GB2437981A

GB2437981A - Measurement of surface adsorption or desorption

Info

Publication number: GB2437981A
Application number: GB0617274A
Authority: GB
Inventors: Neville Freeman; Marcus Swann; Louise Peel; Mark Gostock
Original assignee: Farfield Sensors Ltd
Current assignee: Farfield Sensors Ltd
Priority date: 2006-02-15
Filing date: 2006-09-04
Publication date: 2007-11-14
Also published as: GB0617274D0; GB0603036D0

Abstract

A method of determining a mass loading characteristic of a material of interest. An optical sensor, such as an optical waveguide sensor, is equipped with a measurement surface 4 which mimics a surface of interest such as a ceramic surface. A sample to be measured, such as a cleansing agent or detergent, is introduced from injection module 1 to liquid flow 2 through a frit 3 and into the optical sensor. Waste is recovered in chamber 7. Data recording is performed by data module 5 and outputted to a data output device 8. Once the sample has equilibrated with the measurement surface 4 sample addition is halted and the liquid flow 2 is continued. The desorption time is measured and calibrated using known techniques. The mass loading characteristic may also be a change in surface adsorption. The sensor device may be an interferometric sensor device.

Description

Method The present invention relates to a method for determining a mass

loading characteristic of a material of interest.

There are increasing legislative demands on manufacturers to provide measurable proof of the claimed benefits of a product and to qualify or quantify the surface behaviour and characteristics of a product formulation which are important either to its performance or to the safety of those in contact with it. Formulation chemistry Is concerned with optimising the performance of a material for a certain application. The performance of a specific material S...

formulation must be measured using a reliable technique which gives results which can be related to the observed performance in the intended application. The observed performance may be for example the finish of a paint * formulation, the cleaning propensity of a cleaning formulation or the tendency of a material formulation to be removed from a surface after use. SI *

It is often the case that the materials in a formulation need to be comprehensively and safely removed, in particular if there is a risk associated with contact with a user (eq through skin contact, inhalation or ingestion). For materials in solution, determining what is required to remove the solution to a point where it has reached an acceptably safe level is a straightforward matter. For a formulation which adheres to a surface (which the majority of formulations will do to some extent), determining what is required to remove the formulation is much less straightforward. Surface measurements are notoriously demanding and often expensive to undertake. It can be difficult to determine what the final fate of the material might be, especially in complex environments. Moreover the effects of long-term, low-level exposure to materials which bleed' off surfaces can be difficult to determine with certainty.

In the specific case of a hard surface cleansing formulation, it is desirable for as much of the cleansing agent as possible to be removed (desorbed) from the surface with the minimum amount of water. This helps to avoid long term human exposure to the cleansing agent, to ensure optimum cleanliness of the surface and to minimize water usage. Over the years tests have been developed which are often laborious arid in some cases potentially risk the health of those carrying out the tests. Despite the relatively primitive nature of the tests which have evolved, a number of such tests have become accepted as standard within the industry. S In recent years with increasing emphasis on the safety of.:.

laboratory workers, a number of such tests have become unacceptable leading to an urgent need to replace them with instrumental approaches requiring significantly less human intervention. *:::: The present invention seeks to improve the determination of a mass loading characteristic of a material of interest by exploiting macroscopically the real time sensitivity of certain sensor devices to mass loading at the molecular level. More particularly, the present invention relates to a method for determining a mass loading characteristic of the material of interest by measuring the temporal response of a sensor device to which the material of interest is exposed.

Thus viewed from one aspect the present invention provides a method for determining a mass loading characteristic of a material of interest in a localised environment, said method comprising: (A) providing a sensor device having a sensor component with a measurement surface capable of exhibiting a measurable response to a change in the localised environment caused by mass loading of the material of interest therein; (B) either introducing the material of interest into the localised environment over a temporal range or depleting the material of interest from the localized environment over a temporal range; (C) generating an output from the sensor component over the temporal range; and (D) relating the output from the sensor component over the temporal range to the mass loading characteristic. S...

In the method of the present invention, the temporal output of the sensor component is characteristic of the mass loading of the material of interest at the microscopic level. This has been found unexpectedly to approximate to the macroscopic behaviour of the material of interest permitting the method *w to be exploited in a number of practically and industrially.. *...

useful embodiments.

