GB2441780A

GB2441780A - An apparatus for measuring a signal from a carrier with surface plasmon characteristics

Info

Publication number: GB2441780A
Application number: GB0615649A
Authority: GB
Inventors: Carl Edward Norman; Katja Franziska Kastl
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2006-08-07
Filing date: 2006-08-07
Publication date: 2008-03-19
Anticipated expiration: 2026-08-07
Also published as: GB2441780B; GB0615649D0

Abstract

An apparatus for measuring a carrier 81, 83, 85, said carrier 81, 83, 85 being configured to exhibit surface plasmon resonance said apparatus comprising a source of polarised radiation 87 for irradiating the carrier 81, 83, 85, a receiver for measuring the radiation reflected from the carrier and a polarisation rotator 89 for rotating the polarisation plane of the polarised radiation dependent on the orientation of the carrier 81, 83, 85.

Description

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M&C Folio: GBP94686 2441780

AN APPARATUS FOR MEASURING A SIGNAL FROM A CARRIER

The present invention relates to the field of reading apparatus for reading encoded carriers and for a method for reading a signal from an encoded carrier.

There has always been a need to be able to quickly and efficiently monitor reactions between different chemicals in order to both identify molecules and to study the reactions of both known and unknown molecules. This need is now all the more acute with the substantial and numerous discoveries being made in the biotechnology field.

Currently, there are a number of methods known for performing this analysis. One particular method is taught in GB 2 404 918. In this patent, encoded carriers are formed. The encoded carriers have a first region which is a reaction region and a second region which is a code region. The reaction region has a variation in its refractive index so that a surface plasmon resonance measurement may be made.

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In a surface plasmon resonance measurement, if radiation of a certain wavelength, polarisation angle and incident angle iiTadiates a structure which has a periodic variation in its refractive index, the radiation will couple to the surface plasmon mode causing a decrease in the amplitude of the reflected light. The periodic variation in the refractive index serves to couple radiation with a correct incident angle and properties to the surface plasmon mode.

The incident angle at which surface plasmon resonance occurs will be dependent on any chemical species on the surface. In GB 2 404 918, probe molecules are attached to the surface of the reaction region. If the probe molecules then react with a further chemical species, the incident angle of the radiation necessary to excite surface plasmon resonance will vary. Therefore, it is possible to determine if a reaction has taken place.

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In GB 2 404 918, encoded carriers are provided with a code corresponding to the different types of probe molecules used in the reaction region.

The measurement of the surface plasmon characteristics of the carriers is slightly cumbersome in that it is necessary to ensure that the direction and polarisation of the incident radiation is suitable to measure surface plasmon resonance. Conventionally, surface plasmon resonance measurements are performed by carefully aligning the reaction region with the measurement apparatus to ensure that the orientation of the polarisation is correct. In practice, the carriers are invariably measured while located in a flow cell. Rotating the whole flow cell is slow and cumbersome, hampered by inlet and outlet flow tubing connections and also increases the chance of mechanical dislodgement of particles between measurements as centrifugal forces on the particles away from the centre of rotation could be large. Similarly, it is cumbersome to rotate the entire optics and detectors of the measurement system.

The present invention addresses the above identified problem and, in a first aspect provides an apparatus for measuring a carrier, said carrier being configured to exhibit surface plasmon resonance and said apparatus comprising a source of polarised radiation for irradiating the carrier, a receiver for measuring the radiation reflected from the carrier and a polarisation rotator for rotating the polarisation plane of the polarised radiation.

Thus, the present invention provides a system where the flow cell and carriers will only have to be moved by small steps in the X-Y directions. The need to rotate the flow cell is removed.

In a preferred embodiment, the source is configured to irradiate said carrier with a frusto-conical beam, the narrowest part of said beam being located at the carrier. The carrier is located at an appropriate distance above the effective focal point of the (truncated) cone in order to provide the requisite range of incident angles, since this range of angles effectively decreases to zero at the focal point of the cone. Thus, an area of the carrier, as opposed to a point on the carrier, is irradiated. This area of the

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carrier is thus irradiated with radiation over a range of different incident angles. Therefore, there is no need to sweep the incident angle of the radiation. Preferably the carrier is located between around 300 jam and around 1000 }im above the effective focal point.

