Direct injection Dirty” samples

Hot splitless WCOT 0.5 ppm (FID) without preconcentration Lower injection temperature than split Trace analysis Handles dirty samples Automation Flash vaporisation Optimisation required (splitless time, oven temperature, solvent) Limited number of solvents ( solvent effect ) Thermal degradation possible Discrimination possible Poor direct quantification Unsuitable for very polar substances... 

Specially designed injection port inlet sleeves have been available on the market for direct and hot on-column injection.In the direct injection mode, 2 -mm-lD inlet sleeves are commonly used, which permit a sufficient space for sample evaporation however, the on-column mode is usually performed by inserting a 26-gauge needle inside a 0.53-mm-lD column. Direct injection is more favorable because it is less problematic than the hot on-column mode. Because the liner can trap nonvolatile residues before entering the column, this technique is suitable for dirty samples. Compared to the splitless mode, the direct injection is advantageous, involving less adsorption of the solutes and better sensitivity. However, with this technique, the adsorption of the sample may occur on the inlet sleeve during the evaporation process.In this case, the hot on-coIumn mode offers more benefits. 

The high temperature of the injector, typically 250 °C, means that this method is not suitable for analytes that are subject to thermal degradation. For these materials, an on-column method is preferable, in which the solution of sample is injected directly into the narrow capillary column with a fine needle. On-column injection techniques are also more suitable for extremely dilute samples, as more sample is delivered to the column, but are less suitable for dirty samples containing non-volatile contaminants, which accumulate on the column. 

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