Impact of GC Parameters on The Separation - Part 4: Choice of Film Thickness
By Jaapde Zeeuw, Restek Corporation, Middelburg, The Netherlands.
In part 1, 2 and 3 of this series we focused on the selection of stationary phase, column length and internal diameter. The stationary phase is also deposited as a film or a layer (Figure 1), which is the fourth parameter that we need to understand its impact. The thickness of the layer determines the amount of the stationary phase that is present in the capillary. Retention is directly related to the amount of the stationary phase; thicker films equal more retention. When do we choose thin or thick films and how do they impact the chromatography? Changing film thickness will impact several parameters: retention, loadability, bleed, efficiency and even inertness.
Film Thickness in Capillary Columns.
Film thickness in capillary columns, is in general between 0.1 and 10 μm for liquid stationary phases such as Rtx-1, 17 and wax. Mostly used are the 0.25 micron films. Thin films are used for the analysis of high boiling materials and thick films for volatiles. If adsorbents are used, we do not talk about films but of layers. Such layers are between 5 and 50 μm in thickness and are usually built by particles.
It is essential is that the film is distributed homogeneously over the whole column length. If the phase is not distributed as a film anywhere in the column, the efficiency of the column will be compromised, especially with liquid stationary phases. This happens most often in the inlet section when injecting samples. If the film is broken up, stationary phase may deposit downstream in the column. This can be observed as a kind of droplet (see Figure 2). This will cause peak broadening and activity. That’s also why cutting off a section of 10–30 cm from the inlet will often completely restore column efficiency and peak shape: By removing the contamination or sections with distorted film the performance is restored.