP-MS) and capillary electrophoresis (CE)- ICP-MS,9 are currently popular and in common use. However most of these methods are not applicable to the direct determination of MMA and DMA in water because these species are present at low concentrations. It has also been diYcult to apply these methods to sea-water samples because of the high level of polyatomic interference from 40Ar35Cl on 75As. The purpose of the present investigation was to develop a sensitive and robust method for the determination of these species in water. As the analytes four arsenic species viz.AsIII AsV MMA and DMA were investigated because of their high level of toxicity. Another reason for selecting these species is that they form volatile hydrides. It is also possible to generate volatile hydrides from non-hydride forming species such as AsB and AsC provided that they are decomposed into inorganic forms by microwave digestion10,11 or photooxidation. 12,13 However these techniques were not investigated in the present study. In order to increase the sensitivity a variety of sample introduction methods such as high-eYciency nebulizers,14 ultrasonic nebulizers,15 thermospray nebulizers,16 direct injection nebulizers17 and hydride generation,3,4,9 have been used. Among these hydride generation (HG) resulted in the highest sensitivity for arsenic species.Furthermore since only gaseous species were introduced into the ICP via HG polyatomic ion spectral interference from 40Ar35Cl on 75As was not a factor. Clogging of the sampling cone and non-spectral interference such as space charge eVects were also eliminated. It is necessary not only to increase the sensitivity but also to decrease the background signal in order to improve the detection limit. The detection limits for arsenic with LC-ICP-MS are often determined by the high background signal arising from arsenic impurities in the LC eluent. In particular when a phosphate buVer4,10,16–19 was used as the eluent this phenomenon was pronounced because reagents derived from phosphorus which belongs to the same periodic group as arsenic generally contain significant arsenic impurities.Therefore it is desirable to perform a chromatographic separation of arsenic species via the use of only a high-purity acid solution as the eluent. The LC modes investigated thus far for arsenic speciation include reversed-phase,14 ion-pair reversed-phase,3,14,17–21 size exclusion,20 micellar22 and ion-exchange chromatography. 4,16,23–25 However in so far as we are aware there is no example in which the separation of arsenic species was performed based on ion exclusion. In the present study ion exclusion LC was examined for the separation of such species. Our data show that three arsenic species viz. AsIII AsV and MMA can be separated using only a dilute acid solution but that DMA cannot be eluted.The detection limits obtained with the present method were more than 20 times lower than those reported previously.3,4 This appears to be largely due to the use of dilute acid as the eluent which can easily be obtained in high purity compared with salts. Although deviations in retention times and background signal fluctuation 651 J. Anal. At. Spectrom. 1999 14 651–655 due to co-existing ions were reported for ion-pair reversedphase LC and ion-exchange LC,4,18 these interferences were not observed in the experiments reported herein suggesting that the method is of superior robustness. Because of this the present method is directly applicable to the analysis of seawater samples. 3 4 2O3 in a small portion Preliminary experiments revealed that a sea-water sample of Japan) and dimethylarsinic acid 3)2AsO(OH)] (Tri Chemical Laboratory) in water water containing 1 ng ml-1 of AsV was investigated.The As a result the eVect of adding NaCl or Na2SO4 to pure Results and discussion 4 4 3 3 Column Mobile phase Experimental 4 2 Reagents A stock solution of arsenate (AsV) at 1000 mg ml-1 As was prepared by dissolving sodium arsenate dibasic heptahydrate (Na HAsO ·7H2O) (reagent grade >99% Wako Osaka 1000 mg ml-1 As was purchased from Wako (atomic absorp- Japan) in water. A stock solution of arsenite (AsIII) at tion spectrometry grade). According to the manufacturer the EVect of Na2SO4 concentration on the AsV peak shape solution was prepared by dissolving As of NaOH solution which was then neutralized with HCl AsV gave a sharp peak but that AsV when added to pure solution to pH 5.0.Stock solutions of MMA and DMA at 1000 mg ml-1 As were prepared by dissolving monomethylarwater gave only a small and broad peak. It was speculated that some components contained in sea-water played an sonic acid [CH AsO(OH)2] (Tri Chemical Laboratory important role in the generation of the sharp peak for AsV. Yamanashi [(CH respectively. A 1% sodium tetrahydroborate solution was addition of NaCl had no eVect but added Na2SO4 produced prepared by dissolving high-purity grade NaBH4 (>95% a marked improvement in the