WO2019158878A1

WO2019158878A1 - Gas chromatography detector

Info

Publication number: WO2019158878A1
Application number: PCT/FR2019/050353
Authority: WO
Inventors: Eric Colinet; Philippe Andreucci; Pierre Puget; Serge Junca; Régis BARATTIN
Original assignee: Apix Analytics
Priority date: 2018-02-19
Filing date: 2019-02-15
Publication date: 2019-08-22
Also published as: FR3078164B1; CN215297276U; FR3078164A1

Abstract

The invention relates to a gas chromatography detector comprising: - a nanoelectromechanical system (NEMS) resonator arranged in a fluid line suitable for the circulation of an analyte originating from a chromatography column, said resonator comprising a functional layer, - a reading device configured so as to make the resonator vibrate at the resonance frequency thereof and to measure a variation of said resonance frequency under the effect of adsorption or of desorption of the analyte by the functional layer, wherein said detector is characterized in that it comprises a chamber in which the resonator is encapsulated, said chamber comprising an inlet orifice and an outlet orifice for the fluid line and a temperature-regulating unit configured for varying the temperature inside the chamber according to a given temperature profile.

Description

DETECTOR FOR GAS CHROMATOGRAPHY

FIELD OF THE INVENTION

The present invention relates to a detector for gas chromatography, as well as a gas analysis system comprising such a detector.

STATE OF THE ART

A measurement chain for gas chromatographic analysis comprises an injector (gas valve type or liquid injector), a separation column, and at least one detector.

For the purposes of the analysis, it is necessary to check the temperature of the electrode at any point.

In particular, a commonly accepted dimensioning rule is to maintain the temperature of the injector and the detector at a constant value set at about 50 ° C above the boiling point of the gaseous sample to be analyzed (the point d boiling being the temperature from which the sample is in the gaseous state).

Regarding the column, its temperature control is more subtle. Indeed, the temperature of the column must be adjusted so as to ensure good separation of the different gas peaks constituting the sample to be analyzed while favoring a good analysis speed. It is best to work at a column temperature between the dew point and the boiling point of the sample.

Thus, it is very often necessary to maintain the column in a large temperature range between 20 ° C and 350 ° C.

Two types of analysis are commonly used. The first controls the temperature of the column at a constant value. This isothermal analysis is particularly well suited for simple gaseous samples at atmospheric pressure (small deviation of the boiling points of the analytes constituting the sample) where the analysis cycle (time interval between two successive analyzes) must be reduced to its minimum. strict minimum. The second "temperature programming" approach uses linear temperature ramps consisting of raising the temperature of the column progressively and in stages. This second approach is used for complex samples having a large difference in the different boiling points: for example, complex mixtures which are in the liquid state at room temperature. The temperature ramps (rate of rise in temperature expressed in ° C / min) are adjusted according to the separation capacity requirements or analysis time. These ramps also make it possible to control the phenomena of absorption / desorption between the stationary phase (chemical substance of functionalization of the column) and the mobile phase (gas) in the column. It is this sharing between the stationary phase and the mobile phase which fixes the transit speed of a given analyte and thus makes it possible to separate two different analytes present in the sample to be analyzed.

The detector located at the outlet of the column makes it possible to detect the different molecules and to convert them into chromatographic peaks thus separated.

There are different sensor technologies.

Among the detectors traditionally used, there are catharometers (TCD, acronym for the term "Thermal Conductivity Detector"). An advantage of such a sensor is that it is universal, that is to say capable of detecting any type of gas. Another advantage is that this sensor does not destroy the gaseous compounds of the sample; it can therefore be coupled in series with another type of detector. Finally, this detector is compatible with inert carrier gases (helium, argon, nitrogen), even if its sensitivity is related to the vector gas used. A disadvantage of this detector is that its sensitivity is of the order of ppm for light compounds (carbon chains with less than 7 carbon atoms) and about 10 ppm for heavier compounds (carbon chains with more than 7 carbon atoms). carbon atoms) for the best detectors available to date on the market.

Another type of sensor is called FID (acronym for the term "Flame Ionization Detector"). Such a sensor has a good sensitivity (less than ppm) vis-à-vis hydrocarbons; Moreover, this sensitivity increases linearly with the number of carbon atoms. However, this detector is sensitive only to carbon products (alkanes, alkenes, aromatics) and is therefore not universal. Moreover, its operation requires a flame with hydrogen which implies a high consumption of H ₂ and makes it not very compatible with explosive environments. Finally, this detector has the disadvantage of burning the gaseous compounds forming the sample.

There are other types of detectors that are used more confidentially for specific purposes. Among these, we will mention:

- the PFPD (acronym for the term "Puise Flame Photonic Detector"), which is highly sensitive to sulfur or phosphorus products but which is not universal, involves the use of hydrogen and destroys the sample;

the HID detectors, which are universal detectors with a detection limit of the order of 1 ppm and which are sensitive to the mass of the analyte, but which require a source radioactive and which involves a high consumption of helium; another disadvantage of these detectors is that they destroy part of the sample; the nano-electromechanical system (NEMS) detectors allow mass measurements based on a variation of the resonance frequency of a resonator under the effect adsorption or desorption of an analyte on a functional layer deposited on the resonator. These detectors have a high sensitivity (less than ppm) over a wide range of C1 to C40 molecules (not limited to carbon chains). Being non-destructive to the sample, they can be coupled in series with another detector. Finally, these detectors are compatible with inert carrier gases (helium, argon, nitrogen) without significant impact on sensitivity and with hydrogen (unlike HID detectors). On the other hand, they are not very sensitive to light compounds (C1-C6).

