Characteristics of gas chromatograph and key points of daily maintenance

1 Characteristics of gas chromatograph

When a gas chromatograph fails, in order to find out which system or component the fault lies in, we must first understand the basic structure of the instrument, the division of the system and the mutual connection. This is a necessary condition to gradually narrow the scope of the fault to a system or a component. The gas chromatograph system is divided into a gas circuit system, a circuit system, a detector system, an electronic system (power supply components, temperature measurement and control components, amplifier components, a dedicated microcomputer system, an external event control component), and a data processing system. Correctly understanding the role and requirements of each component of the gas chromatograph is very important for our analysis work.

The gas chromatograph analysis system uses the difference in gas properties. The gas sample is carried into the chromatographic column through a specific carrier gas, and the gas sample components are distributed multiple times between the gas phase and the stationary phase. Although the carrier gas flow rate is the same, the adsorption capacity of the stationary phase to each component is different, resulting in different flow rates of each component in the chromatographic column. After a certain period of flow, they are separated and enter the detector system through the chromatographic column. After amplification, different signals are generated and the chromatographic peaks of each component are displayed on the host through data processing. According to the peak position (time), the name of the component is determined, and the content is determined according to the peak area (or peak height value).

2. Determination and treatment of gas chromatograph failure causes

As an important precision instrument for gas component analysis, it is particularly important to strengthen the observation of the gas chromatograph during operation, especially the correctness of the judgment, analysis and treatment process after the failure, which plays a decisive role in restoring the normal operation of the instrument. Therefore, the following summary is made on the typical daily failures of gas chromatographs and their treatment measures to ensure the accurate and effective operation of the chromatograph.

(1) The gas system is blocked, leaking, has no pressure indication, has no gas output, and other faults. Treatment measures: 1) Check whether the gas cylinder and gas generator are normal; 2) Observe the gas pressure gauge to determine whether the gas input is normal; 3) Check whether the external gas circuit such as the filter is normal; 4) Check whether the gas output of the injection gas circuit is normal; 5) Check whether the part from the pressure regulating valve to the chromatographic column is normal; 6) Check whether the gas input and output of the detector are normal; 7) Properly block the gas system and observe the changes in the indication of the corresponding pressure gauge to determine whether there is a leak.

(2) The temperature control part does not heat up or the temperature is unstable. Treatment measures: 1) When the temperature is unstable, you can lower the column box temperature and observe the temperature of the detector. If it is normal, the fault is caused by the grid voltage or grounding wire; 2) When the temperature is abnormal, check whether the voltage and grounding wire are normal; if the column box temperature control is abnormal, check whether the corresponding relay and thyristor are normal; 3) If a certain temperature control is abnormal, check whether the platinum resistor and heating wire of the corresponding temperature control are normal. If the platinum resistor and heating wire are normal, then the microcomputer circuit board is faulty.

(3) The FID detector fails to ignite or flames out. Treatment measures: 1) Check whether the carrier gas and auxiliary gas enter the detector normally; 2) Check whether the shape of the ignition wire is normal, whether there is a break or short circuit, or poor contact; 3) Check whether the ignition relay is normally attracted and whether the ignition current is normal; 4) Check whether the detector mechanism is contaminated or blocked; 5) Check whether the flow rate setting ratio of various gases is appropriate.

3. Key points for daily maintenance of gas chromatograph

When using an instrument, we often judge whether there is a problem by comparing the instrument startup status with the analysis spectrum. Abnormal spectra usually reflect problems in the operating conditions or instrument system.

During the daily operation of the chromatograph, the temperature change of the external environment may cause abnormal peak spectra of the chromatograph. The specific examples of the treatment process are as follows: (1) The channel peak time is moved forward, indicating that the flow rate in the gas path has increased, which is easy to cause peak overlap. This phenomenon indicates that the corresponding carrier gas flow control is not good. It is necessary to check the carrier gas flow and pressure of the corresponding channel gas path and control to reduce the flow rate; (2) The channel does not produce peaks, indicating that the flow ratio of hydrogen, air and nitrogen in the detector is not appropriate, causing the hydrogen flame to go out. It is necessary to properly adjust the flow ratio of hydrogen, air and nitrogen to maintain hydrogen: air = 1:5-10; (3) The channel peak time is delayed, indicating that the gas path flow rate is low, which is easy to cause incomplete peaks. It is necessary to increase the gas flow rate of the corresponding channel or check whether the gas path filter medium needs to be replaced. Judging the working status of the chromatograph by the peak situation of the spectrum is one of the important bases for operators to deal with initial instrument failures.

When troubleshooting instrument faults, maintenance personnel need to pay attention to the following points: (1) Carefully observe the fault phenomenon and record the details of the fault phenomenon. Check and analyze whether there are any errors in the operating steps; (2) Check the instrument's usage and maintenance records to see if the same or similar phenomena have occurred in the past; (3) Based on the observed phenomena, refer to the drawings, self-diagnosis manual and maintenance guide to analyze the approximate location of the possible distribution of the cause of the fault; (4) Test according to the planned steps, and determine the exact location and extent of the fault distribution based on the test results; (5) During the fault diagnosis process, always pay attention to whether the connectors and connecting wires are open, short-circuited or have poor contact.

In the process of troubleshooting, the accuracy of fault analysis, testing and judgment plays a decisive role in the success or failure of the entire maintenance work.

Gas chromatograph is a relatively expensive precision instrument. In order to use it safely and obtain good working results, it is necessary to understand the basic knowledge about the installation, commissioning and maintenance of the instrument. When the instrument fails, it can be judged in time that a certain component or gas path of the analysis system has a problem based on the phenomenon (instrument pressure, channel flow rate, peak characteristics, etc.). It is necessary to "prescribe the right medicine" for the failure of the chromatograph, avoid unnecessary losses or expand the scope of the failure during the maintenance process, and improve the efficiency of instrument maintenance.