Hello,

I'm trying to develop a LC-ESI-HRMS/MS method for the analysis of triacylglycerols in drying oils and paints, however I struggle with removing TAG contamination in blanks. After trying almost everything, I'm stuck.

I'm working with a Thermo Vanquish UHPLC, Waters 150 x 2.1 x 1.7 BEH C18 column and Thermo Orbitrap Exactive Plus. Mobile phase A: 60:40 H2O:ACN + 0.1%FA + 10 mM NH4Ac mobile phase B: 90:10 IPA:ACN + 0.1% FA + 10 mM NH4Ac

After a few injections of diluted oil samples I started to notice carry over occurring in blank injection. I first thought, this was caused by a weak washing solvent, however when changing to 100% IPA, peaks where still visible.

I have tried the following steps, without success:

- flushed the UPLC with different solvent mixtures according to the Waters LC-MS guide

- flush multiple times with 100% IPA (+ column)

- changing the column multiple times

- bypassing the autosampler/pre-heater/divert-valve to MS

- change the entire HESI-source by another HESI-source

- clean the ion sweep cone, ion capillary with IPA in ultrasonic bath and replace the O-ring

- increase the temperature of the HESI source to 'bake out'

- changing the mobile phase by fresh mobile phase

- changing mobile phase A to 80:20 ACN:H2O

- remove the FA in the mobile phases

- clean needle seat with IPA in ultrasonic bath

However, I still see signal when I run a zero injection while bypassing the autosampler, pre-heater and the divert-valve.

Any suggestions on what could be the cause/solution?