GB2192900A

GB2192900A - Electrophoretic separator

Info

Publication number: GB2192900A
Application number: GB08715487A
Authority: GB
Inventors: Dr Geoffrey Frederick Hewitt; Professor Thomas Franci Irvine
Original assignee: UK Atomic Energy Authority; Research Foundation of State University of New York
Current assignee: UK Atomic Energy Authority; Research Foundation of State University of New York
Priority date: 1986-07-18
Filing date: 1987-07-01
Publication date: 1988-01-27
Anticipated expiration: 2007-07-01
Also published as: GB8715487D0; GB2192900B; GB8617573D0

Abstract

In a method of continuous flow electrophoretic separation comprising injecting a migrant material into a carrier liquid and flowing the liquid through a separation chamber in which it is subjected to an electric field transverse to the flow direction, the carrier liquid includes a viscosity- increasing additive so that convective disturbance of the carrier liquid flow due to Joule heating is prevented. A suitable additive is carboxy-methyl- cellulose. The method may be performed in a separator (10) which comprises a flow channel (12) through which a carrier liquid is caused to flow to emerge through a plurality of outlets (26). Electrodes (18,20) on opposite sides of the channel apply an electric field transverse to the flow direction to separate a migrant material (injected through a port (42) into the carrier liquid flow) into its components. The invention allows separation to be carried out in apparatus which is of simple construction but which enables greater throughputs than separators described for example by Hannig in Z.Anal. Chem., 181, 244 (1961). <IMAGE>

Description

SPECIFICATION Electrophoretic separator This invention relates to a continuous flow electrophoretic separator, and to a method of operation of such a separator.

Hitherto two types of continuous flow electrophoretic separator have been known, in each of which a migrant material and a carrier liquid flow through a region in which an electric field is applied in a direction perpendicular to the flow direction, so as to separate the migrant material into its components. In one type, described for example by K. Hannig, in Z. Anal. Chem., 181, 244 (1961), the separation region is between two flat plates about imm apart or closer, with the electric field applied in a plane parallel to the plates. Both the plates are cooled to minimise the temperature rise due to ohmic heating of the carrier liquid. The through put of this type is however limited to about 5ml/hr.In the other type, described for example in UK Patent Specification No. 1 186184, the separation region lies between a cylindrical stator and a concentric tubular rotor defining between them an annular chamber a few millimetres wide across which the electric field is applied in the radial direction. Convective disturbance is minimised in this type by the creation of a gradient of angular velocity across the annular chamber, by rotating the rotor for example at 150 r.p.m. Although this type of separator enables a much larger throughput to be achieved than does the first type, for example 20 ml/min, it is of course a far more complex piece of equipment.

An object of the present invention is to provide a continuous flow electrophoretic separator of comparatively simple construction, but which will enable much greater throughputs to be achieved than separators of the first type described above.

According to the present invention there is provided continuous flow electrophoretic separator for subjecting a migrant material to electrophoresis comprising a separation chamber, a source of a carrier liquid, an inlet port for inflow of the carrier liquid into the separation chamber, means for injecting the migrant material into the carrier liquid, and a plurality of outlet ports from the separation chamber for outflow of the carrier liquid and the migrant material therethrough, and electrodes arranged to generate an electric field in the separation chamber in a direction transverse to the flow direction of the carrier liquid, wherein the carrier liquid includes a viscosity-increasing additive.

Preferably the carver liquid comprises water to which the additive is added to produce the desired viscous properties. For example the additive might be carboxy-methyl-cellulose. Usually the viscous properties of the carrier liquid will then be non-Newtonian, for example of the power law type that is to say that, at least for shear rates greater than a certain value, the coefficient of viscosity is directly proportional to the shear rate raised to a certain power. The use of a non-Newtonian carrier liquid brings about advantageous changes in the velocity profile within the separation chamber.

The invention also provides a method of operation of a continuous flow electrophoretic separator comprising injecting a migrant material into a carrier liquid, said liquid flowing through a separation chamber in which it experiences an electric field transverse to the flow direction, wherein the carrier liquid includes a viscosity-increasing additive.

As the migrant material passes through the separation chamber, its components are displaced according to their electrophoretic mobilities, so as to follow separate paths, and can therefore be collected from separate outlets from the chamber. However, if the carrier liquid is electrically conducting, as with water, there is also the passage of an electric current through the carrier liquid with concomitant Joule heating. This heating may be non-uniform due to differing resistivities of the different migrant components; due to buoyancy effects this will tend to alter the flow pattern of the carrier liquid, with the possible generation of transverse flow velocities. Such convective disturbance of the flow may decrease the effectiveness with which the components of the migrant material are separated.It has now been found that the use of an increasedviscosity carrier liquid enables such convective disturbances to be substantially prevented.