peak shape although the reason Merck Darmstadt Germany or >98% atomic absorption for this was not apparent.The eVect of Na2SO4 concentration spectrometry grade Kanto Kagaku Tokyo Japan) in a 0.1 mol l-1 NaOH solution immediately prior to the experion AsV and AsIII peak shapes is shown in Fig. 2. The addition 2SO4 at 400 mg ml-1 as the final concentration was found of Na ment. Sodium hydroxide (analytical-reagent grade) and nitric to be suYcient and this concentration had no eVect on the acid (ultrapure grade) were purchased from Merck. equilibrium between AsV and AsIII. The added Na2SO4 did Trifluoroacetic acid (>99% protein sequencing grade) and not increase the blank level due to the presence of low levels sodium sulfate (anhydrous analytical-reagent grade) were of arsenic as impurities. purchased from Wako. Ultrapure water from a Milli-Q Low TOC system (Millipore Milford MA USA) was used EVect of NaBH concentration throughout.Instrumentation A schematic diagram of the LC-ICP-MS instrumentation with hydride generation is described in Fig. 1. An LC-6A liquid chromatograph (Shimadzu Kyoto Japan) equipped with a Shimadzu LC-6A pump and a sample injection valve (9725i Rheodyne Cotati CA USA) with injection volumes of 50 or 200 ml was used. The ion exclusion column was a sulfonated polystyrene type Shim-pack SCR-102H (30 cm long Shimadzu). A 5 cm guard column with the same packing material was also used in a column switching method. The mobile phase was dilute trifluoroacetic acid adjusted to pH 2.1 and a flow rate of 1.5 ml min-1 was used. The eluate from the liquid chromatograph was first mixed with 1.5 mol l-1 HNO3 (flow rate 2.3 ml min-1) and then with a 1% NaBH solution (flow rate 1.8 ml min-1) to generate the hydrides.The mixture was then transferred to a gas–liquid separator (fabricated in our laboratory) through a poly(tetrafluoroethylene) tube Fig. 1 Schematic diagram of the LC-ICP-MS instrumentation with hydride generation. 652 J. Anal. At. Spectrom. 1999 14 651–655 (50 cm long 3 mm id). The details of the gas–liquid separator have been described in a previous paper.26 The waste liquid was removed immediately after the reaction from the phase separator by means of a peristaltic pump to prevent the excess of hydrogen from entering the plasma and to prevent the chromatographic peak from broadening as a result of memory eVects.An ICPM-8500 inductively coupled plasma mass spectrometer (Shimadzu) equipped with a miniaturized torch was used. Operating conditions are listed in Table 1. The data from the ICP-MS instrument were converted into ASCII format and handled with an Excel spreadsheet (Microsoft Cambridge MA USA) for further processing. 4 7 1.2 1.5 5 20 75 0.64 1 1.8 1.5 2.3 4 The eVects of NaBH4 concentration on the signal peak heights and the background equivalent concentrations (BECs) for AsV AsIII and MMA are shown in Fig. 3. The BECs are defined as the concentrations that would give the equivalent peak height to the continuous background signal. Although the signal peak heights increased with increasing NaBH4 concentration the BECs became worse because of the increased background levels arising from impurities present in the NaBH4 solution.In order to obtain both considerable peak heights and better BECs 1% NaBH was chosen as a compromise concentration. Peak broadening by incorporating a hydride generation reaction between the LC and ICP-MS steps was negligible compared with the peak widths obtained by LC-ICP-MS without hydride generation. This is probably Table 1 Optimum operating conditions for LC-HG-ICP-MS ICP-MS parameters— Forward power/kW Ar plasma gas/l min-1 Ar auxiliary gas/l min-1 Ar carrier gas/l min-1 Sampling depth/mm Measured m/z Dwell time/ms flow rate/ml min-1 Hydride generation parameters— NaBH concentration (%) NaBH flow rate/ml min-1 HNO concentration/mol l-1 HNO LC parameters— Flow rate of mobile phase/ml min-1 Sample injection volume/ml Shim-pack SCR-102H Trifluoroacetic acid (pH 2.1) 1.5 50 or 200 Fig.2 EVect of Na2SO4 concentration on peak shapes of AsV and AsIII. A 0 mg ml-1 Na2SO4; B 40 mg ml-1 Na2SO4; C 400 mg ml-1 Na2SO4. Fig. 3 EVects of NaBH4 concentration on (a) signal peak heights and (b) background equivalent concentrations (BECs). because the transportation of gaseous hydrides from the gas– liquid separator was fairly rapid and the unreacted arsenic species in solution were rapidly removed from the gas–liquid separator by the peristaltic pump. Retention behavior 1 The column used was a sulfonated polystyrene type.The mobile phase was dilute trifluoroacetic acid adjusted to pH 2.1. Since the pK values are reported to be 2.25,27 2.6,28 6.3,28 and 9.2327 for AsV MMA DMA and AsIII respectively and the protonation of DMA is reported to occur at pH 3.85,29 AsV partly exists as an anionic form while MMA and AsIII predominantly exist as neutral forms and DMA exists as a cationic form respectively at pH 2.1. A chromatogram of