How to select a GC column for your application
Features to look for in a GC capillary column:
- Low column bleed
- Maximum temperatures
- Column inertness
- Resistance to chemical degradation, air and water
- Produces needed results
When selecting a GC capillary column for an application, four basic parameters need to be considered:
- Stationary phase
- Column internal diameter
- Film thickness
- Column length
You can also refer to these overviews:
1. Stationary phase
- ◄General rules on selecting a phase
- Select the least polar phase that will perform the separation you require.
- Non-polar stationary phases separate analytes predominantly by order of boiling point. Increase the amount of phenyl and/or cyanopropyl content in the phase, and the separation is then influenced more by differences in dipole moments or charge distributions (BP10, BPX35, BPX50, BP225, BPX70).
- To separate compounds that differ more in their hydrogen bonding capacities (for example aldehydes and alcohols), polyethylene glycol type phases are best suited – BP20 (WAX), BP21 (FFAP), SolGel-WAX.
- Wherever possible use published retention indices to assist in your selection. Retention indices are calculated for a range of probe compounds which can highlight specific selectivity characteristics of a stationary phase.
- ◄Retention indices for nine cross-linked phases
The use of retention indices is a valuable tool in assisting selection of the stationary phase, which will provide maximum resolution for the compounds to be analyzed.
The retention indices of the five test compounds indicate the differences and similarities of each stationary phase. The values are calculated in reference to a homologous series of n-alkane hydrocarbons plotted on a logarithmic scale. Each n-alkane has a retention index of 100 times the carbon number (ie. C6, RI=600). Therefore, the retention index for each of the test compounds illustrates the elution position in reference to this n-alkane series.
Each probe compound is selected to represent the interaction characteristics of various organic functionalities.
Probe compound Interactions represented Benzene Aromatics, unsaturated hydrocarbons Butanol Alcohols, diols 2-Pentanone Ethers, esters, ketones and aldehydes Nitropropane Nitro and nitrile derivatives Pyridine Aromatic bases
Retention Indices are calculated using the following formula:
IA = 100N+100n (log t'R(A) - log t'R(N) ) / (log t'R(N+n) - log t'R(N) )
IA is the retention index of compound A (from corrected retention times) which elutes between two n-paraffins separated by either one or two carbon numbers.
Butanol (Y) 2-Pentanone
Average BP1 647 646 666 707 722 678 BP5 667 665 692 743 746 703 BPX5 664 667 697 752 750 706 HT8 680 673 728 796 780 731 BPX35 728 726 763 862 848 785 BP10 709 774 772 862 832 790 BP225 824 931 918 1070 1014 951 BP20 (WAX) 947 1153 998 1217 1185 1100 BPX70 1067 1219 1170 1365 1300 12
The table lists the responses to each test compound and the average value for nine cross-linked phases ranging from the non-polar BP1 to the very polar BPX70. The range has been developed to cover the widest possible range of compound functionality and application areas.
Average retention index values are listed, and provide an indication of the phase polarity. This can assist in selecting a suitable stationary phase for a particular application area. The individual responses to each test compound can further assist in determining the best phase for any specific type or group of compounds.
2. Column internal diameter
- ◄ Effect of column diameter
The smaller the diameter the greater the efficiency and therefore the better the resolution. Reduce the diameter by half and the column efficiency doubles.
As the diameter increases, the film thickness can increase to maintain the same phase ratio. The thicker the film, the greater the loading capacity. Overloading a column will always result in loss of resolution. If the column diameter is halved while maintaining the same film thickness, then the loading capacity will also be halved.
Column ID Recommendations 0.1 mm and 0.15 mm Fast GC columns ideal for FID, ECD. 0.22 mm and 0.25 mm Ideal for MS and high resolution applications. 0.32 mm Provide good resolution for most applications, ample sample loading and are compatible with nearly all detector systems. 0.53 mm Provide large sample capacities and ruggedness.
3. Film thickness
- ◄Sample loading
For samples with a variation in solute concentration, a thick film column is recommended. This will reduce the possibility of broad overloaded peaks co-eluting with other compounds of interest. If the separation of two solutes is sufficient and co-elution is still unlikely, even with large differences in concentration, then a thinner film can be used.
- ◄Volatility of solute
The greater the film thickness the greater the retention of a solute, therefore the higher the elution temperature. As a rule, doubling the film thickness results in an increase in elution temperature of approximately 15-20°C, under isothermal conditions. Using a temperature program, the increase in elution temperature is slightly less.
As well as film thickness, changing the column internal diameter will also effect the elution temperature. To avoid using two parameters that can alter individually, phase ratio is often used as it takes both into account.
The chromatograms demonstrate the effect on elution temperature for a mixture of compounds using 0.32 mm ID columns with film thickness of 0.25 μm, 1.0 μm and 5.0 μm.
An increase in film thickness from 0.25 μm to 5.0 μm needed a change in analysis temperature of 80°C to maintain the same elution time.
- ◄Phase ratio
Phase ratio encompasses both the film thickness and column internal diameter to give a value that can characterize all column internal diameters and film thickness combinations.
Calculate phase ratio using following formula: ß = d/4df
ß = phase ratio
d = column internal diameter (μm)
df = film thickness (μm)
From the phase ratio value, a column can be categorized for the type of application it would best suit. The smaller the ß value, the greater the concentration of phase to the volume of the column, making it better suited for analyzing volatile compounds. Columns which have thin films, are generally better suited for high molecular weight compounds and are characterized by large ß values.
Film thickness (µm) Column ID (mm) 0.1 0.15 0.22 0.25 0.32 0.53 Phase ratio 0.1 250 - 550 625 800 1325 0.15 - - - - - 883 0.25 - 150 220 250 320 530 0.5 - 75 110 125 160 265 1.0 - - 55 63 80 132 3.0 - - - - 27 44 5.0 - - - - 16 26
Keeping a similar phase ratio when changing column internal diameters will ensure that your chromatographic parameters will not need substantial changes.
4. Column length
- ◄ Effect of column length
Always try to select the shortest column length that will provide the required resolution for the application (12-30 m). If the maximum column length available is being used and resolution of the sample mixture is still inadequate, try changing the stationary phase or internal diameter.
Resolution is proportional to the square root of the column efficiency. Therefore, doubling the column length will only increase the resolving power of the column by approximately 40%.
The three chromatograms give an indication of how column length influences the resolution of a mixture.