INSTRUMENTATION from an external reservoir to produce sufficient flow to stabilize the CF-FAB device without inducing mechanical flow in the CE capillary. The CE/MS separation and analysis of peptide mixtures such as those produced from protease digestions are a major focus of this technology. Figure 10 shows a selected ion chromatogram for the peptide of m/z 693.4 from the
analysis of 40 pmol of the tryptic digest of recombinant human growth hormone (17). The mass range scanned was m/z 600-2000 at a rate of 10 s/scan. The peak at half-height is about 12 s wide and represents the practical minimum peak width needed for a mass spectrometer scanning at this speed. The electrophoretic separation was performed at 15 keV (8 /uA) with a buff-
er containing 40 mM sodium citrate and 40 mM NaCl, pH 2.5. The CF-FAB carrier solvent was 92% water, 5% glycerol, and 3% acetonitrile containing 2.3 mM acetic acid and 1 mM ammonium hydroxide to provide electrical conduction. Generally, although high salt concentrations in the CE capillary tend to give optimal separations, this is not used in the anodic reservoir because it results in cationized molecular species in the FAB spectrum. Thus, at high sodium concentrations, one sees ions at (M + H) + , (M + Na)+, (M - H + 2Na) + , and so forth. This decreases the effective sensitivity by distributing the ion current of the molecular species among several ions and complicates interpretation of the spectra, especially in complex mixtures. The limit of detection for peptides undergoing CE was determined using the octapeptide angiotensin II (MW 1045). The mass spectrometer was scanned over a 4-mass-unit-wide window centered on the (M + H ) + ion at m/z 1046. When 368 fmol of the peptide was injected, the measurement of the S/N for this ion was 12:1, and for injection of 75 fmol, about 3:1. This experiment defines only the apparent sensitivity of a single peptide under
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Figure 10. Selected ion chromatogram of m/z 693.4 obtained from the CE/CFFAB separation and analysis of a tryptic digest of recombinant human growth hormone. Data were obtained on a Finnigan MAT 90 mass spectrometer. (Adapted with permission from Reference 17.)
484 A • ANALYTICAL CHEMISTRY, VOL. 62, NO. 8, APRIL 15, 1990
