WO2000062053A1

WO2000062053A1 - Method for cleaning the flash chamber of a gas chromatographic device and injector for the implementation of said method

Info

Publication number: WO2000062053A1
Application number: PCT/FR2000/000936
Authority: WO
Inventors: Pierre Witier
Original assignee: Pierre Witier
Priority date: 1999-04-13
Filing date: 2000-04-12
Publication date: 2000-10-19
Also published as: FR2792413B1; WO2000062053A8; FR2792413A1; EP1169637A1

Abstract

The invention relates to an improvement in gas chromatographic analysis, consisting in burning off the non-volatile products remaining in the injector after an analysis. The invention also relates to an advanced injection device comprising a combustion-flash chamber (7), wherein the sample that is to be analyzed is introduced with the aid of a syringe, and a lateral tube (8) that enables a combustion gas containing oxygen to be introduced through a pressure drop (35). The flash-combustion chamber is connected to the chromatography column (18). When the volatile constituents have been vaporized and analyzed on the chromatography column (18), the combustion gas is introduced in the tube (7) by opening a valve (32) when the valve (31) enabling the vector gas to arrive inside the injector is closed and a valve (5) is open. The injector becomes de-pressurized and the combustion gas flows through the injector, is brought to a sufficient temperature to enable combustion of the non-volatile residues and is then discharged, together with the combustion gas, via the valve (5).

Description

Process for cleaning the vaporization chamber of a gas chromatography apparatus and injector for carrying out this process

The present invention relates to a method for cleaning the vaporization chamber of a gas chromatography apparatus and to an injector for carrying out this method.

Gas chromatography is an analytical technique that allows the volatile constituents of a sample to be separated. To this end, the sample to be analyzed is introduced into a heated injector, where the constituents of the sample which are not already in gaseous form are vaporized. Then, pushed by an inert gas called carrier gas, the mixture of the constituents is introduced into a column filled with a stationary phase (generally a polymer), where interactions between the gas phase and the stationary phase make it possible to obtain, in column outlet, separation of the various constituents. The concentration of these in the carrier gas is then determined using a detector, which allows qualitative and quantitative analysis of the mixture.

In order to obtain quantitative results, it is necessary that the various stages of the process described above proceed satisfactorily. The first step in this process is the introduction and then the vaporization of the constituents to be analyzed in an injector, which must be complete. The injectors commonly used for this purpose include a vaporization chamber heated to an adjustable temperature, which may vary over time. This room has 2 openings. One of them is closed by an elastomer membrane (called a septum), which can be pierced by the needle of the syringe containing the sample. The other allows connection to the chromatographic column. The injector also includes a carrier gas inlet, in order to transfer the vaporized constituents of the sample to the column and to the detector. If the stage of vaporization of the volatile constituents of the sample to be analyzed does not generally pose problems when the latter contains only volatile constituents, it is not the same if non-volatile constituents, therefore non-vaporizable , are here.

The presence of non-volatile constituents in a sample, some volatile constituents of which must be analyzed by gas chromatography creates many difficulties due to the accumulation of these constituents in the injector.

It is thus often observed that the areas of the peaks of the products leaving the analysis column decrease from one injection to another, and this all the more so since the products to be analyzed are less volatile. This is due to the fact that the injected products stick to the non-volatile products accumulated in the injector and must therefore diffuse in this new stationary phase to reach the separation column. This process is slow and not necessarily quantitative.

A second drawback of non-volatile products is that they can decompose giving volatile constituents which interfere with the products to be separated or else give rise to chemical reactions with the constituents of the sample at the high temperature of the injector. . This occurs in particular with samples of biological origin.

The practical consequences of the presence of non-volatile constituents in the samples are numerous. It is often necessary to disassemble the injector periodically to clean it, for example with a solvent. This causes loss of time and prevents the automation of analyzes. In some cases, it is even necessary to clean the injector after each injection. In order to avoid the introduction of non-volatile constituents into the injector, it may possibly be necessary to abandon conventional systems for direct injection of the sample using a syringe and to use more complex methods. and more expensive, such as the headspace analysis method, ie carrying out various preliminary treatments of the sample. It is therefore desirable to automate the cleaning of the injector, while maintaining an injector configuration close to that of a conventional syringe injector, in order to avoid overcomplicating the injection device.

