Because of its intrinsic dependence on a large number of parameters (excitation wavelength, emission wavelength, polarization, concentration, decay time, sample location, etc.), fluorescence intensity is multidimensional and its measurement has become an important analytical tool in environmental chemistry. In addition, fluorescence measurements may often be performed in situ, offering additional benefits of minimal or no sample preparation, generation of chemical waste, or exposure of workers to hazardous substances. In recent years, the excitation-emission matrix, or EEM, a matrix of fluorescence intensities which can also be plotted as a three-dimensional “fingerprint” of a sample, has gained favor over individual emission spectra or synchro-scanned excitation-emission spectra because of its high information content.
Extraction of the information in EEMs has been hampered until recently by the possible contributions to the signal from substances not in the investigator’s spectral library. That limitation has been transcended, at least apparently, by PARAFAC, an acronym for PARAllel FACtor analysis, a form of multi-way analysis recently introduced into the chemical community. The rotational ambiguity in decomposing spectral data using conventional chemometric tools such as principal component analysis is removed by the additional dimension in the data set. In the case of EEMs, which may be viewed as two-dimensional rectangular arrays of intensities, this may be achieved simply by stacking a set of individual EEMs along an axis to form a three-dimensional “data cube”. This third axis may corresponding to sample number; it may also represent a more meaningful dimension such as sample date, pH, etc. (Additional axes may be added to permit the use of PARAFAC and related multi-way analysis methods.) Subject to certain limitations, PARAFAC decomposes a data cube (or hypercube, in the case of additional axes) into a set of component spectra (excitation and emission) along with a set of scores indicating relative concentration of each component in each sample. The intensity at the jth excitation wavelength and kth emission wavelength of the ith sample is modeled as a sum of contributions from R fluorophores:
where air represents the score (relative concentration) of fluorophore r in sample i, br and cr are the excitation and emission vectors of fluorophore r, and the eijk are the unfitted residuals. The conditions required by eq. 1, namely, that the fluorescence contribution of each component at each excitation emission wavelength pair be trilinear, i.e., a product of an extinction coefficient, a fluorescence quantum yield, and the concentration of the component, are met by solutions of sufficient diluteness, with no interactions among the fluorophores.
PAH solutions in nonpolar solvents are susceptible to dynamic quenching of fluorescence by dissolved oxygen. This form of quenching can result in a decrease in the PAH’s quantum yield. The mechanism for dynamic quenching involves the deexcitation of a fluorophore, without emission of photons, via collisions with molecular oxygen (O2) during the fluorophore’s excited-state lifetime. The ratio of the fluorescence measured in the absence of oxygen, F0, to the fluorescence measured in the presence of oxygen, F, is given by the following Stern-Volmer relationship:
where τo is the lifetime of the fluorophore measured in an oxygen-free solution, kO2 is the oxygen quenching rate constant, and [O2] is the solution’s oxygen concentration. Typically fluorophores with longer fluorescence lifetimes will generate larger F0/F values. Solutions can be deoxygenated in order to maximize the fluorescence intensity and improve quantitative analysis.
The largest reservoir in the global carbon cycle is represented by the humic substances, the partially decomposed organic matter from dead plants and animals. These complex materials are a key factor in determining soil fertility and metal bioavailability. When present in natural waters as DOM, they can affect drinking water quality, both by fouling membranes used in water treatment and by providing the chemical precursors that become toxic disinfection byproducts such as trihalomethanes (THMs). There is evidence that both the amount and nature of DOM are likely changing as a result of global climate change. One change that has been reported under controlled conditions of increased temperature and CO2 levels is an increase in the phenolic content of DOM, which has been implicated as one of the most active DOM components leading to THMs and other toxic byproducts.
“Multidimensional Fluorescence Studies of the Phenolic Content of Dissolved Organic Carbon in Humic Substances,” Todd Pagano, Annemarie D. Ross, Joseph Chiarelli, and Jonathan E. Kenny, J. Environ. Monit., 2012, 14 (3), 937-943.
"Study of pH Effects on Humic Substances using Chemometric Analysis of Excitation-Emission Matrices," H. Chen and J. E. Kenny, Annals of Env. Science 1, 1 (2007).
"Application of PARAFAC to determination of distribution constants and spectra of fluorescent solutes in micellar solutions," Hao Chen and Jonathan E. Kenny, Analyst, 2010, 135, 1704–1710.
