Analytica Chimica Acta, 184 (1986) 197-203 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

SPECIATION OF n-BUTYLTIN COMPOUNDS BY ATOMIC ABSORPTION SPECTROMETRY WITH AN ELECTROTHERMAL QUARTZ FURNACE AFTER HYDRIDE GENERATION

LOUISE RANDALL, OLIVIER F. X. DONARDa and JAMES H. WEBER*

Chemistry Department, Parsons Hall, University of New Hampshire, Durham, NH 03824 (U.S.A.)

(Received 15th December 1985)

SUMMARY

A method is described for the speciation of n-butyltin compounds. The compounds are volatilized from aqueous solution, trapped on a chromatographic packing material contained in a teflon column, and separated on the basis of differing boiling points. The column is attached directly to an electrothermal quartz furnace of an atomic absorption spectrometer for detection at 224.61 nm. Absolute detection limits (30) are 11-45 pg as tin, calibration curves are linear to 13 ng, and reproducibility at the 2-ng level ranges from 5% for mono-n-butyltin to 18% for tri(n-butyl)tin.

The extensive use of n-butyltin compounds [l] allows the ready introduc- tion of these compounds into the environment [2]. Extremely low levels of alkylated tin compounds are acutely toxic to non-target organisms [3]. In France, for example, leaching of tri(n-butyl)tin compounds from marine anti- fouling paints is considered to affect oyster larvae adversely [43.

Thompson et al. [l] reviewed techniques for the determination of low concentrations of n-butyltin compounds. Atomic absorption (or emission) spectrometry (a.a.s.) without prior separation has detection limits of 0.3 ng ml-’ tin [ 51 but does not speciate n-butyltin compounds [l] . Gas chromato- graphy of aquated n-butyltin compounds with a modified flame photometer [ 6-811, mass spectrometer [ 91, or flame ionizationquenching [lo] as detec- tor, involves derivatization prior to detection [l] . Ebdon et al. [ 111 speciated tin compounds by coupled high-performance liquid chromatography/flame a.a.s. Hodge et al. [12] used hydride derivatization and separation on a glass wool column, followed by a.a.s. detection. Donard et al. [13] reported the speciation of methyltin and n-butyltin compounds by hydride generation, cryogenic trapping, separation on chromatographic packing material, and detection by a.a.s. with a quartz furnace.

aPresent address: Laboratoire de Chimie Physique A, Universite de Bordeaux I, 33405 Talence Cedex, France.

0003-2670/86/$03.50 D 1986 Elsevier Science Publishers B.V.

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The chromatographic/a.a.s. technique for aqueous solution is further developed here by using a column attached directly to the quartz furnace to obtain simultaneous detection limits (30) of 11-45 pg for n-butyltin com- pounds with linear calibration from sub-nanogram to nanogram quantities. The detection limit of 45 pg for tri(n-butyl)tin is a factor of 4 lower than previously reported [13] and the technique requires a minimum of sample manipulation. Determination of tetra(n-butyl)tin as well as direct and simul- taneous determination of methyl- and n-butyltin compounds are possible.

EXPERIMENTAL

Appam tus Butyltin hydrides were detected with a Perkin-Elmer Model 503 atomic

absorption spectrometer fitted with a Westinghouse electrodeless discharge lamp (EDL) for tin, operated on continuous mode by a Westinghouse EDL power supply. The hydride generator (5 X 7 cm, loo-ml sample size), hydride trap (35 cm, 0.4-cm i.d.), and quartz furnace (10.5 cm, l.l-cm i.d.) are illus- trated in Fig. 1. Butyltin hydrides were trapped at -196°C (liquid nitrogen) on 2.5 g of Chromosorb G AW-DMCS (45-60 mesh) coated with 3% SP-2100 packed in the U-shaped teflon trap. The trap was prepared from Cole-Parmer Type T-6407-44 teflon TFE tubing, and was wrapped with nichrome wire (26 gauge, 0.088 ohm cm-’ resistance). The major inlet of the previously de- scribed furnace [ 131 was wrapped with the same type of nichrome wire. Two heated flexible transfer lines were prepared from teflon PTFE tubing. The inner tube (0.3cm o.d., 0.1~cm wall) was inserted into the outer tube (0.5~cm o.d., 0.05-cm wall), which was wrapped with nichrome wire (28

0 a

-w-

z Y

A 7 O2

A 1

H2 A He

Fig. 1. (a) Apparatus for the speciation of n-butyltin compounds: A, four-way valve; B, hydride generator; C, heated transfer lines; D, separation trap in liquid nitrogen; E, quartz furnace. (b) Dimensions of optimum quartz furnace: W, 10.5 cm; X, 1.2 cm; Y, 1.5 cm; Z, 0.5 cm.

