JP2000065774A - Gas chromatograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas chromatograph equipped with a heat conductivity type detector capable of adjusting the zero point of output without changing the current state in a cell. SOLUTION: A first differential amplifier 11 outputs a voltage (vin) equal to the differential output of a bridge and a second differential amplifier 12 outputs a voltage (vout) having magnitude which is obtained by magnifying the difference between the voltage (vin) and the offset voltage (voff) outputted from a D/A converter 13 by a gain G. A control unit 15 monitors the output voltage (vout) of the second differential amplifier 12 and sends the signal altering the value of the voltage (voff) to the D/A converter 13 when the voltage (vout) is judged to be out of the input range of an A/D converter 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas chromatograph, and more particularly to a gas chromatograph using a thermal conductivity type detector (hereinafter, referred to as TCD) for detecting a sample component.

[0002]

2. Description of the Related Art One of the detectors for detecting sample components separated and extracted by gas chromatography is TCD.
It has been known. FIG. 2 shows a conventional TCD.
It is a figure which shows the schematic structure of. TCD20 shown in FIG.
Is a measurement gas flow path 21 through which a gas eluted from a column of a gas chromatograph (not shown) (mixed gas of a separated component and a carrier gas; hereinafter, referred to as “measurement gas”) flows.
And a cell 23 having a reference gas channel 22 for flowing a carrier gas (reference gas) containing no other components. In the cell 23, two elements are provided for each channel,
A total of four filaments 24 to 27 are provided. These filaments 24 to 27 are connected as shown in FIG. 2 to form a Wheatstone bridge (hereinafter, referred to as a bridge 32), and a constant current source 28 is connected to the bridge 32. Although not shown, the cell 23
Are stored in a thermostat.

When a measurement gas containing a sample component flows into the measurement gas flow path 21, the electric resistance of the filaments 24 and 25 changes according to the thermal conductivity of the sample component, and the two output terminals of the bridge 32 are correspondingly changed. The voltage between P and Q changes. The voltage between the output terminals P and Q is taken out as the output of the bridge 32 (hereinafter, referred to as the differential output of the bridge 32), amplified by the amplifier 29, and amplified by the data processor 30.
Is input to The data processing device 30 includes an A / D converter (not shown), converts an output signal of the amplifier 29 into digital data indicating the intensity of the signal, and identifies a component contained in the measurement gas based on the digital data. Note that TC in FIG.
In D20, all the elements constituting the bridge 32 are filaments, but this is not necessarily required. For example, even if the filament 25 in the measurement gas flow path 21 and the filament 26 in the reference gas flow path 22 are replaced with a constant resistance, a voltage change occurs between the output terminals P and Q, and the component can be identified in the same manner as described above. It is possible.

In general, an input interface of an electronic circuit configured using a microcomputer or the like has a standard of a range of signal strength that can be input (hereinafter referred to as an input range), for example, 0 to 1V. . Therefore, in the TCD 20 of FIG.
Is variable, so that the signal strength sent to the data processing device 30 can be adjusted so as to fall within the input range. Further, a potentiometer 31 is connected in parallel to the bridge 32 as shown in FIG. 2 so that the output of the TCD 20 can be adjusted to a zero point. For example, if the input range of the data processing device 30 is 0
In the case of １1 V, the zero point of the output of the TCD 20, that is, the voltage output from the amplifier 29 when the measurement gas does not contain components other than the carrier gas, is changed by appropriately changing the setting of the potentiometer 31. Be within the input range (for example, 0.
5V).

In the analysis using a gas chromatograph including the TCD 20 as described above, the temperature of the column, the amount of current supplied from the constant current source 28 to the bridge 32, and the like depend on the purpose of the analysis and the type of the component to be detected. When the measurement condition such as the set temperature of the thermostat containing the cell 23 is changed, the differential output of the bridge 32 changes with the change of the measurement condition. At this time, if the zero point of the differential output is within the input range of the data processing device 30 even after the change, the potentiometer 3
It is not necessary to readjust the zero point by 1, and the next analysis can be started only by resetting the gain of the amplifier 29. On the other hand, when the zero point of the differential output goes out of the input range of the data processing device 30, it is necessary to readjust the zero point by the potentiometer 31.

