GB2416913A

GB2416913A - A centrifugal particle mass analyser

Info

Publication number: GB2416913A
Application number: GB0417657A
Authority: GB
Inventors: Mark Geoffrey Rushton; Kingsley Stjohn Reavell
Original assignee: Cambustion Ltd
Current assignee: Cambustion Ltd
Priority date: 2004-08-07
Filing date: 2004-08-07
Publication date: 2006-02-08
Also published as: GB0417657D0

Abstract

A centrifugal particle mass analyser removes particles from an aerosol except those close to a desired mass to charge ratio by holding the desired particles in a rotating flow between two electrodes 17,18 or 24,25 between which an electric field exists. Other particles strike the electrodes. The analyser is constructed so that the electric field is not inversely proportional to the required centripetal acceleration of the particles, thereby providing a stable classification of the particles. In one embodiment (fig 8), the electric field is generated in two channels 13 each of which has a centre of curvature which is not coincident which the axis of rotation of the analyser. Another arrangement is described (fig 9) in which an electric field is generated between two coaxial cylindrical electrodes 24,25 which rotate at different velocities.

Description

CENTRIFUGAL PARTICLE MASS ANALYSER WITH IMPROVED

PERFORMANCE

This patent relates to improvements made to instruments that select particles of a desired mass to charge ratio from an aerosol by using an electric field to provide the centripetal force to maintain particles of only that mass to charge ratio in a rotating flow. Such an instrument may generically be termed a centrifugal particle mass analyser, hereinafter CPMA.

Such an instrument is useful because it allows measurement of the concentration of particles of a given mass in an aerosol of interest. This is important in studying the formation and effects of such aerosols.

One particular CPMA is the Aerosol Particle Mass spectrometer, hereinafter referred to as the APM, described in US Pat no 5,428,220 and Journal of Aerosol Science, 1996, volume 27, number 2, pages 217-234. The instrument described therein includes a classifier consisting of a pair of concentric electrically conducting cylinders, one inside the other, which are caused to rotate at the same angular velocity while having an electrical potential difference applied between them. In this way an annular channel is created between the inside surface of the outer cylinder and the outer surface of the inner cylinder and the surfaces of these cylinders also act as electrodes to generate an electric field in this channel. An aerosol containing charged particles suspended in a carrier gas, hereinafter referred to as the inlet aerosol, is introduced to one end of this annular channel. The output aerosol is removed at the other end of the annular channel. Some of the particles are precipitated on the walls of the channel, thus the output aerosol comprises the gas which entered the classifier and whatever particles remain suspended in it. The particles remaining in the output aerosol are sometime described as being selected by the analyser. In the channel, the carrier gas has an axial component of velocity due to its flow from one end to the other and a uniform angular velocity about the axis of the cylinders equal to the angular velocity of the cylinders (flow straightening means near the entry to the channel may be included in the instrument partly to ensure this angular velocity); thus elements of the carrier gas follow helical paths along the classifier. A particle in the aerosol requires a centripetal force to maintain it on the helical path and this is provided for charged particles by the electric field created by the potential difference between the cylinders. The force applied to a particle by the electric field is: F=qE (1) where F is the force in Newtons, q is the charge in coulombs and E is the electric field strength in volts per metro. The radius of curvature of the trajectory this will cause the particles to follow depends on their mass. For particles to remain on the trajectory followed I, - 2 by the carrier gas, their mass, charge and the electric field strength must be consistent with the relation for centripetal acceleration: an = F = qE (2) rtrnJ where rural is the radius of the trajectory in metros, vat is the azimuthal component of velocity, in metres per second, of the aerosol flow due to its angular velocity and flow and m is the mass of the particle. At a given radius and electric field strength, this relationship is only true for particles with the correct mass to charge ratio, . Particles with a higher mass to charge ratio will tend to follow a larger radius of curvature and will thus spiral out towards the outer cylinder, while those with a lower mass to charge ratio will follow a smaller radius of curvature and spiral in towards the inner cylinder. Particles which hit these cylinders will tend to adhere to them and thus the output aerosol will tend to contain particles with only close to the mass to charge ratio calculated above. Given knowledge of the electrical charge on the aerosol particles, which is well established in many cases (see for example Hinds W C; "Aerosol Technology" 2nd Ed, Wiley Interscience, 1999 pp323-33 1), the mass of the particles in the output aerosol can thus be calculated.

