WO1996016331A1

WO1996016331A1 - Method for depositing a conducting polymer into a tube by vapour phase chemical oxidation and its use as chromatography column

Info

Publication number: WO1996016331A1
Application number: PCT/GB1995/002704
Authority: WO
Inventors: John Merza Watson; Krishna Chandra Persaud
Original assignee: Aromascan Plc
Priority date: 1994-11-18
Filing date: 1995-11-17
Publication date: 1996-05-30
Also published as: AU3876595A

Abstract

There is disclosed a method of depositing conducting polymer onto a tube wherein the deposition is performed by a vapour phase chemical oxidation process.

Description

METHOD FOR DEPOSITING A CONDUCTING POLYMER INTO A TUBE BY VAPOUR PHASE CHEMICAL OXIDATION AND ITS USE AS CHROMATOGRAPHY COLUMN

This invention relates to chromatography performed using capillary columns coated with conducting polymer.

The technique of gas-solid chromatography utilises a mobile phase - a carrier gas - that passes through a column containing the stationary phase, which can be a solid coating on the surface of a capillary column or a fine powder packed into a column. The carrier gas, usually nitrogen or helium, passes through a drying tube and flow controller. The sample is injected into the column by syringe through a septum into the injection unit, which is at a high temperature so as to vaporise the sample. The sample, having been carried through the column by the mobile phase is then detected using a flame ionisation detector or other means.

Inverse gas-chromatography (IGC) differs from normal chromatography in that a probe sample is used to investigate a physical property of the surface. The sample is injected onto the column and is carried down the column by the mobile phase. The probe sample molecules will collide with the surface of the stationary phase at various times during its journey down the column, and will be adsorbed on the surface with a probability of adsorption and a residence time that depend on the nature of the interaction between the surface and the probe molecule. The stronger the interaction the longer the molecule spends on the surface and, since the molecule only travels down the tube in the mobile phase, the longer the solute takes to elute. The elution time is thus a function of the strength of adsorption, and therefore samples may be separated and identified on the basis of their differing elution times. The elution time also depends on the nature of the isotherm, i.e. the adsorption energy sometimes changes with variations in solute concentration.

The relation between the elution time and the strength of adsorption is used to calculate a number of different surface physical parameters, whilst deviations from ideal behaviour, e.g. tailing, can be used to explain kinetic and mass transfer processes.

IGC is capable of rapidly measuring analytes at very low dilutions because equilibration is rapid and a vast surface area can be measured. More than one sample probe can be used at once. The amount of sample used can be very small and is only limited by the sensitivity of the detector. It is also very easy and quick to carry out measurements at room temperature.

When looking at surface interactions, the interaction can be split into two components : the dispersive (van der Waals' forces, γ ) and the specific component (Lewis acid/base forces γ*). Papirer et al (Papirer, E, Vidal, A, and Balard, H, Inverse Gas-Chromatography, Characterisation of polymers and other materials, ACS Symposium Series 391, American Chemical Society, Washington DC, 1989) used this method of splitting the interaction to determine the nature of the adsorbent with IGC using various probe solutes. The fundamental equation they used relates the free-energy of adsorption (ΔG ) to the net retention volume (V_N).

(1)

ΔG_A = -RT.ln(V_N) + B The net retention volumes are calculated from equation 2.

(2) v_N= JJ-J.

where j is the pressure gradient correction factor, F is the flow rate and t'_r is the net retention time (see Figure 1).

Papirer et al ran a homologous series of alkanes from pentane to decane, to investigate the dispersive interactions. The graph RT.lnV_N vs number of carbon atoms gave a straight line, the gradient of which was thought to be the free-energy of adsorption of a methylene group. Equation 3 was used to determine the dispersive component of the surface free-energy of the adsorbent (γ ).

(3)

D 0.25 ΔG ^C"²

Ys =

YCH, N.a C, H,

where N is Avogadro's number, γCH₂ is the surface free-energy of a solid only containing CH₂ groups, e.g. polyethylene γCH₂ = 36.8 - 0.058 T(°C)) and a^ is the cross sectional area of an adsorbed CH₂ group (6 A²). Chehimi et al (Chehimi, M M, Pigois-Landureau, E, Delamar, M, Watts, J F, Jenkins, S N, and Gibson, E M, Bull Soc. Chim, Fr., 1992 122 (2), 137) have examined the dispersive component of a polypyrrole powder surface using a packed column. They also attempted to quantify the specific interactions of a polar probe with the parameter I_sp. This parameter is derived by comparing the retention volume of the polar probe (V_N) and the retention volume of a theoretical solute (V_Nre£) which undergoes no specific interaction, i.e. an alkane (see equation 4).

