Gas chromatography column comprising a porous stationary phase in keeping therewith

Abstract

Gas chromatography column comprising a substrate, a channel formed in said substrate, a cover closing said substrate and a stationary phase covering the walls of said channel, wherein said stationary phase is made of SiOxCyHz with x between 1.6 and 1.8, y between 1 and 2.2 and z between 3 and 4, wherein said stationary phase is porous with a porosity of between 10% and 40%.

Claims

1. Gas chromatography column comprising: a substrate, a channel formed in said substrate, a cap closing said substrate, a stationary phase covering the walls of said channel, said stationary phase being made of SiOxCyHz with x between 1 and 2, y between 0.8 and 3 and z between 2.5 and 4.5, the limits of the ranges being included, said stationary phase having a porosity of between 10% and 40%.

2. Gas chromatography column according to claim 1, being a chromatography microcolumn.

3. Gas chromatography column according to claim 1, in which the stationary phase has a thickness of between 50 nm and 1000 nm.

4. Gas chromatography column according to claim 1, in which the substrate is made of silicon and the cap is made of glass or silicon.

5. Method of manufacturing a gas chromatography column according to claim 1, comprising the steps of: a) forming a channel in a substrate, b) forming a layer of SiOxCyHz on the walls of said channel, a pore-forming agent being implemented during the formation of the layer of SiOxCyHz, for example norbornadiene, c) annealing to eliminate the pore-forming agent, d) closing of the channel by putting in place a cap.

6. Method of manufacture according to claim 5, in which step b) is carried out by chemical vapour deposition.

7. Method of manufacture according to claim 5, in which step b) is carried out by enhanced chemical vapour deposition.

8. Method of manufacture according to claim 5, in which step a) is carried out by photolithography and etching.

9. Method of manufacture according to claim 5, in which during step b) diethoxymethylsilane is used as precursor.

10. Method of manufacture according to claim 5, in which several columns are manufactured collectively on a same substrate, the substrate then being divided so as to separate the columns thereby produced.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be better understood by means of the description that follows and the drawings in which:

(2) FIG. 1A is a top view of a chromatography microcolumn;

(3) FIG. 1B is a schematic sectional view of a chromatography microcolumn represented partially;

(4) FIG. 2 is a photo of a transversal section of a chromatography microcolumn comprising a stationary phase made of SiOCH;

(5) FIGS. 2A and 2B are enlarged views of a lateral edge of the bottom respectively of the channel of FIG. 2;

(6) FIG. 3A represents a chromatogram obtained with a microcolumn with a deposition of porous SiOCH of 120 nm for the injection of a BTEX (benzene, toluene, ethyl-benzene, o-xylene) mixture, having a porosity of 30%;

(7) FIG. 3B represents a chromatogram obtained with a microcolumn without stationary phase for the injection of a BTEX (benzene, toluene, ethyl-benzene, o-xylene) mixture;

(8) FIGS. 3C and 3D represent chronograms obtained with a microcolumn with a deposition of porous SiOCH of 120 nm for the injection of a BTEX (benzene, toluene, ethyl-benzene, o-xylene) mixture, having a porosity of 3% and 1% respectively,

(9) FIGS. 4A and 4B represent chronograms obtained with a microcolumn with a deposition of porous SiOCH for the injection of a C5-C7-Tol-C8 (pentane, heptane, toluene and octane) mixture, having a porosity of 33% and 20% respectively.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(10) In FIG. 1A may be seen a top view of a gas chromatography microcolumn 2. In FIG. 1B may be seen a transversal section of the microcolumn comprising a substrate 4 in which is formed a channel 6 which generally has a rectangular transversal section, a cap 8 closing the channel in a sealed manner. The cap 8 is attached to the substrate 4 for example by anodic sealing.

(11) The column 2 also comprises fluid connection ends to enable the later connection of the microcolumn to external equipment such as an injection system at the inlet and an analysis system at the outlet, via for example capillaries.

(12) In the example represented, the channel 6 has a spiral profile, it is a spiral having a double winding. Thus, the two connection ends of the channel may be situated on one edge of the substrate. The channel may also have a coil shape or any other geometry making it possible to optimise the length of the channel with respect to the surface of the substrate.

(13) Preferably, the channel 6 has a high form factor, i.e. a ratio depth P over width L preferably greater than 1, even more preferably greater than 10.

(14) The microcolumn also comprises a stationary phase 10 covering the walls of the channel 6.

(15) The stationary phase 10 is a compound of SiOCH type. The term designates a compound of formula SiOxCyHz with: x between 1 and 2, preferably between 1.6 and 1.8, y between 0.8 and 3, preferably between 1 and 2.2 z between 2.5 and 4.5, preferably between 3 and 4.

(16) The SiOxCyHz is made porous. The stationary phase layer 10 preferably has a thickness e of between 50 nm and 1000 nm, or even between 50 nm and 2000 nm.

(17) The SiOxCyHz has an open porosity, which enables the access into the pores of the chemical species to be separated.