The variation of electric field strength with radial position in the annular channel and the variation of centripetal force required at different radii in the channel together create problems in the operation of the APM. The variation in the electric field is inherent in the geometry of the APM described: the field between two long (where the length is substantially longer than the radial width of the gap between them) coaxial cylinders at different voltages is given by the relationship: E V (3) r ln( ro'ter inner where His the electric potential difference between the inner and outer cylinders, in volts, r is the radius, in metres, from the centreline of the cylinders at which the field is being evaluated, ro'ter is the radius in metres of the inner wall of the outer cylinder (ie. the outer wall of the annular channel) and r,nner is the radius of the outer wall of the inner cylinder (ie. the inner wall of the annular channel).

The centripetal force required to maintain particles on the helical trajectory of the gas also varies with radial position in the annular channel: F = ma}2r (4) - 3 where axis the angular velocity of the flow (equal to that of the cylinders). Considering these two equations together, it is clear that the electric field reduces in inverse proportion to the radial location whereas the centripetal force required to maintain a particle on a helical trajectory of constant radius increases in proportion with that radius. Combining (1), (3) & (4) gives the variation of the mass to charge ratio of the particle at equilibrium with radial location: m Oc r-2 (5) 4)eqm Therefore, at different radii in the annular channel particles of different mass to charge ratios are selected. In a centrifugal particle mass analyser particles of greater than the equilibrium mass inherently drift to larger radii. Therefore, a variation of equilibrium mass to charge ratio which decreases with increasing radius causes particles perturbed from the radius which would allow them to pass through the classifier to tend to diverge from that radius and be lost to the walls. Such perturbation may be caused by, for example, Brownian motion or turbulence. Thus this classification field may be described as unstable. This is inherent in the design of the APM, and is the case for those other CPMAs that have an equilibrium mass: charge ratio proportional to a negative power of the radius. A stable classification field, in which the equilibrium mass to charge ratio increases with increasing radius, ie. a positive index in equation (5), would lead to particles perturbed from their equilibrium radius to converge back to it. A classification field that produces a constant equilibrium charge to mass ratio across the channel would be termed neutrally stable.

The variation in selected mass to charge ratio across the channel also results in the output aerosol containing particles with a wider variety of mass to charge ratios than would otherwise be the case. The variation of selected mass to charge ratio and the unstable nature of the classification field it produces reduce the proportion of particles of the desired mass to charge ratio that are transferred from the inlet to output aerosols. These two problems broaden the attainable mass selectivity of the instrument and reduce its sensitivity.

According to the current invention, a centrifugal particle mass analyser is constructed with an electric field in the classifier flow channel which varies with radial location in the channel in proportion with, greater than proportionally with, or between inversely proportional with and in proportion with (not including inversely proportional with) the variation of required centripetal acceleration to maintain particles on the same trajectory as the carrier gas at that radial location.

The desired relationship between the electric field strength and required centripetal acceleration may be obtained by modification to the electric field from that in the APM described above or by changing the flow pattern from the uniform angular velocity used m the APM.

The invention is described referring to the accompanying diagrams: Figure 1 shows a simple schematic of a CPMA modified to change the electric field variation

for neutral stability of the classification field.

Figure 2 shows a cross section through the classification channel of the CPMA in figure 1.

Figure 3 shows a cross section through the classifier channel of a CPMA with reduced

instability of the classification field.

Figure 4 shows a cross section through the classifier channel of a positively stable CPMA.

Figure 5 shows the creation of an azimuthal component of force on the particles by interaction of the electrical field and required centripetal force.

Figure 6 shows the cross section through a classifier channel modified to reduce the azimuthal component of force on the particles.

Figure 7 shows a cross section through a classifier comprising a multiplicity of shaped channels.

Figure 8 shows a two view cross section of an implementation of the invention achieving neutral stability by shaping of the electrodes.

Figure 9 shows a cross section through an implementation of the invention producing variable stability of the classification field by allowing controllable variation of the speed of rotation with radial location in the channel by varying separately the speed of the inner and outer electrodes.