In a preferred embodiment, step (B) is introducing the material of interest into the localized environment over a temporal range.

The material of interest may be introduced into the localized environment in a manner sufficient to ensure a linear or non-linear condition gradient. The material of interest may be introduced into the localized environment in a single dose (eq an injection), multiple doses (eq aliquots) or in a Continuous flow. A single dose or multiple doses may be bled into the localized environment over the measurement surface (eg in a parabolic flow front). Preferably the material of interest is introduced in a continuous flow. Typically the flow rate is constant.

In a preferred embodiment, step (B) is injecting material of interest or a rinsing fluid into the localized environment.

Particularly preferably an injection loop is left open after injecting material of interest or rinsing fluid. This advantageously causes spreading and dilution of the sample at the back end of the sample slug.

In a preferred embodiment, step (B) is depleting the material of interest from the localized environment over a temporal range. S *

The material of interest may be depleted from the localized * SS* environment in a manner sufficient to ensure a linear or non-* : linear condition gradient. The material of interest may be depleted (eq desorbed) from the localized environment using a: ..

rinsing fluid (eg a removing liquid) introduced into the localized environment. The rinsing fluid may be introduced in *. S...

a single dose (eq an injection), multiple doses (eg aliguots) or in a Continuous flow. Preferably the rinsing fluid is introduced in a continuous flow.

In a preferred embodiment, step (B) is introducing the material of interest into the localized environment over a temporal range and then depleting the material of interest from the localized environment over a temporal range.

The mass loading characteristic may be qualitative or quantitative.

Preferably the mass loading characteristic is a time or interval (t) in the temporal range at which the output generated from the sensor component attains a predetermined threshold.

The predetermined threshold may be quantitative or qualitative. The output generated from the sensor component at time or interval t may exceed or fall below the predetermined threshold. For example, it may be of interest to identify the time or interval t in the temporal range at which the output generated from the sensor component exceeds a predetermined threshold when mass loading of the material of interest is a desirable occurrence. For example, it may be of interest to identify the time or interval t in the temporal range at which the output generated from the sensor component falls below a predetermined threshold when mass loading of the material of interest is an undesirable occurrence.

In a preferred embodiment, the mass loading characteristic is a change in surface adsorbtion. S...

S S...

In a preferred embodiment, the mass loading characteristic is the rate of desorption.

The measurement surface may be adapted to mimic a surface of interest, preferably to mimic a surface of industrial interest. In this embodiment, the present invention advantageously permits an industrial manufacturer to unambiguously demonstrate the behaviour and associated benefits of a material of interest at a surface of industrial interest.

The adaptation of a measurement surface in this manner is within the capabilities of the man skilled in the art deploying known surface methodologies.

The measurement surface may be keratinous, lipidic or cellulosic. The measurement surface may be charged.

For example, the measurement surface may be adapted to mimic paper and the material of interest may be an ink formulation.

For this embodiment, the measurement surface may be cellulosjc.

For example, the measurement surface may be adapted to mimic a ceramic (eq a household ceramic). For this embodiment, the measurement surface may be or comprise silicon dioxide, silicon, silicon oxynitride or silicon nitride.

The measurement surface may be adapted to mimic bodily::;:; tissue. For example, the measurement surface may be adapted to mimic skin, bone, nails or hair. * .* * S *

The measurement surface may be adapted to mimic a natural or synthetic fibre (eg a textile). For example, the measurement surface may be adapted to mimic wool or cotton. *, The material of interest may be chemical or biological. The material of interest may be liquid or gas. The material of interest may be an active material or a composition or formulation of an active material (eq a solution such as an aqueous solution of an active material).

The material of interest may be a bodily fluid (eq a human or non-human bodily fluid). The bodily fluid may be selected from the group consisting of blood serum, plasma and CSF.

The material of interest may be a surfactant.

The material of interest is preferably a cleansing agent (eg a detergent). The cleansing agent may be a household cleansing agent such as a cleansing agent for cutlery and crockery (commonly referred to as washing up liquid).

