EP1337843A2 - Device for coupling a micro-chromatograph with a mass spectrometer and analysis device - Google Patents

Abstract

The invention concerns a coupling device connecting a micro-chromatograph ( mu CG) output to a mass spectrometer input, said device comprising a capillary tube whereof one end is sealingly connected to the mass spectrometer vacuum source via an interface device, and whereof the other end is loosely connected to the micro-chromatograph output open to the atmosphere; the length and the diameter of the capillary tube being selected so that the flow rate inside the capillary tube is very close to the flow rate at the micro-chromatograph output; the interface device being further provided with heating means for adjusting very accurately the flow rate tapped by the capillary tube.

Description

 DEVICE FOR COUPLING A MICROCHROMATOGRAPH WITH A MASS SPECTROMETER AND ANALYSIS DEVICE

DESCRIPTION

The invention relates to a device for coupling a microchromatograph with a mass spectrometer.

. The invention also relates to an analysis device comprising a microchromatograph and a mass spectrometer connected by means of this coupling device.

The combination of a gas chromatograph (GC) with a mass spectrometer (MS), possibly associated with a computer is one of the most powerful tools used in analytical chemistry and finds its application in particular in the analysis of gas.

This technique has known significant progress, in particular thanks to the development of capillary columns in fused silica of small diameter and flow and the appearance of new mass spectrometer with a high pumping capacity.

Thus, the microchromatograph (μCG) is a device which makes it possible to perform high resolution analysis of complex mixtures quickly, for example in less than three minutes, and efficient.

The detector is a non-destructive microcatharometer, which explains the interest in coupling the microchromatograph (μCG) to a mass spectrometer which adds the possibility of identification certain of each compound separated by the chromatographic column.

An important problem encountered in devices combining a chromatograph and a spectrometer is the choice of the coupling system, realizing the interface between the two devices.

The coupling problem is posed differently depending on the chromatographic device used and. the pumping capacity of the spectrometer. The previous generation of conventional catharometers required the use of filled columns, of large diameter, for example of several millimeters, and of high carrier gas flow rate, for example of a few tens of milliliters / minute. Coupling with a mass spectrometer then required a flow separator and differential pumping, the control of which was always difficult [1].

The catharometer microdetector

(microcatharometer) coupled to capillary columns of low flow (1 to 2 ml / min.) is now perfectly compatible with the pumping capacities of a current mass spectrometer.

Direct online coupling without separator is therefore possible, provided, however, that a permanent pressure drop of 1 bar is maintained between the microdetector and the source of the mass spectrometer: this is the function provided by a tube specially dimensioned capillary.

Assuming this condition is met, it is practically possible to make the connection to the microdetector in two ways: either sealed or open.

The tight coupling is described in FIG. 1. In this coupling, the flux coming from the detector (1) arrives directly in the source (2) via the capillary tube (3) and the interface (4).

This coupling presents the risk of disturbing the operation of the microcatharometer detector, at the mercy of a variation in the pumping capacities of the spectrometer or a modification of the analysis conditions, namely essentially the head pressure and / or the column temperature. .

In other words, the tight coupling has the advantage of an efficiency of 100%, but to the detriment of the optimal operation either of the microcatharometer, which can be in depression, or of the spectrometer, the vacuum of which becomes defective by saturation of the pumping capacities. On the other hand, the open connection or coupling, which is described in FIG. 2, has the advantage of preserving the optimal operating conditions of the two detectors, and the retention times are identical to those obtained in conventional chromatography [2]. This assembly generally works on the following principle: the pressure drop of the transfer line is imposed, it is 1 bar, the diameter and the length of the capillary (3) are chosen so that a near carrier gas flow of the maximum tolerable by the spectrometer (source 2) crosses the transfer line, and this flow is as close as possible to the flow leaving the microdetector [3].

The authors recommend adding additional helium (5) to compensate for a drop in flow, this helium protecting the mass spectrometer from an air inlet. This solution has the advantage of preserving both the mass spectrometer and the catharometer, but it induces a dilution of the solute flow (6). which can be harmful in all cases where the search for traces is the objective.

None of the coupling devices, currently known, allows satisfactory coupling between the output of the microchromatograph, that is to say the catharometer and the source of the mass spectrometer. Such a coupling device must provide several functions and meet several requirements which include the following:

- ensure that the catharometer detector operates at atmospheric pressure, whatever the requirements of the analysis relating in particular to the pressure at the top of the column and to the column temperature.

