**2.2 The ionization techniques**

The ionization techniques such as electron impact (EI), Chemical ionization (CI), and field ionization (FI) have been accessible for several years, which makes the GC–MS the oldest coupling technique [5]. The ionization occurs inside the

**Figure 3***. Schematic representation of a direct coupling system.*

instruments for the three mentioned techniques, namely in the high vacuum of the MS. For the hydrophobic and small hydrophobized molecules to be analyzed with GC–MS, the MS must vaporize undecomposed analytes [6]. The gas is led into the ionization chamber of MS via the outlet of the GC separating capillary. The MS must be kept under a vacuum because of the capillary columns, which operate at low flow rates; for this reason, the carrier gas that emerges from the GC column into the chamber of ionization needs to be pumped out.
