EP4100495A1

EP4100495A1 - Tracers and method of marking liquids

Info

Publication number: EP4100495A1
Application number: EP21700058.7A
Authority: EP
Inventors: Duncan MCCALLIEN
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2020-02-03
Filing date: 2021-01-04
Publication date: 2022-12-14
Also published as: GB202100040D0; GB2592744A; BR112022011539A2; GB202001450D0; WO2021156598A1; GB2592744B; US20230021024A1

Abstract

A method of marking a hydrocarbon fuel, the method comprising adding to said hydrocarbon fuel a tracer compound for marking the hydrocarbon fuel, the tracer compound being a substituted fluorene having a structure of Formula (I): wherein R1 and R2 are the same or different and selected from hydrogen, straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, or R1 and R2 form a single substituent linked intramolecularly to each other, or R1 and R2 are ether groups excluding acetal groups, wherein R3 and R4 are the same or different and selected from hydrogen, straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, and wherein at least one of R1, R2, R3, and R4 is not hydrogen.

Description

Tracers and Method of Marking Liquids

Field

The present specification concerns marking liquids, especially hydrocarbon liquids, with tracer materials. The present specification in particular concerns marking hydrocarbons which are taxable or liable to be subject to tampering or substitution, such as gasoline and diesel fuels for example.

Background

It is well-known to add tracers to hydrocarbon liquids. A typical application is the tagging of hydrocarbon fuels in order to identify the fuel at a subsequent point in the supply chain. This may be done for operational reasons, e.g. to assist in distinguishing one grade of fuel from another, or for other reasons, in particular to ensure fuel quality, deter and detect adulteration and to provide a means to check that the correct tax has been paid. Apart from fuels, other products such as vegetable oils or additive packs may be marked to identify the product is produced at a particular source or certified to a particular standard.

One problem which is known to exist with the marking of fuel liquids in particular is the potential for the tracer to be removed for unlawful purposes such as avoidance of paying tax, by evaporation from the fuel, by degradation of the tracer through ageing or exposure to environmental conditions such as heat, sunlight, air or other methods of deliberate removal. Methods for deliberate removal of tracers include adsorption of the tracer onto common adsorbent materials such as charcoal or clays, exposure to radiation, such as ultraviolet light, oxidation etc. A useful fuel tracer therefore needs to be resistant to removal by these common methods and also to treatment with acids and/or bases or oxidants. It is an aim of the invention to provide tracer compounds and methods of marking hydrocarbon liquids which are more resistant to removal of the tracer than other known tracers.

W02012/125120, US6808542, and CA2365814 disclose the use of photoluminescent fluorene copolymers for marking fuels and other products.

WO2018/182437 discloses a coating material for marking plastics containing a base of the coating material and fluorescent markers to aid identification of the plastics and sorting of plastic waste. The base of the coating material is disclosed as being lacquer, silicone or aqueous dispersion of resins. Numerous possibilities for the fluorescent markers are disclosed as options including fluorene.

W02019/195013 discloses the use of xanthenes as fuel markers.

W02019/195016 discloses the use of substituted dibenzofurans as fuel markers.

Summary of invention

A method of marking a hydrocarbon fuel is provided, the method comprising adding to said hydrocarbon fuel a tracer compound for marking the hydrocarbon fuel, the tracer compound being a substituted fluorene having a structure of Formula I:

Formula I wherein R1 and R2 are the same or different and selected from hydrogen, straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, or R1 and R2 form a single substituent linked intramolecularly to each other, or R1 and R2 are ether groups excluding acetal groups, wherein R3 and R4 are the same or different and selected from hydrogen, straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, and wherein at least one of Rl, R2, R3, and R4 is not hydrogen.

The substituted fluorene tracer compounds as defined above have several advantages over prior art tracers as discussed below.

It has been found that fluorene is susceptible to adsorption by activated charcoal, which is a common laundering agent as mentioned in the background section. It is considered that this is because of p- p interactions between the aromatic rings of the fluorene molecule and the activated charcoal. Another factor contributing to fluorene's lack of resistance to laundering is considered to be the presence of weakly acidic protons at the C-9 position of the fluorene molecule.

