Anal. Chem. 1981, 53, 287-291
Determination of Germanium in Natural Waters by Graphite Furnace Atomic Absorption Spectrometry with Hydride Generation Meinrat 0. Andreae" and Phlllp N. Froelich, Jr. Depatiment of Oceanography, Florida State University, Tallahassee, Florlda 32306
Germanium is determined In aqueous matrlx at the part-pertrlllion level by a combination of hydride generatlon, graphlte furnace atomisatlon, and atomic absorptlon detection. The germanium is reduced by sodium borohydride to germane (GeH,), stripped from solutlon by a helium gas stream, and collected in a liquld-nltrogen-cooled trap. I t Is released by rapld heating of the trap and enters a modified graphite furnace whlch is synchronlsed to reach the analysls temperature of 2000 O C before arrival of the germane peak. The atomic absorption peak is recorded and electronically integrated. The absolute detection limit Is 140 pg of germanium. The concentration limit of detectlon Is 0.50 ng L-' for a 250-mL sample. The dynamic range of the method spans 3 orders of magnltude. The precision of the determlnatlon Is 8 % when peak absorbance Is used; by peak integration in the nanogram range, the preclslon Is 4 %. Results of the analyses of natural waters are presented.
The earliest methods for the determination of germanium in natural waters involved the concentration of germanium from large water samples by coprecipitation and extraction procedures and its spectrophotometric measurement as the phenylfluorone complex (I, 2). At the concentrations typical of natural waters, these methods are working close to their limits of detection and require time-consuming enrichment steps. Due to its tendency to form very stable oxide species, germanium shows relatively poor sensitivity in flame atomic absorption methods (3). The high temperatures and relatively long residence times available in graphite tube atomizers made significant improvement in sensitivity possible: Johnson et al. ( 3 )report an absolute limit of detection of 0.3 ng of germanium obtained with a graphite tube atomizer of their own construction. The restriction to sample volumes in the microliter range, however, results in concentration limits of detection of about 15 pg L-l, several orders of magnitude above those characteristic of natural waters. The reduction of germanium in solution to the volatile germane (GeH4, bp -88.5 "C) by sodium borohydride and the subsequent detection of the gaseous germane by atomic absorption was first used by Pollock and West (4),who achieved a relatively high limit of detection (about 0.5 pg Ge) by injecting the gas into a standard atomic absorption flameSimilar limits of detection are achieved with externally heated silica tube atomizing furnaces (5). Braman and Tompkins (6) combined the borohydride techniques with a dc discharge atomic emission detector and achieved a detection limit of 0.4 ng of Ge. The objective of this study was to develop a system which would be sensitive enough so that relatively small volumes of water could by analyzed directly and which would consist largely of commercially available components. Further design criteria were to minimize sample pretreatment and reagent
additions to reduce the chance of contamination. For this purpose, we investigated the combination of hydride generation and cold trapping for sample enrichment with two detection systems: first, with the quartz cuvette burner atomic absorption detector used by Andreae (7) for the detection of arsenic and tin and, second, with a modification of the Perkin-Elmer HGA 400 graphite furnace atomizer. This paper discusses the applicability of this approach to the analysis of natural waters and the optimization of the analytical procedure. EXPERIMENTAL SECTION Apparatus. The apparatus for the volatilization and trapping of the germane (Figure 1) is constructed of Pyrex glass. The sample is contained in a reaction vessel into which helium is introduced through a fritted bubbler and which has a side port for the injection of the sodium borohydride solution. This port is closed off by a septum held in a Teflon Swagelok union (6.4 mm 0.d.). Different size reaction vessels (25-250 mL) can be attached to the apparatus; for this purpose, the lower part of the bubbling tube is attached to the helium inlet tube by a short Teflon sleeve (6.4 mm 0.d. Teflon tubing, which creates a gastight fit around 6 mm 0.d. glass tubing). All glass components of the system are joined glass-to-glass by such Teflon sleeves. After the reaction vessel, the carrier gas stream passes through a Pyrex U-tube (7 mm i.d., 30 cm long) immersed in a cold bath filled with isopropyl alcohol. This bath is cooled either with dry ice or with an immersed refrigeration probe and serves to remove most