The material of interest may be a complex biological molecule such as a protein, antigen, enzyme or DNA. The term "protein" used herein is intended to cover peptides, polypeptides and amino acids. The localized environment may contain a binding molecule (eg a specific binding partner) of the biological molecule (eg protein). The localized environment may contain an antibody or aptamer. For example, the material of interest may be an antigen and the binding molecule may be an antibody. I... * ***

The method of the invention may further comprise: (E) deducing from the mass loading characteristic the affinity of a protein and a binding molecule. This embodiment: *. ***.

may be usefully exploited in protein assay, quality control and diagnostic or sensor characterisatjon. *::::*

I I I I. *

The measurement surface may be derivatised for the purposes of absorbing, attaching, adhering or immobilizing the binding molecule (or probe). The affinity of a binding molecule and a protein deduced from the measured elution of the protein over time at a given flow rate and temperature. Alternatively the protein may be adhered to the measurement surface and the affinity of a binding molecule and a protein deduced from the measured elution of the binding molecule over time at a given flow rate and temperature. If the concentration profile is known the time can be related to a specific affinity which can be used to identify an unknown or given species.

In a preferred embodiment, the material of interest is a cleansing agent (eg a detergent), the measurement surface is adapted to mimic a ceramic and step (B) is depleting the detergent from the localized environment over a temporal range using an aqueous rinsing fluid liquid (preferably water) introduced into the localized environment in a continuous flow, wherein the mass loading characteristic is a time (t) in the temporal range at which the output generated from the sensor component falls below a predetermined threshold (eg an industrially defined acceptable output). In this embodiment, the time t is an index of the rate of desorption of the detergent and is therefore directly related to industry standard measurements. This embodiment offers several advantages over many existing tests. It is faster, suitable for automation and does not requires less human intervention with test formulations. * S..

S

The measurement surface may be capable of exhibiting a.:.

measurable response in a parameter selected from effective * refractive index, a dielectric constant, a viscoelastic property, a frequency of oscillation, a thermal *.

absorption/desorption parameter, the permeability, the absorption of energy or of energetic particles (such as x-rays, gamma rays, n-rays, electrons, neutrons, ions, light, microwaves, acoustic waves) or the particle size. For example, the sensor device may be one or more of the following types: surface plasmon resonance sensor devices, resonant mirror sensor devices, acoustic sensor devices (such as quartz crystal and surface acoustic wave devices (by using frequency decay techniques for example)) or electrical sensor devices (capable of measuring impedance at (for example) RF or microwave frequencies). Preferably the parameter is the effective refractive index.

In a preferred embodiment, step (C) comprises: (C) irradiating the sensor component with electromagnetic radiation to generate the output over the tec:: range.

Before or after step (B), the method may com::: (1) irradiating the sensor compone: :h electromagnetic radiation to generate a firs: :::ut; (2) measuring the first output; and wherein steps (C) and (D) are: (C') generating an output from the component over the temporal range relative to the firs: :::ut; and (D') relating the output from the se:: component over the temporal range relative to the firs: ::?ut to the mass loading characteristic.

Steps (1) and (2) may be performed at start-e results. :; may be stored electronically (eg as calibrat..c. ta). S.

In a preferred embodiment, the sensor device a: .. S..'

interferometric sensor device. The sensor ccr:-rit of the interferometric sensor device may comprise a _t one waveguide (eg a slab or channel waveguide) er::bre optic component. For example, the sensor component c. e a waveguide structure. The waveguide structure e generally of the planar type disclosed generally or spec.ally in WO-A-98/22807 or WO-A-Ol/36945.

Preferably the sensor component is a wavegu:e::ucture including: either (a) one or more sensing layers capable: inducing in a secondary waveguide a measurable response:: :hange in the localised environment caused by mass loa:.-or (b) a sensing waveguide capable of exhibi: a measurable response to a change in the localised envircza-caused by mass loading.

In this embodiment, mass loading contributes to a change in the effective refractive index of the sensor component. The waveguide structure is particularly sensitive to mass loading and this is advantageously exploited to determine a mass loading characteristic.

