Chromatography Forum

Hello,

For an LC-MS method of mine, I use standards of the four analytes and include an external calibration curve during each run (plus periodic check standards throughout). The calibration curve is made by diluting our stock standard (which is kept in the freezer in amber glass and wrapped in foil and parafilm) and vialling various concentrations from 1 - 100 ppb. This is done fresh for every run.

I have tabulated the MS detector area response to the 100 ppb standard for all runs going back about a year, and I am noticing that the counts for each of the four analytes have significant drift (as much as doubling!) over a period of weeks-to-months. What is most puzzling to me is that the drift is generally *upward* rather than downward, and it is a fairly stable, almost linear, drift over time. I could understand a *downward* drift, e.g. if the standard were unstable, but it's hard for me to understand an upward drift. Again, these are standards that are re-diluted and re-vialled every run. The calibration curves are very nicely linear, for what that's worth.

Is this drift being caused by the instrument, or something else? What investigative or corrective steps could I take to pin down the cause of this drift?

Thank you!

I have traced this problem to standards on more than one occasion: Upward drift of the area can be caused by evaporation of the storage solvent leading to more concentrated solutions every time the standard is pulled from the freezer, accelerating as the remaining volume decreases. Stored QCs pools in matrix or a standard prepared in a different solution will help identify this fault--their area response should be more constant leading to relatively lower concentrations and drifting accuracy. Weighing a fresh standard or buying a fresh standard would also help identify and remedy this.

Maybe its the instrument: Can you run repeated injections of the same mix of 4 analytes and get constant areas from the same vial on a daily basis? On a weekly basis? Does this hold true for a different (maybe more stable) analyte? Sometimes positive and negative ionization modes are possible for the same analyte. If you can run both positive and negative mode for the same analyte, do both modes agree as to the direction & severity of the drift? Is there a change in baseline/blank response? Are the autosampler and column oven temperatures under control? Is the MS regularly cleaned and tuned? Is the probe distance being routinely adjusted based on performance?

Maybe its something else: Aging reagents used to prepare mobile phase could cause a change in response. Possibly the solution used for diluting the standards has changed or expired. Area drift connected to changes in peak shape or RT could indicate a problem in this category or a possible problem with the column. Somewhere along the line I saw a paper claiming sodium adduct response for the author's analyte increased over time as the lab reused the same glass bottles for mobile phase.

Is the instrument gain being adjusted, either manually or through automated tuning routines/detector gain adjustments during this period?

As detectors age and gain is increased to compensate, I have noticed that the baseline noise increases, the signal-to-noise goes down, but the area counts increase. (Have you checked signal-to-noise ratio for a decrease?)

I would recommend investigating evaporation and detector aging as being likely causes.