# Analysis of non-native fluorescent pesticides by thermo-induced and photo-induced fluorescence, a comparative study

Diéry Diouf, Ndeye Diop, Diène Thiaré, Atanasse Coly, Mame Gaye-Seye, Pape Diaw, Bernard Le Jeune, Philippe Giamarchi

► To cite this version:

Diéry Diouf, Ndeye Diop, Diène Thiaré, Atanasse Coly, Mame Gaye-Seye, et al. Analysis of non-native fluorescent pesticides by thermo-induced and photo-induced fluorescence, a comparative study. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy [1994-..], 2023, 302, pp.123036. 10.1016/j.saa.2023.123036. hal-04315482

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https://hal.univ-brest.fr/hal-04315482v1

Submitted on 15 May 2025

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# Analysis of non-native fluorescent pesticides by thermo-induced and photo-induced fluorescence, a comparative study.

Diéry DIOUF<fnref n="1" />,<fnref n="3" />, Ndeye A. DIOP<fnref n="2" />,<fnref n="3" />, Diène D. THIARE<fnref n="1" />, Atanasse COLY<fnref n="1" />, Mame D. GAYE-SEYE<fnref n="1" />,<fnref n="4" />, Pape. A. DIAW<fnref n="2" />, Bernard LE JEUNE<fnref n="3" />, **Philippe GIAMARCHI**<fnref n="3" />

## ABSTRACT

This paper compares the performance of three analytical methods for the determination of pesticides in natural waters. As many pesticides are non-fluorescent, they are transformed into highly fluorescent by-products in two ways: elevated temperature in an alkaline medium (thermo-induced fluorescence - TIF); or UV irradiation in water (photo-induced fluorescence - PIF). The first method studied uses TIF, the second one uses PIF and the third one uses an automatic sampling and analysing PIF system. Analytical applications were carried out using the three methods for the determination of deltamethrin and cyhalothrin, pesticides widely used in Senegal. In both cases, the calibration curves obtained are linear without matrix effects, and the detection limits are good in the ng mL⁻<fnref n="1" /> range. It appears that the analytical performances of the automatic PIF method are better than the two others. The advantages and disadvantages of the three methods are then compared and discussed in term of analytical performance and usability.

## Keywords

Pesticides ; Photo-induced ; Thermo-induced ; Fluorescence

<footnote n="1">¹ Lab. Photochimie et d'Analyse, Univ. C. A. Diop, Dakar, Sénégal</footnote>

<footnote n="2">² Lab. Matériaux, Electrochimie et Photochimie Analytique, Univ. A. Diop, Bambey, Sénégal.</footnote>

<footnote n="3">³ OPTIMAG Lab., Brest Univ., 6 Av. Victor Le Gorgeu, 29285 Brest Cedex, France</footnote>

<footnote n="4">⁴ Lab. Géomatériaux et Environnement, Univ. Paris-Est, 77454 Marne-la-Vallée, France</footnote>

<footnote>*E-mail: philippe.giamarchi@univ-brest.fr</footnote>
# 1. Introduction

Protecting the environment and the quality of water and soil is a worldwide concern. Pollution is often of industrial or agricultural origin due to the lack of appropriate precautions to avoid the leakage of toxic waste such as fertilisers and pesticides. The latter are frequently used in agriculture and thus contribute to the pollution of surface and groundwater [1], and to the contamination of foodstuffs. The toxicity of pesticides is linked to their molecular structure, which is stable and resistant to degradation, or to the fact that they degrade into more dangerous and carcinogenic by-products [2,3,4]. Several hundred pesticide formulations and thousands of active substances are used worldwide for crop protection and yield improvement [5,6]. In Senegal, the agricultural sector employs 60% of the population [7] and plays a key role in the country's socio-economic development. Faced with the growing demand for food, producers are looking for ways to increase their agricultural production and improve their financial income. To achieve this, pesticides are frequently overused with an average of 598 tonnes of solid pesticides and more than one million litres of liquid pesticides per year [8]. For example, in peri-urban agriculture Niayes area, it was shown that during phytosanitary treatments, a large proportion of the pesticides are deposited on the soil, risking reaching the water table in places where it is outcropping or close to the topographical zone [9].