To solve the problem of adsorption and increase the resistance to laundering of tracer compounds based on fluorene, it is possible to modify the aromatic rings of the fluorene molecule and/or the carbon at the C-9 position, ideally with bulky non-planar groups. These bulky non-planar groups inhibit the interaction of the molecule with adsorbents used in laundering. In doing so, these inherently non-polar molecules additionally become non-planar. The net result is a family of molecules which is harder to launder from a hydrocarbon fuel or other hydrocarbon liquid after treatment with acids, alkalis or repeated use of activated charcoal as well as other reagents used in fuel laundering.

It has also been found that while the additional functionalization of fluorene molecules with non- planar groups increases their mass, the molecules are surprisingly quick-eluting by gas chromatography for their mass. The combination of higher mass while remaining relatively quick eluting is a very useful combination of properties as it means the tracer molecules elute at least with some of the components of the hydrocarbon liquid in which they are disposed but can still be resolved from those components by virtue of their mass. For example, the tracer molecules as described herein are heavier than most of the components of a typical fuel (gasoline or diesel fuel) but are still readily distinguishable from the fuel components which elute at a similar rate as the tracer molecules. By way of comparison, known non-polymeric, non-halogenated, non- launderable tracers typically have masses of less than 300 atomic mass units (amu). However, analysis of diesel fuels has shown that there is a broad distribution of molecular weights between 100-400 amu, particularly 100 - 300 amu, but increasingly few components heavier than 300 or 400 amu. Substituted fluorene molecules are readily synthesised with molecular weights over 300, 350, 400, 450, or 500 amu. This makes the molecules readily distinguishable from components of gasoline and diesel fuels. At the same time, the non-polymeric molecules of the present invention are still sufficiently light (e.g. less than 1000, 800, 600 amu) so as to have relatively fast elution times in GC-MS analysis, unlike the fluorene copolymers mentioned in the prior art.

Furthermore, it is often the case that prior art tracer molecules operate best in one or other of gasoline and diesel but not both. Due to the combination of properties as outlined above, the substituted fluorene tracer molecules as described herein can be detected well in both fuels while satisfying the other critical requirement of non-launderability. The tracer molecules can also be used for marking kerosene-based fuels, liquified petroleum gas fuels, bio-diesel fuels, or bio-ethanol fuels.

Further still, the tracer molecules of the present invention can consist of atoms selected only from the group carbon, hydrogen, and oxygen which is a specified requirement for certain fuel marking applications. Additionally, the tracer molecules do not contain reactive functional groups or fused-ring structures which would otherwise decrease their resistance to laundering.

Finally, the basic fluorene structure enables a family of related tracer molecules to be derived. That is, forming a substituted fluorene confers the advantage that a suite of molecular tracers can be produced simply by varying the species that is reacted with the fluorene core. The R groups of the fluorene, while typically being C₃ to C₂₀ groups, can be intentionally varied to provide a suite of tracer compounds. As each substituted fluorene will possess a different mass or affinity to the separation column, they can all be distinguishable from each other by gas chromatography mass spectrometry (GC-MS). Such a suite of tracer compounds is very useful for marking hydrocarbon liquids (e.g. fuels) from different sources and/or for marking a hydrocarbon liquid with a combination of different tracer molecules.

A method of marking a hydrocarbon fuel is thus provided, such as a gasoline fuel, diesel fuel, kerosene-based fuel, liquified petroleum gas fuel, bio-diesel fuel, or bio-ethanol fuel, comprising adding a tracer compound as defined above to the hydrocarbon fuel. It is also envisaged that the tracer compounds as described herein may be used in other applications, particularly other tracer applications. For example, the compounds as described herein may be used in hydrocarbon reservoir tracing methods where the tracer compound is introduced into a hydrocarbon reservoir and then detected in fluids produced from the hydrocarbon reservoir.

