Reasons for column performance degradation

1. Column breakage

The polyimide coating of a fused silica column breaks if it breaks a little. The polyimide coating protects the fragile fused silica tubing. Continued heating or cooling of the column oven, vibration of the column oven fan, and winding the column around the circular column will stress the line. Finally, a break occurs in the weak place. Weakness is caused by lightly scratching or abrading the polyimide coating. Scratches are usually caused when a sharp tip or side is drawn. Column hooks and labels, metal edges of the GC oven, column cutters, and various items on the lab bench have sharp points or edges. There are very few cases where the column itself breaks. Column manufacturing seeks to identify all defective pipelines and avoid using them in already prepared columns.

Larger diameter columns are more susceptible to breakage. This means that it is more prudent to handle the 0.45-0.53 mm id line than the 0.18-0.32 mm id line to prevent breakage. A broken column is not unusable. If the broken column is kept running at high temperatures or running multiple temperature programs, it will be very susceptible to damage. Exposure of the second half of the broken column to high temperature oxygen can quickly damage the stationary phase. The first half of the column will remain intact due to the passage of carrier gas. If the broken column is not heated but exposed to high temperatures or oxygen for a short period of time, the second half will not be subject to any serious damage. The broken column can be attached by installing a fitting. Any suitable fitting can be reconnected to the column. More than 2-3 connectors cannot be loaded on one column. Multiple connectors can cause dead volume (tailing peaks) problems.

2. Thermal damage

Exceeding the upper temperature limit of the column can cause accelerated damage to the stationary phase and the tube surface. This can result in excessive column loss, active component tailing, and/or reduced column efficiency (resolution). Fortunately, thermal damage is a very slow process, so there is still a long time before the column is severely damaged, which can be used above the temperature limit. Thermal damage is greatly accelerated when oxygen is present. Overheating a column with a leak or high oxygen content in the carrier gas can quickly and permanently damage the column. Setting the GC oven's maximum temperature to the column temperature limit or slightly above this temperature limit is the best way to prevent thermal damage. This will avoid accidental overheating of the column. Even if the column is damaged by heat, it can still be used. Remove the column from the detector. Heat the column for 8-16 hours at the constant temperature limit of the column. Cut the column to the end of the detector and cut it 10-15 cm. Install the column as normal and age it. The column will not return to its original performance, but it will still be usable. The life of the column is reduced after thermal damage.

3. Oxygen damage

Oxygen is the enemy of many capillary GC columns. At room temperature or near room temperature, the column will not be damaged, but the column will be severely damaged as the column temperature increases. Generally, for a polar stationary phase, severe damage can occur at lower temperatures and oxygen concentrations. Oxygen damage can occur when exposed to oxygen for a long time. Exposure to oxygen for a short period of time (such as injecting air or quickly removing the septum nut) will not cause any problems.

Leakage in carrier gas flow paths (eg, gas lines, joints, injectors) is often the source of exposure to oxygen. As the column heats up, the stationary phase is quickly damaged. This can result in excessive column loss, active component tailing, and/or reduced column efficiency (resolution). The signs are similar to thermal damage. Unfortunately, the column has been severely damaged when oxygen is damaged. In less severe cases, the column can still be used, but performance is degraded. In severe cases, the column will be completely unusable.

Keeping the system out of contact with oxygen and avoiding leaks is the most effective way to avoid oxygen damage. Good maintenance of the GC system includes periodic inspection of leaks in the gas and pressure gauges, periodic replacement of septa, use of high quality carrier gas, installation and replacement of oxygen traps, and replacement of gas cylinders before they are completely used up.

