Anal. Chem. 1988, 60,733-736
the one hand, the sensitivity of the measurement is high because of the large change in electrical conductivity with temperature for aqueous samples. On the other hand, two factors lead to degradation of precision. First, electrical conductivity is not usually measured with high precission. The major difficulty in conductivity measurements, in fact, is temperature sensitivity. Some of the low-frequency temperature drift which plagues electrical conductivity measurements should be attenuated at the low audio frequencies employed in the TMEC measurements. I t is expected that higher modulation frequencies, employed with narrower gap electrodes, will greatly improve the precision of the measurement. A second source of reduced precision in TMEC measurements is a large background signal associated with illumination of the electrodes by stray laser light. This phenomenon is similar t o that observed in photoacoustic spectroscopy. As in photoacoustic spectroscopy, minimization of the background signal will result from incremental improvements in the laser beam quality and shielding of scattered laser light. One potential solution is to utilize an electrodeless conductivity detector (22). Applications of the thermally modulated electrical conductivity detector will be found whenever high sensitivity measurements of absorbance must be made, particularly in aqueous samples. One advantage of the technique is the small volume in which the measurement may be performed. The volume between the two electrodes employed in our crude instrument is on the order of 10 nL. At the detection limit, only 2 fmol of analyte is present between the electrodes. Improvements in the electrode configuration should produce detection volumes a t least an order of magnitude smaller. Applications of the high sensitivity detector may be envisioned in both capillary liquid chromatography and capillary zone electrophoresis. In particular, the latter technique has been combined with minaturized conductivity detectors ( 2 3 , 2 4 ) . I t should be possible to introduce a laser beam between the electrodes for simultaneous absorbance and conductivity detection in capillary zone electrophoresis. Additional sensitivity for the thermally modulated electrical conductivity may be possible by taking advantage of the shift in equilibrium with temperature for weak acids a t room temperature. Both carbonic acid ( k , ) and boric acid undergo a 2% change in ionization constant per degree (15) and the autoprotolysis constant for water undergoes a 6% increase per degree temperature rise a t room temperature (25). It should be possible to double the sensitivity of the TMEC measurement by utilizing an appropriate buffer, hence combining ionization and conductivity temperature dependence of the solvent system. TMEC forms the first example of a potentially very large class of absorbance determinations based upon thermal modulation of electrochemical processes. For example, the
733
potential of a half-cell undergoes a relative change with temperature which is inversely proportional to the absolute temperature; a t room temperature, the relative change in potential of a cell is about 0.3% per degree, roughly comparable to thermooptical techniques. Either the absorbance associated with colored material in solution or adsorbed directly upon the electrode could give rise to the thermal modulated signal. Application with microelectrodes should result in high-precision, low-volume measurements of both electrochemistry and absorbance. Thermooptical measurements based upon mirage spectroscopy and the gradient-sensitive technique have been performed a t electrodes (26,271. In these experiments, the electrochemical process perturbs the thermooptical signal. However, in thermal modulation of electrochemical processes, the thermal changes associated with absorbance would be related to a perturbation in the electrochemical measurement. Since only a single light beam is employed in the thermal modulation of electrochemical processes, the technique should demonstrate greater simplicity compared with those thermooptical techniques which utilize two laser beams.
