Chapter 9. Liquid Chromatography-Mass Spectrometry of Organotin Compounds

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Liquid Chromatography=Mass Spectrometry of Organotin Compounds L.D. BETOWSKI and T.L. JONES US Environmental Protection Agency, National Exposure Research Laboratory, P.O. Box 93478, Lar Vegas, NV 89193-3478,USA

9.1. INTRODUCTION

The commercial use of organotin compounds has increased by more than an order of magnitude in the past 30 years. Used as active ingredients in wood preservatives, antifouling marine paints, pesticides, fungicides, and stabilizers, they are currently used most extensively for stabilizing PVC polymers. It was estimated that in the 1970s 10% of all PVC was stabilized with organotin compounds, mainly dibutyltin [l]. A major environmental concern is the widespread use of these compounds in marine paints. The discovery that tributyltin was efficient in killing several species of freshwater snails responsible for transmitting schistosomiasis [ 11 led to the organotins being incorporated into marine paints to reduce “fouling”. Fouling results from marine organisms adhering to smooth surfaces in contact with the water. Fouling produces roughness that increases turbulent flow and drag on boat hulls, thereby adding to operating expenses. Tributyltin remains the most common organotin used in these marine paints. The Sn(C4H&+ ion is responsible for the toxic effect on marine species. This ion displays an increased fat solubility over other tin species, which leads to an enhanced penetration of biological membranes [2]. Elemental or other inorganic forms of tin do not show increased toxicological effects in humans and wildlife. There is some evidence for increased toxicity to marine organisms with increasing butyl substituents from one to three, but a decrease with the fourth butyl [3]. There have been reports of sublethal toxicity in mussels and oysters at tributyltin concentrations in seawater of part-per-billion levels or below [4,5]. The fat solubility properties of References pp. 412-414

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tributyltin make bioaccumulation of this compound a problem. Factors from 5000 to 50 000 have been reported for the bioconcentration of this compound [6]. Because of its relatively high sorption coefficients, the measurement of tributyltin and its degradation products is important [7,8]. Thus, the sediment acts as both a sink and, because the sorption process is reversible, a potential source for tributyltin. The degradation of tributyltin is assumed to follow the scheme [9,10] Bu3Sn++ Bu2Sn2++ BuSn3++ Sn(IV) There is the possibility of other degradation pathways for tributyltin, including reactions that result in mixed butylmethyltin species [ 111. Since the toxicity of tributyltin is greater than the other butyltins, it is important to determine individual “species” when analyzing a sample - not just a total concentration of tin. Because of the toxic effect of organotins (especially tributyltin) on marine organisms, the US Environmental Protection Agency has been active in reviewing the registration of pesticides containing tributyltin. A study in 1988 of tributyltin contamination in bivalves from US coastal estuaries showed concentration between 4 and 1560 ng Sdg [12]. Therefore, in certain parts of the country, marine life is in danger because of these high levels of organotins. It is also a global problem, as shown by recent results of high concentrations of tributyltin on the Cadiz coast in Spain [131. The development of analytical methods for speciating organotin with lower detection limits is necessary to effectively monitor this worldwide threat to the environment.

9.2. ANALYTICAL METHODS 9.2.1. Gas chromatographicmethods Recent analytical methods for determining organotins have taken advantage of techniques that enable the speciation of these compounds. Most of the methods in the past 10 years have used some form of gas chromatography (GC) for separating the various organotin species (see, e.g. [11,12]). However, since the mono-, di-, and trisubstituted organotin compounds do not chromatograph well, some form of derivatization is usually required [14]. There are three general types of derivatization used in the GC analysis of samples for organotins. The first is hydride generation, which can be described as follows [151

where x = 0, 1, 2 or 3, and R is methyl, ethyl, or butyl. This technique is usually combined with preconcentration by cryofocussing, chromatographic separation, and detection by atomic absorption spectroscopy (AAS) [ 161. However, detectors such