Further still, there is also provided a hydrocarbon fuel, such as a gasoline fuel, diesel fuel, kerosene-based fuel, liquified petroleum gas fuel, bio-diesel fuel, or bio-ethanol fuel, comprising a tracer compound as defined above.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows the generic structure of a family of substituted fluorene tracer compounds featuring substitution at the C-2, C-7 and C-9 positions;

Figure 2 shows an example of a substituted fluorene tracer compound - 2,7-di-tert-butylfluorene; Figure 3 shows another example of a substituted fluorene tracer compound - 2,7-di-ferf-butyl-

9.9-dipropylfluorene;

Figure 4 shows another example of a substituted fluorene tracer compound - 2,7-di-ferf-butyl-

9.9-di-(2-ethylhexyl)fluorene;

Figure 5 shows a reaction scheme for the synthesis of 2,7-di-ferf-butyl-9,9-di-(2- ethylhexyl)fluorene;

Figure 6 shows the relative elution of 2,7-di-tert-butyl-9,9-dipropylfluorene with components in diesel fuel;

Figure 7 shows the relative elution of 2,7-di-tert-butyl-9,9-di-(2-ethylhexyl)fluorene with components in diesel fuel;

Figure 8 shows the relative elution of 2,7-di-tert-butylfluorene with components in diesel fuel; Figure 9 shows the relative elution of 9,9-di-n-octylfluorene with components in diesel fuel; and Figure 10 shows the relative elution times by GC-MS for 9,9-di-n-octylfluorene, 2,7-di-tert- butylfluorene, 2,7-di-tert-butyl-9,9-dipropylfluorene and 2,7-di-ferf-butyl-9,9-di-(2- ethylhexyl)fluorene.

Detailed Description

As described in the summary section, the present specification provides a tracer compound for marking a hydrocarbon liquid is provided, the tracer compound being a substituted fluorene having a structure of Formula I:

According to certain examples R3 and R4 are not hydrogen. In such examples Rl and R2 can be hydrogen. That is, substitution of the fluorene at the C-9 position is optional but can be advantageous for the reasons described previously in the summary section.

Alternatively, Rl and R2 are not hydrogen. In such examples R3 and R4 can be hydrogen. That is, substitution of the fluorene on the aromatic rings is optional but can be advantageous for the reasons described previously in the summary section.

According to further examples, all of Rl, R2, R3, and R4 are not hydrogen. That is, the fluorene is substituted on the aromatic rings and also at the C-9 position.

Each R group can consist of atoms selected from the group carbon, hydrogen, and oxygen. As such, for applications which specify that the tracer must only contain carbon, hydrogen, and/or oxygen atoms, embodiments of the tracer compound as described herein can fulfil this requirement.

Each R group can be a C3 to C20 group. The R groups can advantageously be straight chain, branched or cyclic alkyl groups. Particularly useful are non-planar, branched alkyl groups such as ferf-butyl, 2-ethylhexyl and neo-pentyl groups.

If Rl and/or R2 are ether groups, Rl and R2 can be selected from a straight chain, branched or cyclic alkyl group, substituted phenyl, or substituted benzyl where each incorporates one or more oxygen atoms so as to form an ether, but where Rl and R2 do not constitute an acetal.

According to certain examples, R3 and R4 are at the C-2 and C-7 positions such that the substituted fluorene tracer compound has a structure of Formula II:

Formula II

Furthermore, while the preceding examples have shown a single substituent on each of the aromatic rings of the core fluorene structure, it is also envisaged that one or both of the aromatic rings of the substituted fluorene tracer compound is substituted with one or more further groups selected from straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups.

