WO2005052560A1

WO2005052560A1 - Apparatus and method for identifying a liquid product

Info

Publication number: WO2005052560A1
Application number: PCT/GB2004/004885
Authority: WO
Inventors: Darrell Green; Clive Antony Marchant; Jon Paul Sturgeon
Original assignee: Johnson Matthey Plc
Priority date: 2003-11-19
Filing date: 2004-11-19
Publication date: 2005-06-09
Also published as: EP1685387A1; GB0326928D0

Abstract

The invention concerns a method for marking a liquid product and identifying a sample of the liquid comprising adding to the liquid at least one fluorescent dye and measuring the fluorescence of an unknown sample of the liquid to determine the amount of dye contained therein. Also disclosed is an identification system comprising at least one dye for marking a liquid and an associated apparatus for detecting the marked liquid.

Description

Apparatus and method for identifying a liquid product

The present invention relates to an apparatus and method of identifying a liquid hydrocarbon product.

It is known to incorporate marker compounds into commercially formulated products in order to identify the source of the product or to trace the flow of a particular chemical composition. Such marking or "tagging" of products may be used to identify counterfeit products or simply to monitor the flow of chemicals through a process or supply chain.

Various marker compounds may be used, for example radioactive materials, dyes, fluorescent materials or other materials which are readily detectable by particular methods.

One particular application for tagging technology is in petroleum products. Petroleum products are often subject to taxes and regulations which require that the amount and composition of the products sold is monitored and recordable. In such situations any deviation from a regulated standard of purity may impact the amount of tax which is recoverable in the region in which it is sold. Petroleum products such as automotive fuel gasoline or diesel may contain hydrocarbon solvents such as toluene, xylene or hexane but the amount of solvent permitted is normally strictly controlled to avoid potential damage to engines and to limit the amount of tax lost on the non-petroleum content of such fuels. It is often difficult to separate and thus identify and measure solvents found in a petroleum mixture because the petroleum is itself a complex mixture of compounds.

The adulteration of petroleum products through the addition of low taxation solvents may be monitored by adding tags to those solvents which are likely to be added to petroleum products. One method of marking or tagging a solvent is to add an isotope of the solvent such as a deuterated version of the solvent chemical. This approach has the benefit of avoiding the introduction of different chemicals into the solvent so that potential problems of incompatibility may be minimised. The disadvantage of this approach is that such isotopes may be expensive and the method for detecting their presence, i.e. mass spectroscopy, requires relatively expensive and sensitive equipment which is not suitable for use "in the field".

Monitoring for the presence of tags or marker compounds in petroleum products provides a means for identification of the product and quantification of adulteration. The addition of a known amount of tag or marker compound to a particular petroleum product itself and subsequent identification and measurement of that tag may be used to confirm its identity or the degree of adulteration of the petroleum product. Petroleum products may therefore be tagged by adding a marker chemical to the petroleum product and comparing the properties of the product with a standard sample of the product containing a known amount of the marker. It is an object of the invention to provide an improved method for marking a petroleum product. It is a further object of the invention to provide an apparatus and method of identifying a petroleum product. It is still a further object of the invention to provide a method and apparatus for dispersing an identifying marker product in a liquid.

According to the invention, we provide a method for marking a liquid petroleum product comprising adding to said petroleum product at least one dye which, when irradiated with light of a first wavelength emits light at a second wavelength, said second wavelength being in the range from 250 - 1200 nm nm. Such emission is known as fluorescence and the second wavel ength will be referred to as the emission wavelength. The amount of fluorescence, i.e. the area of the fluorescence peak when measured using a fluorescence spectrometer, is proportional to the amount of fluorescent dye in the sample measured. Therefore dilution of the marked liqu id with an unmarked or differently marked liquid can be detected by a reduction in the fluorescence of the sample compared with the original marked liquid or a standard sample of the marked liquid.

In a second method according to the invention we provide a method for marking a liquid petroleum product comprising adding to said petroleum product at least one dye which, when irradiated with light of a first wavelength emits light at a second wavelength, said second wavelength being in the range from 250 - 1200 nm, passing a sample of said liquid throu gh an absorbent capable of absorbing said dye, and determining the amount of dye present originally in the sample from the fluorescence energy detected at the emission wavelength. In a preferred embodiment of the invention we provide an apparatus for detecting the presence of a pre-determined amount of a fluorescent material in a liquid, the apparatus comprising:

(a) a sample holder, suitable for receiving and supporting a sample cell containing a sam le of the liquid,

(b) at least one source of excitation radiation, adapted to irradiate the sample cell with light at a predetermined wavelength,

(c) a radiation detector arranged to detect radiation emitted from the sample, said radiation detector being located out of the path of the excitation radiation, (d) a data collection and manipulation device for gathering data from the radiation detector and producing information concerning the intensity and wavelength of the emitted radiation detected by the radiation detector.

In a particularly preferred embodiment of the invention, the apparatus is designed to be used to detect a specific fluorescent marker or combination of markers in a particular type of liquid, e.g. gasoline. In this embodiment we provide an identity detection system for marking and identifying a liquid comprising a pre-determined quantity of a marker composition comprising at least one fluorescent material, said composition being soluble in the liquid, and an apparatus for detecting the marker composition in a sample of the liquid, said apparatus comprising (a) a sample holder, suitable for receiving and supporting a sample cell containing a sample of the liquid,

(b) at least one source of excitation radiation, adapted to irradiate the sample cell with light at a predetermined wavelength, said predetermined wavelength being selected to be of a wavelength capable of being absorbed by and promoting fluorescent emissions from one of said at least one fluorescent materials,

(c) a radiation detector arranged to detect radiation emitted from the sample, said radiation detector being located out of the path of the excitation radiation,

(d) a data collection and manipulation device for gathering data from the radiation detector and producing information concerning the intensity and wavelength of the emitted radiation detected by the radiation detector and data-processing means for comparing the characteristics of intensity and wavelength of the fluorescent light emitted from the sample when sample irradiated by said excitation source with the characteristics of radiation emitted from a standard sample of the liquid containing a known quantity of the fluorescent material, and (e) indication means to indicate whether the characteristics of the light emitted from the sample are within a predetermined range of the characteristics of the light emitted from the standard sample.