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as the flame photometric detector (FPD) [11,14] and the quadrupole mass spectrometer [ 171 have also been used with the hydride derivatization. The second extensively used derivatization technique involves a Grignard reagent to make volatile tetraalkyl substitutes, as follows [15]:

where x = 0, 1, 2 or 3, and R is methyl, ethyl, butyl or phenyl. Methyl, butyl, pentyl or hexyl have been used for R . Recently, the hexylbutyltin derivative has been the most popular with the FPD and mass spectrometer as detectors [ 12,181. GC-MS has often been used in confirmatory work. Since the sensitivity of the traditional quadrupole GC-MS in the full scan mode precludes extensive analysis, the selected ion mode (SIM) of GC-MS has been used as an alternative. Helium atmospheric pressure microwave induced-atomic emission spectrometry (MIPAES) has recently been used with capillary gas chromatography to speciate organotin compounds [19]. This technique is reported to reach detection limits of 0.05 pg for the organotins. Additionally, an off-line complexatiodsupercriitical fluid extraction with GCMIPAES detection was developed for the determination of 1 1 organotin compounds in soil and sediment samples [20-221. This method uses diethylammonium diethyldithio-carbamate to form neutral complexes with ionic organotin compounds and extracts the sample with supercritical carbon dioxide with 5% methanol as a modifier. The extracts were then derivatized with pentylmagnesium bromide and determined by GC-MIPAES. A recent derivatization scheme makes use of sodium tetraethylborate to produce ethylation of organotin compounds [23-2.51. The main advantages of this technique over the previous derivatization schemes are that the Sn-C bond is more stable thermally than the Sn-H bond in the hydrides and the derivatization reaction can be performed in aqueous media, while the Grignard reaction must be performed in a waterfree environment [25]. Detection by GC with FPD [24,25] and with AAS [26] has been used.

9.2.2. High-performanceliquid chromatographicmethods High-performance liquid chromatography (HPLC) offers several advantages over GC separation. With HPLC methods there is no need for derivatization of the organotins. One of the limitations to the various derivatization schemes is that each species must be fully converted to be quantitatively detected. Another drawback to derivatization is that the original oxidation state of the metal is not necessary preserved [27]. HPLC offers the user a variety of separation techniques, such as ion exchange, ion-pairing, and normal- and reversed-phase chromatography. Such an array of separation mechanisms can be used to optimize the resolution of a wide variety of neutral and charged, polar and non-polar species. References pp. 412-414

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Flame atomic spectrometry detection for HPLC does not offer good sensitivity for the most part [15,28]. However, graphite furnace AAS systems have been interfaced to HPLC because of their good detection limits and have been used for the speciation of organotins [29,30]. Detection limits of 0.050mg organic Sn/kg dry matter have been achieved with GFAAS [30]. Long tube flame AAS is used to determine different species of tin because of its better detection than ordinary flame AAS [31,321. Kadokami et al. [33] used HPLC with long tube AAS to separate eight organotin compounds. They found that the AAS response is independent of the form of the tin for the long tube AAS. The detection limits were 5 ng as Sn for the 224.6-nm line and 10 ng as Sn for the 286.3-nm line. Lakata et al. [34] found that the use of 8hydroxyquinoline (oxine) as a complexing agent enabled the ultraviolet absorbance detection of organotin compounds after separation with HPLC. The oxine was added to the mobile phase and the mono- and dialkyltin compounds reacted directly, were separated, and detected by their UV absorbance. The tri- and tetraalkyltin compounds did not react directly in an appreciable manner; therefore, a postcolumn photoreactor was added to degrade these compounds to enable sufficient complex formation before detection. A weak cation exchange column with direct (non-suppressed)conductivity detection was used for the analysis of tributyltin [35]. The authors showed that tributyltin exists in ionic form under the mobile phase conditions of 15 mM tartaric acid and 50:50 methanol/water. The predominant retention mechanism in this case is ion exchange. However, monobutyltin was not eluted under these conditions, and dibutyltin gave unsymmetric peaks. 9.2.2.1.HPLC-ICP-MS Cation exchange chromatography and ion pair chromatography have been widely used for tin speciation with inductively coupled plasma mass spectrometry (ICP-MS) detection [36-381. The ICP source is an argon plasma operated at atmospheric pressure and powered by inductive coupling to a radio frequency electromagnetic field. The sample is usually aspirated by a nebulizer system to the center of the plasma, where it experiences temperatures of 6000-8000 K with a residence time of about 2 ms. Originally, an atomic emission spectrometer was used as a detector for the ICP. As such, the ICP had several advantages over other excitation sources, among them high dynamic linear range, multi-element analysis, excellent detection limits, long term stability, and rapid analysis time. Advances in sampling techniques for flames and plasmas [39] and development of an effective interface led to the first analytical ICP-MS [40]. The interface is usually a water-cooled sampler cone with a circular orifice of 0.5-1 .O mm drilled into its tip. Another orifice is located in the tip of a skimmer located a few millimeters behind the sampler. The pressure between the two orifices measures about 1-2 Torr. Behind the second orifice is a region of higher vacuum (10-*-104 Torr) with electrostatic lenses, a quadrupole mass filter, and a continuous dynode detector. The quadrupole mass filter usually operates over a