A method of marking a hydrocarbon liquid is also provided comprising adding a tracer compound as described herein to the hydrocarbon liquid. The resultant product is a hydrocarbon liquid, such as a gasoline or diesel fuel, comprising the tracer compound. The hydrocarbon liquid may be a pure compound such as hexane or octane or it may comprise a mixture of compounds such as a distillation fraction having a particular range of boiling points. The hydrocarbon liquid may be intended for use as a chemical, a solvent or a fuel. The hydrocarbon liquid may be a biologically derived fuel such as a bio-diesel or bio-ethanol or a mixture of a biologically derived with a mineral oil derived fuel. The tracer compounds as described herein are of particular use for marking liquid hydrocarbon fuels such as gasoline, diesel fuels, kerosene-based fuels or liquified petroleum gas. In one particular application, a low-tax fuel such as an agricultural diesel may be marked in order to detect any subsequent sale and use for purposes such as road-vehicle fuel, which would normally be taxed more highly. In such cases unlawful dilution or substitution of a more highly taxed fuel with the low-taxed fuel may be detected by analysis of the highly taxed fuel to determine whether the tracer is present. Therefore, in these cases, it is highly beneficial to use a tracer compound in the low-taxed fuel which is not easily removed, or laundered, from the fuel to a level at which it can no longer be detected. We have found that compounds as described herein are resistant to removal from hydrocarbon fuels by multiple known methods of fuel laundering. The tracer compound is added to the hydrocarbon liquid in such an amount as to provide a concentration of the tracer compound which is detectable by readily available laboratory methods capable of identifying the tracer compound in the liquid at the concentrations used. Suitable methods include but are not limited to gas chromatography coupled with a suitable detector such as a mass spectrometer. Typical concentrations are within the range 1 pg/l to 10000 pg/l with the specific amount dependent on the detection method and limit of detection of the particular tracer compound used. The tracer compound may be present at a higher concentration than 10000 pg/l although when the product to be marked is a high-volume commodity such as a motor-fuel, economic considerations usually favour lower levels of tracer compound. The tracer compound may be supplied in the form of a concentrated dosing solution (or master-batch) of the tracer compound in a solvent. In this case, the preferred solvent is a liquid which is similar to the liquid to be marked, although a different solvent, e.g. a single or mixed component aliphatic or aromatic solvent may be used, provided the presence of such a solvent can be tolerated in the hydrocarbon liquid to be marked. A preferred solvent is solvent naphtha, optionally C10-C13 low naphthalene aromatic solvent or an equivalent. The concentrated dosing solution can be added to the hydrocarbon liquid to be marked to produce on dilution the required final concentration of the tracer in the liquid. More than one tracer compound may be added to the hydrocarbon liquid or to the hydrocarbon fuel.

Examples of the invention as described herein generate a family of non-polar, non-planar molecules from a core molecule based on fluorene. These molecules are advantageous for use as tracer molecules in hydrocarbon fuels as they satisfy the following criteria: high resistance to laundering; contain only carbon and hydrogen; relatively high molecular weight; relatively quick and generally distinct elution times by GC-MS; non-hazardous; and a similar method of synthesis.

Figure 1 shows the generic structure of a sub-family of substituted fluorene tracer compounds comprising substituents R3, R4 at the C-2 and C-7 positions on the aromatic rings of the fluorene core structure with optional substituents Rl, R2 at the C-9 position. Examples of such tracer compounds include: 2,7-di-tert-butylfluorene as shown in Figure 2; 2,7-di-ferf-butyl-9,9- dipropylfluorene as shown in Figure 3; and 2,7-di-tert-butyl-9,9-di(2-ethylhexyl)fluorene as shown in Figure 4.

Figure 5 shows a reaction scheme for the synthesis of 2,7-di-ferf-butyl-9,9-di-(2-ethylhexyl) fluorene. The experimental protocol, as described below, is based on that obtained from page 151 of "Fluorene-based fluorescent markers: new insights in synthesis and applications into labelling of nucleic acids and imaging of cell membranes", by Janah Shaya at Universite de Nice- Sophia Antipolis, Cote d'Azur. An alternative synthetic method can be found in: G. Saikia, P K Iyer, J Org Chem 2010, 75, 2714-2717.