In a further embodiment of the invention we provide a method of identifying a liquid comprising the steps of:

(a) dissolving in said liquid a pre-determined quantity of a marker composition comprising at least one fluorescent material having a characteristic wavelength of maximum absorption of light, said composition being soluble in the liquid to form a marked liquid,

(b) subsequently taking a sample of a liquid which is to be positively identified as being a sample of said marked liquid into a sample cell and measuring the fluorescence of said sample using an apparatus comprising: i. a sample holder, suitable for receiving and supporting a sample cell containing a sample of the liquid, ii. a source of excitation radiation, adapted to irradiate the sample cell with light at a predetermined wavelength, said predetermined wavelength being selected to be capable of being absorbed by one of said at least one fluorescent materials, iii. a radiation detector arranged to detect radiation emitted from the sample, said radiation detector being located out of the path of the excitation radiation, iv. a data collection and manipulation device for gathering data from the radiation detector and producing information concerning the intensity and wavelength of the emitted radiation detected by the radiation detector and data-processing means for comparing the characteristics of intensity and wavelength of the fluorescent light emitted from the sample when sample irradiated by said excitation source with the characteristics of radiation emitted from a standard sample of the liquid containing a known quantity of the fluorescent material, and v. indication means to indicate whether the characteristics of the light emitted from the sample are within a predetermined range of the characteristics of the light emitted from the standard sample

(c) irradiating the sample in the sample cell with light from said source of excitation radiation at a predetermined wavelength, said predetermined wavelength being capable of being absorbed by one of said at least one fluorescent materials,

(d) detecting the fluorescent light emitted from the sample which impinges upon said radiation detector,

(e) comparing the characteristics of the intensity and wavelength of the light detected by the detector with predetermined characteristics of intensity and wavelength of fluorescence emitted by a standard sample of the marked liquid or of a standard marked liquid, thereby to determine whether the sample of liquid in the sample cell is from a source containing the marker composition.

When the fluorescence is to be measured in liquids having strong absorptions within the UV and /or visible region, such as in many hydrocarbon fuels such as gasoline, it may be preferred to use dyes which absorb and emit at higher wavelengths e.g. above 500nm preferably between 500 and 750 nm. Preferably the marker composition comprises more than one dye. When the fluorescence is measured in a sample of the marked liquid it is preferred that each dye emits fluorescent radiation at an emission wavelength which is different from the emission wavelength of each other dye. When more than one dye is used it is not necessary for the emission wavelength of each dye to be within the range from 500 - 750 nm. In a preferred embodiment the marker composition comprises at least two different dyes in combination, each having a different emission wavelength. More preferably the marker composition comprises three or more dyes.

In one embodiment, where the petroleum product contains an organic polar solvent, especially an alcohol such as methanol or ethanol, the dyes are selected to be soluble in the polar solvent. This increases the range of dyes which may be used. Therefore preferred dyes are polar, most preferred are dyes which are capable of dissolving fully in ethanol at a concentration of up to 500 ppb at ambient temperatures and pressures. Examples of preferred dyes having an emission wavelength in the range 500 - 700 nm are quinone-imine dyes, especially oxazine dyes such as Oxazine 1 chloride, Oxazine 170, Oxazine 750, and xanthene dyes, particularly the Rhodamine dyes (e.g. Rhodamine 6G, Rhodamine F3, Rhodamine 700) and cόumarin dyes. The halide (especially chloride), perchlorate, tetrachlorozincate and tetrafluoroborate forms of these dyes are suitable. Anionic dyes such as disodium Fluorescein or Stilbene 3 disodium salt may also be used. Other fluorescent dyes may be used. Suitable dyes are well known, for example in the field of laser dyes or histopathology and characteristics of suitable dyes may be found in manufacturers' literature, for example "Lambdachrome® Laser Dyes" by U. Brackmann published by Lambda Physik AG.

Light of wavelength greater than about 500nm is at the extreme limit of the visible spectrum. Therefore in order to make the presence of the dye visually undetectable, the dyes are used in low concentrations. Preferably each dye is present at a concentration of less than 1 ppm, more preferably less than 100 ppb w/v based on total volume of the petroleum liquid together with any other liquid components such as solvents.

When more than one dye is used in the method of the invention, the dyes are premixed to a predetermined composition. The composition of the mixture of dyes is then capable of identifying the liquid into which the dye mixture is placed because the concentration of each dye, having a different emission wavelength on irradiation at its appropriate excitation wavelength, may be determined from the magnitude of the emitted radiation.

The dye or mixture of dyes may be solid or liquid. Both solid and liquid forms of suitable dyes are known. Preferably, the dye or dyes are selected to be sufficiently soluble in the liquid to be marked that they are dispersed homogeneously with no or only mild to moderate mixing. In the preferred form of the invention in which petroleum liquids are marked according to the method of the invention, the dyes should preferably be capable of dissolving to provide a homogeneous solution of dye in the petroleum when the mixing action is provided by the movement of the liquid in a road or rail tanker, over a period of a few hours.

