421
Anal. Chem. 1984, 56, 421-424
and may minimize this potential problem.
ACKNOWLEDGMENT The authors are grateful to Mark Bruce for his thoughtful discussions. Registry No. Cr, 7440-47-3;Sr, 7440-24-6; Ti, 7440-32-6; V, 7440-62-2; Cu, 7440-50-8; Zn, 7440-66-6;silicon carbide, 409-21-2; cellulose, 9004-34-6. LITERATURE CITED Lanamvhr. F. J. Analyst (London) 1978, 104, 993. VaLoon, J. C. Anal: Chem. 1880, 52, 955A. Headrldge, J. B. Spectrochlm. Acta, Part B 1980, 3 5 6 , 785. Fassel, V. A. Anal. Chem. 1978, 5 1 , 1290A.
Dagnail, R. M.; Smith, D. J.; West, T. S.; Greenfield, S. Anal. Chlm.
Acta 1971, 5 4 , 397. Salln, E, D.; Horllck, G. Anal. Chem. 1879, 5 1 , 2284. Ng, K. C.; Caruso, J. A. Anal. Chlm. Acta 1882, 143, 209.
(8)
Zerezghl, M.; Mulilgan, K.
J.; Caruso, J. A.
Anal. Chim. Acta
1883,
154, 219.
(9) Mulllgan, K. J.; Zerezhgi, M.; Caruso, J. A. Spectrochlm.Acta, Part B 1983, 3 8 6 , 369. (10) Browner, R. F. In "Applications of Industlvely Complex Plasma Atomic Emission Spectroscopy";Barnes, R. M., Ed.; Franklin Institute Press: Philadelphia, PA, 1978; p 51. (11) Mills, J. C.; Beicher, C. B. Prog. Anal. Atom. Spectrosc. 1981, 4 , 49. Ng, K. C.; Caruso, J. A. Anal. Chem. 1883, 55, 2032. (12) (13) Ng, K. C.; Caruso, J. A. Analyst (London) 1883, 108, 476. (14) Scott, R. H. Spectrochim. Acta, Part 6 1878, 3 3 8 , 123. (15) Young, R. S. Analyst (London) 1982, 107, 721.
RECEIVED for review June 5,1983. Accepted December 2,1983. The authors are grateful to NIOSH which supported this work in part through Grant No. OH-00739. K.C.N. acknowledges the financial assistance from the University of Cincinnati in the form of a summer (1983) research fellowship.
Determination of Methylgermanium Species in Natural Waters by Graphite Furnace Atomic Absorption Spectrometry with Hydride Generation Gordon A. Hambrick 111,' Philip N. Froelich, Jr.,* Meinrat 0. Andreae, and Brent L. Lewis Department of Oceanography, Florida State University, Tallahassee, Florida 32306
Inorganlc and methylgermanlum specles are determined In aqueous matrix at the parts-per-trllllon level by a comblnatlon of hydride generation, graphite furnace atomization, and atomic absorptlon spectrometry. The germanlum specles are reduced by sodlum borohydride to the corresponding gaseous germanes and methylgermanlum hydrides, stripped from solutlon by a hellum gas stream, and collected in a Jiquid-nltrogen+xoied trap. The germanes are released by rapld heating of the trap and enter a modlfled graphite furnace at 2700 'C. The atomic absorption peak is recorded and electronically Integrated. The absolute detectlon limlts are 155 pg of Ge for lnorganlc germanium (Ge,), 120 pg of Ge for monomethylgermanium (MMGe), 175 pg of Ge for dlmethyigermanlum (DMGe), and 75 pg of Ge for trlmethyigermanlum (TMGe). The preclslon of the determination ranges from 6% for TMGe to 16% for MMGe. Results of the analyses of natural waters are presented.