Examples

Example 1

2,7-Di-tert-butylfluorene (1 g, 3.59 mmoles) was weighed directly into a 50 ml round bottom flask. Potassium iodide (195 mg, 1.17 mmole, 0.3 eq.), 1-bromopropane (5.3 g, 43 mmole, 12 eq.) and dimethylsulfoxide (15 ml) were added. Lastly a small quantity of finely ground potassium hydroxide (1.27 g, 22.5 mmole, 6.3 eq.) was added. The flask was fitted with a stirrer bar and condenser and then heated over-night to 80°C in an oil bath under air. The contents turned deep orange and a white precipitate formed.

The crude reaction mix was worked up by addition to iso-octane (50 ml) and water (50 ml). The iso-octane was washed with water (2 x 50 ml) and then dried over anhydrous magnesium sulfate. Evaporation under reduced pressure gave a brown oil - Yield = 0.808 g (69.4%).

GC-MS analysis showed a mono-alkylated impurity (5% peak area) having mass 320.5 amu and the di-alkylated product (95% peak area) having mass 362.6 amu. The oil was purified by column chromatography followed by recrystallisation from ethanol. The 2,7-di-ferf-butyl-9,9- dipropylfluorene so obtained was used in the subsequent work.

Analysis of Example 1 in fuel

The crystallised 2,7-di-tert-butyl-9,9-dipropylfluorene (20.7 mg) was added to a 10 ml volumetric flask and made to the mark with iso-octane. The diluted alkyl fluorene (241 microlitre) was added to diesel fuel (250 ml) to give a tag level of 2 mg/ L. The tagged diesel fuel was analysed by GC MS in selective ion monitoring (SIM) mode at 362 amu. An untagged diesel sample was also analysed in both SIM mode at 362 amu and also in SCAN mode. The results are shown in Figure 6: lower trace = untagged diesel sample in SIM mode; middle trace = tagged diesel fuel in SIM mode; upper trace = untagged diesel sample in SCAN mode. The tagged diesel sample shows a clear signal at a retention time of 7.58 min corresponding to the 2,7-di-ferf-butyl-9,9- dipropylfluorene tracer compound. The chromatogram for diesel fuel not containing this tag molecule has no background at the same mass and time as the tag molecule but a small number of peaks in the same region. It is concluded that 2,7-di-tert-butyl-fluorene can be successfully b/s-alkylated at the 9-position, the product is soluble in organic solvents, and its relatively high mass means it can readily be detected in the diesel fuel.

Example 2

2,7-Di-tert-butylfluorene (1 g, 3.59 mmoles) was weighed directly into a 50 ml round bottom flask. Potassium iodide (60 mg, 0.36 mmole, 0.1 eq.) and dimethylsulfoxide (30 ml) were added. 2-ethylhexylbromide (2.78 g, 14.4 mmole) was added. The reagents all dissolved. Lastly a small quantity of finely powdered potassium hydroxide (0.806 g, 14.4 mmole) was added. The flask was fitted with a stirrer bar and condenser and then heated to 80°C in an oil bath under air. After a few minutes heating the colour began to yellow slightly. The reaction mix was left over-night at room temperature during which time little further colour change occurred.

The reaction mix was poured into water (50 ml) and left to settle. A yellow oil separated to the surface after a few minutes. The aqueous layer was removed, and the oil diluted with iso-octane (50 ml). The iso-octane was washed with water (2 x 50 ml) and then dried over anhydrous magnesium sulfate. Evaporation under reduced pressure gave a yellow oil - Yield = 1.364 g (75.6%).

GC-MS analysis showed a mono-alkylated impurity having mass 390.7 amu and the intended di- alkylated product having mass 502.9 amu. The 2,7-di-tert-butyl-9,9-(2-ethylhexyl)fluorene was used without further purification as it constituted 93% of the total area by GC-MS.

