GC Column Selection
I am often asked about the best way to select GC columns based on analyte or sample properties. The full answer, of course, is a very broad topic, beyond the scope of this short article. However, during a recent consulting exercise, I had to give a very brief summary of the guiding principles for the selection of GC pillars in order to quickly achieve some results. So I answered the question as pragmatically as possible, using the rules of thumb that I have developed in gas chromatography over the past 27 years. I hope it will serve as a primer or reminder the next time you have to make spontaneous decisions about choosing a GC column.
When choosing a suitable stationary phase, it is necessary to take into account four primary interactions between analyte and stationary phase:
Dispersive interactions (& lt; & lt; 1 kJ / mol) - forces of lower energy (Van der Waals) between nonpolar portions of the analyte molecule, that is, C-H bonds, etc.). These will play a role in the use of a stationary phase based on silicon dioxide, since the majority of the phase polymer backbone (polydimethylsiloxane (PDMS)) is inherently non-polar.
Dipole-dipole and dipole-induced dipole interactions (3 and 1 kJ / mol, respectively) are always involved when unsaturated, aromatic or more polar functional groups (i.e. C-Cl or C-N bonds) are present in the stationary phase or in the analyte molecule. Stationary phases with phenyl, cyano or trifluoro functional groups are more polar than PDMS and the more of these functional groups there are, the greater their influence on the separation. For example, consider the increase in the retention of aromatic compounds and the relative decrease in the retention of aliphatic analytes in the transition from a 5% phenylmethyl PDMS phase to a 50% phenylmethyl PDMS phase.
Hydrogen bond interactions (19 kJ / mol) - are the strongest intermolecular forces in capillary GC and occur whenever the stationary phase contains cyano-, trifluoro- or (most especially) hydroxyl-functional groups. This kind of force is at play in the analysis of alcohols with a polyethylene glycol or wax type phase.
The different stationary phases and their interactions are shown in Figure 1.
Figure 1: Most common stationary phase types in capillary GC with their primary interactions.
Pragmatic phase selection rules can be summarized as follows:
- Use the principles of 'Same dissolves same' wherever possible and match the polarity of the analyte to the polarity of the stationary phase
- Remember that there are really only five 'chemistries' that we need to take into account – these are shown in Figure 1. To increase the retention or selectivity based on a certain interaction, increase the amount of the functional group within the phase (i.e. switch from a 14% to a 35% cyanopropyl phase)
- Use the lowest possible polar phase, as more polar phases bleed more (this is due to chemistry)
- A 5% phenyl column should be used to screen unknown samples - the retention and selectivity of the analyte can then be assessed and, if necessary, a more suitable phase can be selected
- A 5% phenyl, 50% phenyl, 14% cyanopropyl and a wax (PEG) column cover the largest range of possible interactions (stationary phase polarities) in the fewest number of columns
This leaves only the physical aspects of the GC column selection - namely length, inner diameter and film thickness. Again, the information below is a gross oversimplification, but rules of thumb are great if you're in the middle of the lab and only have your thumbs for reference...
The column length influences the separation performance and thus the resolution. Doubling the column length doubles the efficiency (number of theoretical separation stages (N)), doubles the analysis time for isothermal separations (1.5 - 1.75 times increase when using gradient temperature programming), doubles the column cost and increases the resolution by a factor of 1.4. Increasing the column length is the worst way to improve the resolution of a separation - however, if you have a sample with many components (100s), sometimes you need a long column . Rule of thumb - Choose the column length according to the number of species that need to be separated in the sample. Two components - 10m column, hundreds of components - 60m or 120m column.
The internal diameter of the column affects the retention and efficiency. Halve the column inner diameter, double the efficiency and increase the resolution by a factor of 1.4. This doubles the residence time only with isothermal separations and only if the film thickness is not changed. The phase ratio (b) is the column radius (mm) divided by 2 x the layer thickness (mm). Keep this constant between columns, and the retention time is approximately constant. Use columns with a smaller internal diameter if the separation depends on the selectivity of the stationary phase, i.e. if sample components are very similar or if several components need to be separated in shorter periods of time. Note that the required column head pressure to achieve a certain carrier flow increases and the column capacity decreases as the column inner diameter is reduced (use Table 1 as a guideline)
|Internal Diameter (mm)||Capacity (ng)||Pressure (psig Helium)|
|0.1μm Film Thickness|
Table 1: Relationship between GC column inner diameter, column capacity (per component)
and exemplary column head pressure, which is required to obtain a carrier flow of 1 ml / min.
Use phase ratios <100 for highly volatile (low M.Wt. analytes). Use phase ratios >400 for high molecular analytes or for trace analysis.
The film thickness influences the retention of the analyte species, the interaction with the silicon dioxide tube (peak forming effects), the phase bleeding and the column capacity. Doubling the layer thickness doubles the retention time for the isothermal analysis and increases the retention by a factor of about 1.5 for the temperature-programmed analysis. Doubling the film thickness increases the elution temperature by about 20oC. Use thin layers (0.1 - 0.25 µm) when an increased signal-to-noise ratio is required or when analytes are relatively volatile.
Use thicker films (1-5 µm) when working with volatile analytes, analytes with a high concentration or with poor peak shape. It should be noted that increasing film thickness may impair the resolution for later eluting analytes (k>5) and that phase bleeding and column capacity increase with increasing film thickness. The upper temperature limits of the column decrease with increasing film thickness.
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