CZ2008783A3 - Gas chromatography detector - Google Patents

Abstract

The object of the invention is a thermally conductive detector in which the temperature of the filament is measured optically. The filament is placed in a cell through which the carrier gas flows and which is provided with a window behind which the light sensitive element is placed. The detector can be used in an arrangement with two cell - specific and reference cells. It can also be used in a feedback circuit where the control condition is that the signal that gives the light sensitive element is constant and the heating current changes. Another variant is to measure non-cumulative radiated energy in the optical element sensitivity band, but to measure in the narrow wavelength range defined by the filter. By combining the monitoring of the defined wavelength in feedback with the glow intensity, the highest detector sensitivity can be achieved.

Description

Detector for gas chromatography

Stanislav-VerkarMtfoslav PedetokTMitoiBofih

Technical field:

The invention relates to the analysis of substances by gas and supercritical chromatography.

BACKGROUND OF THE INVENTION:

Gas chromatographs are equipped with various types of detectors. Among the most popular is the universal thermal conductivity detector (TCD) measuring the change in the thermal conductivity of the carrier gas that occurs in the presence of detected substances. It follows from the measuring principle that the detector is universal and the sensitivity to the analyzed compounds is given by the difference between the thermal conductivity of the pure carrier gas and the carrier gas containing the analyte.

Thermally conductive detectors are traditionally fitted with two or four heated filaments which are located in a specific and reference cell. The heated filaments are situated in a bridge circuit so that their electrical resistance can be measured, which is proportional to the temperature. In the case of an increased (decreased) conductivity of the gas passing through the whole unit, the temperature of the filament of the heated electric current passage decreases at a constant intensity.

The disadvantage of the thermally conductive detector is, in particular, its lower sensitivity compared to, for example, a flame ionization detector, but also the low lifetime of the heated fibers and their sensitivity to the presence of oxygen in the carrier gas or in the sample.

SUMMARY OF THE INVENTION:

The subject of the invention is a thermally conductive detector in which the temperature of the heated filament is measured optically by a suitable light-sensitive element. The heated filament is placed in a cell through which the carrier gas flows and which is provided with a window. If the thermal conductivity of the carrier gas is increased, the filament temperature is lowered and the amount of energy emitted is reduced by constant current heating. The detector can be used in a two-cell arrangement - specific and reference - like a conventional TCD. It can also be used in a closed circuit, where the control condition is that the signal giving the light-sensitive element is constant. To achieve this, a variable glow current value is used, the instantaneous value of which is a measure of the change in the thermal conductivity of the carrier gas. Another variant is to measure not the total radiated energy in the sensitivity band of the optical element, but the measurement in the narrow wavelength range defined by the filter, because when the fiber temperature decreases, its spectral characteristic primarily changes (maximum shifts to higher wavelengths). By combining the defined wavelength in feedback with the glow intensity, the highest sensitivity of the detector can be achieved.

Examples:

Example 1

The thermally conductive detector according to the invention was made of a stainless steel block into which two cylindrical cells of 3 mm diameter and 6 mm length were drilled. The inlet and outlet of the carrier gas was realized tangentially in the cylindrical part of the cells. Two micro-bulbs intended for a voltage of 1.2 V and a nominal current of 0.3 A stripped of glass bulbs were sealed from one side into the bottom of the cells. On the other hand, both cylinders were fitted with a 6 mm diameter shoulder, in which quartz windows with a wall thickness of 1 mm were also fastened with high-temperature sealant. Two Hamamatsu S2386-5K photodiodes were placed 10 mm from the windows. The detector block was equipped with an electrical resistance heater of 250 W and a control electronics with a platinum measuring sensor. The presence of the analyzed substances in the carrier gas was measured as the difference between the specific and the reference photodiode signal.

Example 2

The detector described in Example 1 was used, but the measurement was performed only with a specific whole. The presence of substances in the carrier gas was indicated by a change in the photodiode signal.

Example 3

The detector described in Example 1 was used and the measurement was performed only with the whole cell, with a filter wavelength of 750nm inserted between the cell window and the photodiode. The presence of substances in the carrier gas was indicated by a change in the photodiode signal.

Example 4

A detector as described in Example 3 was used. An electrical circuit was used in which the modified photodiode signal was used to control the incandescent filament current of the bulb through a suitably connected transistor. The presence of substances in the carrier gas was indicated by the filament heating current value.

Claims (4)

PATENT CLAIMS 1. The object of the invention is a thermally conductive detector characterized in that it consists of a measuring cell equipped with an inlet and an outlet of a gas, in which a filament of suitable material is placed, the cell is provided with a transparent window. and the fact that the signal of the photosensitive element, after processing, is used as an indicator of the change in the thermal conductivity of the gas passing through it. 2. A thermally conductive detector according to claim 1, characterized in that it operates in a dual arrangement with two cells through which the specific and regenerative gas passes and the indicator of the change in the thermal conductivity is the difference in the signals of the photosensitive elements. 3. A thermally conductive detector according to claim 1, wherein a filter defining a suitable wavelength range of light incident on the light-sensitive element is interposed between the light-sensitive element and the cell window. 4. A thermally conductive detector according to claim 1 or 2 or 3, characterized in that the signal of the photosensitive element (s) is kept constant by changing the magnitude of the filament current (s). whole or whole.