GB2550479B

GB2550479B - Measuring a SERS-active taggant in a sample of organic liquid

Info

Publication number: GB2550479B
Application number: GB1705500.5A
Authority: GB
Inventors: Adamson Lauren; Elizabeth Ayscough Sophie; Brian Croud Vincent; Mclintock Alison; Lowri Peake Catherine
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2016-05-19
Filing date: 2017-04-05
Publication date: 2019-11-06
Anticipated expiration: 2037-04-05
Also published as: GB201608836D0; WO2017198993A1; GB2550479A; EP3458846A1; EA201892602A1; BR112018072221A2; PH12018502426A1; GB201705500D0; CL2018003280A1; JP2019523861A; CN109154573A; CO2018011735A2; KR20190010597A

Description

Measuring a SERS-active taqqant in a sample of organic liquid

The present invention relates to the analysis of a material to measure the amount of a taggant and identification of a product by adding a known taggant compound to the product and then later analysing a sample of the product or a similar material to determine whether the taggant is present. The method is particularly useful for the analysis of taggant compounds in hydrocarbon fuels by surface-enhanced Raman spectroscopy (SERS). W02008/019161 describes a method of fuel identification with surface enhanced Raman spectroscopy (SERS) tags. This method includes the association of a substance having a known Raman spectrum with a quantity of fuel. In one embodiment, a nanoparticle including a SERS active core may be mixed into a fuel supply. In an alternative embodiment, a SERS active dye including a Raman active reporter molecule may be mixed with a quantity of fuel. If the quantity of fuel is tagged with a dye having Raman active reporter molecules, the process of identifying the quantity of fuel may include mixing into a sample of the fuel a colloid of Raman enhancing metal particles and then acquiring the Raman spectrum of the Raman active reporter molecule associated with the tag. Suitable metals include, but are not limited to, silver or gold. Alternatively, a portion of the sample may be associated with a SERS active substrate. Although a semi-quantitative example of the procedure is described in W02008/019161, we have found that the SERS response of the tags tend to vary such that the results include a significant uncertainty due to non-reproducibility. An improved method of quantitatively analysing a material by means of SERS to determine the amount of a SERS-active taggant in a sample of the material is described in WO2012/052779. That method comprises the steps of adding an isotopically-altered version of the SERS-active taggant compound to the sample as an internal standard, contacting the sample /internal standard mixture with a SERS substrate then subjecting the mixture and SERS substrate to Raman spectroscopy. The concentration of SERS-active taggant compound in the sample was then calculated from the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard. Although the method showed an improvement over prior methods, we have found that variations in the source of the tagged material could affect the result achieved, leading to the requirement to provide additional calibration data for each tagged material. Fuels such as gasoline and diesel are distillate fractions of petroleum and as such they contain a mixture of compounds, the identity of which depends on the source of the petroleum and any treatment it may have received to adjust the composition. Such treatments may include removal of undesirable compounds such as sulphur-containing compounds and heavy metals. Certain fuel additives, such as detergents, corrosion inhibitors, lubricants, metal scavengers, octane improvers and others may also be present in retail fuels. For all of these reasons, fuel compositions vary a great deal between suppliers and overtime. The variation in composition can lead to different samples of fuel giving different SERS responses so that calibration must be undertaken on each different type of fuel which is to be analysed for the concentration oftaggant.

It is an object of the invention to provide a method for identifying a material using a tagging method which overcomes at least some of the disadvantages of such prior methods.

According to the invention, a method of measuring the amount of a particular SERS-active compound in a sample of an organic liquid comprises the steps of> a) mixing said sample with an immiscible liquid; b) separating the mixture into at least a first and second phase, wherein said first phase comprises the immiscible liquid and said second phase comprises the sample of organic liquid; c) treating the first phase to remove trace compounds originating from the organic liquid to provide a treated first phase; d) diluting the first phase before, after or simultaneously with the treating step c); e) contacting said diluted and treated sample of first phase with a SERS substrate f) subjecting said diluted and treated sample of said first phase and SERS substrate to Raman spectroscopy in the presence of an internal standard compound; and calculating the ratio of (i) the Raman spectroscopy detector response to a SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard.