Organogermanium compounds have been reported not to be present in natural waters (1-3). In our preliminary inorganic germanium work we observed germanium compounds in some natural waters which are reduced and trapped similarly to Ge(OH),, but which elute from chromatographic packings after GeH, (4). We suggested that these peaks are unidentified methylgermanium species by analogy with previous observations for arsenic, antimony, and tin (1,4-6, 7, 8 ) . Further work has confirmed the presence of methylgermanium in natural waters and led us to modify our technique in order to optimize the recovery of the methylgermanium species. Present address: Florida Department of Environmental Regu-
lation, Pensacola, FL 32507.
The objective of this paper is to present a modification of the inorganic germanium technique of Andreae and Froelich (2)that is sufficiently sensitive to measure both inorganic and methylgermanium species in a single sample with a minimum of sample pretreatment and reagent additions. Portions of the new technique are unchanged; rather than duplicate information, we refer the reader to the original article ( 2 ) .
EXPERIMENTAL SECTION Apparatus. The apparatus is modified slightly from that described in Andreae and Froelich (2) for the determination of inorganic germanium. Instead of one reaction vessel connected directly to the water trap, we now employ two vessels in parallel via an ALTEX Teflon four-way slider valve (Alltech, Deerfield, IL) connected to the trap with '/e in. Teflon tubing (Figure 1). This configuration reduces analysis time by allowing alternate purging of the sample in one vessel to vent, while reducing, stripping, and trapping germanes from the other vessel onto the cryogenic trap. All other glassware is unmodified except that the cryogenic trap is packed with 22 cm of 15% OV3 silicone oil on 60/80 mesh Chromosorb W-AW-DMCS held in place by two 1-cm plugs of glass wool. Approximately 1.0 m of Nichrome wire (5 a) is coiled around the trap and connected to a variable transformer (Variac, Superior Electric Co., Bristol, CT) set at 16 V. We have found that passivation of glass surfaces is critical in the successful quantitation of monomethylgermanium. Once each week all glassware is cleaned overnight in a hot (-80 "C) 6 N HC1 bath. After the glassware is thoroughly rinsed with deionized water and dried, the water trap, transfer tube, and bubbler head are treated with 5% dimethyldichlorosilanein toluene (Sylon CT, Supelco, Bellefonte, PA), then washed with toluene followed by methanol, and dried by purging with nitrogen gas. The cryogenic trap is treated in a GC oven (nitrogen gas flow) at 150 OC by injecting two 25-wL aliquots of Silyl-8 (Pierce Chemical Co., Rockford, IL) 15 min apart. The trap is then conditioned overnight. Thus all glass surfaces which come in contact with the gaseous germanes are deactivated by silylation to avoid peak trailing and irreversible sorption. The reduction vessels and bubbler tubes are not passivated.
0003-2700/84/0358-0421$01.50/00 1984 American Chemical Society
422
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984 H~
,"
from Second
-.D c
a
E
Reduction
Vessel
Flgure 1. Apparatus for the determination of inorganic and methyl-
germanium species in natural waters. A Perkin-Elmer HGA 400 furnace (Perkin-Elmer Corp., Norwalk, CT) is modified to accept gaseous samples according to the design of Andreae and Froelich (2). The outlet from the hydride generation apparatus is connected to the left internal purge opening of the graphite furnace. The helium carrier stream (50 mL/min) is mixed with argon (100 mL/min) before entering the furnace. The right internal purge is left open to vent, since we use graphite tubes lacking the central injection hole (UltraCarbon, Bay City, MI). The argon flow tube to the unused nipple is clamped to save argon. The graphite furnace is mounted in a Perkin-Elmer 5000 atomic absorption spectrophotometer