26 February / March 2021
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.
o 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.
o Only change one variable at a time to isolate the issue. Changing multiple variables at once may fi x an issue, but the true fi x will not be known to prevent the issue arising again.
o 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
No Peaks 1. Injector:
a. A blocked syringe can be fi xed by cleaning or replacing the syringe
b. Injecting into the wrong inlet can be corrected by resetting the autosampler.
c. The carrier gas fl ow plays a crucial role in the chromatography, therefore, simply checking the gas is fl owing at the prescribed fl ow rate is important.
d. Is the sample in the correct position in the batch? Amend the batch to aspirate from the correct vial position
2. Column:
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.
3. Detector:
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 fl ow rate used, therefore, the detector should be turned on and the fl ow rate adjusted to the correct value.
Tailing Peaks 1. Column:
a. Column installation issues can result in signifi cant peak tailing. The tailing can be reduced by ensuring the column is cut properly, minimise dead volume, or by verifying the correct installation depth.
2. Leak:
a. A leak should be ruled out by checking all the connections for a leak, and also replacing critical seals if required.
3. Adsorption:
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.
Fronting Peaks 1. Column:
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 fi lm 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 fi tted column can also cause voids which exhibit as fronting. This can be remedied simply by reinstalling the column.
Ghost Peaks 1. Injector:
a. A contaminated syringe could easily cause additional peaks to be present in a chromatogram. The rinse solvents should be fi rst replaced and either rinse or replace the syringe if the syringe continues to contaminate.
2. Column:
a. Backfl ash, 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
Peak Broadening 1. Injector:
a. Sample carryover could cause the peaks to appear broader. To resolve these issues, look at the ghost peak injector solutions.
2. Column:
a. The column fi lm selected could be too thick, which would result in the peak to broaden. The retention of compounds could be reduced by decreasing the fi lm 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.
3. Oven:
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 fl ow:
a. An incorrect gas fl ow could be detrimental to the peak shape, where the wrong fl ow can result in broadened peaks. Verify the inlet and detector fl ow rates and adjust accordingly. Also verify the make-up gas fl ow and adjust.
Splitting Peaks 1. Injector:
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.
increasing the fl ow 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 fi nally 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.
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