Organic compound detection fluorescence

Fluorescence spectroscopy is characterized by a greater selectivity when compared with other spectro-photometric techniques because there is an excitation and an emission spectra, with maxima usually quite characteristic of a particular compound. It is also selective because of the limited number of organic compounds that fluoresce. It has greater sensitivity than spectrophotometric methods in solution 10 9-10 12 M can usually be measured while on a thin-layer chromatogram some naturally fluorescent compounds may be detected instrumentally in sub-nanogram amounts. 

Short-wavelength UV radiation (A = 254 nm) is employed for excitation. This allows aromatic organic compounds, in particular, to be detected by fluorescence quenching. Uranylacetate may also be excited at A = 366 nm. 

Sawicki (13) used solid-surface fluorescence techniques extensively in the 1960 s for air pollution research. In 1967, Roth (14) reported the RTF of several pharmaceuticals adsorbed on filter paper. Schulman and Walling (15) showed that several organic compounds gave RTF when adsorbed on filter paper. Faynter et al. (16) reported the first detailed analytical data for RTF and gave limits of detection, linear dynamic ranges, and reproducibilities for the compounds. 

We inferred that these properties might be exploited in a series of unique derivatizing reagents designed specifically for trace analysis of organic compounds using HPLC separation and fluorescence detection. The use of these pyridones for the analytical purposes reported here is based on their acidic properties. Treatment of a lH-2-pyridone with a base converts the pyridone to its salt. 

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. 

Other uses are to produce phosphorescence and fluorescence in organic compounds and for scintillation screens on instruments used to detect radiation. Radium salts were used in the past to paint the dials of luminous clock faces that glow in the dark. 

HPLC analysis of polycyclic aromatic hydrocarbons (PAH) in drinking water is one of the current and classical applications of fluorescence. In this case, the detector contains a fluorescence flow cell placed after the chromatographic column. This mode of detection is specifically adapted to obtain threshold measurements imposed by legislation. The same process allows the measurement of aflatoxins (Fig. 12.11) and many other organic compounds (such as adrenaline, quinine, steroids and vitamins). 

The cerium(IV) oxidation reaction of many organic acids provides a sensitive and selective method for HPLC analysis of these compounds [116,117]. The oxidation of specific classes of organic compounds with cerium(lV), and the effects on the reaction of temperature, acidity, anion and catalyst, have been studied extensively [118-120]. The reaction produces cerium(HI) which is fluorescent and can be measured spectrofluori-metrically. The method has been applied successfully to the post-column reaction and detection of nmole amounts of organic acids by HPLC. 

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. 

Figure 3.13 Two-step grafting process in order to construct a sensor for the detection of volatile organic compounds based on the well-known solvatochromic fluorescent phenoxazinone dyes. Hydrophobizing of the surface of MCM-41 with silazanes leads to better performance of this sensor.

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