obtained by applying a potential difference of 3−6 kV between the tube and a
counter electrode. The drops accumulate charge on their surface, and as they
shrink by evaporation they break into ever smaller charged droplets. The
uncharged solvent molecules are pumped away and the charged ions pass into
the mass spectrometer. A schematic is shown in Figure 1.
These interfaces can deal with a wide range of solvent polarities, although for
ionic mobile phases the electrospray is preferable.F5 – Liquid chromatography-mass spectrometry 303
+3 to 6 kV, ions +ElectrosprayNozzleSkimmerMetallic capillaryLensN 2 (80°C)N 2 (80°C)AnalyzerPumpFig. 1. Electrospray (ES) interface. Reproduced from J. Barker Mass Spectroscopy, 2nd
edn, 1999, with permission from Her Majesty’s Stationery Office. Crown copyright.Applications A typical example of LC-MS is the detection of impurities in synthesized drugs
using an acetonitrile-aqueous ammonium ethanoate mobile phase and a
reversed-phase column, coupled to a quadrupole mass spectrometer using an
electrospray interface. The total ion current (TIC) chromatogram in Figure 2(a)
shows that the parent compound elutes at 17.7 minutes, and impurity peaks
occur before and after this time. The compound eluting at 8.35 minutes gave
the mass spectrum shown in Figure 2(b). The peaks due to [M+H]+occur at
m/z = 225 and 227 in the ratio 3 : 1, indicating the presence of one chlorine
atom. The compound was identified as a trisubstituted quinazoline.