Causes of performance degradation of chromatographic column
Column ruptureIf the polyimide coating of fused silica capillary column breaks a little, it will break. The polyimide coating protects the fragile fused silica capillary. The continuous heating and cooling of the column box, the vibration of the fan of the column box, and winding the chromatographic column on the circular column frame will cause stress on the capillary tube. Finally, it will break at the small weak point, and the scratch or wear place formed by polyimide coating When the sharp edge or sheet touches the capillary tube, it often causes scratches. The hook and label of chromatographic column, the metal edge in the column box, the column cutter and various similar things on the laboratory bench are the sources of the sharp edge or sheet.
The chromatographic column itself is rarely broken. The column manufacturing industry should pay attention to eliminate any weak column, so as to avoid affecting the integrity of the column. The column with large inner diameter is more likely to break.
Thermal damage
Exceeding the upper temperature limit of the column will cause accelerated damage to the stationary phase and the surface of the column, which will cause excessive loss of the column, tailing of active components and decrease of column efficiency. Therefore, it takes a long time to run above the temperature limit before the obvious damage of the chromatographic column. When there is oxygen, the thermal damage will be greatly accelerated. In case of leakage or overheating, the chromatographic column will be accelerated and permanently damaged.
Setting the maximum column temperature of GC at or slightly higher than the high temperature limit of chromatographic column is the best way to avoid thermal damage, which can avoid accidental overheating of chromatographic column. If the chromatographic column suffers heat loss, it still has certain functions. Remove the chromatographic column from the detector and heat it at the limit constant temperature for 8-16 hours. Connect the chromatographic column to the end of the detector, cut off 10-375px, and install the chromatographic column according to normal conditions and conduct aging. However, the performance of the column can not be restored to its original performance, but it still has certain functions. After thermal damage, the life of the column will be shortened.
Oxygen damage
The chromatographic column will not be damaged when the temperature is close to room temperature. Generally, for polar stationary phase, the temperature and oxygen concentration are very low when serious damage occurs. Prolonged exposure to oxygen is problematic. Short term exposure to oxygen, such as air injection or removal of spacer nuts, will not cause problems.
The leakage in the carrier gas channel (such as gas path, connector, injector) is often the source of oxygen. When the column is heated, the stationary phase will be damaged quickly, which will lead to excessive loss of the column, tailing of active compounds and reducing column efficiency. In less serious cases, the column will have a certain separation function, but the performance has declined. In severe cases, the column is completely unusable.
The most effective way to avoid oxygen damage is to make the system avoid contact with oxygen and avoid leakage. The good maintenance of GC system includes regular leak detection of pipeline and pressure regulator, regular replacement of spacer, use of high purity carrier gas, installation of oxygen trap and replacement of carrier gas cylinder when it is empty.
Pan > 5m or longer. If the protective column is used, the length of the damaged column will be reduced. However, if the protective column needs to be treated, the acid or alkali will often destroy the inactivated surface of fused silica tube, which will cause the peak shape of active compounds to deteriorate.
Chemical damage
A few compounds will destroy the stationary phase. The introduction of non-volatile compounds (high molecular weight or high boiling point) into the column will often reduce the performance of the column, but it will not destroy the stationary phase. These deposited residues can be removed by washing the column with solvent to restore the performance of the column.
The main compounds to avoid entering the column are inorganic or mineral bases and acids, which include hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid. Alkali includes potassium hydroxide, sodium hydroxide and ammonium hydroxide. These acids and bases are not volatile and accumulate in the front of the column. If it stays there, the stationary phase will be destroyed, a large amount of chromatographic column will be lost prematurely, the active compounds will be tailed and the column efficiency will be reduced. The symptoms are similar to heat loss and oxygen damage.
Since chemical loss occurs at the front end of chromatographic column, any chromatographic failure can be eliminated by processing or cutting off the front end of chromatographic column by 0,5-1m. In serious cases, 5m or longer section can be cut off. If the protective column is used, the length of the damaged column will be reduced. However, if the protective column needs to be treated, the acid or alkali will often destroy the inactivated surface of fused silica tube, which will cause the peak shape of active compounds to deteriorate.
Column contaminated
There are two basic types of pollutants: non-volatile pollutants and semi-volatile pollutants. Nonvolatile pollutants or residues can not be eluted from the chromatographic column, but accumulated in the chromatographic column, so it becomes the chromatographic column coated with residues, thus affecting the distribution of solute, that is, the normal distribution of solute into and evaporated out of the stationary phase, and the residue will be activated The interaction of the complexes leads to the adsorption problem of the peak (even causes tailing or reducing the peak area). Active solutes are substances containing hydroxyl or amino groups and some thiol groups and aldehydes. Semi volatile pollutants or residues accumulated in the chromatographic column will eventually be eluted. However, it takes several hours or days to completely elute. Like the nonvolatile residues, they can cause the problems of peak shape deterioration and peak area reduction. In addition, they often cause many baseline problems (instability, drift, noise, ghost peak, etc.).
There are many sources of pollutants, among which injection is the most important one. Samples extracted from the worst matrix, such as biological liquid and tissue, soil, waste water and other similar matrix containing a large amount of semi volatile and non-volatile substances, even if careful and thorough extraction method is used, the sample will also contain a small amount of these substances and bring them to the injection sample. Several to hundreds of injections will cause the accumulation of residues. Injection techniques such as on column injection, no split injection and large diameter column directly injecting a large number of samples into the chromatographic column will often cause the pollution of the chromatographic column.
Maintenance measures
Minimizing semi volatile and nonvolatile sample residues is the best way to reduce pollution problems, but the presence of contaminants is often unknown. Strict and thorough purification of samples is the best way to prevent pollution. The use of protective column can reduce or delay the damage of chromatographic column. If the column is contaminated, the best way is to clean it with solvent to remove the contamination. It is recommended not to use the method of heating for a long time to treat the contaminated column.
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