This operation guarantees the optimal sensitivity of the detector and the linearity of the response, as a function of the concentration of the species;

- take the entire flow leaving the microdetector and therefore benefit from the maximum sensitivity at the level of the detection of the mass spectrometer; - keep the separation of the species already detected, and transfer them to the source of the spectrometer, under secondary vacuum;

- This coupling device must be able to be connected indifferently to any of the modules, for example four in number, which can equip the microchromatograph. Going from one microdetector to another must be very simple and rapid, without disturbing the operation of the spectrometer.

The object of the present invention is, inter alia, to provide a coupling device which does not have the drawbacks, limitations, defects and disadvantages of the prior art and which solves the problems of the coupling devices of the prior art.

The object of the present invention is also to provide a coupling device which satisfies, inter alia, the criteria and requirements defined above for such a device.

This object and others are achieved, according to the invention, by a coupling device connecting the output of a microchromatograph (μCG) to the input of a mass spectrometer (SM), said coupling device comprising a capillary tube, one end of which is tightly connected to the vacuum source of the mass spectrometer via an interface device, and the other end of which is connected to the outlet of the microchromatograph, loose manner, open to the atmosphere; the length and diameter of the tube capillary being chosen so that the flow rate inside the capillary tube is very close to the flow rate at the outlet of the microchromatograph; the interface device being, moreover, provided with heating means for very precisely adjusting the flow rate taken by the capillary.

The coupling device according to the invention, provided with a specific, heating interface, ensures the transfer of solutes leaving the microdetector at atmospheric pressure to the source of the spectrometer operating under secondary vacuum.

The coupling device according to the invention provides a solution to the problems posed by the coupling devices of the prior art and meets the criteria and requirements mentioned above.

The coupling according to the invention can be defined as an “open” coupling which makes it possible, in particular, to preserve both the mass spectrometer and the catharometer, while preserving the flux of solute in its entirety, without any dilution.

For the first time, according to the invention, there is a coupling device connecting two detectors whose principle of operation differs fundamentally, working on a common sample taking and whose detection limits are entirely comparable.

In other words, the device according to the invention fully retains the separation obtained in the microchromatograph, and transfers it without altering it or diluting it in the mass spectrometer. During the chromatographic analysis, the flow rate in the column may change due to temperature variations.

According to the invention, the heating of the interface makes it possible to regulate the temperature to ensure the continuity of the flow rate, which is absolutely impossible with the coupling devices of the prior art.

. According to the invention, the flow rate sampled by the capillary tube is very easily adjusted by simple modification of the temperature of the interface.

The temperature of the interface is advantageously adjusted so that the flow taken by the capillary is very close to or equal to the flow at the outlet of the microchromatograph.

Advantageously, the relative intensity of the peaks is compared, on the one hand, with water and, on the other hand, with nitrogen and / or oxygen present in the source of the mass spectrometer and we act by consequence on the heating means of the interface to increase or decrease the temperature of the latter and respectively decrease or increase the flow rate sampled by the capillary tube.

The heating means with which the interface device is provided, according to the invention, make it possible to vary this temperature over a wide range, generally from ambient to 200 ° C.

This temperature depends on the capillary used (diameter and length). Within the above temperature range, for each capillary tube diameter, there is a range of temperatures preferred - for example, 50 to 60 ° C - within the temperature range, above, for which the flow rate taken by the capillary is strictly equal to the flow rate at the outlet of the chromatographic column. According to the invention, the interface temperature can be optimized for each chromatographic column temperature and an adequate operating range for the analysis by coupling can be determined in all circumstances. The invention also relates to an analysis device comprising a microchromatograph and a mass spectrometer, the output of the microchromatograph being connected to the input of the mass spectrometer by the coupling device, as described above.

Such an analysis device has all the advantages linked to the implementation of the coupling device of the invention and, for the first time, it combines two detectors whose principle of operation differs fundamentally, working with a sample taking common and whose detection limits are entirely comparable.

The analysis device according to the invention allows, with a single injection, to carry out two analyzes.

The analysis device according to the invention allows quantitative and qualitative analysis of unknown mixtures without necessarily having to have the corresponding standard mixtures. Advantageously, the analysis device of the invention can be provided upstream of the microchromatograph with a preconcentration or concentration, restitution device, making it possible to accumulate the traces of compounds to be analyzed in a fluid.

This preconcentration device is based on adsorption followed by thermoresorption.