Preferably the dye or prepared mixture of dyes is supplied in the form of a pre-measured amount which is suitable to be added to the liquid to be marked. The pre-measured amount of dye may conveniently be supplied in a container such as a sachet, bottle, ampoule or canister from which the dye is removed, e.g. by tipping into the liquid to be marked. It is preferred that the measured amount of dye in the container is suitable to be added to the liquid as a single dose so that inconsistencies in estimating parts of the measured amount are avoided. Alternatively the pre- measured amount of dye or dyes is supplied in an encapsulated form in which the container is added to the liquid and which then releases the dye into the liquid following addition. Preferably the container is capable of dispersing in the liquid in such a way that no or minimal solid residues of the container remain which could foul the liquid transfer lines in a subsequent transfer of the liquid. A soluble container is preferred. The soluble container may be in the form of a bag or sachet made from a relatively flexible, thin soluble material such as a woven fabric or film of a soluble material such as a suitable polymer, for example. Alternatively the container may be a rigid container, for example made by molding a solid soluble material. When a soluble encapsulation device is employed, it is preferred to supply it in a protective packaging to avoid breakage of the capsule or tampering with its contents.

The marker in the liquid is identified by measuring the fluorescent emission of the dye or dyes contained in a sample of the liquid. The dye is irradiated with light of a wavelength which excites fluorescent emission. The wavelength and magnitude of fluorescent emission from the liquid sample is then determined and compared with the wavelength and magnitude of fluorescent emission from a standard sample of liquid containing a known amount of the dye. A discrepancy between the emission detected from the measured sample and that from the standard sample is then indicative that the liquid from which the measured sample was taken has been changed, e.g. diluted or tampered with.

The fluorescent emission of the dyes may be determined with a standard fluorimeter in which a sample, normally contained in a transparent cell or cuvette, is irradiated with light of known wavelength (the excitation radiation). By transparent, we mean that the sample cell, or at least a portion of it, is substantially transparent to the excitation radiation and the emitted fluorescence. The light emitted by the sample is then collected using a light detecting device and the wavelength and intensity of the emitted light is measured.

The excitation radiation is of a pre-determined wavelength which is chosen to be of a wavelength which is capable of being absorbed by and producing fluorescent emission from one of the fluorescent materials in the sample. The wavelength selected for the excitation radiation depends upon the shape of the absorption peak of the particular fluorescent dye in use. The excitation radiation is selected to be of a wavelength which is known to excite fluorescence in a dye contained in the sample, hereinafter referred to as the excitation wavelength. The excitation wavelength may be a range of wavelengths within which the dye is stimulated to fluoresce. The excitation wavelength is preferably less than or about equal to the wavelength of maximum absorption in order to avoid detection of the excitation radiation and interference with the fluorescence emission spectrum. The wavelength selected for the excitation radiation may depend upon the shape of the absorption peak of the particular fluorescent dye in use. Preferably the wavelength of excitation is within the range between λ_P and λ _₂ where λ_P is the wavelength of the maximum absorption, i.e. the peak wavelength, and λ .₂ is the minimum wavelength at the full width at 20% of the peak height of the absorption peak.of the dye. When the wavelength of excitation is lower than this, the dye may still be excited but the response to the excitation radiation is lower because less energy is absorbed by the dye. More preferably, the wavelength of excitation is within the range between λ_P and λ .₅ where λ _-5 is the minimum wavelength at the full width half peak height of the absorption peak of the dye. More preferably the excitation wavelength is within the range between λ_P and λ _-9 where λ _-9 is the minimum wavelength at the full width nine tenths height of the absorption peak of the dye. When the selected absorption band is not close to the fluorescent emission spectrum of the dye, the excitation radiation may also be within the range the range between λ_P and λ ₊₂ where interference of the excitation radiation with the emitted spectrum is unlikely to occur. In this case the excitation radiation may be within the range the range between λ _₂ and λ ₊₂.

This is illustrated in Figure 4. Normally the excitation radiation is selected to be within about 30nm of the wavelength of maximum absorption of the dye which is to be detected. The wavelength of maximum absorption, and the fluorescent emission wavelength is normally known and documented for known commercial dyes. In case these parameters are not known, it is a matter of routine to measure the spectral absorption and emission for a particular dye using standard spectrometry apparatus.

The excitation radiation may be outside the preferred range of wavelengths if the reduced fluorescence response is acceptable in the application for which the apparatus is used. The excitation radiation may be selected to be of a wavelength which is not within the peak absorption spectrum of the dye, but which falls within a region of the spectrum in which the dye absorbs less strongly. Most dyes absorb radiation in more than one region of the spectrum, e.g. the dye may absorb strongly in the visible region and less strongly in the ultraviolet region of the spectrum. In such a case, it may be more convenient to use a source which produces light in the region of weaker absorption , e.g. in the ultraviolet region, provided the detection system is sufficiently sensitive to detect the weaker fluorescence produced by the dye absorbing less light. Whichever absorption band is selected to excite the dye fluorescence, the excitation radiation is preferably selected to be within the range between λ_P and λ _₂ of the selected band. However, when the selected absorption band is not close to the fluorescent emission spectrum of the dye, the excitation radiation may also be within the range the range between λ_P and λ ₊₂ where interference of the excitation radiation with the emitted spectrum is unlikely to occur. In this case the excitation radiation may be within the range the range between λ _₂ and λ ₊₂.