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range of mlz 1-250 with unit resolution. The quadrupole can be scanned very fast (120 ms) or can be switched between peaks in a few microseconds [41]. Signal averaging techniques can be used to enhance the data. Theoretically, 54 elements can be ionized in an ICP with an efficiency of 90% or more. Even some elements that do not show ionic emission lines should be ionized with reasonable efficiency (viz. As (52%) and P (33%)) [41]. This is one of the advantages of ICP-MS over ICP-AES. Other features of the ICP-MS that make it more attractive than ICP-AES are much lower detection limits, ability to provide isotopic ratio information and to offer isotope dilution capabilities for quantitative analysis, and clean and simple spectra. It has been observed that introduction of organic solvents into an ICP-MS results in decreases in sensitivity due to excessive solvent loading of the plasma [37]. This has also come with an increase in the background due to the formation of molecular ions [36,42]. The use of micelles in the mobile phase was used for HPLC to try to circumvent some of these problems associated with the heavy use of organic solvents [37]. Micelles assist in dissolving solutes that are not easily dissolved in pure water. The use of HPLC with micelles has unique retention mechanisms involving three partition equilibria: between water and the micelle, between the micelle and the stationary phase, and between the water and the stationary phase [37]. The chromatographic efficiency with this system is comparable to organic phases, plus it has the advantages of low toxicity, low cost, time and solvent savings, and no long equilibration time. Suyani et al. [37] tried three different surfactants with a CI8column: sodium dodecyl sulfate (SDS), which is negatively charged; dodecyltrimethylammonium bromide (DTAB), which is positively charged; and non-ionic Brij 35. Only the SDS separated a mixture of organotins. The concentration of micelle used in the mobile phase has to be balanced between two opposing factors: the capacity factor, k’, decreases with increasing micelle concentration to an optimum range of 0.5-20; too high a micelle concentration will lead to the plugging of the ICP-MS orifices. Moreover, butyltins could not be separated because of high k’ values without SDS concentrations exceeding 0.1 M. Separation of methyltins necessitated 0.02 M SDS and KF added to the mobile phase. Detection limits for trimethyltin using micellar LC were better by a factor of 15 over those achieved by ion-pair chromatography as reported by Suyani et al. [36]. McLaren et al. [38] showed the use of a strong cation exchange column (Whatman SCX) with ICP-MS. This procedure featured gradient elution with 0.3 M ammonium citrate in methanovwater (60:40) and a pH change from 6 to 3 after 1 min (to elute monobutyltin). They reported no problems with plasma instability with up to 70% methanol if the plasma was operated at a forward power of at least 1.4 kW,with a nebulizer gas flow rate of 0.75 Ymin and a sample delivery rate of 1 mumin. Detection limits of 200pg Sdml for tributyltin, 400pg Sdml for dibutyltin and 20-40 pg Sn absolute were reported. Dauchy et al. [43] used a TSK gel ODS 80-TM analytical column and a preReferences pp, 412-414

