WO2017137805A1

WO2017137805A1 - Systems for and method of quantitative measure of components of a liquid

Info

Publication number: WO2017137805A1
Application number: PCT/IB2016/050723
Authority: WO
Inventors: Hans Villemoes Andersen; Henrik Vilstrup Juhl
Original assignee: Foss Analytical A/S
Priority date: 2016-02-11
Filing date: 2016-02-11
Publication date: 2017-08-17

Abstract

A system (2) for the optical spectrophotometric assay of components in of a liquid sample (40) comprises an optical spectrophotometer (4) having an inspection zone (18) for receiving a sample for assay (40a); a radiation source (10) configured to generate optical radiation for supply into the inspection zone (18) to impinge on and thereby interact with a received sample for assay (40a); and a nebulizer (24) configured to discharge an aerosol (44) of the liquid sample (40) towards a one of one or more collection surfaces (32) located spaced apart from the nebulizer (24) and disposed to receive discharged aerosol (44) to form the sample for assay (40a).

Description

System for and Method of Optical Spectrophotometry Assay of Components of a Liquid Sample

[0001] The present invention relates to a system for and method of optical

spectrophotometric assay of components of a liquid sample, in particular low concentration, non-volatile components.

[0002] These components have chemical bonds which can be made to vibrate in response to their interaction with radiation of characteristic wavelengths. This phenomenon is linked to absorption of radiation at these

characteristic wavelengths. Generally these characteristic wavelengths will lie in that portion of the electromagnetic spectrum from the ultra-violet, through the visible and into the infra-red regions (Optical radiation') and notably in the infrared region, particularly the mid-infrared region, of the electromagnetic spectrum.

[0003] The characteristic wavelengths will depend on the nature of the chemical bond itself, for example a Carbon-Hydrogen, a Carbon-Oxygen or a Carbon-Carbon bond, and also upon its environment (for example, its location within the liquid matrix). Thus a given molecule may result in absorption at several characteristic wavelengths. The amount of optical radiation absorbed at these characteristic wavelengths is proportional to the amount of the molecule being examined.

[0004] By subjecting the liquid sample to optical radiation comprising some or all of these characteristic wavelengths the spectrophotometer will generate a spectrum containing features that are characteristic of components present in said sample.

[0005] The spectrum generated from the interaction of optical radiation will

therefore show absorptions at certain wavelengths which are characteristic of different molecules present in the liquid. In this way, the generated optical spectrum contains information useful in identifying and quantifying components of interest in the liquid. This technique has found application in the food and beverage industries where components in a liquid matrix, such as a wine, a milk or other potable product such as beer and fruit juices, are assayed using typically their interaction with infrared,

particularly mid-infrared optical radiation.

[0006] It is well known to provide optical spectrophotometer based assay systems for one or both of the quantitative and the qualitative determination of components of a liquid sample. In general such assay systems comprise a monochromator or an interferometer based spectrophotometer for generating optical spectral data from the interaction of optical radiation with a liquid sample.

[0007] The spectrophotometer is configured with an inspection zone for receiving a sample for assay; an optical radiation source configured to supply optical radiation into the inspection zone to impinge on the received sample and a complementary detector for detecting optical radiation after it having impinged on and thus interacted with the received sample. A data processor is provided in operable connection with the spectrophotometer to receive the spectral data and to process it using standard chemometric techniques in order to make therefrom at least a qualitative but preferably a quantitative determination of one or more components of the liquid sample to be assayed.

[0008] One such system is disclosed in US 6885003. Here a broadband infrared Fourier transform interferometric spectrophotometer is employed in the assay of liquid vinification products at various stages of the wine making process by means of chemometric analysis of spectral data in the mid- infrared region obtained by the spectrophotometer. In this manner a wide variety of compounds may be detected and here employed in the determination of a quality index for the sample. Another such system is disclosed in US 5252829. Here a broadband infrared Fourier transform interferometric spectrophotometer is employed in the assay of milk, particularly in the quantitative determination of urea in milk from the application of a chemometric model to infrared absorption data acquired in the wavelength region from 10.0 micrometers to 2.50 micrometers.

