Anal. Chem. 1996, 68, 1883-1887
Determination of Copper in Environmental Matrices Following Vapor Generation R. E. Sturgeon,* J. Liu,† V. J. Boyko, and V. T. Luong
Institute for National Measurement Standards, National Research Council of Canada, Ottawa, Ontario K1A 0R9, Canada
Copper was determined in environmental matrices following generation, separation, and atomization of a volatile species formed by the merging of an acidified sample solution with an aqueous sodium tetrahydroborate solution at room temperature. The copper species, as yet unidentified, was phase separated in a conventional gasliquid separator and directed via a stream of Ar carrier gas to an inductively coupled plasma atomic emission detection system. Optimum conditions for generation were investigated. The efficiency of generation/transfer was estimated to be 50%, and no interference from the presence of 1000 mg/L concentration of As, Cd, Co, Ni, Fe, Cr, Mn, Pb, Se, and Zn concomitants was evident. Simple aqueous standards were used for calibration purposes, and good agreement was obtained with certified values in the analysis of National Research Council of Canada marine sediment BCSS-1 and lobster hepatopancreas tissue TORT-1. The generation of gaseous analytes and their introduction into atomization cells offers several significant advantages over conventional solution phase pneumatic nebulization of samples. These include elimination of the need for a nebulizer, enhancement of analyte transport efficiency (approaching 100%), and the presentation of a homogeneous vapor to the atomizer. In addition to the widely practiced techniques of covalent hydride generation, the successful introduction of volatile chlorides (Bi, Cd, Ge, Mo, Pb, Sn, Tl, As, and Zn), fluorides (W, Mo, U, V, Re, and Ge), β-diketonates (Cr, Fe, Zn, Co, Mn, Cu, Ni, and Pd), and dithiocarbamates (Co and Cu) into atomization cells should also be noted. With such approaches,1 concentration detection limits can be significantly improved while achieving separation of the analyte from the matrix (which usually leads to improved accuracy of determination). Preconcentration is easily achieved, as is development of procedures which are amenable to automation. Use of hydride generation techniques has expanded in scope in the last decade to encompass elements supplementary to the usual suite of semimetals, viz., Pb,2 Tl,3 In,4 and, more recently, Cd.5,6 Additionally, the introduction of readily available ethylating agents7 † On leave from Beijing University of Aeronautics and Astronautics, Peoples Republic of China. (1) Yan, P.-X.; Ni, Z.-M. Anal. Chim. Acta 1994, 291, 89-105. (2) Valdes-Hevia y Temprano, M. C.; Aizpun Fernandez, B.; Fernandez de la Campa, M. R.; Sanz-Medel, A. Anal. Chim. Acta 1993, 283, 175-182. (3) Ebdon, L.; Goodall, P.; Hill, S. J.; Stockwell, P.; Thompson, K. C. J. Anal. At. Spectrom. 1995, 10, 317-320. (4) Liao, Y.; Li, A. J. Anal. At. Spectrom. 1993, 8, 633-636. (5) Sanz-Medel, A.; Valdes-Hevia y Temprano, M. C.; Bordel Garcia, N.; Fernandez de la Campa, M. R. Anal. Proc. 1995, 32, 49-52.
S0003-2700(95)01259-5 CCC: $12.00 Published 1996 Am. Chem. Soc.