(18) Preferably, the porosity is between 10% and 40%, or even between 10% and 60%. This percentage represents the volume of the pores for a given volume of material. Preferably the pores have a radius between 1 nm and 3 nm, or even between 1 nm and 5 nm. These dimensions have been observed by ellipsoporosimetry, the probe molecule being toluene, the measuring device being the EP12 modelSOPRA firm.

(19) The SiOxCyHz may advantageously be deposited by chemical vapour deposition or CVD. The deposition of SiOxCyHz is then conformal, i.e. it has a homogenous thickness over the entire length of the walls.

(20) Moreover, the addition of a pore-forming agent during the vapour phase deposition makes it possible to control the porosity and to have a low dispersion in the pores of the stationary phase. As indicated previously, the fact of controlling the size, the size dispersion and the spatial distribution of the pores makes it possible to make the chromatographic peaks sharper.

(21) In an even more advantageous manner, it may be deposited by chemical vapour deposition, enhanced for example by plasma, a method known by the acronym PECVD (Plasma-Enhanced Chemical Vapour Deposition), which makes it possible to obtain a more conformal deposition than the sputtering techniques of the prior art. Thus, deposition by CVD or enhanced CVD are well suited to channels having a large form factor. It will be recalled that a deposition by PECVD, or, more generally, by enhanced CVD, makes it possible to carry out the deposition at low temperature and thereby conserve the organic character of the material. This favours the homogeneity of the spatial distribution of the pore-forming agents and, in fine, the homogeneity of the spatial distribution of the pores formed during the elimination of the pore-forming agents.

(22) According to an advantageous variant, the deposition may be carried out by filament assisted chemical vapour deposition or FACVD, which makes it possible to optimise the conformity.

(23) According to another variant, the deposition may be carried out by chemical vapour deposition of a first layer comprising SiOCH and the pore-forming agent, followed by chemical vapour deposition of a second layer so as to constitute a second gas tight layer. A step of foaming is then carried out, which enables the formation of pores in the layer of SiOCH. The second layer is then eliminated.

(24) This method is described in the patent application FR2918997. It enables pores of greater size to be obtained than depositions by CVD or enhanced CVD.

(25) Moreover, the formation of the stationary phase by CVD makes it possible to form several columns collectively. Furthermore, better reproducibility is obtained.

(26) After the step of chemical vapour deposition, the pore-forming agent is eliminated by a thermal method (annealing), potentially enhanced by UV irradiation. During this step, the pore-forming agent disappears, which enables the formation of pores in the matrix.

(27) According to an embodiment, whatever the deposition method implemented, the layer of porous SiOCH may undergo a post-treatment to modify the surface chemical functions. This makes it possible to adjust the selectivity of the stationary phase with respect to a given analyte. An example of post treatment is the application of a O.sub.2, He, or N.sub.2O plasma. Another example is a silanisation, that is to say a covalent grafting of organic molecules via a silane function.

(28) In FIG. 2 may be seen a photo of a transversal section of a channel 6, made in a substrate 4, the channel 6 being covered with a stationary phase 5 made of SiOCH. In FIG. 2A may be seen an enlarged view of a lateral edge of the channel of FIG. 2, in which the thickness of the stationary layer is indicated in two spots (0.100 m and 0.106 m) and in FIG. 2B may be seen an enlarged view of the bottom on which the thickness of the stationary layer is 0.140 m. The stationary phase is delimited schematically by a broken line. A low variation of this thickness is thus observed. The calculation of the conformity is 0.7: this is the ratio of the lowest thickness of the layer over the highest thickness, these thicknesses being measured over the section of the channel. By comparison, when a deposition is carried out by liquid deposition method, the conformity observed is less than 0.05. On the bottom.

(29) The formation of the stationary phase by chemical vapour deposition enables a formation of several columns simultaneously, for example in the case of microcolumns formed in a substrate made of silicon. The stationary phases of all the microcolumns formed on a wafer made of silicon may be formed simultaneously.

(30) Moreover, chemical vapour deposition offers good reproducibility from one wafer to the next.

(31) Thanks to the invention, columns may be formed offering good separation efficiency, for example more than 4000 plates per meter.

(32) In FIG. 3B may be seen a chromatogram obtained with a microcolumn comprising a stationary phase made of porous SiOCH of 120 nm thickness, following the injection of a BTEX (benzene, toluene, ethyl-benzene, o-xylene) mixture, the SiOCH having a porosity of 30%.

(33) The injection conditions are the following: the temperature of the column is 60 C., the carrier gas is helium at 20 psi (that is to say a column flow rate of around 0.5 ml/min), 0.01 l of BTEX mixture (i.e. around 3.5 ng of each compound injected into the column).

(34) For the formation of chromatograms, the detector connected at the output of the column is a FID type detector; the column has a length of 1.3 m, a depth of 80 m and a width of 80 m.

(35) In FIG. 3A, the chromatogram is obtained with a microcolumn without stationary phase.

(36) It may be noted that the column with the stationary phase made of SiOCH with a porosity of 30% makes it possible to separate very efficiently the compounds of the BTEX mixture whereas the column without stationary phase does not carry out any separation.