According to a second aspect of the invention, we provide a method of comparing a sample of an organic liquid to a reference organic liquid comprising the steps of> a) mixing said sample with an immiscible liquid; b) separating the mixture into at least a first and second phase, wherein said first phase comprises the immiscible liquid and said second phase comprises the sample of organic liquid; c) treating the first phase to remove trace compounds originating from the organic liquid to provide a treated first phase; d) diluting the first phase before, after or simultaneously with the treating step c); e) contacting said diluted and treated sample of first phase with a SERS substrate f) subjecting said diluted and treated sample of said first phase and SERS substrate to Raman spectroscopy in the presence of an internal standard compound g) calculating the ratio of (i) the Raman spectroscopy detector response to a SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard. h) comparing the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard with a reference value representing the ratio of (iii) the Raman spectroscopy detector response to the SERS-active taggant compound to (iv) the Raman spectroscopy detector response to the internal standard measured in a sample of said reference material containing a known concentration of SERS-active taggant compound.

According to a third aspect of the invention, we provide a method whether a sample of an organic liquid is a sample of a known organic liquid to which a known concentration of a SERS-active taggant compound has been added as a marker comprising the steps of; a) mixing said sample with an immiscible liquid; b) separating the mixture into at least a first and second phase, wherein said first phase comprises the immiscible liquid and said second phase comprises the sample of organic liquid; c) treating the first phase to remove trace compounds originating from the organic liquid to provide a treated first phase; d) diluting the first phase before, after or simultaneously with the treating step c); e) contacting said diluted and treated sample of first phase with a SERS substrate f) subjecting said diluted and treated sample of said first phase and SERS substrate to Raman spectroscopy in the presence of an internal standard compound g) calculating the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard. h) comparing the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard with a reference value representing the ratio of (iii) the Raman spectroscopy detector response to the SERS-active taggant compound to (iv) the Raman spectroscopy detector response to the internal standard measured in a sample of a reference material containing a known concentration of SERS-active taggant compound.

The concentration of the SERS-active taggant compound in the reference material is preferably the same as or has a known relationship to the concentration of SERS-active taggant compound in the known organic liquid. The reference material may be a sample of the known organic liquid. The reference value may be a numerical value provided in a reference source such as a manual, a computer memory, an instrument operating code. The reference value may alternatively be a value measure using a sample of a known organic liquid containing a known quantity of the SERS-active taggant compound.

The method is suitable for identifying a variety of types of organic liquids. Examples of materials for which the method of the invention may be desirably practised, include hydrocarbons, fuels, mineral oils, vegetable oils, pharmaceuticals, agrochemicals, cosmetics, perfumes, liquids that are known to be used to adulterate fuels and oils such as organic solvents, alcohols and other high value or highly-taxed products. The method has been found to be particularly applicable to identifying hydrocarbon fuels by marking the fuels with a taggant and then measuring the concentration of taggant in a sample of fuel to determine whether the sample is a sample of the marked fuel and also whether the fuel has been diluted with an unmarked or differently marked liquid. A taggant, as referred to in this specification, is a compound present in a material by means of which the material may be identified. Thus, if a known amount of taggant compound is added to a material before the material is distributed, to form a marked or tagged material, a sample of the distributed material may be identified by analysing the material to determine the presence, absence or the concentration of the taggant compound and comparison with the concentration of taggant in the material before distribution. Adulteration of the distributed material, for example by dilution with an un-tagged or differently tagged material, may also be detected by comparison of the concentration of taggant in the material before and after distribution.

When we refer to SERS in the present specification we intend to include other forms of surface enhanced spectroscopy (SES) such as SERRS (surface-enhanced resonance Raman spectroscopy). For brevity these methods will all be referred to as SERS.

The SERS-active taggant compound is a chemical compound which can be identified by its Raman signal when in contact with or in close proximity to a SERS substrate. Suitable taggants are therefore capable of exhibiting surface-enhanced Raman scattering. When the material to be marked is a liquid, the taggant is preferably soluble in the liquid which is to be marked with the taggant up to the concentration which is to be used. For use in hydrocarbon liquids such as fuels, the taggant is preferably soluble in the fuel up to a concentration which is measurable using SERS analysis. The taggant may be less soluble in the material to be marked than it is in a solvent used to extract the taggant prior to SERS analysis. When the SERS substrate is an aqueous solution of a metal colloid, the taggant may be capable of adsorbing onto the metal surface whilst maintaining the ability of the colloidal particles to partition into the aqueous phase after mixing with the sample. The choice of taggant must therefore take into consideration its capability of producing an enhanced Raman signal through use of SERS methods, its affinity to a SERS substrate surface, and solubility and partitioning properties. When the internal standard is an isotopically altered version of the SERS-active compound, the taggant must also be available as an isotopically-altered version for use as an internal standard in the method. Methods of producing isotopically altered molecules are well known and typically include replacing at least one hydrogen atom in the taggant molecule with deuterium or replacing a carbon atom with C13. An isotopically altered version of a SERS-active taggant must itself be SERS-active and must produce a SERS Raman signal which is resolvable from the signal produced by the taggant.