Analysis of Example 2 in fuel

The crude 2,7-di-tert-butyl-9,9-di-(2-ethylhexyl)fluorene (28.7 mg) was added to a 25 ml volumetric flask and made to the mark with decalin. The diluted alkyl fluorene (871 microlitre) was added to diesel fuel 500 ml) to give a tag level of 2 mg/ L. The tagged diesel fuel was analysed by GC MS in selective ion monitoring (SIM) mode at 502 amu. An untagged diesel sample was also analysed in both SIM mode at 502 amu and also in SCAN mode. The results are shown in Figure 7: lower trace = untagged diesel sample in SIM mode; middle trace = tagged diesel fuel in SIM mode; upper trace = untagged diesel sample in SCAN mode. The tagged diesel sample shows a large signal at a retention time of 9.37 mins corresponding to the 2,7-di-ferf-butyl-9,9-di(2- ethylhexyl)fluorene tracer compound. The chromatogram for diesel fuel not containing this tag molecule has no background at the same mass and time as the tag molecule and no peaks in the same region. It is concluded that 2,7-di-tert-butyl-fluorene can be successfully b/s-alkylated at the 9-position, the product is soluble in organic solvents, and its high mass means it is readily detected in diesel fuel.

Further Examples

Figure 8 shows the relative elution of 2,7-di-tert-butylfluorene with components in diesel fuel. The tagged diesel fuel was analysed by GC MS in selective ion monitoring (SIM) mode at 263 amu. An untagged diesel sample was also analysed in both SIM mode at 263 amu and also in SCAN mode. The results are shown in Figure 8: lower trace = untagged diesel sample in SIM mode; middle trace = tagged diesel fuel in SIM mode; upper trace = untagged diesel sample in SCAN mode. The tagged diesel sample shows a clear signal at a retention time of 7.80 mins corresponding to the 2,7-di-tert-butylfluorene tracer compound. The chromatogram for diesel fuel not containing this tag molecule has an appreciable background signal at the same mass and time as the tag molecule. The background has approximately one quarter the area of the tag molecule. Flowever, the 2,7-di-tert-butylfluorene can still be measured. It can be seen that although the elution time for 2,7-di-tert-butylfluorene is similar to that of 2,7-di-ferf-butyl-9,9- dipropylfluorene, the latter compound is far easier to identify in the fuel matrix as a result of its molecular ion having a larger mass. The benefit of searching for a tracer compound of larger molecular mass is that although there may be fuel components of similar retention time, they will have a lower mass and so will be 'screened out' of the chromatogram, making the tracer compound far easier to observe.

Figure 9 shows the relative elution of 9,9-di-n-octylfluorene with components in diesel fuel. The tagged diesel fuel was analysed by GC MS in selective ion monitoring (SIM) mode at 390 amu. An untagged diesel sample was also analysed in both SIM mode at 390 amu and also in SCAN mode. The results are shown in Figure 9: lower trace = untagged diesel sample in SIM mode; middle trace = tagged diesel fuel in SIM mode; upper trace = untagged diesel sample in SCAN mode. The tagged diesel sample shows a large signal at a retention time of 9.11 mins corresponding to the 9,9-di-n-octylfluorene tracer compound. The chromatogram for diesel fuel not containing this tag molecule has no background signal at the same mass and time as the tag molecule. The relatively high mass of the tag molecule means it can be easily and selectively identified when it is present in the fuel. The relative retention times of a number of fluorene derivatives analysed by the same GC-MS method are shown in Figure 10. The compounds and their respective retention times were:

2.7-di-tert-butyl-9,9-dipropylfluorene - retention time 7.58 min;

2.7-di-tert-butylfluorene - retention time 7.80 min;

9,9-di-n-octylfluorene) - retention time 9.11 min; and

2.7-di-tert-butyl-9,9-di-(2-ethylhexyl)fluorene) - retention time 9.37 min.

Launder Tests

A range of fuel laundering tests have been performed on individual samples of diesel fuel containing 2,7-di-tert-butylfluorene, 2,7-di-tert-butyl-9,9-dipropylfluorene, 2,7-di-ferf-butyl-9,9- di(2-ethylhexyl)fluorene, and 9,9-di-n-octylfluorene.

