Thermal conductivity detector for gas chromatography

FIELD: gas chromatographic devices; chemical industry; oil industry; metallurgy; medicine; biology etc.

SUBSTANCE: detector can be used in gas chromatographic device as with filled and empty capillaries and micro-nozzle separating columns for analyzing complex mixtures of matters having as natural and artificial origin. Detector has heater made in form of metal thread, two film thermo-sensitive elements mounted in gas channel at output of chromatographic column in parallel to the heater at similar distance. Thermo-sensitive elements are connected into opposite arms of bridge circuit. Electric current source for the heater has automatic controller to keep temperature and resistance of the source to permanent value.

EFFECT: improved precision of detection; improved sensitivity.

2 dwg

 

The invention relates to gas chromatography and can be used in chromatographic devices as filled and capillary and microanatomy separation columns for determination of components of complex mixtures of substances of natural and anthropogenic origin in various industries: chemical, oil, gas, petrochemical, metallurgy, medicine, biology, ecology and other

Known thermal conductivity detectors for gas chromatography, contains massive thermostatic metal case with one or two working and comparative cameras that have sensing elements (made of metal threads or spirals)included in the pavement measuring circuit and heated by the current bridge. In the working chamber of the detector enters the gas flow from the outlet of the chromatographic column. Comparative chamber is purged with pure carrier gas. In the measuring diagonal of the bridge is formed by a differential signal proportional to mainly thermal conductivity and heat capacity of the gas environment in the cells, due to changes in temperature and resistance of the heat-sensitive elements associated with the process of heat transfer to the chamber walls (see Brazhnikov CENTURIES Differential detectors for gas chromatography. M.: Nauka, 1974. P.71-85).

One is about the well-known thermal conductivity detectors have a high inertia, which excludes their use for detection of signals in chromatographic devices with hollow capillary columns. Large inertia of these detectors is caused, on the one hand, the magnitude of the volume of the detector, on the other, the duration of the process of establishment of thermal equilibrium in the gas flow between the heated heat-sensitive element and the wall of the chamber.

The closest to the invention by the combination of essential features is the detector for gas chromatography containing film sensing element included in the pavement measuring circuit and is made, for example, made of a metallized dielectric, parallel to that in the immediate vicinity installed heater connected to a source of electric current. The gas stream emerging from the column is directed into the space between the heat-sensitive element and the heater, and transfers heat generated by electric current in the heater to the heat-sensitive element. The distance between them is of the order of 0.05-1 mm, working length equal to 2 mm In this regard, the volume of gas, which transfers heat and mass metallized film, influencing the time of the establishment of thermal equilibrium, so small that practically no effect on the blur of the chromatographic strip and provide the opportunity and the use of this detector for capillary chromatography (see Buvalo CENTURIES, Berezin VG, Anuchin V.N., Stalnov P.I. // Zavodskaya laboratoriya, 1974. T. No. 10. S-1191).

The disadvantages of the known detector with a film heat-sensitive elements are relatively low sensitivity and accuracy of the measurement (detection) of the concentration of the analyzed substance at the outlet of the chromatographic column.

The objective of the invention is to increase the sensitivity and accuracy of detection.

This problem is solved due to the fact that a heat conductivity detector for gas chromatography, containing a heater connected to a source of electric current, a film heat-sensitive element mounted in the gas channel output separation column parallel to the heater in the immediate vicinity and are included in the bridge measuring circuit, and in parallel to the heater has a second film sensing element at the same distance from the heater, as the first, and the first and second temperature sensitive elements included in the opposite shoulders of the bridge measuring circuit, and a source of electric current of the heater includes an automatic supporting constant temperature and the resistance of the heater, made for example of metal threads.

When solving a task to create the raised effect, which is to improve the sensitivity and accuracy of measuring the concentration of the analyzed substance at the outlet of the chromatographic column.

