摘要:

Determination of Arsenic in Airborne Particulate Matter by Inductively Coupled Plasma Mass Spectrometry CHU-FANG WANG*a, SU-LING JENGa AND FANG-JIR SHIEHb aDepartment of Nuclear Science, National T sing Hua University, Hsinchu, T aiwan, Republic of China bMaterial Science Division, Industrial T echnology and Research Institute, Hsinchu, T aiwan, Republic of China An optimized sample digestion procedure was developed for techniques, ICP-MS has several attractive features, e.g., the determining As in airborne particulate matter by ICP-MS and spectra are simple to interpret and mainly free from interapplied to real sample analysis.A two-step high-pressure bomb element interferences, isotopic information is inherent, the digestion procedure, entailing the digestion of airborne multi-elemental capability allows a large body of chemical particulates with HNO3–HClO4 and then digestion with data throughput, the detection limits are lower than those in HClO4–HF after the removal of the intact filter membrane, most conventional techniques and most of the elements in the appears to be the most effective pre-treatment procedure for Periodic Table can be detected.7 However, interference probdetermining As by ICP-MS.The direct determination of As in lems due to the acid-derived background ions formed during airborne particulate matter by laser ablation ICP-MS was also the ion extraction process in ICP-MS significantly degrade the investigated.Standard filters prepared in the laboratory and sensitivity of As determination.8,9 A survey of the literature10–20 real samples were both analyzed, and the results agreed with revealed that the chloride, introduced via reagents or samples, the certified concentrations and data obtained from the combining with argon from the plasma can give rise to the conventional acid digestion–ICP-MS method. Finally, the formation of 40Ar35Cl+, which may seriously inhibit the deteccapabilities of various analytical techniques applied to real tion of 75As+ (100%).sample analysis were compared. The introduction of a small amount of nitrogen into the plasma has been shown by Branch and co-workers17,18 to Keywords: Arsenic determination; airborne particles ; reduce ArCl formation. However, nitrogen addition was not microwave digestion ; high-pressure bomb digestion ; inductively effective in removing the ArCl interference for samples with a coupled plasma mass spectrometry high chlorine content, which made the technique unsuitable for determining As in airborne particle samples.Anderson et al.11 proposed the use of the hydride generation technique as a means of eliminating chloride interference for determining Arsenic, considered the major ‘marker element’ of air pollution As in soils by ICP-MS. However, the chemistry is not straight- emitted from coal combustion, has been increasingly released forward and care must be taken to ensure that the analyte into the atmosphere in recent periods of industrial develop- elements are in the optimum oxidation states for evolution of ment. Arsenic is one of the most hazardous anthropogenic air the hydrides.21 A mathematical correction method by measur- pollutants affecting humans globally. The Environmental ing blank and sample signals at m/z 77, 78 or 82 to eliminate Protection Agency of Taiwan has recently rated it as a class the chloride interference at m/z 75 has also been suggested by ‘A’ substance to be determined in air.1 The US National various workers.14,19,22,23 Institute of Occupational Safety and Health (NIOSH) has Introducing a laser ablation (LA) technique, involving the recommended a standard method, procedure P&CAM 139, removal of particulate matter directly from the filter media by for determining As in airborne particulate matter.2 The method ablation with laser pulses, in conjunction with ICP-MS involves collecting a sample on a membrane filter for dissolu- (LA–ICP-MS), may help to avoid many of the problems tion in a nitric acid–sulfuric acid mixture, in which As is associated with conventional As determination methods.24,25 determined by hydride generation in an argon–hydrogen flame. The advantages of LA–ICP-MS for determining As over the For a 30 l air sample, the range of the method is from 0.002 conventional aqueous method are that almost no sample to 0.06 mg m-3.However, a much lower concentration level treatment is required, which means that losses due to the of As is expected in ambient air.formation of volatile As compounds are negligible. Further- Our previous experiments involved developing a two-step more, no interference is caused by undesirable polyatomic ions closed-vessel digestion procedure, first entailing the digestion from water and acids during ICP-MS measurement. However, of samples with HNO3–HClO4 followed by adding HF after the major problems that remain are the instability of the laser removal from the intact filter membrane for multi-element output and inhomogeneity of the filter matrix and the loaded analysis of the airborne particles.3–6 However, inherent probparticles.lems arise with the As determination during the acid digestion The objectives of this work were two-fold. The first aim was process. It is known that As may be volatile in the halide form, to establish optimized sample digestion conditions for e.g., as arsenic fluorides which are primarily formed by interdetermining As in airborne particulate matters by ICP-MS, action with digestion agents such as hydrofluoric acid.and the second was to investigate the feasibility of an Determining the formation and volatility of such compounds LA–ICP-MS technique for the quantitative analysis of airborne requires systematic studies under conditions associated with particulate matter loaded on PTFE-membrane filters. In the decomposition of airborne particulate matter samples, but addition, the capabilities of various analytical techniques such studies are not available at present.applied in real sample analysis were compared throughout the ICP-MS was selected in our previous work as the instrumental method of detection. Compared with other analytical investigation. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (61–67) 61EXPERIMENTAL bomb and microwave closed-vessel digestion methods were tested with NIST SRM 1648 Urban Particulate Matter.Sample Preparation Between 1991 and 1993, airborne particulate samples were High-pressure bomb digestion (HPBD) collected from different monitoring stations in Taiwan. Hi-Vol samplers, PM-10 samplers and dichotomous samplers were Glass-fiber or PTFE-membrane filters loaded with airborne particulate matter or around 10 mg of reference standard were employed to collect samples on glass-fiber and PTFE-membrane filters.A total of 566 loaded filters were collected and placed in 25 ml PTFE containers and 10 ml of acid mixture, including HNO3, HClO4 and HF, in different combinations, analyzed. Detailed information regarding the sampling instruments and methods has been given elsewhere.5 were added. The sealed containers were transferred into a pressure bomb and heated on a heating block to facilitate complete dissolution. After digestion, the sample solution was Apparatus transferred into a PTFE beaker and heated gently at about 80°C to allow the HF to evaporate.The residue was then A pressure bomb digestion system consisting of a 25 ml PTFE diluted to 10 ml for measurement by ICP-MS. vessel and a thermostatically controlled heating block supplied by Berghof (Eningen, Germany) was used for sample digestion. The microwave digestion system was a Model MDS 2000 Microwave digestion (MD) 630 W instrument (CEM, Matthews, NC, USA) operating at Teflon filters loaded with airborne particulate matter and a frequency of 2.455 GHz.Details of the instruments were about 5 mg of reference standard were placed in closed PFA given in a previous paper.4 The ICP-MS instrument employed digestion vessels with 5 ml of acid mixture and digested in the was a Perkin-Elmer SCIEX (Thornhill, Ontario, Canada) microwave oven for a total digestion time of 27 min. Detailed ELAN 5000 and the LA instrument was a Perkin-Elmer programme settings for the digestion process were given in a SCIEX Model 320 sampler equipped with a Quanta-Ray previous paper4 and are described here in Fig. 1. An evapor- GCR-11 pulsed Nd5YAG laser. Details of operating conditions ation process was also performed to evaporate the HF before for the LA and ICP-MS instruments are given in Table 1. the ICP measurement. Containers and Reagents ICP-MS and L A–ICP-MS measurements PTFE, polyethylene and polypropylene containers were After digestion, aliquots were analyzed directly using the cleaned by immersion in concentrated HNO3, heated in a ICP-MS instrument.A direct LA technique combined with microwave oven and then washed with de-ionized water. Two ICP-MS was also tested, employing standard filter samples heating cycles of approximately 30 min each with concentrated prepared in the laboratory. NIST SRM 1648 Urban Particulate nitric acid were performed in the microwave system to minimize Matter was deposited on the PTFE-membrane filter by a the blank background.All chemicals employed were of analytdesigned sampling box, and the certified As concentrations on ical-reagent grade from Merck (Darmstadt, Germany), and each filter were verified by the conventional ICP-MS method high-purity water (resistance >16 MV cm) was produced by after the LA–ICP-MS analysis. reverse osmosis and de-mineralization. Stock solutions of the elements of interest (1000 mg ml-1) were prepared from Titrisol concentrates (Merck) by diluting to volume with de-ionized Procedures water.Fig. 1 shows the experimental flow chart for this investigation. Several alternatives, including sample preparation (A, B, C), Analysis two-step digestion (E, F, G, H, I, J), evaporation (K, L), instrument analysis (D, M) and data evaluation (N) were To determine As concentrations in loaded airborne particulate explored to accomplish the following objectives: filters, two different techniques, namely ICP-MS after acid (i) To study the influence of acid mixture on sample dissolu- digestion and direct LA–ICP-MS, were employed. To detertion: Different digestion procedures, including mine the optimum digestion conditions, both the high-pressure (6)�(8)�(10)�(14)�(15)�(17)�(19)�(21), (6)�(8)� (12)�(19)�(21), (6)�(9)�(11)�(14)�(16)�(18)� (20)�(21), and (6)�(9)�(13)�(20)�(21), were per- Table 1 LA–ICP-MS operating conditions formed to study the influence of acid mixtures in different proportions on sample dissolution and to determine the ICP-MS— Instrument Perkin-Elmer SCIEX ELAN 5000 optimum digestion conditions.NIST SRM 1648 Urban Power 1100 W Particulate Matter was utilized for the recovery test. Coolant argon flow rate 14 l min-1 (ii) To evaluate the two-step digestion procedure: A two-step Auxiliary argon flow rate 0.8 l min-1 HPBD digestion procedure [(1) or (4)�(8)�(10)�(14)� Carrier argon flow rate 0.8 l min-1 (15)�(17)�(19)�(21)] was used to determine the As Dwell time 10 ms content of airborne particulate samples in order to achieve Resolution Normal Readings/replicate 100 complete digestion and minimize the blank contributions Sweeps/reading 1 from the filter media.Real samples collected on PTFE- Scanning mode Peak hopping transient membrane filters and also NIST SRM 1648 were utilized Isotope 75As to examine the applicability of the method. (iii) To investigate the feasibility of the LA–ICP-MS method: L aser ablation — Prepared standard filters and real samples were directly Instrument Perkin-Elmer SCIEX Model 320 sampler analyzed by the LA–ICP-MS (D) by procedure [(2)�(23)] Operating mode Free running and the optimum experimental conditions were investi- Ablation type Single pulse gated.Following LA–ICP-MS measurement, samples were Laser flash lamp energy 35 J (laser beam energy #140 mJ) then analyzed by the conventional HPBD–ICP-MS Defocus distance 5.5 mm method. A comparison between the results of the two 62 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Fig. 1 Flow chart for the determination of As in suspended particulates collected on filters by ICP-MS. methods (N) was performed to verify the capability of the followed by treatment with hydrofluoric acid to destroy the difficult-to-digest silicates.26,27 As Table 2 illustrates, the lower proposed method. recoveries of one-step high-pressure bomb digestion with HNO3 and HNO3–HClO4 can be attributed to the incomplete RESULTS AND DISCUSSION dissolution of difficult-to-digest silicates in airborne particles. On the other hand, the high-pressure bomb method with two- Optimized Digestion Conditions for ICP-MS Measurements stepHNO3–HClO4/HF digestion, exhibited excellent precision The data presented in Table 2 were obtained using the high- and accuracy, which may indicate that complete digestion and pressure bomb and microwave digestion methods with different far smaller losses of As were achieved with the addition of HF.combinations of acid mixture. These methods may suffer, to In the digestion of airborne particulate matter with hydroflu- varying extents, from the losses of volatile As compounds, oric acid, complete oxidation of AsIII to AsV under the closed- incomplete digestion and contamination of the reaction mix- system conditions is essential. Several previous studies have ture by impurities from the filter matrix and from reagents verified the volatilization of AsIII, e.g., AsF3 ,from non-oxidizing leached from the walls of the digestion vessels.media of hydrofluoric acid.28–31 Clearly, with digestion with Losses of volatile As, and also incomplete digestion during HNO3–HClO4 at 170 °C for 5 h, only the oxidized AsV the digestion process, for airborne particulates can be mini- remained in the final solution from the high-pressure bomb mized by the two-step digestion process proposed in our method. To restate the position, the losses by evaporating previous work.4 It was designed to accomplish the digestion AsF3 can be avoided if complete digestion is achieved in a first with a mixture of nitric and perchloric acids to dissolve continuous low-temperature closed-vessel digestion system.Arsenic in airborne particulates can also be determined by and separate collected airborne particulates from filter media, Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 63Table 2 Comparison of As determinations in airborne particulate matter by closed-vessel digestion methods under different conditions (n=10) Digestion method Amount of acid mixture/ml Digestion time SRM recovery (%) High-pressure bomb digestion — HNO3 10 5 h 74.5±10.3 HNO3–HClO4 10 (1+1) 5 h 82.0±5.4 HNO3–HClO4/HF 10 (3+5/2) 7 h 96.4±3.8 HNO3–HClO4/HClO4–HF 10 (3+3/2+2) 7 h 101.3±4.5 Microwave digestion*— HNO3–HClO4–HF 5 (3+5+2) 18 min 140.2±10.0 HNO3–HClO4/HClO4–HF 5 (3+3/2+2) 18 min 107.4±10.5 * Only PTFE-membrane filter samples were digested by microwave methods.the more rapid but vigorous microwave digestion method. Minimal blank values may be achieved in a closed vessel microwave digestion system, as the amount of acid is reduced and the contact of the reaction mixture with the laboratory atmosphere is eliminated. The largest and most difficult-todetermine blank contribution in the analysis of airborne particulates may be attributed to the filter matrix dissolving during the digestion process.However, this contribution may also be reduced by the above-suggested two-step acid mixture digestion method. To prevent dissolution of the filter media by hydrofluoric acid, the filter residue after the first digestion step with HNO3–HClO4 was removed from the vessel before adding HF. Generally, complete evaporation of the acid residue cannot be performed by the microwave system. The relatively higher Fig. 2 Single laser pulse transit signal of As ablated from A, blank, recovery of As during the microwave digestion, as indicated in B, standard and C, real PTFE-membrane filter samples.The mass Table 2,of standard and real filter samples are 0.38 and 0.13 mg cm-2, respectively. the residual chloride in the treated sample solution. The magnitude of this interference was checked by measuring the reagent blank signal directly. It was estimated that almost one sixth of the measuring signal could be contributed by ArCl+ reached a maximum, then gradually decreased for around 20 s.interference in the microwave digestion procedure. In this investigation, the dwell time was 10 ms and scans were It is doubtful whether incomplete oxidation of AsIII, which repeated 100 times, extending the total acquisition time to exists in the silicates of airborne particles, occurs during the 1000 ms. Since a total of 22 elements were designed to be second step of digestion in the absence of sufficient oxidizing scanned in the measurement, a replicate time of 35 s for each agent.Adding HClO4 in the second digestion step may over- element was allowed for. Spectra of standards with different come this problem. It also appears that the presence of HClO4 As concentrations and real sample and blank filters were with HF offers the advantage of increasing the boiling temper- plotted to reveal the general variations produced during the ature of the reaction mixture and of improving the efficiency measurement.The fluctuation of the laser beam intensity was of HF. An acid digestion employing the combination of reflected significantly in the signal peak. A delay of the signal HNO3–HClO4 with HClO4–HF (3+3 and 2+2 v/v) in both peak, which was mainly affected by the transport time in the high-pressure bomb and microwave digestion procedures addition to the operational shift from LA equipment to the did indeed exhibit better recovery results in Table 2 for ICP-MS, was also indicated.Fig. 2 confirms that the optimizdetermining As in airborne particles. ation of the laser energy and the time duration of the signal Therefore, losses in As can be prevented by carefully con- peaks could be maintained, and that an accurate analysis could trolling the temperature during both the closed digestion and therefore be achieved by appropriately integrating the signal the open-phase evaporation process.It was concluded that the peaks. highest recovery of As in airborne particulates is achieved if To minimize the inhomogeneous fluctuations of the As the evaporation temperature is maintained below 80°C. In concentrations distributed on the real filter samples, ten ranaddition, the recovery data for NIST SRM 1648 listed in domly selected spots on the same filter were ablated and Table 2 indicate that the proposed two-step acid mixture detected during the measurement. The RSD of the averaged digestion method adequately liberates As from particulate intensities for As was <7% in all cases.matter and achieves complete oxidization. Standard filter samples with different As contents were prepared in the laboratory to assess the capability of LA–ICP-MS. NIST SRM 1648 Urban Particulate Matter was Feasibility Study of Direct LA–ICP-MS Analysis deposited on a PTFE-membrane filter by a designed sampling chamber, and the certified As concentrations on each filter Determination of trace contents of As in airborne particulate matter was also examined by LA–ICP-MS, although major were verified by the conventional ICP-MS method after LA–ICP-MS analysis.All the ablation processes were per- obstacles arose. These included instability of the operating conditions, such as fluctuation of the laser beam intensity and formed in a single-shot, free-running mode with a laser flash lamp energy of 35 J (laser beam energy#140 mJ).Consecutive signal delay, and the preparation of calibration standards. Fig. 2 depicts the response of As when a single laser pulse was laser shots on the thick samples may have given improved precision but seemed an inappropriate approach for membrane fired at blank, standard and real PTFE-membrane filter samples. Several seconds after the shot, the integrated signals filter samples which are usually thinner than 100 mm. A single 64 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12was also employed in a comparative exercise. Sets of airborne particulate-loaded filter samples collected from the heavily polluted metropolitan area of Kaoshiung, Taiwan, and standard filter samples prepared in the laboratory were analyzed. As Fig. 4 illustrates, LA–ICP-MS measurements in a sequence of 12 spot analyses with a crater diameter of 1.5 mm on the filter samples were carried out. The purpose was to minimize the fluctuations due to laser instability and to the inhomogeneity of the particles on the filter.The area of the Teflon filter for particle collection was calculated to be around 7 cm2. An ablated area of about 70 mm2 was cut away with care. The remainder of the filter was then digested by the high-pressure bomb method and analyzed by ICP-MS. Fig. 5 exhibits an Fig. 3 Calibration curves for As obtained from NIST SRM 1648 excellent linear correlation for all the data between Urban Particulate Matter mass loadings from 0 to 3.5 mg per filter.LA–ICP-MS and HPBD–ICP-MS. A total of 14 standard (n= 7) and real filter (n=7) samples with different As concentrations laser shot was confirmed to provide sufficient energy to ablate were analyzed by both methods. The standard filters exhibited samples on the filter, although preventing efficient burning of a larger scattering of data, which might be attributed to the low-b.p. PTFE substrates. loss of loosely attached particulates on the filters during the A calibration curve was then generated from the integration preparation process.of these transient signals. Fig. 3 demonstrates that the measured It is recognized that, in conventional ICP-MS analysis, As intensities are generally enhanced with increasing As concen- may be lost owing to the formation of volatile fluoride com- tration, and reach a plateau with greatly amplified fluctuations pounds, e.g., AsF3 , during the acid digestion process.31 Any at higher concentrations.Possibly the incomplete ablation may remaining HF in the final solutions is liable to attack the be the cause of such ‘saturation’ in the excess mass loading of glassware employed in ICP-MS nebulizers, spray chambers particulates on the filter media. The ablated particulate samples and injectors. Evaporation prior to analysis is suggested as an on the filter are apparently limited to the amount of energy efficient method of removing all HF from the solutions.propagated by each laser shot. As indicated in Fig. 3, efficient However, this may also contribute to the loss of As. Adding ablation can be performed by a single 140 mJ shot with a HClO4 for complete oxidation may form ArCl+, which could particulate mass loading in less than 2 mg on the filter. A introduce significant spectral interferences when determining slight increase in laser energy reveals a shift of saturation to As in airborne particulate matter by ICP-MS measurements.higher concentrations but also increases the possibility of A correction for 40Ar35Cl+ interference with 75As can be incinerating a shallow crater. During the ablation process, the performed by ICP-MS measurement to obtain more reliable focused laser beam induces a plasma of ionized argon, sample data. Since chlorine has isotopes at both mass 35 and 37, the vapor and free electrons above the filter surface. In the extent of 40Ar35Cl interference could then be corrected by using Q-switched mode (at 240 ms delay) or in the higher energy freerunning mode, much of the energy is propagated to the nearby the following equation: region of the ablated surface, and affects the coupling efficiency i(75As)=i(75)-3.08i(77)+2.54i(82) (1) of the energy to the sample.Because the filter sample is generally less than 100 mm thick, an inappropriate energy adjustment may easily burn through the membrane. Therefore, a free-running operation mode with a laser beam energy at 140 mJ was selected.As depicted in Fig. 3, an excellent linear calibration curve was obtained with a mass loading of less than 2 mg, which is considered to be the limit of the loading capacity for collecting airborne particles of a PTFE-membrane filter. It was also found that the color of particulates deposited on the white filter membrane significantly influences the absorbed energy. The amount of ablated particulates depends not only on the laser energy setting but also on the defocused distance of the Fig. 4 Preparation of Teflon filter samples for the determination of samples. A large portion of filter area could be ablated by As by LA–ICP-MS and HPBD–ICP-MS. defocusing and rastering the laser beam over the sample surface. The integral intensities of As generally increase at a defocused distance of <6 mm, which may be attributed to the laser beam evaporating more particles on the filter. However, since only limited energy can be transferred with each laser setting, the amount of particulates ablated becomes limited at higher defocused distances.Therefore, a 5.5 mm defocused distance was selected for this investigation. Consequently, in view of the above mentioned advantages of rapidity and sensitivity, non-destructive LA–ICP-MS is considered an appropriate analytical technique for determining As and other elements in airborne particulate matter on membrane filter samples. Comparison of LA–ICP-MS and Conventional HPBD– ICP-MS methods Fig. 5 Comparison of As concentrations of various standard (+) and To examine the practicability of the proposed LA–ICP-MS real filter ($) samples (n=7) measured by LA–ICP-MS and HPBD– ICP-MS. method for real sample analysis, the HPBD–ICP-MS method Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 65Table 3 Quality assurance/quality control results for the determi- gation of As concentrations in airborne particulate samples nation of As in airborne particulate samples analyzed in the last 3 years collected in Taiwan.They indicate that the high-pressure bomb digestion method may be superior to the microwave digestion QA/QC parameter MD–ICP-MS HPBD–ICP-MS method. Table 3 also indicates that the blank values of As in SRM recovery (%) 107.4±10.5 98.7±8.2 PTFE-membrane and glass-fiber filters were 11.5±1.5 and Blank value (ng g-1) 11.5±1.5 66.0±64.0 66.0±64.0 ng g-1, respectively. Clearly, major factors affecting Relative deviation of 7.8 2.6 the accurate and precise determination of As can only be repeated analysis (%) attributed to the sensitivity of the selected analytical instrument.Restated, the applicabilities of the analytical methods depend mainly on the elemental content to be determined in where i(X) represents the integral counts at mass X and 3.08 airborne particulate matters. is the isotopic ratio of 35Cl/37Cl. Because of the existence of Our laboratory has previously employed several analytical appreciable concentrations of selenium in the airborne par- techniques, such as XRF, INAA, AAS, ICP-AES and ICP-MS, ticles, further correction for Se at mass 77 should be considered.to analyze airborne particulate matter collected on membrane The coefficient 2.54 is therefore the product of isotopic ratios, filters. A general comparison of the various techniques for As 35Cl/37Cl and 77Se/82Se. However, Nixon and co-workers23,24 determination was therefore performed and the results are mentioned that these isobaric fractionation (IBF) methods given in Table 4.The detection limits and other key elements may consistently underestimate the 40Ar35Cl interference and of the methods were compared to examine their capabilities in potentially give rise to a large error in determining As in real sample analysis. biological materials. ICP-MS in combination with the proposed two-step diges- The above problems can also be easily resolved by applying tion method is certainly the most sensitive technique for the LA–ICP-MS.This method allows direct sample analysis and determination of As in airborne particulate matter. It offers therefore introduces little dilution of the sample. Eliminating the added advantage of determining many other elements sample dissolution shortens the sample analysis time markedly. within the same digestion procedure. ETAAS may be an apt Contamination from reagents is minimized since sample hand- choice for determining As, but only if As is the single element ling is minimal.In addition, diminishing the levels of H2O to be determined. XRF, ICP-AES and INAA should be ruled entering the plasma reduces the levels of interfering polyatomic out either because of their insensitivity or their labor- and ions and allows for the low level determination of As. time-consuming procedures. In terms of convenience, rapidity Consequently, LA–ICP-MS is appropriate for determining As and sensitivity, LA–ICP-MS has decided advantages over the in airborne particulate matter collected on thin membrane other methods.Fig. 6 depicts the frequency distribution of the filters (approximately 100 mm thick). As content in the atmosphere of Taiwan from 1991 to 1993. It is estimated from the frequency curve that nearly 50% of the Comparison of the Determination of As in Airborne Samples by samples register As concentrations of less than 0.002 mg m-3 Various Methods [in total suspended particulate (TSP) <100 mg m-3 conditions], and 20% less than 0.001 mg m-3 (TSP<50 mg m-3).In real sample analysis, both glass-fiber filters and PTFE- LA–ICP-MS is recommended for the accurate As determi- membrane filters were employed to collect suspended particu- nation of airborne particulate matter in real sample analysis. lates. This aspect was reported in a previous paper which mentioned that the blank contribution for As from filters and also reagents could be negligible.6 Table 3 gives the quality The authors thank the National Science Council, Republic of China for financially supporting this research under Contract control results obtained in a recent several year long investi- Table 4 Comparison of techniques for As determination in airborne particulate matter* Detection Technique Type of analysis Analysis time limit/mg g-1 Cost Advantages Limitations Atomization of FAAS ~10 h 2† Medium High sensitivity Single-element analyte Low interferences analysis solution after High chemical loss digestion ICP-AES Excitation of ~10 h 350† Medium Low interferences Poor sensitivity analyte High chemical loss solution after digestion ICP-MS Excitation of ~10 h 0.4† High High sensitivity High interference analyte High chemical loss solution after digestion LA–ICP-MS Direct ~10 min 3.2 High High sensitivity Inhomogeneity evaporation Low interferences Poor reproducibility and/or Excellent mapping excitation by laser INAA Direct activation 2–3 d 400 High High sensitivity Time consuming by neutron Low interferences Poor sensitivity source XRF Direct excitation ~10 min — Medium Qualitative Very poor sensitivity by X-ray screening source * A 12 h particulate loading of 1 mg per filter on the PTFE-membrane filter was assumed.† The digested solution, which contains less than 0.1% dissolved particulate material, was diluted to 10 ml for spectrometric measurement. 66 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 126 Wang, C. F., Yang, J. Y., and Ke, C. H., Anal. Chim. Acta, 1996, 320, 207. 7 Kuss, H.-M., Fresenius’ J. Anal. Chem., 1992, 343, 788. 8 Kawaguchi, H., Tanaka, T., Nakamura, T., Morishita, M., and Mizuike, A., Anal. Sci., 1987, 3, 205. 9 Olivares, J. A., and Houk, R. S., Anal. Chem., 1986, 58, 20. 10 Larsen, E. H., and Sturup, S., J. Anal. At. Spectrom., 1994, 9, 1099. 11 Anderson, S.T. G., Robe�rt, R. V. D., and Farrer, H. N., J. Anal. At. Spectrom., 1994, 9, 1107. 12 Coedo, A. G., and Dorado, M. T., J. Anal. At. Spectrom., 1994, 9, 1111. 13 Cleland, S. L., Olson, L. K., Caruso, J. A., and Carey, J. M., J. Anal. At. Spectrom., 1994, 9, 975. 14 Jiang, S. J., Lu, P. L., and Huang, M. F., J. Chin. Chem. Soc., 1994, 41, 139. 15 Sheppard, B. S., Heitkemper, D. T., and Gaston, C .M., Analyst, 1994, 119, 1683. 16 Ebdon, L., Fisher, A. S., and Worsfold, P.J., J. Anal. At. Spectrom., 1994, 9, 611. 17 Branch,., Ebdon, L., and O’Neill, P., J. Anal. At. Spectrom., Fig. 6 Frequency distribution of As content in the atmosphere of 1994, 9, 33. Taiwan from 1991 to 1993 (n=566). 18 Branch, S., Ebdon, L., Ford, M., Foulkes, M., and O’Neill, P., J. Anal. At. Spectrom., 1991, 6, 151. Number NSC84–2621-M007–008EA. Professor M. H. Yang, 19 La�sztity, A., Krushevska, A., Kotrebai, M., Barnes, R. M., and Department of Nuclear Science, National Tsing Hua Amarasiriwardena, D., J. Anal. At. Spectrom., 1995, 10, 505. University, and Professor P. C. Chiang, Institute of 20 Campbell, M. J., Demesmay, C., and Olle�, M., J. Anal. At. Spectrom., 1994, 9, 1379. Environmental Engineering, National Taiwan University, are 21 Imai, N., Anal. Chim. Acta, 1989, 235, 381. appreciated for their valuable help and discussions. The authors 22 Nakahara, T., Spectrochim. Acta Rev., 1991, 14, 95. also gratefully acknowledge the assistance with the ICP-MS 23 Nixon, D. E., and Moyer, T. P., Spectrochim Acta, Part B, 1996, measurements given by Mr. W. F. Chang of the National 51, 13. Science Council’s Regional Instrument Center at Hsinchu. 24 Kershisni, N. M., Kalamegham, R., Ash, K. O, Nixon, D. E., and Ashwood, E. R., Clin. Chem., 1992, 28, 2197. 25 Durrant, S. F., and Ward, N. I., Food Chem., 1994, 49, 317. REFERENCES 26 Maher, W. A., Chem. Geol., 1984, 45, 173. 27 Bajo, S., Anal. Chem., 1978, 50, 649. 1 T he Annual Assessment Report of the Air Pollution Control in 28 Stock, L. W., Fresenius’ Z. Anal. Chem., 1934, 99, 321. T aiwan Area for 1994, Environmental Protection Agency, 29 Portmann, J. E., and Riley, J. P., Anal. Chim. Acta, 1964, 31, 509. Taiwan, 1995. 30 Lounamaa, K., Fresenius’ Z. Anal. Chem., 1955, 146, 422. 2 NIOSH, Manual of Analytical Methods, National Institute of 31 Sulcek, Z., and Povondra, P., Methods of Decomposition in Occupational Health and Safety, Washington, DC, 1979. Inorganic Analysis, CRC Press, Boca Raton, FL, 1989. 3 Wang, C. F., Miau, T. T., Perng, J. Y., Yeh, S. J., Chiang, P. C., Tsai, H. T., and Yang, M. H., Analyst, 1989, 114, 1067. 4 Wang, C. F., Chen, W. H., Yang, M. H., and Chiang, P. C., Paper 6/05658J Analyst, 1995, 120, 1681. Received August 13, 1996 5 Wang, C. F., Chang, E. E., Chiang, P. C., and Aras, N. K., Analyst, 1995, 120, 2521. Accepted September 24, 1996 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 1

