Chromatography Forum

I am using older version of MSD ChemStation. As I am preparing my samples gravimetrically, each has slightly different internal standard concentration. Nominally it is 25 ng/g but ranges from ~ 23.8 to 25.5 ng/g depending on the exact mass of sample dispensed. To account for these diefferences, I need to change the sample ISTD concentration for the ISTD compounds in the calibration table before processing each individual sample. Very tedious. Can anyone suggest a work around perhaps using "sample amount" value?

Maybe I don't understand exactly how you are employing your IS, but when I am using an IS, I will have the IS value for the samples and standards "fixed" at 1 (regardless of what it really is) in the software, since I try to keep everything the same.

In your case, the IS for the standards should be at 25ppb (assuming they are indeed 25ppb), while your samples should have the amount present updated. You "should" be able to add a column in your batch/sequence that allows you to enter the IS amount for each sample. Do you have the version of Chemstation you are using available?

Hello,
If I understand the situation correctly, your sample is diluting your IS concentration.
MSD Chemstation in sequence has a column titled 'Dilution' (if I remember correctly), by altering this value, the final calculated concentration
is multiplied by this number.

Alex

I'm also not sure if I'm understanding correctly what you are doing.

I think chemstation and schmitty are probably on the right lines already, but alternatively:

Are you are adding an extraction solvent containing internal standard to accurately-weighed amounts of material, which vary a little from a particular target mass? (And therefore the ng internal standard per gram sample varies a bit because you don't always weigh out exactly X mg, but you do always know how much you've got)?

If so, you could switch to doing your calibration curve in ng (because the internal standard is the same everywhere), rather than ng/g, and deal with the dividing by amount of sample elsewhere (either in Chemstation or in any favourite spreadsheet package if you've already got the sample amounts there).