772
Anal. Chem. 1986, 58,772-777
Speciation of Inorganic Tin and Alkyltin Compounds by Atomic Absorption Spectrometry Using Electrothermal Quartz Furnace after Hydride Generation Olivier F. X. Donard,' Spyridon Rapsomanikis? and James H. Weber* Chemistry Department, Parsons Hall, University of New Hampshire, Durham, New Hampshire 03824
We speclated lnorganlc tln and methyl- and n-butyltin compounds by volatlllzatlon from water samples by hydride generatlon, separatlon by a chromatographlc packlng materlal, and detection by atomic absorption spectrophotometry In an electrothermal quartz furnace at the 224.61-nm wavelength. Absolute detectlon llmlts of 20-50 pg ( 3 4 as Sn and linearity of calibration curves up to 30 ng as Sn avoid preconcentratlon by extraction and allow direct determlnatlon of methyl- and n-butyltln compounds from envlronmental waters. Reproduclblllty for 15 ng as Sn Is less than 15% for most alkyltln compounds. Thls study dlscusses optlmlratlon of parameters and results from natural waters.
There is growing concern about the presence of anthropogenic alkyltin compounds in the environment. For example, methyl- and n-butyltin compounds, which are widely used as stabilizers, biocides, antifouling agents in paints, and bactericides, escape into the environment (1). Serious problems occur through their impact on the biota. Toxicity of n-tributyltin inhibits development of oyster larvae (2). Correlation between exposure from organotin compounds in antifouling paints and failing oyster fisheries is known (3). Concentrations in the 10 ng of Sn/mL range of various methyl- and n-butyltin compounds are also lethal to fish, crustaceans, and molluscs (1).
Not all aquatic methyltin compounds result from man-made pollution, and it is necessary to understand their global cycles and fate in order to distinguish anthropogenic input and natural processes. Environmental methylation of tin undoubtedly contributes to the occurrence of methyltin compounds in natural waters. Reactions such as oxidative addition of carbocation (CH,+) donors and nucleophilic attack by carbanion (CH,-) donors along with redistribution and disproportionation reactions can modify the distribution of alkyltin species in the environment (4). Methylation processes can also affect man's contributions, since Maguire (5) observed (n-Bu)3MeSn and (n-Bu)2Me2Snmoieties, which are very unlikely to be of anthropogenic origin. Furthermore, various methylated tin species, which can represent up to 90% of total tin present in natural waters (6), show very different adsorption behavior under simulated environmental conditions (7). Thompson et al. ( I ) have reviewed a variety of techniques for determinations of low concentrations of alkyltin compounds (RSn). Most of them represent coupling of gas chromatographic (GC) or liquid chromatographic (LC) separation with electron capture, flame photometric, flame ionization, mass spectrometric, or atomic absorption spectrophotometric (AAS) detectors. Present address: Laboratoire de Chimie Physique A, Universit6 de Bordeaux I, 33405 Talence Cedex, France. 2Presentaddress: Department of Chemistry, University of Essex, Colchester C04359, UK.
Derivatization techniques have been quite successful for volatilization of alkyltin species. For example, Chau and co-workers (8, 9) determined methyl- and n-butyltin compounds by GC separation followed by AAS or flame photometric detection after alkylation by a Grignard reagent. However, this technique involves several steps such as extraction by a solvent, derivatization, and concentrating by evaporation. These manipulations may lead to errors. Many papers have described the experimentally simpler hydride generation approach to volatilization (6, 10-15). Hodge et al. (11) pioneered use of AAS to detect alkyltin compounds, but the method lacked sensitivity in comparison to apparatus using the Sn-H bond emission with a hydrogen-air flame emission detector (6). Later detectors achieved high sensitivity and low detection limits but had less specificity and a narrow calibration range. Furthermore, many techniques address only methyl- or n-butyltin compounds or have poor separation. We further developed the technique by using the ease of volatilization for both methyl- and n-butyltin compounds by hydride generation and the flexibility and specificity of AAS as a detector. Optimization of the process included steps of hydride generation, cryogenic trapping and separation of alkyltin hydrides (RSnH,) on a chromatographic trapping material, and AAS detection using an electrothermally heated quartz furnace. We achieved linear calibration ranges of 2 magnitudes and detection limits ( 3 ~of) ca. 50 pg for inorganic Sn and methyl- and n-butyltin compounds. The sensitivity and dynamic range obtained with the optimized setup followed direct simultaneous determination of both methyl- and nbutyltin compounds in estuarine bulk waters, microlayers, and sediment pore waters.