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gauge, 0.35 ohm cm-’ resistance) and insulated with teflon tape. A Hamilton four-way valve allows the helium carrier gas to by-pass the hydride generator. Teflon-to-teflon and teflon-to-quartz connections were made with Omnifit teflon variable-bore connectors. Power to the transfer lines, trap, furnace inlet, and furnace was individually supplied by Variacs. The output signal (1 V full deflection) was amplified lo-fold, filtered by a low-pass filter, and integrated by a Hewlett-Packard Model 3392 A integrator. The hydrogen and helium flow rates were controlled by Cole Parmer flow meters and the oxygen flow rate by a precise Brooks flow meter.

Operating parameters determined by Simplex optimization were gas flow rates of helium = 400, oxygen = 21, and hydrogen = 833 ml min-‘. The tin EDL was used at 10 W in continuous mode. The spectrometer was operated at 224.61 nm with a l-mm slit and an 0.3-s integration repeat mode. The output signal was amplified lo-fold and filtered at 1.3 Hz. The Variac outputs were set at 50 V for transfer lines, 5-30 V for the trap, 8 V for the furnace inlet and 19 V for the furnace. Temperatures were 150°C in the transfer lines, 20-300°C in the trap, and 750°C in the furnace. Temperatures were measured with a chromel/alumel thermocouple. Integrator parameter settings were 8 for attenuation and threshold, 0.16 for peak width, and a chart speed of 1.5 cm min-’ .

Reagents and standards The water used in all experiments was deionized three times and then dis-

tilled through a Corning Megapure still. All glassware was soaked in 7% (v/v) nitric for at least 12 h and rinsed with water. Nitric acid solutions (5 M) were prepared from Fisher 70% nitric acid. Aqueous 6% (w/v) sodium tetrahydro- borate was prepared from the Aldrich (99+%) reagent. The solution was filtered through a 0.2~pm Nuclepore filter, and allowed to stand for at least 12 h in a glass flask to reduce the tin blank. Methanol was Fisher certified A.C.S. Spectroanalyzed grade.

Mono- and di( n-butyl)tin chlorides, tetra(n-butyl)tin, and triethyltin bromide were from Alfa Ventron, and tri(n-butyl)tin hydride and tri(n-butyl)- tin chloride from Aldrich. Standards (total concentration 50 Erg ml-’ as tin) were prepared (without purification) by dissolving the required compounds in 10 ml of methanol and storing them in lo-ml Hypovials sealed with crimp- on teflon-lined septa. All solutions stored in the dark at 4°C were stable over several months. The standards were injected through the septum of the hydride generator by a Hamilton 0.5~~1 syringe.

Opem ting procedure Water (100 ml) and nitric acid (2 ml, 5 M) were introduced into the

hydride generator. The magnetic stirrer was started at a medium speed, the trap (heating coil set at 5 V) was lowered into the liquid nitrogen, and helium was bubbled through the solution for 2 min. The n-butyltin standards and the triethyltin internal standard, followed by 2.5 ml of the tetrahydroborate

solution, were added via the septum. The stirrer was turned to high for 2 min, the stirrer speed was reduced, and then a further 2.5 ml of the tetrahydro- borate solution was added. After the bubbling had decreased, the stirrer speed was increased and the solution was purged for 4 min. The hydride generator was then by-passed via the four-way valve, the trap was removed from the liquid nitrogen, and after 2 min, 30 V was applied with the Variac.

RESULTS AND DISCUSSION

Hydridegenemtion, optimization of atomization and furnace design The reaction of tetrahydroborate and alkyltin compounds was described

earlier [ 131, but additional information is noted here. The solution pH was initially 1.6, rising to 2.2 after addition of the tetrahydroborate. Additional injections of tetrahydroborate solution did not generate more n-butyltin hydrides at the concentrations studied. A comparison of peak area of (n-Bu)$nH formed from (n-Bu)$nCl and of (n-Bu)3SnH indicated a 100 + 5% recovery from (n-Bu)$nCl.