The invention will now be described by way of example only, and with reference to the accompanying drawings, in which:~ Figure 1 represents graphically the variation of coefficient of viscosity with shear rate for a pseudo-plastic non-Newtonian liquid; Figure 2 shows a diagrammatic medial sectional view, in a vertical plane, through a continuous flow electrophoretic separator; and Figure 3 represents graphically the variation of velocity across the thickness of the flow channel in the separator of Fig. 2, for two different carrier liquids.

For any liquid, the coefficient of viscosity, 11, may be defined as the ratio of tangential stress to rate of shear (i.e. velocity gradient). For a Newtonian liquid the coeffiecient of viscosity is a constant, but for a non-Newtonian liquid it depends upon the value of the rate of shear.

Referring now to Fig. 1, for a pseudo-plastic non-Newtonian liquid at low values of rate of shear, A, the coefficient of viscosity is a constant, but for high values of rate of shear, C, the coefficient of viscosity decreases with increasing rate of shear according to the power law relationship: 71=K (shear rate)fl-1 where K is the consistency of the liquid, and n is the flow index. Between these two regimes, at medium values of shear rate, there is a transitional region B.

One such pseudo-plastic non-Newtonian liquid is water to which a small amount of carboxymethyl-cellulose is added. For values of concentration, C, of carboxy-methyl-cellulose in the range 50 to 1000 parts per million, the values of the consistency K and the flow index n are given by: K 1.045 x 10-4C0"492 n = 1. 193 C-0.0B48 For example, at a concentration of 100 parts per million, K=8.27 x 10-3 n=0.807 so that, in a situation where the velocity changes by 0.1 m/s over a distance of 0.01 m, i.e.

the shear rate is 10 s-1, then ?1=5.26x10-3 Pa s For comparison, the coefficient of viscosity of pure water is P1=1.07x10-3 Pa s Thus in a situation where a shear rate of 10 5- occurs, the addition of carboxy-methylcellulose at 100 p.p.m. to water increases its effective viscosity approximately five-fold. At a greater shear rate the increase will be slightly less; at a shear rate of 40 s~ for example the effective viscosity would be 4.02 x 10--3 Pa s.

Referring to Fig. 2, a continuous flow electrophoretic separator 10 comprises a rectangular flow channel 12 defined between two upright parallel glass plates (not shown), held at a separation of 5 mm, the channel 12 being 0.40 m wide. The base of the channel 12 is defined by a spacer 14 through which are a number of inlet ports 16 for a carrier liquid (only three are shown) arranged so as to ensure the carrier liquid flow is uniform across the width of the channel 12. The spacer 14 also defines the lower parts 15 of the sides of the channel 12, while the upper 0.50 m of each side is defined by a respective electrode assembly 18 or 20. The top of the channel 12 is defined by an outlet plate 22 whose lower surface defines a large number of tapered grooves 24 (only fourteen are shown), and from the top of each groove 24 an outlet pipe 26 extends perpendicular to the plane of the Figure.Each electrode assembly 18 or 20 consists of an electrode chamber 30 in which is located a platinum wire mesh electrode 36, and which is separated from the channel 12 by an ion-permeable membrane 38 supported by a strip 40 of water-permeable resin-bonded cellulose fibre material; each electrode chamber 30 has an inlet duct 32 and an outlet duct 34 for an electrolyte. An inlet port 42 for a migrant material, i.e. the material which is to be subjected to electrophoresis, is provided through one of the glass plates just below the bottom of the two electrode assemblies 18 and 20, but sufficiently far above the inlet ports 16 that the carrier liquid flow is laminar and steady.

In operation of the separator 10 a constant flow of carrier liquid comprising water and a concentration of carboxy-methyl-cellulose of 100 parts per million is supplied through the inlet ports 16, flowing upwardly in the channel 12 and emerging as fourteen separate outlet streams from the fourteen outlet pipes 26. Electrolytes are supplied to each electrode assembly 18 and 20 through the inlet ducts 32; a potential difference is applied between the two wire mesh electrodes 36 so as to generate an electric field across the width of the channel 12; electrolytes and electrolytically generated gases are discharged from the outlet ducts 34. A migrant material is injected into the carrier liquid through the inlet port 42, and is carried upwardly with the carrier liquid. As it passes through the electric field its components are displaced according to their electrophoretic mobilities, so as to follow separate paths (two such paths 50, 51 being indicated by broken lines in Fig. 2) and to emerge in the carrier liquid flow through separate outlet pipes 26. Although an electric current does flow through the carrier liquid, and heat is therefore generated in it, because of the viscous properties of the carrier liquid this heat has substantially no effect on the flow.

It can be shown that the equations describing the flow of the carrier liquid, when expressed in a dimensionless form, include a term corresponding to viscous forces, and a further term corresponding to buoyancy forces. To minimize convective disturbance of the flow the viscous force term should be large and the buoyancy force term small.The viscous force term is proportional to the reciprocal of a generalized Reynolds number, Re, i.e. proportional to:

where p is the density of the carrier liquid, H is the height of the flow channel 12, b is the thickness of the flow channel i.e. the separation of the glass plates defining the flow channel 12, and u is the average flow velocity. [It should be noted that although the shear rate, and hence the coefficient of viscosity, will vary across the thickness of the channel 12, the ratio (u/b) can be taken as a characteristic value for the shear rate, and so the second term in the right hand expression above can be taken as a characteristic value for the coefficient of viscosity w1 ] .

