We are attempting to validate the predicted biosynthesis of a fungal non-ribosomal peptide synthase (NRPS) product via mass spec. The H+ ionized product produces a m/z at 433.2443. Looking at the MS results, there is a noticeable increase in the +1 valine, +2 alanine, and +4 tyrosine intensities for the product. However, these peaks can also be observed (albeit with lesser intensities) in the control as well as within the other amino acid plates. Considering the increased intensities for the plates with the corresponding labeled amino acids as well as the fairly uniform, lower intensities of peaks from the other plates, could the presence of similar ions be explained by the presence of naturally occurring 13C isotopes? Or does their presence simply invalidate the results?