For all of these detectors except NEMS, the operating temperature must be controlled above the boiling point of the gaseous sample to be analyzed to ensure proper transport of the sample to the sensitive part of the detector and the good functioning of the detector.

For NEMS detectors, the temperature must be finely controlled to optimize their detection limit. Indeed, insofar as the adsorption phenomena are minimized with the increase in temperature, it is generally sought to maintain the detector at a sufficiently low temperature. However, if the temperature of the detector is lowered too much, the adsorption reaction is favored to excess, possibly causing condensation of the analyte on the resonator.

SUMMARY OF THE INVENTION

An object of the invention is to design a detector compatible with hazardous environments and having good sensitivity for a wide range of compounds.

For this purpose, the invention provides a detector for gas chromatography, comprising:

an electromechanical nano-system resonator (NEMS) arranged in a fluid conduit adapted for the circulation of an analyte coming from a chromatography column, said resonator comprising a functional layer,

a reading device configured to drive the resonator in vibration at its resonant frequency and to measure a variation of said resonant frequency under the effect of the adsorption or the desorption of the analyte by the functional layer, said detector characterized by comprising an enclosure in which the resonator is encapsulated, said enclosure comprising an inlet port and an outlet port of the fluid conduit and a temperature control unit configured to vary the temperature at the inside the enclosure according to a determined temperature profile. By "temperature profile" is meant a controlled variation of temperature over time. Said profile can be linear (temperature ramp) or not, with more or less rapid variations. Said profile may comprise one or more phases of temperature rise and / or one or more phases of temperature reduction within the enclosure.

The temperature control unit may include a heating element.

According to one embodiment, the heating element is an electrically resistive wire arranged opposite the resonator. Advantageously, said resistive wire may be supported by an electrically and thermally insulating plate closing the volume in which the resonator is arranged. In particular, said plate may comprise a plurality of holes through which the resistive wire passes. The temperature control unit advantageously comprises a computer configured to control the intensity of the electric current flowing in the resistive wire in order to selectively vary the temperature within the enclosure.

According to another embodiment, the heating element is a plane element, mounted on one face of the electrically and thermally insulating plate.

The temperature control unit may also include a cooling system. The system may include a fan, a Peltier cell and / or a cooling fluid circuit.

Advantageously, the temperature control unit is adapted to bring the temperature inside the enclosure to a temperature between 20 ° C and 350 ° C.

According to one embodiment, the detector further comprises a katharometer arranged in the fluid conduit, upstream or downstream of the resonator.

Said catharometer may be arranged inside the enclosure or outside the enclosure.

Advantageously, the resonator is arranged on a portion of a printed circuit made of ceramic or polyimide, the enclosure comprising an orifice adapted to insert into the enclosure the portion of the printed circuit comprising the resonator.

The resonator may include a doped silicon beam.

The functional layer may comprise a SiOC layer.

Advantageously, the fluidic conduit comprises a cavity formed in a substrate, in which the resonator extends, and two capillaries opening into said cavity and extending respectively through the inlet orifice and the orifice of exit from the enclosure.

The detector advantageously comprises a processing system configured to subtract from the baseline of the response of the resonator measured by the reading device the baseline of the response of the resonator, said blank response, previously measured in the absence of circulation of a fluid in the fluid conduit for the same temperature profile inside the enclosure.

Alternatively, the detector comprises at least a second resonator, referred to as reference resonator, encapsulated in the chamber outside the fluid conduit in which the sample circulates, the reading device being configured to drive each of the vibration resonators at its frequency. resonator and for measuring a variation of the resonant frequency of each of said resonators, the detector further comprising a processing system configured to subtract from the response signal of the resonator exposed to the sample the response signal of the reference resonator.

According to one embodiment, the reading device is further configured to measure a variation of the resonance amplitude of the resonator.

Another object of the invention relates to a gas analysis system comprising a gas chromatography column and a detector as described above. The chromatography column is arranged in an enclosure thermally decoupled from the enclosure in which the resonator is encapsulated.

Particularly advantageously, the enclosure containing the chromatography column comprises a unit for regulating the temperature distinct from the temperature control unit of the detector, said control units being configured to vary the temperature in their respective enclosure according to different profiles.

BRIEF DESCRIPTION OF THE FIGURES

Other advantages and characteristics of the invention will emerge from the detailed description which follows, with reference to the appended drawings in which:

FIG. 1 is a scanning electron microscope image of a resonator according to one embodiment of the invention, as well as a sectional view of said resonator;

FIG. 2 is a raw detector response chromatogram as a function of time (in seconds) obtained with a detector according to one embodiment;

FIG. 3 is a chromatogram (detector response as a function of time (in seconds)) obtained after subtraction of the chromatogram of FIG. 2 from the baseline of the blank response of said detector;

- Figure 4 is a block diagram of a differential reading device to overcome the baseline variations related to exogenous phenomena;

FIGS. 5-9 are exploded perspective views of a detector according to one embodiment of the invention;

- Figures 10 and 11 are partial sectional views of an enclosure encapsulating a resonator according to one embodiment of the invention; FIG. 12 is a sectional view of an embodiment of the fluid conduit in which the resonator is arranged;

