Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
  • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
  • G01N21/65—Raman scattering
  • G01N21/658—Raman scattering enhancement Raman, e.g. surface plasmons
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y15/00—Nanotechnology for interacting, sensing or actuating, e.g. quantum dots as markers in protein assays or molecular motors
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/28—Investigating the spectrum
  • G01J3/44—Raman spectrometry; Scattering spectrometry ; Fluorescence spectrometry
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/22—Fuels; Explosives
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/26—Oils; Viscous liquids; Paints; Inks
  • G01N33/28—Oils, i.e. hydrocarbon liquids
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/26—Oils; Viscous liquids; Paints; Inks
  • G01N33/28—Oils, i.e. hydrocarbon liquids
  • G01N33/2835—Specific substances contained in the oils or fuels
  • G01N33/2882—Markers

Definitions

• 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).
• WO2008/019161 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.
• a nanoparticle including a SERS active core may be mixed into a fuel supply.
• 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.
• a portion of the sample may be associated with a SERS active substrate.
• 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
• fuel additives such as detergents, corrosion inhibitors, lubricants, metal scavengers, octane improvers and others may also be present in retail fuels.
• fuel compositions vary a great deal between suppliers and over time. 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 of taggant.
• 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;
• 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;
• 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.
• 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;
• the Raman spectroscopy detector response to the SERS-active taggant compound compared 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.
• 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
• materials for which the method of the invention may be desirably practised include hydrocarbons, fuels, mineral oils, vegetable oils, 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.
• 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.
• SERS surface enhanced spectroscopy
• SERRS surface-enhanced resonance Raman spectroscopy
• 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.
• 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.
• 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.
• the taggant 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.
• the taggant 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.
• 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).
• SERS substrate may be any material showing surface plasmon enhancement.
• SERS substrates typically comprise metals such as silver, gold and copper.
• SERS substrates particularly metals
• transition metals such as Pt, Ni, Ru, Rh, Pd, Co, Fe, Cr.
• 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.
• 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 KlariteTM.
• 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.
• 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.
• 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 ppb to 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.
• the taggant may be added to one ingredient of the composition, such as a dispersing agent, for example.
• 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.
• the taggant may be added to or mixed with a solvent before it is mixed with the material to be marked.
• 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 ⁇ — 10 ml is used for analysis in step (a) of the method. An amount in the range 10 ⁇ — 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.
• 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 are particularly suitable.
• the immiscible liquid is an aqueous solution which may comprise a salt solution.
• the salt solution may comprise a salt which is known for use as an aggregating agent in SERS analysis.
• 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.
• 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).
• an internal standard compound may be added to the mixture which is contacted with a SERS substrate in step (e) of the method.
• the internal standard and the immiscible liquid may be mixed together before mixing with the sample of organic liquid.
• the internal standard and immiscible liquid may be mixed with the sample separately.
• 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 pre- mixed 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.
• 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.
• the pre-mixed reagent may be supplied as a measured quantity in a sealed container, such as a vial.
• a measured amount of the sample to be analysed may be dispensed into the container containing the pre-mixed reagent.
• 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.
• the sample may be diluted with a suitable organic solvent or with a further volume of the liquid comprising the bulk of the sample.
• a suitable organic solvent such as an alkane, for example iso-octane, n-octane, decane, toluene or dodecane.
• 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.
• 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.
• 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.
• a plastic syringe filter for example comprising PTFE, is effective for removing traces of interfering organic compounds.
• 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.
• 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.
• 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.
• the colloidal metal particles 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.
• one or more aggregating agents may be used in order to 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.
• 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.
• 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.
• 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.
• 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.
• 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;
• 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;
• 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;
• 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.
• 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 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.
• 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. 250 ⁇ of sample was added to a vial containing 800 ⁇ 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 ⁇ ) was eluted into GC vial containing 900 ⁇ of 0.1 1 1 M NaCI aqueous solution. 500 ⁇ 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 .
• Example 1 The diesel calibration mixtures prepared in Example 1 were analysed by a different method.
• 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.

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