The apparatus of the present invention measures a carrier which exhibits surface plasmon resonance. Such a carrier preferably has a periodic variation in its refractive index in a direction parallel to a surface or interface where the surface plasmon resonance will occur, in order to allow coupling of the radiation to the surface plasmon mode.

The carrier preferably comprises a diffraction grating. The grating may be a planar grating achieved by forming elongate holes or trenches in the carrier, or the carrier may be corrugated. The grating may be a 1 dimensional or two dimensional grating.

Preferably, the apparatus further comprises means to determine the orientation of the carrier and to control the polarisation rotator in accordance with the orientation of the carrier, such that each carrier is measured using radiation having the same polarisation relative to the orientation of the carrier.

Such means may be provided by a camera used to determine the position of the carrier prior to performing the SPR measurement, for example, the carrier may have a corner missing which allows its orientation to be determined. The position of a plurality of carriers may be measured and the orientation may be stored so that a plurality of measurements may then be made on the plurality of carriers. Alternatively, each carrier may individually have its orientation measured and then its SPR measured.

Preferably the carrier comprises at least one indicator to allow alignment of polarisation between carriers.

In a different method, the carrier may be measured using a number of different polarisations as opposed to having its orientation determined.

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Preferably, the carrier has a direction within which it has a maximum periodic variation in its refractive index, and said polarisation rotator is configured to align the polarisation of the radiation with the direction of maximum periodic variation of the refractive index.

The apparatus may be configured to irradiate the carrier with radiation with its polarisation oriented in a first direction and to irradiate the carrier with radiation with its polarisation oriented in a second direction. Preferably the radiation with its polarisation orientation in the first direction is such as to excite a surface plasmon mode within the carrier and the radiation with its polarisation orientation in the second direction is such as to not excite a surface plasmon mode within the carrier. The apparatus may comprise means to combine the measurements from the first and second directions, and the means to combine the measurements may comprise means to subtract, divide or normalise the measurements from, or with, one another.

In a preferred embodiment a carrier is measured using radiation polarised such that it can excite a surface plasmon resonance. Traditionally, this is achieved using a beam of radiation striking a grating perpendicular to the direction of the grating lines, polarised such that the magnetic component of the radiation is parallel to the direction of the grating lines. This is referred to as "TM mode". Preferably the apparatus further comprises a means to make a measurement in "TE mode" in which the polarisation of the radiation is rotated by 90° with respect to the TM mode. Radiation in the TE mode cannot excite surface plasmons. Preferably, the apparatus further comprises means to combine data from these two measurements. If the TE mode measurement data (which shows no SPR) is used to remove the background signal from the TM mode measurement data (which does show SPR), then only the signal associated with surface plasmon generation can be revealed. Thus it is possible to remove spurious signals due to (e.g.) reflections from the carrier, stray objects or impurities in the surrounding liquid medium. Also, by performing two measurements in quick succession, it is possible to correct for longer term variations due to intensity fluctuations of the radiation source.

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Preferably the apparatus further comprises analysis means to determine the position of a surface plasmon resonance signal.

The. applicant has discovered that although the polarisation needs to be aligned within a certain error range in order to observe surface plasmon resonance, this error range is large.

However, in order to maximise the signal to noise ratio in SPR measurement, it is desirable to align the polarisation as closely as possible to the TM mode. Therefore, preferably the polarisation is aligned to within a tolerance of 10°or less.

Generally, the receiver will be configured to measure the amplitude of the reflected radiation as a function of the angle of incidence of the radiation.

The apparatus will preferably further comprise means to move the carrier into the path of the beam of radiation. More preferably, this will be in the form of a flow cell.

In a second aspect, the present invention provides a method of measuring a carrier, said carrier being configured to exhibit surface plasmon resonance, said method comprising: irradiating said carrier with a source of polarised radiation;

measuring the radiation reflected from the carrier;

the method further comprising rotating the polarisation plane of the polarised radiation dependent on the orientation of the carrier.