The analysis device according to the invention is advantageously a transportable device, in particular when it is provided with the preconcentration system mentioned above, which makes it possible to conduct analyzes relating to traces.

The device according to the invention finds its application in all the fields where trace analysis proves to be essential or not: environment, refineries, storage and distribution of natural gas, confined atmosphere, reactor sky, etc.

The invention will be better understood on reading the description which follows, given by way of illustration and not limitation, with reference to the accompanying drawings, in which: FIG. 1 is a schematic sectional view illustrating a coupling device in line, without separator, of the type with tight coupling, of the prior art; Figure 2 is a schematic sectional view illustrating an in-line coupling device, without separator, of the open coupling type, of the prior art; - Figure 3 is a top view, schematic of a microchromatograph capable of being connected to a mass spectrometer by the coupling device of the invention; - Figure 4 is a schematic sectional view of the coupling device according to the invention; Figure 5 is a schematic sectional view of the mounting device for optimizing the diameter and length of the capillary tube of the coupling device according

The invention;

- Figures 6A and 6B represent the background noise in real time of the mass spectrometer, for a temperature of the analytical column of 130 ° C and interface temperatures of 50 ° C and 60 ° C respectively; on the ordinate, the relative abundance in% is plotted and on the abscissa the ratio m / z;

- Figures 7A, 7B and 7C respectively represent the chromatogram, the chromatogram built on the total ion current (CIT) and the mass spectrum obtained during the search for traces of ethylene oxide in surgical bags; FIG. 7D represents the mass spectrum, reference of ethylene oxide, stored in the spectrum library;

- Figures 8A, 8B and 8C illustrate the method, called "deconvolution" and respectively represent the chromatogram, the chromatogram built on the current of ions m / z = 69 and 119 (C ₂ F ₆ ) and the chromatogram built on the ion current m / z = 28 and 32 (0 ₂ + N ₂ ); - Figures 9A, 9B, 9C and 9D illustrate the identification of unknown products present in a trace mixture.

- Figure 9A is a chromatogram (abscissa time and seconds) of the unknown mixture; FIG. 9B is the chromatogram constructed on the total ion current of this same mixture, FIGS. 9C and 9D are respectively the mass spectra of the two unknown products detected in the mixture, identified by comparison with the mass spectra stored in the library. spectra.

The coupling device according to the invention connects the output of a microchromatograph (μCG) to the input of a mass spectrometer (SM), in particular a quadrupole mass spectrometer.

Thus, this coupling combines two detectors based on completely different analytical principles and operating from the same sample. The coupling, carried out and successfully tested, consists of three distinct elements, each having a specific function during the analysis: the microchromatograph allows the separation of the different constituents of the gas mixture. Each compound is detected by the microcatharometer, which leads to the emission of a first chromatogram;

- the quadrupole mass spectrometer makes it possible to obtain a second chromatogram based on the variation of the total current of ions, as well as the spectrum mass of each of the compounds, this spectrum allowing identification; the specific interface of the invention ensures the transfer of solutes leaving the microdetector at atmospheric pressure to the source of the spectrometer operating under secondary vacuum.

In the following, we will successively describe each of the three elements.

The first element is therefore a microchromatograph; is described hereafter a particular microchromatograph, such as that marketed by the Company SRA INSTRUMENTS ^®, whose modulus is shown in Figure 3, but it is obvious that the device according to the invention enables coupling of all μCG with any SM.

The microchromatograph is a gas analyzer that is both fast and efficient. The heart of the system is the analytical module which consists of an automatic injection valve, the analytical column equipped with a heating device and the detector which is thermally conductive, also called "microcatharometer".

The micro-CG is generally equipped with two to four chromatographic modules (31), each of these modules being in itself a chromatograph having its detector and being able to operate with its carrier gas. It is possible to adjust the head pressure and the column temperature (isotherm), the analysis time and, finally, the volume injected. A membrane micropump, common, for example, to the two modules, is capable of sucking a sample as long as its pressure is at least close to, for example, 600 mbar (absolute pressure) and withstands a suction pressure ranging, for example, up to 4 bar. The automatic injector (33), specific to each module, receives the samples (at 32) and makes it possible to introduce programmable volumes of samples comprised, for example, between 0.33 and 15 μL in the columns. Associated with capillary or microcapillary columns, the assembly allows very short analysis times, for example from 30 to 160 seconds.

