Abstract : The TCD detector is the most widely used type of universal detector, but there are many factors that make the TCD detector unstable. Since the factors that affect baseline instability are related to most parts of the entire chromatograph, there are interactions between the various unstable factors. The following describes the maintenance methods and causes of the TCD failure phenomenon.
Introduction TCD detectors are the most widely used type of universal detectors, but there are many factors that make TCD detectors unstable. Since the factors that affect baseline instability are related to most parts of the entire chromatograph, there are interactions between the various unstable factors. The following describes the maintenance methods and causes of the TCD failure phenomenon.
When the thermal conductivity, the baseline appears a regular and wavy oscillating motion with a period of about 0.5 min.
Maintenance method:
1. When the flow rate increases, the fluctuation cycle decreases accordingly.
2. Block the air outlet by hand and slowly lower the rotor to zero.
3. There is no corresponding fluctuation in the accuracy of column room and detection room temperature control.
4. The phenomenon remains unchanged after replacing the regulator valve.
5. After the temperature of the test chamber was reduced from 180 degrees to 150 degrees, the fluctuations completely disappeared.
Analysis of the causes: A small amount of condensate volatilizes in the test chamber, causing the baseline to fluctuate. “In the process, the condensate volatilizes to form the base flow. The base flow is related to the flow of the air flow. When the flow rate is large, it volatilizes much, and the base flow is large. Flow is small. Usually the flow is slowly fluctuating, about 1% or less. When the air path is clean and free of pollution, this change has little effect on the baseline response. And when the airway is not clean, it can cause large fluctuations. When the temperature decreases, the amount of condensate volatilization decreases. "Even if the flow rate fluctuates, there is no observable effect on the baseline.
The baseline is unstable at the thermal conductivity zero adjustment! The noise manifests as irregular beat detection method:
1. When the attenuation increases, the noise peak-to-peak value decreases.
2. The baseline is normal after preheating the instrument for 2 hours.
Analysis of reasons: The instrument is not used for a long period of time and the wall is adsorbed. Released during warm-up, affecting baseline stability. After the instrument is fully warmed up, the baseline is normal.
No peak and low sensitivity. Maintenance method Repeated inspection of operating conditions. It should be verified whether the operating conditions are close to the originally known conditions. This includes the flow rate of each airway! Column temperature and detector temperature; Output attenuation position; Bridge flow size; Power is 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 in a timely manner some of the unfavorable factors that affect the recovery of the operating value. Analysis of the reasons There are only two factors that should be suspected at this time. One is that the connection of the hot wire is incorrect, and the other is the serious contamination of the surface of the hot wire. The former should focus on whether to reconnect the heat sink lead. For the heat sink connection, in addition to the four hot filaments forming a bridge flow, it must also be noted that the hot wire elements of the thermal bridge should be in the same air path. If the bridge wiring is reversed, there will be little or no thermal conductivity. In this case, bi-directional peaks are often generated. For the serious contamination of the hot wire surface, first try to clean the thermal conductivity tank, and then consider removing the hot wire cleaning and replacing it completely.
Vaporization Room Temperature Uncontrolled Maintenance Method Remove the vaporization heating plate and observe whether the gasification chamber is still below the maximum temperature. If it still remains out of control, it indicates that the thyristor organic breakdown, the heating wire or lead collided with the chassis. At this time, the total power of the instrument is cut off, and then the multimeter is used to test the SCR and the insulation of the furnace wire. When testing the thyristor, the anode lead can be disconnected and the forward and reverse resistance between the thyristor anode and cathode can be directly checked. Normally a few megohms. This value indicates that the thyristor has broken down and needs to be replaced. Checking the insulation of the furnace wire can test the resistance of the chassis separately 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 touching the shell.
Analysis of the reasons: 1. SCR yin and yang between the two poles breakdown; 2. heating wire or heating wire and the casing collision.
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