Organic compounds fluorescence determination

Figure 9.3 Fluorescent character in organic compounds is often determined by the presence of a planar aromatic ring system. The fluorescent compounds differ only in the closure of their central ring system, which produces the required constraints to create a planar triple-ring configuration.

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

Recent developments in the determination of elements in this group have been very much linked to the use of atomic fluorescence detection systems rather than AAS (see section 8.7). ICP-AES and ICP-MS can also be used but they are generally inferior in sensitivity. Best sensitivity is obtained from AFS detection. It should also be noted that the analysis may also be required to detect and measure organic compounds of these elements because of the toxicity in the organic form. Separation by one of the methods reviewed in Chapter 4 may thus be used in sample processing prior to analysis. 

Waggot and Britcher [38] have discussed experimental considerations in the determination of organic carbon content of sewage effluent. Close attention is paid to the determination of particular classes of organic compounds in sewage including carbohydrates, amino acids, volatiles, steroids, phenols, surface active materials, fluorescent materials, organochlorine pesticides and ethylene diamine tetracetic acid. 

The S2 state fluorescence of metalloporphyrins was first noticed by Bajema et al. ( ) and later the photophysical parameters concerned with the S2 state were determined on several metalloporphyrins (9,10). S2 - Sq emission from large molecules in condensed phases has also been recognized in many other organic compounds, e.g., azulene (11), thiocarbonyl compounds (12), and several polyenes (13). However, in some metalloporphyrins one can observe the S2 state fluorescence even after the excitation to the state (14-17), that is, the blue... 

The eventual products in reaction (1) have been identified as SO and MSA from experiments involving the steady photolysis of mixtures of DMS and a photolytic precursor of OH (4-91 Absolute measurements of lq have been obtained using the discharge-flow method with resonance fluorescence or electron paramagnetic resonance (EPR) detection of OH (10-141. and the flash photolysis method with resonance fluorescence or laser induced fluorescence (LIF) detection of OH (14-181. Competitive rate techniques where Iq is measured relative to the known rate constant for a reaction between OH and a reference organic compound (18-211 have also been employed to determine k at atmospheric pressure of air. 

X-ray fluorescence,1516 surface acoustic waves (SAW) for determining volatile organic compounds (VOCs),17 18 and immunoassays19-21 are examples of direct analytical techniques (in which a sample preparation step is unnecessary) that are environmentally friendly. In addition, there are environmentally benign procedures from which reagents and solvents have been eliminated or their quantities minimized (calculated per analytical cycle) ... 

The identification and quantitative determination of specific organic compounds in very complex samples is an area of intense current research activity in analytical chemistry Optical spectroscopy (particularly UV-visible and infrared absorption and molecular fluorescence and phosphorescence techniques) has been used widely in organic analysis. Any optical spectroscopic technique to be used for characterization of a very complex sample, such as a coal-derived material, should exhibit very high sensitivity (so that trace constituents can be determined) and extremely great selectivity (so that fractionation and separation steps prior to the actual analysis can be held to the minimum number and complexity). To achieve high analytical selectivity, an analytical spectroscopic technique should produce highly structured and specific spectra useful for "fingerprinting purposes," as well as to minimize the extent of overlap of spectral bands due to different constituents of complex samples. 

The principal analytical disadvantage of MI fluorescence spectrometry is the obvious one that fluorescence is not a universal analytical technique. Many organic compounds of interest fluoresce weakly and can not be determined by any form of fluorescence spectrometry at realistic concentration levels. Accordingly, it is often necessary to use MI fluorometry in conjunction with other techniques which, though less sensitive... 

Solvatochromic fluorescent probe molecules have also been used to establish solvent polarity scales. The solvent-dependent fluorescence maximum of 4-amino-V-methylphthalimide was used by Zelinskii et al. to establish a universal scale for the effect of solvents on the electronic spectra of organic compounds [80, 213], More recently, a comprehensive Py scale of solvent polarity including 95 solvents has been proposed by Winnik et al. [222]. This is based on the relative band intensities of the vibronic bands I and III of the % - n emission spectrum of monomeric pyrene cf. Section 6.2.4. A significant enhancement is observed in the 0 0 vibronic band intensity h relative to the 0 2 vibronic band intensity /m with increasing solvent polarity. The ratio of emission intensities for bands I and III serves as an empirical measure of solvent polarity Py = /i/Zm [222]. However, there seems to be some difficulty in determining precise Py values, as shown by the varying Py values from different laboratories the reasons for these deviations have been investigated [223]. 

The use of aminophenyl fluorescein [28] as the organic compound allows the selective localization of the secondary oxidation reaction with confocal fluorescence microscopy (Fig. 25.3a) [29]. In a similar way as described for a-chymotrypsin, the enzyme was immobilized in an agarose matrix and fluorescence intensity time traces were recorded at a position where an enzyme was found. Time traces with exceptional signal-to-noise ratio were obtained (Fig. 25.3b) and a histogram of time-averaged single enzyme activities was constructed (Fig. 25.3c) which allowed the determination of the average activity of the analyzed enzymes. 

The emission lifetimes of the bipy and phen complexes of ruthenlum(II) at 77°K are generally in the range t = 0.5-10 ps. (Table 7). Since these values are intermediate to those generally observed for the fluorescence and phosphorescence of organic compounds, the radiative transition in the ruthenium complexes was suggested to be a heavy-atom perturbed spin-forbidden process (168,169). From a determination of the absolute quantum yields as well as lifetimes of a series of ruthenium(II) and osmium(II) complexes, the associated radiative lifetimes were calculated (170). The variations in these inherent lifetimes within the series could be rationalized with a semi-emipirical spin-orbit coupling model thus affording further evidence that the radiative transitions are formally spin forbidden in these systems. 

Titrimetric luminescence methods record changes in the indicator emission of radiation during titration. This change is noted visually or by instruments normally used in luminescence analysis. Most luminescence indicators are complex organic compounds which are classified into fluorescent and chemiluminescent, compounds according to the type of emission of radiation. As in titrimetry with adsorption of colored indicators, luminescence titration makes use of acid-base, precipitation, redox, and complexation reactions. Unlike color reactions, luminescence indicators enable the determination of ions in turbid or colored media and permit the detection limit to be lowered by a factor of nearly one thousand. In comparison with direct luminescence determination, the luminescence titrimetric method is more precise. 

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