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Anal. Chem. 1988, 58, 1140-1143
this regard. Improved fiber optic ammonia sensors are currently being developed based on new sensor designs and different indicators.
ACKNOWLEDGMENT We wish to thank John Peterson for his assistance with this project. Registry No. NH,, 7664-41-7;p-nitrophenol, 100-02-7.
LITERATURE CITED
'
(1) Peterson, J. I.; Vurek, G. G. Sclence (Washington,D . C . ) 1984,224, 123-127. (2) Seitz, W. R. Anal. Chem. 1984, 5 6 , 16A-34A. (3) Luebbers, D. W.; Opitz. N. 2.Naturforsch., C.: Biochem., Biophys., BIOI. VlrOl. 1975,30C,532-533. (4) Peterson, J. I.; Goidstein, S. R.; Fitzgeraid, R. V.; Buckhold, D. K. Anal. Chem. 1980, 5 2 , 864-869. (5) Saari, L. A.; Seitz, W. R. Anal. Chem. lS82, 5 4 , 823-824. (6) Zhujun, 2.; Seitz, W. R. Anal. Chlm. Acta 1984, 160, 47-55. (7) Zhujun, 2.; Seitz, W. R. Anal. Chlm. Acta 1984, 760, 305-309. (8) Opitz, N.; Luebbers, D. W. Adv. Exp. Med. Biol. 1984, 769, 907-912.
(9) Kirkbright, 0. F.; Narayanaswamy, R.; Welti, N. A. Analyst (London) 1984, 109. 1025-1028. (IO) Saari, L. A.; Seitz, W. R. Anal. Chem. 1983, 5 5 , 667-670. (11) Peterson, J. I.; Fltzgeraid, R. V.; Buckhold, D. K. Anal. Chem. 1984, 5 6 , 62-67. (12) Arnold, M. A. Anal. Chem. 1985, 578 565-566. (13) Freeman, T. M.; Seitz, W. R. Anal. Chem. 1977, 5 0 , 1242-1246. (14) Uwira, N.; Opitz, N.; Luebbers, D. W. Adv. Exp. Med. Biol. 1984, 769, 913-021. (15) Guiiiani, J. F.; Wohitjen, H.; Jarvis, N. L. Opt. Leff. 1983, 8 , 54-64. (16) David, D. J.; Willson, M. C.; Ruffin, D. S. Anal. Leff. 1976, 9 , 389-404.
RECEIVED for review November 18,1985. Accepted January 2, 1986. T. J. Ostler is visiting from the Department of Chemistry, Randolph-Macon Woman's College, Lynchburg, VA, as a University of Iowa, Department of Chemistry, 1985 Summer Undergraduate Research Fellow. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This work was also partially supported by a Northwest Area Foundation Grant of Research Corporation.
Hydride Generation Atomic Absorption Determination of Selenium in Marine Sediments, Tissues, and Seawater with in Situ Concentration in a Graphite Furnace S. N. Willie, R. E. Sturgeon,* and S.,S. B e r m a n Analytical Chemistry Section, Division of Chemistry, National Research Council of Canada,] Ottawa, Ontario, Canada KIA OR9
Methods are descrlbed for the determinatlon of total Se in marlne tissues and sediments and Se( I V ) in seawater based on the generation of SeH, uslng NaBH, with its subsequent trapping in a graphite furnace at 600 'C. Cailbratlon is achleved with a slmpie aqueous working curve. The absolute detection ilmit Is 70 pg with a concentration detection limit ( 3 4 of 1.4 pg/g. Corresponding precision of 5-10% Is typical for analyses of these samples. Results are reported for the determination of Se in a sulte of marlne reference materials.
