摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 439 Verification of a Correction Procedure for Measurement of Lead Isotope Ratios by Inductively Coupled Plasma Mass Spectrometry* Michael E. Ketterer United States Environmental Protection Agency National Enforcement Investigations Center Box 25227 Building 53 Denver Federal Center Denver CO 80225 USA Michael J. Peterst and Preston J. Tisdaleg ICF/Kaiser Engineers 165 South Union Boulevard Suite 802 Lake wood CO 80228 USA Inductively coupled plasma mass spectrometry (ICP-MS) is a suitable method for determining Pb isotope ratios. This study investigates the effectiveness of a mass discrimination correction technique which is based upon the addition of TI to the sample and measurement of the 205TI:203TI ratio. The TI correction method has been found to be highly effective for minimizing bias and drift in Pb isotope ratios under a variety of operating and sample matrix conditions and for various samples independently analysed by thermal ionization mass spectrometry (TIMS).Biases for ratios with respect to *04Pb are generally controlled at <0.5% and relative standard deviations of 0.2% are attainable. Results using ICP-MS for environmental samples compare favourably with those obtained by TIMS. This correction technique is sutiable for routine analysis of environmental samples. Keywords Lead isotope ratio determination; inductively coupled plasma mass spectrometry; thallium based mass discrimination correction; reference material analysis; environmental samples analysis The determination of Pb isotope ratios and their use in geochemical and environmental studies is a subject that has evoked much interest since the first measurements were reported by Nier in 1938.' The details of theories and applications relating to this subject have been extensively Variations in Pb isotope ratios have been used in environmental studies1*-18 to determine the source of Pb contamination in specimens of interest.Typically Pb in environmental samples reflects contributions from multiple sources and thus isotope ratios are observed that are weighted averages of individual ore sources. Large varia- tions in 2oaPb:204Pb and 208Pb:204Pb are expected to be based upon consideration of the major worldwide sources of lead ores. Lead isotope ratios have been determined using induc- tively coupled plasma mass spectrometry (ICP-MS).19-27 Systematic biases have been observed for reference ma- terials such as the National Institute for Standards and Technology (NIST) Standard Reference Material (SRM) Common Lead and for environmental samples compared with those obtained by thermal ionization mass spectrome- try (TIMS). A method of correcting biases due to mass discrimination has been proposed by Longerich et The Longerich study demonstrated that T1 could be added as an internal standard for mass discrimination correction and that this correction technique could possibly lead to the removal of mass discrimination effects. For applications related to enforcement of environmental regulations the precisions attainable by ICP-MS for Pb isotope ratio measurements are generally satisfactory.How- ever the need to produce ratios that are minimally biased so that samples can be compared with published ore results and to ensure between-batch and interlaboratory compara- bility is of great concern. The purpose of the present study was to investigate the effectiveness of the use of T1 as an internal standard correction. *Presented in part at the 32nd Rocky Mountain Conference ?Present address J. F. Sat0 Associates 1667 Cole Boulevard $ Present address US Department of Agriculture Agricultural Denver CO USA July 28th-August 2nd 1990. Suite 175 Golden CO 8040 1 USA. Research Center NAL Building Beltsville MD 20705 USA. Experimental Materials and Reagents Trace-metal grade HNO (16 mol drn-,) and HCl (12 mol dm-,) were used without further purification.Distilled de-ionized water was the solvent for all solutions. Thallium stock solution (1000 mg 1-l) was obtained from Spex Industries (Edison NJ USA). The following chemicals also obtained from Spex were used to prepare matrix element solutions Na2C03 RbNO CsNO and U03.H20. A stock NIST SRM 981 Common Lead solution (1500 mg 1-l of Pb) was prepared by dissolving a portion of the metal in 1 mol dm- HNO,. Solutions analysed in this study were prepared using the above reagents as needed. Preparation of Environmental Samples Lead ore smelter fly ash and environmental samples were prepared for ICP-MS analysis by open- or closed-vessel digestion with a mixture of HN0,-HCl. Prior to ICP-MS measurement digestates were diluted with 0.16 mol dm- HNO to give solutions of 0.5- 1 .O mg 1-l of Pb and were spiked with 0.5 mg 1-l of T1.Instrumentation A Sciex Elan Model 250 ICP mass spectrometer equipped with mass flow meters for all gas streams and a peristaltic sample delivery pump was used in these studies. A refrigerated circulating bath was used to maintain the nebulizer spray chamber at a temperature of 10 "C. Meinhard TR-C concentric glass nebulizers (J. E. Meinhard Associates Santa Ana CA USA) were used. The ion optics of the spectrometer are the updated version; voltage adjustments consist of a B lens (barrel) P lens (plate) El (Einzel) and S2 (photon stop). The instrument was operated in the multichannel (peak-hopping) mode with single measurements being taken at the nominal mass value of each peak.For all scans an equal measurement time was used for each isotope. The low resolution mode was used producing peak widths of 1.0-1.1 mlz at 10% height. The program 'Spectrum Display' was used to collect data which were directed to a personal computer for storage and manipulation.440 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 Table 1 Partial factorial design for investigation of the effects of multiple parameters upon Pb isotope ratio bias and precision Experiment Nebulizer type B Lens setting Measurement Cycle Sample flow Nebulizer flow time/s time/s rate/ml min-' rate/l min-l TR-C-0.5 TR-C-0.5 TR-C-0.5 TR-C-0.5 TR-C-3.0 TR-C-3.0 TR-C-3.0 TR-C-3.0 25 25 30 30 25 25 30 30 20 20 100 100 100 100 20 20 0.5 2.0 0.5 2.0 0.5 2.0 0.5 2.0 1 .o 2.0 1 .o 2.0 2.0 1 .o 2.0 1 .o 1 .oo 1.12 1.12 I .oo 1 .oo 1.12 1.12 1 .oo Isotope Ratio Measurements by ICP-MS The influence of the parameters of nebulizer flow rate nebulizer type sample pump speed total measurement time cycle time and €3 lens setting upon the precisions and biases of 206Pb:204Pb 207Pb:204Pb and 208Pb:204Pb measure- ments for NIST SRM 981 were investigated (Table 1).For each set of conditions three scans of a blank solution (0.5 mg 1-l of T1 in 0.16 mol dm-3 HN03) and ten scans of a 0.75 mg 1-l NIST SRM 981 Pb solution containing 0.5 mg 1-l of T1 in 0.16 mol dm3 HN03 were obtained. The precision and bias of 206Pb:204Pb 207Pb:204Pb and 208Pb:204Pb for three blank-corrected scans of NIST SRM 981 were measured at ion optics settings which produced varying degrees of mass discrimination.Settings for lens B of 10 20 25 30 35 40 and 50 were used with the lens values E1=42 P=07 and s2=37 being held constant. Also investigated were measurements of 206Pb:204Pb 207Pb:204Pb and 208Pb:204Pb for five blank-corrected scans of NIST SRM 981 in a reference matrix of 0.16 mol dm-3 HN03 and in matrices containing 1000 mg 1-l of Na Rb Cs and U. Isotope ratios were measured in environmental samples using a mass integration time of 100 s and with correction for reagent blank contributions and 204Hg isobaric interference. Blank and Mass Discrimination Correction Equations It was necessary to measure and correct for the influence of reagent blanks. Furthermore for the ICP-MS instrument used in this laboratory the addition of T1 to the solution to be analysed generally produced a signal at m/z 204 that is 1 x 10-3-1 x that of the 205 m/z signal (for a low resolution of 1 .O- 1.1 m/z).For the sample signals 2204-i208 and the corresponding blank signals i204b-i208b the following equations apply i204c= 2204 - (i204b)(i205)/(2205b) (1) i206c= 2206 - (i206b)(i205)/(~205b) (2) i207c= 2207- (i207b)(i205)~(~205b) (3) Z208c= 2208 - (Z208b)(~205)~(~205b) (4) The terms i204c i206c i207c and z20gc are blank corrected signals. The term (i205)/(i205b) corrects for time-dependent and sample-derived differences in analyte sensitivity be- tween the blank and sample solutions in a similar way to internal standardization in quantitative analysis. The term i205b is the intensity of a reagent blank spiked with TI.No additional correction is performed for intensities obtained at m/z 203 and 205 in the absence of TI. Inclusion of the (i205)/(i205b) term has been found to be significant under some circumstances and can alter the resulting ratios by up to 1 %. The isotope ratios corrected for mass discrimination effects are obtained from (6) (7) The terms (i206c/i204c) etc. are the isotope ratios uncorrected for mass discrimination effects. The naturally occurring 205Tl:203Tl ratio is 2.3871 and (i205/i203) is the same ratio measured in the sample. Eqns. (1)-(7) were applicable to all experiments investigating the isotope ratios of NIST SRM 98 1. The 204Hg isobar was readily corrected for in environ- mental samples using the following equations (additional isobaric corrections for Os Ir and Pt polyatomic ions were unnecessary) 207Pb:204Pb = ( i207c/i204c)[2.387 1 /( i20s/i203)]1.5 208Pb:204Pb = ( i208c/i204c)[2. 387 1 /( i2o5/i2O3)l2 Z204hc= i2O4c-0.5 [i201 -(i201b i20S/i205b)l (8) The term i204hc is the intensity at m/z=204 which is corrected for both reagent blank Pb contribution and the 204Hg isobar. The term 0.515 is the 204Hg:201Hg natural abundance ratio iZol is the reagent blank signal at m/z 201 and i204c is from eqn. (1). The isotopic ratios are then computed by using i204hc in place of i204c in eqns. (5)-(7). Results and Discussion Effect of Operating Parameters on Isotope Ratios Table 2 lists the means and statistics of both the uncor- rected Pb and T1 isotope ratios and the ratios corrected for mass discrimination effects obtained using the instrumental parameters shown in Table 1.It is evident that the uncorrected isotope ratios are significantly positively bi- ased; this bias is nearly eliminated by applying eqns. (5)-(7). In the corrected ratios a small amount of residual negative bias is seen with the 207Pb:204Pb measurements in experiment 1 having the largest bias of -0.96%. The residual bias appears to diminish with time; experiments 1-8 were run in order. The precision values are generally comparable between the uncorrected and corrected ratios. Table 2 shows the isotope ratios of NIST SRM 981 corrected for bias using eqns. (1)-(7) and that this correc- tion is applicable under a variety of instrumental operating conditions. Statistical analysis of the results of Table 2 demonstrates that measurement time is the most influential variable among those investigated and the higher flow rates of the nebulizer argon supply and sample solution are also associated with improved precision.These two variables produce effects which are related to the counting statistics as increasing them also raised the ion-count rates for all isotopes. The B lens setting nebulizer type and cycle time all produced insignificant effects upon the precision of the ratio measurements. The isotope ratio bias is influenced by the B lens setting and by the nebulizer type but neither of these factors is of great significance. The effect of the nebulizer type is convoluted with the experimental run order in this study as seen in Table 1 since it was most practical to run experiments 1-8 in numerical order.The run order parameter is probably the more influential givenJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 44 1 Experiment 1 2 3 4 5 6 7 8 Experiment 1 2 3 4 5 6 7 8 Uncorrected Corrected Uncorrected Corrected Table 2 Uncorrected and mass discrimination corrected results for the determination of NIST SRM 981 lead isotope ratios with varying nebulizer type B lens setting measurement time cycle time sample flow and nebulizer flow values * Certified values for 206Pb:204Pb 207Pb:204Pb and 20sPb:204Pb are 16.937 f 0.0 19 15,497 k 0.0 10 and 36.722 f 0.037 respectively. 17.111 20.103 17.332 f 0.039 17.341 f 0.034 17.225 f 0.040 17.316 k0.035 17.413f0.048 17.263 f 0.048 17.384 -t 0.098 16.879 f 0.089 16.862 k 0.059 16.9 10 k 0.044 16.922 f 0.042 16.905 -t 0.043 16.939 +- 0.037 16.9 15 +- 0.056 16.935-tO.108 15.661 kO.101 15.928 f0.042 16.02 1 f 0.033 16.036 k 0.032 15.883 k 0.034 15.999 k 0.033 16.138 k 0.057 16.094 k 0.089 15.344 f 0.086 15.457 f 0.048 15.442 f 0.036 15.450 f 0.056 15.376 f 0.065 14.440 f 0.045 15.480 k 0.044 15.475 +O.11 7 Uncorrected 37.36 1 f 0.2 10 38.304 k 0.147 38.464 f 0.093 38.449 k 0.083 37.992 k 0.078 38.355k0.083 38.761 f0.103 38.540f0.213 Corrected 36.355 -t 0.2 11 36.546 k 0.241 36.61 3 f 0.135 36.611 k0.147 36.593 k 0.110 36.705 k 0.120 36.575 f 0.21 1 36.578 f 0.318 Uncorrected Corrected 2.4 199 f 0.0054 2.4439 f 0.0044 2.4467 f 0.0027 2.4323 k 0.0024 2.4402 k 0.0027 2.4574 k0.0067 2.4503 f 0.0058 2.4463 k 0.0036 (2.3871) (2.387 1) (2.3871) (2.3871) (2.3871) (2.3871) (2.3871) (2.3871) Table 3 Uncorrected and mass discrimination corrected results of the determination of NIST SRM 98 1 lead isotope ratios for several B lens settings B Lens setting 10 20 25 30 35 40 50 Uncorrected Corrected 16.592f0.102 16.923 k 0.099 16.846 k 0.070 16.923 k 0.06 1 16.83 1 f 0.026 16.855 f0.035 17.269 f 0.133 16.938k0.148 17.097 f 0.034 17.2 12 f 0.042 17.56 1 f 0.060 16.928 k 0.070 16.939 f 0.075 16.920 f 0.024 Uncorrected Corrected 15.005 k0.108 15.455 k0.106 15.322 f 0.050 15.427 f 0.033 15.350 k 0.024 15.382 -t 0.037 16.656 k 0.01 9 15.425 f 0.077 15.791 k0.042 15.4 16 k 0.087 15.936 k0.130 15.480k 0.151 16.273 f 0.022 15.390 f0.025 B Lens setting 10 20 25 30 35 40 50 Uncorrected 35.171 k0.167 36.13 1 f 0.089 36.373 f 0.049 37.175 f 0.037 37.599f 0.054 38.085 f 0.320 39.1 17 f 0.086 Corrected 36.584k 0.203 36.459 f 0.046 36.474 f 0.065 36.447 f 0.225 36.41 5 f 0.184 36.637 k 0.376 36.3 13 k 0.063 Uncorrected 2.3405 2 0.0054 2.3763 f 0.0023 2.3838 k 0.0033 2.4 109 f 0.0069 2.4256 f 0.0048 2.4776 -t 0.0049 2.4338 f 0.0040 Corrected (2.3871) (2.3871) (2.3871) (2.3871) (2.8371) (2.3871) (2.387 1) the tendency of ICP-MS instrumentation to show drift over time.29 In Table 3 precisions and biases are shown for uncor- rected and TI corrected Pb isotope ratios at various B lens settings. As previously shown by Longerich et al.,28 the B lens has the effect of shifting the mass response curve; shifts in the setting from 10 to 50 produce uncorrected results ranging from strongly negatively biased (setting 10) to strongly positively biased (setting 50).The mag- nitude of the effect clearly increases in the order 2MPb:204Pb<207Pb:204Pb<208Pb:204Pb. It was also noted that the zero-bias B lens setting is affected by the other ion optics settings; however the type of behaviour seen in Table 3 usually exists at other sets of ion lens conditions. The correction is thus capable of alleviating instrumentally induced bias over a small range of ion optic conditions. On a practical note use of this correction procedure alleviates the requirements for careful tuning prior to analysis; drift during analysis in the degree of mass discrimination is tolerable also. Effect of Sample Matrix Table 4 shows the influence of 1000 mg 1-l matrices upon the raw and corrected isotope ratios.It is evident that the addition of matrix elements produces large negative shifts in the uncorrected Pb ratios with respect to 204Pb. However this trend is the opposite of the expected sample matrix effect which is a preferential attenuation of the lighter isotopes. An explanation for this result is not apparent at present. It is further noted that the experiments were run in the matrix addition order (none Nay Rb Cs and U) followed by a repeat of the no-matrix NIST SRM 981 solution which produced biased results (prior to TI based442 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 Table 4 Uncorrected and mass discrimination corrected results for the determination of NIST SRM 98 1 lead isotope ratios under varying matrix conditions 206pb:204pb 207pb:204pb Matrix Uncorrected Corrected Uncorrected Corrected None 16.91540.057 16.974 k 0.064 15.480 k 0.042 15.56 1 k 0.053 1000 mg I-' of Na 16.830 k0.064 16.9 15 f 0.03 1 15.374 f 0.049 15.490f 0.044 1000 mg I-' of Rb 16.770 k 0.056 16.887k0.054 15.28 1 k0.064 15.443 f 0.06 1 1000 mg I-' of Cs 16.605 k 0.087 16.853 k0.074 15.120 f 0.088 15.460 k 0.071 1000 mg 1 - I of U 16.608 k 0.01 9 16.856 k 0.053 15.163 k 0.027 15.505 -t- 0.076 208pb 204pb 205T1:203Tl Matrix Uncorrected Corrected Uncorrected Corrected None 36.485 f 0.122 36.761 rf~ 0.167 2.3788 k0.0033 (2.3871) 1000 mg 1-I of Na 36.186k0.119 36.550k0.167 2.3752k0.0077 (2.3871) 1000 mg I-' of Rb 35.942 4 0.172 36.449 f 0.126 2.3704 k 0.0053 (2.387 1) 1000 mg I-' of Cs 35.446 f 0.233 36.5 12 k 0.174 2.3520 k 0.0027 (2.3871) 1000 mg 1-1 of U 35.538 k 0.032 36.610k0.124 2.3519k0.0049 (2.3871) Table 5 Comparison of ICP-MS and TIMS lead isotope ratio results for ore tailings soil and smelter fly ash samples Sample Technique 206pb:204Pb 207Pb:*04Pb 20sPb:204Pb [Pb]/mg kg-I Galena ICP-MS (n=3)* TIMS (n=2)* Tailings No.1 ICP-MS (n=3) TIMS (n=2) Tailings No. 2 ICP-MS (n=3) TIMS (n=3) Soil no. 1 ICP-MS (n= 3) TIMS (n=2) Soil no. 2 ICP-MS (n= 3) TIMS (n=3) Soil no. 3 ICP-MS (n= 5 ) TIMS (n=2) TIMS (n=2) Fly Ash ICP-MS (n=3) 16.26 k 0.07 15.33 k 0.07 16.25 k 0.01 15.40 k 0.02 18.13 k 0.07 15.53 4 0.05 18.11 50.02 15.58k0.03 16.32f0.06 15.37k0.06 16.27k0.01 15.39k0.02 18.18 4 0.05 15.50 4 0.09 18.17 k 0.02 15.58 k 0.02 17.64 k 0.05 1 5.52 4 0.05 17.5740.01 15.53 k0.02 16.65 k0.13 15.36 k0.12 16.60 k 0.01 15.40 -t 0.01 18.1 1 k0.02 15.4720.02 18.13 k 0.03 1 5.58 t- 0.04 35.79 k 0.13 388 000 36.01 f 0.05 37.58 k 0.18 26 000 37.67 -t- 0.10 35.95 k0.19 36 300 35.99 rt 0.06 37.57 k0.24 5 470 37.72 k 0.08 37.2620.24 1 1 100 37.32 f 0.06 36.24 +- 0.32 600 36.30 f 0.01 37.47 f 0.06 63 1 000 37.71 f0.19 * Replicates are of the digestion and measurement stages for ICP-MS and the measurement stage for TIMS. correction) similar to the U reference matrix. Following a 2 d idle period the uncorrected reference matrix ratios with respect to 204Pb returned to the initial values. Thus some time dependency exists. Regardless of origin and mecha- nism the sample-induced bias effect is amenable to remedy using T1 to correct for mass discrimination effects.Environmental Samples In Table 5 comparative results are shown for the determi- nation of 206Pb:204Pb 207Pb:204Pb and 20sPb:204Pb for seven environmental samples using ICP-MS and TIMS. Gener- ally excellent agreement is seen for the two analytical methods for this group of samples; in some instances biases of 0.5-1 .O% exist. The relative standard deviations (RSDs) of the results of the analyses of environmental samples using ICP-MS are similar in most instances to those obtained with NIST SRM 981. These samples have been developed as in-house batch quality control materials and isotope ratios for these samples measured by ICP-MS with T1 based correction exhibit long-term reproducibility.Thus data obtainable by the methods presented here are of a suitable quality for the enforcement of environmental regulations. Conclusions Measurements of Pb isotope ratios by ICP-MS can be significantly affected by both instrumental and sample- induced sources of bias. It was observed that substantial changes in mass discrimination could be brought about by the presence of 1000 mg 1-' of concomitant elements. Both sources of bias are readily corrected for by using the value of 205Tl:203Tl measured in the sample. The T1 correction method first proposed by Longerich et is apparently successful because T1 closely emulates the mass discrimina- tion level of Pb under a variety of instrumental and sample conditions. For a measurement time of 100 s the RSDs for 206Pb:204Pb 207Pb:204Pb 20sPb:204Pb and 205Tl:203Tl measure- ments are apparently limited to about 0.20 0.25 0.30 and 0.10% respectively.These values are larger by a factor of 2-3 than are predicted by counting statistics considerations. Inductively coupled plasma MS is a robust and practical method of measuring Pb isotope ratios in environmental samples where data quality requirements are of the order of 0.5% acceptable bias and 0.2-0.5% precision. The authors thank R. E. Zartman and L. M. Kwak of the US Geological Survey for providing comparative TIMS results for the environmental samples. This work was supported by the US Environmental Protection Agency. Specific vendors are mentioned for information purposes only. References 1 Nier A.O. J . Am. Chem. Soc. 1938 60 1571. 2 Bate G. L. and Kulp J. L. Science 1955 122 970. 3 Cahen L. Eberhardt P. Geiss J. Houtermans F. G. Jedwab J. and Signer P. Geochim. Cosmomchim. Acta 1958,14 134.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 443 4 Russell R. D. and Farquhar R. M. Lead Isotopes in Geologjj Interscience New York 1960. 5 Cannon R. S. Pierce A. P. Antweiler J. C. and Buck K. L.. Econ. Geol. 1961 56 1. 6 Brown J. S. Econ. Geol. 1962 57 673. 7 Stacey J. S. Zartman R. E. and NKomo I. T. Econ. Geol. 1968 63 796. 8 Doe B. R. and Delevaux M. H. Econ. Geol. 1972 67 409. 9 Doe B. R. and Stacey J. S. Econ. Geol. 1974 69 757. 10 Saager R. and Koppel V. Econ. Geol. 1976 71 44. 11 Koppel V. and Gruenfelder M. in Lectures in Isotope Geologv eds.Jager E. and Hunziker J. C. Springer-Verlag Berlin 1979 p. 134. 12 Chow T. J.. and Johnstone M. S. Science 1965 147 502. 13 Ault W. U. Senechal R. G. and Erlebach W. E. Environ. Sci. Technol. 1970 4 305. 14 Rabinowitz M. B. and Wetherill G. W. Environ. Sci. Technol. 1972 6 705. 15 Gulson B. L. Tiller K. G. Mizon K. J. and Merry R. H. Environ. Sci. Tech. 1981 15 691. 16 Facchetti S. Mass Spectrom. Rev. 1988 7 503. 17 Gulson B. L. Mizon K. J. Korsch M. J. and Noller B. N. Environ. Sci. Tech. 1989 23 290. 18 Facchetti S. Garibaldi P. in Proceedings of the International Symposium on Environmental Health Aspects of Lead Com- mission of the European Communities Luxembourg 1973 p. 995. 19 Gray A. L. Analyst 1975 100 289. 20 Houk R. S. Fassell V. A. Flesch G. D. Svec H. J. Gray A. L. and Taylor C. E. Anal. Chem. 1980 52 2283. 21 Data A. R. and Gray A. L. Analyst 1981 106 1255. 22 Date A. R. and Gray A. L. Analyst 1983 108 159. 23 Date A. R. and Gray A. L. Int. J. Mass Spectrum. Zon Phys. 1983 48 357. 24 Smith R. G. Brooker E. J. Douglas D. J. Quan E. S . K. and Rosenblatt G. J. Geochem. Explor. 1984 21 385. 25 Date A. R. and Cheung Y. Y. Analyst 1987 112 1531. 26 Hinners T. A. Heithmar E. M. Spittler T. M. and Henshaw J. M. Anal. Chem. 1987 59 2658. 27 Russ G. P. and Bazan J. M. Spectrochim. Acta Part B 1987 42 49. 28 Longerich H. P. Fryer B. J. and Strong D. F. Spectrochim. Acta Part B 1987 42 39. 29 Houk R. S. and Thompson J. J. Mass Spectrom. Rev. 1988 7 425. Paper 0/04859C Received October 29th I990 Accepted May 2Ist 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600439

