Abstract:
Stable isotope-labelled tetra-n-butyl(124)tin was synthesized by the reaction of n-butylmagnesium bromide with  124  SnCl 4  and tri-n-butyltin bromide was synthesized by bromine cleavage of the tetraalkyltin. The tri-n-butyl(124)tin compound labelled with the stable isotope  124  Sn is useable in environmental, chemical and biological applications to determine the fate, uptake and metabolism of tributyltin compounds. This determination is made by tracing the chemical, biological and physical processes by using gas chromatography(GC) and electron impact low resolution mass spectrometry(MS). The simplified spectra of the isotopically enriched compounds leads to a twofold increase in detection sensitivity by the GC and MS techniques.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     Tri-n-butyltin compounds are used widely as antifoulant coatings and, as a consequence, have caused increased environmental concern. Their environmental fate is not completely understood and their degradation pathways are uncertain. 
     Tri-n-butyltin compounds which contain the radioisotopes  3  H,  14  C and  113  Sn have been synthesized. Tri-n-butyltin oxide, and some related compounds, labelled with  14  C have been used for metabolism and accumulation studies in mammals, in fish, and in marine invertebrates. Studies have looked at complete mineralization of tri-n-butyltin oxide in soil using  14  C-labelled compounds.  113  Sn-labelled tri-n-butyltin oxide was used for metabolic studies in mice and in fish. 
     Although radioactive isotopes are easy to detect, there are hazards and regulatory complications associated with their use. In addition, in the case of  14  C- and  3  H-labelled compounds, the detection and localization of radioactivity does not ensure its association with the tin atom. 
     Thus, there is a continuing need in the state of the art for a means for determining tri-n-butyltin chemical and biological pathways which does not impose a physiological or political hazard associated with their use and which accurately ensures the localization and detection of the compound of interest. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to providing a compound tri-n-butyl(124)tin that is synthesized by reaction of bromine with tetra-n-butyl(124)tin that is labeled with the stable isotope  124  Sn to be used in environmental, chemical and biological applications to determine the fate, uptake and metabolism of tributyltin compounds. The compound has the properties to enable such determination by gas chromatography(GC) and mass spectrometry(MS) detection techniques. 
     An object of the invention is to provide a tracer associated with the tri-n-butyltin compounds used, for example, in antifoulant coatings. 
     An object of the invention is to provide a stable isotope tracer associated with the tri-n-butyltin compounds used, for example, in antifoulant coatings. 
     Another object of the invention is to provide a tracer for an antifoulant compound which introduces no additional hazard associated with its use. 
     Yet another object of the invention is to provide a tracer having the property that products resulting from chemical and biological process are readily identified and distinguished from naturally occurring tin compounds. 
     Yet another object of the invention is to provide a tracer for tri-n-butyltin that enables a simplified spectra that enhances the sensitivity of detection by GC and/or MS techniques. 
     A further object of the invention is to provide a tracer for a tri-n-butyltin that affords an increased sensitivity for a selected detection means. 
     A further object is to provide a tracer for tri-n-butyltin compounds that does not impose a radiation hazard. These and other objects of the invention will become more readily apparent from the ensuing specification with taken in conjunction with the claims and the associated drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a mass spectrum of natural tetrabutyltin. 
     FIG. 2 shows the mass spectrum of tetrabutyl(124)tin. 
     FIG. 3 is the mass spectrum of natural tributylhexyltin. 
     FIG. 4 shows the mass spectrum of tributylhexyl(124)tin. 
     FIG. 5 is the mass spectrum of natural tributyltin bromide. 
     FIG. 6 shows the mass spectrum of tributyl(124)tin bromide. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Tri-n-butyl(124)tin bromide and its intermediate tetra-n-butyl(124)tin were synthesized with the compounds enriched to 97.4% in  124  Sn. A low abundance stable isotope at either the heavier or lighter end of the mass range of naturally occurring tin has been found to be most desirable as a tracer. Although  112  Sn is lower in natural abundance than  124  Sn, it is also much more expensive. Largely, for this reason,  124  Sn was selected for the synthesis. 
     These labelled compounds can be used to trace chemical, biological and physical processes in environmental and metabolic studies. An MS is necessary to identify the compounds of interest. The mass spectra of the synthesized compounds and a derivative of tributyltin bromide demonstrate the utility of mass spectrometric detection and identification of these compounds when used in tracer studies. 
