I. Introduction
The TCD detector is the most widely used general-purpose detector, but there are quite a few factors for the instability of the TCD detector. Because the factors that affect the baseline instability involve most parts of the entire chromatograph, and various instability factors interact with each other. The following introduces several maintenance methods and the analysis of the reasons why TCD often fails.
2. During the thermal conductivity, the baseline appears to have regular and smooth wavy swings with a period of about 0.5 min.
2.1 Inspection method
1. As the flow rate increases, the fluctuation period decreases accordingly.
2. Block the outlet of the gas path with your hand, and the rotor slowly drops to zero.
3. Check the accuracy of the column room and the detection room temperature control, there is no corresponding fluctuation.
4. The phenomenon remains the same after replacing the regulator valve.
5. After reducing the temperature of the detection room from 180 degrees to 150 degrees, the fluctuation completely disappears.
Reason analysis: There is a small amount of condensate volatilization in the detection chamber, which causes the baseline to fluctuate. The process is that the condensate volatilizes to form a base flow. The base flow is related to the gas flow rate. When the flow rate is large, the base flow is large, and the base flow is large Flow is small. Usually the flow rate fluctuates slowly, about 1% or less. When the gas path is clean and free of pollution, this change has little effect on the baseline response. But when the gas path is not clean, it can cause greater fluctuations. When the temperature decreases, the condensate volatilization decreases. "Even if the flow rate fluctuates, there is no observable effect on the baseline.
3. The baseline is unstable at the thermal conductivity zero adjustment! The noise appears as irregular jitter
3.1 Maintenance method
1. As the attenuation increases, the peak-to-peak noise decreases accordingly.
2. The baseline is normal after 2 hours of warming up the instrument.
Reason analysis: The instrument is not used for a long time, and the wall has adsorption. It is released during warm-up and affects baseline stability. After the instrument is fully warmed up, the baseline reaches normal.
4. No peak and low sensitivity
The overhaul method carries out repetitive inspection of operating conditions. It should be verified whether the operating conditions are close to those previously known. This includes the flow values ​​of each gas circuit! Column temperature and detector temperature; the position of the output attenuation gear; the size of the bridge flow; whether the power is turned on. If you find that the operating conditions are abnormal, you should try to make the operating value close to the original given value, and find out some adverse factors that affect the recovery of the operating value in time. Cause analysis There are only two factors that should be suspected at this time. One is that the position of the hot wire is incorrectly connected, and the other is the serious contamination of the surface of the hot wire. The former should focus on understanding whether to reconnect the lead of the thermal conductivity cell. For the connection of the thermal conductivity cell, in addition to the four hot wires to form a bridge flow, it must also be noted that the hot wire elements of the opposite arm of the thermal conductivity bridge should be in the same air circuit. If the bridge wiring is reversed, it will cause the phenomenon that the thermal conductivity sensitivity is very small or even not peaked. In this case, there are often two-way peaks. For the serious contamination of the surface of the hot wire, you should first try to clean the thermal conductivity pool. When it is invalid, consider removing the hot wire for cleaning and completely replacing it.
Fifth, the temperature of the gasification chamber is out of control
The maintenance method removes the vaporization heating plate and observes whether the vaporization chamber continues to be below the maximum temperature. If it still keeps out of control, it means that the thyristor has broken down organically, and the heating wire or lead wire has collided with the cabinet. At this time, cut off the total power of the instrument, and then use a multimeter to test the insulation of the thyristor and the furnace wire. When testing the thyristor, the anode lead can be disconnected to directly check the forward and reverse resistance between the thyristor anode and the cathode. Normally it is several megohms. This value indicates that the SCR has broken down and needs to be replaced. To check the insulation of the furnace wire to the shell, the resistance of the shell can be tested at both ends of the heating iron core lead. If the resistance at one end is very small, it means that the heating circuit is at the shell.
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