FLAME IONISATION DETECTOR AND METHOD FOR THE ANALYSIS OF AN OXYGEN-CONTAINING MEASURING GAS

Abstract

A flame ionization detector includes a burner to combust an oxygen-containing sample gas in a gas flame in the presence of air and while supplying a hydrogen-containing combustion gas. A measurement device measures an ion current from the gas flame to an electrode, and a temperature sensor measures a temperature of the gas flame. An evaluation device evaluates the measured ion current and compensates during evaluation of the measured ion current for a cross-sensitivity of the ion current to oxygen in the sample gas using the measured temperature of the gas flame.

Claims

1.-7. (canceled)

8. A flame ionization detector, comprising: a burner combusting an oxygen-containing sample gas in a gas flame in the presence of air and while supplying a hydrogen-containing combustion gas; a measurement device configured to measure an ion current from the gas flame to an electrode; a temperature sensor configured to measure a temperature of the gas flame; and an evaluation device configured to evaluate the measured ion current and to compensate during evaluation of the measured ion current for a cross-sensitivity of the ion current to oxygen in the sample gas using the measured temperature of the gas flame.

9. The flame ionization detector of claim 8, further comprising absolute pressure regulators configured to regulate a supply of the hydrogen-containing combustion gas and the air, respectively.

10. The flame ionization detector of claim 8, further comprising a sample gas pump configured to regulate a supply of the sample gas.

11. The flame ionization detector of claim 8, further comprising: relative pressure regulators configured to control a supply of the hydrogen-containing combustion gas and the air; and an air pressure sensor configured to measure an ambient pressure, wherein the evaluation device is configured to correct the measured temperature of the gas flame using the measured ambient pressure.

12. A method for analyzing an oxygen-containing sample gas on the basis of flame ionization, said method comprising: combusting a sample gas in a gas flame in the presence of air and while supplying a hydrogen-containing combustion gas; measuring an ion current from the gas flame to an electrode; measuring a temperature of the gas flame; evaluating the ion current; and compensating during evaluation of the measured ion current for a cross-sensitivity of the ion current to oxygen in the sample gas using the measured temperature of the gas flame.

13. The method of claim 12, wherein the hydrogen-containing combustion gas is pure hydrogen.

14. The method of claim 12, further comprising regulating a pressure of the hydrogen-containing combustion gas, the air and the sample gas to a constant absolute pressure.

15. The method of claim 14, further comprising: measuring an ambient pressure; regulating the pressure of the hydrogen-containing combustion gas, the air and the sample gas to a constant relative pressure in relation to the ambient pressure; and correcting the measured temperature of the gas flame using the measured ambient pressure.

Description

[0015] The invention is described in more detail in the following by way of example on the basis of the drawings, in which, shown in detail:

[0016] FIG. 1 shows a flame ionization detector, and

[0017] FIG. 2 shows an example of the dependency of the ion current and the flame temperature upon the oxygen content of a zero gas.

[0018] FIG. 1 shows a simplified schematic representation of a flame ionization detector with a combustion chamber 1 and a burner (burner nozzle) 2 arranged in or protruding into said combustion chamber 1. The burner 2 is supplied with an oxygen-containing sample gas 3 and a hydrogen-containing combustion gas 4, preferably pure hydrogen, on the input side, these being mixed before or in the burner 2 and combusted in a flame 6 at an output-side outlet 5 of the burner 2. In addition, oxygen-containing combustion air 7, preferably purified hydrocarbon-free ambient air, with a constant approx. 21 percent by volume, is conducted into the combustion chamber 2.

[0019] During the combustion of the sample gas 3, the organically bound hydrocarbon content is ionized. In order to measure this hydrocarbon content, a measurement device 8 is provided with two electrodes 9, 10, between which the gas flame 6 burns. As an alternative to the example shown, one of the electrodes may be formed by the burner 2 and the other electrode may be arranged above the flame 6. By means of a voltage source 11, an electrical field is generated between the electrodes 9, 10, on the basis of which the ions flow out of the gas flame 6 to one of the electrodes 9, 10 in an ion current I. The ion current I is measured by means of a highly sensitive amplifier 12, digitized (ADC 13) and evaluated in an evaluation device 14 to determine the overall hydrocarbon concentration in the sample gas 3.

[0020] Arranged in or immediately above the gas flame 6 is a temperature sensor 15, e.g. a thermistor or thermocouple, which captures the flame temperature T and supplies it to the evaluation device 14 via a temperature measurement circuit 16.

[0021] The flame ionization detector has a cross-sensitivity to the oxygen in the sample gas 3, because the measured ion current I is not only dependent upon the hydrocarbon content of the sample gas 3, but also upon the oxygen content thereof.

[0022] By way of example, FIG. 2 shows the dependency of the measured ion current I and the flame temperature T upon the oxygen content O.sub.2% of a hydrocarbon-free zero gas, wherein the oxygen content O.sub.2% here is increased in ten stages from left to right from 0% to 21%. As the oxygen content O.sub.2% increases, the flame temperature T decreases, whereas the ion current I rises.

[0023] In the case of a hydrocarbon-containing sample gas 3, the ion current I and the flame temperature T decrease as the oxygen content O.sub.2% of the sample gas 3 increases. This means that the FID measurement signal indicating the overall hydrocarbon concentration of the sample gas 3 is too low in the case of an oxygen content O.sub.2% which is higher compared to the oxygen content during calibration, and is too high in the case of an oxygen content O.sub.2% which is lower. Furthermore, however, it has been possible to establish that a change in the hydrocarbon content of the sample gas 3 has no or only a negligible influence on the flame temperature T. The flame temperature T therefore enables an indirect measurement of the oxygen content O.sub.2% and thus a correction of the FID measurement signal. If the flame temperature T is therefore lower during measurement operation than during calibration, then the overall hydrocarbon concentration of the sample gas 3 ascertained from the ion current I is corrected upwards, and downwards in the reverse case.

[0024] The evaluation device 14 therefore contains a correction device 17, in which, during the evaluation of the measured ion current I, the cross-sensitivity thereof to the oxygen in the sample gas 3 is compensated using the measured flame temperature T. The correspondingly corrected evaluation result from the measured ion current I is output as a result 18 of the determination of the overall hydrocarbon concentration of the sample gas 3. Instead of a hydrogen/helium mixture, which is comparatively expensive, it is therefore possible to use pure hydrogen as combustion gas 4.

[0025] The inflows of the sample gas 3, combustion gas 4 and combustion air 7 are kept constant with the aid of a system consisting of a sample gas pump 19, pressure regulators 20, 21 and capillaries. The pressure regulators 20, 21 may involve absolute pressure regulators or relative pressure regulators. In the case of the latter, the regulation takes place in relation to the ambient pressure, meaning that a volumetric flow rate is produced which is independent of the ambient pressure. The mass flow, however, is dependent upon the gas density and thus upon the ambient pressure, wherein in the event of a decrease in the ambient pressure, the mass flows of the gases also decrease, and the flame temperature thus also decreases. An air pressure sensor 22 captures the ambient pressure p.sub.atm and supplies this to the evaluation device 14 via a pressure measurement circuit 23. In the correction device 17, the measured flame temperature T is corrected using the measured ambient pressure p.sub.atm. If the ambient pressure p.sub.atm measured in measurement operation is therefore lower than during calibration, then the value of the measured flame temperature T used for the correction of the FID measurement signal is corrected upwards, and downwards in the reverse case.