TIME OF FLIGHT – SECONDARY ION MASS SPECTROMETRY AND FORENSICS

Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) is a surface–analytic technique that provides detailed elemental, chemical state, and molecular information from surfaces of solid materials. It can be both used as a quantitative and qualitative analysis method. It uses a pulsed ion beam of caesium or micro-fused gallium to probe the surface of the solid. The secondary ions generated from the surface of the solids are separated and detected using a ToF analyser. It has been proved to be successful in the analysis of a wide range of forensic evidence such as Toxicological samples, Gunshot residues, Fingerprints, Hairs, Glass, Dyes & Inks, Questioned documents, and cosmetics. It significantly improves analyses in areas such as microbial forensics, organic analysis, biological imaging, etc.

ToF–SIMS uses a primary ion beam to focus on the sample which generates secondary ions from the atomic monolayers (uppermost layer) of the sample. These secondary ions are then accelerated to ‘Flight Path’ and are separated and analyzed using mass spectrometry according to their mass to charge ratio. This is achieved by measuring the time of flight. The lighter ions move much quicker than the heavier ions from the sample surface to the detector. So the charge to mass ratio is related to the number of time ions takes to reach the detector. ToF-SIMS is fitted with other gas ions for etching the materials away depending on the sample analyzed. It also comes with a Bismuth Liquid metal ion gun for generating the spectra. The whole system is under a vacuum.

ToF-SIMS holds high sensitivity, accuracy, is less destructive, and can be used for in-situ analysis. It can identify all elements in the periodic table even in low detection levels such as parts per million and billion. ToF-SIMS has three operational modes: surface spectroscopy – for detection of all elements and molecules on the surface of the sample, surface imaging – produce both 2D and 3D images and depth profiling – in-depth analysis. No extraction or sample preparation is needed for ToF-SIMS. ToF-SIMS has a wide range of applications when comes to Forensics. It has proved its role in identifying and comparing evidence samples such as ballpoint ink, Pencil markings, dyes, hair analysis, related to toxicological samples, GSR, and so on. ToF-SIMS have greater sensitivity and potential when comes to the elemental analysis which in turn makes it be used in the Analysis of Gunshot Residue (GSR). It can be used to analyze GSR collected from fingers and also from secondary substrates such as tables, glass, or windows.

It can do more when comes to the field of Document examination. Ink examination is the recent one among them. A ballpoint pen is most used for writing and hence the characterization colour, drying time, etc., which differ depending on the ink and its individualization is so important for a forensic expert. The resultant spectra of ToF–SIMS will be highly characteristic to the particular ink composition which hence differentiates among two samples. This technique provides the advantage of conducting the examination directly on the document itself. It can also be used for the analysis of sealing inks. The sequence of strokes in the questioned document and the age of the document can be correctly identified. Since ToF–SIMS is a non-destructive technique even the oldest documents of historical importance can be analyzed.

Pencil markings are the field in question documents that are not studied much. But experiments show that ToF–SIMS can be used successfully for the characterization and comparison of pencil markings on a paper without causing destruction. This technique can differentiate between the markings from two sources depending on the composition offered by each manufactures to make their product peculiar.

Analysis of different types of propellants is also done using ToF–SIMS. Depth analysis mode of it with high-resolution imaging provides the spatial distribution of elements in each propellant.

Analyzing Fingerprints from curved surfaces such as bullet casings are always a challenge for the experts. ToF–SIMS made it possible. The high sensitivity and high-resolution imaging capacity of ToF-SIMS can analyze and detect the presence of compounds even from the small area of fingerprints.

Recent studies show that the distribution of calcium deposits in Hair can be analyzed using this technique. Scalp hairs are mostly preferred for examination. It can also be useful for identifying the exogenous components on hair by studying the cuticle since it accumulates all exogenous products on it. Other applications of ToF–SIMS identifying the origin of cosmetics, to study the properties of porous silicon layer used in various fields, drug of abuse and toxin analyses, Bioagents such as spores of Bacillus, explosives, and so on.

Time of Flight Secondary Ion Mass Spectrometry is expensive and time-consuming. The small inlet system also adds to disadvantages. The ultra-vacuum system makes the inability to analyze volatile explosives such as nitro-glycerine. Till, its high resolution, sensitivity, and less destructive nature makes ToF–SIMS an unavoidable analysis technique especially in the field of Forensics.

About The Author

Megha Therassa George is a dedicated third-year graduate student in Forensic Science (H) at Jain (deemed to be).

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