It looks like you have conversion of the N-terminal Gln to pyroglutamic acid
which would yield [M+H]+ = 961.5 (monoisotopic or most abundant mass).
That peptide may give you a minor amount of non-covalent or proton-bound
dimer (possible mass spec artifact of too high concentration or inefficient
declustering) that appears at [M+H]+ = 1923.0 (average mass). If you
can tune your instrument to show some resolution of the isotope peaks
for m/z 961, you should be able to verify that the charge is +1. Otherwise,
operation with a higher orifice or nozzle-skimmer voltage should make
the 1923 peak disappear (if 961 were +2, the higher voltage should cause
a decrease in 961 and an increase in 1923).
John Stults
Genentech, Inc.