I'm not too firmiliar with this ASTM but if you are using an internal standard that is not a deuterated version of the analyte, you maybe finding that there is drift in the MS.
Another possible explanation is that with this being in gasoline for analysis, that there is a slight build up of any residue in the inlet or front end of the column there tends to be a change in response, especially if the internal standard isn't a deuterated version of the analyte as the properties are different.
You could try installing a backflush method or have a higher post injection split flow.
One other possible explanation is that hte split vent trap in hte instrument is oily/full. If you method is a split method this could possibly be the reason why.
Herb gives an excellent video on how to change this on youtube:
Hope this helps.
D59769 method needs a high split ratio on the gc. This is determined by looking at signal to noise of .01% 1,4-diethylbenzene. The signal to noise should be 5:1 but no more than 6:1.
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