ISSN:0267-9477    

DOI:10.1039/a605658j

出版商:RSC    

年代:1997

数据来源: RSC

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Simultaneous Determination of Aluminium, Titanium and Vanadium in Serum by Electrothermal Vaporization– Inductively Coupled Plasma Mass Spectrometry LIJIAN YU*a, S. ROY KOIRTYOHANN†a, MELVIN L. RUEPPELa, ANASTASIA K. SKIPORb AND JOSHUA J. JACOBSb aEnvironmental T race Substances L aboratory, Center for Environmental Science and T echnology, University ofMissouri, Rolla,MO 65409, USA bDepartment of Orthopedic Surgery, Rush Presbyterian St. L uke’sMedical Center, Chicago, IL 60612, USA The systemic distribution of corrosion and wear products from With Al, Ti and V concentrations ranging from sub-ppb to tens of ppb in biological fluids, the choice of analytical total joint replacement devices in the human body needs to be better understood.This requires the measurement of metal techniques is limited. NAA, ETAAS and ICP-MS have all been used extensively in medical and biological research. ions, especially Al, Ti and V, at ppb and sub-ppb levels in serum and other organic matrices.Both ETAAS and NAA A large amount of sample (about 10 ml) is needed for NAA, and a pre-irradiation separation is necessary to alleviate inter- have been reported for determining some of the elements, but neither is completely suitable for determining all three ferences from the serum matrix.6 The pre-treatment is not only time consuming but also, more importantly, it greatly increases elements in serum at the levels encountered in control subjects and in patients with well-functioning devices.A method is the chance of contamination. NAA is expensive. It requires access to a nuclear reactor, and the disposal of irradiated described for the simultaneous determination of Al, Ti and V in serum using ETV–ICP-MS. Spectral interferences from the samples becomes an environmental concern when dealing with large numbers of samples. Finally, the analytical cycle time serum matrix were circumvented or alleviated by the careful selection of analytical masses, chemical modifiers and the may exceed 4–6 weeks.ETAAS is widely used for trace element analysis.7–9 In temperature programme of the electrothermal vaporizer. The serum matrix was found to influence the transport of analytes addition to the excellent sensitivity achievable with the technique, it has the advantage of requiring only about 10–50 ml (carrier effect) from the furnace to the plasma. The carrier effect was corrected by using major component matrix of sample.With careful selection of chemical modifiers and temperature programmes, matrix effects can be minimized or matching with internal standardization, and good recovery was obtained. Detection limits of 0.7, 0.4 and 0.1 ppb in serum alleviated, and analyses can be performed without sample pretreatment. The technique has been successfully applied to the were obtained for Al, Ti and V, respectively. NIST SRM 1598 Bovine Serum and a pooled human serum were analysed. The determination of Al, Ti and V in biological fluids.Nevertheless, the reported detection limits for Ti and V are above concen- Al concentration found was 3.8±0.8 ppb while the V concentration was below the detection limit of 0.1 ppb, trations encountered in normal (unimplanted) individuals as well as in many patients with well-functioning devices. The compared with the certified value for Al of 3.7±0.9 ppb and the information value for V of 0.06 ppb (no value is given for high atomization temperatures necessary for Ti and V, owing Ti in SRM 1598).The Ti value obtained for the pooled human to their refractory nature, lead to an unacceptably short tube serum sample was within the concentration range previously life (about 50 firings9). The technique is further complicated reported for normal human serum. by a build up of carbon residue in the graphite tube for biological samples such as serum. The residue has to be Keywords: Electrothermal vaporization; inductively coupled removed periodically in order to maintain the desired repro- plasma mass spectrometry ; chemical modifier; matrix ducibility.8 Finally, ETAAS is basically a single element tech- matching; internal standard nique.The resulting low throughput is unsatisfactory for the large numbers of samples generated in medical research. Developed in the last three decades, total joint replacement ICP-MS with pneumatic nebulization generally offers better arthroplasty (TJA) has physically enabled and liberated thou- detection limits than ETAAS for Al, Ti and V in simple sands from debilitating hip and knee diseases or injuries.For matrices. However, routine sample digestion is necessary, and most patients, the short and intermediate term (2–7 year) the large amounts of Ca, C and Cl in biological fluids render results have been satisfactory. On the other hand, there is an it extremely difficult to determine low concentrations of Ti increasing recognition that, in the long term, TJA might be and V without prior separation of the interfering elements associated with adverse local and remote tissue responses, and from the sample matrix.For trace element analysis, each that these effects may be mediated by the degradation products sample manipulation step introduces the possibility of conof prosthetic materials.1,2 The most commonly used titanium- tamination. The increased likelihood of contamination can based alloy in TJA devices is Ti6Al4V.The toxicity of Ti, Al ultimately undermine the robustness of the analytical and V has been studied.3–5 With the introduction of newer methodology; therefore, sample manipulation needs to be kept prosthetic designs that are intended for use in younger more to a minimum. active individuals, it has become more imperative to under- ICP-MS equipped with an ETV sample introduction system stand the wear and corrosion of TJA devices in patients. offers several advantages over nebulization ICP-MS for the determination of Al, Ti and V in serum.First and foremost, sample digestion can be avoided. Secondly, by careful selection † Present address: Department of Chemistry, University of Missouri, Columbia, MO 65211, USA. of instrument parameters, interfering species of the matrix may Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (69–74) 69be removed inside the instrument; therefore, Al, Ti and V may washed poly(propylene) tubes and frozen at about -80 °C until needed.be determined without external chemical separation from the sample matrix. Thirdly, ETV–ICP-MS offers about an order of magnitude lower detection limit than pneumatic nebuliz- Procedure ation ICP-MS. The higher sensitivity offered by ETV–ICP-MS There are several sources of potential interference from the makes it possible to use less interference prone, albeit less serum matrix. These include the organic material (containing abundant, isotopes of analytes to achieve the desired limit of carbon), major inorganic components and trace elements with detection.Finally, ETV–ICP-MS is capable of multi-element isobars at analytical masses. A 10% v/v solution of Triton analysis using only a few microlitres of sample, an important X-100 in water was used as a sample to study the interferences asset to clinical studies. These characteristics of ETV–ICP-MS caused by carbon. Counts at masses 24, 25, 26, 27, 28, 47, 48, make it the most suitable technique for these studies. 49, 51 and 52 were measured. For studies of interferences from inorganic components, EXPERIMENTAL three series of solutions were prepared. Solutions of P at 3, 6, 13, 25 and 50 ppm were prepared from ammonium phosphate Instrumentation and used to study the interference caused by PO+ at Ti masses. A Perkin-Elmer SCIEX ELAN (Thornhill, Ontario, Canada) Chloride in the form of NaCl at 240, 480, 960, 1900 and Model 5000 ICP-MS instrument was used.The electrothermal 3900 ppm was used to study the chloride related interferences vaporizer was a Perkin-Elmer (Norwalk, CT, USA) at Ti and V masses. Magnesium as its nitrate at 1.2, 2.5, 5, 10, HGA-600MS equipped with an AS-60 autosampler. The elec- 16, 20, 31, 63, 130 and 250 ppm was used to study the Mg2+ trothermal vaporizer was connected to the base of the plasma interference at Ti and V masses. All the solutions were prepared torch with a 1.5 m×6 mm id PTFE tube.Pyrolytic graphite in 0.1% v/v nitric acid, and 10 ppb of Rh was added to all the tubes from Perkin-Elmer were used for all experiments. A solutions as an internal standard to correct for any differences Clestra Cleanroom Components (Syracuse, NY, USA) Super II in the efficiency of transport to the plasma. Solutions were Model 771031–595 filter, capable of providing Class 10 clean analysed as samples, and counts at masses 47, 48, 49, 51 and air, was mounted above the electrothermal vaporizer and 103 were measured.No chemical modifier was used for autosampler. A plastic curtain was attached to all four sides interference studies. of the filter so that the sampler tray was under the protection For sample analysis, a solution containing 0.1% m/v of a laminar downflow of filtered air. Optimization of ion-lens NH4NO3 and 50 ppm Mg as its nitrate was used as chemical settings was accomplished by using a pneumatic nebulizer for modifier.Spike solutions containing 0, 10, 20, 30 and 40 ppb the introduction of standard solutions. Instrumental conditions of Al, Ti and V were prepared in 0.1% v/v nitric acid with 10 are listed in Table 1. ppb Rh as an internal standard. Samples and spikes were Reagents prepared by diluting the pooled serum (1+1) with correspond- De-ionized water (18 MV) was prepared by passing distilled ing spike solutions in autosampler cups. Standards were pre- water through a Barnstead (Dubuque, IA, USA) NANOpure II pared by diluting a 100 ppm Ca solution (1+1) with de-ionized water system. All stock standard solutions were corresponding spike solutions in autosampler cups.The auto- from High Purity Standards (Charleston, SC, USA) except for sampler cups were covered with Parafilm (Fisher Scientific) P, Na and Ca Plasma Emission Standards, which were from and rigorously shaken. A 20 ml aliquot of sample or standard Solutions Plus (Fenton, MO, USA).Electrophoresis-grade plus 10 ml of chemical modifier were injected into the furnace Triton X-100 and Optima-grade acids were from Fisher for each analysis. Scientific (Pittsburgh, PA, USA). The pooled human serum HypoClean 100 latex gloves (Fisher Scientific) were worn sample used was produced by mixing about 2.5 ml of serum throughout sample preparation. All solutions were prepared collected from each of 12 individuals. After thorough mixing, on an Envirco (Albuquerque, NM, USA) Model LS 630 the pooled sample was transferred in 1.5 ml aliquots into acid- laminar flow clean bench and then stored in high-density polyethylene bottles (Wheaton, Millville, NJ, USA).All pipette tips, autosampler cups and plastic bottles were cleaned by Table 1 Instrumental operating conditions and data acquisition soaking in 10% v/v sub-boiling distilled nitric acid for at least parameters 24 h. They were rinsed with de-ionized water immediately before use.Inductively coupled plasma mass spectrometer: Rf power: 1000 W Plasma flow rate: 15.0 l min-1 Auxiliary flow rate: 800 ml min-1 RESULTS AND DISCUSSION Carrier flow rate: 1150 ml min-1 Analytical Masses Electrothermal vaporizer: Among the three elements, only Al is monoisotopic. Vanadium Sample volume: 20 ml has two isotopes at masses 50 (0.25%) and 51 (99.75%). The Modifier volume: 10 ml low abundance of 50V renders this isotope of little use. Internal Ar flow: 300 ml min-1 before vaporization step Therefore, 27Al and 51V had to be used.Titanium has five stable isotopes; of those, 46Ti (8.0%), 48Ti (73.8%) and 50Ti ET V temperature: Dry (ramp, hold): 100 °C (10 s, 30 s) (5.4%) are isobars of 46Ca (0.001%), 48Ca (0.187%) and 50Cr 200 °C (30 s, 10 s) (4.35%). The average content of Ca in serum (Table 2) is about Char (ramp, hold): 800 °C (30 s, 20 s) 93 ppm, which would be expected to produce a signal equival- 1400 °C (20 s, 20 s) ent to about 60 and 300 ppb of Ti at masses 46 and 48, Vaporization (ramp, hold): 2650 °C (0 s, 5 s) respectively.These concentrations are much higher than the Purge (ramp, hold): 2650 °C (1 s, 2 s) normal Ti level of about 3 ppb in serum.8 Therefore, masses Data acquisition: 46 and 48 could not be used. Similarly, the normal Cr content Dwell time: 20 ms in serum (Table 2) will give rise to an equivalent concentration Scan mode: peak hop transient of about 0.2 ppb Ti as an additive interference at mass 50.Points per spectral peak: 1 Since Cr concentrations as high as 180 ppb have been Signal measurement: integrated signal pulse reported14 in serum, a signal equivalent to about 130 ppb of 70 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 2 Elemental composition of normal human serum minimum Mg, Al, Ti and Cr content was sought as a substitute for the serum matrix. Care was taken in choosing the reagent Element Concentration/mg ml-1 Ref.so that it did not completely evaporate prematurely but charred Na 3200 10 in a manner similar to the serum matrix. A solution of 10% Mg 21 11 Triton X-100, which was sufficiently free of inorganic impurit- Ca 93 10 ies, was a reasonable choice for this purpose. Counts at masses K 170 10 24 and 48 were measured as a check on any Mg and Ti P 34 10 impurities in the Triton X-100 solution. A run with chemical Cl 3700 10 Cr <0.03–0.25* 12, 13 modifier only (dry run) was performed to measure the carbon Ti 3.3* 8 related interferences from the furnace itself.Counts collected at relevant masses are summarized in Table 4. Note that counts * Values in ng ml-1. of Triton X-100 solution at masses 26 and 52 were within 1 standard deviation of that of a dry run at each respective mass, Ti at mass 50 could result. Mass 50 is, therefore, not a good and vice versa. This indicates that CN+ and ArC+ resulting choice either. This leaves 47Ti (7.3%) and 49Ti (5.5%) as viable from the organic matrix were statistically insignificant com- analytical isotopes.Mass 49 was chosen for reasons to be pared with that from the electrothermal vaporizer itself. As discussed later. discussed earlier, the species from ETV would not cause an additive interference to blank-subtracted data. The upper limits Interferences of noises caused by 13C14N+ and 36Ar13C+ at masses 27 and 49 were estimated. In the worst CN+ case, assuming that Major background features of ICP-MS with nebulizer and counts at mass 26 were largely from 12C14N+, the magnitude ETV sample introduction systems have been studied by Horlick of the 13C14N+ contribution to the standard deviation at mass and co-workers15,16 and by Gregoire and Sturgeon.17 27 would be 5 counts or 0.5 ppt with a 27Al+ sensitivity of 104 Compared with nebulization, ETV–ICP-MS exhibited a back- counts per ppb.This is negligible compared with the detection ground with decreased counts due to oxide ions and increased limit of Al to be discussed later.Similarly, assuming that the levels of carbide ions. Gregoire and Sturgeon17 also showed counts at mass 52 were mainly from 40Ar12C+, the standard that the count rate for the carbon dimer was low deviation resulting from 36Ar13C+ was 2 counts or 2 ppt with (300 counts s-1) and increased only slightly with tempera- a 49Ti+ sensitivity of 1200 counts per ppb. Again, this is a ture. Molecular ions involving three or more carbon atoms negligible amount compared with the detection limit of Ti to were not observed.It was suggested that species with multiple be discussed below. Carbon from the serum matrix will not, carbon atoms were very reactive and therefore scavenged by therefore, interfere with Al, Ti and V determination. the entrained ambient oxygen. The carbon related background Interferences from Cl, P and Mg in the form of CCl+, ClO+, was, therefore, composed of molecular ions containing a single ClN+, PO+ and Mg2+ were studied by analysing a series of carbon atom.Considering the serum matrix specifically, all the concentrations of interfering elements as samples. The ranges likely isobaric interferences for Al, Ti and V are listed in of the concentrations were chosen to bracket the normal Table 3. They can be classified as C, Mg, Cl or P related concentrations of Mg, Cl and P in human serum. Instrument interferences. Interferences resulting from CCl+ will be parameters listed in Table 1 were used.The correlations discussed with Cl related interferences. between the counts at masses of interest and the concentration There are two carbon sources in serum analysis. One is the of interfering species are included in Table 5. The weak corre- electrothermal vaporizer furnace itself and the other the organic lation at masses 47, 49 and 51 suggests that counts from matrix of serum samples. Carbon from the electrothermal 12C35Cl+, 12C37Cl+, 35Cl14N+, 37Cl14N+ and 35Cl16O+ species vaporizer is mainly a result of increased vaporization of are indistinguishable from background noise.Similarly, the graphite at high temperatures.17 Since the same temperature weak correlation at mass 48 excluded the possibility of Mg2+ programme was used here for an analysis, the amount of interferences. For P, the strong correlation at mass 47 suggested carbon from the electrothermal vaporizer was approximately the presence of 31P16O+ at mass 47.A strong correlation was constant, which would affect every reading to nearly the same not observed at mass 49 at the concentrations used. extent. This additive interference to a sample was cancelled Presumably, the counts due to 31P18O+ were below the out when the blank was subtracted. The only adverse effect of background fluctuation owing to the low abundance of 18O. the interference was an elevated level of baseline fluctuation or white noise.Carbon from the serum matrix would only be present in readings from serum samples. Carbon related mol- Table 4 Effect of polyatomic carbon interferences (raw counts) at Al, ecular ions from this source may cause additive interferences Ti and Cr masses* that cannot be easily corrected for in sample analysis. Triton X-100 Dry run The potential carbon related interferences are 13C14N+ on 27Al+ and 36Ar13C+ on 49Ti+. The extent of these interferences 24Mg (×103) 120±3 92±10 can be estimated by the intensity at mass 26 (12C14N+) and at 26Mg (×103) 110±0.3 110±0.5 27Al (×103 ) 120±5 48±2 mass 52 (40Ar12C+), respectively.Serum cannot be used to 48Ti (×103) 28±6 29±9 study the interferences because of the high concentration of 49Ti (×102) 16±2 16±2 Mg which has an isotope at mass 26. An organic reagent with 52Cr (×104) 94±3 94±7 Table 3 Abundance of interfering species at Al, Ti and V masses (%) * For the dry run, n=7 for Mg and Al, n=20 for Ti and Cr.For Triton X-100, n=2 for Mg and Al, n=10 for Ti and Cr. 27Al (100) 47Ti (7.3) 49Ti (5.5) 51V (99.8) CN+ 1.1 Table 5 Correlation coefficient at Ti and V masses ArC+ 0.0037 Mg2+ 16.1 2.26 Mass 47 48 49 51 CCl+ 75.0 23.9 ClN+ 75.0 24.1 Cl 0.44 -0.12 -0.48 P 0.98 -0.49 PO+ 99.8 0.2 ClO+ 75.65 Mg -0.04 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 71Because of this PO+ interference, mass 49 was chosen over programme in Table 1. Intensities of Al, Ti and V were measured in solutions of the three analytes with various mass 47 for Ti determination.amounts of Mg as its nitrate (Fig. 3). Intensities for all three analytes increased initially with increasing amounts of Mg Pyrolysis Temperature until a maximum was reached at about 0.16 mg of added Mg, There are about 3700 ppm of Cl and about 34 ppm of P in after which they started to decrease. The increase in the Ti serum, the molecular ions of which, viz., ClO+, CCl+, ClN+ and V signals at up to 0.16 mg of Mg was probably caused by and PO+, may interfere with analyses at masses 47, 49 and the carrier effect of the chemical modifier, since no loss of 51.With ETV–ICP-MS, sample vaporization and ionization either element was