Overview of mass spectrometric measurement procedures. The first step in obtaining mass spectrometric measurements include preparing the sample, which might involve extraction and addition of any internal standards or other reference materials (Figure 1). The sample is introduced into the ion source of the mass spectrometer by various means, such as direct probe, gas reservoir inlet, direct liquid infusion, gas chromatography, liquid chromatography, or application to a plate followed by irradiation (Matrix Assisted Laser Desorption Ionization; MALDI). Once the sample components are in the ion source, ionization must be induced before mass spectrometric analysis can be performed, and this can be accomplished by electron ionization (EI), chemical ionization (CI), negative ion electron capture (NIEC), electrospray ionization (ESI), and matrix assisted laser desorption (MALDI).
Ionization: the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons. In Mass Spectrometry, sample analytes in the source become (+ or -) charged molecules or molecule fragments and the mass spectrometer detects their abundance on the basis of their mass-to-charge (m/z) ratios.