Mass Spectrometry

In forensic testing, GC-MS has been the "gold standard" for confirmation technique. The advances in LC-MS instrumentation design have resulted in many laboratories adopting this newer MS technology. This section gives only a brief overview of MS. Interested readers are encouraged to refer two excellent book chapters on MS (47,48).

A mass spectrometer consists of the following basic components: inlet, ion source, vacuum system, mass analyzer, and computer (Fig. 1): The "inlet" is a device through which samples are introduced into the mass spectrometer. The most common inlets are those interfacing GC or LC with the mass spectrometer. Because the molecules must be in the gas phase and the mass analyzer operates under vacuum, the bulk of the mobile phase—carrier gas in GC and liquid solvent in LC—delivering the molecules to the mass spectrometer must be removed, hence, the challenge of designing the interface, particularly for LC-MS, because the mobile phase is liquid. The "ion source" is where molecules in the gas phase are ionized (the different modes of ionization are discussed in sections 4.2 and 4.3). The ions formed are delivered to the "mass analyzer" (the

Vacuum System

Fig. 1. Basic components of a mass spectrometer

Vacuum System

Fig. 1. Basic components of a mass spectrometer different mass analyzer types are discussed in Section 4.4.) under vacuum where the ions of specific mass are selected and directed to the detector, usually an electron multiplier device.

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