The electric field is dependent on the geometry of the electrodes that generate it. Between coaxial cylindrical electrodes, the field strength varies proportionally with,/ where r is the distance from the centre of curvature of the electrodes. In the APM, this centre is also the centre of rotation of the classifier. If the annular classifier channel is replaced by a smaller channel not fully encircling the axis of rotation then a different cross section can be selected to make the electric field strength vary in a different manner. Figure 1 shows schematically an analyser with the fully annular classifier channel replaced by a classifier channel, 1, which is a small sector of an annulus curved in the opposite direction. A cross section through such an analyser in a plane perpendicular to the axis is shown in Figure 2. The centre of rotation, 2, is clearly no longer coincident with the centre of curvature, 3. In this design, the inner electrode, - 5 - 4, and outer electrode, 5, are cylindrical sectors rather than full cylinders and there are side walls, 6, between them. The electrodes are conductive and the side walls must be sufficiently resistive or insulating to allow an electric field to be applied between the inner and outer electrodes. It is preferable that the side walls are not perfect insulators but have sufficient conductivity to prevent the accumulation of static charges on their surface which would

interfere with the desired electric field.

In the design of Figure 2, with the centre of curvature of the electrodes lying on the extension of the radial line from the centre of rotation through the centre of the channel, the electric field increases with increasing distance from the centre of rotation. When the centre of curvature is an equal and opposite distance from the channel to the centre of rotation (as shown in figure 2), and the gap between electrodes is relatively small compared with the distance from the centre of rotation, the electric field strength varies approximately in proportion to the distance from the centre of rotation. This approximation is improved as the radial gap between the electrodes is reduced. This is equal to the variation of required centripetal force and thus the equilibrium mass to charge ratio across the gap is constant and

the classification field is neutrally stable.

Other variations, lying between the design of Figure 2 and the APM, which has coincident centres of rotation and curvature, are possible. These produce classification fields which are unstable, but less so than the APM. Figure 3 shows a such situation produced by a longer radius of curvature than radius of rotation. The equilibrium charge to mass ratio for this design will approximate proportionality with a power of radius (from the centre of rotation) less negative than the -2 for the APM in equation 5 and thus the classification field is less unstable. Flat plate electrodes are another example, with curvature effectively centred around a point infinitely far out from the centre of rotation: for these the electric field in the gap and the equilibrium mass to charge ratio will be inversely proportional to the radius (a power of -I in the equivalent of equation 5).

For channels curved in the opposite direction from the APM, as in figure I, the radius of curvature need not be equal in magnitude to the radius of rotation. A centre of curvature which is further from the channel than the centre of rotation gives a classification field less unstable than the flat plate electrodes mentioned previously. A centre of curvature which is closer to the channel than the centre of rotation, (figure 4) gives an electric field which varies more strongly with distance from the centre of rotation than the required centripetal force and thus produces an equilibrium mass to charge ratio which increases with distance from the centre of rotation, giving a stable classification field. - 6

In practice, the electrical field in such an annular sector varies not only with distance from the centre of rotation, but also azimuthally. This is because it does not preserve the rotational symmetry of the annular classifier of the APM. Towards the side walls, 6, of the sector, the electrical force, 7, is not in line with the required centripetal force, 8, and has an additional azimuthal component, 9, as shown in figure 5. This creates a tendency for particles to drift towards these edges where they may be lost. This effect is minimised as the angles subtended by the sector at the centres of curvature and rotation are reduced. To further minimise this effect, the shapes of the electrodes may be modified from the coaxial cylindrical shape described. A simple improvement is for the two electrodes to be centred on points slightly displaced from on another as shown in figure 6. Here the inner electrode, 4, is centred on one centre of curvature, 10, and the outer electrode, 5, centred on one closer to the sector, 11.

Further improvements may be obtained by iterative design processes for the electrode shapes utilising available numerical software packages for calculating the electric fields produced by electrodes of arbitrary shapes.

The overall classifier may consist of one of more of these annular sector channels. Given that high speeds of rotation are required, using two or more channels equally distributed around the centre of rotation may be preferred to improve mechanical balance. Alternatively an annular channel may be modified by forming the inner and outer walls with a number of scallops, as shown in figure 7, such that it effectively comprises a multiplicity of such sectors curved around centres further out from the centre of rotation. The regions between these sectors may be occupied with components (12) manufactured from a conductive, resistive, material in order to prevent artefacts caused by departure from the desired electric field variation in these regions.

Instead of, or along with, modifying the electric field from that in the APM, the flow pattern in the classifier may be modified from the uniform angular velocity used in the APM. If the angular velocity of the gas flow in the annular classifier channel is made to vary with radial location then the variation of required centripetal force can be other than proportional to the radius. Such a variation of angular velocity can easily be arranged by rotating the cylinders forming the inner and outer walls of the channel at different speeds.