The present invention will now be described with reference to the following non-limiting embodiments in which:

Figure la is a schematic of an encoded carrier undergoing a surface plasmon resonance measurement prior to reaction of the encoded carrier with a target species, Figure lb is a plot of reflected amplitude against the angle of incidence indicating the resonance angle; Figure 2a is a schematic of the encoded carrier of Figure la undergoing a surface plasmon resonance measurement after reaction of the encoded carrier with a target

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species, Figure 2b is a plot of the reflected amplitude against the angle of incidence indicated in the resonance angle;

Figure 3 is schematic of a surface plasmon resonance measurement apparatus in accordance with the prior art;

Figure 4 is an encoded carrier having a planer strip diffraction grating on a barcode;

Figure 5(a) is a schematic showing in perspective the TE and TM polarisation modes in a reflection SPR measurement using a fanned angle arrangement; figure 5(b) is a side view of the schematic of figure 5(a); and figure 5(c) is a plan view of the schematic of figure 5(a);

Figure 6a is an image of the reflected radiation from a carrier, where the polarisation is oriented so that no surface plasmon resonance is observed; Figure 6b is an image of the reflected radiation from a carrier, where the polarisation of the incident radiation is oriented so that surface plasmon resonance is observed; Figure 6c is the result of subtracting the image of Figure 6b from the image of Figure 6a; and Figure 6d is a plot of the intensity recorded across Figure 6c;

Figure 7a is a side view of an apparatus with a flow cell in accordance with an embodiment of the present invention; figure 7b is a plan view of a flow cell of an apparatus; and figure 7c is a plan view of a flow cell of an apparatus in accordance with an embodiment of the present invention;

Figure 8 is a plot of the measured intensity for the carrier of figure 6 for four different polarisation settings, each subtracted from the intensity measured at TE polarisation;

Figure 9 is a plot showing how the height of peaks such as those shown in figure 8 vary as a function of polarisation angle; and

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Figure 10 is a plot of the separation of SPR peaks such as those in figure 8 against time over which the polarisation rotation angle is varied, the plot also shows the conversion of the x-axis in time to a rotation angle.

Figure la is a schematic cross-section of an encoded carrier 1. The encoded earner 1 comprises a first dielectric layer 5 of SiC^ and a second metal layer 7 of Au. The order of these two layers may be reversed.

Region 3 is the reaction region of the encoded carrier 1, in this region, the first dielectric layer 5 and second metal layer 7 are corrugated to form a diffraction grating.

A beam of radiation 10 is incident on said carrier 1 at angle 0. Radiation 10 enters the dielectric layer 15 and the first layer 5. When the radiation has the appropriate wavelength, polarisation and angle of incidence, the energy from the incident radiation couples to the electrons of the metal layer 5 to excite surface plasmons at the interface 17 between dielectric layer 15 and metal layer 5. At this "resonance" condition, there is a steep fall in radiation reflected from the reaction region since this radiation is now absorbed by coupling to the surface plasmon mode.

The surface plasmon mode travels along the plane of the interface and extends for approximately 200nm on either side of the interface 9. The characteristics of the surface plasmon mode can thus be governed by physical properties of layers which are within the range of the surface plasmon. Thus chemical species 11 adhering to the surface of the reaction region may affect the surface plasmon characteristics.

The carrier is primarily intended for determining if molecules of a first type or "probes", 11, react with molecules of a second type or "targets", 13.

In figure 2a, the probes 11 have adhered to the targets 13 and thus both chemical species are attached to metal layer 7. The presence of the targets 13 will further affect the angle at which incident radiation couples to the surface plasmons, or the "resonance angle".

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In figure la, where just the probes 11 adhere to the surface of the reaction region, the resonance angle is 6 to the surface normal. In figure 2b, where both the probes 11 and targets 13 adhere to the surface of the reaction region, the resonance angle is 6 to the surface normal. This is illustrated in figures lb and 2b respectively which show a plot of the reflected amplitude of radiation against the incident angle of radiation.

Thus, since the reaction between the probes 11 with the targets 13 causes a variation in resonance angle, measurement of the "resonance" angle, can be used to determine if the second type of molecules 13 has reacted with the first type of molecules 11.

