Anyone who has injected a sample into a GC and then pulled the plunger back has seen that some of the sample remained in the needle. It is logical to think that by chasing the sample with solvent, the solvent should force the entire sample out of the needle and then only clean solvent would remain. However, in practice, the process of evaporation from the needle involves solvent and volatile components being preferentially evaporated from the needle and less volatile sample components disproportionately being left behind (i.e., causing high-end needle discrimination).
We have discussed in prior articles that injections into hot inlets are fraught with problems. The problems are influenced by many variables, which is why it is problematic to generalize from one set of conditions or limited optimization space. Some of the variables are listed below. Changing any one of them can change the results.