986
Anal. Chem. 1991, 63,986-989
Determination of Germanium Species by Hydride Generation-Inductively Coupled Argon Plasma Mass Spectrometry Kazuo Jin,*J Yasuyuki Shibata, a n d Masatoshi Morita*
National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba-shi, Ibnraki 305, Japan
Inorganic and methylated germanium species were determlned at sub parts per trllllon levels by a comblnatlon of hydrlde generatlon and Inductively coupled argon plasma mass spectrometry (ICP-MS). The germanium species In solution were reduced to the correspondlng hydrides by sodium tetrahydroborate, transferred with a helium gas stream, and trapped In a liquid nitrogen cooled U-trap. The hydrides were evaporated and introduced into the ICP torch, and the Ion count at m / z = 74 was monitored. The reduction efficiencies for methylated germanium species in a malic acid matrix were more than 97 % . The absolute detection limits were 0.08 pg of Ge for lnorganlc germanlum (Ge,), 0.1 pg of Ge for monomethylgermanium (MMGe) and dlmethylgermanium (DMGe), and 0.09 pg of Ge for trimethylgermanium (TMGe). The dynamic ranges of the detection span 4 orders of magnitude. The proposed method was applied to natural waters and wastewaters, and Ge,, MMGe and DMGe were detected In all of the samples studied.
INTRODUCTION Chemical speciation of elements is indispensable to an understanding of the biological cycles of some elements in the environment (I). Recent studies have revealed that germanium in natural waters exists as both inorganic and organic chemical species a t sub-ng to ng L-' levels (2-9). Two organogermanium species, monomethylgermanium (MMGe) and dimethylgermanium (DMGe), have been identified in natural waters by methods based on the hydride generation technique (5-7), and it should be noted that MMGe is the major germanium species in seawater. However, information on organogermanium species in the environment is still limited because of the very low concentrations of the element. Although the hydride generation technique has improved the detection limit of germanium in various atomic spectrometric methods, the absolute detection limits so far reported are still at ng to sub-ng levels (2, 3, 10-15), and few studies have been done for methylated germanium species. Hambrick et al. (5) achieved detection limits of 75-150 pg of Ge for inorganic germanium (Gei), MMGe, DMGe, and trimethylgermanium (TMGe) by hydride generation-graphite furnace atomic absorption spectrometry (HG-GFAA). In this method, 100-250 mL of natural water sample is required for the analysis. Inductively coupled argon plasma mass spectrometry (ICP-MS) is a highly sensitive and increasingly used technique for elemental analysis (16-19). In the present study, we combined a hydride generation technique with ICP-MS to determine methylated germanium species as well as inorganic germanium at sub-pg levels (sub ng L-' leveLs in concentration)
* Correspondence author. 'Present address: Hokkaido Institute of Public Health, North 19, West 12, Sapporo 060, Japan. 0003-2700/91/0363-0986$02.50/0
with minimum sample treatment, a sample size of less than 10 mL, and an analysis time of about 10 min for a measurement. The choice of the reaction matrix is important to achieve a high reduction efficiency; we used a malic acid matrix. The proposed method was applied to natural waters and wastewaters. Treated wastewaters from a sewage plant contained two unknown germanium species in addition to Gei, MMGe, and DMGe.
EXPERIMENTAL SECTION Apparatus. Figure 1shows the schematic construction of the hydride generation and ICP-MS system. The ICP-MS instrument used was a PMS 100 manufactured by Yokogawa Electric Co. Ltd. The solution spray chamber was removed, and the gaseous hydrides were introduced into the plasma through a quartz capillary tube (1-mm i.d.) inserted into the plasma torch. The flow rate of helium gas was regulated with a SEC-400 MK3 mass flow controller from Stec Ltd. Each part of the hydride evolution system was connected with Teflon tubing (i.d. 2 mm). The reaction vessel of 30-mL volume was placed inside a clean bench (class 1OOO). Clean bench condition was necessary to avoid contamination. A sodium tetrahydroborate solution was injected by syringe attached to a repeat kit (Drummond Scientific Co., Broomall, PA). Generated germanium hydrides were passed first through a boat-shaped glass tube (5-mm i.d. and 150 mm) immersed in an ice bath for the removal of water and then through a carbon dioxide trap (4-mm4.d. and 50-mm-long Teflon tube packed with finely crushed sodium hydroxide pellets). The carbon dioxide trap was changed daily. A U-shaped quartz tube (i.d. 4 mm), with one arm packed with 60/80-mesh quartz beads to a height of 200 mm and wound with Nichrome wire (250 mm long; 20 a),was used for trapping and subsequent volatilization of the hydrides. Optimization of ICP-MS Parameters. Parameters of the ICP-MS instrument were optimized by introducing a trace amount of germanium hydride gas from a cylinder (5.2 ppm (v/v) as GeH, diluted with helium; Takachiho Chemical Ltd., Tokyo) through the three-way valve to the plasma torch (Figure 1). The helium flow rate was adjusted primarily to get optimum separation of the four germanium species, and the carrier argon flow rate was controlled to attain the highest sensitivity. Typical operation conditions after optimization were as follows: argon flow rate, carrier 1.05 L/min, auxiliary 0.5 L/min, plasma 14.0 L/min; helium flow rate 50 mL/min; forward power 1.2 kW; sampling height 3 mm from the end of the induction coil; dwell time 0.5 s; monitoring ion mlz = 74. Standards. An inorganic germanium standard solution (lo00 mg/L; germanium dioxide dissolved in water) was obtained from Wako Pure Chemical Ltd. Monomethylgermanium trichloride (purity, >98%) and dimethylgermanium dichloride (>98%) were purchased from Alfa Products. Trimethylgermanium chloride (>98%) was obtained from Tokyo Kasei Co. By diluting these reagents in water (inorganic germanium) or methanol, primary standards of about 1000 mg/L (as Ge) were made and were standardized by ICP-AES. Since the standardization of the TMGe solution without digestion was unreliable due to the high vapor pressure, TMGe was subjected to analysis after nitric acid digestion using a digestion bomb with a Teflon double vessel (20) and further decomposition by digestion with perchloric acid. From the primary standards, secondary standards (1.0 mg/L), tertiary 0 199 1 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
987
Xld 2
8,
1
-2
Hydride trap Liquid N2 bath
clean bench
; :
mgnetic stirrer
N~BH,
soh.
0
He
MMGe
50
0
Schematic diagram of the hydride generation -1CP-MS system for the determination of inorganic and methylated germanium species in natural waters and wastewaters: A, four-way valve; B, three-way valve. Figure 1.
standards (10 pg/L), and quaternary standards (100ng/L) were made by sequential dilution. Tertiary and quaternary standards were prepared every week. Reagents. Commercially available highest grade reagents were used after checking the blanks. The hydrochloric acid used was ultrapure grade (Kanto Chemical Co.). A 0.5 M malic acid solution was prepared by dissolving the reagent (Nacalai Tesque Ltd., Kyoto) in water. A 2% (w/v) sodium tetrahydroborate solution was prepared by dissolving the reagent (Kodak Ltd.) in 0.1% sodium hydroxide solution and filtering through a 0.45-fim membrane filter. A 0.1 M EDTA solution was prepared by dissolving Na2(EDTA)(Dotite Ltd.) in water. All of the reagents were prepared inside a clean bench (class 1OOO) in a clean room (class 1OOo). Water was prepared by a Milli-Q water purification system (Millipore Ltd.). Sample Treatment. Water samples were filtered through a 0.45-pm membrane filter, acidified to pH 2 with hydrochloricacid, stored in Teflon bottles that had been steeped in 6 M nitric acid for several days and rinsed with water, and kept at 4 OC until analysis. Sewage samples were analyzed within a week. Procedure. A sample solution (up to 7.5 mL), 2 mL of 0.5 M malic acid, and 0.5 mL of 0.1 M EDTA solution were placed in the reaction vessel; the volume was adjusted to 10 mL with water. One drop of n-octyl alcohol was added to seawater samples to suppress foaming. A spin bar was added to the vessel, which was placed in the hydride generation unit, and the three-way valve (Figure 1) was set to the ventilation position. Helium gas was passed through the system for 2 min to purge air. The U-tube was immersed in the liquid nitrogen vessel and cooled for 1min. Sodium tetrahydroborate solution (1mL) was added to the sample solution over 15 s, and the generated germanium hydrides were collected for 4 min. The four-way and three-way valves (Figure 1) were then turned to the bypass position and sample introduction position, respectively. After 1 min, the U-trap was removed from the liquid nitrogen, and the hydrides were evaporated and separated according to their boiling points. After all the germanium species had been evaporated, the trap was warmed with the Nichrome wire heater to remove condensed water.
RESULTS AND DISCUSSION Optimization of Parameters for Hydride Generation. Reaction Matrix. Hambrick et al. (5) recommended a tris(hydroxymethy1)aminomethane hydrochloride buffer system (Tris-HC1) as the reaction matrix for methyl germanium species as well as for Gei. In addition to Tris-HC1, we tested oxalic acid and malic acid, which have also been recommended for Gei (2, 12). These matrices gave little difference in sensitivity for Gei and TMGe a t pH 2-8.5, whereas notable differences were observed for MMGe and DMGe. The sensitivities for MMGe and DMGe with the malic acid matrix were 4-5 times superior to those with the Tris-HC1 matrix
150
100 Malic acid,
cylinder
mM
Figure 2. Relative sensitivities of inorganic germanium (Ge,),monomethylgermanium (MMGe), dimethylgermanlum (DMGe), and trimethylgermanium (TMGe) as a function of malic acid concentration. Twenty milligrams of NaBH, was used. Reaction matrix was 100 mM malic acid.