(4)

Chehimi et al used this parameter to investigate the interaction of polypyrrole with various polar probes, and determined that polypyrrole did interact significantly with the polar organics that were tested.

The present invention provides a novel chromatographic means of separating a wide range of polar and non-polar samples involving the use of capillary columns coated with conducting polymer. The invention also embraces a method of producing said columns.

According to one aspect of the present invention there is provided a method of depositing conducting polymer onto a tube wherein the deposition is performed by a vapour phase chemical oxidation process. The tube may be a capillary column and may comprise fused silica.

The capillary column may be exposed to an oxidising agent followed by exposure to the monomer vapour.

The monomer vapour may be present in combination with water vapour.

The oxidising agent may be FeCl₃ and may be present in solution in a polar solvent. Other oxidising agents may be used. The solvent may be l-methoxy-2-propanol or another polar solvent which may be selected having regard to its viscosity.

The monomer may be pyrrole or N-methyl pyrrole but other monomers such as anilines, thiophenes and indoles may be used.

The capillary column may be treated with chromic acid prior to exposure to the oxidising agent.

A second layer of conducting polymer may be deposited by electro¬ chemical means.

According to another aspect of the present invention there is provided a chromatograph comprising a tube coated with at least one conducting polymer.

The tube may comprise a capillary column. The conducting polymer may be deposited by a vapour phase chemical oxidation process. An extra layer of conducting polymer may be deposited by electrochemical means.

The conducting polymer may be polypyrrole or poly-N-methyl pyrrole.

The chromatograph may be a gas-solid chromatograph.

The chromatograph may comprise a flame ionisation detector.

The chromatograph may be used for gas chromatography in which instance the stationary phase comprises the coated tube.

The chromatograph may be used to perform inverse gas chromatography.

An electric field may be applied to the conducting polymer and this application may be used to selectively desorb chemical species. The electric field may be applied via a plurality of electrodes inserted in the capillary column. The applied electric field may be static, alternating, pulsed or swept along the column.

Methods of depositing conducting polymer and a chromatograph in accordance with the present invention will now be described with reference to the accompanying drawings, in which :- Figure 1 shows the determination of the net retention time;

Figure 2 shows the apparatus used for coating the capillary;

Figure 3 shows the chromatograms of a series of alkanes;

Figure 4 shows the chromatograms of two polar probes;

Figure 5 is a graph of RTlnV_R against number of carbon atoms;

Figure 6 is a graph of RTlnV_R against lgPo; and

Figure 7 shows a capillary column equipped with electrodes.

a) Deposition of Conducting Polymer

The first aspect of the present invention is a method of depositing conducting polymer onto a tube by a vapour phase chemical oxidation process.

Of particular relevance to chromatography is the example where the tube is a fused silica capillary column.

The vapour phase chemical oxidation process comprises exposure of the capillary column to an oxidising agent followed by exposure to the monomer vapour. The monomer vapour may be present in combination with water vapour. The examples described hereinbelow involve the deposition of conducting polypyrrole and poly N-methyl pyrrole using FeCl₃ as the oxidising agent. Other conducting polymers known to those skilled in the art, such as polyanilines and other substituted polypyrroles, and other oxidising agents, such as potassium dichromate, sulphuric acid and potassium ferricyanide, are within the ambit of the invention.

Fused silica capillary tubing (J & W Scientific) of internal diameter 0.53 ram, outside diameter 0.15 mm and length 200 cm is employed; the tubing is externally coated with a protective layer of polyimide. Nitric acid and chromic acid are used to clean the capillary tubing. 1M nitric acid is prepared from concentrated nitric acid (BDH). Chromic acid is prepared by adding 97.99% sulphuric acid (Vickers Labs) (200 ml) to potassium dichromate (20 g) in water (40 ml). Pure deionised water and 99.8% methanol (BDH) are used to wash the capillary tubing.

