Separation ofo-,m- andp-phthalic acids by high-field asymmetric waveform ion mobility spectrometry (FAIMS) using mixed carrier gases

Baseline separation of the three isomers of phthalic acid was achieved in a mixed gas system containing a 95 : 5 mixture of N(2) and CO(2), even though the acids could not be distinguished by high-field asymmetric waveform ion mobility spectrometry (FAIMS) when either pure N(2) or pure CO(2) was used as the carrier gas. Pseudomolecular anions of o-, m- and p-phthalic acids were generated by electrospray ionization and detected, following separation by FAIMS, using a quadrupole mass spectrometer. Addition of small amounts of CO(2) to an N(2) carrier gas also caused the compensation voltages to increase by as much as 12 V, accompanied by 2-7-fold improvements in the measured ion current and dramatic reductions in both adduct ion formation and parent ion fragmentation.