[0009] Unfortunately, some of the components that may be of interest to assay using such spectrophotometer based assay systems are present in the liquid matrix in amounts just above or less than the limit of detection of the systems. Such components may remain undetected or at least are detected with an associated large uncertainty (often quoted as a Root Mean Squared Error of Prediction or RMSEP). Furthermore, in a liquid matrix predominantly of water, such as a vinification or a milk product or other potable products, the water produces a large background absorption across a broad band which interferes with the mid-infrared absorption bands of the components of interest. Generally, the liquid matrix

containing the components of interest for assay may often create an interfering background absorption signal and it is therefore desirable to separate at least a portion of the interfering liquid matrix from the components to be assayed.

[0010] It is known to enhance the sensitivity of spectrophotometer based assay systems based by adding a sample concentrator to the system for concentrating components of the liquid to be assayed. A typical sample concentrator is an extraction column on which one or more of the components of interest are bound. These bound components, separated from their liquid matrix, are subsequently released as a concentrate by eluting with a suitable solvent. One such example is given by C. CHILLA et al. "Automated on-line-solid-phase extraction— high-performance liquid chromatography-diode array detection of phenolic compounds in sherry wine"; Journal of Chromatography A. 1996, vol.750, no.1 -2, p.209-214. However such a method requires addition of a solvent and results in a relatively mechanically complex and expensive arrangement which is commercially undesirable and addition of a solvent is furthermore related to extra cost, and is generally environmentally undesired.

[001 1] Another sample concentrator used in a spectrophotometer assay system is disclosed in EP 2009437 and comprises a freezer. Water and similarly volatile components (here components with a freezing point around or higher than the freezing point of water) are frozen out from a water containing potable sample such as wine or milk to leave behind a liquid sample for assay having a higher concentration of relatively non-volatile components than the original water containing liquid sample. However, freezing generally takes a relatively long time and a separation step is necessary to remove the frozen material from the sample to be assayed.

[0012] A further sample concentrator used in a spectrophotometer based assay system is described by N.K. AFSETH et al. "Predicting the Fatty Acid Composition of Milk: A Comparison of Two Fourier Transform Infrared Sampling Techniques"; Applied Spectroscopy 2010, vol. 64, no. 7 p.700- 707. Here liquid milk samples, diluted with water in a milk:water ratio of 75:25, are deposited in sample well plates and dried for approximately one hour at room temperature in an exicator using silica gel. The resulting dry film samples were suitable for analysis using a Fourier Transform Infrared ('FTIR') analyser in transmission mode. However, the resulting surface tends to be in-homogeneous with poor thickness repeatability. There is also a tendency for sample to concentrate at the edges of the wells.

[0013] According to a first aspect of the present invention there is provided a system for the optical spectrophotometric assay of components in a liquid sample, particularly a water containing potable sample, the system comprising an optical spectrophotometer having an inspection zone for receiving a sample for assay, a radiation source configured to generate optical radiation for supply into the inspection zone to impinge on and thereby interact with the received sample for assay and a complementary detector for detecting an intensity of optical radiation after it having interacted with the received sample for assay; the system further comprising a concentrator for concentrating non-volatile components of the liquid sample wherein the concentrator comprises a nebulizer configured to discharge an aerosol of the liquid sample and wherein the system further comprises a collection surface spaced apart from the nebulizer and disposed to receive discharged aerosol to form the sample for assay. In this manner, a thin layer of sample liquid droplets may be deposited for evaporation on the collection surface which may either be located in or external of the inspection zone, the former location reducing the amount of handling of the sample required before assay. Evaporation of at least a portion of the more volatile components, including in some cases the liquid matrix, leaves behind a sample for assay that comprises a relatively high concentration of non-volatile components of the liquid sample distributed evenly in a thin, reproducible layer.

[0014] In some embodiments the liquid sample may be a water based sample, such as a wine, a milk or other potable product such as beer and fruit juices, and the system adapted for the mid-infrared optical assay of the water based liquid sample.

[0015] In some embodiments the collection surface may be formed of a

translucent material through which may pass radiation supplied from the radiation source at least at wavelengths absorbed by components of the liquid sample to be assayed. In other embodiments the collection surface may be formed of a reflective material, such as gold deposited onto the collection surface, to receive the discharged aerosol. In this manner the optical radiation may pass through the sample for assay to be reflected from the reflective collection surface to pass through the sample for assay again before being incident on the detector. The amount of sample interacted with the optical radiation is therefore effectively doubled which can increase the accuracy of the assay or reduce the time needed to deposit sufficient sample on the collection surface.

