Nov. 18, 2019
ScienceEnvironment

Identification of the Fluorescent Photoproducts

Analysis of Two Phenylureas by Photo-induced Fluorescence (PIF) and GC-MS

  • Fig. 1: Evolution of the photoproducts DL-PIF emission spectra of a FEN aqueous solution (initial concentration = 2.5 μg mL-1) vs. the irradiation time. Laser beam: 240 nm, 1 mJ,10Hz – Figure reprinted with authorization from reference [2].Fig. 1: Evolution of the photoproducts DL-PIF emission spectra of a FEN aqueous solution (initial concentration = 2.5 μg mL-1) vs. the irradiation time. Laser beam: 240 nm, 1 mJ,10Hz – Figure reprinted with authorization from reference [2].
  • Fig. 1: Evolution of the photoproducts DL-PIF emission spectra of a FEN aqueous solution (initial concentration = 2.5 μg mL-1) vs. the irradiation time. Laser beam: 240 nm, 1 mJ,10Hz – Figure reprinted with authorization from reference [2].
  • Fig. 2: Evolution of the excitation/emission fluorescent matrix with laser irradiation (at 240 nm) for DFB after 1 min of irradiation – Figure reprinted with authorization from reference [2].
  • Fig. 3: Comparison of the PIF emission spectra of FEN and DFB photoproducts (PIF 1, 2 and 3) with the fluorescence emission spectra of the standard compounds (phenol and p-hydroxyaniline) – Figure reprinted with authorization from reference [3].

Quantitative analysis of diflubenzuron (DFB) and fenuron (FEN) pesticides was successfully performed using photo-induced fluorescence (PIF) methods. The analytical conditions were optimized for the determination of traces of these pesticides in Senegalese natural water samples. Mean recoveries were satisfactory, ranging between 80 and 120%. Also, gas chromatography-mass spectrometry (GC-MS) was combined with the PIF methods, in order to identify the formed fluorescent photoproducts.

Phenylurea pesticides, including diflubenzuron (DFB) and fenuron (FEN) are widely used in agriculture to improve productivity and, consequently, they can produce residues in crops, soils and surface waters. Due to the photochemical reactivity of these pesticides, photo-induced fluorescence (PIF) methods were developed, based either on UV irradiation (classical PIF) or on direct laser irradiation (DL-PIF), for determining their residues [1,2]. Therefore, in order to validate the PIF methods, analytical applications were performed in Senegal natural waters. Moreover, gas chromatography-mass spectrometry (GC-MS) was combined to PIF to separate and identify the fluorescent DFB and FEN photoproducts by comparing them to standard compounds, including phenol and p-hydroxy-aniline [3].
 
Procedures
Fluorescence measurements were carried out at room temperature on a Kontron SFM-25 spectrofluorimeter, interfaced with a microcomputer. Phototransformation of both pesticides into strongly fluorescent photoproducts was realized under UV or DL irradiation of the DFB and FEN working standard solutions (10-5 M). Liquid extraction of 3 mL of DFB and FEN irradiated solutions was carried out three times with 10 mL of ethyl acetate. Then, the organic phase was dried with anhydrous magnesium sulfate (MgSO4) in order to remove the traces of water, and afterwards was evaporated to dryness at 45 °C with a rotavapor. The dried residues were dissolved in 300 μL of ethyl acetate. A 200-μL sample was supplemented to 1 mL, and a 1.5 μL volume sample was studied by GC-MS. A Nist library X calibur software was utilized to interpret the mass spectra (m/z values ranging from 50 to 650).

All experimental conditions were optimized in our previous work [1].

 
Analytical Performances and Applications
First, the analytical performances of both PIF methods were studied. For the classical PIF method, strongly fluorescent photoproducts were obtained at λex/λem = 331/405 nm for DFB in pH4 water-methanol (30:70, v/v) mixture and at 282/343 nm for FEN in pH4 aqueous solution, with low limit of detection (LOD) values of, respectively, 9 and 28 ng mL-1. In the case of the DL-PIF method, the excitation/emission fluorescence matrix also revealed very fluorescent photoproducts at λex/λem = 240/342 nm for DFB in pH4 water/methanol (30:70, v/v) mixture and at 240/308 nm for FEN in pH4 aqueous solution, with LOD values of 4.5 and 1.5 ng mL-1, respectively. A  LOD value of 5 ng mL-1 in propanol-2-ol was also reported by Coly and Aaron [4] for the PIF determination of DFB in technical formulations. For analytical applications, a liquid-liquid extraction was applied using dichloromethane as extracting solvent. Standard addition method and direct spiking procedure were applied to determine mean recoveries. Standard addition slopes were found to be very close to those measured for the calibration curves. For both methods, the mean recoveries of DFB and FEN in Senegal natural water samples (tap, river and sea water) ranged between about 80 and 120% with relative standard deviation values below 10 %, according to the procedure and type of water sample [1,2].
 
