Excitation-emission matrix spectra

Figure 4.5. Fluorescence excitation-emission matrix spectra of humic acids (HAs) isolated from sewage sludge (B) and two soils either unamended (TH1 and TH2, respectively) or amended with 3901 ha-1 of sewage sludge (THB1 and THB2, respectively) sampled from the surface (s, 0-25cm) and subsurface (ss, 25-50cm) layers. Reprinted from Bertoncini, E. I., D Orazio, V., Senesi, N., and Mattiazzo, M. E. (2005). Fluorescence analysis of humic and fulvic acids from two Brazilian oxisols as affected by biosolid amendment. Anal. Bioanal. Chem. 381,1281-1288, with permission from Springer.

Figure 16.41. Fluorescence excitation-emission matrix spectra of humic acids (HA) isolated from municipal solid waste compost (MSWC), from soil amended with MSWC at 40tha 1yr 1 (MSWC40), and from the corresponding unamended control soil (MSWC0), in the absence and presence of Cu2+, Zn2+, Cd2+, and Pb2+ ions at a total concentration of 40 xmol liter-1. EEWPmax denotes the excitation/emission wavelength pairs at maximum fluorescence intensity (Plaza et al., 2006).

The excitation-emission matrix spectra (EEMS) of CDOM fluorescence also are altered on exposure to solar radiation, generally with reductions in fluorescence that approximately parallel absorption losses [36-38]. Hypsochromic shifts (shifts to shorter wavelengths) occur in both excitation and emission maxima on irradiation [36-38]. Interactions between photochemical and microbial degradation [38] are involved. 

DVM, diel vertical migration E, downwelling irradiance EEMS, excitation-emission matrix spectra ENSO, El Nino-Southern Oscillation ph, euphotic zone (1% of surface PAR)... 

Room-temperature fluorescence (RTF) has been used to determine the emission characteristics of a wide variety of materials relative to the wavelengths of several Fraunhofer lines. Fraunhofer lines are bands of reduced intensity in the solar spectrum caused by the selective absorption of light by gaseous elements in the solar atmosphere. RTF studies have recently included the search for the causes of the luminescence of materials and a compilation of information that will lead to "luminescence signatures" for these materials. For this purpose, excitation-emission matrix (EEM) data are now being collected. 

More fluorescence features than just the emission intensity can be used to develop luminescent optosensors with enhanced selectivity and longer operational lifetime. The wavelength dependence of the luminescence (emission spectmm) and of the luminophore absorption (excitation spectrum) is a source of specificity. For instance, the excitation-emission matrix has shown to be a powerful tool to analyze complex mixtures of fluorescent species and fiber-optic devices for in-situ measurements (e.g. 

Molecular fluorescence spectroscopy is a commonly employed analytical method that is sensitive to certain chemical properties of FA (9-13). Fulvic acid s molecular fluorescence is principally due to conjugated unsaturated segments and aromatic moieties present in the macromolecule (14). Several types of fluorescence spectra can be measured, including an excitation emission matrix or total luminescence spectrum, constant offset synchronous fluorescence, excitation spectra, and emission spectra, furnishing the researcher with useful data. The ability to resolve and select multiple fluorescent species makes these approaches extremely useful for studying FA relative to its chemical reactivity. 

Another type of luminescence spectrum is shown in Figure I5 h. The total luminescence spectrum is cither a three-dimensional representation or a contour plot. Both simultaneously show the luminescence signal as a function of excitation and emission wavelengths. Such data aie often called an excitation-emission matrix. Although the total luniinescence spectrum can be obtained on u normal coni])U(cri/ed instrument, it can be acquired more rapidly with array-detector-based systems (see next seel ion). 

There are several other chemometric approaches to calibration transfer that will only be mentioned in passing here. An approach based on finite impulse response (FIR) filters, which does not require the analysis of standardization samples on any of the analyzers, has been shown to provide good results in several different applications.81 Furthermore, the effectiveness of three-way chemometric modeling methods for calibration transfer has been recently discussed.82 Three-way methods refer to those methods that apply to A -data that must be expressed as a third-order data array, rather than a matrix. Such data include excitation/emission fluorescence data (where the three orders are excitation wavelength, emission wavelength, and fluorescence intensity) and GC/MS data (where the three orders are retention time, mass/charge ratio, and mass spectrum intensity). It is important to note, however, that a series of spectral data that are continuously obtained on a process can be constructed as a third-order array, where the three orders are wavelength, intensity, and time. 

Up to this point, regression has been restricted to two blocks of two-way data Y and X. In chemical analysis, however, a growing number of problems can be cast as three-way regression analysis. Consider the calibration of chemical constituents on the basis of their fluorescence excitation/emission spectrum or of gas chromatography/mass spectrometry (GC/MS) data. For each sample, two-dimensional measurements are available that constitute a three-way data array, X. This data array has to be related to sample concentrations of one, vector y, or several analytes, matrix Y. Cases can be imagined where even the matrix Y constitutes a three-way data array. 

FRET is a nonradiative process that is, the transfer takes place without the emission or absorption of a photon. And yet, the transition dipoles, which are central to the mechanism by which the ground and excited states are coupled, are conspicuously present in the expression for the rate of transfer. For instance, the fluorescence quantum yield and fluorescence spectrum of the donor and the absorption spectrum of the acceptor are part of the overlap integral in the Forster rate expression, Eq. (1.2). These spectroscopic transitions are usually associated with the emission and absorption of a photon. These dipole matrix elements in the quantum mechanical expression for the rate of FRET are the same matrix elements as found for the interaction of a propagating EM field with the chromophores. However, the origin of the EM perturbation driving the energy transfer and the spectroscopic transitions are quite different. The source of this interaction term... 

Spectrofluorometry underpins all luminescent techniques for characterizing the target analyte and the associated sample matrix. LIF occurs from the vacuum UV through the NIR range, but is most common between 250 nm and 800 nm. A typical fluorescent profile contains an absorption spectrum and an emission spectrum as depicted in Figure 11.3. Spectrofluorometric excitation profiling at emission maxima and syn-chronons scanning are common methods to characterize a fluorophore. 

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

Fig. 3. Emission spectra of Pd(2-thpy)2 (a) in an n-octane Shpol skii matrix (line spectrum) and (b) in butyronitrile (broad band spectrum) at T = 1.3 K, Aexc = 337.1 nm (N2-Laser). The energies of the vibrational satellites are specified relative to the electronic origin at 18,418 cm f The structures marked by asterisks on the high energy side of the electronic origin result from other sites and vanish with a site-selective excitation, e.g. at 19,113 cm (18,418 cm i -1-695 cm vibrational satellite). Concentration of Pd(2-thpy)2 10 mol/1. Note Fora better comparison, the broad band spectrum is shifted by 200 cm to lower energy. (Compare Ref. [56])...

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