Mass spectrometry

Mass spectrometer is capable of analyzing charged particle based on their mass. A typical mass spectrometer consists of an inlet system, which supplies the pure compound (separated from complex biological matrix by GC or HPLC) to the mass spectrometer, an ion source, a mass analyzer, and a detector. The ion source is responsible for fragmentation pattern of the compound of interest in characteristic pattern depending on the functional groups and other structural features of the molecule. The detector plots a chromatogram listing all ions generated and separated by their mass to charge ratios as well as abundance. Mass spectrum is usually considered as a molecular fingerprint of a compound, not only for relatively small molecules but also for macromolecules such as proteins and DNA (11,12). Mass spectrometer is often used as a detector for compounds eluting from a gas chromatograph or column of an HPLC. Gas-chromatography-mass spectrometry (GC/MS) is widely used in clinical toxicology laboratories for detection and quantification of DAU in biological matrix such as urine because of its specificity, sensitivity, and the availability of larger number of mass spectra in standard drug libraries. Coupling of HPLC with mass spectrometry enables the analysis of thermally labile compounds, polar compounds, or compounds with high molecular weights that cannot be analyzed by GC or a combined GC/MS.

Electron ionization (EI) at 70 eV produces reproducible mass spectrum, which is a common ion source used in GC/MS analysis of therapeutic drugs especially DAU. As far as combining HPLC with a mass spectrometer, only moving belt and particle beam interfaces are compatible with EI (13). The electrospray interface is very common in HPLC/MS analyzer used in clinical laboratories. The electrospray interface produces singly or multiple charged ions directly from a solvent system by creating a fine spray of highly charged droplets in the presence of a strong electric field with assistance from heat or from pneumatics. In this process, nonvolatile and polar compound can be ionized. The atmospheric pressure chemical ionization interface produces sample ions by charge transfer from reagent ions. The reagent ions are produced from solvent vapor of the mobile phase.

After producing charged particles from the analyte eluting from the column, a mass spectrum is produced by detecting these charged particles (positive or negative ion) in the detector of the mass spectrum. The major types of mass spectrometric analyzers are quadrupole analyzer, ion trap analyzers, and time-of-flight analyzer. Quadrupole detector can be used for quantification of drugs (14). Mass spectrometer is usually operated in a selected ion-monitoring mode or single ion-monitoring mode.

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