Flow-injection chemiluminescence determination of formaldehyde in water
Introduction
Formaldehyde (HCHO) is an important air pollutant. It is the most abundant carbonyl compound in the atmosphere and plays an important role as a free radical source [1]. Formaldehyde is emitted to the atmosphere by natural or anthropogenic sources. The most significant anthropogenic sources are automobile exhaust gases and industrial emissions. Formaldehyde is also formed during a photooxidation of methane, as well as other hydrocarbons, directly in the atmosphere [2], [3]. Formaldehyde is often found in indoor air at levels typically several times higher than in outdoor air as a result of emission from a variety of materials and products. As a suspected human carcinogen, formaldehyde is an important toxic compound [4]. Atmospheric deposition is a significant source of HCHO in aquatic systems, since concentrations in rainwater are expected to be up to three orders of magnitude higher than in surface water [5]. Furthermore, rainwater HCHO could significantly contribute to dissolved organic carbon budget [6].
For the determination of formaldehyde in water a lot of methods have been proposed [7]. Chromatographic methods including gas chromatography of oxazolidine [8], O-alkyloxime [9] or dinitrophenylhydrazone [10] derivates, ion chromatography of formate after alkaline oxidation [11], and liquid chromatography of the 1,3-cyclohexanedione [12], dansylhydrazone [13] or, especially, 2,4-dinitrophenylhydrazone (DNPH) [14], [15] derivates have been shown to provide good sensitivity for formaldehyde detection. The capillary zone electrophoresis was also applied for the separation of dansylhydrazone [16] and 4-hydrazinobenzoic acid derivates [17]. However, chromatographic and capillary zone electrophoresis methods are slow and cannot be simply adapted for a continuous analysis. Further, several colorimetric methods, chromotropic acid [18] and modified pararosaniline method [19], for the determination of formaldehyde have been developed, unfortunately, the poor sensitivity limits their application for the detection of HCHO in water. Therefore, more sensitive methods have been developed, based on a fluorescence detection of derivates produced by reaction of formaldehyde with β-diketone (the Hantzsch reaction), when 2,4-pentanedione [20], [21] or 1,3-cyclohexanedione [22] have been used as reagents. These methods are continuous and very selective for formaldehyde because other aldehydes fail to produce strong fluorescence. Another fluorescent method is based on formatedehydrogenase catalyzed production of reduced nicotinamide adenine dinucleotide [23]. In spite of high sensitivity and selectivity, the method has some shortcomings. The major problem arises from the application of enzyme that is expensive and not very stable.
This problem is eliminated in a chemiluminescence method for the determination of formaldehyde based on its inhibiting effect on the chemiluminescence (CL) emission of lucigenin–hypochlorite–H2O2 system [24]. Moreover, the chemiluminescence methods, in comparison with the fluorescence methods, offer simpler instrumentation as no excitation source is required. Thirty years ago, another chemiluminescence method for the determination of formaldehyde in natural and waste water based on the Trautz–Schorigin reaction involving formaldehyde, hydrogen peroxide and gallic acid in a strong alkaline solution has been developed [25]. However, the method in its original setting requires bubbling of nitrogen through a detection cell to assure good mixing of reagents, therefore, the original method is not convenient for the flow-injection application.
We modified the original configuration of the Trautz–Schorigin reaction into form suitable for flow-injection analysis (FIA) application. This paper describes two different approaches applied for the optimization of flow-injection system for the chemiluminescence determination of formaldehyde in water based on the Trautz–Schorigin reaction.
Section snippets
Flow-injection detection system
A schematic diagram of employed flow-injection analysis system is presented in Fig. 1. The sample is introduced into a carrier stream utilizing a manually operated six-way injection valve with the sample-loop volume of 200 μl. The carrier stream (distilled-deionized water) is first merged in a PTFE tee with mixture of gallic acid (0.015 M) and EDTA (0.001 M), then with hydrogen peroxide solution (0.6 M), and finally the mixture is merged with potassium hydroxide solution (0.1 M) directly in a CL
System optimization
The parameters of the FI system were optimized by two different approaches—by traditional way using the one-variable-at-a-time method (OVATM) and the modified simplex method. In both approaches, five variables, i.e., total reagents and carrier flow rates, as well as the concentrations of gallic acid, hydrogen peroxide and potassium hydroxide, were optimized. The optimisation procedure was designed to include relatively large changes in variable levels in order to cover a large portion of
Conclusion
Flow-injection chemiluminescence determination of formaldehyde in water based on Trautz–Schorigin reaction is described. To compare both used optimization procedures, it is evident that the simplex method is much faster (about one day) in contrast with several days procedure using the one-variable-at-a-time method. Both optimization procedures led to the comparable limits of detection. On the other hand, the simplex method requires special software.
Due to simple instrumentation, speed,
Acknowledgements
This work was supported by a Grant No. TD7033 from the Ministry of Science and Environmental Protection of the Republic of Serbia and by an Institutional research plan of the Institute of Analytical Chemistry of the Academy of Sciences of the Czech Republic No. AV0 Z40310501.
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