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column of the same stationary phase to separate butyltins before determination by ICP-MS. Separation of these species in an actual environmental matrix was difficult compared with the separation of pure standards. In the more complicated matrix, coextracted compounds can cause peak broadening and deformation. They found that an acetic acid extraction led to the best separation for monobutyltin, dibutyltin, and tributyltin in two reference sediments. They reported a detection threshold of 5 ng Sn/g for a test sample of 3 g of dry sediment. 9.2.2.2.HPLC-MS (thermospray) Cullen et al. [44] used a Kratos MS 80 RFA mass spectrometer equipped with a Vestec Kratos thermospray interface to determine butyltins in marine samples. These authors used a 0.6 mumin flow of 2% tetrahydrofuran/98% acetone containing 2% acetic acid through a CI8 HPLC column with 0.2% trifluoroacetic acid in water added post-column at a rate of 0.4 mumin. With this mobile phase they were able to separate tributyltin chloride from dibutyltin chloride. However, they were not able to separate dibutyltin from monobutyltin. The thermospray temperatures were as follows: vaporization, 182°C; probe, 117°C; ion source, 225°C; jet, 213-215°C. Most of the major peaks in the thermospray mass spectra of the butyltin chlorides, (C4H9),SnC14- n, were attributed to solvent adduct or replacement ions of the butyltin species, For example, for tributyltin chloride, the base peak was the acetone species, [(C,H,),SnCO(CH,),]+, and for dibutyltin chloride, the base peak was the acetic acid species, [(C4H9)2Sn00CCH3]+.The other reported ions for the butyltins retained the basic butyltin structure; for the tributyltin ions, all ions had three butyl groups and one tin atom, whereas for the dibutyltin ions, all ions included two butyl groups and one tin atom. Since thermospray involves a soft ionization process, there were no ions reported originating from Sn-C4H9 cleavage. Recognizable spectra were reported for solutions of these compounds at concentrations of 36 pg/ml; monobutyltin chloride gave the weakest response. No butyltins were identified in environmental marine samples by HPLC-MS with the thermospray interface. Dimethyltin chloride was identified in one of the samples by HPLC-GFAAS, but none was observed with thermospray. One feature of thermospray that these authors [44] recognized was the revelation of isotopic patterns in some of the samples indicating the presence of tin. The naturally occurring isotopes of tin are shown in the histogram in Fig. 9.1. The cluster of peaks around m/z 369 in a marine extract sample, as seen in Fig. 9.2, shows a pattern similar to the isotopic ratio of tin. The species responsible for these peaks went unidentified.

9.2.2.3.HPLC-MS (electrospray, ion spray) Since sensitivity was a problem for analysis with thermospray, several groups investigated the potential of electrospray for organotin analysis. Many of the advantages of electrospray have been applied to the determination of large biomolecules

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1 26

6

0 C

C

-

r

-

r

mh

Fig. 9.1. Natural isotopic abundances of tin.

[45].Large and fragile polar molecules can be introduced to the mass spectrometer by electrospray and show intact ions with multiple charges (thereby satisfying the mass-to-charge requirements of mass spectrometers). However, the promise of better sensitivity than offered by thermospray and the lesser need to optimize for specific samples [46] sparked an interest in electrospray for smaller molecules. Electrospray has been called the “purest” form of transfer of ions from solution to the gas phase with little internal energy imparted to the ions [47].The mechanisms of the electrospray process are still not completely understood. In electrospray, a flow in the usual range of 1-20puYmin enters the chamber through a stainless steel hypodermic needle. The needle is typically maintained at a voltage of 2-3 kV with respect to a surrounding cylindrical electrode. The field at the tip of the needle enables the emerging liquid to be charged. The liquid becomes dispersed by coulombic forces into a fine spray of charged droplets [45,48].A countercurrent flow of bath-gas hastens the evaporation of the charged droplets. As the droplets get smaller, the charge density on their surface increases until the Rayleigh limit is reached. At the Rayleigh limit the coulombic repulsion overcomes the surface tension on the droplet resulting in an explosion that produces smaller charged particles. Kebarle and Tang has characterized these explosions as uneven fissions [47]producing different sized and charged particles. After multiple explosions of this sort, eventually isolated solute ions appear in the gas phase. In the true electrospray process the positive and negative ions observed in the spectra should References pp. 412-414

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100

-

so 60

-

70 -

L I0

0

t

a08

aos

1.7

asD

aoD

a70

mh

Fig. 9.2. Thermospray mass spectrum of an unidentified compound in a marine sample (adapted from Ref. [a]).

always be the positive and negative ions of the electrolytes present in solution. Ion spray, which could be termed aeroelectrospray, uses aerodynamic forces to assist in the production of charged particles. A flow of nitrogen is added coaxially to the sample flow to assist in the nebulization of the liquid. A practical advantage of ion spray over electrospray is to allow higher solution flow rates [49,50].In addition, solutions of higher conductivity can be nebulized in ion spray than in unassisted electrospray. Although some uncertainly exists, it appears that pure electrospray produces smaller droplets than either thermospray or ion spray [51]. There is also some evidence that the ratio of charge to analyte in droplets from pure electrospray is always higher than in droplets produced by thermospray or ion spray [51]. Thus, the combination of small droplet and high charge is most likely responsible for the high sensitivity observed in electrospray. Both electrospray and ion spray are two techniques of atmospheric pressure ionization (API). There are several advantages to the API interfaces that are not present in other interfaces [52]. Operating conditions for both the LC separation and MS detection can be optimized separately. Secondly, the droplets are desolvated more efficiently at atmospheric pressure than at reduced pressured. Ion evaporation or