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 445 Quantitative Analysis of Trace Elements in Carbonates Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry William T. Perkins Ronald Fuge and Nicholas J. G. Pearce Geochemistry and Hydrology Research Group institute of Earth Studies University College of Wales Aberystwyth UK Laser ablation inductively coupled plasma mass spectrometry has been applied to the analysis of carbonate materials. Multi-element synthetic standards prepared both as pressed powders and fused glass discs were used for calibration. The elements Mg Mn Sr Ba and Pb were added to the pressed powder standards and these elements together with U were added to the fused glass standards. Calibration graphs which are linear over at least three orders of magnitude were produced using both types of standard but the fused glass discs gave better precision.The accuracy of the technique was evaluated using reference materials. Acceptable values were obtained using the pressed powders [e.g. BCS 393 (limestone) certified values of 905 ppm (Mg) 77 ppm (Mn) and 160 ppm (Sr)] but better accuracy was achieved with fused glass discs [e.g. BCS 393; 957 ppm (Mg) 79.6 ppm (Mn) and 167 ppm (Sr)]. The technique is applied to the analysis of carbonate shell material and demonstrates its potential in environmental monitoring. Keywords Laser ablation inductively coupled plasma mass spectrometry; quantitative analysis; carbonates; trace element; environmental monitoring The use of laser ablation (LA) was first proposed in the early 1960s following the publication of work on laser action in ruby.The subsequent development of laser microanalysis has been reviewed by Moenke-Blankenburg.2 The tech- nique which has been applied to atomic emission spectro- metry (AES) and inductively coupled plasma (ICP) AES,2-s was first coupled with ICP mass spectrometry (ICP-MS) by Gray,6 who used a JK Type 2000 ruby laser and demon- strated the applicability of this technique to geological materials both for trace element determination and isotope ratio measurements. Arrowsmith7 reported the use of a Nd:YAG laser with ICP-MS for the analysis of microprobe reference materials and Cu standards More recently van Heuzen* described procedures for quantitative analysis using both fused glass and pressed powder materials.In this latter study the importance of matrix matching of samples and standards was emphasized. The laser ablation system has the potential to perform spatial analysis with the laser spot size being to a certain extent controlled by the power output of the laser; typical craters are about 100 pm in diameter. The aim of this work was to demonstrate the capability of LA-ICP-MS as a quantitative analytical tool for the analysis of trace components in carbonate materials covering a compositional range of CaC03-MgC03. A comparison was made between pressed powder samples and fused glass discs in terms of accuracy and precision. The merits of different methods of internal standardization are discussed. Experimental Instrumentation A VG Instruments PQII+ ICP mass spectrometer and VG LaserLab were used during this work.The VG LaserLab is based on a Spectron Laser Systems 500 mJ Nd:YAG laser operating at 1064 nm. This may be run in either fixed Q- or Q-switched mode. The output of the laser at 10 Hz is rated at 500 mJ in fixed Q- and 250 mJ in the Q-switched mode. It is however possible to run the laser at repetition rates greater than 10 Hz and in some applications a repetition rate of 15 Hz was employed. A standard VG sample chamber was used throughout this study. The ablated material was transferred from the LaserLab to the PQ1I-t using 2 m of 4 mm i.d. poly(viny1 chloride) (PVC) tubing. This configuration is similar to that used by Gray.6 The general operating conditions of the ICP mass spectrometer are given in Table 1.Pressed Powder Standards The first attempt at standardization was based on pressed powder discs. Specpure (Johnson Matthey) carbonates or oxides of the desired constituents were accurately weighed and mixed with a known mass (1 0 g) of the CaC03 matrix to produce a series of standards. Specpure In203 powder was also added as an internal standard. The powder mixture was transferred into glass jars which contained synthetic leucite [K(AlSi,O,)] balls. The jars were closed and the mixtures shaken in a laboratory mixer-mill for 5 min. The powder was then mixed with a 10% m/v poly(viny1 alcohol) binder and pressed at 25 tons into 30 mm diameter discs. As this work was concerned with the analysis of shell material which contains a significant proportion of organic matte^,^,^^ it was decided to use such material as the basis for a set of pressed powder standards.It was considered that using such material would overcome any ablation effects in the shells caused by the presence of the organic matter. For this reason the first series of standards produced were based on crushed shell material and contained added components (Mn Sr Ba and Pb) up to 10 000 p g g - l . The ablation of each standard was performed in fixed Q- mode with a focused laser. The system was set at 750 V giving a laser output of approximately 200 mJ. A 2 x 10 raster pattern with single laser shots at each position and a repetition rate of 1.33 Hz was used. A repetition rate of more than 1 Hz should produce an almost constant signal Table 1 Operating conditions for ICP-MS Forward power Gas flow rate Cool gas Auxiliary Carrier gas Reflected power No.of sweeps Mass range Dwell time 1250 W 12.75 1 min-l 0.5 1 min-' 1.00 1 min-l ow 400 22-246 rnlz 160 p s Institute of Earth Studies No. 161.446 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 I VOL. 6 (cf. Gray6). In this configuration the amount of material removed by each set of 20 shots averaged 720 pg with a standard deviation of 83 taken over five determinations. Each standard was analysed five times and the raw data were collected using a peak .width of 0.7 u with valley integration. The data were reported as counts per second for the given area [normally quoted as area counts per second (ACPS)]. The data were processed using Microsoft Excel which is a spreadsheet package. Calibration graphs for Mn Sr Ba and Pb are presented in Fig.1. These graphs were produced using In as an internal standard. In general linear calibration graphs were pro- duced over this wide concentration range although the degree of scatter shown by the 20 error bars is consider- able. Careful observation of the pressed discs revealed variation in the grain size of the carbonate matrix despite every effort to ensure homogeneity. This variation could account for much of the scatter observed in the calibration graphs. In an attempt to overcome this problem a second series of pressed powder standards were produced. The second series of standards were prepared using AnalaR CaCO (Merck) as the matrix material. This has the advantage over the shell matrix of being fine-grained and homogeneous.These standards were analysed in the focused fixed Q-mode and in a further attempt to over- come small scale inhomogeneity in the de-focused Q- switched mode. The standards were produced with a range of concentrations up to 5000 pg g-l. As with the first series of standards this set was analysed five times and the data collected as ACPS. The data were processed using both the added In internal standard and 44Ca as a true internal standard. Calibration graphs were produced using a least- squares regression and the correlation coefficients for Mn Sr Ba and Pb are presented in Table 2. The data produced good correlations by both methods of internal standardiza- tion although the 44Ca isotope produced marginally better data overall.This might again relate to the homogeneity of the standard since the In,03 is added as a powder and there is a problem when trying to mix thoroughly small additions of powders prior to pressing. As a result of this test subsequent analyses made use of the 44Ca isotope for internal standardization. E 12 ;; (c) E" 1 0 - v) al * C * 6 - 0 ru 4 - Y 2 - - 5 8 - L cc E al a O A 0 2500 5000 7500 10000 Concentration of element added/pg g-' Fig. 1 Calibration graphs for the first series of pressed powder standards using crushed shell material as the base ( a ) 55Mn; (b) %r; (c) 137Ba; and (d) 207Pb. Indium is used as an internal standard. Plots show the best-fit linear regression line with ? 20 error bars Table 2 Correlation coefficients for pressed powder CaC0,-based standards using both fixed Q- and de-focused Q-switched lasers with In and 44Ca as internal standards Correlation coefficient Fixed Q-mode Q-switched mode Element In 44Ca In 44Ca 0.9805 Mn 0.9355 0.9851 0.9509 0.989 1 Sr 0.9963 0.9763 0.9982 Ba 0.9355 0.9809 0.9906 0.9999 Pb 0.9726 0.9123 0.9473 0.9799 Fused Glass Standards The work on pressed powder standards suggests that there are problems in trying to produce homogeneous mixtures of powders If the mixtures were fused into a glass disc such as those commonly produced for X-ray fluorescence analysis then a solid solution should result and the standard would then be homogeneous.In this study three matrix compositions were chosen and a series of standards produced for each.The three matrix compositions were CaCO,; (Mg,Ca)CO,; and MgC03 (calcite/aragonite; dolomite; and magnesite respectively). AnalaR CaCO and Specpure MgO were used to give the correct Ca and Mg concentrations. These formed the matrix for the CaC0 and MgC0 standards respectively. The (Mg,Ca)CO standard was produced using a mixture of these two in order to produce a Ca Mg ratio close to that of natural d01omite.l~ The flux used was lithium tetraborate LiZB407 (Johnson Matthey Spectroflux loo) with a flux to sample ratio of 5 1 for the calcite and dolomite and 10 1 for the magnesite (2.5 g Li2B407:G.5 g matrix and 5.0 g Li2B407:0.5 g matrix respectively). Solutions of trace elements Mg Mn Sr Ba Pb and U were added in small volumes (1 000 pg ml-I or diluted Aldrich standard solu- t ions) using a gravimetrically calibrated Labsystems elec- tronic Finnpipette.The matrix flux and additions were mixed in a Pt-Au crucible and dried at 110 "C. The Fig. 2 Scanning electron microscope image of a typical ablation crater produced by ablation of a fused glass disc. The central crater is approximately 250 pm in diameter and is surrounded by a raised area 500 pm in diameter. The outer zone shows deposits of material ejected from the ablation crater this zone is approxi- - - '4 mately 1.5 mm in diameterJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 447 mixtures were then fused for 30 min over a Meker burner. The resulting melts were allowed to cool in the crucibles and following a period of annealing were removed and stored prior to analysis.The system must be operated in the focused Q-switched mode in order for the laser to couple with a glass. It was found that the efficiency of ablation was much poorer than with the pressed powder materials and it was necessary to use a rapid repetition rate (15 Hz) in order to achieve a usable signal. Because of the homogeneity of the glass material it was not necessary to raster the sample surface and the data were collected from a single spot. The laser power was set at approximately 125 mJ. The sample surface was subjected to pre-ablation for a period of 20 s before the spectra were obtained. The craters produced during Q- switched laser ablation of these glasses were examined using a scanning electron microscope (SEM) as shown in Fig.2. The SEM image shows a crater of approximately 250 pm diameter similar to those reported by van Heuzen.* The ablated hole is surrounded by a raised area of smooth material extending 250 pm beyond the crater. This area is interpreted as being either melted during the ablation period or deposited by sputter of molten glass from the crater. Outside the smooth zone is a region characterized by fragmented material extending over a diameter of more than 1.5 mm. This is almost certainly produced by the deposition of both fragmented and molten material. Results Reference Materials Three reference materials [Geological Survey of Japan (GSJ) JLS-1 Limestone Institute of Geophysical and Geochemical Prospection (IGGE) Ministry of Geology China GSR-6 Limestone and British Chemical Standards (BCS) Certified Reference Material (CRM) 393 Limestone] were analysed as pressed powders against a calibration graph produced from the CaCO blank and a 500 pg g-l multi-element standard.The results are tabulated in Table 3. There is general agreement between the values deter- mined in this work and the recommended and proposed values although the accuracy varies from 8% at best to 102% at worst. If this inaccuracy results from the inhomo- Table 3 Comparison of LA-ICP-MS data with recommended or proposed values for certified reference materials; calibration and analysis using pressed powders Sample BCS 393 (Limestone)- Mg* Mn* Sr* Ba Mg Mn Sr GSJ JLS- 1 (Limestone)- IGGE GSR-6 (Limestone)- m * Mn* Sr* Ba* Pb* Recommended/ LA-ICP-MS proposed (PPm) (PPm) 905 77 160 53 3 739 15 296 31 298 46 5 913 120 18.3 1043/730? 24/44? 1 1 1/123? 49/82? 5 472 11 26 1 51 146 394 604 191 37 * Recommended value all others are proposed values. t Duplicate determination.geneous distribution of the small amount of powder added to the matrix then it was considered that fusing the mixture into a glass disc would be one way to overcome the problem. However for a comparative study of the distribu- tion of elements in carbonate shell material these pressed powder standards were adequate and in any event represent an improvement on the qualitative data previ- ously attained. Shell Walls Two specimens of Arctica islandica a shallow-burrowing marine bivalve living in sands and muds were examined as a test of the calibration graphs produced.One specimen was collected from the beach at Borth just north of Aber- ystwyth. The sample was a single valve washed up after a winter storm and the age of the specimen was unknown. The second sample set were collected as live specimens by the Scottish Universities Marine Biological Station Mill- port Isle of Cumbrai. Analyses of the trace element composition from the inner to the outer shell were taken from two of the Scottish specimens and compared with the profile of the Borth specimen. The results for typical distributions of Mg Sr and Pb are presented in Fig. 3. The shells show parallel trends for Mg and Sr although the abundance of the trace elements is different between the two sample localities. This variation is a reflection of the Sr:Ca and Mg:Ca ratios since 44Ca is used as the internal standard.These ratios have been shown to be dependent on salinity and temperature at the time the organism secreted the shell although the ratios may be subject to change following death and burial of the shell material.'' The Pb levels in the specimen from Borth are elevated when compared with the Scottish samples. The rivers draining into Cardigan Bay are known to be polluted with Pb from the Pb mining activities in the area during the last century. This pollution is reflected in the distribution of heavy metals in Cardigan Bay.12 It seems probable therefore that Pb contamination of the marine environment is reflected in 400 300 200 - '0 100 0 0 $ 3000 f - 0 3 2000 C 0 .- 5 1000 C C I - E 0' I I I I 0 0 1 2 3 4 Distance/m m Fig.3 Profiles across three specimens of Arctica isfandica from the inner to the outer shell margin for the elements (a) Mg (b) Sr and (c) Pb. The sample from Borth (A) shows significantly higher Pb levels than the Millport samples (B and C)448 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 the chemistry of the shells of marine organisms. This is the subject of further research at Aberystwyth. Fused Glass Standards Glass discs of six reference materials (GSJ JLS-1 Lime- stone JDo-1 Dolomite; GSR-6; BCS CRM 393 Limestone 368 Dolomite and 389 High Purity Magnesite) were also produced. Four analyses were obtained from different points on the disc surfaces and the data collected as ACPS. These data were transferred to a spreadsheet and the calibration graphs and calculations performed off-line.Minor isotopes of the major elements were used as internal standards (44Ca for the calcite and dolomite and 24Mg for the dolomite and magnesite). By using either Ca or Mg as the internal standard the effects of volatile loss can be overcome since the trace element to major element ratio will remain constant. The correlation coefficients for the calibration graphs generated by least-squares linear regres- sion are presented in Table 4. Good linear calibration graphs (i. e. with a least-squares regression correlation coefficient of better than 0.95) are produced using the glass discs although the slope for a given element varies in the three matrices because of the different concentrations of the major element internal standard present.When the true concentration of the internal standard is used (z.e. ACPS divided by the mass fraction of the element present) the calibration graphs give similar slopes. The slope values obtained after this correlation are presented in Table 5. There is no apparent matrix effect throughout the range of compositions analysed although this might in part be a function of the dilution by the flux (5:l for the calcite and dolomite 10 1 for the magnesite). The fused glass standards produced from the reference materials were analysed using the synthetic standards to calibrate the instrument. The values obtained during this study are presented in Table 6. These data are in agreement with the recommended values the exceptions being the Sr data for GSR-6 after correction for the CaC0 concentration and BCS CRM 368.It is not obvious why these data are poor when the other Sr values are in good agreement. In general however the data give an accuracy of better than k 10% and in many instances better than ?5%. This is excellent especially when the range of values concerned is from 15 ppm of Mn in JLS-1 to 3 I 298 ppm of Mg in GSR-6. These data are similar to the values reported by van Heuzens although this work deals with a carbonate rather than a silicate matrix. Comparison of Calibration Methods This work has demonstrated that linear calibration graphs can be produced using pressed powder standards based both on crushed shell material and AnalaR CaCO,. Calibra- tions based on crushed shell materials gave relatively large errors (Fig.1) which have been attributed to a combination of inhomogeneous grain size in the matrix and an uneven distribution of the powder additions. When these standards are compared with a series of standards based on AnalaR CaC0 there is no significant improvement in the precision for Sr [Fig. 4(a)-(c)]. However when the calibration data obtained using added In as an internal standard are compared with the data obtained using a minor Ca isotope as internal standard there is an improvement in the precision of the results. This supports the conclusion that the problem lies with the mixing of powder additions in a powder matrix. When the laser is de-focused in the Q- switched mode so that a larger area of the specimen is ablated the calibration graphs for the shell- and CaCO based standards are almost parallel the offset being caused by the higher level of Sr in the shell material.The level of precision is comparable to that obtained in the focused fixed Q-mode despite the larger area sampled. These data illustrate a fundamental problem in producing standards as mixtures of powders which are homogeneous. In Fig. 4(d) a calibration graph for Sr in the fused glass standards is presented. This demonstrates the improvement in precision gained when fused glass standards (true solid solutions) are used to calibrate the instrument. Table 4 Table of correlation coefficients for the fused glass standards internal standards in parentheses Correlation coefficient Limestone Dolomite Dolomite Magnesite (“Ca) (44Ca) (24Mg) (24Mg) Element - - - Mg 0.9980 Mn 0.9999 0.9992 0.9996 0.9854 Sr 0.9997 0.99 13 0.9744 0.98 14 Ba 0.9988 0.993 1 0.9860 0.9807 Pb 0.9974 0.9874 0.984 1 0.9467 U 0.9993 0.9969 0.9945 0.9997 Table 5 Values for the slope of least-squares regression analysis of the fused glass standards following correction for the mass fraction of the major element internal standard; internal standards in parentheses Slopelpg - Limestone Dolomite Dolomite Magnesite (“Ca) (“Ca) (24Mg) (24Mg) Element Mn 1 .4 2 6 ~ 1 . 6 0 8 ~ 1 . 4 7 6 ~ 1 . 4 3 4 ~ Sr 3.599 x 10-6 3.437 x 2.929 x 2.875 x Ba 3 . 4 2 7 ~ 2 . 9 1 3 ~ 2 . 8 1 6 ~ lop7 4.781 x low7 Pb 3 . 5 8 4 ~ 4.931 x 4 . 5 3 4 ~ 5.081 x U 8.948 x 9.144 x 8.460 x lo-’ 7.252 xJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL.6 449 '/ ~~ ~ _ _ _ _ ~ ~ _ _ _ _ Table 6 Comparison of LA-ICP-MS data with recommended or proposed values for certified reference materials; calibration and analysis using fused glass discs Sample BCS CRM 393 (Limestone)- Mg* Mn* Sr* Ba Mg Mn Sr GSJ JLS- 1 (Limestone)- IGGE GSR-6 (Limestone)- Mg* Mn* Sr* Ba* IGGE GSR-6t- Mg* Mn* Sr* Ba* Mn* Sr Pb Mn Sr Mn* BCS-CRIM 368 (Dolomite)- GSJ JDO- 1 (Dolomite)- BCS-CRM 389 (Magnesite)- Recommended proposed LA-ICP-MS (PPm) (PPm) 905 957 160 167 77 79.6 53 66.5 3 739 3 867 296 324 15 19.6 31 298 39 598 465 690 913 849 120 192 31 298 34417 465 439 913 540 120 122 465 39 1 67 117 61 59.8 46 44. I 119 138 62 51 *Recommended value all others are proposed values. 7 IGGE GSR-6 is a slightly dolomitic limestone (MgO = 5.19%) and the values for this standard were recalculated using the recommended value for Ca.2.5 2.0 1.5 C - g 1.0 0.5 0 1 .o 0.8 8 0.6 ,,f) 0.4 0.2 0 s L 1.2 0.8 0.4 0 0 1000 2000 300( 0.03 0.02 0.01 0 1 0 1000 2000 3000 Additions of Sr/pg g-' Fig. 4 Comparison of calibration graphs for Sr produced using different standards and ablation conditions. Solid lines represent best-fit linear regression lines for the AnalaR CaCO based standards and broken lines join the blank and 2500 pg g-I crushed shell-based standards. Graphs illustrate (a) curves obtained in focused fixed Q-mode using added llsIn as the internal standard; (b) curves obtained in focused fixed Q-mode using 44Ca as the internal standard; (c) curves obtained in de-focused Q-switch mode using 44Ca as internal standard; and (d) calibration graph for the fused glass standards using 44Ca as internal standard Conclusion Laser ablation ICP-MS is capable of producing quantitative analytical data for a carbonate matrix providing the standards are matrix matched.Pressed powder standards give reasonable calibration graphs when a true internal standard is used but there are problems in producing homogeneous standards by the mixing of powders. One of the advantages of the laser system when com- pared with sample dissolution is the ability to obtain spatial information about element distribution. This aspect has been demonstrated for the bivalve Arctica islandica and illustrates the potential of the technique as a pol- lution monitor this being an area of active research at Aberystwyth.The production of fused glass standards has the potential to obtain accurate determinations of trace constituents in carbonate materials. Given the dilution of the flux to sample ratio used there is no apparent matrix effect for the range of compositions studied in this work. This is in agreement with the work of van Heuzen8 although the accuracy demonstrated here is somewhat better than that quoted by van Heuzen. Since the fused glass discs are solid solutions there is no requirement to raster the surface in order to achieve analytical precision. Thus only a small 1 2 3 4 5 6 7 8 9 10 11 12 13 portion of the surface needs to be used in an analysis and these durable discs can be stored for repeated use. References Maiman T. H. Nature (London) 1960 187 493. Moenke-Blankenburg L. Laser Microanalysis Chemical Ana- lysis Wiley New York 1989 vol. 105. Thompson M. Goulter J. E. and Sieper F. Analyst 1981 106 32. Carr J. W. and Horlick G. Spectrochim. Acta Part B 1982 37 1. Ishizuka T. and Uwamino Y. Spectrochim. Acta Part B 1983 38 51 9. Gray A. L. Analyst 1985 110 551. Arrowsmith P. Anal. Chem. 1987 59 1437. van Heuzen A. A. Ph.D. Thesis University of Amsterdam 1990. Taylor J. D. Kennedy W. J. and Hall A. Bull. Brit. Mus. (Nut. Hist.) Zool. 1969 Suppl. 3. Taylor J. D. Kennedy W. J. and Hall A. Bull. Brit. Mus. (Nat. Hist.) Zool. 1 973 22 253. Brand U. and Morrison J. O. Geosci. Can. 1987 4 85. Abdullah M. I. Royle L. G. and Morris A. W. Nature (London) 1972 253 158. Deer W. A. Howie R. A. and Zussman J. An Introduction to the Rock-forming Minerals Longman London 1975 part 5 p. 474. Paper I /OO 7098 Received February 14th 1991 Accepted April 25th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600445