equipped with a germanium electrodeless discharge lamp (EDL). The monochromator is set at 265.2 nm and the slit width at 0.7 nm. The deuterium background corrector is not necessary for routine analyses. Standards and Reagents. Inorganic germanium standards are made from a commercially available 1000 ppm Ge standard solution in the form of sodium hexafluorogermanate (Alfa-Ventron, Danvers, MA). Serial dilutions are made down to the 1ppb range and stored in polyethylene bottles. These were periodically checked against other standards and have proved to be stable for over 2 years. Methylgermanium trichloride, dimethylgermanium dichloride, and trimethylgermanium bromide (Alfa-Ventron,Danvers, MA) are used to make methylated germanium primary standards. These are prepared in the range 600 ppm to 800 ppm (as Ge) in double-deionized water (DDW) and are kept refrigerated. The primary standards were validated in our lab by flame AAS, direct-injection flameless AAS,hydride-generation flameless AAS, analyses of halides in the standards, and analyses of inorganic germanium resulting from persulfate oxidation of the methylated standards. From the primary standards, secondary and tertiary standards are prepared by 1000-fold serial dilutions into DDW. The tertiary standards (-0.6-0.8 ppb) are used as the daily working standards and are prepared periodically. We have not observed instability in any of these standards for over a year. Sodium borohydride (Fisher Scientific Co., Orlando, FL) is dissolved in DDW to make a 20% solution, to which 3 mL of 2 N NaOH/100 mL is added. This solution is prepared at least 6 h before use to permit stabilization (5). It is stable for several days. The Tris buffer solution is identical with that used by Andreae and Froelich (2). Sodium chloride is dissolved in DDW to make a saturated solution (-300 g L-'). NazEDTA is dissolved in DDW to make a 0.2 M solution. Methods. The reaction vessel is filled with the appropriate volume of sample (50 to 250 mL). Five milliliters of Tris reagent, 10 mL of NaCl reagent, and 1mL of EDTA reagent per 100 mL of sample are added to the reaction vessel. One drop of 1-decanol is added to estuarine and seawater samples to suppress foaming. The reaction vessel is then attached to the apparatus and purged for 15 min per 100 mL of sample to remove enclosed air. After the apparatus is purged, the sample trap is immersed in liquid nitrogen and 6.0 mL per 100 mL of reaction volume of 20% Na13H4is injected slowly with a hypodermic syringe. The reaction and stripping time for a 100 mL sample is 25 min. The subsequent procedure is the same as for Gei (2). The graphite furnace program cycle is lengthened to match the longer peak elution times required. The fiit step allows complete purging of the graphite tube with argon (step 1: temperature,
e e .-
O
r
4
1 300
400 500 600
700
NaBH4 (mM)
Flgure 2. Relative sensitivities of inorganic germanium (Ge,),monomethylgermanium (MMGe), dimethylgermanium (DMGe), and trimethylgermanium (TMGe) as a function of initial NaBH, strength. The 300 mM NaBH, corresponds to 15 mL of a 20% NaBH, solution inlected into a 250-mL reaction volume. This concentration is chosen
as a compromise that best optimizes all four species. 30 OC; ramp, 1 s; hold, 5 8). The second step removes contamination in the graphite tube (step 2: temperature, 2900 O C ; ramp, 1S; hold, 3 s; full purge gas flow). The tube is then quickly brought to the analysis temperature (step 3: temperature, 2700 "C; ramp, 1s; hold, 75-90 s). The tube is then allowed to cool down slowly (step 4: temperature, 20 "C; ramp, 15 s; hold, 1 s). In the last two steps the internal purge argon flow (which is mixed into the carrier gas flow) is reduced to 100 mL/min (miniflow). The analog output of the AAS is connected to a strip chart recorder (Soltec 1241: Soltec, Sun Valley, CA) and an integrating chart recorder (3390A Integrator, Hewlett-Packard, Avondale, PA).