FIG. 13 shows three chromatograms obtained for a mixture of gasoline and C28H58 with a chromatography column encapsulated in a chamber distinct from that of the resonator, for different temperature differences between the column and the resonator, showing the effect of the decoupling between the two speakers.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention uses a detector based on at least one NEMS resonator. This detector is intended to be arranged at the outlet of a gas chromatography column in order to detect one or more analytes contained in a sample and previously separated by the column. The sample may be in the gaseous state at room temperature; alternatively, it may be liquid at ambient temperature but brought to a temperature above its boiling point so as to be injected into the column in the vapor state, the column and the resonator as well as the fluid circuit which makes them It is also maintained at a temperature above this boiling point in order to avoid any condensation of the sample. The sample is transported in the chromatography column and the detector by a carrier gas.

The resonator is in the form of a beam of which at least one main surface is covered with a functional layer which has a chemical affinity with the analytes of interest.

Depending on the targeted applications, the functional layer may be polar or apolar.

Optionally, the detector may comprise several resonators, comprising the same functional layer or a different functional layer, chosen according to the analytes of interest.

The beam is suspended relative to a substrate. According to one embodiment, one end of the beam is embedded in the substrate and the opposite end is free, but other solutions for suspending the beam are possible, for example a recess at both ends of the beam.

As a purely indicative, the dimensions of the beam of such a resonator are of the order of a few micrometers in length, a few hundred nanometers in width, and a hundred nanometers in thickness.

Thus, according to an embodiment given by way of example, the beam has a length of 1 to 100 pm, a width of 50 to 500 nm or even a few pm and a thickness of 50 to 500 nm.

The resonator is slaved to an electronic reading device configured to drive the resonator into vibration at its resonance frequency and to measure a resonator. variation of said resonance frequency under the effect of the adsorption or desorption of an analyte by the functional layer.

When an analyte of interest is adsorbed on the functional layer (or desorbed), the effective mass of the resonator is modified, which causes a change in the resonance frequency of the resonator. Thus, the measurement of the variation of the resonant frequency by the reading system makes it possible to measure the variations in the mass of the resonator and to deduce therefrom the concentration of the gas or of the vapor to be analyzed. For a given analyte concentration, the adsorbed gas mass and therefore the sensitivity of the measurement depends on the equilibrium constant between the vapor phase and the adsorbed phase of the analyte. This equilibrium constant depends on the temperature of the surface and the physicochemical characteristics of the sensitive surface. The functional layer therefore advantageously has an adsorbed phase / vapor phase equilibrium constant at a given operating temperature.

For a given concentration and nature of the functional layer, a decrease in detector temperature increases the adsorbed analyte mass, which increases the signal delivered by the detector. However, if the detector temperature is too low, the adsorption reaction is favored to excess, possibly leading to condensation on the surface of the detector. There is therefore an optimum operating temperature of the detector for an analyte of interest. This optimum operating temperature increases approximately like the boiling temperature of the analyte.

The manufacture of an NEMS resonator is known per se and therefore does not need to be described in detail in the present text. In particular, reference may be made to documents [Mile2010], EP 2,008,965, WO 2012/034990 and WO 2012/034951, which describe NEMS resonators that may be implemented in a detector according to the invention.

Note that, instead of a single NEMS resonator, the detector may include one or more NEMS resonator arrays.

The advantages of a network of resonators compared to an individual resonator are multiple. On the one hand, the network of resonators offers a total area for the capture of the species to be analyzed, which is greater the greater the number of beams. This makes it possible to detect more finely species contained in low concentration in the sample to be analyzed. Furthermore, the use of a resonator network makes it possible to minimize the impact of the failure of one of them, which is compensated by the operation of the other resonators of the network, thus improving the robustness of the detector. Finally, for a network of N NEMS resonators, one should theoretically achieve a detection limit gain of the order of V / V in terms of signal (or of the order of N in terms of power). Reference can be made to WO 2014/053575 for the description of a network of NEMS resonators that can be implemented in a detector according to the present invention.

Moreover, in the case where several NEMS resonator networks are used within the detector, it is possible to functionalize these networks with a different functional layer from one network to another.

Although the term "NEMS resonator" is used in the remainder of the text, it is understood that the description also applies to a plurality of NEMS resonators, arranged or not networked.

FIG. 1 is a scanning electron microscope view of a resonator

NEMS may be implemented in a detector according to the invention, and a schematic cross-sectional view.

Said resonator is advantageously formed on a semiconductor substrate 1000, for example silicon. The substrate 1000 is advantageously covered by an electrically insulating layer 1003 (for example, made of silicon oxide) and a silicon layer 1002, to form a silicon-on-insulator substrate (SOI). Silicon On Insulator ")

The resonator comprises a beam 1001 of length L and width w.

The beam 1 is suspended relative to the support substrate 2, with the exception of a recess of one of its ends 1001a in a portion of the substrate 1000, projecting from the plane of the substrate which extends under the beam.

The other end 1001b of the beam is free.

In a manner known per se, such a beam may be formed in the layer 1002, by means of an etching for delimiting the beam and for removing the portion of the electrically insulating layer 1003 located under the beam 1001, in order to release this one.

On either side of the beam extend two strain gauges 1004, for example piezoresistive, which are also suspended relative to the substrate 1000.