(e.g., sodium tetraethylborate) has opened up several applications of this reagent for the more efficient production of Pb8,9 and Cd.10 There is little doubt that further research in this area will yield conditions conducive to the generation of volatile forms of other elements. To the best of our knowledge, this work reports on the first generation of a volatile species of copper resulting from the merging of an acidified sample stream with sodium tetrahydroborate reductant. The aim of this study was to undertake a parametric optimization of the variables influencing vapor generation in an effort to acquire an understanding of the process and to illustrate its analytical utility in an application to the determination of copper in environmental matrices (i.e., marine sediment and lobster tissue certified reference materials). Although the generation system was interfaced to an ICP-AES spectrometer in order to effect convenient detection, this analytical measurement technique was not specifically optimized in itself for best performance. Hence, the derived limit of detection may not compare favorably with direct solution phase nebulization; such comparison was not the aim of this study. EXPERIMENTAL SECTION Instrumentation and Reagents. Continuous vapor generation was accomplished using a four-channel Minipuls II peristaltic pump (Gilson Instrument Co.) to deliver acidified sample and borohydride solutions to a gas-liquid phase separator and to remove waste liquid from the chamber. A schematic diagram of the vapor generation manifold is shown in Figure 1. The system is similar to that described earlier11 except that minor but significant alterations were made to the central glass channel, where the reacting solutions first mix after being pumped into the cell. This is illustrated in more detail in Figure 2. Rather than permitting the NaBH4 and acidified sample solutions to mix in a T-piece external to the gas-liquid separator, they were conducted into the cell through 1 mm i.d. Teflon lines inserted into the glass tubing. Subsequently, contact was only as thin films flowing down the interior walls of the central glass channel. This promoted intimate mixing, reduced excessive frothing due to hydrogen generation, and, most importantly, permitted the process of phase separation to begin immediately through a thin liquid phase only. The U-tube and lower portion of the generator were (6) Xiao-Wei, G.; Xu-Ming, G. J. Anal. At. Spectrom. 1995, 10, 987-991. (7) Rapsomanikis, S. Analyst (London) 1994, 119, 1429-1439. (8) Sturgeon, R. E.; Willie, S. N.; Berman, S. S. Anal. Chem. 1989, 61, 18671869. (9) Ebdon, L.; Goodall, P.; Hill, S. J.; Stockwell, P.; Thompson, K. C. J. Anal. At. Spectrom. 1994, 9, 1417-1421. (10) D’Ulivo, A.; Chen, Y. J. Anal. At. Spectrom. 1989, 4, 319-322. (11) Tao, H.; Boyko, V. J.; McLaren, J. W. Spectrochim. Acta 1993, 48B, 13391345.
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Table 1. Operating Parameters rf power reflected power argon flow rates outer gas intermediate gas observation height wavelength vapor generation sample acidity sample flow rate NaBH4 concentration NaBH4 flow rate carrier gas flow rate
1.2 kW 1 µg/ mL), the efficiency decreases. At 5 µg/mL Cu(II), formation of a visible precipitate of reduced copper in the reaction vessel is evident, and this may serve to scavenge any further analyte by serving as a decomposition site in solution, thereby reducing efficiency. It is clear that optimal application of this technique will lie with the analysis of copper at the trace level. It should be emphasized that the measured efficiency may not correspond to the actual efficiency with which copper reaches the plasma due to unknown losses which may occur in the transfer tubing. Additionally, as these measurements were made in a batch generator and not in the actual merging flow thin-film reactor, the efficiency may be biased on the high side, as it is likely that the gas-liquid sparging process may be more complete in this system than in the experimental arrangement used. Collection of the waste from the U-tube arm of the flow system would likely not produce results which are any more reliable, as the reaction is not quenched at this point and, consequently, timing between collection and analysis becomes too significant. Figure 7 illustrates a typical calibration curve obtained with this technique. The range of linearity spans only about 2 orders of magnitude 1886 Analytical Chemistry, Vol. 68, No. 11, June 1, 1996
Figure 7. Typical calibration curve obtained using conditions cited in Table 1.