The SERS substrate is a substrate having a surface which is capable of enhancing the spectroscopic response of a molecule which is close to or in contact with the surface, i.e. it is capable of promoting surface-enhanced spectroscopy (SES). The SERS substrate may be any material showing surface plasmon enhancement. SERS substrates typically comprise metals such as silver, gold and copper. The use of other SERS substrates, particularly metals, may be possible, including Na and Al and transition metals such as Pt, Ni, Ru, Rh, Pd, Co, Fe, Cr. As new methods of surface-enhanced spectroscopy are developed, different SES-promoting substrates may become available and may be useful for the method of the invention. The SERS substrate may take the form of small particles, usually nanoparticles, typically used as colloidal solutions, especially aqueous colloidal solutions. Alternatively the SERS substrate may take the form of a planar material having a metallic surface comprising microstructure in the form of an immobilised metal colloid or a patterned surface made from or coated with a metal such as gold, silver or copper. Suitable SERS substrates are widely available commercially, either as colloidal gold or silver solutions or as specialist planar materials for SERS having plasmonic surfaces, such as Klarite™.

The SERS-active taggant compound and / or the internal standard may be incorporated in a “nanotag” including a SERS substrate. Suitable nanotags include SERS-active composite nanoparticles which are described, for example, in WO01/25758 and comprise a SES (surface enhanced spectroscopy) metal nanoparticle, a layer of a SES-active species in close proximity to the metal surface and an encapsulating shell comprising a polymer, glass or another dielectric material. The internal standard may comprise a nanotag incorporating an isotopically-altered version of the SERS-active taggant compound. For example, the internal standard may comprise a SES metal nanoparticle, a layer of a SES-active species in close proximity to the metal surface and an encapsulating shell comprising a polymer, glass or another dielectric material in which the SES-active species is used as an internal standard.

The SERS-active taggant is added to the material to be marked by standard means, for example when the material is held in bulk volumes at a manufacturing or distribution location. Different taggants may be added to different volumes of material of the same bulk composition which are intended for different purposes or for use in different territories or which originate from different batches of material. Alternatively, different taggants may be added to different bulk volumes of material having different compositions, for example to distinguish between fuels of different grades. The concentration of taggant in the material may be in the range from 1 ppt (i.e. 0.001 ppb)to 100 ppm, more preferably from 0.1 ppbto 10 ppm. The taggant is normally added to the material in amounts less than 10 ppm. “ppt” means parts per trillion, “ppb” means parts per billion, “ppm” means parts per million. More than one taggant may be added to a single material and the use of combinations of different taggants in varying relative amounts may provide a large number of uniquely tagged materials using relatively few taggant compounds.

The taggant may be added to the organic liquid in bulk form or it may be added to a component of the liquid to be marked. For example, when the material is an agrochemical, the taggant may be added to one ingredient of the composition, such as a dispersing agent, for example. When the material to be marked is a fuel then the taggant may be added to a fuel additive which is then incorporated into the bulk fuel before it is distributed. In these cases, the material becomes marked with the taggant when the ingredient incorporating the taggant is added to the material composition during preparation or manufacture. Alternatively, the taggant may be added to or mixed with a solvent before it is mixed with the material to be marked.

When it is required to determine the amount of a particular SERS-active taggant in a sample of an organic liquid the method of the invention is used.

The amount of sample used for analysis by the method of the invention is usually accurately known. Suitable methods of sampling are known and may involve the use of a syringe, volumetric flask or a sampling loop. The amount of sample used for analysis may vary. In a typical method, 10 pi - 10 ml is used for analysis in step (a) of the method. An amount in the range 10 pi — 1 ml may be used in step (a) of the method.