RESULTS AND DISCUSSION Optimization of Hydride Generation Process Buffering. We continue to use the Tris-HC1 system for reaction pH control (2). A typical sample has a pH of about 6.5 before NaBH, addition and a final pH of about 8.5. Braman and Tompkins (1)use an oxalic acid buffer system to maintain sample pH between 1.5 and 2.0 after NaBH, addition. Comparison of the ratios of sensitivities for MMGe and DMGe to Gei in our data and in that of Braman and Tompkins (1)shows that in both cases the recovery of DMGe is about half that of Gei and that of MMGe is about the same to half that of Gei. We have previously shown that the oxalic acid buffer is only about one-third as effective in reducing inorganic Ge ( 2 ) ;thus the Tris-HC1 buffer is superior to the oxalic acid system in the reduction of methylgermanium species. Borohydride Concentration. We use 20% NaBH4 to provide a convenient volume of reagent for injection. This results in an initial NaBH, concentrat,ion of approximately 0.30 M in the sample. This is ten times the NaBH, concentration used in the inorganic technique and was found to be necessary to optimally reduce MMGe and DMGe which are more difficult to reduce than Gei. Sensitivity for the methylgermanium species does not increase if NaBH, concentration is increased above this level (Figure 2). NaC1. We routinely add salt to enhance the stripping rate of the methyls in freshwater samples. This permits a shorter stripping time and minimizes a slight salt effect when analyzing estuarine samples. The salt addition is not necessary with seawater samples which all have similar salinities.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
423
Table I. Sensitivities and Detection Limits peak height
compound @i
MMGe DM Ge TM Ge
sensitivity, A/ng
abs detection limit, pg
concn detection limit (250 mL), Pg L-l
0.009 86 0.008 30 0.005 65 0.013 5
155a 120b 175 75b
620 480 710 300
peak area abs detection sensitivity , (A s)/ng limit, pg 0.0278 0.0293 0.0149 0.0426
195a
Based on twice the standard devia-
a Based on twice the standard deviation of replicate blank determinations ( n = 6). tion of the base line noise (0.0005 A).
Table 11. Typical Concentration Ranges of Germanium Species in Natural Waters concentration, ng L-' no. of sample location analyses Geia MMGe DMGeb TMGe
totalGe
0 3 2.2-3 7.8 10.0-11.5 11 0.4-1.8 21.9-24.5 Atlantic, Sargasso Sea 2 8.4 -4 0.6 8.0 0 18.2-20.3 21 2.2-12.3 Bering Sea, Geosecs 219 1.7-7.3 1.0-5.9 0.2-1.0 0 5 0.5-0.7 rain water, Tallahassee, FL 7.1-29.3 0 0-18.6 0-10.0 30 0.7-7.1 Peace River and Charlotte Harbor Estuary, FL 0-12.0 0 5.7-3 3.9 0-19.5 20 2.4-5.7 Ochlockonee River and Bay Estuary, FL 0-6.6 tr 7.4-26.8 0-19.6 11 0.6-7.4 Tejo River and Estuary, Lisbon, Portugal a Gei increases in concentration with increasing water depth in the oceans and with decreasing salinity in estuaries. Thus the ranges reported here reflect natural gradients in surface and deep-sea water and along the salinity gradient of estuaries. MMGe and DMGe are vertically homogeneous in the oceans. Thus the ranges for each ocean reflect analytical variability. In estuaries, MMGe and DMGe vary from nondetectable in the riverine freshwater to near the seawater values at the ocean end. Thus these ranges reflect a gradient in the estuary.
a
3i b
I
F
min
I
IO
io
io
40
io
Reduction and Stripping Time (min)
Flgure 3. Relative sensltivities of germanium species as a function of reduction and stripping times for a 100-mL reaction volume.