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 45 1 Comparison of Refractive Index Energy Dispersive X-Ray Fluorescence and Inductively Coupled Plasma Atomic Emission Spectrometry for Forensic Characterization of Sheet Glass Fragments Robert D. Koons Forensic Science Research Unit FBI Laboratory FBI Academy Quantico VA 22135 USA Charles A. Peters and Pamela S. Rebbert Elemental and Metals Analysis Unit FBI Laboratory Washington DC 20535 USA Fragments (in the milligram size range) from 81 tempered sheet glasses were used in order to evaluate the source discrimination capabilities of refractive indices (RI) and elemental composition by using energy dispersive X-ray fluorescence (EDXRF) and inductively coupled plasma atomic emission spectrometry (ICP- AES). The X-ray intensities of five elements were determined by EDXRF with precisions of between 1 and 25%.The concentrations of nine elements were determined using ICP-AES and precisions of from less than 1 to about 10% were obtained. Both methods offer improved discrimination capability over RI measurements alone. The technique of EDXRF provides rapid non-destructive testing and is widely available in forensic laboratories. The ICP-AES method offers the advantages of providing quantitative data on the concentration of elements applicability to a greater number of elements and improved discrimination. Keywords X-ray fluorescence; inductively coupled plasma atomic emission spectrometry; elements in glass; forensic examination Methods for determining the concentration of elements in glass fragments both quantitative and qualitative have been explored in the forensic characterization of glass for more than 20 years.Two approaches have been suggested depending upon whether the point in question is one of classification i.e. the placing of a glass fragment into a product-use category or discrimination i. e. the distinction among sources of glass within a product class. Methods used for classification must provide good accuracy but not necessarily good precision since the composition of a sample is compared with a database consisting of fairly broad class definitions. Methods used for discrimination among similar glasses must provide a high degree of precision for as many measured parameters as possible however a high degree of accuracy is not required as long as the samples in question are compared under the same analytical conditions.The long-standing interest in elemen- tal characterization of forensic glass samples is indicated by the number of methods that have been suggested for this purpose including neutron activation analysis l v 2 atomic absorption spe~trometry,~ d.c. arc emission spectro- g r a p h ~ ~ spark source mass spectr~rnetry,~-~ scanning elec- tron microscopy/)<-ray fluore~cence,~~~ energy dispersive X- ray fluorescence (EDXRF),8-10 inductively coupled plasma atomic emission spectrometry (ICP-AES)' '-l3 and induc- tively coupled plasma mass spectrometry.14 Of these ICP- AES and EDXRF are the only methods that have been applied to a significant number of samples and are currently utilized in forensic laboratories.Several recent papers provide a good overview of the use of ICP-AES and EDXRF for forensic glass analysis.8~11.12~15~16 Most procedures for elemental determination were devel- oped for the purpose of classification either as a means of eliminating alibi sources8J2J5J6 or in investigations involv- ing the contamination of or tampering with products.11 However the majority of criminal cases involve the comparison of glass fragments i. e. discrimination is required. Methods developed for classification may not be directly applicable to source discrimination because of the different analytical requirements of the two questions. Currently most examiners of forensic glass rely predomi- nantly upon physical and optical properties when compar- ing glass fragments found at the scene of a crime with those associated with a Recently there has been renewed interest in elemental analysis as a means of improving discrimination because there is some evidence of tighter manufacturing controls on the physical and optical properties of glass.Clearly properties such as refractive index (RI) and dispersion will remain the principal methods of comparing glass samples because they can be determined non-destructively with commonly avail- able instrumentation offer relatively high levels of discrim- ination and are well established in the forensic laboratory and judicial systems. Elemental composition is considered by glass examiners in those instances where additional discrimination is desired The results of a study to evaluate the relative capabilities of RI EDXRF and ICP-AES to discriminate between sheet glasses are reported.Samples for this study consist of 81 tempered side window glasses taken from automobiles produced in the years 1974-1987. Fragments used for comparison of the methods were in the ( 5 mg size range in order to be typical of those occurring in materials trans- ferred from the scene of a crime. Experimental Sample Collection and Description Samples of tempered glass were collected from the side windows of automobiles from automobile salvage yards in the Washington DC USA area. Each sample consisted of several hundred grams of broken glass. The make model year and vehicle identification number of the automobile were recorded for each sample.The 8 1 samples collected for this study represent 19 makes and 60 models from the period 1974-1 987. There are eight instances of two samples from the same make model and year of automobile. Refractive index determinations n (656.3 nm) n (589.3 nm) and nF (486.1 nm) were made on single fragments from each sample using a procedure similar to the Emmons double variation method adopted by the Association of Official Analytical Chemists.17-19 Sheet thickness visual estimate of colour and floathon-float characteristics were also determined using large pieces of glass. Only one sample (1978 Dodge Colt) was non-float glass.452 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 Sample Handling and Preparation for Analysis Samples were broken and individual pieces (1-3 g) of the diced glass were selected for analysis.Prior to elemental analysis individual pieces of glass were washed for 30 min with concentrated HN03 and then rinsed three times each with de-ionized water in order to remove surface contami- nation. After drying each piece of glass was placed between polyethylene sheets and crushed so as to provide fragments of appropriate size ( ( 5 mg). Replicate samples for analysis were selected from three separate pieces which had been carried through the washing and crushing procedure. Reagents and Standards Multi-element standard sclutions for ICP-AES were pre- pared by dilution of commercially available single element stock solutions with concentrations of 1000 ,ug rnl-l using distilled de-ionized water.High-purity HCl and HF (J. T. Baker or Fisher) were used for dissolution of the samples. National Institute of Standards and Technology glass Standard Reference Materials (SRMs) 62 1 (Glass Con- tainer) 1830 (Soda-Lime Float Glass) and 1831 (Soda- Lime Sheet Glass) were analysed by ICP-AES as checks on the accuracy of results. No standards were required for EDXRF analysis. However SRMs were used in order to provide multiple fragments of a homogeneous sample needed for optimization of the conditions used for the acquisition of X-ray data. EDXRF Instrumentation and Analytical Conditions X-ray fluorescence measurements were made on individual fragments of glass using a collimated X-ray source and energy dispersive X-ray analyser. Instrument operating conditions were as listed in Table 1.Samples were posi- tioned in the X-ray beam by suspending them upon an approximately 0.4 mm wide strip of adhesive tape contain- ing negligible levels of all the elements of interest. By using the conditions given in Table 1 reproducible results were obtained for a sample positioned within a region 1 mm on either side of the centre of the beam. The accelerating voltage size of the collimator and composition and thick- ness of the filter were optimized in order to obtain the highest peak-to-background ratios for Fe K a X-rays from glass fragments in the milligram size range and allow measurement of X-ray intensities in the energy range up to 20 keV. X-ray spectra were collected until the intensity of the Ca K a X-ray plus background equalled 40000 counts.For a 1 mg sample with the X-ray tube current set to obtain 50% dead time the counting time was about 150 s. Acquisition time was a function of sample size shape and orientation since for the limited range of glasses in this study all concentrations of Ca were about the same. Generally larger samples required longer acquisition times because of the need to reduce the X-ray tube current in order to maintain 50% dead time. Samples requiring acquisition times of more than 300 s resulted in relatively poor precision so were further crushed in order to obtain smaller fragments and shorter counting times. Glass frag- ments of a size that is useful forensically are incompletely penetrated by both the primary and fluorescent X-rays. As a result quantitative elemental concentrations could not be determined with sufficient accuracy and precision for samples of this small size and irregular shape. However in comparing fragments of varying mass and shape from a homogeneous glass standard the intensity ratios of two X- ray lines of similar energies were relatively constant.Therefore the discrimination capability of EDXRF for glass samples was evaluated using X-ray peak intensity ratios after correction for Si escape peaks and continuum background subtraction. In all samples the elements Si K Ca Fe Sr and Zr were determined. Additionally Mn and Rb were determined in some samples. Triplicate determi- nations were made for each sheet glass sample using fragments obtained from separate pieces of glass. Dissolution Procedure for ICP-AES Dissolution of samples was similar to procedures reported previou~ly.~~J~ Fragments of approximately 5 mg in size were weighed on a microbalance to the nearest 0.01 mg and placed in 15 ml polyethylene screw top tubes for dissolu- tion.To each sample tube 500 pl of HF were added. The tubes were placed in an oven at 80 "C for 1 h removed and individually placed briefly in a sonic bath then returned to the oven and dried. After drying 500 p1 of concentrated HC1 were added to each tube. The samples were again dried overnight in the oven at 80 "C. Upon removal from the oven and cooling 500 pl of concentrated HCl 500 pl of a 1000 ppm Sc solution and 9.00 ml of water were added to each tube. The tubes were capped mixed on a vortex mixer and returned to the oven for 1 h.The samples were cooled to room temperature and analysed by ICP-AES using multi- element standards prepared from stock standard solutions. ICP-AES Instrumentation and Analytical Conditions Instrument and operating conditions for the ICP-AES instrument are shown in Table 2. Calculation of the concentration of the elements was carried out using multi- Table 1 Instrumental and analytical conditions for EDXRF Instrumentation- Spectrometer model X-ray source Multichannel analyser Computer Signal processing Analytical conditions- Operating voltagelkv Operating current Source collimator diametedmm Source filter Path Spectral Ijnes/keV Kevex 0700 1200 W Rh anode in direct mode 1024 channels 0-20 keV at 2 eV per channel DEC LSI 11/23 Quantex version 4x21 35 As needed to give 50% dead time 3 0.15 mm aluminium Vacuum Si 1.740 K 3.312 Ca 3.690 Mn 5.895 Fe 6.400 Rb 13.375 Sr.14.142 Zr 15.746JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 453 Table 2 Instrumental and analytical conditions for ICP-AES Instrumentation- Spectrometer model Dispersing system Torch Nebulizer Spray chamber R.f. generator Argon flow rate/l min-' Analytical conditions- Sample uptake rate/ml min-' Observation height/mm Spectral line/nm Background correction Signal compensation Integration time/ms Perkin-Elmer Plasma IT Monochromator A 3600 grooves mm-* resolution ~ 0 . 0 0 9 nm 160-400 nm Monochromator B 1800 grooves mm-I resolution ~ 0 . 0 18 nm 160-800 nm Fassel-type Perkin-Elmer high solids Scott design 27.12 MHz 1.2 kW forward power Plasma 15 Auxiliary 1 .O Nebulizer 1.2 1 .o 15 above load coil Monochromator A Fe 238.204 Monochromator B Ca 393.366 Mn 257.610 Mg 279.553 Ti 334.941 A1 396.152 Sr 407.77 1 Ba 455.403 Na 589.592 Auto On all except Na 100 Table 3 Frequency of indistinguishability of 8 1 sheet glass samples taken in pairs (3240 comparisons) Comparison parameter and criteria No.of indistinguishable pairs Frequency n +- 0.0002 n +- 0.000 1 (1) and n,+-0.0004 and nF+0.0004 (2) and n,-k0.0002 and nFk0.0002 EDXRF (see text for criteria) ( 5 ) and (3) ( 5 ) and (4) (8) and (3) (8) and (4) TCP-AES (see text for criteria) 648 418 487 178 305 81 33 3 3 2 1:5.0 1 :7.8 1:6.7 1:18.2 1:10.6 1 :40 1 :98 1:1080 1:1080 1:1620 element external standard solutions for all elements and Sc as an internal standard for all of the elements except for Na.A standard response graph was constructed and SRMs were analysed as a check on accuracy before each set of approximately 25 samples. Triplicate measurements were made on each solution and on triplicate samples from each sheet glass. Triplicate samples were not run sequentially so that a good estimate of total precision over the course of a set of samples would be obtained. Results and Discussion Optical and Physical Properties Discrimination among sources based on thickness colour and floathon-float characteristics provides some sub-divi- sion of automobile sheet glass samples. However since these parameters cannot be measured on small fragments they are not considered in this study.Refractive indices are generally considered to provide the best discrimination capability of the commonly measured optical and physical properties of glass. The RI values for the 81 sheet glasses in this study occur within the ranges of n (1.51 11-1.5201) n D (1.5138-1.5231) and nF (1.5 193- 1.5287). Although these ranges suggest that good discrimination of sources is possible 5 1 of the 8 1 samples had n D values in the range 1.5 180- 1.5 194 and the 65 glasses produced in the US had n values in the range 1.5 1 72- 1.5 1 99. Examiners of forensic glass samples have generally considered glass fragments to originate from different sources when n values differ by more than 0.0002 or n or nF values differ by more than 0.0004.17 These values were derived from interlaboratory studies and are greater than the combined heterogeneity of modern sheet glasses and short-term analytical precision. Using auto- mated imaging systems for match point determination it has been shown that RI variation across most sheet glasses is less than 0.00003 for all three indices.*O For the measurement procedure used in this study more appropri- ate criteria for source discrimination based on within- laboratory precision are 0.0001 for and 0.0002 for n and nF.The worst instance of discrimination using n criteria is 1.5 185 k 0.0002 which includes 32 samples or 1.5 187 k 0.000 1 which includes 22 samples. For 81 samples taken in pairs there are 3240 possible comparisons. Of these the number of indistinguishable pairs of sheet glasses using several RI criteria are shown in the first four lines of Table 3.As shown the discrimination capability increases to some extent when nc and nF are included in the comparison in addition to n,. This conclusion supports the observations made in other foren- sic laboratories in the US and contrasts with those from the UK where dispersion is generally considered to offer little added discrimination. The discrimination capability im-454 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 Table 4 Summary of EDXRF results for sheet glass fragments. Results shown are range and mean X-ray intensities or intensity ratios of 243 fragments and mean relative standard deviation (RSD) of triplicate measurements of 8 1 samples Element Range Mean RSD (Yo) Si 3 41 1-5 407 4 338 4.3 Ca 36 155-38717 37482 1 .O Fe 4 989-38 262 27 099 4.3 Sr 534-10791 3080 20.1 Zr 1226-20055 6910 22.4 Si:Ca 0.090-0.144 0.1 16 4.3 Fe:Ca 0.134-1.01 8 0.723 4.8 Sr:Zr 0.102-1.533 0.514 11.5 proves only moderately as the overlap criteria are made more stringent.There are no consistent relationships between automobile make and model and the likelihood of indistinguishable RI values. Of the eight pairs with match- ing make model and year of automobile three pairs have indistinguishable TZ values at the kO.0002 level and two pairs are indistinguishable at all RI levels. The only trend related to place of manufacture is that all glasses made in Japan are at the low end of the RI ranges and the highest three RI values are for glasses made in Europe. Also the non-float glass in this study has the lowest RI values of any sample.EDXRF The discrimination capability of any variable is determined in large part by the precision of the measurement (random error plus sample heterogeneity) and the range of values observed across sources. As a measure of the combined precision of the X-ray intensities obtained and the within- sample variability the standard deviation of the measure- ments from the triplicate fragments of each glass sample relative to the sample mean relative standard deviations (RSDs) were calculated and then averaged over the 81 glass samples. The RSD measurements for the intensity of each element and selected intensity ratios are shown in Table 4. The results are generally similar to those reported by Ryland* using different instrument operating condition^.^ Fragment sizes in the range 0.5-3 mg were used to obtain the data given in Table 4.The RSD values of the intensity of individual elements range up to about 25%. The RSD of the Sr:Zr intensity ratio shows about a 2-fold improvement over either element individually as expected for two X-rays of similar energy from small samples of irregular size and shape. A comparison of the RSD values with the ranges shown in Table 4 indicates that the Fe:Ca and Sr:Zr ratios are the parameters which provide the greatest degree of discrimination using the EDXRF method. Painvise comparison of glass samples to determine the discrimination capability of EDXRF was carried out using a two-step method. Firstly samples containing measurable levels of uncommon elements Mn and Rb and high levels of K were separated as a sub-group of the sheet glasses.Secondly comparison of two samples within a group was made by the method of range overlap ie. for the variables Si:Ca Fe:Ca and Sr:Zr the ranges of triplicate measure- ments for each of two samples were compared. For samples in which a particular ratio exhibited an unusually small range of values the range was increased to a level based on two times the mean RSD before comparison was made. If all variables exhibited overlapping ranges then the samples were considered to be indistinguishable. Use of range overlaps instead of statistically based confidence intervals was selected as being more realistic to the forensic ex- aminer. In forensic comparison of trade evidence the need to limit type I1 errors i.e.incorrectly attributing two samples to a common source is more important than the need to minimize type I errors i.e. incorrectly attributing two samples to different sources since the former tend to associate a suspect with the scene of a crime. Clearly overlapping ranges of all parameters between two samples render them analytically indistinguishable and hence dif- ferent sources are not indicated. As shown in Table 3 of the 3240 paired comparisons 305 indistinguishable pairs were found when the EDXRF criteria were used. In the worst instance one sample (1 984 Ford EXP) was indistinguishable from 26 others. Discrimi- nation of EDXRF is better than that obtained by TZ alone and all three indices using the wider RI discrimination criteria but slightly worse than all three RI using the more stringent criteria.In general the samples that were indistin- guishable by EDXRF were not the same ones that were indistinguishable by RI. This is evidenced by the significant improvement in discrimination when EDXRF and RI results are combined as shown in lines 6 and 7 of Table 3. There were no readily discernable relationships between EDXRF spectra and manufacturers except for the automo- biles made in Japan. All Japanese glasses contained higher amounts of IS Mn and Rb than other automobiles in this study. The eight pairs of samples from the same make model and year were all distinguishable using EDXRF criteria. No other elements such as Mn Ti As or Ba reported by other workers9J0 were observed in the samples of limited composition used in this study.One likely contributing factor to the success of EDXRF in discriminating among similar samples such as those produced within a model year is the heavy reliance upon concentrations of Zr. Zirconium occurs in glass both as a result of erosion of the refractory material from the walls of the melting tank during manufacture and the presence of zircon grains in the sand used in the making of glass. Measurable variations in the concentrations of Zr over the course of large production runs within a manufacturing facility are thus likely to occur and can serve as a disciminator of otherwise indistinguishable glasses. ICP-AES In contrast to EDXRF ICP-AES provides quantitative measurements of the concentrations of elements in glass.In order to avoid biases that might occur between day-to-day analyses of sets of samples SRMs were analysed together with each sample set. For each set of samples the results obtained for the SRMs were compared with their certified values and whenever the results were outside accepted limits recalibration was performed prior to analysis of samples. Table 5 Summary of ICP-AES results for sheet glass fragments. Figures are range and mean of concentrations of 243 analysed fragments and mean RSD (O/O) of triplicate measurements of 81 samples Element Range Mean RSD (O/O) A1 (Yo m/m) Ba/pg g-l Ca (O/o m/m) Fe (Yo m/m) Mg (O/o m/m) Mn/pg g-l Na (Yo m/m) Sr/,ug g-l Tilpg g-I 0.032 1-0.985 7.6-205 4.94-6.76 0.0589-0.484 2.02-2.40 7-262 6.9- 1 3.3 19.0- 1 68 4 1.2-480 0.213 3.2 35.0 5.4 5.97 0.7 0.309 2.1 2.24 1.2 32 8.0 10.1 12.0 46.5 1.1 121 6.5JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL.6 45 5 Table 6 Data for four sheet glass samples Manufacturer model and year Parameter Optical data- nc nD HF EDXRF- Si (counts) Ca (counts) Fe (counts) Sr (counts) Zr (counts) Si:Ca Fe:Ca Sr:Zr Al/pg g- * Ba/,ug g-I Ca (O/o m/m) Fe (O/o m/m) Mg (O/o m/m) Mnlpg g-I Na (O/o m/m) Sr/,ug g-' Ti/pg g-* ICP-AES- Chevroiet Nova 1978 1.5161 1.5186 1.5246 4291 4 167 4757 37663 37484 37 160 29 033 20 834 28 037 1997 2083 2 312 12 131 13 154 15431 0.114 0.111 0.128 0.77 0.82 0.76 0.16 0.16 0.15 526 515 528 16.1 19.1 17.9 5.96 6.03 6.07 0.330 0.337 0.339 2.17 2.21 2.20 14.5 15.5 11.5 8.7 9.3 10.1 31.0 31.6 31.1 120 112 114 Pontiac Grand Prix 1977 1.5159 1.5187 1.5247 4 279 4045 4 123 36303 38 121 37 146 27 884 28 232 28 987 3261 2122 2118 10 286 5 765.7 338 0.118 0.106 0.111 0.77 0.74 0.78 0.31 0.36 0.29 548 533 582 14.2 16.4 17.8 5.92 6.01 6.1 1 0.302 0.312 0.313 2.26 2.28 2.30 11.4 7.5 11.0 8.7 9.9 9.4 30.0 30.6 30.3 69 49 54 Lincoln Mark VI 1980 1.5160 1.5186 1.5248 3 975 4 142 4 262 36 849 37 248 37 791 31 850 30 576 26 742 1485 1873 1201 6 294 6 976 8 386 0.108 0.1 1 1 0.1 13 0.86 0.82 0.71 0.24 0.27 0.31 540 527 56 1 11.8 11.3 11.4 5.93 5.98 6.02 0.349 0.334 0.335 2.37 2.34 2.34 14.7 15.4 13.9 7.8 10.1 9.9 25.2 26.1 25.6 65 54 53 Chrysler Cordoba 1979 1.5158 1.5186 1.5246 4 460 4 735 4 503 37 986 37 023 37 986 31 706 28679 31,199 3 013 3 687 2 389 7 270 12 063 6 967 0.1 17 0.128 0.1 19 0.83 0.78 0.82 0.41 0.31 0.34 5 12 472 490 22.5 22.5 22.2 5.95 5.97 5.98 0.364 0.355 0.362 2.22 2.16 2.18 13.1 11.7 11.9 8.0 12.6 9.7 49.3 48.8 48.0 107 113 105 The precisions of the concentrations of elements ob- tained by ICP-AES were calculated using the measurements from triplicate fragments in the same manner as discussed previously for EDXRF results.These results are given in Table 5. In comparison with EDXRF measurements the ICP-AES results clearly offer a greater number of par- ameters upon which discrimination can be based and better precision in the determination of these parameters. Most elements were determined with an RSD of 1-5% of the mean concentration. Sodium exhibits greater uncer- tainty than the other elements because of its ubiquitous presence in reagents possible heterogeneity in the glass and the fact that an internal standard could not be accommo- dated by the programme used by the instrument manufac- turer for the calculation of Na concentration.The results for Mn had slightly worse precision than other elements because the concentrations of Mn in the glass sample digests were close to the detection limits. The values for precision for the elemental ranges given in Table 5 indicate that all elements except Na offer good discrimina- tion capability. Pairwise comparison of the 81 sheet glass samples using the ICP-AES results was performed using range overlap criteria similarly to those used with EDXRF data. Two samples were judged to be indistinguishable when for all nine elements measured the concentration ranges of the two samples overlapped.Results of painvise comparison of the 81 samples (Table 3) resulted in discrimination of all but three combinations of the samples. There are three samples that are indistinguishable from one another. These are from a Ford Thunderbird a Lincoln Mark IV and a Lincoln Mark V all produced in the model year 1977. Since these models were all made by the Ford Motor Company in the same year the indistinguishable compositions of the glasses may mean that they represent a single source of production. Of the eight pairs of samples representing the same model and year of production six are readily discriminated and two are similar but distinguishable having only one element in each with non-overlapping concentration ranges.The values of frequency of indistinguishability of pairs of samples using each method singly and in various combina- tions shown in Table 3 must be interpreted carefully. The frequencies listed should not be interpreted as population distribution frequencies since the samples were not ran- domly selected as being representative of the entire popula- tion of tempered automobile glass. However they do provide a measure of the relative discrimination capabili- ties of RI EDXRF and ICP-AES results. Clearly both EDXRF and ICP-AES offer improved discrimination whether used alone or in combination with RI measure- ments. It is also to be noted that the composition ranges for glass used in this study were fairly limited. Compositional analysis particularly by ICP-AES becomes a much more powerful method for discrimination among glasses having a different end use but which may have similar optical and physical properties despite widely different elemental compositions.Example Data for Four Sheet Glass Samples In order to provide an example of the use of EDXRF and ICP-AES for discrimination among glass sources four samples having similar optical properties were selected. Results for these samples are given in Table 6 . Individual EDXRF and ICP-AES results are given in order to illustrate the range overlaps which exist. The four glasses represent different makes models and years of automobile yet they all have indistinguishable RI values. The EDXRF results clearly indicate that the glass from the 1978 Chevrolet Nova is distinguishable from the other three glasses because of its lower Sr:Zr values.The other three samples are not discriminated using the EDXRF criteria. Results for ICP- AES indicate clearly distinguishable compositions among the four glasses.456 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 Conclusions Both EDXRF and ICP-AES procedures along with RI measurements and other physical and optical properties have a place in the forensic laboratory for discrimination among sources of sheet glass. Energy dispersive X-ray fluorescence offers the advantages of speed non-destructive testing wide availability reasonably good discriminating capability and provides points of comparison which are relatively independent of optical measures.Inductively coupled plasma atomic emission spectrometry offers the advantages of providing quantitative element concentra- tion data applicability to a greater number of elements and improved discrimination at the cost of destruction of the sample. The quantitative multi-element capability of ICP- AES is also advantageous for classification purposes and for comparison of glasses of a wider range of compositions than shown in the limited product use category of glasses studied here. The authors thank S. Brixey for assistance in sample collection and refractive index measurements and D. Ward C. Fiedler B. Hall and L. Deremer for helpful suggestions concerning this research. 1 2 References Coleman R. F. and Goode G. C. J. Radioanal. Chert?. 1973 15 367.Goode G. C. Ward G. A. Brooke N. M. and Coleman R. F. Atomic Weapons Research Establishment Report 02417 1 Aldermaston UK 197 1. 3 Hughes J. C.. Catterick T. and Southeard G. Foresnsic Sci. 1976 8 217. 4 Blacklock E. C. Rogers A. Wall C. and Wheals B. B. Forensic Sci. 1976 7 12 1. 5 Dabbs M. D. G. German B. Pearson E. F. and Scaplehorn A. W. J. Forensic Sci. Soc. 1973 13 281. 6 Haney M. A. J. Forensic Sci. 1977 22 534. 7 Terry K. W. van Riessen A. and Vowles D. J. Micron 1982 3 293. 8 Ryland S. G. J. Forensic Sci. 1986 31 1314. 9 Reeve V. Mathiesen J. and Fong W.. J. Forensic Sci. 1976 21 291. 10 Dudley R. J. Howden C. R. Taylor T. J. and Smalldon. K. W. X-Ray Spectrum. 1980 9 119. 1 1 Wolnik K. L. Gaston C. M. and Fricke F. L. J. Anal. At. Spectrom. 1989 4 121. 12 Koons R. D. Fiedler C. and Rawalt R. C. J. Forensic Sci. 1988 33 49. 13 Catterick T. and Hickman D. A. Forensic Sci. lnt. 1981 17 253. 14 Zurhaar A. and Mullings L. J. Anal. At. Spectrum. 1990 5 611. 15 Hickman D. A. Harbottle G. and Sayre E. V. Forensic Sci. lnt. 1983 23 189. 16 Hickman D. A. Forensic Sci. lnt. 1983 23 213. 17 Miller E. J. in Forensic Science Handbook ed. Saferstein R. Prentice-Hall Englewood Cliffs NJ 1982 ch. 4. 18 OfJicial Methods of Analysis Association of Official Analytical Chemists Arlington VA 14th edn. 1984 p. 936. 19 McCrone W. C. J. Assoc. Off Anal. Chem. 1973 56 1223. 20 Locke J. and Underhill M. Forensic Sci. Znt. 1985 27 247. Paper 1 /00 71 OF Received February 14th 1991 Accepted May 14th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600451