Several sensitive methods are available for the determination of Se in environmental samples including fluorometry (I), gas chromatography (2),neutron activation analysis (3), and various atomic absorption spectrometric (AAS) procedures (4-8). Of these, hydride generation AAS has become a well-established technique (9) because of its high sensitivity and simplicity. Three types of atomizers are currently used in conjunction with hydride generation systems (IO): an argon (or N2)/hydrogen diffusion flame, a flame-in-tiihe, and an externally heated quartz (or graphite) tube, with the latter being the most popular (e.g., ref 11). Improved detection limits may be relized by collection and concentration of hydrides prior to their introduction into the atomization cell. Cryogenic condensation in a U-tube im'NRCC No.25377. 0003-2700/86/0358-1140$01.50/0
mersed in a liquid N2 trap (12) has proved extremely useful both for speciation and concentration determinations. Trapping SeH, in AgN03 solution followed by injection of discrete aliquots of this solution has also been utilized (13). A considerably simpler and more elegant approach is the use of a graphite furnace as both the hydride trapping medium and atomization cell (14-1 7). Although the mechanisms by which trapping and atomization occur are not understood as yet, this arrangement has been used for the detemination of As (14), Sb (16,17), and Bi (15,16) in environmental samples, and its application to Se appears obvious. This study reports on the application of such in situ trapping to the determination of Se in seawater as well as marine biological tissues and sediments by hydride generation-graphite furnace AAS. EXPERIMENTAL SECTION Apparatus. Atomic absorption measurements were made with a Perkin-Elmer 5000 spectrometer fitted with a Model HGA 500 graphite furnace and Zeeman effect background correction system. Peak absorbance signals were recorded with a Perkin-Elmer PRS-10 printer-sequencer. A Se electrodeless discharge lamp (Perkin-Elmer Corp.) operated at 6 W was used as the source. Absorption was measured at the 196.0-nm line. The spectral band-pass was 0.7 nm. Standard Perkin-Elmer pyrolytic graphite-coated tubes were used in all studies. A custom-made Pyrex cell (17)was used to generate SeH,. The internal purge gas supply line to the furnace was routed through a stopcock made of Teflon that permitted the operator to select gas flow into either the bottom of the hydride cell or into the furnace, as illustrated in Figure 1. In this manner an Ar flow could be used to strip the generated hydride from solution and carry it out the top of the cell where
Published 1966 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
A
Table I. Furnace Program internal program generationcollection
1
step temp, O C 1 2
3
n
Figure 1.
atomization
H
IJ
INTERNAL PURGE
Schematic of gas distribution system.
it was directed, via a 1 mm i.d. X 1.5 mm 0.d. quartz tube, into the sample introduction port of a preheated furnace. Prior to use, the cell and transfer line were silylated to deactivate the internal surfaces (18). A 10% solution of dimethyldichlorosilane (Pierce Chemical Co.) in toluene was used to rinse the cell, followed by successive rinses in toluene and methanol. The surfaces were then dried at room temperature in a stream of Nz. Sodium borohydride solution was pumped into the cell using a rack-mounted Ismatec peristaltic pump (Cole Parmer Instrument Co.). Reagents and Standards. Stock solutions (1000 mg/L) of Se(1V) and Se(V1) were prepared by dissolution of NazSeOBand NazSeOl (BDH Chemicals, L a . , Poole, England) in 1M HCl and 1 M HN03, respectively. Working standards of lower concentration were prepared by serial dilution of the stocks using distilled, deionized water (DDW). High-purity subboiling distilled HC1, "OB, and HClO,, prepared in-house, were used for sample decompositions. A 1% (m/v) solution of NaBH4 (Alfa Inorganics) was prepared daily, or more frequently if required, by dissolution of 0.25-g pellets in DDW. Several marine samples were analyzed for Se, including National Research Council of Canada (NRCC) open ocean seawater (NASS-l), NRCC marine sediment (BCSS-l), NRCC lobster hepatopancreas (TORT-l), and NBS oyster tissue (SRM 1566). Procedures. All sample and analytical manipulations were conducted in a routine laboratory environment. Aliquots of NASS-1 (20-50 mL) were transferred directly to the hydride cell and acidified to 0.5 M HCl (reagent grade) for analyses of Se(1V). Nominal 0.5-g samples of BCSS-1 sediment were decomposed by acid digestion in a PTFE bomb according to the procedure described in ref 19 and diluted to 50 mL in 1 M HCl. Total Se was determined by using 500-~Laliquots delivered into the hydride cell containing 5 mL of 0.5 M HC1. Nominal 0.5-g samples of TORT-1 and SRM 1566 were dry ashed according to ref 20 using Mg(N0J2 as an ashing aid to prevent volatilization losses of Se (21). The solution was diluted to 50 mL. Total Se was determined by using 