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Film Thickness and Phase Ratio in GC Capillary Columns

28 Nov 2023

The phase ratio (beta, ß) of a WCOT capillary column (Wall Coated Open Tubular) is dependent on the internal diameter and the film thickness of the stationary phase and is defined as the ratio between the mobile and stationary phases.

Phase Ratio Equation

The phase ratio is important when it comes to the retention of a substance.

Experience has shown that increasing the film thickness of a GC capillary column considerably increases the retention of the analytes. We benefit from this, especially when it comes to the analysis of volatiles.

A thicker film can lead to better separations by increasing retention due to more interactions, but it also leads to broader peaks (efficiency decreases with increasing film thickness). You should not use unnecessarily thick films, unless you need them because of their higher loadability.

Rule of Thumb: Volatile compounds need columns with thick film (1-10 µm) while high boiling compounds need columns with thin film (0.1 – 0.5 µm).
BUT: The retention of a compound depends not only on the film thickness, but also the phase ratio. Therefore, the inner diameter is also an important factor. For example, a 1 µm thick film is not thick regarding retention for a 0.53 mm ID column but is for a 0.25 mm ID column.
Rule of Thumb: Increasing β = shorter retention
Decreasing β = longer retention

If you want to maintain a separation while changing the inner diameter of the column, it is most useful to consider the phase ratio  e.g. to be able to work faster (smaller ID) or to have more loading capacity (larger ID).

  • In this case, you can choose a column dimension with a similar phase ratio, making it more likely to obtain a similar separation.

The following table shows the phase ratios of different column dimensions (ID/film thickness):

  Film Thickness [µm]
ID (mm) 0.10 0.25 0.5 1.0 1.4 1.8 3.0 5.0
0.18 450 180 90 45 32 25 15 9
0.25 625 250 125 63 45 35 21 13
0.32 800 320 160 80 57 44 27 16
0.53 1325 530 265 133 95 74 44 27

The dimensions in blue were used in the following example.

A typical example are the different column dimensions of the 624 phases for volatiles, e.g. Rxi-624Sil MS. The theory “more retention with thicker film" does not apply here, as shown in the following figure:

Solvent mix on Rxi-624 Sil MS

Figure 1 - Solvent mix on Rxi-624 Sil MS. Conditions were adapted to the inner diameter respectively. Use the Restek Method Translator to help. Analytes from 1 to 16: methanol, acetonitrile, methylene chloride, trans-1,2-dichloroethene, cis-1,2-dichloroethene, tetrahydrofuran (THF), cyclohexane, methylcyclohexane, 1,4-dioxane,  toluene, chlorobenzene, ethylbenzene, m-xylene, p-xylene, o-xylene, isopropylbenzene (cumene)

If only the film thickness increases, the phase ratio would become smaller (as the film thickness is in the denominator) and the retention would increase significantly.

  • However, since the inner diameter increases as well, the phase ratio remains almost constant. Consequently, the retention remains almost constant and - most importantly - the separation remains almost unchanged.

This is the background of the different column dimensions of the 624 phases:
The chosen phase ratio is optimal for this kind of volatile analytes, resulting in sufficient retention and good separation, but without keeping them for an unnecessarily long time on the column. The different inner diameters offer different possibilities in terms of loadability and optimization of run time.

Summary:

The phase ratio of a WCOT GC column affects the retention of the analytes.

  • If you want to increase the retention, you have to use a column with a smaller phase ratio (see table), e.g.
    • same inner diameter, but thicker film or
    • same film thickness, but smaller inner diameter
  • If you want less retention, you have to use a column with a higher phase ratio (see table), e.g.
    • same inner diameter, but smaller film thickness or
    • same film thickness, but larger inner diameter
  • If you want to maintain the separation, but need more loading capacity, use a column with the same phase ratio, but with a larger inner diameter. The film thickness will increase accordingly.
  • If you want to accelerate the separation, keep the phase ratio but reduce the inner diameter. The film thickness will decrease accordingly.