LITERATURE CITED Pardue, H. L.; Rodriguez, P. A. Anal. Chem. 1987, 3 9 , 901-903. Kaye, W. Anal. Chem. 1981, 5 3 , 369-374. Dovichi, N. J.; Martin, J. C.; Jett, J. H.; Trkuia, M.; Keiier, R. A. Anal. Chem. 1984, 56, 348-354. Hirschfeid, T. Appl. Spectrosc. 1977, 3 1 , 238. Briimyer, G. H.; Bard, A. J. Anal. Chem. 1980, 5 2 , 685-691. Kreuzer, L. B. Anal. Chem. 1974, 4 6 , 235A-244A. Rosencwaig, A. Anal. Chem. 1975, 4 7 , 592A-604A. Harris, J. M.; Dovichi, N. J. Anal. Chem. 1980, 5 2 , 695A-706A. Whinnery. J. R. Acc. Chem. Res. 1974, 7 , 225-231. Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier, D. Appl. Opt. 1981, 2 0 , 1333-1344. Dovichi, N. J. CRC Crif. Rev. Anal. Chem. 1987, 17, 357-423. Dovichi, N. J.; Keiler, R. A. Anal. Chem. 1983, 5 5 , 543-549. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 5 3 , 106-109. Nolan, T. G.; Dovichi, N. J. I€€€ Circuits Devices Mag. 1988, 2 , 54-56. CRC Handbook of Chemisfry and Physics, 51st Ed.; Weast, R . C., Ed.; CRC Press: Boca Raton, FL, 1970; Sections D and F. Dovichi, N. J.; Harris, J. M. Proc. S P E - l n f . SOC. Opt. Eng. 1981, 288, 372-375. Nolan, T. G.; Weirner. W. A.; Dovichi, N. J. Anal. Chem. 1984, 56, 1704-1707. Brasweii, E. H. J . Phys. Chem. 1984, 88. 3653-3658. Rabinowitch, E.; Epstein, L. F. J . Am. Chem. SOC.1941, 6 3 , 69-78. Brasweii, E. J. Phys. Chem. 1988, 7 2 , 2477-2483. Wetsei, G. C.; Stotts, S . A. Appl. Phys. Lett. 1983, 4 2 , 931-933. Shaw, R.; Light, T. S . ISA Trans. 1982, 2 1 , 63-70. Huang. X. H.; Pang, T. K. J.; Gordon, M. J.; Zare, R. N. Anal. Chem. 1987, 5 9 , 2747-2749. Mikkers, F. E. P.; Everaerts, F. M.; Verheggen. Th. P. E. M. J . Chromafogr. 1979, 169, 1-10, Light, T. S.;Licht, S. L. Anal. Chem. 1987, 5 9 , 2327-2330. Tarnor, M. A.; Hetrick, R. E. Appl. Phys. Lett. 1985, 4 6 , 460-462. Pawiiszyn, J.; Weber, M. F.; Dignarn, M. J.; Mandeiis, A,; Venter, R. D.; Park, S . U. Anal. Chem. 1988, 5 8 , 239-242.
RECEIVEDfor review August 3,1987. Accepted November 24, 1987. This work was funded by the National Sciences and Engineering Research Council of Canada.
Determination of Selenium and Tellurium in Copper Standard Reference Materials Using Stable Isotope Dilution Spark Source Mass Spectrometry E. S. Beary,* P. J. Paulsen, and G. M. Lambert
National Bureau of Standards, Center for Analytical Chemistry, Inorganic Analytical Research Division, Gaithersburg, Maryland 20899 The levels of certain trace elements in high-purity copper, including selenium (Se) and tellurium (Te), are important because even small variations can significantly alter such
properties as conductivity and malleability. In the middle 1970s, an isotope dilution spark source mass spectrometric (IDSSMS) procedure was developed for the certification of
This article not subject to US. Copyright. Published 1988 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
Se and T e in unalloyed copper Cu(0) through Cu(II1) (1). This method involved the reduction of Se4+,Te4+,and a n added gold (Au2+)carrier t o the zero oxidation state with H(H2P02). Se is reduced with the T e in this procedure and was, therefore, determined simultaneously. The analytical procedure reported here was modified t o enhance recoveries and t o minimize contamination. Difficulties in Se determinations, particularly at low levels, are well-known. The volatility of Se as Se02.2HC1 and H2Se03 can be a serious analytical problem (2). A 75Setracer was used t o evaluate dissolution techniques and t o address the possibility of Se losses due to volatility. Other researchers have used 75Seto assess volatility problems and separation schemes (3, 4 ) . A modified Carius tube technique (closed system) dissolution and equilibration was applied in this work. This procedure was compared with a n open system dissolution of the same standard reference material (SRM).