Particularly preferably the sensor component is a waveguide structure including: either (a) one or more sensing layers capable of inducing in a secondary waveguide a measurable response to a change in the localised environment caused by mass loading and an inactive (eg deactivated) secondary waveguide in which the sensing layer is incapable of inducing a measurable response to a change in the localised environment caused by mass:; loading or (b) a sensing waveguide capable of exhibiting a measurable response to a change in the localised environment.:.

caused by mass loading and an inactive (eg deactivated) : .. S...

waveguide substantially incapable of exhibiting a measurable response to a change in the localised environment caused by mass loading.

Preferably each of the sensing waveguide or secondary waveguide (or any additional waveguides such as reference waveguides) of the sensor component is a planar waveguide (ie a waveguide which permits light propagation in any arbitrary direction within the plane). Particularly preferably each planar waveguide is a slab waveguide.

Preferably the sensor component constitutes a multi-layered structure (eg a laminated waveguide structure) of the types disclosed in WO-A-98/22807 and WO-A-Ol/36945. In a preferred embodiment, each of the plurality of layers in the multi-layered sensor component are built onto a substrate (eg of silicon) through known processes such as ?ECVD, LPCVD, etc. I0 Intermediate transparent layers may be added (eg silicon dioxide) if desired. Typically the sensor component is a multilayered structure of thickness in the range 0.2-10 microns. A layered structure advantageously permits layers to be in close proximity (eq a sensing waveguide and an inactive (reference) waveguide may be in close proximity to one another so as to minimise the deleterious effects of temperature and other environmental factors). Preferably the sensor component comprises a stack of transparent dielectric layers wherein layers are placed in close proximity.

Preferably each layer is fabricated to allow equal amounts of electromagnetic radiation to propagate by simultaneous excitation of the guided modes in the structure. * a..

Preferably the output is a pattern of interference fringes which may be measured by a conventional measuring means (see *.

for example WO-A-98/22807) eg one or more detectors such as: .. *

photodetectors (eq in an array) which measure the intensity S...

of electromagnetic radiation. Preferably step (C) comprises: measuring movements in the pattern of interference fringes over the temporal range. Particularly preferably step (C) further comprises: calculating the phase shift from the movements in the pattern of interference fringes over the temporal range.

In a preferred embodiment, the output is the contrast of a pattern of interference fringes (eg the difference in intensity between the outer fringe envelope and the inner fringe envelope). For example, the contrast may be the difference in intensity between the outer fringe envelope and the inner fringe envelope at a corresponding position in the pattern. Preferably the contrast may be the difference in intensity between the maxima of the outer fringe envelope and the maxima of the inner fringe envelope.

II

Preferably the sensor component is adapted so as to be usable in evanescent mode or whole waveguide mode.

Thus in a first embodiment, the sensor component includes one or more sensing layers capable of inducing in a secondary waveguide a measurable response to a change in the localised environment caused by mass loading. In this first embodiment, the sensor device is advantageously adapted to optimise the evanescent component so as to induce in the secondary waveguide a measurable response. The sensor component may comprise a plurality of separate sensing layers to enable mass loading at different localised environments to be determined. * **. S * *

S S

In a preferred embodiment, the sensing layer comprises an.:.

absorbent material (eq a polymeric material such as polymethylmethacrylate, polysiloxane, poly-4-vinylpyridine) or a bioactive material (eg containing antibodies, enzymes, DNA fragments, functional proteins or whole cells). The absorbent material may be capable of absorbing a gas, a liquid or a vapour containing a chemical material of interest. The bioactive material may be appropriate for liquid or gas phase biosensing. For example, the sensing layer may comprise a porous silicon material optionally biofunctionalised with antibodies, enzymes, DNA fragments, functional proteins or whole cells.

In a preferred method of the invention, the secondary waveguide comprises silicon oxynitride or silicon nitride.

In a second embodiment, the sensor component includes a sensing waveguide capable of exhibiting a measurable response to a change in the localised environment caused by mass loading. In this second embodiment, the sensor device is adapted to minimise the evanescent component and may be used advantageously in whole waveguide mode.

