Application of the effects of solvent and dissolved oxygen on the determination of benzo[a]pyrene by constant-wavelength synchronous spectrofluorimetry in smoke-flavouring
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are noted for their environmental persistence and ubiquity, some are suspected carcinogens and/or mutagens, necessitating monitoring of PAH levels in diverse matrices (e.g. sediments, biota, and foodstuffs). Often, benzo[a]pyrene (BaP) is sought as an indicator of the presence of other PAHs, for example in drinking water [1], smoked food and smoked-flavour agents [2].
Analysis of PAHs or BaP in foods and smoked-flavour agents is problematic owing to the inherent complexity of these matrices, and the low analyte levels to be detected. To overcome these problems, selective analytical techniques with very low detection limits must be used, such as UV absorption spectrophotometry or spectrofluorimetry, the latter being 10–1000 times more sensitive than the former. For highly complex matrices, however, these techniques are not usually sufficiently selective, and so are used in combination with liquid chromatography to clean up the samples. In the case of spectrofluorimetry, adequate selectivity, with no loss in sensitivity, can be obtained by using synchronous excitation. This technique first developed by Lloyd [3], increases selectivity while maintaining sensitivity, and reduces experimental errors; it is rapid, simple and particularly adequate as a method for routine monitoring of organic pollutants, and it has been used by various investigators for PAH analysis 4, 5, 6, 7, 8, 9, 10, 11, 12.
The validity of any qualitative or quantitative analytical method that employs fluorimetry is highly dependent on the solvent used 13, 14, which should comply with a series of requirements. It must have high purity, be transparent at the excitation and emission wavelengths and be free of interfering Raman or Rayleigh bands 7, 8, 9, and, preferably be inexpensive and non-toxic, have high extractive capacity, and enhance the fluorescence of the analyte, and maintain it stable in solution.
The PAHs, especially BaP, are photosensitized oxidation by dissolved oxygen 15, 16, 17. Furthermore, the high efficiency of oxygen as a quencher of excited states means that it can also interfere with analysis by reducing fluorescence intensity.
In this work, we have studied the effects of more commonly used solvents 18, 19, 20, 21, 22and dissolved oxygen on the determination of BaP by constant-wavelength synchronous spectrofluorimetry in smoked-flavour agents and its possible use as confirmation and quantification technique.
Section snippets
Apparatus
All spectrofluorimetric measurements were performed in a Perkin-Elmer LS-50 luminescence spectrometer equipped with a xenon lamp, Monk-Gillieson monochromators and 1 cm quartz cuvettes with teflon stoppers. Spectral data acquisition and processing were carried out by means of the program Fluorescence Data Manager (v. 2.5 and 3.5) on a PC (with 4 RAM) serially interfaced (RS232C) to the LS-50.
Reagents and standards
BaP was supplied by Aldrich. Residue analysis grade n-hexane, organic trace analysis grade
Effect of the solvent
First, the 3-dimensional and contour plots of the spectra of each solvent studied were obtained [24]. The optimum excitation–emission wavelength interval for synchronous spectrofluorimetric analysis of BaP in each solvent (Δλ), together with the wavelength at which the most intense analyte peak appeared, is given in Table 1.
The 3-dimensional and contour plots were very useful for the sensible choice of optimum Δλ for the analysis of BaP in each solvent (i.e. figures in n-hexane and DMSO, Fig. 1
Conclusions
- 1.
Change in the solvent. The sensitivity of BaP determination by constant-wavelength synchronous spectrofluorimetry depends on the solvent used: the lowest DL and QL are obtained with DMSO, and the highest with n-hexane.
- 2.
The effects of oxygen quenching on the sensitivity of BaP determination by this technique also depend on the solvent, these effects are smaller in benzene or DMSO and greatest in n-hexane, for which roughly 8-fold improvements in DL and QL can be obtained by deoxygenation.
- 3.
The
Acknowledgements
The authors thank the Consellerı́a de Educación y Ordenación Universitaria of the Xunta de Galicia (N.W. Spain), for the concession of bursary to M.S. Garcı́a-Falcón.
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