of the water vapor from the carrier gas stream. (The type of refrigeration used is only a matter of convenience and does not influence the operation of the system.) The germane is trapped in a 6 mm 0.d. glass U-tube which is immersed in liquid nitrogen. The length of the U is 30 cm, about 8 cm of which are packed with silanized glass wool (Supelco, Inc., Bellefonte, PA). About 1m of Chrome1 wire (- 3 Q) is coiled around the outside of this cold trap and connected to a variable transformer. The outlet of the cold trap is connected to the detection system, which consists either of the quartz cuvette burner described by Andreae (7) or of a Perkin-Elmer HGA 400 graphite furnace atomizer. All glass surfaces which come in contact with the germane are deactivated by silyation. The water trap and the transfer tube are treated with a commercial silyation reagent (Sylon CT [E;% dimethyldichlorosilane in toluene]; Supelco, Bellefonte, PA) and the sample trap is treated by heating to 150 "C after inserting the glass wool, injecting two times 25 fiL of Silyl-8 (Pierce Chemical Co., Rockford, IL), and conditioning overnight. This passivatiion is important to avoid tailing and irreversible sorption of the germane to the walls of the apparatus. The quartz cuvette burner consists of a quartz tube, 9 mm i.d. and 7 mm long, which is mounted in the beam path of the atomic absorption spectrophotometer and aligned so that the beam follows its central axis. It has an inlet at its back, where a flow of hydrogen is introduced, and one at its front, where the carrier gas premixed with air enters. A hydrogen-rich flame burns inside the tube and produces a germanium atom population from the germane carried in the gas stream. As will be described below, this sytem did not have adequate sensitivity due to the low atomization yield encountered for germanium at the relatively low flame temperatures achieved with this burner. Therefore we are now using exclusively the HGA 400 graphite furnace as (detector.
0003-2700/81/0353-0287$01.00/0 . 0 1981 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
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Figure 1. Apparatus for the generation and collection of germane. We modified the Perkin-Elmer HGA 400 furnace for the introduction of gaseous samples by disconnecting the two inlets for the internal purge flow which normally supply the purge gas to both ends of the graphite tube. In the normal configuration, the gas escapes through the hole in the center of the tube, which also serves for the introduction of liquid samples. We connected the carrier gas stream from the hydride generation apparatus to the left purge gas inlet port by a bent piece of thick-wall glass tubing (1 mm i.d.1 and left the right inlet port open. The germane is thus swept by the carrier gas stream through the graphite furnace. When the normal graphite tubes are used, the carrier gas can escape both through the right purge gas port and through the central hole in the graphite tube. We attempted to improve the detection limit by using graphite tubes without the central injection hole (Ultra-Carbon, Bay City, MI), forcing all the gas to go completely through the graphite tube. This substitution did not result in a significant improvement in sensitivity, however. Graphite tubes with interior pyrolytic coating often gave highly erratic results for a considerable number of burn cycles after installation, an effect not observed with the uncoated tubes. The detector is mounted in a Perkin-Elmer 5000 atomic absorption spectrophotometer equipped with a germanium electrodeless discharge lamp operated at 6.5 W. The monochromator is set at 265.2 nm, slit at 0.2 nm, and the deuterium background corrector is selected. Standards and Reagents. Two types of standards were used a commercially available 1000 ppm Ge standard solution which contains germanium in the form of sodium hexafluorogermanate (Alfa-Ventron, Danvers, MA), and a solution prepared by dissolving germanium dioxide (ultrapure, Alfa-Ventron) in base and by acidifying and diluting this solution to lo00 ppm. No difference between these two standards could be detected. These stock solutions were diluted and acidified daily to form intermediate standards with concentrations down to 1 pg L-', from which aliquots were pipetted into 100 mL of deionized water in the reaction vessel to form the final standard solution in the nanogram per liter concentration range. Comparisons of fresh dilutions with intermediate standards up to 3 months old showed no detectable change. Sodium borohydride (Fisher Scientific, Pittsburg, PA) was dissolved to make a 4% solution, to which 1mL of 2 N NaOH/100 mL of reagent was added. This solution is stable for several days. All other chemicals