observed at the experimental pyrolysis become independent of each other; hence, the relative volatility temperature. The increase in the Al signal in this region was of each component in a sample can be explored to facilitate a largely due to the stabilizing effect of Mg(NO3)2 on Al at high separation of analytes from the interfering species in the matrix pyrolysis temperatures in addition to the carrier effect.The by way of temperature programming. The volatility of Cl and signal intensity decreased for all three elements with increasing P was studied by measuring the counts of a 3900 ppm chloride amounts of Mg when more than 0.16 mg of Mg was added. solution and a 500 ppm P solution at mass 47 at various This phenomenon was similar to that observed by Ediger and pyrolysis temperatures.The pyrolysis curves for Cl and P Beres.19 This trend of decreasing analyte intensities with showed (Fig. 1) that ClO and PO signals decreased sharply increasing amounts of Mg can be attributed to the transport with increased pyrolysis temperature until a point was reached loss due to matrix particle coalescence or space-charge effects. beyond which no appreciable decrease in ClO and PO signals With the use of 0.5 mg of Mg as a chemical modifier, the loss was observed with increased temperature.This occurred at of Al was prevented at temperatures below 1600 °C (Fig. 2). about 1200 °C for ClO and at about 800 °C for PO. Note that Ammonium nitrate was also incorporated in the chemical the ClO curve did not reach zero at 1400 °C: this was probably modifier to facilitate the removal of chloride. A pyrolysis caused by trace amounts of Ti contaminant in the NaCl used temperature of 1400 °C was therefore chosen.for the experiment. The PO curve also did not reach zero at A carbon deposit was observed when serum samples were 1400 °C, probably because of the incomplete removal of P analysed. Significant carbon build-up was observed when about which was subsequently released at the vaporization tempera- ten serum samples were run consecutively. Although there was ture of 2650 °C. These arguments are supported by the results no noticeable degradation in the analyte signal, the large amount from the interference studies discussed earlier.of carbon build-up did eventually cause difficulty in sample The volatility of the analytes was then studied to maximize introduction. The deposit could be eliminated by introducing the removal of interfering species and to minimize the loss of air as an alternate gas for about 10 s at 800°C; however, the the analytes. No loss of Al, Ti or V was observed in serum at life of the graphite tube was reduced by about 50%, to about pyrolysis temperatures below 1600 °C, but Al from aqueous 150 analytically useful firings.More than 100 firings at the standards was lost at temperatures higher than 1000 °C (Fig. 2). vaporization temperatures were necessary before Al, Ti and V It became apparent when the matrix of serum was compared in a new graphite tube decreased to levels that were acceptable against that of a standard that Mg, often used as a universal for sample analysis.A shortened tube life translates into an chemical modifier in ETAAS,18 was present in the serum matrix increased percentage of time spent on conditioning the tube but not in that of the standard. The presence of Mg in the and a decreased efficiency. Pyrolysis with air was, therefore, not serum probably acted as an agent that delayed the vaporization used. By running a 0.1% nitric acid solution between samples, of Al until a higher temperature was reached. The optimum the carbon build-up in the graphite furnace was reduced to an amount of Mg as a modifier was studied by using the sampling extent that it did not affect analyses for the lifespan of the tube.Carry-over of refractory elements between runs, which was observed in this work, was also minimized by running a dilute (0.1%) nitric acid solution between samples. Recovery and Detection Limit Electrothermal vaporizer sampling for ICP-MS introduces new variables associated with sample vaporization and transportation into the analytical equation.More often than not, the transport efficiency of samples differs from that of standards, resulting in poor analytical recovery. Chemical modifiers have frequently been used to alleviate chemical interferences and to enhance the transport efficiency of analytes.19,20 For samples with simple matrices, the use of a Fig. 1 Effect of pyrolysis temperature on P and Cl related interferences. A, PO and B, CCl. chemical modifier frequently masks the differences in transport Fig. 2 Pyrolysis curves of 20 ppb Al standards and a 20 ppb Al serum Fig. 3 Effect of magnesium nitrate on A, Al; B, Ti; and C, V signal. spike. A, 0.5 mg Mg; B, 50% serum; and C, 0.1% HNO3 . 72 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12efficiency between samples and standards so that uniform the analyte. This explains the greater loss of Al among the three analytes of interest. The poor result with internal stan- transport efficiency is achieved.Calibration against external aqueous standards has become feasible.21,22 For complex dardization probably originates from vapor loss also, because a single internal standard cannot approximate the volatility samples, a modifier alone may not be adequate. Some researchers have used standard additions21,23,24 to correct for the and hence the vapor loss of three analytes simultaneously. The loss of analyte vapor in the transport process can be matrix effects while others have used isotope ratio25,26 methods to circumvent them.Isotope ratio and isotope dilution are by minimized by the introduction of foreign particles (a physical carrier), which provide surfaces to compete with the tube wall far the preferred methods because not only are matrix problems solved, but also the effects of firing-to-firing variation of the for vapor condensation. With the use of maximum power heating for ETV, the heating rate generally exceeds electrothermal vaporizer26 are minimized.Although isotope ratio-based methods apply only to elements with at least two 1000°C s-1. At such fast heating rates, even analytes with different volatilities evaporate almost simultaneously. The sim- isotopes, the principle and some of the advantages of the methods can be extended to the determination of monoisotopic ultaneous evaporation of all analytes makes it possible for a uniform condensation of the analytes on the physical carrier.and other elements by the internal standardization technique. Serum has a complex matrix consisting of about 10% of The subsequent loss of a portion of carrier particles in the transport process represents a proportional loss of all analytes, organic components and over 1% of inorganic salts. In order to investigate the effect of the serum matrix on analytical and hence the ratio of analytes remains constant. This loss is therefore corrected for by an internal standard.Therefore, figures of merit, recovery studies were performed. A chemical modifier of 0.5 mg of Mg and 10 mg of ammonium nitrate was with the vapor loss minimized, the results for internal standardization are likely to improve. used for all measurements. Five-point calibration graphs and spike curves with analyte concentrations ranging from 0 to The cause for the high transport efficiency of the serum matrix was investigated. Al, Ti and V are refractory elements 40 ppb, run in duplicate, were constructed.Recoveries were calculated by dividing the slope of a spike curve by that of a that have high vaporization temperatures; therefore, refractory components in the serum matrix are most likely to evaporate calibration graph. Direct calibration of serum samples against aqueous standards resulted in apparent recoveries of over with the analytes at high temperatures and to act as physical carriers for the analytes. An examination of the elemental 190% (first row of Table 6) for the three elements.This high recovery suggested a higher transport efficiency from serum composition of the serum matrix was carried out (Table 2). With a melting- and boiling-point of 2580 and 2850°C, respect- than from aqueous standards. In order to account for the discrepancy in transport efficiency, internal standardization ively, CaO was considered to be the most likely component responsible for the high transport efficiency of analytes in serum.was considered. Ideally, the chemical and physical properties of an internal standard should closely match those of the A unified approach towards the selection of a chemical modifier for ETV was studied by Hughes et al.29 They found analyte, and therefore three internal standards were needed for Al, Ti and V. In order to avoid severe signal distortion of HCl to be an effective carrier. Consequently, they suggested that an ‘ideal’ chemical modifier could be made by incorporat- transient ETV signals, a maximum of four elements is recommended27 for simultaneous determination, and, therefore, ing in the chemical modifier various chlorides that hydrolyse and release HCl at various temperatures. The effectiveness of the number of internal standards was reduced to one.Rhodium was chosen as a viable compromise internal standard for all the chloride modifier was demonstrated29 with NASS-3, a seawater reference material, in the determination of ten elements three analytes and 0.1 ng of Rh was added to each sample and standard.This improved recoveries slightly, but they were still with volatilities ranging from volatile (Cd) to involatile (Co). There are several reasons why a chloride modifier would poor (second row of Table 6). According to Kantor,28 efficient transport of analyte species not be a wise choice for the determination of Al, Ti and V. The first is that chlorine is the element to avoid in the occurs when stable nuclei are formed, and the formation of these nuclei depends on the magnitude of supersaturation of determination of V because of the ClO+ interference at mass 51.Secondly, Al, Ti and V are refractory elements. There are the species under consideration. After the analyte has been vaporized, it starts to travel downstream along the transport no known refractory chlorides that may co-volatilize with the analytes. Thirdly, a general purpose chemical modifier such as line.A portion of the analyte vapor is lost by condensation on the walls of the transport tubing, and therefore the transport NASS-3 is very effective for samples with varied matrices. Serum has a well defined and consistent matrix; therefore, a efficiency of the vapor is low. As the vapor travels further downstream, the temperature decreases to a point at which a simpler approach of major component matrix matching can be applied. In addition to the Rh internal standard, 50 ppm of critical degree of supersaturation of the vapor is reached.A high concentration of stable nuclei is formed from the vapor, Ca was added to each calibration standard and the recovery improved to 94% for Al and 100% for Ti and V (fourth row and the supersaturated vapor rapidly condenses on these nuclei. Since the transport efficiency of these fine particles is of Table 6). Matrix matching of Ca without the Rh internal standard resulted in recoveries of about 130% for Al and Ti, much higher than that of the vapor, transport loss is greatly reduced. After the formation of the fine particles, loss of analyte and about 120% for V (third row of Table 6).Theoretically, by matching each component in the serum matrix, a perfect is largely due to gravitational or streaming force separation of large particles formed from Brownian coagulation. recovery can be obtained without internal standardization. This was, however, neither possible nor necessary in the light Therefore, analyte loss in the transport process is attributed to vapor and particle losses.As regards the vapor loss, it of the good recoveries obtained with major component matrix matching combined with internal standardization. All of the follows that the greater the volatility the greater the loss of calibration graphs and spike curves were linear as indicated by the correlation coefficients (>0.99) shown in Table 7. Table 6 Recovery (%) of Al, Ti and V from serum samples The detection limit of the method was calculated as three times the standard deviation of 21 measurements of the three Relative to standards Al Ti V analytes in a zero calibration standard and in serum, respect- Containing 0.1% HNO3 330 210 190 ively.A reference material, NIST SRM 1598 Bovine Serum, Containing Rh 280 180 160 and a pooled human serum from Rush Presbyterian St. Luke’s Containing Ca 130 130 120 Medical Center were analysed. The data are listed in Table 8.Containing Ca and Rh 94 100 100 For the Bovine Serum the Al value is comparable to the Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 73Table 7 Correlation coefficients of calibration graphs and spike standard makes the ETV–ICP-MS measurement less suscep- curves* tible to variations of instrument sensitivity, which should reduce the variation of analytical results both within- and Al Ti V between-runs. This increases the homogeneity of the data Calibration 0.999 0.998 1.000 obtained at different times, thus allowing the study of metal Spike 1.000 1.000 1.000 concentration levels in individual patients as well as group comparisons. * Correlation coefficients were calculated with five-point curves.Studies are now underway to extend this analytical approach to other biological fluids including urine and cerebrospinal Table 8 Results and limit of detection (LOD) of serum samples fluids, as well as to tissue digests.The ease and accuracy of (ng ml-1). The stated intervals are at the 95% confidence level multi-element determinations by ETV–ICP-MS suggests that such techniques may become the standard practice for the Al Ti V determination of metals released from TJA. Pooled human serum (n=8) 5.5±0.4 1.3±0.3 0.3±0.1 The authors thank Dr. Jonathan Black for helpful discussions. SRM 1598 (n=16) 3.8±0.4 0.7±0.1 <0.1 Certified value for SRM 1598 3.7±0.9 NA* (0.06)† The support of Rush Presbyterian St.Luke’s Medical Center LOD in serum (n=21) 0.7 0.4 0.1 and the National Institute of Health through grant number LOD in a blank containing Ca (n=21) 0.3 0.4 0.08 AR39310 for this work is gratefully acknowledged. * NA=Not available. REFERENCES † NIST information value. 1 Jacobs, J. J., Gilbert, J. L., and Urban, R. M., Adv. Oper. Orthop., 1994, 2, 279. certified value, while the V value is below the detection limit 2 Jacobs, J. J., Shanbhag, A., Glant, T.T., Black, J., and Galante, of 0.1 ppb compared with the NIST information value of 0.06 J. O., J. Am. Acad. Orthop. Surg., 1994, 2, 212. 3 Williams, D. F., in Systemic Aspects of Biocompatibility, ed. ppb. A certified value of Ti is not available for the Bovine Williams, D. F., CRC Press, Boca Raton, FL, 1981, vol. 1, p. 169. Serum. The normal concentration of Ti in human serum has 4 Aluminum and Health, A Critical Review, ed. Gitelman, H. J., been reported to be 3.3 ppb with ETAAS detection.8 The value Marcel Dekker, New York, 1989.quoted was an average of 15 out of 21 measurements with 5 Jandhyala, B. S., and Hom, G. L., L ife Sci., 1983, 33, 1325. values above the detection limit of 2.1 ppb. Although the 6 Morris, S., University of Missouri Research Reactor, personal communication. average was the only piece of statistical information available 7 May, J. C., Rains, T. C., Yu, L., and Etz, N. M., Vox Sang., 1992, from the paper,8 a normal Ti range of <2.1–7.9 ppb was 62, 65.reported in a separate paper by the same research group.30 8 Skipor, A. K., Jacobs, J. J., Schavocky, J., Black, J., and Galante, The Ti value of this work is therefore comparable to the J. O., At. Spectrosc., 1994, 15, 131. normal Ti concentration reported in the literature. 9 Stroop, S. D., Helinek, G., and Greene, H. L., Clin. Chem., 1982, 28, 79. 10 Van Steirteghem, A. C., Robertson, E. A., and Young, D. S., Clin. CONCLUSIONS Chem., 1978, 24, 212.ETV–ICP-MS is one of the most sensitive techniques for trace 11 Williams, G. Z., Harris, E. K., and Widdowson, G. M., Clin. Chem., 1977, 23, 100. element analysis. The minimum amount of sample required 12 Skipor, A. K., Jacobs, J. J., Paprosky, W. P., Patterson, L. M., for an analysis and the multi-element analysis capability make Black, J., and Galante, J. O., Orthop. T rans., 1996, 21, 46. it an attractive analytical technique for clinical and biological 13 Brune, D., Altio, A., Nordberg, G., Vesterberg, O., and studies.By separating the sample vaporization from ionization, Gerhardsson, L., Scand. J. Work Environ. Health, 1993, 19, Suppl. ETV–ICP-MS provides the possibility of separating and 1, 39. 14 Versieck, J., and Cornelis, R., Anal. Chim. Acta., 1980, 116, 217. removing the interfering species in the matrix by means of 15 Tan, S. H., and Horlick, G., Appl. Spectrosc., 1986, 40, 445. chemical modification and temperature programming of the 16 Vaughan, M.A., and Horlick, G., Appl. Spectrosc., 1986, 40, 434. vaporization cell. Spectral interferences can therefore be mini- 17 Gregoire, D. C., and Sturgeon, R. E., Spectrochim. Acta, Part B, mized. On the other hand, owing to the extensive use of 1993, 48, 1347. graphite parts in the electrothermal vaporizer, carbon- 18 Slavin, W., Carnrick, G. R., and Manning, D. C., Anal. Chem., 1982, 54, 621. containing species appear as the main spectral feature in ETV– 19 Ediger, R.D., and Beres, S. A., Spectrochim. Acta, Part B, 1992, ICP-MS and can interfere with the analysis in some cases. 47, 907. Efficient and uniform transport of sample and standard from 20 Ediger, R. D., At. Absorpt. Newsl., 1975, 14, 127. the ETV cell to the ionization source is the key to the success 21 Santosa, S. J., Tanaka, S., and Yamanaka, K., Anal. L ett., 1995, of an analytical method. 28, 509. 22 Sturgeon, R. E., Willie, S. N., Zheng, J., Kudo, A., and Gre�goire, In this study, a method was developed to determine Al, Ti D. C., J. Anal. At. Spectrom., 1993, 8, 1053. and V in serum simultaneously with ETV–ICP-MS. Sample 23 Gre�goire, D. C., Goltz, D. M., Lamoureux, M. M., and preparation was kept to a minimum, and the risk of contami- Chakrabarti, C. L., J. Anal. At. Spectrom., 1994, 9, 919. nation was greatly reduced. Interferences from Cl and P in the 24 Richner, P., Evans, D., Wahrenberger, C., and Dietrich, V., serum matrix were eliminated by chemical modification and Fresenius’ J. Anal. Chem., 1994, 350, 235. 25 Whittaker, P. G., Lind, T., Williams, J. G., and Gray, A. L., careful selection of pyrolysis temperature. Poor recoverycaused Analyst, 1989, 114, 675. by the difference in transport efficiencies between serum and 26 Gre�goire, D. C., J. Anal. At. Spectrom., 1988, 3, 309. standards was successfully addressed with major component 27 Byrne, J. P., Gre�goire, D. C., Goltz, D. M., and Chakrabarti, matrix matching and internal standardization. It appeared that C. L., Spectrochim. Acta, Part B, 1994, 49, 433. the use of some form of physical carrier was critical for an 28 Kantor, T., Spectrochim. Acta, Part B, 1988, 43, 1299. 29 Hughes, D. M., Chakrabarti, C. L., Goltz, D. M., Gregoire, D. C., internal standardization technique to be successful, be it a Sturgeon, R. E., and Byrne, J. P., Spectrochim. Acta, Part B, 1995, chemical modifier or a matching matrix component of the 50, 425. samples. Sub-ppb level detection limits were attained. 30 Jacobs, J. J., Skipor, A. K., Black, J., Urban, R. M., and Galant, The procedure developed and reported here appears to be J. O., J. Bone Joint Surg., 1991, 73A, 1475. particularly well suited for use in long-term prospective studies, in which specimens obtained from patients at various times Paper 6/04797A must be analysed in different analytical campaigns over a Received July 8, 1996 Accepted September 13, 1996 period of years. The use of a physical carrier and an internal 74 Journal of Analytical Atomic Spectrometry, January 1997,

