Figure 3: Comparison of C-peptide internal standard (IS) mass spectrograms for pure IS (A) and IS mixed with the serum sample (B). A: 50 pg of pure stable isotope labeled C-peptide (retention time 13.5 min, S/N 165) B: 50 pg on column of the same isotope as in A spiked in serum (retention time 13.6 min, S/N 86). The lower signal intensity observed for IS (Mw/z=1017.7) mixed with serum is due to both matrix effect and analyte losses during purification steps.