Thus in FIG. 3, the chromatographic module (31) represented includes an inlet (32), an injector (33), connected to an analytical column and to a reference column (34, 35) and, finally, a microcatharometer detector (36) connected to the output (37) of the module.

The catharometer (or microcatharometer) discriminates gases based on their thermal conductivity. The principle consists in comparing the thermal conductivity of the pure carrier gas in the reference column with that of the solute / carrier gas mixtures which leave the analytical column at a given time t. The measurement principle of the catharometer (or microcatharometer) is based on the Wheatstone bridge.

One of the major characteristics of this type of detector is the linearity of the response as a function of the concentration. The interest of the microdetector is its sensitivity associated with a response time, for example of only 10 milliseconds: most compounds are detected in a concentration range from ppm to 100%.

The “response” of any compound, detected using a catharometer, is proportional to the difference in thermal conductivity that this compound has with the carrier gas. It is therefore possible to carry out an estimate of the concentration of this compound in a mixture, from the surface of the peak which corresponds to it, provided however that the compound in question has been identified, since knowledge of its thermal conductivity is essential for calculation. Thus, even in the absence of a standard mixture, it is possible to conduct a first estimate of the concentrations which remains very acceptable.

The detection, by means of the catharometer being non-destructive, the coupling with a mass spectrometer opens the possibilities of quantitative analysis relating to unknown mixtures, without necessarily having the corresponding standard mixtures available.

According to the invention, the coupling device is connected, on the other hand, to a mass spectrometer. The mass spectrometer used according to the invention generally consists of an electronic impact source, a quadrupole analyzer, an electron multiplier detector and the pumping system making it possible to obtain a secondary vacuum. The mass spectrometer can be considered, in the case of coupling with the microchromatograph, like a detector whose purpose is to continuously analyze the composition of the eluate leaving the microcatharometer.

The mass spectrometer used according to the invention allows rapid scanning of the mass domain, for example 5,200 uma / second, in order to maintain the initial resolution obtained by chromatography.

The spectrometer indistinctly records mass spectra at a predetermined rate. Each spectrum has a series of mass peaks whose intensities are automatically added together and this sum is called "total ion current (CIT)". The mass spectrometer thus makes it possible to obtain a second chromatogram based on the variation of the intensity of the CIT.

In Figure 4 is shown the coupling device according to the invention. This coupling device comprises a capillary tube (41), the length and internal diameter of which are chosen such that the flow rate which circulates inside the tube is substantially equal, or is as close as possible to the flow rate (42) at the output of the microchromatograph (43).

By way of example, the internal diameter of the capillary tube will be, for example 0.15 mm, and, in this case, the length of the capillary tube will be close to 1.20 m. The capillary tube can be made of any material suitable for this use. Preferably, the capillary tube is made of deactivated fused silica.

One end of the capillary tube is tightly connected, by means of an interface device (44) to the source (45) under secondary vacuum (10 ^{~ 5} - 10 ^"6 mbar) of the spectrometer.

The interface device is known and already fitted to many spectrometers on the market: the function of this interface device is to conduct the capillary tube to the source of the spectrometer.

Sealing with respect to the atmosphere is ensured, for example, by means of a ferrule which may be of variable composition (for example, graphite, wespel-graphite, etc.).

According to the invention, the interface device is provided with heating means (46) making it possible to adjust and adjust the temperature of the interface and therefore of the capillary tube.

These means can be easily determined by a person skilled in the art, they can comprise, for example, a heating resistor (46). These means of heating the interface make it possible to adjust, to regulate, the flow rate sampled by the capillary tube, so that it is as close as possible, but by default the flow rate at the outlet of the chromatograph: the manner in which is adjusted this flow rate, by means of the interface heating, is described in detail below. The other end (47) of the capillary tube is loosely connected, open to the atmosphere, at the outlet of the microdetector (48), i.e. this end of the capillary tube is brought as close as possible the output of the microdetector (48), but with a loose connection, open to the atmosphere.

By closer, we mean that the end of the capillary tube is at a distance of a few millimeters from the microdetector.

In FIG. 4, it can be seen that on the side of the microchromatograph, the capillary tube penetrates over a certain distance, for example 5 cm, into a tube (49) of metal, for example of stainless steel, with which it is surrounded.

