Metals luminol reaction

Burdo and Seitz reported in 1975 the mechanism of the formation of a cobalt peroxide complex as the important intermediate leading to luminescence in the cobalt catalysis of the luminol CL reaction [116]. Delumyea and Hartkopf reported metal catalysis of the luminol reaction in chromatographic solvent systems in 1976 [117], while Yurow and Sass [118] reported on the structure-CL correlation for various organic compounds in the luminol-peroxide reaction. 

The luminol reaction has been used for the determination of oxidizing agents such as hydrogen peroxide, for enzymes such as peroxidase and xanthine oxidase, and for metal ions such as copper or cobalt that catalyze this CL reaction [24],... 

Fig.4.72. Diagram of the luminol reaction detector and chromatograph for analysis of trace amounts of metals (see text for details). (From ref. 200 with permission of Marcel Dekker, New York.)...

Hartkopf, A., Delumyea, R. Use of the luminol reaction for metal ion detection in liquid-chromatography. And. Lett 7, 79 (1974)... 

Cu, Co, Fe, Mn, Zn, Cd Mineral, tap, drinking, seawater, estuarine Use of the catalytic or inhibitory effect of metal traces on the luminol reaction with hydrogen peroxide or dissolved oxygen Kinetic discrimination and masking strategies are widely used for selectivity improvement Low detection limits are frequently achieved... 

In an aqueous medium, luminol reacts with a potent oxidizing agent in the presence of a catalyst (generally a metal, a metal-containing compound or an enzyme) in alkaline solution to yield 3-aminophthalate in an excited electronic state, which returns to ground state with the production of light. Metal ions such as Co(ii), Cu(n) and Fe(ii) or enzymes like HRP and microperoxidase catalyse the luminol reaction efficiently (Figure 27.3). The CL reaction can be summarized as follows ... 

The 3-aminophthalate product ion is in an excited electronic state and shows luminescence in the visible region. Reaction [31] is catalysed by the presence of certain metal ions such as Cr(IIl) and the intensity of the chemiluminescence has been found to be proportional to the concentration of the specific metal ion, usually in the concentration range of parts per billion (ppb). Thus, the luminol reaction can be used to determine the concentration of selected metal ion species at very low concentrations. 

Boyle and co-workers [27] undertook a study to determine whether an HPLC method based upon luminol chemiluminescence detection could be used for the analysis of Co(II) in seawater and freshwater samples at the 10 mol kg level. Their preliminary results showed weak chemiluminescent signals for those chromatographic systems in which cobalt eluted from the column in a complexed form chemiluminescence from the luminol reaction is dependent upon catalysis by free metal ions. It was also observed that the chemiluminescent signal was not severely depressed when moderately strong complexing agents were used in the mobile phase. These results ultimately led the researchers to use weakly complexing eluents in order to elute cobalt in a relatively uncomplexed form, and thus ensure maximum sensitivity from the chemiluminescence reaction. 

A strong influence is exerted by the base used in the chemiluminescent oxidation. It was observed [38-40] that only when quaternary ammonium hydroxides are used as base in the luminol reaction does there exist a one-to-one correlation between luminol chemiluminescence and aminophthalate fluorescence. With alkah metal hydroxides (NaOH, KOH) in DMSO, containing 10% water, the 425 nm maximum is more pronounced in 3-aminophthalate fluorescence than in luminol chemiluminescence. 

Classical metal ions with a positive catalytic effect on the luminol reaction are Fe-(III)-(CN)6-ions or the hemin-Fe-complex. 

Divalent copper, cobalt, nickel, and vanadyl ions promote chemiluminescence from the luminol—hydrogen peroxide reaction, which can be used to determine these metals to concentrations of 1—10 ppb (272,273). The light intensity is generally linear with metal concentration of 10 to 10 M range (272). Manganese(II) can also be determined when an amine is added to increase its reduction potential by stabili2ing Mn (ITT) (272). Since all of these ions are active, ion exchange must be used for deterrnination of a particular metal in mixtures (274). 

Iron(II) can be analy2ed by a luminol—air reaction in the absence of hydrogen peroxide (276). Iron in the aqueous sample is reduced to iron(II) by sulfite other metals which might interfere are also reduced to valence states that are inactive in the absence of hydrogen peroxide. The detection limit is 10 ° M. 

