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- Posts: 1862
- Joined: Fri Aug 08, 2008 11:54 am
The internal standard peak area varies quite a bit on repeated injections from a single vial, in a bad case from peak area 100,000 to 170,000. Is this normal?
Some compounds, in some cases, seem to behave similarly to the internal standard, and give better calibrated results than raw peak areas, but in other cases I get the impression (from too few replicates) that late-eluting compounds behave independently, or perhaps even opposite to the internal standard. The internal standard elutes quite early, and the most variable offender is the last peak to elute, almost at the end of the gradient.
These are the things I'm worrying about:
(1) What is the normal repeatability of multiple injections in GC?
(2) If ours are worse, is there anything obviously wrong in our method?
(3) When repeat injections suggest that the internal standard is behaving differently to the things we're measuring, surely there's no justification in using it? But if anything is varying a lot, and we can't compensate using an internal standard, surely we're utterly sunk: we just have unusable results?
Help! Any comments greatly appreciated!