JPH06308071A - Heat conduction type detector - Google Patents

Abstract

PURPOSE:To provide a heat conduction type detector wherein response speed is rapid and a filament is difficult to produce secular change. CONSTITUTION:Four sides of a bridge circuit is constituted by connecting reference resistance R1, R2, R3 and filament resistance F exposed to gas. Two opposed connection points O, P out of four connection point among pieces of these resistance are coupled to two input terminals of an operational amplifier CA, its output terminal is connected to one end of two residual connection points Q, the other connection point R is grounded and amplifier output from introduction of measuring gas for the bridge circuit up to equilibrium is detected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal conductivity type detector used for gas analysis, for example, for gas chromatography.

[0002]

2. Description of the Related Art In general, a gas separated and discharged from a column of a gas chromatograph is often measured by a thermal conductivity type detector. An example of this thermal conductivity type detector will be described with reference to FIG. To do. In the figure, a filament F and reference resistors R1, R2 and R3 form a bridge circuit. The temperature of the filament changes due to the gas flowing to the filament F, and as a result, its resistance value changes, the bridge becomes unbalanced, and the unbalanced output is detected by the bridge voltage detection circuit N. Causes the components in the sample gas to be detected. In this case, the filament was housed in a cell formed of a metal block maintained at a constant temperature, and a constant voltage circuit or a constant current circuit M was generally used as a drive power source for the bridge.

[0003]

In the conventional thermal conductivity detector described above, the volume of the cell becomes large because it is necessary to store the filament in the cell (miniaturization is limited). There has been a problem that the heat capacity of the cell formed of the metal block is large, that is, the thermal time constant is large, and the response speed is deteriorated. Also,
When a gas component with low thermal conductivity flows, the temperature of the filament rises too much, resulting in the metal forming the filament being oxidized and sometimes corroded by gas, which causes the filament to change over time. Then, there is a problem that the performance as a detector, that is, as a gas chromatograph is not stable. In this case, it is possible to keep the current flowing through the filament low so that the temperature of the filament does not rise too much even if a gas with low thermal conductivity comes in, but this not only lowers the detection accuracy. However, the types of measurable gas are limited, and this method is rarely used.

The present invention aims to solve these problems (deficiencies), and a bridge circuit and an operational amplifier are combined so that the temperature of the filament exposed to the measurement gas incorporated in the bridge circuit is always constant. As described above, the current due to the unbalanced output of the bridge circuit is fed back to the filament to suppress the temperature change and the bridge circuit is always balanced, and the concentration of the measurement gas component can be measured from the output of this operational amplifier. The purpose is to obtain a highly accurate and fast response thermal conductivity type detector.

[0005]

In order to solve the above problems, the present invention provides reference resistors R1 and R having a constant resistance value.
2, R3 and the filament resistor F exposed to the gas are connected to form four sides of the bridge circuit, and 4 between these resistors are connected.
Among the connection points of the two points, connect two opposing connection points to the two input terminals of the operational amplifier, connect the output terminal to one of the remaining two connection points, and ground the other connection point. It is intended to detect the amplifier output from the introduction of the measuring gas to the bridge circuit to the balance.

[0006]

The resistance of the filament changes in proportion to the temperature of the filament, and this temperature changes in proportion to the current flowing through the filament. Therefore, the reference resistance and the filament resistance are installed in the bridge so that the ratio of the reference resistance and the filament resistance is kept constant, for example, when the filament resistance becomes smaller than the reference resistance, the current flowing in the filament is increased so that the filament resistance becomes the reference value. When it exceeds the resistance, the output of the operational amplifier is fed back to the bridge circuit so as to reduce the current flowing through the filament, and the operation is performed so that the ratio of the reference resistance and the filament resistance becomes constant. By preparing a reference resistance that always has a constant value (for example, winding resistance), the filament resistance value will always be constant. As a result, the equilibrium output at this time will be measured to measure the sample gas. Is possible.

[0007]

EXAMPLE An example of a thermal conductivity type detector according to the present invention is shown in FIG. 1, and an example of a gas chromatograph system using this detector will be described with reference to FIG.

In FIG. 2, 1 is a carrier gas cylinder filled with an inert gas such as helium, 2 is a carrier gas flow controller for controlling the carrier gas to flow at a predetermined flow rate, and 3 is separated and analyzed. A sample injection part for injecting a sample, 4 is a separation column, 5 is a thermal conductivity type detector, and a portion 6 shown by a dotted line is a thermostatic chamber, and the sample injection part 3, the separation column 4, the thermal conductivity type detection described above. Each of the containers 5 is constantly maintained at a predetermined temperature. The sample gas to be separated and analyzed, which is introduced into the sample introduction unit 3 in the above configuration, is sent to the column 4 by a carrier gas whose flow rate is controlled, and is separated into each gas component, and the thermal conductivity type detector 5 is separated.
Flow out to.

FIG. 3 is a schematic view of this thermal conductivity type detector, and 7 is a container (cell) having a small volume made of metal (or ceramic or the like), which is placed in a constant temperature bath 6 shown by a dotted line. It is stored and kept at a constant temperature. The column 4 described in FIG. 2 is connected to the upper end of the cell 7, and the carrier gas from the column and the separated sample gas flow into the cell in the direction of the arrow and flow out from the other.
A metal filament F made of, for example, tungsten or a tungsten rhenium alloy is housed in the cell 7, and the filament F is connected to one side of the bridge circuit described in detail in FIG.