x ~I ~ 5 MMGe
x
d
oh
10
20
30
40
Amount of NaBHa, mg
Figure 3. Relative sensitivities for Gel, MMGe, DMGe, and TMGe as a function of NaBH, amount. Amounts of standards were 105, 102, 94, and 91 pg of Ge for Ge,,MMGe, DMGe, and TMGe, respectively. Twenty milligrams of NaBH, was chosen after considering the reagent blank for Gel.
at p H 2-8.5. The malic acid matrix was also superior to the oxalic acid matrix for the determination of MMGe; the sensitivity with malic acid was 10 times higher than that with oxalic acid in the same pH range. Therefore, we chose malic acid for the reaction matrix. Figure 2 shows the effect of malic acid concentration on sensitivity for the four species. Maximum sensitivities were attained at 75-125 mM malic acid for Gei and 100-150 mM for MMGe and DMGe. The sensitivity for TMGe was constant between 25 and 150 mM malic acid. Thus, we used 100 mM malic acid. Amount of Sodium Tetrahydroborate. The signal intensity was constant for TMGe in the investigated range (10-40mg of NaBH,) and was a maximum for MMGe and DMGe a t 20-30 mg of the reagent, whereas the intensity for Gei increased from 10 to 40 mg (Figure 3). T o reduce the reagent blank for Gei, we used 20 mg of the reductant (1.0 mL of 2% NaEiH4). The reduction efficiencies estimated by repeating the NaEiH4 injection were 75.9%,97.0%,96.8%,and 99.9% for Gei, MMGe, DMGe, and TMGe, respectively. Collection Time. Four minutes was enough for the collection of the four species (Figure 4). Reduction of Gei takes slightly longer than reduction of methylated germanium species under the reaction conditions. Carbon Dioxide Trap. A Teflon tube packed with sodium hydroxide powder was adopted for the trapping. When the carbon dioxide trap was removed from the system, the peak for MMGe on the chromatogram was distorted and sometimes
988
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
Table I. Detection Limits for Hydride GenerationInductively Coupled Argon Plasma Mass Spectrometry ~
compd Gei MMGe DMGe TMGe
limit of detection: pg graphite present furnace method AAS' 0.08 0.10 0.10 0.09
155 120 175 75
~
A I"
3-
limit of determinationb concn (7.5 abs, pg mL), ng L-I 0.26 0.34 0.32 0.31
0.035 0.05 0.04 0.04
1
'-
I
n
a Based on triple the standard deviation of replicate blank measurements (n = 6; peak area with background correction). bBased on 10 times the standard deviation of replicate blank measurements (n = 6). (Reference 5; based on twice the standard deviation of blank measurements (GeJ or of the baseline noise (MMGe, DMGe,and TMGe).
was split into two. However, the reason for this was not identified. Trapping and Separation. Of the three tested trap packings, Le., quartz beads (60-80 mesh), silanized quartz beads, and 5% OV-17 on Chromosorb AWW-DMCS (60-80 mesh), the quartz beads gave the best separation of germanium hydride and methylated germanium hydrides. The helium flow rate also affected the separation. While an increase in the flow rate shortened the retention times and provided sharper peaks, the separation of the methylated germanium hydrides became less. We adopted a helium flow of 50 mL/min to minimize measurement time and to obtain baseline separation of all four standards. Typical chromatograms are shown in Figure 5. Electrical heating of the U-tube for the rapid evolution of methylgermanium hydrides gave no analytical advantages in our system. Detection Limits and Linearity. The limits of detection achieved by the present method are shown in Table I together with those previously reported ( 5 ) . Because of the high sensitivity of the ICP-MS detector and modification to the reaction conditions, the limits are 3 orders of magnitude better than those previously reported for graphite furnace AAS (5). The calibration curves by peak area are linear over 4 orders of magnitude, that is, from the detection limits (sub-pg amounts) of the Gei, MMGe, DMGe, and TMGe species to ng amounts. Precision. The relative standard deviations at blank levels (0.24-0.63 pg of Ge) and 10 pg of Ge by peak area (n = 6) were 21% and 5.2% for Gei, 8.1% and 1.5% for MMGe, 14% and 2.1% for DMGe, and 6.1% and 2.9% for TMGe. Peak Characteristics. The retention times were 39 s for Gei, 64 s for MMGe, 82 s for DMGe, and 97 s for TMGe with
t
i
II
Retention time, s
