profile of 1 nmol of total antibiotic for both UV and MS detection. The elution rate was 2 ML/min, and a makeup solvent containing glycerol, water, and methanol (2:1:1) was added postcolumn at 1 jtL/min. Six different antibiotic compounds were identified in the analysis. Teicoplanin A2-2 [(M + H)+ = m/z 1880.6] was the major component. Although full-bore columns have been used with CF-FAB, high split ratios of 1:50 to 1:100 are generally necessary and therefore preclude their use where analyte concentrations are very low. In some cases, even the use of microbore and capillary bore columns is not a great advantage because the benefit of a splitless interface is offset by the lower injection volumes used. Kokkonen and co-workers (15) have used a technique for target compound analysis, termed phase system switching, that uses full-bore columns and CF-FAB without splitting. Valving and column trapping techniques accomplish this goal. Basically, the compound of interest, which is eluted off a large-bore column, is trapped onto a short microbore column. After washing, this column is eluted with the appropriate solvent at the flow rate suitable for the MS interface. For example, these investigators determined dextromethorphan in plasma using an RP-2 analytical column (3 X 100 mm) at a flow rate of 1.0 mL/min in combination with an XAD-2 trapping column (1 X
50 mm) that was subsequently eluted into the source of the mass spectrometer at 5 fiL/min. A concentration of about 0.5 ng/mL of the drug in plasma could be detected at a S/N ratio of 10:1. This procedure is best suited for target compound analysis.
CE/MS CE is a high-resolution separation technique that is well suited for lowflow-rate liquid introduction MS. The principal advantages of the technique are its ability to achieve very high resolution separations of several hundred thousand theoretical plates or better, a very low flow rate on the order of tens of nanoliters per minute in most cases, and its ability to achieve separations based primarily on charge. Several investigators have reported the coupling of CE and CF-FAB MS (16-18). An interface is needed between the end of the CE capillary and the CF-FAB probe because of flow rate incompatibility; CF-FAB requires a minimum flow rate of about 1-2 /uL/ min to maintain stability, whereas any pressure-driven flow through the CE capillary would lead to band broadening and a loss of resolution. Basically, interfaces are of two types: a liquid junction of the type first reported by Minard et al. (16) and Caprioli et al. (17), and the coaxial type described by Tomer et al. (18). The liquid junction interface is illustrated in Figure 9. In both cases, carrier solution is supplied
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Figure 9. Instrumental setup for coupling CE and CF-FAB. The interface, shown in the magnified area, allows 5 jtL/min of solution from the CF-FAB reservoir to be supplied to the mass spectrometer without inducing mechanical flow in the CE capillary. (Adapted with permission from Reference 17.)
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 8, APRIL 15, 1990 • 483 A