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER I99 1 VOL. 6 457 Rapid Stopped-flow Microwave Digestion System Vassili Karanassios,* F. H. Li B. Liu and Eric D. Salint Department of Chemistry McGill University 80 I Sherbrooke Street West Montreal Quebec H3A 2K6 Canada A prototype system for stopped-flow microwave assisted wet digestions has been developed. A coiled Teflon PFA tube serves both as a sample container and as a digestion vessel. A sample plug consisting of a water slurry mixed with an acid mixture is pumped into the coil. Sample flow is stopped the coiled tube is sealed (by closing an input and an output valve) and microwave power is applied for 2 min for digestion of the sample. Methodology was developed using powdered botanical reference samples and was tested with powdered botanical and biological reference materials.The digests were analysed by inductively coupled plasma atomic emission spectrometry. In addition to comparisons with certified values the results were compared with those obtained by conventional open-vessel hot-plate digestions by open-vessel microwave digestions and by digestions taking 32 min by using the coiled tube system. Precise and in many instances quantitative digestions were obtained using a net digestion time of 2 min. Elemental recoveries were sample type and digestion time dependent and were found to be comparable with and sometimes superior to those obtained when using a 3 h long hot-plate digestion. In this preliminary study characteristics limitations and future directions are discussed. Keywords Microwave digestion; flow system; elemental analysis; powdered sample; inductively coupled plasma atomic emission spectrometry Microwave assisted wet digestions offer an alternative to traditional (i.e. open vessel hot-plate) time-consuming sample dissolution techniques. Since the first description’ of the use of microwave radiation as an energy source in acid the method has attracted considerable attention and has been successfully applied to a variety of sample types. Included in these are botanical biological and geological materials and foodstuffs. For more details the book by kngston and Jassie3 and recent reviews4y5 may be consulted. Although open- and closed-vessel acid digestions have been developed the high-pressure sealed-bomb approach is the most widely The success of bomb digestions is exemplified by the range of applications reported and the availability of commercial instrumentat ion.7 4 In spite of the advantages offered by closed-vessel digestions sample preparation still remains a multi-step and labour-intensive procedure. The labour involved per digestion is the same whether Parr bombs are heated in electrical furnaces for hours or sealed Teflon vessels are exposed to microwave energy for minutes. In an attempt to increase sample throughput further reduce labour and cost and facilitate automation wet digestions can be forced to occur as a sample stream slowly flows through a microwave oven. This is an approach that gives rise to the concept of flowing stream digestions which has been successful with blood samples.’ These were digested in 30 s while slowly flowing through a microwave oven.’ In order to accommodate powdered samples that require much longer digestion times (i.e.32 min) the flow may be interrupted for a period of time resulting in stopped-flow digestions. Ease of automation is perhaps the most important advantage of this approach. The goal of this work is the development of stopped-flow digestion instrumentation and simple methodology for the dissolution of powdered samples. Of particular interest are rapid (ie. less than 5 min) and reproducible extractions of ‘environmentally available’ elements* in botanical samples of environmental concern. In our system a Teflon PFA tube serves both as a sample container and a digestion vessel.Besides the advantages mentioned above this closed-vessel approach minimizes the risk of sample cross * Present address Department of Chemistry University of t To whom correspondence should be addressed. Waterloo Waterloo Ontario N2L 3G1 Canada. contamination when digesting multiple samples simultane- ously and of acid fumes attacking the oven components. Therefore it eliminates the need for special precautions such as oven coatings acid scrubbers or evacuated oven chamber^.^ It also offers additional safety protection to personnel by limiting exposure to hazardous acid fumes and by minimizing reagent handling. Experimental Instrumentation A schematic diagram of the instrumentation required for an ‘ideal’ stopped-flow microwave digestion system is shown in Fig.1 . The prototype system developed in this work consists of a peristaltic pump two high pressure valves a microwave oven a ‘tube assembly’ a pressure gauge and a temperature transducer. A list of equipment suppliers is provided in Table 1. The safety precautions required for routine operation of the ‘ideal’ system shown in Fig. 1 for example installation of waveguide attenuators in all access ports and of a pressure release valve are currently being addressed. The heart of this system is a conventional microwave oven. A slightly modified domestic-type commercially avail- able microwave oven was used. The oven has an internal volume of 0.9 ft3 (25.5 1) and operates at the standard 2.45 GHz frequency. The power is adjustable in 9 steps from 1 (‘low’ corresponding to a power of approximately 72 W) to 9 (‘high’ or ‘full’ which is equivalent to about 720 W) in I I Pump I tube I I Sample I oven I Perista I t ic I collector Waveguide Waveguide pump I attenuator attenuator Fume hood Slurry 1 Schematic diagram of an ‘ideal’ stopped-flow digestion L - - - - - - - - - - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Fig.1 system458 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 Table 1 List of instrument suppliers Digestion system- Microwave oven Microwave meter Pump tubing Digestion tube Pressure gauge ICP spectrometer- Spectrometer Spray chamber Nebulizer Peristaltic pump Toshiba Model EXR- 1690C Toshiba Minato-Ku Tokyo Japan Holaday Model HI-1 800 Holaday Industries Eadon Prairies MN USA Mandel Scientific Guelph Ontario Canada Teflon PFA Cole-Palmer Chicago IL USA Cole-Palmer Thermo Jarrell-Ash Model ICAP-6 I Franklin MA USA Technical Service Laboratories Ontario Canada Technical Service Laboratories Gilson Miniplus3 Model 3 12 Gilson Medical Electronics Middleton WI USA increments of 81 W. The power delivered from the magnetron to the oven was not calibrated for this work and was assumed from the manufacturer’s specifications.Partial power is delivered to the oven by automatically adjusting the amount of time full power is applied to the magnetron. For example for this oven and for power level 2 full power is applied to the magnetron for about 3 s power is then turned off for about 12 s and this 15 s cycle is repeated often as required to complete the desired heating time (i.e.2 min). The heating time is programmable via front panel push- buttons and can be set from 1 s to 99 min 99 s. The oven was modified by placing an electrical fan on its side as shown in Fig. 2. This was done in order to vent hot air during operation and to help cool the tube at the end of a digestion. In addition two 3/8 in ( e 9 . 5 mm) holes were drilled in the back of the oven. These serve as entrance and exit ports for the digestion tube. As a further safety precaution the oven was operated inside a fume hood. Two types of tubing constitute the ‘tube assembly’. These are identified as ‘pump tube’ and as ‘digestion tube’ in Fig. 1. The pump system consists of two conventional peristaltic pump tubes with an internal diameter (i,d.) of 0.090 in (-2.3 mm) and an outer diameter (0.d.) of 0.1 575 in (-4.0 mm).These are connected to the digestion tube with a T- joint. Slurry and acid(s) are pumped into the digestion tube (Fig. 1) utilizing two channels of a six channel peristaltic pump- A Teflon PFA tube (perfluoroalkoxy a chemically inert non-porous tetrafluoroethylene with a fully fluorinated alkoxy side chain7 with an 0.d. of 1/4 in (-6.35 mm) and an i d . of 5/32 in (-4.0 mm) serves as the digestion tube. According to manufacturer’s specifications it can withstand pressures of 42 1 psi (1 psi-6.894 x lo3 Pa) and tempera- tures of 260 “C. The tube 13.8 ft ( ~ 4 2 0 cm) in length between the input and output valves was coiled in six turns (the diameter of the inner turn of the coil was about 9 cm) and was placed facing the magnetron as shown in Fig.2. This arrangement was chosen in order to take full advan- tage of the power delivered from the magnetron to the sample. A sample plug is pumped into the centre of the coil leaving about 50 cm of air on both ends of the tubing. No Slurry and acid Waveguide in Solution out \ Grounded fan screen 3‘ ietron Fig. 2 Actual microwave oven and coiled-tube set-up portion of the sample acid slug extends outside the microwave cavity. During digestion the sample slowly rotates inside the coiled tube either clockwise or anti- clockwise due to pressure differences developed on either side of the sample plug. Fortuitously this rotation serves as a stirring mechanism and also helps to reduce the effects of non-uniform heating due to ‘hot spots’.These are due to inhomogeneous microwave fields and are typically ob- served in domestic microwave oven^.^^^ Furthermore ow- ing to self-rotation the need for a sample container rotating device for example a rotating carousel typically used with conventional sealed-vessel digestion system~,~-~ is also eliminated. After repeated use for example about 100 times the external wall of the digestion tube showed visible signs of ageing such as yellowing particularly at the T- and valve- joints and was replaced for safety reasons. In addition a dark yellow-brown coating and black spots were visually observed inside the tube in particular in tube segments that are near the input and output ports and in the interior of the valves and the tube joints. Because these were primarily observed in parts of the tube assembly that are the furthest from the microwave energy flux or are external to the oven they were attributed to undigested sample.Some preliminary results indicate that the tube may have to be replaced more frequently owing to accumulation of undi- gested material (which may give rise to memory effects) rather than ageing or thermal/mechanical stress. The peristaltic pump tube was replaced depending on the work load about once a week. All measurements were performed using a Thermo Jarrell-Ash Model ICAP-6 1 inductively coupled argon plasma optical emission spectrometer. The ICAP-61 is a 0.75 m 34 channel polychromator system providing a spectral resolution of 0.048 nm (in the first order) and is capable of determining 32 elements simultaneously.The spectrometer was controlled by an IBM PC-compatible microcomputer using standard ThermoSpec software. A spray chamber a pneumatic V-type nebulizerlo and a peristaltic pump were used to introduce the digested samples into the inductively coupled plasma (ICP). Typical operating conditions were 1 .OO kW applied forward power 16 and 1.0 1 min-l for coolant and auxiliary argon flow rates respectively. The solution uptake rate was 2.0 ml min-l and the observation height was 15 mm above the load coil. Reagents and Procedures Standards reagents and standard reference materials All standards were prepared by serial dilution with distilled de-ionized water (1 8 MR cm-l specific resistivity prepared by feeding in-house distilled water through a Millipore purification system) of 1000 ppm (certified to k 1%) standard stock solutions (standards for atomic absorption spectrometry) of the respective element.Baker Instra-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 459 Analyzed inorganic acids were used throughout. A list of suppliers of chemicals and samples is given in Table 2. The methodology developed in this work relies heavily on the use of botanical reference materials provided by the Ontario Ministry of the Environment (MOE Table 2). Included in these are those designated as V85-1 Norway maple and White birch by the MOE. Particle sizes averaged 160 pm. Although there are no certified values for these samples the concentrations reported here are the average quoted by the MOE.These were obtained using a 3 h open- vessel hot-plate acid digestion procedure and were analysed by ICP atomic emission spectrometry (AES)." The MOE1l and other environment monitoring laboratories8 are inter- ested in trace elements that are environmentally available (i.e. not bound) so their digestions are typically not brought to completion. Their procedure 'quantitatively and repro- ducibly recovers most to all of the more toxic transition elements'. The methodology was tested using standard reference materials (SRMs). These include National Institute of Standards and Technology (NIST) (formerly National Bureau of Standards Table 2) Bovine Liver (SRM 1577) and Orchard Leaves (SRM 1 57 1). Certified values reported here were obtained either from certificates of analysis or from literature r e p o r t ~ .~ J ~ All concentrations reported here are the average of five repeats and refer to dried samples. Digestion procedure Approximately 0.35 g of powdered sample is accurately weighed in a glass beaker and 20 ml of de-ionized water are added. The resultant slurry is continuously and rigorously stirred and is pumped with the input and output valves open (Fig. I) into the digestion coil simultaneously with about 20 ml of an acid (i.e. HC1 or HN03) or an acid mixture [ie. aqua regia HN03+HC1 (1 +3 v/v) or HN03+H202 (4+ 1 v/v)]. The valves are then closed and the sample is heated either at high power for 2 rnin or using a 'power programme' for a pre-determined period of time for example full power for 2 rnin immediately followed by power level 5 for 3 min.After digestion the sample is allowed to air cool for several minutes. The output is then opened to relieve the pressure and the effluent is quantita- tively transferred into the collection vessel. Samples with residues were filtered (using a Whatman filter paper No. 42) the filter was washed with distilled water and the filtrate was diluted to volume (i.e. 50 ml). The solutions were then analysed using an acid-containing blank. Acidified multi-element standard solutions contain- ing the elements of interest were used for calibration. Depending on sample type power programme and digestion time the digest may contain an unspecified amount of residue. The amount of residue remaining in the digest and the digestion tube increases with decreasing digestion time for a given power programme.The residue which was attributed to undigested organic material such as amino acid^,^>^ does not typically contain any trace ele- Table 2 List of suppliers of reagents. SRMs and samples Standards and reagents- Standards Acids NIST SRMs Fisher-Scientific Fair Lawn NJ USA J. T. Baker Phillipsburg NJ USA National Institute of Standards and Technology Gaithersburg MD USA Ontario Ministry of the Environment (MOE) Rexdale Ontario Canada SRMs and samples- MOE samples m e n t ~ . ~ > ~ J ~ However unless the material is allowed to settle before analysis it may clog the nebulizer thus necessitating an extra filtration step. In addition because some of the material adheres to the inner walls of the tube the input and output valves and the T-joints trace elements accumu- late and eventually give rise to memory effects.When sequentially digesting the same sample type undigested material was removed from the tube by a high- speed flush with a water plug. The digestion tube was then cleaned by pumping through an acid plug followed by multiple rinses with a water plug. The total time for the digestion cooling and cleaning cycles was about 5 min. When changing sample types the tube was cleaned by exposing a water-acid mixture to microwave energy for 2 rnin followed by multiple rinses with a water plug. The total time for this cleaning cycle was about 5 min. These procedures reduce analyte concentrations in blanks to levels below the detection limit of the ICP spectrometer.For the open-vessel work approximately 0.35 g of sample was accurately weighed in an Erlenmeyer flask and mixed with 20 ml of distilled de-ionized water and 20 ml of acid. The flask covered with a watch-glass was placed in a Pyrex vacuum desiccator (without the desiccant) which was subsequently placed in the microwave oven where it was exposed to full power for a net exposure time of 6 min. Power was applied in 2 min intervals followed by a 2 rnin cooling period for a total time of more than 10 rnin per digestion. In order to vent acid fumes and to provide additional protection of the oven and its associated elec- tronics the vacuum port in the lid of the desiccator was connected to Tygon tubing. The tube was routed through one of the holes drilled in the back of the oven to the fume hood.An open 100 ml beaker containing 50 ml of water was placed inside the oven in order to protect the magnetron from reflected power17*J2J4 and to ensure the use of experimental conditions that are the same as those reported by other w o r k e r ~ . ~ J ~ Safety Owing to the use of potentially hazardous microwave energy and strong acids at elevated pressures and tempera- tures safety was a key consideration in operating this system. The following safety criteria were set minimum radiation leakage (ie. 1 mW cm-2) measured at 5 cm from the oven15 or any tube or cable emerging from it,3 no acid fumes in the microwave cavity and maximum operating pressure and temperature of 125 psi and 230 "C. Although the tube assembly was safely tested and briefly operated at pressures as high as 200 psi the choice of a lower operating pressure for routine operation was dictated by the weakest links in the tube assembly more specifically the tube to valve and the tube to T-joint connections (Fig.1). Accord- ing to the manufacture's specifications this is only 125 psi. Microwave oven Owing to the modifications to the oven (ie. fan duct and access holes) radiation leakage was an important concern. In addition the tube emerging from the oven might act as an antenna transmitting microwave radiation. Microwave leakage was extensively tested using a sensitive microwave power meter. The measured values were found to be below the levels mentioned previously typical values were below 0.2 mW cm-2. Temperature Unlike other workers who measured temperature in real- time during the course of digestion^,^^^^^ the initial concern of this study was the final temperature of the samples rather than a temperature versus time profile.The final tempera-460 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 ture was determined in order to avoid thermal degradation of the tube. Temperature was measured using a copper-constantan thermocouple encapsulated within a thin layer of glass and was held in place in the middle of the digestion tube with a T-joint (Fig. 1). The thermocouple leads were attached to a shielded grounded cable. Regardless of power programme sample type and digestion time the final temperature was below the safety limit. This is as expected considering the maximum allowable operating pressure (1 25 psi) and the boiling-points of the acids and acid mixtures used in this work.Pressure Pressure was monitored in real-time using a conventional pressure gauge in order to establish the operating conditions at which the tube assembly can be operated safely. Readings were taken manually at 1 min intervals. The results for various acids 0.35 g of V85-1 botanical sample and full power are shown in Fig. 3. The steep increase in pressure observed when a mixture of HNO and H202 is utilized indicates rapid formation of gaseous products and al- though it may have to be correlated with temperature and gaseous H202 de-composition products it also suggests a faster attack of botanical samples. However it also limits the digestion time to less than 2 min.From these results it can be concluded that the maximum time for which V85-1 botanical samples can be safely digested at full power is less than 3 min. In summary pressure rather than temperature is the limiting parameter in digestions involving botanical samples in nitric acid or its mixtures as has also been reported by other worker^.^^^^^ As mentioned earlier depending on power programme and digestion time an unspecified amount of undigested organic material remains in the digestion tube and settles in the collection vessel. This necessitates an extra time- consuming and labour-intensive filtration step. In order to eliminate this step and to obtain more complete digestions (i.e. those that result in clear and colourless solutions with no visible signs of residue) longer digestion times may have to be used.250 200 .- 150 \ 2? cn 100 L 50 0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 Digestion ti melmi n Fig. 3 Digestion tube pressure versus heating time for MOE V8 5- 1 botanical samples high power and different acids (see text for discussion). A HN03+H202; B aqua regia; and C HNO,. Horizontal broken line is the safety limit Results and Discussion From the preceding discussion it can be concluded that microwave energy effectively couples to acidified slurries in the digestion tube. The key question then becomes what is the best set of operating conditions (i.e. power exposure time and acid mixture) that will provide safe rapid and complete digestions? Digestion Time and Safety Pressure Digestion times can be extended without exceeding the safety pressure.For example as shown in Fig. 4 a digestion time of 5 rnin can be achieved by applying full power for 2 rnin followed by the application of power level 5 for 3 min. In order to reduce the amount of undigested material remaining in the tube further digestion times can be extended to over 30 rnin by applying full power for 2 min followed by continuous application of power level 3 for 30 min. Almost clear and lightly coloured solutions signifying more complete digestions have been obtained using this programme V85-1 samples and aqua regia. Acid Mixtures As has been amply demon~trated,~*~>~J~ a variety of acid mixtures can be used for the destruction of organic matter.16 In this work HNO HC1 a mixture of HNO and H202 and aqua regia were tested with V85-1 botanical samples with various degrees of success.Because aqua regia resulted in the least amount of undigested material (on visual inspection) in the shortest time without exceeding the safety pressure it was the reagent of choice and was used throughout. Even with aqua regia and the use of a power programme complete digestions require over 30 min an excessively long time for a rapid digestion system. However it is questionable whether complete digestions are necessary in order to obtain 100°/o recovery of trace elements in botanical samples. Elemental Recoveries and Digestion Time In order to address this question V85-1 botanical samples were digested with aqua regia for 2 4 8 16 and 32 min. The digested samples were analysed for Al Ba Cd Cu Fe Mg Mn and Zn. The average recovery was found to be about 100% irrespective of digestion time. Therefore trace elements are fully recovered in 2 rnin when digesting V85-1 samples at full power.However in order to substantiate this conclusion further recoveries of individual elements must be investigated. The effect of the matrix (i.e. sample type) on elemental recoveries will be examined subsequently . In order to obtain ‘total’ digestions and to establish a ‘reference’ value MOE V8 5- 1 botanical slurries were digested for 32 rnin by applying a power programme. These results their respective MOE values and an inter-compari- son (presented as Yo recovery with respect to the MOE values) are shown in Table 3. With the exception of A1 and Ba which show relatively high recoveries with respect to the MOE values the average concentrations and the standard 200 180 - 160 - .- 60 I I I - 2.0 3.0 4.0 Digestion time/min 5.0 Fig.4 Tube pressure at various power levels values in paren- theses are applied power in W A 9 (720); B 8 (639); C 7 (558); D 6 (477); E 5 (396); F 4 (315); G 3 (234); and H 2 (153)JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 I VOL. 6 46 1 Table 3 Analysis of 0 5 - 1 vegetation samples Concentratiodpg g- Recovery (O/O) Element A1 Ba Ca c u Fe Mg Mn Zn Coiled Coiled tube L* tube St 1 1 4 k l l 5 4 k 5 15k0.9 13k0.4 12 000 2 550 1 2 k 2 14-t 1 234-t 15 255-t 13 13 000 + 400 2 3 0 0 k 71 2300k88 61 + 2 67 f 2.5 1 5 0 f 8 140 f 6.4 *L Long 32 rnin digestion time. t S Short 2 rnin digestion time.$Average I all elements. §Average 2 excluding A1 and Ba. MOE value l o o k 11 10k0.7 12 000 k 580 13k0.8 230k 18 2 1 O O k 130 6 0 k 4.6 140k11 Average 1$ Average 24 Tube L MOE 114 150 108 92 102 109 102 107 110 103 Tube S MOE 54 130 100 108 111 109 111 103 103 107 Tube L tubes 21 1 115 108 86 92 100 91 114 115 99 deviations (SDs) reported here are in agreement with the values quoted by the MOE; elemental recoveries for most elements tested were about 100% (Table 3). In order to test for the recovery of trace metals when short digestion times are used V85-1 samples were digested for 2 rnin at high power. The results are also shown in Table 3. With the exceptions of Al which is under-recovered and of Ba which is over-recovered (both with respect to the MOE values) the concentrations (Table 3) and SDs are in agreement with those quoted by the MOE.A comparison of elemental concentrations obtained when using long ( i e . 32 min) and short (ie. 2 min) digestion times suggests that recovery of A1 depends on digestion time. This was verified by digesting V85-1 samples for 2 4 8 16 and 32 min. The results are shown in Fig. 5. The recovery of A1 increases with digestion time and plateaus at about 16 min. From these results it can be concluded that the hot-plate digestions used by the MOE under-recover A1 and that an increase in digestion time increases its recovery with this system. Similar conclusions can also be drawn for Ba (Table 3) an increase in digestion time is expected to result in a small increase in recovery.In order to substantiate further the results obtained when short digestion times are used and to provide a basis for compa~-ison,~J* V85-1 samples were digested in the micro- wave oven using open vessels. It is worth mentioning that when using open vessels about 100°/o recoveries are obtained from botanical and biological samples in less than 6 min.3J2 The recoveries obtained when using short digestions with the tube system and open vessels show striking similarities. In both instances [Fig. 6(a) and (b)] A1 is under-recovered Ba is over-recovered as are all other 120 I 1 40 ' I I I 1 I I I 0 5 10 15 20 25 30 35 Digestion time/min Fig. 5 Effect of digestion time on A1 recovery from MOE V8 5- 1 botanical sample using aqua regia and coiled tube digestion elements.From the results shown in Fig. 6(c) it can be concluded that recovery in the tube system in 2 rnin is almost as much as in open vessels in 6 min thus demonstrating the validity of short digestion times in a tube-based system. A comparison of short and long digestion times reveals that with the exception of A1 and Ba short digestion times provide quantitative recoveries for most elements tested the average recovery of Ca Cu Fe Mg Mn and Zn was 99% (Table 3). A comparison of short digestion times with the MOE values also reveals that except for Al the tube digestion system recovers more in 2 rnin than the hot-plate method does in hours thus providing substantial time savings. From the low SDs it can be concluded that highly reproducible digestions are obtained with this system as has also been reported for other microwave digestion ~ y s t e m s ~ * ~ - ~ Therefore for an element that is difficult to digest such as Al a correction factor may be applied with a reasonable degree of confidence. This is of prime impor- tance in environmental monitoring where total digestions are not required but reproducible recoveries short sample preparation times and straightforward and rugged digestion procedures are crucial.8 These demands have been met by the system and the methodology described here. In summary from the results presented thus far it can be concluded that for V85-1 a 2 rnin digestion provides elemental recoveries of about 100% for most elements. This is further documented and the effect of sample matrix on individual elemental recoveries is investigated by consider- ing other MOE botanical samples and NIST botanical and biological SRMs.Analysis of MOE Botanical Samples and NIST SRMs In order to evaluate 2 rnin digestions further this study was expanded to include other MOE botanical samples and NIST botanical and biological materials. The results for the botanical samples are shown in Table 4 for MOE White birch and Norway maple and in Table 5 for NIST SRM 1571 (Orchard Leaves) and NIST SRM 1577 (Bovine Liver). It should be borne in mind that a key objective in this work was elemental extractions which are on average compatible with those obtained when using hot-plate digestions. Based on previously drawn conclusions for V85- 1 poor recoveries were expected for A1 [Fig.6(a)]. This is so for all462 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 I5O;1 5 ; r ; y j 15;;l 100 100 100 50 50 50 0 Al Ba Ca Cu Fe Mg Mn Zn Al Ba Ca Cu Fe Mg Mn Zn Al Ba Ca Cu Fe Mg Mn Zn L- 100 100 > 50 50 50 CC 150 100 50 Al Ba Ca Cu Fe Mg M n Zn " Al Ba Ca Cu Fe M g Mn Zn Al Ba Ca Cu Fe Mg Mn Zn ( i ) 100 100 = m . . . . . di B'a C'a C l Fk h;g d n Element Fig. 6 Elemental recoveries for V85-1 Norway maple and White birch. (a) MOE V85-1 tube/MOE recovery; (b) MOE V85-I open- vessel/MOE recovery; (c) MOE V85-1 tubelopen-vessel recovery; (d) MOE Norway maple tube/MOE recovery; (e) MOE Norway maple open-vessel/MOE recovery; cf) MOE Norway maple tube/open-vessel recovery; (g) MOE White birch tube/MOE recovery; ( h ) MOE White birch open-vessel/MOE recovery; and (i) MOE White birch tube/open-vessel recovery ~~~ Table 4 Analysis of MOE Norway maple and White birch botanical samples Norway maple White birch Coiled Open MOE Coiled Open MOE tube/ vessel/ value/ tube/ vessel/ value/ Element Pg g-' Pg g-' P g g-' Pg g-' Pi? g-' Pg g-' A1 Ba Ca Cd c u Fe Mg Mn Pb Zn 130k 19 18 k 0.76 28 000 k 1 700 9.8 k 0.31 420 k 16 2 500 k 140 5 6 k 10 120 f 4.1 42 f 1.9 - 100 k 5.8 18 k 0.59 28000k 1 100 460k 13 2 6000 f 82 5 3 f 18 130 k 3.4 4 4 k 1.6 - 9.6 k 0.44 180 14 28 000 8 420 2 200 47 95 40 - 24 k 2.6 84 k 0.95 14 000 k 450 0.8 a 1 5.6 f 0.77 5.7 k 1.1 61 f 2 .5 68 k 2.4 2 500 k 67 24 k 2 100 k 3.4 16000 k 370 1 kO.07 2 100 k 75 611 .t 16 180 f 8.5 704 f 19 220 k 7.3 - - 42 86 14 000 0.9 5 59 2 100 600 200 - Table 5 Analysis of NIST SRM Orchard Leaves and Bovine Liver Orchard Leaves Bovine Liver Coiled tube/ Element Pg g-' A1 120k21 Ba 41 k0.96 Ca 21 OOOk650 Cd cu 11 k l .l Fe 240 & 13 Mg 6200k 150 Zn 25 20.63 - Pb 41 +15 * Reference 12. NIST value/ 410* 46.3* Pg g-' 20 900 k 300 1 2 k 1 300 k 20 6 200 k 200 25 k 3 - - Recovery (tube/NIST) (Yo) 29 89 100 92 80 100 100 - - Coiled tube/ 12.4 f 2.02 0.61 k O . l l 140k6.6 193k8.2 288 f 6.3 669 k 27.1 10k0.24 128 + 3.3 Pg g-' 1.01 k0.17 NIST value/ Pfz g-' 35.5* 12426 270 k 8 605 k 9 10.3 2 1 130k 13 1.24* - 193 f 10 Recovery (tube/NIST) (Yo) 35 50 113 100 107 111 97 98 -JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 463 sample types analysed [Fig. 6(d) and (g) and Table 51.In addition a recovery higher than 100% was expected for Ba. However this is true only for Norway maple [Fig. 6(6)]. Barium was fully recovered from White birch [Fig. 6(g)] and it was under-recovered from both NIST samples (Table 5). In order to verify the results obtain using the tube system and short digestion times MOE samples were also digested in open vessels. Elemental recoveries are shown in Fig 6(e) for Norway maple and in Fig. 6(h) for White birch. Much like the tube digestion [Fig. 6(d)] similar recoveries are obtained when using open-vessel digestions for Norway maple [Fig. 6(e)]. For White birch however the observed recoveries are considerably different [Fig. 6(g) and (h)]. No explanation can be offered at this time for this discrepancy. By comparing elemental recoveries obtained by the tube system with those of open vessels some subtle points begin to emerge.For example with the exception of Al both digestion procedures provide about equal recoveries for all elements for Norway maple [Fig. 6 0 1 . For White birch [Fig. 6(i)] open-vessel digestions recover more than the tube-based system suggesting that longer digestion times may be required for this sample. The data shown in Fig. 6(c) (f) and (i) indicate that the digestion time required for White birch [Fig. 6(i)] is longer than that required for V85- 1 [Fig. 6(c)] and Norway maple [Fig. 6 0 1 . This is much like other microwave digestion systems3y4 in which elemental recoveries are sample type dependent. For example the low Fe recovery obtained when digesting Orchard Leaves (Table 5 ) could be attributed to the siliceous material present in the leaves and the low Ba recoveries (Table 5) could be attributed to sulphur-containing species.The latter may produce sulphate which causes Ba to precipitate. Clearly more work remains to be done to substantiate these conclusions further and to establish the extent of the dependence of elemental recoveries on sample type and on operating system parameters. Conclusions Precise and with some exceptions quantitative extractions of trace elements in botanical and biological samples of environmental concern are obtained rapidly using straight- forward methodology simple and inexpensive instrumenta- tion and net digestion times of 2 min. Recoveries were found to be comparable to and sometimes superior to those obtained when using a 3 h hot-plate digestion.Elemental recoveries were found to be sample type and digestion dependent. The present shortcomings of the system stem from the use of fairly large volumes of acids and from delays arising during cooling of the digests and cleaning of the tube. These are under study and appear to be easy to solve. It is worth pointing out that if the tube is not thoroughly cleaned memory effects become an important consideration. The additional safety precautions required for routine oper- ation for example installation of waveguide attenuators in all access p0rts~9~ and of a pressure release valve,17J8 are being addressed as shown in Fig. 1. In the future the dependence of pressure temperature accuracy and elemental recoveries on sample/particle size acid mixture digestion time and power programme will be documented for a variety of botanical biological and geological SRMs.Furthermore computer control of the prototype system described here an implementation in- volving multiple digestion tubes and even a direct interface to the ICP are envisaged. Financial assistance from the Ontario Ministry of the Environment Project 45 3G is gratefully acknowledged. References 1 Abu-Samra A. Morris J. S. and Koirtyohann S. R. Anal. Chem. 1975 47 1475. 2 Barrett P. Davidowski L. J. Jr. Penaro K. W. and Copeland T. R. Anal. Chem. 1978 50 1021. 3 Introduction to Microwave Sample Preparation. Theory and Practice eds. Kingston H. M. and Jassie L. B. American Chemical Society Washington 1988. 4 Matusiewicz H. and Sturgeon R. E. Prog. Anal. Spectrosc. 1989 12 21. 5 Matusiewicz H. Spectroscopy 1991 6 38. 6 Sulcek Z. and Povondra P. Methods of Decomposition in Inorganic Analysis CRC Press Boca Raton FL 1989 ch. 6. 7 Burguera M. Burguera J. L. and Alarcon 0. M. Anal. Chim. Acta 1986 179 351. 8 Millward C. G. and Kluckner P. D. J. Anal. At. Spectrom. 1989 4 709. 9 Kingston H. M. and Jassie L. B. Anal. Chem. 1986 58 2534. 10 Legere G. and Burgener P. ICP Znj Newsl. 1985 11 447. 1 1 Boomer D. personal communication July 1990. 12 Nadkarni R. A. Anal. Chem. 1984 56 2233. 13 Schelkoph G. M. and Milne D. B. Anal. Chem. 1988 60 2060. 14 Aysola P. Anderson P. and Langford C . H. Anal. Chem. 1987 59 1582. 15 Toshiba Microwave Oven Service Data File No. 330-353 Toshiba Minato-Ku Tokyo Japan. 16 Gorsuch T. T. The Destruction of Organic Matter Pergamon NY 1970. 17 Kratochvil B. and Mamba S. Can. J. Chem. 1990 68 360. 18 Gilman L. and Grooms W. Anal. Chem. 1988 60 1624. Paper 1 /0053 7E Received February 5th 1991 Accepted May lst 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600457