ISSN:0267-9477    

DOI:10.1039/a604797a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Copper, Cadmium and Lead in Biological Samples by Electrothermal Vaporization Isotope Dilution Inductively Coupled Plasma Mass Spectrometry CHAO-CHIANG CHANG AND SHIUH-JEN JIANG* Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, T aiwan 804, Republic of China Electrothermal vaporization isotope dilution inductively concentrations of Cu, Cd and Pb in several biological samples. The pyrolytic graphite platforms of the graphite tubes were coupled plasma mass spectrometry (ETV-ID-ICP-MS) was applied to the determination of Cu, Cd and Pb in several pretreated with palladium to delay the Cd vaporization by three different methods, thermal pretreatment, electroplating biological samples.The isotope ratios for each element in each analytical run were calculated from the peak areas of each and sputtering, and the analytical characteristics of these pretreatment methods were compared. The influences of non- isotope. Various chemical modifiers were tested to obtain the optimum signal of cadmium and palladium was selected.The spectroscopic and spectroscopic interferences resulting from the matrix on the accuracy of isotope ratio determination were pyrolytic graphite platforms of the graphite tubes were then pretreated with palladium by three different methods, thermal also investigated. The ETV-ID-ICP-MS method was applied to the determination of Cu, Cd and Pb in dogfish liver reference pretreatment, electroplating and sputtering, and after careful evaluation of the analytical performance of these methods, material DOLT-1, dogfish muscle reference material DORM-1 and freeze dried urine standard reference material NIST thermal pretreatment of the platform with Pd before each analysis was selected.The ETV-ID-ICP-MS method was SRM 2670. applied to the determination of Cu, Cd and Pb in dogfish liver reference material DOLT-1, dogfish muscle reference material EXPERIMENTAL DORM-1 and freeze-dried urine standard reference material NIST SRM 2670.The results agreed satisfactorily with the Apparatus and Conditions certified values. A Perkin-Elmer SCIEX (Thornhill, Ontario, Canada) ELAN Keywords: Electrothermal vaporization; chemical modifier; 5000 ICP-MS instrument equipped with an HGA-600MS isotope dilution; inductively coupled plasma mass spectrometry ; electrothermal vaporizer was used. Pyrolytic graphite coated copper; cadmium; lead; biological samples graphite tubes with platforms of the same material were used throughout.The transfer line consisted of 80 cm×6 mm id PTFE tubing. The sample was introduced with a Model AS-60 Most analyses by ICP-MS are carried out on solutions using autosampler. Teflon autosampler cups were used. The exper- a conventional pneumatic nebulizer. However, the type of imental conditions for ICP-MS and ETV are given in Tables analytical tasks that can be solved by ICP-MS can be extended 1 and 2, respectively.using a number of other sample introduction techniques which The ICP operating conditions were selected to maximize can be easily adapted to ICP-MS, e.g., electrothermal vaporiz- the sensitivity for the isotopes of interest to obtain the best ation (ETV) .1–15 Although ETV-ICP-MS has been applied to precision and accuracy for isotope ratio determination. The several types of sample analysis, it still suffers from analyte ICP conditions were selected to maximize ion signals while a loss during transportation and poor reproducibility problems.Chemical modifiers are commonly used in ETV-ICP-MS in Table 1. Equipment and operating conditions order to reduce losses of analyte due to condensation on different parts of the ETV cell or the transfer line that connects ICP mass spectrometer Perkin-Elmer SCIEX ELAN 5000 the ETV system to the ICP-MS set-up.1–7 This effect increases Plasma conditions— Outer gas flow rate 15.0 l min-1 the transport efficiency between the graphite furnace and the Intermediate gas flow rate 0.60 l min-1 ICP-MS torch.The improvements in analyte transport caused Carrier gas flow rate 1.08 l min-1 by physical carriers depend on the types and amounts of the Rf power 1100 W physical carriers that are used and also the type and concen- Sampler/skimmer Nickel tration of the analyte. Ediger and Beres5 suggested that, although most effects are believed to be physical, it is likely Mass spectrometer settings— Bessel box barrel +10.95 V that chemical effects could also be involved in specific instances. Bessel box plate lens -72.50 V Isotope dilution (ID) techniques have been applied in several Photon stop lens -10.05 V previous ICP-MS applications.16–27 Isotope dilution is well Einzel lenses 1 and 3 -0.04 V recognized as a definitive analytical technique for the determi- Resolution Normal, 0.7 u at 10% nation of trace elements.Since another isotope of the same signal maximum element represents the ideal internal standard for that element, Dwell time 10 ms Sweeps per reading 1 ID results are expected to be highly accurate even when the Readings per replicate 150 sample contains high concentrations of concomitant elements Number of replicate 1 and/or losses occur during sample preparation or transport Points per spectral peak 1 from the sample introduction device to the ICP.Scan mode Peak hopping In this study, ETV-ID-ICP-MS was used to determine the Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (75–80) 75Table 2. HGA-600MS temperature programmes Programme I, for sample analysis (sample volume 20 ml)— Step Drying Pyrolysis Cooling Vaporization Cooling Cleaning Cooling Temperature/°C 80 130 700 20 2300 20 2500 0 Ramp time/s 35 10 5 5 0 5 1 5 Hold time/s 10 10 20 5 5 5 10 5 Signal acquisition — — — — ON — — — Programme II, for modifier thermal pretreatment (modifier volume 10 ml)— Step 1 2 3 Temperature/°C 90 120 1200 Ramp time/s 5 5 5 Hold time/s 10 20 10 solution containing 10 ng ml-1 of Cu, Cd and Pb in 1% HNO3 electroplating and the composition of electrolytic bath solution are given in Table 3.After all electrodes had been fixed on a was continuously introduced with a conventional nebulizer. The sensitivity of the instrument sometimes varied slightly plastic holder, they were immersed carefully in a cylindrical glass cell containing 10 ml of electrolytic bath solution.To from day to day. The ICP operating conditions used throughout this work are summarized in Table 1. obtain good repeatability of the process for each electroplating, a fresh electrolytic bath solution was used each time. When Mass spectrometer parameters used for isotope ratio measurements are given in Table 1. The measurements were the electroplating cycle was finished, the platform was washed in a stream of pure water and dried at room temperature, then made by peak hopping rapidly from one mass to another, remaining for only a short time (dwell time) at each mass.For fixed in the graphite tube, slowly dried at about 80°C and then heated to 2000 °C. After this procedure, the modified the best accuracy and precision for isotope ratio determination, a 10 ms dwell time was used in subsequent experiments. The platform was ready to be used in further investigations. ion lens voltages were set so as to obtain the best ion signals for the elements studied simultaneously. Sputtering The graphite platforms were arranged in a circle on the Pretreatment of the Platform substrate holder in the sputtering system and sputtered in a single experiment at room temperature.A commercially avail- T hermal pretreatment able palladium plate (75 mm diameter, 0.5 mm thick, 99.9% A solution containing 500 mg ml-1 of Pd in the form of pure) was used as a target for the sputtering source. When the palladium nitrate was used for introduction on to the platform sputtering system had been set up, an argon flow was turned inside the graphite tube.A 10 ml aliquot of the Pd solution on to maintain a pressure of about 0.05 Torr, which was just was first injected, dried and pyrolysed (Table 2, programme sufficient for sputtering to occur. The plasma forward power II) to obtain Pd in its reduced form. After this procedure, was set to 250Wwith sputtering for 60 min. Then the platform the modified platform was ready to be used in further was fixed in the graphite tube and slowly heated to 2000°C.investigations. After this procedure, the modified platform was ready to be used in further investigations. Electroplating The apparatus depicted in Fig. 1 was used to electroplate Pd Reagents on the surface of the platform. The operating conditions for Trace metal grade HNO3 (70% m/m) was obtained from Fisher (Fair Lawn, NJ, USA),palladium powder from Kojundo Chemical Laboratory (Saitama, Japan), rhodium AA standard solution from Aldrich (Milwaukee, WI, USA), NH4H2PO4 from TEDIA (Fairfield, OH, USA) and (NH4)2HPO4 from Merck (Darmstadt, Germany).Enriched isotopes purchased from the Oak Ridge National Laboratory (Oak Ridge, TN, Table 3. Conditions for electroplating Operating conditions— Operating mode Constant potential Working electrode Graphite platform Reference electrode Ag/AgCl Counter electrode Pt wire Deposition potential -0.6 V Deposition time 60 min Composition of electroplating bath solution— Na2HPO4 0.5 mol l-1 (NH4)2HPO4 0.1 mol l-1 Pd standard 4000 mgml-1 Fig. 1 Schematic diagram of the electroplating apparatus. 76 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12USA) included 65CuO, 111CdO and 204Pb(NO3)2. Stock standard solutions of approximately 500 mg l-1 of each were prepared by dissolution of an accurately weighed amount of the material in nitric acid and dilution to volume. The concentrations of the spike solutions were verified by reversed spike isotope dilution ICP-MS.This was done by spiking the enriched isotope with suitable amounts of natural isotope of each element and measuring the altered isotope ratio by ICP-MS. The concentrations of the enriched isotope were then calculated using the equation described in previous papers.26,27 Sample Preparation The applicability of the method to real samples was demonstrated by the analysis of NIST SRM 2670 Toxic Metals in Freeze-Dried Urine and National Research Council of Canada Fig. 2 Effect of pyrolysis temperature on ion signals of Cu, Cd and (NRCC) DORM-1 (Dogfish Muscle Reference Material for Pb. The amounts of A, Cu; B, Cd; and C, Pb injected were 0.2 ng Trace Metals) and DOLT-1 (Dogfish Liver Reference Material each. No modifier was used in this experiment. for Trace Metals). The freeze-dried urine reference materials were reconstituted as indicated on the certificate. Aliquots of 10 ml of the reconstituted solution and appropriate amounts of enriched isotopes were transferred into closed Teflon PFA vessels.After 5.0 ml of nitric acid had been added, these solutions were heated inside an MDS-2000 microwave digester (CEM, Matthews, NC, USA) to decompose the organic components. The digests were then diluted to the desired volume after cooling to room temperature. A blank was carriedthrough the digestion procedure, as outlined above, to correct for any analyte contaminants in the reagent used for sample preparation.The marine biological samples were dried as instructed on the certificate. A 0.5 g amount of dogfish muscle reference material DORM-1 and 0.1 g of dogfish liver reference material DOLT-1 were weighed into closed Teflon PFA vessels and digested as described below. To each vessel, 10.0 ml aliquots of nitric acid and appropriate amounts of enriched isotopes Fig. 3 Effect of various modifiers on Cd signal. NH4H2PO4 concen- were added.These mixtures were heated inside a CEM tration, 2.0 mol l-1; volume injected, 10 ml. The amounts of Rh and MDS-2000 microwave digester to decompose the organic Pd injected were 2.5 and 5.0 mg, respectively. A, Without modifier; B, components. The digests were then diluted to the desired Pd; C, NH4H2PO4; and D, Rh. volume with pure water after cooling to room temperature. A blank was carried through the digestion procedure to correct analysis in subsequent experiments.Palladium has been used for any analyte contaminants in the reagent used for sample as a chemical modifier to improve the signals of volatile preparation. The analyte concentration in the sample was then elements in many ETV-ICP-MS applications.4–7 In the follow- calculated using the equation described in previous papers.26,27 ing, the pyrolytic graphite platforms of the graphite tubes were Owing to the mass discrimination effect, the intensities pretreated with palladium by three different methods, thermal obtained during isotope ratio determination of each solution pretreatment, electroplating and sputtering, and the analytical were used to calculate the isotopic abundance of each element.characteristics of these pretreatment methods were compared. RESULTS AND DISCUSSION Pretreatment of Platform Selection of Chemical Modifier Thermal pretreatment Fig. 2 shows the effect of the pyrolysis temperature on the ion signals of Cu, Cd and Pb. Since no modifier was used in this Fig. 4 shows the effect of the amount of Pd on the ion signals of Cu, Cd and Pb. As can be seen, Pd not only delays the ETV-ICP-MS analysis, Cd was evaporated and its ion signal decreased rapidly when the pyrolysis temperature was higher vaporization of Cd, but is also a good carrier for the other elements. The signals of Cu, Cd and Pb were enhanced by a than 200 °C. Owing to the volatility of Cd, a chemical modifier is needed for the simultaneous determination of Cu, Cd and factor of 2–4 when the platform was thermally pretreated with about 5 mg of Pd.On the other hand, the ion signals decreased Pb by ETV-ICP-MS. In this study, several modifiers were tested to obtain the optimum signal of Cd and the results are with increase in the amount of Pd used above 10 mg. The use of too much Pd modifier may increase the total mass trans- shown in Fig. 3. As can be seen, when the graphite platform was thermally pretreated with 5.0 mg of Pd, Cd did not vaporize ferred into the ion optics to an amount that is sufficient to result in space charge and/or other ion optic perturbations until 800 °C.NH4H2PO4 could also delay the vaporization of Cd until 900 °C. Further, the signal of Cd could be enhanced that resulted in a signal decrease. Fig. 5 shows the ion signals and isotope ratios of Cu, Cd threefold when NH4H2PO4 was used as modifier. However, a significant amount of PO2+ ion was formed, which interfered and Pb for 50 consecutive determinations.Since the platform was freshly pretreated with Pd before each analysis of the with the determination of 63Cu. Hence NH4H2PO4 modifier was not suitable in this study. sample solution, the ion signals and the isotope ratios remained steady for 50 consecutive determinations. The RSDs of the Rhodium was also tested as the modifier for the delay of Cd vaporization but, as shown in Fig. 3, it did not work. After isotope ratio measurement were in the range 1.1–2.0% for the elements studied (Table 4).evaluation, Pd was selected as the modifier for real sample Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 77Fig. 4 Effect of the amount of Pd injected on ion signals. The amounts of A, Cu; B, Cd; and C, Pb injected were 0.2 ng each. Fig. 6 Effect of number of firings of a Pd electroplating pretreatment platform on (a) ion signal and (b) isotope ratio determinations. The signal was measured relative to the first point. Fig. 5 Effect of number of firings of a Pd thermal pretreatment platform on (a) ion signal and (b) isotope ratio determinations. The amounts of Cu, Cd and Pb injected were 0.2 ng each. The signal was measured relative to the first point. Electroplating Although the isotope ratios were kept at a stable value, as Fig. 7 Effect of number of firings of a Pd sputtering pretreatment shown in Fig. 6, after 50 consecutive firings the ion signals of platform on the (a) ion signals and (b) isotope ratio determinations.Cu and Pb decreased to 90% of those after the first firing and The signal was measured relative to the first point. the Cd ion signal decreased to only about 75% of that after the first firing when the platform was pretreated by electroplat- Sputtering ing with Pd. As shown in Table 4, the RSDs of the isotope ratio measurement were in the range 1.1–4.0% for the elements As shown in Fig. 7, although the isotope ratios did not change significantly, the ion signals of the elements studied decreased studied.This precision was slightly worse than that obtained with the Pd thermally pretreated platform. rapidly with increase in the number of firings when a Pd Table 4. Effect of various Pd pretreatment methods on the reproducibility of the ion signal and isotope ratio determinations* (n=10) Pretreatment 63Cu 114Cd 208Pb method (counts) 63Cu/65Cu (counts) 114Cd/111Cd (counts) 208Pb/207Pb Thermal Mean 269 000 2.09 52 500 2.33 565000 2.40 pretreatment RSD(%) 3.9 1.1 5.9 2.0 3.4 1.6 Electroplating Mean 236 000 2.08 45 100 2.37 508000 2.49 RSD(%) 5.4 1.1 5.6 4.0 4.5 2.5 Sputtering Mean 196 000 2.03 39 200 2.36 428000 2.4 RSD(%) 7.7 2.2 9.7 3.4 5.1 2.8 * The amounts of Cu, Cd and Pb injected were 0.2 ng each. 78 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12sputtering coated platform was used. The ion signal of Cd ions ArNa+ and Ar25Mg+, which interfere with the determination of 63Cu+ and 65Cu+, respectively.The extent of the decreased to less than 40% of that after the first firing signal after only 12 firings. The signals of Cu and Pb also decreased interferences in the determination of the 63Cu/65Cu ratio was studied in the following experiments. Solutions containing Cu to 70–80% of the first firing levels. The RSDs of the isotope ratio measurement were in the range 2.2–3.4%. During this and various concentrations of matrix elements were analysed by ETV-ICP-MS and the isotope ratio was determined for experiment, the accumulation of the coated Pd could be observed.The coated Pd collected together into a few small comparison. The results are shown in Figs. 8 and 9. As can be seen, the 63Cu/65Cu ratio increased with increase in Na concen- droplets, which decreased the surface area of Pd which could interact with Cd. This could be due to the fact that the graphite tration when the latter was higher than 1000 mg ml-1 (Na concentration in urine #2620 mg ml-1), which indicated an surface was contaminated with air and water vapour before sputtering, which degraded the interaction of the graphite interference at m/z 63 from ArNa+.However, the 63Cu/65Cu ratio did not change even when 100 mg ml-1 of Mg (Mg surface and Pd vapour.28 The reproducibilities for the isotope ratio determination concentration in urine #63 mg ml-1) was added, which indicated that the interference of Ar25Mg+ with 65Cu+ was not using these three platform pretreatment methods are given in Table 4.It can be seen that the determination of the isotope significant. Although several modifiers were tested for the removal of Na before the vaporization step, Na could not be ratios by ETV-ICP-MS with a thermal pretreatment platform gives the best precision (1–2%). This could be due to a larger evaporated completely with the modifiers tested. In order to alleviate the molecular ion interference, the samples were and more stable ion signal being obtained when a thermally pretreated platform was used.Although not illustrated here, diluted to a concentration (Na concentration <1000 mg ml-1) at which the formation of ArNa+ ion did not affect the analysis the detection limits were similar when the elements were studied with different platform pretreatment methods. As is of Cu significantly. The sulfate ion in the biological samples usually the case with ETV sample introduction, the precision of isotope ratio measurement tended to be worse than that with a conventional pneumatic nebulizer.25–27 In summary, although the analysis time is slightly longer, the thermal pretreatment method has the advantages of being easy to use, giving good repeatability and with no need for extra equipment or skilled operators.After careful evaluation of these factors, thermal pretreatment of the platform with Pd was selected for subsequent experiments.Selection of ETV Operating Conditions Thermal pretreatment of the platform with Pd was selected for real sample analysis after careful evaluation of the performances of the three platform pretreatment methods. In order to remove the sample matrix as completely as possible, the pyrolysis temperature was set at 700 °C. The vaporization Fig. 8 Effect of the concentration of Na on Cu isotope ratio. The temperature was set at 2300 °C to evaporate the elements being samples injected contained 0.2 ng of Cu and various amounts of Na.studied completely and simultaneously. A summary of the A, Found; and B, expected value. ETV operating conditions is given in Table 2. Calibration parameters for the elements studied are given in Table 5. Under the selected ETV-ICP-MS operating conditions, the calibration plots for the elements studied were linear with correlation coefficients better than 0.995 at levels near the detection limits up to at least 5 ng ml-1.The detection limits, calculated from the calibration plots, were 0.04, 0.02 and 0.02 ng ml-1 for Cu, Cd and Pb, respectively, based on the conventional definition of the concentration of the analyte yielding a signal equivalent to three times the standard deviation of the blank signal. Better detection limits would be expected if a higher purity of Pd was used. Determination of Cu, Cd and Pb in Biological Samples by ETV-ID-ICP-MS In order to validate the ETV-ID-ICP-MS method, concentrations of Cu, Cd and Pb were determined in DORM-1, Fig. 9 Effect of the concentration of Mg on Cu isotope ratio. The DOLT-1 and NIST SRM 2670 reference samples. Samples samples injected contained 0.2 ng of Cu and various amounts of Mg. A, Found; and B, expected value. with high concentrations of Na and Mg form the molecular Table 5. Calibration parameters (0.05–5 ng ml-1) of Cu, Cd and Pb with ETV-ICP-MS and thermal pretreatment of Pd modifier Reagent blank/ Correlation Relative detection Absolute detection Element ng ml-1 coefficient limit/ng ml-1 limit/pg 63Cu 0.51 0.995 0.04 0.8 114Cd 0.13 0.995 0.02 0.4 208Pb 0.35 0.997 0.02 0.4 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 79Table 6. Determination of Cu, Cd and Pb in biological samples by 4 Gregoire, D. C., and Sturgeon, R. E., Spectrochim. Acta, Part B, 1993, 48, 1347. ETV-ID-ICP-MS (n=3) 5 Ediger, R. D., and Beres, S. A., Spectrochim. Acta, Part B, 1992, 47, 907. Concentration/mg g-1 6 Gregoire, D.C., Lamoureux, M., Chakrabarti, C. L., Al-Maawali, S., and Byrne, J. P., J. Anal. At. Spectrom., 1992, 7, 579. Sample Element Found* Certified† 7 Gregoire, D. C., Al-Maawali, S., and Chakrabarti, C. L., NRCC DORM-1 Cu 5.42±0.14 5.22±0.33 Spectrochim. Acta, Part B, 1992, 47, 1123. Cd 0.077±0.008 0.086±0.012 8 Wei, W. C., Chen, C. J., and Yang, M. H., J. Anal. At. Spectrom., Pb 0.40±0.02 0.40±0.12 1995, 10, 955. NRCC DOLT-1 Cu 19.0±1.0 20.8±1.2 9 Gregoire, D.C., and Naka, H., J. Anal. At. Spectrom., 1995, 10, 823. Cd 4.08±0.18 4.18±0.28 10 Sparks, C. M., and Holcombe, J., Spectrochim. Acta, Part B, 1993, Pb 1.42±0.03 1.36±0.29 48, 1607. NIST SRM 2670 Cu 0.134±0.006 0.12±0.02 11 Hasting, D. W., Emerson, S. R., and Nelson, B. K., Anal. Chem., (normal level) Cd 0.0005±0.0001 (0.0004) 1996, 68, 371. Pb 0.010±0.003 (0.01) 12 Lamoureux, M. M., Gregoire, D. C., Chakrabarti, C. L., and NIST SRM 2670 Cu 0.38±0.03 0.37±0.03 Golt, D.M., Anal. Chem., 1994, 66, 3208. (elevated level) Cd 0.091±0.003 0.088±0.003 13 Byrne, J. P., Hughes, D. M., Chakrabarti, C. L., and Gregoire, Pb 0.106±0.005 0.109±0.004 D. C., J. Anal. At. Spectrom., 1994, 9, 913. 14 Becker, S., and Hirner, A. V., Fresenius’ J. Anal. Chem., 1994, * Values are means of three measurements±standard deviation. 350, 260. † Certified values with 95% confidence limits. 15 T. Kantor, Spectrochim. Acta, Part B, 1988, 43, 1299. 16 Dean, J. R., Ebdon, L., and Massey, R., J.Anal. At. Spectrom., 1987, 2, 369. could form 32S16O2H and 33S16O2 molecular ions in the ICP, 17 Gregoire, D. C., and Lee, J., J. Anal. At. Spectrom., 1994, 9, 393. which interfere with 65Cu.29 In a separate experiment, we found 18 van Heuzen, A. A., Hoekstra, T., and van Wingerden, B., J. Anal. that the 63Cu/65Cu ratio did not change when 1000 mg ml-1 of At. Spectrom., 1989, 4, 483. SO42- (SO42- concentration in urine #1300 mg ml-1) was 19 Beary, E.S., Brletic, K. A., Paulsen, P. J., and Moody, J. R., added, which indicated that the interferences of 32S16O2H and Analyst, 1987, 112, 441. 33S16O2 molecular ions with 65Cu were not significant when 20 McLaren, J. W., Beauchemin, D., and Berman, S. S., Anal. Chem., 1987, 59, 610. an ETV sample introduction device was used. 21 Bowins, R. J., and McNutt, R. H., J. Anal. At. Spectrom., 1994, Analytical results for the reference materials are given in 9, 1233. Table 6. The determined concentrations are in good agreement 22 McLaren, J. W., Mykytiuk, A. P., Willie, S. N., and Berman, S. S., with the certified values. These results indicate that Cu, Cd Anal. Chem., 1985, 57, 2907. and Pb could be readily quantified by the proposed method. 23 Beauchemin, D., McLaren, J. W., Mykytiuk, A. P., and Berman, Other applications of ETV-ICP-MS for the determination of S. S., J. Anal. At. Spectrom., 1988, 3, 305. 24 McLaren, J. W., Beauchemin, D., and Berman, S. S., J. Anal. At. trace elements in biological samples are under investigation. Spectrom., 1987, 2, 277. 25 Lu, P.-L., Huang, K.-S., and Jiang, S.-J., Anal. Chim. Acta, 1993, This research was supported by a grant from the National 284, 181. Science Council of the Republic of China under Contract NSC 26 Hwang, T.-J., and Jiang, S.-J., J. Anal. At. Spectrom., 1996, 11, 353. 86–2113-M-110–021. 27 Liaw, M.-J., and Jiang, S.-J., J. Anal. At. Spectrom., 1996, 11, 555. 28 Huang, H.-J., Department of Chemistry, National Sun Yat-Sen University, Taiwan, personal communication. REFERENCES 29 Vanhoe, H., Goossens, J., Moens, L., and Dams, R., J. Anal. At. Spectrom., 1994, 9, 177. 1 Hughes, D. M., Chakrabarti, C. L., Goltz, D. M., Gregoire, D. C., Sturgeon, R. E., and Byrne, J. P., Spectrochim. Acta, Part B, 1995, Paper 6/05655E 50, 425. 2 Gregoire, D. C., Miller-Ihli, N. J., and Sturgeon, R. E., J. Anal. Received August 13, 1996 At. Spectrom., 1994, 9, 605. Accepted October 2, 1996 3 Fonseca, R. W., and Miller-Ihli, N. J., Appl. Spectrosc., 1995, 49, 1403. 80 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a605655e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Reduction of Background Absorption in the Measurement of Cadmium, Lead and Selenium in Whole Blood Using Iridiumsputtered Graphite Tubes in Electrothermal Atomic Absorption Spectrometry CORNELIUS J. RADEMEYERa, BERNARD RADZIUKb, NATALYA ROMANOVAc , YNGVAR THOMASSEN*c AND PAOLO TITTARELLId aDepartment of Chemistry, University of Pretoria, Pretoria, South Africa bBodenseewerk Perkin-Elmer GmbH, P.O. Box 101761, D-88662 U� berlingen, Germany cNational Institute of Occupational Health, P.O.Box 8149 DEP, N-0033 Oslo, Norway dStazione Sperimentale per i Combustibili, V iale A. De Gasperi 3, I-20097 San Donato Milanese, Italy The thermal behaviour during pyrolysis and of the vapour stant for more than 700 atomization cycles. It was also shown that the pyrolysis temperature of Cd, Pb and Se in acidified phase during atomization for Cd, Pb and Se in acid-digested whole blood using Ir-sputtered tubes is described. The aqueous solutions is increased to that obtainable with an ordinary pyrolytic graphite-coated graphite tube in conjuction performance of Ir as a permanent modifier was affected unfavourably by the complex matrix compared with with modifiers added in solution form.In this work, the suitability of such a permanent modifier conventional modifiers. Background absorption was measured using an atomic absorption spectrometer in addition to a for the measurement of Cd, Pb and Se in acid-digested whole blood was assessed. The effects on thermal stabilization and diode-array spectrometer and compared with the background obtained in pyrolytic graphite-coated graphite tubes.Both background absorbance were investigated for the three elements. The contribution of the background was assessed by methods of measurement indicated that the background was much reduced in the Ir-sputtered tubes. The decrease in monitoring the background absorption detected by a Zeemaneffect atomic absorption spectrometer and also by monitoring background absorption improves conditions for the measurement of these elements. Background molecular molecular absorption in the UV range using a diode-array spectrometer.The latter approach provided insight into the absorption was also measured as a function of time. Molecular species such as NO were detected in the vapour phase using evolution of the molecular species deriving from the whole blood sample decomposition. pyrolytic graphite-coated tubes, whereas CS and CO were detected using Ir-sputtered tubes.Keywords: Electrothermal atomic absorption spectrometry; iridium permanent modifier; background reduction; cadmium; EXPERIMENTAL lead; selenium; whole blood Instrumentation A Perkin-Elmer (Norwalk, CT, USA) Zeeman 5100 atomic The concept of chemical modification has become essential in the measurement of elements at trace and ultratrace concen- absorption spectrometer equipped with an HGA 600 atomizer and an AS-60 autosampler was used for the thermal stabiliz- trations using ETAAS.1 It has been demonstrated that a variety of reagents, including metals such as palladium, can thermally ation and background absorption investigations.Standard tubes and iridium sputtered pyrolytic graphite coated tubes stabilize many analyte elements, making possible the selective removal of major portions of the sample matrix. Chemical (Perkin-Elmer Part No. B009–1504) were used to assess the pyrolysis characteristics of the sputtered surface for Cd and modification by Pd may be accomplished by heating a Pd-containing solution in the graphite tube to 1200 °C prior Pb in whole blood and integrated platform tubes with and without sputtered Ir (Perkin-Elmer Part.No. B300–1260) were to addition of the sample. The resulting metallic form of Pd has proved to be a reliable modifier for the measurement of also used for Se and background measurements. A Jasco (Tokyo, Japan) KS-100M diode-array spectrometer Cd, Pb and Se in whole blood.This has the additional advantage that any Cd and Pb contained in the modifier is equipped with a Perkin-Elmer HGA-400 atomizer and an AS-1 autosampler was used for the recording of the vapour- volatilized, eliminating the risk of contamination. However, such procedures can increase the analysis time by a factor of phase spectra collected during the atomization step of the elements. This spectrometer has been used previously for the almost two.It would therefore be advantageous if the modifier were ‘permanent’, i.e., added only once to each tube rather study of the thermal behaviour of slurries.4 The diode array of the spectrometer is a Reticon 512 S (512 elements 25 mm wide). than before every sample and used throughout the lifetime of the tube.2 The UV range (190–350 nm) is focused on the diode array by a concave holographic grating (500 linesmm-1) blazed at Recently, we showed that the inside surface of a graphite tube may be coated with a homogeneous Ir layer by means of 230 nm.The calculated resolution is about 0.3 nm per diode. The data processor allows the collection of spectra with a sputtering process in a low-pressure argon discharge.3 When sufficient Ir was deposited, this metallic modifier surface collection times varying from 0.05 to 1 s and with interval times between the start of consecutive spectra varying from changed only slightly during the entire lifetime of the graphite tube and the integrated absorbance remained essentially con- 0.05 to 25 s.Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (81–84) 81Acid Homogenization of Human Whole Blood A 2.5 ml volume of sub-distilled 65% nitric acid (Chemscan, Elverum, Norway) was added to 2 ml of reconstituted lyophilized human whole blood (Seronorm 404108, NycoMed, Oslo, Norway) in a polypropylene digestion tube. After degasification at room temperature overnight, the tube was heated at 95°C for 1 h in a laboratory oven.The sample was allowed to cool to room temperature before final dilution to a previously calibrated volume (13.7 ml). De-ionized water (18 MV cm) obtained from a Milli-Q water-purification system (Millipore, Watford, UK), was used throughout. Recording of Absorption Spectra UV absorption spectra were continuously collected during the atomization step using the diode-array spectrometer. The collection time of each spectrum and the interval between the starts of consecutive spectra were set to 0.2 s.Hence the spectra were recorded continuously without delay between the end of Fig. 2 Pyrolysis curves for Se in aqueous solution (15 ml injection) a spectrum and the start of the next one. Particular attention (%) and whole blood (15 ml injection) (2) using an Ir-sputtered was paid to the molecular absorption occurring at the 196.0 nm intergrated platform graphite tube under normal internal gas flow analytical wavelength of Se.It was not possible to employ (R) and stopped flow (2---2) conditions. The background collection times shorter than 0.2 s owing to the combined effect absorbance was also measured under normal (&) and stopped flow conditions (Q). of poor efficiency of the diodes and low emission intensity of the deuterium lamp at this wavelength. A faster measurement rate may have shed light on transient phenomena. stop conditions, volatile Se species released from the platform above 400 °C are likely to be collected and thermally stabilized by the Ir-modified inner surface of the graphite tube rather RESULTS AND DISCUSSION than being deposited at the cooler ends or flushed out of Figs. 1 and 2 show the pyrolysis curves for Cd, Pb and Se in the tube. aqueous solutions and whole blood using Ir-sputtered tubes. It can be expected for a complex matrix such as acid The maximum pyrolysis temperature for each of these elements homogenized whole blood, which contains undigested organic in aqueous standard solutions is increased to a temperature compounds, that the surface modified by iridium may not be similar to that reported for modifiers such as Pd added to the fully available for reaction with the analyte.The reason for analyte in solution form.5 The data for whole blood displayed this is that the permanent modifier is not dissolved in the in Fig. 1 show that the maximum pyrolysis tperature is solution.It was demonstrated previously that, e.g., Sb was approximately 500 °C for Cd, 750 °C for Pb and only 400°C stabilized to a comparable extent by Ir added to aqueous for Se. solutions and to whole blood and urine.6 With pre-reduced A unique feature of a graphite tube fully coated by Ir noble metal modifiers, the modifier is distributed on the surface sputtering can be seen in Fig. 2. Selenium is retained in the of the tube in finely dispered form. On addition of the sample tube at much higher pre-treatment temperatures under solution on the surface, it will mix intimately with the modifier stopped-flow conditions compared with a flow rate of 300 ml and the latter may even be partly redissolved in the solution.min-1, which is normally used during pyrolysis. Under gas- While the permanent Ir modifier is not as effective in the thermal stabilization of the analyte in this matrix, it is very effective in modification of the matrix itself. Fig. 1 also shows the background absorption measured at the analytical wavelengths of Cd and Pb.In both cases background absorption decreases to an almost constant level at pyrolysis temperatures higher than 300 °C. The background absorption recorded during atomization at the analytical wavelengths of Cd, Pb and Se is shown in Fig. 3. The solid curves (Ir-sputtered) were obtained using the same type of tubes for the respective elements which had been sputtered by Ir and with no further addition of modifier in any case.The background is considerably smaller for all three elements when Ir-sputtered tubes are employed. The effect is particularly remarkable for Se. As long-term tests of Ir-sputtered tubes have shown that the modifier is little affected by age, it may be presumed to be chemically inert. Therefore, a possible explanation for the decrease in background involves a catalytic role of the Ir-sputtered surface of the tube while sample components are reduced during the thermal decomposition of the organic part of the blood matrix.This modification of the matrix by the Ir-sputtered surface is also indicated in Fig. 1 Pyrolysis curves for Cd and Pb in aqueous solution (15 ml the shift of appearance time of the background absorption injection) (%) and whole blood (15 ml injection) (2) using Ir-sputtered in Fig. 3. graphite tubes and wall atomization. The background (&) from whole blood is measured for both elements. The molecular absorption spectra that were obtained using 82 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Fig. 4 Absorption spectra obtained at different times for 15 ml of nitric acid-digested whole blood after commencement of the atomiz- Fig. 3 Background absorbance–time profiles obtained for Cd, Pb ation cycle in ordinary pyrolytic graphite-coated graphite tubes as (wall atomization) and Se (platform atomization) for 15 ml of nitric measured with a diode-array spectrometer between 190 and 340 nm.acid-digested whole blood for ordinary pyrolytic graphite-coated Collection and interval time, 0.2 s. Scale marks on the time scale also graphite tubes (broken lines) compared with those obtained in represent zero absorbance for the corresponding spectrum. Ir-sputtered graphite tubes (solid lines). Pyrolysis temperatures: Cd and Pb, 100 °C; Se, 200 °C. Atomization temperatures: Cd and Pb, 2000 °C (ramp 1); Se, 2300°C (maximum power). the diode-array spectrometer under the same experimental conditions as employed for the recording of background profiles are shown in Figs. 4 and 5. Fig. 4 shows the absorption of the vapour phase when an ordinary pyrolytic graphitecoated integrated platform tube is employed, and Fig. 5 shows the absorption when the tube is sputtered with Ir. As the period at the beginning of the atomization step is particularly affected by the evolution of molecular species, these spectra were recorded from the onset of this step for a period of 1.8 s.In the case of the pyrolytic graphite-coated tubes (Fig. 4), the background absorption is very strong below 220 nm with a maximum around 200 nm. The behaviour of the molecular absorption as a function of time at the analytical wavelengths of Cd, Pb and Se, recorded with the diode-array spectrometer, is similar to that shown in Fig. 3 and recorded using the atomic absorption spectrometer. The collection time of the diode-array spectrometer (0.2 s) does not allow the characterization and distinction of very fast phenomena such as those evidenced at 196 nm.The absorbance values measured were also lower than those recorded with the atomic absorption spectrometer. This difference can be attributed to the different spectral and time resolution between these spectrometers. The Se line is located between various overlapping molecular bands whose features are changing very rapidly, as can be seen Fig. 5 Absorption spectra obtained at different times for 15 ml of nitric acid-digested whole blood after commencement of the atomiz- in the first three spectra in Fig. 4. For the vapour-phase spectra ation cycle in Ir-sputtered graphite tubes as measured with a diode- obtained with the Ir sputtered tube (Fig. 5), it is clearly array spectrometer between 190 and 340 nm. Collection and interval noticeable that the molecular absorption is greatly reduced. time, 0.2 s. Scale marks on the time scale also represent zero absorbance Further, the molecular absorption decreases to a minimum for the corresponding spectrum.value at a much earlier stage during the atomization step than in the case of the ordinary pyrolytic graphite-coated tube. Fig. 6 shows the molecular absorption spectrum collected 214.3, 225.9 and 235.3 nm belong to the d-system of NO.7 The appearance of NO species at this stage is due to the decompo- between 0.4 and 0.6 s from the beginning of the atomization step using a pyrolytic graphite-coated tube (third spectrum in sition of the nitric acid employed in the dissolution of the blood sample resulting in the transformation of the bulk of Fig. 4). The well defined bands with maxima located at 204.2, Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 83lines of Pd, Mg and Na can be observed in the lower spectrum in Fig. 7. After 2.8 s only the lines of Pd can be identified. While Na and Mg are major components of the matrix, no Pd was contained or added to the samples in Ir-sputtered tubes.The absorption is therefore attributed to the volatilization of Pd from the tube ends contaminated by the graphite cones which had been used in previous experiments in which Pd was added. CONCLUSIONS It has been found that Cd, Pb and Se, when contained in acidified aqueous solutions, are thermally stabilized to at least the same extent in Ir-sputtered tubes as by conventional modifiers. However, the performance of the permanent modifier Fig. 6 Absorption spectrum obtained between 0.4 and 0.6 s using a was affected unfavourably by the complex matrix contained in pyrolytic graphite-coated graphite tube (third spectrum in Fig. 4). nitric acid-digested whole blood samples. Maximum pyrolysis temperatures were lowered, compared with those for acidified solutions, from 800 to 500°C for Cd, from 1200 to 750°C for Pb and from 1400 to 400 °C for Se. The background absorption that was measured during the atomization phase was dramatically lowered in comparison with that measured when reduced palladium was used as the modifier, in spite of the much lower permissable pyrolysis temperatures.This decrease in background absorption may be explained in terms of a catalytic effect of the Ir-treated surface on the dissociation of volatile components originating from proteins contained in the whole blood matrix. The decrease in background absorption improves conditions for the measurement of these elements in blood, in particular for Se, which is measured at an analytical wavelength situated among molecular bands that are rapidly changing in intensity and spectral position during the atomization step.Fig. 7 Absorption spectra obtained after 1.6 (lower spectrum, ninth spectrum in Fig. 5) and 2.8 s (upper spectrum) using an Ir-sputtered graphite tube. REFERENCES 1 Tsalev, D. L., and Slaveykova, V. I., J. Anal. At. Spectrom., 1991, the inorganic residue to nitrate form during drying.The 7, 147. appearance of NO bands is also observed during the heating 2 Tsalev, D. L., D’Ulivo, A., Lampugnani, L., Di Marco, M., and Zamboni, R., J. Anal. At. Spectrom., 1995, 10, 1003. or atomization of samples treated with nitric acid as in the 3 Rademeyer, C. J., Radziuk, B., Romanova, N., Skaugset, N. P., case of sea water.8 The presence of NO in the vapour phase is Skogstad, A., and Thomassen, Y., J. Anal. At. Spectrom., 1995, observed to a much smaller extent when Ir-sputtered tubes are 10, 739.used (Fig. 5), indicating that nitrates are reduced at the sputt- 4 Tittarelli, P., and Biffi, C., J. Anal. At. Spectrom., 1992, 7, 409. ered surface during atomization. This behaviour is in agreement 5 Tsalev, D. L., Slaveykova, V. I., and Mandjukov, P. B., Spectrochim. with a general decrease in the appearance of vapours of Acta Rev., 1990, 13, 225. 6 Dahl, K., Thomassen, Y., Martinsen, I., Radziuk, B., and Salbu, monoxides during the atomization step when a noble metal B., J. Anal. At. Spectrom., 1994, 9,1. such as Pd is added to slurries.4 7 Pearse, R. W. B., and Gaydon, A. G., T he Identification of The appearance of CS molecules is enhanced in the case of Molecular Spectra, Chapman and Hall, London, 3rd edn., 1963. Ir-sputtered tubes after 1.6 s and CO after 2.8 s (Fig. 7). This 8 Katskov, D. A., Marais, P. J. J. G., and Tittarelli P., Spectrochim. may be a result of reduction of sulfur compounds of the matrix. Acta, Part B, 1996, 51, 1169. Both CS and CO are observed at high temperatures when 9 Tittarelli, P., Biffi, C., and Kmetov, V., J. Anal. At. Spectrom., 1994, 9, 443. mixed oxides and sulfates are atomized, as a result of the decomposition of the matrix on the graphite surface.9 The Paper 6/04783A appearance of CS and CO occurs at the end of the atomization Received July 8, 1996 cycle and does not affect the background absorption measured Accepted October 4, 1996 during the atomization of Cd, Pb and Se. Apart from the molecular bands of CS, the atomic absorption 84 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a604783a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Comparison of the Suitability of Various Atomic Spectroscopic Techniques for the Determination of Selenium in Human Whole Blood O. MESTEK*a, M. SUCHA� NEKa, Z. VODIC¡ KOVA� a, B. ZEMANOVA� b AND T. ZI� MAc aDepartment of Analytical Chemistry, Prague Institute of Chemical T echnology, T echnicka� 5, 166 28 Prague 6, Czech Republic bMasaryk WaterManagement Research Institute, Podbabska� 30, 160 30 Prague 6, Czech Republic cFirst Institute ofMedical Chemistry and Biochemistry, First Faculty of Medicine, Charles University, Kater¡inska� 32, 120 00 Prague 2, Czech Republic A method was developed for the determination of selenium in mineralization is essential, because the recovery of the process is affected most by this step, as has been demonstrated by human blood by ICP-MS. The procedure involves sample dilution with Triton X-100 and HNO3 (1 ml of whole Welz and Melcher.7 Mineralization procedures usually involve the use of various mixtures of acids such as nitric, sulfuric and blood+1 ml of 1% Triton X-100+1.5 ml of 0.14 mol l-1 HNO3+0.2 ml of 5 mg l-1 In solution+water to 10 ml) and perchloric acids (e.g., Refs. 8–10). The effects of digestion acid and possible interferences caused by Na, K, Ca, Mg, Cu, Fe determination using the 77Se line. Standard samples were modified with a solution containing NaCl, Fe, Ca, Br, S, and and Zn have been discussed.11 A mixture of nitric and perchloric acid is recommended and no interferences at the usual K.No corrections were made for interferences from polyatomic ions. The applicability of the ICP-MS, ICP-OES, levels of interferent concentrations were found. The average recoveries were 99–103% and the RSD was 2–3%. ETAAS and HGAAS techniques to the determination of Se in human blood was compared. Limits of detection (3s) of these ETAAS is routinely used to determine selenium in human blood. Although procedures for the direct analysis of blood methods were 6, 76, 14 and 8 mg l-1 Se, respectively.The ICP-MS method was found to be well suited to routine without mineralization have been developed (e.g., Refs. 12–14), pyrolysis and atomization conditions in which graphite does analysis. not deposit in the atomizer are difficult to establish. Solutions Keywords: Blood; selenium; inductively coupled plasma; of Ni 15 or Pd15–17 are commonly used as chemical modifiers optical emission spectrometry; mass spectrometry ; atomic in the determination of Se.Background correction is a major absorption spectrometry ; hydride generation step in the ETAAS determination of Se. As approximately 40 iron lines can be found within the region 196.0±1 nm, with the closest line occurring at 196.014 nm, Zeeman-effect back- Selenium is an essential trace element for mammals, other ground correction is recommended; the use of deuterium arc animals and many bacteria.1 Selenium may perform several correction can easily lead to signal overcompensation.18 roles in the human body, e.g., it appears to be involved in the ICP-OES is not very sensitive for the determination synthesis of hormones by the thyroid gland and it is necessary of selenium.The sensitivity can be enhanced by coupling for the production of prostaglandins. It works closely with with hydride generators19 or by using an ultrasonic vitamin E in some of its metabolic actions and in the promotion nebulizer. of growth and fertility. Selenium is a natural antioxidant and ICP-MS allows mineralization to be omitted from the blood appears to preserve elasticity of tissue by delaying oxidation analysis procedure without causing appreciable difficulties.of polyunsaturated fatty acids. Selenium is probably present For instance, Delves and Campbell20 determined lead in at the active site as selenocysteine, the amino acid cysteine in blood diluted with phosphate buffer with addition of which the normal sulfur atom has been replaced by a sel- (NH4)2H2EDTA, and Stroh21 determined lead in blood diluted enium atom.1–3 with ammonia and a dilute solution of Triton.Dilution with Several diseases have been linked with a low selenium status: Triton and dilute nitric acid has also been applied to this Keshan cardiomyopathy, cancer, hypertension, arthritis, mus- analysis.22 The direct determination of selenium in human cular dystrophy, infertility, ageing and cataracts. Deficiency blood by ICP-MS has not been reported so far, but Goossens leads to cardiomyopathy, congestive heart failure and striated et al.,23 for example, reported determination of this element in muscle degeneration.Recent evidence has indicated that sel- blood plasma diluted with 0.14 mol l-1 HNO3 in the ratio of enium deficiency is a risk factor for atherosclerosis and death 1+9, using the lines of 76Se and 78Se for the standard additions from cardiovascular disease.1,2,4 method; they derived relationships to make a correction for The recommended daily allowance of selenium is interference from the Ar2+ ions, and attained a detection limit 50–200 mg2,5 and its concentration in human serum is of 1 mg l-1. 80–200 mg l-1 (1.0–2.5 mmol l-1).6 The half-life of selenium in humans is on average 65–115 d4 and it is eliminated from the body via urine (60%), faeces (30%), lungs, saliva and perspir- EXPERIMENTAL ation.4 Trace amounts of selenium are needed in the diet, Blood Sampling and Treatment although large amounts are poisonous.Atomic spectroscopic methods such as HGAAS, ETAAS Blood samples were taken from healthy volunteers, viz., university students after overnight fasting. All of them have given and ICP-OES are widely used for the determination of Se in human whole blood, and ICP-MS has been introduced more their informed consent prior to entering the study. Blood was obtained from the cubital vein by venepuncture and hep- recently. In this work, these methods were compared for the determination of selenium in human blood. arinized (0.5 ml of heparin per 10 ml of blood).Selenium was measured in whole blood. The samples taken were stored in a HGAAS is currently the most widely used technique for the determination of selenium in human blood. Thorough sample refrigerator at 4°C. Before analysis the blood samples, were Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (85–89) 85allowed to adjust to room temperature and were carefully any ‘clear’ isotope.Whereas the 80Se isotope was totally obscured by noise, the 77Se isotope gave the most convenient shaken. It is recommended that the analysis be performed on the same day as the blood sample is taken or the next day. S/N separation in systems containing interferents. The latter isotope was therefore selected for subsequent measurements, Check analyses performed in duplicate by HGAAS showed that by the fifth day after sampling the Se concentration was despite its low abundance. As the signal of 77Se is influenced by the presence of about 9% lower.This decline in Se concentration could not be explained by experimental error; it was caused probably by accompanying compounds in human blood, all standard solutions and the reagent blank were spiked with a solution adsorption of Se on the surface of the vial. modelling the blood (see above). This composition of standards corresponds to the average composition of human whole ICP-MS blood. However, as the actual concentrations can differ from this average, the tests also included an assessment of the effect Instrumentation of this associate component in human blood.The test was Measurements were performed on an ELAN 6000 instrument carried out by analysing a reagent blank containing different (Perkin-Elmer, Norwalk, CT, USA). amounts of the above mentioned model solution. The apparent Se contents caused by changes in the concentrations of the associate components are shown in Fig. 1 (full line). An amount Reagents and standard solutions of associate component of 100% corresponds to the average A 1000 mg l-1 selenium stock standard solution (Analytika, concentration in human blood and the Se concentration is Prague, Czech Republic) was used for the preparation of expressed as the concentration in whole blood. The king standard solutions containing 10, 20, 40 and is not very marked and small changes in the blood composition 100 mg l-1 Se.The working standard solutions and the reagent against the model standards do not affect the results of analysis blank were spiked (1+99) with a model solution represent- appreciably. The differences in results are comparable to the ing associated components in human whole blood,24 as experimental errors; see Table 4 for the repeatability of the follows: 8 g NaCl, 0.4 g KCl, 0.6 g Ca(NO3 )2 ·4H2O, 6.6 g results. cysteine·HCl·H2O, 2.23 g FeCl3·6H2O, 0.07 g KBr and H2O to The 40Ar37Cl ion is the major interferent for 77Se.This 100 ml. All these reagents were obtained from Merck suggests that a correction should be made by measurement to (Darmstadt, Germany), expect for cysteine hydrochloride, the 40Ar35Cl line and subtraction: which was supplied by Sigma (St. Louis, MO, USA). I(77Se)=I(77)-I(75) (37Cl abundance)/(35Cl abundance) Indium solution used as an internal standard was prepared (1) from a 1000 mg l-1 indiumstock standard solution(Analytika).Triton X-100 (Aldrich, Milwaukee, WI, USA) was used for where I(xx) is the intensity related to the proper mass xx or blood stabilization. ion. However, this procedure led to signal over-correction De-ionized water obtained from a Milli-Q system (Millipore, (Fig. 1, dashed line), which indicates that it is not only the Bedford, MA, USA) was used for the preparation of all 40Ar37Cl ions that contribute to the I (75) signal but also that solutions and for the preparation of solutions used for other other components are present as well.Arsenic-75 is presumably measurement techniques. All acids used during the study (nitric, the major additional component, probably being present in hydrochloric and perchloric) were of analytical-reagent grade negligible amounts in the chemicals employed for the prep- (Lachema, Brno, Czech Republic). aration of the model solution. No correction was made during subsequent measurements. Procedure HGAAS The blood samples were subjected to the following procedure.A 1 ml volume of 1% Triton X-100 was pipetted into a 10 ml Instrumentation calibrated flask, 1 ml of blood was diluted with approximately Blood decomposition was accomplished in a microwave min- 7 ml of water and agitated, 1.5 ml of 0.14 mol l-1 HNO3 and eralizer with a focused field BM-1S/II instrument (Plazmo- 0.2 ml of a 5 mg l-1 solution of In (internal standard) were tronika, Wroc�aw, Poland). AAS measurements were pro- added and the mixture was diluted to volume.This system cessed by a GBS 932AA instrument connected to an HG 3000 was stable for a minimum of 2 d. hydride unit (both from GBC, Dandenong, Victoria,Australia). Measurements were carried out on a Perkin-Elmer ELAN 6000 instrument applying the conditions shown in Table 1. The 77Se intensity was measured. Selection of a suitable Reagents and standard solutions selenium isotope for measurement is very difficult.An overview A 1000 mg l-1 selenium stock standard solution (Analytika) of stable Se isotopes, along with their relative abundances and was used for the preparation of working standard solutions interferences from polyatomic ions (if any), is given in Table 2. containing 5, 10, 20 and 40 mg l-1 Se. The working standard Table 2 also includes the S/N ratios for systems where Se was solutions and the reagent blank were carried through the present at a concentration of 100 mg l-1 in 0.028 mol l-1 HNO3 procedure involving evaporation with perchloric acid and and in a solution containing Triton X-100 and components reduction of SeVI by hydrochloric acid (see below).modelling dilute blood. Apparently, selenium does not have Procedure Table 1 Optimum ICP-MS operating conditions A blood sample (2 ml) was decomposed with concentrated Rf power 1000 W HNO3 (5 ml) in the microwave mineralizer for 10 min, the Dwell time 50 ms resulting solution was transferred into a beaker, HClO4 (2 ml) Sweeps per replicate 20 No.of replicates 3 was added and the mixture was evaporated to white fumes. Total acquisition time 3 s The SeVI was then reduced to SeIV with of 8 mol l-1 HCl Acquisition mode Peak hopping (15 ml) at 80°C for 30 min. After reduction, the solution was Ar nebulizer flow rate 0.825 l min-1 transferred into a 25 ml calibrated flask and diluted to volume. Sample uptake rate 1 ml min-1 The atomic absorption measurement was performed at 86 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 2 Overview of selenium isotopes S/N ratio S/N ratio Isotope Abundance (%) Mass interferences (0.028 mol l-1 HNO3 ) (model solution) 74Se 0.87 ArS, Cl2 Not measured Not measured 76Se 9.02 ArS, Ar2 1.4 1.4 77Se 7.78 ArCl, Ar2H 61.1 15.0 78Se 23.52 Ar2 5.8 6.2 80Se 49.82 Ar2, BrH, SO3 1 1 82Se 9.19 Ar2H, BrH, SO3 116.8 5.0 Table 3 Temperature programme for the determination of Se by ETAAS Step Temperature/°C Time/s 1 110 30.0 2 120 10.0 3 1200 7.0 4 2400 1.5 5 2900 1.5 ing a model solution (see ICP-MS) with the amounts encountered in human blood.Comparison of the slopes of the calibration straight line obtained by using the model samples and by applying the standard additions method revealed that the latter approach suits this analysis better. Therefore, the Fig. 1 Apparent concentration of Se caused by changes in concen- standard addition methods (10 ml of 1000 mg l-1 Se solution trations of associated components.A, without correction; and B, with was mixed with 1 ml of sample solution in an automatic correction. sampler) was applied in all subsequent measurements. Palladium was also tested as a chemical modifier. It induces 196.0 nm with no background correction. A quartz T-tube a sensitivity increase but the graphite atomizers wear out faster. heated with an air–acetylene flame served as the atomizer. The hydride was fed into the atomizer with a stream of nitrogen.ICP-OES Hydride was generated by using HCl (10 mol l-1) and sodium tetrahydroborate prepared by dissolving NaBH4 (3 g) and Instrumentation NaOH (3 g) in water (500 ml). A model 24 instrument (Jobin Yvon, Longjumeau, France) in The recovery of the process was checked by analysing blood conjunction with a U 500 AT ultrasonic nebulizer (Cetac, with additions of Se (prior to mineralization); the average Omaha, NE, USA) was used for emission measurements.A value was 98.2%. BM-1S/II microwave mineraliser (Plazmotronika) was used for sample decomposition. ETAAS Instrumentation Standard solutions A model 939 AA instrument equipped with a GF 90 graphite The reagent blank and working standard solutions containing atomizer (both from Unicam, Cambridge, UK) was used for 10, 20, 40 and 50 mg l-1 Se were prepared as described for atomic absorption measurements. A BM-1S/II microwave ICP-MS. The last standard also served for method optim- mineralizer (Plazmotronika) was used for sample decom- ization.position. Procedure Reagents Blood samples were decomposed and prepared as described Ni(NO3)2·6H2O (Lachema) was used for the preparation of for ETAAS. The measurement itself was carried out at wave- the chemical modifier solution. A 1000 mg l-1 selenium stock length 196.09 nm, plasma input power 800 W, argon carrier standard solution (Analytika) was used for the preparation of flow rate 0.4 l min-1 and sample uptake 0.9 l min-1. a working standard solution of 50 mg l-1 Se, which was used during optimization of the method.This working standard COMPARISON OF RESULTS OBTAINED BY solution and the reagent blank were spiked with the model THE VARIOUS METHODS solution as described for ICP-MS. For a comparison of the methods, blank analyses were performed repeatedly (n=7) with each method to determine the Procedure detection limit. The values obtained (3s) (where s is blank The blood samples were mineralized in the microwave min- standard deviation), converted to the final Se content in blood, eralizer (2 ml of blood+3 ml of concentrated HNO3, volume are given in Table 4.Eleven blood samples were collected from completed to 25 ml). Atomic absorption measurements were various donors and analysed in duplicate using each method. performed at 196.0 and background correction was applied The standard deviation of the repeatability was calculated for by means of a deuterium discharge lamp.A volume of 20 ml each method as of sample was injected on to a pre-dried amount of 10 ml of a 1% solution of nickel nitrate. The temperature programme se2= .k i=1 (xi1-xi2)2/(2k) (2) was optimized (Table 3) by means of a model sample contain- Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 87Table 4 Detection limits and standard deviations of repeatibility of the methods tested Parameter HGAAS ICP-MS ICP-OES ETAAS Detection limit/mg l-1 0.6 0.6 6.1 1.1 (solution measured) Detection limit/mg l-1 8 6 76 14 (blood) se /mg l-1 2.9 8.4 24 37 (blood) where k (=11) is the number of samples tested and xi1 and xi2 are the results of repetitions for the ith sample.The results are also included in Table 4. As HGAAS is the most widely used technique with minimum Fig. 4 Correlation between results obtained by HGAAS and ETAAS. interferences and also exhibits the lowest detection limit and standard deviation of repeatability, this method can be taken as the reference, to which the results of the other methods are related by the regression equation nation of selenium in blood.The parameters of the ICP-MS technique are slightly poorer but they can be easily improved [Se]method=a+b [Se]HGAAS (3) by increasing the number of analyses because the sample The parameters a and b in this equation, along with the preparation and measurement procedures are rapid. This confidence intervals and correlation coefficients, are given in method gives a good correlation with the HGAAS results, the Table 5. Plots of the regression straight lines are shown in parameter a (bias) being not significantly different from zero Figs. 2–4. and the parameter b reasonably approaching unity. The The results indicate that HGAAS is the best method with remaining two techniques, ICP-OES and ETAAS, in the respect to the detection limit and repeatability of the determi- arrangement used, are not very well suited to the determination of selenium in blood.Lower blood dilution may improve the Table 5 Parameters of regression eqn. (3) results but problems associated with solution acidity or nonmineralized blood analysis then arise. The detection limit of Parameter ICP-MS ICP-OES ETAAS ICP-OES is very poor and the sensitivity can be increased a 4±30 -0.13±70 62±95 only by using a hydride generator. The use of an ultrasonic b 0.96±0.30 0.90±0.69 0.31±0.94 nebulizer was insufficient. The results of ETAAS were very r 0.963 0.731 0.262 unconvincing. Its parameters should be improved by analysing less diluted non-mineralized samples. However, a suitable temperature programme for such an analysis could not be found.Zeeman-effect background correction can also improve the characteristics of the results. CONCLUSIONS This comparison of the determination of selenium in human blood by various atomic spectroscopic techniques supports the current approach in analytical chemistry in which direct sample analysis omitting unnecessary sample treatment procedures is preferred.The benefits of this approach include reduction of the hazard of sample contamination and loss of analyte, and also well as reduced time and equipment demands, resulting in lower costs. The availability of sufficiently sensitive and matrix-independent methods such as ICP-MS, however, is a prerequisite. Fig. 2 Correlation betweenresults obtained by HGAAS and ICP-MS.REFERENCES 1 Robinson, M. F., in Clinical, Biochemical, and Nutritional Aspects of T race Elements, ed. Prasad, A. S., Alan R. Liss, New York 1982, pp. 325–343. 2 Dunne, L. J., Nutritional Almanack. McGraw-Hill New York, 3rd edn., 1990. 3 Halliwel, B., and Gutteridge, J. M. C., Free Radicals in Biology and Medicine, Oxford University Press, New York, 2nd edn., 1995. 4 Holec¡ek, V., and Racek, J., Klin. Biochem. Metab., 1993, 2, 137. 5 Harrison’s Principles of Internal Medicine, ed.Wilson, J. D., Brownswald, E., Isselbacher, K. J., Petersdorf, R. G., Martin, J. B., Fauci, A. S., and Root, R. K., McGraw-Hill, New York, 12th edn., 1991, pp. 434–445. 6 Wallach, J., Interpretaion of Diagnostic T ests, Little, Brown, Boston, 5th edn., 1992, p. 773. 7 Welz, B., and Melcher, M., Anal. Chim. Acta, 1984, 165 ,131. Fig. 3 Correlation between results obtained by HGAAS and ICP- 8 Dogan, P., Dogan, M., and Klockenkaemper, R., Clin. Chem., 1993, 39, 1037.OES. 88 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 129 McLaughlin, K., Dadgar, D., Smyth, M. R., and McMaster, D., 18 Radziuk, B., and Thomassen, Y., J. Anal. At. Spectrom., 1992, 7, 397. Analyst, 1990, 115, 275. 19 Recknagel, S., Bra�tter, P., Tomiak, A., and Ro�sick, U., Fresenius’ 10 Hansson, L., Petterson, J., and Olin, A., T alanta, 1987, 34, 829. J. Anal. Chem., 1993, 346, 833. 11 Cui, X., Jiang, H., and Han, G., Determination of Selenium in 20 Delves, H. T., and Campbell, M. J., J. Anal. At. Spectrom., 1988, Human Serum by Hydride Generation, Varian Instrument in Work 3, 343. AA-82, Vanam, Palo Alto, CA, 1988. 21 Stroh, A., At. Spectrosc., 1993, 14, 141. 12 Eckerlin, R. H., At. Spectrosc. 1987, 8, 64. 22 La�sztity, A., Viczia�n, M., Wang, X., and Barnes, R. M., J. Anal. 13 Morisi, G., Patriarca, M., and Menotti, A., Clin. Chem., 1988, At. Spectrom., 1989, 4, 761. 34, 127. 23 Goossens, J., Moens, L., and Dams, R., T alanta, 1994, 41, 187. 14 McMaster, D., Bell, N., Anderson, P., and Love, A., Clin. Chem., 24 Vanhoe, H., Vandecasteele, C., Versieck, J., and Dams, R., Anal. 1990, 36, 211. Chem., 1989, 61, 1851. 15 Knowles, M. B., and Brodie, K. G., J. Anal. At. Spectrom. 1989, 4, 305. 16 Johannessen, J. K., Gammelgaard, B., Jøns, O., and Hansen, S. H., J. Anal. At. Spectrom., 1993, 8, 999. 17 Knowles, M., Selenium Determination in Blood Using Zeeman Paper 6/04950H Background Correction and Palladium/Ascorbic Acid Chemical Received July 15, 1996 Modification, Varian Instrument in Work AA-70, Vanam, Palo Alto, CA, 1987. Accepted October 11, 1996 Journal of Analytical Atomic Spectrometry, January 1997, Vo