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chemical ionization requires partial or complete vaporization of the solvent droplets to achieve optimal sensitivity. Finally, ions to be sampled by the MS undergo a free jet expansion as they go from atmospheric pressure to vacuum. This adiabatic cooling helps maintain the integrity of labile compounds for mass analysis. The electrospray or ion spray mass spectrometry of organotins is characterized by many adduct ions that appear in both the positive and negative ion modes. Siu et al. [53] used an API mass spectrometer (Sciex TAGA 6000 prototype) equipped with an atmospheric pressure chemical ionization source (APCI) and with a laboratory built ion spray source to investigate organotins. They found that methyl, propyl, butyl, and phenyltin chlorides formed intense adduct ions under negative ion APCI and ion spray conditions. They did not observe tin-containing ions for organotin hydrides and tetraalkyltins. The most frequently observed adducts were formed with the parent molecules (e.g. (C4H9)3SnC1,plus oxide, superoxide, halide, acetate, and formate). The specfic adduct ion was more dependent on the availability of the type of anion rather than mode of introduction. The addition of buffer to the mobile phase, for example, was important in the formation of adduct ions (formate and acetate). Structures containing pentacoordinated tin atoms are known to exist in solution [53,54]. These ions have never been reported to exist in the gas phase, but these authors postulated the pentacoordinated tin in a trigonal-bipyramid structure. Identical ions, when the anions are available, are generated in both the APCI and ion spray sources. The mechanisms of formation, however, are different: in APCI, the adduct reaction takes place in the gas phase after the anions are formed in a corona discharge; in ion spray, the adducts must be preformed in solution. While the adduct ions formed in the negative ion mode depend on the matrix and other environmental conditions, the ions that appear in the positive ion mode are the same irrespective of the counter ions and the presence of other anions. With this in mind, Siu et al. [55] used the positive ion mode for quantification of tributyltin. Ion spray gives intense positive ions for trimethyl, tripropyl, tributyl, and triphenyltin chloride. However, no tin-containing positive ions are seen below mlz 450 under APCI [53]. In ion spray, the trialkyl and triphenyltin cations are evaporated into the gas phase with solvent molecules such as methanol and water. An ion, such as (C4H9)3Sn*CH,0H+,may easily lose methanol in the lens region of the ion spray source via collision induced dissociation (CID) to form the respective alkyltin (e.g. and C4H9Sn3+have been observed. The singly charged (C4H,)3Sn+).No (C4H9)2Sn2+ adduct ion (C4H&SnC1+ is present when a source of chloride ions is available, but this ion is observed with low sensitivity. Jones and Betowski [56] used a commercial electrospray mass spectrometer (Vestec) to investigate a group of organotin compounds. These compounds were analyzed by flow injection in a mobile phase of 99% methanol and 1% acetic acid at a flow of 2-4,uVmin. Spectra were acquired with two different repeller voltages on skimmer cone 1 (1 2 V and 30 V) with skimmer cone 2 set to 5 V. In the high pressure region between skimmers 1 and 2, ions are susceptible to CID processes, espeReferences pp. 412414

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408 100

80 70

eo

40

rt a0 20

10

0

5 8 5 : 5 R 8 4 8 8 8 8 $ f Q Q mlz

Fig. 9.3. Electrospray mass spectrum of tributyltin at a repeller voltage of 12 V.

cially at the higher voltage. Figures 9.3 and 9.4 show the electrospray mass spectra of tributyltin at 12 V and 30 V, respectively. The base peak in both cases was attributed to Sn(C4H9)2H2+ at mlz 235. Other major ions were Sn(C4H&+ at mlz 291 and Sn(C4H,)H2+at mlz 179. In the 30 V spectrum there was also a contribution from SnH3+at mlz 123. The spectra were consistent with CID processes being stronger at 30 V and producing more product ions. Two other ions that were observed deserve some mention. The high mass cluster in Fig. 9.4 contained an additional contribution to mlz 293 over the natural abundance of Sn122(C4H9)3+. The authors attributed this ion to Sn(C4H9)20COCH3+, which arises from the presence of acetic acid in the mobile phase. They showed the effect of acid modifier on the spectra by using formic acid in place of acetic acid. This same mlz 293 appeared in the spectrum of dibutyltin dichloride. With the formic acid in the mobile phase, mlz 279 appeared in place of mlz 293. This ion, then, has the form Sn(C4H9)20COH+.In the electrospray mass spectrum of dimethyltin di-