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 465 Investigations on the Determination of Chloride and Bromide by Furnace Atomic Non-thermal Excitation Spectrometry and Furnace Ionic Non-thermal Excitation Spectrometry* Klaus DittrichJ Bernard Radziuk and Bernhard WelzS Department of Applied Research Bodenseewerk Perkin-Elmer GmbH 0- W-7770 Uberlingen Germany The determination of chloride and bromide by non-thermal excitation spectrometry in a graphite furnace using both atomic and ionic spectral lines was investigated. The most sensitive determinations could be made at the ionic lines CI II 479.545 nm and Br II 470.486 nm. The addition of an ionization buffer provided constant plasma conditions resulting in improved linearity of the calibration function. Under optimum conditions and in the presence of appropriate buffers detection limits of 0.6 ng for chloride and 2 ng for bromide were obtained.Keywords Furnace atomic non-thermal excitation spectrometry; halide determination; atomic and ionic lines; helium plasma; ionization buffer Non-metals are much less commonly determined using atomic spectrometric techniques than are the metallic elements. This is because the most sensitive resonance lines of the non-metals are found in the vacuum ultraviolet (VUV) spectral range and can therefore be monitored only at the cost of increased instrumental complexity. Also of significance is the fact that the less sensitive lines which result from transitions between higher excited states of atoms or ions and can be measured in the UV or visible range have as those in the VUV excitation potentials greater than 9 eV thus requiring more energy than is available in most of the commonly used electrically gener- ated plasmas such as inductively coupled plasmas (ICPs) and sparks.In addition to the requirement for high excitation energy considerable energy is needed for mole- cular dissociation because non-metals often form very stable compounds with metals or other non-metals. On the other hand the energies needed for vaporization are often not very large since halides are among the more volatile compounds. Considering the above it is not surprising that the microwave-induced plasma (MIP) with high electronic temperature and low gas temperatare has in most instances been used for the determination of non-metals by atomic spectrometric methods.Furnace atomic non-thermal excitation spectrometry (FANES) which was developed by Falk and co-~orkers,~-~ uses a non-thermal excitation source based on a hollow cathode discharge. The possibilities of this technique for the determination of fl~oride,~ chl~ride,~ sulphate,6 phos- hate,^^^ ammonium and nitrate9 using atomic emission have been described previously. For chloride molecular non-thermal excitation spectrometry (MONES) a particu- lar variation on FANES which made use of the non- thermal excitation of the stable diatomic molecule MgCl was developed and a detection limit of 0.24 ng of C1 was ~btained.~ This was a considerable improvement over FANES for which the best detection limit that could be measured was 8 ng.5 On the other hand it has previously been reported that 80 pg of chloride could be detected using FANES.4 Other techniques for the determination of halides based on molecular-emission cavity analysis (MECA) and electrothermal evaporation molecular absorption spectro- metry (ETE-MAS) have been described by Dittrich.l0 * Presented in part at the 1990 Winter Conference on Plasma -f Present address Institute for Analytical Chemistry University 8 To whom correspondence should be addressed.Spectrochemistry St. Petersburg FL January 8th-l3th 1990. of Leipzig D-0-70 10 Leipzig Germany. Detailed investigations of the determination of chloride and bromide by FANES and furnace ionic non-thermal excitation spectrometry (FINES) were undertaken in the present work.A helium plasma was used with the aim of developing a sensitive analytical technique for chloride and bromide in aqueous solution and of studying the factors influencing the determination with a view to resolving the discrepancy mentioned above. Experimental Instrumentation A 1 m off-axis Ebert monochromator (Perkin-Elmer Nonvalk CT USA) with an 1800 lines mm-I grating a reciprocal linear dispersion of 0.72 nm mm-I and a wavelength positioning resolution of 3.5-4 pm obtained by means of a stepping motor under microcomputer control was used. The central portion of a tube cross-section located midway between the sampling hole and the tube end adjacent to the anode was imaged 1:l on the entrance slit (50 pm x 2.5 mm) using f/l 1 optics in order to match the aperture of the monochromator.For measurements above 400 and 700 nm cut-off filters WG 320 and GG 475 (Schott Mainz Germany) respectively were used. The FANES source with gas control unit was manufac- tured in the Central Institute for Optics and Spectroscopy of the Academy of Sciences of the German Democratic Republic Berlin Germany (see also references 2-10 and 0 Fig. 1 Schematic diagram of the FANES source 1 and 2 inlets for internal gas flow (gas entering at inlet 1 flows through the tube in all instances); 3 and 4 inlets for external gas flow which reaches the outer surface of the tube through channels in the graphite cones; 5 connection for the evacuation of the furnace (3 and 4 may also be used for evacuation but at a slower flow rate than through 5); and 6 sample introduction port466 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL.6 Table 1 Temperature-pressure-discharge programme for CI and Br FANES and C1+ and Br+ FINES measurements Time/s Temperature/"C 0-1 1 20- 152 (12 "C s-1) 11-14 152 14-18 152-352 (50 "C s-') 18-3 1 31-32 32-35 35-45 45-60 60-63 63-65 65-70 70-7 1 352 352 352 352 352 15001 Maximum power 2200 Maximum power 20 20 Phase Pressure/flow* 1 +2+3+4 inlet 6 outlet 5 closed Atmospheric/ Drying At mospherid 1 +2+3+4 inlet 6 outlet 5 closed Vacuum/ 1 +2 closed 5+6 closed Evacuation through 3+4 1 +2 closed 6 closed Evacuation through 3 + 4 + 5 Reduced pressure/ 1 +2 inlet 3+4+6 closed Evacuation through 5 Reduced pressure/ 1 +2 open 3+4+6 closed Evacuation through 5 Reduced pressure/ 1 + 2 open 3 + 4 + 6 closed Evacuation through 5 1 +2+3+4 inlet 5 + 6 closed Atmospheric/ 1 +2+3+4 inlet 6 outlet 5 closed Vacuum/ Pyrolysis Close lid Evacuation? Evacuation3 Pressure stabilization§ Atomization Discharge on )I Purge Discharge on (1 Filling Open lid * The designations 1-6 refer to Fig.1. t During this phase water-solvent residues are removed by means of the sample introduction port in order to keep the anode $ During this phase the maximum rate of evacuation is applied for the removal of residual gases. 9 The optimum pressure and flow conditions are set during this phase. fl Atomization temperatures between 1300 and 2000 "C were investigated and the optimum was selected. I/ Discharge current was also investigated and optimized (in the range 25-1 50 mA).The measurement took place for 8 s between 58 and compartment clean. 66 s. Fig. 1). An HGA-600 power supply (Perkin-Elmer) was used for the electrical heating of the graphite tube. The power supply was controlled from an Epson PCe microcom- puter (Seiko Epson Corporation Nagano Japan) by means of a National Instruments (Austin TX USA) PC2A GBIB interface corresponding to the IEEE-488 standard. Pyro- lytic graphite coated graphite tubes (Perkin-Elmer Part No. B089 1 504) were used exclusively. Samples were introduced using a Model AS-60 furnace autosampler (Perkin-Elmer). The hollow cathode discharge was generated using a current stabilized high voltage power supply (NCL 1200-1 50 pos Heinzinger Rosenheim Germany) also under microcomputer control. The photomultiplier current was converted into voltage using an operational amplifier circuit with a time constant of 60 p s and digitized at a rate of 100 Hz using a 12 bit analogue to digital ( N D ) converter.For data acquisition and manipulation an Epson AX microcomputer (Seiko Epson) and custom software were used. Evaluation was based exclusively on peak height. Procedure Samples 10 p1 in volume were pipetted onto the graphite tube of the FANES source dried and pyrolysed (see Table 1). At this point the FANES head was closed and evacuated. A flow rate of helium gas was selected such that a pressure of 3.5 x lo3 Pa was established (for a discussion of the influence of gas flows and directions see under Optimiza- tion of Pressure and Flow Conditions and of the Tempera- ture Programme).The ignition of the discharge and the electrothermal vaporization/atomization should occur as nearly as possible at the same time. In addition to electrothermal atomiza- tion sputtering contributes to the transport of sample intoJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY. SEPTEMBER 199 1 VOL. 6 46 7 the gas phase. However detailed investigations into this phenomenon were not carried out. Stock Solutions Stock solutions with concentrations of 1 and 10 mg ml-I of halide were prepared from KC1 KBr and NaCl (Suprapur Merck Darmstadt Germany). The analyte and ionization buffer solutions were obtained from the stock solutions by dilution. Results and Discussion Wavelength Selection Intensities of spectral lines measured in a helium MIP have been reported by Tanabe et aLl1 Since it could be assumed that excitation mechanisms in the FANES hollow cathode glow discharge would be similar to those in the MIP the most intense of these lines were investigated.From Table 2 it can be seen that the relative intensities measured for different sources vary significantly. It is noteworthy that under the conditions described atomic lines were found to be the most intense for both chlorine and bromine. The large discrepancy between the results of this study and those of Tanabe et was due to differences in the excitation conditions. Tanabe et al. introduced gaseous samples into the MIP under constant conditions; thus a particular electron pressure and degree of ionization was established resulting in the reported intensities. In the present study the electron pressure was determined not only by the discharge parameters but also particularly by the presence of the easily ionized cationic component of the analyte salt.Thus the electron pressure was higher than that in an MIP so that the ionization equilibrium was shifted in the direction of neutral atom formation resulting in a partial inversion of the relative intensities. The values obtained within the same level of ionization are similar for FANES/FINES and the MIP. The following lines were chosen as given in Table 2 for further investigations C1 I (C1 FANES) 725.662 nm; C1 11 (C1+ FINES) 479.545 nm; Br I (Br FANES) 635.074 nm; and Br I1 (Br+ FINES) 470.486 nm. Whereas the pair of ion lines chosen arise from analogous transitions the C1 I line at 725.662 nm does not correspond to the Br I line at 635.074 nm but rather to a Br I line at 655.981 nm.The Br I line at 635.074 nm is analogous to the C1 I line at 741.412 nm. On the basis of intensities for the chlorine lines it would be expected that the Br I line at 655.98 1 nm would be the most intense for the determination of bromide. This line however is overlapped by the very strong Ha spectral line at 656.279 nm so that it was not possible to confirm this theoretical assumption experimentally. Wavelength positions were found using the stepping motor with the aid of lines emitted by a barium hollow cathode lamp Ba I1 493.409 nm and Ba I1 455.403 nm for the spectral range between 400 and 500 nm. For the spectral range 650-800 nm the lines of a potassium hollow cathode lamp K I 769.898 nm and K I 766.491 nm or the Li I line at 670.784 nm were used.Optimization of Pressure and Flow Conditions and of the Temperature Programme As shown in Fig. 1 there are a variety of possible flow conditions. Firstly the conditions required to maintain a stable discharge were investigated. This was possible when helium was introduced through inlets 1 and 2 and pumped out through outlet 5. Inlets 3 and 4 were closed. The flow rates of the gas through the two inlets were measured. Optimum discharge conditions were obtained with flows of 27 and 8 ml s-l through inlets 1 and 2 respectively. These values refer to atmospheric pressure. The actual flow rates at the discharge pressure of 3.5 x lo3 Pa are significantly higher.It is surprising that the discharge is stabilized by such a strong gas flow through the tube. Further optimization by for example reducing the cross-section of the vacuum outlet would be useful since the residence time of the atoms would thus be increased. This however could not be carried out for practical reasons. Based on the optimization described in detail below the temperature-pressure-dis- charge programme given in Table I was used for all further measurements. Table 2 Relative intensities for selected spectral lines of chlorine and bromine as measured in the FANES source compared with the MIT wavelength tables and with the MIP Intensities MIPT FANES/FINES$ Element Wavelength/ Energy levels/ MIT* and line nm Transition cm-' ( 4 9 (a) @Ill (a) (b) c1 I 741.412 4s 4P=j/2-4p 'Pg/2 71 954-85 438 150 3 21 18 18 c1 I 725.662 4s 4P5,2-4p 4S9/2 7 1 954-8 1 73 1 200 14 100 100 100 Cl I 754.709 4s 4P3/2-4p 4Sg12 72 484-85 73 1 25 6 42 44 44 c1 I1 479.545 4s 5Sq-4p 'P3 107879-128 730 250 100 100 47 100 c1 11 48 1.006 4s 'Sq-4~ 'P2 107 879-128 663 200 76 76 37 18 c1 I1 48 1.946 4s 5Sq-4p 5P1 107 879-128 622 200 46 46 26 55 Br I 635.074 5s 4P5/2-5p 'P9,2 63 430-79 172 200 19 100 100 100 Br I1 470.486 5s 5sp-5p 5P3 93927-115176 250 100 100 86 100 Br I1 487.550 5s 5pp-5p 5P3 93 927-1 14 818 400 60 60 38 44 Br I 734.856 5 s 4p3/2-5P 2D$2 64 90 1-78 805 500 15 75 76 76 Br I 700.521 5 s 4P3/2-5p 'P9/2 64901-79 121 200 14 73 83 83 * See reference 12.j. See reference 11. # Results from the present study. Discharge current 100 mA no ionization buffer amount of material 100-200 ng of chloride or fj Measured relative intensities.1 Intensities normalized relative to the highest intensity in the respective ionization state. bromide. These were not optimized conditions (see text).468 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 CI- concentrationhg per 10 pl Fig. 2 Dependence of net line intensity (IN) for C1 FANES at 725.662 nm on the C1 concentration (as NaC1). Discharge current 100 mA; atomization temperature 1500 "C Optimization of Excitation Conditions for the Determination of Chloride General investigations of Cl FANES The spectral line 725.662 nm was used in the investigation of C1 FANES. The dependence of the measured intensity value on concentration is shown in Fig.2. The concave shape of the curve can be interpreted only in terms of a relative increase in the concentration of chlorine atoms with the amount of sodium chloride. This is a result of the suppression of ionization due to the increasing concentra- tion of electrons arising from the increased sodium content. The conditions in the plasma are described by the following equations Na+Na++e- E1=5.14 eV C1 *C1+ +e- E,=13.01 eV NaCle + II (1) where E is the ionization energy. Both ionizations may be caused by interaction with helium but the degree of ionization is higher for sodium. Thus an increase in the over-all concentration of sodium chloride in the plasma shifts the equilibria in the direction shown by the thicker arrows. The effect of potassium bromide on chlorine emission intensities is presented in Fig.3. It is evident that two competing processes determine the effect of KBr on chlorine emission. These are signal enhancing effects resulting from the increase in electron pressure and thus in the concentration of chlorine atoms NaeNa+ +e- El= 5.14 eV + I1 K e K + +e- Ei=4.34eV II NaCl- + KBr .- CleCl+ +e- Ei= 13.01 eV + II BreBr+ +e- Ei= 11.84 eV and a signal depressing effect resulting from the presence of the salt in increasing amounts. This affects the volatiliza- tion process and can also result in further changes in the structure of the plasma. It was concluded based on the results presented in Figs. 2 and 3 that an ionization buffer may be necessary for the application of FANES to the determination of chloride.Optimization of current and atomization temperature for Cl FANES with and without KBr as the ionization buffer In Fig. 4(a) and (6) the effects of current and atomization temperature (tube temperature) respectively on net inten- sity are shown. The results confirm the effect of ionization 0 500 1000 K- concentration/ng per 10 pi Fig. 3 Dependence of net line intensity (IN) for C1 FANES at 725.662 nm on the K+ concentration of the solution (as KBr). Discharge current 100 mA; atomization temperature 1500 "C; analyte 50 ng of C1 as NaCl 25 50 100 150 ilm A m W B W I 1400 1600 1800 2000 TPC Fig. 4 Effects of (a) discharge current and (b) atomization temperature on the net line intensity (IN) for Cl FANES in the presence (A) and absence (B) of the ionization buffer 200 ng of K+ (as KBr).(a) Atomization temperature 1500 "C; and (b) discharge current A 125 and B 45 mA. Analyte 100 ng of C1 (as NaCI) on C1 FANES. Without an ionization buffer there is an initial increase in net intensity with the discharge current followed by a rapid decrease due to increasing ionization. The atomization temperature has virtually no effect on Cl FANES in the absence of a buffer because it has no influence on the extent of ionization of the chlorine atoms. In the presence of the ionization buffer KBr the ionizing effect of the discharge current is suppressed and the signal enhancing effect dominates. The results indicate that for currents greater than 150 mA which were not achievable using the present apparatus the same ionization effect as in the absence of the buffer might be observed.Also the maximum net intensity for C1 FANES was not obtained in the presence of the buffer. This may be a result of the change in plasma characteristics due to an increase in the concentration of charge carriers. The atomization tempera- ture has an effect in the presence of the ionization buffer. At first the net intensity increases because the volatilization of the large amounts of material is improved. At higher temperatures the intensity decreases since the extent of ionization of the easily atomized K atoms is also increased resulting in a change in plasma characteristics. Optimization of discharge current and atomization tempera- ture for CI+ FINES with and without ionization buffer These experiments were carried out at the 479.545 nm spectral line.Increasing concentrations of the ionization buffer gave the anticipated signal depression resulting fromJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 - ( b ) - B - PA I 1 I 469 1500 h c v) C .- = 1000 e ,Z 500 c ; .- v 0 100 300 500 700 900 K- concentrationhg per 10 pI Fig. 5 Dependence of the net line intensity (I,) for C1+ FINES at 479.545 nm on the K+ concentration of the solution (as KBr). Discharge current 100 mA atomization temperature 1500 "C; analyte 50 ng of C1 as NaCl (A) or KCI (B) 2000 - c v) c .- 1500 c s .- $ 1000 v ,z 500 I I 2500 3000k 0 50 100 150 1400 1600 1800 2000 ilmA TPC Fig. 6 Effects of (a) discharge current and ( b ) atomization temperature on the net line intensity (I,) for C1+ FINES at 479.545 nm in the presence (A) and absence (B) of the ionization buffer 200 ng of K+ (as KBr).(a) Atomization temperature 1500 "C; and ( b ) discharge current 110 mA an increase in electron pressure (Fig. 5). There is no significant difference between the results for NaCl and KCI. As for FANES (Fig. 3) it is apparent that the volatilization impeding effect dominates for larger amounts of KBr. The effects of discharge current and atomization temper- ature on the net C1+ FINES line intensity are shown in Fig. 6(a) and (b). The results are as expected. The very strong increase of signal with current results from increased excitation and ionization. The ionization suppressing effect of the K+ ions in the presence of the ionization buffer affects only the magnitude of the increase.The atomization temperature has no significant effect [see also Fig. 4 (b)]. A discharge current of 1 10 mA was used because the stability of the discharge was reduced at higher currents. A comparison of Figs. 4 and 6 confirms that ionization is the major factor affecting the relationships presented. It was not possible to measure the effect of atomization temperatures greater than about 2000 "C in the presence of the ionization buffer as the discharge became unstable. Optimization of Excitation Conditions for the Determination of Bromide For the determination of bromide the spectral lines Br I 635.074 nm and Br I1 470.486 nm were used. By analogy with the studies on C1 FANES and C1+ FINES the effect of the ionization buffer KC1 on Br FANES and Br+ FINES was investigated.The results are given in Fig. 7. The same 1000 r 0 500 1000 K- concentration/ng per 10 pl Fig. 7 Dependence of the net intensities (Z,) for Br FANES (A) and Br' FINES (B) on the additional K+ concentration in the solution (as KC1). Discharge current 100 mA; atomization temper- ature 1500 "C; and analyte 100 ng of Br+ (as KBr) I 1 - 100' 50 100 150 1400 1600 1800 2000 ilmA TPC Fig. 8 Effects of (a) discharge current and (b) atomization temperature on the net intensity (Z,) for Br FANES at 635.074 nm in the presence (A) and absence (B) of the ionization buffer 224 ng of K+ (as KCl). (a) Atomization temperature 1500 "C; (b) discharge current B 30 and A 1 10 mA. Analyte 100 ng of Br- (as KBr) I I 50 100 150 ilm A 2000 1000 Fig.9 Effects of (a) discharge current and (b) atomization temperature on the net intensity (ZN) for Br+ FINES at 470.486 nm in the presence (A) and absence (B) of the ionization buffer 224 ng of K+ (as KCl). (a) Atomization temperature 1500 "C; (b) discharge current B 30 and A 110 mA. Analyte 100 ng of Br- (as =r) relationships as for chlorine (see Figs. 3 and 5) are valid for bromine i.e. the depression of the Br+ FINES signal is much stronger than that of the FANES signal. This can be explained in terms of the ionization equilibria KeK+ +e- E,=4.34eV Br*Br+ +e- KBre + II (3) E = 1 I .84 eV and the increasing volatilization interference. On the other hand the increase in the Br+ FINES signal at low concentrations of K+ is similar to that for Br FANES and not as strong as for C1 FANES.This is because the analyte with 100 ng of Br- as KBr already contains 50 ng of K+,470 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 thus shifting the origin of the curves. Moreover small amounts of the additives appear to improve the volatilization behaviour of the analyte. Optimization exper- iments were accordingly carried out with and without the ionization buffer containing 224 ng of K+ as KC1. The results of a study on the optimization of the discharge current and atomization temperature for Br FANES and Br+ FINES with and without an ionization buffer are presented in Figs. 8 and 9. The effects of current and temperature are evidently very similar to those for C1 FANES (Fig. 4) and C1+ FINES (Fig. 6). For Br FANES in the absence of an ionization buffer the signal decreases with increasing current [Fig.8(a)]. In the presence of a buffer the enhancement of excitation due to the current is dominant. The net intensity for Br+ FINES increases in all instances with current. Also by analogy with C1+ FINES the signal for Br+ FINES decreases somewhat in intensity in the presence of the buffer. The maximum achievable intensity is limited by the stability of the plasma at high currents. The effect of temperature on Br FANES corresponds exactly to that on C1 FANES. When Br+ FINES measurements were made in the presence of the ionization buffer the rapid introduction of relatively large amounts of easily ionized material resulted in discharge instabilities showing that the reported values were not comparable to those obtained for C1+ FINES.Results A typical series of FINES measurements is depicted in Fig. 10. It can be seen that the half-widths of the emission peaks are of the order of 0.1 s. Thus high time resolution and exact selection of integration times are required for the evaluation of peak areas. Since both C1 and Br can be brought into the gas phase by sputtering heating of the tube and ignition of the discharge must be synchronized to within a few tenths of a second. The background emission intensity varies with the tube temperature. Prior to the analytical signal tube tempera- tures are between 300 and 500 "C. After the signal the temperatures reach 1600 "C. The following improvements in instrumentation would be desirable based on the above considerations (i) sub- millisecond time resolution; (ii) optimized integration intervals (between appearance time and end time); (iii) background correction by means of wavelength modula- 0 8 t / s Fig.10 A typical series of C1- determinations using C1+ FINES for samples containing A 10; B 20; C 50; and D 100 ng of C1-. Discharge current 1 10 mA; atomization temperature 1600 "C; ionization buffer 200 ng of K+ as KBr; integration time as marked 0.7 s immediately after the start of atomization 4000 - c v) .- 5 3000 .= 2000 ; 5 v 1000 0 50 100 0 50 100 CI- concentration/ng per 10 pI Fig. 11 Calibration graphs for (a) C1 FANES at 725.662 nm; and (b) C1+ FINES at 479.454 nm in the presence (A) and absence (B) of the ionization buffer (KBr) ; .= 2000 e ,z v 1000 20 100 200 20 100 200 Br concentratiodng per 10 pl Fig.12 Calibration graphs for (a) Br FANES at 635.074 nm; and (b) Br+ FINES at 470.086 nm in the presence (A) and absence (B) of the ionization buffer (KCl) tion; and (iv) provision for gas stop and reduced flow in order to increase the residence time of atoms in the tube. Growth curves were generated for chlorine and bromine using atomic and ionic lines under the conditions described in Tables 1 and 3. The results for both chloride [Fig. ll(a) and (b)] and bromide [Fig. 12(a) and (b)] show an improvement in linearity in the presence of an ionization buffer. There is not much difference in sensitivity for atomic emission despite the fact that considerably higher currents were found to be optimum in the presence of the buffer.This is apparently balanced by the degradation in volatilization conditions caused by the buffer. Since NaCl was used for chlorine measurements at higher concentrations of analyte the sodium acts as an ionization buffer and the addition of KBr results only in poorer volatilization. Chloride is detected with higher sensitivity using FINES when no buffer is used whereas the opposite behaviour was recorded for bromide. This is partly because a higher current could be maintained for all bromide concentrations measured with the buffer whereas for chloride the currents were identical both with and without the buffer and because ion line intensities vary with the square of the discharge current. Although the use of an ionization buffer did not always result in higher sensitivity the plasma conditions became less dependent on the amount of analyte added as evidenced by the improved linearity. Thus the use of an ionization buffer for practical applications is in general advantageous.Conclusions Chlorine and bromine atoms generated from the respective halides can be determined successfully using FANES and the ions using FINES.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 47 1 Table 3 Conditions used for the measurement of analytical growth curves Discharge current/ Atomization temperature/ mA "C Wavelengthl With Without With Without Ionization buffer Method nm buffer buffer buffer buffer per 10 pl C1 FANES 725.662 125 45 1600 1500 200 ng of K+ (as KBr) C1+ FINES 479.545 110 110 1600 1600 200 ng of K+ (as KBr) Br FANES 635.074 110 30 1600 1100 224 ng of K+ (as KCl) Br+ FINES 470.486 125 100 1600 1500 224 ng of K+ (as KCl) Table 4 Results for FANES and FINES Method C1 FANES without IBt C1 FANES with IB C1+ FINES without IB C1+ FINES with IB Br FANES without IB Br FANES with IB Br+ FINES without IB Br+ FINES with IB Wavelengthl nm 725.662 725.662 479.545 479.545 635.074 6 3 5.074 470.486 470.486 Working range of calibration graph Z,=37.2 x ng of C1- 50-100 ng of C1- 1,=29.3 xpg of c1- 0-100 ng of C1- Ih=65.7xpg of C1- 0-20 ng of C1- I,=36.1 x p g of C1- 0- 100 ng of C1- I =4.9 x ng of Br- 100-200 ng of Br- I,= 5.2 x ng of Br- 0-200 ng of Br- I,=9.7 x ng of Br- 40-200 ng of Br- I,=19xng of Br- 0-200 ng of Br- Blank value (arbitrary units) 194 239 289 284 215 295 452 46 3 * I,/Z,=Ratio of intensity for 100 mg of halide to intensity for blank solution.'f IB = Ionization buffer. Relative standard deviation of blank (%) 10.6 12.7 10 7.4 8.5 10 15 13 lS1IB* 19 12 23 13 2.3 1.8 2.1 4.1 Limit of detection/ ng 0.85 1.3 0.46 0.6 5.1 5.8 4.6 2 The ionization buffer containing potassium e.g. as KBr for C1 FANES should be used in the practical analysis of unknown samples in order to keep the electron pressure in the plasma constant. As a result and in particular at low concentrations of analyte the maximum sensitivity for atomic lines is increased and that for ionic lines decreased. Thus the linearity of the calibration function is good and a first order polynomial fit can be made. Under optimum conditions i.e.in the presence of the ionization buffer and using the required parameters atomization temperature 1600 "C and discharge current 110 mA the ion lines for both elements are the most sensitive. The ratios measured between atom and ion line intensities in the presence of the ionization buffer (calculated from the sensitivities Table 4) are 8 1 100 for Cl I:C1 I1 and 27 100 for Br 1:Br 11 which are nevertheless shifted in the direction of atomic line intensity as compared with the MIP (see Table 2). There is an improvement of an order of magnitude in detection limit compared with our previous measurements on C1 FANESS The results for C1+ FINES are similar to those for MgCl MONES which were however obtained in an argon p l a ~ m a . ~ In general in the evaluation of FANES FINES and MONES it must be remembered that depending on the composition of the plasmas and owing to the relatively low gas and high electron temperatures it is essential to study and to account for the dissociation and ionization equilibria MeM++e- X e X + +e- M X s + II (4) in order to obtain accurate results. In the present work this was achieved using potassium (K+K++e-) as the ioniza- tion buffer. References 1 2 3 4 5 6 7 8 9 10 11 12 Falk H. Hoffmann E. and Ludke Ch. Fresenius Z. Anal. Chem. 1981 307 362. Falk H. Hoffmann E. and Liidke Ch. Spectrochim. Acta Part B 1981 36 767. Falk H. Prog. Anal. '42. Spectrosc. 1980 3 181. Falk H. Hoffmann E. and Liidke Ch. Prog. Anal. At. Spectrosc. 1988 11 417. Dittrich K. and Fuchs H. J. Anal. At. Spectrom. 1987 2 533. Dittrich K. Fuchs H. Berndt H. Broekaert J. A. C. and Schaldach G. Fresenius J. Anal. Chem. 1990 336 303. Dittrich K. and Fuchs H. J. Anal. At. Spectrom. 1989 4 705. Dittrich K. and Fuchs H. in preparation. Dittrich K. Fuchs H. Mermet J. M. and Riviere B. J. Anal. At. Spectrom. 1991 6 313. Dittrich K. CRC Crit. Rev. Anal. Chem. 1986 16 223. Tanabe K. Harauchi H. and Fuwa K. Spectrochim. Acta Part B 1981 36 119. Wavelength Tables with Intensities in Arc Spark or Discharge Tubes of the Massachusetts Institute of Technology (MIT) ed. Harrison G. E. John Wiley New York 1939. Paper 0/0395 7H Received August 30th 1990 Accepted May 3rd. 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600465

