(2M-1)-

[MarkK]

I've been having this specie, (2M-1)- show up in a couple of my standards. No dimerizing. There is an explaination but it escapes me. Thanks in advance, Mark

[JMB]

You are seeing the -ve ion equivalent of [2M + H]+, that will often be observed at higher sample loadings. Depending on the sample loading and structure, the "dimer" signal may be anywhere in the range 5-50% of [M + H]+.

Regard it as confirmation of MW determined from [M+H]+ or [M-H]-.

JMB

[james little]

The amount of dimer, 2M-1, depends heavily on the entrance voltage of the skimmer. For example, look at the ratio of OAc- at m/z 59 to m/z 119 for OAc- . HOAc as a function of the voltage. Makes a big difference in electrospray..