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- Posts: 1862
- Joined: Fri Aug 08, 2008 11:54 am
If you can include a good internal standard (heavy-isotope labelled things are best, as they will very nearly coelute and have nearly identical chemistry to your analyte, meaning that they will be measured at the same efficiency) your problem will go away. Your calibration will be of amount, or amount ratio, plotted versus area ratio (ratio of analyte : internal standard), so if both signals decline together, the ratio remains the same. Incidentally, internal standards also correct for issues during sample prep, a nice bonus.
If you can't include an internal standard, there are still things you can do: for example, I have resorted to paired runs, running the sample twice, with and without an addition of a spike of a known amount of analyte. This tells me the instrument's efficiency for that sample. I still use external calibration in this case, but correct the final amount by multiplying up based on the known efficiency of measurement of the spike. It's a bit of a fudge but in a research environment it's been a helpful approach from time to time. It's a sort of trimmed-down "method of standard addition".