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 Determination of Lead in Soil by Slurry-Electrothermal Atomic Absorption Spectrometry With a Fast Temperature Programme 473 Michael W. Hinds Royal Canadian Mint 320 Sussex Drive Ottawa Ontario KIA OG8 Canada Kathryn E. Latimer Department of Chemistry University of Saskatchewan Saskatoon Saskatchewan S7N OWO Canada Kenneth W. Jackson Wadsworth Center for Laboratories and Research New York State Department of Health and School of Public Health State University of New York Albany NY 12201-0509 USA A fast temperature programme for the determination of Pb in soil by slurry-electrothermal atomic absorption spectrometry was developed by omitting the charring step and drying at a higher temperature for a shorter time.The effects of drying temperature and time were systematically studied. Work with phosphate and Mg-Pd as chemical modifiers indicates that better results can be obtained without the use of modifiers. Keywords Electrothermal atomic absorption spectrometry; slurry; fast temperature programme; soil analysis A long analysis time is a major disadvantage of trace element determinations by electrothermal atomic absorp- tion spectrometry (ETAAS) owing to the temperature programme steps needed for sample drying charring furnace cooling atomization and furnace cleaning. Halls' showed that analysis time could be reduced significantly by drying at a fast rate and omitting the charring step. Subsequent ~ o r k ~ - ~ applied this fast temperature pro- gramme technique to other elements in a variety of biological matrices.Slurry-ETAAS reduces the sample preparation time required for the determination of a number of different elements in various It also decreases the risks of sample contamination and high blank values that can occur during conventional sample digestion procedures. Accurate results can be obtained with aqueous calibration standards provided that slurry particle diameters are less than 50 This method has been further enhanced by the introduction of an autosampler for ~lurries.~ Recently there have been reports of the application of a fast temperature programme to slurry samples.10-12 Brad- shaw and Slavinll determined a variety of elements in coal and coal fly ash using a rapid temperature programme and automated slurry sample introduction. They reported ana- lysis times of less than l min per sample which allowed for high sample throughput.Bendicho and de Loos-Volle- bregt12 found that omission of the charring step made no difference to the determination of trace metals in glass by the slurry technique. The work presented here is a systematic study of the effect of drying time and temperature on Pb atomization from aqueous solutions and soil slurry samples without a charring step. The effccts of two common chemical modifiers Mg-Pd13 and phosphate,14 are described. Experimental Apparatus Two spectrometers were used in this work. One was a modified Perkin-Elmer Model 2280 with fast-response signal recording and measurement of the graphite furnace temperature with an optical pyrometer (Series 1 100 Ircon Niles IL USA).A programme written in Turbo Pascal (Borland Scotts Valley CA USA) correlated absorbance temperature and time data.15 The other spectrometer was a Perkin-Elmer Model 5000 with signals collected via a Perkin-Elmer Model 3600 data station. In both instances the atomizer was a Perkin-Elmer HGA- 500 with pyrolytic graphite coated graphite tubes and solid pyrolytic graphite L'vov platforms. The purge gas was Ar. A Pb hollow cathode lamp (Perkin-Elmer Part No. 303-5039) was operated at 8 mA. The wavelength used was 283.3 nm and the spectral bandwidth was 0.7 nm. Deuterium arc background correction was used throughout this study. Samples and Reagents Three Soil Certified Reference Materials from the Canada Centre for Mineral and Energy Technology (CANMET) were used.16 They were oven-dried at 105 "C for 24 h prior to use and did not require further grinding.The following stock solutions were prepared from analyt- ical-reagent grade chemicals 1000 mg 1-l of Pb prepared from Pb(N@& (Analchemia Mississauga Ontario Canada) 10% m/v Mg(N03)2-6H20 (BDH Toronto Ontario Canada) 10% m/v (NH,),HP04 (BDH) and 3000 mg 1-1 of Pd prepared from Pd(N03)2 (Johnson Matthey/ Aesar Toronto Ontario Canada). Solution and Slurry Preparation Aqueous solutions The stock Pb solution was serially diluted with l0/o HN03. Appropriate concentrations were introduced into the fur- nace with a 10 or 20 pl micropipette (Socorex ISBA Renens Switzerland). Chemical modifiers were pipetted on top of the aqueous standards so that the total volume of sample and modifier did not exceed 20 pl.Slurries Slurries were prepared by adding exactly 20 ml of distilled water to weighed amounts of soil (typically 45 mg) in a 50 ml beaker. The slurry was stirred magnetically for 5 min before sampling. Appropriate amounts of the modifier were added to the slurry. The volumes added were small so dilution effects were negligible. Results and Discussion Optimized Fast Temperature Programme The conventional drying and charring stages were replaced by a modified fast drying stage. It was considered that solutions and soil slurries might have different optimum fast drying temperatures and times hence this was investi-474 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 a c 0 0.5 $ $ gated.Table 1 lists the fast temperature programme used in this study. The drying temperature (A') and drying time (r) were studied systematically by setting the drying tempera- ture (at 200 "C initially) and varying the drying time (10 s increments to a maximum of 60 s). Once a series was completed the temperature was increased by 100 "C (600 "C maximum) and the experiment was repeated. A record was kept of the absorbance signal peak area and peak profile. The drying process was also monitored visually through the use of a dental mirror. Occurrences of incom- plete drying and liquid sputtering off the platform were noted. The whole experiment was carried out for 10 and 20 pl aliquots of aqueous solution and soil slurry. When incomplete sample drying was observed the temperature programme was interrupted in order to prevent damage to the tube and platform.A programme that would completely dry the sample was then used in order to clear the atomizer ready for the next injection. The data for the entire optimization experiment are summarized in Fig. 1 which shows the minimum time with respect to a given drying temperature for a specified aliquot size to dry and which gives the same peak area disturbance for Pb runs with longer drying times. Peak area absorbances for both solution and slurry samples gave a relative standard deviation (RSD) of 4% with manual pipetting. The area above the curves indicates conditions where the solutions and slurries are completely dried. The area below each curve shows conditions where drying was incomplete.As expected the larger aliquots required longer . Table 1 gate drying temperatures and times Outline of the temperature programme used to investi- Step Temperature/"C Ramp time/s Hold time/s Drying 'P 1 r* Cooling 50 1 10 Atomization 1700 O f 5 Clean 2 700 1 5 * X and Y varied. t Gas flow stopped. read - 1 s. 50 40 30 20 10 v) . .- E o .w (a) .'\ 30 - 20 10 - - I * I I 0 100 200 300 400 500 600 TemperaturePC Fig. 1 Minimum time and temperature required for an aliquot of (a) aqueous solution and (b) slurry to dry without sputtering. A 10 pl aliquot; and B 20 p1 aliquot A A 0 1 2 3 4 Timels Fig. 2 Comparison of absorbance peak profiles for 0.6 ng of Pb using a fast temperature programme and 20 pl aliquots A aqueous solution (0.274 A s); and B soil slurry (0.252 A s) drying times.Temperatures above 400 "C resulted in changes in peak shapes (shoulders and double peaks). When drying at 600 "C instances of sputtering were observed and the peak areas were irregular (RSD >4%) probably owing to unobserved sputtering. However work carried out with a Perkin-Elmer 5 100 AA spectrometer at a different location (Ottawa) at 600 "C showed none of these effects. This is probably owing to differences in cooling-water temperature which varies with location and season. l 7 The experiments were carried out in winter (Perkin-Elmer 5 100 instrument) and in summer (Perkin-Elmer 2850 instrument). It has been observed that in Canada temperature differences in municipal water can range from 20 to 25 "C between summer and winter.This implies that analysts wishing to adopt this technique may have to adjust drying parameters to account for differences in cooling-water temperature. The peak shapes of Pb from aqueous solutions and soil slurries are shown in Fig. 2. The peaks are nearly coinciden- tal. Little variation was observed for various soil types contrary to previous observations1* when a charring step was used. The peak position did not vary with changes in the drying programme. The optimum fast temperature programme is shown in Table 2. This programme is suitable for aliquots of up to 20 p1 and results in single-peak profiles for aqueous standards. The temperature programme cycle time was reduced from 161 (typical soil slurry programme) to 43 s. Effect of Chemical Modifiers Chemical modifiers typically change the atomization char- acteristics of Pb.The optimum atomization temperatures were established to be 1800 "C for phosphate and 2000 "C for Mg-Pd. Phosphate Phosphate either as HP042- or H2P04- was shown previously to be an effective chemical modifier for Pb in Table 2 Optimized fast temperature programme for solutions and slurries (20 pl aliquots) Step TemperaturePC Ramp time/s Hold time/s Drying 400 1 20 Cooling 50 1 10 Atomization 1700 O* 5 Clean 2700 1 5 * Gas flow stopped read - 1 s.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 475 H I I 0 1 2 3 4 Timels Fig. 3 Comparison of absorbance peak profiles for 0.6 ng of Pb plus 14 pg Of POA3- using a fast temperature programme and 10 p1 aliquots A aqueous solution (0.279 A s); and B slurry (0.262 A s) 8 0.3 0 x 0.1 - a 0 1 2 3 4 5 6 7 Timels Fig.4 Comparison of absorbance peak profiles for 0.6 ng of Pb plus 15 pg of Pd+lO pg of Mg(N03)*.6H20 using a fast temperature programme and 20 pl aliquots A aqueous solution (0.156 A s); B slurry (0.154 A s); and C background absorbance soil s1urTies.l8 It was considered that this modifier would delay the Pb signal without a charring step because the modifier is reported to act either in the gas phase19 and/or in the condensed phase,20 which would occur after drying. The experimental results support this contention as shown in Fig. 3 for both Pb in aqueous solution and Pb from a soil slurry. However with the addition of phosphate the Pb signal from the soil slurry is delayed more than the aqueous Pb signal.Without the phosphate the solution and slurry Pb signals coincide which is advantageous (Fig. 2). Work by Falk et a1.21 showed that temporal non-isothermal- ity exists within the graphite furnace during atomization. This suggests that the aqueous standard and soil slurry peaks ought to coincide in order to minimize the effect of furnace temperature differences on peak area measure- ments. Magnesium and palladium Peak profiles for Pb using Mg-Pd as a modifier are shown in Fig. 4. The modifier combination delays Pb atomization from the aqueous solution and soil slurry which was expected.22 However peak profiles of Pb absorbance from aqueous solution and slurry do not coincide. The slurry peak appears later than the solution peak.As discussed previously this detracts from the usefulness of the modifier combination. Another disadvantage in using this modifier is that the peak area absorbances for Pb in both aqueous solution and slurries are lower with the modifier than for Pb alone. The sensitivity is reduced using this modifier combination as shown in the differences in the characteristic masses (pg per 0.0044 A s) of 12.5 for Pb alone and 18.2 with the Mg-Pd modifier. This was not so when a 900 "C charring step was used previously.22 Table 3 Determination of Pb in soil with a fast temperature programme Concentration of Pb in soillpg g- I Soil Experimental* Certified? so- 1 $ 18.1 kO.01 21 k 4 so- 1 + Pod3- 16.1 k2.9 SO- 1 + Mg-Pd 19.3k 1.1 SO-34 SO-3 + Po43- SO-3 + Mg-Pd SO-47 SO-4 P043- SO-4 + Mg-Pd 12.3 + 0.1 1 4 k 3 l l s k 1.4 14.0 k 0.4 14.6k 0.7 1 6 f 3 12.9k 1.7 17.1 k 0 .7 SO-4 + 10% humic acid 13.9 f 0.8 21 + 4 SO-4 + 10% humic acid (with charring step) 9.6k 1.0 * +One standard deviation. t 95% confidence interval. $ Regosolic Clay Soil. 4 Calcereous C Horizon Soil. 7 Chernozemic A Horizon Soil. Quantitative determination Three CANMET Soil Certified Reference Materials were analysed using the fast temperature programme. Chemical modifiers (phosphate and Mg-Pd) were also used in some determinations (Table 3). Values for the determination of Pb without chemical modifiers are within the 95% confi- dence limits of the certified concentrations for each soil. From previous using a 900 "C charring step the addition of 10% humic acid reduced the Pb recovery substantially. However without the charring step much better results were obtained.It appears that large amounts of organic matter enhance volatilization losses during the charring step but do not interfere with the atomization of Pb from soil slurries. Experimental values obtained using an Mg-Pd modifier overlap with the 95% confidence limits of the certified concentrations although the peak profiles of aqueous solutions do not coincide with soil slurry profiles. The determinations with phosphate as a modifier tended to be low compared with the certified values. Precision was somewhat poorer for these determinations. Overall the addition of a modifier does not appear to assist in the determination of Pb in soil by slurry-ETAAS with a fast temperature programme.The risks of higher blanks temporal differences between standard and sample peaks and reduced sensitivity (for Mg-Pd) are not suffici- ently offset by any substantial benefits. Conclusion This study makes it evident that ETAAS temperature programmes for the direct determination of Pb in soil slurries may be shortened by drying at higher temperatures and eliminating the charring step. Ultimately this means that the analysis time can be reduced by a factor of four. Volatilization losses from organic-rich soil can be decreased by removing the charring step. For the determination of Pb in soil by slurry-ETAAS chemical modifiers do not appear to be of benefit in combination with the fast temperature programme used in this study. M. W. H. thanks the University of Saskatchewan and the Royal Canadian Mint for supporting this work.476 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL.6 1 2 3 4 5 6 7 8 9 10 11 12 References Halls D. J. Analyst 1984 109 1081. Halls D. J. and Fell G. S. Analyst 1985 110 243. Halls D. J. Mohl C. and Stoeppler M. Analysf 1987 112 185. Halls D. J. Black M. M. Fell G. S. and Ottaway J. M. J. Anal. At. Spectrom. 1987 2 305. Keating A. D. Keating J. L. Halls D. J. and Fell G. S. Analyst 1987 112 1381. Hinds M. W. Jackson IS. W. and Newman A. P. Analyst 1985 110 947. Miller-Ihli N. J. J. Anal. At. Spectrom. 1988 3 73. Hoenig M. and Van Hoeyweghen P. Anal. Chem. 1986 58 2614. Miller-Ihli N. J. J. Anal. At. Spectrom. 1989 4 295. Hinds M. W. Allen K. and Jackson K. W. paper presented at XVI Federation of Analytical Chemistry and Spectroscopy Societies Meeting Chicago IL 1st-6th October 1989 paper No. 465. Bradshaw D. and Slavin W. Spectrochim. Acta Part B 1989 44 1245. Bendicho C. and de Loos-Vollebregt M. T. C. Spectrochim. Acta Part B 1990 45 679. 13 Schlemmer G. and Welz B. Spectrochim. Acta Part B 1986 41 1157. 14 May T. W. and Brumbaugh W. G. Anal. Chem. 1982 54 1032. 15 Allen E. and Jackson IS. W. Anal. Chim. Acta 1987 192 355. 16 Steger H. F. Bowman W. S. and McKeague J. A. Geostand. Newsl. 1985 9 213. 17 Slavin W. personal communication. 18 Hinds M. W. and Jackson K. W. J. Anal. At. Spectrom. 1987 2 441. 19 Czobik E. J. and Matousek J. P. Talanta 1977 24 837. 20 Bass D. A. and Holcombe J. A. Anal. Chem. 1987 59 974. 21 Falk H. Glismann A. Bergann L. Minkwitz G. Schubert M. and Skole J. Spectrochim. Acta Part B 1985 40 533. 22 Hinds M. W. and Jackson IS. W. J. Anal. At. Spectrom. 1990 5 199. 23 Hinds M. W. and Jackson K. W. J. Anal. At. Spectrom. 1988 3 997. Paper 1/00717C Received February 15th 1991 Accepted April 30th I991

ISSN:0267-9477    

DOI:10.1039/JA9910600473

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 477 Platform in Furnace Zeeman-effect Atomic Absorption Spectrometric Determination of Arsenic in Beer by Atomization of Slurries of Sample Ash Maria Luisa Cervera Ascensio Navarro and Rosa Montoro lnstituto de Agroquimica y Tecnologia de Alimentos (CSIC) Jaime Roig 7 7 4601 0 Valencia Spain Miguel de la Guardia and Amparo Salvador Departamento de Quimica Analitica Universidad de Valencia Dr. Moliner 50 467 00 Burjassot Valencia Spain A precise and accurate procedure is proposed for the determination of As in beer samples at the ng g-I level by electrothermal atomic absorption spectrometry. The sensitivity of this technique is enhanced by a factor of 2.5 by introducing a preconcentration step consisting of preliminary dry ashing of the samples and direct injection into the L'vov platform of a slurry of the ashes.The high background caused by the beer sample matrix is compensated for by the use of Zeeman correction. The arsenic is atomized under stabilized temperature platform furnace conditions with the addition of a nickel-ascorbic acid chemical modifier. The effect that each of these compounds has on the determination of As was studied both separately and in combination. The methodology developed has a characteristic mass of 35 pg which corresponds to a concentration of 0.7 ng of As per g of beer a recovery of 102 f 2% and a relative standard deviation of 4.5% for six independent analyses of a sample containing 8.5 ng g-' of As. The accuracy of the method was confirmed by the analysis of a certified reference material sample.The proposed procedure was used to analyse real samples of beer and the results are comparable to those found by hydride generation atomic absorption spectrometry. Keywords Arsenic determination; beer sample; platform in furnace Zeeman-effect atomic absorption spectrometry; slurry atomization; chemical modifier The determination of arsenic in vegetable matrices by electrothermal atomic absorption spectrometry is hindered by severe suppression of the analyte signal owing to the sample matrix constituents.' In addition to this chemical interference spectral interference has been reported in the determination of arsenic when deuterium background correction is used.2 Use of the stabilized temperature platform furnace (STPF) concept3 in combination with Zeeman-effect background correction can overcome these limitation^.^.^ Nevertheless for direct determination of arsenic in previously digested food samples a preconcen- tration step is required when the arsenic content in the original sample is at the ng g-' level.Solvent extraction6 or the coprecipitation of arsenic with an ammonium pyrrolidine dithiocarbamate-nickel com- plex have been used to preconcentrate arsenic.' These procedures also allow the elimination of matrix effects but they are time consuming and laborious. Slurry atomization is an alternative technique for the introduction of solid samples in atomic spectrometry.8-10 Direct injection of slurries of sample ash makes it possible to use a lower dilution than would be required to dissolve the ash totally,ll thus improving the detection limits. However the suppres- sion effect caused by the sample matrix is enhanced by the introduction of the slurry and it is necessary to use chemical modification to minimize this interference Zeeman-effect background absorption correction is also required.The addition of nickel avoids the loss of arsenic during the ashing stage and permits higher pyrolysis temperatures so that the removal of matrix components is easier and more complete.12 Moreover nickel has a supplementary role in preventing the formation of phosphorus molecular species which cause a structured background.* The effectiveness of ascorbic acid in minimizing signal suppression in the determination of other elements using electrothermal atom- ization is also well known.I3J4 No method has been published in the literature for the direct determination of low levels of arsenic in food samples. In this work a method has been developed for determining arsenic in beer at the ng g-' level.Slurries of the ashes obtained from the sample were used to preconcentrate the arsenic with a combina- tion of STPF conditions with Zeeman-effect background correction with nickel-ascorbic acid as the chemical modifier. The analytical parameters of the method were established and a series of real beer samples analysed. The results were compared with those found by hydride generation atomic absorption spe~trometry.~~ Experimental Equipment A Perkin-Elmer Zeeman/3030 atomic absorption spectro- meter equipped with an HGA-600 graphite furnace and an AS 60 autosampler was used throughout this study.The instrument includes a graphics display. The highly time- resolved signals were plotted with a Perkin-Elmer PR- 100 printer. Pyrolytic graphic coated graphite tubes with an inserted pyrolytic graphite L'vov platform Perkin-Elmer Part No. 112660 were used exclusively. A Heraeus Model 1 100/3 muMe furnace fitted with a Jumo DPG-4411 digital microprocessor was used to ash the samples. Reagents Analytical-reagent grade water (1 8 MR cm-l specific resistivity) was used throughout. All reagents used were of the highest purity available and at least of analytical-reagent grade. An aqueous stock solution of As111 was prepared from arsenic 111 oxide (Riedel de Haen).The ashing aid was prepared by stirring 2.5 g of Mg(N03)2-6H,0 and 0.25 g of MgO in 100 ml of water until homogeneous. The chemical modifier used was Ni in the form of Ni(N03)2 and ascorbic acid. The ascorbic acid together with nitric acid was used as the suspending agent. A National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) NIST SRM 1573 Tomato Leaves was also used. Optimization of Analytical Parameters Slurries of the sample ashes stabilized with nitric acid and the surfactant Gandax SX (Molins KAO Barcelona Spain) or alternatively with nitric and ascorbic acids were478 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 injected onto the L'vov platform. Nickel and mixtures of nickel and ascorbic acid were assayed as chemical modifiers.Pyrolysis and atomization parameters were optimized for both samples and standards in order to obtain the best sensitivity and highest precision and accuracy. In all instances the integrated peak area absorbances were used in accordance with STPF conditions. Recommended Procedure Dry ashing of the samples Add 2.5 ml of ashing aid containing 2.5% m/v of Mg(N0,)2 and 0.25% m/v of MgO to 25.00 Ifr 0.0 1 g of previously de- gassed beer samples and mix well. Evaporate in a sand- bath until totally dry controlling initial foam formation. Ash in a muffle furnace as described in an earlier report,15 at a temperature of less than 450 "C until white ash is obtained. Sometimes it may be necessary to wet the ash with HN03 (1 0% v/v) and repeat the ashing step.Slurry formation and determination of arsenic Add 100 pl of HN03 to the white ash and place in an ultrasonic bath to remove any solids adhering to the glass wall. Add 10 ml of the chemical modifier solution of ascorbic acid (2% m/v) and nickel (0.5% m/v) and homogenize the sample slurries shaking vigorously in the ultrasonic bath. Take volumes of 2 ml of slurry and add 20 pl of standard aqueous solutions containing 0 4 6 8 and 10 pg m1-I of As in order to obtain the various standard additions levels. Analyse by platform in furnace Zeeman-effect atomic absorption spectrometry using the analytical parameters indicated in Table 1 and a slurry injection of 20 pl. Results and Discussion Stabilization of Beer Ash Slurry Two alternative methods were employed to stabilize the beer ash slurry.Firstly the use of nitric acid and the surfactant Gandax SX permits the dispersal of the ash from 25 g of beer in 5 ml and secondly the use of nitric and ascorbic acids provides ash slurries of 25 g of beer in 10 ml. Both types of slurries are homogeneous and stable for more than 5 h without the use of any mechanical or ultrasonic stirrer and so the use of a conventional autosampler to inject the slurries into the graphite furnace is possible. In order to test the stability of the slurry small amounts of a sample were placed in the autosampler rack and injected into the graphite furnace at fixed time intervals. The absorbance values obtained were compared with those found for samples stirred just prior to injection.Selection of the Chemical Modifier The high background correction capacity and the accuracy of the Zeeman effect provides great flexibility in the selection of chemical modifiers and in the concentrations of Table 1 Analytical parameters for the determination of arsenic in beer. Wavelength 1 93.7 nm; arsenic electrodeless discharge lamp 8.5 W; slit width 0.7 nm Internal Time/s Ar flow Step "C Ramp Hold ml min-' Temperature/ rate/ 1 Drying 90 10 20 300 2 Drying 120 10 20 300 3 Ashing 800 10 10 300 4 Ashing 1400 10 60 300 5 Atomization 2300 0 5 0 6 Cleaning 2650 1 5 300 7 Cooling 20 10 10 300 those used. Accordingly nickel nitrate and 'mixtures of nickel nitrate and ascorbic acid were assayed in this work for use as chemical modifiers. The Zeeman-effect corrected absorbance of stabilized ash slurry samples and standards is plotted in Fig. 1 as a function of the amount of nickel added.In the standards nickel nitrate prevents pre-atomization analyte loss but also reduces the absorbance of arsenic. Increased amounts of nickel nitrate have a supplementary effect on samples the suppression effect and background are reduced by using between 100 and 200 pg of nickel in the form of Ni(N03)2. In the analysis of real samples using slurries containing 100 pg of Ni poor accuracy (results not shown) were obtained and the results found were different to those obtained by hydride generation atomic absorption spectro- metry.15 Consequently the use of mixtures of nickel nitrate and ascorbic acid as chemical modifiers was assessed. The effect of the ascorbic acid concentration on background and also on the sensitivity and accuracy of the arsenic determi- nation is shown in Table 2.The best results were obtained with a 2% m/v ascorbic acid concentration because it provides a higher absorbance and a lower background. Samples containing nickel nitrate and ascorbic acid provide better results than those containing only nickel. This can be explained by the relative availability of active carbon provided by ascorbic acid in the atomization process,I4 which causes a reduction of suppressive interfer- ence and enhancement of the arsenic signal. 2'2 I 2.1 - 2.0 - 1.9 - 1.8 - 1.7 - al c 4 1.6 - 0.5 1 ZAA 0.4 0.3 0.2 0.1 ~ 100 200 300 400 ! 0 Mass of Ni/pg Fig. 1 Effect of mass of nickel on the arsenic and background response in slurries of 25 g of beer ash stabilized with Gandax SX ashing temperature 1100 "C; ashing time 30 s; atomization temperature 2 300 "C; and injection volume 20 pl.A Real sample with 100 ng ml-l of As; and B 100 ng ml-' As standardJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 479 Table 2 Effect of ascorbic acid concentration on the arsenic determination carried out in 10 ml of a nitric acid slurry of 25 g of beer ash also containing 100 p g of nickel Ascorbic acid concentration (Yo m/v) Parameter 1 2 3 Absorbance* 0.1 17 0.174 0.169 Background* 0.922 0.715 0.841 As concentration/ng g-' 22.6 24.8 23.8 Sensitivity (absorbance*/ng ml-I) 1.99 x 10-3 2.88 x 10-3 2.79 x 10-3 Relative differencet (O/O) - 6.2 2.9 - 1.2 *Integrated peak area absorbance units.t Between values found by Zeeman-effect corrected atomic absorption spectrometry and hydride generation atomic absorption spectrometry1s (24.1 ng g-I). Background Correction The atomization of slurries of ashed food samples provides high background readings. For arsenic determination the use of a magnesium salt as the ashing aid also increases background. In order to measure and compensate for the background effect the use of Zeeman correction is neces- sary. In this instance a Perkin-Elmer Zeeman/3030 with the magnetic field perpendicular to the optical beam enabled good background correction to be achieved and thus reproducible measurements of the arsenic samples and standards were obtained. A typical background absorbance value for an aqueous arsenic standard is of the order of 0.160.However reagent blank solutions containing magnesium provide background absorbance readings of about 0.450. The atomization of real samples presents background absorbance values of 0.600. Optimization of the Temperature-Time Programme The ashing and atomization temperature-time programme was optimized in order to provide maximum matrix decomposition (without loss of arsenic); minimum back- ground; and maximum sensitivity. 1.4 1.2 1 .o 0.8 8 0.6 m 0.5 C fJ 2 2 0.4 0.3 0.2 0.1 BG 800 1200 1600 2000 2 400 Temperatu rePC Fig. 2 Optimization of the temperature-time programme. Ashing temperature was established for a real sample using an atomiza- tion temperature of 2300 "C. The atomization temperature was determined after an ashing step of 60 s at I400 "C.A Real sample; and B 100 ng ml-I As standard The effects of ashing and atomization temperatures on the absorbance of an aqueous standard solution of arsenic and a real sample of ash slurry are indicated in Fig. 2. It can be observed that the optimum ashing and atomization temperatures correspond to 1400 "C and 2300 "C respec- tively. The background was reduced between 1000 and 1400 "C in the ashing step. On the other hand it was observed that an ashing time of 60 s provided better results than the use of 40 s. Comparison Between Direct Calibration and Standard Addi- tions Method A typical comparison between the analytical and standard additions graphs is shown in Fig. 3. It can be seen that the matrix provides a negative effect of 40% on the sensitivity obtained by electrothermal atomization.In order to com- pensate for this effect it is necessary to use the standard additions method in order to obtain accurate results. Besides the use of spiked samples provides high absor- bance readings which are more reproducible. Analytical Characteristics of the Proposed Method The analytical characteristics such as sensitivity detection limit precision and accuracy for the determination of arsenic in beer samples were evaluated from a study of the calibration graphs the recovery assays and from the analysis of the certified vegetable sample. 0.6 0.5 0.4 a) C 0.3 s D 6 0.2 0.1 (20) 0 20 40 60 80 100 [Asl/ng ml-' Fig. 3 A Aqueous arsenic calibration graph; and B standard additions graph for arsenic in a sample of beer ash slurry (stabilized with nitric and ascorbic acids and containing 1 OOpg of Ni).A 2Opl sample volume was used throughout480 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 Sensitivity The sensitivity was established using the mean value of the slopes of 13 standard additions graphs. It corresponded to 2.7 x loe3 (50.1 x integrated area absorbance units for each ng ml-I of As. obtained at this level. For this reason and as the character- istic mass is always very similar to the absolute detection limit this can be used for the estimation of the lowest concentration that can be routinely measured in real samples. 16* Detection limit The detection limit corresponded to 2.7 ng g-l this value was evaluated using three times the standard deviation of the blank measurements divided by the sensitivity.The characteristic mass for the aqueous standard (the arsenic mass that provides an absorbance value of 0.0044) using 20 pg of Ni was 18 pg (0.4 ng g-l) and the characteristic mass for the procedure (using 100 pg of Ni 2% ascorbic acid and applying the analyte additions technique) was 35 pg (0.7 ng g-'). These parameters are expressed in nanograms of arsenic per gram of beer sample taking into account sample size and the dilution carried out in the procedure recommended. The detection limit calculated from the standard devia- tion of the blanks which are almost indetectable in all instances is too high owing to the low reproducibility Precision The precision expressed as the relative standard deviations (standard deviationlmean) of six independent analyses of a sample containing 8.5 ng g-' of arsenic is 4.5%.In order to evaluate the accuracy of the method recovery assays were performed on a matrix of beer and a certified sample of vegetable origin was analysed. Percentage recovery A recovery of 102 k 2% was obtained for a known amount of 25 ng g-I of arsenic added to a real sample containing 8.5 ng g-'. The good recovery obtained proves that the loss of arsenic and contamination do not occur during the various stages of analysis. Table 3 Determination of arsenic in real samples by hydride generati~n'~ and electrothermal atomic absorption spectrometry Hydride generation Electrothermal As found ng g-' 73.7 66.2 As found/ 65.0 67.9 61.0 47.3 49.7 6.5 6.1 67.1 57.8 61.8 7.3 7.3 10.5 13.3 11.7 12.9 9.3 6.5 8.5 4.4 6.1 5.3 4.0 2.0 4.0 8.9 8.5 7.8 8.8 8.5 8.5 26.2 24.8 22.2 ng g-* Sample 1 Mean k SD* 7 0 a 5 Mean f SD* 65+3 46.0 48.8 6.3 6.9 59.9 62.4 4 7 f 2 6.6 f 0.4 61 a 2 4 8 k 2 6.3 k0.3 62+5 5 8.5 7.9 8.2 2 0.4 8 2 2 6 11.9 12.5 12.2 f 0.4 12.6 f 0.8 7 7.2 7.5 7.3 a 0.2 8 + I 8 9 10 5.5 5.5 5.5 k 0.0 5.3 k 0.8 3.9 3.5 3.7 k 0 .3 8.5 k0.5 3 5 1 8.0 8.2 8.8 9.0 8.5k0.4 25.1 24.0 24.9 22.5 24.0 2 4 k I 11 24+2 * SD = standard deviation.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 48 1 Accuracy The NIST SRM 1573 Tomato Leaves (containing 0.27 k 0.05 pg g-lof arsenic) was analysed by the proposed method in order to determine the accuracy of this method for the determination of arsenic in vegetable samples.The results obtained 0.28 f 0.04 pg g-l are in close agreement with the certified values and this testifies to the accuracy of the method and its applicability to the analysis of other food samples of vegetable origin. Analysis of Real Samples The determination of arsenic in real beer samples was carried out by both quartz tube atomization of arsenic hydride' and electrothermal atomic absorption spectrome- try using the recommended procedure for preliminary dry ashing of the samples in all instances. The results obtained by the two methods assayed are comparable as can be seen in Table 3. The regression equation value calculated between the concentration values1*J9 obtained by electrothermal atomi- zation and those found by hydride generation demon- strates that the proposed method does not require the use of blank corrections and constant relative errors are not obtained.Conclusions The proposed method allows the determination of arsenic at the ng g-l level by electrothermal atomic absorption spectrometry. This is the first time that such a low concentration of arsenic has been determined accurately by electrothermal atomic absorption spectrometry with direct injection of digested food matrices. The use of the STPF concept Zeeman-effect background correction and the nickel-ascor- bic acid chemical modifier to avoid the high background effect and matrix interference allows arsenic atomization in digested food samples. The use of the slurry approach to inject beer sample ash also allows preconcentration of the samples which improves the detection limit of the method. The results obtained in the analysis of real and certified samples show the suitability of the proposed method for this type of determination.Funds to carry out this work were provided by the Cornission Interministerial de Ciencia y Tecnologia (CI- CyT) Project AL189-0521 for which we are deeply indebted. References 1 Hoenig M. and Van Hoeyweghen P. Spectrochim. Acta Part B 1982,37 81 7. 2 Hoenig M. and Van Hoeyweghen P. Intern. J. Environ. Anal. Chem. 1986 24 193. 3 Slavin W. Manning D. C. and Carnrick G. R. At. Spectrosc. 1981 2. 137. 4 Fernandez F. J. Bohler W. Beaty. M. M. and Barnett W. B. A4t. Spectrosc. 1981 2 73. 5 Fernandez F. J. and Giddings R. At. Spectrosc. 1982 3 61. 6 Ishizaki M. Buseki Kagaku 1977 26 667. 7 Dabeka R. W. and Lacroix G. M. A. Can. J. Spectrosc. 1985 30 154. 8 Langmyhr F. J. and Wibetoe. G. Prog. Anal. At. Spectrosc. 1985 8 193. 9 Ebdon L. Foulkes M. E. and Hill S. Microchem. J. 1989 40 30. 10 Littlejohn D. Stephen S. C. and Ottaway J. M. Anal. Proc. 1985 22 376. 1 1 de Benzo Z. A. Fernandez M. Carrion N. Eljuri E. At. Spectrosc. 1988 9 87. 12 Zhumanova K. M. Beilina A. Z. and Muldakhmetov Z. M. Deposited Doc. 1977 VINITI 37 13-77 13 pp. (Russ); Chem. Abstr. 1979 91 203435~. 13 Regan J. G. T. and Warren J. Analyst 1978 103 447. 14 Hoenig M. Scokart P. O. Van Hoeyweghen P. Anal. Lett. 1984 17 1947. 15 Cervera M. L. Navarro A. Montoro R. Catala R. J. Assoc. Off Anal. Chem. 1989,72 282. 16 Grobenski Z. Lehmann R. Radziuk B. and Voellkopf U. At. Spectrosc. 1986 7 61. 17 de Loos-Vollebregt M. T. C. Koot J. P. and Padmos J. J. Anal. At. Spectrom. 1989 4 387. 18 de la Guardia M. Salvador A. and Berenguer V. An. Quim. Ser. B 1981 77 129. 19 de la Guardia M. Salvador A. and Berenguer V. An. Quim. Ser. B 1983 79 446. Paper 0/04 732E Received October 22nd I990 Accepted May 23rd 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600477