Removal of the trap and connection of the hydride generator directly to the furnace allowed optimization of atomization conditions. The (n-Bu)$n+ peak area and peak height were observed for changing values of helium, oxygen and hydrogen. (For brevity, the cationic form (n-Bu),Sn+ is used here for all tri(n-butyl)tin species detected as the hydride.) The gas flow rates were initially checked by trial and error, and then fixed by using Simplex optimization [14]. Peak areas yielded slightly better reproducibility than peak heights. The reproducibility of the three highest peak areas was checked. The helium flow rate was critical for (n-Bu)$n+, but a high flow rate resulted in excessive back-pressure. A flow of 400 ml min-’ was most suitable. Opti- mum oxygen and hydrogen flow rates were 21 and 833 ml min-‘, respec- tively .

The furnace design yielding the highest atomization efficiency is illustrated in Fig. 1. The low volatility of (n-Bu)$n+ (b.p. 80°C at 0.4 mm Hg) required minimization of “cold spots”. A white deposit on the furnace inlet and a smaller (n-Bu),Sn’ peak were noted for furnaces having the oxygen inlet at a distance of more than 2 cm from the atomizing region. Heating the inlet also increased the (n-Bu)$n+ peak. In the previous experiments [ 131, a stream of inert gas was used to blanket the furnace to extend the furnace lifetime. A stream of inert gas was not used here because the furnace temperature was 200” C lower.

Development of the method Several systems based on glass rather than teflon were investigated. The

technique previously described [13] had a detection limit of 200 pg for (n-Bu)&r+ based on 3a of background and loo-ml samples. In an attempt to reduce this limit, Chromosorb T 45-60 mesh was used to replace the Chromo- sorb G packing, but the column was easily blocked under the conditions

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used. Silanized glass wool gave a separation of the n-butyltin compounds, but after three experiments, sensitivity decreased rapidly. Similarly, glass beads did not give a reproducible separation. An empty g.c.-type column (l-mm i.d., 150-cm length) coated with 3% SP-2100 gave an excellent separation with high sensitivity, but was again irreproducible. Silanizing the column prior to coating did not improve reproducibility. Use of a two- column system incorporating a U-tube containing glass wool or 5A molecular sieve and a g.c.-type column also suffered problems. The U-tube and g.c.-type column did not give reproducible results and the molecular sieves resulted in no detection of n-butyltin compounds. Activation of the wet (and heated) glass surface by the alkaline sodium tetrahydroborate and subsequent loss of n-butyltin compounds is a possible reason for the lack of reproducibility. The teflon columns used gave a reproducibility of 18% for 2 ng of (n-Bu)$n+ and detection limit of 45 pg as Sn (3~). However, the sensitivity decreased after 15 experiments because of deterioration of the SP-2100coated Chromo- sorb G.

Separation conditions The Chromosorb G with a mesh size of 45-60 was the most suitable for

the helium flow rate used, and a coating of 3% provided a good separation of the n-butyltin compounds (Fig. 2). Separation of stannane (1.2 ng), tri- methyltin (0.2 ng), mono-n-butyltin (2 ng), triethyltin (2.2 ng), di(n-butyl)- tin (2.6 ng), tri(n-butyl)tin (1.8 ng), and tetra(n-butyl)tin (2.2 ng) occurs within 4 min (Fig. 2). Teflon transfer lines allowed a high temperature to be maintained through the lines. Typical retention times of the hydrides (3% reproducibility) for 2-min heating at a rate of 0.8”C s-l followed by heating at 2.4’C s-’ were: stannane (0.50 min), mono-n-butyltin (2.08 min), triethyl- tin (2.63 min), di(n-butyl)tin (2.85 min), tri(n-butyl)tin (3.35 min) and tetra(n-butyl)tin (3.43 min). Redistribution of the compounds did not occur. The stannane results mainly from contamination of the sodium tetrahydro- borate by inorganic tin. The tin blank of 1.2 ng can be further reduced by purging the tetrahydroborate solution with nitrogen [13]. Splitting of the inorganic tin peak occurred occasionally as before [13]. Splitting of the tri(n-butyl)tin hydride peak, noted when a 2% coating of SP-2100 was used, probably indicates desorption from different sites on the packing material.