EXPERIMENTAL SECTION Apparatus. The optimized setup (Figure 1) includes a 5 X 7 cm round-bottom hydride generator (100-mL sample size) with an injection port on the side for a septum. The optional water trap is a U-shaped (45 X 6 mm i.d.) Pyrex tube with a small bulb on the hydride generator side of the U to prevent clogging by ice. Organotin hydrides were trapped at -196 "C (liquid N,) on 2.5 g of Chromosorb G AW-DMCS (45-60 mesh) coated with 3% SP-2100 packed in a U-shaped (45 cm, 6 mm id.) Pyrex trap. The trap was wrapped with Nichrome wire (26 gauge, 17.5 Q resistance). The 11cm X 1.2 cm quartz furnace (Figure 1)has one major inlet located in the middle and a gas premixing chamber (2 X 1 cm). It was wrapped with a 1-mm layer of asbestos, coiled with a doubled strand of 26-gauge Nichrome wire (7 R resistance), insulated by asbestos cord (0.8 cm diameter), and mounted on a custom stainless-steel frame placed on the AAS burner head (CAUTION: Asbestos is a known carcinogen.) Heated flexible tubing transfer lines (2.5 mm i.d.) made of Teflon were inserted in 3-mm-i.d. Tygon tubing. Nichrome wire (28 gauge, 65 Q resistance) was coiled around the Tygon tubing and insulated by tape made of Teflon. A Hamilton four-way valve allows the He carrier gas to bypass the hydride generator. Teflon-to-Teflon and Teflon-to-Pyrex connections were made with Omnifit Teflon variable-bore connectors. Power to transfer lines, trap, and furnace was individually supplied by Variacs. Organotin hydrides were detected with a Model 503 Perkin-Elmer AAS. The Westinghouse tin electrodlessdischarge lamp (EDL) is operated on continuous
0003-2700/86/0358-0772$01.50/00 1986 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
E
IG
HQ Flgure 1. (a)Apparatus for the speciation of Inorganic tin and alkyltin compounds: A, four-way valve; B, hydride generator; C, heated transfer lines; D, optional water trap in dry idacetone bath; E, separation trap in liquid N;, F, quartz furnace; G, burner head; I.G., inert gas. (b) Quartz furnace, top view.
mode by a Westinghouse EDL power supply. The output signal (1-V full deflection) is amplified 10-fold, filtered by a low-pass filter, and integrated by a Hewlett-Packard Model 3392 A integrator. The H2and He flow rates are controlled by Cole-Parmer flow meters and that of O2 by a precise Brooks flowmeter. The optimum operating conditions follow. The tin EDL is used at 12 W in the continuous mode. The AAS operates at 224.61 nm with a 1-mm slit width and 0.3-s integration repeat mode. The AAS output signal is amplified 10-fold and filtered at 1.3 Hz. The integrator parameters are as follows: attenuation, 8; threshold, 8; peak width, 0.16; and chart speed, 1.5 cm/min. Gas flow rates are 90 mL/min for 02,400 mL/min for He, and 1200 mL/min for H2. The Variacs' outputs are to the transfer line (45 V), trap (5 V), and furnace (30 V). Temperatures are 95 "C in the transfer lines, 20-200 "C in the trap, and 950 "C in the furnace. Temperatures were measured with a chromel/alumel thermocouple. Reagents. Laboratory water, which was deionized 3 times and then distilled through a Corning Mega-pure still, was used in all experiments. All glassware used for preparation, storage, and dilutions was soaked in 7 % HNO, for at least 12 h and rinsed with water. HN03 solutions (5 M) were prepared from either J. T. Baker Ultrex or Alfa Ultra pure HN03 acid to ensure low tin blanks. Aqueous 4% NaBH, was prepared from Ventron 99% sodium borohydride. The solutions were filtered with a 0.2-pm