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 483 Determination of Lead by Hydride Generation Atomic Absorption Spectrometry With In Situ Concentration in a Zirconium Coated Graphite Tube Yan Xiu-ping and Ni Zhe-ming" Research Center for Eco-Environmental Sciences Academia Sinica P. 0. Box 934 Beijing China A method is described for the determination of lead by the in situ concentration of lead hydride on a Zr coated graphite tube with subsequent detection by electrothermal atomic absorption spectrometry. The method gives a six-fold enhancement of sensitivity with respect to that using a pyrolytic graphite coated graphite tube for the sorption of lead hydride. The characteristic mass (i.e. the mass of analyte which provides a defined peak absorbance of 0.0044 A) is 52.8 pg.The relative standard deviation for ten replicate measurements is 2% at the level of 3 ng of lead. An absolute detection limit (30) of 242 pg is obtained. The proposed method has been applied successfully to the determination of lead in some reference materials and a tap water sample. Keywords Electrothermal atomic absorption spectrometry; in situ concentration; zirconium coated graphite tube; lead hydride generation The generation of volatile covalent hydrides of a number of elements (ie. As Bi Sn Se Sb Te and Pb) for determina- tion by atomic absorption spectrometry (AAS) has proved extremely useful because it serves to separate the metal from other potentially interfering matrix components in the sample and can also be used as a method of concentra- tion.l12 However owing to the low yield of lead hydride and its poor thermal stability the conditions for lead hydride generation are critical and the sensitivity is low compared with that for other hydride-forming elements.Several papers3-l0 have been published on the determination of lead by hydride generation AAS (HG-AAS) in which oxidants such as dichromate hydrogen peroxide and peroxodisul- phate in acidic solution have been used in order to improve the generation efficiency of lead hydride and yield better results. Recently a niore sensitive method was developed by using nitroso-R salt medium for the generation of lead hydride. In situ trapping procedures which utilize the graphite furnace as both the concentration medium and atomization cellI2-l7 have been proved to be the most sensitive atomic spectrometric methods available for the detection of As Se Sb and Sn,18 particularly wherl a Pd coated graphite tube is used for the absorption of the hydride~.l~-*~ The combina- tion of hydride generation in the presence of oxidants with in situ concentration in the graphite tube is expected to give greater enhancement of sensitivity for the determination of lead compared with the method using HG-AAS.However few papers have been published on the determination of Pb by use of this technique. Aroza et al. 23 used HG electro- thermal atomic absorption spectrometry (HG-ETAAS) with in situ concentration in the conventional graphite tube in order to determine Pb and found it was five times more sensitive than a method based solely on HG-AAS.9 Stur- geon et aL2 proposed a method for the determination of Pb based on the generation of Pb(C2H,) using NaB(C2H,) with its subsequent trapping in a graphite furnace which gave a lower blank signal and detection limit.However the method included the synthesis of NaB(C,H,) which is not commercially available. Graphite tubes treated with compounds of refractory metals such as Zr La Mo and W have considerable advantages over conventional tubes for example enhanced sensitivity and decreased matrix interference^.^^-^^ In this work an attempt has been made to use a Zr coated graphite tube for the sorption of lead hydride generated in an H202-HN03 medium. The sorption temperature of lead hydride in such a tube was substantially lower than that reported previously and also the sensitivity was greatly enhanced.23 Experimental Apparatus A hydride generator (HG-100) made by the Research Center for Eco-Environmental Sciences Beijing China was used.Its construction and function have been described previously. Hydride generation was accomplished in a continuous mode in a mixing cell by using two channels of a peristaltic pump to deliver the sample and NaBH solu- tions. Silicone rubber tubing 4 mm i.d. x 36 cm was used to connect the outlet of the hydride generator with the quartz tube. The tip of the quartz tube was inserted into the sample introduction port at the centre cif the graphite tube and held in contact with the opposite interior wall. The hydride generated was stripped from the solution by an argon gas stream and was absorbed on the inside wall of the Zr coated graphite tube.A Perkin-Elmer Model 4000 atomic absorption spectro- meter equipped with an HGA-400 graphite furnace and a Model 056 chart recorder was used for the measurement of Table 1 Recommended experimental conditions Parameter Value Parameter Value Wavelength Bandwidth Lamp current Carrier gas flow rate Uptake rate of sample solution 283.3 nm Uptake rate of NaBH solution 3.7 ml min-' 0.7 nm HN03 concentration 0.5% v/v 6% m/v 480 ml min-' H,Oz concentration 1.5% m/m 3.7 ml min-' Sorption temperature 300 "C 3 mA NaBH concentration * To whom correspondence should be addressed.484 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 1.0 0.8 $ 0.6 0.4 al -e a .- +- - cr" 0.2 Table 2 Furnace programme for coating zirconium - - - - - Step Temperature/"C Ramp time/s Hold time/s Internal gas flow 1 120 5 150 Normal 2 8 00 10 30 Normal 3 2000 5 10 Normal analyte absorbance in the peak height mode under gas stop and maximum power conditions.A lead hollow cathode lamp was used. The sample introduction port was enlarged to a diameter of 2.5 mm with a drill bit. Argon was used as both the purge and carrier gas. The recommended experi- mental conditions are listed in Table 1. All data obtained from sample analysis were based on the standard additions method. Reagents All the reagents were of analytical-reagent grade. De- ionized water was used throughout. A Pb stock solution 1000 pg ml-l was prepared by dissolving metallic Pb in 20 ml of 1 + 1 nitric acid and diluting to 1 1 with water.Working solutions were prepared fresh every day by diluting appropriate aliquots of the stock soh t ion. Sodium tetrahydroborate solutions of 4 and 6% m/v were prepared daily or as required in de-ionized water and were used without further filtration or stabilization. Coating the Graphite Tube With Zirconium Soak a pyrolytic graphite coated graphite tube in a 5% ZrOC12.8H20 solution for 48 h and dry it with an infrared lamp. Then repeat the following procedure five times on the dried tube inject 100 pl of 5% ZrOCl2.8H20 solution into the graphite furnace with an Eppendorf microlitre pipette fitted with a disposable polypropylene tip and heat the furnace through use of the furnace temperature programe given in Table 2.A tube treated in this manner can stand 150-200 firings. Sample Decomposition Accurately weigh 0.100 or 0.250 g of a national standard reference material (Research Centre for Eco-environmental Sciences Chinese Academy of Sciences) into a polytetra- fluoroethylene (PTFE) container add 1 ml of concentrated nitric acid and allow to stand overnight. Add 1 ml of 72% perchloric acid and 1 ml of concentrated hydrofluoric acid. Cover the container with a PTFE cover place it in a stainless-steel bomb (62 x 36 mm id. 44 mm 0.d.) and seal the bomb tightly with a screw closure in order to prevent gas leakage. Place the bomb in an oven heat to 180 "C and maintain at this temperature for about 6 h. Remove the bomb from the oven cool to room temperature remove the PTFE container take off the cover and heat the container on a hot-plate at about 200 "C and evaporate to near dryness.Finally add a suitable volume of 0.1 mol dm-3 hydrochloric acid and dissolve the residue by warming for about 10 min. Transfer the solution into a 10 ml calibrated flask and dilute to volume with 0.1 mol dm-3 hydrochloric acid. Prepare a reagent blank in parallel. Hydride Generation and GFAAS Measurement The Zr coated graphite tube was heated to 100 "C and kept at this temperature for 5 s. The tip of the quartz tube was inserted from the outlet of the hydride generator into the sample introduction port at the centre of the tube and was held in contact with the opposite interior wall. The furnace was heated to the adsorption temperature the peristaltic pump was then started and the sample and NaBH Table 3 Furnace temperature programme Step Temperature/"C Ramp time/s Hold time/s Internal gas flow 1 100 2 5 Normal 2 300 5 110 Normal 3 2200 0 5 Gas stop solutions were pumped into the cell.The hydride and hydrogen generated were swept into the furnace with argon. Collection of the hydride continued for 100 s at the adsorption temperature the quartz tube was withdrawn from the graphite furnace and the analyte was atomized at 2200 "C for 5 s. The quartz tube was inserted into the graphite tube and withdrawn from it automatically. The furnace temperature programme is summarized in Table 3. Results and Discussion Optimization of Operating Conditions In order to achieve maximum sensitivity two types of parameter were optimized by studying the effect of one variable at a time while keeping the others constant (i) parameters that may affect the efficiency of the lead hydride generation; and (ii) those that may affect lead hydride adsorption on the Zr coated graphite tube.Parameters Affecting Efficiency of Lead Hydride Generation The influence of various concentrations of HN03 on the efficiency of lead hydride generation is shown in Fig. 1. As can be seen the optimum concentration of HN03 lies within the range of 0.4-0.6% when the concentrations of H202 and NaBH were kept at 1.5 and 4% respectively. Thus a concentration of 0.5% v/v HN03 was selected for further experiments. The effect of the concentration of H202 on lead absor- bance is shown in Fig. 2. The maximum sensitivity was .L 0.4 - l t 0.2 1 I I I I 1 0 0.2 0.4 0.6 0.8 1 .o Concentration of HNO (% v/v) Fig. 1 Effect of HN03 concentration on the peak absorbance of 2.5 ng of Pb with 4% m/v NaBH and 1.5% m/m H202 I I I I I I L I 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Concentration of H,O (YO m/m) Fig.2 Effect of H202 concentration on the peak absorbance of 2.5 ng of Pb with 4% m/v NaBH and 0.5% v/v HN03JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 1.0 c" 0.8 e 2 0.6 n $ 0.4 al m .- 4- - u" 0.2 485 - - - - - 0 1 2 3 4 5 6 7 8 9 1 0 Concentration of NaBH (% m/v) 0 30 60 90 120 1 Time/s Fig. 3 Effect of NaBH concentration on the peak absorbance of 2.5 ng of Pb with 0.5% v/v HNO and 1.5% m/m H202 obtained at 1.5- 1.7% m/m H202 when the concentrations of NaBH and HN03 were kept at 4 and 0.5% respectively.Fig. 3 shows the effect of the concentration of NaBH on the efficiency of hydride generation at 0.5% v/v HN03 and 1.5% m/m H202. The optimum concentration of NaBH lies within the range 5-7% m/v (Fig. 3). Therefore a concentra- tion of 6% m/v NaBH was chosen for further study. The absorbance increases with an increase in the carrier gas flow rate up to 460 ml min-* above which the absorbance remains unchanged. A flow rate of 480 ml min-l was used. Parameters Affecting Lead Hydride Adsorption The sensitivity increases by a factor of 6.3 for the Zr coated graphite tube compared with that obtained by using the uncoated tube. The Pd coated tube gives similar sensitivity to the Zr coated tube. However the latter tube is preferred because zirconium carbide is a refractory material and after coating once with zirconium the tube can stand more than 100 firings without losing sensitivity.As palladium volatilizes at a temperature close to the atomization temperature (2200 "C) it should be added every time before collecting lead hydride. Aroza et al.23 found that the optimum sorption tempera- ture for lead hydride was higher than 600 "C when an uncoated graphite tube was used. However lead hydride can be adsorbed on the Zr coated graphite tube at much lower temperatures. Fig. 4 shows the influence of sorption temperature on lead absorbance. As can be seen the signal remains constant in the sorption temperature range of 100-500 "C. Note that even at 100 "C lead hydride can be effectively adsorbed on the Zr coated graphite tube. An adsorption temperature of 300 "C was selected for further experiments in order to avoid prolonged heating of the graphite tube at higher temperatures. After the hydride generation a period of time is required for sweeping the residual hydrides into the graphite tube.The effect of collection time was investigated under the I 4 I TemperaturePC I I 0 200 400 600 800 Fig. 4 Effect of trapping temperature on the peak absorbance of 2.5 ng of Pb with 1.5% m/m H202 0.5% v/v HNO and 6% m/v NaBH 0 Fig. 5 Effect of collection time on the peak absorbance of 3 ng of Pb with 1.5% m/m H202 0.5% v/v HN03 and 6% m/v NaBH Table 4 Interferences of foreign ions on lead absorbance (5 ng ml-l of Pb) Added as [Interfering ion] Relative [Pb"] absorbance SeO (HCl) 2 0.84 5 0.68 20 0.55 NaAsO 20 1 .oo Sb203 (HCl) 20 1 .oo K,TeO 2 I .oo Cd(N03)2 10 1 .oo 20 0.79 50 0.66 WN03 13 10 1 .oo 20 0.79 ZnSO 20 1 .oo 80 0.92 Fe203 (HNO3) 20 1 .oo 100 0.86 CoC12'6H20 20 1 .oo Ni(N03)2 20 1 .oo 80 0.88 (NH4)6M07024*4H20 10 1 .oo CU(N03)2 5 1 .oo mo3 5 00 1 .oo NaNO 500 1 .oo CaCO (HN03) 500 1.00 MgO (HNO3) 5 00 1 .oo K2S04 500 1 .oo NaCl 100 1 .oo 5 00 0.90 NaF 50 000 1.09 NH,C10 70 000 0.95 50 0.84 recommended conditions.The results shown in Fig. 5 indicate that above 60 s the absorbance of the analyte remains unchanged. A retention time of 100 s was chosen for further experiments in order to ensure that all of the lead hydride had been adsorbed completely. Interferences The interferences of different ions on the determination of Pb were studied and the results are shown in Table 4.Interferences due to hydride-forming elements such as Se and Te may be explained by competitive reactions with NaBH to form the corresponding hydrides. The transition metals Cd Cr Fe Ni and Cu inhibit hydride generation probably owing to coprecipitation of insoluble interfering corn pound^.^^^^^ Alkali alkaline earth metals and Sod2- do not interfere with the determination of Pb up to a concentration ratio of interferent to PbI1 of 500:l. The interferences of F- and C104- are not serious (see Table 4).486 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 Table 5 Determination of lead in reference materials Concentration/pg g-' Sample Determined' Certified GBW 08501 Tea 1.04 k 0.06 1.06 2 0.10 (China) Tea leaves 1.02 k 0.05 1 .OO * 0.04 (China) GBW 08401 Coal fly ash 34.0 2 0.12 33.8 * 4.4 (China) GBW 08571 Mussel 1.90k 0.08 1.96 * 0.09 (China) GBW(E) 08001 * Precision expressed as standard deviation based on three determinations.Since the concentration ratios of interferent to Pb" in the sample are usually lower than those used in the interference study and the standard additions method is used no separation procedure was necessary. Sample Analysis The proposed method was applied to the determination of Pb in different types of reference materials and a tap water sample. The results in Table 5 show that the values of Pb determined in the reference materials are in good agree- ment with the certified values. A Pb concentration of 1.59kO.04 ng ml-l determined in a tap water sample by using the recommended method is in excellent agreement with that of 1.63 k 0.13 ng ml-'obtained by an independent determination using ETAAS.32 Analytical Figures of Merit The characteristic mass of the proposed method (ie.the mass of analyte which provides a defined peak absorbance of 0.0044 A) is 52.8 pg. The absolute detection limit based on the variability of the blank (ie. 30) is 242 pg. This leads to a detection limit of 0.44 ng ml-l for a solution flow rate of 3.7 ml min-* pumped for 9 s. The detection limit can be improved by introducing larger volumes of sample solution into the continuous hydride generator. The precision is 2% relative standard deviation for ten replicate measurements at the level of 3 ng of Pb.The regression equation of y=O.O833x (where y=the peak absorbance x=the analyte mass in ng) with a regression coefficient of 0.9986 is obtained from the calibration graph. The linear working range spans about two decades extending to 15 ng. The reagent blank was found to be 0.925 f 0.059 ng. Conclusion Lead hydride can be effectively adsorbed on the surface of a Zr coated graphite tube at relatively low temperatures. The sensitivity accuracy and precision are significantly im- proved compared with those obtained with the uncoated tube. The proposed method can also be used for the determination of other hydride-forming elements such as tin.31 The authors thank D. T. Wu for sample preparation. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 References Robbins W.B. and Caruso J. A. Anal. Chem. 1979 51 889A. Godden R. G. and Thomerson D. R. Analyst 1980 105 1137. Schmidt R. At. Spectrosc. 1981 2 155. Jin K. and Taga M. Anal. Chim. Acta 1982 143 229. Thompson K. C. and Thomerson D. R. Analyst 1974 99 595. Vijan N. P. and Wood G. R. Analyst 1976 101 966. Kumamaru T. Nakata F. Haras M. and Kiboki M. Bunseki Kagaku 1984 33 624. Tao R. and Zhou H. Fenxi Huaxue 1985 13 283. Bonilla M. Rodriguez L. and Chmara C. J. Anal. At. Spectrom. 1987 2 157. Madrid Y. Bonilla M. and Camara C. J. Anal. At. Spectrom. 1988 3 1097. Zhang S.-Z. Han H.-B. and Ni Z.-M. Anal. Chim. Acta 1989 221 85. Brovko I. A. Tursunov A. Rish M. A. and Davirov A. D. Zh. Anal. Khim. 1984 39 1768. Sturgeon R. E. Willie S . N. and Berman S.S. Fresenius 2 . Anal. Chem. 1986 323 788. Sturgeon R. E. Willie S. N. and Berman S. S. Anal. Chem. 1985 57 231 1. Willie S. N. Sturgeon R. E. and Berman S. S. Anal. Chem. 1986 58 1140. Sturgeon R. E. Willie S. N. and Berman S. S. J. Anal. At. Spectrom. 1986 1 115. Sturgeon R. E. Willie S. N. and Berman S. S. Anal. Chem. 1987 49 2441. Vien S . H. and Fry R. C. Anal. Chem. 1988 60 465. Zhang L. Ni Z.-M. and Shan X.-Q. Spectrochim. Acta Part B 1989 44 75 1. Zhang L. Ni Z.-M. and Shan X.-Q. Spectrochim. Acta Part B 1989 44 339. Sturgeon R. E. Willie S. N. Sproule G. I. Robinson P. T. and Berman S. S. Spectrochim. Acta Part B 1989 44 667. Doidge P. S. Sturman B. T. and Rettberg T. M. J. Anal. At. Spectrom. 1989 4 25 1. Aroza I. Bonilla M. Madrid Y. andchmara C. J. Anal. At. Spectrom. 1989 4 163. Sturgeon R. E. Willie S. N. and Berman S. S. Anal. Chem. 1989 61 1867. Muller-Vogt G. and Wendel W. Anal. Chem. 1981 53,651. Fritzsche H. Wegscheider W. Knapp G. andortner H. M. Talanta 1979 26 219. Vickrey T. M. Harrison G. V. Ramelov G. J. and Carver J. C. Anal. Lett. 1980 A13 781. Gao Y.-Q. and Ni Z.-M. Acta Chim. Sin. 1982 40 1021. Welz B. and Melcher M. Spectrochim. Acta Part B 1981,36 439. Verlinder B. and Deelstra H. Fresenius 2. Anal. Chem. 1979 296 253. Ni Z.-M. Han H.-B. Li A. He B. and Xu F.-Z. J. Anal. At. Spectrom. 1991 6 385. Shan X.-Q. and Ni Z.-M. Can. J. Spectrosc. 1982 27 75. Paper I /O I2 7.50 Received March 18th 1991 Accepted May 9th I991

