Method and thermal conductivity detector
A thermal conductivity detector includes a heatable resistive detector configured to be physically arranged in an analytes flow eluting from a chromatography column and electrically arranged with resistors in separate arms of a measuring bridge, an amplifier which detects differential voltage between two opposite nodes of the bridge and applies an output voltage to other opposite nodes of the measuring bridge to maintain the detector at a constant operating temperature, and an additional resistor with a controllable switch in parallel connected in series with the detector or resistor arranged in one arm of the bridge, where the switch is periodically turned on and off at a predetermined duty cycle and/or controlled by information on characteristic times-of-arrival of analytes at the detector to compensate for operating temperature uncertainties due to manufacturing variations of the resistors and/or to allow for processing small and large peaks of a chromatogram with highest available resolution.
1. A thermal conductivity detector for a gas chromatograph comprising:
a heatable resistive detector element configured to be physically arranged in a flow of analytes eluting from a chromatography column and electrically arranged with resistors in separate arms of a measuring bridge;
an additional resistor with a controllable switch in parallel and connected in series with the heatable resistive detector element or a resistor in one arm of the measuring bridge;
an amplifier configured to detect a differential voltage between two opposite nodes of the measuring bridge and to apply an output voltage to other opposite nodes of the measuring bridge to maintain the heatable resistive detector element at a constant operating temperature; and
a controller configured to provide a control signal to the switch to adjust the operating temperature;
wherein the control signal is a pulse-width modulated voltage with a period lower than a thermal time constant of the detector element.
2. The thermal conductivity detector of claim 1 , wherein the pulse-width modulated voltage has a momentary change in duty cycle at characteristic times-of-arrival of predetermined ones of the flow of analytes at the detector element.
3. The thermal conductivity detector of claim 1 , wherein the controllable switch is connected at one end to one of the other opposite nodes of the measuring bridge.
4. The thermal conductivity detector of claim 1 , wherein the heatable resistive detector element is connected at one end to one of the other opposite nodes of the measuring bridge.
5. The thermal conductivity detector of claim 1 , wherein the switch is a semiconductor device.
6. The thermal conductivity detector of claim 5 , wherein the semiconductor device is a transistor selected from the group consisting of a bipolar transistor and a field effect transistor.
7. The thermal conductivity detector of claim 5 , further comprising:
a second switch coupled in series with the controllable switch, the controllable and second switches being controlled together; and
a voltage divider connected between the other opposite nodes of the measuring bridge, a tap of said voltage divider being coupled to a node between the serially coupled controllable and second switches, and the voltage divider being configured to provide a tap voltage having a level which corresponds to a voltage drop across the additional resistor.
8. The thermal conductivity detector of claim 6 , further comprising:
a second switch coupled in series with the controllable switch, the controllable and second switches being controlled together; and
a voltage divider connected between the other opposite nodes of the measuring bridge, a tap of said voltage divider being coupled to a node between the serially coupled controllable and second switches, and the voltage divider being configured to provide a tap voltage having a level which corresponds to a voltage drop across the additional resistor.
9. The thermal conductivity detector of claim 7 , wherein the tap of said voltage divider is coupled to the node between the controllable and second switches via a buffer.
10. A gas chromatograph comprising at least one thermal conductivity detector of claim 1 .