Restek GC Column Selection Guide

Section IV: Film Thickness

Film thickness has a direct effect on the retention and elution temperature for each sample compound. Thicker films retain compounds longer by maximizing the amount of time the compounds spend in the stationary phase. Thinner films retain compounds less by minimizing the amount of time the compounds spend in the stationary phase. Therefore, very volatile compounds should be analyzed on thick filmed columns to increase the time the compounds spend in the column and allow them to separate. High molecular weight compounds such as triglycerides must be analyzed on a thin film column. This minimizes the amount of time the analytes stay in the column and provide low bleed at elevated temperatures which are required when analyzing high molecular weight compounds.

Film thickness directly effects phase ratio (beta) which is an important consideration when changing internal diameter. When internal diameter increases, film thickness (df) must increase in order to provide the similar resolution and retention. Table IV (below) shows beta values for common dimensions of columns. Similar values indicate similar elution for different IDs.

Table IV: Common ß Values

Column ID df 0.1µm 0.25µm 0.50µm 1.00µm 1.50µm 3.00µm 5.00µm 0.18mm 450 180 90 45 30 15 9 0.25mm 625 250 125 63 42 21 13 0.32mm 800 320 160 80 53 27 16 0.53mm 1325 530 265 128 88 43 27

The chromatograms in this figure show a sample containing low boiling compounds analyzed on a 0.25, 1.0, and 5.0µm column with all other variables held constant. Notice that the 0.25µm column does not resolve butanol from benzene (peaks 1 & 2). The 1.0µm column provides about 80% resolution of this pair. Note that the retention times of the compounds eluting on the 0.25µm column more than double on the 1.0µm column. Now, compare the 5.0µm to the 0.25 and 1.0µm columns. The resolution between butanol and benzene (peaks 1 & 2) is not any better than the 1.0µm column, and the retention times have increased six times over the 0.25µm. For this particular sample, the 1.0µm column is best. The resolution is better than the 0.25µm column and the 5.0µm column does not offer any additional improvements. If our true interest was in resolving the compounds prior to butanol (peak 1), then the 5.0µm column would be the preferred film thickness.

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