Certain solutions proposed previously do not make it possible to keep a conventional syringe injector, either because they use a modified glass needle injector to pyrolyze the non-volatile constituents, or because they eliminate these by dissolution in a solvent, which considerably complicates configuration.

The present invention makes it possible to solve the problems posed by the introduction of samples containing non-volatile constituents in gas chromatography (GC) in a particularly simple and efficient way which consists in burning these constituents.

Thus, the present invention relates to an improvement in methods of analysis by GC. It also relates to an injector adapted to the implementation of this process.

According to a first advantage, the invention makes it possible to solve in a particularly effective and simple manner all of the problems exposed above and this with relatively minor modifications of the existing devices. This improvement notably allows a considerable saving of time since it allows in particular to avoid dismantling the injector to rid it of the undesirable products which accumulate there.

According to another advantage, the method of the invention is easily adaptable to all types of injectors where the injection is not made directly into the column.

According to another advantage, the invention makes it possible to improve the quality of the analyzes while avoiding all the drawbacks resulting from the possible interference of the non-volatilized products during the preceding analyzes.

According to another advantage, the invention provides, by virtue of its simplicity, a means which can be easily automated to solve the problems described above and linked to the deposition of non-volatile products in the injection chamber.

The invention applies to the analysis by gas chromatography of all the mixtures liable to contain non-volatile products at the temperature of the injection chamber and very particularly:

- to the analysis of petroleum products containing heavy fractions (bitumens or others),

- the determination of residual monomers or additives in polymers,

- the dosage of pesticides or other pollutants in the environment. Thus, according to one of its essential characteristics, the invention relates to an improvement to the methods of analysis by gas chromatography of samples containing non-volatile products at the temperature imposed in the vaporization chamber during the analysis of said samples, intended to clean the vaporization chamber of the chromatograph after one or more analyzes, characterized in that it comprises a stage of cornbustion of the non-volatilized fraction of the samples introduced into said vaporization chamber during said analysis or analyzes.

According to a particularly advantageous variant which makes it possible to prevent the combustion products from polluting the chromatography column, the combustion is carried out by introducing the oxidizing gas against the current with respect to the direction of injection of the sample.

The gas used for combustion is a gas containing oxygen. Except in exceptional cases, air will generally be chosen to carry out combustion. However, it is not excluded to use other gases, in particular air enriched with oxygen, or even pure oxygen, which could however only be achieved by adapting the nature of the constituent materials. of the entire injection device, so as to avoid damaging them during this combustion.

The combustion step is advantageously preceded by a step of depressurizing the vaporization chamber so as to avoid sending the oxidant gas and / or the combustion products to the chromatography column.

It is indeed well known that most of the columns are sensitive to the effect of oxidizing products. This is why it is particularly recommended in the process of the invention to avoid sending the combustion gas (oxidizer) as well as the combustion products to the column. Furthermore, when the combustion step is completed, a scanning of the vaporization chamber is carried out against the current.

This step is carried out under the action of an inert gas, preferably under the action of the gas used as carrier gas during the analysis of the samples.

The invention relates, according to one of its variants, to a method as defined above and which is implemented in an injection device comprising a vaporization chamber, connected upstream to a device through which the samples are injected to be analyzed and downstream to a chromatography column, said chamber being provided with a heating and / or cooling device making it possible to adjust its temperature during the vaporization of the sample as well as during the combustion step, said device being provided with a supply of carrier gas and means for circulating this gas towards the downstream part of said device and with a supply of combustion gas as well as means for circulating said combustion gas against the flow of said carrier gas to the upstream part of said device. This device advantageously further comprises means for depressurizing the vaporization chamber as well as means for introducing a current of inert gas in its downstream part and for evacuating this inert gas in its upstream part. To implement the method described above in its main lines, the improved injection device used, also called hereinafter injection-combustion device, advantageously comprises an injector with a vaporization chamber, also called below. sample vaporization-combustion chamber, temperature adjustable by means of heating and cooling devices, a septum or similar device allowing the introduction of the sample into the vaporization chamber using a syringe, whether or not part of an automatic injection system, a septum purge and a carrier gas inlet to allow the transfer of volatile constituents from the sample to the chromatographic column. At the outlet of the vaporization chamber, a first tube arranged laterally allows, as desired, to introduce either a combustion gas, which may for example be air, oxygen or a mixture of oxygen and inert gas, or carrier gas.

