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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Electrospray Ionization (ESI)

The most common LC/MS/MS ionization technique in the clinical laboratory is electrospray ionization (ESI). In contrast to electron ionization (EI) used in GC/MS, ESI produces much simpler mass spectra because ionization and fragmentation is produced with much lower energy (soft ionization). In addition, the highest m/z is called the quasimolecular ion instead of the molecular ion. This is because the molecule is ionized by the addition of a proton or another adduct (for example NH4+) or the loss of a proton or adduct. In electron impact ionization the loss of an electron didn’t affect the mass of the molecule because electrons have negligible mass. However, a proton does have negligible mass (1.00). Therefore, the addition of or loss of protons will increase or decrease the mass of the molecule in ESI resulting in the quasimolecular ion.
In ESI, the liquid sample stream exiting the column enters a heated probe that has a voltage applied to it. The difference in potential creates the ions, either positive ions or negative ions, depending on the mode selected. Opiates, amphetamines, benzodiazepines, and tricyclic antidepressants are analyzed in positive mode, whereas barbiturates, THC, and ethyl glucuronide are analyzed in negative mode. A heated nitrogen flow surrounds the probe and nebulizes the mobile phase as it exits. There are also heaters external to the probe that helps speed up evaporation. As the mobile phase is nebulized, it is converted to very minute droplets of liquid containing ions. As the droplets proceed into the source, they begin to get smaller and smaller as they evaporate. The ions get closer together as the droplets get smaller. As the ions get closer together, the repulsive charges increase to the point that the droplet explodes or the analyte ions evaporate from the small droplets.