Thermal conductivity detector (TCD) is a concentration-type detector that responds by using different thermal conductivity coefficients of the measured component and the carrier gas. Some are also called hot wire detectors (HWD) or thermal conductivity meters and katherometer Or Catherometer), it is a well-known overall performance detector, which belongs to the method of physical constant detection.
First, the working principle
TCD consists of a thermal conductivity cell and its detection circuit. Figure 3-2-1 shows the connection between the TCD and the injector and the column, and the upper part shows the detection circuit of the Wheatstone bridge. The carrier gas flows out of the measurement cell cavity through the reference cell cavity, the sampler, and the chromatography column.
R1 and R2 are fixed resistors; R3 and R4 are the heating arms of the measuring arm and the reference arm, respectively.
Principle and precautions of thermal conductivity detection When the carrier gas flow rate, bridge current and TCD temperature are adjusted to a certain value, the TCD is in working condition. The current I flowing from the power source E is divided into two paths i1, i2 to point B at point A, and then returns to the power source. At this time, both hot wires are in a heated state, maintaining a certain wire temperature Tf, and the pool body is at a certain pool temperature Tw. Generally, the difference between Tf and Tw should be greater than 100 ℃ to ensure that the hot wire conducts heat to the pool wall. When only the carrier gas passes through the measuring arm and the reference arm, because the two arms have the same gas composition, the heat transmitted from the hot wire to the cell wall is equal, so the temperature of the hot wire remains constant; the resistance value of the hot wire is a function of temperature, and the temperature does not change. , The resistance value does not change; the bridge is in equilibrium at this time: R1 • R3 = R2 • R4, or written as R1 / R4 = R2 / R3. The potentials of M and N are equal, the potential difference is zero, and no signal is output. When the sample is injected from 2 and separated by the column, the components flowing out from the back of the column enter the measuring arm. Since the gas at this time is a mixture of carrier gas and components, its thermal conductivity is different from that of pure carrier gas. The heat conducted by the pool wall is different, which causes the temperature of the hot wire of the two arms to be different, which in turn makes the resistance value of the hot wire of the two arms different, and the balance of the electric bridge is destroyed. The M and N two-point potentials are not equal, that is, there is a potential difference, and a signal is output.
Second, the thermal conductivity cell is composed of a thermal element and a cell body
1 Thermal element
Thermistors are sensing elements of TCDs. Their resistance changes with temperature. They can be thermistors or hot wires.
(1) Thermistor The thermistor is made of oxide semiconductors such as manganese, nickel, and cobalt. The beads are approximately 0.1 to 1.0 mm in diameter and sealed in a glass envelope.
The thermistor has three advantages:
① The thermistor has a large resistance (5-50kΩ) and a large temperature coefficient, so the sensitivity is quite high. It can be used for trace analysis of μg / g level directly;
② Thermistor has a small volume and can be made into a ball with a diameter of 0.25mm, so that the cell cavity can be as small as 50μL; Corrosive and resistant to oxidation.
The thermistor also has three shortcomings:
- The response value of the thermistor # $% decreases rapidly with increasing temperature. Therefore, the thermistor is usually used below 120 ° The use range is greatly restricted;
② Compared with hot wire, the temperature coefficient of the thermistor is large, which shows that its response value is very sensitive to temperature changes. For example, at 60 ° C, when the cell temperature changes by 1 ° C, the baseline drift of the thermistor and the hot wire are 10.4mV and 5.0mV, respectively. The former is more than double the latter, so the stability of the thermistor is poor, especially at Cheng Sheng is particularly prominent during operation;
③ The thermistor is very sensitive to the reduction conditions, so hydrogen cannot be used as a carrier gas.
At present, there are only the following two cases where a thermistor can be used as a thermal element; first, low-temperature trace analysis; and second, a small cell volume is required for a capillary column. In other cases, thermistors are rarely used, and hot wires are more often used. In addition, in recent years, the advantages of the thermistor as a small cell volume have gradually decreased.
(2) Hot wire has an excellent performance TCD. The requirements for the hot wire are mainly considered in four points: ① high resistivity, so that high resistance can be obtained within the same length; ② large temperature coefficient of resistance, so that it can be obtained after the bridge current is heated. High resistance; ③ Good strength; ④ Resistant to oxidation or corrosion. ① and ② are to obtain high sensitivity, and at the same time, the volume of the silk is small, the volume of the pool can be reduced, and the micro TCD is made. ③ and ④ are for high stability. Table 3 -2-3 lists the hot wire properties commonly used in commercial TCDs.