Figure 3 schematically illustrates a measurement apparatus. Here, an encoded carrier 21 is provided on a substrate 23. The substrate 23 may be a glass or other type of substrate. The encoded carrier 21 may be the same encoded carrier described with reference to Figures la and 2a.

Laser 25 is rotatably mounted about a rotation axis which is roughly in the plane of the interface between the first and second layers of the encoded carrier 21 (the first and second layers are layers 5 and 7 respectively described with reference to Figures la and 2a). Laser 25 is rotatably mounted so that the angle of incidence 6, may be varied.

Similarly, the detector 31 may be moved in order to detect the reflected radiation.

Laser 25 emits a beam of radiation which impinges on beam splitter 27. Beam splitter 27 sends part of the beam to a reference detector 29. Reference detector 29 ensures that the laser 25 produces a known output The remainder of the light is transmitted through beam splitter 27 and impinges on encoded carrier 21 at an angle to the surface normal. Light reflected from encoded carrier 21 is then reflected at an angle to the surface normal away from laser 25 and towards detector 31. Detector 31 is also rotatably mounted about a rotation axis which is roughly in the plane of the first layer of encoded carrier 21.

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When the angle of incidence is such that the incident radiation couples to the surface plasmon mode, there is a sharp decrease in the amount of radiation reflected and thus detected by detector 31. As this angle is dependent on the chemical species attached to the surface of reaction region 3 (Figure 1), it is possible to determine whether or not the probes have reacted with the targets.

Figure 4 illustrates an encoded carrier. The encoded carrier has a reaction region 43 and a coding region 45 comprising a code 47.

The reaction region 43 comprises a diffraction grating 49. In this embodiment, the diffraction grating comprises elongate rectangular holes 51 which are oriented parallel to one another and extend through the whole of the encoded carrier 41. Thus, the diffraction grating 49 comprises a plurality of stripes. Although in this particular embodiment, the holes 51 extend through the whole of the encoded carrier, they may also extend just part of the way into the reaction region, such that the diffraction grating 49 comprises alternating stripes of different thickness in the direction into the plane of the encoded carrier 41. Alternatively, the carrier may be corrugated, possessing a series of ridges in the upper surface, each of which correspond exactly to a groove in the lower surface, and vice-versa. Thus the overall thickness of the carrier may be approximately constant, with corresponding undulations in the two larger surfaces of the carrier providing the diffraction grating.

The code 47 is a bar code provided at the edge of the encoded carrier 41 and extends over the edge of the carrier 41. The code has been designed so that if the encoded carrier 41 is read from the opposite side, i.e. from underneath the plain of paper, each numeral can still be uniquely identified.

Corner 53 of encoded carrier 41 has been removed so that the orientation of the carrier can be determined. Confirmation of the orientation of the carrier 41 is necessary in order to correctly measure the reaction area since the orientation of the diffraction grating 49 with respect to the laser should be the same for all encoded carriers 41. The orientation of the carrier 41 may also be useful for reading the code 47.

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Typically, the encoded carrier 41 will be 50 to 100 pm square. Each of the characters of the bar code will typically have a length between 5 and 10 jam.

In use, probes will be attached to reaction region 43. The molecules may be attached using a number of methods, but will preferably be attached by placing the encoded carriers in a solution of probes.

The encoded carriers 41 will then be introduced to molecules of a different type, "target molecules". Typically, this will again be performed by placing the encoded carriers with the probes in a solution or suspension of the target molecules.

If the probes attached to reaction area 43 react with the targets, the surface plasmon resonance characteristics of the reaction area will change as described with reference to Figures 1 to 3.

In practice, a plurality of encoded carriers 41 will be prepared, by attaching a first type of probes to a first plurality of encoded carriers having the same "first" code, a second type of probes (different to the first type of probes) to a second plurality of encoded carriers having the same "second" code. The second code is different to the first code.

Further encoded carriers with third, fourth, fifth etc codes and types of probes will be prepared. The reaction region 43 for each different type of encoded carrier having a different probe will be analysed as explained with reference to Figure 3 to determine the "resonance angle" prior to reaction. The different types of encoded carriers 41 will then be introduced into a solution containing the target molecules. The encoded carriers may then be removed from the solution and measured. Alternatively, the carriers may be read while in the solution.

