Gas Chromatography Instrument Troubleshooting: Thermal Pool Detector

1, bridge current failure

Under the premise that the thermal conductivity cell carries the carrier gas, open the bridge current switch and adjust the bridge current control knob. The bridge current should be steadily adjusted to a predetermined value. If the current is not adjusted during the adjustment process, especially if the thermal conductivity cell is at a high temperature, the bridge current is not adjusted to the maximum rated value, and it can be considered that the bridge current is not adjusted to a predetermined value.

The occurrence of such faults is as follows: the thermal conduction unit is not connected; the hot wire in the thermal conductivity cell is disconnected or the lead is open; the bridge power supply is faulty; the bridge configuration circuit is disconnected or the ammeter is faulty.

2, baseline zeroing fault

After the bridge current is adjusted and stabilized, adjust the knobs of the thermal conduction zero to return the baseline indication on the recorder to zero. If the knob is not changed or adjusted to zero regardless of the knob, the thermal conduction is considered to be faulty.

The reason why the thermal conduction cannot be zeroed is caused by the following reasons: the heat wire resistance is asymmetrical or the lead wire is wrong; the hot wire hits the wall or the pollution is serious; the zero potentiometer leads the open circuit; the recorder is open or no reaction; the double gas path The flow rates are too different.

To eliminate the fault that the heat guide cannot be zeroed, follow the steps below:

(1) Attenuation gear test: When it is found that there is an offset from the zero point of the baseline, the attenuation gear is adjusted from small to maximum, and whether the baseline deviation is gradually reduced.

(2) Zero adjustment knob function check: Rotate the coarse, medium and fine adjustment knobs respectively to observe whether the baseline reacts.

(3) Two-way flow check: On the basis of the gas path test leak, the flow values ​​of the two gas paths are respectively tested with a soap membrane flowmeter to observe whether the difference is too large.

(4) Error check between the resistance of the hot wire: The resistance value of the hot wire outlet socket of each stage of the thermal conductivity cell is measured. Generally speaking, the difference between the resistance values ​​of the hot wires of each group should not exceed 0.2~0.5Ω. If it exceeds this value, it should be treated as (6).

(5) Hot wire hitting the wall or staining: The hot wire hitting the wall can be confirmed by measuring the insulation resistance between the hot wire and the cell body. Severe contamination of the hot wire can be eliminated or partially eliminated by cleaning the cell of the thermal cell. See the Cleaning section of the detector for specific steps.

(6) Hot wire asymmetry or wire misconnection: This usually occurs after repairing the thermal conductivity cell circuit. In this case, the connection between the hot wire lead wires should be carefully checked. The correct connection is that the four hot wires form a bridge, and the heat of the two upper arms in the bridge is exactly on the same gas path.

(7) Treatment of double flow difference or gas path leakage: The difference between the two flow rates can be solved by adjusting the gas path control valve, but there should be no leakage between the two gas paths.

(8) The zeroing circuit has an open circuit.

(9) The recorder is open or non-responsive.

3. Baseline noise and drift

There are many reasons for the instability of the thermal conductivity detector baseline. There are dozens of them. Common ones are:

(1) The power supply voltage is too low or fluctuating too much, and the power load on the same phase changes too much;

(2) There are condensate or foreign matter in the gas outlet pipe;

(3) The instrument is poorly grounded;

(4) The column temperature control is unstable, and the room temperature control has fluctuation or drift;

(5) The carrier gas is not clean, the gas path is polluted, the gas in the carrier gas path is leaking, the carrier gas pressure is too low or is running out;

(6) The stability of the control valve and the steady flow valve are poor;

(7) The difference between the two-column gas path is too large and the compensation is poor;

(8) There is soap in the soap film flowmeter at the air outlet or outlet of the carrier gas outlet;

(9) the column packing is loose;

(10) The mechanical vibration is too large;

(11) Bridge DC power supply is unstable;

(12) Loss of the stationary phase in the column;

(13) The carrier gas flow rate is too high;

(14) The bridge configuration potentiometer has poor contact;

(15) Thermal conductivity pool pollution;

(16) The thermal element is locally overheated;

(17) Poor contact of power plug and lead, poor contact of shift band switch;

(18) The tungsten wire is not aged, and the thermosensitive element tungsten wire hits the wall;

(19) The bridge current is too large.

