Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
  • G01N30/02—Column chromatography
  • G01N30/62—Detectors specially adapted therefor
  • G01N30/72—Mass spectrometers
  • G01N30/7206—Mass spectrometers interfaced to gas chromatograph
  • G01N30/7213—Mass spectrometers interfaced to gas chromatograph splitting of the gaseous effluent
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
  • G01N30/02—Column chromatography
  • G01N30/26—Conditioning of the fluid carrier; Flow patterns
  • G01N30/28—Control of physical parameters of the fluid carrier
  • G01N30/30—Control of physical parameters of the fluid carrier of temperature
  • G01N2030/3038—Control of physical parameters of the fluid carrier of temperature temperature control of column exit, e.g. of restrictors
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
  • G01N30/02—Column chromatography
  • G01N30/62—Detectors specially adapted therefor
  • G01N30/72—Mass spectrometers
  • G01N30/7206—Mass spectrometers interfaced to gas chromatograph
  • G01N2030/7226—OWTC, short capillaries or transfer line used as column

Definitions

• 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.
• GC gas chromatograph
• MS mass spectrometer
• 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.
• microchromatograph 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 ( ⁇ CG) to a mass spectrometer which adds the possibility of identification certain of each compound separated by the chromatographic column.
• 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].
• connection to the microdetector in two ways: either sealed or open.
• 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. .
• 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.
• 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].
• This operation guarantees the optimal sensitivity of the detector and the linearity of the response, as a function of the concentration of the species;
• 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.
• a coupling device connecting the output of a microchromatograph ( ⁇ CG) 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.
• 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.
• the flow rate in the column may change due to temperature variations.
• 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.
• 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.
• 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 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 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.
• 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.
• 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
• FIGS. 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;
• FIG. 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 9A, 9B, 9C and 9D illustrate the identification of unknown products present in a trace mixture.
• FIG. 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 connects the output of a microchromatograph ( ⁇ CG) to the input of a mass spectrometer (SM), in particular a quadrupole mass spectrometer.
• ⁇ CG microchromatograph
• SM mass spectrometer
• 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.
• 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 ⁇ CG 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 ⁇ L in the columns. Associated with capillary or microcapillary columns, the assembly allows very short analysis times, for example from 30 to 160 seconds.
• 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 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 is based on the Wheatstone bridge.
• 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.
• 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)".
• CIT total ion current
• 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).
• 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.
• 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.
• a ferrule which may be of variable composition (for example, graphite, wespel-graphite, etc.).
• 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.
• the end of the capillary tube is at a distance of a few millimeters from the microdetector.
• 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.
• 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.
• 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).
• the helium flow in the line, upstream of the tapping, is constant: for example, about 5 L / min.
• 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.
• 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.
• 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.
• 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.
• 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 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.
• 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.
• 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.
• a gas analysis device comprising a microchromatograph and a mass spectrometer connected by the coupling device of the invention to search for traces of ethylene oxide.
• 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.
• 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.
• 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:
• the microchromatograph used for coupling, is equipped with two chromatographic modules A and B.
• the two corresponding capillary columns have the following technical characteristics:
• the volume injected into the PoraPLOT ® U column is fixed at its maximum value, ie approximately 15 ⁇ L.
• 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 ⁇ L, 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.
• 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.
• 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.
• the difference in retention time between the chromatogram ( ⁇ CG) and the CIT chromatogram is less than a second.
• 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.
• the bottle is isolated and the line placed under vacuum, the argon-sample mixture is then relaxed to be then analyzed.
• the temperature of the PoraPLOT U ® column is adjusted to 160 ° C to separate the two unknown products.
• the volume injected is approximately 15 ⁇ L.
• 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.
• 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 ⁇ L.
• 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.

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