Combining Capillary Electrochromatography with Ion Trap Accumulation and Time-of-Flight Mass Spectrometry
Simpson, David C
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Capillary electrochromatography (CEC) is a rapidly developing liquid chromatographic technique in which electroosmotic flow (EOF) is used to propel mobile phase through the chromatographic column. The use of EOF results in reduced band dispersion when compared with pressurised flow, but narrow capillaries are required to avoid dispersion due to heating that arises from the required application of high electrical potentials. Measurement of UV absorbance in these narrow capillaries is therefore relatively insensitive, demanding improved detection methods. This work presents an alternative strategy that is based on the combination of ion trap accumulation with time-of-flight mass spectrometry. Electrospray is most often used to transfer analytes from solution to the gas phase, concomitant with ionisation, when interfacing CEC to mass spectrometry. The small volumetric flow rates encountered in CEC, however, raise the possibility of other types of interface being effective. The work presented here describes the development of a novel interface in which a pulsed IR laser is used to vaporise chromatographic eluent, followed by ionisation using a pulsed UV laser. Vaporisation and ionisation both occur within the ion trap to remove the possibility of transmission losses. Ionisation laser wavelength is varied to impart a degree of selectivity. The presence of vaporised solvent and analyte ions inside the trap offers the possibility of performing ion-molecule chemistry. In developing this instrument, the electrochromatographic column was separated from the interface by an electrically grounded junction and a transfer capillary. To preserve chromatographic efficiency, the fluid dynamics of this junction between the column and the transfer capillary were investigated both computationally and experimentally. Simulations of the fluid dynamics of the junction are presented. In order to test the interface without the intermittent, chromatographic, delivery of analyte, a continuous leak inlet was employed. The performance of the instrument was evaluated with polycyclic aromatic hydrocarbons because they are important environmental pollutants and because they are amenable to laser ionisation at 266 nm. Expressed as a number of theoretical plates per metre, an average chromatographic efficiency of 95,000 was obtained with a test mixture that consisted of acenaphthene, biphenyl, fluorene, naphthalene and phenanthrene. Furthermore, using the leak inlet, naphthalene was detected as a 100 nM solution in acetonitrile.