JOURNAL OF MASS SPECTROMETRY J. Mass Spectrom. 2005; 40: 807–814 Published online 10 June 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jms.855 Interpretation of butyltin mass spectra using isotope pattern reconstruction for the accurate measurement of isotope ratios from molecular clusters Juris Meija, 1 Giuseppe Centineo, 2 J. Ignacio Garcia Alonso, 2 Alfredo Sanz-Medel 2 and Joseph A. Caruso 1* 1 Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, USA 2 Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, Julian Claveria 8, 33006 Oviedo, Spain Received 18 October 2004; Accepted 17 February 2005 The fragmentation patterns of butyltin compounds (mono-, di-, and tributyltin) in an electron impact ion source were studied using an isotope pattern reconstruction algorithm with emphasis on isotope ratio measurements from molecular clusters. For this purpose, standards of natural tin isotope abundance and a 119 Sn-enriched mixture of the three compounds were both ethylated and propylated using sodium tetraalkylborates. The corresponding mass spectra of the various tetraalkyltin compounds prepared were obtained by GC/MS after their extraction with hexane. The results showed that pure interference-free molecular clusters were obtained only for certain R 3 Sn + ions where no isobaric overlap with R 2 SnH + ions occurred (e.g. BuEt 2 Sn + overlaps with Bu 2 SnH + ). These ions are ideal candidates for accurate Sn isotope ratio measurements, while isotope pattern perturbing interferences are observed for other molecular fragments down to Sn ž+ . Isotope pattern reconstruction algorithm thus can be used as an analytical tool to ensure the absence of molecular interferences — a requirement for accurate isotope ratio measurements from molecular clusters. The relevance of these studies for the determination of butyltin compounds in environmental samples by isotope dilution GC/MS is also discussed. Copyright  2005 John Wiley & Sons, Ltd. KEYWORDS: isotope pattern reconstruction; butyltin compounds; isotope ratio measurements; gas chromatography/mass spectrometry; modular representation of mass spectra INTRODUCTION Since the 1960s, various organotin compounds have been extensively used as broad-spectrum biocides, and lately the need to measure butyltin species in the environment on a routine basis has increased because of the global ban on the application of tributyltin. 1 To meet the legislative criteria, several analytical methods have been developed. Inductively coupled plasma mass spectrometry (ICP-MS) based methods with isotope dilution gas chromatography (ID-GC/ICP- MS) 2,3 or liquid chromatography (ID-HPLC/ICP-MS) 3 are ideal for research purposes, however, their cost may be high for routine testing laboratories. Isotope dilution GC/MS is regarded as an attractive alternative to this. 4 However, electron impact ionization produces molecular ions, in contrast to atomic spectrometric methods, and this complicates the use of ID methods since elemental isotope ratios need to be measured from molecular clusters. With L Correspondence to: Joseph A. Caruso, Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, USA. E-mail: joseph.caruso@uc.edu Contract/grant sponsor: Spanish Ministry of Science and Technology; Contract/grant number: BQU2000-0221, BQU2000-0468 and BQU2003-03438. respect to this, EI ionization creates two challenges. First, any molecular ion signal at a particular m/z value represents not only a single Sn isotope abundance but also includes contributions from other lower-mass Sn isotopes due to the presence of 13 C and/or 2 H in the molecular ions. This precludes any direct use of the measured signal to obtain Sn isotope ratios for isotope dilution analysis. Secondly, a particular molecular ion signal can also have contributions from isobaric molecular interferences as different species could overlap in the mass spectrum. Perhaps the biggest concern in accurate isotope ratio measurements is the loss or gain of protons. It is known that many mass spectral rearrangements involve loss or transfer of hydrogen atoms resulting in various fragment ions differing only in one or two hydrogen atoms. For example, ethyl groups are eliminated from diselenides both as C 2 H ž 5 and neutral CH 2 D CH 2 . 5 Such phenomena lead to the distortion of the natural Se isotope pattern due to the isobaric superimposition of several molecular ions. This is easy to recognize for monoisotopic elements (such as As or P), where distinct signals [M š nH] appear without overlapping with the primary signal of interest [M]. In this case, the identity and the amount of such molecular interferences can be readily estimated. For polyisotopic element species (Sn, Hg, Cd etc.), however, Copyright  2005 John Wiley & Sons, Ltd.