Dye-sensitized continuous photochemical analysis. Identification and relative importance of key experimental parameters

Abstract

Important operating conditions for a new analytical technique, based on dye-sensitized reactions carried out in flowing streams, are evaluated. The photoredox reaction used is a form of triplet energy transfer wherein a colored organic dye absorbs light to generate the excited state triplet, which then abstracts electrons from a suitable substrate. The products of the reaction are colorless reduced dye and substrate oxidation products; spectrophotometric measurement of dye photobleaching is employed. The rate and stoichiometry of photoredox reactions are dramatically affected by the structures of dye and substrate used, dissolved oxygen, and reaction pH. Other important parameters, largely associated with the apparatus used, are dyecarrier flow rate, photolysis source power, and reaction temperature. Data for DCPA measurements, using Methylene Blue and Rose Bengal, are reported for the model substrates ascorbic acid, epinephrine, nicotine, and caffeine. Appropriate operating conditions yield low micromolar limits-of-detection and 1 YO precision.

Novel photochemistry of leuco-Methylene Blue

“…( 2) is well illustrated by using a combination of visible and UV light to irradiate an aqueous solution containing MB + (10 25 mol dm 23 ) and ascorbic acid (8 3 10 25 mol dm 23 ) at pH 2. Visible light of this system photogenerates LMB as expected, since it is well established [5][6][7][8][9][10] that visible light irradiation of a solution containing MB + in the presence of ascorbic acid leads to the efficient generation of LMB via a photoreductive quenching mechanism, i.e. In conclusion, experiments show that leuco-Methylene Blue has a very active, triplet state photochemistry particularly with regard to its oxidative quenching by dissolved oxygen.…”

“…4 Finally, the Methylene Blue-leuco Methylene Blue redox indicator is commonly used in the quantitative analysis of a large number of mild reducing agents, such as glucose and ascorbic acid. [5][6][7][8][9][10] In the detection of ascorbic acid using Methylene Blue a number of methods have been developed based on the photoreduction of the MB to LMB by ascorbic acid, via a photoreductive quenching mechanism and the subsequent measurement of the extent of reduction either by spectrophotometry [5][6][7] or amperometry. 9,10 Curiously, although this method works perfectly well if visible light is used as the irradiation source, it has been noted 7 that UV light is ineffective, despite the fact that MB absorbs both visible and UV light.…”

“…Thus, although UV light does promote the photoreduction of MB + to LMB by AA via eqn. (3),[5][6][7][8][9][10] the reverse reaction, i.e.the previously unrecognized photooxidation of LMB by O 2 (i.e. eqn.…”

Flow injection Fluorimetric Determination of Ascorbic Acid Based on its Photooxidation by Thionine Blue

“…AA has been determined by photobleaching Methylene Blue using continuous 38 and FI systems, 39,40 both procedures involving spectrophotometric detection. The chemiluminometric determination of AA based on its photooxidation sensitized by Toluidine Blue has also been described.…”

Pumping pressure and reagent consumption in continuous flow analysis with unsegmented reaction streams

“…At least three independent groups have recently described systems for chemical analysis based on continuous flow in an unsegmented fluid stream. The system described by White and Fitzgerald (1, 2) employs a photochemical reaction in a flowing stream, with the sample introduced by gravity (1) or by injection via a motor-driven syringe (2). The reagent stream is pumped by applying a small air pressure (5 psi) to the reagent reservoir.…”