Basic Theory

Every MS instrument has 3 essential components: detector, mass analyzer and the ion source. Technically, the MS formulates gas phase ions, while separating the ions according to their m/z ratio. It then detects every ion and their quantity. MS can ionize compounds according to numerous ways with the EI – Electron Ionization and the CI – Chemical Ionization being the commonest ones. The EI is a conventional and commonly used method in forensic sciences.

Moreover, a number of designs can be configured and used for mass analyzers. These devices tend to measure m/z in time and space (quadruple ion trap) but their working is technically same. The ions pass through the mass analyzer via electrostatic or magnetic forces. When these forces are changed or altered, it can detect a wide range of m/z. This is relevant to the design of the analyzer and is known as the dynamic range that the instrument possesses. A dynamic range of 10 to 700 m/z is present in a conventional GC/MS analyzer.

The ability of an analyzer to divide the compounds by their integer ratio m/z is known as the Unit resolution of an analyzer. This measures the specificity of the analyzer and functions for a low resolution analyzers (less than 1000) like quadruple ion trap instruments. The unit resolution of high resolution systems such as TOF-MS and FTICR-MS is known as:

R= (m)/(∆m)

In which, m = mass of the ion and ∆m = difference in the two peaks of mass spectrum. This resolution measures the selectivity and the specificity of the instrument.

MS runs in high vacuum from 10-4 to 10-6 torr such that the ions en route to the detector, should not collide with each other. The high vacuum of the MS makes sure that the ions travel through their destined path. It also prevents contaminants such as air and water out of the MS system. Moreover, it also removes uncharged particles and prevents them from interfering.

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