In a preferred embodiment, the sensing waveguide comprises an absorbent material (eg a polymeric material such as polymethylmethacrylate, polysiloxane, poly-4-vinylpyridjne) or a bioactive material (eg containing antibodies, enzymes, DNA fragments, functional proteins or whole cells). The absorbent material may be capable of absorbing a gas, a liquid or a vapour containing a chemical material of interest. The bioactive material may be appropriate for liquid or gas phase biosensing. For example, the sensing S...

waveguicie may comprise a porous silicon material optionally biofunctjonaljsed with antibodies, enzymes, DNA fragments, functional proteins or whole cells. .. * * I. S

Where the sensor component comprises a sensing waveguide adapted for use in whole waveguide mode, an absorbent layer in the form of an overcoating may be present for use as a membrane (for example) to separate out certain stimuli.

To optimise the performance of the first embodiment, the sensor component may further comprise an inactive secondary waveguide in which the sensing layer is incapable of inducing a measurable response to a change in the localised environment caused by mass loading. The inactive secondary waveguide is capable of acting as a reference layer. It is preferred that the secondary waveguide and inactive secondary waveguide have identical properties with the exception of the response to the change in the localised environment caused by mass loading. By way of example, the secondary waveguide and inactive secondary waveguide are made of silicon oxynitride.

To optimise the performance of the second embodiment, the sensor component may further comprise an inactive (eg deactivated) waveguide substantially incapable of exhibiting a measurable response to a change in the localised environment caused by mass loading. The inactive waveguide is capable of acting as a reference layer. The physical, biological and chemical properties of the sensing waveguide and inactive waveguide are as similar as possible (with the exception of the response to the change in the localised environment caused by mass loading). Typically the inactive waveguide is made of silicon oxynitride.

As a consequence of mass loading, changes in the dielectric properties (eg the effective refractive index) of the sensing

S

waveguide or sensing layer occur. This causes a measurable response (ie a change in the transmission of electromagnetic radiation down the sensing waveguide (or waveguides) in whole: . $. S waveguide mode or the secondary waveguide in evanescent field mode) which (in one embodiment) manifests itself as a p.., movement of interference fringes. This differs according to.. : whether the sensor component is interrogated in TE or TM mode.

In a preferred embodiment of the method of the invention, step (C) is carried out with electromagnetic radiation in TM mode.

In a preferred embodiment of the method of the invention, step (C) is carried out with electromagnetic radiation in TE mode.

In a preferred embodiment of the method of the invention, step (C) comprises: (Cl) irradiating the sensor component with electromagnetic radiation in TE mode to produce a first pattern of interference fringes; (C2) irradiating the sensor component with electromagnetic radiation in TM mode to produce a second pattern of interference fringes; (C3) measuring movements in the first pattern of interference fringes; and (C4) measuring movements in the second pattern of interference fringes.

Particularly preferably step (C) of the method of the invention further comprises: (C5) calculating the phase shift of the sensor component in TM mode from the movements in the first pattern of interference fringes; (C6) calculating the phase shift of the sensor component in TE mode from the movements in the second pattern of, ..

interference fringes; and step (D) is relating the phase shift of the sensor component in TM mode and the phase shift of the sensor component in TE mode to the mass loading characteristic.

More preferably step (C) of the method of the invention further comprises: (C5) calculating the phase shift of the sensor component in TM mode from the movements in the first pattern of interference fringes (C6) calculating the phase shift of the sensor component in TE mode from the movements in the second pattern of interference fringes; (C7) calculating the phase shift of the sensor component in TM mode relative to the phase shift of the sensor component in TE mode; and step (D) is relating the phase shift of the sensor component in TM mode relative to the phase shift of the sensor component in TE mode to the mass loading characteristic.

Preferably the phase shift of the sensor component in TM mode relative to the phase shift of the sensor component in TE mode is a ratio of the phase shift of the sensor component in TM mode to the phase shift of the sensor component in TE mode.

Step (C) may comprise: generating an output from the sensor component on at least two occasions over a temporal range.

Preferably step (C) comprises: generating an output from the * S..

sensor component continuously over a temporal range. * ,

S

Preferably the sensor device further comprises: : .. **

means for intimately exposing at least a part of the (or each) sensing layer or the sensing waveguide of the sensor S...

component to the localised environment.

In a preferred embodiment, the means for intimately exposing at least a part of the sensing layer or the sensing waveguide to the localised environment is integrated onto the sensor component.

Preferably the means for intimately exposing at least a part of the (or each) sensing layer or the sensing waveguide of the sensor component to the localised environment is as described in WO-A-Ol/36945. The means may be automated in order to reduce the requisite degree of user intervention.

