Guidelines to Improve Repeatability of Gas Chromatography

While using a gas chromatograph for analysis, good quantitative repeatability is very important. However, repeatability often occurs during the experiment. Which components have problems that will lead to poor repeatability of the analysis results? Let’s discuss it here.

Common issues that lead to poor reproducibility

According to the structure of GC equipment, the components and locations that are prone to problems are as follows:

1. Sample problem
2. Gas source. Carrier gas hydrogen carrier gas (hydrogen, split) valve is unstable.
3. The injector. If it is clogged, contaminated, and has poor air tightness, it cause problem.
4. Inlet, septum aging, air leakage, liner adsorption contamination, split plate contamination
5. Loss of bonded phase from the chromatographic column or contamination of the chromatographic column
6. Air leakage at the detector
7. Inject samples outside the linear range of injection

Knowing where the problem occurs, we can analyze why the problem occurs and how to improve it.

Poor Repeatability of Gas Chromatography Causes and troubleshooting guidelines

Guidelines to Sample inspection

• Sample uniformity inspection. The repeatability of the peak sensitivity will be affected if the prepared sample is mixed unevenly in the sample bottle or the syringe contaminates the sample every time the sample is taken, as well as sample volatilization.
• Sample bottle inspection. If headspace sampling is used, confirm the wear or contamination of the headspace bottle. If the cap of the headspace bottle is not tight, it can be easily solved. Just replace the headspace bottle with a new one and pay attention to the strength of the bottle cap.
  Effect of sample solvent on instrument repeatability. When analyzing samples, in addition to instrument hardware reasons, sometimes sample processing has a negative impact on reproducibility.
• When analyzing samples, in addition to instrument hardware, sometimes sample processing also has an impact on reproducibility. For example, improper solvent selection can cause severe tailing, which will affect reproducibility.

Guidelines to Inspection of injector related problems

• Syringe inspection. Check whether the syringe itself is blocked or leaking. If necessary, replace the syringe with a good one and re-inject the test.
  Inspection of inlet pollution and system air leakage. Remove the inlet septum and observe whether there is contamination or accumulation in the inlet. If there is any, it needs to be removed and cleaned. After cleaning and installing the septum, the gas line system needs to be tested for leaks.
• Inspection of sampling technology. Poor sampling technology is one of the important reasons for poor reproducibility of experimental results. It usually manifests itself as peak height/peak area suddenly increasing or decreasing, and peak height/peak area changes irregularly. Manual injection requires relatively high requirements for operators.

Guidelines to Inlet inspection

• Reasons for sample vaporization caused by vaporization liner. The glass liner is the central component of the gas chromatography sampling system. In order to obtain good analysis results, the liner must be maintained regularly and used correctly. Glass liners must be stored in a clean environment to avoid contamination. Glass liners need to be inertized before use. During use, if the liner begins to exhibit activity, causing peak tailing and poor sensitivity and reproducibility, the liner must be inerted or replaced. When the peak shape broadens, produces an irregular baseline, and reduces resolution and response, you should check whether the inner wall of the glass liner is contaminated or has injection pad debris. If any abnormality is found, clean the glass liner.
• The shape of the glass liner is abnormal and the sample cannot be injected for normal analysis. A new liner needs to be installed. When replacing the liner, wait until the column inlet pressure reaches 0, then place the removed parts on clean paper or a pallet. No contact with moisture is allowed. Maintenance work should be performed after cooling down and stopping the gas to prevent damage to the interface screws. When a splitless liner is installed in the chromatographic split inlet, the repeatability may be deteriorated due to the influence of the liner volume; not filling the liner with quartz wool or filling the quartz wool in an incorrect position or quantity may also cause The sample vaporizes unevenly, resulting in poor repeatability.

The injection pad is not properly installed.

• The injection pad is one of the most commonly used consumables in gas chromatographs and needs to be replaced regularly to prevent leakage, decomposition, sample loss, ghost peaks, etc. In severe cases, it may cause blockage of the capillary column. When the number of injections reaches about 100, the carrier gas leaks, resulting in poor reproducibility of retention time and peak area or ghost peaks, and the injection pad must be replaced.
• When injecting, it feels like there is no resistance when injecting, and at the same time, there seems to be a feeling of gas recoil when the needle is pulled out. This is a phenomenon that the injection pad is installed too loosely. There will be a small amount of air leakage when the syringe is pulled out after injection, resulting in unstable injection volume and poor parallelism. Too tight will also cause poor repeatability. When the sample pad is too tight, it will be more difficult for the injection needle to insert into the injection pad to inject samples, and it will also easily cause the needle to bend.
• Generally, when replacing a new injection pad, the injection port nut should not be over-tightened until the injection needle can smoothly penetrate the septum and no significant resistance is felt. As the number of uses increases, the inlet nut will become too loose. You can tighten the inlet nut 1/4 to 1/2 turn when the injector pad is used 1/3 of the times.

Guidelines to inspect GC Column

• Capillary chromatography column is a core part in gas chromatography analysis. The correct use of capillary chromatography column is crucial to ensure the accuracy of gas chromatography analysis results and extend the service life of capillary chromatography column.
• The capillary column must be fully aged before installation: when aging the column, connect the inlet end, disconnect the detector end, keep the carrier gas flowing at an initial temperature of 50°C for 30 minutes, then raise it to the aging temperature at a rate of 3°C min, and keep it for 12~ Just 24 hours is enough. The aging temperature is about 20°C lower than the maximum operating temperature of the column and 10~30°C higher than the normal temperature. Avoid allowing difficult-to-volatile components to enter the column. It is best to use the column at a temperature 20°C lower than the maximum operating temperature of the capillary column. When not in use, seal both ends of the column to prevent oxidation or entry of debris.
• When loading the column, attention should be paid to cutting off the injection end of the column by 1 to 2 cm, and the cut should be smooth to prevent the column from adsorbing the sample. If false peaks and frontier peaks appear, the column should be reinstalled.

Guidelines to inspect the detector

• In addition to air leakage caused by improper installation of the graphite tough ring at the detector end, there may be other special reasons causing poor repeatability.
• Special cause inspection: For some detectors, the malfunction abnormalities associated with certain causes are not obvious and can easily be ignored. Therefore, this inspection should be carried out so as not to miss the factors that may cause the failure.

a) For FID, low polarization voltage and unstable hydrogen flow may cause sensitivity changes without other obvious abnormalities. For this, you can first test the polarization voltage to determine whether the polarization voltage is too low. Too low a polarization voltage and no polarization voltage are both faults. Normally the polarization voltage is 150~300V. If the polarization voltage is normal, you should turn to the sensitive setting of the amplifier to observe the changes in the base current of the hydrogen flame. When the hydrogen flow is unstable, the base flow should be able to show swing and drift phenomena.

b) For any detector, certain components in the sample gradually condense and accumulate in the detector, which will affect the sensitivity after the next injection. In serious cases, it may also cause blockage of the gas path. A common solution is to increase the temperature of the detector appropriately to reduce or eliminate condensation in the sample chamber.