Anal. Chem. 1088, 60, 465-472 (24) Davis, Willlam D. Environ. Sci. Techno/. 1077. 1 1 , 587. (25) Karmen, Arthur In Advances in Chromatography;Giddlngs, J. C., Keller, R. A., Eds.; Marcel Dekker: New York, 1966; Vol. 2. pp 293-336. (28) Svojanovsky, V.; Krejcl, M.: Tesarlk, K.; Janak, J. In Chromatographlc Reviews ; Lederer, Michael, Ed.; Elsevier: Amsterdam, 1986; Vol. 8, pp 151-157. (27) Gudzinowicz, B. J. In The Practice of Gas Chromatography: Ettre, L. S., Zlatkls, A., Eds.; Wiky: New York, 1967; pp 254-258. (28) Blades, A. T. J . Chromatogr. Sci. 1076, 14, 45. (29) Wagner, J. H.; Lillie, C. H.; Dupuis, M. D.; HIII, H. H., Jr. Anal. Chem. 1080, 52, 1614. (30) McAlllster, T.; Scott, J. D. Int. J . Mass Spectrom. Ion Phys. 1080, 33, 63. (31) Flytzanl-Stephanopoulos, M.; Wong, S.; Schmidt, L. D. J . Catal. 1977, 49, 51. (32) Rhead, G. E.; Mykura, H. Acta Metall. 1082. IO, 843. (33) Chen, M.; Schmidt, L. D. J . Catal. 1070, 56, 198. (34) Chen, M.; Schmidt, L. D. J . Catal. 1078, 55, 348. (35) Chen, M.; Wang, T.; Schmldt, L. D. J . Catal. 1970, 6 0 , 356. (36) Burch, R.; Garla, L. C. React. Klnet. Catal. Lett. 1081, 16, 315. (37) Kell, D. G.; Gill, R. J.; Olson, D. 6.; Calcote, H. F. ACS Symp. Ser. 1984, No. 249, 33.
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(38) Eyler, John R. ACS Symp. Ser. 1984, No. 249, 49. (39) Calcote, H. F. NATO Conf. Ser., 1 1083, 6 (no. 7), 197. (40) Homann, Klaus H.;Strofer, Eckhard NATO Conf. Ser., 1 1083, 6(no. 7), 217. (41) Howard, Jack B.; Bittner, James D. NATO Conf. Ser., 1 1063, 6 (no. 7), 57. (42) Warnatz, Jurgen NATO Conf. Ser., 1 1083, 6(no. 7), 127. (43) Weinberg, Felix J. NATO Conf. Ser., I 1083, 6(no. 7), 243. (44) Keil, D. G.; GIII, R. J.; Olson, D. 6.; Calcote, H. F. Int. Symp. Combust. 1084, 2 0 , 1129. (45) Calcote, H. F.; Manos, D. M. Combust. &me 1083, 49, 289. (46) Olson, D. 8.; Calcote, H. F. I n t . Symp. Combust. 1081, 18, 453. (47) Pergament, H. S.; Calcote, H. F. I n t . Symp. Combust. 1088, 1 1 , 597. (48) A I f Handbook, 2nd ed.; McGraw-Hill: New York, 1963; pp 4-13.
RECEIVED for review July 15, 1987. Accepted November 6, 1987. This work was financed by a CooperativeResearch and Development Grant from the National Science and Engineering Research Council of Canada.
Ultrasensitive, Simultaneous Determination of Arsenic, Selenium, Tin, and Antimony in Aqueous Solution by Hydride Generation Gas Chromatography with Photoionization Detection Steve H. Vien and Robert C. Fry*
Department of Chemistry, Willard Hall, Kansas State University, Manhattan, Kansas 66506
An optlmlzed hydrlde generation gas chromatography system for analysls of aqueous solutlons Is presented that employs an Intermedlate cold trap but Is tolerant of water vapor, CO,, vdatlle boranes, and other spurious byproducts from a BH4reactor w I t M requlrlng drylng agents or COPscrubbers. The system Ls free of background and byproduct Interference. The overall hydride generatlon gas chromatography system Is Inert toward SeH, and does not suffer significant loss of this compound. A specially conditioned Tenax-GC column suppresses unwanted byproduct elution and separates volatile hydrides of As, Se, Sn, and Sb at room temperature (Isothermal). A photoionization detector Is used for generalized muRlelemenl hydride response to these four elements. Detectlon llmlts In the orlglnal water mlutlon are as low as 0.025 ng (0.001 ppb In a 28-mL sample). Once sample analysis Is complete, byproduct desorption may be Induced by heating the column briefly to -100 O C .
