Detection of low molecular weight aldehydes in aqueous solution by membrane introduction mass spectrometry

Membrane introduction mass spectrometry (MIMS) is used to detect low molecular weight aldehydes in aqueous solutions. The best sensitivity was obtained by aqueous phase derivatization of aldehydes with O-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine (PFBOA) and electron capture detection. This negative chemical ionization mass spectrometry procedure allowed the measurement of C(1)C(6) aldehydes at low concentrations in mixtures. The characteristic ion signals in the mass spectrum of the mixture were verified by examining the full mass spectra and product ion MS/MS spectra of the derivatives of individual aldehydes. A reaction scheme is proposed to explain the fragmentation pattern of the molecular anions (M(-.)) of the derivatives. The processes observed include loss of HF to form (MHF)(-.) ions which then competitively fragment by elimination of H(R)CN and NO(.) to produce ions of m/z 178 and (M-50)(-.), respectively. Multiple reaction monitoring was applied to establish the lower limits of detection. Formaldehyde could be detected without preconcentration at 1 ppb with S/N = 3/1. The detection limits of acetaldehyde, propanal and butanal were found to be 10 ppb and that of pentanal and hexanal were found to be 20 ppb. Response curves vs. concentration are linear in the ppb range. This method is not as readily applicable to the corresponding ketones.