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Spectrophotometric Detection of Trace Copper Levels in Jet Fuel Greg E. Collins,*,† Robert E. Morris,† Jing-Fong Wei,‡ Matthew Smith,§ Mark H. Hammond,§ Veronique Michelet,| Jeffrey D. Winkler,⊥ Pamela M. Serino,# and Yan Guo# Naval Research Laboratory, 4555 Overlook Ave., S.W., Chemistry Division, Code 6116, Washington, DC 20375-5342, Southern University, Department of Chemistry, Baton Rouge, Louisiana 70813, Nova Research, Inc., 1900 Elkin Street, Suite 230, Alexandria, Virginia 22308, Laboratoire de Synthe` se Organique, E.N.S.C.P., 11, rue P. et M. Curie, 75231 Paris Ce´ dex 05, The University of Pennsylvania, Department of Chemistry, Philadelphia, Pennsylvania 19104, and Defense Energy Support Center, 8725 John J. Kingman Rd., Fort Belvoir, Virginia 22060-6222 Received November 15, 2001
To sensitively and selectively monitor jet fuel for trace levels of copper, two spirobenzopyran dyes, quinolinospiropyranindoline (QSP) and nitrospiropyranindoline (NQSP), have been investigated for their application in a spectrophotometric method that is amenable to rapid, on-site detection of µg/L levels of copper in jet fuel. Both NQSP and QSP respond sensitively to various metal ions in jet fuel, including copper, nickel, and zinc, with NQSP demonstrating the best performance in terms of selectivity of spectral response and sensitivity. Principal component analysis (PCA) was applied to the spectral data, and the individual contributions to the absorption spectra by copper, nickel, and zinc in JP-5 fuel can be readily separated using this technique. The effect of metal deactivating additives (MDA) in jet fuel on the sensitivity of NQSP for copper was investigated. Silica-bound diethylenetriamine was investigated as an effective solid-phase extraction agent for defining the baseline absorbance of NQSP in jet fuel samples being analyzed by standard addition methodologies.
Introduction It has been well established that trace quantities of certain metals will promote autoxidation of organic compounds in the presence of oxygen. While it has been observed that iron, zinc, and lead can promote the oxidative degradation of hydrocarbon fuels,1 copper and its compounds have been shown to be one of the most active and available oxidation initiator instability promoters.2,3,4 Trace amounts of copper are commonly found in fuels, having been introduced during copper sweetening processes or from contact with copper-bearing piping, brass fittings, admiralty metal, and other copperbearing alloys. In gas-drive fuel coker tests,2 as little as 15 to 25 µg/L of added copper or iron, and 100 to 250 µg/L of added zinc or lead were shown to have deleterious effects on JP-7 thermal stability. There is also * Author to whom correspondence should be addressed. † Naval Research Laboratory. ‡ Southern University. § Nova Research, Inc. | Laboratoire de Synthe ` se Organique, E.N.S.C.P. ⊥ The University of Pennsylvania. # Defense Energy Support Center. (1) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; ASTM Monograph 1; American Society of Testing and Materials: Philadelphia, PA, 1991; Chapter 8. (2) Schenk, L.; Johnson, R.; Monita, C. Investigation of Effects of Trace Metals in the Thermal Stability of JP-7 Fuels; Southwest Research Institute, Report AFAPL-TR-71-98, Dec. 1971. (3) Pederson, C. J. Ind. Eng. Chem. 1949, 41, 924. (4) Smith, J. D. J. Aero. Eng. 1967, 33 (4), 19.