Some liquids which are dark-coloured have an optical density which affects significantly the amount of light transmitted through the sample. When the optical density of the liquid is increased the amount of excitation energy transmitted to the dye fluorophores is reduced and also the amount of energy emitted from the fluorophores which reaches the detector is reduced. Because the amount of emitted energy measured is proportional to the amount of excitation energy received by the fluorophore and also to the amount of dye in the sample any variation in optical density between the sample measured and any standard or calibration sample may lead to an incorrect result. For example, if the optical density of the standard sample is lower than the optical density of the "unknown" then less fluorescent energy is emitted from the unknown and so the unknown may appear to contain less dye than was added to the standard, i.e. it may appear to have been diluted. In order to overcome the above problem the sample may be contacted with an absorbent capable of absorbing the dye in preference to the liquid to absorb the dye. The absorbent is preferably a solid and may be selected from for example a shape-specific absorbent, a functionalised polymer absorbent, an ion exchange resin such as polyamino vinyl benzene, or a functionalised silica such as a cyano-, amino- or hydroxyl- functionalised silica. Suitable absorbents for anionic dyes include amine-functionalised absorbents such as ethylenediamine-N-propyl compounds (e.g. 1SOLUTE™ PSA), a quaternary ammonium absorbent (e.g. ISOLUTE SAX). Suitable absorbents for cationic dyes include carboxypropyl -type absorbents such as ISOLUTE CBA for example. ^* Other absorbents are well-known to the skilled person. The dye may then be washed from the absorbent using a suitable solvent having a known optical density. A sample of the resulting solution of the dye in the solvent may then be measured using the apparatus of the invention to determine the concentration of the dye in the solvent sample and thereby the amount of dye in the original liquid sample. Alternatively, when the dye has been removed from the liquid sample by contact with the absorbent, the optical density of the liquid sample may be measured using apparatus similar to the apparatus of the invention so that a correction may be applied to the measured emitted fluorescence of the liquid sample containing the dye, based upon the optical density of the liquid medium in the absence of the dye.

In an alternative embodiment of the method of the invention, more than one fluorescent dye having similar excitation and emission wavelengths may be used if each dye may be separated from the liquid sample using a physical or chemical absorbent. For example a mixture of a cationic dye and an anionic dye may be used where the fluorescence of the dyes is excited by similar wavelengths of light and the emitted radiation of each dye is of a similar wavelength. The dyes may be separated by contacting the sample with an appropriate absorbent (e.g. a suitable ion exchange resin) to extract either the cationic dye or the anionic dye from the liquid sample. The fluorescence of the absorbed dye may then be determined by extraction of the dye from the absorbent into a solvent followed by measurement of the fluorescence of the resulting solution; or alternatively by measurement of the fluorescence of the liquid sample with and without the presence of the absorbed dye followed by a comparison or subtraction of the resulting fluorescence spectra. As a further alternative, each dye may be of different polarity or have another characteristic which provides each dye with a different affinity for an absorbent material which allows one of or each dye to be separated from the liquid by contact with an absorbent.

When an absorbent is used to extract a dye from the liquid, it is preferably supplied in a cartridge which is adapted for a liquid to flow through a bed of absorbent contained within the cartridge. The cartridge preferably has an input port into which a sample of liquid may be injected and an output port from which the liquid may emerge. The liquid may be collected in a container or alternatively the liquid may be caused to flow from the absorbent cartridge directly into a sample cell of the apparatus of the invention. The cartridge of absorbent is preferably integrated with the apparatus of the invention but is removable from the apparatus to allow for the cartridge to be changed. Preferably the cartridge is re-usable, optionally with means for flushing the absorbent with a solvent to remove traces of absorbed dye before the liquid sample is introduced into the absorbent. In one embodiment, the apparatus includes an automated system comprising a solvent reservoir, pump means and control apparatus to automate the passing of the sample through the absorbent and flushing of the absorbed dye into a solvent prior to the introduction of the liquid or solvent containing the dye into the sample cell of the apparatus.

When more than one fluorescent compound is present in the sample within the sample cell, it is preferred that the apparatus comprises a separate light source tuned to produce excitation radiation within the absorption spectrum of each fluorescent compound. The fluorescent compounds should preferably be selected so that their absorption spectra do not overlap, that is, so that each light source excites only one fluorescent compound at any time.

The excitation radiation may be produced by any known source, e.g. a full-spectrum light source with a suitable band-pass filter or, more preferably, by a single or narrow-band source such as a laser. Laser light sources are available at many different wavelengths and may be selected or tuned to provide light of the correct wavelength to excite fluorescence in at least one of the fluorescent materials. It is preferred to provide a separate laser source for each fluorescent marker material present in the sample, each laser being selected and tuned to emit radiation at or near the frequency of maximum absorption of a respective fluorescent marker. In order to provide for simple portability of the apparatus, solid laser diode sources are preferred to dye lasers. When more than one light source is present, means are provided to indicate to the data- processor which radiation source is being used.

The wavelength is moderated using known methods such as by means of a standard grating or filter or by using a laser or other form of monochromatic light. The radiation emitted by the source should have a centre peak wavelength which does not vary by more than ± 2nm over the period of time taken to test the sample in order to ensure that the absorption spectrum does not vary. When using a light source which produces output radiation whose wavelength varies with temperature, for example, then a stabilised source should be selected, and temperature stabilising means such as a Peltier module or alternative heat-sink should be provided. The temperature stabilizing means may also be useful to warm the light source if necessary to bring the source to its optimum operating temperature. It is preferred to divert a portion of the excitation radiation' directly to a radiation detector in order to determine the centre peak wavelength and/or the intensity of a particular wavelength of the emitted light and its variability. Where the centre peak wavelength or the intensity of a particular wavelength of the excitation radiation changes between samples, it may be possible to calculate an enhancement or reduction factor for the sample fluorescence spectrum to compensate for changes in the excitation radiation which cause the sample to absorb more or less energy. In order to differentiate between light transmitted through the sample and fluorescent emission by the sample, it is usual to place the light detector out of the path of transmitted light. The light detector may be a photo cell or photodiode, including a charge-coupled device.