[0016] In some embodiments there may be provided a plurality of such collection surfaces located, spaced apart, on a support, such as a planar or disc-like support. Thus a plurality of samples may be analysed using the optical spectrophotometer without removing the support between analyses.

[0017] In some embodiments the concentrator further comprises a housing

enclosing the collection surface and at least a portion of the nebulizer from which aerosol is discharged and a gas supply connected to the interior of the housing for generating an overpressure therein. The entrainment of dust from the atmosphere in the aerosol is thereby mitigated.

[0018] A heater, for example as may be formed using a resistive heater element or an infra-red radiation source, may be provided in some embodiments and is configured to supply heat to the collection surface. This speeds up the evaporation process.

[0019] According to a second aspect of the present invention there is provided a method of spectrophotometric assay of components in a liquid sample comprising the steps of: generating an aerosol of the liquid sample;

directing the aerosol of the liquid sample onto a collection surface;

allowing liquid to evaporate from the aerosol on the collection surface to form a sample for assay and performing an optical spectrophotometric assay of the so formed sample for assay.

[0020] In some embodiments of the method according to this second aspect the optical spectrophotometric assay is a mid-infrared spectrophotometric assay.

[0021] The step of allowing liquid to evaporate from the aerosol may, in some applications of the method, include supplying heat to the collection surface. This enhances the rate of evaporation of liquid from the aerosol.

[0022] These and other features and advantages will become apparent and further understood through a consideration of the following detailed description of specific exemplary embodiments made in relation to the drawings of the accompanying figures of which:

Fig. 1 A schematic representation of one embodiment of a system

according to the present invention;

Fig. 2 A schematic representation of another embodiment of a

system according to the present invention;

Fig. 3 Illustrates a nebulizer of the venturi type for use in a system

according to the present invention;

Fig. 4 Illustrates a nebulizer of the ultrasound type for use in a

system according to the present invention; and

Fig. 5 Shows comparative transmission mid-infrared spectra of milk

obtained using A) a system according to Fig. 1 and B) a

conventional spectrophotometric system.

Fig. 6 Shows C) a reflection mid-infrared spectrum of milk obtained

using a system according to Fig.1 and D) a transmission

spectrum using a conventional spectrophotometric system [0023] One embodiment of a system for the optical spectrophotometric assay of components in a liquid sample is illustrated in Fig. 1. The system 2 comprises an optical spectrophotometer 4 and a concentrator 6.

[0024] The optical spectrophotometer 4 of the present embodiment comprises a Fourier Transform (FT) interferometer 8 and includes a radiation source 10 configured to generate optical radiation. The radiation source 10 is here illustrated as being contained within the interferometer 8 but in other embodiments it could be provided external of the interferometer 8 and the optical radiation it generates directed towards the interferometer 8, for example via a fiber optic or lens system. The FT interferometer 8 is constructed to operate in a manner well known in the art and thus comprises a beam splitter 12 on to which, in use, optical radiation generated by the radiation source 10 is initially supplied to be split into two beams, a first beam which is reflected by the beamsplitter 12 to travel towards a first mirror 14 and a second beam which is transmitted through the beamsplitter 12 to travel towards a second mirror 16. One or both of the first mirror 14 and the second mirror 16 (here illustrated as being the second mirror 16) is movable along the direction of propagation of the respective first and or second beam. Optical radiation is reflected from the first mirror 14 and the second mirror 16 to recombine at the beamsplitter 12 and pass through an inspection zone 18 into which, in use, a sample for assay 40a is received so that the recombined optical radiation from the beamsplitter 12 impinges on and interacts with it. A detector 20 is provided to detect optical radiation after it having passed through the inspection zone 18 and is configured to monitor wavelength dependent intensity variations of the incident optical radiation and to transmit a signal representative of the monitored intensity variations to a data processor 22, here illustrated as being located external of the FT interferometer 8.

Indeed, in some embodiments the data processor 22 may be located at a remote location and connected to the detector 20 via a

telecommunications system. In other embodiments the detector 20 may additionally or alternatively be located remote of the FT interferometer 8 and the optical radiation communicated to it via fiber optics. [0025] The data processor 22 is, in a manner well known in the art, configured to apply to the signal passed from the detector 20 of the optical

spectrophotometer 4 a chemometric model linking a component of interest to be assayed in the liquid sample with wavelength dependent intensity variations and thereby obtain a prediction of a quantitative measure of the component of interest in the liquid sample.

