Molecular fluorescence is typically reduced or "quenched" by dissolved oxygen present in solutions. Quenching mechanisms are broadly categorized as dynamic quenching, and static quenching. Dynamic quenching involves collisions between oxygen and the fluorophore. Static quenching occurs when the fluorophore forms a nonfluorescent complex with oxygen. In a previous study, oxygen quenching of naphthalene showed evidence of static quenching, although the mechanism was not known. The earlier work was limited to low oxygen concentrations (∼2 mM) in cyclohexane, leading to relatively large uncertainties in the value of the static quenching constant. In the present study, oxygen concentrations up to ∼50 mM were used, resulting in a more accurate determination of the static quenching constant. Absorption spectra obtained at higher oxygen concentrations revealed the spectrum of a new species, which we interpret as that of a naphthalene-oxygen charge-transfer complex. The binding energy of the naphthalene-oxygen complex was estimated and compared with that of the benzene-oxygen complex. In the method described herein, the equilibrium (or association) constant for the naphthalene-oxygen complex was assumed to be the static quenching constant, as determined from graphs of relative fluorescence intensities vs oxygen concentration using the Stern-Volmer formalism. The values of the complex molecular absorption coefficients were determined from graphs of absorbance vs oxygen concentration using the value of the static quenching constant from fluorescence data. This method differs from a commonly used method in which graphs of absorbance vs oxygen concentration are used to determine the product of the equilibrium constant and the complex absorption coefficient. The latter method can lead to substantial errors in both these quantities if the spectrum of the free molecule and complex overlap significantly.

A fluorescence quenching model using copper(II) ion (Cu2+) ion selective electrode (Cu-ISE) is developed. It uses parallel factor analysis (PARAFAC) to model fluorescence excitation–emission matrices (EEMs) of humic acid (HA) samples titrated with Cu2+ to resolve fluorescence response of fluorescent components to Cu2+ titration. Meanwhile, Cu-ISE is employed to monitor free Cu2+ concentration ([Cu]) at each titration step. The fluorescence response of each component is fit individually to a nonlinear function of [Cu] to find the Cu2+ conditional stability constant for that component. This approach differs from other fluorescence quenching models, including the most up-to-date multi-response model that has a problematic assumption on Cu2+ speciation, i.e., an assumption that total Cu2+ present in samples is a sum of [Cu] and those bound by fluorescent components without taking into consideration the contribution of non-fluorescent organic ligands and inorganic ligands to speciation of Cu2+. This paper employs the new approach to investigate Cu2+ binding by Pahokee peat HA (PPHA) at pH values of 6.0, 7.0, and 8.0 buffered by phosphate or without buffer. Two fluorescent components (C1 and C2) were identified by PARAFAC. For the new quenching model, the conditional stability constants (logK1 and logK2) of the two components all increased with increasing pH. In buffered solutions, the new quenching model reported logK1 = 7.11, 7.89, 8.04 for C1 and logK2 = 7.04, 7.64, 8.11 for C2 at pH 6.0, 7.0, and 8.0, respectively, nearly two log units higher than the results of the multi-response model. Without buffer, logK1 and logK2 decreased but were still high (>7) at pH 8.0 (logK1 = 7.54, logK2 = 7.95), and all the values were at least 0.5 log unit higher than those (4.83 ∼ 5.55) of the multi-response model. These observations indicate that the new quenching model is more intrinsically sensitive than the multi-response model in revealing strong fluorescent binding sites of PPHA in different experimental conditions. The new model was validated by testing it with a mixture of two fluorescing Cu2+ chelating organic compounds, i.e., l-tryptophan and salicylic acid mixed with one non-fluorescent binding compound oxalic acid titrated with Cu2+ at pH 5.0.

Over the past several decades, dissolved organic carbon (DOC) in inland natural water systems has been a popular research topic to a variety of scientific disciplines. Part of the attention has been due to observed changes in DOC concentrations in many of the water systems of the Northern Hemisphere. Shifts in DOC levels, and changes in its composition, are of concern due to its significance in aquatic ecosystem functioning and its potential and realized negative effects on waters that might be treated for drinking purposes. While it may not be possible to establish sound cause and effect relationships using a limited number of drivers, through long-term DOC monitoring studies and a variety of laboratory/field experiments, several explanations for increasing DOC trends have been proposed, including two key mechanisms: decreased atmospheric acid deposition and the increasing impact of climate change agents. The purpose of this review is three-fold: to outline frequently discussed conceptual mechanisms used to explain DOC increases (especially under a changing climate), to discuss the structure of DOC and the impact of higher levels of DOC on drinking water resources, and to provide renewed/sustained interest in DOC research that can encourage interdisciplinary collaboration. Understanding the cycling of carbon from terrestrial ecosystems into natural waters is necessary in the face of a variable and changing climate, as climate change-related mechanisms may become increasingly responsible for variations in the inputs of allochthonous DOC concentrations in water.