Figure 5. Chromatograms showing peaks for germanium species: (A) standards (Ge,, MMGe, DMGe, and TMGe) with the same amounts as in Figure 3; (6) 5 mL of surface seawater from Ishikari Bay (Japan Sea); I, germanium hydride; I I, monomethylgermanium hydride; I1 I, dimethylgermanium hydride: IV, trimethylgermanium hydride. relative standard deviations less than 7% ( n = 22). The peak width at half-height for these germanium species at 10 and 100 pg were 1.3 and 1.6 s for Gei, 2.9 and 3.8 s for MMGe, 3.3 and 3.8 s for DMGe, and 3.8 and 4.2 s for TMGe. Interferences. We checked interferences by the standard addition of known amounts of the four germanium standards into seawater and sewage water and obtained 92-103% recoveries. Selenium(1V) is a possible interfering ion, as the element has an isotope of mass 74 (abundance, 0.87%), but the most
Table 11. Determination of Inorganic a n d Methylated Germanium Species in N a t u r a l Water a n d Wastewater Samples sample coastal surface seawater" Ishikari Bay, Japan Sea (Aug 1989) Funka Bay, Pacific Ocean (Aug 1989) Australia, South Indian Ocean (Aug 1989) Australia, South Indian Ocean (-50 m; Aug 1989) river water Ishikari River, Sunagawa, Hokkaido (Nov 1989) Ishikari River, near estuary (Nov 1989) tap water, Sapporo city (Sept 1989) wastewaterb (Nov 23, 1989) 1, untreated 2, treated in a sewage plant 3, treated in a sewage plant
Gei 0.51 0.84 0.48 0.46
concentration of Ge, ng L-I MMGe DMGe TMGe 22.3 20.4 23.4 24.0
total Ge
5.2 5.5 6.2 6.3
n.d. n.d. n.d. n.d.
28.01 26.74 30.10 30.76
24.6 30.0 55.0
0.36 1.40 0.21
0.10 0.34 0.07
n.d. n.d. n.d.
25.06 31.74 55.28
68.0 58.0 56.0
0.34
0.88
0.41 0.56
0.79
0.50
n.d. n.d. n.d.
68.75 59.44' 57.29'
"Salinity is 31.79-33.75% except the sample from South Indian Ocean (50-m depth) whose salinity is 29.26% "Taken treatment plant in Sapporo city on the same day. 'Two known species are eliminated.
from a wastewater
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
two unknown germanium species in the wastewaters were produced by microbial activity during the sludge treatment under aerobic condition (5000 microbes/mL, dominant species is Vorticella microstoma). Recently, Lewis et al. (9,211 reported the presence of TMGe in the liquid fraction of sludge samples taken from both aerobic and anaerobic digesters of a sewage treatment plant. However, we could not find TMGe in wastewaters from a similar plant. Environmental water samples we have analyzed so far contained no detectable amount of TMGe.
lo3
1.6
VI
c
c
3
u
Ii3 50
100 150 Retention time, s
Q8Q
200
Flguro 6 . Chromatograms of a wastewater treated in a sewage treatment plant (A) and that spiked wlth 5 pg of Ge as TMGe (B). -1, Ge,;2, MMGe; 3, DMGe; 4 and 5, not identified.
sensitive single ion monitoring at m/z = 74 is possible for the determination of germanium because hydrogen selenide produced by sodium tetrahydroborate reduction would have been trapped by sodium hydroxide beads (carbon dioxide trap) if present. Application. Analytical results for germanium species in natural waters and wastewaters are shown in Table 11. In addition to Gei, MMGe and DMGe were detected from all of the water samples including tap water. The concentration of Gei in coastal surface seawater was a t the sub ng L-' level, which was in accordance with the previous study (8). MMGe in the seawater samples accounted for 76-8070 of the total germanium determined. The concentration of DMGe in the seawater samples, 5-6 ng L-*, was slightly lower than that of a previous report (6). The concentrations of MMGe and DMGe in wastewaters from a sewage treatment plant in Sapporo, where sewage-sludge treatment had been carried out, were slightly increased compared to those of untreated water. Furthermore, the treated wastewaters contained two unknown germanium species (Figure 6A). The peak that appeared after DMGe (peak 4) was different from TMGe as shown in Figure 6B. Molecular interferences are unlikely in these cases because the peaks also appeared in the same position when m / z = 72 instead of m / z = 74 was monitored with identical relative intensity to other peaks and because the isotope ratio 72Ge/74Gecorresponded roughly to the correct natural ratio. These data suggest that MMGe, DMGe, and
ACKNOWLEDGMENT We thank Dr. J. S . Edmonds of the Western Australian Marine Research Laboratories for supplying seawater samples of South Indian Ocean and Dr. K. Okamoto of National Institute for Environmental Studies for helpful discussions. LITERATURE CITED (1) (2) (3) (4) (5) (6)
(7) (8) (9) (10) (1 1) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
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RECEIVED for review October 15,1990. Accepted February 27, 1991.