If the ratio of the angular velocities of the inner and outer walls of the channel is controlled to be in inverse proportion to their radii, and the radial gap is small relative to the radii, then the angular velocity of the gas will be approximately inversely proportion to the radial location.

From equation 4 it can be seen that this requires a centripetal acceleration approximately inversely proportional to radial location, the same as the variation of electrical field strength with radius. Therefore the equilibrium mass to charge ratio across the channel can be uniform and the classification field stable. If the difference in angular velocities of the inner and outer walls is greater than this condition, the classification is stable: if the difference is smaller, the classification becomes increasingly unstable until it reaches the limit of the APM when both walls are rotating at the same angular velocities.

In designing a centrifugal particle mass analyser with different rotation speeds for the inner and outer walls of an annular classifier, the fluid mechanic stability of such a flow pattern must be considered. The stability of such flows is well investigated and published in the literature (for example: Andereck D, Liu S S. Swinney H L; Journal of Fluid Mechanics, 1986, volume 164, pages 155-183). The relevant mode of instability is the formation of Taylor vortices and the applicable condition for flow stability is the Rayleigh Criterion which can be expressed as: )nner < roadster (6) 61)outer runner where Dinner and Her are the angular velocities of, respectively, the inner and outer walls of the classifier. The neutrally stable condition discussed above is clearly within this limit and thus the flow is stable to this instability. There is scope for some stable classification fields without exceeding this limit.

In addition, the shear in the sample aerosol flow due to the difference in speeds of rotation should not be sufficient to cause a transition to turbulent flow. Where the radial gap between the classifier walls is small relative to their radius, the behaviour can be assumed to follow that between parallel plates moving relative to one another. The established parameter (see for example "Turbulence",Hinze J 0, 2nd Ed. McGraw-Hill 1975 pp76-77) determining this transition is the Reynolds' number, defined as: Reshear = - (7) where Res'enr is the relevant Reynolds' number, p is the density of the gas, u is the shear velocity between the walls, d is the gap between the plates and,u is the viscosity of the gas.

For this annular geometry with a small gap, this can be taken to be equivalent to: R P(Vnner Coder)ro'ter (ro'ter runner) (8) Threshold values of Reshear for transition to turbulence are reported from 288 to 1900 or even higher. For practical centrifugal particle mass analysers, this limit may be approached, especially when designing to select small particles, and must be considered. - 8

The CPMA may be combined with well known equipment for Imposing a known charge on the particles in the input aerosol, such as a bipolar neutraliser or unipolar diffusion or field charger. With or without the charger, it may be combined with an instrument to indicate the concentration of particles, or total electrical charge, in the output aerosol. Instruments such as Condensation Particle Counters or Aerosol Electrometers are particularly appropriate. Such a combination may then be used to measure the concentration of particles of all sizes selectable by the CPMA in the input aerosol. This is the size spectrum of the aerosol and the combination may be termed a Centrifugal Particle Mass Spectrometer. This may be operated by changing the parameters (rotation speed, flow rate and voltage) of the CPMA between a number of fixed conditions and measuring the output concentration when the output aerosol is stable at each, or by progressively changing these parameters of the CPMA while continuously monitoring the output of the concentration measuring instrument.

In the detail design of the classifier, it is important to consider the field in the transfer channel, 35, shown in figure 1, which transfers the aerosol flow from the inlet tube to the classifier channel. In the transfer channel, components of both the electrical force and the required centripetal force that are perpendicular to the flow direction must be in balance for particles of the desired mass to charge ratio otherwise these particles may be deposited on the wall of this channel. In the examples shown in this patent, this is achieved by ensuring this channel is free of electric field and the flow is radial. Other designs are possible; for example a transfer channel at an angle intermediate between the radial direction and parallel to the classifier channel could be used if arranged to have a progressive increase in electric field proportional to the required local centripetal force. Such a variation in electric field could be produced by varying the cross section of the channel or by controlling the voltage at either end of the channel and constructing either or both of the electrodes from a resistive material so that the voltage varies progressively along either or both of them.

Two alternative examples of the invention, shown respectively in figures 8 and 9 will now be described.

