Injection #

An important issue when selecting an injector is the properties of the analyte, such as potential for chemical instability, thermal degradation, or discrimination of high-boiling-point compounds within the injector.

In splitless mode, the split outlet remains closed during the so-called splitless period so that sample vapors are transferred from the vaporizing chamber into the column. Flow through the split outlet is turned on again to purge the vaporizing chamber after most of the sample has been transferred. Transfer into the column is slow (e.g., 30–90 s), resulting in broad initial bands that must be focused by cold trapping or solvent effects. Although splitless injection is >30 years old, the vaporization process in the injector continues to be investigated and debated.28,29 Splitless injection, however, is frequently performed incorrectly for a large number of reasons; vaporizing chambers can be too small, syringe needles too short, carrier gas supply systems poorly suited, sample volumes too large, needle technique inappropriate (cool versus hot) by slow instead of rapid injection with too low carrier gas flow rates, incorrect column temperature during the sample transfer, splitless periods that are too short, and liner packings at the wrong site. Some of these problems relate to a lack of understanding of the mechanisms involved (e.g., evaporation by “thermospray” (TSP) and “band formation”). 30 Although compromises have to be made when dealing with multiresidue determinations, there is considerable benefit in evaluating each step of the injection process.

The limitations of splitless injections, small injection volumes (i.e., up to 2 mL), the potential to thermally degrade components, and incomplete transfer of compounds with high boiling points, can be overcome somewhat by using pressure- pulsed splitless injection. The pulsed splitless technique uses high pressure (high column flow rate) during injection to sweep the sample out of the inlet rapidly.