Nuclepore filter, purged with N2for 30 min, and allowed to stand 12 h in a glass flask to reduce the tin blank and ensure optimum reproducibility. Methanol was Fisher certified ACS Spectroanalyzed grade. Fulvic acid (FA) solutions were made from characterized fulvic acid extracted from soil (16). Fe(II1) solutions were diluted from 1000 pg/mL Fisher AAS standards. All other reagents unless stated otherwise were of analytical grade. Standards. All RSn standards were purchased from Alfa Ventron, and all except MeSnCla were used without further purification. MeSnCl, was purified by three successive recrystallizations from petroleum ether (bp 100-110 "C). GC analyses showed that 98% of the methyltin compounds in the purified sample is MeSnC13. A 1000 pg/mL stock solution of inorganic tin was prepared by dissolving 1g of 99.9% 20 mesh Sn powder (Alfa) in 100 mL of concentrated HCl and adjusting the volume to 1 L with €320. Methyltin chloride stock solutions (1000pg/mL as Sn) were diluted with water and acidified with 1 mL of 5 M HN03 in 100-mL volumetric flasks. All stock solutions except Me3SnC1,when kept at 4 "C in the dark, were stable over several months. Me3SnC1,however, was stable in Hypo-vials sealed with crimp-on Teflon-lined septa. Butyltin chlorides, ( n - B ~ ) ~ s nand H, Me,Sn were prepared by dissolving them in 20 mL of methanol (total concentration 100 pg/mL as Sn) and keeping them in 20-mL Hypo-vials sealed with crimp-on Teflon-lined septa. Individual and mixed stock solutions were stable over several months.
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Calibrations were done differently for various tin compounds. Standards for inorganic tin and ionic methyltin compounds were made by dilution of their stock solutions to 100 mL. Stock solutions of Me4Sn and n-butyltin compounds were directly injected through the septum of the reaction vessel using a Hamiltonian 0.5-pL syringe. Operating Procedure. Samples of 50-100 mL were introduced into the reaction flask, They were acidified by 0.2 mL of 5 M HN03 for detbrmination of MeSn compounds and by 1 mL of 5 M HNO, for simultaneous determination of MeSn and nBuSn compounds. The magnetic stirrer was started, and He bubbled through the sample for 4-5 rnin to strip O2 or any other volatile gas from solution. First the Me4% internal standard and then 1.5 mL of 4% NaBH, were injected through the septum. Four minutes later 1.5 mL more of NaBH, was added, and the sample was purged 4 min more. The hydride vessel was then bypassed by use of the four-wayvalve; the trap was removed from the liquid nitrogen, and 5 V was applied with the Variac. After elution of the last MeSn compounds (Me4Sn),the current was boosted to 20 V to allow elution of n-butyltin hydrides. Environmental Sampling. A variety of samples was collected during different seasons from the Great Bay Estuary. Microlayer samples of 40-70 pm thickness were collected by use of the glass plate technique (8). Bulk water samples were collected in acidcleaned Pyrex 1-L bottles, filtered immediately aboard ship using acid-cleaned 0.4-pm Nucleopore filters, and kept in Pyrex containers at 4 "C in the dark. All determinations were done in duplicate within 2 days after collection. Pore water samples were collected from the oxic layer of sediments (1-2 cm depth), centrifuged for 10 min at 12 000 rpm and 4 "C, and filtered at 0.4 pm. Samples were spiked with 1mL of 5 M HNO, and digested for 1 2 h before determinations. Salinities were determined by using a refractometer.
