Effect of high performance liquid chromatography mobile phase (methanol versus acetonitrile) on the positive and negative ion electrospray response of a compound that contains both an unsaturated lactone and a methyl sulfonegroup

During method development for the quantitative determination of a compound in human plasma by electrospray high performance liquid chromatography/mass spectrometry (HPLC/MS) we found that the positive ion electrospray respons e of the compound was dramatically higher (about twentyfold) in a mobile ph ase that contained methanol rather than acetonitrile. We then decided to in vestigate the electrospray behavior of the compound under different mobile phase conditions. The positive and negative ion electrospray response of th e compound in mobile phases that contained methanol or acetonitrile at neut ral and low pH (low pH achieved with formic acid), and with and without amm onium acetate, was investigated. The positive ion response at [M + H](+) or [M + NH4](+) was consistently higher in a methanolic mobile phase under al l conditions studied. The comparatively lower response at [M + H](+) or [M + NH4](+) in the acetonitrile mobile phase was not due to the formation of adducts or fragment ions not seen in the methanolic mobile phase. However, the negative ion response at [M - H](-) or at an adduct of [M - H](-), obta ined in a methanolic mobile phase that contained formic acid and/or ammoniu m acetate was the same as that obtained in the corresponding acetonitrile m obile phase. The positive ion electrospray results obtained for this compou nd, with an exocyclic alpha,beta-unsaturated lactone as the only group that can potentially be protonated, are instructive and may serve as an example of the positive electrospray behavior of weakly basic compounds. The negat ive ion response, attributed to the methyl sulfone group, the only acidic g roup of the compound, may also serve as an example of the electrospray beha vior of weakly acidic compounds. It should be noted that the weakly acidic methyl sulfone group gives good negative ion electrospray response in mobil e phases of neutral and acidic pH in which the compound is not expected to be in the ionized form. (C) 1999 John Wiley & Sons, Ltd.