This study provides a detailed investigation of the fluorescence quenching mechanisms of the fluorophore, 1-aminoanthracene, by dissolved oxygen in cyclohexane. Dynamic/collisional quenching dominates in the system studied, but there is also a small component of static quenching. Stern-Volmer plots revealed that the dynamic quenching constant is 0.445 ± 0.014 mM(-1) and represents ∼95% of total quenching in the system. The static quenching rate constant is 0.024 ± 0.001 mM(-1), and mechanisms by complex formation and "sphere of action" static quenching were examined. Compensation of steady-state fluorescence data for solvent loss during the gradual deoxygenation period of the experiment was found to be important in order to conduct a thorough evaluation of the different quenching mechanisms of the system. The enhancement factors, (F(o)/F) and (τ(o)/τ), for 1-aminoanthracene were determined to be 2.20 ± 0.01 and 2.08 ± 0.01, respectively, and the diffusion-controlled bimolecular rate constant was found to be 2.1 × 10(10) ± 0.2 × 10(10) M(-1) s(-1). The work involved the development of a novel instrumental setup that simultaneously measures several important spectroscopic parameters (steady-state fluorescence intensity, absorbance, fluorescence lifetime, and dissolved oxygen concentration) for the careful study of oxygen quenching mechanisms of 1-aminoanthracene in a cyclohexane solution.

We report on trace gas and major atmospheric constituents results obtained by the Vehicle Cabin Atmosphere Monitor (VCAM) following almost two years of operation aboard the International Space Station (ISS). VCAM is an autonomous environmental monitor based on a highly compact gas chromatograph/quadrupole ion trap mass spectrometer. It was flown to the International Space Station (ISS) on shuttle mission STS-131 and commenced operations on June 2010. VCAM is capable of providing measurements of both parts-per-billion (ppb) levels of volatile trace-gas constituents, and of the atmospheric major constituents (nitrogen, oxygen, argon, and carbon dioxide) in a space vehicle or station. It is designed to operate autonomously and maintenance-free, approximately once per day, with a self-contained gas supply sufficient for a one-year lifetime. VCAM’s performance is sufficient to detect and identify 90% of the target compounds at their 180-day Spacecraft Maximum Allowable Concentration levels.

Indicators suggest that the amount of dissolved organic carbon (DOC) in natural waters may be increasing. Climate change has been proposed as a potential contributor to the trend, and under such a mechanism, the phenolic content of DOC may also be increasing. This study explores the assessment of the phenolic character of DOC using multidimensional fluorescence spectroscopy as a more convenient alternative to traditional wet chemistry methods. Parallel factor analysis (PARAFAC) is applied to fluorescence excitation emission matrices (EEMs) of humic samples to analyze inherent phenolic content. The PARAFAC results are correlated with phenol concentrations derived from the Folin-Ciocalteau reagent-based method. The reagent-based method reveals that the phenolic content of five International Humic Substance Society (IHSS) samples varies from approximately 5.2 to 22 ppm Tannic Acid Equivalents (TAE). A four-component PARAFAC fit is applied to the EEMs of the IHSS sample dataset and it is determined by PARAFAC score correlations with phenol concentrations from the reagent-based method that components C2, C3, and C4 have the highest probability of containing phenolic groups. The results show the potential for PARAFAC analysis of multidimensional fluorescence data for monitoring the phenolic content of DOC.

Parallel factor analysis with soft independent modeling by class analogy (PARAFAC-SIMCA) was used to analyze fluorescence data from shrimp extracts (organic and aqueous phases) to create classification schemes for two species of shrimp from four different countries. Twenty-four shrimp (six from each location: Ecuador, Philippines, Thailand, and United States) were studied; two were classified as statistical outliers. Using PARAFAC scores from the two aqueous fluorescent components and the strongest four components from the organic phase, country of origin was correctly identified at the 95% confidence level for all 22 remaining specimens; three false positives, at lower confidence levels than the true positives, were also indicated. A classification scheme which used all eight fluorescent components reproduced the 22 correct classifications and reduced the number of false positives to one. Finally, a scheme using PARAFAC scores from the two aqueous fluorescent components and the strongest four components from the organic phase, designed to classify according to species, produced 22 correct matches with no false positives. Spectral similarities between known chemical species and the components identified by PARAFAC are suggested for most cases. The results indicate that environmental effects appear in the fluorescence fingerprints of shrimp collected in different locations; therefore, fluorescence measurements on shrimp have the potential to permit geographical classification of shrimp or, conversely, to permit inferences to be made about the animal's environment.