[0026] The concentrator 6 comprises a nebulizer 24 for generating and

discharging an aerosol of liquid sample and, as illustrated in this present embodiment, optional heater plate 26 and/or housing 28. In the present embodiment a support 30 is provided as an element of the system 2 and has a number of sample collection surfaces 32 (illustrated as one surface) at which the sample for assay 40a will be formed. In the present embodiment this one or more collection surface 32 is translucent to optical radiation generated by the source 10, at least for those wavelengths sensitive to components of the liquid sample to be assayed, the

quantitative presence of which is to be determined by the system 2. This translucent collection surface 32 may, for example, be formed of a calcium fluoride disk in embodiments of the system 2 adapted for mid-infrared spectrophotometric assay. The support 30 may be releasably located in a holder (not shown) or, when present, releasably located on the heater plate 26 (as illustrated) so as to be able to receive (at least on the collection surface 32) liquid sample aerosol which, in use, is discharged from the nebulizer 24. In other embodiments the support 30 may be dispensed with. The collection surface 32 may be divided to provide different sections, each of which may act as a separate collection surface for receiving a different sample aerosols.

[0027] When located in the inspection zone 18 the support 30 may be movable so that the optical radiation passing through the measurement zone 18 can impinge on and interact with different portions of the collection surface 32 (or indeed with different collection surfaces should the support 30 be provided with more than one) on which the sample for assay 40a has been deposited. In this manner a plurality of quantitative measures of the component of interest can be made using different aliquots of the sample for assay or the component of interest in a plurality of different samples (each being deposited as a different sample for assay on its own associated sample collection surface) may be measured. In an

embodiment the support 30 may be a disc having a plurality of separate collection surfaces 32 located circumferentially separated around the disc. A rotary motor (not shown) may then be provided as an element of the optical spectrophotometer 4 to rotate the disc 30 between measurements to move a different collection surface 32 into the path of the optical radiation passing through the inspection zone 18.

[0028] In an embodiment of the present invention each of the number of

collection surfaces 32 on the support 30 at which a sample for assay 40a will be formed is a reflective surface, such as a gold surface when mid- infrared optical radiation source 10 is employed. The detector 20 is then located, for example as illustrated by the broken construction 20', to detected optical radiation that has passed through the sample for assay 40a twice as a result of being reflected from the reflective collection surface 32.

[0029] The housing 28 of the present embodiment is configured with an interior space 48 for containing the collection surface 32, the nebulizer 24 (or at least the portion at which the aerosol is generated) and, where present, the heater plate 26. A pressurised gas (typically air) supply 50 is connected to the interior space 48 for generating an overpressure therein. A pressure release valve 52 may also be provided in connection with the interior space 48 for safety. This pressurised housing 28 helps prevent dust from the surroundings becoming entrained in the aerosol and thus from contaminating the collection surface 32.

[0030] The nebulizer 24 of the present embodiment is provided with a gas inlet 34 for connection to a pressurised gas (typically air) supply 36 and a liquid inlet 38 for connection to a supply of liquid sample 40 to be assayed. The nebulizer 24 is here illustrated by way of example as being of a venturi (or so-called 'airbrush') type and is thus configured to, in use, pass a stream of fast moving pressurised gas from supply 36 past a venturi which is provided in liquid connection with the inlet 38 (or in some embodiments a local reservoir of the liquid 40 to be assayed connected to supply of the same). This creates a local reduction in pressure which causes liquid sample to be assayed to be pulled through the venturi where upon it forms an aerosol 44 with the fast moving gas from supply 36. This aerosol 44 is discharged from a spray nozzle 42 of the nebulizer 24 towards the collection surface 32.

[0031] An example of such a venturi type nebulizer 24 is illustrated in Fig. 3 and comprises a coaxial arrangement of outer gas conduit 24a intended for connection to gas inlet 34 and an inner liquid sample conduit 24b intended for connection with liquid inlet 38. Each of the gas conduit 24a and the liquid sample conduit 24b terminate in the spray nozzle 42. The liquid sample conduit 24b is terminated in the spray nozzle 42 with a small diameter venturi opening 24c and the gas conduit 24a terminates in the spray nozzle 42 with a section 24d shaped to direct a flow of gas across the venturi opening 24c in order to suck liquid there through and thereby to generate the aerosol 44 for discharge from the spray nozzle 42.