The fluorescent emission of each dye is preferably measured independently of each other dye. This is achieved by irradiating the sample with light of a wavelength which is at or near the wavelength of maximum absorption of one of the dyes but which does not significantly excite fluorescence in any other dye or in the liquid medium within the sample cell.

In a particular embodiment of the invention a mixture of at least a first and a second dye is used to mark a petroleum liquid. The first and second dyes have, respectively first and second excitation wavelengths, and first and second fluorescence emission wavelengths. The dyes are premixed and a pre-determined amount of the mixture is added to a known volume of petroleum liquid. When the marked liquid is to be identified, a sample of liquid is placed in a sample cell and introduced into the sample holder of a suitable fluorescence spectrometer.

The apparatus may comprise a means by which a sample is caused to flow through an absorbent material before entering the sample cell. Such means may comprise a housing for an absorbent cartridge, such means enabling a cartridge of a suitable absorbent to be inserted into and removed from said housing by the user of the apparatus. The means may also comprise input and out means for introducing the sample liquid into the absorbent bed and removing it from the bed. The input and output means may be provided on an absorbent cartridge. The cartridge of absorbent is preferably integrated with the apparatus of the invention but is removable from the apparatus to allow for the cartridge to be changed. Preferably the apparatus further comprises means for eluting absorbed species from the absorbent into the sample cell. The apparatus may further comprise means for flushing the absorbent with a solvent to remove traces of absorbed dye before the liquid sample is introduced into the absorbent. In each case the liquid used for flushing and/or eluting absorbed species may be provided from a liquid storage means associated with the apparatus. In one embodiment, the apparatus includes an automated system comprising a solvent reservoir, pump means and control apparatus to automate the passing of the sample through the absorbent and flushing of the absorbed dye into a solvent prior to the introduction of the liquid or solvent containing the dye into the sample cell of the apparatus.

The sample cell is transparent to the excitation radiation and to the emitted radiation, at least in the portions of the sample holder through which this radiation is to pass. Preferably the sample cell formed entirely from a transparent material, such as a silica. The sample cell may be provided with a lid or stopper. This is preferred when the sample is a liquid, especially a flammable and/or volatile liquid such as gasoline. Suitable sample cells are well known in the field of spectroscopy and are available from various manufacturers as standard items. For example, a suitable sample cell has a rectangular or square cross-section, supplied with a push- fit stopper. The sample cell dimensions determine the path length of the sample. A short path length is beneficial to reduce absorption of the radiation by the bulk liquid. Therefore a suitable sample cell may be e.g. 5mm x 5mm x 40mm to provide a path length of about 5mm although smaller or larger sample cells could be used, providing a path length of from about 1mm to about 10mm.

The sample holder is adapted to receive the sample cell and to support the sample cell securely in a particular orientation with respect to the sample holder and the radiation source(s). The sample holder also provides an enclosure for the sample wherein light may be excluded from the sample or light from a source of excitation radiation may be admitted to the sample. Means to allow emitted radiation from the sample to pass out of the sample holder to the light detector are also provided. The sample holder is adapted to allow light from one (and only one) radiation source to pass into the sample cell at any time. The sample holder therefore comprises a sample cell retaining portion and a wall portion, the wall portion surrounding the sample cell and being located between the sample cell and the or every radiation source. The wall portion is formed from a material which does not transmit light. The sample holder includes at least one aperture in the wall portion of the sample holder between the sample cell and any one radiation source. When the sample holder includes more than one such aperture, then shutter and interlock means are present to provide that light from only one radiation source may pass into the sample cell through one aperture at any time. The shutter and interlock means may comprise a number of different arrangements. As a first example, the sample holder may comprise two concentric circular sleeves at least one of which may be rotated relative to the other. One sleeve has more than one aperture or a single aperture through which radiation from more than one radiation source may pass, while the other sleeve has one or more apertures arranged such that apertures in the inner and outer sleeve coincide in the path of only one of the radiation sources at one time. The sleeves may also be moved to a position in which no light is admitted to the sample, i.e. in which the sample is shielded from light from the or all radiation sources. In an alternative arrangement, the wall of the sample holder comprises a sleeve having a single aperture arranged to allow light from only one radiation source to pass into the sample cell through one aperture at any time. The sleeve may then be moved to align the aperture with the path of any one radiation source or no radiation source in order to admit light from the selected radiation source or alternatively to admit no light into the sample. As a further variation, the sleeve may have more than one aperture, positioned with respect to the or each radiation source such that no more than one aperture and radiation source may be aligned at any time. The provision of more than one aperture in the sleeve may be convenient for the operation of the instrument. The sleeve is conveniently circular in cross-section, i.e. cylindrical in shape, so that the sleeve may be rotated relative to the sample, and/or the radiation sources in order to bring the aperture(s) in the sleeve into and out of alignment with the radiation path. Other methods of shuttering and interlock means which are known to the skilled person may be used.

The light source(s) are arranged around the sample holder in such a way that the light paths through the sample are different from each other and preferably do not cross. This may be achieved by arranging each of the light sources at an angle from each other light source and optionally at a different height relative to each other light source so that each irradiates a different part of the sample cell.

As is well known, care must be taken in handling laser sources and this is why it is greatly preferred that the sample holder is arranged with respect to the or each laser source so that in at least one position all laser light sources are prevented from entering the sample holder. In this condition the user may safely open or otherwise gain access to the sample holder in order to insert or remove a sample or to clean the sample holder. Preferably a safety interlock is provided to prevent the sample holder from being opened or accessed by the user when a laser light source is shining into the sample holder. The instrument is designed to avoid the possibility of laser light contacting a user during normal operation and maintenance.

