Barb,
The easiest explanation may be provided if you reduced and
carboxymethylated the protein prior to analysis. If my guess about
reduction/carboxymethylation is correct, the peak is a peptide which
contains carboxymethylated/degraded methionine (mass decrease = 48 Da).
See Anal.Biochem. 216 (1994) 135-146.
Jacek Mozdzanowski
Bioanalytical Sciences
SmithKline Beecham