Ferric chloride solutions are prepared by adding anhydrous FeCl₃ (Aldrich) to l-methoxy-2-propanol and stirring overnight with a magnetic stirrer. The solutions are filtered through celite and stored in reagent bottles wrapped in aluminium foil to inhibit photochemical oxidation. Said oxidation necessitated weekly preparation of fresh solutions.

Pyrrole and N-methyl pyrrole (Aldrich) are distilled under vacuum before use.

The procedure for coating the internal surface of the capillary tube with polypyrrole comprises in the first instance cleaning said internal surface with chromic acid (1 minute) followed by washing with water (30 seconds) and methanol (30 seconds) and drying with air (5 minutes). Subsequent procedure is depicted in Figure 2, which shows the capillary tubing 20 attached to a Buchner flask 22 via a reducing union 24 equipped with a graphite ferrule.

The other end of the capillary tubing is placed (a) into a saturated solution of FeCl₃ in l-methoxy-2-propanol 26 for 5 minutes, the vacuum created by the water pump (not shown) thereby drawing said solution through the capillary and into the Buchner flask. Next, the capillary tubing is placed (b) into a flask containing pyrrole 28 and water 30, and pyrrole/water vapour is pumped through the capillary for 20 minutes. Finally the FeCl₃ is washed off with methanol for 5 minutes and the tube dried.

Viewed under a microscope, the surface is revealed to be totally covered with black polymer. The experiment was successfully repeated with capillary tubes 60, 110 and 210 cm long.

The coating with poly-N-methylpyrτole is substantially similar to the above. The capillary was 20 cm long and a 3.04 M FeCl₃ solution in l-methoxy-2- propanol is used. The N-methylpyrrole/water vapour is pulled through for an hour, and a dark brown film is observed on the surface of the capillary.

(b) Gas Chromatography on a Conducting Polymer Coated Capillary

The second aspect of the present invention is a chromatograph comprising a capillary column coated with at least one conducting polymer. The capillary column may comprise fused silica and the conducting polymer may be deposited by the vapour phase chemical oxidation process hereinabove mentioned. It may be desirable to deposit a second layer of conducting polymer by electrochemical means.

The example described hereinbelow comprises inverse gas-solid chromatography on a polypyrrole coated capillary. The gas chromatograph used is a Hewlett Packard 437 with a control pad which enables the temperature of the oven to be set. The detector is a flame ionisation detector (FID) which requires hydrogen and air supplies. The mobile phase is dry nitrogen. The conditions for every chromatogram are: injection unit at 150°C, oven at 40°C and FID at 250°C. A Hewlett Packard 3390A integrator is used to record the chromatograms.

The polypyrrole coated fused silica capillary column (210 cm length; 0.53 mm internal diameter) is cut down to 2 metres and joined with a quick fit connector to a 2 metre fused silica guard column, also of 0.53 mm internal diameter. The other end of the coated capillary is attached to the injection unit via a 0.53 mm to —inch adapter o equipped with a graphite ferrule. The nitrogen supply is turned on, the pressure is set at 150 kPa, and the column is conditioned at an oven temperature of 120°C for 12 hours. After conditioning the capillary column is attached to the detector via a graphite ferrule containing adapter.

The nitrogen and hydrogen flow rates are measured using a soap film flow meter which is attached to the outlet of the FID. It is not possible to measure the flow rate of air, but the air pressure is kept at a constant 100 kPa for all measurements. The required flow rate should separate pentane and hexane. The nitrogen flow rate is set to 0.8 ml min'¹ and the hydrogen flow to 42 ml min^"1. The retention times of hexane and pentane are significantly different, indicating that work may be carried out using these flow rates.

Samples of a homologous series of alkanes are kept in 10 ml sample bottles with septa sealed crimped tops. The experiment employed n-pentane, n-hexane, n- octane, n-nonane and n-decane. The hexane and pentane samples are kept at around 37 °C and the remainder of the sample bottles are placed in an oil bath for at least 3- minutes before sampling. N-octane is kept at 80°C, n-nonane at 110°C and n-decane at 120°C.

The same procedure is used for each solute. A sample of the head space is taken using a gas tight syringe, the syringe is rapidly brought to the injection unit and the septum is penetrated. The needle is left for 3 seconds, the injection is made and the needle is rapidly withdrawn. The injection volumes vary: 2 μl for n-octane, n-nonane and n-decane; 4 μl for pentane; and 5 μl for hexane.

