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- Posts: 2
- Joined: Mon Aug 07, 2017 7:54 am
In our lab we use HPLC-MS/MS to quantify drugs of abuse in biological samples. In most of them (almost forty) the precursor ion breaks into three different daughter ions, i.e. we identify the quantifier ion and two confirming ions. Again, the ion ratio of these two confirming ions are within the proper range in most of the drugs. However, we notice that ion ratio is below this range at low concentrations.
We know that lower concentrations means lower sensibility but I don't think that is the cause because we sometimes get a fine confirmation of both ions. When we cannot confirm one ion, its ionic ratio is usually below the range (90% of times) but it can be higher sometimes (10% of times). Also, this problem is VERY common in THC, to the point that the confirmation peak doesn't even appear in the chromatogram.
In some occasions, the ionic ratio of the confirming ion of calibrator 1 is too low but the ionic ratio of the confirming ion of calibrator 2 is too high. In this scenario we can slightly modify the mean of ion ratios so that the range deviates towards one of them, thus confirming it. However confirming both of them is impossible without opening the window of tolerance.
In other occasions, the ion ratio in one of the calibrators is so high/low that the average of ion ratios is not representative. Thus, the ion ration range (which is calculated from the average of ion ratios in all calibrators) deviates to the point that we cannot confirm any calibrator or samples.
Sorry for the long post, I wanted to give as much information as possible. Does anyone have a similar problem? How can we get a fine confirmation at low concentrations (now we only get it from time to time)? Why does this thing happen: the detector, the lens, etc? Thanks in advance!
Carlos.