by
lmh » Mon Feb 03, 2020 2:28 pm
There might be things in the oral fluid that elute at the same time as your drugs of interest, and compete with them to ionize. This is called cosuppression. It means that you get less signal from a fixed amount of drug than you should.
If you use an internal standard that is chemically almost identical to the drug, it will elute at the same place, and suffer the same level of cosuppression. This means that while the peak area for the drug will go down, the ratio of peak areas of drug and internal standard will remain the same.
Internal standard calibration therefore accounts for even the most awkward of changes in sensitivity of your method. It also accounts for general changes in sensitivity, for example the instrument becoming dirty over the course of running a batch of hundreds of samples, and it accounts for any losses during sample preparation, in all stages after you added the standard.
The point of a deuterated standard is that it will be chemically very similar to the drug, but can be distinguished from the drug by its increased mass.
Ideally, you should have individual internal standards for each analyte. If this is impossible, remember that you will no longer be accounting properly for cosuppression, because your internal standard will not coelute with all your target compounds. If the compounds are chemically reasonably similar, the internal standard will probably still account for general losses during extraction, and general variations in the instrument's sensitivity.
An internal standard that is wildly different to the target compound is often a bad idea as the internal standard may behave opposite to the target compound and actually increase the errors; or at best it behaves differently, and merely introduces another random factor.
So why are you spiking and adding the internal standard? The answer is that you're spiking so that you have a known sample, to test your method. The internal standard isn't a test that the method's working, it's actually part of the method. It's one of the things that makes the method reliable. If you use the method on oral fluid from someone where you have no idea what the result should be, the internal standard helps you get the right result. The spiked samples were the control that reassured you that your method would give the right result when used on true unknowns.
