Acylcarnitine screening MS/MS

[M.C.]

Dear scientists,
I've a question for you:
I'm involved in acylcarnitine newborn screening using dried blood spots.
I use API 2000 ESI-MS/MS and PE autosampler 200 coupled with PE micropump 200.
The analysis are run with a commercial kit that wants butylation as derivatisation step.
Acylcarnitines are calculated with precursor scan of 85 m/z.
In the last month I've noticed in the spectra of all sample (not in the blanks: only internal standards) the increased presence of an ion (before was present but only in small traces) : 419.5 m/z
now It appears the ion with the highest intensity among the others.
does someone of you know it ?
_an unkonwn (for me) blood metabolite with the same fragment (85 m/z)?
_release from somewhere inside the LC system?
_other
please give me a suggestion
many thanks in advance M.C.

[MG]

If it isn't showing up in your blanks, then we can rule out your LC system as a source.

Has something changed in your preparation procedure? Or could it be coming from certain lots of your reagents? It might be useful to run a "reagent blank" by running the prep method on some surrogate material (e.g. water) in place of the blood.