The need for chromatographic and mass resolution in liquid chromatography/tandem mass spectrometric methods used for quantitation of lactones and corresponding hydroxy acids in biological samples

Because of the potential in-source conversion between a lactone and the cor responding hydroxy acid, it has been recognized that a liquid chromatograph y/tandem mass spectrometric (LC/MS/MS) method developed for quantitation of a lactone drug in the presence of its hydroxy acid metabolite (or vice ver sa) must incorporate chromatographic separation between the two compounds, unless in-source conversion between the two compounds has been eliminated b y the appropriate selection of the LC/MS/MS parameters. We now report that chromatographic separation between a lactone and its hydroxy acid will be r equired under certain LC/MS/MS conditions used even in the absence of in-so urce conversion. This is due to the fact that the 18-mass-unit difference b etween a lactone and its hydroxy acid is, by coincidence, different by only one mass unit from the 17-mass-unit difference between the [M + H](+) and [M + NH4](+) ions of the lactone or the hydroxy acid. Thus, the [M + H](+) ion of a hydroxy acid is higher than the [M + NH4](+) ion of its lactone by only one mass unit. Therefore, in a method developed for quantitation of a hydroxy acid drug utilizing a selected-ion-monitoring (SRM) scheme that in corporates its [M + H](+) ion as the precursor ion, the quantitation would be inaccurate due to the interference by the contribution of the A + 1 isot ope response from the [M + NH4](+) ion of the lactone metabolite present in the sample, unless there is a chromatographic separation between the two c ompounds. This is true even if Q1 is operated under a unit-mass resolution. The implication of this type of interference, arising from the presence of both the [M + H](+) and [M + NH4](+) ions of a drug and its metabolite, to the selection of LC and MS conditions (including mass resolution) will be discussed using the data obtained with a model lactone drug and its hydroxy acid metabolite. Copyright (C) 2000 John Wiley & Sons, Ltd.