2.7: Manganese

Manganese (VII) in the form of potassium permanganate has been used as a chemiluminescence reagent for several decades. A broad band of red light is emitted on reaction with over 270 compounds in acidic solution^[1]. Among the organic analytes are morphine and a wide range of other pharmaceuticals, phenolic substances, amines and hydrazines in addition to well-known reductants such as ascorbic acid and uric acid. Proteins and amino-acids are also known to reduce permanganate with chemiluminescence. Inorganic analytes include sulfur dioxide and sulfites, hydrogen sulfide, hydrogen peroxide, hydrazine and iron(II). Chemiluminescence intensity is a linear function over a very wide range of concentration, but varies considerably for different analytes. It is also affected by anions present so that acidification with sulfuric acid gives a better signal than hydrochloric, nitric or perchloric acids. Considerable signal enhancement occurs in the presence of polyphosphates; these are unstable at low pH but hexametaphosphate is more stable than the others. In a number of cases, chemiluminescence is enhanced by the presence of an ancillary reductant such as formic acid or, especially, formaldehyde. Manganese(II) is sometimes a useful signal enhancer. Fluorophores such as quinine, riboflavin or rhodamine B have also been used but sometimes give a high blank signal and a reduced signal to noise ratio.

The emitting species is an electronically excited manganese(II) species, as has been confirmed by a direct comparison of the laser-induced photoluminescence of manganese(II) chloride with the chemiluminescence from reaction of sodium borohydride with acidic potassium permanganate^[2]. In many cases where permanganate is used in the presence of fluorescent compounds, e.g. enhancers or reaction products, energy transfer to the efficient fluorophore has been proposed on the basis of spectral distributions that match those obtained using other oxidants; in most cases, however, the red emission characteristic of manganese(II) is also produced and can make a significant contribution to the total light output^[3], especially in the presence of polyphosphate.

More recently, manganese(III) and manganese(IV) have been explored as chemiluminescence reagents^[4]. As with the +VII oxidation state, these produce on reaction with a wide range of molecules an excited manganese(II) species that emits light, but differ markedly in terms of selectivity. They also possess characteristics that provide new avenues for detection, such as the immobilisation of solid manganese dioxide, the production of colloidal manganese(IV) nanoparticles and the electrochemical generation of manganese(III).

A brown, transparent, stable solution of manganese(IV) can be prepared by dissolving freshly precipitated manganese dioxide in 3M orthophosphoric acid. Using this reagent at about 1 x 10-4 M, analytically useful chemiluminescence has been reported for a growing list of compounds, often with nanomolar detection limits. Light emission is enhanced by up to 2 orders of magnitude in the presence of 0.2 – 3.0 M formaldehyde. Numerous pharmaceuticals have been determined in commercial formulations by this reaction in flow-injection assays. Detection of drugs and biomolecules in more complex matrices such as urine or serum requires coupling with an initial separation step such as HPLC.

Manganese(III) can be obtained by oxidation of manganese(II) or reduction of manganese(IV); it readily disproportionates into the +II and +IV states but can be stabilised by acidification, by complexation with anions or by adding manganese(II). The reduction of manganese(III) produces excited manganese(II) leading to emission of light of the same spectral characteristics as that emitted in permanganate or manganese(IV) chemiluminescence. On-line electrochemical generation of manganese(III) from manganese(II) has been applied to the chemiluminescence determination of a wide range of analytes, especially pharmaceuticals, with satisfactory selectivity and typically sub-micromolar limits of detection.