RESULTS AND DISCUSSION Hydride Generation. Alkyltin compounds react with NaBH, under acidic conditions to yield the corresponding hydrides (eq 1). In agreement with Andreae and Byrd (13) NaBH,
R,Sn(4-,)
H+
R,SnH,,-,,
+ H2 t
(1)
n = 1, 2, 3 studies on a variety of NaBH, concentrations and pH values showed that the highest absorbance for methyltin compounds resulted from acidification of the sample to pH 2 followed by addition of 1.5 mL of 4% NaBH,. The absence of a buffer minimized possible interferences and contamination of samples. Injection of NaBH, to a pH 2 sample yields a very large volume of H2, which contributes significantly to increased scrubbing of organotin hydrides from samples. We studied the removal efficiency as a function of pH by recording simultaneously the AAS absorbance and the change in pH during hydride generation by connecting the hydride generator directly to the furnace and using a pH electrode in the reaction flask. The p H typically increases from 2 to 8, but scrubbing of RSn is completed at an intermediate pH value of 3-4. Scrubbing under these conditions is most efficient, and inorganc tin and methyltin hydrides effervesce from the solution within 4 min. The use of a magnetic stirrer and injection of NaBH, below the liquid level of the sample improved the reproducibilty of the hydride generation step. A second 1.5-mL NaBH4 injection after 5 min did not generate further methyltin hydride. We compared peak areas of (n-Bu),SnH formed from ( n - B ~ ) ~ S nby c lhydride generation and of (nBuI3SnH placed in the hydride generator and obtained 100 +/- 5% recovery of (n-Bu),SnCl. We observed no butyltin hydride decomposition products. However, for n-butyltin compounds or samples in complex matrices such as microlayers an additional injection of NaBH, in acidified samples ensured complete removal of alkyltin hydrides from solution. In organic-rich matrices generation of alkyltin hydrides presumably competes with reduction of
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
organic matter present. It is likely that the organic matter (L) complexes inorganic tin(1V) (eq 2) and/or reduces it to tin(I1) (eq 3). Several consequences of eq 2 and 3 are possible.
+L Sn(1V) + L Sn(1V)
--
SnIVL
(2)
SnIILox
(3) First, the decreased concentration of hydrated Sn(1V) due to complexation could make SnH, formation more difficult. Second, SnH, formation from SnI1LoXor Sn(I1) released from it could be a slower process. Transfer Lines and Water Traps. The small 2.5 mm diameter of the transfer lines contributed to the overall high sensitivity of the system by reducing considerably the dead volume. Heating to 95 "C significantly increased the recovery and diminished tailing in the signal of high-boilingcompounds, especially (n-Bu),SnH (bp 78 "C at 0.5 mmHg). The water trap is necessary only to extend the packing-phase lifetime. For determination of methyltin compounds, the U-shaped Pyrex tube immersed in dry ice/acetone (-78 "C) very efficiently removes water and allows measurements on 60 samples before the necessity of changing the packing phase. However, the trap also removes most n-butyltin hydrides. We investigated several systems including an ice bath and room-temperature traps containing Drierite, 5A molecular sieves, and 2.5 g of 5% Ethofat 60/25 on 40/60 mesh Chromosorb T. In all cases the water trap caused partial or complete removal of n-butyltin compounds from a mixed standard of methyland n-butlytin compounds. Therefore, best results for optimum recovery of n-bultytin hydrides occurred without a water trap. Condensation of water in the separating trap does not interfere with the signal during the elution of alkyltin compounds. However, deterioration of the packing phase occurs rapidly and only 15 experiments are possible before its necessary replacement. Furnace Optimization. Increased atomization efficiency will contribute to overall improved sensitivity. For this purpose, we tested three configurations of quartz furnaces to optimize absorbance signals by minimizing dead volume in the furnace (17). Furnace efficiencies based on absorption signals from a mixed inorganic tin and Me3SnCl standard a t 950 "C showed that furnace configuration is important. The configurations and relative efficiencies are as