The sample of organic liquid which is to be analysed to determine the presence and/or quantity of a SERS-active compound is first mixed with an immiscible liquid. The liquids should be immiscible to the extent that when mixed together they separate into at least first and second phases which may be distinguished from each other. Some combinations of liquids may form more than two phases. The immiscible liquid may comprise water or an aqueous solution. In this case, the first phase is an aqueous phase and the second phase is an organic phase. Suitable immiscible liquids which may be useful include compounds which affect the ionic concentration in the mixture which is in contact with a SERS substrate. Such compounds and solutions of such compounds may be known for use as SERS aggregating agents. These include active or passive salts, acids and bases; polymers or long-chain ions which may affect the surface charge of a colloidal metal SERS substrate or otherwise alter the colloidal properties e.g. by affecting steric interaction or stability of the colloid by displacement of or interaction with colloidal stabilisers. Particularly suitable include solutions of sodium chloride, sodium sulphate, sodium nitrate, potassium nitrate, potassium chloride, calcium chloride, nitric acid, sulphuric acid, sulphurous acid, hydrochloric acid, spermine, and poly(L)lysine. In a preferred embodiment, the immiscible liquid is an aqueous solution which may comprise a salt solution. In this case the salt solution may comprise a salt which is known for use as an aggregating agent in SERS analysis.

The use of internal standards in analytical methods is widely practised. The relative response of the target compound and internal standard to the analytical technique is likely to be insensitive to inconsistencies in carrying out the method or in the nature of the sample and so using an internal standard can reduce the error in the analysis caused by such factors. Use of an internal standard can overcome the error in analysing for a SERS active taggant in fuel because the relative response of the internal standard and the taggant should be dependent only on the relative concentration of the internal standard and the taggant. However, SERS is very dependent on the adsorption of the SERS active compound to the SERS substrate, aggregation of the gold particles to which they have adsorbed and, in the case of analysis of organic liquids, on the partitioning of the SERS active compound and the gold nanoparticles between organic and aqueous phases. An isotopically altered (or “isotopically edited”) version of the taggant compound is as chemically similar to the SERS-active taggant compound as possible, so that it behaves in the same way. The concentration of internal standard used may be greater or less than the concentration of SERS-active taggant expected to be present in the sample. The concentration of internal standard mixed with the sample is preferably in the range from 1 ppt (i.e. 0.001 ppb) to 100 ppm, more preferably from 0.1 ppb to 10 ppm. The concentration of internal standard mixed with the sample may be the same as the concentration of internal standard in the reference sample, if a reference comparison is made.

The internal standard compound is present in the mixture in which is in contact with the SERS substrate. The internal standard may be added to the mixture at any stage in the method up to that point. It may therefore be added to the sample before step (a) or during any of steps (a) -(f). For example, an internal standard compound may be added to the mixture which is contacted with a SERS substrate in step (e) of the method. In certain embodiments of the method, the internal standard and the immiscible liquid may be mixed together before mixing with the sample of organic liquid. As one alternative, the internal standard and immiscible liquid may be mixed with the sample separately. In an embodiment of the invention, a predetermined quantity of internal standard and a predetermined quantity of an immiscible liquid is mixed together and supplied as a pre-mixed reagent for use in the method of the invention. The premixed reagent comprising a predetermined quantity of internal standard and a predetermined quantity of immiscible liquid may be supplied as a bulk liquid which may be dispensed in aliquots for mixing with the sample in step (a) of the method of the invention. In a particular embodiment of the invention, a predetermined quantity of internal standard and a predetermined quantity of an aqueous salt solution is mixed together and supplied as a pre-mixed reagent for use in the method of the invention. Alternatively, the pre-mixed reagent may be supplied as a measured quantity in a sealed container, such as a vial. In this case, a measured amount of the sample to be analysed may be dispensed into the container containing the pre-mixed reagent. There are several advantages to supplying the immiscible liquid and internal standard as a measured quantity of pre-mixed reagent. In this form the method becomes more streamlined for the operator because the requirement to identify the correct salt solution, internal standard, look up the amounts of each reagent to use and measure and dispense each reagent separately is avoided. The use of a pre-mixed reagent also reduces the opportunity for the analysis to be affected by contamination. The internal standard may be present in or added to a diluent which is used in step (d) of the method. A pre-mixed diluent containing internal standard may be provided for use in step (d). Such a premixed diluent may be supplied in bulk or in measured quantities as described above.