were reagent grade. No blank contribution was detectable from any of the reagents used. The buffer solution used to control the reaction pH was made by dissolving TRIS (tris(hydroxymethy1)aminomethane)in water to form a 1.9 M solution and titrating this solution with concentrated HC1 until a pH of about 6 was indicated by a glass electrode. Methods. The sample, 25-250 mL, is pipetted into the reaction vessel. As the method provides an absolute determination of the amount of germanium contained in a given sample, the volume has to be adjusted so that this amount falls into the useful range of detection from about 100 pg to 100 ng. Five-milliliters of 1.9 M TRIS-HCl buffer solution are added per 100 mL of sample. The reaction vessel is then attached to the apparatus and purged with helium (50 mL/min) for 3 min to remove enclosed air. The sample trap is then immersed in liquid nitrogen and 3 mL of 4% NaBH, solution is injected with a hypodermic syringe. The
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Figure 2. Influence of the solution pH on the yield of the reduction of germanium wlth sodium borohydride (50 ng of Ge In 30 mL of solution). The buffersystems and their concentrations are (0)TRISHCI (0.095 M), (A)oxallc acid-KOH (0.05 M), (A) malic acid (0.17 M), (0) strong acids (1 M HCI or 0.3 M H2S04),and (X) deionized water only. reaction and stripping time is 4 min for a 30-mL sample, 7 min for 100 mL. After the reaction is complete, we simultaneously (1) remove the liquid-nitrogen bath, (2) switch on the variac which supplies voltage to the heating coil around the sample trap (ca. 10 V, 3 A), and (3) initiate the program cycle of the HGA 400. With the configuration and heating rate used in our system, the germane peak occurs 13-14 s after heating of the trap has been started. The furnace program cycle has to be designed accordingly. The first step consists of a purge burn to remove any accumulations that may be present in the graphite tube (temperature, 2900 OC;ramp, 1 e; hold, 7 s; full purge gas flow). The tube is then cooled again for 3 s (temperature, 20 "C; ramp, 1 s; hold, 3 s), and finally brought to analysis temperature (temperature 2600 "C, ramp, 1 s; hold, 10 8). In the final step the internal purge argon flow is reduced to 100 mL/min; the argon is mixed into the carrier gas flow (50 mL/min) as indicated above. The analog output of the AAS is connected to a chart recorder and the germanium peak is recorded. Peak areas are available either by using the "peak area" mode of the AAS 5000 or by using an integrating chart recorder.
RESULTS AND DISCUSSION Optimization of the Hydride Generation Process. Buffering. The efficiency of the hydride generation is strongly dependent on the pH at which the reaction is performed. Since the borohydride solution acts as a base, the pH after the addition of the borohydride can be much higher than the initial pH. This is indicated in Figure 2 as horizontal bars, the left end of which represents the pH of the sample solution before the addition of the borohydride reagent, the right end the pH at the end of the reaction. A number of buffer systems were used: TRIS, oxalate, maleate, and strong acids (HC1, H2S04). Oxalate buffers were adjusted to the desired initial pH by mixing solutions of potassium oxalate and oxalic acid. Figure 2 shows that a broad maximum of the reaction yield exists in the near-neutral pH range. This maximum is independent of the buffer system used; in addition to the TRIS and oxalate systems shown in Figure 2 we also experimented with a pH 7 phosphate buffer system and obtained the same yield. Phosphate buffers are, however, impractical for seawater analysis, since precipitation of calcium phosphate will occur. The reaction efficiency was significantly lower at both low and high pH or in unbuffered solutions, which attained a pH of 10.3 upon addition of the borohydride reagent. We have selected the TRIS-HC1 system a t a concentration of 0.095 M in the final solution for routine work because it displays adequate buffering capacity in the desired pH range and because the high solubility of TRIS-HC1 allows the convenient addition of this buffer in the form of small volumes of concentrated solution. Other workers have chosen much more acidic systems for the generation of germane: Braman and Tompkins (6)use oxalic acid a t an initial pH of 1.5-2.0, Robbins et al. (8)work in 1.2-2.4 N HC1, and Thompson and Pahlavanpour (9) use
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