EXPERIMENTAL SECTION Apparatus. In the “open system” dissolution, beakers made of Teflon FEP resin with lids made of Teflon TFE resin were used. A modified Carius tube was used for the “closed system” dissolution. These thick-walled (3-mm) digestion tubes were fabricated from borosilicate glass and annealed a t 560 “C. The tube had a narrow neck about 3 cm long. The inside diameter of the neck is 0.7 cm and the outside diameter is 0.9 cm. The base of the tube is 20 cm long with an inside diameter of 1.3 cm and an outside diameter of 1.9 cm. These digestion tubes were sealed and placed into steel shells ( 5 ) . The centrifuge tubes used to collect the reduced Se and Te were standard 15-mL Pyrex tubes. The isotope ratios were measured with a JEOL JMS-OlBM-2 spark source mass spectrometer using an ion multiplier in the current integration mode. Reagents. NBS high-purity HzO,“OB, HC1, and HF reagents were used (6). H(HzPO2)ACS analytical reagent was used in the reduction step for those SRMs in which the Se level was >10 pg/g. A purified H(HzPOZ)/HClreduction mixture was used for the lower level SRMs. This purified reagent was prepared by adding HCl to the reagent grade H(HzPOz)and Au in solution to reduce and coprecipitate possible contaminants. The %e and lzsTe enriched isotopes were obtained from Oak Ridge National Laboratory (ORNL) as the elements, dissolved in high-purity HNO, and stored as a 0.5 N H N 0 3 solution. The radioactive isotope 75Seused in the tracer study was obtained from Amersham Co. as NazSeOBand diluted so that 100 p L of the working solution gave the desired radioactivity. Tracer Study. The original purpose of this tracer study was to develop a chemical separation suitable for the determination of Se by thermal ionization mass spectrometry (TIMS) (7). Information about the oxidation state of Se during dissolution was of interest since equilibration is best assured when the sample is fully oxidized. In addition, the problems of the volatile losses of Se were addressed. The first tracer experiment was designed to determine if higher temperature (150-200 “C) dissolution/evaporation causes appreciable volatile Se losses. About 100 pL of the 75Seworking solution was added to each of two glass beakers with a mixture of 2 g of HNO, and 0.1 g of HC104, and the beakers were then covered. Both beakers were heated for about 6 h to simulate the time involved in a “real” sample dissolution. Wet oxidation at 150-200 “C and at 85-90 “C with 75Sedemonstrated that there were no detectable losses of Se from either sample under these conditions. The sample that had been heated at the higher temperature was then evaporated at temperatures about 25C-300 “C (the boiling point of HC104 is 200 “C). This sample lost 95% of the added 76Seduring this evaporation. The second sample was evaporated by heating to about 85 “C to remove the H N 0 3 and then the temperature was raised until the HC104 fumed off (about 200 “C). This evaporation recovered about 90% of the Se tracer. Thus, the covered dissolution procedures resulted in no substantial Se losses, and, since isotopic equilibration occurs during dissolution, these results were encouraging for low-temperature dissolutions. Care must be taken when the sample solution is evaporated in order to prevent significant Se losses.
