Ambient ionization mass spectrometry (AMS): A new forensic tool for adhesive tape evidence discrimination

Date

2020-05

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Abstract

Adhesive tapes are often found at crime scenes of all types, and these tapes supply numerous investigative leads. Trace evidence (hair, fibers, etc.) are often found on tapes, along with DNA, and fingerprints. Physical tear matches can also be performed to link a piece of tape to the original roll it from which it came. There is also one other way that forensic professionals analyze adhesive tapes, and that is via chemical analysis. Chemical analysis of adhesive tapes is where the adhesive layer and/or the backside of the tape are analyzed to get its chemical composition. The chemical information can be used to differentiate the tapes by type, and or brand, which is important because it allows investigators to make connections between suspects and the tape evidence. Several chemical analysis techniques (including FTIR, Py-GC-MS, IRMS, LAICPMS, NMR, EDX) have been shown to yield different accuracy in distinguishing tape brands, but, while they offer several advantages, they may require intensive sample-preparation, complex instrumentation with limited accessibility, or are destructive in nature. Ambient mass spectrometry (AMS) is a relatively new, easy-to-use, cost-effective, and portable technique that allows direct desorption/ionization from sample substrates with little-to-no sample preparation to yield chemical analysis in real-time. In this study, direct desorption flowing atmospheric pressure afterglow (FAPA) MS and laser-assisted desorption (LD) FAPAMS were studied for their potential to discriminate adhesive tapes. The two AMS sources provided enhanced desorption and pyrolysis of the different types of tapes to obtain the mass spectral fingerprint from the backing and adhesive sides The three types of tapes studied (duct, electrical, and masking tape) were selected due to the high frequency of forensic cases where they are seen. FAPA mass spectra are characterized predominantly by ions at lower m/z. LD-FAPA mass spectra features ion peaks in a wider range, extended toward larger m/z, and this is due to the heating effects of the laser. The increasingly high temperatures aid analyte desorption processes and eventually will introduce pyrolysis processes to yield fragments of larger analytes, e.g. polymers. The spectra showed the difference in ionization of the two desorption/ionization methods, as well as characteristic peaks to differentiate types and brands of tape. The classification of tapes by brand was performed with principle component analysis (PCA) of the mass spectra. It was determined that complete differentiation was accomplished if, taking into account the first three principal components, the normal distribution ellipsoids (1 standard deviation) of a certain type/brand of tape did not overlap with others. The FAPAMS technique was able to differentiate 60% of samples, and LD-FAPAMS was able to differentiate 66% of samples. It was noted that the two different ionization techniques were complimentary, or able to distinguish different brands of tapes, and the differentiation power can be combined to have a discrimination power of 93.3% for the three different types of tape. This research provided a technique that is fast (~5s analysis), semi-nondestructive, can analyze both the backing and adhesives sides of tape, does not require specialized personnel, and is cost effective. While there is still a lot of development work to perform (optimization of experimental parameters, assessment of performance of different classification algorithms, more comprehensive tape sample population, and studying the effects of contamination/weathering on tape classification) these techniques have ultimately become the standard technique to analyze adhesive tape in a forensic crime laboratory.

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Keywords

Adhesive tape, Ambient mass spectrometry, Flowing atmospheric pressure afterglow (FAPA), Laser desorption, Brand

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