Mass Spectrometry in Pharmaceutical Analysis

Abstract

Mass spectrometry (MS) applied to the biological sciences in the pharmaceutical industry covers a broad range of application topics, such as quantitative analysis of drugs in physiological fluids, qualitative analysis of noncovalent complexes, structural analysis to determine chirality and protein sequence analysis. Regardless of the application, electrospray ionization (ESI) is the primary ionization method for transfer of nonvolatile biologics like proteins and peptides into the gas phase for analysis. It is most often used as an interface between high‐performance liquid chromatography (HPLC) separation methods and MS. The ESI process converts analytes in solution to ions in the gas phase via electrostatic nebulization at atmospheric pressure. It is characteristic of ESI to produce multiply charged analytes in a concentration‐dependent manner that is sensitive to changes in solvent, pH and ionic strength. A high voltage is applied to a liquid containing analytes as they traverse a capillary, and when the liquid droplets exit this capillary at atmospheric pressure they are directed into the source region of a mass spectrometer where desolvation is completed. Ions are then focused by an electronic gradient across a series of lenses and separated by mass/charge ( m / z ) using various mass spectrometer designs. Currently, the two most popular mass separation devices are quadrupoles (either single or triple quadrupole (TQ)) and ion traps (ITs). Both types of mass spectrometers are used with ESI to achieve sensitivities as low as attomoles (10 −18 mol) of analyte injected onto a microcapillary HPLC column. The type of analyte, as long as it is capable of accepting a charge (addition of H + or loss of charge in negative ion mode) is relatively transparent to ESI and so a diverse range of analyte classes with disparate molecular weights can be ionized, from peptides (500–5000 amu) to proteins (10 000–100 000 amu). Depending on the construction of the source, ESI can be carried out with reversed‐phase HPLC at flow rates from 200 nL min −1 to 2 mL min −1 and also with capillary electrophoresis. The ease of use of ESI compared to prior techniques like fast atom bombardment has made it very popular with biologists, chromatographers and chemists who might not otherwise have been interested in MS. In fact, for many scientists ESI has made MS almost as “user‐friendly” and thus accessible as an ultraviolet (UV)–visible detector for HPLC.

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