4. Chemical damage

There are quite a few compounds that can damage the stationary phase. Non-volatile compounds (high molecular weight or high boiling point) entering the column typically degrade column performance without damaging the stationary phase. Flushing the column with solvent usually eliminates residue and restores column performance. The main compounds to avoid entering the column are inorganic acids and bases. The acid includes hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and the like. Bases include potassium hydroxide, sodium hydroxide, and ammonium hydroxide. Most of these acids and bases are less volatile and accumulate at the front of the column. If you do not remove them, the stationary phase will be damaged. This can result in excessive column loss, active component tailing, and/or reduced column efficiency (resolution). Its signs are similar to thermal damage and oxygen damage. Hydrochloric acid and ammonium hydroxide are the least harmful of this class of compounds. These two substances are readily soluble in the water in the sample. If the water does not stay or stays in the column, the HCl and NH ₄ OH will stay in the column for a short time. This eliminates or reduces the likelihood of damage from these compounds. Therefore, if the sample contains HCl or NH ₄ OH, use a non-retaining environment or column to reduce the hazard of these compounds to the column. Only perfluoric acid is an organic compound that can damage the stationary phase. These examples include trifluoroacetic acid, pentafluoropropionic acid, and heptafluorobutyric acid. They need to be destructive at high concentrations (eg 1% or higher). Most problems occur during splitless or large diameter direct injection, where a large amount of sample is deposited at the front of the column. Since chemical damage is common at the front end of the column, trimming or cutting the front end of the column 1/2-1 m usually eliminates all chromatographic failures. In more severe cases, it may be necessary to cut a section of 5 meters or more. Using a guard column or a retention gap column will minimize damage to the column, but the guard column needs to be trimmed frequently. The acid or base often destroys the deactivated surface of the fused silica line, causing the peak shape of the active compound to deteriorate.

5. The column is contaminated

Contamination of the column in capillary GC is a common problem. Unfortunately, it is similar to various common problems and is often misjudged as other failures. Usually, contaminated columns are not damaged, but they can no longer be used. There are two basic types of pollutants: non-volatile and semi-volatile. Non-volatile contaminants or residues do not elute and accumulate in the column. This column becomes the column to which the residue is applied, thus affecting the correct distribution of the solute in the dissolved stationary phase and the eluted stationary phase. Moreover, the residue also interacts with the active solutes, causing peak adsorption problems (such as tailing peaks or reduced peak areas). Active solute refers to a substance containing a hydroxyl group (-OH) or an amino group (-NH) and certain thiol groups (-SH) and an aldehyde. Semi-volatile contaminants or residues that accumulate in the column will eventually elute. It takes hours or even days to completely elute from the column. Like non-volatile residues, they can cause problems with peak shape and peak area. In addition, they often cause many baseline problems (stability, deviation, drift, ghost peaks, etc.). There are many sources of pollutants, of which injection is the main source. Extract samples from complex matrices. For example, physiological fluids and tissues, soil, wastewater, groundwater and similar substrates contain large amounts of semi-volatile and non-volatile substances. Even with a careful and thorough extraction method, the sample will contain a small amount of these substances. After several injections up to several hundred injections, the accumulated residue can cause problems. Injection techniques such as on-column injection, splitless injection, and direct injection of large-bore columns involve large amounts of sample into the column, so these injection methods often cause column contamination.

Sometimes contaminants are derived from materials in the gas path and traps, gaskets and septum particles, or any material that comes in contact with the sample (vials, solvents, syringes, pipettes, etc.). If there is a sudden contamination problem, but the similar samples in the previous months or a few years have not caused any problems, the problem comes from these kinds of pollutants.

Minimizing semi-volatile and non-volatile sample residues is the best way to reduce contamination problems. However, the presence or absence of contaminants and the presence of contaminants are generally unknown. Strict and thorough purification of samples is the best way to prevent contamination problems. The use of guard columns or retention gap columns can often alleviate the severity of problems caused by column contamination or delay the occurrence of these problems. If the column is contaminated, the best method is to use a solvent to flush the column to remove contaminants. It is not recommended to use long-term heating (often referred to as a bake column) to treat contaminated columns. Because baking columns can turn certain contaminants into insoluble materials and cannot be removed from the column by solvent cleaning. If this happens, it is usually impossible to recover the column. Sometimes the column can be cut into two sections and the second half may still be used. When baking the column at the constant temperature limit of the column, the time should not exceed 1-2 hours.