In the first two scenarios, the separation canbeallowed toproceednormally. In the
4th scenario, different column chemistries shouldbe selected. Stop-FlowGC can be
used to significantly improve the resolutiondescribed in the3rd scenario.
For compounds that resolveon the first column, but coeluteat theendof the second
column, a stop-flowpulse canbe introducedbetween the two compounds as they
elute from the first column. This is doneby opening the valve to theexternal gas
sourceafter the first compoundhas passed the junctionpoint. The first compound
continues on through the second column, while the second compound is stopped (or
moved slightly backward) on the first column. In thisway, the separationbetween
the two components canbe increased, as demonstrated inFigure5. InFigure5a,
no stop-flowpulses havebeenapplied, resulting in the coelutionof AandB, aswell
asCandD. InFigure5b, onepulseat 28 seconds has beenapplied, effectively
resolvingAandBat theoutlet of the second column. InFigure5c, both sets of
coeluting compounds havebeen resolvedby using stop-flowpulses at 28and43
seconds.
InFigures 6and7, the stop-flow techniquewas applied toamixtureof volatile
organic compounds. Inparticular, pyridine, p-xylene, andm-xylenewere difficult to
resolveon the selected stationary phases. Usinga series of 3 stop-flowpulses, the
resolutionof these compounds canbegreatly improved. The timinganddurationof
the stop-flowpulses, aswell as the resulting chromatogram, are shown inFigure7.