AsV AsIII and MMA is shown in Fig. 4. The separation between AsV and AsIII appears to be based on an ion exclusion mechanism. However the separation between AsIII and MMA is thought to be based on hydrophobic adsorption by the polymeric resin. DMA is thought to be retained on the column by electrostatic attraction and hydrophobic adsorption.This retention behavior of AsV AsIII and MMA is unique compared with that reported thus far in which the elution of AsIII is usually earlier than AsV. Attempts to elute MMA and DMA more rapidly by using a higher pH eluent and by adding methanol to the eluent were not successful. Calibration graphs detection limits and repeatability Six-point calibration graphs for AsV AsIII and MMA were obtained by plotting the peak areas against the concentration of arsenic for each species in the range 0–10 ng ml-1. The calibration graphs were linear within this range. The slopes and regression coeYcients of the calibration graphs are given in Table 2. The detection limits defined as three times the standard deviation of the peak areas for seven replicates of the blank are also listed in Table 2.These values are more than 20 times lower than those reported to date.3,4 The use of a high-purity acid as the mobile phase is thought to contribute to this improvement in the detection limits. The repeatability for the three arsenic species was evaluated from five replicates using a standard containing 1 ng ml-1 As of each species. The relative standard deviations are given in Table 2. Recovery test In order to verify the reliability of the present method recovery tests were carried out with diVerent types of water such as riverine water sea-water and tap water by adding standards. JAC0031 and SLRS-1 are riverine reference water samples of the Japanese Society for Analytical Chemistry (JSAC) and of The National Research Council of Canada (NRCC) respectively.CASS-3 is a reference sea-water sample of the NRCC. Each arsenic species standard was added to the reference water samples to increase the concentrations by 1 ng ml-1 As per species. The averages and standard deviations of four recovery tests are given in Table 3. Since the tap water is chlorinated AsIII added to it is rapidly oxidized to AsV by hypochlorite. Therefore the tap water used for the recovery test was boiled for 10 min to remove chlorine species. Recoveries of the three arsenic species which ranged from 94 to 108% are thought to be acceptable. Fig. 4 Chromatograms for AsV AsIII and MMA. A Blank; B 0.1 ng ml-1 As; C 0.5 ng ml-1 As; D 1.0 ng ml-1 As of each species.653 J. Anal. At. Spectrom. 1999 14 651–655 Table 2 Calibration graphs detection limits and repeatability AsV 349 0.9993 1.1 0.8 Slope/counts per pg ml-1 As Detection limit/pg ml-1 As Regression coeYcient (R2) Repeatability (n=5) at 1 ng ml-1 As (%) Table 3 Recovery of added AsV AsIII and MMA from water samples (%) (n=4 at 1 ng ml-1 As)a MMA AsIII AsV 94.3±1.9 96.7±1.5 101.0±0.5 JAC0031 riverine water 106.3±7.7 SLRS-1 riverine water 105.0±4.0 CASS-3 sea-water 108.0±5.3 Tap water 97.5±0.7 95.7±4.6 95.0±0.5 96.3±2.3 99.5±3.5 107.5±6.3 a±values are standard deviations. Fig. 5 Schematic diagram of LC-HG-ICP-MS with a column switching system. Column switching method to shorten analysis time Approximately 18 min was required to complete one analysis because of the long retention time of MMA.In order to shorten the analysis time a column switching method was applied. A guard column (SCR-102H 5 cm long) and a sixway motorized valve (Rheodyne Model 9750) were inserted before the separation column (SCR-102H 30 cm) as shown in Fig. 5. Initially the valve was positioned so that AsV and AsIII eluted from the guard column were introduced into the separation column. The valve position was then changed at 1.2 min so that MMA eluted from the guard column could be introduced directly into the hydride generator. Finally after the detection of MMA by ICP-MS the valve was returned to the original position at 2.6 min to complete the separation of AsV and AsIII by the separation column.Peak broadening of AsV and AsIII as a result of stopping the flow in the separation column from 1.2 to 2.6 min was negligible. A better separation was obtained for AsV and AsIII because of the longer total column length (35 cm) compared with the result shown in Fig. 4. These modifications permitted the analysis time to be shortened to 9 min. Chromatograms for SLRS-3 and CASS-3 obtained using an injection volume of 200 ml are shown in Fig. 6 along with the standard containing each species at 1 ng ml-1 As. The AsIII peak was not observed in Table 4 Analytical results for various reference waters and tap water (ng ml-1 As) (n=3) AsV JAC0031 riverine water SLRS-1 riverine water SLRS-3 riverine water SLEW-2 estuarine water CASS-3 sea-water Tap water 0.22±0.014a 0.30±0.014 0.49±0.031 0.80±0.065 1.11±0.07 0.10±0.010 aPrecision expressed as the standard deviation.bBelow detection limit. cUncertainties for the certified values are 95% confidence intervals. 