Retention Time Variability

Retention Time Variability

Consistent retention times are essential for correct identification of target analytes; therefore, variability must be kept to a minimum.

The algorithms that lie behind automated peak assignment will take account of everyday minor variations. But larger shifts (or steadily increasing shifts over a period of time) need to be resolved.

A sudden shift in retention time could be due to method parameters being inadvertently changed. Where possible, compare the current instrument readings to the SOP or old method file and look for differences in the method parameters.

When developing a method, unsuitable temperatures and flows will cause problems. The following parameters need to be optimised for the application: 

- Column temperature and oven temperature program.

- Carrier gas linear velocity.

- GC oven equilibration time.

Once you’re happy with the method, begin troubleshooting other possible causes beginning with the inlet:

- Look for leaks in the inlet and column connections. Replace septa and/or o-ring if required.

- Analyte adsorption due active sites being created by contamination can be an issue. In this case, perform inlet and column maintenance. Use properly deactivated inlet liners and the GC columns.

- Avoid potential sample overloading by diluting the sample or increasing the split ratio.

- Make sure the carrier gas flow is consistent. Repair or replace parts, such as the flow controller, if necessary.

- Make sure the oven temperature is stable at the beginning of the analysis. Increase the equilibration time if required.

- If you’re using manual injection, check your procedure carefully, and consider moving to an autosampler or using an internal standard to compensate for the retention time variability.

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