[0006]

SUMMARY OF THE INVENTION Potentiometer 31
Is changed, each filament 24 to
Although the amount of current flowing through 27 changes, a change in current flowing through the filament can lead to a temperature change in the filament. As described above, the TCD is stored in a thermostat, in order to maintain a constant temperature at which the sensitivity of the TCD becomes highest. Therefore, changing the current flowing through the filament as described above is not preferable especially when performing highly accurate analysis. The present invention has been made to solve such a problem, and an object of the present invention is to provide a heat source capable of performing zero point adjustment of an output without changing a state of a current inside a cell. It is an object of the present invention to provide a gas chromatograph having a conductivity type detector.

[0007]

SUMMARY OF THE INVENTION A gas chromatograph according to the present invention, which has been made to solve the above-mentioned problems, has a heat conduction having a bridge including a filament disposed in each of a measurement gas flow path and a reference gas flow path. In a gas chromatograph provided with a rate detector, a first differential amplifier receiving a differential output of the bridge and outputting a voltage corresponding to the magnitude of the differential output, and a voltage generating means for generating a constant voltage And receiving an offset voltage generating means capable of changing the value of the constant voltage, an output voltage of the first differential amplifier and a constant voltage generated by the offset voltage generating means, A second differential amplifier that outputs a voltage corresponding to the difference; and a data processing unit that performs a process of identifying a component in the measurement gas based on an output voltage of the second differential amplifier. It is characterized by a door.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a gas chromatograph according to the present invention, a first differential amplifier receives a differential output of a bridge (more precisely, each voltage taken out from two output terminals of the bridge) as an input signal. And outputs a voltage corresponding to the magnitude of the differential output as a single-ended signal. Here, it is not particularly necessary to amplify the differential output of the bridge (that is, the magnitude of the output voltage of the first differential amplifier may be equal to the magnitude of the differential output of the bridge). The second differential amplifier receives as input the output voltage of the first differential amplifier and the voltage generated by the offset voltage generating means, and outputs a voltage corresponding to the difference between the two voltages. Here, the voltage is amplified as needed (that is, the second differential amplifier is obtained by multiplying the difference between the output voltage of the first differential amplifier and the offset voltage by a predetermined gain). Output a large voltage). The output voltage of the second differential amplifier is sent to data processing means after A / D conversion. The data processing means may be configured using a microcomputer, a personal computer, or the like as in the related art. In addition, the process of identifying the components by the data processing means may be basically performed in the same manner as in the related art.

In the gas chromatograph according to the present invention, the zero point of the output of the TCD is adjusted as follows.
That is, the output voltage of the second differential amplifier is checked with the carrier gas not containing the sample flowing through both the measurement gas flow path and the reference gas flow path. The offset voltage generated by the offset voltage generating means is appropriately changed in accordance with the output voltage so that the output voltage of the second differential amplifier falls within the input range of the data processing means (more preferably, the center of the input range). Position).

In a preferred embodiment of the gas chromatograph according to the present invention, it is determined whether or not the output voltage of the second differential amplifier is within a predetermined range. And an offset voltage setting means for obtaining a digital signal indicating the offset voltage, receiving the digital signal, converting the digital signal into an analog signal, and sending the digital signal to the offset voltage generating means. A conversion means, and the offset voltage generation means generates an offset voltage specified by the analog signal. Here, the predetermined range of the output voltage is the same as or included in the input range of the data processing means. In this gas chromatograph, analysis conditions (column temperature, amount of current supplied to the bridge, temperature of thermostat, etc.)
When the zero point of the output of the TCD changes due to the change of the value or the drift, the offset voltage is automatically reset, and the output voltage of the second differential amplifier comes within a predetermined range. This eliminates the need for the operator to monitor the output voltage of the second differential amplifier and manually reset the offset voltage as needed.

[0011]

According to the present invention as described above, the zero point of the output of the TCD can be adjusted without changing the state of the current flowing through the bridge in the cell of the TCD. It is possible to stably maintain such a condition as to increase. Further, in the preferred embodiment of the present invention, since the zero point adjustment of the output of the TCD is automatically performed, the trouble of manually performing the zero point adjustment every time the analysis conditions are changed is not only required, but also it is unnecessary. Also, the effect that the input range of the data processing means is substantially increased can be obtained.

[0012]

FIG. 1 is a diagram showing a schematic configuration of an embodiment of a TCD used in a gas chromatograph according to the present invention.
In the following description, the same components as those of the conventional TCD described above with reference to FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted as appropriate.