[0032] Liquid and the more volatile components in the aerosol 44 that impinges the collection surface 32 will evaporate to leave behind as the sample for assay a concentrated layer 40a of less volatile components of the liquid sample 40 which is to be located (here manually) in the inspection zone 18 of the optical spectrophotometer 4.

[0033] In some embodiments, and as illustrated in this Fig. 1 a pump, here a

piston pump 46, is provided in liquid communication between the supply of liquid sample 40 to be assayed and the inlet 38 to the nebulizer 24 to provide a pressurised feed of liquid to the venturi opening 24c. This has an advantage that the flow of liquid through the venturi opening 24c can be better controlled than if only the gas pressure from supply 36 is regulated.

[0034] Another embodiment of a system for the optical spectrophotometric assay of components in a liquid sample is illustrated in Fig. 2. As with the system 2 of Fig.1 the system 60 of Fig. 2 comprises an optical spectrophotometer 62 and a concentrator 64.

[0035] The optical spectrophotometer 62 is in the present embodiment illustrated as a conventional monochromator here having a fixed dispersion element 66 for generating a spatially dispersed spectrum 68 which is incident on diode array detector 70. As is known, the diode array 70 is configured with an array of individually addressable detector elements, each of which will receive a different wavelength portion of the spatially dispersed spectrum 68 and will generate a signal representative of wavelength dependent intensity variations for transmission to a data processor 72. The data processor 72 is configured, as per the data processor 22 of the

embodiment of Fig. 1 , to apply an appropriate chemometric model to the transmitted signal in order to determine a quantitative measure of a component of interest.

[0036] The concentrator 64 comprises a nebulizer 74 for generating and

discharging an aerosol 78 of liquid sample to be assayed; a heater plate 82; and a collection surface 84 for receiving discharged aerosol 78 from which liquid and other relatively volatile components will evaporate to form a concentrated layer 92 as a sample for assay. The nebulizer 74 is, in use, connected to a source of liquid sample for assay 80 via an inlet 77 and to a source of pressurised gas 76 via an inlet 75. The collection surface 84 is here illustrated, by way of example only, as an outer surface of a conventional Attenuated Total internal Reflection (ATR) crystal 86 which is located in thermal connection with the heater plate 82 in an inspection zone 88 associated with the optical spectrophotometer 62.

[0037] A radiation source 90 is provided to generate optical radiation having

appropriate wavelength components for interacting with components of interest to be assayed in the liquid from the supply of the liquid sample 80 and to direct this optical radiation into the ATR crystal 86. This optical radiation undergoes total internal reflection within the ATR crystal 86 to be reflected at least once from the surface 84 onto which the layer 92 has been formed. Upon exit from the inspection zone 88 the optical radiation which has traversed the ATR crystal 86 is focussed onto an entrance slit of the monochromator constituting the spectrophotometer 62.

[0038] The nebulizer 74 here is of the ultrasound type generally known in the art, such as is illustrated in Fig. 4. The nebulizer 74 comprises a piezoelectric crystal 74a for generating ultrasound waves into a buffer solution 74b which provides for efficient coupling thereof into a reservoir 74c of liquid sample. The reservoir 74c is connectable to the source of liquid sample 80 via inlet 77 and is provided with a narrow opening 74d through which liquid droplets 74e are ejected by the ultrasound into a chamber 74f. The chamber 74f is connectable at a first end 74g to the inlet 75 for the pressurised gas source 76 and terminates at a second end in a spray nozzle 74h through which the liquid droplets 74e entrained in a gas stream from the pressurised gas source 76 will be discharged as the aerosol 78.

[0039] The systems according to the present invention may be usefully employed in the assay of low concentration non-volatile components of water based liquid samples such as a wine, a milk or other potable product such as beer and fruit juices. The radiation sources 10, 90 are then mid-infrared radiation sources and the spectrophotometers 4, 62 are mid-infrared spectrophotometers.

