Mobile phase influence on electrospray ionization for the analysis of smokeless powders by gradient reversed phase high-performance liquid chromatography-ESIMS

Effect of methanol quality on the ionisation of herbicides, insecticides and fungicides using gradient elution liquid chromatography

This paper explores the changes in the electrospray signal response of 39 structurally different compounds caused by the quality of the methanol, when used as a component in a gradient elution mobile phase. When three batches of LC-MS grade methanol from one manufacturer were evaluated, the largest variation in the electrospray signal responses of the 39 compounds examined was 18%. However, significant enhancement of the electrospray signals of up to 106% were observed among different brands of LC-MS grade methanol from different manufacturers. The effect of methanol quality on signal response was found to be compound dependent. This study also demonstrated that the senescence of the methanol was important. Using an expired batch of LC-MS grade methanol, electrospray signals were suppressed by as much as 95% for all compounds measured using positive mode electrospray. Conversely, the negative mode electrospray signals of compounds such as 4-octyl benzoic acid showed an enhancement of up to 96% when using the same batch of methanol. Linuron was used as a model compound to examine the change in the electrospray response, during gradient elution, when the proportion of an expired batch of methanol was varied. An infusion experiment showed that the linuron signal intensity decreased as the proportion of expired methanol increased in the mobile phase, which was in direct contrast to the increase in linuron signal observed with a non-expired batch of methanol. A series of isocratic experiments also showed that the linuron signal decreased as the proportion of expired methanol increased in the mobile phase. The ion ratios of several of the compounds studied changed significantly when using the expired batch of LC-MS methanol. The change in the ion ratios accentuates the difficulty of identifying compounds from in-source spectral libraries. A protocol is recommended for assessing the quality of methanol for LC-MS applications.

Study on the formation of an amoxicillin adduct with methanol using electrospray ion trap tandem mass spectrometry

The possibility of using the protonated methanol-adduct of antimicrobial amoxicillin for its identification and quantification at residue levels has been investigated, since it is impossible to completely suppress the formation of these adducts when methanol is present in the solvent system. This process has been monitored over time and as a function of concentration. It was determined that adducts were instantly formed and that the abundance of the protonated methanol-adduct at m/z 398 increased at the expense of the protonated molecule m/z 366 with storage time. The effect of several common solvents and mobile-phase additives on the ionization efficiency of amoxicillin and the formation of the methanol adduct has also been investigated. It was shown that the mass spectra of amoxicillin were strongly influenced by the solvent in which the analyte is dissolved and by the analyte concentration, as well as by the composition of mobile phase. Methanol was determined to be the best spray solvent, as it provided spectra with the lowest abundance of dimer ions. It was also determined that acetic acid as the mobile-phase additive provided the highest signal intensities, while ammonium acetate should not be used as an additive for the determination of amoxicillin at residue levels. Using high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC/ESI-MS/MS), fragmentation of the protonated molecules and the protonated methanol-adduct ions, in both positive and negative ion mode, has been performed. The fragmentation was stable and strong product ion spectra were obtained. The linearity of the MS detector response, and that of the chromatographic method, was tested. Due to the linear behaviour it was concluded that the protonated methanol-adduct ion can be used for analytical purposes, i.e. for identification and quantification of amoxicillin at trace levels.