Hydride generation is a procedure commonly used for sensitivity enhancement in a variety of instrumental methods for measuring trace levels of As, Se, Sb, Sn, Ge, Te, and Bi (and sometimes Pb) in aqueous solutions and wet-ashed solid samples. For atomic spectral analysis, the hydride technique typically results in several orders of magnitude improvement in concentrationsensitivity over conventional nebulizer sample introduction. The two instrumental methods most commonly coupled to the hydride preconcentration are atomic absorption and plasma emission spectrometry. When either of these two detectors are used with the most recent commercially available, state-of-the-art, automated, continuous-flow hydride generators, solution-phase concentration detection limits in the range of 0.2-0.4 ppb As, Se, and Sn (1,2)can be routinely achieved. 0003-2700/88/0360-0485$0 1.50/0
Limits of quantitation ((LOQ)defied as 5X detection limit (3))for recent commercially available continuous-flow hydride generation atomic absorption (AA) and plasma emission systems are 1-2 ppb As,Se, and Sn (computed from the data of ref 1and 2). Although this range of LOQ is adequate for screening contaminated water samples near or above the current maximum concentrations permitted by the US Environmental Protection Agency (10 ppb Se, 50 ppb As), for testing spiked EPA water standards, and for analyzing wetashed samples of solid food, biological tissue, soils, and environmental sediments, it should be noted that uncontaminated, unspiked natural water samples often exhibit substantially lower As, Se, and Sn levels. Typical “base line” As, Se, and Sn levels in natural water systems are in the range 0.02-8 ppb (4,5). The organic forms of As, Se, and Sn in natural waters are even lower in concentration (0.002-1 ppb). Commercially available, continuous-flow hydride generation atomic absorption and plasma emission systems are not sufficiently sensitive to accurately determine the concentrations of As, Se, and Sn in natural water samples. Several “homemade”hydride generators have been reported for further sensitivity improvement. Pierce et al. (6) utilized a “Chu” furnace and heated stripping column with detection limits of 0.01-0.02 ppb for As and Se, and LOQ’s of 0.05-0.1 ppb. A major drawback was that this particular automated analyzer only acidified the sample after adding the BH4reagent. When Cu2+was present, this order of acidification resulted in considerable interference with selenium hydride evolution. It was therefore necessary to perform extensive sample dilution to the point where copper no longer interfered. Unfortunately, the extra dilution required to avoid metal interference cancelled out the extra sensitivity that otherwise would have been gained by this AA system. It is well-known that cryogenic trap collection with subsequent temperature-jump injection directly into the atomic 0 1988 Amerlcan Chemlcal Society
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absorption or plasma emission detector can substantially improve the hydride detection limits for As and Sn over the values achieved with a continuous-flow BH4- reactor. Unfortunately, in all but four of the reported cold-trapped BH, reactors, drying agents have been found necessary to remove water vapor and avoid ice clogging in the cold trap. Of the four systems that did not employ a drying agent prior to the trap, two employed a tube of NaOH pellets after the trap to remove CO,, which otherwise interfered with the emission discharge tube response to arsine. Selenium hydride could not be determined with these two systems, because it is removed from the gas stream by NaOH pellets (7, 8). Carbon dioxide scrubbing is aLso sometimes performed prior to the cold trap to avoid clogging it with dry ice when residual carbonates are acidified and degassed from solutions as CO, (9, 10). Arsine can be readily determined with such a system, but any procedure involving NaOH pellets will immobilize and prevent detection of selenium hydride emerging from the BH, reactor. Of the remaining two cold-trapped hydride generation systems that did not employ a drying agent, one used a dryice-cooled water trap, but a selenium response was not reported ( 4 ) . The other was relatively insensitive to selenium (LOQ 2 35 ppb (11)). All of the many other cold-trapped hydride generators reported in the literature have employed some type of drying agent prior to the cold trap. The drying agents have been silica gel, Drierite (CaS04), CaC12, and Mg(C104), (with NaOH pellets for CO, removal), all of which can react irreversibly with major amounts of selenium hydride. CaC1, appears to be somewhat less reactive toward selenium hydride than the other solid drying agents. Although some disagreement exists as to the extent of anhydrous CaClz reactivity toward selenium hydride, it is clear that those few authors who have reported any selenium response from a CaCl, drying tube were still not able to get a selenium detection limit below