The radiation detector is arranged to detect radiation emitted from the sample and is located out of the path of the excitation radiation in order to avoid, so far as possible, detection by the detector of the excitation radiation. The emitted radiation from the sample may be directed into the path of the radiation detector by one or more lenses or mirrors or a combination thereof as is known to the skilled person. It is preferred that the emitted radiation is collected over the whole of the path length of the sample for maximum sensitivity to changes in the emission between samples. The radiation may pass through a slit or aperture to reduce the divergence of light reaching the detector and thus increase the resolution of the spectrum. The aperture is preferably of similar dimension to the path length. The detector may be any of those used in standard spectroscopy apparatus, including, for example, a photocell, a charge-coupled device etc. Normally the radiation emitted from the sample is split into individual wavelengths, e.g. by a diffraction grating, a prism or a concave holographic mirror and the intensity of the light at each wavelength is then measured by the detector. The optical system comprising the mirror, lenses diffraction grating and detector are preferably designed as a close-coupled optical system.

The data from the detector is collected and analysed using a data processor which produces the information required by the user concerning the wavelength profile of the emitted light. Normally the user requires information concerning the light emitted at each wavelength and the data processing device may produce this information in the form of a chart or graph etc. The data processor may be linked to a data storage device so that information on previous plots may be retrieved and compared.

The apparatus is preferably portable, having all components located within a single housing. Controls are provided to open / close the sample holder and operate the instrument to allow light from the selected radiation source to enter the sample. The housing preferably also comprises a power source such as a battery pack. The housing may incorporate a display to indicate the results from a sample, the status of the instrument or instructions on its use.

The indication means may comprise a message display or some other indicator such as a light. Preferably a source of excitation radiation is provided for each fluorescent material contained in the marker composition, each source being selected and/or tuned to provide light at or near the wavelength of maximum absorption of one of the fluorescent materials. The marker composition may comprise one or more than one fluorescent material. Preferably more than one fluorescent material is present, for example from 1 to 4 fluorescent materials, each being present in the marker composition in a known amount which may be different from the amount of each other material in the composition. Other materials may also be present, such as a solvent, other types of marker compound, non-fluorescent materials etc. The marker composition is added to the liquid in a predetermined amount which contains a known amount of each fluorescent material contained within the composition. The marker composition is preferably supplied in a container containing a measured amount of the composition, calculated to provide a selected concentration of the or each fluorescent material when dissolved in a measured amount of the liquid.

It is an important feature of the identity detection system and method of the invention that the excitation radiation source(s) comprised in the apparatus are matched to the fluorescent material(s) contained in the marker composition so that the marker composition and the apparatus for measuring the fluorescence form an integrated identifying system. This facilitates operation of the identity detection system and apparatus by unskilled persons. In this way it is also possible to provide an identity detection system which is more robust against fraud because the nature of the fluorescent materials and their excitation and emission wavelengths need not be known by the operators of the system. Therefore preferably the apparatus is provided with a sealed housing to shield the radiation sources from interference and to prevent adjustment of the wavelength of emitted radiation. Therefore it is preferred that the apparatus does not indicate the wavelength of excitation or emission or provide an emission spectrum as would be expected in a conventional fluorescence spectrometer.

The characteristics of the fluorescence spectrum which are calculated by the data processor may include peak emission wavelength, peak height and peak area. It is preferred that the characteristics which are measured and compared with a standard sample are not known by the user when the identification system is intended to detect fraudulent tampering with the liquid to be measured. For example, the data processor may be programmed to determine the emission peak area between two pre-selected wavelengths so that the presence of a fluorescent dye which is not contained in the standard sample may be detected, even when the dye has a similar peak emission wavelength to the dye contained in the standard, if the emission spectrum is of a different shape.

The invention will be further described with reference to the accompanying drawings, which are:- Figure 1 : a plan view of an apparatus according to the invention Figure 2: a schematic view of a transverse section through the apparatus Figure 3: a schematic view of a longitudinal section through the apparatus Figure 4: a dye absorption spectrum illustrating λ_P, λ .₂, λ _-5 and λ _₉ Figure 5: fluorescence spectra from Example 2.

The apparatus shown in fig 1 comprises a housing 10, having a power indicator light 11 , a "battery low" warning light 12 and a display screen 13. A sample holder lid 14 is opened by sliding handle 15 in the direction of the arrow. Sliding knob 16 is movable between three positions O, A & B. In the O position, the sample holder lid 14 may be opened. Referring to Fig 2, the sample cell 17 is retained within the sample holder 18. Sample holder 18 is cylindrical and has an aperture' 19 in the side wall, vertically aligned to allow light from lasers 20 and 21 to pass through the aperture 19. The sample holder is supported upon a splined base 22 which cooperates with the toothed surface of pawl 23, operated by knob 16. Thus when knob16 is moved to position A, the pawl 23 moves to the right and splined base 22 rotates clockwise to bring aperture 19 into alignment with^'light from laser 21. In Fig 3, the light path may be seen. Laser 20 emits light at a predetermined excitation wavelength λe shown by the solid arrow. If the light is of a suitable wavelength to be absorbed by a fluorescent compound in the sample cell 17 then fluorescent light is emitted by the sample, shown by the dotted line. The emitted fluorescent light of wavelength(s) λf is deflected by a mirror 24 to pass through a slit 25, lens 26 and diffraction grating 27 to be split into its component wavelengths. The intensity of each wavelength impinging upon charge coupled device 28 is analysed by data processor 29. The data processor compares characteristics of the spectrum with characteristics of a standard sample and an indication of the degree of similarity is shown on the display 13.

