[vipergts2207]

So in a lab I'm running gasoline through a GC/MS. When I add an internal standard to my calibration standards and samples does it need to be added to each in a constant volume or a constant concentration? For instance if I'm using toluene and ethanol for 2 of my standards, do I need to calculate how much internal standard to add to each to have the same ppm or can I just add a constant volume to both of them. Thanks.

[Arkcon]

If you're going to report concentration for your analyte, and you want to use response factor calculated from the internal standard as an answer, then you probably should also use internal standard as a concentration.  If you're just going to report the response factor as some sort of dimensionless number you get from a ratio of analyte and internal standard, then I suppose you can get away with constant volume.

[fledarmus]

The whole point to using an internal standard is that you know how much of that standard is going onto the column, so you can compare the output with a known input. That means you need to know the concentration of the internal standard in your sample, and the volume of the sample that is added.

You can use a constant volume for your internal standard if you also use a constant volume for everything else. That way the concentration of the internal standard in your sample also remains constant, which is what you are really concerned with.