Figure 8 shows two cross sections through a practical implementation of the concept shown in figures 1 and 2. This design uses two classification channels, 13, for simplicity and inherent mechanical balance. The electrodes in the classifier channels are curved around centres further away from the centre of rotation than the channels themselves, with the distance from the centre of the channel to the centre of curvature being the same as the average distance from the centre of the channel to the centre of rotation. Thus, the instrument is approximately neutrally stable as discussed above. - 9-

For clarity, only the rotating assembly and its bearings, 14, are shown: fixed support mountings for the bearings and aerosol and electrical connections via standard sliprings or equivalent are also required. The rotating assembly must also be rotated at controllable speed, by, for example, an electric motor and controller which may be coupled to the assembly by a belt drive or other means of transmission.

The inlet aerosol enters the CPMA through a hollow axle, 15, which it leaves through a number of radial holes to enter the input transfer channel, 16, where the aerosol flows outwards to the classifier channels. The hollow axle and outer walls of the diffusing space are manufactured from metal or some other conductive material and are held at electrical ground either by contact through its bearings or a standard slip ring (not shown) A single centre electrode, 17, integral with the hollow axles, 14 and 22, serves as the inner electrode for both channels: these are thus held at electrical ground. The outer electrodes, 18, are supported on mounts, 19, which also serve as the side walls for the classifier channels.

These mounts should preferably be manufactured from highly resistive material which allows a strong electric field to be imposed between the inner and outer electrodes but which prevents the accumulation of static charges on the side walls. Materials such as statically dissipative plastics, with a resistivity between 109 and 10'2 ohm cm are suitable for the mounts. Corner elements, 20, are also manufactured from a similar resistive material to provide a progressive increase in electric field from the field free diffusing space to the classifier channel. Electrical connections must be provided from a controllable high voltage supply to the outer electrodes by insulated wires and a standard slip ring.

On exiting the classifier channel, the aerosol enters an output transfer channel, 21, identical to the diffusing space on the inlet side and via radial holes into the lower hollow axle, 22.

For the instrument described here, the classifier channels are designed on a mean radius of 30mm from the centre of rotation and the centres of curvature for each channel, common for their inner and outer electrodes, are displaced 60mm from the centre of rotation. The classifier channels are formed as 40 sectors around the centres of curvature. The classifier channel length is approximately 50mm, and the spacing between inner and outer electrodes is 2.5mm.

From equation 2, it is clear that the voltage applied to the outer electrodes and the rotation speed of the analyser determine the charge to mass ratios of the particles which will be selected by the CPMA. In practice, a range of charge to mass ratios around this nominal value will be selected. The range of charge to mass ratios selected depends on the flow rate of aerosol through the analyser and the electrode voltage and speed of rotation. For a given mass to charge ratio, the ratio of voltage and speed is fixed, but increasing the two together results in a narrower range of particle mass to charge ratios being selected. Increasing the sample - 10 aerosol flow through the analyser increases the range of mass to charge ratios that will be selected. If too narrow a range of mass to charge ratios is selected, then the concentration of particles in the output aerosol may be too small to be useful, whereas if the range of mass to charge ratios were too wide, then the instrument would not be performing its primary role of identifying particles of a specific mass to charge ratio.

The standard way of describing the range of particle sizes in an aerosol is by the geometric standard deviation (GSD) of the distribution of sizes, where a GSD of 1 corresponds to particles all of precisely the same size and larger numbers to wider range of particle sizes. The operating parameters below are chosen to select singly charged particles with diameters distributed with a GSD of 1.1: this corresponds to a GSD of 1. 331 for the distribution of mass to charge ratio (because mass is proportional to diameter cubed). All the speeds and voltages assume an aerosol flow rate of 0.1 litres per minute.

For the described above, a speed of 16500 rpm and a voltage of 5.9 V will select particles with a mass to charge ratio of 0.026 kg/C which corresponds to a singly charged particle with the same density as water and a diameter of 20 nm. The practical limit on the minimum particle size is set by the practical maximum rotation speed. A speed of 6500 rpm and voltage of 112 V selects particles with 3.3 kg/C, corresponding to singly charged particle of lOOnm. A speed of 960 rpm and a voltage of 2.5 kV selects particles with a mass to charge ratio of 3300 kg/C, corresponding to singly charged particles of 1 micron diameter. The maximum particle size limit of the instrument is set by the maximum voltage without breakdown between the electrodes: at atmospheric pressure, this is of the order of I kV/mm.

