Elsevier

Analytica Chimica Acta

Volume 400, Issues 1–3, 22 November 1999, Pages 257-264
Analytica Chimica Acta

Very fast peroxyoxalate chemiluminescence

https://doi.org/10.1016/S0003-2670(99)00626-1Get rights and content

Abstract

Peroxyoxalate chemiluminescence (PO-CL) detection offers an advantage in chromatographic detection, by the virtue of its multiple unique selectivities and high sensitivity. However, many of the analytical separation techniques available today require observation times in the millisecond range to preserve the band resolution, and as the reaction kinetics of the PO-CL reaction is considerably slower, extra flow elements are needed to observe the reaction in a time window at maximum emission intensity. Since these flow elements increase the complexity of the system and contribute to band-broadening, the rational way to adapt PO-CL detection to miniaturised separation systems is to speed up the reaction, so that it emits an initial burst of light within the acceptable detection time-frame. Although this may result in a lower overall quantum yield, the actual detection sensitivity could be equal to, or better than slower PO-CL systems. By making careful selections of oxalic reagent and catalyst(s) the reaction can be fine-tuned to maximise the intensity. In this work, the time-dependent light emission from the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) was studied under the catalytic influence of imidazole, 1,2,4-triazole, 4-dimethylaminopyridine (DMAP), and 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) in acetonitrile. Both DMAP and DBU accelerated the reaction substantially, but the best combination of reaction speed and intensity was found for a mixture of 0.5 mM DBU and 5 mM 1,2,4-triazole, which reached its maximum emission after only 40 ms and had an emission intensity comparable to that seen with 5 mM imidazole as catalyst.

Introduction

The pursuit of lower detection limits is always a major objective in trace analysis. There is also a continuous demand for faster analysis times and higher selectivity. These are all factors that call for fast, high efficiency separations, where the demands on dead volume and response time in the detection system are high [1]. Chemiluminescent (CL) detection techniques have proven to be valuable tools for sensitive measurement at ultra-trace levels, and have consequently been successfully applied to the detection of several classes of analytes [2], [3]. Due to the lack of external excitation sources, the detection limits attainable with CL techniques are not limited by excitation light filtering and light scattering [4] or photobleaching [5] (as in fluorescence detection), but rather by reagent purity and lack of understanding of the sometimes rather complicated reaction chemistry [6]. Since the light intensity from a CL reaction is a manifestation of the chemical reaction rate, fundamental studies of fast CL reactions are of prime importance to achieve the objectives outlined above for CL analyses in flowing systems. Improvements in terms of light efficiency and light intensity thus require detailed knowledge of the reaction chemistries and the reacting species.

Over the last few years, we have been working with peroxyoxalate chemiluminescence (PO-CL) [6], a reaction that involves an oxalate ester (or amide), hydrogen peroxide, a luminophore capable of accepting the energy from a metastable intermediate in the reaction, and a base/catalyst. An important role of the latter is to buffer the system by converting hydrogen peroxide to the hypernucleophilic hydroperoxide ion. Equally important in the pursuit of fast reactions is that certain heterocyclic bases possess a catalytic action that goes beyond mere general base catalysis, through a nucleophilic reaction mechanism [7], [8], [9], [10]. We were thus interested in evaluating new catalyst systems, where a good nucleophilic catalyst with proven efficiency in acylation reactions [11], [12] was used alone, or where a catalyst from the azole class more commonly employed in PO-CL [13] was used in conjunction with a non-nucleophilic base sufficiently strong to promote elimination of HX from haloalkanes [14]. Preliminary results from these experiments are presented in this paper.

Section snippets

Reagents and solutions

Bis(2,4,6-trichlorophenyl) oxalate (TCPO) was synthesized as described previously [15]. 4-Dimethylaminopyridine (DMAP; 99%), 1,2,4-triazole (98%), 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU; 98%), 3-aminofluoranthene (3-AFA; 97%), 2,4,6-trichlorophenol (TCP; 98%) and oxalyl chloride (98%) were purchased from Aldrich (Steinheim, Germany) and used as received. Hydrogen peroxide (30% in water, ‘Perhydrol Suprapur’) was acquired through Merck (Darmstadt, Germany). The ‘HPLC-grade’ acetonitrile

Results and discussion

The first step in the PO reaction, schematically described in Fig. 1, can be viewed as an acylation (i.e., oxalylation) of a hydroperoxide anion, forming a singly substituted monoperoxyoxalate. A requirement for an oxalate to work in this reaction is that it is activated, i.e., the carbonyl carbons must be active towards nucleophilic substitution. In other words, the leaving groups must have electron-withdrawing properties. Furthermore, the leaving group properties of the phenol (or amine)

Conclusions and future trends

Although the overall quantum yield was reduced considerably with the 1,2,4-triazole/DBU system, the actual detection sensitivity in a miniature detection cell is comparable to the slower imidazole catalysed PO-CL reactions. The mixture of base and nucleophilic catalysts thus appears to be an advantageous alternative in applications of modern separation techniques which demand a detector time constant of 100 ms or less. For other applications with less stringent demands on the band-broadening of

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