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FIGURE 1.5 (A) Base-peak reconstructed ion chromatogram from a mixture of five standard peptides from a 20 femtomole/peptide on-column injection using gradient elution from a 75-|im-ID tip column as shown Figure 1.4. (B) MS spectrum of peak number 1 showing a doubly charged molecular ion of high S/N. (C) MS/MS spectrum of peak number 3 showing labeled N-terminal (a, b) and C-terminal (y) fragment ions. Scale expansion factors are denoted at the top of the figure. (Data courtesy of William S. Lane, Harvard University.)

for a through-column flow rate of approximately 200 nL/min. A 200-nL on-column injection of a standard mixture of five peptides (20 femtomole/peptide) was delivered prior to water/acetonitrile (5% to 95% acetonitrile in 0.05 M acetic acid) gradient elution. The column was positioned within 1-2 mm of an ion trap mass-spectrometer inlet. High voltage (1-2 kV) was applied on the inlet side of the column directly to the mobile phase through a wire electrode assembly. Excellent peak shape (no tailing) and exceptional MS and MS/MS S/N of greater than 50:1 at the 20 femtomole level was obtained. In contrast to the single-use nanospray tips, such columns can perform well for hundreds of injections. The approach has proved itself in extended time scale experiments, such as large-scale proteome complex analysis [54,55] and quantitative proteomics [56,57].

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