l ppb or a limit of quantitation below 5 ppb (12-15). The dry-ice-cooledwater trap has worked for arsine (4),but no reports of selenium determination with this drying method have yet appeared. The only drying agent we are aware of that has been demonstrated not to give any significant selenium hydride loss is concentrated sulfuric acid (16).When kept clean and thoroughly deaerated, this particular drying agent will quantitatively pass traces of selenium hydride. An atomic absorption detection limit of 0.08 ppb selenium (LOQ = 0.4 ppb) was reported. Unfortunately, this is only a factor of 3 improved over the most recent commercially available continuous-flow (untrapped) BH, reactors with flame heated quartz tube absorbance cells. In addition, concentrated H 8 0 4 can also act as a collection site for NO and NOz contaminants (from wet-ashed solids) which can irreversibly oxidize and trap SeH, (16). The HzS04drying agent is furthermore inconvenient to replenish and requires a relatively complicated glassware design. In addition to their simplicity of design and use, one of the biggest advantages of continuous-flow BH4- reactors (over cryogenic entrapment methods) is that drying agents are unnecessary and selenium hydride loss is therefore avoided. However, since natural water samples are often too low in concentration for quantitative analysis by this approach to atomic absorption or plasma emission analysis, a more sensitive method is called for, especially for selenium. Pyrolytically coated graphite furnaces have been used as both hydride-trapping medium (-600 "C) and as the subsequent atomization cell a t 2600 OC. The volatile hydrides initially enter the graphite furnace through a quartz tube inserted through the normal pipet delivery hole in the top of the furnace. The other end of the quartz tube is directly
connected (without intermediate cold traps, drying agents, or C02scrubbers) to a batch or continuous-flow BH4- reactor. The limit of detection for selenium was 0.0017 ppb (17). To our knowledge, this is the most sensitive selenium method reported in the literature to date. The LOQ would be -0.01 ppb which is reasonable for total element determinations (0.02-8 ppb) in natural water systems. Similar values were obtained for As, etc. (18-20), but only one element could be determined during any given furnace atomization cycle. Separate preconcentration and atomization cycles would be required for each additional element. An electrodelessdischarge lamp (EDL) excited flame atomic fluorescence system was reported based on a simple batch hydride reactor without cold trap (21). The detection limits were only a factor of 2- to 4-fold improved over values obtainable with commercially available continuous-flow BH4reactors coupled to conventional atomic absorption and plasma emission systems. In the present paper, a photoionization approach to hydride detection replaces atomic absorption and plasma emission detectors. A cryogenic entrapment procedure that requires no drying agents or carbon dioxide scrubbers is also introduced. This paper is a study of the potential for combined usage of a photoionization detector (PID) and cold trap to provide several orders of magnitude of sensitivity improvement over the best existing continuous-flow hydride generation atomic absorption and plasma emission systems for As, Se, and Sn determination (without suffering any significant selenium hydride loss). The paper also includes sensitivity comparisons between this new cold-trapped PID method and earlier ultrasensitive graphite furnace wall preconcentration methods of hydride generation atomic absorption. The discussion to this point has been primarily directed to the question of how to improve on the sensitivity limitations of conventional hydride generation atomic absorption and plasma emission methods. Another major drawback of existing systems based on the hydride preconcentration lies in the area of cost-effective, simultaneous multielement determination. In principle, the BH4- reactor is capable of simultaneously generating volatile hydrides of up to eight elements during a single reaction. The graphite furnace wall collection method for hydride preconcentration atomic absorption is ultrasensitive, but it can only respond to one of these eight hydride-forming elements at a time and cannot readily be adapted for simultaneous multielement determination. A GC method with detector based on a sequentially slew-scanned monochromatorsystem with wavelength change only occurring during the time interval between base line resolved chromatographic peaks has been proposed (14))but it would not be readily adaptable to the sensitive furnace wall collection AA detector. Also, the final LOQ's for the slewscanned system were above 1 ppb As and above 5 ppb Se. Plasma emission is inherently better suited to simultaneous multielement determination, but only when an expensive, multichannel, photomultiplier-based, direct reading polychromator and data system are employed. Even then, LOQ values below 1ppb have not been reported for As and Se with this type of multichannel plasma emission system. As a simpler, less expensive alternative, hydride generation gas chromatography with precolumn cryogenic trap collection of the hydrides and subsequent temperature-jump injection has been suggested for multielement hydride detection by a number of authors (4,5,8,12-15,22,23). Previous hydride generation GC methods have been summarized in the first part of Table I along with indications of whether each method (a) was sensitive enough to analyze natural water samples (LOQ5 1ppb), (b) exhibited good chromatographic resolution of the hydrides in question, (c) provided a multielemental