evidence that dissolved copper can promote certain prerequisite chemical reactions in jet fuel during storage,5 which enhance the autoxidation process and result in further thermal degradation. Low levels of dissolved copper in jet fuel initiate chemical processes that may lead to the formation of gums and sediments in aircraft fuel systems that are deleterious to optimal performance. In a shipboard JP-5 fuel survey conducted by Southwest Technical Institute,6 out of over 200 samples, the copper ranged between 0 and 838 µg/L, with approximately 10% of the samples containing more than 50 µg/L. The majority of the fleet fuel samples tested were found to fail the standard JFTOT test for thermal stability when they contained 50 µg/L of dissolved copper, and several samples failed with 25 to 50 µg/L copper. We have seen fuels fail the JFTOT test with as little as 15 µg/L copper, although it is generally agreed that levels above 25 µg/L should be avoided. The determination of copper in fuel to that level is nontrivial. Graphite furnace-atomic absorption spectroscopy analysis is currently the only method available to reliably quantify copper in fuel at these low levels. Thus, there is no field method currently available to detect low (5) Pande, S. G.; Hardy, D. R. Energy Fuels 1995, 9, 177-182. (6) Cuellar, J. P., Jr.; Russell, J. A. Additive Depletion and Thermal Stability Degradation of JP-5 Fuel Shipboard Samples; Report NAPC-PE-141C, Southwest Research Institute on contract with the Naval Air Propulsion Center, Trenton, NJ, June 1985.
10.1021/ef010271a CCC: $22.00 © 2002 American Chemical Society Published on Web 07/13/2002
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threshold levels of dissolved copper in jet fuel. While there are numerous metallochromic spectrophotometric methods for quantitating copper levels in aqueous media,7,8 there have been no reports to date dealing with the direct application of these dyes to fuels. The objective of this study is to develop a field test that is capable of detecting 25 to 50 µg/L copper in jet fuel, as a means of ensuring the purchase and application of clean, safe jet fuels for aircraft operations, while avoiding costly laboratory analysis, problems with shipping samples, and long turnaround times. Spirobenzopyran dyes9-11 are an interesting class of metal complexation indicators which have been shown to exhibit extremely sensitive absorptivity changes following complexation of transition metal ions. These dyes undergo photoreversible spectral shifts that are unique for different metal ions. Moreover, their performance is enhanced in nonaqueous solvents. Absorption spectroscopy is very amenable to field implementation, although it is typically hampered by low sensitivities (µg/L parts per million regime). The spirobenzopyran metal complexes investigated here, however, have such high molar absorptivities for the metal complexes formed, that low µg/L level detection of copper in jet fuel is feasible in a simplistic manner. We examine here the spectroscopic properties of two structurally similar spirobenzopyran dyes, NQSP and QSP, which differ only in the covalent attachment of a nitro functional group for NQSP, for the detection of trace levels of copper in jet fuel. These dyes have significantly different solubility and color change responses to metal complexation. Colorimetric changes observed are from colorless to red for QSP and violet to red for NQSP upon exposure to Cu(II). NQSP and QSP have some very unique properties that separate them from other dyes. First, these dyes are comprised of two photoisomers in solution, and can be forced to photoreversibly release complexed metal ions by shining intense light on them (a parameter which need not be exploited for this application).11 Second, NQSP has already been shown to complex different metal ions with distinctly different spectroscopic signatures in the visible wavelength region.9 Although there is overlap of the absorbance spectra for various NQSP-metal complexes, partial least-squares analysis has been successfully used to selectively identify six different transition metal ions in the plasticizer solvent, dicapryl phthalate, in both single and binary component mixtures.9 Most importantly, NQSP has enabled the detection of copper in the presence of low levels of interferents such as nickel and zinc. In a survey of various jet fuel samples, the highest zinc and iron concentrations found were 30 and 16 µg/L, respectively, while the majority of fuel samples had undetectable levels. Nickel concentrations, on the other hand, from exposure to copper-nickel piping, were as high as 52 µg/L, with one sample (7) Richter, P.; Toral, M. I.; Castro, H. Anal. Lett. 2002, 35, 635646. (8) Foerster, J. W.; Lamontagne, R. A.; Ewing, K. J.; Ervin, A. M. Field Anal. Chem. Technol. 1999, 3, 3-18. (9) Evans, L. III; Collins, G. E.; Shaffer, R. E.; Michelet, V.; Winkler, J. D. Anal. Chem. 1999, 71, 5322-5327. (10) Winkler, J. D.; Bowen, C. M.; Michelet, V. J. Am. Chem Soc. 1998, 120, 3237-3242. (11) Collins, G. E.; Choi, L.-S.; Ewing, K. J.; Michelet, V.; Bowen, C. M.; Winkler, J. D. Chem. Commun. 1999, 4, 321-322.
