Comparison of Temperature Programmable Split/Splitless and Cool On-Column Inlets for the Determination of Glycerol and Glycerides in Biodiesel by Gas Chromatography with Flame Ionization Detection

The European Standard EN 14105:2011-07 is an analysis method for quantifying free glycerol and residual mono-, di- and triacylglycerides impurities in biodiesel by gas chromatography. The method specifies an "on-column injector or equivalent device" as the means of sample introduction. Cool on-column (COC) would appear to be an ideal choice, particularly for quantifying triacylglycerides, as it provides high quantitative accuracy and precision with minimal mass discrimination. However, there are a few drawbacks in using COC for this application. The relatively high concentration of the biodiesel in the prepared samples impedes solvent focusing of early eluting compounds such as glycerol, causing band broadening and shifts in retention time compared with the external calibration standards. More problematic is method robustness when using a metal retention gap. Repeated injections onto the retention gap cause the method control specification to fail within relatively few injections. As an alternative, a temperature programmable split/splitless (TPSS) inlet was investigated for performance equivalency. The results demonstrate that the TPSS yields concentration measurements indistinguishable from the COC inlet at the 95% confidence level. In addition, the robustness of the TPSS far exceeds that of the COC inlet by eliminating the performance control failure and providing solvent focusing for the early eluting peaks.