Auréa Andrade-Eiroa

Environmental Applications of Excitation-Emission Spectrofluorimetry: An In-Depth Review I

Abstract: This review attempts to cover the principles and environmental applications of excitation-emission spectrofluorimetry (EES). The article is divided into two parts and each part is divided into several sections. The first part includes the following: Introduction, advantages and drawbacks of EES, comparative EES versus other techniques, factors influencing EES signals, representation of EES spectra, relationships and changes in intensity of fluorescence, multivariate calibration combined with EES, quenching of fluorescence, and conclusions. Theoretical and practical considerations are included, and the possibilities and limitations are evaluated. The second part is devoted to the environmental applications of EES: Characterization of chromophoric dissolved organic matter (CDOM) in drinking waters, rivers, fog water, lakes, oceans, leachates, wastewaters, sludge, bioreactor membrane foulants and soils; characterization of extracellular polymeric substances in sluges; study of interactions between CDOM and organic pollutants in soils; and quantification of organic pollutants in waters and soils. This part includes the pretreatment of samples, and the environmental applications of the technique are discussed, including its application to biorremediation of wastewaters. More than 500 references (focusing mainly on the last 10 years) from all kinds of journals (environment, analytical chemistry, biotechnology) have been critically reviewed and included in this article.

Improved optimization of polycyclic aromatic hydrocarbons (PAHs) mixtures resolution in reversed-phase high-performance liquid chromatography by using factorial design and response surface methodology

A new procedure for optimizing PAHs separation in very complex mixtures by reverse phase high performance (RPLC) is proposed. It is based on changing gradually the experimental conditions all along the chromatographic procedure as a function of the physical properties of the compounds eluted. The temperature and speed flow gradients allowed obtaining the optimum resolution in large chromatographic determinations where PAHs with very different medium polarizability have to be separated. Whereas optimization procedures of RPLC methodologies had always been accomplished regardless of the physico-chemical properties of the target analytes, we found that resolution is highly dependent on the physico-chemical properties of the target analytes. Based on resolution criterion, optimization process for a 16 EPA PAHs mixture was performed on three sets of difficult-to-separate PAHs pairs: acenaphthene-fluorene (for the optimization procedure in the first part of the chromatogram where light PAHs elute), benzo[g,h,i]perylene-dibenzo[a,h]anthracene and benzo[g,h,i]perylene-indeno[1,2,3-cd]pyrene (for the optimization procedure of the second part of the chromatogram where the heavier PAHs elute). Two-level full factorial designs were applied to detect interactions among variables to be optimized: speed flow, temperature of column oven and mobile-phase gradient in the two parts of the studied chromatogram. Experimental data were fitted by multivariate nonlinear regression models and optimum values of speed flow and temperature were obtained through mathematical analysis of the constructed models. An HPLC system equipped with a reversed phase 5 microm C18, 250 mm x 4.6mm column (with acetonitrile/water mobile phase), a column oven, a binary pump, a photodiode array detector (PDA), and a fluorimetric detector were used in this work. Optimum resolution was achieved operating at 1.0 mL/min in the first part of the chromatogram (until 45 min) and 0.5 mL/min in the second one (from 45 min to the end) and by applying programmed temperature gradient (15 degrees C until 30 min and progressively increasing temperature until reaching 40 degrees C at 45 min).

Determination of Polycyclic Aromatic Hydrocarbons in kerosene and bio-kerosene soot

Here we report a new, efficient and reliable analytical methodology for sensitive and selective quantification of Polycyclic Aromatic Hydrocarbons (PAHs) in soot samples. The methodology developed is based on ultrasonic extraction of the soot-bound PAHs into small volumes of acetonitrile, purification of the extracts through C(18) Solid Phase Extraction (SPE) cartridges and analysis by Reverse Phase Liquid Chromatography (RPLC) with UV and fluorimetric detection. For the first time, we report the convenience of adapting the SPE procedure to the nature of the soot samples. As a matter of fact, extracts containing high percentage of unpolar material are recommended to be cleaned with acetone, whereas extracts poor in unpolar compounds can be efficiently cleaned with methanol. The method was satisfactorily applied to kerosene and bio-kerosene soot from atmospheric open diffusion flames (pool fires) and premixed flames achieving Quantification and Detection limits in the range ng mg(-1) soot and recoveries about 90% for most of the PAHs studied.