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Fig. 9.4. Electrospray mass spectrum of tributyltin at a repeller voltage of 30 V (adapted from Ref. [561).

chloride with formic acid, the Sn(CH3)20COCH3+ion at mlz 209 is replaced by Sn(CH,),OCOH+ at mlz 195. It was noted that the spectra acquired with formic acid had a lower overall sensitivity than those spectra acquired with acetic acid in the mobile phase. The second unusual ion from tributyltin chloride appeared in the 12 V spectrum at mlz 373. This appears to be an adduct ion since the molecular weight of tributyltin chloride is 326 Da, and this ion is present only in the 12 V spectrum and not the 30 V spectrum; that is, this ion undergoes CID easily. This ion may be related to the mlz 293 ion, which appears in the tributyltin chloride spectrum only in the 30 V case. The mlz 293 ion may be a CID product of mlz 373. In the spectra of both dibutyltin dichloride and butyltin trichloride, clusters of peaks above mk 350 were present with the appearance of multiple tin patterns. These were not identified. References pp. 412-414

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The spectra of the phenyl tin chlorides were relatively simple. Triphenyltin chloat mlz ride showed just two ions, Sn(C6&)3+ at m/z 351 and S~IOCOCH~(C&~)~+ 333. The diphenyltin dichloride spectra showed the same mlz 333 ion, which was the base peak. In addition, the ion SnOCOCH,C6H5Cl+at rnlz 291 was present. The 30V spectrum showed a rnlz 179 peak, which was attributed to Sn(II)OCOCH,+, ion at rnlz 309. The spectrum for while the 12 V spectrum showed the Sn(C6H5)2C1+ phenyltin chloride contained the SnOCOCH3C6H5Cl+at rnlz 291 and the SnOCOCH3(C6H5),+ion at rnlz 333. This latter ion was due to the instability of phenyltin chloride in methanol solution; it tends to form the diphenyltin structure. Also present in the 30V spectrum were Sn(II)OCOCH3+at m k 179 and Sn(C6H5)HCl+at rnlz 233. The 12 V spectrum also contained the Sn(OCOCH3)2C6H5+ ion at mlz 315. 9.2.2.4.HPLC-MS-MS Besides effecting CID processes in the lens region of the ion spray or electrospray mass spectrometer, researchers have used tandem mass spectrometers in conjunction with LC-MS to investigate these processes in the collision cell of a triple quadrupole mass spectrometer. Siu et al. [53,55] (see above) used a Sciex triple quadrupole mass spectrometer to properly assign the organotin chloride adducts in the negative ion mode and to perform both structural analysis and quantification in the positive ion mode. The most intense ions in the CID of the adduct ions formed in the negative ion mode of the ion spray source were the chloride ion and other basic anions (e.g. acetate and formate, depending on the buffer composition). In contrast, the tincontaining species were less abundant. For example, for the chloride adduct of tributyltin chloride, the CID spectrum of Sn(C4H9)3C12-showed the Sn(C4H9)C12ion and the SnC1,- ion, besides an intense C1-. The CID spectrum of the tributyltin cation, Sn(C4H&+,showed successive losses of butene (C4H8)to give Sn(C4H9),H+,Sn(C4H9)H2+, and SnH3+.The authors [53] reported high product-ion yields for tripropyl, tributyl, and triphenyltin cations at collision energies of 7-9 eV. However, for trimethyltin cations, a collision energy of 20 eV was needed to achieve a comparable yield. For the triphenyltin cation, the first loss appeared to be the neutral loss of biphenyl to give SnC6H5+. A sensitive method for the detection of tributyltin [55] was the selected reaction (mlz 179). This reaction proved monitoring of Sn(C4H9)3+(mlz 291) + Sn(C4H9)H2+ to be so specific for the determination of tributyltin that no liquid chromatographic separation was needed after extraction of sediment samples. The determination of tributyltin in a certified reference material was 1.29 k 0.07 p g of Sn/g of sediment by this method, in excellent agreement with the certified value of 1.27 & 0.22pg of sn/g. Although they did not use HPLC introduction, Lawson and Ostah [57] used a VG Trio triple quadrupole mass spectrometer to investigate the ion-molecule reactions that lead to organotin adduct formation that often occurs in HPLC interfaces. Since both electrospray and ion spray are atmospheric pressure ionization sources, chances

