As the mass of an organic molecule increases so does the probability of the presence of 13C and the other heavier isotopes of the elements. For carbon, 13C is roughly 1% of the 12C; therefore, for every 100 carbon atoms, 1 of them is 13C. Bradykinin contains 50 carbon atoms. As seen in Figure 1, approximately 50% of the molecules of bradykinin contain a 13C isotope.
When the number of carbons increases to 131, as in the peptide melittin from bee venom, the probability that each molecule will have a 13C isotope is >100% and the 13C peak is the most abundant in the spectrum (see Figure 2).
The molecules of insulin (bovine) and cytochrome c (equus) contain 254 and 558 carbon atoms, respectively. Figures 3 and 4 contain the isotopic profile of bovine insulin and cytochrome c respectively. For insulin the monoisotopic peak is approximately 15% in relative intensity; whereas, for cytochrome c, the monoisotopic peak is 0.03% in relative intensity.
As the monoisotopic peak decreases in relative intensity (e.g., bovine insulin and greater mass) it becomes difficult to obtain a signal from which an accurate center of mass can be determined. When such a problem occurs it is often better to use the average mass of the peak. As discussed earlier, the average mass is limited to five or six significant figures. Therefore the accuracy is limited to approximately 100 ppm.