ISSN:0267-9477    

DOI:10.1039/a604950h

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Arsenobetaine in Manufactured Seafood Products by Liquid Chromatography, Microwaveassisted Oxidation and Hydride Generation Atomic Absorption Spectrometry DINORAZ VE� LEZ, NIEVES YBA� N� EZ AND ROSA MONTORO* Instituto de Agroquý�mica y T ecnologý�a de Alimentos (CSIC), Apdo. Correos 73, 46100 Burjassot, Valencia, Spain. E-mail: Rmontoro@iata.csic.es A study was carried out to develop and optimize a method for sition. Microwave decomposition of organoarsenicals, separated by HPLC using flow injection systems coupled to determining arsenobetaine (AB) in seafood products by coupling HPLC, microwave-assisted oxidation and HGAAS.HGAAS, has recently been reported for determining AB in urine samples,9,10 water, synthetic fish and sediment extracts,11 Conditions were established for the extraction and instrumental determination of AB in popular seafood products. and water and sediments,12 but there are no reports of the application of this method to seafood products.The analytical features of the method were as follows: the detection limit was 0.68–27.20 ng g-1 As (fresh mass), the The objective of this study was to develop and optimize a method for determining AB in real seafood products by HPLC- relative standard deviation ranged from 0.4 to 6%, and the recovery was 104±5%. The analysis of DORM-1 (Dogfish microwave-assisted oxidation (MO)–HGAAS. AB was extracted quantitatively from commercial seafood Muscle, National Research Council of Canada, Certified Reference Material ) provided an AB value of 16.5±0.6 mg g-1 products in a simple and rapid way.The suitability of the clean-up and chromatographic procedure for separating AB As (dry mass), in agreement with results obtained by other workers using HPLC–ICP-MS. The proposed procedure was from other arsenical species was investigated in extracts of real seafood samples. The detection limit, precision and accuracy used to analyse canned seafood products purchased at local retail market outlets.The contents of total As represented by of the method were also evaluated. The proposed procedure was used to analyse real seafood samples. AB ranged from 5 to 75%. The lowest percentages of AB corresponded, in general, to the bivalve group. Keywords: Arsenobetaine; high-performance liquid EXPERIMENTAL chromatography; microwave-assisted oxidation; hydride Instrumentation generation atomic absorption spectrometry; manufactured The hyphenated HPLC–MO–HGAAS system is shown in seafood Fig. 1 and details of the operating conditions for the system are given in Table 1. The equipment used included a highperformance liquid chromatograph (Hewlett-Packard Model Arsenobetaine (AB) has been reported as being the organoarsenical most frequently found in marine animals,1 and the major 1050), equipped with a Model HP 79852A quaternary pump with on-line de-gassing system; a Rheodyne valve fitted with form of As in fish and crustaceans.1,2 We have reported in a previous study,3 in which total As was determined by HGAAS a 100 ml loop; and data station: viz., a Hewlett-Packard personal computer, Vectra 486/33N Model 170 with 486 micropro- and AB by HPLC–ICP-AES, that AB contents vary over a very broad range, viz., from <0.1 to 5.4 mg g-1 As, fresh mass cessor rated at 33 MHz (Hewlett-Packard Espan�ola, Madrid, Spain).(fm), and consequently they are sometimes below the limit of detection of the method.This shows the need to develop For AB determination, the chromatographic system was connected to a Moulinex Super Crousty domestic micro- methodologies with detection limits for AB lower than those offered by the ICP detector. The total As and the percentage wave oven with a maximum power of 1100 W and an operating frequency of 2450 MHz. A loop of PTFE tubing of total As represented by AB calculated by using these methodologies cannot, alone, evaluate the toxicity or otherwise (1.6 m×0.5 mm id) was placed inside the microwave oven through the ventilation holes.An additional oven load of of seafood products, but both can be indicators of their possible toxicity.3 400 ml of water was placed inside the oven to prevent overheating. The effluent from the microwave oven passed through HGAAS coupled to HPLC as a post-column derivatization method is one of the most inexpensive and convenient ways PTFE tubing (0.5 m×0.5 mm id) and was cooled in an icebath before it reached the hydride generator, to avoid over- of improving the sensitivity of As determination.One major limitation to using HGAAS as part of an HPLC system for pressure and decomposition of the sodium tetrahydroborate. The Perkin-Elmer (Norwalk, CT, USA) Model 5000 atomic determining AB is that this arsenical compound does not form volatile hydrides. This problem can be solved by two different absorption spectrometer was equipped with a Perkin-Elmer FIAS-400 system operating as a hydride generator in approaches, viz., UV4–7 or microwave on-line decomposition of organoarsenicals to forms suitable for generating arsines.continuous-flow mode. A drainage system (flow 9 ml min-1) was incorporated for the waste solution from the hydride Both systems provide appropriate detection power but the UV-HG attachment is time consuming. The efficient heating generation, working at constant pressure through a peristaltic pump.An electrothermally heated quartz cell was employed. of the microwave oven, which can rapidly decompose the organoarsenicals,8 possesses advantages over the UVdecompo- The HGAAS system was controlled by the software of a Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (91–96) 91Fig. 1 Hyphenated HPLC–MO–HGAAS system. Table 1 Operating conditions for HPLC–MO–HGAAS P-Selecta Vibromatic-340 mechanical arm shaker (Selecta, Barcelona, Spain). High-performance liquid chromatography— Column Hamilton PRP X-100, 10 mm polymer-based Reagents and Samples anion-exchange column (25.0 cm×4.1 mm Analytical-reagent grade water (QRG), 18 MV cm, obtained id) (Teknokroma, Barcelona, Spain) Guard column Hamilton PRP X-100, 10–20 mm polymer- with a Milli-Q water-purification system (Millipore Ibe�rica, based anion-exchange guard column Madrid, Spain), was used for the preparation of reagents and (25×2.3 mm id) (Teknokroma) standards.All chemicals including standards and solutions Mobile phase Phosphate buffer (Na2HPO4–H3PO4), were of pro analysi quality or better, viz., hydrochloric acid, 3 mmol l-1 at pH 5.0.Flow rate, 1 ml nitric acid, ammonia solution 32% extra pure, sodium hydrox- min-1 ide and HPLC phosphate buffers (99.5% Na2HPO4 ·2H2O and Injection volume 100 ml Temperature 28 °C 85% H3PO4 ). The stock standard solutions were: arsenite solution Hydride generation— (1000 mg l-1 AsIII), prepared by dissolving 1.320 g of arsenic Cell temperature: 900 °C trioxide (Riedel-de Hae�n, Hanover, Germany) in 25 ml of 20% Sample solution: Phosphate buffer, 3 mmol l-1 at pH 5.0; m/v KOH solution, neutralizing with 20% v/v H2SO4 and 1 ml min-1 flow rate diluting to 1 l with 1% v/v H2SO4; arsenate solution (1000 mg Reducing agent: 2.0% m/v NaBH4 in 0.7% m/v NaOH; l-1 AsV) (Titrisol, Merck, Darmstadt, Germany).Solutions of 1.9 ml min-1 flow rate HCl solution: 3 mol l-1; 1.9 ml min-1 flow rate monomethylarsonic acid (1000 mg l-1 MMA) and dimethylar- Carrier gas: Argon; 45 ml min-1 flow rate sinic acid (1000 mg l-1 DMA) were prepared by dissolving appropriate amounts of CH3AsO(ONa)2 ·6H2O (Farmitalia Atomic absorption spectrometer— Carlo Erba, Milan, Italy) and (CH3 )2AsNaO2·3H2O (Fluka Wavelength 193.7 nm Chemika Biochemika, Alcobendas, Madrid, Spain) in water.Spectral bandpass 0.7 nm AB solution (973 mg l-1) and arsenocholine (AC) solution Lamp power 8.5 W (electrodeless discharge lamp) (1000 mg l-1) were obtained from the Service Central Microwave oxidation— d’Analyse du CNRS-SCA, Vernaisson, France.The CRM Power 1100 W DORM-1 (Dogfish Muscle) was obtained from the National Oven load 400 ml water Research Council of Canada, Institute for Environmental DiE tubing (1.6 m×0.5 mm id) Chemistry (Ottawa, Canada). Refrigeration coil PTFE tubing (0.5 m×0.5 mm id) For the clean-up of the methanol–water extracts, columns Oxidizing solution 1% m/v K2S2O8 in 2.5% m/v NaOH; packed with Dowex 50W-X8 cation-exchange resin (H+ form, 0.6 ml min-1 flow rate 1×6 cm) (Bio-Rad, Barcelona, Spain) were employed.Oxidizing potassium persulfate (Probus, Barcelona, Spain) solutions (1, 3, and 5% m/v) were prepared daily in 2.5% m/v separate programmable PC system. The spectrometer signal NaOH. As reducing solutions for hydride generation coupled was acquired by a Hewlett-Packard Model 35900 C analogueto HPLC, sodium tetrahydroborate(III ) (Probus) solutions (1, to-digital converter, using the chromatograph software.Peak 2, 3, 4 and 5% m/v) were prepared daily by dissolving NaBH4 area signals were recorded. powder in 0.7% m/v NaOH solution and filtering through Determination of total As in previously dry-ashed samples Whatman No. 42 filter-paper. was performed with an atomic absorption spectrometer All glassware was treated with 10% v/v HNO3 for 24 h and (Perkin-Elmer Model 5000) equipped with a flow injection then rinsed three times with QRG water before use.When not system (Perkin-Elmer FIAS-400) operating as a hydride generin use, glassware was placed in 10% v/v HNO3 for 24 h. ator in continuous-flow mode. Various canned seafood products were purchased at local A lyophilizer equipped with a microprocessor controlling retail outlets. Descriptions are given in Table 2. the lyophilization process (FTS Systems, New York, USA) was employed. The microprocessor was connected to an Epson Sample Preparation Equity I+ computer.Other equipment used included a Janke & Kunkel A10 The brine or sauce in the canned seafood products was removed by the method for determining the drained mass of water-cooled mill (Schott Glaswerke, Mainz, Germany) and a 92 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 2 Seafood products analysed for AB extraction process was repeated three times and the extracts were combined.Sample Description (source) Seafood product Clean-up Fish— The methanol–water extract of the lyophilized samples was Salmon 01 Smoked salmon, skinless and boneless, water, salt, 150 g drained mass evaporated (T=55°C) to dryness, redissolved in 25 ml of 0.1 (Chile) mol l-1 HCl and adjusted to a pH less than 2 (1.8±0.2) with L amellibranchs— 4 mol l-1 HCl. The acidified solution was passed through a Razor 02 Pacific razor clams, water, salt, strong cation exchanger (Dowex 50W-X8, 1×6 cm).The Clams stabilizer (E-450) and antioxidant column was washed with 25 ml of reagent grade water and the (H-3246), 65 g drained mass (Chile) AB sorbed was eluted with 75 ml of 4 mol l-1 ammonia. The 03 Pacific razor clams, water, salt, 78 g drained mass (Chile) eluate was evaporated to dryness and the residue was redis- Cockles 04 Cockles, water, salt, 130 g drained solved in 3 ml of QRG water, filtered through Whatman No. 1 mass (The Netherlands) and No. 42 filter-paper and finally through a 0.45 mm filter.Langostillo 05 Langostillo, water, salt, 60 g drained mass (Pontevedra, Spain) AB Determination by HPLC–MO–HGAAS Mussel 06 Mussel in brine, water, salt, 70 g drained mass (Spain) For chromatographic separation, samples and standard solu- Clams 07 Clams, water, salt, 65 g drained mass tions were loaded into a 100 ml sample loop and injected onto (Spain) the chromatographic column. The eluate from the column was Gastropods— mixed with the persulfate solution before entering the micro- Snails 08 Sea snails, water, salt, stabilizer (E-450) and antioxidant (E-300, wave oven.The thermo-oxidized effluent, cooled in an ice- H-3246), 110 g drained mass (Chile) bath, was in turn interfaced via PTFE tubing to the continuous Crustaceans— HGAAS system. The HPLC separation was performed by Shrimp 09 Peeled shrimp, water, salt, citric acid, isocratic elution with 3 mmol l-1 phosphate buffer of pH 5. 120 g drained mass (Thailand) Quantifications were made by the method of standard 10 Peeled shrimp, water, salt, citric acid, additions and peak area signals were measured.The peak 120 g drained mass (Thailand) Crab 11 Fancy pacific crab, water, salt, areas recorded were the average of at least two injections of stabilizer (E-450) and antioxidant each solution. Spikes of AB were added to samples prior to (H-3246, E-300), 120 g drained mass injection. The amounts of AB added were approximately equal (Chile) to two and three times the AB contents determined previously by comparison with the calibration graph.In order to correct the data for reagent contamination, reagent blanks were canned foods.13 The samples obtained were pressed between taken through the total procedure (extraction, clean-up, two sheets of filter-paper, cut into pieces and frozen at-20 °C; HPLC–MO–HGAAS). The instrumental conditions and ana- they were then freeze-dried for 20 h. Sublimation heat was lytical parameters used are listed in Table 1.supplied by conduction from heating plates at 20°C. The lyophilized samples were crushed and homogenized to a fine RESULTS AND DISCUSSION powder in a water-cooled mill. The resulting powder was stored in previously decontaminated twist-off flasks and stored Extraction at 4°C until analysis. The AB recovery efficiency of the methanol–water extraction was evaluated by spiking three sub-samples of lyophilized Determination of Total As squid samples (mass of sample 2.00±0.01 g; As content 0.19 mg g-1, fm) with 1 ml of 2 mg ml-1 AB as As (0.28 mg g-1 As) The seafood products were dry-ashed, applying the dry minbefore performing the extraction process and determining AB eralization methodology developed previously,14 and total As as As by HGAAS after an ashing step.The mean recovery of was determined by HGAAS. The ash from the mineralized AB was 101±3%. samples was dissolved in 5 ml of 50% v/v HCl, washed with water and filtered through Whatman No. 1 filter-paper into a 25 ml calibrated flask. Determinations of total As were also Clean-up made in dry-ashed solid residues resulting from the methanol– In order to avoid the overlapping of AB and AsIII on the water extraction procedure. Calibration graphs made with AsV chromatographic column and to fractionate the arsenical were employed in each instance. The instrumental conditions species, a clean-up procedure is necessary. Preliminary assays used for the determination of As by HGAAS in continuous- were made with a Hypersep IC-OH anionic cartridge, as flow mode were as follows: atomic absorption spectrometer: employed by Lo�pez-Gonza�lvez et al.11 With this cartridge, wavelength, 193.7 nm; spectral bandpass, 0.7 nm; lamp power, AsIII, AsV, MMA and DMA were retained, while AB and AC 8.5 W (electrodeless discharge lamp).Hydride generation: cell passed through. It was observed that the cartridge does not temperature, 900 °C; sample solution, 1 ml min-1 flow rate; permit on-line separation of AB in samples with a high AB reducing agent, 1.5% m/v NaBH4 in 0.7% m/v NaOH, 1 ml content since the amount of phase employed is low.The need min-1 flow rate; HCl solution, 1.5 mol l-1, 2.5 ml min-1 flow to assay highly diluted samples in order to avoid overloading rate; carrier gas, argon, 45 ml min-1 flow rate. the cartridge would involve greater complexity in the treatment of the sample.Consequently, it was decided not to use the Methanol–Water Extraction cartridge. The quantitative recovery of an aqueous standard of AB The lyophilized seafood products were extracted by applying the methodology developed previously.15 The lyophilized after a Dowex clean-up procedure was described in a previous study.16 In this work, 32.185 ng of AB, expressed as As, sample (2.00±0.01 g) was weighed into a 50 ml centrifuge tube with screw top and conical base. A 40 ml volume of methanol– obtained from an extract of a sample of DORM-1 was quantitatively recovered after passing the extract through the water (1+1 v/v) was added and the tube was agitated for in in a mechanical arm shaker.The extract was collected Dowex resin. This was shown by a comparison of the AB contents found in DORM-1 samples without clean-up after centrifugation at 2000 rev min-1 (378g) for 10 min. The Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 93(16.10±0.82 mg g-1 As, dm) and with clean-up (16.52±0.62 mg were selected on the basis of giving a good separation for AB and DMA while providing the shortest retention times for g-1 As, dm) (dm=dry mass).After the Dowex clean-up, of the six species analysed (AsIII, both species. Fig. 2 shows a typical chromatogram, obtained under AsV, MMA, DMA, AB and AC), only AB and DMA were present in the extracts. Consequently, the overlapping of AsIII the optimum conditions described above and using HPLC–MO–HGAAS, of (a) a standard mixture of AsIII, AsV, and AB, tested in this work and frequently described in the literature, was avoided.9,17,18 The AsIII content in DORM-1 is MMA, DMA, AB and AC (10 ng As, respectively) spiked in aqueous solution, and chromatograms of a cleaned aqueous <50 ng g-1.19 Because of this, and given the high dilution (1+716) employed for the DORM-1 lyophilized sample analy- clam extract (b) and a cleaned aqueous clam extract spiked with standards of AB and DMA (c) (5 ng As both).As can be sis, there will be no overlap of AB with AsIII in the uncleaned sample. The Dowex resin employed in this work has a higher seen, AB is well separated from DMA, which co-elutes with AB in the clean-up step. exchange capacity than the anionic cartridge and can separate AB from AsIII in samples with a wide range of AB concentrations (0.5–16.5 mg g-1 As, dm). The clean-up procedure MO–HGAAS makes possible the isolation of AB from other arsenical species Preliminary MO was performed with an aqueous AB standard (MMA and AsV) with a high retention time (MMA, tR= (100 mg l-1 As), connected on-line with the HGAAS detector. 26 min; AsV, tR=26 min), thereby decreasing the time of analy- Initially, 1% persulfate and a 1.6 m coil (residence time 12 s) sis for each sample.were employed in order to establish the working power and oven load necessary for MO. The maximum power of the Evaporation Temperature Study for the AB Solutions microwave oven (1100 W) was selected as the working power.In order to speed up the process of evaporating the extract to Lower power levels led to intermittent suspension of microwave dryness after extraction and clean-up, recovery assays were radiation, giving rise to chromatographic peak splitting. An carried out with an AB standard (100 mg l-1 As), spiked in additional oven load was necessary to avoid overheating the methanol or ammonia solutions and evaporated at differ- system.A load of 400 ml of water was the minimum volume ent temperatures. AB was subsequently determined using that prevented the water from boiling during the time required HPLC–MO–HGAAS. At a temperature of 80°C, the mean (4 min) for determining AB by HPLC–MO–HGAAS. recovery of AB was 77±15%. AB can readily be converted to With a 1.6 m coil and 1% persulfate in 2.5% NaOH, signals trimethylarsine oxide and/or DMA by heating in the presence of aqueous standards (of 100 mg l-1 As of AsIII, AsV, MMA, of a base.20,21 A temperature of 55°C did not cause losses DMA, AB, AC) were compared by MO–HGAAS.The during the process of evaporating to dryness and the recovery absorbance signals obtained were of the same order for all the of AB was 100±9%. species. Le et al.8 obtained the same result with a 3 m coil and 2.7% potassium persulfate in 1.2% NaOH. The AsIII, AsV and HGAAS Conditions AB signals in 3 mmol l-1 phosphate buffer of pH 5.0 were compared, and it was observed that they provided the same The HCl concentration (3 mol l-1) and flow rate (1.9 ml absorbance readings as those obtained for the aqueous stan- min-1) and the NaBH4 flow rate (1.9 ml min-1) used here dards.Thus, under the conditions selected, complete conversion were the same as those proposed by Lo�pez-Gonza�lvez et al.11 of AB to hydride-forming arsenicals was achieved. in a previous work. Subsequently, different concentrations of NaBH4 (1, 2, 3, 4 and 5%) were tested for aqueous standards HPLC–MO–HGAAS of AsV and DMA (100 mg l-1 of each arsenical compound as As), in order to study the efficiency of AsH3 generation.For The effect of coil length (residence time) and persulfate concen- both arsenical species the signal increased until the NaBH4 tration on MO was studied with aqueous standards of AB and concentration reached 3%, and then remained constant up to concentrations of 4 and 5%.However, the use of NaBH4 concentrations above 2% involves difficult and tedious cleaning of the gas–liquid separator system after analysing each sample, owing to the excessive hydrogen bubbling produced during hydride generation. This makes it impractical to use NaBH4 concentrations above 2% for on-line AB determination. HPLC–MO–HGAAS Conditions HPLC separation of AB The ionic nature of organic and inorganic As compounds makes them amenable to anion-exchange HPLC. This type of chromatography uses buffers compatible with the HGAAS detection system.After the Dowex clean-up used in this work, only AB and DMA were present in the extracts injected onto the chromatographic column. In order to separate AB and DMA, a phosphate buffer and a polymer-based column (PRPX100) were selected which were also used in a previous study to determine MMA and DMA.15 In order to optimize the chromatographic separation of AB and DMA in isocratic mode, the phosphate buffer concentration was varied from 3 Fig. 2 HPLC–MO–HGAAS chromatograms: (a) standard mixture of to 20 mmol l-1 and the pH from 5 to 7. Low-concentration AsIII, AsV, MMA, DMA, AB and AC (10 ng As, respectively) spiked phosphate buffers produced a better resolution for AB and in aqueous solution; (b) cleaned aqueous clam extract; (c) cleaned DMA. A higher pH also improved the resolution of AB and aqueous clam extract spiked with standards of AB and DMA (5 ng DMA. Optimum chromatographic conditions were obtained As both).Peak identification: 1: AC; 2: AB; 3: AsIII; 4: DMA; 5: MMA; and 6: AsV. with a 3 mmol l-1 phosphate buffer of pH 5.0. These conditions 94 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12AsIII (100 mg l-1 As) injected separately onto the chromato- The detection limit, established as the AB concentration in the seafood product that provides an absorbance area reading graphic column and subsequently oxidized in the microwave oven.The AB MO efficiency, expressed in per cent., was statistically different from that of the blank, was calculated by dividing three times the standard deviation of the absorbance obtained by comparing peak area signals from AB with those obtained with AsIII. The data obtained are shown in Fig. 3. area readings, at the retention time of AB, of nine reagent blanks by the slope of the standard additions graph and taking For comparison with the AB signals, AsIII signals were chosen rather than AsV because the former has a shorter retention into account the sample mass and dilution employed.The dilutions used for the analysis of the AB water sample extracts time (3.4 min) than the latter (26 min) and because, as stated above, AsIII and AsV give the same absorbance signals with that provided good peak separation ranged from 1+1 to 1+79, hence, the detection limits for AB vary in accordance MO–HGAAS. Tests with coil lengths of 0.8, 1.0 and 1.3 m showed that with the dilution employed (from 0.68 to 27.20 ng g-1 As, fm).The detection limits for AB on a fresh mass basis were when the persulfate concentration was increased from 1 to 3 or 5% there was an increase in the peak area signals obtained calculated by taking the average moisture content of seafood products to be 75%.3 The precision of the method, calculated for AB, but the signals obtained were always lower than the signals obtained for AsIII. Using a 1.6 m coil, for all the by independent analysis of 11 canned seafood products, analysing each in trcate, ranged from 0.4 to 6%.For DORM-1, persulfate concentrations tested, complete decomposition of AB to arsine-forming species was observed, as shown by the the precision obtained from three sub-samples was 4%. The mean recovery, evaluated by spiking three canned seafood sub- equality of the AB and AsIII peak area signals. In order to reduce consumption of reagents, the definitive working con- samples of shrimps, snails and mussels with AB, was 104±5%.ditions selected were a coil length of 1.6 m (residence time 12 s) and a persulfate concentration of 1%. AB Determination in Real Samples The levels of total As and AB expressed in terms of fresh mass Analytical Features of the Method for all the seafood samples analysed are given in Table 4. For The analytical characteristics (Table 3), such as detection limit, total As, the range found was 0.55–2.65 mg g-1 As (fm). precision and recovery, were evaluated in samples of seafood Samples of cockles, mussels, crab and clams gave values over products prepared as described under Experimental. 2 mg g-1 As (fm). For AB, the levels found ranged from 0.03 to 1.61 mg g-1 As (fm). As can be seen, in all instances AB levels in the samples analysed were above the detection limit of the method. The percentages of total As represented by these contents are also shown in Table 4. The highest percentages of AB (75 and 61%) were found in crab and salmon, respectively.The remaining samples showed an AB content ranging from 5 to 49% of total As. The lowest percentages of AB corresponded, in general, to the bivalve group, with contents of AB ranging from 8 to 36%. These results are similar to those reported by us for percentages of total As represented by AB in canned lamellibranchs in a previous study.3 Other workers have also reported low percentages of As represented by AB: Shibata and Morita22 in NIES (National Institute of Environmental Studies) CRM No. 6 Mussel (13%), and Larsen et al.19 in NIST SRM 1566 Oyster Tissue and NIES CRM No. 6 Mussel Fig. 3 Microwave oxidation efficiency versus residence time of AsIII and AB (100 ng ml-1 As both). A, 1% Potassium persulfate; B, 3% (11 and 14%, respectively). potassium persulfate; C, 5% potassium persulfate; D, arsenite. By considering the levels of AB and total water-soluble As we were able to obtain the As levels that would correspond to water-soluble species other than AB.These levels were around Table 3 Analytical characteristics of the method 1 mg g-1 As (fm) for razor clams and mussels. A proportion of AB as As this As could be due to AsIII, AsV, MMA, DMA, AC, TMAO (trimethylarsine oxide) and TMA+ (tetramethylarsonium ion), Detection limit*/ng g-1 2.72 (dm)#0.68 (fm) 108.80 (dm)#27.20 (fm) and also to arsenosugars. Shibata and Morita22 and Larsen23 have stated that bivalves Precision (RSD) (%)— generally contain not only AB but also As-containing ribofur- Seafood† 0.4–6 anosides as major As species.The relatively large content of DORM-1‡ 4 arsenosugars found in the mussel and oyster samples can be explained by the fact that shellfish feed on marine algae, which Recovery§ (%)— Shrimps 102±7 (0.03, 0.04) are the arsenosugar-synthesizing organisms in the marine food Snails 109±3 (0.62, 0.34) chain.23 There is good agreement for the arsenosugar levels Mussel 100±5 (0.48, 0.22) obtained by the above-mentioned workers in NIES CRM Mean 104±5 No. 6 Mussel, employing a water–methanol–chloroform extraction system23 or a methanol–water system.22 The latter * Nine reagent blanks were employed; the first set of values corre- extraction technique, as modified by us,15 i.e., by changing the spond to a 1+1 dilution and the second set to a 1+79 dilution; results expressed as As; dm=dry mass; fm=fresh mass. † RSD interval volume of methanol–water and reducing the number of extracobtained from independent analysis of 11 canned seafood products.tions, was also employed in this work for the quantitative Each seafood was analysed in triplicate. ‡ Mean RSD obtained from extraction of AB. three independent analyses of the DORM-1 sample. § Percentage The mean As concentration corresponding to species other recoveries expressed as mean±s from three independent analyses. than AB in the canned seafood samples analysed was 1.02 Values in parentheses are the species average concentration of the mg g-1 As (fm), which represents 70% of the mean total As unspiked samples (first value) and concentration of added species (second value) in mg g-1 As (fresh mass).(1.45 mg g-1 As, fm). Obviously, more data on the arsenical Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 95Table 4 Total As, total water-soluble As, water-soluble As corresponding to species other than AB, AB contents and percentages of total As represented by AB in canned seafood samples Total water-soluble Water-soluble As Seafood product Sample Total As* AB* AB (%)† As*,‡ other than AB* Fish— Salmon 01 0.23 0.14 61 0.17 0.03 L amellibranchs— Razor clams 02 1.55 0.13 8 0.96 0.83 03 0.62 0.07 11 0.35 0.28 Cockles 04 2.06 0.10 5 0.50 0.40 Langostillo 05 1.92 0.69 36 1.35 0.66 Mussel 06 2.65 0.48 18 1.22 0.74 Clams 07 2.06 0.44 21 1.02 0.58 Gastropods— Snails 08 1.27 0.62 49 0.78 0.16 Crustaceans— Shrimps 09 0.87 0.40 46 0.62 0.22 10 0.55 0.03 5 0.33 0.30 Crab 11 2.15 1.61 75 2.01 0.40 * Results expressed in mg g-1 As, fresh mass.† Percentages of total As. ‡ Total As in samples-As in solid residue resulting from the methanol– water extraction. species contained in seafood are needed.24 Also, studies of the REFERENCES possible effects of these compounds on humans are required. 1 Morita, M., and Edmonds, J. S., Pure Appl. Chem., 1992, 64, 575. The AB content detected in DORM-1 (16.5±0.6 mg g-1 As, 2 Phillips, D.J. H., Aquat. T oxicol., 1990, 16, 151. dm) using HPLC–MO–HGAAS is compared in Table 5 with 3 Ve� lez, D., Yba�n� ez, N., and Montoro, R., J. Agric. Food Chem., results reported in the literature. The value obtained here was 1995, 43, 1289. 4 Atallah, R. H., and Kalman, D. A., T alanta, 1991, 38, 167. very close to those obtained by other workers using HPLC– 5 Violante, N., Petrucci, F., La Torre, F., and Caroli, S., ICP–MS,19,20,25,26 and also to the result obtained by us in a Spectroscopy, 1992, 7, 36. previous study to determine AB by HPLC–ICP.16 6 Howard, A.G., and Hunt, L. E., Anal. Chem., 1993, 65, 2995. 7 Albertý�, J., Rubio, R., and Rauret, G., Fresenius’ J. Anal. Chem., 1995, 351, 415. CONCLUSIONS 8 Le, X.-C., Cullen, W. R., and Reimer, K. J., Appl. Organomet. Chem., 1992, 6, 161. The proposed method permits the sensitive, precise and accu- 9 Le, X.-C., Cullen, W. R., and Reimer, K.J., T alanta, 1994, 41, 495. rate determination of AB in real samples representative of 10 Lo�pez-Gonzalvez, M. A., Go�mez, M. M., Ca�mara, C., and seafood products. Moreover, the methodology described here Palacios, M. A., Mikrochim. Acta, 1995, 120, 301. provides the performance required for use with the instrumen- 11 Lo�pez-Gonza�lvez, M. A., Go�mez, M. M, Ca�mara, C., and tation available in many control laboratories. The high level Palacios, M. A., J. Anal. At. Spectrom., 1994, 9, 291.of arsenical species other than AB found in some samples 12 Martin, I., Lo�pez-Gonza�lvez, M. A., Go�mez, M. M., Ca�mara, C., and Palacios, M. A., J. Chromatogr. B, Biomed. Appl., 1995, emphasizes the need to obtain a better understanding of the 666, 101. As species composition of seafood. The hyphenated technique 13 Presidencia de Gobierno, Orden 21 de noviembre de 1984 por la proposed here could make a substantial contribution to this que se aprueban las normas de calidad para conservas, BOE aim.(Boletý�n Oficial del Estado), 30/11, 1–3/12/1984, 287, 34574. 14 Yba�n� ez, N., Cervera, M. L., and Montoro, R., Anal. Chim. Acta, Funds to carry out this work were provided by the Comisio�n 1992, 258, 61. 15 Ve� lez, D., Yba�n� ez, N., and Montoro, R., J.encia y Tecnologý�a (CICyT), Project 1996, 11, 271. ALI92-0147. D.V. received a Research Personnel Training 16 Yba�n� ez N., Ve�lez, D., Tejedor, W., and Montoro, R., J. Anal. At. Grant from the Ministerio de Educacio�n y Ciencia. We are Spectrom., 1995, 10, 459. grateful to Maite de la Flor for her assistance in the perform- 17 Lo�pez-Gonza�lvez, M. A., Go�mez, M. M., Palacios, M., and ance of analytical work. We also thank the Service Central Ca�mara, C., Fresenius’ J. Anal. Chem., 1993, 346, 643. d’Analyse du CNRS (SCA) of Vernaisson (France) for provid- 18 Caroli, S., La Torre, F., Petrucci, F., and Violante, N., Environ. Sci. Pollut. Res., 1994, 4, 205. ing the arsenobetaine and arsenocholine standards. 19 Larsen, E. H., Pritzl, G., and Hansen, S. H., J. Anal. At. Spectrom., 1993, 8, 1075. 20 Beauchemin, D., Bednas, M. E., Berman, S. S., McLaren, J. W., Siu, K. W. M., and Sturgeon, R. E., Anal. Chem., 1988, 60, 2209. Table 5 Determination of AB in DORM-1 sample; results in mg g-1 21 Kaise, T., Yamauchi, H., Hirayama, T., and Fukui, S., Appl. As dry mass Organomet. Chem., 1988, 2, 339. 22 Shibata, Y., and Morita, M., Appl. Organomet. Chem., 1992, 6, 343. AB Analytical technique ref. 23 Larsen, E. H., Fresenius’ J. Anal. Chem., 1995, 351, 582. 15.7±0.8 HPLC–ICP-MS 20 24 Vather, M., Clin. Chem., 1994, 40, 679. 15.7 HPLC–ICP-MS 25 25 Shibata, Y., and Morita, M., Anal. Chem., 1989, 61, 2116. 14.2 HPLC–ICP-MS 19 26 Branch, S., Ebdon, L., and O’Neill, P., J. Anal. At. Spectrom., 15.1±0.6* HPLC–ICP-MS 26 1994, 9, 33. 16.1±0.4* HPLC–ICP-MS 26 16.5±0.9 HPLC–ICP-AES 16 Paper 6/02769E 16.5±0.6 HPLC–MO–HGAAS This work Received April 22, 1996 Accepted October 2, 1996 * Values obtained by employing two extraction procedures. 96 Journal of Analytical Atomic Spectrometry, Jan

