3) Peak area 100 times larger

Problem:

In the lab you have a standard for 2,4-DNT (crystals with a purity of >98%), which shows 5 peaks of equal areas when injected in its gaseous phase into a GC.

The analytical data sheet for the standard (provided by the company that delivered the standard) also shows 5 peaks with the same column, but the third peak has an area 100 times larger than the other peaks. Obviously, this large peak is the standard. In this analysis the standard was injected not in the gaseous phase but as a solution. What is the explanation for the difference in peak areas between injection in a gaseous phase and injection as a solution?

Answer:

Note that the answer here is somewhat different from the answer in the previous question. In the previous case, the impurities had to have a higher vapor pressure than the standard chemical, which makes up for their smaller concentration so that the gas phase concentration would still be similar. In that case, it was save to assume that the standard chemical corresponds to the last peak because it is the least volatile. Here in the new question, this line of argument is not valid because we have single crystals of pure compounds. Hence, even the chemicals that make up the 2% impurities will exist as an own pure phase exhibiting a saturation vapor pressure. Thus, the impurities that one sees in the chromatogram cannot necessarily be expected to be more volatile that the standard compound itself. An allocation of the peaks is not possible.