Sodium adduct resonse vs concentration

[jclark]

This is a follow-up to my inquiry from 10-14-09 on "MS/MS of peruvoside". I'm trying to confirm that a particular ion is indeed a sodium adduct. For a set of standards, the [M+H]+ response is quite linear but the [M+Na]+ response falls off at higher concentrations in somewhat of a quadratic form. Is the non-linear response what is expected from a Na adduct? I was unable to find any info in this regard in this forum or on-line.
Sorry, I wasn't able to insert the plot itself, due to lack of computer savvy.
jc

[H.Thomas]

Maybe adding a little Na to the mobile phase improves linearity?