After the volatile constituents of the sample to be analyzed have been transferred to the chromatographic column and have been analyzed, the combustion gas can be introduced through this side tube in order to burn the non-volatile constituents which have deposited in the injector.

In order to avoid introducing combustion gas onto the column, which would damage it, the injector is depressurized beforehand by opening a valve located near the inlet of the injector, so that the combustion sweeps the vaporization-combustion chamber against the current.

The temperature of the vaporization-combustion chamber is then brought to a value allowing the combustion of the non-volatile constituents of the sample. The products of combustion are evacuated by the valve located in the upstream part of the injector. This valve is advantageously protected from fouling by a filter. When the combustion is complete, the introduction of the combustion gas is stopped and the combustion gas is removed from the injector by sweeping with the carrier gas or an inert gas. The temperature of the combustion-vaporization chamber is then readjusted for the next injection and the initial conditions of vector gas flow restored.

To avoid diffusion of the combustion gas towards the column, the circulation is maintained against the current with a small flow of carrier gas introduced by a second lateral tube located downstream from the point of arrival of the combustion gas.

In order to introduce only a part of the volatile constituents of the sample onto the chromatographic column, according to the so-called split injection technique, a flow control valve, in series with a stop valve and a filter , is placed at the bottom of the injector. These valves allow you to adjust the amount of sample transferred to the column.

The gas flows introduced at the various points of the device can be optionally regulated by means of fixed or variable pressure losses, associated with pressure regulators, or else by flow regulators, in particular electronic flow regulators. The gas flows evacuated by the septum purge or the valve located in the upper part of the injector can be regulated by fixed or variable pressure losses.

According to another of its essential characteristics, the invention also relates to a device for injecting samples, with a view to their analysis by gas chromatography on a column connected downstream of this device. Such a device comprises a vaporization chamber provided in its upstream part with a septum-type device through which the sample is introduced and with a supply of carrier gas and further comprises means for injecting an oxidizing gas into the chamber vaporization and means for depressurizing said chamber so as to circulate said oxidizing gas against the current in said chamber.

This device advantageously comprises means for injecting carrier gas against the current in the vaporization chamber. According to an advantageous variant, the device of the invention further comprises means making it possible to inject only a fraction of the vaporized part of the sample onto the chromatographic column, so as to perform an analysis of samples in division mode.

A device particularly suitable for implementing the method of the invention is described below in more detail, by way of illustration and without this being able to make any limitation whatsoever to the scope of the invention. This device is shown in FIG. 1.

The device shown in Figure 1 corresponds to a vertical injection system which is the most commonly used type of injector currently. It is however very obvious that the skilled person will have no difficulty in transposing the following in the case of a horizontal injection system. We will then speak, of course, of input or upstream part of the device (and not of the upper part) and of output or downstream part of the device (and not of the lower part).

The device, represented in FIG. 1, comprises the following elements: - a radiator nut 1 making it possible to maintain an elastomer septum 2, through which the sample can be introduced using a syringe (not shown) ;

- a septum purge 3 and a valve 4 for adjusting the septum purge flow;

- a stop valve 5, allowing the depressurization of the injector in the open position and the evacuation of the combustion gas;

- a filter 6 which prevents fouling of the valves 4 and 5;

- A tubular part 7, where will be carried out the vaporization of volatile products from the sample then the combustion of non-volatile products, called vaporization-combustion chamber, inserted in a cylindrical part 9, threaded at the bottom, provided with means 10 allowing to heat or cool the room and to quickly vary its temperature. These means are means conventionally used for heating or cooling, such as a metal part surrounding the part 9, in good thermal contact with it. On this metal part are fixed electrical resistances, a metallic coil, in which one can make circulate a cooling fluid (air or other), and a temperature probe. The electrical supply to the resistors and / or the coolant flow are adjusted by a regulator to obtain the desired temperature.

