Internal Standard Fluctuation Problem

When running a calibration on the GCMS the internal standard increases the entire time for all of our standards. It goes from around 1.9 million for the bottom of the calibration to 3.2 million. Recent things that we have done that should help eliminate this problem:

  • The EM Horn was replaced about 6 months ago (it should last a lot longer than this)
  • The temperature for the ramp parameters was decreased that included the IS
  • The column was changed about a month ago

When running clean outs the IS stays stable and therefore when the samples are ran on the daily calibrations it fails. One of the internal standards coelute with a compound but the other two do not. This is for an instrument that is specific to the TO15 and APH method. When we run the APH method with the same IS, the increase in IS does not happen as dramatically.

Does anyone have any idea on why this could be occurring?

Thank you!

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