An alternative to trial and error methodology in solid phase extraction: an original automated solid phase extraction procedure for analysing PAHs and PAH-derivatives in soot

This paper introduces the generalization of reverse-phase HPLC fundamentals to normal-phase liquid chromatography and Automated Solid Phase Extraction (A-SPE). This paper upholds that the same fundamentals and principles of reverse-phase HPLC are also applicable in normal phase HPLC and A-SPE. Based on these fundamentals, we could overlook the trial and error method and accomplish a rational and fast selection of the most suitable SPE procedure for analyzing aromatic hydrocarbons and polar-aromatic-hydrocarbons in soot from methyloctanoate–kerosene blends. These fundamentals and procedure could be applied to any sample. Application to soot samples was carried out for demonstrating the high efficiency of the procedure to very complex matrices. The analytical methodology introduced here consists of: (i) creating a solution of the soot sample in hexane, (ii) extraction of the target analytes by filtration twice through PTFE filters, (iii) A-SPE fractioning through a silica column by using an HPLC pump and hexane at a flow rate of 0.05 ml min−1 as the mobile phase, and (iv) analysis of the fractions collected by reverse-phase HPLC with a photodiode array detector (PDA). The methodology developed was successfully applied to the identification of 56 PAHs and 44 PAH-derivatives (belonging to 14 different chemical classes) in soot from methyloctanoate–kerosene blends in a reproducible, simple, and environmental friendly way. The aforementioned procedure can be implemented in any lab with an HPLC system.

Photodegradation of pyrene on Al2O3 surfaces: a detailed kinetic and product study.

In the current study, the photochemistry of pyrene on solid Al2O3 surface was studied under simulated atmospheric conditions (pressure, 1 atm; temperature, 293 K; photon flux, JNO2 = 0.002-0.012 s(-1)). Experiments were performed using synthetic air or N2 as bath gas to evaluate the impact of O2 to the reaction system. The rate of pyrene photodegradation followed first order kinetics and was enhanced in the presence of O2, kd(synthetic air) = 7.8 ± 0.78 × 10(-2) h(-1) and kd(N2) = 1.2 ± 0.12 × 10(-2) h(-1) respectively, due to the formation of the highly reactive O2(•-) and HO(•) radical species. In addition, kd was found to increase linearly with photon flux. A detailed product study was realized and for the first time the gas/solid phase products of pyrene oxidation were identified using off-line GC-MS and HPLC analysis. In the gas phase, acetone, benzene, and various benzene-ring compounds were determined. In the solid phase, more than 20 photoproducts were identified and their kinetics was followed. Simulation of the concentration profiles of 1- and 2-hydroxypyrene provided an estimation of their yields, 33% and 5.8%, respectively, with respect to consumed pyrene, and their degradation rates were extracted. Finally, the mechanism of heterogeneous photodegradation of pyrene is discussed.

Mineral oxides change the atmospheric reactivity of soot: NO2 uptake under dark and UV irradiation conditions.

The heterogeneous reactions between trace gases and aerosol surfaces have been widely studied over the past decades, revealing the crucial role of these reactions in atmospheric chemistry. However, existing knowledge on the reactivity of mixed aerosols is limited, even though they have been observed in field measurements. In the current study, the heterogeneous interaction of NO2 with solid surfaces of Al2O3 covered with kerosene soot was investigated under dark conditions and in the presence of UV light. Experiments were performed at 293 K using a low-pressure flow-tube reactor coupled with a quadrupole mass spectrometer. The steady-state uptake coefficient, γ(ss), and the distribution of the gas-phase products were determined as functions of the Al2O3 mass; soot mass; NO2 concentration, varied in the range of (0.2-10) × 10(12) molecules cm(-3); photon flux; and relative humidity, ranging from 0.0032% to 32%. On Al2O3/soot surfaces, the reaction rate was substantially increased, and the formation of HONO was favored compared with that on individual pure soot and pure Al2O3 surfaces. Uptake of NO2 was enhanced in the presence of H2O under both dark and UV irradiation conditions, and the following empirical expressions were obtained: γ(ss,BET,dark) = (7.3 ± 0.9) × 10(-7) + (3.2 ± 0.5) × 10(-8) × RH and γ(ss,BET,UV) = (1.4 ± 0.2) × 10(-6) + (4.0 ± 0.9) × 10(-8) × RH. Specific experiments, with solid sample preheating and doping with polycyclic aromatic hydrocarbons (PAHs), showed that UV-absorbing organic compounds significantly affect the chemical reactivity of the mixed mineral/soot surfaces. A mechanistic scheme is proposed, in which Al2O3 can either collect electrons, initiating a sequence of redox reactions, or prevent the charge-recombination process, extending the lifetime of the excited state and enhancing the reactivity of the organics. Finally, the atmospheric implications of the observed results are briefly discussed.

Investigation of the photochemical reactivity of soot particles derived from biofuels toward NO2. A kinetic and product study.

