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Continuous-flow fast atom bombardment liquid chromatography-mass spectrometry in bioanalysis study

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Development of an on-line coupling of liquid chromatography (LC) with fast atom bombardment (FAB) mass spectrometry (MS) by using continuous-flow fast atom bombardment (CF-FAB) or CF-frit-FAB interfaces, has opened up new opportunities in the analysis of polar, ionic and thermolabile compounds often encountered in bioanalysis study.

The parameters affecting the performance of CF-FAB were studied. Stable CF-FAB ionization conditions are a matter of importance for reproducible performance of CF-FAB. A uniform film of the liquid on the CF-FAB target with equilibrium between the rate of solvent delivery and evaporation was found to be essential for maintenance of stable CF-FAB conditions. Various parameters such as the target material and shape, mobile phase composition, flow-rate and ion source temperature affected the properties of the liquid film on the target. Of those targets tested, the stainless-steel target with a gold-plated drain channel showed the best overall performance.

Intact glucosinolates were separated and identified in standard mixtures and in a plant extract from Brussels sprouts using CF-frit-FAB LC-MS and LC-MS/MS. The mass spectra of glucosinolates in the negative ion FAB LC-MS mode mainly contained molecular weight information. The daughter ion spectra after collision-induced dissociation of the characteristic (M-H) $\sp-$ anion gave structural information about the compound, based on the group- and compound-specific fragmentation and group-characteristic neutral losses.

The applicability of the CF-FAB technique in quantitative bioanalysis study was demonstrated. Specific CF-FAB LC-MS methods for quantitative determination of dextromethorphan and erythromycin 2$\sp\prime$-ethylsuccinate in plasma were developed.

The most serious limitation of CF-FAB technique in LC-MS operation is the maximum flow-rate of ca. 15 $\mu$l/min, necessitating the splitting of the eluent from conventional LC columns. The solute splitting was avoided completely in target-compound analysis by using the phase-system switching approach prior to CF-FAB LC-MS. Using the phase-system switching approach, a lowering of the flow-rate from 1.0 ml/min in the liquid chromatography system to 15 $\mu$l/min going into the mass spectrometer was achieved without splitting. Optimal mobile phases and flow-rates were used for the liquid chromatographic separation as well as for the mass spectrometric detection. A specific method for quantitative determination of erythromycin 2$\sp\prime$-ethylsuccinate in plasma using phase-system switching CF-FAB LC-MS is described.

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