This metal tube with an internal diameter obviously larger than the external diameter of the capillary tube, for example greater than 1/10 to 2/10 mm in diameter than the external diameter of the capillary, is connected to the output of the detector of the microchromatograph. The gas flow of solute, leaving the detector of the microchromatograph (microcatharometer) at atmospheric pressure, is represented by the arrow (42) in the figure. It is therefore seen that an annular space (410) exists between the capillary tube (41, 47) and the metal tube (49), space through which atmospheric air can penetrate. This is why we are talking about a “loose” link open to the atmosphere. According to the invention, the length and the diameter of the capillary tube are chosen in such a way that the flow circulating inside the tube, fixed by the pressure drop, is substantially equal to or approaches as closely as possible the flow leaving the column of the microchromatograph. In order to optimize the diameter of the capillary tube and its length, the inventors have produced a specific assembly (cf. FIG. 5) in which the conditions for circulation of the gas flow inside the capillary are reproduced: the two ends of the capillary tube are tightly connected, one of these ends (51) enters the source of the mass spectrometer (52), and the other end of the tube is connected to the line (54) through which a flow of helium flows at atmospheric pressure (arrow 55), via a nozzle (56).

In the source of the spectrometer, there is a secondary vacuum, due to the action of the secondary (57) and primary (58) pumps.

The helium flow in the line, upstream of the tapping, is constant: for example, about 5 L / min. While the capillary sucks a certain amount of the flow, the flow (59) downstream of the tapping at the outlet of line (510) is measured, for example by a bubble flow meter (511). This measurement makes it possible, by difference, to deduct the flow taken by the capillary.

Different tube diameters, for example in deactivated fused silica, have been tested, for example, 0.10 mm, 0.15 and finally 0.25 mm in diameter. The suction flow is measured after each successive reduction in the length of the capillary, until a value similar to the flow rate measured at the output of the microdetector is obtained, ie approximately 1.5 mL / min.

By way of example, it has appeared that the best compromise is found in the use of the intermediate diameter of 0.15 mm. Indeed, a length of 120 cm is then necessary to obtain a flow rate close to the optimum. This length makes it possible to maintain the normal operating conditions of the microcatharometer, namely atmospheric pressure, while keeping a "practical" distance between the two devices.

The use of a smaller tube diameter can thus be envisaged. It would reduce the length of the capillary, which goes in the direction of preserving the quality of the separation obtained by the chromatographic column.

It is also essential, according to the invention, that the flow rate taken at the outlet of the microchromatograph is optimized.

The role of the coupling device and the interface is to create the necessary pressure drop between atmospheric pressure and the secondary vacuum in the source of the spectrometer, while maintaining the separation of the species already detected by the microcatharometer. This pressure difference assumes that the flow passing through the capillary is viscous, until the entry of the source and then becomes molecular. A viscous flow is therefore to be considered for the micro-CG / coupling-interface device assembly, to which the kinetic theory of gases can therefore apply.

Viscosity is thus designated as being a property of transport of gases, in the same way as thermal conductivity and the diffusion coefficient which are all three linked to the stirring movement of the molecules. The gas transport coefficients are expressed in terms of integrals reflecting the dynamics of the collision of molecules. However, these collision integrals are a function of the temperature, so that the speed of transport of a gas in a capillary decreases when the temperature and therefore the viscosity of the gas increases. Thus, for a given and constant column head pressure, the flow rate at the column outlet of the micro-CG is higher the lower the analysis temperature, just as the flow rate taken by the capillary is all the more important as the temperature of the interface is close to ambient. By controlling this last parameter, namely the temperature of the interface, the device according to the invention offers the possibility of very precisely adjusting the flow rate taken by the capillary and of bringing this flow rate in the immediate vicinity, but slightly by default, from that leaving the chromatographic column.

The observation, in real time, of the “background noise” of the mass spectrometer allows this precise adjustment. Indeed, the relative abundance of the detected ions, of m / z equal to 4, 18, 28 and 32, corresponding respectively to the helium, water, nitrogen and oxygen present, can be viewed in time real. These ions reflect the quantities of air and carrier gas arriving in the source of the spectrometer. In the absence of air leakage, the relative intensities of the peaks m / z = 18 (H ₂ 0), m / z = 28 (N ₂ ) and m / z = 32 (0 ₂ ) must be classified in this order Iι ₈ > I ₂ β »I ₃ 2- Conversely, the presence of a majority N in front of that of H ₂ 0 indicates the presence of a leak.