In the TCD 10 of this embodiment, the cell 23
The output terminals P and Q of the bridge 32 provided inside are
It is connected to two input terminals of the first differential amplifier 11, respectively. Here, the first differential amplifier 11 has an output terminal P,
It is configured to output a voltage equal to the voltage between Q (that is, the gain of the first differential amplifier 11 is 1). The output terminal of the first differential amplifier 11 is connected to one input terminal of the second differential amplifier 12. The other input terminal of the second differential amplifier 12 is connected to an output terminal of a D / A converter 13 that generates an offset voltage voff.

The second differential amplifier 12 is connected to the first differential amplifier 1
A voltage vout having a magnitude obtained by multiplying the difference between the voltage vin output from 1 and the offset voltage voff output from the D / A converter 13 by a gain (G) is output. That is, the following equation holds for the above variables: vout = G × (vin−voff) (1) Here, it is assumed that the gain of the second differential amplifier 12 can be changed. The output voltage vout of the second differential amplifier 12 is converted into digital data by the A / D converter 14 and sent to the control device 15. Control device 15
Is based on the digital data received from the A / D converter 14, as will be described later,
And outputs a data signal to the data processing device 30.

The adjustment of the zero point of the output of the TCD 10 is performed in the following procedure. First, the gas chromatograph is started, and the measurement gas flow path 21 and the reference gas flow path 2 of the cell 23 are started.
2 only the carrier gas flows. At the time of zero point adjustment, it is preferable to set the gain of the second differential amplifier 12 to 1. In such a state, the control device 15 sends a voltage instruction signal for setting the offset voltage voff to zero to the D / A converter 13, and based on the digital data output from the A / D converter 14, The value of the output voltage vout of the amplifier 12 is checked to determine whether or not the value is within the input range of the A / D converter 14. More preferably, a range further limited than the input range of the A / D converter 14 defined in the standard is set in advance as an allowable range, and the control device 15 determines whether vout is within the allowable range. It is good to judge. In the following description, it is assumed that such an allowable range is set.

When it is determined that the output voltage vout of the second differential amplifier 12 is out of the allowable range, the control device 15 outputs a voltage instruction signal for changing the offset voltage voff to D.
/ A converter 13. For example, when vout exceeds the upper limit of the allowable range, the difference between vin and voff is reduced by increasing the offset voltage voff,
vout is centered in the tolerance. Conversely, vout
Is smaller than the lower limit of the allowable range, the difference between vin and voff is increased by reducing the offset voltage voff so that vout is at the center of the allowable range.
The offset voltage thus reset is defined as Voff.

After the offset voltage is set as described above, the value of the gain is set appropriately, and the sample is analyzed. As long as the analysis is performed under the same conditions, the differential output of the TCD 10 can be obtained by the following equation in which the equation (1) is modified and Voff is substituted for voff. vin = vout / G + Voff (2)

After that, when the analysis conditions (the temperature of the column, the amount of current supplied to the bridge 32, the temperature of the thermostat, etc.) are changed, the control device 15 again operates the TCD1 as described above.
The zero point of the output of 0 is adjusted (that is, the offset voltage is reset).

The form of the TCD used in the chromatograph according to the present invention is not limited to the above. For example, the present invention can be implemented by using a TCD in which a part of the filament constituting the bridge 32 is replaced with a constant resistance. In addition, the present invention can be variously modified within the spirit and scope.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of an embodiment of a thermal conductivity type detector (TCD) used in a gas chromatograph according to the present invention.

FIG. 2 is a diagram showing a schematic configuration of a conventionally known TCD.

[Explanation of symbols]

 10, 20: thermal conductivity type detector (TCD) 11: first differential amplifier 12: second differential amplifier 13: D / A converter 14: A / D converter 15: control device 24-27: filament 30 Data processing device 32 ... Bridge

Claims (1)

[Claims] 1. A gas chromatograph comprising a thermal conductivity type detector having a bridge including a filament disposed in each of a measurement gas passage and a reference gas passage, wherein the gas chromatograph receives a differential output of the bridge, A first differential amplifier that outputs a voltage according to the magnitude of the output, and voltage generating means for generating a constant voltage, wherein an offset voltage generating means such that the value of the constant voltage is changeable, A second differential amplifier that receives an output voltage of the first differential amplifier and a constant voltage generated by the offset voltage generating unit, and outputs a voltage corresponding to a difference between the two voltages; A data chromatograph comprising: data processing means for performing a process of identifying a component in a measurement gas based on an output voltage.