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988 INJECTOR VALVE
Table I LOQ 1 ppb) to help optimize the hydride generation conditions and to confirm the elemental identity of analyte peaks emerging from the column and PID. Since the PID has a low internal detector volume and is virtually nondestructive, the atomic absorption spectrophotometer could be placed directly in series with the gas outlet port of the PID. Reagents. A stock 1000 ppm Se(1V) solution was prepared by dissolving 1.40 g of Se02 (Ventron, Danvers, MA) in 1.00 L of deionized distilled water. The Se(1V)working standards were prepared by further diluting the stock solution in 6 N HCl (Baker Instraanalyzed grade). Certified atomic absorption standards containing 1000 ppm As(II1) (as As203in dilute nitric acid) and
1000 ppm tin (as SnC1, in dilute HC1) were obtained from Fisher Scientific. Stock As(II1) standard was further diluted to the desired concentration with 6 N HC1. Treatment of the stock tin solution is mentioned later in the results and discussion section. Stock 5% NaBH4was prepared by dissolving 5.0 g of analytical grade NaBH4 (Ventron)in 100 mL of 0.5% KOH (Fisher, certified ACS grade). The NaBH4 solution was stored frozen and used within 3 weeks. This solution was further diluted with 0.5% KOH to the 0.5% NaBH4 level for use in the VGA-76 reactor. The diluted reagent was stored at 5 OC and used within a week. Procedure. In continuous-flow experiments,all three solutions were at room temperature. Each sample run was immediately followed by a blank run. The Tenax GC material was preconditioned in a laboratory oven at 200 "C for 2 h in air and allowed to cool briefly to room temperature before packing into the Teflon GC column. The packed column was further conditioned with N2 (50 mL/min) at room temperature for 30 min, immediately followed by a temperature ramp from room temperature to 120 "C at a rate of 3 OC/min. The final column preconditioning temperature of 120 "C was maintained for not less than 6 h before use. For comparison purposes, a second column of the same material was packed and preconditionedat 120 "C in nitrogen atmosphere only. To operate the system, the empty sample loop was precooled with liquid nitrogen for 30 s ("INJECT" configuration, Figure 2b). The injector valve pair was then switched to the "COLLECT" configuration (Figure 2a). Volatile hydrides were generated continuously from the VGA-I6 reactor for 2 min. The VGA-76 reagent pump was then turned "OFF" to stop generating the hydride. The injector valve pair remained in the "COLLECT" configuration for another 30 s to collect residual hydrides from the gas-liquid phase separator. The injector valve system was then switched to the "INJECT", confiation (Figure 2b) immediatelyfollowed by a rapid thawing of the sample loop by removal of the liquid nitrogen Dewar and immersion of the loop in a water bath at room temperature. This results in a rapid injection of any trapped hydride onto the Tenax GC column. One minute later, the injector was reset to the "COLLECT" position. By this time, all ice (which had cocondensed earlier with the volatile hydrides when the loop was at liquid nitrogen temperature) in the trap has melted, but most of the resulting water has not yet had a chance to evaporate into the carrier gas stream passing over it. Most of the water vapor from the original BH, reaction is thereby separated from the volatile hydride and is not injected onto the GC column. Room temperature nitrogen from the VGA-76 outlet now evaporatively dries out the sample loop and carries any remaining water vapor to the vent and hood (bypassing the GC column) within 5 or 10 min. Meanwhile, the volatile hydride separation is in progress on the Tenax GC column. The column temperature was maintained at 30 "C for 8 min, or until all hydrides eluted. Major byproducts of the BH4- reaction (HCl vapor, some residual water vapor, C02, and volatile boranes) that would otherwise give a large interference signal in the PID are completely and irreversibly trapped by the Tenax GC column at 30 OC. After the analyte hydrides have eluted, the trapped byproducki were desorbed by increasing the column temperature to 110 "C at a rate of 24 "C/min. The column temperature was kept at 110 "C for about 