The size range of the instrument can be increased if the particles are multiply charged. In particular, unipolar diffusion charging may be used. This is common practice is other types of aerosol instrumentation.

An alternative embodiment is shown in cross section in figure 9. In this analyser, an annular classifier channel, 23, is formed between inner, 24, and outer, 25, cylindrical electrodes which are caused to turn at different rotational velocities. An inner spool, 26, passes through the instrument and incorporates hollow sections, 27 and 28, for inlet and exhaust of the aerosol and flanges, 29 to support the inner electrode. The inner spool is supported on bearings, 30, which are mounted in fixed mounts (not shown), and is driven from an electric motor with speed controller via a belt and pulley (not shown). It should also be connected to electrical ground via its bearings or with a standard slip ring and should be conductive: due to the high speeds of rotation, a strong metal such as stainless steel is an appropriate material for the inner spool. 11

The inner electrode, 24, is mounted on support flanges, 34, which are made of a resistive material such that the inner electrode can be controlled to a high voltage which is dropped progressively across the support flange sothat the electric field in the classifier channel builds up progressively. Materials such as statically dissipative plastics, with a resistivity between 109 and 102 ohm cm are suitable for the support flanges. The inner electrode is connected to a slip ring (not shown) via an insulated wire (not shown) buried in a channel (not shown) in the inner spool. The slip ring provides connection to a controllable high voltage supply.

The outer housing, 31, is mounted on bearings, 32, supported on the inner spool. It is driven by a pulley (not shown) from another electric motor with a speed controller. The outer housing is connected to electrical ground by a standard slip ring (not shown). The outer housing should be made of conductive material. To reduce the inertial forces at high speeds, a low density metal such as aluminium is an appropriate material for the outer housing.

The inlet aerosol enters through the hollow pipe, 27, and exits through radial holes, 33, into the space between the flanges on the inner spool and the outer housing. It then flows into the classifier channel, 23, where particles of less than or more than the desired mass to charge ratio are deposited on the electrodes. The output aerosol containing only particles of close to the desired mass to charge ratio then flows inwards through the space between the lower flange on the inner spool and the outer housing into the hollow shaft, 28, via the radial holes.

The outer diameter of the inner electrode is 50mm, and its length is 50mm. The classifier channel has a radial gap of 1.5mm so the inner diameter of that part of the outer housing is 53mm.

The annular geometry of the classifier yields an electric field strength in inverse proportion to the radius. Therefore, for neutral stability, the flow field must be such that the required centripetal acceleration is also in inverse proportion to the radius. From equation 4, it is clear that this requires the angular velocity of the flow to be in inverse proportion to the radial location. This is achieved by rotating the inner and outer electrodes at speeds inversely proportional to their radii. Thus, for the instrument described here the, outer electrode should rotate at 0.943x the speed of the inner electrode.

For singly charged particles, to select aerosols distributed in diameter with a Geometric Standard Deviation of approximately I.1, the following conditions are required: Particle diameter 20nm, corresponding to mass to charge ratio of 0.026 kg/C, inner speed = 19800 rpm, outer speed = 18670 rpm, voltage = 4.2 V. Particle diameter lOOnm, corresponding to mass to charge ratio of 3.3 kg/C, inner speed = 7760 rpm, outer speed= 7320 rpm, voltage = 81 V. - 12 Particle diameter 1 micron, corresponding to mass to charge ratio of 3300 kg/C, inner speed = 1 160 rpm, outer speed = 1090 rpm, voltage = 1800 V. Intermediate conditions can be used to select particles of intermediate mass to charge ratios.

The advantage of this style of CPMS is that the degree of stability can be adjusted by changing the ratio of rotation speeds of the inner and outer electrodes. In particular, increasing the difference in the speeds can broaden the range of particle mass to charge ratios selected without reducing the transmission efficiency of the selected particles.

Claims

1. A centrifugal particle mass analyser constructed with an electric field in the classifier which varies with radial location in the classifier in proportion with, greater than proportionally with, or between inversely proportional with and in proportion with (not including inversely proportional with) the variation of required centripetal acceleration to maintain particles on a trajectory at that radial location in the channel.

2. An instrument as described in Claim I where the electric field in the classifier flow channel varies with radial location in proportion with or greater than proportionally with the variation of required centripetal acceleration to maintain particles on a trajectory at that radial location.