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containing 170 µg/L. Selective detection of copper in the presence of these metal interferents is a critical issue to the application of any indicator dye. NQSP and QSP have demonstrated improved spectroscopic behavior in nonaqueous solvents, e.g., benzene, tetrahydrofuran, acetonitrile, and dicapryl phthalate, indicating the applicability of these dyes to jet fuel, a characteristic certainly not common for most aqueous based indicator dyes.9,11 Experimental Section Nitroquinolinospiropyranindoline (NQSP) was prepared by the condensation of Fisher’s base 1,3,3-trimethyl-2-methyleneindoline with 7-formyl-8-hydroxy-5-nitroquinoline as described elsewhere.10 A similar synthetic procedure for quinolinospiropyranindoline (QSP) has also been detailed elsewhere.12 The metal salts utilized were copper(II) ethylacetoacetate (Pfaltz & Bauer), copper(I) chloride (Fischer Scientific), nickel(II) 2-ethylhexanoate (Alfa Aesar), zinc(II) 2-ethylhexanoate (Alfa Aesar), and iron(III) ethoxide (Alfa Aesar). N,N′-disalicylidene-1,2-propanediamine, the active ingredient in the widely used metal deactivating additive (MDA) was obtained from Pfaltz & Bauer. Metal complexation agents ethylenediamine tetraacetic acid, tetrasodium salt (Na4EDTA) was supplied by Aldrich and diethylenetriamine bound to silica gel (DETA-Si) was prepared by a previously discussed method.13 The Jet A and JP-5 fuels were in accordance with applicable specifications and contained less than 15 µg/L copper. All spectrophotometric results were recorded on a Hitachi U-3000 dual beam spectrophotometer. Data analysis was performed using MATLAB (version 6.1, Mathworks Inc., Natick MA). Principle component analysis routines were used from the PLS_toolbox (version 2.1.1, Eigenvector Technologies Inc., Manson, WA). Prior to chemometric analysis, the NQSP absorbance spectrum for each series was subtracted from the metal-NQSP complexation spectrum. This background subtraction was found to improve the reproducibility of the spectra.
Results and Discussion QSP and NQSP differ only in the presence of an -NO2 functional group para to the phenolate ion, but the two indicator dyes have significantly different spectroscopic and chemical characteristics. Investigations were carried out to determine which dye was more appropriate for the quantitation of trace copper levels in jet fuel in the presence of possible impurity metal ions such as zinc and nickel. As can be seen from Figures 1 and 2, the addition of various metal ions to solutions of NQSP and QSP in Jet A results in extremely sensitive spectroscopic changes in the absorbance spectra. For the purposes of the following discussion, we will emphasize first the indicators’ response to copper(II). Note that NQSP responds to the addition of Cu(II) with the formation of a blue-shifted absorbance peak at 440 nm, while the primary absorption peak at 572 nm is reduced (color change observed is from purple to red). QSP, unlike NQSP, complexes copper(II) with a small increase in the absorbance band from 525 to 625 nm, and the formation of a blue-shifted absorbance peak at 425 nm, causing a color change in solution from colorless to red with the addition of Cu(II). (12) Przystal, P.; Phillips, J. P. J. Heterocyclic Chem. 1967, 4, 131. (13) Morris, R. E.; Cheng, E. L. Petrol. Sci. Technol. 2000, 18, 11471159.
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Figure 1. Spectrophotometric response of QSP in Jet A jet fuel with 20% ethanol to the addition of 300 µg/L Cu(II), Zn(II), Ni(II), Cu(I), and Fe(III).