5 min. RESULTS AND DISCUSSION Byproduct Elimination. Undesirable byproducts of hydride generation reactions such as water vapor, volatile boranes, C02, and HCl vapors are trapped along with arsine, stannane, selenium hydride, and stibine in the liquid-nitrogen-cooled sample loop. When the injector valve is switched and the sample loop warmed to room temperature, most of the water remains liquefied in the trap, but some of it is injected as water vapor along with the other byproducts and the analyte hydrides onto the GC column. At least nine different major byproducts of the BH4- reaction were found to elute from the GC column. With several unsuccessful GC columns that were initially tried (e.g. glass beads and Chromosorb T (Teflon powder)), a number of the
ANALYTICAL CHEMISTRY, VOL. 80, NO.
byproducts eluted during the same time period as the analyte hydrides. On glass beads or Chromosorb T, the column resolution at room temperature was insufficient to prevent the major reaction byproducts from obscuring the PID response to analyte hydrides. The interference was so great that no useful data could be obtained for As, Sn, Se, or Sb with these two columns. By use of a direct current plasma emission spectrometer as detector, several of the major interfering byproduct peaks eluting from a column of glass beads were tentatively identified as CO, and a series of volatile boranes (via their time resolved carbon and boron atomic emission spectra). Confirmation of the tentative COz byproduct peak assignment was made by verifying the retention time with a known COZ injection. No attempt was made to confirm the volatile borane assignment or to identify the remaining unassigned interfering byproduct peaks. As a result of byproduct elution and PID response to these interfering compounds, no chromatographic base line could be established in the region where analyte hydrides elute a t room temperature from columns of glass beads or Chromosorb T. The Tenax GC column remedied this difficulty by irreversibly trapping the byproducts onto the stationary phase at room temperature while separating and passing the analyk hydrides on to the detector. The undesired byproducts remain trapped as long as the Tenax GC column is maintained at room temperature. The byproducts are not desorbed until later when the Tenax GC column temperature is raised above 80 "C. A "blank" run of the new hydride generation Tenax GC system yielded a flat base line during the room temperature region of the chromatogram (where the analyte hydrides would normally appear). This result demonstrated that the new Tenax GC system can tolerate both water vapor and COz non column". To our knowledge, this is unique among hydride generation/GC systems reported in the literature. External traps or scrubbers (prior to, or in addition to the cold trap) for water vapor and COPappear to be no longer necessary if Tenax GC is employed at room temperature as the stationary phase and the experimental procedure, conditions, reagents, and apparatus of this report are duplicated. BH, reaction times and sample sizes greater than 4 min and 28 mL, respectively, will clog the trap with ice. At first, the high-temperature byproduct desorption step (GC column cleanout) was performed after each room temperature isothermal analyte chromatogram. It was later determined that at least 10 sample chromatograms could be run in sequence a t room temperature before it was necessary to thermally desorb the undesired volatile byproducts from the column a t 110 "C. Corrosive byproducts such as HC1 and diborane may shorten the column life. The columns lasted approximately 3 months in daily use under these conditions. After 3 months, the retention times decreased, the chromatographicresolution became unacceptable, and a new Tenax GC column would have to be packed. Hydride Retention and Separation. A typical chromatogram of hydrides generated from an aqueous solution which originally contained ions of As, Se, Sn, and Sb is shown in Figure 3. The analytes are seen to elute during the isothermal room-temperature period of the chromatogram, and a byproduct desorption cycle is shown at the end of the chromatogram when the temperature was raised above 80 "C. The degree of hydride separation achieved in the isothermal room temperature region of Figure 3 is excellent for SeH, and SbH3 and is reasonable for AsH3 and SnH4 (though not base line resolved). Our injection from the cold trap onto the Tenax GC column is not quite as rapid as it could be. Some minor
6
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,,,?;
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5, MARCH 1, 1988
BY-PRODUCTS
12
;
18
24
TlME(min)
Figure 3. Typical chromatogram derived from BH,- reductlon of 14-ml, acidified sample (6 N HCl) containing 5 ppb As(III), 1 ppm Sn(II), 300 ppb Se(IV), and 400 ppb Sb(II1). Order of elution: ASH,, SnH,, SeH,, and SbH,. GC oven temperature program: initial 30 "C isothermal (15.9 min), final 110 OC, rate 24 "C/min.