When there is a baseline instability in the chromatograph, first check the chromatogram gas path for contamination. This is not only because the airflow in the gas path is not clean, but also directly affects the stability of the baseline. More commonly, in the unclean condition of the gas path, many factors that have little effect on the baseline stability when the gas path is clean are small. The effects on the stability of the baseline (such as changes in airflow, temperature fluctuations, etc.) will suddenly increase. This is the interaction of airway pollution with other instability.

The following steps are a series of measures taken on the gas path under the premise of determining the pollution of the gas path. There are three reasons for the pollution, that is, the stationary phase is lost, the gas pipeline is contaminated by impurities, and the carrier gas is impure.

In order to further distinguish the root cause of the fault, the following check steps can be carried out:

(1) Reduce the column temperature. The amount of fixative in the column is exponentially related to the column temperature. Therefore, lowering the column temperature will greatly reduce the loss of the fixed solution. If the baseline becomes stable when the column temperature drops, the column loss is too large and needs to be further processed according to the specific analysis conditions.

(2) Whether to allow the column to have a large loss. Under certain analytical methods, the column has to be allowed to have some loss. In this case, it is considered to properly improve the stability of other parts of the instrument, so that the entire analysis method can be realized.

(3) Dealing with the large column loss. First, it should be suspected that the column is fully aged, which can be confirmed by observing the baseline at operating temperature after further aging the column at elevated column temperature. If the aging treatment has no obvious effect, a few needles of distilled water can be injected for cleaning test under the condition that the column temperature is above 150 °C (the amount of water per needle can be about 10-20 μl). After cleaning with steam, if it is effective, it can be considered that the column is contaminated with impurities; if the steam cleaning has no effect, it is necessary to consider replacing the new column.

(4) Post-column gas path test leak. There should be no leakage in the piping between the column and the thermal conductivity detector, including the thermal conductivity detector itself. If there is a leak there, the oxygen in the air will seep into the gas path from the leak, affecting the stability of the baseline, and severely corroding the tungsten wire, causing permanent damage. The method of post-column leak test is very simple, as long as the outlet of the heat guide cell is blocked, it is only necessary to observe whether the flowmeter rotor of the corresponding gas path drops to zero.

(5) Replace the filter and purifier. The filter and purifier on the gas carrier gas path of the chromatograph should be activated or replaced after a period of use. When the carrier gas source is not clean, it should be replaced in time. Observe the change in baseline stability after the filter and purifier are replaced. If the baseline is significantly better, the carrier gas purity is not enough, or the filter and purifier fail.

(6) Impurity of carrier gas: Although the gas source with low purity can pass through a good filtration and purifier, it can be used as a high-grade gas source with less impurity content. However, this will affect the life of the filter and purifier, and the more impurities contained in the gas source, the shorter the period of use of the filter and purifier. Therefore, a thorough approach is to use a high purity carrier gas source with an effective filter and purifier. This will ensure that the baseline is as stable as possible, and its normal application period can be as long as one year.

(7) Cleaning the gas pipeline is dirty. When cleaning the gas pipeline, the sample may be cleaned by distilled water or ethanol. The method is to raise the temperature of the whole system to above 150 ° C, and then inject 10 to 20 microliters of distilled water or ethanol into the injector several times, and observe the stability of the baseline after the corresponding peaks are finished. If the baseline is significantly better, it can be considered that the pipeline is only slightly stained and can continue to be used; if the baseline stability is unchanged or does not change much, the pipeline should be thoroughly cleaned. In the gas path, the inlet, the connection between the column and the thermal conductivity cell, and the cell of the thermal conductivity cell are easily contaminated, so it is important to treat it during cleaning.

(8) Air infiltration into the detector. A small leak in the gas path behind the column is the root cause of the infiltration of oxygen from the air into the thermal conductivity detector. Most of this occurs at the installation of the connecting pipe joint and the tungsten wire component. For the repair method of the gas leakage in this part, refer to the above-mentioned inspection and elimination of the gas path leakage.

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