Each type of encoded carrier may be determined from its code. An SPR measurement can then be performed to determine if the probes on the carrier have reacted with the target molecule.

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The encoded carriers 41 may also be used to monitor the progress of a reaction between a molecule attached to the carrier and a target molecule. At the start of the reaction, only a few of the probes attached to the encoded carrier will react with target molecules.

As mentioned in relation to Figures 1 and 2, the results of the single plasmon resonance are dependent on the angle of incident of the radiation impinging on the carrier its wavelength and polarisation. Whereas the wavelength of the radiation will remain fixed and the angle of incidence of the radiation to the surface of the carrier will be swept through the same range of angles for each carrier using the equipment of Figure 3, the incident polarisation angle of the radiation on the carrier will vary dependent on the orientation of the carrier. Previous attempts to address this problem have used a rotation controller located underneath the carrier in order to ensure that each carrier has the same rotation when examined.

In the preferred embodiment of the present invention, the SPR of a carrier is measured using a fanned angle arrangement where the carrier 61 is illuminated with a cone of light 63.

In figure 5a, the carrier is of the type described with reference to figure 4. In figure 5a, the cone of light is polarised with the optimum polarisation orientation for an SPR measurement, namely such that the magnetic component of the radiation is parallel to the direction of the grating lines, whilst the electric component of the radiation is perpendicular to the direction of the grating lines and thence parallel to the direction of the greatest periodic variation of refractive index of the carrier surface. This is referred to above as the "TM mode".

Figure 5b shows a cross section of figure 5a. The orientation of polarisation 65 of the radiation 63 is parallel to the direction of greatest variation of refractive index of the carrier 61. In this figure, the carrier 61 is corrugated, but a planar stripped carrier may also be used.

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Figure 5c is a plan view of figure 5a, showing the orientation of polarisation 65 of the radiation 63 to be parallel to the direction of greatest variation of refractive index of the carrier 61 for TM polarisation and 90° rotated for TE polarisation.

By irradiating the carrier with a cone of radiation, there is no need to sweep the angle of incident radiation in the manner described with reference to figure 3. The cone is a continuum of beams of radiation. The beams which impinge on the carrier at the appropriate angle (and the correct polarisation) will couple to the surface plasmon resonance mode, be absorbed and hence not reflected.

Figure 6a is an image of a carrier taken using a fanned angle arrangement of the type described with reference to figures 5a, 5b and 5c. In the image of figure 6a, the radiation is TE polarised, hence the incident radiation does not interact with the surface plasmon mode of the carrier and surface plasmon resonance is not observed.

Figure 6b shows the same carrier as that of Figure 6a. However, in this case, the cone of incident radiation is TM polarised. Two distinct plasmon resonance lines 105 and 107 can be easily seen due to radiation which is incident at an appropriate angle (either side of the surface normal, thence two resonance lines) coupling to the surface plasmon mode. By measuring the position of lines 105 and 107 it is possible to determine if the probe molecules on the carrier have reacted with target molecules.

Figure 6c shows an image obtained by subtracting the result of image B (TM polarisation) from image A (TE polarisation). Therefore, the signal due to the carrier itself (and any background level signal which this causes) has been removed but two vertical lines due to the single plasmon resonance 105 and 107 can be clearly seen. Also, as a result of subtracting the TM image from the TE image, the lines now appear as bright (i.e. regions of higher intensity).

Figure 6d shows a plot of the intensity of the optical signal against distance for line A -A' of Figure 6c. The intensity plot shown in figure 6d could be obtained via either a single line scan across the central region of the plasmon resonance lines in figure 6c, or

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by compressing part, or the whole of, the image shown in figure 6c in the vertical direction and thence recording an "average" linescan with lower bit-wise noise, effectively across the whole image of the plasmon resonance lines. Two peaks corresponding to the lines caused by the single plasmon resonance 105 and 107 are clearly seen. Measuring the separation distance of these two peaks 105 and 107 or measuring their separation from a central point can be used to determine if the probe molecules have reacted. A change in the separation of the peaks (or their distance from a central point) will indicate whether the probe molecules have reacted with target molecules. The position of the peaks may be determined using known analysis software.