ISSN:0267-9477    

DOI:10.1039/a602769e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Analysis of Geological Materials for Bismuth, Antimony, Selenium and Tellurium by Continuous Flow Hydride Generation Inductively Coupled Plasma Mass Spectrometry Part 1. Mutual Hydride Interferences GWENDY E. M. HALL* AND JEAN-CLAUDE PELCHAT Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8 A study was made of mutual interferences in the determination dearth of data for the low abundance elements, Bi and Te, in soils (e.g., Table 1) indicates an absence of sufficiently sensitive of Bi, Sb, Se and Te by hydride generation inductively coupled plasma mass spectrometry (HG–ICP-MS).The design of the and robust analytical methods for their measurement. Although also an element of low abundance, the geochemistry study was based upon the valency states in which these elements would occur following two acid digestion procedures of Se is better known because it has received considerable attention in agricultural and epidemiology studies, being an commonly used for geological materials (aqua regia and HF–HClO4–HNO3–HCl), that is, as BiIII, SbV, SeIV and TeIV. essential element which exhibits a narrow range between deficiency (e.g., leading to Keshan and Keshan-Beck disease) Arsenic in its two valency states (III and V ) was also examined as a potential interferent but Ge, Sn and Pb were not, as and toxicity (selenosis, capable of causing death).4 In order to improve our understanding of the processes controlling the formation of their hydrides at the concentration of HCl used (2–4m) would be negligible.Interferents were investigated at distribution of these elements in the surficial environment and to search for anomalous concentrations in mineral exploration, concentrations up to 2000 mg l-1 while the analyte concentration was held at 0.2 mg l-1. AsV and SeIV severely analytical methods must provide the capability of accurate and precise detection at, and indeed below, the natural back- suppressed the signal for Te in both 2 and 4 m HCl, though less so in the stronger acid medium.These interferences were ground concentrations of the various media examined. At the Geological Survey of Canada (GSC), we have used negated by reduction of AsV to the non-interfering state AsIII and SeIV to SeO by addition of KI and ascorbic acid at a final hydride generation quartz tube atomic absorption spectrometry (HG–QTAAS) to determine these elements in tens strength of 0.005%.The preferred acid medium was selected as 4 m HCl for two reasons: interferences were reduced; and of thousands of geological samples (mainly rocks, soils and sediments) since 1975.5 While this technique has served well in wash-out time between samples was shorter (i.e., decreased memory effect). Mutual interferences of possible concern analysis for As and Sb, it has shortcomings of both inadequate sensitivity and numerous interferences for the other three comprise the effect of: 2500-fold excess of Bi on the measurement of Te; a 5000-fold excess of Te on Bi; a elements.Given the wide range of matrix composition expected amongst these samples, separation of the analytes from the 2500-fold excess of Bi on Sb; and a 5000-fold excess of Bi on Se. However, these relative abundances in geological materials well documented liquid phase interferents (e.g., Cu, Ni, Co) has been used routinely in preference to addition of complexing (rocks, sediments, soils, vegetation) would be extraordinary and hence implementation of the recommended scheme should reagents such as tartrate, citrate, cysteine or ethylenediaminetetraacetic acid.6 Coprecipitation with La(OH)3 is used and, if produce analysis for Bi, Sb, Se and Te by HG–ICP-MS that is free of mutual interference.high concentrations of transition metals are present (i.e., >2% in Ni, Cu), a second precipitation may be necessary.7 Keywords: Bismuth; antimony; selenium; tellurium ; hydride Suppression by precious metals is not negated by this separa- generation inductively coupled plasma mass spectrometry ; tion but their natural background concentrations are very low geological ; interferences (<5 mg kg-1).This leaves gas phase interferences of major concern, both The fact that group VA and VIA elements form gaseous in transport to the measurement cell (in transport kinetics and hydrides at ambient temperature (e.g., AsH3, SbH3 , BiH3, SeH2 efficiency) and in the atomiser itself, an area of considerable and TeH2) has long been used as a mode of improving their debate in terms of mechanisms.8–11 Two mechanisms were method of measurement in many different sample types, includ- proposed by Dedina8: the interferents may reduce the popuing geological materials.1 These elements are chalcophile in lation of radicals responsible for the atomisation of the analyte nature, that is, they have an affinity for S and hence are found by accelerating their decay (radical population interference); concentrated in sulfides. They are used in mineral exploration as pathfinders for epithermal precious metal and base metal Table 1 Average natural ranges in mg kg-1 reported for As, Bi, Sb, Se and Te in various geological media3 deposits.2 The following average concentrations, in mg kg-1, of these elements in igneous rocks provide a general guide to Element Magmatic rocks Shale Limestone Soil their expected background values: As, 2; Bi, 0.1; Sb, 0.1; Se, As 0.5–2.5 5–13 1.0–2.4 <1–95 0.1; and Te, 0.002.However, the compositions of many types Bi 0.001–0.15 0.05–0.50 0.10–0.20 of rocks differ, and perhaps a better appreciation of element Sb 0.1–1.0 0.8–1.5 0.3 0.05–2.3 ranges can be gleaned from Table 1, taken from summary Se 0.01–0.05 0.6 0.03–0.1 0.02–2.3 tables by Kabata-Pendias and Pendias.3 Note the enriched Te 0.001–0.005 0.01 background of these elements in sedimentary shale media.The Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (97–102) 97and the interferents accelerate the decay of the analyte atoms themselves (analyte decay interference). Using thermodynamic calculations coupled with spectroscopic and thermal investigations, Dittrich and Mandry11 found the main cause of gas phase interference to be the formation of diatomic molecules (e.g., AsSb) and therefore suggested the use of a higher atomisation temperature (>2000 °C).They classified mutual hydride interferences into the following three categories: (1) those which involve only molecule formation in the gaseous phase (As in Sb matrix, Sb in As); (2) those which include both molecule formation in the gaseous phase, and chemical reaction and adsorption in the liquid hydride generation phase (As, Sb and Se in Bi, Se, Ge, Sn and Pb matrices); and (3) those which originate by chemical reaction and adsorption in the liquid phase (Te in As, Sb, Bi, Se, Ge, Sn and Pb matrices, and As, Fig. 1 Schematic diagram of the GSC hydride generation system. Sb and Se in Te matrices). With consideration to the relative Introduction of KI is optional (see text). natural abundances of these elements, the determination of Te in samples containing significant amounts of As would appear to be the most challenging. Indeed, using the GSC deliver solutions to the HG system and for this work the HG–QTAAS method5 the presence of 2000 mg l-1 of As was sample tubing was placed in test-tubes manually (rather than found to suppress the signal for 5 mg l-1 of Te by 60%, a ratio using the Gilson 222 autosampler). The hydrides were trans- of interferent to analyte which is to be expected in geoanalysis.ported via Tygon tubing (about 3.2 mm id) directly to the ICP Clearly, the combination of hydride generation with induc- torch without the spray chamber in place. Stability of the tively coupled plasma mass or atomic emission spectrometry hydride signal was improved by the additional flow of Ar at (ICP-MS, ICP-AES) should be much freer of these gas phase 0.1 l min-1 to the gas–liquid separator (Fig. 1) and by consist- interferences than HG–QTAAS. Wickstrøm et al.,12 for ent pumping of the liquid waste to the drain. Hardware and example, reported significant reduction of gas phase inter- operating conditions are given in Table 2. Two isotopes each ferences from Sn and As in the analysis of Ni alloys and of Se, Sb and Te were measured in these tests to ensure low-alloy steels for Se by HG–ICP-AES compared with consistent response. A period of about 50 s was allowed to HG–QTAAS.Uggerud and Lund13 studied the effect of 10 lapse between initial uptake of sample solution and actual and 100 mg l-1 of As and Sb on the HG–ICP-AES signal measurement. intensity of 100 mg l-1 of As, Sb, Bi, Se and Te. The signals for Bi and Se were mostly enhanced by As and Sb (up to 128%), Reagents and Solutions whereas that for Te was strongly depressed (e.g., to 2 and 22% in 100 and 10 mg l-1 of As, respectively). All reagents were of analytical-reagent grade, purchased mostly Since the early preliminary report by Powell et al.14 demon- from Baker (Phillipsburg, NJ, USA); distilled, de-ionised water strating the detection capability of continuous flow was used throughout for dilution of standard solutions.The HG–ICP-MS in the ng l-1 range for As, Se, Sb, Bi and Te, NaBH4 reducing agent, stabilised in 0.1 M NaOH and filtered, there have been relatively few applications of this technique in was prepared daily.Standard solutions containing 1000 mg l-1 the literature, and very few indeed focusing on the analysis of geological materials.15 Mutual interference (amongst others) in Table 2 HG–ICP-MS hardware and operating conditions the measurement of Se by flow injection HG–ICP-MS was studied recently:16 at an interferent5Se ratio of 1000, As and Inductively coupled plasma Sb at the III or V valency state had no effect, while at an Rf generator 2.5 kW, frequency 27.12 MHz interfereent5Se ratio of 100, BiIII and TeIV had no effect.Nebuliser Meinhard C concentric glas The intention of this paper is to report the mutual inter- Torch SCIEX, ‘long’ Distance from load coil to ferences encountered in continuous flow HG–ICP-MS and to sampler orifice 16 mm design appropriate methodology to counteract them for accu- Rf power 1.1 kW rate and efficient analysis of geological materials.Mutual Plasma Ar flow rate 12 l min-1 interferences from Group IVA elements were not investigated Intermediate Ar flow rate 2.1 l min-1 as their hydride formation would be minimal at the strength of HCl (2–4 M) used in the generation medium here. Part 2 Mass spectrometer Sample Nickel, 1.14-mm orifice will describe the decomposition and separation procedures Skimmer Nickel, 0.89-mm orifice developed, together with results for geological standard Ion lens settings B=17, El=83, P=13, S2=28 reference materials.Resolution High (0.6 u 10% peak height) Data acquisition parameters Measurement time/isotope= 1.8 s; dwell time=200 ms in EXPERIMENTAL multichannel mode (peak hopping), 3 sweeps/3 Instrumentation readings/3 replicates each solution The ICP mass spectrometer employed was a SCIEX Perkin- Isotopes measure 78Se, 82Se, 121Sb, 123Sb, 128Te, Elmer (Thornhill, Ontario, Canada) ELAN Model 250, 130Te, 209Bi upgraded to the 500.Matheson mass flow controllers were used for the plasma, intermediate and carrier gases. [This Hydride generation Reducing agent 1% NaBH4 in 0.1 mol l-1 instrument was replaced recently with the VG PlasmaQuad NaOH PQII+ (VG Elemental, Winsford, Cheshire, UK) and all Flow rate of reducing agent 1.1 ml min-1 previous results were verified with this equipment.] The hydride Flow rate of acidified sample 3.6 ml min-1 generation system was built at the GSC and is shown sche- Flow rate of carrier gas (Ar) 1.9 l min-1 matically in Fig. 1. A Minipuls 3 peristaltic pump was used to 98 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12each of BiIII, SeIV and TeIV were purchased from Plasmachem (Bradley Beach, NJ, USA). AsIII and AsV stock solutions at 100 mg l-1 were both made by using the primary standard As2O3 dissolved in a small amount of NaOH and then acidified with HNO3. To prepare the AsV standard, a portion of this solution was oxidised by addition of H2O2 and boiled in an HNO3 medium.AsIII was kept in 2% HCl whereas AsV was kept in 2% HNO3. SbIII and SbV stock solutions at 100 mg l-1 were prepared from Sb2O3 and Sb2O5, respectively; both were kept in 1 M HCl. Fe and La were added as their trivalent chlorides in the interference studies. Procedure The element of lowest natural abundance, Te, was used for optimisation studies of: carrier gas flow rate; plasma power; Fig. 2 Optimisation of Ar carrier gas flow rate for 0.2 ppb (mg l-1) HCl concentration; and NaBH4 concentration. Response under of Te by HG–ICP-MS. these conditions, optimal for Te, was then tested for Bi, Sb and Se. The acid decomposition procedures to be used (aqua regia and HF–HClO4–HNO3–HCl) result in the following valency states: AsV; Bi2I; SbV; SeIV and TeIV (discussed in Part 2 of this series). Thus, Bi, Se and Te are in their reactive forms (cf., SeVI, TeVI), whereas SbV must be converted to SbIII for greatest sensitivity, a reduction usually carried out with KI and ascorbic acid.5 The effects of SeVI and TeVI as possible interferents therefore were not required to be evaluated but both valency states of As and Sb were included in the tests.The following interferents, at levels of 50, 100, 250, 500, 1000 and 2000 mg l-1, were examined for their effect on the signal for 0.2 mg l-1 of TeIV: AsIII, AsV, BiIII, SbIII, SbV and SeIV . This study was carried out at two concentrations of HCl, 2 and 4 M, and was repeated in 4 M HCl in the presence of KI at various concentrations.Care was taken to measure the interferent solutions alone for their possible Te content. Fig. 3 Optimisation of plasma power for 0.2 ppb (mg l-1) of Te by The signal intensity for 0.2 mg l-1 of SeIV in 4 M HCl was HG–ICP-MS. then examined in the presence of 50–2000 mg l-1 of AsV, AsIII, SbV, SbIII , Bi and Te. KI was not included in the test for Se as it is known to reduce Se to the unreactive SeO state.Bi as an analyte was examined similarly, with and without KI present. Finally, SbIII as an analyte was examined, using both SbIII directly from the standard solution (made as SbIII) and SbIII prepared by reducing SbV with KI, which would be required after either acid decomposition. The effect of Fe, at concentrations of 100–2000 mg l-1, was also examined on the signal of 0.2 mg l-1 of Bi, Sb, Se and Te.Fe can be a major constituent of geological samples and comes through the separation procedure with the analytes. Any influence from La, at concentrations of 0.1–1% in the analyte solution, was also checked for the four analytes. RESULTS AND DISCUSSION Fig. 4 Effect of HCl concentration in analyte solution on the 0.2 ppb Optimisation of Hydride Generation Conditions (mg l-1) 130Te signal intensity. Dependence of the Te signal on carrier gas flow rate, plasma power and HCl concentration is shown in Figs. 2–4, respectively. The concentration of NaBH4 in the range 0.5–3% was 5 M. An HCl concentration of 4 M was chosen but interference studies for Te were also carried out at 2 M HCl to test for not critical but signals tended to be noisier at 3%; 1% NaBH4 was selected. Fortunately, the other three analytes behaved in changes in severity with acid strength. The washout time required in 4 M HCl appears to be in the order of 100 s (Fig. 4), a similar manner to Te and therefore optimum conditions for all four elements were chosen to be 1.9 l min-1 carrier gas but this can be decreasedsubstantially to about 60 s if solutions containing analyte at similar concentrations are being run.flow rate and 1.1 kW power. Increase in signal intensity with HCl concentration levels off at 2 M (Fig. 4), but a higher Under these operating conditions, the net intensity (blank subtracted) obtained by hydride generation in 4 M HCl was concentration of 4 M was found to be beneficial in decreasing the amount of wash-out time required between samples (Fig. 5). compared with that found by nebulisation of the analytes in 4% HNO3 solution. Solutions containing 0, 0.2 and 0.5 mg l-1 Again, the behaviour of Te was typical of all analytes. The percentage relative standard deviation (RSD) for 10 readings analyte were used for HG, whereas standard solutions of 0, 5, 10 and 20 mg l-1 analyte were used for nebulisation ICP-MS. of a 0.2 mg l-1 Te solution at various concentrations of HCl was: 2.9 at 1 M; 2.7 at 2 M; 1.7 at 3 M; 1.8 at 4 M; and 2.1 at Summary data are given in Table 3, showing a maximum Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 99Fig. 5 Effect of HCl concentration on wash-out time for 2 ppb (mg l-1) of Te. Table 3 Comparison of sensitivity by hydride generation (HG) versus nebulisation (NEB) ICP-MS for Bi, Sb, Se and Te (10 readings for each solution) (Net c s-1/mg l-1)HG/ Analyte (Net c s-1/mg l-1)NEB c s-1 for 0.2 mg l-1 by HG Bi 264 57 040±548 Sb 359 69 660±727 Se 1110 13 270±240 Fig. 6 Effect of AsV on 130Te at 0.2 ppb (mg l-1): (a), in 2 M HCl; (b), Te 897 13 480±160 in 4 M HCl. improvement for Se of over a 1000-fold increase in sensitivity by HG. Mutual Interferences T ellurium An interference in this work is defined as an effect causing a positive or negative change in the analyte signal of 10% or more. No interference is evident in the measurement of 0.2 mg l-1 of Te, in 2 or 4 M HCl, in the presence of up to 2000 mg l-1 of AsIII, SbIII or SbV.However, AsV causes a dramatic decrease of the Te signal in 2 M HCl [Fig. 6(a)]. This decrease, though certainly still a problem, is considerably less severe in the 4 M HCl medium [Fig. 6(b)]. Thus, at a ratio of 500051 As to Te (not uncommon in geological materials), a suppression of about 35% can be expected. This interference was not reduced by changing the strength of NaBH4.Increasing concentrations of Bi beyond 500 mg l-1 led to a severe suppression of the signal for 0.2 mg l-1 Te [Figs. 7(a) and (b)]. As with AsV, the degree of suppression is greater in 2 M HCl. Furthermore, once these high concentrations of Bi had been run, the 0.2 mg l-1 Te alone did not regain its initial intensity of about 26×103 c s-1 (counts s-1), which is unlike the situation in the interference by AsV. The generation system and torch required cleaning to restore performance. Bismuth hydride is known to be both thermally and kinetically unstable, Fig. 7 Effect of Bi on 130Te at 0.2 ppb (mg l-1): (a), in 2 M HCl; (b), in even at 25°C.A rate constant of 0.24 min-1 at room tempera- 4 M HCl. ture has been determined17 for the first order reaction BiH3�Bi+3/2H2 intensity of the 0.2 mg l-1 Te signal and again the effect was more severe at 2 M rather than 4 M HCl [Figs. 8(a) and (b)]. Thus, the plating out of Bi on the walls of the tubing, separator and torch is causing sequestering and decomposition of TeH2.Unlike the situation with Bi, the original sensitivity was obtained for 0.2 mg l-1 of Te alone when run at the end of the Fortunately, this suppression is not evident at levels of Bi below about 500 mg l-1 (i.e., a 2500-fold excess), allowing interference study. The tolerance limits for both Se and Bi appear to be at about 250051 excess over Te but the suppres- accurate determination of Te in the majority of geological samples.sion by Se is less drastic than that of Bi [cf., Figs. 7(b) and 8(b)]. Of the three mutual interferences discussed above, that from Selenium at levels above 500 mg l-1 also decreased the 100 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12(by way of their precipitation as TeO and BiOI) as well as from Se but the strength of KI was much higher in that case (0.25%). Given the reduced degree of interference generally in 4 M rather than 2 M HCl, studies of Bi, Sb and Se were carried out in the higher strength acid.Other workers have recommended high acid strengths to reduce interferences. For example, using HG–QTAAS, Hershey and Keliher19 studied the effect of progressively increasing HCl strength from 1.2 to 7.2 M and found this to reduce the interferences of: Se on Bi; Au and Ir on Se and Bi; W on Se; and Ir and Pt on Sb. Bismuth There was no interference on 0.2 mg l-1 Bi from AsIII, AsV, SbIII, SbV or Se at levels up to 2000 mg l-1.In one of the trials there was an indication of a slight enhancement of the Bi signal in the presence of 500 mg l-1 or more of SbIII, but the change in intensity was only about 7–9%. A marked suppression is evident for 0.2 mg l-1 of Bi in the presence of greater than 1000 mg l-1 of Te (Fig. 9); the intensity is reduced by about 50% at a Te concentration of 2000 mg l-1 (10000-fold excess). In fact, analysis of the 0.2 mg l-1 Bi alone following the solution containing 2000 mg l-1 of Te produces a signal of only 70×103 c s-1 (cf., 80×103 c s-1 at the beginning of the run), suggesting memory effects of Te in the generation system and possibly the central channel of the torch.However, this residual effect is much stronger in the reverse situation, i.e., Fig. 8 Effect of Se on 130Te at 0.2 ppb (mg l-1): (a), in 2 M HCl; (b); the influence of Bi on the determination of Te. This interference 4 M HCl. pattern was unchanged in the presence of KI (at a level of 0.005%).The apparent enhancement of the signal for 0.2 mg l-1 AsV, the most abundant of the elements, is the most serious. of Bi below about 1000 mg l-1 of Te is due to the Bi content However, the simple solution is to reduce AsV to AsIII by of the Te interferent solution added (see 1000 mg l-1 Te blank, addition of KI and ascorbic acid. This was carried out on-line Fig. 9). Given the expected relative abundances of Bi and Te using 5% KI but was rejected for two reasons: firstly, blank in geological media, interference from Te does not pose a values for Bi rose to unacceptably high levels; and secondly, a problem.long coil was needed to provide sufficient time for the reduction to occur. It was decided instead to add a much smaller quantity Antimony of KI manually several hours prior to analysis. Test solutions containing 0.2 mg l-1 of Te, 50 mg l-1 of As and 10 mg l-1 of There was no interference on 0.2 mg l-1 SbIII from AsIII, AsV, Bi in 4 M HCl were made up to the following concentrations Se or Te at levels up to 2000 mg l-1.However, a slight in KI and ascorbic acid: 0.001; 0.002; 0.005; 0.0075; 0.01; 0.0125; enhancement was evident in the presence of 250 mg l-1 of Bi, 0.02; and 0.025%. By monitoring the As intensity at 75 u, it which became essentially constant from 500 to 2000 mg l-1 was found that the AsV was fully reduced to AsIII at a minimum (Fig. 10). This enhancement, of the order of 10%, is seen KI concentration of 0.005%, whereas the Te signal began to whether the test is carried out with SbIII (from SbIII stock decrease steadily at KI concentrations above 0.0125%.solution) or SbV is used and the solution made 0.005% in KI. Presumably this decrease was due to the formation of unreac- This relatively small effect on the Sb signal contrasts sharply tive TeO. The signal for Bi was unaffected by any of these with the severe suppression created by 1000 mg l-1 or more of concentrations of KI.Thus, an optimum KI and ascorbic acid Bi on the Te signal. Excess amounts of Bi over Sb, by several concentration in the 4 M HCl of 0.005% was chosen and added orders of magnitude, would be extremely rare in geoanalysis 3 h prior to the determination of Te. Repeated interference studies under these conditions showed no interference from AsV (or Sb) and, furthermore, no interference from Se, presumably having been reduced to the unreactive SeO state (a more favourable reduction than for Te).Subsequent analyses of numerous samples have shown that these solutions should not be left for more than 12 h before analysis as reductions in Te signals have been observed. Using this scheme, the remaining interference from Bi (unaffected by the presence of KI), shown in Fig. 7(b), is tolerable for the vast majority of geological samples. The associated mineralogy of these elements suggests that where the concentration of Bi is high, it is likely that Te is also anomalous and hence dilution could be used to negate this suppression.Clearly, the presence of KI would negate the possibility of measuring Se in t same solution as that for Te but Bi and Sb could both be determined with Te. It should be noted that Barth et al.18 have suggested the addition of KI in the Fig. 9 Effect of Te on 209Bi at 0.2 ppb (mg l-1) in 4M HCl. determination of Sb to eliminate interferences from Te and Bi Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 101addition of a small amount of KI and ascorbic acid, at a strength of 0.005% in 4 M HCl. Other mutual interferences of possible concern comprise the effect of: a 5000-fold excess of Te on Bi; a 2500-fold excess of Bi on Sb; and a 5000-fold excess of Bi on Se. However, the likelihood of these concentration ratios arising in geological materials is minimal. If the levels of low abundance elements, such as Te and Bi, were high (i.e., in mineralised samples), then it is probable that Se and Sb would also be elevated and dilution could be used to negate interference.Attempts have not been made here to elucidate the mechanisms of interference, other than to distinguish between the mode of suppression by Bi on Te and Se (by action of deposited Bi sequestering the hydride) and that of AsV on Te (by action during hydride generation). Rather, our goal has been to obviate such interferences by sample treatment (addition of Fig. 10 Effect of Bi on 121Sb at 0.2 ppb (mg l-1) in 4M HCl. KI) and hence, using this scheme, mutual interferences in HG–ICP-MS are not of concern for the analysis of geological materials. Clearly, analysis for these four elements must be carried out in two runs, one for Se without KI and the other for Te, Bi and Sb with KI. Arsenic may be determined with either run, as AsV with Se or alternatively as AsIII with the other elements. The authors are grateful to Nimal de Silva for converting the original continuous flow HG scheme to a miniaturised version with improved conditions of mixing and flow.Thanks are also given to Gilles Gauthier and Alice MacLaurin for assistance in this development. REFERENCES 1 Nakahara, T., Spectrochim. Acta, Rev., 1991, 14, 95. 2 Rose, A. W., Hawkes, H. E., and Webb, J. S., Geochemistry in Fig. 11 Effect of Bi on 78Se at 0.2 ppb (mg l-1) in 4M HCl. Mineral Exploration, Academic Press, New York, 1979. 3 Kabata-Pendias, A., and Pendias, H., T race Elements in Soils and and, if so, calibration by standard addition would appear Plants, CRC Press, Boca Raton, Florida, 1985. appropriate. 4 Merian, E., Clarkson, T. W., and Fishbein, L., Metals and their Compounds in the Environment, UHC Verlagsgesellchaft, Germany, 1991, pp. 1153–1158. Selenium 5 Aslin, G. E. M., J. Geochem. Explor., 1976, 6, 321. 6 Wickstrøm, T., Lund, W., and Bye, R., J. Anal. At. Spectrom., Neither As nor Sb, in the III or V valency state at concentrations 1995, 10, 803.up to 2000 mg l-1, interfered in the determination of 0.2 mg l-1 7 Bedard, M., and Kerbyson, J. D., Anal. Chem., 1975, 47, 1441. of Se. Tellurium had no effect on the Se signal. Bi appeared to 8 Dedina, J., Anal. Chem., 1982, 54, 2097. 9 Welz, B., and Stauss, P., Spectrochim Acta, Part B, 1993, 48, 951. produce an enhancement initially, but there was as much as 10 Walcerz, M., Bulska, E., and Hulanicki, A., Fresenius’ J.Anal. 0.06 mg l-1 of Se in the 1000 mg l-1 Bi test solution (Fig. 11). Chem., 1993, 346, 622. Clearly, there was a suppression at levels of Bi greater than 11 Dittrich, K., and Mandry, R., Analyst, 1986, 111, 277. 1000 mg l-1, probably with decomposition of SeH2 with 12 Wickstrøm, T., Lund, W., and Bye, R., J. Anal. At. Spectrom., elemental Bi, as with Te. It is interesting that this suppression 1995, 10, 809. is much less severe for Se compared with Te, indicating greater 13 Uggerud, H., and Lund, W. J. Anal. At. Spectrom., 1995, 10, 405. 14 Powell, M. J., Boomer, D. W., and McVicars, R. J., Anal. Chem., stability of its hydride. As is the case for Te and Sb, this 1986, 58, 2867. interference by Bi does not create a problem in geological 15 Hall, G. E. M., J. Geochem. Explor., 1992, 44, 201. samples. 16 Quijano, M. A., Gutie�rrez, A. M., Conde, M. C. P., and Camara, Concentrations of Fe (added as FeCl3 ) and La (added as C., J. Anal. At. Spectrom., 1995, 10, 871. LaCl3) of up to 2000 mg l-1 and 1%, respectively, in the 4 M 17 Fujita, K., and Takada, T., T alanta, 1986, 33, 203. HCl test solution did not have any effect on the signal 18 Barth, P., Krivan, V., and Hausbeck, R., Anal. Chim. Acta, 1992, 263, 111. intensities of Bi, Sb, Se or Te (each at 0.2 mg l-1). Thus, with 19 Hershey, J. W., and Keliher, P. N., Spectrochim. Acta, Part B, a dilution factor of 40 in the analytical preparation scheme, 1986, 41, 713. up to8% Fe in the sample can be toleratedwithout interference. Paper 6/05398J Received August 1, 1996 CONCLUSIONS Accepted October 25, 1996 Of the four analytes studied, Te is the most susceptible to mutual interference, significant suppression beginning at interferent5analyte ratios of 1000 for AsV and 2500 for Bi and Se. However, interference from AsV and Se can be negated by 102 Journal of Analytical Atomic Spectrometry, January 1997, Vol.

