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Forensic Material Identification Techniques, Quadrupole Mass Spectrometer

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Generally, the operation principle of most of the analytical instruments used in modern forensic identification and analysis of materials function by changing some feature or property of the substance being tested, the analyte, into a photometric or an electronic form or signal, which a machine can detect and read (Skoog, 2007). The accuracy of forensic instruments to detect and characterize a material or its components is however hampered by factors such as fluctuating concentrations of target and intruder substances, random electronic or spurious photon transmissions. That is, these factors interfere with the accurate and reproducible measurement of the substance of interest.

Among the most common techniques used to identify and characterise materials in forensic science officers in police departments is Gas Chromatography, which entails Quadrupole Mass Spectrometer (GC– QMS) and GC-MS-MS. Quadrupole Mass Spectrometer (GC– QMS) and GC– MS– MS The GC– QMS or the GC– mass selective detector (GC– MS) involves the use of analytes, which are often extracted from blood, urine, or other matrices through chemical means. In many cases, forensic scientists are forced to create derivatives of the said substances to obtain them in a volatile before they are inserted into the instrument for identification and characterisation.

In its basic sense, the GC technique separates compounds and elements in a substance on the basis of their solubility in the liquid, gaseous and solid phases. In addition, elements and compounds could be separated based on their volatility. The figure below shows a Quadrupole Mass Spectrometer, a gas chromatography instrument. In this instrument, ions of a specified mass-to-charge ratio are sorted and detected in a quadrupole mass spectrometer (QMS) after which a constant AC current produces a radiofrequency field, which selects the resonant ion.

Basically, in the QMS, molecules get ionized by electron ionization (EI) technique as they enter the source of ion in a sequence. As they leave the GC into the QMS, molecules are bombarded by an electron beam after which the electrons are removed from the molecules. The result is an unstable positive ion, also referred to as molecule ion, which breaks into numerous unstable fragments. The charged fragments then move with carrier gas molecules into the mass detector at lower pressure.

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