how to minimize retention time drifts in Gas Chromatography?

Retention time of a chromatographic peak

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.