In recent years, successful interfacing of a LC to a mass spectrometer has led to a gradual movement to performing confirmation of drugs of abuse using LC-MS. This is particularly true for confirmation testing on specimens of alternate matrices (hair, sweat, and oral fluid). One advantage of liquid chromatographic over gas chromatographic separation is that as the mobile phase is aqueous, sample extraction procedures are less extensive. Unlike GC, LC does not require vaporization of the compounds at high temperature for chromatographic separation. LC can analyze the more polar and nonvolatile compounds without resorting to chemical derivatization to make these compounds suitable for analysis by GC. As LC can be performed at much lower temperature, compounds that are unstable at the high operating temperature of GC can be analyzed by LC.
A technical challenge for the design of a LC-MS instrument is the large volume of liquid (mobile phase) that must be removed before the analytes can enter the mass spectrometer, which is under vacuum, for ionization and detection. In recent years, techniques have been developed for the ionization of analyte molecules to take place outside of the mass analyzer at atmospheric pressure (i.e., atmospheric pressure ionization). The ability to have the ionization process occurring at atmospheric pressure outside of the mass analyzer is a major advantage of LC-MS over GC-MS. Two atmospheric ionization techniques in common use for drugs of abuse testing are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI).
ESI involves the passage of the LC effluent though a small capillary nozzle to which a voltage has been applied. This results in a spray of small solvent droplets inside the ionization chamber and the transfer of energy (charge) to the droplets. As the droplets evaporate (assisted by gas flowing through the chamber), they decrease in size until they break apart, and a charge is transferred to the molecules inside. The charged molecules are then delivered to the mass analyzer for analysis.
APCI uses a fine spray of LC effluent that is vaporized in a high temperature tube. At the exit of the tube is applied a high voltage, resulting in the formation of ions between the solvent molecules and the sheath gas. These ions, in turn, react with analyte molecules to form ions, which are drawn into the mass analyzer.
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