Mass difference of 59 between pos and neg mode

[smit4227]

Hello all,
In my research, I have isolated a small peptide and have observed an unusual mass discrepancy between the positive and negative ion modes. In positive, the mass is 59 Da higher than in negative. This is true for several other related peptides I have observed. I have read that loss of a trimethylamine can result in this mass difference and I believe this modification is possible, but all the sources I have read that described this witness both parent and derivative peaks in positive mode. Has anyone else encountered this phenomenon before?
~Tom