Preferably the means for intimately exposing at least a part of the sensing layer or the sensing waveguide to the localised environment is adapted to permit the continuous introduction of an analyte containing a material of interest (ie a dynamic system). For example, it may permit the continuous introduction of the material of interest in a discontinuous flow (eq as a train of discrete portions) into the localised environment. This may be achieved by capillary action or by a separate urging means. The means for intimately exposing at least a part of the sensing layer or the sensing waveguide to the localised environment may be a flow cell or cuvette.

Viewed from a further aspect the present invention provides a sensor assembly for determining a mass loading characteristic of a material of interest in a localised environment * .** * ** comprising: a sensor device as hereinbefore defined; and an ancillary apparatus operably connected to the measurement surface for either introducing the material of interest into the localised environment over a temporal range or depleting **.* the material of interest from the localized environment over * * * ** * a temporal range.

The ancillary apparatus may comprise a reservoir of material of interest. The ancillary apparatus may comprise a reservoir of rinsing fluid. The ancillary apparatus may comprise a connector (eg tubing) connecting the reservoir of material of interest or the reservoir of rinsing fluid to the measurement surface. The ancillary apparatus may comprise a flow generating device adapted to cause the material of interest or the rinsing fluid to flow into the localized environment.

Preferably the connector is adapted to permit slow flow of the materialof interest to ensure a linear or non-linear condition gradient in the localized environment.

Preferably the ancillary apparatus further comprises: an injection module operatively connected to the reservoir of material of interest or the reservoir of rinsing fluid, wherein the injection module is adapted to inject material of interest or rinsing fluid through the connector to the localized environment.

Preferably the assembly further comprises: a frit upstream of the ancillary apparatus. The frit may serve to spread the material of interest.

Preferably the connector is chemically modified so as to * * interact chemically with the material of interest. This may * * S..

spread the material of interest and thereby serve to control * . or enhance the concentration profile of the material of *5* interest. * S. * * . S...

In a preferred embodiment, the sensor assembly further S...

comprises: irradiating means for irradiating the sensor component with electromagnetic radiation; and measuring means for measuring the output of the sensor component.

Electromagnetic radiation generated from a conventional source may be propagated into the sensor component in a number of ways. In a preferred embodiment, electromagnetic radiation is simply input via an end face of the sensor component (this is sometimes described as 5an end firing procedure). Preferably the electromagnetic radiation source provides incident electromagnetic radiation having a wavelength falling within the optical range. Propagating means may be deployed for substantially simultaneously propagating incident electromagnetic radiation into a plurality of waveguides. For example, one or more coupling gratings or mirrors may be used. A tapered end coupler rather than a coupling grating or mirror may be used to propagate radiation into the lowermost waveguide. The amount of electromagnetic radiation in the sensing waveguide/inactive waveguide or in the secondary waveguide/inactive secondary waveguide is typically equal. The incident electromagnetic radiation may be oriented (eg plane polarised) as desired using an appropriate polarising means. The incident electromagnetic radiation may be focussed if desired using a lens or similar micro-focussing means.

The sensor assembly may further comprise: relating means *

...DTD: capable of relating the output of the sensor component to the * * : mass loading characteristic. *a S **

Preferably the sensor assembly further comprises: a synchronising means for synchronising the measuring means S...

with the irradiating means so as to correlate the measurement of the output of the sensor component with the irradiation of the sensor component with electromagnetic radiation.

The present invention will now be described in a non-limitative sense with reference to the Example and accompanying Figures in which: Figure 1 illustrates schematically a first embodiment of the method and assembly of the invention; Figure 2 illustrates schematically a second embodiment of the method and assembly of the invention.

Example I

1) An embodiment of the present invention is shown schematically in Figure 1 with an optical sensor of the type disclosed in WO-A-98/22807 equipped with a measurement surface 4 which mimics a ceramic surface. A liquid 2 is flowed along a fluidic flow line through a frit 3 and into the optical sensor. Waste is recovered in recovery chamber 7.

Data recording is started by the data module 5 and outputted to a data output device 8.

2) A sample to be measured is introduced from an injection module 1 to the liquid flow 2 upstream from the measurement surface 4. The data reading immediately prior to adding the sample is noted. * S..

