Preserving the First Dimension Separation in GCxGC

I mentioned in a recent blog post that to maximize peak capacity in GCxGC the first dimension separation needs to be preserved by having a very short second dimension separation (short modulation time), often on the order of 2 sec or less, even.  While maximizing peak capacity can be very important when trying to characterize a complex sample (e.g., for metabolomics or discovering emerging contaminants in the environment), maintaining the first dimension separation through short modulation times is even more critical when isomers are to be determined individually, including for mass spectrometry.  That’s because isomers that coelute in the first dimension for GCxGC are very rarely separated in the second dimension.  The second dimension column, no matter the alternate selectivity, is just too short.

The problem outlined above, first dimension coelution caused by a longer modulation time in GCxGC, and its solution, a faster modulation time to preserve the first dimension separation, are illustrated in the figure below.  See how the tetrachlorobenzenes coelute when the modulation time is 2 sec?  The peaks eluting from the first dimension column are “piling up” at the modulator, which might be OK if they were separated in the second dimension, but they are not.  Modulate faster = preservation of the first dimension separation, in GCxGC.  The general rule of thumb is modulate (slice) the first dimension peak at least 3 times.

Mod time

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