Fraction collection from capillary liquid chromatography and off-line electrospray ionization mass spectrometry using oil segmented flow.

Off-line analysis and characterization of samples separated by capillary liquid chromatography (LC) has been problematic using conventional approaches to fraction collection. We demonstrate collection of nanoliter fractions by forming plugs of effluent from a 75 mum inner diameter LC column segmented by an immiscible oil such as perfluorodecalin. The plugs are stored in tubing that can then be used to manipulate the samples. Off-line electrospray ionization mass spectrometry (ESI-MS) was used to characterize the samples. ESI-MS was performed by directly pumping the segmented plugs into a nanospray emitter tip. Critical parameters including the choice of oils, ESI voltage, and flow rates that allow successful direct infusion analysis were investigated. Best signals were obtained under conditions in which the oil did not form an electrospray but was siphoned away from the tip. Off-line analysis showed preservation of the chromatogram with no loss of resolution. The method was demonstrated to allow changes in flow rate during the analysis. Specifically, decreases in flow rate were used to allow extended MS analysis time on selected fractions, similar to "peak parking".