Figure 2. Spectrophotometric response of NQSP in Jet A jet fuel with 2% ethanol to the addition of 300 µg/L Cu(II), Zn(II), Ni(II), Cu(I), and Fe(III).
NQSP and QSP are nearly insoluble in jet fuel without the addition of at least 2% ethanol, a quantity which can be easily added by spiking the jet fuel sample with a concentrated aliquot of NQSP or QSP in ethanol stock solution. As Figure 3 demonstrates, the ethanol concentration in jet fuel is a critical parameter influencing NQSP and QSP’s sensitivity of response to Cu(II). In this figure, the sensitivity of QSP and NQSP to Cu(II), i.e., the slope of the absorbance change as a function of copper concentration, is plotted versus increasing levels of ethanol in JP-5 jet fuel. To more accurately represent the sensitivity of this approach to an actual field application, the concentration of copper in the jet fuel sample prior to any dilution effects by the addition of ethanol/dye as a cosolvent, has been utilized for all sensitivity calculations. With respect to QSP at 555 nm, the sensitivity to Cu(II) increases until the addition of approximately 20% ethanol, at which point the dilution effect resulting from the addition of ethanol forces a continual drop in sensitivity. At 425 nm, however, the addition of ethanol immediately causes a linear decrease in sensitivity to Cu(II), although the decline in sensitivity lags behind the expected dilution effect from ethanol addition. NQSP, on the other hand, responds to the
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Figure 3. Sensitivity of response for QSP and NQSP to Cu(II) as a function of the total ethanol concentration in JP-5 jet fuel: O, NQSP at 572 nm; 9, NQSP at 440 nm; 3, QSP at 425 nm; 4, QSP at 555 nm.
addition of ethanol to JP-5 with a continual and immediate drop in sensitivity at both 440 and 572 nm, that, for the most part, can be directly ascribed to a dilution effect by the added ethanol. QSP and NQSP are solvatochromic dyes that are extremely sensitive to polarity, responding to the addition of ethanol with an equilibrium shift of the molecule from a spirobenzopyran or “closed” state to a merocyanine or “open” isomeric state.9 These states correspond to the breaking and forming of a phenolate bond within the molecule. The merocyanine form opens the molecule up, exposing the phenolate anion and dramatically improving metal complexation, while the spirobenzopyran state removes this binding site, effectively eliminating the metal binding capabilities of this molecule. QSP, when dissolved in pure jet fuel, is nearly colorless. This is a direct indication that the spirobenzopyran-merocyanine equilibrium for QSP has shifted to the colorless spirobenzopyran isomer due to the nonpolar character of this medium. Under these conditions, metal binding of Cu(II) requires a shift in the equilibrium to the merocyanine form, a condition which is not favored until the addition of a polar solvent such as ethanol. NQSP, in contrast, equilibrates to a significantly higher fraction of merocyanine dye following the addition of only 2% ethanol. The reason for this is the presence of the nitro functional group, which sufficiently stabilizes the phenolate anion.11 In this way, NQSP is significantly more sensitive to Cu(II) in the presence of minimal amounts of ethanol (the sensitivity of NQSP at 440 nm for Cu(II) is nearly 2.5 times more sensitive than QSP at 425 nm, for example), and more importantly, the baseline spectrum of NQSP in the absence of any metal ions is significantly less sensitive than QSP to slight polarity changes, a feature which should make quantitation more straightforward. Both QSP and NQSP respond linearly and sensitively to the addition of Cu(II) in jet fuel, albeit in two different spectroscopic regions. Typical data obtained are shown in Figure 4, which shows the response of NQSP in JP-5 fuel with 2% ethanol following the sequential addition of Cu(II). The inset plot shows the change in absorbance at 437 nm as a function of Cu(II) concentration (the
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Figure 4. Spectral response of NQSP in Jet A jet fuel containing 0 mg/L MDA to increasing concentrations of Cu(II). The inset graph demonstrates the linearity of response obtained for increasing Cu(II) concentrations from 0 to 300 µg/L at 440 nm in the presence and absence of 2 mg/L MDA.