ISSN:0267-9477    

DOI:10.1039/a605398j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Analysis of Geological Materials for Bismuth, Antimony, Selenium and Tellurium by Continuous Flow Hydride Generation Inductively Coupled Plasma Mass Spectrometry Part 2.† Methodology and Results GWENDY E. M. HALL* AND JEAN-CLAUDE PELCHAT Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8 A new method to determine Bi, Sb, Se and Te, at These elements and As were determined previously at the Geological Survey of Canada (GSC) by the much less sensitive concentrations as low as their natural concentration ranges in geological samples, has been developed based on continuous technique of hydride generation quartz tube atomic absorption spectrometry (HG–QTAAS), suitable for the element of higher flow hydride generation inductively coupled plasma mass spectrometry.Samples are dissolved by digestion in either aqua natural abundance, As, but of limited capability for the other four elements.4 It was considered desirable in developing the regia or HF–HClO4–HNO3–HCl acid mixtures and analytes separated from potential interferences (e.g., Cu, Ni and Co) by new method to base it on the two acid digestions in common use today in geochemistry laboratories: hot aqua regia and the coprecipitation with La(OH)3.Addition of a small amount of KI and ascorbic acid, to a concentration of 0.005% in the final so-called ‘total’ attack of HF–HClO4–HNO3–HCl.5 It is highly unlikely that these elements would exist with minerals (e.g., analysis medium of 4 m HCl, negates the most serious mutual interference of AsV on Te.Analysis is carried out in two tourmaline, chromitite, spinel and barite), which would not be fully decomposed by the HF–HClO4–HNO3–HCl attack. batches, the first for Se alone, and the second for the other three elements in the presence of KI. The method was applied Thus, a large suite of elements could be determined from these two digestions by other techniques, making this new method to 18 stream sediment and rock standard reference materials from the People’s Republic of China (GSD and GSR series).cost-effective. Precipitation with La(OH)34,6 has been used routinely at the GSC to separate the analytes from interferents Precision is typically in the range 3–8% relative standard deviation, the higher values usually being associated with such as Cu, Ni and Co and is incorporated here. Lanthanum is added to the acid leachate as the nitrate, followed by concentrations in the very low mg kg-1 range.Detection limits were achieved of 1 mg kg-1 for Bi and Te, and 6 mg kg-1 for ammonia solution, thereby precipitating La(OH)3, which carries down the analytes. Elements such as Cu2+ remain in Sb and Se. Bismuth was completely extracted from all samples by both digestions but some matrices required the more solution, presumably as the amine Cu(NH3)42+. The desirability of this choice, rather than complexation for example, comprehensive decomposition of HF–HClO4–HNO3–HCl for full recovery of Te, Se and, to a lesser degree, Sb.has been confirmed recently, when it was demonstrated that elements such as Ni and Co can be transported to various Keywords: Bismuth; antimony; selenium; tellurium ; hydride degrees as an aerosol with the hydrides and hence could cause generation inductively coupled plasma mass spectrometry ; a potential problem beyond the liquid phase.7 Furthermore, a geological ; acid digestion; reference materials flow injection procedure has been developed elsewhere where this precipitation/redissolution step is carried out on-line, using Part 1 of this series1 described the development of an analytical a filterless knotted Microline reactor, to preconcentrate Se.8 scheme, designed to be free of mutual interferences, to deter- The analytes must be in their most reactive states prior to mine Bi, Sb, Se and Te in geological materials by continuous measurement by HG–ICP-MS and these are BiIII, SbIII flow hydride generation inductively coupled plasma mass (cf., SbV), SeIV (cf., SeVI) and TeIV (cf., TeVI).Earlier work at spectrometry (HG–ICP-MS). The potential interference of the GSC using HG–QTAAS to measure these elements followmajor concern was that of AsV on Te, which can be negated ing an aqua regia leach had shown that Se and Te were indeed by reduction to non-interfering AsIII by the addition of a very in their reduced states (IV, not VI) but Sb (and As) was present small amount of KI and ascorbic acid, to a final concentration in its oxidised form (V, not III).Thus, KI would be required to of 0.005% in an analyte medium of 4 M HCl. Tellurium is not reduce Sb. It was not known whether application of the reduced to an unreactive form (TeO) by this concentration of HF–HClO4–HNO3–HCl digestion would also result in KI. Hydride generation from this high strength of HCl was the lower IV valency state for Se and Te, and hence this needed favoured, firstly to minimise not only mutual interferences but investigation. Reduction of Se to the IV state can be also those from other elements,2–3 and secondly to hasten accomplished by addition of KBr or by heating in 4–6 M HCl; clean-out of the HG system between samples.The analytical care should be exercised to prevent further reduction to the procedure was designed to comprise two separate runs: one SeO state, which is unreactive with NaBH4.The reduction in for Se alone, without KI; and the other with KI in solution HCl is as follows: for the determination of Bi, Sb and Te. Instrumental detection limits, based on 3s of 10 readings of the 4 M HCl background HSeO4-+3H++2Cl-=H2SeO3+Cl2(aq)+H2O solution, were 1 ng l-1 for 209Bi and 121Sb, 12 ng l-1 for 78Se and 2 ng l-1 for 130Te. Hill et al.9 calculated the activation energy for this reaction to be 90.4 kJ mol-1 and found the reduction to be complete within 6 min at 70°C.† For Part 1 of this series, see ref. 1. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (103–106) 103The majority of applications of HG–ICP-MS to date in the sample is highly organic, first add 2 ml of 16 M HNO3 and take to insipient dryness.) Next add 10 ml of HF–HClO4–HNO3 literature have not been focused on geological materials5 but the technique has been employed to determine total (by fusion (5+3+2) and cap the tube. Heat for 1 h. Shake twice during this period.Transfer all of the contents of the tube to a Teflon with Na2O2 ) and leachable (by 1 M NH4NO3 ) As and Se in soils.10 Results were presented for three reference materials beaker (with thorough rinsing using water) and evaporate to insipient dryness on a hot-plate (do not bake). Then dissolve (GXR–5, MESS–1 and NIST 1633a), with detection limits based on the fusion preparation of 0.1 and 0.4 mg kg-1 for As the solid in 1 ml of 12 M HCl, with the addition of heat; add 3 ml of 16 M HNO3 and heat gently again.Finally, transfer the and Se, respectively. Reduction of these elements to their reactive valency states was accomplished by heating with KBr solution to a calibrated test-tube and, when cool, make up the volume to 20.0 ml with H2O. prior to analysis. The methods described in this paper were applied to a suite of 18 geological standard reference materials (GRM) from the Separation with L a(OH)3 Institute of Geophysical and Geochemical Prospecting (IGGE), People’s Republic of China.These GRMs have been Take 15.0 ml of either leachate and add 4 ml of 2% La(NO3)3 solution. Add 20 ml of 1+1 ammonia solution. Allow the found, during previous work in the authors’ laboratory, to be exceptionally well prepared and homogeneous.11 solution to stand for 20 min. Cap the test-tube, centrifuge (10 min at 2800 rpm) and decant. Next add 10 ml of 5% ammonia solution to the precipitate, cap the tube, centrifuge EXPERIMENTAL and decant. Then dissolve the precipitate in 4 M HCl and make up the volume to 15.0 ml, ready for analysis.The dilution Part 1 of this series described the hardware and operating conditions used in the analysis by HG–ICP-MS, based on the factor for either leach is 40. The analysis was carried out in two batches: one for Se GSC hydride generation system.1 The hydrides were generated from an analyte solution 4 M in HCl, which was mixed at alone, measuring at 78Se and 82Se, and the other 3 h after 10 ml of a solution 5% in KI and 5% in ascorbic acid had been 3.6 ml min-1 with a solution of 1% NaBH4 (stabilised in 0.1 M NaOH) pumped at a rate of 1.1 ml min-1.From the gas– added to 10 ml of the 4 M HCl solution containing the analytes. For this second run, measurements were made at 121Sb, 123Sb, liquid separator, the hydrides were swept directly into the central channel of the torch at a flow-rate of 1.9 l min-1. The 128Te, 130Te and 209Bi.Sampling from a Gilson 222 autosampler was set up such that analyte solution was pumped through plasma was operated at a power of 1.1 kW. In addition to the reagents listed previously, SeVI and TeVI the system for a total of 50 s prior to actual reading and a similar period was used for the wash-out time (when 4 M HCl stock solutions at 1000 mg ml-1 were made from Na2SeO4 and Na2TeO4 salts (from K&K Chemicals, Plainsview, NY, USA) was being pumped through the system).Calibration solutions in 4 M HCl were at concentrations of 0, 0.2, 0.4 and 1.0 ng ml-1 and stored in 1 M HCl. These were employed to test whether Se and Te were in their reduced valency states (IV) upon of analyte. A 0.2 ng ml-1 standard solution was interspersed every 10 samples to check for drift. completion of the HF-based acid digestion. Procedure RESULTS AND DISCUSSION Signals for the eight blank solutions derived from the two acid The Chinese GRMs selected for study were the drainage sediment series GSD 1–12 and the rock series, GSR 1–6, all digestions (i.e., four from each) were indistinguishable from those for 4 M HCl alone (i.e., the zero calibration solution) for of which had been ball-milled during their preparation to pass a 200-mesh screen. The rock matrices are as follows: GSR-1, Bi and Te.For Sb and Se, respectively, blank values by the aqua regia digestion averaged the equivalent of 8±2 and granite; GSR-2, andesite; GSR-3, basalt; GSR-4, sandstone; GSR-5, shale; and GSR-6, carbonate.These 18 GRMs were 4±1 ng g-1 in an unknown sample. The corresponding values by the HF–HClO4–HNO3–HCl digestion were 9±2 and taken in triplicate through each acid decomposition, as described below, and analysed for Bi, Sb, Se and Te by 4±2 mg kg-1 for Sb and Se. These averages were subtracted from results for samples themselves in the final calculations. It HG–IC-MS following separation with La(OH)3. Intensities for blank solutions taken through both digestions and the was found desirable to monitor the level of Kr in the Ar supply, especially towards the end of a tank, as it had a separation procedure were compared with those for 4 M HCl alone, used as a wash between samples and as the zero point dramatic effect on the background of 82Se.Recoveries for the 40 ng spikes of analytes taken through for calibration. Recoveries through the two methods were determined for three 40 ng spikes (equivalent to 80 mg kg-1 in both procedures were in the range 94–98% with typical relative standard deviations (RSD) of 2–4%.Again, results for the a sample) of BiIII, SbIII, SbV, SeIV, SeVI, TeIV and TeVI. GRMs were adjusted to offset these recovery factors. Of note is the fact that SeVI and TeVI showed 96±3% and 95±4% Aqua regia digestion recovery by the HF–HClO4–HNO3–HCl digestion, reflecting their conversion to the active valency state IV. The final To 0.5 g of sample in a test-tube, add 20 ml of aqua regia (HCl–HNO3, 3+1 v/v).Leave overnight or for several hours solubilisation step, where the residue is heated in HCl and then in a medium of reversed aqua regia, is probably critical (to avoid excessive reaction during the later heating stage). Place the tube in a water- or sand-bath and slowly increase here in ensuring Se and Te are in their reduced state. Similarly, Kuldvere12 reported that Se extracted from geological samples the temperature to 90°C, heating for 3 h.Mix by vortexing several times. Next, cool the tube to room temperature and by either aqua regia or reversed aqua regia was present in the IV state. make the solution up to 20.0 ml with water. Shake it and allow to settle. With a pipette, take 15.0 ml of solution for separation Results of the triplicate analyses of the 18 GRMs by both digestions are presented in Tables 1 (Bi and Sb) and 2 (Se and of the analytes. Te). The measurements of isotopes 78Se, 121Sb and 130Te were used for the information in these tables but the alternative HF–HClO4–HNO3–HCl digestion isotopes of these elements, also monitored, produced identical results.Literature values13,14 are taken from compilations To 0.5 g of sample in a 50 ml polycarbonate (‘pressure’) tube, add 2 ml of 12 M HCl and heat for 20 min at 90°C. (If the published by the host organisation, IGGE; parentheses indicate 104 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 1 Results in triplicate (mean±s in mg kg-1 for 3 separate 0.5 g aliquots) for Bi and Sb in 18 GRMs by HG–ICP-MS following digestion in (1) HF–HClO4–HNO3–HCl (HF) and (2) aqua regia (AR) Bi Sb GRM Literature13,14 HF AR Literature13,14 HF AR GSD-1 0.66±0.11 0.64±0.03 0.58±0.02 0.22±0.1 0.25±0.02 0.16±0.01 GSD-2 1.64±0.17 1.63±0.07 1.64±0.07 0.46±0.17 0.40±0.05 0.41±0.01 GSD-3 0.79±0.14 0.79±0.01 0.80±0.03 5.4±0.8 5.02±0.12 5.11±0.18 GSD-4 0.64±0.15 0.63±0.02 0.61±0.03 1.84±0.27 1.73±0.04 1.66±0.09 GSD-5 2.4±0.4 2.45±0.07 2.40±0.08 3.9±0.7 3.30±0.16 3.25±0.12 GSD-6 5.0±0.6 4.59±0.02 4.61±0.03 1.25±0.33 1.63±0.06 1.53±0.05 GSD-7 0.66±0.14 0.60±0.001 0.61±0.03 2.6±0.3 2.50±0.17 2.49±0.08 GSD-8 0.19±0.04 0.19±0.001 0.18±0.01 0.24±0.09 0.24±0.01 0.22±0.02 GSD-9 0.42 0.42±0.01 0.39±0.01 0.81 0.96±0.02 0.91±0.03 GSD-10 0.38 0.38±0.002 0.32±0.01 6.3 6.19±0.26 6.25±0.25 GSD-11 50 48.6±1.6 49.6±2.0 14.9 14.3±0.4 14.0±0.3 GSD-12 10.9 10.7±0.1 10.5±0.2 24.3 22.0±0.7 21.5±0.3 GSR-1 0.53 0.54±0.02 0.54±0.01 0.21 0.22±0.01 0.14±0.01 GSR-2 0.081 0.07±0.001 0.07±0.01 0.12 0.20±0.01 0.11±0.02 GSR-3 (0.045) 0.007±0.001 0.008±0.001 0.083 0.09±0.01 0.07±0.01 GSR-4 0.18 0.18±0.004 0.17±0.01 0.60 0.72±0.01 0.54±0.01 GSR-5 0.23 0.23±0.003 0.23±0.01 0.17 0.17±0.01 0.14±0.01 GSR-6 0.16 0.16±0.01 0.16±0.01 0.43 0.56±0.03 0.41±0.02 Table 2 Results in triplicate (mean±s in mg kg-1 for 3 separate 0.5 g aliquots) for Se and Te in 18 GRMs by HG–ICP-MS following digestion in (1) HF–HClO4–HNO3–HCl (HF) and (2) aqua regia (AR) Se Te GRM Literature13,14 HF AR Literature13,14 HF AR GSD-1 (110±90) 37±1 39±2 19±2 20±1 GSD-2 (210±100) 137±12 141±4 (31±17) 17±1 13±2 GSD-3 1060±200 836±45 878±61 (150±20) 187±17 98±5 GSD-4 (280±130) 209±9 218±11 (80±20) 58±3 48±3 GSD-5 (360±100) 319±21 338±24 (120±30) 168±13 126±8 GSD-6 (300±130) 220±9 231±10 (140±40) 178±16 157±11 GSD-7 (310±80) 212±10 228±9 (65±20) 94±4 69±3 GSD-8 (150±100) 67±16 49±5 8±1 4±1 GSD-9 160 145±5 125±10 (40) 41±2 31±1 GSD-10 280 293±5 264±12 (90) 75±5 66±2 GSD-11 200 180±6 203±17 (380) 445±20 333±33 GSD-12 250 240±6 224±10 290 350±14 239±10 GSR-1 (59) 10±2 12±2 21 8±1 7±2 GSR-2 (63) 11±1 9±2 17 <1 <1 GSR-3 (86) 37±2 38±1 (22) <1 <1 GSR-4 (98) 76±4 52±2 38 38±1 35±1 GSR-5 (84) 39±2 42±2 (22) 13±2 14±1 GSR-6 99 78±3 68±5 (23) 13±2 16±2 too few data points were available to justify formulation of being lower in Bi and higher in Sb than those recommended by IGGE.recommended values. Uncertainties associated with the recommended values are given where available (i.e., for GSD Of the 18 GRMs, only six have recommended values for Se and even fewer, four, for Te (Table 2), a reflection of the need 1–8). Clearly, despite the number of manipulations involved in either decomposition, reproducibility is excellent for all four for improvement in analytical capabilities in this area. In these 10 cases, agreement with values obtained by the elements.Precision is comparable by either digestion. Taking the HF–based results, the average RSD is 3.1% for Bi, 4.7% HF–HClO4–HNO3–HCl digestion is good except for Te in GSR-1 and GSR-2, and Se in GSR-6. However, the range of for Sb, 6.7% for Se and 8.1% for Te, this trend probably reflecting the elements’ concentration ranges. As expected, data upon which the recommended values are based can be wide, as for example, 50–140 mg kg-1 for Se in GSR-6.14 Given precision is inferior at lower concentrations, for example, at: 14% for Bi in GSR-3 (at 7 mg kg-1); 10% for Sb in GSR-3 these wide ranges in the compilations used to produce recommended values, the disparity between results in this work (at 90 mg kg-1); 20% for Se in GSR-1(at 10 mg kg-1); and 15% for Te in GSR-5 (at 13 mg kg-1).and proposed values (e.g., Se in GSD-1, GSD-2, GSR-1 and GSR-2; Te in GSD-4, GSD-11 and GSR-3) is not of concern. For Bi, agreement with recommended values of results by both digestions is excellent, even at the lower concentration For example, the spread in the six data points for assignment of Te in GSD-4 is from 30 to 90 mg kg-1, and the analytical range of 0.1–1 mg kg-1 (Table 1).Agreement remains good for Sb but there is a tendency for lower results to be obtained methods used are almost exclusively polarographic.13 Note in Table 2 the tendency for the aqua regia data in some samples by the aqua regia digestion, specifically in GSD-1 (0.16 versus 0.25 mg kg-1 ), GSR-1, GSR-2, GSR-4 and GSR-6.Given the to be lower than those by the HF-based attack, particularly for Se in GSR-4 and Te in GSDs 3, 5, 7, 11 and 12. These very low standard deviations associated with the mean values, these differences appear to be significant (Table 1). Results for differences could indicate that portion of element bound as or encapsulated in a silicate matrix (which is liberated by HF).GSD-6 by this method, using either decomposition, tend to Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 105Table 3 Comparison of values for Te by this method, using aqua The behaviour of Bi shown in this study of being liberated regia (n=3), and by the method of Kontas et al.15 using Br2–HBr (n=3) completely by both acid digestions without exception, is consistent with the findings of Kuldvere.12 He examined the Te/mg kg-1 extraction of Bi, As, Sb and Se from geological materials by six different acid combinations (mainly of HCl and HNO3 , GRM Aqua regia Br2–HBr alone and in combination). While Bi was extracted fully by all GSD-9 31±1 28±1 media, Se was hardly dissolved by HCl and Sb was completely GSD-10 66±2 77±1 extracted by only two media, aqua regia and reversed aqua GSD-11 333±33 460±10 GSD-12 239±10 250±10 regia.The finding that an element such as Te may be present GSR-1 10±1 7±1 in a mineral occluded in or protected by siliceous material is GSR-2 <1 1±0.5 not altogether surprising, but detection limits for this low GSR-3 <1 1±0.5 abundance element have hitherto been inadequate to demon- GSR-4 35±2 29±2 strate this occurrence.Table 3 compares Te data by the aqua GSR-5 14±1 12±1 regia leach with concentrations reported by Kontas et al.15 GSR-6 16±2 15±2 who determined Te by graphite furnace AAS following decomposition with Br2–HBr and separation by reductive REFERENCES coprecipitation.With one exception, both the mean values and 1 Hall, G. E. M., and Pelchat, J. C., J. Anal. At. Spectrom., 1997, the precision obtained compare well. This indicates that the 12, 97. proposed values shown in Table 2 are indeed high. Although 2 Thompson, M., and Walsh, J. N., in Handbook of Inductively difficult to explain, it is noteworthy that the higher concen- Coupled Plasma Spectrometry, Blackie, London, 1989, tration found by the Br2–HBr leach for GSD-11 agrees with pp. 161–182. that by the HF-based attack used here. 3 Welz, B., and Schubert-Jacobs, M., J. Anal. At. Spectrom., 1986, 1, 23. 4 Aslin, G. E. M., J. Geochem. Explor., 1976, 6, 321. 5 Hall, G. E. M., in Manual of Physico-Chemical Analysis and Bioassessment of Aquatic Sediments, eds. Mudroch, A., and Azcue, J., Lewis, Boca Raton, FL, USA, in the press. CONCLUSIONS 6 Bedard, M., and Kerbyson, J. D., Anal. Chem., 1975, 47, 1441. Based on the variability in the blank solutions taken through 7 Wickstrøm, T., Lund, W., and Bye, R., Analyst, 1996, 121, 201. 8 Nielsen, S., Sloth, J. J., and Hansen, E. H., Analyst, 1996, 121, 31. either an aqua regia or HF–HClO4–HNO3–HCl digestion, the 9 Hill, S. J., Pitts, L., and Worsfold, P., J. Anal. At. Spectrom., 1995, proposed method using HG–ICP-MS is capable of detecting 10, 409. Bi and Te to levels of 1 mg kg-1 and Sb and Se to levels of 6 10 Anderson, S. T. G., Robe�rt, R. V. D., and Farrer, H. N., J. Anal. mg kg-1 in geological samples. This methodology now provides At. Spectrom., 1994, 9, 1107. the detection limits required to measure these elements at their 11 Hall, G. E. M., and Pelchat, J. C., Geostand. Newsl., 1990, 14, 197. 12 Kuldvere, A., Analyst, 1989, 114, 125. natural ranges in rocks and sediments, which can be as low as 13 Xie, X., Yan, M., Li, L., and Shen, H., Geostand. Newsl., 1985, 9, 83. several mg kg-1 for the element of lowest geogenic abundance, 14 Xie, X., Yan, M., Wang, C., Li, L., and Shen, H., Geostand. Newsl., Te. The method’s expected precision is in the range 3–8% 1989, 13, 83. RSD, perhaps slightly better or worse dependingupon extremes 15 Kontas, E., Niskavaara, H., and Virtasalo, J., Geostand. Newsl., in analyte concentrations and the homogeneity of the samples 1990, 14, 477. under investigation. Dissolution by aqua regia provides complete extraction of Bi but some sample matrices may require Paper 6/05399H the more total decomposition of HF–HClO4–HNO3–HCl for Received October 1, 1996 Accepted October 25, 1996 full recovery of Te, Se and, to a lesser degree perhaps, Sb. 106 Journal of Analytical Atomic Spectrometry, January 1997, Vol.

ISSN:0267-9477    

DOI:10.1039/a605399h

出版商:RSC    

年代:1997

数据来源: RSC