In the case of our “open” coupling, an increase in the characteristic peaks of air (N and 0 ₂ ) compared to that of water, implies that the flow taken up by the capillary is slightly too strong and consists of the entire flow leaving the detector to which is added an undesirable complement, coming from the air present around the non-tight connection. The temperature of the interface is then adjusted accordingly - by acting on the heating means of the latter - so as to slightly decrease the speed of the flow inside the capillary and to bring the flow rate taken to a level slightly lower than the one leaving the microchromatograph, or rather the detector thereof. Conversely, it is possible to increase the flow rate withdrawn if it is too low, by decreasing the temperature of the interface, by acting in the same way on the heating means of the latter. The invention will now be described, with reference to the following examples, given by way of illustration and not limitation:

Example 1. figure 6)

In this example, the capacity for adjusting the flow rate sampled is illustrated by simple modification of the temperature of the interface, by means of the heating means provided in the coupling device according to the invention.

The temperature of the PoraPLOT U ^® chromatographic column was set at 130 ° C for the specific needs of an analysis. Under these conditions, the helium flow through the column is close to 1.5 ml / min.

Figures 6A and 6B show this “background noise” in real time for the two chosen temperatures (50 ° C and 60 ° C). The interface temperature is initially set at 50 ° C (Figure 6A). The peak of N ₂ is then greater than that of H ₂ 0, which means that the flow circulating in the capillary is greater than that at the outlet of the column and causes a dilution of the gas flow which leaves it and a parasitic "pollution" by ambient air.

The temperature of the interface is then raised to 60 ° C. (FIG. 6B), which has the effect of increasing the viscosity of the carrier gas, consequently, slightly decreasing the speed of the flow inside the capillary. and bring the flow taken at a level slightly lower or almost equal to that leaving the micro-CG detector. This results in a relative abundance of ions m / z = 18 (water), in the source which becomes again the majority in front of that of air.

It therefore appears that the interface temperature, corresponding to a “yield” of sampling of 100% for a column temperature of 130 ° C., lies between 50 and 60 ° C. At this temperature, the flow rate taken off by the capillary is strictly equal to the flow rate at the outlet of the chromatographic column.

This example demonstrates that, thanks to the coupling device according to the invention, the interface temperature can thus be optimized for each temperature of the chromatographic column and a suitable operating range for the analysis by coupling can be determined in all circumstances: the Sensitivity of the catharometer detector is preserved and the optimal operating conditions of the mass spectrometer are saved.

Example 2 (Figures 7 and 8.

In this example, a gas analysis device is used comprising a microchromatograph and a mass spectrometer connected by the coupling device of the invention to search for traces of ethylene oxide. Indeed, certain surgical procedures require the use of a freon type C ₂ F ₆ . This freon is stored at pressure close to atmospheric pressure in “bags” or flexible envelopes, previously sterilized by means of treatment with ethylene oxide. An international standard defines a tolerated ethylene oxide concentration threshold, after filling the bag. This is to measure these traces of sterilant.

a) Mass spectrometer

The mass spectrometer used is a mass spectrometer of the quadrupole type, provided with a conventional interface system. This interface is provided, in accordance with the invention, with a heating system allowing temperature regulation from the ambient to 200 ° C.

Before each series of analyzes, the mass spectrometer is calibrated. The calibration is commonly carried out with masses of 10 to 500 uma, with a scanning duration of 0.4 seconds per spectrum, or 1,225 uma / s. This spectral acquisition speed constitutes, thereafter, the limit not to be exceeded in order to remain in the calibration range.

The parameters of the spectrometer source, for all the analyzes by coupling, are the following:

- temperature: 200 ° C;

- ionization mode: IE ⁺ ;

- ionization energy: 70 eV; - ionization current: 200 μA. b) Microchromatograph

The microchromatograph, used for coupling, is equipped with two chromatographic modules A and B. The two corresponding capillary columns have the following technical characteristics:

Since ethylene oxide and freon are separated by the PoraPLOT ^® U column, the temperature is adjusted to 110 ° C., which corresponds to the best compromise between separation and resolution for these two peaks.

Since ethylene oxide is present in trace amounts in the bags, the volume injected into the PoraPLOT ^® U column is fixed at its maximum value, ie approximately 15 μL.

The second column (molecular sieve) separates only the constituents of the residual air present in the bag (approximately 2%), the column temperature is therefore set at 40 ° C and the volume injected is 1 μL, as if it were it was a conventional air metering.