- a device for fixing the part 7 to the part 9 by a nut 11 and a ferrule 12;

- A side inlet 8 on the tube 7, which is connected to the gas supplies by a part 13 provided with 2 nuts 14 and 15 and 2 ferrules 16 and 17;

a connection of the part 7 to the chromatographic column 18 by means of a part 19, provided with 2 nuts 20 and 21 and with 2 ferrules 22 and 23. The part 19 is connected to the supply of carrier gas to through a loss load 24 and is also provided, in series, with a filter 25, an adjustment valve 26 and a stop valve 27, in order to be able to perform injections in division mode;

- pressure regulators 28 and 29, respectively for the carrier gas and the combustion gas;

- a pressure gauge 30 for controlling the pressure in the vaporization-combustion device;

a stop valve 31 for controlling the arrival of the carrier gas in the upper part of the injector; a stop valve 32 for controlling the arrival of the combustion gas;

- pressure drops 33 to 36, the pressure drop 35 regulating the rate of introduction of the oxidizing gas.

This injection system allows, as is conventional to do in gas chromatography, the introduction onto the column of all or part of the volatile constituents of the sample introduced into the injector. It is thus possible, by opening the valve 27 and adjusting the valve 26, to vary the quantity of sample which reaches the column.

The particular device described above makes it possible to obtain the advantages of the invention with a simple and economical configuration. It is easy to design a more efficient system and an operation that is easier to optimize by replacing the fixed pressure drops in FIG. 1 with electronically controlled flow regulators (combining an electronic flow measurement and a flow control by piloted valve), which makes it possible to impose the flow rates at the points of the system where these must be controlled. An example of application is given below in which all of the volatile constituents of the sample are transferred to the column. In this case, the valve 27 is permanently closed.

The operation, in the different stages of the analysis, can be schematized as follows: In a first stage, the valves 5 and 32 are closed and the valve 31 is open. The sample is introduced using a syringe inside the injector, into part 7, by penetration of the septum. During this time, the body of the injector is maintained at a temperature T1 sufficient to obtain complete vaporization of the volatile constituents of the sample. In a variant of the invention, this temperature T1 can vary from a temperature below a higher temperature, in order to avoid certain disadvantages associated with too brutal vaporization of the sample. The separation of the constituents on the chromatography column is then normally carried out with an inlet pressure of the carrier gas maintained with the pressure regulator 28. During this vaporization step, the non-volatile constituents of the sample are deposited in the 'injector.

In a second step, which constitutes the improvement of the invention, the non-volatile constituents are removed from the sample by combustion, avoiding the introduction of the combustion gas onto the chromatographic column. To this end, the valve 31 is closed and the valves 32 and 5 are opened. The oxidizing gas enters the injector through the lateral tube 8 through the pressure drop 35 and, due to the depressurization of the injector caused by the opening of valve 5 and the closing of valve 31, sweeps the injector against the current and is then evacuated by valve 5. During this operation of back-sweeping the injector by the oxidizing gas, the temperature of the injector is brought to a temperature allowing complete combustion of the products to be eliminated. With air as oxidizer, this temperature is generally between 350 and 450 ° C. Pure oxygen is more efficient but places severe constraints on the materials that make up the injector. In order to avoid damaging the column, which generally contains an oxidizable and combustible stationary phase, it is necessary to avoid any introduction of oxidizing gas thereon. To this end, carrier gas is introduced at the bottom of the injector, through the pressure drop 24. Thus, a permanent flow of carrier gas flows from the bottom of the injector upwards and prevents the diffusion of the oxidizing gas. towards the injector provided that the linear speed of the carrier gas is sufficient. The flow rate of this shielding gas must not however be excessive in order not to dilute the oxidizing gas too much with an inert gas, which would reduce the efficiency of the combustion process.