In the current study, the heterogeneous reaction of NO2 with soot and biosoot surfaces was investigated in the dark and under illumination relevant to atmospheric conditions (J(NO2) = 0.012 s(-1)). A flat-flame burner was used for preparation and collection of soot samples from premixed flames of liquid fuels. The biofuels were prepared by mixing 20% v/v of (i) 1-butanol (CH3(CH2)3OH), (ii) methyl octanoate (CH3(CH2)6COOCH3), (iii) anhydrous diethyl carbonate (C2H5O)2CO and (iv) 2,5 dimethyl furan (CH3)2C4H2O additive compounds in conventional kerosene fuel (JetA-1). Experiments were performed at 293 K using a low-pressure flow tube reactor (P = 9 Torr) coupled to a quadrupole mass spectrometer. The initial and steady-state uptake coefficients, γ0 and γ(ss), respectively, as well as the surface coverage, N(s), were measured under dry and humid conditions. Furthermore, the branching ratios of the gas-phase products NO (∼80-100%) and HONO (<20%) were determined. Soot from JetA-1/2,5-dimethyl furan was the most reactive [γ0 = (29.1 ± 5.8) × 10(-6), γ(ss)(dry) = (9.09 ± 1.82) × 10(-7) and γ(ss)(5.5%RH) = (14.0 ± 2.8)(-7)] while soot from JetA-1/1-butanol [γ0 = (2.72 ± 0.544) × 10(-6), γ(ss)(dry) = (4.57 ± 0.914) × 10(-7), and γ(ss)(5.5%RH) = (3.64 ± 0.728) × 10(-7)] and JetA-1/diethyl carbonate [γ0 = (2.99 ± 0.598) × 10(-6), γ(ss)(dry) = (3.99 ± 0.798) × 10(-7), and γ(ss)(5.5%RH) = (4.80 ± 0.960) × 10(-7)] were less reactive. To correlate the chemical reactivity with the physicochemical properties of the soot samples, their chemical composition was analyzed employing Raman spectroscopy, NMR, and high-performance liquid chromatography. In addition, the Brunauer-Emmett-Teller adsorption isotherms and the particle size distributions were determined employing a Quantachrome Nova 2200e gas sorption analyzer. The analysis of the results showed that factors such as (i) soot mass collection rate, (ii) porosity of the particles formed, (iii) aromatic fraction, and (iv) pre-existence of nitro-containing species in soot samples (formed during the combustion process) can be used as indicators of soot reactivity with NO2.

Advances in PAHs/nitro-PAHs fractioning

The aim of this work was to develop an efficient methodology for the reliable fractioning of nitrated-polycyclic aromatic hydrocarbons (nitro-PAHs) and polycyclic aromatic hydrocarbons (PAHs). Unlike what usually occurs under pressures developed by HPLC (high performance liquid chromatography) systems (above 11 bar) we observed that when normal phase chromatographic fractioning procedures are accomplished under very low pressures (about 1 bar), dipole molecules (nitro-PAHs) elute much faster than non-polar organic molecules (PAHs). This finding allowed developing an original and very efficient methodology for fractioning nitro-PAHs and PAHs. This method is based on normal-phase liquid chromatography through a home-made phenyl column by using hexane as mobile phase at very low speed flow (0.05 ml min−1). Unlike typical HPLC methodology, the fractioning of nitro-PAHs and PAHs was accomplished as a function of their polarity (first the polar compounds as a unique peak and further, the non-polar compounds, PAHs) rather than as a function of their medium polarizability.

Spectrofluorimetric method for monitoring fluorene in rivers

This paper introduces an accurate, sensitive, easy and rapid method for monitoring fluorene in river waters. Compared to other analytical methodologies, synchronous spectrofluorimetry offers rapidity, low detection limits, and high recoveries as we show in the results section. Constant-wavelength spectrofluorimetric signals of fluorene undergoes an inner-filter effect in the presence of other Polycyclic Aromatic Hydrocarbons (PAHs) such as IP which makes linear calibration models not valid for the quantification of fluorene. However, alternative mathematical models facilitate the quantification of fluorene in the presence of IP. The mathematical model here introduced includes a term of interaction between the two compounds aforementioned and allowed the accurate quantification of fluorene in waters. In fact the application of the methodology proposed to real samples (tap and river waters, including a highly polluted urban river) provided us with excellent recovery percentages (around 95% in all the cases). An exhaustive comparative between our methodology and other methodologies reported in the recent literature was also carried out. This comparative showed that synchronous fluorimetry provides highly satisfactory results and in most of the cases lower detection limits and higher recoveries than other time- and solvent-consuming methodologies.