     The constituents for the labelled compounds were gathered from reliable sources to assure purity and suitability for the intended use. Anhydrous tetrachloro(124)tin was obtained from U.S. Services, Summit, N.J. Florisil, 100/200 mesh, was obtained from Supelco Inc., Bellefonta, Penna. Tetra-n-butyltin (98% purity), 2.0 M n-butylmagnesium bromide in diethylether and 2.0 M n-hexylmagnesium bromide in diethylether were purchased from Aldrich Chemical Company, Milwaukee, Wis. Tri-n-butyltin bromide was purchased from Alfa Products, Danvers, Mass. 
     The retention times and mass spectra of synthesized and purchased compounds were obtained with a Hewlett-Packard Model 5890A Gas Chromatograph directly connected to a Hewlett-Packard Model 5970 Mass Selective Detector. A Hewlett-Packard 9000-300 Computer using Model 59970C ChemStation software collected the data. Samples were analyzed using splitless injection onto a 12.5 m by 0.1 mm i.d. Hewlett-Packard HP-1 fused silica capillary column with 0.33 um coating thickness. Helium carrier gas was used at a head pressure of 40 kPa. The oven was programmed, after an initial 2 minute hold at 50° C., to 230° C. at 30° C./min. Injector, transfer line and detector were held at 250° C. Masses were scanned between 100 and 450 amu to obtain mass spectra. Detection limits were established using optimal selected ions. 
     Tetra-n-butyl(124)tin was prepared in accordance with the following procedure. Approximately 1 g anhydrous tetrachloro(124)tin which was enriched in  124  Sn to 97.4% purity was dissolved in 10 ml hexane. The hexane solution was added dropwise to a threefold excess of 2.0 M n-butylmagnesium bromide in diethylether which was cooled to 0° C. in an ice bath. When addition was complete, the reaction mixture was refluxed in a hot water bath for 3 hours. The reaction mixture was again cooled to 0° C. and then hydrolyzed with 3% Hcl. The separated organic layer was shaken with 5% aqueous KF to precipitate contaminating n-butyltin halides as insoluble fluorides. The organic layer was dried with anhydrous Na 2  SO 4 , and the solvent and low boiling impurities were removed by vacuum distillation at room temperature. Purity of the tetra-n-butyl(124)tin product was determined by GC/MS. It was found that yield of the tetra-n-butyltin was approximately 80%. 
     Preparation of tri-n-butyl(124)tin bromide was as follows. Approximately 1 g tetra-n-butyl(124)tin was suspended in 10 ml reagent grade anhydrous methanol. A stoichiometric amount of bromine dissolved in methanol was added dropwise to the organotin solution at room temperature and in dim light to reduce possible free radical reactions. Upon completion of the bromine addition, the solvent was removed by vacuum distillation. The crude product was cleaned by column chromatography using a 1.5×30 cm Florisil column. The reaction product mixture was eluted first with hexane to remove unreacted tetra-n-butyltin, then with 1:4 (v/v) ethyl acetate/hexane to recover the tri-n-butyltin bromide. Side products with two or more bromine groups remained on the column. The halide was produced in a 80% yield. 
     Butyltin halides were dissolved in hexane. An excess of 2.0 M hexylmagnesium bromide in diethylether was added and the samples were allowed to react for approximately 20 minutes at 50° C. The samples were hydrolyzed with 1.0 N H 2  SO 4 , and the organic layer was recovered for GC/MS analysis. 
     Tetraalkyltin compounds exhibit excellent chromatographic characteristics on non-polar columns. Compounds are not lost due to thermal degradation; the compounds are nonpolar, so better peak shape is attained. Simpler spectra, and hence greater sensitivity, are obtained because isotopic contribution from the halides is eliminated. Although trialkytin halides chromatograph fairly well, the mono- and di-substituted alkyltins do not if there is a need to determine their presence. When this is the case, the di-substituted alkyltins require derivatization. Quantification without prior derivatization of alkyltin halides can also be a problem since anion exchange in the hot GC injector can occur. 
     The tri-n-butyltin bromide spectra are identified. For quantitative analysis, the alkyltin halides routinely are derivatized with an excess of commercially obtained Grignard reagent such as 2.0 M n-hexylmagnesium bromide in diethylether. 
