Gas chromatography (GC) column maintenance can often seem a little daunting. While some maintenance tasks can be fairly time consuming, others are easily done in less than a few minutes. The frequency of column maintenance is application and sample specific, which is why many labs are uncertain about how often to carry it out. This article will look at the most obvious signs that maintenance is needed, and advise on how to care for your columns efficiently to minimize downtime.
The best way to ensure that your GC column is running at optimal performance is to perform a quality control (QC) check. By saving a high resolution, clear separation as a reference chromatogram, you should then periodically compare separations, monitoring multiple reference points.
The following signs may indicate that maintenance is needed:
– Peak size or analyte response changes (usually occur gradually over time).
– Peak symmetry changes (most commonly observed as peak tailing).
– Ghost peaks.
– Baselines rising.
Check the sample introduction system
The QC check provides peace of mind that the GC system is running as expected. It should be carried out periodically at a frequency that fits with your lab workflow. If QC results are unacceptable, performing inlet maintenance can often resolve the preceding problems and takes far less time than trimming the column.
For example, the sample introduction system could be developing active sites or concentrated samples could be contaminating your system. Replacing the septum, liner, and the gold seal if contaminated with matrix can usually solve any issues. To learn more about picking the right liner and gold seal for your GC application, check out our previous article. If a noisy baseline and matrix interference persists and peak symmetry is still poor, consider trimming or replacing the column. If the baseline is still high, the GC/MS ion source may need to be cleaned. The baseline is usually indicative of the maintenance required.
A few common problems can cause column bleed. Ask yourself the following questions to help troubleshoot the problem:
– Did you condition the column after installing it?
– Are you exceeding the maximum temperature limit of your system?
– Are you using the right film thickness for your analysis?
– Are leaks present in your flowpath or are your carrier gasses contaminated with air? Oxygen damages the cross-linking in the column, meaning the stationary phase is stripped and responding in the detector. In this case, you will need to replace the column.
– Do you need to change your split vent trap?
Top tip: save a chromatogram of the bleed profile from immediately after the column was installed and conditioned. Overlay the problematic chromatogram with your reference chromatogram to determine whether column bleed may be a problem. It’s important to note that repeating intense peaks are likely caused by septum bleed, rather than column bleed. Column bleed always manifests as an elevated baseline, not as a unique peak.
Trimming the column
Cutting too much off your column can result in massive retention time shifts and ultimately, more work. On the other hand, if you do not trim enough then you risk the chance of still having active sites. Trim between 0.5–1 m from the inlet side of the column and remember to adjust to the new column length in your software. As always, run the QC standard after you trim to check the pass criteria.
Agilent columns now all come on a 7 inch cage, meaning that one loop is approximately 0.55 m to make column trimming easier. An easy way to verify that the approximate column length is to count the number of coils and divide that number by 2.
Still fed up of spending too much time on column maintenance?
Utilize a guard column or a retention gap, which is a piece of deactivated fused silica, without any stationary phase. It catches the matrix and prevents it from entering and damaging the analytical column. These should be greater than or equal to the column ID and typically between 1 to 5 m long. It is useful to note that retention gaps can act as guard columns, but not all guard columns are retention gaps. Retention gaps are a larger ID than the analytical column and focus on the analytes onto the head of the analytical column. Guard columns can be equal or larger ID than the analytical column.
There are other tools available, such as the Agilent self-tightening column nut, which eases the burden of column maintenance and prolongs column lifetime.
Find out more about Agilent column guards here.
For Research Use Only. Not for use in diagnostic procedures.