Figure 7a shows a cross sectional schematic of an apparatus in accordance with an embodiment of the present invention. The apparatus comprises a flow cell 71; the flow cell has an inlet tube 73 and an outlet tube 75. The flow cell may be constructed from two spaced-apart microscope slides 77 and 79. The spacing between the microscope slides must be large enough to allow the flow of fluid over the carriers 81, 83, 85 of the type described with reference to figure 4.

The fluid is introduced into the space between the slides 77 and 79 through inlet tube 73. The carriers 81,83, 85 are settled in the space between the microscope slides 77, 79 such that the carriers are arranged with their plane parallel to the flow direction.

The flow cell 71 guides carriers underneath cone of radiation 87 which allows an SPR - measurement to be made in the manner described with reference to figures 5 and 6. In figure 7a, the middle carrier 83 is in the position where an SPR measurement can be made.

Figure 7b is a plan view of the flow cell 71 of figure 7a. The three carriers 81, 83 and 85 can be seen arranged in the plane of the flow cell 71. However, the carriers 81, 83 and 85 are in different rotational orientations. This means that the polarisation of the radiation cone 87 will be not be optimally aligned with the diffraction grating on the carriers and, in many cases, it will not be possible to correctly measure an SPR signal.

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In figure 7a, a polarisation rotator 89 is provided to rotate the polarisation of radiation cone 87 in order to correctly align the polarisation to TM, meaning the polarisation direction is aligned with the direction on the carrier having the greatest periodic modulation in refractive index as shown in figure 7c.

Thus, using the apparatus of the present invention, it is possible for the polarisation orientation of the cone to be correctly aligned in order to measure SPR from each carrier, regardless of the carrier's rotational orientation.

The orientation of each carrier may be easily determined using standard imaging techniques. A corner is missing from the carrier illustrated in figure 4 which allows the orientation of the carrier to be determined. Sensing means (not shown) may be provided which allow the apparatus to determine the orientation of the carrier and hence alter the polarisation rotator as required. Alternatively, the apparatus may be configured so that the polarisation is rotated if no plasmon resonance is determined.

Figure 8 shows a plot of the corrected SPR resonance measured for the same carrier for different angles of rotation of the polarisation of the radiation cone. The equivalent of the TM image (figure 6b) was measured using a variable polarisation setting. This will be referred to as the first reflected signal (imge). The TE image (figure 6a), the second reflected signal (img-re), was measured in TE polarisation. (For the data of figure 8, this was at an angle of 140° for the polarisation rotator).

The corrected signal (equivalent to figure 6c) was obtained by subtracting the first measured reflected signal from the second measured reflected signal. By correcting the signal in this manner, it is possible to remove any artefacts due to the carrier and also conect for any longer timescale variations in the intensity of the radiation source which is providing the cone of radiation.

In figure 8, four measurements are taken where the first reflected signal has a polarisation rotation of 0°, 50°, 100° and 140°. Where a 0° polarisation rotation is

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applied, an SPR signal can be clearly detected. However, when the polarisation rotation is increased to 50°, the SPR signal becomes much stronger indicating that the TM polarisation direction is much more closely aligned with the direction of change of refractive index of the carrier.

Rotating the polarisation for measuring the first reflected signal to 100° gives a corrected SPR signal of similar strength to that measured with a polarisation rotation of 0°. Finally, using a polarisation rotation of 140° for measuring the first signal gives a roughly flat line in the corrected SPR signal, unsurprising since this angle is equivalent to the TE polarisation, so the first reflected signal is recorded at the same polarisation as the second signal.

Thus, rotating the polarisation direction of the incident radiation to correct for the rotational orientation of the carrier allows the optimum SPR signal to be measured. Although it is the position of the peak and not the size of the peak which is important when determining if the probe molecules have reacted, optimising the SPR signal increases the signal to noise ratio which provides more reliable data.

Figure 9 shows a plot of the corrected peak height for each of the two peaks shown in figure 8 as a function of polarisation angle 9. A different data set is shown for each peak. Where the maximum peak height is measured, the polarisation should be aligned with the direction on the carrier which has the greatest variation in refractive index, i.e. TM polarisation.