5O-wL aliquots diluted to 5 mL with 0.5 M HCl. Reagent blanks were processed through identical steps for decompositions of sediments and biological tissues. The sequence of operations describing SeH, generation, collection, and atomization is given below. During collection the stopcock was closed to direct internal purge gas through the hydride cell, and the furnace was preheated for 10 s at 600 OC. The NaBH, solution was pumped into the cell at a rate of 4 mL/min for 30 s for 5-mL samples (60 s for 50-mL seawater samples), during which time the SeH, was swept, via the generated H2gas stream (=50 mL/min under these conditions), into the furnace where it was trapped. Internal purge gas flow was automatically initiated at the end of NaBH4 addition, and the cell purged for 120 s at a flow rate of 100 mL/min for 5-mL sample volumes (190-210 s for 50-mL seawater samples). A t the end of this cycle, thermal programming of the furnace was terminated, the quartz transfer line removed from the sample introduction port, and the stopcock opened to permit internal purge gas to flow through the furnace. The sample was then atomized at 2600 OC using maximum power heating and internal gas stop, followed
1141
2
1 2
purge,
time, s
mL/min
10 30
0 0 100
600 600 600
120
2600
4
0
2700
2
300
by a cleaning cycle at 2700 OC with 300 mL/min internal purge gas flow. The furnace program is shown in Table I. Internal purge gas was again diverted through the reaction cell, which was emptied and rinsed with DDW. The NaBH, solution was withdrawn from the injector tip by reversing the direction of the peristaltic pump. The next sample aliquot was then added to the cell and the measurement process repeated. Replicate measurements could be made every 3-4 min. Standard calibration curves prepared from spikes of Se(1V) added to 5 mL of 0.5 M HC1 were constructed and used for sample analysis.
RESULTS AND DISCUSSION Optimization of Signal. In contrast to SbH, (In,SeHz could not be introduced into the preheated furnace using the internal purge gas lines. It was completely removed from the system by deposition on the metal components of the furnace housing and the tube contact rings. It was thus necessary to deliver the hydride directly from the generation cell into the furnace tube via its sample introduction port. The ease with which SeH, was sequestered from the gas phase permitted the use of pyrolytic graphite-coated tubes. A large absorber surface area and/or significant concentration of active surface sites required for SbH3 deposition (17) was unnecessary for Se. Glassy carbon tubes were found to be equally satisfactory for collection of selenium, revealing that neither the surface area nor surface-active site concentration influenced the deposition process. Typical lifetime of a graphite tube exceeded 1200 firings and permitted "used" tubes, not suitable for other analyses, to be put to good service. Approximately 75% of the optimum Se signal was recovered by using a furnace "trap" temperature of 100 "C. Recoveries rose to 100% a t 400 "C. At 700 "C signal recovery dropped to =go%, and at 900 "C recovery was still =75%. The danger of explosive combustion of Hz exists when the furnace trap temperature exceeds 700 "C. Trapping can be safely accomplished if the cell and furnace are purged with Ar for 20 s at a flow rate of 150 mL/min prior to raising the trap temperature and generating SeH,. Atomization temperatures above 2400 "C (maximum power heating) gave constant, optimum response. Generation of SeH, at HCl concentrations above 0.3 M was quantitative. A 0.5 M HC1 medium was found satisfactory for all samples used in this study and is in the range reported by other investigators (8). The generation cell could be used for approximately 130 sample determinations before resilylation was required. This is clearly marked by the onset of a decrease in the generation-transfer efficiency. It was found to be most expedient to resilylate the cell each day, prior to undertaking analyses. Resilylation was preceeded by briefly rinsing the cell with 5% HF solution. Analytical Blanks. Absolute blanks were assessed for each of the sample matrices. Although this was straightforward for sediment and biological materials, where a real reagent blank was processed with the samples, only a synthetic blank was available for seawater. For this purpose, 5 mL of DDW served as a "carrier" medium for the HC1 and NaBH4 reagents. Absolute blanks of 0.054 f 0.023 ng were obtained.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
Table 11. Analyses of Se(1V) in NASS-1 Seawater concn, ng/mL
run no. 1
4 5
mean
matrix
TORT-1,pg/g NBS 1566, pg/g
AU/ng
seawater 0.061 f 0.014 sediments 0.061 f 0.014 biologicals 0.061 f 0.014
0.024 f 0.002
LOD,” ng/g 0.0014 30 40
X
LODIb
8 (20) 10 (15)
5 (85)
linear range, ng 6 6 6
” Limit of detection defined as 3 times the standard deviation of
the procedural or method blank. bNumber in parentheses is concentration factor above LOD at which samde analvsis was run.