Sensitivity and selectivity Table 1 illustrates calibration data for triethyltin and n-butyltin com-

pounds at 224.61 nm in water. The sensitivity to the n-butyltin compounds decreases with increasing numbers of n-butyl groups. The linear calibration range extends from subnanogram quantities to ca. 12 ng. The range can be extended by using a less sensitive wavelength such as 235.48 nm [ 131. The detection limit of 45 pg for (n-Bu)&r+ is among the lowest reported, and a minimum of sample manipulation is needed. Triethyltin was chosen as an internal standard because it undergoes the same reduction process as mono-,

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I I 0 TIME (MN) 4

Fig. 2. Chromatogram of inorganic tin, trimethyl-, Methyl- and n-butyl-tin compounds. Peaks (and amounts as ng Sn): (1) inorganic tin (1.2); (2) trimethyltin (0.2); (3) mono-n- butyltin (2); (4) triethyltin (2.2); (5) di(n-butyl)tin (2.6); (6) tri(n-butyl)tin (1.8); (7) tetra(n-butyl)tin (2.2).

TABLE 1

Data for linear range, detection limits, and reproducibility for triethyltin and n-butyltin compounds

Compound

( n-Bu)Sn3+ Et,Sn+ (n-Bu)$n*+ (n-Bu),Sn+

Linear range8 (ng)

0.03-12 0.06-5 0.04-13 0.10-8

Slopeb x lo-’

0.747 (5) 0.321 (7) 0.558 (12) 0.159 (18)

IntercepV d se3t(Y) Detection

x lo-’ limite (PP)

-0.107 (6) 0.005 11 -0.305 (1) 0.003 22 -0.056 (14) 0.009 14 -0.096 (21) 0.004 45

aFrom 100 of blank to limit of linearity. bCorrelation coefficients are greater than 0.99. Slope values are ng Sn (arbitrary relative units)-‘. RSD (%), given in parentheses, is for 2 ng, as tin (n = 5). =Arbitrary relative units. dStandard error of estimate. eBased on 30 of background and loo-ml samples.

di-, and tri-(n-butyl)tin, hasa retention time between mono- and di-(n-butyl)- tin, and is not known to occur in the environment. By using a slower rate of heating, determination of methyltin compounds is possible, but with a slight loss of sensitivity for n-butyltin compounds.

Conclusion The method allows the simultaneous speciation of n-butyltin compounds

with little sample manipulation at detection limits of 11 pg for mono-, 14 pg for di-, and 45 pg for tri-(n-butyl)tin. The detection of methyltin compounds is also possible. The technique incorporates the flexibility and selectivity of a.a.s. as the detector, with detection limits lower than those reported previ- ously [8, 12, 131. The internal standard for the proposed method confirms proper operation and allows direct calibration for each individual compound. Speciation of n-butyltin compounds from complex matrices is under study.

This research was funded in part by the Office of Sea Grant, National Oceanic and Atmospheric Administration, and in part by EPA grant R 809416. It is contribution UNHMP-JR-SG-85-18 from the University of New Hampshire Sea Grant Program.

REFERENCES

1 J. A. J. Thompson, M. G. Sheffer, R. C. Pierce, Y. K. Chau, J. J. Cooney, W. R. Cullen and R. J. Maguire, Organotin Compounds in the Aquatic Environment, 1985, NRCC/ CNRC 22494 Ottawa, Canada.

2P. J. Craig, in G. Wilkinson (Ed.), Comprehensive Organometallic Chemistry, Vol. 2, Pergamon Press, Oxford, 1982, p. 979.

3 R. B. Laughlin, Jr. and W. J. French, Bull. Environ. Contam. Toxicol., 25 (1980) 802. 4 G. E. Walsh, L. L. McLaughlan, E. M. Lores, M. K. Louie and C. H. Deans, Chemo-

sphere, 14 (1985) 383. 5 P. Hocquellet, At. Spectrosc., 6 (1985) 69. 6 R. J. Maguire and H. Huneault, J. Chromatogr., 209 (1981) 458. 7 R. J. Maguire, Environ. Sci. Technol., 18 (1984) 291. 8 R. J. Maguire and R. J. Tkacz, J. Chromatogr., 268 (1983) 99. 9H. A. Meinema, T. Burger-Wiersma, G. Versluis-de Haan and E. Ch. Gevers, Environ.

Sci. Technol., 12 (1978) 288. 10 D. R. Hansen, C. H. Lillie and H. H. Hill, Jr., J. Chromatogr. Sci., 23 (1985) 208. 11 L. Ehdon, S. J. Hill and P. Jones, Analyst (London), 110 (1985) 515. 12 V. F. Hodge, S. L. Seidel and E. D. Goldberg, Anal. Chem., 51 (1979) 1256. 13 0. F. X. Donard, S. Rapsomanikis and J. H. Weber, Anal. Chem., 58 (1986) 772. 14 S. Rapsomanikis, 0. F. X. Donard and J. H. Weber, Anal. Chem., 58 (1986) 35.