How to choose chromatographic column?

One
Chromatographic column material
This material must be as inert as possible, especially for trace analysis or easily tailed compounds, such as mercaptans or similar active compounds. Fused quartz is an optional material for capillary columns.
There are two types of fused silica capillary columns: wall coated open column (wcot) and porous layer open column (plot). Wcot column is a stationary phase liquid membrane coated on the wall of the column. This is the most commonly used column in gas chromatography. The stationary phase in plot column is a solid substance coated on the column wall.
The columns can be glass or metal, usually stainless steel. Although metals are more active, they are more stable to non-polar substances. However, if there are polar components in the sample to be analyzed, please select glass column. If the glass column is still active (causing tailing of peak, loss of sample, etc.), please inactivate it.
Two
stationary phase
When selecting capillary column, it is necessary to determine whether PLOT column is needed. The following are typical applications of three plot columns:
Molecular sieves are non volatile and sensitive to water
Divinylbenzene (DVB) - hp-plot Q, separation of all isomers from C1 to C3, separation of C4 and higher isomers (up to C14), polar compounds, volatile solvents may allow water
The separation of C1 to C10 isomers of alumina alumina is sensitive to water
If the application mentioned above is not of interest, you can choose a wcot type column.
When faced with an unknown sample, first try the current column on GC. If satisfactory results cannot be obtained, consider the sample information you know. The basic principle is that analytes interact more easily with stationary phases with similar chemical properties. This means that the more information about the sample, the easier it is to find the best separation stationary phase.
The most important step is to determine the polarity of the analyte
Nonpolar molecules - usually containing only hydrocarbon atoms - have no dipole distance.
Straight chain hydrocarbons (n-alkanes) are examples of nonpolar compounds.
Polar molecules - mainly hydrocarbons, but also nitrogen, oxygen, phosphorus, sulfur, or halogen atoms. For example, alcohols, amines, mercaptans, ketones, nitriles, organic halides, etc.
Polarizable molecules - mainly hydrocarbons, but also unsaturated bonds. For example, alkenes, alkynes and aromatic compounds.
Provide the correct stationary phase for a specific separation: is the sample a mixture of nonpolar substances of the same chemical type? For example, hydrocarbons in most petroleum fractions? Try a non-polar column, such as hp-1, which can be separated in (approximate) boiling point order. If you suspect that there are some aromatic compounds, try HP-5 or HP-35 column suitable for phenyl compounds.
Polar or polarizable compounds are usually separated on the stationary phase containing stronger polarity and / or polarizable groups of phenyl, and the resolution is better. For example, hp-210 or hp-225 column. If a stronger polar stationary phase is needed, polyethylene glycol (PEG) stationary phase, commonly known as wax stationary phase, can be selected.
Bonding can produce chemical bond between stationary phase and chromatographic column. Crosslinking polymerizes the stationary phase in proper position to increase the molecular weight. These two processes occur simultaneously in the preparation of bonded / crosslinked columns, which can increase thermal stability and reduce column loss. The bonded / cross-linked column can be flushed to remove contaminants that may accumulate over time and allow a larger injection volume. If there is a choice, we recommend that the bonded / cross-linked column should be selected among the standard coated column types.
Three
Film thickness
The general rule is that the separation speed of thin liquid membrane is faster than that of thick liquid membrane, and higher separation degree can be obtained at relatively low temperature. This indicates that it is suitable for high boiling point compounds, close compounds or heat sensitive compounds.
The "standard" film thickness is 0.25 ~ 0.5mm. These thicknesses are good for most samples (including paraffin, triglycerides and steroids) separated at temperatures up to 300 ° C. For substances that need to be separated at higher temperatures, a thin liquid membrane (0.1 mm) column can be used.
Standard or thin liquid membrane chromatographic column is used to separate high boiling point substances, while thick liquid membrane column is used to separate low boiling point substances. The 1 ~ 1.5mm membrane can be used to separate substances with boiling point of 100 ~ 200 ° C. Ultra thick liquid film (3 ~ 5mm) is used for gas, solvent and purge to improve their interaction with stationary phase.
Another reason for using ultra thick liquid membrane chromatographic column is that the resolution and retention time can be maintained when the large pore size chromatographic column is replaced. For this reason, thick liquid membranes are used in large aperture chromatographic columns.
A thick liquid film means that there is more material in the column, so there is more loss. With the increase of film thickness, the maximum temperature that can be reached will decrease.
Four
Column length
In general, a 15 m column is used for rapid separation, simple mixtures or high molecular weight compounds. For most analyses, a 30 m column is the most commonly used one. Very long columns (50, 60 and 105m) are used for very complex samples.
Column length is not a very important parameter in the performance of chromatographic column. For example, when the column length is doubled, the isothermal analysis time is doubled, but the peak resolution is only increased by 40%. If the analysis is not ideal, there are more effective ways to improve it than column length. The flow rate of a thin column can be optimized by using a thin column temperature.
A special case is the analysis of samples containing very active components. If they come into contact with the column material, their tailing can be very serious. For relatively short columns, the use of thick film can reduce the chance of contact, because the contact time of column material is relatively less, and the contact between analyte and activation point can be closed with stationary phase.
Five
internal diameter
Increasing the inner diameter means an increase in the number of stationary phases. Even if the thickness remains constant, a larger number of samples can be analyzed. It also means less separation and greater loss.
Thin columns are used for complex sample separation, but split injection is usually required because of the small sample size allowed. If a reduction in resolution can be allowed, this can be avoided by using a thicker column. When sample size is the main factor, such as gas, volatile sample, purge and trap or headspace injection, large inner diameter or even PLOT column is more suitable.
At the same time, the limitations and needs of the instruments used should also be considered. A suitable injection port of a packed column can use a large diameter capillary column, but not a thin column. Injection ports specially designed for capillary columns are usually suitable for all internal diameter columns. The direct combination of GC / MS and MSD may require a fine column because the vacuum pump cannot handle the high flow rate in the coarse column. Examine the entire system to find out which parts limit the selection of column inner diameter.
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