"Estuarine water classification using EEM spectroscopy and PARAFAC-SIMCA," G.J. Hall and J.E. Kenny, Analytica Chimica Acta 581 (2007), 118-124.
"Estuarial Fingerprinting through Multidimensional Fluorescence and Multivariate Analysis," G.J. Hall, K.E. Clow, and J.E. Kenny, Environ. Sci. Technol., 39 (19), 7560 -7567, 2005.
"Spectral fingerprinting and classification by location of origin of natural waters by multidimensional fluorescence," Clow, K.E.; Hall, G.J.; Chen, H.; Kenny, J.E. Proc. SPIE (2004), 5586 (Advanced Environmental, Chemical, and Biological Sensing Technologies), 107-115.
"Application of PARAFAC to a two-component system exhibiting Fluorescence Resonance Energy Transfer: from theoretical prediction to experimental validation," Hao Chen and Jonathan E.Kenny, Analyst, 2012, 137, 153-162.
Evidence from fluorescence quenching experiments suggests that naphthalene and oxygen form a weakly bound complex. Earlier work in this laboratory lead to the interpretation that this complex is a “contact charge-transfer” (CCT) complex, one in which an electron from naphthalene is transferred to a nearby oxygen molecule. Spectral evidence for CCT complexes is often observed in the absorption spectra of pure liquids such as hexane or benzene having dissolved oxygen; the CCT absorption band disappears when oxygen is removed. More recent studies of naphthalene absorption and fluorescence in solvents with higher dissolved oxygen concentrations favor the interpretation that the complex is more likely a van der Waals complex bound by dispersion forces.
An understanding of oxygen interactions with aromatic compounds is important for several reasons. Oxygen quenches the fluorescence of most polynuclear aromatic hydrocarbons. Polynuclear aromatic hydrocarbons (PAH) are by-products of petroleum combustion and common environmental pollutants that can be analyzed by fluorescence methods. By removing dissolved oxygen from solution, PAH concentrations can be determined with higher sensitivity.
"Fluorescence intensities and lifetimes of aromatic hydrocarbons in cyclohexane solution purged with nitrogen," J. Thomas Brownrigg and Jonathan E. Kenny, J. Phys. Chem. A, 2009,113 (6), 1049-1059.
"Pushing Pyrazine to the (Statistical) Limit," W.R. Moomaw, T.-Y. Liu and J.E. Kenny, J. Phys. Chem., 99, 7320 (1995).
"Improvement of Inner Filter Effect Correction Based on Determination of Effective Geometric Parameters Using a Conventional Fluorimeter," Qun Gu and Jonathan E. Kenny,Anal. Chem., 2009, 81 (1), 420-426.
"Nitrogen Gas Purging for the Deoxygenation of Polyaromatic Hydrocarbon Solutions in Cyclohexane for Routine Fluorescence Analysis," T. Pagano, A. J. Biacchi, and J.E. Kenny, Appl. Spectrosc. 62, 333-336 (2008).
"A laser induced fluorescence dual fiber optic array detector applied to rapid HPLC separation of polycyclic aromatic hydrocarbons," S. J. Hart, G. J. Hall and J. E. Kenny, Anal. Bioanal. Chem. (2002) 372: 205-215.
"In Situ Measurements of Subsurface Contaminants with a Multi-channel Laser-Induced Fluorescence System," J. W. Pepper, A. O. Wright, and J. E. Kenny, Spectrochimica Acta A58, 2002, 317.
"Speciation of Aromatic Compounds with Excitation-Emission Matrix Measurements," Pepper, Jane W.; Chen, Y.-M,; Wright, Andrew O.; and Kenny, Jonathan E., Proceedings of SPIE. 1999, 3856, 252-260
"Assessment of Inner Filter Effects in Fluorescence Spectroscopy using the Dual- Pathlength Method– a Study of Jet Fuel JP-4", Pagano, Todd E. and Kenny, Jonathan E., Proceedings of SPIE. 1999, 3856, 289-297.
"Two-Fiber Spectroscopic Probe with Improved Scattered Light Rejection," Andrew O. Wright, Jane W. Pepper, and Jonathan E. Kenny, Analytical Chemistry 71, 2582-2585 (1999).