[0040] An example of a mid-infrared transmission spectrum obtained for a milk sample 40 using a system according to Fig. 1 is shown at Fig. 5 as the upper trace A. A concentrated layer 40a of 37 micrometers (microns) thickness is built up on the translucent collection surface 32 of the planar support 30 and introduced into the inspection zone 18 of the FT

interferometer 8. The transmission spectrum A is that recorded by detector 20. A comparative transmission spectrum B for an aliquot of the milk sample 40 placed between transparent windows of a 37 micron

transmission cuvette is also illustrated in Fig. 5. As can be seen, the transmission spectrum A shows features around 1600 cnv¹ and around 3000-3500 cnr¹ which are regions of 'noise' in the spectrum B. It is known that urea and beta-hydroxybutyrate both produce absorbances around the 1600 cnr¹ region and that Cis-double bond is responsible for absorbances around 3000-3500 cnr¹. Since these features are better distinguished in the spectrum A, obtained from a concentrated layer 40a which is generated by the system according to the present invention, it is expected that a quantitative measure of the aforementioned components of the milk sample 40 may be better made using a system according to the present invention. The spectrum A is 'lifted' relative to the spectrum B i.e. the system according to the present invention produces generally higher absorbance values. This is probably due to some scatter of the dried sample layer 40a: some of the light is scattered away from the detector 20. Without this 'lifting' it can be seen that the actual peak heights are similar - which is to be expected since both the concentrated layer 40a and the liquid sample comprise the same amount of non-volatile component, although the thickness of the concentrated layer 40a is much less than that of the liquid milk aliquot.

[0041] An example of a mid-infrared reflection spectrum obtained for the same milk sample 40 using a system according to Fig.1 adapted to make reflection measurements on a 37 micron layer of a sample for assay 40a located in the inspection zone 18 is shown at Fig. 6 as the upper trace C. A transmission spectrum D is shown for comparative purposes. Again it can be seen that the reflection spectrum C exhibits features around 1600 cnr¹ and around 3000-3500 cnr¹ which are not distinguishable in the spectrum D. Moreover it can be seen that the signal at the detector 20' is almost twice that measured by the detector 20 in transmission.

[0042] It is convention that the quantitative measure of most components of a liquid sample is provided in units of milligrams per litre (mg/l) and therefore in order to provide a comparable measure using the system according to the present invention a 'concentration factor' may be provided which links the quantitative measure obtained using the present system (i.e. after evaporation) to the amount present in the original liquid sample. This factor may be obtained in many ways, such as, for example:

1 A reproducible procedure where the same sample

always gives the same spectra. A concentration factor

can then be calculated using a sample with a known concentration of a component where a calibration exists.

2 The same sample is measured before and after

evaporation. A major component that can be measured under both conditions is used for calculation of a

calibration factor required to convert the measurement

made after evaporation to that before evaporation. If all

samples are measured on both setups then the

concentration factor can be individual.

3 A tracer is added to the sample in a known

concentration. The prediction of this component after evaporation can be used for calculation of the

concentration factor.

[0043] A notable exception to this convention is the measure of free fatty acids

(FFA) in milk. Here it is convention to quote the concentration in milligrams per gram (mg/g). The concentration can therefore be obtained directly as the prediction made after evaporation (i.e. made using the system according to the present invention) and no concentration factor needs be calculated.

[0044] By way of example a calibration is established for urea in milk and is

illustrated with reference to Figs. 7 and 8. Measurements were made using the system according to the present invention on a milk sample set having had reference analysis made for urea content. Fig. 7 represents a conventional full spectrum PLS calibration established using cross validation based on measurements on the system according to the present invention on the milk sample set using two replicates. Variation in the thickness of the deposited layers between samples of sample set are corrected for using a fat absorption peak relative to the same peak using the conventional system. Fig. 8 represents a conventional full spectrum PLS calibration established using cross validation based on

measurements on a conventional Fourier Transform interferometer based system on the milk sample set, again using two replicates. The straight lines through the points represent the best fits of the calibrations

established according to the above. The Root Mean Squared Error of Prediction (RMSEP) for the calibration of Fig. 7 is 8.7 whilst the RMSEP for the calibration of Fig. 8 is 30.3. This indicates an accuracy

improvement for the system according to the present invention of at least a factor of 3 compared to the conventional interferometer based

measurements on liquid samples.

