interfaces used in lcms
Answers
Answer:
Currently, the most common LC-MS interfaces are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo-ionization (APPI).
Answer:
ESI generates ions by first drawing and spraying sample solutions at the tip of a capillary tube, where a high voltage of about ± 3 to 5 kV is applied. This generates a fine mist of charged droplets with the same polarity as the applied voltage. To accommodate a larger LC flow rate, the nebulizer and heating gas flows from outside the capillary to speed up the solvent evaporation process. As this process continues, the electric field on the droplet surface increases. When the mutual repulsive force of the charges exceeds the liquid surface tension (i.e. repulsion), fission occurs. It is thought that as this evaporation and fission cycle is repeated, the droplets eventually become small enough that the sample ions are liberated into the gas phase (based on the ion evaporation model). A schematic representation of the generation and desolvation processes in ESI for positively charged ions are illustrated in Figure 7. Similarly, negatively charged ion are generated by applying a negative voltage on the ESI probe.
ESI is one of the softest ionization method available, which means fragmentation is minimal and it can be used for highly polar, least volatile, or thermally unstable compounds. Since most of the compounds result in protonated (or deprotonated) molecular ions and adduct ions, without generating complicated fragment ions, determination of the molecular mass of compounds is very simple.