Tungsten wire has a low resistivity. The resistance value of the same length is only half that of iron wire. It is difficult to improve the sensitivity. In addition, the strength of tungsten wire is poor, and it is easy to oxidize at high temperature, which results in increased noise and decreased signal-to-noise ratio.
Compared with tungsten wire, thoriated tungsten wire has higher resistivity and slightly lower temperature coefficient of resistance. Because the value of S is roughly proportional to α√ρ. The α√ρ values of 3%, 5% rhenium-tungsten wire and tungsten wire were 12.2 × 103, 11.7 × 103, and 10.29 × 103, respectively. It can be seen that the value of α√ρ of thorium tungsten wire is higher than that of tungsten wire. Therefore, the former is conducive to improving sensitivity.
Thermal conductivity detection principle and precautions In addition, compared with tungsten wire, thoriated tungsten wire has a significantly improved breaking force and high temperature characteristics, so its performance is stable. But it still has the problem of easy oxidation at high temperature. Now high-performance TCDs use thorium tungsten wire. Such as HP6890 type, Shimadzu GC-17A type μ-TCD hot wire.
There are two series of thorium tungsten wires: pure tungsten plus thorium (W-Re) alloy wire and doped tungsten plus thorium (Wal2-Re) alloy wire. In terms of resistivity, processability and high temperature strength, the latter is better than the former. Therefore, under the same structural design and operating conditions, the latter can be used to obtain higher resistance values. The resistance value and TCD sensitivity of doped tungsten and rhenium alloy wires increase with the increase of erbium content, see Table 3-2-4.
It can be seen that simply changing the ratio of Re can double the sensitivity.
Gold-plated thorium tungsten wire refers to thorium-tungsten tungsten wire that is welded with unplated thorium-tungsten wire on the bracket first, and after strict cleaning, it is directly plated with gold in the electrolytic cell. Although the resistance value is reduced by about 11% and the sensitivity is reduced by about 30% under the same bridge current, its oxidation resistance and corrosion resistance are significantly improved, and sensitivity and stability are taken into consideration. The gold-plated thoriated tungsten wire was first plated and then welded to the bracket, and the effect was poor.
In recent years, Valco has introduced iron-nickel alloy wires, which are said to greatly improve sensitivity and avoid the oxidation of thorium-tungsten wires.
The installation of the hot wire is usually to fix it on a bracket and put it into the channel of the pool body. The bracket can be made into various forms, see Figure 3-2-3.
The pool body is a metal body that is internally processed into the pool cavity and channels. Pool materials used copper in the early days because of its good thermal conductivity but poor corrosion resistance. Therefore, it has been replaced by the stainless steel form in recent years. The total volume of the internal cell cavity and channels is usually referred to as the cell volume. The pool volume of early TCDs was mostly 500-800 μL, and then reduced to 100-500 μL, which is still called normal TCD. It is suitable for packed columns. In recent years, micro-TCD has been developed. Its cell volume is below 100 μL, and some reaches 3.5 μL. It is suitable for capillary columns.
Principle and precautions of thermal conductivity detection (1) Usually TCD cells Generally, TCD cells can be divided into straight-line type (a), diffusion type (b) and semi-diffusive type according to the flow mode of carrier gas to the hot wire (see Figure 3-2-4). For formula (c), the comparison of the three flow patterns is shown in Table 3-2-5.
2) Since the volume of the micro-TCD cell has been reduced to a few microliters, or even 200 nL, the flow mode of the carrier gas in μ-TCD is not as obvious as the usual TCD. It can basically be divided into two types: straight-through and collimation. Figure 3-2-5 lists several μ-TCD pool structures.
It can be seen that the volume of the μ-TCD cell cavity is only a few microliters or tens of microliters, and a standard capillary column can be directly connected to it, which basically does not cause peak expansion. Of course, if the sensitivity permits, it is still very beneficial to improve the peak shape by properly adding makeup gas.
Although the volume of the μ-TCD cell cavity is small, in order to make it work stably, the cell block should also have an appropriate quality to ensure the constant temperature effect and thus stabilize the baseline.
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