ISSN:0267-9477    

DOI:10.1039/JA9910600483

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 487 Behaviour of Various Organic Solvents and Analytes in Electrothermal Atomic Absorption Spectrometry Emil Tserovsky and Sonja Arpadjan Faculty of Chemistry University of Sofia 1 Anton lvanov Boulevard I126 Sofia Bulgaria Factors affecting the determination of trace metal impurities in organic solvents using electrothermal atomic absorption spectrometry have been studied. The behaviour of Cd Co Cu Fe Ni and Pb in various organic solvents (isobutyl methyl ketone butyl acetate toluene and xylene) was investigated. The influence of the nature of the solvent complexing agent atomizer type chemical form and amount of chemical modifier used was evaluated. Optimum conditions with respect to the analyte solvent and atomizer were established.A comparison of the sensitivities using organic and aqueous solutions is presented. Keywords Organic solvent; trace element; electrothermal atomic absorption spectrometry The introduction of organic solvents in flame atomic absorption spectrometry (FAAS) provides various advan- tages. Unfortunately this does not apply when organic solvents are utilized in electrothermal atomic absorption spectrometry (ETAAS) as drawbacks and restrictions are encountered. Nevertheless analytical practice sometimes imposes the use of organic solvents for ETAAS for example in the direct determination of elements in organic materials and when determination by ETAAS is combined with extraction procedures in preconcentration and separa- tion stages which include the application of organic reagents.In recent years comprehensive publications deal- ing with the problems of using organic solvents in ETAAS have appeared but there are still some aspects which have not been investigated thoroughly. A review which discussed the analyses of petroleum and petroleum products by AAS has been published by Sychra et aE.' The determination of Cu Fe Ni Pb and V in petroleum fractions by ETAAS has been reported by Gonzalez et aL2 Volynsky et u I . ~ have summarized publications devoted to organic extracts in ETAAS and emphasized the basic problems of these analyses. Analytical instrumentation has improved drama- tically in recent years hence it is timely to check and reassess the basic conclusions of these earlier studies. The present work was an attempt to examine the most important aspects of the analyses of organic solvents in accordance with current experiences with ETAAS.The behaviour of Cd Co Cu Fe Ni and Pb dissolved in butyl acetate (BA) isobutyl methyl ketone (IBMK) toluene and xylene was investigated. These solvents are widely used in hybrid extraction-AAS methods. Preliminary experiments on the behaviour of the same analytes in water miscible organic solvents (such as methanol and ethanol) in ETAAS showed that the differences were insignificant in compari- son with aqueous solutions and hence will not be discussed further. The results obtained allow conclusions to be drawn on the influence of the nature of the organic solvents the chemical form of the analytes the role of the atomizer surface and the requirement for a chemical modifier.The optimum temperature programmes for particular analytes and solvents were also established. Experimental The measurements were carried out on a Perkin-Elmer 1 1 OOB atomic absorption spectrometer with an HGA-700 graphite furnace. The light sources were hollow cathode lamps for the specific elements investigated. The atomic absorption signals were recorded on an Epson 80MX printer. The spectral bandpass and wavelengths used (228.8 nm for Cd 240.7 nm for Co 324.8 nm for Cu 248.3 nm for Fe 232.0 nm for Ni and 283.3 nm for Pb) were as recommended by Perkin-Elmer. Pyrolytic graphite coated graphite tubes (PGT) (Perkin-Elmer Part No. 091 504) tungsten impregnated graphite tubes (W-t~bes)~ and tubes with a platform were used.Micropipettes (Eppendorf) were used for injections and dilutions. The temperature pro- grammes used are summarized in Table 1. Stock standard solutions (1000 pg ml-l Merck Darm- stadt Germany) were used for the preparation of working aqueous solutions by appropriate dilution. Organic working standard solutions were prepared by chelate extraction. The required amount of the aqueous standard solution was mixed with an acetate buffer of pH 4.66 (Merck No. 7827) along with a chelating agent ammonium pyrrolidin- 1- yldithioformate (APDC) dithizone (Dz diphenylthiocar- bazone) or oxine (Ox quinolin-8-01). A specific volume of the organic solvent was then added and a 5 min extraction was carried out. Elemental standards (1000 pg ml-l) of Co and Pb dissolved in oil (Merck No.15061 and No. 15501) as cyclohexane butyrates (CHB) were also used. The working standard solutions were prepared by appropriate dilution with the various organic solvents. Organic solvents (pro analysi Merck) were used without purification. Doubly distilled water was used throughout. Table 1 Temperature programme Step Parameter 1 2 3 4 5 Temperature/"C Var* 300 Var? Vart 2650 Ramp time/s 15 10 5 0 1 Hold time/s 10 10 30 5 3 Read Internal Ar flow/ml min-' 300 300 300 300 300 - - - On - *Variable; see text. Variable; see Table 4.488 c-_--______ A Stopped flow JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 t Results and Discussion 8 0.500 rn Influence of Physical Properties of Solvents The different viscosities densities surface tensions and wetting abilities of the solvents affect the sampling and injection steps of the procedure. These effects have been described in detail el~ewhere,~g~ hence only new observa- tions will be discussed here.Undoubtedly the use of autosampler devices ensures better precision and they are more convenient to use. However most autosamplers are not adapted for use with organic solvents. If care is taken manual sampling can give acceptable results as seen in Table 2. The injection of 10 pl of organic solution into the furnace is optimal. Spreading of the sample makes injection of more than 20-25 pl into the furnace and 30 pl on to the platform (Perkin-Elmer platforms with V-like profiles and with enlarged capacities were used) practically impossible.In order to introduce a larger amount of solvent multiple sample injections and drying is recommended. The negative influence of the physical properties of the organic solvent may be compensated for to some extent by injection of the sample into a pre-heated No significant difference was found on using this approach; enhancement of the sensitivity was about 20%. The pyrolytic graphite coated graphite tubes give better results than those from the normal electrolytic graphite tubes. In the former penetration into the graphite is probably less. n (,.I Non-specific Light Absorption It is likely that the high volatility of the organic solvents leads to their complete removal from the furnace at relatively low charring temperatures over a short period of time. In practice however it has been shown that organic solvents penetrate deeply into the graphite and their removal requires a high charring temperature (at least 1200 "C) and a long pre-treatment step (30-40 s).Fig. 1 shows the background absorption throughout the charring and atomization stages. As can be seen even at a charring temperature (Tchar)= 1100 "C and 20 s duration of the charring step part of the organic solvent or products of its pyrolysis are retained in the furnace. It is not clear whether any chemical bonds exists between the molecules of the organic solvent and the carbon of the atomizer similar to those established for chlorine containing organic solvents.* As is anticipated the background absorption is more in evidence at shorter wavelengths. The background absorp- tion depends on the nature of the ~olvent,~ solvents containing a benzene ring such as xylene and toluene give higher non-specific light absorption in comparison with IBMK and BA (Fig.2). The contribution of the ligands studied (APDC Ox Dz and CHB) to the non-specific light absorption was not ascertained. The complexing agents are probably pyrolysed and fully removed from the atomizer during the charring step. Table 2 Relative standard deviation of sampling (five injections PGT) RSD (Yo) Solvent Manual micropipette Autosampler Water 2.8 1.8 IBMK 8.5 3.5 Toluene 10.6 7.2 Xylene 6.3 4.8 Y 0 fl 0.500 (4 I U rl f 0 Fig. 1 Background absorbance as a function of charring tempera- ture using PGT with xylene as the organic solvent. (a) Peak area=1.739 A s peak height=0.798 A Tchar=900 "C; (b) peak area=0.696 A s peak height=0.302 A T,,,,=lOOO "C; (c) peak area= 1.419 A s peak height=0.125 A Tchar= 1100 "C; and (6) peak area=0.274 A s peak height=0.082 A Tc,,,=12O0 "C.Atomization temperature ( Ta,)= 2300 "C and A= 193.7 nm 0.500 a 1 I (a' Time/s Fig. 2 Effect of the nature of the solvent on the background absorbance using PGT; Tchar= 1000 "c Ta,=2300 " c and ,I= 193.7 nm. (a) IBMK peak area=0.233 A s peak height=0.092 A; (b) toluene peak area=3.264 A s peak height=0.646 A; and (c) xylene peak area= 3.445 A s peak height = 1.028 A Influence of Atomizer Type The experiments showed that normal uncoated graphite tubes are unsuitable for work with organic solvents. As mentioned above the use of PGTs reduces the detrimental effects of the organic solvents but they cannot be elimi- nated completely. Hence the possibility of using other types of atomizers was investigated tubes with a plat- form and W-tubes.The results for the relative sensitivity A,=A,/A (A is the integrated absorbance for the organicJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 489 Table 3 Comparison of the relative sensitivity (A,) for organic ('4J and aqueous (A,) solutions. ilr=A,/A Analyte Solvent Atomizer Cd Co Cu Fe Ni Pb IBMK PGT 0.88 1.02 1.00 0.95 0.97 0.92 W-tube 1.26 1.21 1.11 0.96 1.13 1.36 Platform 1.34 - - 1.00 - 1.02 Xylene PGT 0.79 1.00 0.94 0.93 0.86 0.71 W-tube 1.38 1.19 1.26 0.91 1.25 1.47 Platform 1.31 - - 1.00 - 0.72 Table 4 Optimum charring and atomization temperatures ("C) for various solvents Solvent Water IBMK Analyte Atomizer Cd PGT W-tube Platform W-tube Platform W-tube Platform W-tube Platform W-tube Platform W-tube Platform Co PGT Cu PGT Fe PGT Ni PGT Pb PGT Tchar 400 700 500 1400 1600 1500 1300 1400 1400 1100 1600 1300 1300 1600 1500 700 900 800 T a t 1200 1800 1300 2200 2400 2300 1900 2100 2300 2000 2500 2300 2400 2500 2400 1100 1500 - Tchar Ta 5 50 8 50 700 1800 600 1000 1400 2200 1600 2400 1500 2300 1300 1900 1400 2100 1500 2300 1100 2000 1700 2500 1300 2300 1400 2400 1600 2500 1500 2400 7 00 1000 900 1500 900 - Xylene Tchar T a t 550 850 700 1800 600 1000 1400 2200 1600 2400 1500 2300 1300 1900 1400 2100 1500 2300 1100 2000 1700 2500 1500 2300 1400 2400 1600 2500 1500 2400 700 1000 900 1500 1000 - solution and A for the aqueous standard at the same concentration of analyte in various organic solvents) are illustrated in Table 3.It is evident that atomization from the platform gives better sensitivity and decreases the difference in absorbance signals between the aqueous and the organic solvents. Atomization from the W-tubes gives interesting results. In all instances (except for Fe) an enhancement in the sensitivity and also an improvement in the reproducibility were observed. The influence of the W-tube on the atomization is greater for the organic solvents than for aqueous solutions the extent of this also depends on the age of the W-tube. The shapes and positions of the absorbance signals obtained from the W-tube and the unimpregnated tube are different. With the W-tube the peaks appear 0.2-0.4 s later than with the unimpregnated tube.The use of W-tubes permits higher Tchar and higher atomizing temperature (Tat) values are obtained (Table 4). Temperature Programme Parameters The main task in the present investigation was to define the parameters relevant to the temperature programme appro- priate for each element solvent and atomizer and to compare these data with those used for aqueous solutions. Thermal pre-treatment-atomization curves were prepared for this purpose. For most elements they appear to be similar except for Cd and Pb with a PGT (Figs. 3 and 4) but these differences were eliminated when a W-tube was used. The optimum Tchar and Tat values are given in Table 4. There are pronounced decreases in the atomization temper- atures for Cd and Pb in organic solvents in PGTs.For the other elements the differences are insignificant. The most suitable drying temperature was that equal to the boiling- point of the solvent. The Tchar can be reached very quickly (about 5 s) but a long period of heating at that temperature (30-40 s hold time) is necessary. The reason for this long pre-treatment step was explained above. A 'cooling stage' prior to the atomization did not give any benefits. Influence of Nature of Complexing Agents and Solvent on AA Signal Information in the literature about the effects of the complexing agent on atomization is ~ o n t r a d i c f o r y . ~ ~ ~ J ~ - ~ Ligands with different donor atoms were investigated APDC Dz Ox and CHB. The signals recorded had almost the same characteristics height area shape and appearance time (Fig.5). Thermal pre-treatment curves for different complexes also have a similar appearance. There are no differences between the AA signals for the chelate com- plexes and the CHB (Fig. 6). The type of organic solvent changes the AA signal. Peaks obtained with organic solvents are slightly lower and wider than those for aqueous solutions [Fig. 6(a)]. The differences are greater for toluene and xylene in comparison with490 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 BG 0.500 d I I 1 I I I 1000 1200 1400 300 500 700 900 1100 1300 1500 1700 1900 Tern peraturePC Fig. 3 Pre-treatment and atomization curves for Cd (a) from PGT and (b) from tungsten-impregnated tube.A. Cadmium in aqueous solution and B Cd in IBMK ( a ) A V I I I I 500 700 900 1100 1300 & o l 300 4 BG 0.500 a 0 1 I I I 1 1 I 1 400 600 800 1000 1200 1400 1600 1800 TemperaturePC Fig. 4 Pre-treatment and atomization curves for Pb (a) from PGT and (b) from tungsten-impregnated tube. A Lead in aqueous solution and B. Pb in IBMK (b) A IBMK and BA. These differences are more obvious for the easily volatilized Cd and Pb and less so for Ni Co Fe and Cu. The sensitivity for organic and aqueous solutions is different and depends not only on the analyte and the solvent but also on the atomizer (Table 5). It usually changes in the following order water>IBMK = BA>tolue- ne=xylene. The differences in sensitivity are not high and could be explained by physical observations the greater spreading of the sample in the furnace leading to lower and wider peaks,14 the deeper penetration of the organic solvents into the graphite exceeding the diffusive loss through the walls of the furnace.Chemical Modification Chemical modification is now a routine practice in ETAAS determinations of volatile elements. Modification in or- ganic media raises questions such as whether the known modifiers for aqueous solutions are still suitable; in what form should they be used; in what amount? To answer these questions Cd and Pb were investigated with Pd being chosen as the modifier. The AA signals for Pb with and without modifier are shown in Fig. 7. The Pd stabilizes Cd to 850 "C and Pb to 1250 "C in a PGT using an organic medium (Fig. 8). The modifying effect does not depend on AA 0.500 AA 0.500 Time/s Fig.5 Absorbance profiles for Cu in IBMK with various ligands. (a) APDC peak area (AA-BG)=O. 132 A s peak area (BG)=0.008 A peak height (AA-BG)=0.295 A and peak height (BG)=0.005 A; (b) dithizone peak area (AA-BG)=0.120 A s peak area (BG) = 0.008 A s peak height (AA - BG) = 0.270 A and peak height (BG)=0.004 A; and (c) oxine peak area (AA-BG)=O.119 A s peak area (BG)=0.008 A s peak height (AA-BG)=0.266 A and peak height (BG)=0.005 A 0 4.0 Time/s Fig. 6 Absorbance profiles for Co in A IBMK and B aqueous solution with the same concentration; Co is extracted as (a) the APDC complex and (b) organometallic standard for Co (Co-cyclo- hexane butyrate) the chemical form of the Pd. No differences were observed for Pd added as PdClz and for soluble Pd added in an organic solvent ion associate complex PdC14R2.It is interesting that in an organic medium the stabilizing effect Table 5 Characteristic masses (pg) Solvent Analyte Atomizer Cd PGT c o PGT c u PGT Fe PGT Ni PGT Pb PGT W-tube W-tube W-tube W-tube W-tube W-tube Water 0.32 0.3 1 9.5 10.0 4.5 4.8 8.1 8.2 13.7 17.2 16.1 15.7 IBMK 0.36 0.27 9.4 8.5 4.6 4.3 8.5 8.6 14.1 15.2 17.5 11.6 Xylene 0.41 0.24 9.6 8.7 4.8 3.9 8.7 9. I 15.9 13.8 22.6 10.8JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 49 1 AA 0.300 A '0° I B 5.0 Tim e/s Fig. 7 Absorbance signals for Pb-CXB in IBMK using PGT; A without and B with a chemical modifier (2 jig of Pd as PdClJ Bg 1000 1300 1600 1900 700 0 Temperature/"C Fig. 8 Thermal pre-treatment and atomization curves using PGT for A Cd and B Pb in IBMK with a chemical modifier I 2 jig of Pd as PdC12) 0 1 2 3 Logk (ppm)l Fig.9 Effect of the concentration of the modifier (Pd) on the absorbance of Cd-APDC in IBMK is reached with smaller amounts of the modifier with 0.2 pug of Pd (Fig. 9) while 10-15 pg of Pd are necessary for modification in aqueous solutions. Conclusion The determination of trace elements in organic solvents by ETAAS has some interesting aspects. An attempt to determine the more important factors affecting the analyses has been made. The detrimental effects due to the physical properties of the solutions are relatively minor. A high charring temperature is necessary to reduce the 'non- specific' absorbance. The influence of the complexing agent is insignificant.The effect of the nature of the solvent is more pronounced for toluene and xylene in comparison with IBMK and BA. It is also important to choose the right type of furnace tube as for example W-impregnated tubes give some benefits. An aqueous solution of Pd may be used as a chemical modifier in the organic medium. The optimum working conditions for determining all the ele- ments of interest were established. References 1 Sychra V. Lang I. and Sebor G. Prog. Anal. At. Spectrosc. 1981 4 341. 2 Gonzalez M. C. Rodrigues A. R. and Gonzalez V. Micro- chem. J. 1987 35 94. 3 Volynsky A. B. Spivakov B. Ya. and Zolotov Yu. A. Talanta 1984 31 449. 4 Arpadjan S. Karadjova I. Tserovsky E. and Aneva Z. J. Anal. At. Spectrom. 1990 5 195. 5 Hulanicki A. and Bulska E. paper presented at 25th Colloquium Spectroscopicum Internationale July 2-7 1 989 Sophia Bulgaria. 6 Apostoli P. Alessio L. Dal Farra M. and Fabbri P. L. J. Anal. At. Spectrom. 1988 3 471. 7 Kunwar U. K. Littlejohn D. and Halls D. J. J. Anal. At. Spectrom. 1989 4 153. 8 Hulanicki A. and Bulska E. Can. J. Spectrosc. 1983,29 148. 9 Betz M. Gucer S. and Fuchs F. Fresenius 2. Anal. Chem. 1980 303 4. 10 Volynsky A. B. Spivakov B. Ya. and Zolotov Yu. A. Zh. Anal. Khim. 1987 XLIII 1835. 11 Komarek J. and Sommer L. Talanta 1982 29 159. 12 Sturgeon R. S. Berman S. S. Desaulniers A. and Russell D. S. Talanta 1980 27 85. 13 Krakowska R. Bulska E. Baranat K. A. and Hulanicki A. Chem. Anal. (Warsaw) 1980 25 1043. 14 Guell 0. A. and Holcombe J. A. Spectrochim. Acta. Part B 1988 43 459. Paper 1 /00828E Received February 21st 1991 Accepted April 22nd I991

ISSN:0267-9477    

DOI:10.1039/JA9910600487

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 493 INTER-LABORATORY NOTE Inserted Injector Tubes for Inductively Coupled Plasma Spectrometry Lyne S. Gervais and Eric D. Salin" Department of Chemistry McGill University 80 1 Sherbrooke Street West Montreal Quebec H3A 2K6 Canada Experiments are described in which an inductively coupled plasma is operated with an injector tube that is inserted into the body of the plasma. The system is highly tolerant to pressure fluctuations from the sample introduction system. Both wide (6 mm o.d. 3 mm i.d.) and narrow (2 mm o.d. 1 mm i.d.) injector tubes were studied. Interestingly the intensities observed with the two different injector tubes were fairly similar. Alumina was found to be a superior material since graphite appeared to reduce the power of the plasma.Keywords Inductively coupled plasma; torch; design; injector; sample introduction Work has been carried out on the development of sample introduction systems for inductively coupled plasma (ICP) spectrometry. 1-5 Recently studies have been undertaken on a system that utilizes a Beckman burner with a hydrogen- oxygen flame as a sample n e b ~ l i z e r . ~ * ~ With a conventional ICP torch this sytem produced severe fluctuations in the aerosol flow rate. By the use of an 'inserted injector' torch design this sample introduction system could be stablized. The inserted injector system is extremely tolerant to pressure fluctuations from a sample introduction system which might otherwise extinguish the plasma. The design may be of interest to researchers involved with novel sample introduction systems.All work was carried out on a Jarrell-Ash Model 61 direct reading spectrometer system with a 27 MHz generator operated at 1.0 kW unless otherwise indicated. An outer (plasma) gas flow of 16 1 min-l was used for all experiments. The auxiliary gas flow was varied as described in the given experiment but never exceeded 1.0 1 min-'. A Ligere V-type nebulizer was used with a 0.9 1 min-l argon flow. A heated spray sample introduction system was used with modifications to a conventional torch as de- scribed in the various sections. All solutions were prepared from 1000 ppm standards obtained from Fisher Scientific. Multi-element solutions of Cd Cu Ni Pb and Zn at the 20 ppm level were used for the experiments. Conventional torches use a configuration which places the injector tip several millimetres below the plasma.Since the injector tubes are commonly made of quartz this is 10 mm - 5 mm H essential to their survival. Other materials of course are possible including boron nitride and alumina. Alumina is particularly attractive owing to its ability to tolerate higher temperatures. An undesirable feature is the more demand- ing requirements for shaping this material. Alumina cannot be conveniently shaped in machine or glass shops but must be fabricated by the manufacturer in a given form. In attempts to develop a torch system which was relatively immune to aerosol flow fluctuations initially small extensions were placed on a conventional injector tube.The design is illustrated in Fig. 1. The materials used were boron nitride and graphite. The second set of experiments used the tubular system illustrated in Fig. 2. The tube was commercially available alumina in two dimensions 6 mm o.d. 3 mm i.d. (6/3) and 2 mm o.d. 1 mm i.d. (2/1). The height reference system used for insertion heights is illustrated in Fig. 3. The bottom of the load coil was selected as a logical 0 (zero) reference point. The inner torch tube was located 2 mm below the 0 reference point (- 2 mm in this reference system). Viewing heights were measured conventionally from the top of the load coil. All numeric data (as compared with visual observations) presented were collected from the tubular system. Background corrected signals were calculated for Standard injector tube If )I- Fig.1 Cap insertion configuration * To whom correspondence should be addressed.494 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 Coil 0 O / - - _ _ - - Reference point for viewing height measurements I 0 “I 12mm Reference point for insertion mm height measurements Auxiliary gas Plasma gas Conventional torch cross-section Fig. 3 Measurement reference system I 1 I I 4 6 8 10 12 14 16 18 Observation height/mm Fig. 4 Comparison of profiles for ‘hot’ versus ‘normal’ systems using a 2/1 injector tube 2 mm below the load coil (see Fig. 3 for observation height reference point). A Cd ‘normal’; B Cd ‘hot’ C Cu ‘normal’ D. Cu ‘hot’; E Zn ‘normal’; and F Zn ‘hot’ each element using a two point background correction method.Signals were obtained from the average of 30 1 s integrations. A small gap between the alumina injector tube and the graphite plug allowed an auxiliary flow to be introduced. Previous experience with direct sample insertion (DSI)l0 suggested that the graphite a conducting material might affect the plasma. Experiments with graphite extensions (Fig. 1) confirmed that larger masses of graphite affected the power required to start the plasma. In a similar vein an insertion of a 6/3 graphite tube 3 mm into the plasma required 1.5 kW for plasma stability versus 1.0 kW for a 1 mm extension of the same material. Contrary to our DSI experience the graphite extension does not heat within seconds. It reaches an apparent ‘red heat’ after about 1 min depending on the height and mass of the system.Interestin- gly the graphite does not become sufficiently hot that it affects the quartz injector tube on which it is resting. Boron nitride on the other hand becomes sufficiently hot after 15 min that a visible orange Si emission is present in the plasma from the injector tube. The length of the extension material into the plasma will also affect the amount of auxiliary gas required to place the plasma in its conventional position directly above the auxiliary tube. With an extension of 6 mm into the load coil region no auxiliary gas is necessary. Further extensions cause the plasma to move up. The adverse effects of longer graphite extensions led to the development of the tubular system. The tubular system was made of alumina a non- B ,c 10 - 1 oh -I- - - -*- 4 6 8 10 12 14 16 18 30t 20 “4 6 8 10 12 14 16 18 Zn E (4 60 - “ U 4 6 8 10 12 14 16 18 ,Y :‘ H ‘I 4 6 8 10 12 14 16 18 Observation heighvmm Fig.5 Profiles of relative intensity versus observation height. (a)-(c) 6/3 Injector tubes at insertion heights of A 3; B 8; and C 12 mm. (4-m 2/1 Injector tubes at insertion heights of D -2; E 1; F 3; G 5 ; H 8; and I 12 mm. See Fig. 3 for reference pointsJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 495 8 - 7 - x. c 1 - 0 4 8 12 16 18 1 L Observation heighthm Fig. 6 Reproducibility of the signal for the 2/1 injector tube system using ‘hot’ and ‘normal’ sample introduction systems at various insertion heights A ‘normal’ -2 mm; B ‘hot’ -2 mm; C ‘hot’ + 3 mm; D.‘hot’ + 8 mm; and E ‘hot’ + 12 mm. See Fig. 3 for reference points conducting ceramic material and consequently allowed a 1.0 kW operation; however with insertion heights greater than 8 mm one must apply more power to light the plasma. The amount of auxiliary gas required to position the plasma depends on both the tube size and the insertion height. With the 6/3 tube a gas flow of 0.3 1 min-’ was required at both 6 and 12 mm. In contrast the smaller 2/1 tube required 1.0 1 min-I at 6 mm and 0.3 1 min-’ at 12 mm. Apparently the tubes exert an upward force related to their diameters and insertion heights. The auxiliary flow pushes the plasma upwards in a manner similar to a normal auxiliary flow. There is no apparent sheathing effect on the injector tube. When the injector tube is low in the plasma the appearance of the plasma is as anticipated from normal operation.When the injector tube is high in the plasma the central channel wedges outward slightly and is much darker than the usual central channel. The inserted injector is in part a byproduct of research performed with a heated sample introduction system called the ‘hot’ ~ y s t e m . ~ . ~ The system uses a heated spray chamber and a condenser. Data from what is called the ‘normal’ system have been included for comparison purposes. This system differs from a conventional system only in one respect it has a condenser between the spray chamber and the torch. Fig. 4 contains both ‘normal’ and ‘hot’ system data. For clarity of presentation the ‘normal’ system data have been scaled so that the largest value in each ‘normal’ set matches the largest value in each ‘hot’ set.In both instances the injector is at the conventional position 2 mm below (-2 mm) the load coil using the reference system illustrated in Fig. 3. As previous work demon~trated,~ the differences in performance between this and a conventional system are minimal with the exception that the ‘hot’ system does provide higher signal levels (approximately a factor of 4) due to an increased mass transfer rate.’19 Since all data were obtained under the same conditions (power data acquisition etc.) the primary differences seem to be simply an increase in signal level and a degradation in precision by a factor of approximately 2 as discussed below. For a given element the profiles are fairly similar.This leads to speculation that the results may be comparable to those that would be obtained with a conventional nebulizer-spray chamber system. Height profiles for a range of elements are presented for the 613 tube [Fig. 5(a)-(c)] and the 2/1 tube [Fig. 5(d)-(f)]. It is not surprising that the intensitiey would fall off with deep insertions however it is surprising that the 6/3 system with its large central channel produces intensities equivalent to those of the much narrower 211 system at lower insertion depths. This indicates that some type of pinching effect may be channelling the analyte into the viewing zone. Unfortu- Table 1 Differential pressures Insertion height/ Differential pressure mm above atmospheric/atm -2 + 1 +3 +5 +8 + 12 0.0 17 0.023 0.028 0.029 0.029 0.036 nately no profiles are available to verify this.Certainly this is not what one would expect from simple geometric and optical considerations. Fig. 6 shows a noise profile of the 2/1 system using both ‘hot’ and ‘normal’ sample introduction systems. While the ‘normal’ system is a factor of 2 lower in noise its behaviour is otherwise similar to the ‘hot’ system with -2 and + 3 mm insertion heights. At higher insertion levels 8 and 12 mm the noise distribution is fairly flat indicating an independence between viewing height and noise. This is interesting because it suggests that some noise reduction process probably drying of the droplets is taking place in the tube. Furthermore it indicates that the process takes place more efficiently in the tube than in the plasma.Considering the variety of mechanisms available for energy transfer this does not seem unreasonable. Another noteworthy facet of the inserted injector system is the pressure induced in the sample introduction system. Whenever a gas is forced through an orifice one would expect pressure differentials. When using a 613 system the differential pressure in the spray chamber was very low approximately 0.0025 atm (1 atm= 9.86923 x lo5 Pa). This was used for the flame sample introduction system. Using a 2/1 system the pressures increased as indicated in Table 1. The inserted injector system allows the ICP to be used with sample introduction systems which have large pressure fluctuations. For the flame sample introduction system described previou~ly,~*~ the inserted injector system allowed a system which could not be maintained for more than 30 s to be run continu- ously with approximately 1 O/o relative standard deviation. The delay in introducing the analyte stream into the plasma body may allow other types of experiments which could not be carried out with a conventional system. References 1 Blain L. Salin E. D. and Boomer D. W. J. Anal. A?. Spectrom 1989 4 721. 2 Sing R. L. A. and Salin E. D. Anal. Chem. 1989 61 163. 3 Monasterios C. J. and Salin. E. D. Anal. Chem. 1986 58 780. 4 Habib M. M. and Salin E. D. Anal. Chem. 1985 57 2055. 5 Gervais L. S. and Salin E. D. J. Anal. Ai. Spectrom. 1991,6 41. 6 Usypchuck L. Undergraduate Honours Thesis McGill University 1989. 7 Usypchuck L. Moss P. Karanassios V. and Salin E. D. in preparation. 8 LCgere G. and Burgener P. ICP Inf Newsl. 1985 11 447. 9 Gervais L. S. M.Sc. Thesis McGill University 1989. 10 Salin E. D. and Horlick G. Anal. Chem. 1979 51 2284. Paper 0/04983B Received November 5th 1990 Accepted April 29th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600493

出版商:RSC    

年代:1991

数据来源: RSC