Table 111. Analytical Results”
NASS-1, ng/g BCSS-1, pg/g
precision, % (at a
sensitivity,
0.0214 0.0240 0.0236 0.0230 0.0273
2 3
sample
Table IV. Figures of Merit
determined
accepted value
0.024 f 0.002 0.46 f 0.04 6.74 f 0.17 2.13 f 0.10
(0.025 i O.OO1)b (0.43 f 0.06)c 6.88 f 0.47 2.1 & 0.5
Precision expression as standard deviation based on five determinations. buncertified value (17). Uncertified value (22).
It was verified that the DDW contributed insignificantly to the measured Se, and the blank thus represents contamination introduced via manipulation as well as from reagents. No attempt was made to purify the NaBH4 solution, although this is the most probable source of Se. (Assuming the NaBH4 pellets account for the total blank necessitates Se be present only at 1.4 pg/g as an impurity). Absolute procedural blanks for analysis of the remaining samples were BCSS-1, 13 f 5 ng; TORT-1, 13 f 7 ng; and SRM 1566, 13 f 7 ng. Blank corrections to the analytical results were acceptable, contributing 4% to the Se(1V) in seawater, 6% of the total Se in BCSS-1,0.4% of total Se in TORT-1, and 1% of total Se in SRM 1566. Analytical Results. Analytical results are summarized in Tables I1 and 111. Table I1 presents blank-corrected data for the determination of Se(1V) in NASS-1 seawater, illustrating the precision of replicate determination that can be achieved with this technique. It should be noted that a concentration of 0.02 ng/mL is the typical detection limit achieved with normal hydride generation techniques. Results for the determination of total Se in BCSS-1, TORT-1, and NBS SRM 1566 are given in Table 111. In all cases, calibration was achieved with a simple working curve prepared by generating SeH, from 5-mL aliquots of spiked (with Se(1V)) DDW. The accuracy of the method is evident from a comparison of the results with accepted or certified values of these materials. Interference Study. Interelement interference effects have been reported for the hydride generation of Se (9,11, 23, 24). The more commonly recognized species include transition metals of groups VI11 and IB, notably excesses of Cu, Fe, Ni, and Co as well as excesses of other hydride-forming elements such as Sn, Pb, As, Sb, Bi, Te, and Hg. Interferences have been found not to depend upon the analyte-to-interferent ratio but upon the concentration of interfering element in the sample solution (25). Analytical data presented in Table I11 substantiate the conclusion that no interferences are present in this study. Interference levels of Fe, Cu, Ni, and As selected for study were arbitrarily established by first identifying the “worst case” ratio of Se:interferent that would likely be encountered with this suite of samples and subsequently examining the effect of an interferent mass excess 50-fold greater than the above ratio. In this manner, the influence of 1000 pg of Fe, 6 pg of Cu, 15 pg of Ni, and 2.5 pg of As on the determination of 2 ng of Se in 5 mL of 0.5 M HC1 was undertaken and found to have no effect on the signal.
It should be noted that the above levels of interferents are considered tolerable; no attempt was made to establish the maximum interferent concentration levels. The advantage of using the furnace to trap the SeHz is that it eliminates the effects of those interferences causing variable rates of hydride evoluation. Additionally, the range of interference-free determination can probably be increased significantly when the hydride generation is undertaken in 5 M HCl instead of the 0.5 M used here ( I I , 2 4 ) . The methodology should therefore prove suitable for the trace determination of Se in a wide variety of biological and geological materials. Analytical Figures of Merit. Figures of merit are presented in Table IV. The absolute sensitivity, as determined from the slopes of calibration curves run in a number of tubes over several months, is 0.061 f 0.014 AU/ng (i.e., 70 pg/O.O044 AU), comparable to that obtained by direct injection of aqueous solutions. This suggests that the generationtrapping process is =loo% efficient. Estimated procedural detection limits, based on the variability of the blank (3a), are 1.4 pg/g of Se(1V) in seawater and 30-40 ng/g in sediments and biological samples and are adequate for all but the “cleanest” of samples (8). The estimated LOD of the measurement process itself (not procedural LOD) is 1.4 pg/g. Precision of determination is approximately 10% (relative standard deviation) on determinations 20-80-fold above detection limits. The linear working range spans 2 decades, extending to 6 ng, comparable to that reported by Piwonka et al. (8). Higher analyte concentrations are accessible by simply working with smaller sample aliquots or by introducing a purge gas flow during atomization.