When compounds are mixed together, it is usually sufficient to mix the components by agitating the mixture, for example by shaking a container into which the components of the mixture have been dispensed. Alternative methods of mixing may be used. Microfluidic methods may be used.

The sample may be diluted with a suitable organic solvent or with a further volume of the liquid comprising the bulk of the sample. In the case of analysis of hydrocarbon liquid samples, especially fuel samples, we have found that it may be useful to mix the sample with a non-polar solvent, such as an alkane, for example iso-octane, n-octane, decane, toluene ordodecane.

The volume ratio of sample to solvent used is typically in the range 1 :1 - 50. The use of a solvent may enhance the partitioning of the taggant into an aqueous phase. A suitable solvent may assist in the separation of the mixture into first and second phases. When a solvent is used, it is preferably added to the sample before the sample is contacted with the SERS substrate. It is further preferred to add the internal standard to the sample before adding a solvent.

After mixing in step (a), the mixture is separated into first and second phases, for example an aqueous phase and an organic phase. The mixture may be separated by allowing it to stand after mixing. A portion of the first phase is preferably removed from the mixture for use in steps (c) - (g) of the method. This may be achieved by any practical method, including for example, by withdrawal of aqueous phase by means of a syringe inserted into the mixture or by microfluidics.

The first phase (or a portion of it) resulting from step (b) is treated to remove trace compounds originating from the organic liquid. We have found that liquids such as hydrocarbon fuels such as gasoline or diesel may contain interfering compounds which interfere with the measurement of the Raman spectrum by SERS. The identity of these compounds is not known and may vary between fuels according to their source or type. The interfering compounds may affect the binding of the SERS-active taggant compound or of the internal standard to the SERS substrate, thereby reducing or blocking the SERS response. Other types of interfering compound may produce a SERS response or may affect the aggregation of a colloidal SERS substrate. The treatment may comprise or consist of a method which is capable of removing interfering compounds, such as organic components for example, which have been retained in the aqueous phase separated instep (b). Such treatments include passing the first phase through a filtration medium. Certain filtration media have an affinity for organic components. We have found that a plastic syringe filter, for example comprising PTFE, is effective for removing traces of interfering organic compounds. In preferred embodiments, the selected treatment does not remove the SERS-active taggant compound or internal standard from the aqueous phase.

The first phase is diluted in step (d) of the method. The diluting step may take place before, after or simultaneously with the treating step (c). The diluting may be achieved by placing an aliquot of the first phase into a volume of a diluting liquid. The diluting liquid may be water, or another aqueous liquid, for example it may be a salt solution. The diluting liquid may be non-aqueous. If the diluting liquid is a salt solution, it may contain similar salts to an aqueous salt solution used in step (a). The diluting liquid may function as an aggregating agent for colloidal metal particles. The diluting liquid may be selected from the aggregating agents mentioned above. The diluting liquid may be a quantity of the immiscible liquid used in step (a) or it may be different. As an example, when the immiscible liquid used in step (a) is an aqueous salt solution, the first phase from step (b) may be present in a syringe fitted with a plastic syringe filter, and then a drop of the first phase may be passed out of the syringe through the filter and into a volume of an aqueous salt solution which may be the same as or different from the aqueous salt solution used in step (a).

Step (e) comprises contacting the diluted and treated aqueous phase with a SERS substrate. When the SERS substrate is a metal colloid in solution, the contact of the SERS-active taggant compound and the internal standard in the sample with the SERS substrate is carried out by mixing, e.g. by shaking, the colloid solution with the sample for sufficient time to allow the molecules in the sample to adsorb on the metal surfaces. The metal particles carrying the adsorbed SERS-active molecules normally form aggregates. The sample and SERS substrate is then subjected to Raman spectroscopy to obtain the Raman spectrum using known methods.

It is normally beneficial for the colloidal metal particles to form aggregates comprising several particles in the presence of the SERS-active taggant compound. Aggregation may take place spontaneously, depending on the nature of the colloid and the compounds present in the mixture to be analysed. As an option, one or more aggregating agents may be used in orderto improve the aggregation of the colloidal metal particles in the presence of the SERS-active taggant compound. Suitable aggregating agents may be selected according to the nature of the colloidal metal and/or the SERS-active taggant compound. In certain embodiments of the methods of the invention, it is advantageous if the immiscible liquid used in step (a) and optionally used as the diluent in step (d) is active as an aggregating agent. In that case, it may be unnecessary to add additional aggregating agent in of after step (e).