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for adduct formation are high. Using the triple quadrupole mass spectrometer, the authors were able to study reactions with specific ions in the collision cell. The compounds were introduced by direct insertion probe; spectra were recorded under electron impact (EI) ionization, and the appropriate ion was selected in quadrupole one for further reaction. Cluster reactions were only observed from a trisubstituted cation. From the EI spectrum of butyltin trichloride, the SnC13+was selected. At low pressures in the collision cell, only CID processes take place with consecutive loss of two chlorine atoms. When the collision cell gas pressure was raised to 10 mTorr, in addition to the CID ions, other ions were observed that were attributed to addition ions that were dependent upon the components in the collision gas mixture. There were four adduct ions observed with water and methanol in the collision cell: SnC13(H20)+,SnC13(CH30H)+,SnC13(CH30H)(H20)+,and SnCl,(CH,OH),+. For triphenyltin chloride, the Sn(C6H5),+suffered the CID loss of biphenyl and a subsequent loss of C6H5at low collision pressures. At higher pressures both the CID reactions and the formation of the adduct species took place. The adduct species of Sn(C6H5),(CH30H)+and Sn(C6H5)3(CH30H)2+ were observed when the collision gas was methanol. Lawson and Ostah [57] also investigated tetraalkyltin compounds. The tetrabutyltin compound showed a loss of butyl followed by sequential loss of two butene groups under EI ionization, which was similar to that seen by Siu et al. [53]in the CID process of the Sn(C4HJ3+ion. When Sn(C,H9)H2+was chosen for further reaction with methanol as the collision gas, it underwent CID to produce SnH3+which then reacted with methanol to produce SnH3(CH30H)+.Sn(C4H9)H2+also directly produced the adduct ion Sn(C4H9)H2(CH30H)+.

9.3. SUMMARY The combined use of HPLC with mass spectrometric detection has greatly enhanced the suite of methods available for the specific analysis of environmental samples containing organotin compounds. The use of chromatographic separation and mass spectrometric detection provided the specificity needed for tin compounds. Speciation of organotin compounds is important because of the greater toxic effects of tributyltin compared to inorganic tin and other organic forms of the metal. The use of HPLC precludes the need for derivatization in the sample workup. Derivatization has been used widely in organotin analysis, but the procedure has several drawbacks, including incomplete reaction, introduction of additional complexity in an already complex sample, and additional sample workup time. The use of ICP-MS or electrospray (or ion spray) MS gives the analyst the sensitivity to investigate environmental samples for organotin compounds at environmentally significant levels. The use of MS-MS techniques further adds to the specificity of detection. Whether to elucidate structures or provide a specific parent-daughter ion pair for quantification, tandem mass spectrometry serves as a helpful adjunct to traditional References pp. 412414

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mass analysis. Since electrospray and ion spray involve soft ionization processes, MS-MS is often necessary to confirm the ionic species. Emerging technology that should be adapted to organotin speciation and detection in the future are the ion trap mass spectrometer and capillary electrophoresis. Both have had some applications to environmental analysis. The ion trap MS has the potential to improve the detection limits for organotin analysis compared with the traditional quadrupole mass filter. Recent developments have been geared towards the fabrication of external ion sources for the ion trap to provide an interface for liquid chromatography. Capillary electrophoresis has emerged as an important tool in biochemical analysis and is starting to be used in the environmental analytical sciences. This technique has the potential to separate neutral and ionic species in the same electrophoretic run. Recent advances have led to the interfacing of capillary electrophoresis to mass spectrometers (see Chapter 12 and references therein). Such a system would be an important addition to instrumentation used for organotin analysis.

9.4.NOTICE The US Environmental Protection Agency (EPA), through its Office of Research and Development (ORD), funded and performed the research described here. It has been subjected to the Agency's peer review and has been approved as an EPA publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

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