GC analysis technique: How to prevent damage to Capillary columns
Capillary GC
columns are capable of producing highly reproducible chromatograms provided right
operational conditions are maintained and steps are taken from time t
o 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
Contamination
Accidental
damage
Accidental
Breakage
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.
High-Temperature
Damage
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.
Oxygen
Damage
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
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.
Contamination
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.
Column
Storage
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.
No comments