The relative amount of the SERS-active taggant compound and the internal standard may be calculated from the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard. The detector response ratio may be the ratio of selected peaks (peak height, peak area) of the SERS spectrum. Alternatively the whole SERS spectrum may be used in calculating the response ratio. Preferably the concentration of said SERS-active taggant compound in said sample is calculated from the ratio calculated in step (g) of the method. The ratio calculated in step (g) may be compared with a reference value representing the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard from a reference sample containing a known concentration of SERS-active taggant compound. The SERS spectrum obtained from a reference sample containing only the target SERS-active taggant or the internal standard may be used to identify suitable peaks which are characteristic of either the taggant or the internal standard, which may be selected for comparing the relative response of the compounds. The relative response may be calculated from the relative intensity or area of one peak attributable to each compound or from more than one peak. As an alternative, the whole spectrum, or a portion of it, obtained from the Raman spectroscopy of the sample in contact with the SERS substrate may be compared, preferably in vector form, to a spectrum obtained from a reference sample containing a known concentration the SERS-active taggant compound in contact with the SERS substrate and a spectrum obtained from a reference sample containing a known concentration the internal standard compound in contact with the SERS substrate. A calculated property of the spectrum, such as the relative response compared to a reference spectrum of one or each compound present, may be used to represent the detector response due to the SERS-active taggant and/or the internal standard. It is not always necessary to collect and display a Raman spectrum. Since the identity of the taggant and internal standard are known, it may be sufficient to measure the detector response at one or more predetermined Raman shift wavenumbers or ranges of wavenumbers and calculate a concentration of the taggant from the measured response. The result of the calculation may be displayed to the user as a concentration value, a relative concentration, for example expressed as a percentage of the amount of taggant known to have been added to an authentic liquid, a “pass/fail” result or as an arbitrary value of quality or similarity based upon a value for a solution containing a standard amount of the taggant.

Methods of comparing spectra and calculating relative response and peak ratios are well-known and are typically carried out using a suitable computer programmed with spectroscopic data handling software. The relationship between the concentration of the SERS-active taggant compound and the Raman detector response ratio may be determined by calibration.

Examples of specific embodiments of the claimed methods include:

Firstly, a method of measuring the amount of a particular SERS-active compound in a sample of an organic liquid comprising the steps of:- a) mixing said sample with an internal standard comprising an isotopically-altered version of said SERS-active compound and an aqueous salt solution; b) separating the mixture into an aqueous phase and an organic phase; c) treating the aqueous phase to remove trace compounds originating from the organic liquid to provide a treated aqueous phase; d) diluting a sample of said aqueous phase or treated aqueous phase; e) contacting said diluted and treated sample of aqueous phase with a SERS substrate f) subjecting said sample and SERS substrate to Raman spectroscopy; and g) calculating the ratio of (i) the Raman spectroscopy detector response to a SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard.

Secondly, a method of comparing a sample of an organic liquid to a reference organic liquid comprises the steps of> a) mixing said sample with an internal standard comprising an isotopically-altered version of said SERS-active taggant compound and an aqueous salt solution; b) separating the mixture into an aqueous phase and an organic phase; c) treating the aqueous phase to remove trace compounds originating from the organic liquid to provide a treated aqueous phase; d) diluting a sample of said treated aqueous phase e) contacting said diluted sample with a SERS substrate f) subjecting said sample and SERS substrate to Raman spectroscopy; and g) calculating the ratio of (i) the Raman spectroscopy detector response to a SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard. h) comparing the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard with a reference value representing the ratio of (iii) the Raman spectroscopy detector response to the SERS-active taggant compound to (iv) the Raman spectroscopy detector response to the internal standard measured in a sample of said reference material containing a known concentration of SERS-active taggant compound.

Thirdly, in a specific embodiment, we provide a method of determining whether a sample of an organic liquid is a sample of a known organic liquid to which a known concentration of a SERS-active taggant compound has been added as a marker, comprising the steps of obtaining a sample of said organic liquid then; a) mixing said sample with an internal standard comprising an isotopically-altered version of said SERS-active taggant compound and an aqueous salt solution; b) separating the mixture into an aqueous phase and an organic phase; c) treating the aqueous phase to remove trace compounds originating from the organic liquid to provide a treated aqueous phase; d) diluting a sample of said treated aqueous phase e) contacting said diluted sample with a SERS substrate f) subjecting said sample and SERS substrate to Raman spectroscopy; and g) calculating the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard. h) comparing the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard with a reference value representing the ratio of (iii) the Raman spectroscopy detector response to the SERS-active taggant compound to (iv) the Raman spectroscopy detector response to the internal standard measured in a sample of a reference material containing a known concentration of SERS-active taggant compound; the concentration of the SERS-active taggant compound in the reference material preferably being the same as or having a known relationship to the concentration of SERS-active taggant compound in the known organic liquid.