FIG. 1 shows an embodiment of the thermal conductivity type detector of the present invention. In a bridge circuit having one side of the filament F, the connection point O of the reference resistors R1 and R2, the filament F and the reference resistor R3. Is connected to the positive and negative input terminals of the operational amplifier CA, and the output side thereof is connected to the connection point Q of the reference resistors R2 and R3, for example. Note that R is a connection point between the filament F and the reference resistor R1, and S is a power source. The operation of this thermal conductivity type detector will be described below. A voltage determined by the voltage division ratio of the resistance values of the reference resistor R1 and the reference resistor R2 is applied to the positive input of the operational amplifier, and a voltage determined by the voltage division ratio of the resistance values of the reference resistor R3 and the filament F is applied to the negative input of the operational amplifier. Given.
When a gas with a large heat capacity flows into the filament F,
The temperature of the filament F decreases, and as a result, the resistance of the filament F decreases. Then, comparing the potentials at the points O and P, the potential at the point O becomes higher. Although the potential at the point P decreases, the point O is connected to the positive input end of the input of the operational amplifier and the point P is connected to the negative input end of the operational amplifier. A large amount of current flows through the filament F and the reference resistor R3, the filament temperature rises, and the lowered filament temperature returns to the initial temperature. As a result, the output voltage of the operational amplifier settles to a stable point, and the voltages at the positive and negative inputs of the operational amplifier become equal. This calming effect is the same when the output voltage of the operational amplifier is decreasing. That is, by controlling the output voltage of the operational amplifier, the positive input voltage and the negative input voltage of the operational amplifier are always equal. The sample gas can be measured by detecting the output of the operational amplifier or the signal from the input terminal of the operational amplifier in these processes.

As described above, by using the control circuit which prepares the reference resistance and keeps the filament voltage constant, as a result, it is possible to construct a gas chromatograph with high accuracy and high speed response. This circuit can be used with only one, or can be used to obtain the differential output by obtaining the difference between the sample gas and the reference gas as shown in FIG. Is the carrier gas and sample gas, and only the carrier gas is flown as the reference gas.

In FIG. 5, a filament F1 is a filament for a sample gas. When a gas having a high heat capacity flows into this filament, the voltage of the negative input of the operational amplifier CA1 decreases, so that the output voltage of the operational amplifier CA1 rises. . On the other hand, since a constant amount of reference gas is flowing through the filament F2, a constant output is obtained by the operational amplifier C.
It becomes the output of A2. Further, since the output of the operational amplifier CA1 is the positive input of the operational amplifier CA3 and the output of the operational amplifier CA2 is the negative input of the operational amplifier CA3, the output difference between the operational amplifier CA1 and the operational amplifier CA2 is the operational amplifier CA.
3 output. In the case of this circuit, since the output when the carrier gas is flowing through both filaments is subtracted by the operational amplifier CA3, the chromatogram of the detection output is a chromatogram with a background close to zero. When a gas having a small heat capacity flows, the voltage of the negative input of the operational amplifier CA1 rises, so that the output voltage of the operational amplifier CA1 falls and, as a result, the operational amplifier CA3.
Output is negative and operation is the reverse of the above.

[0013]

By carrying out the present invention, since the temperature of the filament does not substantially change, the detection output becomes independent of the thermal time constant due to the heat capacity of the cell formed by the filament and the metal block, The volume of the block to be formed can be reduced and the response speed can be increased. Therefore, since the response speed increases, the spread of the skirt on the chart of the chromatogram, that is, the tailing phenomenon decreases, and the resolution of the spectrum increases. The raw data of this is shown in the experimental example
FIG. 7 and FIG. FIG. 6 is a conventional chromatogram for the detector of FIG. 4, and FIG. 7 is a chromatogram for the detector of FIG. 1 according to the present invention. As is clear from FIGS. 6 and 7, in the detector of the present invention, the response of the filament is fast because the response of the filament is fast, so that the response of the filament is improved and the peak value is improved. Will be sharp. In Figure 6, propionic acid (PROPI
ONICACID) and isobutyric acid (ISO-BUTYRIC ACID) overlap, but in the example of the chart of the present invention, these two peaks become sharp separated peaks as shown in FIG. It has a remarkable effect. Also,
Electric current always flows through the filament, but even when a gas with a low heat capacity flows, the temperature of the filament is not overheated, does not settle to a certain value, and is not oxidized or corroded. Therefore, the filament does not change with time, and the performance as a detector is stable. Also,
Since it is not necessary to keep the current value of the filament low, the sensitivity can be increased, the types of measurable gases increase, and the types of gas are not limited.

[Brief description of drawings]

FIG. 1 is a control circuit of a thermal conductivity type detector of the present invention.

Fig. 2 Gas chromatograph system using a thermal conductivity detector

FIG. 3 Thermal conductivity type detector

FIG. 4 is a detection circuit having a bridge structure according to the related art.

FIG. 5 is a combination example of the thermal conductivity type detector of the present invention.

FIG. 6 is an example of a chart of a chromatograph using a conventional thermal conductivity detector.

FIG. 7 is a chart example of a chromatograph using the thermal conductivity detector of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Carrier gas cylinder, 2 ... Carrier gas flow control part, 3 ... Sample injection part, 4 ... Separation column, 5 ... Thermal conductivity type detector, 6 ... Constant temperature bath, 7 ... Cell, 8 ... Detector control circuit,
Filament: F, F1, F2, reference resistance: R1 to R
6, operational amplifier ... CA, CA1, CA2, CA3, power supply ... S, constant current or constant voltage circuit ... M, bridge voltage detection circuit ... N.

Claims (1)

[Claims] 1. A bridge circuit is configured by connecting four resistors, two opposing connecting points of the four connecting points and two input terminals of an operational amplifier are connected, and this output terminal is connected. A thermal conductivity type detector characterized in that it is connected to one of the other connection points and one of the resistors is a filament resistor exposed to gas, and the remaining resistor is a reference resistor having a constant resistance value.