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22 May / June 2021 Retention Time Variability


Changes in the retention times of target analytes can lead to incorrect identifi cation especially when using non-selective detectors such as a Flame Ionisation Detector (FID).


1. Leaks


a) Leaks in any part of the system can affect the fl ow of the carrier gas which will cause variability in the retention time of the target analytes. Leak check the inlet and any column connections.


b) Replace parts such as the septa and O-rings if they show signs of wear. 2. Analyte adsorption


a) Increased activity in the inlet and column may result in adsorption of the target analyte. This may affect its transfer to the column and retention on the column. This can be overcome by routine inlet and column maintenance.


b) Use of properly deactivated liners and columns will reduce the potential for active sites.


3. Resolution and integration issues.


a) Adding too much sample can overload the column. This can cause variability in the retention time as well as poor resolution and peak fronting. This can be avoided by diluting the sample, reducing the injection volume and/or increasing the split ratio.


4. Incorrect column/oven temperature program


a) If the column temperature and the oven temperature program are not appropriate for the analysis, there may be variability between runs. Verify that the column temperature and oven temperature program are optimised and set correctly in the acquisition method.


5. Incorrect or variable carrier gas linear velocity.


a) The retention times of the target analytes may be affected by variations in the linear velocity of the carrier gas. Verify the carrier gas linear velocity is optimised and set correctly in the acquisition method.


b) If the instrument is unable to maintain the correct linear velocity, there may be a problem with the fl ow controller or a leak in the system. Check the system is functioning correctly and repair or replace parts if necessary.


6. Poor control of oven temperature programming.


a) If the oven temperature program in the method is outside the capability of the instrument, it will be unable to accurately maintain the desired temperature. This can cause variability between runs and affect the analyte retention times. This can be resolved by confi rming GC oven program falls within instrument specifi cations.


7. Incorrect oven equilibration time.


a) The oven may not have enough time to stabilise before the run starts. This can be more of an issue if the oven temperature at the start of the run is close to room temperature. This can be overcome by extending the GC oven equilibration time in the acquisition method.


8. Manual injections.


a) When carrying out manual injections, inconsistencies between the injection and pressing the start button can lead to retention time variability. Where possible, standardise the manual injection procedure. Alternatively, using an autosampler will improve reproducibility between injections.


These systems, like all chromatographic techniques, have a list of quite common problems seen by analysts. To aid your troubleshooting, Shimadzu have developed a free troubleshooting guide which discusses these common problems and a list of helpful actions that can be taken. It offers guidance on how to approach certain problems in a methodical, scientifi c manner. Patience is required and don’t be disheartened if your fi rst course of action doesn’t resolve the issue. Remember that the most obvious answers are usually the correct ones, and there is nothing wrong with checking the basics fi rst.


Finally, if you do have a problem please remember to call your instrument provider. Ultimately, they are here to help, and all want to see instruments up and running as much as you!


The free trouble shooting guide is available from: www.shimadzu. co.uk/form/gc-poster


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