Samples of the tagged fuel were subjected to a series of launder tests where the fuel was subjected to commonly used laundering reagents. In the procedure that follows, a sample of tagged fuel that has been subjected to laundering is referred to as 'laundered fuel'; a sample of tagged fuel that had not been subjected to laundering is referred to as 'tagged reference'. In order to assess the degree of removal of the taggant by the laundering reagent, the concentration of the taggant in laundered fuel was compared after a particular launder test with the concentration of the taggant in a sample of the same fuel which had not been subjected to any launder test. A typical GC sequence included tagged reference, untagged fuel, samples of laundered fuel, tagged reference and finally untagged fuel. Reference samples were run at the beginning and end of any GC sequence to help eliminate instrument drift over the course of the sequence.

Analytical conditions Injection size: lpL.

Solvent wash: 2 x 10 pL solvent A, 2 x 10 pL solvent B.

Sample rinse: 2 x 10 pL solvent A, 2 x 10 pL solvent B.

Inlet: split; temperature: 270°C; pressure 11.8 psi; spilt ratio 40:1; spilt flow 96.6 ml/min; total flow 100.9 ml/ min; carrier gas helium.

Column: Agilent HP-5MS; 30 m x 0.25 mm i.d. x 0.25 pm; stationary phase (5% phenyl)- methylpolysiloxane.

Mode: constant flow. Oven temperature: 80°C for 0.5 min, 25°C/ min up to 325°C, hold 1.70 min.

Mass spectrometer conditions Transfer line temperature: 280°C.

Quadrupole temperature: 150°C.

Source temperature: 230°C.

Operating mode: Selective Ion Monitoring (SIM) with ions as appropriate for the molecule being analysed.

Dwell time: 100 msec.

All those launder tests involving a washing procedure were carried out in sealed brown glass bottles to minimise evaporation over the four-hour stirring period. All launder tests involving stirring were allowed to separate before sampling. The fuel layer from any launder test containing an aqueous reagent was separated into a scintillation vial where it was dried over anhydrous magnesium sulfate or potassium carbonate before being filtered through a cotton wool plug and finally transferred to a GC vial. All tests involving the passage of fuel through a column of solid adsorbent were carried out by applying reduced pressure to the outlet of the column rather than a positive pressure to the mouth of the column. This was achieved by fitting the column outlet via a close-fitting seal to a receptacle, such as a Buchner flask and collecting the liquid that elutes from the column into the flask by attaching a vacuum pump to the side arm. Fuels containing obvious particulate matter were filtered before dispensing into a GC vial. The fuel from all other launder tests was sampled into GC vials without further clean-up.

Launder tests included the following:

Hydrochloric acid wash -hydrochloric acid (25 mL, 10% w/w) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Sulfuric acid wash - Sulfuric acid (25ml, 10% w/w) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Sodium hydroxide wash - Sodium hydroxide solution (25 ml, 10M) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Methanolic potassium hydroxide wash - potassium hydroxide (1 M) in methanol (25 ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Methanol wash - methanol (25 ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature. Acetonitrile wash - acetonitrile (25 ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Hydrogen peroxide wash - hydrogen peroxide solution (27%, aqueous, 25 ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

60°C stir - tagged fuel (50 ml) was placed in a beaker and stirred at 60°C for four hours.

Aeration - tagged fuel (50ml) in a brown glass bottle was bubbled with air at about 200 ml/ minute. The fuel was analysed periodically.

UV treatment - two samples of fuel (25 ml) in clear glass scintillation vials were stored under a bench top UV light. One vial was open to the light; the other was sealed and laid on its side under the light. The samples were analysed periodically.

Activated charcoal stir - activated charcoal (2.5 g, Norit SX plus F Cat, p/no. 901933, Sigma Aldrich) was mixed with tagged fuel (50 ml) and stirred for four hours at room temperature.

Fuller earth stir - Fullers earth (2.5 g) was mixed with tagged fuel (50 ml) and stirred for four hours at room temperature.

