Capillary GC columns are capable of producing highly reproducible chromatograms provided right operational conditions are maintained and steps are taken from time to time to prevent damage and performance degradation.
The factors which can result in deterioration are discussed in the present article and preventive measures are suggested:
- Accidental breakage
- High-temperature damage
- Oxygen damage
- Chemical attack
- Accidental damage
Columns appear to be very delicate but the outer polyimide coating on fused silica tubing contributes to their physical stability. However, repeated heating – cooling cycles, vibrations produced by the cooling fan and careless mounting on the cage can lead to breakages. Sudden breakages are not common but over the use weak spots develop which can result in cracks or breakages.
The remedy is installation of unions to join the broken ends but multiple unions can contribute to dead volume which results in complications like peak tailing.
Every column has a specification on upper temperature operation. Exceeding the limit accelerates the degradation of the stationary phase. However, significant damage like loss of resolution or peak tailing becomes apparent over prolonged operation at temperatures above prescribed limit. Overheating a column with leaks result is in exposure to oxygen which can cause irreversible damage.
Thermal damage can be reversed to an extent by removal of a segment of the detector end of the column. Heating for about 8– 10 hours at its isothermal temperature limit and removing about 10 cm length from the detector end. Re-install and condition as prescribed before reusing.
Continuous use of a leaking column at high-temperature leads to fast deterioration of the stationary phase due to oxygen damage. The damage is lower for stationary phases with polar characteristics.
The leaks can result from gas lines or injector fittings. Early symptoms of oxygen damage are excessive column bleed, loss of resolution, peak tailing, etc. It is best to ensure leak free operation to prevent onset of oxygen damage. This can be achieved by regular leak checking of gas lines and regulators, periodic septa changes and use of high purity grades of gases and installation of oxygen traps in gas lines. Remember to always replace gas cylinders before they run out of supplies completely.
Chemical attack is less serious than oxygen attack. Nonvolatile compounds have greater potential of damage to the stationary phase. The influence of nonvolatile compounds can be reversed to an extent by solvent rinsing.
Apart from non-volatile compounds mineral acids or alkalis can attack severely the stationary phases. Organic acids such as perfluoro acids can also result in column damage. Fortunately the damage is confined to around the front end of the column and removal of the front-end (say,1 – 2 m) can improve the column performance.
Column contamination can result from introduction of semi-volatile or non-volatile impurities present in the sample matrices. While semi-volatile impurities are easily eluted over a period of time non-volatile impurities impair stationary phase performance and may or may not elute out of the column even on prolonged use. In addition to sample induced contamination such contamination can also result from foreign solid micro particles from gas lines or traps, septa and ferrules, etc.
Freedom of such contamination can be achieved by proper prior treatment of samples and frequent changes of ferrules and septas. Use of guard columns can also reduce problems arising from contamination. The column performance can be reversed by removal of a length of the front end of the column and using the other section and baking the column at its isothermal temperature limit for about 1 to 2 hours.
Among the other performance recovery solutions solvent rinsing technique is popular. Inject a few ml of solvent to dissolve soluble residues and remove them. However, this approach works with bonded or cross-linked phases and on the contrary can result in severe damage to non-bonded stationary phases.
On removal columns should be stored in their original boxes. Septas should be affixed to the open ends to prevent entry of any debris. At time of reuse around 2 – 4 cm of end tubing should be removed to prevent entry of septa debris, if any.
It should be remembered to keep carrier gas flow on if column is left inside a heated GC oven. Without the carrier gas flow damage can occur to the heated stationary phase.