CONCLUSIONS The combination of hydride generation with subsequent trapping in the graphite furnace provides a rapid, simple, accurate, and precise method for the determination of Se in a variety of environmental samples. Utilization of the furnace as an atomization cell is advantageous in that a single system can be used for both hydride samples and conventional aqueous samples. This technique should prove suitable for other hydride-forming elements and thus offers the potential for multielement capability. Registry No. Se, 7782-49-2; HzO, 7732-18-5; graphite, 778242-5.
LITERATURE CITED Koh, T.-S.; Benson, T. H. J. Assoc. Off. Anal. Chem. 1983, 6 8 , 918-926. Shimoishi, Y.; TBei, K. Anal. Chlm. Acta 1978, 100, 65-73. Knab, D.; Gladney, E. S. Anal. Chem. 1980, 52, 025-028. Carnrick, G. R.; Manning, D. C.; Slavin, W. Analyst (London) 1983, 108, 1297-1312. Julshamn, K.; Ringdal. 0.; Slinning, K.-E.; Braekkan, 0. R. Spectrochlm. Acta, Part 8 1982, 378,473-482. Verlinden, M.; Deelstra, H.; Adriaenssens, E. Talanta 1981, 2 6 , 637-846. Raptis, S. E.; Kaiser, G.; Tolg, G. Fresenius’ Z . Anal. Chem. 1983, 316, 105-123. Piwonka, J.; Kaiser, G.; Tolg, G. Fresenlus’ 2. Anal. Chem. 1985, 321, 225-234.
Anal. Chem. 1986, 58, 1143-1140 (9) W e n , R. G.; Thornerson, D. R. Analyst (London) 1080, 705. 1137-1156. ( I O ) Dedina, J. Fortschr. Atomspektrom. Spurenanal. 1084, 7 , 29-47. (1 1) Meyer, A.: Hofer, Ch.; Knapp, 0.;Tag, G. Fresenlus ' 2.Anal. Chem . 1081, 305, 1-10,
(12) Andreae, M. 0.; A s m e , J.-F.; Foster, P.: Van't dack, L. Anal. Chem. 1081, 5 3 , 1766-1771. (13) Branch, C. H.; Hutchinson, D. Analyst(London) 1085, 770, 163-167. (14) Drasch, G.;Meyer, L. V.; Kauert, 0. Fresenlus' Z . Anal. Chem. 1080, 304, 141-142. (15) Lee, D. S. Anal. Chem. 1982, 5 4 , 1682-1686. (16) Brovko, I. A.; Tursunov, A,; Rlsh, M. A.; Davlrov, A. D. Zh. Anal. Khim. 1084, 39, 1768-1772. (17)Sturgeon, R. E.; wiiiie, s. N.; Berman, s. s. Anal. chern.1085, 5 7 , 2311-2313.
1143
(18) Reamer, D. C.; Veillon, C.; Tokousbalides. P. T. Anal. Chem. 1081, 5 3 , 245-248. (19) Slu, K. W. M.; Berman, S. S. Anal. Chern. 1083, 5 5 , 1603-1605. (20) Siu, K. W. M.; Berman. S.S. W a n t s 1084, 3 7 , 1010-1012. (21) Reamer, D. C.; Veillon, C. Anal. Chem. 1981, 5 3 , 1192-1195. (22) de Oliveira, E.; McLaren, J. W.; Berman. S.S.Anal. Chern. 1083, 55,
2047-2050. (23) Vijan. P. N.; Leung, D. Anal. Chlm. Acta 1080, 720. 141-146. (24) Welz, 6.; Melcher, M. Analyst (London) 1084, 709, 569-572; 577-579. (25) Welz, 6.; Melcher, M. Vom Wasser 1084. 62, 137-148.
