Gas Chromatography- Troubleshooting

 

Introduction:

Gas chromatography is one of the most popular chromatography techniques to separate volatile compounds or Substances. Here the moving phase is gas such as Nitrogen, helium and stationary phase is high-boiling liquid that is adsorbed on a solid. There are many problems arises during GC analysis. GC is a very sensitive instrument and need to be handle with more care since the gases are used. So, safety is must in Gas Chromatography analysis.  

Maintaining GC routinely will prevent many problems. Many GC problems can be prevented if the column is properly installed and GC is maintained routinely. For example, replacing septa or liner at regular intervals and keeping the injector and detector clean and well-maintained should solve many problems. Regular preventive maintenance depends on particular model of GC and you should consult required operations.

The starting point of investigation is depending on the problem, generally need to investigate on the following,

1.      Gas Cylinders

2.      Purification Panel

3.      Gas lines which connected to instrument

4.      Injector

5.      Column

6.      Detector

The above compartments should take into the investigation when problem comes.

Commonly facing problems,

• Baseline Problems
• Unusual peak shapes
• Irreproducible peak response
• Peak not eluting
• Split Peaks
• Tailing Peaks
• Shifting of Retention time

Above mentioned problems are routinely facing issues in the day-to-day analysis. The root cause for these issues can to identified easily and resolved. Let’s see the possible root cause and solution for above mentioned problems.

Baseline Problems:

Baseline problems could be divided into 5 categories: drift, noise, offset, spiking and wander.

·         Drift means slow baseline movement in one direction

·         Noise is rapid and random movement of the baseline position

·         Offset is sudden unexplained change of the baseline position

·         Spiking is presented by peaks with no width, either positive or negative

·         Wander is low frequency noise

Possible Cause	Solution
Drift: Downward drift for a few minutes is normal after installing a new column.	Increase the oven temperature to close to the maximum continuous operating temperature for the column. Maintain the temperature until flat baseline is observed. If the detector signal does not drop in 10 minutes, immediately cool the column and check for leaks.
Unequilibrated detector.	Allow sufficient time for temperature equilibration of the detector.
Downward drift is frequently due to the "back-out" of contaminants from the detector or other parts of the GC.	Clean out contamination.
Damage to the stationary phase of the GC column.	Determine the cause of the damage. It may be due to impurities in the carrier gas or to excessive temperatures. Replace column.
Drift in gas flow rates.	Clean or replace flow or pressure regulator(s). Adjust pressure.
Noise: The column may be inserted too far into the flame of an FID.	Reinstall the column. Be sure to insert the column into the detector exactly the correct distance specified in the instrument manual.
An air leak can result in noise in ECD and TCD detectors.	Eliminate the leak.
Incorrect combustion gases or flow rates can generate noise in FID, NPD or FPD detectors. Drift in gas flow rates.	Clean injector. Replace inlet liner, septa and seals.
Contaminated column.	Bake out the column. Cut off first 10 cm of column. If it does not help, replace the column.
Defective detector.	Clean and/or replace parts as necessary.
Defective detector board.	Consult GC manufacturer.
Spiking: Electrical disturbances entering the chromatograph through power cables, even shielded cables	Try to correlate spikes with events in equipment near the chromatograph. Periodicity is often a clue. Turn off equipment or move it. If necessary, install a voltage regulator.

Unusual peak shapes:

Possible Cause	Solution
Ghost peaks: Remnants of previous samples in the inlet or column. Ghost peaks due to remnants are most likely to occur when increasing inlet or column temperatures.	Increase the final temperature and lengthen the run time to allow for complete elution of previous samples. If ghost peaks continue to occur, clean the inlet. Condition the column at temperature higher than has been used but lower than the maximum continuous operation temperature for the column.
Bleed from the septum or fragments of the septum lodged in the inlet or liner.	Cut 10 cm off the inlet end of the column and/reverse it before reconditioning it. If it does not help, replace the column. Clean the inlet. Replace the inlet liner, glass wool and seals.
Fronting peaks: Fronting peaks are usually the result of column overloading. In this case, the effect should be a function of injection volume.	Solutions include reducing the injection volume or using a column with greater capacity. Columns with larger diameter or thicker stationary phase coatings generally have larger sample capacities, however, resolution may be reduced.

Irreproducible peak response:

Possible Cause	Solution
Inconsistent injection.	Develop a reproducible injection technique. Use autosampler.
Distorted peak shapes can adversely affect quantitative determinations.	Correct any problems that result in the distortion of peak shape. See Peak shape problems.
Variations in GC operating parameters.	Standardize operating parameters.

Peak Not Eluting:

Possible Cause	Solution
Defective syringe.	Try a new or proven syringe.
"Blown" septum or massive leaks at the inlet.	Find and fix leaks.
Problems with carrier gas flow.	Adjust gas flow. Check the column flow and the column outlet by immersion to methanol.
Broken column or column installed in the wrong way.	Replace or reinstall the column.
The detector is not functioning or not connected to the recorder or integrator.	Ensure that detector is working properly. E.g.: Is the flame in FID on? Check connection to the output device.

Split Peaks:

Possible Cause	Solution
Fluctuations in column temperature.	Repair temperature control system
Mixed sample solvent for splitless or on-column injections.	Use single solvent

Tailing Peaks:

Possible Cause	Solution
Contaminated or active injector liner, seal or column.	Clean or replace injector liner. Do not use glass wool in the liner. If necessary, replace the column.
Dead volume due to poorly installed liner or column.	Confirm by injecting inert peak (methane). If it tails, column is not properly installed. Reinstall liner and column as necessary.
A bad match between the polarities of the stationary phase and the solvent.	Change the stationary phase. Usually polar analytes tail on no-polar columns, or dirty columns.
Debris in the liner or column.	Clean or replace the liner. Cut 10 cm off the end of the column and reinstall it.
Injection takes too long.	Improve injection technique.
Split ration is too low.	Increase split ratio to at least 20:1
Overloading the inlet.	Decrease the sample volume or dilute sample
Some types of compounds such as alcoholic amines, primary and secondary amines and carboxylic acids tend to tail.	Try a more polar column. Make a derivative of the sample.

Shifting of Retention time:

Possible Cause	Solution
Change in column temperature.	Check GC oven temperature
Change in gas flow rate (linear velocity).	Inject a detectable unretained sample such as methane to determine the linear gas velocity. Adjust gas pressure or flow to obtain proper values for your analytical method.
Leak in the injector.	Check the septum first. Change, if necessary. Find the leak and fix it.
Change of solvent.	Use the same solvent for standards and samples.
Contaminated column.	Bake out the column. Cut 10 cm off the end of the column. If necessary, replace the column.