Minor losses during evaporation (after equilibration) would not alter IDMS results. A second tracer study was performed in an effort to determine the oxidation state of the Se after dissolution @e4+or Se6+),since selective losses of isotopes and/or species were possible. From ion exchange separation it was apparent that the Se in the 75Se tracer was in two different forms or oxidation states. The first experiment employed the 75Seworking solution in a HN03/HC104mixture. The solution was covered and heated at 85 “C for 6 h and evaporated. The oxidized tracer was then added to an anion column to separate the Se4+and Se6+. The first portion @e4+) contained about 70% of the added Se. Nineteen percent of the Se as Se6+ was found in the second portion. The second experiment was identical with the first, except that it was heated to fuming HCIOl for 6 h. It was uncovered and heated to evaporate the acids. Fifty-five percent of the Se was found as Se4+and 36% as Se6+. In both samples the Se losses (-10%) occurred during the evaporation of HC104, which is after isotopic equilibration in an IDMS experiment. As a result of these experiments, harsh evaporations were avoided for all Se determinations. In addition, the Carius tube was used which promotes oxidation/equilibration because of the high temperature/ pressure conditions achieved by this technique. The closed system also precludes volatile losses of Se prior to equilibration. Sampling and Dissolution: “Open System” Analyses. Samples ranging from 0.25 to 1 g of each SRM were taken depending upon chip size and the concentration level of Se and Te expected. Sufficient spike (a stable enriched isotope) was added to alter the %e/%e ratio from 0.18 to 1.6. ‘ q e was added to alter the ‘?I’e/’qe ratio from 0.21 to about 2.3. These samples were dissolved in covered Teflon FEP beakers using 10 g of HzO, 5 g of “OB, and 1 g of H F (7). Although the initial dissolution was rapid, samples were heated for 48 h to promote isotopic equilibration. Care was taken to keep the temperature near 85 “C during evaporation to minimize volatization of Se as noted in the tracer study. “Closed Systems” Dissolution Procedure. Se in Cu(XI), SRM 454, was determined with all critical steps including dissolution and equilibration carried out in sealed Carius tubes (5). These included the spike calibration as well as dissolving and equilibrating the spiked SRM 454. With this experimental design we could show that IDMS results were not biased due to Se volatility or nonequilibration. The general procedure consisted of adding each component stepwise to a heavy-walled Carius tube. The ?3e spike, 4 g of concentrated “OB, and 6 g of HzO were added to each tube and the contents were frozen ( 5 ) . Next, 0.25-g samples of SRM 454 were added to each sample tube. All tubes were then sealed by use of a gas/oxygen torch. Since the contents of the sample tubes were frozen until the tubes were sealed, no chemical reaction occurred between the Cu and the “OB, thus preventing any possibility of Se losses. The tubes were placed in a steel shell, pressurized with 50 cm3 of solid COz, closed by use of a steel cap with a Cu gasket, and heated to 185 “C for 16 h (6). This system permits complete equilibration of the spike and natural Se isotopes without any possibility of Se losses. After cooling, the contents were again frozen, the tubes were opened, and the contents further processed by using the same techniques as with the “open system” samples. Reduction of Se and Te. After dissolution, both sets of samples (“open system” and “closed system”) were uncovered and evaporated gently. The samples in which the Se levels were >10 pg/g were redissolved in 30 g of 5 N HC1, and 25 pg of Au solution was added. Five grams of H(HzPOz)was added and the samples were warmed. The lower level Se samples were redissolved by using 15 g of 5 N HCl followed by 25 pg of Au in solution and 20 g of the purified H(H2POz)/HCl mixture. The resulting concentration of the reduction mixture for the lower level Se was the same as for the higher levels. Hypophosphorous acid is used in the reduction because it selectively reduces Se, Te, and the added Au (in addition to a few other easily reduced metals) to the zero oxidation state without reducing the copper matrix to Cuo. The copper is reduced from Cu2+to Cu+ but remains in solution. Complete separation from the Cu matrix is essential since 65Cu2+interferes with ‘,@I’e, the major natural Te isotope.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
Table 111. Se and Te in SRM C1251a (Cu(VIII)), p g / g
Table I. Se in SRM 454, Cu(X1) ( p g / g ) Carius tube
'open" beaker
481 478 473 491 476 478
483 481 476 474
480 6.2
479 4.2
bottle 1 2 3 4 4
2
av SQ Os,
Se
Te
composite sample a composite sample b chill cast cross section 1-X chill cast cross section 12-X
11.44 11.52 11.63 10.68
14.5 14.8 14.9 15.0
av
11.32 0.43
14.8 0.22
S
Table IV. Se in SRM 394 (Cu(I)), p g / g
standard deviation.