Advantageously, said gages are, like the beam, etched in the SOI substrate and have at least one plane in common with the beam.

These gauges are advantageously doped semiconductor material, preferably having a dopant concentration greater than 10 ¹⁹ atoms / cm ³ .

Preferably, said doped semiconductor material is doped silicon.

The intersection between each of the gauges and the beam is at a distance h from the flush mounting region of the beam, chosen to maximize the stress exerted on the gauge during the deflection of the beam.

Each of the gauges 1004 is connected to an electrode 1005, said electrodes permitting respectively the application of constant potentials of opposite signs. In other embodiments of the resonator, it is possible to use a strain gauge of doped semiconductor material.

The resonator further comprises an electrostatic actuator of the beam 1001 which, as shown here, may comprise two electrodes 1006 extending in the same plane as the beam and disposed on either side thereof. a determined distance.

The electrodes 1006 are intended to receive respectively an electrical excitation signal and a signal of opposite sign, and therefore constitute two inputs of the resonator.

Under the application of an electrical signal having a frequency corresponding to the vacuum resonance frequency of the beam, the beam is vibrated in a plane parallel to the substrate.

By vacuum resonance frequency of the beam is meant the resonance frequency of the beam in the absence of the sample to be analyzed.

According to one embodiment, the measurement of the electrical resistance variation of the piezoresistive gauges is performed between the embedded end of the beam and the junction between the beam and the gages.

The output signal of the resonator is thus provided to a connection electrode 1007 located at the embedded end of the beam, for reading said signal.

This measurement method is however not exclusive and the output signal can be provided by other means; for example, it is possible to apply a bias voltage at the electrode and to measure the voltage across the two gauges to deduce the variation of their electrical resistance

Those skilled in the art can therefore adjust the design of the polarization of the strain gauge (s) and the measurement of their response without departing from the scope of the present invention. Furthermore, another mode of operation can be used without departing from the scope of the invention.

In a particularly advantageous manner, the NEMS resonator may be formed on a chip of a few millimeters on a side, said chip being able to be embedded on a printed circuit as will be described in detail below.

To control the temperature of the NEMS resonator, it is encapsulated in a temperature controlled enclosure.

The volume of the enclosure is chosen to be just sufficient to encompass the resonator and the chip on which it is formed, minimizing voids to optimize the power consumption required to regulate the temperature in the enclosure.

The chip on which the NEMS is formed being of small dimensions, the internal volume of the chamber can be particularly small (of the order of a few mm ³ or tens of mm ³ ). This has several advantages. On the one hand, the electric power to implement to regulate the temperature within the enclosure is reduced. On the other hand, for a given power, the rate of heating and cooling is increased compared to a larger volume enclosure. Finally, the thermal decoupling of the chamber of the detector vis-à-vis the chromatography column is also easier.

The control of the temperature in the enclosure is provided by a temperature control unit.

Said control unit may comprise heating means and / or cooling means, as well as a temperature sensor and a servo loop for applying inside the enclosure a temperature according to a determined profile. . Said temperature profile can be defined by a user depending on the composition of the sample to be analyzed and the desired analytes.

The servocontrol loop typically comprises a computer communicating with a user interface and configured to receive the temperature profile to be applied and the measurement data of the temperature sensor, and to, from these input elements, control the means of heating and / or cooling to reach the desired temperature within the enclosure over time.

The temperature sensor may be a platinum resistance thermometer commonly used in electronic devices, in particular of the Pt100 type.

Particularly advantageously, the control unit comprises a heating element arranged inside the enclosure. For reasons of compactness, said heating element may be a heating resistor powered by an electric current.

Preferably, the control unit further comprises a cooling system. Different cooling technologies are possible: fan, Peltier cell, fluidic cooling circuit, possibly combined. Those skilled in the art are able to choose and size the cooling system according to the arrangement of the enclosure and expected performance. In particular, the cooling system makes it possible to quickly cool the internal volume of the chamber after a high temperature detection phase, and thus allows the rapid implementation of a new phase of detection at a lower temperature.

The enclosure is not necessarily hermetically sealed or thermally insulated from the outside. On the contrary, it can include vents that allow a faster evacuation of heat when it is desired to cool the resonator. Moreover, the interior volume of the enclosure can be at least partly heated by passive heat transfer from outside the enclosure (for example its proximity to the chromatography column which is itself heated).

NEMS resonators are capable of having significant interactions (called fluidic interactions) with a surrounding gas.

Therefore, in addition to the resonant frequency variation of the resonator, it is possible to measure the resonance amplitude variation due to the fluidic interactions between the resonator and the sample.

As described in EP 2 878 942, it is possible to deduce from this amplitude variation a fluidic characteristic of the sample. This fluidic characteristic is advantageously a viscosity, an effective viscosity, an average free path of the molecules, a flow rate and / or a thermal conductivity of the sample. By "effective viscosity" is meant in the present text a viscosity parameter taking into account the rate of rarefaction of the gases in the Reynolds equation simplifying the Navier-Stokes equation (cf paragraph 5.1 of [Bao2007]).

It has furthermore been demonstrated that the contrast between the fluidic characteristics of the carrier gas and those of the sample to be analyzed increases as the temperature to which the NEMS resonator is subjected is high. In the system described in EP 2 878 942, the NEMS resonator is heated by Joule effect in order to minimize the influence of the adsorption phenomena with respect to the fluidic interactions.