EDTA. We routinely add EDTA to sequester trace metals known to interfere with the borohydride reduction of some metalloids (ref 9 and references therein). In occasional seawater samples, we have observed a significant decrease in response for MMGe that is restored by addition of EDTA. Reaction and Stripping Time. For a sample size of 100 mL, recovery of the methylgermanium species does not increase significantly with reduction and stripping times greater than -25 min (Figure 3). Note that this reduction and stripping time is 4-fold longer than that used in the original inorganic germanium technique. Trapping and Separation. Several trap packings were tested. Finally, 15% OV3 on Chromosorb W-AW-DMCS was chosen because i t traps germanes without irreversible ad-
-
0
I
min
Flgure 4. Chromatograms show the peaks of (a) germane, monomethylgermane, dlmethyigermane, and trimethylgermane resuiting from reduction of a standard soiutlon containing 1 ng of Ge,, 1.85 ng of Ge as MMGe, 2.33 ng of Ge as DMGe, and 2.27 ng of Ge as TMGe and (b) germane, monomethylgermane,and dimethylgermaneresulting from reductlon of 100 mL of deep-ocean seawater from a hydrographic station in the Sargasso Sea (North Atlantic Ocean).
sorption losses, provides good separation of the germane and methylgermane peaks, and minimizes tailing. Typical chromatograms are shown in Figure 4. Graphite Furnace Temperature Selection. Peak absorbance is somewhat dependent upon atomization temperature. Response rises sharply to approximately 2400 "C to 2500 OC and plateaus (2). We have chosen a temperature of 2700 OC. Graphite tubes last at least 50 determinations under the furnace conditions we use. Analytical Figures of Merit: Sensitivity and Detection Limits. The sensitivities of our system for the different germanium species, as determined by peak heights and peak areas, are given in Table I. Our detection limits are about
424
Anal. Chem. 1904, 56,424-427
half of that reported by Braman and Tompkins (1)for Gei, MMGe, and DMGe. There are generally no detectable reagent blanks for the methylgermanium species in DDW or in artificial seawater. There is a small Gei blank (-0.5 ng/L of sample) originating from the NaBH,. Precision and Accuracy. We estimate the variability of the determination of the germanium species by calculating the relative standard deviation of the slopes of several calibration curves. By peak height ( n = 13) and peak area (n = 5), respectively, the relative standard deviations are 11% and 9% (Gei), 16% and 8% (MMGe), 10% and 14% (DMGe), and 6% and 9% (TMGe). The accuracy of the method cannot be assessed directly due to the lack of certified standards for aqueous germanium and methylated germanium species. We have made standard additions of known amounts of the germanium species into seawater and have obtained quantitative recovery. Linearity. The calibration curves are linear from the detection limits of the Gei, MMGe, and DMGe species to approximately 0.4 A (50 ng as Ge; 500 ng L-’ in the 100-mL reaction vessel). Appropriate dilutions can be made for samples of higher germanium concentration. Peak Characteristics. With the configuration and heating rate used in our system, the germane and methylated germane peaks elute in the following sequence and approximate retention times after heating of the trap has begun: Gei (17 s), MMGe (38 s), DMGe (59 s), and TMGe (67 8). Since the integrator is generally started about 10 s after heating of the trap has begun, we do not routinely obtain a retention time for Gei. We thus check reliability of the separation by retention times measured from the Gei peak. The half-height peak widths for the four germanium species at 1000 pg of Ge are 1.5 s for Gei, 1.9 s for MMGe, 1.3 s for DMGe, and 1.3 s for TMGe. These values are provided as guides, since peak characteristics are likely to vary with minor alterations in the details of each system. Interferences. There appear to be no spectral interferences when using atomic absorption spectrometry. We have checked for interferences in deep North Atlantic water both by using the deuterium background corrector and by analyzing at a secondary germanium wavelength and found none. We have checked for interferences in the borohydride reduction of all germanium species by NOz- (9) and found none a t concentrations up to 5 ppm NO2-.