It can be seen from figure 9 that there is approximately a 40° range over which a very strong SPR peak is seen. This figure therefore shows the advantages of correctly aligning the polarisation with the rotational orientation of the carrier.

The variation in the separation of the peaks as a function of rotating the polarisation is shown in Figure 10. In figure 10, the distance between the two peaks of figure 8, AD, is plotted as a function of time over which the polarisation is rotated.

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The solid line on Figure 10 shows how to convert the X axis which is given in terms of time as the polarisation is rotated to changes in the actual polarisation angle. It can be seen that once the resonance peaks start to form as the polarisation angle is varied, the separation of those peaks remains approximately constant. This is to be expected as the polarisation direction should not affect the position of an SPR peak, only its magnitude.

It will be understood that the invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

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Claims

CLAIMS:

1. An apparatus for measuring a carrier, said carrier being configured to exhibit surface plasmon resonance said apparatus comprising a source of polarised radiation for irradiating the carrier, a receiver for measuring the radiation reflected from the carrier and a polarisation rotator for rotating the polarisation plane of the polarised radiation.

2. An apparatus according to claim 1, the apparatus further comprising means to determine the orientation of the carrier and to control the polarisation rotator in accordance with the orientation of the carrier such that each carrier is measured using radiation having the same polarisation.

3. An apparatus according to either of claims I or 2, wherein said source is configured to irradiate said carrier with a frusto-conical beam, the narrowest part of said beam being located at the carrier with the carrier being located above the focal point of said beam.

4. An apparatus according to any preceding claim, wherein the polarisation is controlled to a tolerance of 10 degrees or less.

5. An apparatus according to claim 2, wherein the carrier comprises at least one indicator to allow alignment of the polarisation between carriers.

6. An apparatus according to any preceding claim, wherein the carrier has a direction within which it has a maximum periodic variation in its refractive index, and said polarisation rotator is configured to align the polarisation of the radiation with the direction of maximum periodic variation of the refractive index.

7. An apparatus according to any preceding claim, configured to irradiate the carrier with radiation with its polarisation oriented in a first direction and to irradiate the carrier with radiation with its polarisation oriented in a second direction.

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8. An apparatus according to Claim 7, wherein the radiation with its polarisation orientation in the first direction is such as to excite a surface plasmon mode within the carrier and the radiation with its polarisation orientation in the second direction is such as to not excite a surface plasmon mode within the carrier.

9. An apparatus according to claim 7 or 8, further comprising means to combine the measurements from the first and second directions.

10. An apparatus according to claim 9, wherein the means to combine the measurements comprises means to subtract, divide or otherwise normalise the measurements from, or with, one another.

11. An apparatus according to any preceding claim, further comprising analysis means to determine the position of a surface plasmon resonance signal.

12. An apparatus according to any preceding claim, wherein the apparatus further comprises means to move the carrier into the path of the beam of radiation.

13. An apparatus according to claim 12, the apparatus further comprising a flow cell configured to move a carrier into the path of the beam of radiation.

14. A method of measuring a carrier, said carrier being configured to exhibit surface plasmon resonance, said method comprising:

irradiating said carrier with a source of polarised radiation;

measuring the radiation reflected from the carrier;

15. A method according to claim 14, further comprising:

determining the orientation of the carrier.

16. A method according to either of claims 13 or 14, further comprising:

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irradiating the carrier with radiation with its polarisation oriented in a first direction; and irradiating the carrier with radiation with its polarisation oriented in a second direction.

17. A method according to Claim 16, wherein the first direction is such that the radiation with its polarisation oriented in the first direction excites a surface plasmon mode within the carrier and the second direction is such that the radiation with its polarisation oriented in the second direction does not excite a surface plasmon mode within the carrier.

18. A method according to claim 17, further comprising subtracting, dividing or otherwise normalising the data measured with the polarisation in the first direction from the data measured with the polarisation in the second direction.

19. An apparatus substantially as described herein, with reference to the accompanying drawings.

20. A method substantially as described herein, with reference to the accompanying drawings.