On the other hand, heating the resonator NEMS reduces the effective viscosity of the sample and the carrier gas and thus increase the amplitude variation measured by the reading device.

The present invention can therefore take advantage of the fact that the resonator is heated in the chamber to implement, with good sensitivity, a measurement of at least one fluidic characteristic according to the principle described in document EP 2 878 942, while dispensing with the Joule heating system used in this system.

The combination of the frequency variation measurement and the amplitude variation measurement makes it possible to obtain more information on the sample, which makes it possible to more precisely differentiate analytes having a close response.

Due to the low level of response of the NEMS resonator to light compounds

(C1-C6), it may be desirable, depending on the application requirements, to couple it in series to a micro-katharometer (also called TCD detector in the rest of the text), which is more sensitive to these species. Since the two detectors are nondestructive, the TCD detector can be arranged in the same fluid duct as the NEMS resonator, upstream or downstream thereof.

The TCD detector may be placed in the same enclosure as the NEMS detector, to facilitate the integration of said detectors, for example on the same chip or the same printed circuit. Alternatively, the TCD detector can also be implemented outside the enclosure of the NEMS detector, its sensitivity not being directly influenced by its working temperature.

The TCD detector is known in itself and will not be described in more detail in this text.

In addition to the effective mass of the resonator, the resonance frequency of the resonator

NEMS also depends on detector temperature, carrier gas flow, and other exogenous factors.

As a result, the baseline of the measurement signal varies with temperature. There is therefore a superposition between the signal useful to the measurement (ie the resonance frequency variations of the NEMS resonator linked to a gas adsorption / desorption) and a background signal (temperature variation of the NEMS resonator, and other exogenous factors varying the resonance frequency of the resonator) not useful.

By way of example, FIG. 2 shows a crude chromatogram obtained with an NEMS resonator whose operating temperature varies according to a linear temperature profile varying between 40 and 250 ° C. at a rate of 20 ° C. per minute. The variation in frequency and the low signal-to-noise background ratio linked to the temperature ramp applied to the NEMS, which penalizes the identification of the different peaks, are well observed.

Advantageously, a so-called differential reading of the NEMS resonator is thus used, which makes it possible to reduce these independent variations of the mass variation created by adsorption of an analyte of interest and to bring out only the variations in the resonant frequency of the analyte of interest. NEMS resonator by gas adsorption / desorption.

For this purpose, a first approach consists in carrying out a so-called blank analysis for which no sample is injected into the chromatography column while applying the temperature profiles on the NEMS column and detector necessary for the analysis. For this analysis, it is possible to inject only the carrier gas into the system, or to work with an empty gas circuit. In this way, one can collect variations of the sensor baseline related to any phenomenon other than gas adsorption and desorption. Then, an analysis is performed by injecting the sample and applying the same temperature profiles as during the blank analysis. Finally, in a so-called offline processing, subtracting the measured baseline from the measured baseline with the sample injection so as to retain only detector baseline variations related to the baseline. adsorption / desorption of different gas peaks. FIG. 3 (NEMS curve) illustrates the result of such a treatment carried out on the chromatogram of FIG. 2. By way of comparison, the response of an FID detector is displayed on the chromatogram (FID curve), which makes it possible to check the correct match of the detected peaks with both techniques. The chromatograms of Figures 2 and 3 were obtained for a simple mixture of hydrocarbons. Figure 3 shows that compounds up to 32 carbon atoms could be detected by the NEMS resonator, which confirms the ability of the NEMS resonator to analyze hydrocarbons.

Another differential reading approach consists in simultaneously measuring, during the same analysis phase, the variation in the resonance frequency of two differential resonators (or, if a TCD detector is associated with the NEMS detector), two pairs each formed of a NEMS resonator and a TCD detector). As illustrated in FIG. 4, one of these pairs is arranged on the analysis path A and thus measures the different peaks of analytes and any other exogenous phenomenon. The other pair is arranged on a so-called reference R channel and therefore measures only the exogenous phenomena. The system comprises, upstream of the two channels, an injector I which receives on the one hand the sample S in gaseous and vapor form and the carrier gas V and which mixes them before injecting them, on the one hand into the column of chromatography denoted GC, which is on the analysis path A, and secondly in a DR path presenting a pressure loss identical to that of the column, on the reference path R. Through an electronics differentiating the measurement signals from the two pairs of detectors, it retains only the signals from the adsorption / desorption of different gas peaks. This approach is preferred because it does not require a blank scan or offline processing.

Figures 5 to 9 illustrate an embodiment of a detector comprising a first temperature-controlled chamber in which are embedded a NEMS detector and a TCD detector. Figure 5 is a perspective view of the detector arranged in its enclosure; Figures 6 to 9 are partial sectional views at different angles.

The detectors TCD and NEMS are mounted in series on a printed circuit board 3. The detectors are arranged in the form of modules 31, 32 electrically connected to the printed circuit 3. For example, the module 31 comprises one or more NEMS detectors and the module 32 comprises one or more TCD detectors. Each module may comprise two detectors of the same type, one serving as a reference and the other being used for the analysis for the differential measurement mentioned above.

According to another embodiment (not shown), a first module comprises a network of NEMS resonators with a polar functional layer and a second module comprises a network of resonators with a different functional layer, for example apolar. Any other configuration of the modules is naturally conceivable.

