2) Which peak corresponds to 1,2,4-TCB?

Problem:

The gas phase above a commercial standard of 1,2,4-trichlorobenzene (liquid with a purity of 97%) is sampled with a gas-tight syringe and is subsequently injected into a GC. The resulting chromatogram shows 3 distinct peaks of equal area (equal means same concentration in the injected sample). How is that possible? Which peak corresponds to 1,2,4-TCB?

Answer:

Note that the gas phase was sampled and not the liquid. This must mean that two of the impurities in the standards must have a higher tendency to partition from the 97% pure 1,2,4-TCB into the gas phase than the 1,2,4-TCB itself. The two compounds therefore must have smaller van der Waals interaction abilities than 1,2,4-TCB, because otherwise the higher tendency to partition into the gas phase could not be explained (note that 1,2,4-TCB is an apolar phase). For a series of apolar compounds of different size, the smaller ones will elude first from a non-polar GC column. If the impurities were polar compounds and a polar column was used for separation, then a clear conclusion is not possibe. However, the impurities most likely are other chlorobenzenes (e.g., dichlorobenzenes), which are also polar. Hence, one can expect in any case that 1,2,4-TCB was the compound that eluted last.