bottle 1 3 4
previous analysis
Cu(1) Cu(II1)
Se
n
S
2.00 0.62
4 4
0.12 0.02
1.86 1.90 1.87 1.82 1.87 1.91
2
Table 11. Se in SRMs 394 (Cu(1)) and 496 (Cu(II1)) ( p g / g ) (1977)
735
5
present work Se n s 1.87 0.59
6 9
6
0.03 0.01
When these mixtures are heated at 80-90 "C for 1 h, the Au, Se, and Te are reduced and coprecipitated with the formation of relatively large particles of Au. Without the Au, low levels of Se and T e would form colloidal mixtures that are impossible to separate by centrifugation and difficult to manipulate. CAUT I O N Extreme care must be taken when using H(HZPO2). Explosive metal phosphites may form if the solution evaporates to dryness (8). Collection of t h e Reduced S e a n d Te. The Au-Se-Te precipitates were HzO washed and centrifuged a t 2500 rpm for 2 min and collected with an added 50 mg of 99.9999% Au powder, 100 mesh. Centrifugation eliminated the problems and possible contamination caused by filtration of the precipitates. The washed Au-Se-Te precipitates were oven dried a t 80 "C for 2 h.
INSTRUMENTAL ANALYSIS Each Au-SeTe precipitate served as the electrode material for instrumental analysis. The precipitate was homogenized with an added 50 mg of high-purity Au powder in a stainless steel (Fe-Cr) .vial using a 0.64 cm diameter stainless steel (Fe-Cr) ball bearing for 3 min on a Wig-L-Bug mixer. The homogenized sample was then pressed into a disk approximately 5 mm X 5 mm X 0.2 mm according to the procedure previously reported (9). The pressed Se-Te-Au disk, was cut in half and loaded into the source of the SSMS. The system was pumped so that the source vacuum was less than 2 X Torr (3 x Pa), and the ratios of '%efmSe and 'WTef130Te were measured as the singly charged ion using magnetic peak switching controlled by a Hall probe gaussmeter. Three sets of 25 ratios were measured for both elements. Each measurement set required approximately 30 min. The concentrations of Se and T e were calculated from the measured ratios by using the isotope dilution equation (9). RESULTS AND DISCUSSION IDSSMS has been successfully applied to the simultaneous determination of Se and T e in copper benchmarks. The good agreement between "open beaker" and the sealed Carius tube dissolution in the analysis of SRM 454 demonstrated that losses of Se do not occur under the conditions employed (Table I). The good agreement between the Cu(1) and Cu(II1) analyzed in 1977 (1) and the present work also demonstrate analytical control (Table 11). These separate analyses involved the use of different spikes, calibrations, reagents, and sample bottles, as well as variation in the sample collection procedure after precipitation. Careful monitoring of the blank in all analyses eliminates the possibility of contamination problems. Blanks were measured and blank corrections were made for
Table V. Se in Copper Benchmarks
SRM
(pg/g)
IDSSMS value Se n s
updated certified value, April 1986
Se
Cu(1) Cu(II1) Cu(II1) Cu(1V) Cu(VI1)
1.87 0.59 0.58 3.98 210
6 6 3 9 6
0.02 0.06 12
C1251, Cu(VII1) C1251a, Cu(VII1)
11.26 11.32
4 4
0.48 0.43
11.4
C1252, Cu(1X) C1252a, Cu(1X)
53.71 59.13
4 4
0.33 1.14
53.6 f 1.5
394, 396, 496, 457, 400,
0.03
0.01
2.00 f 0.15" 0.62 f 0.04 0.62 f 0.04 4.2 f 0.2 214 f 10
+ 0.8
b b
C1253, Cu(X) C1253a, Cu(X)
167 142
4 4
5 2
164 f 6
454, Cu(X1)
479
4
4
479 f 16
b
Estimated uncertainty is based on statistical analyses of the data obtained from at least two independent analytical methods and uncertainties associated with their measurement systems. *Reserve stock.
all samples analyzed. The blanks associated with samples having a Se and T e concentration >10 pgfg were found t o be less than 0.5% of the measured concentration. The blanks associated with Cu(I), Cu(III), and Cu(1V) (all Se concentration