The drawings illustrate an apparatus comprising two laser light sources. This apparatus is suitable for use with an identity detection system or method according to the invention in which two fluorescent dye materials are provided in the marker composition and in which the lasers and dye compounds are selected such that the lasers produce radiation which is absorbed by the dye compounds to excite the dye compounds to fluoresce. The fluorescence characteristics of the dye compounds in a standard sample of the liquid at a standard concentration are input into the data processor so that comparison between the standard sample and the measured sample may be made. In a preferred form of the apparatus, identity detection system or method according to the invention more than two fluorescent compounds and light sources are provided, each light source being matched to produce radiation within the absorption spectrum of one fluorescent compound.

Example 1

A gasoline sample was diluted with ethanol to produce a gasoline solution containing 25% ethanol and 75% gasoline v/v. Rhodamine 6G dye (a cationic dye giving a fluorescence emission at approximately 550 nm)was dissolved in the gasoline solution to a concentration of 50ppb. 5 ml of the resulting solution was then passed through a cartridge containing a carboxypropyl absorbent (ISOLUTE^(TM)CBA from International Sorbent Technology Limited). The cartridge was then dried by running air through the cartridge. The Rhodamine compound was then eiuted from the absorbent using 5 ml of a 1 % solution of benzoic acid in methanol. The cartridge was then dried and washed with methanol prior to re-use. A second sample of the gasoline solution containing the Rhodamine 6G dye was then passed through the cartridge and eiuted as before. The fluorescence of both of the eiuted samples was analysed using a fluorescence spectrometer. The fluorescence emission peak (peak maximum at 550nm) of both samples was identical, showing that consistent fluorescence spectra may be obtained in this manner using an absorbent cartridge more than once.

Example 2

A gasoline sample was diluted with ethanol to produce a gasoline solution containing 25% ethanol and 75% gasoline v/v. Rhodamine 6G dye was dissolved in the gasoline solution to a concentration of 50ppb. Additionally disodium Fluorescein (an anionic dye giving a fluorescence emission at approximately 550 nm) was dissolved in the solution to a concentration of 50ppb. 5 ml of the resulting solution was then passed through a cartridge containing a carboxypropyl absorbent (ISOLUTE ^(TM) CBA from International Sorbent Technology Limited). The Rhodamine compound was then eiuted from the absorbent using 5 ml of a 1 % solution of benzoic acid in methanol. The sample was then passed through a second cartridge containing a quaternary amine-modified absorbent (ISOLUTE SAX) in order to isolate the Fluorescein dye. The Fluorescein was eiuted from the absorbent as before.

The fluorescence spectra are shown superposed in Fig 5 and show that each dye was isolated and produced a characteristic fluorescence emission spectrum using the method described. When the dyes were not separated (spectrum marked B in Fig 5), the Rhodamine peak masked the Fluorescein peak so the presence of Fluorescein could not be detected. When each dye was separated prior to measurement of the emission spectrum, the Rhodmine 6G spectrum (marked C) and the Fluorescein spectrum (marked A in the drawing) are clearly identifiable.

Claims

1. An identity detection system for marking and identifying a liquid comprising:

(i) a pre-determined quantity of a marker composition comprising at least one fluorescent material having a characteristic wavelength of maximum absorption of light, said composition being soluble in the liquid, and

(ii) an apparatus for detecting the marker composition in a sample of the liquid, said apparatus comprising (a) a sample holder, suitable for receiving and supporting a sample cell containing a sample of the liquid, (b) at least one source of excitation radiation, adapted to irradiate the sample cell with light at a predetermined wavelength, said predetermined wavelength being selected to be of a wavelength capable of being absorbed by and promoting fluorescent emissions from one of said at least one fluorescent materials, (c) a radiation detector arranged to detect radiation emitted from the sample, said radiation detector being located out of the path of the excitation radiation, (d) a data collection and manipulation device for gathering data from the radiation detector and producing information concerning the intensity and wavelength of the emitted radiation detected by the radiation detector and data-processing means for comparing the characteristics of intensity and wavelength of the fluorescent light emitted from the sample when sample irradiated by said excitation source with the characteristics of radiation emitted from a standard sample of the liquid containing a known quantity of the fluorescent material, and (e) indication means to indicate whether the characteristics of the light emitted from the sample are within a, predetermined range of the characteristics of the light emitted from the standard sample.

2. An identity detection system as claimed in claim 1 , wherein the predetermined wavelength of the excitation radiation of at least one source is selected to be between λ _₂ and λ ₊₂, where λ .₂ and λ ₊₂are respectively the minimum and maximum wavelength at the full width at 20% of the peak height of the absorption peak of the at least one fluorescent material.

3. An identity detection system as claimed in claim 1 or claim 2, wherein said marker composition comprises more than one fluorescent material and a source of excitation radiation is provided for each fluorescent material contained in the marker composition, each source being selected and/or tuned to provide light selected to be between λ _₂ and λ₊₂ of one of the fluorescent materials.

4. A method of identifying a liquid comprising the steps of:

(i) dissolving in said liquid a pre-determined quantity of a marker composition comprising at least one fluorescent material having a characteristic wavelength of maximum absorption of light, said composition being soluble in the liquid to form a marked liquid, (ii) subsequently taking a sample of a liquid which is to be positively identified as being a sample of said marked liquid into a sample cell and measuring the fluorescence of said sample using an apparatus comprising: a) a sample holder, suitable for receiving and supporting a sample cell containing a sample of the liquid, b) a source of excitation radiation, adapted to irradiate the sample cell with light at a wavelength capable of being absorbed by and promoting fluorescence in one of said at least one fluorescent materials, c) a radiation detector arranged to detect radiation emitted from the sample, said radiation detector being located out of the path of the excitation radiation, d) a data collection and manipulation means for gathering data from the radiation detector and producing information concerning the intensity and wavelength of the emitted radiation detected by the radiation detector and data-processing means for comparing the characteristics of intensity and wavelength of the fluorescent light emitted from the sample when sample irradiated^" by said excitation source with the characteristics of radiation emitted from a standard sample of the liquid containing a known quantity of the fluorescent material, and e) indication means to indicate whether the characteristics of the light emitted from the sample are within a predetermined range of the characteristics of the light emitted from the standard sample (iii) irradiating the sample in the sample cell with light from said source of excitation radiation at said wavelength,

