JPH087199B2 - Gas chromatograph - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention introduces a sample gas transferred by a carrier gas to a column to separate it into respective gas components, which are detected by a thermal conductivity detector for analysis. The present invention particularly relates to a gas chromatograph having a function of preventing disconnection due to overheating of the microdiaphragm sensor when the microdiaphragm sensor is used as a thermal conductivity detector thereof.

[Conventional technology]

A gas chromatograph has been generally used as a detector for performing component analysis on process gas in a petrochemical process or a steel process and monitoring each process step or performing various controls based on the analysis result. .

FIG. 6 is a view showing the basic configuration of this type of gas chromatograph, which is an analyzer main body 1, which forms a thermostatic chamber and is maintained at a predetermined temperature, a sample valve 2, a column 3, and a detector arranged in the analyzer main body 1. Equipped with a pressure reducing valve 6 for reducing the pressure of the carrier gas CG consisting of an inert gas such as helium and the measuring vessel 4, to a predetermined pressure, and switching the flow path from the solid line to the broken line in the sample valve 2 during measurement. Thus, the sample gas SG to be measured, which is separated by the measuring pipe 5, is sent into the column 3 by the carrier gas CG. Column 3 varies depending on the sample gas SG,
Powders with a uniform particle size such as activated carbon, activated alumina, and molecular sieves are packed as the stationary phase. The difference in the moving speed based on the difference in the adsorption and partition coefficient between this stationary phase and each gas component in the sample gas SG. Is used to separate each gas component from each other, which is detected by a detector 4 such as a thermal conductivity detector and converted into an electric signal. This electric signal is proportional to the gas component concentration, and is subjected to waveform processing by the controller 7 or recorded on a recording sheet.

On the other hand, at the time of non-measurement, the flow path of the sample valve 2 is switched to the state shown by the solid line to guide the carrier gas CG to the column 3 and the detector 4.

FIG. 7 shows an example of the measuring circuit of the detector 4, and here shows the case where a thermal conductivity detector (hereinafter referred to as TCD sensor) using a filament or thermistor is used.

As shown in FIG. 7, this circuit has one circuit at the position where it contacts only the carrier gas CG on the carrier gas channel.
Provided with a TCD sensor 4 ₂ as a reference side,
The outlet of the column 3 provided TCD sensor 4 ₂ as the measurement side, it connects these sensors 4 _1, 4 _2, to each side of the bridge circuit 8 which includes a fixed resistor R _1, R _2. Then, by previously heating each TCD sensors 4 _1, 4 ₂ by a constant current from the current source 9, and detects the concentration of a component of the sample gas SG in the carrier gas CG at the column outlet through amplifier section 10 .
That, TCD sensor 4 ₂ Reference is detected at all times the carrier gas CG, also measuring TCD sensor 4 _1, usually detects the carrier gas CG. At this time, the bridge circuit 8 is in a balanced state, and no potential difference is generated between the two input voltages of the amplifier section 10.
Has an output voltage V of 0v. Then, when the sample gas SG is transferred after the carrier gas CG, it is used for measurement.
The resistance value changes according to the thermal conductivity of the components of the sample gas SG in TCD sensor 4 _1, a potential difference occurs between the two input voltages of the amplifier unit 10 the bridge circuit 8 becomes unbalanced. As a result, as shown in FIG. 8, the output voltage V of the amplifier section 10 rises from 0v, and the sample gas components A, B, C
Will show the value according to.

By the way, in such a gas chromatograph, TC
As a D sensor, there is a method of using a micro diaphragm sensor instead of a normal filament or thermistor.
This micro-diaphragm sensor is one in which a fine mechanical structure is formed on the substrate such as silicon using the semiconductor manufacturing process in the order of micron, and the pattern shape is very small, so the space factor is good and the debt of the TCD detector is good. There are advantages such as a small space and a small thermal time constant.

[Problems to be Solved by the Invention]

In a gas chromatograph using such a microdiaphragm sensor, nitrogen (N ₂ ) is often used as a carrier gas in addition to helium (He) and hydrogen (H ₂ ) which have a large difference in thermal conductivity from the sample gas. However, since He has high thermal conductivity, for example, the current, that is, the heating current, must be increased in order to keep the temperature of the microdiaphragm sensor high. However, there was a problem that when N ₂ with a low thermal conductivity came in with the current of He flowing, the microdiaphragm sensor became too hot and burned out.