The method of the invention will be demonstrated in the following examples, with reference to the drawings:-

Fig 1: Graph of ratio Tag A/deuterated Tag A vs Tag A concentration in diesel (Example 1).

Fig 2: Graph of ratio Tag A/deuterated Tag A vs Tag A concentration in gasoline (Example 2).

Fig 3: (comparative) Graph of ratio Tag A/deuterated Tag A vs Tag A concentration in diesel (Example 3).

Fig 4: : (comparative) Graph of ratio Tag A/deuterated Tag A vs Tag A concentration in gasoline (Example 4).

Fig 5: (comparative) gasoline results from Example 4 plotted with diesel calibration line from Example 3.

Fig 6: gasoline results from Example 2 plotted with diesel calibration line from Example 1.

The taggant used in these examples is an organic compound which is SERS-active, i.e. yields a readily identifiable SERS response and is miscible with hydrocarbon fuels in the concentration range studied and capable of partitioning into an aqueous liquid. The compound is referred to as Tag A. The internal standard used is a deuterated version of Tag A.

Example 1 A diesel fuel containing 0.9ppm of Tag A was prepared. The mixture was used to prepare further mixtures by dilution with more of the diesel fuel to different concentrations for the purpose of calibrating the SERS response to the concentration of Tag A. A sample of each mixture was analysed by the following method. 250μΙ of sample was added to a vial containing 800μΙ 0.281 ppm of a deuterated Tag A in 10 wt% NaCI aqueous solution and shaken for 30 seconds. When the aqueous and organic layers had separated, 0.2ml was taken from the aqueous layer using a blunt needle and disposable syringe. The sample in the syringe was passed through a 0.45 micron PTFE syringe filter and one drop (approximatelyl Opl) was eluted into GC vial containing 900pl of 0.111M NaCI aqueous solution. 500μΙ of Au colloid was added and the sample was shaken. The resulting mixture was analysed using a Raman spectrometer using an excitation wavelength of 785 nm and an integration time of approximately 1 second. The ratio of response to Tag A to response to deuterated Tag A is plotted against Tag A concentration in Fig 1. The parameters of the calibration line are y = 0.00107x + 0.00391, with an R2 value of 0.9978.

Example 2

The calibration method carried out in Example 1 was repeated using a commercial gasoline.

The resulting plot is shown in Fig 2. The parameters of the calibration line are y = 0.00109x + 0.00489, with an R2 value of 0.9985.

These results show that the calibration is remarkably similar for gasoline and diesel, which enables the same calibration to be used when analysing both types of fuels. The method therefore offers the possibility of analysing different fuels using the method, without the requirement to perform a calibration with each fuel to be analysed. This is a particular benefit if the original source or identity of the liquid to be analysed is unknown, as may be the case when analysing fortaggants in samples collected from retail sources.

Example 3 (comparative)

The diesel calibration mixtures prepared in Example 1 were analysed by a different method. 500μΙ of sample was added to a GC vial containing 400 μΙ 1.125ppm deuterated Tag A in 0.25M NaCI aqueous solution. The mixture was shaken for 30 seconds. The mixture separated into aqueous and organic layers in the vial on standing. 500 μΙ Au colloid was added to the aqueous layer and the mixture was Inverted to ensure mixing. The resulting mixture was analysed using a Raman spectrometer using an excitation wavelength of 785 nm and an integration time of approximately 1 second. The ratio of response to Tag A to response to deuterated Tag A is plotted against Tag A concentration in Fig 3. The parameters of the calibration line are y = 0.00108x + 0.01146, with an R2 value of 0.9994.

Example 4 (comparative)

The calibration method carried out in Example 3 was repeated using a commercial gasoline.

The resulting plot is shown in Fig 4. The parameters of the calibration line are y = 0.00106x + 0.36919, with an R2 value of 0.9883.