Proposed a heat conductivity detector for gas chromatography is characterized by a new set of essential features that ensure the achievement of the technical result. So, installing parallel to the heater of the second thermosensitive element and include it along with the first in the opposite shoulders of the bridge measuring circuit allows to increase the sensitivity of the detector is approximately two times. In addition, the use of a source of electric current of the heater special automatic, supporting a constant resistance, and hence the temperature of the heater, as this eliminates the influence of the flow and composition of the sample gas at the temperature of the heater relative to which we measure the change of the temperature (resistance) of the film heat-sensitive elements is proportional to the concentration of an analyte in a gas.

The invention is illustrated by drawings.

Figure 1 shows the electrical circuit of a heat conductivity detector for gas chromatography. Figure 2 shows the gas circuit of the detector.

thermal conductivity detector includes a housing 1, two film termokos is a valid element 2, included in the opposite shoulders of the measuring bridge, which has connection terminals power supply 3 and the measuring input terminals 4, the heater 5, the power source of the heater with automatic temperature 6, two permanent resistance 7 for adjustment of the measuring bridge, instead of which can also be used sensing elements of the second comparative detector included in gas line clean eluent, insert 8 for supplying the auxiliary gas nozzle 9 with locking nut 10 for attaching a capillary column.

thermal conductivity detector works as follows: the eluate emerging from the capillary chromatographic column enters the gas space between the heater 5 and the two temperature-sensitive elements 2 and transfers heat from the heater to each of the heat-sensitive elements. As a result, the temperature sensitive element is increased and at the same time increasing their ohmic resistance.

It is known (see Brazhnikov CENTURIES Differential detectors for gas chromatography. M.: Nauka, 1974. P.76)that the signal detector thermal conductivity U0(voltage unbalance of the measuring bridge output terminals 4) or the sensitivity of the detector is determined by the geometrical characteristics of temperature-sensitive item, and camera detector (length, the diameter, the thickness of the metal film, and others), electrical parameters of the sensitive element and the measuring bridge (current or voltage of the bridge U at the terminals 3, the temperature and ohmic resistance of the sensing element, the temperature of the heater or the chamber walls of the detector), and thermal conductivity of the pure carrier gas (eluent) and a mixture of the carrier gas with the analyzed substance (eluate).

The measuring bridge is in equilibrium, when the voltage at the output terminals 4 is missing and U0=0. This condition is observed when the equality of the ratio of the ohmic resistance of the measuring bridge

whereand- resistance film heat-sensitive elements 2 that are included in the opposite shoulders of the measuring bridge; R3and R4- fixed resistance 7 which serve for the balance of the measuring bridge.

When changing the ohmic resistance of thermosensitive elements by the value of ΔR, the process results in the transfer of heat from the heater 3 in the gas environment eluent, balance measuring bridge is broken, resulting at the output terminals 4 there is a potential difference U0that can be calculated approximately by the equation:

where U is the supply voltage of the measuring bridge at terminals 3;- load resistance measuring bridge.

From equation (2) shows that the use of the design of the detector one thermosensitive element allows to increase the sensitivity of the detector is approximately two times.

Experimental verification of the sensitivity and accuracy of detection of known and proposed a heat conductivity detector was carried out on the example of the chromatography was carried out oxygen and nitrogen in the air on a capillary column with molecular sieves CAA length L=1500 cm and an inner diameter of dC=0,025 see the column Temperature TC=50°C. the Volume of injected sample crane dispenser VCR=0,125 cm3. The volumetric rate of the carrier gas of hydrogen at the inlet to column Pi=to 73.55 kPa. The division of the flow into the node input sample 1:64. As an auxiliary gas used hydrogen, which is fed into the gas cavity insert 8 with a flow rate of 12 cm3/min

The results of analysis of oxygen and nitrogen were calculated:

- the average value of the detector signal at the output of the measuring bridge, mV; n=10 is the number of consecutive analyses;

- standard deviation of a single measurement;

The results of the experiment are summarized in table Comparative data experimental verification of known and proposed detectors".

