Poor precision/repeatability of standards through run

[jennifercho]

I am experiencing large (20%-50%) variability in the reproducibility of standard peak areas compared to the continuous calibration verification standard or even the end standards for a pesticide MRM method that switches between positive and negative mode. I am using a Thermo Accela pump and TSQ Quantum Ultra. What could be possible causes for this? There does not seem to be any distortion with the peak shape, baseline, etc. The same standard can be injected in a row and give different area counts for each reading. I am using an acidified water and acidified methanol mobile phase gradient with a 9 minute run time. The injection volume is 10 ul using a UPLC waters accuity c-18 column. Any thoughts would be greatly appreciated.

[Or Not]

We're running a large pesticide screeing method on the TSQ Quantum Ultra and we have found that the position of the ESI probe can have a significant effect on the consistency of the response. We usually see 2-5% RSD for most of our compounds when we run replicate injections near our limit of quantitation. Which probe are you using? What is the micrometer setting that you're using for the probe (front/back position)? Have you verified that the spray is stable? If you're using the non-heated probe with a fused silica sample tube, that might also be a problem if the polyimide coating has eloganted. Have you been able to run this method with good reproducibility in the past?