Activated charcoal columns - a 10 cm glass column with 1 cm internal diameter was packed with activated charcoal (Norit RBAA-3 rod). Tagged fuel (50 ml) was passed through the column. Two repeat passes of the fuel through the coloumn were carried out using fresh activated charcoal each time. A similar procedure was carried out using columns containing Fullers earth (100-200 mesh, p/no. F200, Sigma Aldrich), sepiolite (no supplier details) and Davisil silica (grade 710, 50-76A, p/no. 236756, Sigma Aldrich).

Results of the launder tests in diesel fuel are summarized in the tables below indicating fuel type, launder test, and amount of tag molecule or tracer remaining after the test in terms of a percentage of the initial concentration of the tracer/tag in the fuel.

As can be seen for the results table, even when reference samples were run at the beginning and end of the GC sequence to help eliminate instrument drift over the course of the sequence, some of the results indicate tracer concentrations above 100% after the launder test. This could arise from an imperfect correction of instrument drift or from the removal of components from the fuel by the laundering process leading to an increase in tracer molecule concentration. That said, it should be noted that no problem or interference was experienced in analysing by GC-MS for the taggant molecules. For the launder tests, when analysing for: 2,7-di-tert-butyl-9,9-dipropylfluorene the mass spectrometer was set to detect m/e = 362 amu; when analysing for 2,7-di-ferf-butyl-9,9-di(2- ethylhexyl)fluorene the mass spectrometer was set to detect m/e = 502; when analysing for 2,7- di-ferf-butyl fluorene the mass spectrometer was set to detect m/e = 263 amu; and when analysing for 9,9-di-n-octyl-fluorene the mass spectrometer was set to detect m/e = 390 amu. Results show that all of the fluorene derivatives can be readily measured and they are all resistant to the laundering tests in diesel fuel. It was found that the mass of the molecular ion is often the most convenient mass to analyse however, when analysing for 2,7-di-tert- butylfluorene the ion at 263 amu was significantly more intense.

While this invention has been particularly shown and described with reference to certain embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of marking a hydrocarbon fuel, the method comprising adding to said hydrocarbon fuel a tracer compound for marking the hydrocarbon fuel, the tracer compound being a substituted fluorene having a structure of Formula I:

2. A method according to claim 1, wherein R3 and R4 are not hydrogen.

3. A method according to claim 1 or 2, wherein Rl and R2 are not hydrogen.

4. A method according to any preceding claim, wherein Rl, R2, R3, and R4 are selected such that the substituted fluorene tracer compound consists only of atoms selected from the group carbon, hydrogen, and oxygen.

5. A method according to any preceding claim, wherein R1 and R2 and/or R3 and R4 are selected from the same or different C₃ to C₂₀ group.

6. A method according to any preceding claim, wherein R1 and R2 and/or R3 and R4 are selected from the same or different, straight chain, branched or cyclic alkyl groups.

7. A method according to claim 6, wherein R1 and R2 and/or R3 and R4 are selected from the same or different branched or cyclic alkyl groups.

8. A method according to claim 7, wherein R1 and R2 and/or R3 and R4 are selected from the same or different branched alkyl groups.

9. A method according to claim 8, wherein R3 and R4 are ferf-butyl groups.

10. A method according to any preceding claim, wherein R3 and R4 are at the C-2 and C-7 positions of the aromatic rings of the substituted fluorene tracer compound such that the substituted fluorene tracer compound has a structure of Formula II:

Formula II

11. A method according to any preceding claim, wherein one or both of the aromatic rings of the substituted fluorene tracer compound is substituted with one or more further groups selected from straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups.

12. A method according to any preceding claim, wherein the hydrocarbon fuel is a gasoline fuel, a diesel fuel, a kerosene-based fuel, a liquified petroleum gas fuel, a bio-diesel fuel, or a bio-ethanol fuel.

13. A hydrocarbon fuel comprising a tracer compound as defined in any preceding claim.

14. A hydrocarbon fuel according to claim 13, wherein the hydrocarbon fuel is a diesel fuel, a gasoline fuel, a kerosene-based fuel, a liquified petroleum gas fuel, a bio-diesel fuel, or a bio ethanol fuel in which the tracer compound is disposed.