Analysis of sexual assault evidence: statistical classification of condoms by ambient mass spectrometry

Abstract

Desorption electrospray ionization mass spectrometry (DESI-MS) and easy ambient sonic-spray ionization mass spectrometry (EASI-MS) are employed here in the forensic analysis of chemical compounds found in condoms and relative traces, and their analytical performances are compared. Statistical analysis of data obtained from mass spectra only was applied in order to obtain classification rules for distinguishing ten types of condoms. In particular, two supervised chemometric techniques [linear discriminant analysis (LDA) and soft independent modeling of class analogy (SIMCA)] were carried out on absolute and relative intensity values to test the performances of statistical models in terms of predictive capacity. The achieved classification of samples was excellent because of the high prediction percentages of the method used both for DESI and EASI mass spectrometry analyses, confirming these two as potential ambient ionization techniques for forensic analyses in case of sexual assault crimes. EASI-MS showed 99% prediction ability for LDA using relative data and 100% prediction ability for SIMCA using both absolute and relative ones, while DESI showed 94% prediction ability for both LDA and SIMCA. The absence of any sample preparation technique gives advantages in terms of sample preservation and reduced contamination, allowing successive analyses to be performed on the same sample by other techniques.

Using mass spectrometry to transform the assessment of sexual assault evidence

“…All of these are chemicals known to be found in condoms. [27] The Pasante warming and Amor lemon-flavoured condoms produced similar mass spectra.…”

“…[23][24] Furthermore, van Helmond et al demonstrated the classification of condom lubricants in treated fingerprints using DESI-MS. [25] Furthermore, chemometric strategies have been developed capable of differentiating between condom brands based on their chemical profiles. [26], [27] In light of this, it is therefore important to demonstrate that any techniques developed for the direct analysis of suspected semen, would also be capable of detecting components pertinent to condoms and lubricants, in order to develop a technique more widely applicable to sexual assault evidence analysis.…”

DESI-MS analysis of human fluids and tissues for forensic applications

“…The compounds commonly used in lubricated condoms formulations such as polymers (nonoxynol-9, polyethylene glycol, polydimethylsiloxane) and small molecules additives (N-methylmorpholine, N-octylamine, N,N-dibutyl formamide, isonox 132) were successfully determined by DESI-MS [27]. Moreover, a successful statistical classification of ten types of condoms by DESI-MS with 94% prediction ability was presented [30]. When handling, chemical components of condoms might be transferred and deposited with the fingerprint.…”

Forensic analysis of water‐based lubricants using liquid extraction surface analysis high‐resolution tandem mass spectrometry

“…Spectroscopic methods, however, lack the selectivity to identify low‐abundance species in the presence of large excesses of PDMS or PEG, and the power of chromatography‐based analyses is negated by the time‐consuming nature of the methodology. To circumvent these limitations while maintaining the sensitivity and selectivity afforded by mass spectrometry, direct sampling ion sources have been successfully deployed for lubricant analysis, including matrix‐assisted laser desorption/ionisation (MALDI), desorption electrospray ionisation (DESI) and easy ambient sonic‐spray ionisation (EASI) . MALDI is ideally suited to the analysis of polymeric components in lubricants, but is susceptible to interference from matrix ions with mass‐to‐charge ratios ( m/z ) less than 400, potentially obscuring analysis of low‐mass components present at trace levels.…”