The buoyancy force term is proportional to: g p O b2 H k u2 where g is the gravitational field strength, ss is the thermal expansivity coefficient, k the thermal conductivity, and Q the average rate of heat generation per unit volume within the carrier liquid.

These quantities are substantially unaffected by the addition of additives such as carboxy-methylcellulose to the water of the carrier liquid.

Thus convective disturbance of the flow can be reduced by the addition of an additive such as carboxy-methyl-cellulose, as described above, as this increases the viscous force term. Convective disturbance of the flow can also be reduced by decreasing b, so increasing the viscous force term and also reducing the buoyancy force term. Increasing the average flow velocity u (as is necessary if the same throughput is to be achieved at a smaller value of b) although it does decrease the buoyancy force term, also decreases the viscous force term.

Not only does the addition of carboxy-methyl-cellulose produce the desirable consequence of an increase in the viscous force term, and hence a reduction in convective disturbance of the flow, it also changes the velocity profile across the thickness of the flow channel 12. The flow velocity at a positive distance y from the mid-plane of the flow channel 12 is equal to the average flow velocity u multiplied by a flow factor f given by:

where d is the ratio of the distance y to half the channel thickness b. Thus the shape of the velocity profile depends only upon the flow index n. The addition of carboxy-methyl-cellulose to water leads to a reduction in the value of n, as indicated by the equation for n quoted earlier, from 1 for water alone, to 0.807 at a concentration of 100 p.p.m. to 0.664 at a concentration of 1000 p.p.m.By way of example Fig. 3 shows the variation in flow velocity across the thickness of the flow channel for two values of n: 1 and 0.5. In each cased f is plotted against d. It will be observed that the reduction in n leads to a flatter velocity profile across the central portion of the flow channel 12. There is consequently less variation in the time a portion of the migrant material is exposed to the electric field in the flow channel, for portions of the migrant at different distances from the mid-plane. Consequently there is an improvement in the degree of separation of the different migrant components.

It will be appreciated that the invention is applicable to electrophoretic separators different from that described with reference to Fig. 2. For example an electrophoretic separator might have a different number of outlet ducts than shown, or might have a different shape of flow channel for example an annular flow channel with a radially directed electric field. Furthermore it will be appreciated that the additive concentration in the carrier liquid might be different from that specified above, and that alternative high molecular weight polymers might be utilised as the additive, the molecular weight of such polymers typically lying in the range 50000 to 200000.

Where the additive is carboxy-methyl-cellulose, the concentration may be between about 30 p.p.m. and 1000 p.p.m.

Claims

1. A continuous flow electrophoretic separator for subjecting a migrant material to electrophoresis comprising a separation chamber, a source of a carrier liquid, an inlet port for inflow of the carrier liquid into the separation chamber, means for injecting the migrant material into the carrier liquid, and a plurality of outlet ports from the separation chamber for outflow of the carrier liquid and the migrant material therethrough, and electrodes arranged to generate an electric field in the separation chamber in a direction transverse to the flow direction of the carrier liquid, wherein the carrier liquid includes a viscosity-increasing additive.

2. A continuous flow electrophoretic separator as claimed in Claim 1 wherein the carrier liquid has non-Newtonian viscous properties.

3. A continuous flow electrophoretic separator as claimed in Claim 2 wherein the carrier liquid has a flow index, n, less than one.

4. A continuous flow electrophoretic separator as claimed in any one of the preceding claims wherein the additive comprises a high molecular weight polymer.

5. A continuous flow electrophoretic separator as claimed in Claim 4 wherein the additive comprises carboxy-methyl-cellulose.

6. A continuous flow electrophoretic separator as claimed in any one of the preceding Claims wherein the additive is provided at a concentration of between 30 p.p.m. and 1000 p.p.m.

7. A method of operation of a continuous flow electrophoretic separator comprising injecting a migrant material into a carrier liquid, said liquid flowing through a separation chamber in which it experiences an electric field transverse to the flow direction, wherein the carrier liquid includes a viscosity-increasing additive.

8. A method as claimed in Claim 7 wherein the additive is such as to give the carrier liquid non-Newtonian viscous properties.

9. A method as claimed in Claim 8 wherein the additive is such as to give the carrier liquid a flow index, n; less than one.

10. A method as claimed in any one of Claims 7 to 9 wherein the additive comprises a high molecular weight polymer.

11. A method as claimed in Claim 10 wherein the additive comprises carboxy-methyl-cellulose.

12. A continuous flow electrophoretic separator substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

13. A method of operation of a continuous flow electrophoretic separator, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.