Sample Treatment and Storage. Sample contamination does not appear to be a serious problem for any of the germanium species. Samples are stored in polyethylene bottles which have been rinsed with acid and DDW. Samples are acidified to pH -2 with HC1 a t the time of sampling. Analysis of Environmental Samples. We have applied this method of analyses to marine, estuarine, freshwater, and rain water samples. The results are given in Table 11. We have confirmed the existence of methylgermanium species in natural waters based on (1)coelution with standards on two different packings (glass wool and OV3), (2) detection with both atomic absorption and flame emission ( 5 ) ,(3) absence of interferences (background absorption) during AAS detection, (4) detection a t two different wavelengths, and (5) recovery of “excess” germanic acid after persulfate oxidation of natural seawater containing the methyl species. We have found that the major germanium species in seawater is monomethylgermanium. Trimethylgermanium is not found in seawater. Our total Ge values for seawater are slightly lower than those of El Wardani ( 1 0 , l l ) (50 ng L-l), Johnson and Braman (12) (42 ng L-l), Burton et al. (13)(60 ng L-l), and Braman and Tompkins (1) (79 ng L-l). Registry No. Ge, 7440-56-4; MMGe, 88453-53-6; DMGe, 74963-95-4; TMGe, 21941-60-6; water, 7732-18-5. LITERATURE CITED Braman, R. S.; Tompkins, M. A. Anal. Chem. 1979, 51, 12. Andreae, M. 0.; Froelich, P. N. Anal. Chem. 1981, 53, 287. Froellch, P. N.; Andreae, M. 0. Sclence 1981, 213, 205. Hambrick, G. A.; Froelich, P. N. Trans., Am. Geophys. Union 1982, 63, 71. Andreae, M. 0.; Byrd, J. T. Anal. Chim. Acta, in press. Andreae, M. 0. Anal. Chem. 1977, 4 9 , 820. Andreae, M. 0. I n “Trace Metals In Sea Water”; Wong, C. S., Boyle, E., Bruiand, K. W., Burton, J. D., Eds.; Plenum: New York, 1983. Andreae, M. 0.; Asmode, J.; Foster, P.; Van’t dack, L. Anal. Chem. 1981, 53, 1786. Brown, R. M.; Fry, R. C.; Moyers, J. L.; Northway, S.J.; Denton, M. B.; Wilson, G. S. Anal. Chem. 1981, 53, 1560. Ei Wardani, S. A. Geochim. Cosmochim. Acta 1957, 13, 5 . El Wardani. S. A. Geochim. Cosmochim. Acta 1958. 15. 237. i12j Johnson, D. L.; Braman, R. S. Deep-sea Res. 1975, 22, 503. (13) Burton, J. D.; Culkln, F.; Riley, J. P. Geochlm. Cosmochim. Acta 1959, 10, 151.
RECEIVED for review October 17,1983. Accepted December 5,1983. This work was supported by NSF Grant No. OCE8200929 to P.N.F. and M.O.A.
Side Line Indexing for Peak Search in Scanning Inductively Coupled Plasma Emission Spectrometry D. D. Nygaard,* D. S. Chase, D. A. Leighty, and S. B. Smith Instrumentation Laboratory Inc., Analytical Instrument Division, One Burtt Road, Andover, Massachusetts 01810 The slde ilne Indexing method Is introduced as an alternate method of emission llne peak search In scannlng inductively coupled plasma emlsslon spectrometry. I t Is shown to Improve analytlcai accuracy whenever elther background molecular emission or sample matrlx emission obscures the anaiyte emisslon line and thereby Interferes with the conventlonai peak search routlne.
Sequential scanning ICP emission spectrometers utilize a peak search routine to locate analyte emission lines before they 0003-2700/84/0356-0424$01.50/0
are measured (1). The peak search is necessary because, without it, scanning mechanisms cannot position the monochromator wavelength accurately enough to center on an emission line. In a previous paper (2), it was shown that the peak search can sometimes cause an inaccurate analytical result by incorrectly positioning the monochromator at some wavelength other than the analyte peak wavelength. This occurs when there are molecular emission fine structure peaks near the analyte emission line, and when the analyte peak is either very small, as in dilute solutions, or nonexistent, as in the blank. Two suggestions were made to minimize the frequency of such incorrect peak identification: (1)use a high 0 1984 American Cherpal Society