The scope of CAR-CLS in analytical determinations has been expanded with one other type of CL reaction (luminol-based CL reactions are restricted to direct determinations of metal ions and some indirect ones). The so-called energy transfer CL is one interesting alternative, with a high analytical potential. As stated above, PO-CL systems based on the reaction between an oxalate ester and hydrogen peroxide in the presence of a suitable fluorophore (whether native or derivatized) and an alkaline catalyst are prominent examples of energy transfer CL. This technique has proved a powerful tool for the sensitive (and occasionally selective) determination of fluorophores its implementation via the CAR technique is discussed in detail later. 

Much of the study of ECL reactions has centered on two areas electron transfer reactions between certain transition metal complexes, and radical ion-annihilation reactions between polyaromatic hydrocarbons. ECL also encompasses the electrochemical generation of conventional chemiluminescence (CL) reactions, such as the electrochemical oxidation of luminol. Cathodic luminescence from oxide-covered valve metal electrodes is also termed ECL in the literature, and has found applications in analytical chemistry. Hence this type of ECL will also be covered here. 

The CL reaction of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) (1) is one of the more commonly used nonenzymatic CL reactions and has been extensively studied [49, 57-59], It is well known that the luminol CL reaction is catalyzed by many kinds of substances, e.g., ozone, halogen-Fe complex, hemin, hemoglobin, persulfate, and oxidized transition metals. The most acceptable scheme is shown in Figure 10. Luminol forms a six-membered ring of peroxide (3) from a diazaquinone intermediate (2) and then, by the decomposition of 3, N2 gas and the Si-excited state of the phthalate dianion are produced, yielding... 

Luminol derivatives produce emission of light by oxidation with oxygen and hydrogen peroxide under alkaline conditions. By utilizing this reaction, peroxides such as hydrogen peroxide and lipid hydroperoxides can be determined after HPLC separation. Metal ions [e.g., iron(II), cobalt(II), etc.] catalyzing the luminol CL reaction can also be determined. 

Other studies in this specific area are also based on the catalytic effect of a variety of metal ions such as copper (II), cobalt (II), nickel (II), iron (III), and manganese (II) on the luminol-hydrogen peroxide reaction providing a rapid and efficient detection mode for these five ions, when an online CL detector is used before separation by CE [88], This contribution combines capillary ion analysis (CIA) and CL detection by means of a postcapillary reactor similar to the one originally developed by Rose and Jorgenson [80] and finally modified by Wu... 

CL reaction can be catalyzed by enzymes other than HRP (e.g., microperoxidase and catalase) and by other substances [hemoglobin, cytochrome c, Fe(III), and other metal complexes]. The presence of suitable molecules such as phenols (p-iodophenol), naphthols (l-bromo-2-naphthol), or amines (p-anisidine) increases the light production deriving from the HRP-catalyzed oxidation of luminol and produces glow-type kinetics [6, 7], The use of other enzymes, such as glucose-6-phosphate dehydrogenase [38-41], P-galactosidase [42], and xanthine oxidase [43-46], as CL labels has been reported. 

The enhanced chemiluminescence associated with the autoxidation of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) in the presence of trace amounts of iron(II) is being used extensively for selective determination of Fe(II) under natural conditions (149-152). The specificity of the reaction is that iron(II) induces chemiluminescence with 02, but not with H202, which was utilized as an oxidizing agent in the determination of other trace metals. The oxidation of luminol by 02 is often referred to as an iron(II)-catalyzed process but it is not a catalytic reaction in reality because iron(II) is not involved in a redox cycle, rather it is oxidized to iron(III). In other words, the lower oxidation state metal ion should be regarded as a co-substrate in this system. Nevertheless, the reaction deserves attention because it is one of the few cases where a metal ion significantly affects the autoxidation kinetics of a substrate without actually forming a complex with it. 

The best known and most nsefnl of the chemiluminescent reactions involving electron transfer is the oxidation of luminol (3.100) or its derivatives in alkaline medium. The oxidant can be hydrogen peroxide, sodium ferricyanide or hypochlorite, usually with a catalyst that can be a transition metal ion, such as Cu " Co +, Fe + and Mtf+, or haem and haemproteins, e.g. peroxidases. The reaction mode is shown in Figure 3.22.4"... 

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