Also I've seen signal of some phosphopeptides almost completely quenched in
presence of other peptides. Enrichment of phosphopeptides by iron
chelating chromatography eluting the phosphopeptides directly onto reverse
phase and then developing the reverse phase has been useful, providing
signal for phosphopeptides where none was seen in the total digest--even at
the same protein loading.
I haven't been very happy with nanospray for phosphopeptides--sometimes it
works and sometimes it doesn't. Maybe due to the quenching phenomenon.
Katheryn Resing