Table
Comparative data experimental verification of known and proposed detectors
№ p/pThe solutesKnownOffer
SxiΔXiSxiΔXi
1Oxygen128,7531,0222,17243,346,144,39
2Nitrogen142,6529,5121,09271,036,45br4.61

As can be seen from the table, proposed a heat conductivity detector for gas chromatography in the analysis of the same sample of air provides increased sensitivity of detection of approximately 1.9 times:

for oxygen

243,43/128,75=1,89,

for nitrogen

217,03/142,65=1,9.

In addition, the proposed detector has a smaller measurement error of the output signal. Thus, the bound of the confidence interval for oxygen decreased from 22,17 mV to 4,39 MB, and for nitrogen with 21,09 MB to 4.61 MB.

The proposed use of a heat conductivity detector for gas chromatography allows to increase the sensitivity of the detector when working with capillary chromatographic columns about two times and increase the accuracy of the measurement of the output signal due to temperature stabilization of the heater.

thermal conductivity detector for gas chromatography, containing a heater connected to a source of electric current, a film heat-sensitive element mounted in the gas channel output separation column parallel to the heater and is included in the pavement measuring circuit, characterized in that parallel to the heater has a second film sensing element at the same distance from the heater, as the first, and the first and second film heat-sensitive elements included in the opposite shoulders of the bridge measuring circuit, and a source of electric current of the heater includes an automatic supporting constant temperature and the resistance of the heater, the implementation of the military, for example, metallic thread.



 

Same patents:

The invention relates to gas chromatography and can be used for qualitative and quantitative analysis not identified components of complex mixtures of substances belonging to different classes of organic compounds

The invention relates to the field of analytical techniques, namely, devices for measuring gas flow in capillary gas chromatography

FIELD: gas chromatographic devices; chemical industry; oil industry; metallurgy; medicine; biology etc.

SUBSTANCE: detector can be used in gas chromatographic device as with filled and empty capillaries and micro-nozzle separating columns for analyzing complex mixtures of matters having as natural and artificial origin. Detector has heater made in form of metal thread, two film thermo-sensitive elements mounted in gas channel at output of chromatographic column in parallel to the heater at similar distance. Thermo-sensitive elements are connected into opposite arms of bridge circuit. Electric current source for the heater has automatic controller to keep temperature and resistance of the source to permanent value.

EFFECT: improved precision of detection; improved sensitivity.

2 dwg

FIELD: physics.

SUBSTANCE: thermochemical detector comprises operating and comparative sensitive elements in the form of platinum filaments, one of which - the operating sensitive element - is coated with a thin layer of an oxidation process catalyst. The operating and comparative platinum filaments are placed in one cylindrical housing in which two diametrically lying holes are drilled for a gas stream; one of the holes is intended for inserting a capillary which feeds eluate from a column opposite the electric lead with a welded middle point of series-connected platinum filaments, which are connected in a bridge measuring circuit. The eluate-feeding capillary is provided with two baffles arranged in parallel to the platinum filaments, and has a special cross-sectional shape for uniform distribution of the gas stream from the column to the platinum filaments, and the feed capillary is sealed in the cylindrical housing with an inorganic silicate adhesive.

EFFECT: high sensitivity and precision of detection.

1 dwg, 2 tbl

FIELD: fire safety.

SUBSTANCE: method for determining component concentration in a two-component gas mixture placed in a measuring chamber based on the use of the thermal conductivity of a controlled gas mixture, at first the mass m of the controlled component id calculated in the gas mixture according to the formula m=ρ vccm1cm2cm12)/λcm2, where ρis the density of the controlled component, vc is the volume of the chamber, λcm1 is the thermal conductivity of the first component, λcm2 is the thermal conductivity of the second controlled component, λcm12 is the thermal conductivity of the gas mixture. Then, taking into account the mass of one molecule of the second controlled component, the concentration of the required parameter is determined.

EFFECT: increased accuracy of measuring the concentration of a component in a two-component gas mixture.

1 dwg

Up!