Particularly advantageously, each module forms a second temperature-controlled chamber in which the resonator or resonator network is encapsulated. The first enclosure is formed by assembling a cylindrical shell 1 and two flanges 10, 11 arranged at the ends of the shell. The shell 1 has an opening 13 for the passage of a connector 30 mounted on the printed circuit 3 and for electrically connecting the sensors to an external processing system. The flanges 10, 1 1 have openings (vents) 100, 1 10 for faster evacuation of heat. Naturally, one could choose any other structure for the enclosure without departing from the scope of the present invention.

A fan 4 is arranged at one end of the enclosure, the plane of rotation of the blades being perpendicular to the longitudinal axis of the shell 1.

At the end opposite the fan is arranged a mandrel 21 around which is arranged the heating element 22 which is in the form of a heating filament wound around the mandrel. The mandrel 21 and the heating element 22 are arranged in a tube 2.

Inside the mandrel 21 pass capillary conduits adapted to be fluidly connected to the chromatography column via a connector 20. This connector comprises two inputs 201, 202 and two outputs 203, 204. The two inputs supply the two detectors (measurement and reference). The two outputs coming from the two detectors make it possible to connect in series and downstream of these other detectors or a vent. Thus, the capillary ducts do not have a cold point likely to cause condensation of the sample.

Inside the first enclosure is arranged a support 12 for supporting the printed circuit 3 and the capillary conduits 52, 53, 54 which provide a fluid connection between the column and the detectors. The conduit 52 makes it possible to introduce the sample into the module 31 via the input device 51. The conduits 53 and 54 are arranged symmetrically on either side of the modules 31, 32. They allow the sample to be transferred from the NEMS detector to connector 20.

The support 12 also supports a heating block 23 comprising a heating cartridge and a temperature probe for monitoring in real time the temperature in the first chamber.

Figures 10 and 11 show two partial sectional views of a module 31 containing an NEMS detector. The arrangement of the components in the first enclosure is slightly different from that of FIGS. 5-9, but the module of FIGS. 10-11 can be used in this embodiment with some adaptations within the reach of those skilled in the art. With respect to the embodiment of FIGS. 5-9, the heating block 23 is arranged in the first enclosure on the flange 10, facing the tube 2 containing the mandrel and the heating wire wound thereon. The capillary ducts arranged in the mandrel pass through the block 23 in which they are heated to a suitable temperature, in order to open into the module itself. The module 31 comprises a stack of electrically and thermally insulating plates, for example made of mica, which form a second enclosure around the NEMS resonator and capillary ducts which are in fluid connection with it. The resonator is arranged on a chip electrically connected to the printed circuit 3, in the center of the rectangle designated by the reference 31. Only a portion of said printed circuit is visible in Figure 10, the rest being masked by an insulating plate 35 which defines a volume interior in which are arranged the resonator and the capillary ducts. This volume is closed by a plate 33 visible in Figure 1 1. Said plate 33 is provided with a plurality of holes 330 through which a heating wire 34 having a high electrical resistivity is passed. The holes 330 are arranged on the surface of the plate 33 so that the path of the heating wire allows a homogeneous heating of the capillary ducts and the NEMS resonator. Although not visible in FIGS. 10 and 11, a temperature sensor is arranged in the module 31 in order to measure in real time the temperature inside said module. Said temperature is slaved to a temperature profile determined by means of a control loop which comprises a computer (not shown) connected on the one hand to the temperature sensor, from which it receives the measurement data, and to an electrical source connected to the heating wire, to which it sends instructions of intensity of the electric current to pass through the heating wire to reach the desired temperature. Advantageously, the computer is embedded on the printed circuit 3, so that the detector is entirely autonomous. The internal volume of the second chamber being extremely small, the temperature can be controlled in real time very finely over a wide temperature range.

Naturally, these particular arrangements of the detector are given solely by way of non-limiting examples. In particular, the heating element may be in a form other than a resistive wire, for example in the form of a bar or a flat plate, which simplifies its assembly on a wall of the second enclosure , especially with a view to production on an industrial scale. Any other heating element adapted to the dimensions of the second enclosure may be used.

In order to cover a wide range of working temperature of the detector, the materials that constitute it are chosen to withstand a temperature of the order of 300 ° C.

The shell and the flanges which define the envelope of the first enclosure are typically made of metal.

Regarding the printed circuit board that supports the resonator, the preferred material is a ceramic or polyimide (Kapton ™) for example.

The NEMS resonator and, if appropriate, the TCD detector, are advantageously made of doped silicon or silicon nitride providing mechanical rigidity and electrical conduction. The doped silicon is indeed able to support temperatures of more than 400 ° C without modification of its electronic and mechanical properties. The TCD detector is coated with a platinum layer that retains all its physical properties well above 300 ° C.

With respect to the functional layer, polymers that are conventionally used to functionalize NEMS resonators are generally not usable in the present invention because they are poorly heat resistant. Indeed, the deposition temperature of these polymers is of the order of 100 ° C to 200 ° C depending on their nature. Therefore, in the present invention, the NEMS resonator is advantageously functionalized with a layer of a porous oxide derived from microelectronics deposited at high temperature (of the order of 400 to 500 ° C.) which offers good chemical responses. on a wide range of molecules (alkane, alkene, alcohol, aromatic compounds, etc.). Advantageously, the composition of said oxide is of general formula SiO _x C _y H _z (with x> 0 and y and z> 0), for example SiOC, SiO ₂ , SiOH ... Such a porous oxide is described in particular in WO 2015/097282. An alumina oxide of the general formula Al x O y (with x and y> 0), for example Al ₂ O ₃ , could also be used.

