The ESI source is used for the measurements of singly charged samples (small molecules) and multiple charged samples such as proteins, and peptides. The sample solution is introduced through the nebulizer assembly into the spray chamber, where it is subjected to the ESI process by means of an electrical field between the inner chamber wall and the spray shield, and with the aid of a nebulizer gas (nitrogen). Heated drying gas, flowing in the opposite direction of the stream of droplets is used to aid volatilization and ionization, and to carry away any uncharged material. The source assembly delivers the pressurized drying gas and guides it past the spray shield into the spray chamber. Ions are attracted by the electrical field strength between the spray chamber (ground potential) and the negatively biased metal-coated glass capillary, the inlet to the vacuum system (positive mode). A potential difference of about 400 V between the spray shield and the tip of the glass capillary with the spray shield at a lower voltage acts as a further ion pull into the vacuum system.
The APCI source is best used for the analysis of polar and non-polar analytes. The nebulization process for this ion source is similar to that of the ESI source. However, APCI nebulization takes place in a heated vaporizer tube. The heat evaporates the spray droplets resulting in gas-phase solvent and sample molecules. On leaving the vaporizer tube, gas phase solvent molecules are ionized by a current regulated discharge from a corona needle at a voltage of 1-4 kV. By transferring their charge, the solvent ions convert sample molecules to sample ions.
A MALDI sample is prepared by mixing an analyte with a suitable matrix compound on a metal sample plate. Evaporation of the solvent causes a co-crystallization process of both matrix and analyte material. The incorporation of the sample molecules into the lattice structure of the matrix is a pre-condition for a successful laser desorption/ionization process. The crystallized surface of the sample-matrix mixture is then exposed to an intensive pulse of short-wave laser irradiation. The photo-ionized radical matrix molecules cause a high yield of electrically charged sample molecules by transferring protons.
In electron ionization process, an electron from the analyte molecule is expelled during the collision of a molecule with highly energetic (70 eV) bombarding electrons produced by a heated filament. The radical cation products are then directed towards the mass analyzer. Due to the high energy of bombarding electrons some bond dissociation reactions can be observed. These ions are known as second-generation product ions. The ionization process often follows predictable cleavage reactions that give rise to fragment ions which convey structural information about the analyte.