The inlet of the gas phase is the first fortress for the sample to enter the system, and it is also the place where the sample is vaporized. In the normal gas phase maintenance process, it is easy to neglect the maintenance of the gas phase inlet, resulting in contamination. It can be said that more than 80% of the problems of gas chromatography are related to the inadequate maintenance of the sample port. Today, the editor will take you to summarize the aspects of gas phase inlet maintenance.
From the cross-sectional view of the gas injection port structure, the maintenance of the injection port mainly involves four aspects: injection septum, O-ring, liner, and injection pad.
1. Maintenance of the injection pad
Injection pad, also known as injection septum. Among the abnormal problems of gas chromatography, the contamination, loss and leakage caused by the injection septum account for a large proportion. In gas phase analysis, the injection septum is the most frequently replaced consumable. After the septum is used for a long time, the puncture will become larger and larger, and the material will age, resulting in air leakage, septum loss, and shortened life.
- Extra abnormal peaks, bulging peaks;
- Abnormal post-peak baseline;
- Baseline fluctuations;
- retention time drift;
- ghost peak;
- Peak tailing/forward delay, no peak;
- Air/oxygen leakage;
- Inlet pressure abnormal.
Recommended operation steps for replacement of injection pad:
- Close the injection port, column temperature chamber and detector temperature, and close the pressure at the injection port when the temperature returns to normal temperature;
- Screw out the nut of the spacer, take out the spacer with tweezers, scrape off the residual part of the spacer with tweezers, and finally blow it clean with nitrogen (air);
- Insert the new spacer with tweezers and press it tightly;
- Screw on the nut until the C-ring protrudes 1mm.
(There are some differences between products of different brands. If there are differences, please follow the replacement requirements of the instrument manufacturer for the injection spacer)
In addition, the maintenance of the injection septum should pay attention to the following points:
- The slight leakage of the injection septum needs to be tested by the injection port pressure test or leak test to be seen;
- If you hear a slight air leakage sound, hissing sound from the injection port, and the sound disappears when you block the injection port with your fingers, it is recommended to replace the injection septum;
- It is recommended to turn on septum purging to avoid contamination of samples by septum volatiles;
- After the septum is screwed on, it should not be over-tightened, and a sampler with a sharp needle should be used;
- The temperature of the injection port should not exceed the tolerance temperature of the septum, otherwise the aging and loss will be very fast;
- In principle, the septum must be replaced after the number of injections exceeds 100 needles, but in more cases, the maintenance of the injection port is carried out according to the nature of the sample itself and the pollution of the injection port, such as the gas phase running all day long, or the high temperature. For boiling point samples, daily septum replacement is recommended;
- In addition to directly replacing the septum, the coagulated sample remaining on or below the septum can be wiped with absorbent cotton dipped in acetone, toluene and other organic substances; for the contaminants that cannot be wiped off, the septum can be soaked in an organic solvent for a period of time. Then wipe or gently scrape off, or place it in an organic solvent to sonicate. Be careful not to damage the instrument parts.
2. Maintenance of O-ring
The function of O-ring is to seal the liner, separate the carrier gas and split flow at the same time, and form an independent flow path between them. The O-ring contains a plasticizer that makes it flexible. Long term temperature rise and drop will cause rubber aging, loss of plasticizer and hardening of O-ring, resulting in its failure to seal the top, bottom and liner of injection port.
Daily maintenance of O-ring shall pay attention to:
- It is recommended to replace the O-ring when replacing the spacer;
- O-ring is a consumable, which shall be inspected regularly and replaced as required;
- During the installation and replacement of O-ring, make it 3-5mm away from the upper end of liner pipe and 1-2mm away from the lower base;
- When the temperature of the injection port does not exceed 300 ℃, the rubber O-ring can be used (as shown in the left figure below); When the temperature exceeds 300 ℃, the graphite O-ring shall be used (as shown in the right figure below);
Welch Materials has been focusing on the research and development and production of gas chromatographic columns for many years. Each chromatographic column is strictly tested before leaving the factory, and a column evaluation report is attached. It has ultra-inert, low bleed, high column efficiency, high selectivity and stability advantages such as reproducibility and long life.
In recent years,Welch Materials has successively launched 22 kinds of organochlorine, 37 kinds of fatty acids, liquor composition analysis and other popular applications. With excellent product performance and perfect after-sales service system, the company’s gas chromatography columns have been widely used in major universities, scientific research Institutes, pharmaceuticals, petrochemicals, brewing, environmental protection and other industries.
Today, I will introduce you to the maintenance of the septum and O-ring of the GC injection port. We have also prepared for you the maintenance of the high-frequency parts of the GC injection port, which will be brought to you in the near future. Please look forward to it~
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