How to minimize retention time drifts in analytical Gas Chromatography
Retention time drifts contribute to wastage of time and undue
frustration during analytical chromatographic runs. It is defined as the time
that the particular compound spends in the column between its injection and
detection. In other words it is the time
that elapses between its injection and
the maximum of its peak response. It helps to confirm presence or absence of a
particular compound .Ideally the retention time should remain unchanged if
experimental conditions are maintained between chromatographic runs.
At this point of time it would be helpful to
understand the problems that can arise due to shifts in retention times:
- In a
sample containing several components with different boiling points the
peak of analyte of interest can merge with the peak of another boiling
point component. A partial overlap can give rise to shoulder peaks and a
complete overlap can lead to mistaking it for another component.
- In
case the shift becomes significant the peak may get displaced outside the
chromatogram and you may conclude that the component is absent in the
sample
It becomes all the more important to have a tight control on operating
parameters so that the problems outlined above can be overcome. Let us
therefore go into the factors that lead to variations in retention times and
how to minimize them.
Change in flow rate of carrier gas
Flow rate of carrier gas is dependent on several factors such as
incorrect setup of flow control parameters, changes in carrier gas composition
when carrier gas cylinders are changed, fluctuations in temperature of column,
minor changes in column dimensions on replacement, etc.
The GC pressure controller decides the carrier gas flow rate on the
basis of nature of the carrier gas and column dimensions. If a column is
replaced and there are even minor changes in dimensions this can result in a
different flow rate.
Leakages in carrier flow path
Leakages in flow path can occur due to wear and tear of fittings.
Injector leakage is mainly due to septum coring, column bleed or minor changes
in split rent ratio. Regular checks on leaks and timely replacement of worn out
damaged fittings, ferrules and septas can overcome retention time drifts on
this account.
Damage to Stationary Phase
The column packing can get contaminated due to use of aggressive
solvents, sample overloading or injection of dirty samples. The problem gets
aggravated if columns are operated at higher temperatures for extended periods
of time. A temporary solution is to trim the inlet end as maximum damage to
column stationary phase takes place at this portion
Time between Injections
In gradient temperature operation mode the column needs sufficient time
(say about a minute) to equilibrate to room temperature in between successive
injections. If sufficient time is not allowed between injections this can cause
retention time drifts.
It may not be possible for you to exercise the required control over
the operational parameters discussed above so another approach is recommended.
Instead of absolute retention time make use relative retention time. An
internal standard is added to the sample. The ratio of the retention time of
complement of interest to that of the standard takes care of the common
influence that all operational parameters would have on the component of
interest as well as on the standard. This approach can alleviate confusion due
to retention time shifts and save your valuable time.
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