Gas Chromatography(GC) Column Selection Guide

-Published by Welch USA, 10/03/2021

Gas chromatography(GC) is a simple, fast, efficient and sensitive modern separation and analysis technology, which is widely used in scientific research and inspection departments such as environment, medicine, chemistry and chemical industry, and it is also an indispensable detection method for the determination of pesticide residues. Gas chromatograph has the advantages of fast analysis speed, high separation efficiency, high sensitivity, high selectivity, etc. However, due to the complexity of the sample matrix, operator error, and the instrument itself during the analysis process, the gas chromatograph will have various problems, which will affect the results of chromatographic analysis.  Thus, it is critical to have in-depth understanding of the common solutions for gas phase analysis. 

Basic composition of GC Column:

It is very important for chromatographic analysts to grasp the separation and analysis principle and the function of each component. The gas chromatograph mainly consists of seven subsystems: 

• Gas supply system 
• Sample injection system
• Separation system
• Detection system
• Temperature control system
• Signal amplifier
• Recording data and processing system 

Common problems and solutions in chromatographic analysis:

1. Incomplete separation   

The chromatographic peaks overlap and cannot be separated: You can solve the problem by reducing the injection volume and reducing the column heating rate; for the components that can be completely separated after a period of time, the separation effect is not good. It is very likely that the chromatographic column Column efficiency declines. At this time, the chromatographic column can be aged to remove difficult-to-volatile and semi-volatile impurities in the column. It is also possible that the column stationary fluid has been lost seriously, and the service life has expired, and the chromatographic column needs to be replaced.

The separation time is too long, resulting in the peak level of late distillation: Increase the temperature of the chromatographic column to solve it.

The sample does not show peaks, and the components with low content cannot be detected: It may be that the chromatographic column is installed incorrectly, so it is necessary to reinstall the chromatographic column and detect the leakage of the instrument system; The injection needle is blocked; The temperature of chromatographic column and injection port is too low; It is also possible that the sensitivity of the detector is low. The sensitivity of the detector can be improved by increasing the injection volume.

2. Irregular peak shape

• Trailing peak: Contamination of the injector and column requires cleaning and maintenance of the gas-phase injector, such as replacing or cleaning the liner; Remove contaminants by aging the column, or remove 1-2 rings from the inlet end of the column and reinstall.
• Leading peak: Too much injection volume leads to column overload, so the injection volume needs to be reduced; The temperature of chromatographic column and injection port should be increased due to condensation of sample components.
• Flat peak: It can be solved by reducing the sample amount and increasing the carrier gas flow rate and column temperature. A flat peak occurs when the signal amplifier input is saturated and the column is overloaded.

3. The influence of the detector on the analysis results

Take the thermal conductivity detector(TCD) as an example. Its detection principle is mainly to use the difference in thermal conductivity between the carrier gas and the component gas to be measured, and the unbalanced voltage formed in the detection bridge is proportional to the concentration of the component to be measured. Quantitative analysis of sample components.

• Baseline drift (or a stepped baseline) and high noise: The detector is contaminated and needs to be cleaned and maintained.
• The baseline drops to zero: The TCD thermal resistance wire is broken.
• Pulse interference peak: It may be that the TCD power supply voltage is unstable, and the power supply needs to be checked.

4. The carrier gas factors

Unlikely liquid phase analysis, the gas phase uses gas as the mobile phase, and the carrier gas carries the sample components through the stationary phase. After continuous adsorption and desorption, the carrier gas flows out of the chromatographic column successively and reaches the detector to be detected. In this process, the carrier gas flow rate, gas pipeline sealing, carrier gas pressure will also affect the separation efficiency of chromatography.

• Low carrier gas flow rate: will result in prolonged retention time, reduced sensitivity, trailing peak, round peak.
• High carrier gas flow rate: will cause baseline drift, high noise.

Based on the above situations, it is necessary to check whether the pressure reducing valve is beyond the range of use, replace it if necessary, and check the tightness to avoid any leakage. 

5. Circuit problem

If a sine wave occurs at the baseline, it is most likely due to a failure of the amplifier circuit. When the power supply cannot start normally, the column temperature box and the injection port cannot be heated. Then, the root cause could be the circuit.

6. FID detector ignition failure

First of all, it is necessary to confirm whether the hydrogen and air are turned on and whether the ignition coil is intact. In addition, check the column and detector end for air leakage.

7. Inverted peak

If there is an inverted peak, you need to check whether the host or processor of the instrument is connected reversely.

 Welcome to contact us at sales@auroraprosci.com for any questions or need helps.