One of the conventionally accepted requirements for parallel factor analysis (PARAFAC) to handle the fluorescence excitation emission matrices (EEMs) is the independence of each component's absorption and emission spectra in simple mixtures of fluorophores. EEMs of samples in which Förster resonance energy transfer (FRET) occurs between fluorophores seem to fail to meet this requirement. A rigorous theoretical treatment of the steady-state kinetics in the present work indicates that the fluorescence in the presence of FRET, excited by relatively weak excitation light intensity, can be reasonably separated into additive contributions from three parts: donors, acceptors and FRET. This prediction is for the first time verified experimentally in sodium dodecyl sulfate micellar solutions containing biphenyl as the energy donor and 2,5-diphenyloxazole as the energy acceptor. The experimental EEMs were well fitted to three components as predicted. A well accepted diagnostic test called core consistency (CC), specifically designed for modeling simple mixtures of fluorophores with PARAFAC, was found to be negative for the 3-component model in the present study. The simultaneous occurrence of good model fit and significantly negative CC when modeling fluorophore mixtures by conventional PARAFAC would be indicative of the presence of physical/chemical processes (e.g., FRET) that deviate from the conventional working requirements for PARAFAC. The extent of FRET has been independently measured or calculated by three methods: 1) decrease in steady state fluorescence of donor; 2) lifetime measurements with population analysis; and 3) Poisson statistics based on PARAFAC-determined distribution constants. The results of the three methods are consistent. The normalized scores of the three components found by PARAFAC also agree to within a few percent with relative concentrations in aqueous and micelle phases determined from distribution constants for the solutions prepared with nine different combinations of total donor and acceptor concentrations. Our theoretical treatment also for the first time spells out in detail the relationship between the PARAFAC scores and concentrations of components, in terms of photophysical constants of the components and spectral shape factors.

Steady state fluorescence excitation emission matrices (EEMs) and time-resolved fluorescence of biphenyl (BP) and 2,5-diphenyloxazole (PPO) dissolved in aqueous solutions of sodium dodecyl sulfate (SDS) were measured as a function of micelle concentration. Parallel factor analysis (PARAFAC) was applied for the first time to the determination of the distribution of solutes between the bulk aqueous phase and the SDS micellar phase, as well as their excitation and emission spectra in both phases. The raw scores of the PARAFAC analysis, which contain concentration information, are also dependent on photophysical parameters which can change upon micellization of the solute; we have developed a method of isolating accurate concentration values from the scores. The distribution constant of PPO, previously unreported, was found to be (8.8 +/- 0.7) x 10(5)M(-1), while the distribution constant of BP, for which various values have been reported, was found to be (4.5 +/- 0.2) x 10(4) M(-1). BP and PPO both showed weakly phase-dependent excitation and emission spectra. The method was validated by determining distribution constants for anthracene, phenanthrene, naphthalene, and pyrene, all of which showed good agreement with consensus values available in the literature.

ADVERTISEMENT RETURN TO ISSUEPREVBook ReviewStandardization and Quality Assurance in Fluorescence Measurements I: TechniquesJonathan E. KennyView Author Information Tufts UniversityCite this: J. Am. Chem. Soc. 2009, 131, 10, 3789–3790Publication Date (Web):February 17, 2009Publication History Published online17 February 2009Published inissue 18 March 2009https://doi.org/10.1021/ja901039qCopyright © 2009 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1088Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (67 KB) Get e-AlertsSUBJECTS:Colorimetry,Drinking water,Fluorescence,Quality assurance,Sensors Get e-Alerts

The fluorescence lifetimes and intensities of naphthalene, phenanthrene, chrysene, and pyrene were measured as a function of oxygen concentration in cyclohexane solution. Stern-Volmer plots of intensities and lifetimes versus oxygen concentrations revealed smaller Stern-Volmer quenching coefficients for lifetimes than for intensities. Analysis of the data in terms of static quenching models suggests that the Stern-Volmer slope differences result from a charge-transfer interaction between the aromatic and oxygen molecules in the cases of naphthalene and pyrene. A method of estimating dissolved oxygen concentration using the cyclohexane-oxygen charge-transfer band is also reported.

The most widely used correction of fluorescence intensities for inner filter effects in conventional (90 degrees ) fluorimeters fails at high absorbance values. We have critically examined this failure, which is caused by the difference between the geometrical parameters (GPs) of the excitation and emission beams in the typical instrument (focused beams) and in the theoretical picture on which the correction is based (collimated beams). We provide two types of experimental measurement of GPs and show that their substitution in the correction equations leads to significant improvements in the linear range of corrected fluorescence. We also demonstrate that mathematical optimizations give greater improvements and that the optimizations yield GPs consistent with experimental measurements. For solutions exhibiting primary inner filter effect only, we have extended the range of linearity of corrected fluorescence to a(ex) (absorbance per cm) up to 5.3; for systems with both primary and secondary inner filter effects we have achieved linearity for a(ex) + a(em) = 6.7. In all cases linear fits have slopes which agree well with the dilute limit. Different series of one- and two-solute solutions were used to demonstrate effectiveness of our correction methods. We also provide a rationale for the unexpected independence of GPs on excitation and emission bandwidths.