Linearity problems pyridoxine (vitamin B6) LCMS

[Ascorbic acid]

Hi!
I'm having problems with linearity for my calibration curve of pyridxoine. I only get an R^2 value of 0.934 and the curve looks really bad. I am using ESI and single quadropole MS. The column is sunniest rp-aqua (2.0 i.d. x 150 mm) and I use a gradient of ACN and buffer (20 mM ammonium formate/ 0.05% formic acid). Starting with 100% buffer to 70% buffer for 15 minutes. I'm running SIM mode for m/z 170. I am also analyzing ascorbic acid and biotin with this method and they both show great linearity.

Any ideas or tips for improving linearity?

[Arne]

You should describe in more detail HOW the curve for vitamin B6 looks bad, or better yet, post a picture of the curve.

The compound has a phophate group on it, which can be problematic.

Agilent has a medronic acid solution out that has helped me for improving peak shapes and linearity of chelating compounds. Give that a try maybe?

Anal. Chem. 2018, 90, 15, 9457–9464