3) The surface loading of the optical sensor is measured * during the initial part of the method. After the sample has equilibrated with the measurement surface 4, sample addition is halted but liquid flow 2 is continued. Data continues to be recorded. The sample is rinsed from the surface and the *. S.

depletion of sample from the measurement surface 4 is measured as a function of time (which is a function of flow rate and residual concentration).

4) The desorption time is the difference between the time at which rinsing started (either the time at which sample addition is halted or the time at which sample begins to be removed from the measurement surface 4) and the time at which the measurement drops below a predetermined level. The predetermined level may be a fixed response above the previously noted level in step 2).

The data may be plotted on a calibration plot containing samples of known performance in order to correlate the results with desorption times measured using standard industrial measurement techniques where applicable. The time taken to reach a given surface loading can be easily correlated with such tests. This approach leads to a method by which calibration and calculation of industrial test values can be obtained instrumentally.

Example 2

In an alternative embodiment shown in Figure 2 (which is largely the same as the embodiment of Figure 1 and retains the same numbering), the sample delivery system can itself be used instead of a frit 3 to modify the concentration of the liquid 2. An injection loop is left open after injection of liquid 2 to cause spreading and dilution of a sample at the back end of the sample slug to produce a concentration gradient over the measurement surface. * ** * S S... * S I... S * * S. S

Claims

Claims 1. A method for determining a mass loading characteristic of a

material of interest in a localised environment, said method comprising: (A) providing a sensor device having a sensor component with a measurement surface capable of exhibiting a measurable response to a change in the localised environment caused by mass loading of the material of interest therein; (B) either introducing the material of interest into the localised environment over a temporal range or depleting the material of interest from the localized I C*O environment over a temporal range; (C) generating an output from the sensor component over the temporal range; and (D) relating the output from the sensor component over the temporal range to the mass loading characteristic.

2. A method as claimed in claim 1 wherein step (B) is introducing the material of interest into the localized environment over a temporal range.

3. A method as claimed in claim 1 or 2 wherein the material of interest is introduced into the localized environment in a manner sufficient to ensure a linear or non-linear condition gradient.

4. A method as claimed in claim 1, 2 or 3 wherein the material of interest is introduced into the localized environment in a single dose, multiple doses or in a continuous flow.

5. A method as claimed in claim 4 wherein the material of interest is introduced in a continuous flow.

6. A method as claimed in claim 1 wherein step (B) is injecting material of interest or a rinsing fluid into the localized environment.

7. A method as claimed in claim 6 wherein an injection loop is left open after injecting material of interest or rinsing fluid.

8. A method as claimed in claim 1 wherein step (B) is depleting the material of interest from the localized environment over a temporal range. * * **** * **

9. A method as claimed in claim 8 wherein the material of * :..

interest is depleted from the localized environment in a * manner sufficient to ensure a linear or non-linear condition ** S gradient. : S... * S **.

10. A method as claimed in claim 8 or 9 wherein the materials,, of interest is depleted from the localized environment using a rinsing fluid introduced into the localized environment.

11. A method as claimed in claim 10 wherein the rinsing fluid is introduced in a single dose, multiple doses or in a continuous flow.

12. A method as claimed in claim 11 wherein the rinsing fluid is introduced in a continuous flow.

13. A method as claimed in claim 1 wherein step (B) is introducing the material of interest into the localized environment over a temporal range and then depleting the material of interest from the localized environment over a temporal range.

14. A method as claimed in any preceding claim wherein the mass loading characteristic is a time or interval (t) in the temporal range at which the output generated from the sensor component attains a predetermined threshold.

15. A method as claimed in any preceding claim wherein the mass loading characteristic is a change in surface adsorbt ion.

16. A method as claimed in any of claims 1 to 14 wherein the mass loading characteristic is the rate of desorption.

17. A method as claimed in any preceding claim wherein the: measurement surface is adapted to mimic a surface of industrial interest.

18. A method as claimed in any preceding claim wherein the: measurement surface is keratinous, lipidic or cellulosic.

19. A method as claimed in any preceding claim wherein the measurement surface is charged.

20. A method as claimed in any preceding claim wherein the measurement surface is adapted to mimic paper and the material of interest is an ink formulation.