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0.1 M HCl or 0.067 M tartaric acid (pH near 2). Tompkins (IO)has investigated the use of phthalate buffer at pH near 4 and unbuffered distilled water (which he plots at pH 7, but which actually has a pH above 10 after the reagent addition). While he obtains a low yield from the unbuffered solution in agreement with our observations, he observes a better yield with the oxalate (pH 1.5) than with the phthalate system (pH 3-4). It is not clear which effects are responsible for these apparent differences in the optimization of the reaction medium composition for the different systems. It may be due to the rapid increase in the rate of hydrolysis of sodium borohydride in acid solution near pH 1, so that the hydride formation reaction is outcompeted by the hydrolysis reaction. Under such conditions small experimental details (e.g., the rate of addition of the borohydride reagent) strongly influence the yield of the reaction. In order to obtain an estimate of the actual efficiency of the hydride generation process, we injected measured amounts of tetraethylgermane vapor directly into the carrier gas stream before the graphite furnace. From injections of 25 ng of Ge in the form of tetraethylgermane, we obtained a sensitivity of 23.3 counts/ng of Ge. (The results are expressed in peak integral units rather than absorbance to correct for differences in the peak shape between the direct injection and the GeH4 peaks eluting from the cold trap.) This compares within the experimental error with the sensitivity obtained for aqueous germanium by the borohydride method, which we found to be 21.8 counts/ng of Ge. Borohydride Concentration. The reagent concentration of 4% NaBH4 was chosen to provide a convenient volume of reagent for injection. This results in an initial concentration of about 0.03 M NaBH4 in the reaction solution. An increase in the amount of borohydride above this value was not found to increase the reaction yield. Reaction and Stripping Time. We determined the time required to reduce all the germanium(1V) in solution to germane, to strip it out of solution, and to deposit it in the cold trap by varying the time interval between the injection of the borohydride reagent and the heating of the cold trap. The results are plotted in Figure 3. A plateau was reached after 3 and 5 min for the 30 mL and the 100 mL reaction vessels, respectively. For routine analysis, we use 4 and 7 min, respectively. The fact that, for the smaller sample volume, the total time required for reaction, stripping, and transport is practically identical with the deadspace flushing time of our system suggests that the reaction occurs almost instantaneously. Trapping and Volatilization. In contrast to the determination of arsenic and tin in natural waters by an analogous method (9, the determination of germanium does not require the differentiation of inorganic and organometallic species, since organogermanium compounds have not been observed in the environment (6). Therefore, the separation properties of the cold trap packing are of little importance, and the objective of the design of the cold trap and the choice of the
289
heating rate is to obtain as sharp a peak as possible and thus to increase the signal-to-noise ratio. A relatively short plug of glass wool (5-8 cm) is enough to retain the germane in the liquid-nitrogen-cooledtrap. Earlier tests with an unpacked 6 mm 0.d. glass U-tube as a cold trap did not give quantitative retention. Following a suggestion by Braman and Foreback (11),the first half of the U-tube is left empty; this prevents clogging of the sample trap by water vapor freezing out of the head of the cold trap. The heating rate of the cold trap after removal of the liquid-nitrogen bath is controlled by the voltage applied to the heating wire around it. Increasing this voltage improves the peak sharpness up to about 13 V. Further sharpening of the peak is limited by diffusional peak broadening in the cold trap outlet and the connections to the graphite furnace. The width of the peak at half-maximum at our standard conditions is 2 s and increases slightly with increasing amounts of germane. Detector Optimization. Quartz Cuvette Burner. We intended originally to adapt the quartz cuvette burner used by Andreae for the determination of arsenic (7) and tin (Andreae, 1980, unpublished data) to the determination of germanium. Initial experiments showed that only very poor detection limits could be obtained for germanium with this device (3 ng of Ge, compared to 45 pg for As); the sensitivity was 8.1 ng of Ge/0.0044 Abs. This low sensitivity for germanium as compared to other “hydride” elements has been observed by Fernandez (12) who found a detection limit of 270 ng for Ge vs. 5 ng for As using hydride generation and detection in an argon-hydrogen-entrained air flame. The manufacturer’s specifications for the Perkin-Elmer externally heated quartz tube furnace claim detection limits of 200 ng of Ge and 3 