Atmospheric pressure ionisation interface considerations for micro column separation with mass spectrometric detection

Sammanfattning: Two miniaturised atmospheric pressure chemical ionisation (APCI) interfaces were developed for micro column separation and their performances were investigated in high temperature open tubular column liquid chromatography and capillary supercritical fluid chromatography (SFC). The most influential parameters, for example, the interface probe position relative to the sampling orifice and the reagent ion plasma composition, were investigated to optimise the ion current signal and stability. The optimisation procedure was greatly simplified when the corona discharge needle was mounted directly on the interface probe. Detection limits in the low to sub pg range were demonstrated. Further, an interface for capillary column SFC electrospray ionisation (ESI) massspectrometry (MS) was developed. ESI mode gave rise to less fragmentation than APCI,when evaluated using some model compounds. In ESI, the ion signal was found to beoptimal at a spray potential of 4 kV for which a mixed ionisation process of ESI andAPCI was found to be present. The controlling variables involved in this mixedmechanism were studied in detail. By introducing vapour from a solvent, such as benzene, with a low ionisation energy into the nebulising gas, it was found that theappearence of the corresponding molecular ion was correlated with a change in the slopeof the spray current versus spray potential curve. This indicates that the latter change iscorrelated with the onset of corona discharges.Using the APCI interface, energy resolved APCI-MS was utilised for the differentiation of some hydroxy steroid isomers by monitoring the loss of water from the isomers as a function of the collision energy.In another study, an ultraviolet (UV) radiated sunscreen absorber was analysed by capillary column SFC with APCI-MS detection. The presence of proton bounded and covalently bounded isobaric "dimers", generated by a high concentration of the neutral compound in the gas-phase or a (2+2) cycloaddition reaction initiated by UV radiation, respectively, could be confirmed, by the use of "up-front" fragmentation. Finally, the ion source chemistry was controlled in a study on neutral uncharged compounds in ESI-MS. When the ion sampling conditions were manipulated in combination with primary amine addition to the electrosprayed solution, multimer formation of the uncharged neutral compounds was suppressed which greatly improved the quantitative aspects of ESI-MS.