Fig 5 shows the points obtained in the gasoline calibration of Example 4 plotted together with the diesel calibration line from Example 3. This shows that using the diesel calibration would give inaccurate results for gasoline when the comparative method of analysis is used. Figure 6 shows the points obtained in the gasoline calibration of Example 2 plotted together with the diesel calibration line from Example 1. This shows that using the diesel calibration would give accurate results for gasoline when the method of analysis according to the invention is used.

Claims

1. A method of measuring the amount of a particular SERS-active compound in a sample of an organic liquid comprising the steps of:- a) mixing said sample with an immiscible liquid; b) separating the mixture into at least a first and second phase, wherein said first phase comprises the immiscible liquid and said second phase comprises the sample of organic liquid; c) treating the first phase to remove trace compounds originating from the organic liquid to provide a treated first phase; d) diluting the first phase before, after or simultaneously with the treating step c); e) contacting said diluted and treated sample of first phase with a SERS substrate f) subjecting said diluted and treated sample of said first phase and SERS substrate to Raman spectroscopy in the presence of an internal standard compound; and g) calculating the ratio of (i) the Raman spectroscopy detector response to a SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard.

2. A method of comparing a sample of an organic liquid to a reference organic liquid comprising the steps of:- a) mixing said sample with an immiscible liquid; b) separating the mixture into at least a first and second phase, wherein said first phase comprises the immiscible liquid and said second phase comprises the sample of organic liquid; c) treating the first phase to remove trace compounds originating from the organic liquid to provide a treated first phase; d) diluting the first phase before, after or simultaneously with the treating step c); e) contacting said diluted and treated sample of first phase with a SERS substrate f) subjecting said diluted and treated sample of said first phase and SERS substrate to Raman spectroscopy in the presence of an internal standard compound g) calculating the ratio of (i) the Raman spectroscopy detector response to a SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard h) comparing the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard with a reference value representing the ratio of (iii) the Raman spectroscopy detector response to the SERS-active taggant compound to (iv) the Raman spectroscopy detector response to the internal standard measured in a sample of said reference material containing a known concentration of SERS-active taggant compound.

3. A method of determining whether a sample of an organic liquid is a sample of a known organic liquid to which a known concentration of a SERS-active taggant compound has been added as a marker comprising the steps of; a) mixing said sample with an immiscible liquid; b) separating the mixture into at least a first and second phase, wherein said first phase comprises the immiscible liquid and said second phase comprises the sample of organic liquid; c) treating the first phase to remove trace compounds originating from the organic liquid to provide a treated first phase; d) diluting the first phase before, after or simultaneously with the treating step c); e) contacting said diluted and treated sample of first phase with a SERS substrate f) subjecting said diluted and treated sample of said first phase and SERS substrate to Raman spectroscopy in the presence of an internal standard compound g) calculating the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard. h) comparing the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard with a reference value representing the ratio of (iii) the Raman spectroscopy detector response to the SERS-active taggant compound to (iv) the Raman spectroscopy detector response to the internal standard measured in a sample of a reference material containing a known concentration of SERS-active taggant compound.

4. A method according to any one of the preceding claims, wherein said organic liquid comprises a hydrocarbon, fuel, mineral oil, vegetable oil, organic solvent, alcohol, pharmaceutical, agrochemical, cosmetic or perfume.

5. A method according to any one of the preceding claims, wherein said immiscible liquid is an aqueous liquid.

6. A method according to any one of the preceding claims, wherein the internal standard compound is an isotopically altered version of the SERS-active compound.

7. A method according to any one of the preceding claims, wherein the SERS-active compound is a taggant compound which is present in the liquid for identification purposes.

8. A method according to any one of the preceding claims, wherein the treatment in step (c) comprises contacting the sample, optionally after dilution, with a filtration medium.

9. A method according to any one of the preceding claims, wherein the SERS substrate comprises silver, gold or copper.

10. A method according to any one of the preceding claims, wherein the SERS substrate is in the form of a colloidal solution.

11. A method according to any one of claims 1 to 9, wherein the SERS substrate is in the form of a planar material having a metallic surface comprising microstructure.

12. A method according to any one of the preceding claims, wherein an organic solvent is added to the sample before step (e).

13. A method according to any one of the preceding claims, wherein the concentration of said SERS-active compound in said sample is calculated from the ratio calculated in step (g).