In view of the above points, the present invention has been made to solve the above conventional problems, and provides a gas chromatograph capable of preventing deterioration and disconnection due to overheating of a sensor when a micro diaphragm sensor is used as a TCD sensor. The purpose is to do.

[Means for solving the problem]

In order to achieve the above object, the gas chromatograph of the present invention uses a microdiaphragm sensor composed of a thin film heater on a substrate, and a constant current is passed through the thin film heater of the microdiaphragm sensor to heat the gas component. Detecting means for detecting a change in the resistance value of the thin film heater caused by the thermal conductivity of the output voltage.
A determination means for comparing the baseline voltage included in the sensor signal detected by the detection means with a predetermined baseline cancellation voltage for each carrier gas to determine the type of carrier gas, and the determination means Current switching means for switching the current supplied to the thin film heater to an optimum value according to the type of carrier gas.

[Action]

In the present invention, by automatically switching the current flowing through the thin film heater of the microdiaphragm sensor according to the type of carrier gas, deterioration and disconnection due to overheating of this sensor can be prevented, and the output is always maximized. You can take it out.

〔Example〕

Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 1 is a circuit configuration diagram of a measurement system showing an embodiment of a gas chromatograph according to the present invention. In the figure, 11 is a constant voltage diode connected in series to the power supply voltage + V _B via a resistor 12, and voltage dividing resistors 13 and 14 are connected between both ends of this diode 11, and a point a Voltage (+5
v) is through the first contact 15 ₁ of the analog switch 15, b
The voltage at the point (+ 2.5v) is the same as the analog switch 15
The signal is input to the buffer 16 composed of an operational amplifier through the contact point 15 _{2 of the} , and the output current thereof is input to the sensor detection circuit 17. This sensor detection circuit 17 has an input resistor 18 and
Op amp 19 with non-inverting input grounded and op amp 19
It is composed of a microdiaphragm sensor 20 described later that is inserted between the inverting input and the output side of the
Is inputted to the inverting input side of the operational amplifier 19, and a voltage corresponding to a change in the resistance value of the thin film heater 32 constituting the microdiaphragm sensor 20 is detected as an output from the output side through the buffer 21. ing.

Further, 22 is an adder circuit composed of an operational amplifier 23 and resistors 24 to 27, 28 is a baseline cancellation voltage generator, and these are included in the output V _{1 of the} buffer 21 detected by the sensor detection circuit 17, that is, the sensor signal. It constitutes a circuit for canceling the baseline voltage. 29 is an A / that converts the output voltage of the adder circuit 22 into a digital signal.
The D converter, 30 is a CPU corresponding to the control 7 in FIG.

Here, when measuring the sample gas, the CPU 30 first applies the current i to the microdiaphragm sensor 20 so as to match the detection of the sample gas SG with the control control signal CON ₁
Or by CON ₂ , each contact of analog switch 15 15 ₁ , 15 ₂
Are selectively driven and supplied. While the carrier gas CG arrives at the microdiaphragm sensor 20, the output voltage of the sensor detection circuit 17 becomes a constant value and is applied to the adder circuit 22, and this voltage signal is added by the adder circuit 22 to obtain A It is input to the CPU 30 via the / D converter 29. As a result, the CPU 30 receives the digital value converted by the A / D converter 29 as an input and compares the voltage value due to the arrival of the carrier gas CG, that is, the baseline voltage, with a predetermined baseline cancellation voltage for each carrier gas. The type of carrier gas is discriminated based on the result, and the baseline cancel voltage generator 28 is driven based on the result to apply the baseline cancel voltage generated thereby to the adder circuit 22 to add only the discriminated voltage value. It is controlled to subtract from 22.

FIG. 2 is a schematic plan view of a pattern structure showing an example of the micro diaphragm sensor 20. In this microdiaphragm sensor 20, a thin film heater 32 made of permalloy or the like is formed as a thermal conductivity element in a central portion on a single crystal silicon substrate 31 as shown in FIG. Independent upstream and downstream temperature sensors 33 and 3 are provided on both sides of the thin film heater 32.
4 is formed as a flow rate detecting element. A large number of thin slits 35 for etching are provided on the surface of the silicon substrate 31, and the thin film heater 32 and the upstream temperature sensor 3 are provided.
3, by etching the lower side and the periphery of the downstream temperature sensor 34 by an anisotropic etching method such as an etching solution such as potassium hydroxide through a large number of narrow slits 35 provided on the surface of the silicon substrate. A void is formed to form a void 36 having a pattern in which the cross-sectional shape is a substantially inverted trapezoid. As a result, the upper portion of the void 36 is spatially separated from the silicon substrate 31 in a diaphragm shape, and the thin film heater 32, the upstream temperature sensor 33, and the downstream temperature sensor 34 are thermally insulated from the substrate. A supported diaphragm 37 is formed. The thin film heater 33 and the temperature sensors 33 and 34 are covered with a protective film such as silicon nitride. The arrow indicated by reference numeral 38 in FIG. 2 indicates the flow of gas.

