Means for Improving Internal Standard Responses


I run a VOA system using an EST Analytical autosampler and concentrator as well as Agilent GC/MS (7890B and 5977A). This system is used for drinking water. Does anyone have any advice for improving internal standard responses? I have been running this system relatively nonstop for the past month and as expected, ISTD responses have dropped moderately from the last time the calibration curve was ran (~62% of the mid level calibration standard at the moment).

Some have said to bake out the GC for a certain amount of time to reduce the water in the system, but I have done this and seen very marginal increases (~3-5%). I have also heard that increasing the voltage to the MS would likely help that as well, but I don't want to risk throwing off my tune and lose more time trying to retune the instrument. I honestly just don't want to lose an excessive amount in recalibrating the instrument. 

Let me know if you guys need any more information.


Nicholas Davis

  • Baking is highly overrated.  In the MS, water is gone in eight to twelve hours no matter the temperature as long as no more is introduced. Water in the sampling system and GC is typically reduced by just not running samples overnight, too. Moisture management is a pain, right?

    Has your BFB response gone down at the same rate as your ISTDs?  Are all ISTD responses decreased about the same?

    How long since the ion source was properly cleaned and the insulators and filaments were replaced?  How many runs on the system?  As a system ages the adhered organic layer on every surface inside the vacuum chamber continuously redistributes. This means that a cleaned source gets dirty just from the stuff already inside, so the time between source cleanings gets shorter and shorter. The solution for that is a remedial cleaning including wiping out the entire analyzer manifold, the analyzer side plate, the quad radiator, and the source radiator with solvent-dipped lint-free cloths, and removing and cleaning the source radiator.

    How many runs on a system?

    • 8890 and Intuvo: Service Mode, Instrument, Usage
    • 7890: Service Mode , Diagnostics, Instrument Status
    • 6890: Options, Diagnostics, Instrument Status

  • As the filament gets older and the source dirtier areas will go down. Just bump up the EM volts a little  by say 24 and see what you get. Keep bumping it up until you get areas you want. It should not change the tune much. You can always set it back to where it was.

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