I~C!EIVF,D for review September 17,1985. Accepted November 12, 1985.
Comparison of Furnace Atomization Behavior of Aluminum from Standard and Thorium-Treated L'vov Platforms Thomas W. Brueggemeyer* and Fred L. Fricke Elemental Analysis Research Center, U S . Food and Drug Administration, 1141 Central Parkway, Cincinnati, Ohio 45202
A standard pyrolytlc graphite L'vov platform was compared to one treated by soaking In a 10% thorlum nitrate solution. The half-wldhs d fwnace atomk absorptbn peak profiles for Ai were reduced by as much as a factor of 4 by the modlfled surlace, and the maxlmum pennWble drarrlng temperature was extended by more than 800 O C . The peak area preckh (1.5% relative standard devlatbn for 200 pg of AI) and sensltivHy were comparable to those obtained with untreated platforms. Thls behavlor was stlli observed afler 500 flrlngs of a treated platform at 2450 O C .
Improvements in trace-metal analyses using graphite furnace atomic absorption spectrometry (GFAAS)have been hindered by the lack of fundamental knowledge concerning the high-temperature chemistry taking place in the furnace itself. Due to this uncertainty many analysts have resorted to an empirical approach in their attempts to modify the behavior of analyte in the furnace. Such research necessarily has a trial-and-error appearance-various reagents and conditions examined until success is reached. This has been particularly true in the area of graphite atomizer surface modification. The treatment of graphite with various metals or solutions of their salts has produced new surfaces exhibiting properties markedly different than those of the original graphite. The chief pattern that has emerged is the fact that most metals used to modify graphite are known to form metal carbides. It is not known whether the carbide-forming metals not reported in the literature as surface modifiers have yet to be tried or have been found ineffective. The most commonly reported advantage of a metal-treated surface is an increase in analyte sensitivity (1-8). I t should be pointed out that in some instances various authors explicitly mentioned that peak heights were used in preference to integrated peak areas or did not mention which mode of quantification was used. It is possible in some of these cases that the reported sensitivity improvement for peak heights was in fact a peak sharpening, leaving integrated area un-
changed. Theory (9) predicts that integrated peak area is the better indicator of the extent of free atom formaton. It should also be pointed out that most of the reported sensitivity enhancements were in comparison to nonpyrolytic graphite tubes, which have now been largely supplanted by pyrolytically coated graphite. In some instances the advantages indicated might not have been realized over the newer graphite. The benefits of metal-modified graphite surfaces have extended beyond signal enhancement. It was reported by Norval et al. (10) that a tungsten- or tantalum-treated surface resulting from sputtering of the metal led to an improved resistance of the graphite surface toward oxidizing acids. A similar result was found by Sotera et al. (11) for a borontreated surface. Vickrey et al. found that a zirconium carbide surface allowed organolead (12) and organotin (13) compounds to be determined by using aqueous standards in the furnace while this could not be accomplished on normal graphite. It was reported by Thompson et al. (14) that a La treatment of graphite led to reduced matrix interferences in the determination of P b and Cd, while Hodges (15) stated that a phosphate-induced molecular absorption signal was greatly lowered through the use of a molybdenum-treated surface. In view of the somewhat tentative nature of the proposed mechanisms for the atomization of metals from graphite surfaces, it is understandable that little work has been reported dealing with the modification of these mechanisms upon going to metal-treated surfaces. Muller-Vogt and Wend1 explained that for both Si ( 4 ) and Sn (16) a role of the metal-treated surface may be to inhibit the carbon-induced formation of volatile metal suboxide species. Greater sensitivity and charring stability are thus afforded. Wahab and Chakrabarti (17, 18) discussed the mechanism for Y atomization from a graphite surface modified by Ta, La, and Zr, in addition to investigating atomization from a metal foil surface. The most frequently made mechanistic statement concerning the use of metal carbide atomization surfaces is that the metal additive inhibits the formation of analyte-carbide refractory compounds by preferentially forming these carbides itself. When the analyte is not a known carbide former, however, the role of the surface modifier becomes less clear.
This article not subject to U S . Copyrlght. Published 1986 by the American Chemical Society
