Switched elution order

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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Hi _Urgent help needed

I am analyzing petroleum samples using a GCxGC-TofMS instrument. I am specifically focusing on the n-alkanes and aromatic hydrocarbons. In a recent analysis I noticed the two hydrocarbon classes elution order switched in the 2nd dimension ie. aromatic peaks seen below hydrocarbon peaks. I am using a normal phase setup ( non polar column in 1st dimension, midpolar column in 2nd dimension) so this was unexpected. I then injected an n-alkane C8-C20 standard just to check and the peaks eluted after 2 sec in the 2nd dimension.

Might this be an instrumentation problem or perhaps the GC conditions? Any advice would be highly appreciated
I am more likely to suspect the column. It could be aged and thus have a different selectivity than initially. As long as the chromatographic conditions are the same (flow, temperatures...) I would try a new column.
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