precolumn peak broadening may be occurring as a result. It is therefore possible that some additional resolution between AsH3 and SnH4 peaks could be obtained by future use of a warmer water bath to obtain a more rapid temperature-jump injection from the cold trap into the GC. In an attempt to optimize the separation, Tenax GC column lengths of 1.22, 1.52,1.83, and 2.13 m were tested. The 1.52-m Tenax GC column proved to be the most effective column length in overall terms of resolution, reasonable retention time, and gas pressures compatible with thisparticular injector valve system. The pressure limits of this particular injector valve were the primary consideration in selecting a 1.52-m column length rather than a longer column. The 1.52-m column was used to generate the chromatogram of Figure 3. Effect of Preconditioning the Column Packing. In an attempt to further improve the separation of arsenic and tin hydrides, several different preconditioning treatments of the Tenax GC material were tried before packing it intothe Teflon column. The initial results of Figure 3 were obtained with a column in which the Tenax GC was preconditioned at 200 "C in air for 2 h and then packed into the column (with final conditioning performed as described in the procedure section). When the Tenax GC was simply packed without hot-air exposure into the Teflon column before final conditioning under a Nz atmosphere (as described in the procedure section), the results were much worse. The retention times of all four hydrides were significantly shortened, and the SnH4and SeHz overlapped with the ASH, peak, so no separation occurred. Prior to packing, The Tenax GC changed from white to a light brown color after preconditioning with air exposure for 2 h at high temperature (e.g. 200 "C). Some surface oxidation presumably occurred. Some shrinkageof the packing material was also noted. Under Nzatmosphere, subjecting the Tenax GC packing to reasonably high temperature did not induce a color change, but shrinkage was still observed. Preconditioning the Tenax GC packing material with hot air resulted in a reasonable hydride retention and separation (Figure 3), whereas preconditioning with hot nitrogen yielded
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
less retention and little or no separation. Surface oxidation of the Tenax GC packing material (the bulk is a hydrophobic polymer of 2,6-diphenylphenylene oxide) induced during preconditioningtherefore appears to have played a major role in determining the success of this separation. In addition to the air exposure at 200 OC, two other preconditioning temperatures with air exposure were tried (250 and 300 ‘C). This did not result in any observable improvement in the separation beyond that achieved in Figure 3. It is possible that future investigation involving variation of the air exposure time and partial pressure of oxygen employed to precondition the Tenax GC could improve the AsH3/SnH4separation. For the present, preconditioningwith 2 h of air exposure at 200 “C was employed. The corresponding room temperature analytical hydride separation observed in Figure 3 represents the current state of development with this Tenax GC system. Column Reactivity toward SeH,. Arsine, stannane, and stibine are relatively unreactive and have always been relatively easy to chromatograph. Selenium hydride is quite a different story (it is considerablymore reactive). As indicated in the introduction section, the irreversible reactivity of drying agents, CO, scrubbers, and GC column packing materials toward SeHz has long been a major hindrance preventing the successful elution of this compound with high yield and adequate GC sensitivity (LOQ