A preliminary calibration is carried out in static mode: different air-ethylene oxide mixtures are prepared and then analyzed. The resulting calibration curve is a straight line representing the response of ethylene oxide, as a function of its concentration from 0 to 100%. This line confirms one of the specific properties of the microcatharometer detector: the linearity of the response, as a function of the concentration.

c) Coupling device

The coupling device is, according to the invention, constituted by a capillary tube with a diameter of 0.15 mm and a length of 120 cm connected, on the one hand, to the outlet of the microcatharometer, loosely , open to the atmosphere and, on the other hand, to the source of the mass spectrometer, via the interface.

The interface temperature is optimized, after fixing the temperature of the chromatographic column, as described above.

Optimizing the interface temperature at 50 ° C makes it possible to obtain an almost complete sampling of the carrier gas flow, without "pollution" by the ambient air.

d) Analysis

The coupling is carried out with the column

PoraPLOT ^® U. The scanning speed is approximately six spectra per second, with a mass of 10 to 200 uma. The time required to acquire a spectrum is 0.16 seconds, which corresponds to an acquisition of 1188 uma / s.

Current, energy and ionization mode remain unchanged, as does the temperature of the source.

e) Results

On the chromatogram resulting from the analysis conducted on the PoraPLOT ^® U column (Figure 7A) show the peak Pi of the air (0 ₂ + N ₂₎ coelutes with freon C ₂ F _6, followed by peaks P ₂ and P ₃ water and ethylene oxide. The retention times t _R are respectively 30, 68 and 93 seconds. The average area, measured for ethylene oxide (P ₃ ), corresponds, according to the calibration, to a concentration of 60 ± 4 ppmV.

The chromatogram, built on the total ion current (CIT) (Figure 7B), also reveals three peaks (Pi, P ₂ , P ₃ ) corresponding to the constituents of the mixture.

The spectrum of ethylene oxide (figure

7C) is obtained by subtracting the background noise from the residual gases also ionized. The comparison of this spectrum (Figure 7C) with those stored in the library (Figure 7D) leads to the identification of the desired product, with a similarity index of 82.7% between the two spectra. f) Discussion

The variations in the total ion current lead to obtaining a chromatogram very close to that obtained with the microcatharometer. The separation of the various constituents of the mixture by the chromatographic column is preserved during the crossing of the gas flow in the interface.

. The difference in retention time between the chromatogram (μCG) and the CIT chromatogram is less than a second.

The ethylene oxide, already highlighted on the chromatogram, by comparing the retention time with that of the standard, is confirmed by mass spectrometry. The spectrum, obtained by subtracting the background noise, is in fact recognized at 82.7%, as being that of the desired compound. This result is very satisfactory, knowing that this identification relates to a trace compound. The content of ethylene oxide in the bag was evaluated at 60 + 4 ppmV using the microcatharometer. The area of the peak corresponding to this compound allows us to announce a detection limit of the order of ppmV. It should be noted that the area of this peak, evaluated on the CIT chromatogram, allows us to consider that this limit is preserved.

Thus, for the first time, thanks to the device of the invention, there is a coupling using two detectors whose operating principle differs fundamentally, working on a common sample taking and whose detection limits are completely comparable.

Demonstration of the coelution of the air peaks and of the C ₂ F ₆ freon on the chromatogram of FIG. 8A was carried out from the search for traces of the characteristic ions. These specific ion currents (ions m / z = 69 and 119 in FIG. 8B; and 0 + N ₂ in FIG. 8C) make it possible to demonstrate the presence of several compounds, insofar as their retention times differ slightly (0.49 min. In FIG. 8C for air -N ₂ and 0 ₂ not separated on this column - against 0.51 min. For C ₂ F ₆ in FIG. 8B). Although the ion current is limited to the sum of the two majority fragments (69 + 119), the freon peak obviously appears to be much more intense than that of air. These relative intensities reflect the respective abundances in the pocket (≈ 2% air versus majority C ₂ F ₆ ). This well-known deconvolution method can therefore prove to be very useful during the analysis by coupling of complex mixtures. The search for traces of ions makes it possible to confirm the presence of compounds poorly separated by the “coupled” column.

Example 3 (Figure 9.

The objective of this analysis is

1 identification of two unknown products present in a gaseous mixture in a trace state and highlighted following a first separation chromatography conducted on micro-CG. The mixture comes directly from an oven used for controlled thermal degradation, at very high temperature and protected from air, of polyurethane matrices. The sampling in a gas bottle is carried out during the step of cooling the furnace under a nitrogen flow, the pressure in the bottle is of the order of 850 mbar.