21. A method as claimed in any of claims 1 to 17 wherein the measurement surface is adapted to mimic a ceramic.

22. A method as claimed in claim 21 wherein the measurement surface is or comprises silicon dioxide, silicon, silicon oxynitride or silicon nitride.

23. A method as claimed in any of claims 1 to 17 wherein the measurement surface is adapted to mimic bodily tissue.

24. A method as claimed in any of claims 1 to 17 wherein the measurement surface is adapted to mimic a natural or synthetic fibre.

25. A method as claimed in any preceding claim wherein the material of interest is a cleansing agent.

26. A method as claimed in any of claims 1 to 24 wherein the material of interest is a complex biological molecule.

27. A method as claimed in claim 26 further comprising: S.....

(E) deducing from the mass loading characteristic the: :..* affinity of a protein and a binding molecule. S..

S

28. A method as claimed inclaim 27 wherein the measurement surface is derivatised for the purposes of absorbing, attaching, adhering or immobilizing the binding molecule.

29. A method as claimed in claim 1 wherein the material of interest is a detergent, the measurement surface is adapted to mimic a ceramic and step (B) is depleting the detergent from the localized environment over a temporal range using an aqueous rinsing fluid liquid introduced into the localized environment in a continuous flow, wherein the mass loading characteristic is a time (t) in the temporal range at which the output generated from the sensor component falls below a predetermined threshold.

30. A method as claimed in claim 29 wherein the time t is an index of the rate of desorption of the detergent.

31. A method as claimed in any preceding claim wherein the measurement surface is capable of exhibiting a measurable response in the effective refractive index.

32. A method as claimed in any preceding claim wherein step (C) comprises: (C) irradiating the sensor component with electromagnetic radiation to generate the output over the temporal range.

33. A method as claimed in any preceding claim wherein the sensor device is an interferometric sensor device. * *.*. * **

34. A method as claimed in any preceding claim wherein the * :..

sensor component is a waveguide structure. S..

S

S S. *

35. A method as claimed in any preceding claim wherein the sensor component is a waveguide structure including: S...

either (a) one or more sensing layers capable of inducing in *,, a secondary waveguide a measurable response to a change in the localised environment caused by mass loading or (b) a sensing waveguide capable of exhibiting a measurable response to a change in the localised environment caused by mass loading.

36. A sensor assembly for determining a mass loading characteristic of a material of interest in a localised environment comprising: a sensor device as defined in any preceding claim; and an ancillary apparatus opeiabiy connected to the measurement surface for either introducing the material of interest into the localised environment over a temporal range or depleting the material of interest from the localized environment over a temporal range.

37. An apparatus as claimed in claim 36 wherein the ancillary apparatus comprises a reservoir of material of interest.

38. An apparatus as claimed in claim 36 or 37 wherein the ancillary apparatus comprises a reservoir of rinsing fluid.

39. An apparatus as claimed in claim 36, 37 or 38 wherein the ancillary apparatus comprises a connector connecting the reservoir of material of interest or the reservoir of rinsing fluid to the measurement surface. 4s. *

40. An apparatus as claimed in any of claims 36 to 39 * wherein the ancillary apparatus comprises a flow generating device adapted to cause the material of interest or the rinsing fluid to flow into the localized environment. *... * a...

41. An apparatus as claimed in claim 39 wherein the connector is adapted to permit slow flow of the material of interest to ensure a linear or non-linear condition gradient in the localized environment.

42. An apparatus as claimed in any of claims 39 to 41 wherein the ancillary apparatus further comprises: an injection module operatively connected to the reservoir of material of interest or the reservoir of rinsing fluid, wherein the injection module is adapted to inject material of interest or rinsing fluid through the connector to the localized environment - 43. An apparatus as claimed in any of claims 36 to 42 further comprising: a frit upstream of the ancillary apparatus.

44. An apparatus as claimed in claim 39 wherein the connector is chemically modified so as to interact chemically with the material of interest.

45. An apparatus as claimed in any of claims 36 to 44 wherein the sensor assembly further comprises: irradiating means for irradiating the sensor component with electromagnetic radiation; and measuring means for measuring the output of the sensor component. S.'. * e * a,

SI S S.

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