Section F – Combined techniques
F5 LIQUID CHROMATOGRAPHY-
MASS SPECTROMETRY
Principles The wide variety of modes of liquid chromatography available and the separa-
tions that may be achieved have been described in Topic D7. Since these employ
liquid mobile phases, sometimes containing inorganic salts, the most difficult
problem is how to transfer the separated component of the analyte to the mass
spectrometer without interference from the solvent.
Materials of high relative molecular mass are readily separated by liquid
chromatography and, consequently, ionization methods that produce less
fragmentation in the mass spectrometer may have to be employed.
Instrumentation The interfacebetween the liquid chromatograph and the mass spectrometer is
the most vital part of the combined instrument. Early systems using a moving
belt interface have been superseded by spray devices and interfaces, which
operate near atmospheric pressure.
In atmospheric pressure chemical ionization (APCI) interfaces, nitrogen is
introduced to nebulize the mobile phase producing an aerosol of nitrogen and
solvent droplets, which are passed into a heated region. Desolvation occurs, and
ionization is achieved by gas-phase ion-molecule reactions at atmospheric
pressure, electrons and the primary ions being produced by a corona discharge.
Since the pressure is close to atmospheric, the collision frequency is high and
pseudomolecular ions, (M + H)+and (M −H)+, are formed with high efficiency by
chemical ionization. The analyte ions are accelerated into the mass spectrometer
and the uncharged solvent molecules are removed by vacuum pumps.
In the electrospray (ES) interface, also operating at atmospheric pressure, the
liquid mobile phase is ejected from a metal capillary tube into an electric field
Key Notes
The components of a mixture, after separation by liquid chromatography,
may be identified and quantified by mass spectrometry.The removal of the liquid mobile phase, while allowing the analytes to be
transferred to the mass spectrometer has presented difficulties, and the
design of the interface is critical.The analysis of mixtures of pharmaceuticals and drugs, the detection of
degradation pathways using isotopic labeling, and the separation and
analysis of peptides using soft ionization methods are typical of the
application of LC-MS.Related topics High-performance liquid Mass spectrometry (E14)
chromatography: modes,
procedures and applications (D7)PrinciplesInstrumentationApplications