Sampling

Knowing the initial pressure of the sample, it is brought to the vicinity of atmospheric pressure by completing the mixture with argon. The inert gas is therefore introduced into the line in slight suppression, the bottle is then connected and the argon progressively expanded inside until it reaches the desired pressure. This dilution is then taken into account in the calculation.

The bottle is isolated and the line placed under vacuum, the argon-sample mixture is then relaxed to be then analyzed.

Analysis conditions

The temperature of the PoraPLOT U ^® column is adjusted to 160 ° C to separate the two unknown products. The volume injected is approximately 15 μL. The scanning speed is approximately 8 spectra per second, from mass 10 to 150 uma. The acquisition of a spectrum requires 0.12 seconds, which corresponds to an acquisition of 1167 uma / second. Current, energy and ionization mode remain unchanged from the previous example, as does the temperature of the source.

In parallel, the Molecular Sieve column can separate H ₂ and N ₂ present in the mixture, the temperature is therefore set at 60 ° C and the volume injected is close to 1 μL.

Results

At a temperature of 160 ° C, the PoraPLOT U ^® column only very poorly separates air, argon, C0 ₂ , NH ₃ and H ₂ 0 (see the large Pi peak poorly resolved at the top of the chromatogram (see Figure 9A)) . Then appear the two peaks (P ₂ and P ₃ ) corresponding to the desired compounds, perfectly resolved. Several analyzes confirm their presence and the retention times are reproducible, respectively t _R2 = 81 and T _R3 = 141 seconds.

These two compounds are also found on the “CIT” chromatogram (FIG. 9B). The spectra obtained by subtracting the background noise and compared with the library (FIG. 9C and 9D) allow the identification of the corresponding compounds: acetonitrile, and propanenitrile. The rate of similarity of the spectra gives a probability of identification of 90.4% and 73.2%. Discussion

Given the peak areas measured using the microcatharometer, the overall content for these two compounds, evaluated at a few ppmV, is confirmed.

The mass spectrometer allows these two products to be identified with almost certainty. The first, at the retention time t _R2 = 81 seconds, is recognized at .90.4% as being acetonitrile. The second at t _R3 = 141 seconds, is identified at 73.2% as being the propanentrile. Given the nature and the retention time of the first, lighter compound, and considering the origin of the mixture (cooling under flow of N2 and therefore possible formation of c - N bonds), this probability index is sufficient to accept the proposal.

REFERENCES

[1] Message, G. M. Practical aspects of gas chromatography / mass spectrometry. John WILEY & Sons ed., 1984, New York.

[2] HENNEBERG D.; HENRICHS U .; HUSMANN H.; SCHOMBURG G., High-performance gas chromatograph-mass spectrometer interfacing: investigation and optimization of flow and temperature. Journal of chromatography, 1978, 167, 139-147.

[3] HENNEBERG D.; HENRICHS U .; SCHOMBURG G., Special techniques in the combination of gas chromatography and mass spectrometry. Journal of chromatography, 1975, 112, 343-352.

Claims

1. Coupling device connecting the output of a microchromatograph (μCG) to the input of a mass spectrometer (SM), said coupling device comprising a capillary tube one end of which is tightly connected to the vacuum source of the mass spectrometer via an interface device, and the other end of which is loosely connected to the outlet of the microchromatograph, open to the atmosphere; the length and diameter of the capillary tube being chosen so that the flow rate inside the capillary tube is very close to the flow rate at the outlet of the microchromatograph; the interface device being, moreover, provided with heating means for very precisely adjusting the flow rate taken by the capillary. 2. Device according to claim 1, in which the temperature of the interface is adjusted so that the flow rate taken by the capillary tube is very close to or equal to the flow rate at the outlet of the microchromatograph. 3. Device according to any one of claims 1 and 2, in which the relative intensity of the peaks is compared, on the one hand, water and, on the other hand, nitrogen and / or l oxygen from the mass spectrometer and action is accordingly taken on the interface heating means to increase or decrease the temperature thereof, and respectively decrease or increase the flow rate taken off by the capillary tube. 4. Device according to any one of claims 1 to 3, in which the heating means make it possible to adjust the temperature of the interface of the ambient to 200 ° C. 5. Analysis device comprising a microchromatograph and a mass spectrometer, the output of the microchromatograph being connected to the input of the mass spectrometer by a coupling device according to any one of claims 1 to 4. 6. Analysis device according to claim 5, further provided, upstream of the microchromatograph, a preconcentration device based on an adsorption followed by a thermoresorption.