3. An instrument as described in Claim 1 or Claim 2 where the electrical field is arranged to vary other than inversely proportionally with the distance from the centre of rotation of the classifier.

4. An instrument as described in Claim 3 where the electrical field strength in the classifier is arranged to increase with the distance from the centre of rotation of the classifier.

5. An instrument as described in Claim 4 where the classifier consists of one or more flow channels and the walls forming said channels include at least two conductive areas acting as electrodes which are curved around centres on the extension beyond the channel of the radial line from the centre of rotation through the centre of the channel.

6. An instrument as described in Claim 4 where those parts of the classifier walls acting as electrodes are in the form of cylindrical sectors curved about a point or points at a distance from the centre of rotation greater than the distance from the centre of rotation to the classifier flow channel.

7. An instrument as described in Claim 5 or Claim 6 where some parts of the classifier walls between those parts acting as electrodes are made of a conductive but highly resistive material.

8. An instrument as described in Claim 4 where the classifier consists of an annular flow channel but with the inner and outer walls modified to comprise a multiplicity of cylindrical arcs centred on points further from the centre of rotation than the classifier flow channel.

9. An instrument as described in Claim 8 where conductive but highly resistive elements are interspersed between the annular arcs forming the classifier channel.

10. An instrument as described in Claim I or Claim 2 where the angular velocity of the aerosol flow is caused to reduce with increasing distance from the centre of rotation.

11. An instrument as described in Claim 10 including an annular classifier channel with inner and outer cylindrical walls which are controlled to different voltages and are rotated at different angular velocities.

12. An instrument as described in Claim 11 where the inner and outer walls of the classifier are rotated at angular velocities inversely proportional to their radii. Is

Amendments to the claims have been filed as follows I. A centrifugal particle mass analyser comprising one or more classifier channels rotating around an axis or axes through which an aerosol consisting of solid or liquid particles suspended in a fluid is caused to flow and the centripetal force required to maintain those particles of a particular mass to charge ratio at a constant radial distance from said axis is provided by an electric field created by controlling parts of the classifier to different electric potentials where the said mass to charge ratio of the particles which are maintained at constant radial distance reduces with said radial distance less strongly than in inverse proportion to the square of the radial distance or is invariant or increases with said radial distance, this being achieved by the geometry of the classifier channel or channels or by causing different parts of the classifier to rotate at different speeds.

2. An analyser as described in Claim 1 where mass to charge ratio of the particles which are maintained at constant radial distance is invariant or increases with said radial distance.

3. An analyser as described in Claim I or Claim 2 where the electrical field is arranged to vary other than inversely proportionally with the distance from the centre of rotation of the classifier.

4. An analyser as described in Claim 3 where the electrical field strength in the classifier is arranged to increase with the distance from the centre of rotation of the classifier.

5. An analyser as described in Claim 4 where the classifier consists of one or more flow channels and the wails formmg said channels include at least two conductive areas acting as electrodes which are curved around centres on the extension beyond the channel of the radial line from the centre of rotation through the centre of the channel.

6. An analyser as described in Claim 4 where those parts of the classifier walls acting as electrodes are in the form of cylindrical sectors curved about a point or points at a distance from the centre of rotation greater than the distance from the centre of rotation to the classifier flow channel.

7. An analyser as described in Claim 5 or Claim 6 where some parts of the classifier walls between those parts acting as electrodes are made of a conductive but highly resistive material. lb

8. An analyser as described in Claim 4 where the classifier consists of an annular flow channel but with the inner and outer walls modified to comprise a multiplicity of cylindrical arcs centred on points further from the centre of rotation than the classifier flow channel.

9. An analyser as described in Claim 8 where conductive but highly resistive elements are interspersed between the annular arcs forming the classifier channel.

10. An analyser as described in Claim] or Claim 2 where the angular velocity of the aerosol flow is caused to reduce with increasing distance from tile centre of rotation.

11. An analyser as described in Claim l or Claim 2 where the angular velocity of the aerosol flow is caused to reduce with increasing distance from the centre of rotation less than an angular velocity inversely proportional to the square of the distance from the centre of rotation.

12. An analyser as described in Claim 10 or 11 including an annular classifier channel with inner and outer cylindrical walls which are controlled to different voltages and are rotated at different angular velocities.

13. An analyser as described in Claim 12 where the inner and outer walls of the classifier are rotated at angular velocities inversely proportional to their radii.