To make the fluidic vein including the NEMS resonator and the TCD detector, the substrate 1000 which supports them is assembled with a structured glass or silicon 2000 cover with sintered glass 2001 ("fried glass" according to the English terminology). heat sealing is performed at around 400 ° C. Figure 12 is a sectional view of said fluidic vein.

The capillaries 3000, 3001 for feeding the gases onto the chip are made of glass and bonded using a high temperature crosslinked epoxy adhesive, for example sold under the name EPO-TEK® 731, designated by the reference mark 2002.

The respect of these precautions in the choice of materials makes it possible to ensure a good behavior of the detector at high working temperatures.

The detector which has just been described is particularly suitable for use in a gas analysis system comprising a gas chromatography column and a detector arranged at the outlet of said column.

The chromatography column is arranged in a temperature-controlled enclosure and thermally decoupled from the enclosure in which the resonator is encapsulated. Upstream of the chromatography column, an injector makes it possible to vaporize the sample and mix it with the carrier gas.

A fluidic conduit (for example in the form of a capillary tube) connects the outlet of the chromatography column to the detector inlet.

To maximize the thermal decoupling between the two speakers, it is preferable to increase the distance between the speakers and to heat the connecting duct to avoid any cold spots in which condensation may occur. Thanks to this thermal decoupling, it is possible to independently adjust the temperature profiles in each of the two chambers, which makes it possible to control the temperature of the NEMS resonator regardless of the temperature of the chromatography column.

Unlike other detectors used in chromatography, a resonator

NEMS has an optimal detection temperature that depends on the analyte of interest.

If the resonator is at too high a temperature, gas adsorption is less efficient and therefore the measurement sensitivity is lower. Moreover, a too low temperature of the resonator can cause a deformation of the peaks (which are, for a well-dimensioned analytical system, of Gaussian form) at the exit of column (resulting in a drag of the greater peak), decreasing the power of separation of the chromatography column, and / or fouling of the functional layer (the adsorption sites not being released), decreasing its effectiveness over time.

To optimize the performance of the NEMS resonator, it is necessary to finely control its working temperature to keep it close to this optimum value for each analyte of interest. It is therefore useful to dynamically adapt the temperature of the resonator depending on the analytes leaving the column, as the chromatography column is temperature controlled to control the analysis speed and separation. To cover a wide range of gaseous compounds to be detected, it is desirable to control the resonator in as wide a temperature range as possible, typically between room temperature (20 ° C) and 350 ° C.

For many applications for analyzing complex mixtures, it is often appropriate to produce a temperature rise profile, for example in the form of a linear ramp, on the chromatography column to separate the gaseous compounds retained by the column. The working temperature of the NEMS resonator need not be identical to that of the column to optimize its detection performance. In most cases, it is also preferable that the temperature of the NEMS resonator is lower than that of the column, which allows the thermal decoupling between the two enclosures.

According to one embodiment, the temperature regulation of the NEMS resonator can be based on the temperature in the chamber of the chromatography column. For this purpose, the heating element of the chamber containing the chromatography column comprises a temperature sensor (Pt100 or thermocouple type), in order to measure the temperature of the chromatography column at any time. The temperature of the chamber containing the NEMS resonator can be adjusted in real time as a function of the temperature in the chamber containing the chromatography column. In other words, the temperature T_NEMS of the resonator NEMS is adapted according to a law T_NEMS = f (T_GC) where f is a programmable analytical function and T_GC the temperature in the chamber of the chromatography column, measured by the aforementioned temperature sensor.

An example of a temperature profile of the NEMS resonator is, for example, to apply a constant temperature difference between the temperature of the column and that of the NEMS resonator. The law of regulation of the temperature is then T_NEMS = T_GC - DT, where DT is a constant.

Figure 13 shows three chromatograms obtained with a chromatography column encapsulated in a chamber separate from that of the resonator, showing the effect of thermal decoupling between the two enclosures.

The sample to be analyzed is a relatively light petrol solution comprising a concentration of C28H58, which is a relatively heavy hydrocarbon, of 5000 mg / L.

The temperature profile in the chamber of the chromatography column is identical in the three cases: it is a linear ramp of temperature rise of 20 ° C per minute, between 50 ° C and 300 ° C (noted GC on the temperature profiles shown to the right of the curves).

The chromatogram A was carried out by applying in the chamber of the resonator a ramp of temperature rise (denoted NEMS) identical to that of the column (denoted GC) (difference between the two ramps = 0 ° C).

Chromatogram B was produced by applying a temperature ramp of 20 ° C per minute in the resonator chamber, but started with a delay of one minute, so that the difference between the two temperature ramps is 20 ° C. ° C.

Chromatogram C was carried out by applying in the resonator chamber a temperature ramp of 20 ° C. per minute, but started with a delay of three minutes, so that the difference between the two temperature ramps is 60 ° C. C.

Comparison of the chromatograms A, B and C shows that the highest temperature difference between the temperature profile of the column and that of the resonator provides the best peak sensitivity of C28H58.

It should be noted, however, that an excessive temperature difference no longer provides such an improvement.