follows: 60 mm x 9 mm i.d. (l.O), 110 mm X 12 mm i.d. (1.5), and 110 mm x 15 mm i.d. (1.0). The 110 mm X 12 mm i.d. furnace allows a residence time of alkyltin hydrides of 0.3 s. Use of a chamber for mixing He, 02,and H2 gases before the AAS beam path increased the signal-to-noise ratio. The furnace has a stable flame burning on either side as described by Nakahara (18). Blanketing the furnace by a stream (5 L/min) of inert gas via the burner head of the AAS contributed to extending the furnace lifetime and stabilizing the flame, thus decreasing noise. Argon gave better results than nitrogen. Deterioration of the quartz surface decreases sensitivity as documented for other volatile hydride-forming elements (19). After 2 months of operation full sensitivity can sometimes be regained by soaking the inside surfaces of the quartz cell in a solution of 10% HF, but sometimes replacement is necessary. Influence of Gas Flow. Changes in He carrier gas flow rates influence both scrubbing efficiency and speciation of alkyltin hydrides. Higher gas flows are necessary for less volatile n-butyltin compounds than for methyltin ones. A 400 mL/min flow rate is the best compromise for both alkyl groups. Addition of Hz and O2 to the furnace is known to enhance drastically absorbance signals (19,20). H2 alone did not significantly influence atomization of hydrides, but addition of O2 was determinant in increasing the sensitivity in an H2-rich flame. Figure 2 shows absorbances obtained for 15 ng as Sn of Me4Sn, (n-Bu)SnH3,and (n-BuJ3SnHin a
0
100
5 )O O 2 Flow rnL/min
Figure 2. Effect of oxygen flow rate on peak area of 15 ng as Sn of a mixed standard of Me4Sn ( O ) ,(n-Bu)Sn3+(A)and (n-Bu),Sn+ (0). The hydrogen flow is 1200 mLlmin, and that of He is 400 mL/min.
mixed standard as a response to O2 flow rate using 1200 mL/min H2 and 400 mL/min He. Optimum response as a compromise for both methyl- and n-butyltin compounds is 90 mL/min. The significant and determinant contribution of O2 in increased absorbance responses suggests that atomization processes within the furnace may occur through collision with H atoms in a rich hydrogen/oxygen flame (18, 19). This hypothesis is further supported by the fact that in a cool (200 "C) hydrogen/oxygen flame in the absence of electrothermal heating, the absorbance signals of alkyltin hydrides are still 15% of their optimum sensitivity. The processes may be similar to those postulated for arsine in which H atoms successively remove hydrogen (18). For example, a dialkyltin hydride would react as in eq 4. Collisions with H. should break weaker 193 kJ/mol Sn-C bonds as well as stronger 252 kJ/mol Sn-H ones and form tin atoms. R2SnH2 4H. 2RH 2H2 Sn" (4)
+
-+
+
+
Separation Conditions. Our previous study (7) showed that optimum sensitivity for methyltin compounds occurred with 2.5 g of 10% SP-2100 on Chromosorb G AW-DMCS (80-100 mesh). Retention times are as follows: SnH4 (0.44 min), MeSnH, (1.59 min), MezSnH2(2.48 min), Me3SnH (3.34 min), and Me4Sn (4.12 min). Reproducibility of retention time is less than 3% relative standard deviation (RSD) for inorganic tin and methyltin compounds using a heating rate of 0.8 OC/s. This work reports optimized separation conditions for both methyl- and n-butyltin compounds. Good resolution for nbutyltin compounds occurred by decreasing the SP-2100 loading to 3% and increasing the mesh size of the support to 40-60 mesh. The former change decreases the binding strength to the SP-2100 phase, and the latter change allows higher He flow rates. Typical retention times for a 3-min heating at a 0.8 OC/s rate followed by a 1.2 "C/s one are as follows: SnH, (0.44 min), MeSnH, (1.10 rnin), Me2SnH2(1.95 min), Me3SnH (2.32 min), Me,Sn (2.90 min), (n-Bu)SnH3(3.15 min), ( ~ - B U ) ~ (5.50 S ~ Hmin), ~ and (n-Bu),SnH (7.43 rnin). Typical retention times vary slightly among different batches of packing phase. Figure 3 represents a typical simultaneous separation for four methyltin and three n-butyltin compounds. As observed by Braman and Tompkins (6) occasional splitting of the Me2SnHzpeak may occur. Peak splitting is more common for inorganic tin. Sometimes a peak occurs 0.30 min after elution of SnHk The sum of the two areas fitted inorganic tin calibration curves. This second peak area is minor (