and is certainly limited by the reaction processes in the generator rather than by intrinsic limitations of the ICP source. Precision of replicate measurement (expressed as a relative standard deviation) of the steady-state intensity arising from the use of a 100 ng/mL and a 1 µg/mL feed of Cu(II) was 8.7% and 11.5% using the JY48P with integration of the wavelength scanned output and 9.7% and 8.6% with a 20 s integration window on the JY38, respectively. The 1.2% (m/v) NaBH4 reductant proved to be a significant source of copper contamination. Separate analysis of several such solutions yielded a concentration of 19.2 ( 3.8 ng/mL Cu(II), corresponding to a level of 1.6 µg/g in the solid reagent. Attempts to purify the solution using such techniques as filtration and coprecipitation with Zr(OH)4 were unsuccessful in lowering the copper content. A blank-limited concentration detection limit (3σ) of 8 ng/mL was obtained, based on the repeated analysis (n ) 8) of a single batch of the reductant. It is evident that the LOD obtained in this study is not as good as can be achieved with direct solution nebulization sample introduction. The objective of this study was not to optimize the LOD for ICP-AES detection of copper but simply to utilize the technique for the characterization of the vapor generation process and briefly investigate its potential application to real analysis. Without a doubt, the LOD can be further improved by cleaning the reductant or, for that matter, changing the reductant [e.g., to Na(C2H5)4 or change supplier] or improving the design of the generator-phase separator to improve reproducibility as well as generation efficiency. Interferences from Concomitant Elements. One major shortcoming associated with the use of the NaBH4-acid reduction technique for conventional hydride generation is its susceptibility to transition metal interferences. It is generally believed that the transition metal ions interfere with the hydride generation process after being reduced to metals or after being converted to metal borides in that they scavenge the analyte or otherwise effect a decomposition before phase separation can be accomplished. The catalytic effect of colloidal metals on decomposition of BH4- has also been considered as another source of interference. In the present study, the effect of 1000 µg/mL concentrations of As (III), Cd(II), Co(II), Ni(II), Fe(III), Cr(IV), Mn(IV), Pb(II), Se(IV), and Zn(II) present in a 100 ng/mL solution of Cu(II) was examined. No alteration of response was noted in any instance. Despite the formation of precipitates in some cases, the intensities in the presence of these concomitants averaged 100 ( 5%. It is not
known at this time why the generation process is so markedly free of interference. Analysis of Reference Materials. The methodology was applied to the determination of copper in two certified reference materialsslobster hepatopancreas TORT-1 and marine sediment BCSS-1. Digested samples (0.25 g TORT-1 and 0.5 g BCSS-1) were made to 50 mL volume, and aliquots taken for analysis were diluted 20-fold in 0.3 M (2% v/v) HNO3. Quantitation was achieved using a calibration curve prepared from aqueous standards in 2% HNO3. A value of 17.3 ( 1.9 µg/g (dry weight) was obtained for BCSS-1 on the basis of the separate analysis of 10 different sample digests. The material is certified at 18.5 ( 2.7 µg/g. For TORT1, a value of 432 ( 18 µg/g (dry weight) was obtained. The material is certified at 439 ( 22 µg/g. It is clear that the methodology yields accurate and precise results with real samples. CONCLUSIONS This study has reported the first successful generation of a volatile copper species formed by the merging of an acidified sample solution with an aqueous sodium tetrahydroborate solution at room temperature. The identity of the species remains (15) Sturgeon, R. E.; Gregoire, D. C. Spectrochim. Acta 1994, 49B, 1335-1345. (16) Madrid, Y.; Gutı´errez, J. M.; Ca´mara, C. Spectrochim. Acta 1994, 49B, 163170.
unknown, but it is unlikely to be atomic in nature. As the aim of this investigation was not to impliment a new analytical methodology for the determination of copper by ICP-AES but rather to demonstrate that copper vapor generation can be put to analytical use, the reported limit of detection achieved here is not as low as is currently possible using direct solution nebulization. If the copper contamination problem in the reagents can be alleviated, the LOD can be significantly improved. The more promising use of the technique lies with its application to in situ collection of the volatile analyte in the graphite furnace for preconcentration and ultratrace detection by atomic absorption or electrothermal vaporization ICPMS, as is currently used for the determination of conventional hydride-forming elements.15 In this manner, the relative detection limits can be enhanced by several orders of magnitude. It remains to optimize the generation efficiency, possibly through application of micelle-mediated reactions,16 and to extend the field of application to additional transition elements.
Received for review December 29, 1995. Accepted March 21, 1996.X AC951259G X
Abstract published in Advance ACS Abstracts, May 1, 1996.
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