654 J. Anal. At. Spectrom. 1999 14 651–655 MMA AsIII 877 0.9999 0.5 2.8 966 0.9998 0.5 2.5 Fig. 6 Top chromatogram of reference sea-water CASS-3; bottom chromatograms of a standard solution containing 1 ng ml-1 As of each species and riverine reference water SLRS-3. The baseline for the chromatogram of CASS-3 has been shifted so that the peak can be compared readily. The backgrounds of the three chromatograms were nearly identical. 3 3 either of the reference water samples because of acidification with HNO for sample storage.It was confirmed that AsIII was rapidly converted to AsV at low concentrations by adding small portions of concentrated HNO solution at 1 ng ml-1 (final HNO concentration 0.1 mol l-1). 3 to the AsIII standard Therefore it is not recommended that HNO3 is added to the sample solutions when the speciation of arsenic is required. The peak of MMA for SLRS-3 was easily distinguishable and the fluctuation of the background for CASS-3 was much smaller compared with the chromatograms reported previously. 3,4 Although care must be exercised in comparing the chromatograms obtained with the diVerent ICP-MS instruments it appears that the smaller fluctuations and the less noisy background observed here are due to the robustness of the ion exclusion column and the high purity of the mobile phase.It has also been reported that since the high chloride concentration of a reference sea-water sample (NASS-4 of the NRCC salinity 31.3‰) causes peak splitting for MMA the sample must be diluted 1+3 with distilled water in order to be analyzed accurately with an anion exchange column.4 No such problems were encountered with the present method. Certified value MMA AsIII 0.28±0.04c 0.55±0.08 0.72±0.05 0.792±0.082 1.09±0.07 — <d.l. 0.05±0.008 0.08±0.010 0.03±0.005 0.01±0.005 0.06±0.012 <d.l.b <d.l. <d.l. <d.l. <d.l. <d.l. Since DMA could not be eluted from the 5 cm guard column DMA seemed to be retained strongly on the column.Neither a deterioration of the column performance nor elevated background signals due to the retained DMA were observed during the experimental period viz. about 5 months but the guard column should be replaced if such phenomena are observed. Analysis of reference water samples In order to validate the present method three riverine reference water samples (JAC0031 SLRS-1 and SLRS-3) one estuarine reference water sample (SLEW-2 salinity 11.6‰) and one reference sea-water sample (CASS-3 salinity 30.2‰) were analyzed. Analytical results are shown in Table 4 along with the certified values. Arsenite (AsIII) was not detected in any of the samples because of the acidification with HNO3 as mentioned above.The sum of the concentrations of AsV and MMA for CASS-3 and SLEW-2 showed good agreement with the certified values. In contrast those for riverine waters were out of the range of the certified values. In particular the deviations from the certified values were large for SLRS-1 and SLRS-3. The reason for this is not clear at present but one possible reason might be the existence of other species such as DMA which cannot be detected with the present method. A relatively large peak of DMA as large as that of AsV was observed in the SLRS-2 riverine reference water sample (NRCC).3 Conclusions A highly sensitive and robust method for the speciation of arsenic in water was developed by coupling ion exclusion chromatography to ICP-MS with hydride generation as the sample introduction technique.The separation based on ion exclusion and hydrophobic adsorption permitted a unique elution order of AsV>AsIII&MMA by using only a dilute acid as the eluent. The use of a high-purity acid decreased the background level and improved the detection limit. The retention times of the three species were not aVected by the seawater matrix. The method was free from interference from chloride ions and was easily applied to sea-water samples. The main disadvantage of the present method is its failure to elute DMA. This is probably because DMA is retained on the column via hydrophobic adsorption on the polystyrene resin. The use of a more hydrophilic resin should help to decrease this eVect.Methods for the purification of sodium tetrahydroborate should be pursued to achieve a better detection limit since the detection limit was determined by impurities in this reagent. References 1 M. Vahter and E. Marafante in Metal Ions in Biological Systems. Vol. 29. Biological Properties of Metal Alkyl Derivatives ed. H. Sigel and A. Sigel Marcel Dekker New York 1993 p. 161. 2 H. Yamauchi and B. A. Fowler in Arsenic in the Environment Part II ed. J. O. Nriagu Wiley New York 1994 p. 35. 3 C.-J. Hwang and S.-J. Jiang Anal. Chim. Acta 1994 289 205. 4 M. L. 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Anal. At. Spectrom. 1999 14 651&ndash

ISSN:0267-9477    

DOI:10.1039/a809606f

出版商:RSC    

年代:1999

数据来源: RSC