influence of MDA will be discussed later). The detection limit for Cu(II) under these conditions, assuming a signal-to-noise ratio of 3:1, is 9.3 µg/L. Although the absorbance change occurring at 572 nm is significantly more sensitive to Cu(II) than that observed at 437 nm, we have had difficulty defining a true baseline for this red-shifted wavelength band (spectrum for jet fuel containing 0.0 µg/L Cu(II)) following a metal ion extraction step via silica-bound diethylenetriamine, as will be discussed later. In comparing the two spirobenzopyran dyes, NQSP and QSP, with respect to their capability for selectively and sensitively detecting trace levels of Cu(II) in jet fuel, NQSP was specifically chosen for further study on the basis of its (1) enhanced sensitivity to Cu(II) which is nearly 2.5 times that of QSP, (2) more dynamic and unique spectral response following the addition of interfering metal ions, and (3) lower sensitivity to interfering metal ions of concern, Zn(II) and Ni(II). For each of the three primary metal ions of interest, Cu(II), Ni(II), and Zn(II), a set of seven concentrations varying from 25 to 300 µg/L were prepared in JP-5 fuel, and the spectra collected following the addition of NQSP. From simply an observational point of view, it is evident from Figure 2 that each of these three metal ions engenders a characteristic spectrum that is unique from the other two. In the wavelength band 500 to 650 nm, there are two primary peaks at 546 and 577 nm which define this band. Metal complexation of Zn(II) by NQSP causes a dramatic decrease in the peak at 577 nm, while enhancing the 546 nm peak. The net result is an easily measurable decrease in absorptivity and a wavelength shift of 23 nm toward the blue. For Ni(II), the peak at 546 nm is enhanced, while the peak at 577 nm is diminished slightly. Finally, metal complexation of Cu(II) by NQSP causes a significant decrease in both spectral peaks, maintaining the intensity ratio between these two peaks. In the wavelength band from 420 to 500 nm, all three metals report an increase in absorbance at 439 nm, with only minimal spectral wavelength shifts differentiating these three metals. Finally, in the wavelength band of 330 to 425 nm, each metal ion has its own unique spectral response that further differentiates one metal complex from the other. The different
Figure 5. Principal component analysis (PCA) plot for NQSP in JP-5 showing good separation between Cu(II), Zn(II), and Ni(II) at concentrations varying from 25 to 300 µg/L.
spectroscopic signatures obtained by NQSP for these different metal ions indicates that the application of chemometric methodologies should successfully enable the selective identification of metal impurities found in a given jet fuel sample. Preliminary treatment of these data by a principal component analysis (PCA) indicates that the contributions to the absorption spectra by these metals in JP-5 can be readily separated with these techniques. The PCA plot for Cu2+, Zn2+, and Ni2+ (Figure 5) shows a large separation of the different principal components (i.e., metal spectra) for each of the different concentrations examined. Of critical note with respect to the selective detection of Cu(II) in jet fuel, it is apparent that of the three metal ions examined here, the Cu(II) spectral response is most easily differentiated from the remaining two metal ions due to its orthogonal score as shown in Figure 5. From an analytical point of view, we are more concerned with selectively detecting Cu(II) concentrations, as opposed to selectively differentiating between various benign impurities, such as Zn(II) and Ni(II). Future studies will examine the capability for differentiating binary mixtures, in a fashion similar to that achieved previously with NQSP in the plasticizer, dicapryl phthalate. Metal-deactivating additives (MDA) are occasionally added to jet fuel in order to help prevent any thermal problems associated with copper contamination. Because MDA is simply a metal chelating agent, we can presume that a competition will exist between MDA and NQSP for complexation of metal ions in jet fuel. Typical levels for MDA in jet fuels range from 0 mg/L to approximately 2 mg/L, although it is still allowed as high as 5.8 mg/L. When Jet A fuel was dosed with up to 600 µg/L of copper from copper ethylacetoacetate in the presence of 2 mg/L MDA, the absorption wavelength maximum at 439 nm was unaffected. In addition, the linearity of the NQSP response to dissolved copper was retained, although the slope was decreased by 37% (see Figure 4). MDA was determined to have minimal impact on the NQSP baseline spectrum in Jet A in the absence
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Figure 6. Effect of increasing MDA concentration on the absorbance change recorded for 100 µg/L Cu(II) at 437 nm for NQSP in Jet A jet fuel.