Some polar probes are also employed, namely diethyl ether, methanol and acetonitrile. The acetonitrile and methanol sample bottles are kept in an oil bath at 110°C and 70°C respectively, whilst the ether sample is stored at 37°C. The same injection procedure as the alkanes is followed for the chromatograms of these solutes. A 1 μl sample of ether is run four times and a 2 μl sample of acetonitrile is run three times. Figures 3 and 4 show chromatograms for n-alkanes and polar probe molecules respectively. Figure 3 shows that as the number of carbons increase, so the retention times and extent of tailing increases. Figure 4 shows that the acetonitrile peak displays much more tailing than the less polar ether. Methanol, on the other hand, appears to be too polar and the bands tail to the extent that the peak is not resolved.

The principal cause of tailing is the mass transfer between phases, although there will also be band broadening by longitudinal diffusion due to the slow flow rate of the mobile phase. The tailing by mass transfer is caused by the slow equilibration between phases, and this can be caused by the solute molecules being caught in the pores of the polypyrrole phase. Once molecules are trapped in the pores they take some time to diffuse out, by which time the bulk of the band has passed by, resulting in solute molecules left tailing behind the band. The stronger a solute binds to a surface, i.e. with the high alkanes and polar solutes, the greater the tailing.

It should be noted that the calculations described below have been made using equations that assume a linear isotherm, i.e. normally distributed elution bands. The tailing indicates non-ideal behaviour which should be considered when undertaking the calculations.

The net retention time is calculated by subtracting the retention time of the unretained solvent from the solute retention time. The chromatograms carried out with only the guard column give the same retention times regardless of the solute. It is assumed that none of the solutes are retained by the silica surface, so if the retention times are doubled to allow for the polypyrrole column the resulting retention time is equivalent to the retention time of an unretained solute. The linear retention volume is then calculated using equation 2, where the flow rate is determined to be 0.75 ml/min. The correction for the vapour pressure of the water is not significant and is thus ignored.

Table 1 shows the results and calculations of V_R so that RtlnV_R of the homologous series of carbons may be plotted against the number of carbons in the solute (Figure 5). A linear model can be fitted to the data, a better fit being obtained when pentane is rejected. The basis for rejecting the pentane result is that there is insufficient interaction with the polymer surface for it to be considered in IGC calculations.

^* Cj C₆ C, >Cf ^■■ C_j (QflΛO CH₃CN t_R(secs) 51.8 56.0 70.4 106.7 161.0 50.9 105 _R (sees) 0.50 4.7 19.1 55.4 109.7 0 53.7

V_R (m³) 6.25x10^"9 5.88x10^"8 2.39xl0^"7 6.93x10-⁷ 1.4x10^"° - 6.7xl0-⁷ lnV_R -18.89 -16.65 -15.25 -14.18 -13.50 - -14.21

RTlnV_R -49157 -43328 -39685 -36900 -35131 - -36978 (kJmol-¹) lnp° - 2.0486 0.9555 0.4902 0.1075 - 1.8438 (@20^«C)

Table 1. The results of inverse chromatography with alkanes and polar probes (p° is the vapour pressure of the probe).

The gradient of the graph is equal to the surface free energy of adsorption of a methylene group which from Figure 5 is 2086 KJ/mol. The dispersive component of the surface free-energy (γ°) is calculated from equation 3, using the result from Figure 5, and found to be 23.4 mJ/m². Chehimi et al calculated y° for powdered polypyrrole at various temperatures and found results between 30 to 40 mJ/m². The disparity between the results of Chehimi et al and the present results may be caused by the difference in the stationary phases. Chehimi et al used columns packed with polypyrrole powder as opposed to coated a capillary column. The preparation of the polypyrrole powder may give different surface characteristics to the polymer.

Figure 6 is a plot of RTlnV_R against the vapour pressure of solutes at 20 °C, for alkanes and acetonitrile. The ether retention time is the same as the unretained retention time so it is not used in this plot. The parameter I_sp is calculated for acetonitrile from equation 4 by comparing the RTlnV_R of acetonitrile with that of a theoretical alkane with the same p° @ 20°C. The I_sp for acetonitrile is calculated to be 5.8 KJ/mol.