The micro diaphragm lancer 20 having such a structure is
The thin-film heater 32 is heated to a predetermined temperature, and the concentration of the gas can be detected by utilizing the fact that the resistance value of the thin-film heater 32 changes depending on the thermal conductivity of the gas component. Further, while the thin film heater 32 is heated, when the gas 38 flows, the upstream temperature sensor 33 is cooled and its resistance value decreases, whereas the downstream temperature sensor 34 is heated and its resistance value increases. It is also possible to detect the flow rate from the difference by utilizing.

Next, the operation of the configuration of the embodiment shown in FIG. 1 will be described using the sequence flow for judging the carrier gas shown in FIG. Here, helium (He) having a high thermal conductivity and nitrogen (N ₂ ) having a lower thermal conductivity are used as the carrier gas, respectively, and analog signals are generated by the control control signals CON ₁ and CON ₂ corresponding to these He and N _2. Switch 15
When each contact 15 ₁ and 15 _{2 of} is turned on, the optimum current i to the microdiaphragm sensor 20 is, for example, when He is i _a = 4.5 m
An example will be described in which a current of i _b = 2.25 mA flows when A and N ₂ .

In the measurement of sample gas in Fig. 3, CPU30
First, set the control control signal CON ₂ to analog switch
Sending to the second contact 15 _{and second} 15 turning on its contact 15 _2.
Then, the current i _b corresponding to the voltage at the point _b is supplied to the microdiaphragm sensor 20 of the sensor detection circuit 17, and this current i _b is, for example, 2.25 mA as an optimum value corresponding to N ₂ of the carrier gas. (Step 50). At this time, since the thermal conductivity of N ₂ is almost the same as that of air, there is no risk of overheating of the microdiaphragm sensor 20 even if the carrier is not connected. In this state, the sensor detection circuit 17 detects a change in the resistance value of the thin film heater 32 of the microdiaphragm sensor 20 corresponding to the thermal conductivity of N ₂ , and the output voltage thereof is added to the CPU 30 through the addition circuit 22 and the A / D converter 29. Entered in. Then CPU30 is baseline cancel voltage corresponding to N ₂ to a predetermined baseline voltage e _c indicated by the solid line in the output voltage, that a fourth view of a constant level obtained from the sensor detection circuit 17 (a) e _C (N ₂ ) ± ε (where ε is the error) is compared (step 51), and if it is e _C > e _C (N ₂ ), the error is judged (step 53) and e _C = e _C (N _{If 2} ), it is judged as N _{2 of} carrier gas. At the same time, the baseline cancellation voltage generator 28 is driven to operate the gas chromatograph using this N ₂ as a carrier gas (step 5
2). As a result, the adder circuit 22 detects the baseline voltage e _C of the sensor detection circuit 17 and the baseline cancellation voltage generator.
The reverse line baseline cancellation voltage e _C (N ₂ ) shown in FIG. 5 (a), which is generated from 28, is added, and its output voltage V _OUT is shown by the solid line in FIG. 5 (a). A chromatographic signal having a peak value corresponding to the sample gas components A and B as shown by the chain double-dashed line in FIG. 10A fixed at the baseline voltage e _C or the zero point (0 v) can be taken out.

On the other hand, if e _C <e _C (N ₂ ) in step 51 (in case of He as shown by the broken line in FIG. 4A), the process proceeds to the next step 54. The CPU 30 controls the baseline cancel voltage e _C (He) ± corresponding to the predetermined He as the baseline voltage e _C indicated by the broken line in FIG. 4 (a) is indicated by the alternate long and short dash line in FIG. 5 (a). If not equal to ε, (e _C ≠ e _C (He) ±
ε), the error is judged (step 55), and if they are equal (e _C
= E _C (He) ± ε), and judge He as the carrier gas.
After that, the control signal CON ₁ is switched to analog switch.
Sent to the first contact 15 ₁ of 15 on the contacts 15 _1.
Therefore, the current i _a corresponding to the voltage (+ 5v) at the point _a is supplied to the microdiaphragm sensor 20, and the current i _a is, for example, 4.5 mA as an optimum value corresponding to He of the carrier gas (step 56). Then, the gas chromatograph is operated using this He as a carrier gas (step 57).