(37) FIGS. 3C and 3D represent chromatograms obtained on the same microcolumns as previously, with respectively a deposition of porous SiOCH with 3% porosity and a deposition of porous SiOCH with 1% porosity for the injection of a BTEX (benzene, toluene, ethyl-benzene, o-xylene) mixture.

(38) It may be noted that a layer of SiOCH having a porosity of 3% does not assure satisfactory separation of the compounds. A column comprising a layer of SiOCH having a porosity of 1% is equivalent to a column without stationary layer.

(39) In FIGS. 4A and 4B may be seen chronograms obtained with microcolumns according to the invention. The inner surface of the column used for the chronogram of FIG. 4A is covered with a stationary phase of SiOCH the porosity of which is 33%. The inner surface of the column used for the chronogram of FIG. 4B is covered with a stationary phase of SiOCH, the porosity of which is 20%.

(40) A C5-C7-Tol-C8 (pentane, heptane, toluene and octane) mixture is injected, with a flow rate of 0.5 ml/m, into the columns in which the temperature of the column is 40 C.

(41) FIGS. 4A and 4B also illustrate the efficiency of the columns with porous SiOCH according to the invention in the separation of the analytes, in the present case, those of the C5-C7-Tol-C8 mixture.

(42) Consequently, the choice of SiOxCyHz, the values of the coefficients x, y and z and the value of the porosity make it possible to form a stationary layer with great efficiency and does so in an unexpected manner.

(43) An example of method of formation of the microcolumn according to the invention will now be described.

(44) In this embodiment example, the microcolumns are formed in a silicon wafer. Several columns are formed on a same wafer, for example 35 columns may be formed on a same wafer.

(45) The formation of one column will be described, but it will be understood that several columns may be formed simultaneously.

(46) During a first step, the channel is etched in the wafer made of silicon.

(47) For example, a photolithography is carried out then an etching, for example a deep reactive ion etching or DRIE.

(48) During a following step, the stationary phase made of SiOCH is deposited, for example by PECVD, for example with a Producer SE installation of the firm Applied Materials.

(49) DEMS (diethoxymethylsilane) is used for example as precursor for the matrix. Potentially it also possible to use other organosilane precursors such as tetramethyl-cyclotetrasiloxane, dimethyl-dioxiranyl-silane, diethoxy-methyl-oxiranyl-silane, etc.).

(50) The pore-forming agent is for example norbornadiene (NBD). Other pore-forming agents may be used, for example organic molecules such as norbornene, alpha terpinene, cyclohexene oxide, cyclopentene oxide, trivertal.

(51) As an example, the conditions used for the deposition by PECVD are: temperature of the substrate: 300 C. pressure in the chamber 7 Torr, power of the plasma 630 W, flow of O.sub.2 175 sccm, flow of DEMS (100 to 2000 sccm), flow of NBD (0 to 2000 scm).

(52) The deposition step is followed by an annealing step, which enables the elimination of the pore-forming agent and the formation of the pores, since it does not withstand high temperature, and to cross-link the matrix. The substrate is for example subjected for several minutes to UV at a temperature of the order of 400 C.

(53) During a following step, a cap 8 for example made of glass or made of silicon is sealed onto the substrate so as to close the channel 6. This may be sealed for example by anodic sealing, adhesive serigraphy, lamination of a polymer film, molecular sealing. A step of pre-treatment of the sealing surfaces may potentially take place before the sealing. This may be a plasma treatment (helium, oxygen, etc.) or chemical treatment designated by the term Piranha (application of a liquid H.sub.2O.sub.2+H.sub.2SO.sub.4 mixture). This enables a surface cleaning and consequently improves the contact between the substrate and the cap during sealing.

(54) In the case where several columns are formed simultaneously on a same wafer, the columns thereby formed equipped with their cap 8 are cut up so as to obtain individual microcolumns.

(55) As an example, the dimensions of the channels are: a depth h of between 50 m and 200 m, a length of between 0.5 m and 3 m, and a width L of between 20 m and 120 m.

(56) It will be understood that the invention is not limited to gas chromatography microcolumns but to any gas chromatography column. Nevertheless, it has an additional advantage for microcolumns.

(57) In the case of a capillary column, the SiOCH layer may in particular be applied by sol-gel, the pore-forming agent being initially mixed in the sol, then eliminated after the gelling.

(58) The chromatography column according to the invention is intended to form part of an analysis system, serving to separate the compounds of a mixture. The mixture is transported inside the column by a carrier gas, for example helium.

(59) The column is thus intended to be arranged between an injector connected at the inlet of the column to inject the mixture to be analysed, a system for regulating the carrier gas which injects the carrier gas into the column and a detector that is connected at the outlet of the column and analyses the compounds separated by the column. A system for regulating the temperature of the column may be provided.

(60) The analysis system thereby formed may be used for gas analysis in different application areas such as the environment, safety, health, the pharmaceuticals industry, the food processing industry, petrochemicals, etc.