Likewise, in order to prevent oxidizing gas situated between the valve 32 and the injector from diffusing therein during the analysis, a permanent flow of carrier gas is maintained by scanning this line of gas, by playing on pressure drops 34 and 36.

When the combustion of the non-volatile products present in the injector is finished, the valve 32 is closed and then a sufficient time is waited for all the oxidizing gas contained in the injector to be evacuated by the valve 5. Then the valve is closed the valve 5 then the valve 31 is opened in order to restore the pressure in the injector.

The temperature of the injector is also adjusted for the next injection.

The operation of the injector is described above in the mode of total transfer of the volatile constituents from the sample to the column. If it is desired to transfer only part of the volatile constituents of the sample to the column, it will suffice to open the valves 26 and 27 during the period of partial transfer of the volatile constituents of the sample to the column and then to close valve 27 before introducing combustion gas. This valve will then be reopened for the next analysis.

Claims

1. Improvement of the methods of analysis by gas chromatography of samples containing non-volatile products at the temperature imposed in the vaporization chamber during the analysis of said samples, intended to clean the injection chamber of the chromatograph after a or more analyzes, characterized in that it comprises a step of combustion of the non-volatilized fraction of the samples introduced into said vaporization chamber during said one or more analyzes.

2. Method according to claim 1, characterized in that said combustion is carried out under the effect of an oxidizing gas introduced against the current with respect to the direction of injection of said samples.

3. Method according to claim 1 or 2, characterized in that said combustion is carried out in the presence of air.

4. Method according to claim 3, characterized in that said combustion is carried out at a temperature between 350 ° C and 450 ° C.

5. Method according to one of claims 2 to 4, characterized in that the step of injecting the oxidizing gas is preceded by a step of depressurizing the vaporization chamber, so as to avoid sending the oxidizing gas and / or the combustion products to the chromatography column.

6. Method according to one of claims 1 to 5, characterized in that it further comprises a step of scanning the vaporization chamber against the current by an inert gas, preferably by the gas used as carrier gas, at the end of the combustion stage.

7. Method according to one of claims 1 to 6, characterized in that the combustion is carried out in the presence of a backflush by means of an inert gas.

8. Method according to one of claims 1 to 7, characterized in that it is implemented in an injection device comprising a vaporization chamber (7), connected upstream to a device (2) through which the samples to be analyzed are injected and downstream to a chromatography column (18), said chamber being provided with heating and cooling means (10) allowing its temperature to be adjusted during the vaporization of the sample as well as during the step combustion device, said device being provided with a supply (28) of carrier gas and means for circulating this gas towards the downstream part of said device and with a supply (29) with combustion gas as well as means to circulate said combustion gas against the flow of said carrier gas to the upstream part of said device.

9. Method according to claim 8, characterized in that said device further comprises means for depressurizing said vaporization chamber (5).

10. Method according to one of claims 8 or 9, characterized in that a stream of inert gas is introduced into the downstream part of said device and it is ensured by the upstream part of said device.

11. Device for injecting samples, for their analysis by gas chromatography on a column (18) connected downstream of this device, comprising a vaporization chamber (7) provided in its upstream part with a device septum type, through which the sample is introduced, of a carrier gas supply (28), characterized in that it further comprises means for injecting an oxidizing gas into the vaporization chamber (29, 32, 35 , 8) and means for depressurizing said chamber (5) so as to circulate said oxidizing gas against the current in said chamber.

12. Device according to claim 11, characterized in that it comprises means for injecting carrier gas against the current in the vaporization chamber (28, 31, 24).

13. Device according to claim 11 or 12, characterized in that it comprises means (26, 27) making it possible to inject only a fraction of the vaporized part of the sample onto the chromatographic column, so as to perform analysis of samples in division mode.