Unbiased injection of liquid samples in gas chromatography is not a trivial task. The main problem is illustrated in the (programmed-temperature) chromatograms shown below. It is generally known as discrimination.
The top (light blue) chromatogram is obtained using hot injection (using a split/splitless injector in split mode). The bottom (pink) chromatogram is obtained for the same sample using cold injection (using a programmed-temperature-vaporizer or PTV injector).
Retention and selectivity (i.e. peak positions) are seen to be identical in the two chromatograms, confirming presence of the same analytes in what is, after all, the same sample. The early parts of the two chromatograms also show identical peak-height ratios. However, the further into the chromatograms (higher retention times, i.e. higher temperatures and lower analyte volatility), the lower the relative heights of the peaks in the top chromatogram. Relatively low-volatile analytes shown much smaller peaks. In other words, these “high-boilers” are discriminated against.
The cause is “needle discrimination”, due to a temperature gradient arising along the length of the injection syringe, from the metal needle that warms up quickly when inserted in the hot injector, to the glass syringe barrel that essentially stays cold. Such a temperature gradient causes high-volatility analytes to evaporate to a greater extent during injection than low-volatility ones, and eventually injection bias or discrimination in the chromatogram.
To avoid discrimination cold injection can be used. Cold on-column injection, in which the sample is brought directly into an empty pre-column (“retention gap)”, is fundamentally the best solution, but the process is rather intricate and not tolerant for “dirty samples” (that contain non-volatiles). PTV injection is more, flexible and robust, with a high tolerance for “dirty” or aqueous samples.
For more on GC injectors see Module 2.4