Therefore, for the detection of hydrocarbons having long carbon chains, a temperature difference between the chamber of the resonator and the chamber of the chromatography column of between -5 and -150 ° C. is preferably chosen, preferably between -30 and -100 ° C, or between -30 and -70 ° C, and more preferably between -40 and -60 ° C, for example equal to 50 ° C.

Of course, other temperature profile strategies of the NEMS resonator may be adopted depending on the intended applications. In particular, the temperature difference between the chromatography column and the NEMS resonator is not necessarily constant over time. It will be noted that heating the NEMS resonator, and in particular dynamically according to a temperature profile, goes against the usual practice in the field of gas analysis. Indeed, in the case of a gravimetric detection whose sensitivity is directly related to an adsorption mechanism, which is penalized by heat, it is in fact the custom to keep the NEMS resonator at a relatively low and stable temperature. . Moreover, the polymer functional layer usually used to functionalize the NEMS resonator is not adapted to operate at temperatures as high as those provided in the present invention.

Thus, the system according to the invention provides a particularly relevant alternative to FID detectors for the analysis of hydrocarbons, especially heavy hydrocarbons. Indeed, the NEMS detector has performance at least equal to that of a FID detector while overcoming its defects (destructive character, limitation to carbon chains, presence of hydrogen and a flame), which allows use in a constrained environment.

Furthermore, given the reduced size of the detector and the chromatography column, it is possible to miniaturize the gas analysis system, for example to embed it in a transportable device. Thus, the system can be used to perform analyzes in inaccessible locations, including in hazardous environments.

REFERENCES

[Mile2010] E. Mile, G. Jourdan, I. Bargatin, S. Labarthe, C. Marcoux, P. Andreucci, S. Hentz, C. Kharrat, E. Colinet, L. Duraffourg, ln-plane nanoelectromechanical resonators based on Silicon nanowire piezoresistive sensing, Nanotechnology 21, (2010) 165504

[Bao2007] M. Bao, H. Yang, Squeeze film air damping in MEMS, Sensors and Actuators A 136 (2007) 3-27

EP 2,008,965

WO 2012/034990

WO 2012/034951

WO 2014/053575

EP 2 878 942

WO 2015/097282

Claims

1. Detector for gas chromatography, comprising:

a reading device configured to drive the resonator in vibration at its resonant frequency and to measure a variation of said resonant frequency under the effect of the adsorption or the desorption of the analyte by the functional layer, said detector characterized by comprising an enclosure in which the resonator is encapsulated, said enclosure comprising an inlet port and an outlet port of the fluid conduit and a temperature control unit configured to vary the temperature at the inside the enclosure according to a determined temperature profile.

The detector of claim 1, wherein the temperature control unit comprises a heating element.

The detector of claim 1 or claim 2, wherein the temperature control unit comprises a cooling system.

4. Detector according to claim 3, wherein the cooling system comprises a fan or a Peltier cell.

5. Detector according to one of claims 3 or 4, wherein the cooling system comprises a fluidic cooling circuit.

6. Detector according to one of claims 1 to 5, wherein the temperature control unit is adapted to bring the temperature inside the chamber at a temperature between 20 ° C and 350 ° C.

7. Detector according to one of claims 1 to 6, further comprising a katharometer arranged in the fluid conduit, upstream or downstream of the resonator.

8. Detector according to claim 7, wherein said catharometer is arranged inside the enclosure.

9. Detector according to claim 7, wherein said catharometer is arranged outside the enclosure.

10. Detector according to one of claims 1 to 7, wherein the resonator is arranged on a portion of a printed circuit ceramic or polyimide, the enclosure comprising an orifice adapted to insert into the enclosure the portion of the circuit printed matter comprising the resonator.

1. A detector according to one of claims 1 to 10, wherein the resonator comprises a doped silicon beam.

12. Detector according to one of claims 1 to 1 1, wherein the functional layer comprises a SiOC layer.

13. Detector according to one of claims 1 to 12, wherein the fluidic conduit comprises a cavity in a substrate, in which the resonator extends, and two capillaries opening into said cavity and extending respectively through the inlet port and the outlet port of the enclosure.

14. The detector according to one of claims 1 to 13, further comprising a processing system configured to subtract from the baseline of the response of the resonator measured by the reading device the baseline of the response of the resonator, so-called blank response, previously measured in the absence of circulation of a fluid in the fluid conduit for the same temperature profile inside the enclosure.

15. The detector according to one of claims 1 to 13 further comprising at least a second resonator, said reference resonator, encapsulated in the chamber out of the fluid conduit in which the sample circulates, the reading device being configured to drive. each of the resonators in vibration at its resonant frequency and for measuring a variation of the resonant frequency of each of said resonators, the detector further comprising a processing system configured to subtract from the response signal of the resonator exposed to the sample the signal response of the reference resonator.

16. Detector according to one of claims 1 to 15, wherein the reading device is further configured to measure a variation of the resonance amplitude of the resonator.

17. A gas analysis system comprising a gas chromatography column and a detector according to one of claims 1 to 16, wherein the chromatography column is arranged in an enclosure thermally decoupled from the chamber in which the resonator. is encapsulated.

The system of claim 17, wherein the chamber containing the chromatography column comprises a temperature control unit separate from the temperature control unit of the detector, said control units being configured to vary the temperature. in their respective enclosure according to different profiles.