By lower do you mean the section that is run earlier? It might be matrix evaporation. In long runs (> 2hrs) matrix begins to sublimate this gives a “graded” intensity shift. It happen to us when we were using 2,6 DHA.
We sometime observe a sudden shift in intensity, making a “break” in intensity i.e. a bottom half is lighter than the top half for example. We can’t proof it, but we think it’s an instrument problem.
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