ng of As. This low sensitivity for Ge is attributed to the formation of highly stable oxide species under the conditions prevailing in flames or the externally heated quartz tubes (3). As the detection limits available with the quartz tube furnace were much above those required for the analysis of most natural waters, we abandoned further investigations with this device for germanium. Graphite Furnace. The peak absorbance is somewhat dependent upon atomization temperature, rising sharply between 2400 and 2500 “C, and remaining almost constant above this temperature. As the lifetime of the tube decreases with the burn temperature, we decided to use 2600 “C a6 the analysis temperature. We found that the addition of a short high-temperature (2900 “C) burn cycle with full purge gas flow preceding the analysis burn cycle improved the blank values and removed memory effects which were sometimes encountered when going from large to small analyte amounts. Under these conditions, tubes lasted for at least 100 determinations. Carrier Gas Composition. For the hydride generation process, helium is preferred as carrier gas over nitrogen or argon, since it does not condense in the liquid-nitrogen-cooled trap. The flow rate of helium through the hydride generation system influences the stripping rate and peak sharpness; it should be held between 50 and 100 mL/min. Helium is not, however, a suitable purge gas for the graphite furnace: it decreases the sensitivity by a factor of 5 as compared to argon and leads to a very rapid disintegration of the graphite tubes. We therefore operate the hydride generator at the minimum adequate helium flow (50 mL/min), and add argon to the helium stream at the outlet of the cold trap. The argon inlet gas line is connected to the graphite furnace purge gas, so that the Ar flow through the furnace is controlled by the furnace programmer. The dependence of the peak absorbance on the amount of argon added to a flow of 50 mL/min helium is shown in Figure 4 for a sample size of 55 ng of Ge. A flat response maximum is found at about 100 mL/min argon; this flow rate is used for analysis.
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
Table 11. Germanium in Natural Waters sample location
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Figure 4. Peak absorbance for 55 ng of Ge as a function of the amount of argon added to the gas stream. The flow rate of helium is held constant at 50 mL/min.
Table I. Within-Run and Day-to-Day Variability of Replicate Determinations of Germanium 5 ng 0.5 ng of Ge/100 mL of Ge/100 mL absorbance absorb- integral day 2 ance counts day 1 0.0046 0.0048 0.0039 0.0040 0.0042 0.0048 0.0047 0.0043 mean 0.00442 S.D. (n - 1) 0.00037 %RSD 8.3 n 8
0.0050 0.0048 0.0047 0.0041
0.00465 0.00039 8.4 4
0.0454 0.0446 0.0430 0.0512 0.0490 0.0510 0.0421 0.0470 0.0467 0.0035 7.5 8
1049 1031 1062 1111
1148 1045 1053 1031 1066 42 3.9 8
Analytical Figures of Merit. Sensitivity and Detection Limits. The sensitivity of our system is 430 pg/0.0044 Abs. This is in excellent agreement with the manufacturer’s specification of 400 pg/0.0044 Abs. The standard deviation of the base-line noise is about 0.0007 Abs, resulting in a noise-limited detection limit of 140 pg of Ge a t the 95% confidence level. There is no detectable blank when the analysis is performed on deionized water, so that the noiselimited detection limit is the actual lower limit of determination at which quantitative analysis can be carried out. This corresponds to a concentration detection limit of 1.4 ng L-’ for the 100-mL and 0.56 ng L-I for the 250-mL reaction vessel. This is well below typical concentrations of Ge in natural waters (see below). Precision and Accuracy. The variability of the germanium determination was investigated both within a given run and between different days at the 5 ng L-’ level (500 pg of Ge in 100 mL). Relative standard deviations of 8.3% and 8.4% were obtained for two different days (Table I). The pooled estimate of the relative standard deviation is thus 8.4%. The difference in the mean of the absorbance values is not statistically different between days one and two. Integration of the peak areas does not improve precision a t analyte amounts as low as 500 pg, mainly due to the variance of the integration of the base-line noise. At somewhat higher analyte levels, e.g., 5 ng of Ge (Table I), the precision can be improved significantly by electronic integration. While the relative standard deviation of the peak height was still
amt of Ge, no. of ng/L analyses
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surface ocean seawater, < 0.56 4 Central NW Pacifica deep ocean seawater, 7.64 5 0.27 Central NW Pacifica Ochlockonee River 4.09 3 0.45 (FL) water coastal surface ocean