The results described above demonstrate that it is possible to produce a capillary chromatography column with a conducting polymer phase, and that such a column can separate a homologous series of alkanes and retain polar compounds with high affinity.

A further desirable aspect of the present invention is the possibility of active chromatographic supports which utilise the conducting properties of the polymers employed. By applying an electric field it may be possible to modulate the amount of charged bipolaron species in the conducting polymer and to thereby modulate the retention times of adsorbed analytes on the polymer. There are several possible variations within such a scheme : (a) It is possible to modulate the surface charge of the polymer by inserting electrodes at strategic points along the column. Figure 7 shows a sequence of electrodes 70 on a capillary 72. The electrodes are in contact with conducting polymer 74, and may be produced from an inert metal such as gold, stainless steel or nickel. A potential difference may be applied longitudinally along the capillary or across specific regions thereof. The electric field may be static, alternating, pulsed or swept along the capillary via the sequence of electrodes.

(b) It is further possible to vary the nature of the deposited polymer. The variations may be achieved by changing the counter-ion employed: in the examples described above the counter-ion is the chloride anion, but others, such as tosylate, dichromate or ferricyanide, may be envisaged. Because of the presence of electrodes, electrochemical deposition of a new active surface containing a different conducting polymer is also possible: examples of suitable polymers are polyanilines, polythiophenes, polyindoles and polypyrroles.

The use of such active chromatographic supports would permit rapid separations through the use of much shorter columns than presently used in the art.

Because the surfaces of the polymers employed are charged, non-polar analytes are retained for much less time than polar analytes, the latter being adsorbed strongly due to dipole-dipole, ion-dipole and ionic interactions. By applying an electric field the local charge on the polymer surface may be modulated to stimulate active release of adsorbed analytes, and the precise potential employed may preferentially desorb certain chemical species.

Claims

•CLΔIMS

1. A method of depositing conducting polymer onto a tube wherein the deposition is performed by a vapour phase chemical oxidation process.

2. A method according to claim 1 in which the tube is a capillary column.

3. A method according to claim 2 in which the capillary column comprises fused silica.

4. A method according to any one of claims 1 to 3 in which the tube is exposed to an oxidising agent followed by exposure to monomer vapour.

5. A method according to claim 4 in which the monomer vapour is present in combination with water vapour.

6. A method according to claim 4 or claim 5 in which the oxidising agent is FeCl₃.

7. A method according to any one of claims 4 to 6 in which the oxidising agent is present in a polar solvent.

8. A method according to claim 7 in which the polar solvent is l-methoxy-2- propanol.

9. A method according to any one of the previous claims in which the monomer is pyrrole or N-methyl pyrrole or an aniline, thiophene or indole.

10. ^■ A method according to any of the previous claims in which the tube is treated with chromic acid prior to exposure to the oxidising agent.

11. A method according to any of the previous claims in which a second layer of conducting polymer is deposited by an electrochemical method.

12. A chromatograph comprising a tube coated with at least one conducting polymer.

13. A chromatograph according to claim 12 in which the tube comprises a capillary column.

14. A chromatograph according to either claim 12 or claim 13 in which the conducting polymer is deposited by a vapour phase chemical oxidation process.

15. A chromatograph according to claim 14 in which a second layer of conducting polymer is deposited by electrochemical means.

16. A chromatograph according to any one of claims 12 to 15 coated with polypyrrole or poly-N-methyl pyrrole or an aniline, thiophene or indole.

17. A chromatograph according to any of claims 12 to 16 comprising a flame ionisation detector.

18. ^" A chromatograph according to any of claims 12 to 17 for gas chromatography wherein the stationary phase comprises the coated tube.

19. A chromatograph according to claim 18 for inverse gas chromatography.

20. A chromatograph according to any of claims 12 to 19 in which an electric field is applied to the conducting polymer or polymers.

21. A chromatograph according to claim 20 in which the application of an electric field selectively desorbs chemical species.

22. A chromatograph according to claim 20 or claim 21 in which the electric field is applied via a plurality of electrodes inserted in the capillary column.

23. A chromatograph according to any of claims 20 to 22 in which the applied electric field is static, alternating, pulsed or swept along the column.