Gas Chromatography Troubleshooting Part I – Peak Shape Issues
Feb 28 2021
Author: Shimadzu UK on behalf of Shimadzu UK Ltd
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While analytical instrumentation continues to evolve and develop, technology is not yet at the stage where total automation of analytical chemistry is possible. One such area where this is evident is in the field of troubleshooting and maintenance. Analytical techniques are more common than ever, and the number of analytical services available online, and instruments to perform them is booming. However, all these instruments have down time, whether this be due to quality control failures, routine maintenance, or general wear and tear. Therefore, all systems require an analytical mind to act as a ‘doctor’ when needed.
In this article, Shimadzu UK gives insights into effective troubleshooting for abnormal peak shapes in gas chromatography (GC), namely no peaks, fronting or tailing peaks, split peaks, broad peaks and ghost peaks. It will also provide hints and tips for successful troubleshooting. The next article (Part 2) shall investigate potential causes and solutions for other typical issues, including baseline issues, changes in response and retention time variability.
It can be incredibly frustrating when issues arise during a chromatographic run, and the cause and fix are unknown. Usually, there are a number of common problems which could have caused the issue. A basic knowledge of troubleshooting can help to reduce instrument downtime and identify potential issues to get a system back up and running without too much interruption.
Quite frequently, attempts to troubleshoot can be hindered by attempting to change too many variables at once, which can create confusion regarding the symptoms and causes. Alternatively approaches, such as using past experience can help to provide a good starting point for identifying the error, but it may not help in preventing the problem. The most appropriate troubleshooting approach is to investigate problems systematically and prevent errors from recurring.
The process of troubleshooting varies from system to system, but all should follow the same methodical process. Specific tests are often performed, first to confirm the reproducibility of the error, and then slowly eliminating different parts of the instrument in order to chase the fault to its root cause. Troubleshooting can often feel like a game of hide and seek, with intermittent, random errors being the hardest find.
Hints and Tips
These key hints and tips can help towards beginning to identify errors:
- It is important to not overlook the obvious solutions first. For example, check for disruptions to power and other connections to ensure the communication of the system is intact.
- Review the method parameters. Ensure the method has the correct gas flow rate, and the required column and liner are used. Although this sounds obvious, it is often ignored, with time spent trying other more complicated ‘fixes’.
- Record the sample preparation and ensure changes haven’t been made to solvent batches. Confirm the sample vial matches the position in the batch table.
Good documentation can quickly and simply assist in identifying errors. A log should be kept of any instrument error to identify patterns as well as the solution to the error. In addition, logs of maintenance should also be kept, such as when routine care was performed, or the last time a liner or septum was changed.
- A system suitability test mixture which is regularly performed on the instrument can provide valuable information regarding the functionality of the instrument. Previous data can be compared against current performance to assist in identifying issues.
- Only change one variable at a time to isolate the issue. Changing multiple variables at once may fix an issue, but the true fix will not be known to prevent the issue arising again.
- Remember that preventing an issue from occurring is simpler than trying to solve a problem. For example, simple preventative maintenance can help to reduce downtime caused by worn seals. Also, using appropriate seal and rinse washes can dramatically improve reproducibility and prevent contamination of the system.
Common Peak Issues and Solutions
a. A blocked syringe can be fixed by cleaning or replacing the syringe
b. Injecting into the wrong inlet can be corrected by resetting the autosampler.
c. The carrier gas flow plays a crucial role in the chromatography, therefore, simply checking the gas is flowing at the prescribed flow rate is important.
d. Is the sample in the correct position in the batch? Amend the batch to aspirate from the correct vial position
a. The column could simply be broken or installed in the wrong inlet or detector. In these circumstances, replacing or reinstalling the column will resolve the issue.
a. The signal may not be recorded, thus, the detector cables should be checked and verify the detector is on.
b. Alternatively, the detector gas could be turned off or the wrong flow rate used, therefore, the detector should be turned on and the flow rate adjusted to the correct value.
a. Column installation issues can result in significant peak tailing. The tailing can be reduced by ensuring the column is cut properly, minimise dead volume, or by verifying the correct installation depth.
a. A leak should be ruled out by checking all the connections for a leak, and also replacing critical seals if required.
a. Adsorption due to surface activity or contamination can be combatted by using a properly cleaned and deactivated liner and column. It can also be reduced by trimming the inlet end of the column or replacing the column if it is damaged.
b. Adsorption due to chemical composition of the compound can also result in peak tailing. The compound could be derivatised to reduce this effect.
a. Overloading can cause the peak to front. By reducing the injection volume, diluting the sample or increase the split ratio, the degree of fronting can be diminished. The column inner diameter or the film thickness could also be increased to reduce the fronting of the peak.
b. An incompatible stationary phase should be replaced with an appropriate stationary phase.
c. A poorly fitted column can also cause voids which exhibit as fronting. This can be remedied simply by reinstalling the column.
a. A contaminated syringe could easily cause additional peaks to be present in a chromatogram. The rinse solvents should be first replaced and either rinse or replace the syringe if the syringe continues to contaminate.
a. Backflash, where the same volume exceeds the liner volume, can also cause ghost peaks to appear. Some potential solutions include injecting a smaller amount, use a liner with a larger internal diameter, increase the head pressure (such as by increasing the flow rate) to contain the vapour cloud and/or use a slower injection rate. Other solutions include using a liner with packing, use pressure pulse injections and finally using an online calculator to check expansion volumes.
3. Analysis time:
a. If the analysis time is too short in comparison to peak elution, the analysis time can be simply extended to allow all components to elute within the retention window.
a. Sample carryover could cause the peaks to appear broader. To resolve these issues, look at the ghost peak injector solutions.
a. The column film selected could be too thick, which would result in the peak to broaden. The retention of compounds could be reduced by decreasing the film thickness and length.
b. An increased dead volume could also result in wider peak widths. The dead volume should be minimised in the GC system, where the column should be reinstalled, ensure proper connections are used and appropriate liners.
a. A slow GC oven program can increase the peak width leading to broadening, however, it can be overcome by increasing the GC oven programming rate.
b. Alternatively, poor analyte / solvent focussing might require a lower GC oven start temperature.
4. Gas flow:
a. An incorrect gas flow could be detrimental to the peak shape, where the wrong flow can result in broadened peaks. Verify the inlet and detector flow rates and adjust accordingly. Also verify the make-up gas flow and adjust.
a. Split peaks can sometimes be caused by a fast autosampler injection into an open liner, therefore use a wool liner or reduce the injection speed.
b. Incomplete vaporisation. Try adding surface area, such as with wool, to the inlet liner to enhance vaporisation.
a. A mismatched solvent / stationary phase polarity can be amended by adjusting the solvent or stationary phase to allow wetting.
b. If the sample loading capacity is exceeded, inject less sample onto the column by either diluting the sample, using a split injection or reduce the injection volume.
These systems, like all chromatographic techniques, have a list of quite common problems seen by analysts. Missing peaks, tailing, carryover, fronting, inconsistent peak area, baseline noise, to name a few. If any of these terms seem familiar, don’t worry, you’re not alone. These issues have been around ever since Martin and James invented the technique in 1952!
A free trouble shooting guide by Shimadzu 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, scientific manner. Patience is required and don’t be disheartened if your first 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 first.
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 guide is available from: www.shimadzu.co.uk/form/gc-poster
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