It will be appreciated that whilst the system according to the present invention has been illustrated with reference to two exemplary

embodiments it is not intended that the present invention be limited be these. For example other nebulizers of known type for generating aerosols can be substituted for those disclosed herein and the nebulizers disclosed herein can be interchanged without departing from the invention as claimed. Other mid-infrared spectrophotometers can be substituted for those disclosed herein and those disclosed herein can be interchanged without departing from the invention as claimed. Indeed the mid-infrared spectrophotometer may be substituted with spectrophotometer operating in any optical wavelength range appropriate for the absorption by nonvolatile components of interest in the liquid sample. Furthermore the collection surface may be provided in other ways, for example a solid opaque plate may be used. Also equivalent or complementary

spectrophotometer methodologies, such as reflection or absorption in place of transmission; and heater arrangements other than the resistive element heater plate type illustrated, such as infrared radiant heaters, are intended to fall within the scope of the invention as described and as claimed.

Claims

1. A system (2;60) for the optical spectrophotometric assay of components of a liquid sample (40;80), the system comprising an optical spectrophotometer (4;62,86) having an inspection zone (18;88) for receiving a sample for assay (40a;92); a radiation source (10;90) configured to generate optical radiation for supply into the inspection zone (18;88) to impinge on and interact with a received sample for assay (40a;92); and a concentrator (6;64) for

concentrating non-volatile components of the liquid sample (40;80) to form the sample for assay (40a;92) wherein the concentrator (6;64) comprises a nebulizer (24;74) configured to discharge an aerosol (44;78) of the liquid sample (40:80) and wherein the system (2;60) further comprises a one or more collection surface (32;84) spaced apart from the nebulizer (24;74) and disposed to receive discharged aerosol (44;78) to form the sample for assay (40a;92).

2. A system as claimed in claim 1 wherein the collection surface (32) comprises a material translucent to the optical radiation supplied from the radiation source (10) at least at wavelengths absorbed by components of the liquid sample (40) to be assayed.

3. A system as claimed in claim 1 wherein the collection surface (32) comprises a material reflective to the optical radiation supplied from the radiation source (10) at least at wavelengths absorbed by components of the liquid sample (40) to be assayed.

4. A system as claimed in claim 1 wherein the concentrator (6;64) further

comprises a heater (26;82) configured to supply heat to the collection surface (32;84).

5. A system as claimed in claim 1 wherein concentrator (6) further comprises a housing (28) configured with an interior space (48) for containing the collection surface (32) and at least a portion (42) of the nebulizer (24) from which aerosol (44) is discharged; and a pressurised gas supply (50) connected to the interior space.

6. A system as claimed in claim 1 wherein the nebulizer (74) is located to

discharge the aerosol (78) of the liquid sample (80) into the inspection zone (88) for receipt onto the collection surface (84) as the sample for assay (92).

7. A system as claimed in claim 1 wherein the nebulizer (24) is located to discharge the aerosol (44) of the liquid sample (40) outside of the optical spectrophotometer (4) for receipt onto the collection surface (32) as the sample for assay (40a).

8. A system as claimed in claim 1 wherein the nebulizer is a venturi- type

nebulizer (24).

9. A system as claimed in claim 8 wherein a pump (46) is provided in operable communication with the nebulizer (24) to supply a pressurised liquid sample (40) thereto.

10. A system as claimed in claim 1 wherein the radiation source (10;90) is a mid- infrared radiation source and the optical spectrophotometer (4;62,86) is a mid- infrared optical spectrophotometer.

1 1. A method of optical spectrophotometric assay of components in a liquid

sample (40;80) comprising the steps of: generating an aerosol (44;78) of the liquid sample (40;80); directing the aerosol (44;78) of the liquid sample (40:80) onto a collection surface (32;84); allowing liquid to evaporate from the aerosol (44;78) on the collection surface (32;84) to form a sample for assay (40a;92) and performing an optical spectrophotometric assay of the so formed sample for assay (40a;92).

12. A method as claimed in claim 1 1 wherein the step of allowing liquid to

evaporate from the aerosol (44;78) includes supplying heat to the collection surface (32;84).

13. A method as claimed in claim 1 1 wherein the step of supplying heat to the collection surface (32;84) comprises placing the collection surface (32;84) in thermal communication with one or other of a resistive heater (26;82) or an infrared radiant heater.

14. A method as claimed in claim 1 1 wherein the step performing an optical

spectrophotometric assay of the so formed sample for assay (40a;92) consists of performing a mid-infrared spectrophotometric assay.