(iv) detecting the fluorescent light emitted from the sample which impinges upon said radiation detector,

(v) comparing the characteristics of the intensity and wavelength of the light detected by the detector with predetermined characteristics of intensity and wavelength of fluorescence emitted by a standard sample of the marked liquid or of a standard marked liquid, thereby to determine whether the sample of liquid in the sample cell is from a source containing the marker composition.

5. A method as claimed in claim 4, wherein the predetermined wavelength of the excitation radiation of at least one source is selected to be between λ _₂ and λ ₊₂, where λ _-2 and λ ₊₂ are respectively the minimum and maximum wavelength at the full width at 20% of the peak height of the absorption peak of the at least one fluorescent material.

6. A method as claimed in claim 4 or claim 5, wherein said marker composition comprises more than one fluorescent material and a source of excitation radiation is provided for each fluorescent material contained in the marker composition, each source being selected and/or tuned to provide light selected to be between λ _₂ and λ ₊₂, where λ _-2 and λ ₊₂ of one of the fluorescent materials.

7. A method as claimed in any of claims 4 to 6, further comprising passing a sample of said liquid through an absorbent capable of absorbing one of said at least one fluorescent materials, whereby to capture one of said at least one fluorescent materials on the absorbent before the sample of said liquid is introduced into the sample cell.

8. A method as claimed in claim 7 wherein the fluorescence characteristics of said liquid, having passed through said absorbent, are compared with a similar sample of said liquid which has not been passed through the absorbent.

9. A method as claimed in claim 7, wherein a solvent is passed through the absorbent such that the absorbed fluorescent material is eiuted from the absorbent.

10. A method as claimed in claim 9, wherein the solvent containing the eiuted material is introduced into the sample cell.

1 1. A method of identifying a liquid comprising the steps of:

(i) dissolving in said liquid a pre-determined quantity of a marker composition comprising at least one fluorescent material having a characteristic wavelength of maximum absorption of light, said composition being soluble in the liquid to form a marked liquid,

(ii) subsequently passing a sample of a liquid which is to be positively identified as being a sample of said marked liquid through an absorbent material, said absorbent material being capable of absorbing one of said at least one fluorescent materials, whereby to capture one of said at least one fluorescent materials on the absorbent, (iii) eluting said absorbed material from said absorbent by passing a solvent through said absorbent,

(iv) introducing the solvent and eiuted material into a sample cell and measuring the fluorescence of said sample using an apparatus comprising: f) a sample holder, suitable for receiving and supporting a sample cell containing a sample of the liquid, g) a source of excitation radiation, adapted to irradiate the sample cell with light at a wavelength capable of being absorbed by and promoting fluorescence in one of said at least one fluorescent materials, h) a radiation detector arranged to detect radiation emitted from the sample, said radiation detector being located out of the path of the excitation radiation, i) a data collection and manipulation means for gathering data from the radiation detector and producing information concerning the intensity and wavelength of the emitted radiation detected by the radiation detector and data-processing means for comparing the characteristics of intensity and wavelength of the fluorescent light emitted from the sample when sample irradiated by said excitation source with the characteristics of radiation emitted from a standard sample of the solvent containing a known quantity of the fluorescent material, and j) indication means to indicate whether the characteristics of the light emitted from the sample are within a predetermined range of the characteristics of the light emitted from the standard sample (v) irradiating the sample in the sample cell with light from said source of excitation radiation at said wavelength, (vi) detecting the fluorescent light emitted from the sample which impinges upon said radiation detector,

(vii) comparing the characteristics of the intensity and wavelength of the light detected by the detector with predetermined characteristics of intensity and wavelength of fluorescence emitted by a standard sample of the fluorescent material in a solvent or of a standard marked liquid, thereby to determine whether the sample of fluorescent material in the sample cell is from a source containing the marker composition.

12. An apparatus for detecting the presence of a pre-determined amount of a fluorescent material in a liquid, the apparatus comprising: (a) a sample holder, suitable for receiving and supporting a sample cell containing a sample of the liquid,

5 13. An apparatus as claimed in claim12, wherein said data collection and manipulation device is adapted to compare the characteristics of the intensity and wavelength of the light detected by the detector with predetermined characteristics of intensity and wavelength of fluorescence emitted by a standard sample of the marked liquid or of a standard marked liquid, thereby to determine whether the sample of liquid in the sample cell is from a source 10 containing the marker composition.

14. An apparatus as claimed in claim 12, further comprising means by which a sample is caused to flow through an absorbent material before entering the sample cell.

15 15. An apparatus as claimed in claim 14, wherein such means comprise a housing for an absorbent cartridge, such means being adapted to enable a cartridge containing an absorbent material to be inserted into and removed from said housing by the user of the apparatus.

20 16. An apparatus as claimed in claim 14, further comprising a solvent reservoir, pump means and control apparatus to automate the passing of the sample through the absorbent and flushing of the absorbed dye into a solvent prior to the introduction of the liquid or solvent containing the dye into the sample cell of the apparatus.

25 17. A method for marking a liquid petroleum product comprising adding to said petroleum product at least one dye which, when irradiated with light of a first wavelength emits light at a second wavelength, said second wavelength being in the range from 500 - 700 nm.