     Tetraalkyltins exhibit mass spectra characterized by the successive loss of alkyl groups from the tin atom. The parent ion is weak or nonexistent. There is low abundance of tin-containing ions that have lost portions of the original alkyl chain. The most favorable ions are trisubstituted tin; monosubstituted tin atoms are also relatively abundant. Disubstituted tin ions are not very favorable. 
     For tetra-n-butyltin (Bu=butyl group), prepared from naturally occurring tin, the resulting ion fragments are shown in FIG. 1. Clusters of tin-containing ions center at m/z 121 (SnH + ), 179 (BuSnH 2   + ), 235 (Bu 2  SnH + ), and 291 (Bu 3  SnH + ). The clusters are formed by the ten stable tin isotopes, their associated alkyl groups and one or more abstracted hydrogen atoms. 
     When naturally occurring tin is replaced with  124  Sn, the mass spectrum is greatly simplified. Major ions occur at m/z 124 (Sn + ), 125 (SnH + ), 127 (SnH 3   + ), 181 (BuSn + ), 183 (BuSnH 2   + ), 239 (Bu 2  SnH + ) and 295 (Bu 3  Sn + ) in FIG. 2. The low intensity ions at m/z 184, 240 and 296 are  13  C-containing fragments. 
     For tri-n-butyltin-n-hexyltin (Hx=hexyl) prepared from naturally occurring tin, the resulting fragment ions are shown in FIG. 3. The clusters of tin-containing ions center at m/z 121 (SnH + ), 179 (BuSnH 2   + ), 207 (HxSnH 2   + ), 235 (Bu 2  SnH + ), 263 (BuHxSnH + ), 291 (Bu 3  Sn + ) and 319 (Bu 2  HxSn + ). The  124  Sn-containing compound has the simplified spectrum shown in FIG. 4. Major ions occur at m/z 124 (Sn + ), 125 (SnH + ), 127 (SnH 3   + ), 181 (BuSn + ), 183 (BuSnH 2   + ), 211 (HxSnH 2   + ), 239 (Bu 2  SnH + ), 267 (BuHxSnH + ), 295 (Bu 3  Sn + ) and 323 (Bu 2  HxSn + ). 
     Trialkyltin halides in general and tri-n-butyltin bromide in particular show a similar pattern of fragmentation to tetraalkyls, in that successive loss of the alkyl groups is favored over halide loss. As shown in FIG. 5, fragment ion clusters occur at m/z 121, 175, 199, 257, and 313 for SnH + , BuSn + , SnBr + , BuSnHBr + , and Bu 2  SnBr + , respectively. The largest peak occurring between 100 and 350 m/z is Bu 2  SnBr + . The simplified spectrum for  124  Sn, in FIG. 6, shows SnH +   at m/z 125, BuSn +   at m/z 181, SnBr +   at m/z 203 and 205, BuSnHBr +   at m/z 261 and 263, and Bu 2  SnBr +  at 317 and 319. 
     There are several advantages of using  124  Sn compounds as tracers. First, products resulting from chemical and biological processes can be readily identified and distinguished from naturally occurring tin compounds. For this reason, mass balance calculations are simplified and possible redistribution reactions may be resolved. Since these are stable isotopes, there is no additional hazard associated with their use. 
     The simplified spectra exhibited by these labelled compounds enhances the sensitivity of detection by mass spectrometry. In both full scan and selected ion monitoring, the tin-containing ions reaching the detector are limited to only a few discrete m/z values, rather than being dispersed over the ten stable tin isotopes in each cluster as in the natural compounds. The most abundant natural isotope ( 120  Sn) is 32.4% of the total. In the labelled compound the  124  Sn is enriched to 97.4% of the total. Therefore, the theoretical improvement in sensitivity for a single isotope replacing the ten isotope cluster is about a factor of three. However, since several ions are used to insure positive identification of labelled and unlabelled butyltin compounds, the actual increase in sensitivity is only twofold because of computer limitations. 
     To determine detection limits the method of specified assurance probabilities was used. With the calibration design 1.1 micromolar tri-n-butyltin-n-hexyltin were detected with 95% confidence. With a similar design, 0.4 micromolar tri-n-butyltin-n-hexyltin(124)tin could be detected. With planned refinements in calibration design the detection limit for tri-n-butyltin-n-hexyltin should be reduced to about 0.2 micromolar and that for tri-n-butyltin-n-hexyltin(124)tin to about 0.1 micromolar. Where organotin compounds comprised of naturally occurring tin are present, a five percent spike of labelled organotin must be added to perform a tracer experiment. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically claimed.