At this time, the CPU 30 controls the baseline voltage of a constant level obtained from the sensor detection circuit 17 as shown by the broken line in FIG.
e _C ′ (H, which is the reverse polarity baseline cancel voltage e _C shown by the alternate long and short dash line in FIG. 5 (b) corresponding to a predetermined He.
e) ± ε (step 58), which is e _C = e _C ′ (H
e) If it is ± ε, it is judged as He, the baseline cancellation voltage generator 28 is driven, and the gas chromatograph is continuously operated using this He as the carrier gas. e _C ≠ e
If it is _C (He), it is judged as an error (step 59).
As a result, the adder circuit 22 causes the sensor detection circuit 17 to show the baseline voltage e _C indicated by the broken line in FIG. 4 (b) and the dashed-dotted line in FIG. 5 (b) generated from the baseline cancellation voltage generator 28. Baseline cancellation voltage indicated by
e _C ′ (He) is added, and as its output voltage V _OUT , the baseline voltage e _C shown by the solid line in FIG. 5 (b) is fixed to 0v, as shown by the chain double-dashed line in FIG. 5 (b). Sample gas composition
A chromatographic signal showing a peak value corresponding to A and B can be taken out.

〔The invention's effect〕

As described above, according to the present invention, the micro diaphragm sensor is used as the TCD detector of the gas chromatograph, and the current flowing through the thin film heater of the micro diaphragm sensor is automatically switched according to the type of carrier gas. By this self-determination, deterioration and disconnection due to overheating of the microdiaphragm sensor can be prevented, and the output can be maximized and taken out. Further, there are excellent effects such as a connection error of the carrier gas and the presence / absence of the carrier gas (whether or not the cylinder is empty).

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of a measurement system showing an embodiment of a gas chromatograph according to the present invention, FIG. 2 is a schematic plan view showing an example of a pattern structure of a micro diaphragm sensor in FIG. 1, and FIG. FIG. 4 is a diagram showing an example of a sequence flow for determining the carrier gas used for explaining the operation of the example, and FIG. 4 is obtained from the sensor detection circuit when an optimum current is passed through the sensor detection circuit including the microdiaphragm sensor of FIG. It shows the baseline voltage according to the carrier gas, and Fig. 4 (a) shows 2.25mA as the optimum current.
Figure 4 shows the baseline voltage corresponding to each N ₂ and He when flowing current, and Figure 4 (b) shows the baseline voltage according to He when 4.5 mA is flowing as the optimum current. Figures (a) and (b) are respectively Figures 4 (a) and (b).
FIG. 6 is an explanatory view showing a situation in which each of the baseline voltages is corrected by the baseline cancellation voltage, FIG. 6 is a basic configuration diagram of a gas chromatograph, and FIG. 7 is an example of a measuring circuit of the detector of FIG. FIG. 8 is a waveform diagram of the output signal. 11 …… Constant voltage diode, 13,14 …… Voltage dividing resistor, 15…
… Analog switch, 17 …… Sensor detection circuit, 20 …… Micro diaphragm sensor, 22 …… Adding circuit, 28 …… Baseline cancellation voltage generator, 29 …… A / D converter, 3
0 …… CPU, 31 …… Silicon substrate, 32 …… Thin film heater.

Claims (1)

[Claims] 1. A gas chromatograph in which a sample gas transferred by a carrier gas is introduced into a column to be separated into respective gas components, which are detected by a thermal conductivity detector for analysis, wherein the thermal conductivity detector is It consists of a micro diaphragm sensor composed of a thin film heater on a substrate, a constant current is applied to the thin film heater of the micro diaphragm sensor to heat it, and a change in the resistance value of the thin film heater caused by the thermal conductivity of the gas component. And a determination means for comparing the baseline voltage contained in the sensor signal detected by the detection means with a predetermined baseline cancellation voltage for each carrier gas to determine the type of carrier gas. Means and the thin film according to the type of carrier gas discriminated by the discriminating means. Gas chromatograph, characterized in that a current switching means for switching to an optimum value the current supplied to the over data.