"Subsurface Contaminant Monitoring by Laser Excitation-Emission Matrix/Cone Penetrometer," J. Pepper, Y.-M. Chen, A. Wright,R. Premasiri, J.E. Kenny. Proc. SPIE 3534, pp. 234-242 (1998).
"A Fiber Optic Laser Induced Fluorescence Excitation Emission Detector Applied to Flow Injection Analysis of PAHs," S. J. Hart, G. J. Hall, and J. E. Kenny, Proc., SPIE 3534, pp. 601-611 (1998).
"Laser-Induced Fluorescence and Fast Gas Chromatography/Mass Spectrometry with Subsurface Thermal Extraction of Organics: Field Analytical Technologies for Expediting Site Characterization and Cleanup," A. Robbat Jr., J. E. Kenny, S. Smarason, J.W. Pepper, and A. O. Wright, Remediation Winter 1998, 95-111
"Field Demonstration of a Multichannel Fiber Optic Laser Induced Fluorescence System in a Cone Penetrometer Vehicle," S. J. Hart, Y.- M. Chen, J. E. Kenny, B. K. Lien and T. W. Best, Field Analytical Chemistry and Technology, 1, 343 (1997).
"Improved Two-Fiber Probe for In Situ Spectroscopic Measurements," J. Lin, S. J. Hart, and J.E. Kenny, Analytical Chemistry 68, 3098 (1996).
"A Fiber Optic Multichannel Laser Spectrometer System for Remote Fluorescence Detection in Soils," S.J. Hart, Y.-M. Chen,B.K. Lien, and J.E. Kenny, Proc. SPIE 2835, 73 (1996).
"Subsurface Contaminant Monitoring by Laser Fluorescence Excitation-Emission Spectroscopy in a Cone Penetrometer Probe," J.Lin, S.J. Hart, W. Wang, D. Namychkin and J.E. Kenny, Proc. SPIE 2504, 59 (1995).
"Spectroscopy in the Field: Emerging Techniques for On-Site Environmental Measurements," A.Henderson-Kinney and J.E. Kenny, Spectroscopy, 10, No. 7, p. 32 (1995).
"Laser Fluorescence EEM Probe for Cone Penetrometer Pollution Analysis," J. Lin, S. J. Hart, T. A. Taylor and J. E. Kenny,Proc. SPIE 2367, 70 (1994).
"Evaluation of Nd:YAG-Pumped Raman Shifter as a Broad-Spectrum Light Source," G.B. Jarvis, S. Mathew and J.E. Kenny, Appl. Opt. 33, 4938 (1994).
The PI is a member of the faculty advisory board for the Green Chemistry Commitment (GCC) program of Beyond Benign, a non-profit organization which promotes Green Chemistry. The board is working on a statement of commitment to green chemistry learning goals; departments of chemistry and members of higher administration of colleges and universities will be invited to commit to adoption and implementation of these goals. The program will be rolled out in late 2012.
"Reassignment of the vibrational spectra of CHF2CH3 (HFC-152a), CF3CH3 (HFC-143a), CF3CHF2 (HFC-125), and CHCl2CF3 (HCFC 123)," S. Tai, S. Papasavva, J. E. Kenny, B. D. Gilbert, J. A. Janni, J. I. Steinfeld, J.D. Taylor and R.D. Weinstein, Spectrochimica Acta 55A, pp. 9-24 (1999).
"Infrared Radiative Forcing of CFC-Replacements and their Atmospheric Reaction Products," S. Papasavva, S. Tai, K.H. Illinger, and J.E. Kenny, J. Geophys. Research, 102, 13643 (1997).
"Molecular Properties of CFC Substitutes from Ab Initio Calculations: CFCl2CH3, CF2ClCH3, CHCl2CF3, and CHFClCF3," S. Papasavva, K.H. Illinger, and J.E. Kenny, J. Mol. Structure: THEOCHEM, 393, 73 (1997).
"Ab Initio Calculations on Fluoroethanes: Geometries, Dipole Moments, Vibrational Frequencies and Infrared Intensities," S. Papasavva, K.H. Illinger and J.E. Kenny, J. Phys. Chem., 100, 10100 (1996).
"Ab Initio Calculations of Vibrational Frequencies and Infrared Intensities for Global Warming Potential of CFC Substitutes: CH2FCF3 (HFC-134a)," S. Papasavva, S. Tai, A. Esslinger, K.H. Illinger, and J.E. Kenny, J. Phys. Chem., 99, 3438 (1995).
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