of Cu(II), indicating that there is no interaction between NQSP and MDA. The decrease in sensitivity is likely associated with the incomplete extraction of copper from the copper-MDA complex. In Figure 6, the change in NQSP-copper absorbance at 437 nm with increasing MDA concentration in a Jet A sample containing 100 µg/L Cu(II) is shown, when corrected for the absorbance signal obtained in the absence of copper. Although an easily measured absorbance change is observed for 100 µg/L copper even in the presence of 10 mg/L MDA, the change in sensitivity observed with MDA concentration complicates the quantification of copper in jet fuels containing unknown MDA concentrations. Real-world fuel samples may vary in their dielectric properties, and levels of absorbing chromophores (color bodies), MDA, copper, and other metals. Each of these parameters will ultimately influence the baseline absorbance of the NQSP indicator dye, affecting the quantitative capability of this dye. By removing all of the dissolved copper from a given jet fuel sample, we obtain a reference sample, which upon the addition of NQSP, will best represent the spectroscopic behavior of the indicator in this jet fuel in the absence of any metal contamination. This reference spectrum can then be utilized to correct each of the spectra recorded in a typical standard addition method. We have investigated the viability of this approach by removing the copper from jet fuel with powerful sequestering agents, such as EDTA or silica-bound diethylenetriamine (DETA-Si). The Navy is currently investigating the application of similar solid-phase extraction agents for use in a copper removal cartridge for jet fuel.13 It is noteworthy that several hours were
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necessary for the EDTA to remove all the available copper, while the DETA-Si worked on a much faster time scale. For a Jet A sample containing 220 µg/L copper, 2% (w/v) of DETA-Si enabled the complete extraction of all the copper in under two minutes, and the extraction of 70% of the dissolved copper in one minute. By doubling the amount of DETA-Si to 4% (w/ v), a complete extraction of the copper was complete in under a minute, demonstrating the strong adsorptivity of this extraction material. A 50 µg/L Cu(II) in jet fuel sample was prepared and quantitatively determined by standard addition after the background spectra for NQSP with 0.0 µg/L copper was established via the solid-phase extraction of copper with DETA-Si. After four trials, the calculated concentration was 46.9 µg/L with a standard deviation of 3.0 µg/L. Some jet fuels, depending upon their storage history and included additives, have displayed kinetic effects associated with the competition of NQSP with a small concentration of ligands dissolved in the jet fuel which compete for metal complexation of copper. These are effects which we are currently investigating and attempting to overcome. The detection limit for this method, based upon a signal-to-noise ratio of 3:1, is estimated to be 9.3 µg/L copper in the absence of MDA. In the presence of 2 mg/L MDA, we estimate a detection limit of 12.7 µg/L copper. The limit of quantification meets the originally defined goal of determining copper levels in jet fuel above 25 µg/L. Conclusions We have demonstrated the viability of two spirobenzopyran dyes for quantitating low µg/L levels of copper in jet fuel via spectrophotometry. NQSP appears to have the most promise with respect to the sensitive and selective detection of copper in the presence of other metal contaminants such as zinc and nickel. Chemometric algorithms are currently being developed to enable both single and binary component mixture identification. While MDA does compete with NQSP for metal binding of copper, standard addition methods and solid phase extraction methods appear to have successfully dealt with these obstacles. It is our goal to further develop the protocol for applying these indicator dyes in order to develop a field test kit for identification of potentially deleterious copper levels in jet fuel. Acknowledgment. The authors gratefully acknowledge the Defense Energy Support Center for funding support of this study. EF010271A