Interference Studies of Added Foreign Species
Since several commonly used pesticides, namely fluometuron, monolinuron, linuron, carbaryl, pendimethalin and propanil, as well as various inorganic ions (Ca2+, (PO4)23-; K+, NO3-, Na+, CO3-), were generally found in the Senegal natural waters, their possible interference effects on the determination of DFB and FEN was inverstigated. The DFB and FEN  concentrations were respectively fixed at 0.1 mg mL-1 and at 0.015 mg mL-1. The tolerance limit of the interfering foreign species was defined as the concentration limit of these interfering species for which the percentage of PIF signal variation did not exceed ± 5% in the determination of DFB and FEN. Addition of foreign species neither changed the shape of DFB and FEN PIF emission spectra, nor shifted the maximum emission wavelength. But, significant PIF intensity changes occurred with increasing concentrations of foreign species.     In the case of the DL-PIF method, the addition of increasing concentrations of DFB (up to 2 µg mL-1) to a FEN solution (1 µg mL-1) did not affect the FEN PIF spectra, because the DFB photoproducts were only formed in a water/methanol mixture, but not in pure water. In contrast, FEN produced relatively high interference effects, since a FEN concentration as low as 0.04 μg mL-1 increased the DFB PIF signal above the tolerance limit. It might be explained by the formation of the same PIF FEN photoproduct at λem = 342 nm. To overcome the interference, the PIF signal obtained in water/methanol mixture was corrected by the FEN fluorescence obtained from PIF measurements in pure water [2]. Therefore, a correction factor was applied to take into account the fact that the fluorescence quantum yield of the FEN photoproduct was higher in water/methanol mixture than in pure water. This correction minimized the interference effects and improved the PIF selectivity. 
 
Identification of the Fluorescence Photoproducts
Only one fluorescent photoproduct was detected using the classical PIF method at λex/λem = 331/405 nm for DFB and 282/343 nm for FEN. In contrast, the DL-PIF method revealed the formation of three fluorescent photoproducts at λex/λem = 225/308 nm (PIF1), 280/342 nm (PIF2) and 295/420 nm (PIF3) for FEN (Fig. 1), and of two fluorescent photoproducts at λex/λem = 230/342 nm (PIF’1) and 220/422 nm (PIF’2) for DFB (Fig. 2) [1,2]. GC-MS allowed the identification of these photoproducts by comparing their fluorescence spectral characteristics (Fig. 3) to those of standard compounds (p-hydroxyaniline and phenol). The two later compounds are known to contribute to the photodegradation pathways of benzoyl- and phenylurea pesticides. Indeed, both pesticides presented the same PIF photoproduct at λem = 342 nm than p-hydroxyaniline. This fluorescence emission wavelength might also correspond to the formation of 3-[4-(4-aminophenyl)] phenyl]-1,1-dimethylurea, taking place during the photo-rearrangement of benzoyl and phenylureas [5]. In the case of DL-PIF, it was found that the FEN PIF1 presented the same fluorescence characteristics than phenol (λem = 308 nm).
 
Conclusion
In this work, a simple, inexpensive, sensitive and precise PIF method has been developed for the determination of two benzoyl- and phenylurea pesticides, namely diflubenzuron and fenuron, in Senegal natural water samples. Classical PIF and DL-PIF methods were found to be of great analytical interest for monitoring both pesticides in natural waters. Also, it can be concluded that the combination of the PIF and DL-PIF methods and of GC-MS should be suitable to confirm the presence of both pesticides DFB and FEN and their photoproducts in natural waters, and to monitor their evolution in the environment.
 
 
Authors
P. A. Diaw1,2,3,4, O. M. A. Mbaye2,3,4, D. D. Thiaré2, N. Oturan3, M. D. Gaye-Seye2,3, A. Coly2, B. Le Jeune2, P. Giamarchi4, M. A. Oturan3, J.-J. Aaron3
 
 
Affiliations
1 Equipe des Matériaux, Electrochimie et Photochimie Analytiques, Université A. Diop, Bambey, Sénégal
2 Laboratoire de Photochimie et d’Analyse, Univ. Cheikh. Anta Diop, Dakar, Sénégal
3 Laboratoire Géomatériaux et Environnement (LGE), Université Paris-Est Marne-la-Vallée, Paris, France
4 Laboratoire Optimag, EA 938, Faculté des Sciences, Université de Brest, Brest Cedex, France

 

Contact
Prof. Dr. Jean-Jacques Aaron

Laboratoire Géomatériaux et Environnement (LGE), Université Paris-Est Marne-La-Vallée,
Paris, France
jeanjacquesaaron@yahoo.fr

 

Further articles on chromatography!

 

References

[1] P. A. Diaw, O.M. A. Mbaye, M. D. Gaye-Seye, J. J. Aaron,  A. Coly, A. Tine,  N. Oturan,  M. A. Oturan. Photochemically-induced fluorescence properties of two benzoyl- and phenylurea pesticides and determination in natural waters. J. Fluorescence, 2014, 24, 1319–1330.

[2] P. A. Diaw, A. Maroto, O. M. A. Mbaye, M. D. Gaye-Seye, L. Stephan, A. Coly, L. Deschamps, A. Tine, J.J. Aaron, P. Giamarchi. Determination of phenylurea pesticides by direct laser photo-induced fluorescence, Talanta, 2013, 116, 569–574.

[3] P. A. Diaw, O. M.A. Mbaye, D. D. Thiaré, N. Oturan, M. D. Gaye-Seye, A. Coly, B. Le Jeune, P. Giamarchi, M. A. Oturan, J.-J. Aaron. Combination of photoinduced fluorescence and GC–MS for elucidating the photodegradation mechanisms of diflubenzuron and fenuron pesticides, Luminescence, 2019, 34, 465-471. https://doi.org/10.1002/bio.3612.

[4] A. Coly, J.-J. Aaron. Photochemical–spectrofluorimetric method for the determination of several aromatic insecticides, Analyst, 1994, 119, 1205-1209.

[5] J.P. Aguer, C. Richard, Transformation of fenuron induced by photochemical excitation of humic acids, Pesticide Sci., 1996, 46(2), 151-155.

Contact

Laboratoire Géomatériaux et Environnement (LGE), Université Paris-Est Marne-La-Vallée,


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