Faced with these numerous risks, it is essential to develop simple, sensitive and rapid analytical methods to detect pesticide residues in natural waters. In this work, we have chosen to focus on fluorescence methods which are inexpensive, easy to implement and therefore well suited to developing countries like Senegal. Many pesticides do not fluoresce, but they can be transformed into highly fluorescent by-products for sensitive analysis. This transformation can be achieved by two methods: The Thermo-chemically Induced Fluorescence method (TIF) uses high temperature in an alkaline medium to transform native compounds into fluorescent thermoproducts; The Photo-Induced Fluorescence method (PIF) uses intense UV irradiation to create the fluorescents photoproducts.

TIF was recently proposed to quantify pyrethroids in tap water and well water and detection limits (DL) of 4.40 ng.mL⁻¹ was reached [10]; Mendy et al. have determined metolachlor and buprofezin in water pH13 (LOD = 2.6-9.5 ng mL⁻¹) [11]. TIF in micellar media is more classically used, as an example Mendy et al. have determined metolachlor in SDS and Brij-700 micellar media (LOD = 0.06-0.7 ng mL⁻¹) [11]; buprofezin in DM-β-CD (pH 7 and 13; LOD = 0.05-0.1 ng mL⁻¹) [12] and metolachlor in DM-β-CD and HP-γ-CD pH 7 (LOD = 0.06-0.09 ng mL⁻¹) [13].
PIF methods were applied by numerous authors for the analysis of pesticides [14, 15, 16, 17, 18, 19]. For example, Diaw et al. determined fenuron and diflubenzuron by the classical-PIF method using UV irradiation [20], and by an innovative PIF method using laser irradiation [21], in Senegalese natural water with satisfactory mean recovery percentage values in the range 95-105%. Fenvalerate has been studded by PIF by Thiare et al. in river water and a DL of 12 ng.mL⁻¹ has been obtained [22]. In previous work, we have also developed an automatic early warning monitoring systems using photo-induced fluorescence (namely “AUTOPIF”) to detect accidental pollution of surface waters by pesticides [23]. For example, we obtained a detection limit of 12 ng.mL⁻¹ of fluometuron in river water [24] and of 8.43 ng.mL⁻¹ of acetamiprid in tap water [25].

In this paper, we have chosen to compare the analytical performance of the three methods TIF, PIF and AUTOPIF. We selected deltamethrin and cyhalothrin, two pyrethroids pesticides widely used in Senegal. The major danger of these persistent products and their metabolites in various environmental matrices (soil, water, ...), is their ability to bioaccumulate in the food chain and endanger the life of living beings [26]. For example, Abdulbashir Femi Salakoe et al [27], noted the acute toxicity of deltamethrin, and cyhalothrin on guppy fish. Firstly, we studied and compared the physicochemical properties of the thermo-induced and the photo-induced compounds of these two pesticides. Next, we worked on spiked tap and well waters collected in the Niayes area (Senegal), using the standard addition procedure. The analytical performances of the three methods are then compared.

# 2. Material and methods

## 2.1. Reagents

Technical-grade deltamethrin and cyhalothrin (Table 1) (purity > 99%) and spectroscopy solvent Methanol were obtained from Sigma Aldrich (analytical reagent grade). Ultra-pure water (Millipore Mro-MQ System) was used as working solutions.
## 2.2. Materials

For photo-induced methods, an irradiation chamber is used (PVC cylinder, diameter of 16 cm, height of 24 cm). Three UV-C fluorescent lamps (E27 15W 240V, TUV HNS 55x230mm, Bailey Electric & Electronics bv, Oosterhout, the Netherlands) are mounted in a triangle inside the chamber, the upper part of the chamber is closed by a PVC ring. For classical PIF method, three holes in the chamber lid allow the insertion of three 30 mL quartz irradiation tubes (internal diameter 1 cm) 3 cm from 2 adjacent lamps, and 8 cm from the third lamp [23]. Fluorescence measurements are performed on a commercial spectrofluorometer (Cary Eclipse, Agilent, Stevens Creek Blvd., Santa Clara, CA, United State). For AUTOPIF method, samples are irradiated in a Teflon coil (1.5 m long, 0.3 cm internal diameter), arranged along the inner walls of the irradiation chamber [28]. The flow is adjusted by a peristaltic pump (Minipuls III, Gilson, Middleton, WI, USA), to obtain the optimum irradiation time. The sample is then sent through a 0.5 mL cylindrical quartz flow cell, and irradiated by a 250 nm UV LED (Thorlabs, France). The fluorescence signal is collected by an optic fiber and sent to a diode array miniature spectrometer for detection (200–800 nm, Flame series spectrometers, Oceans Insight, Geograaf, Netherland).

For thermo-induced method, the pH of the sample is adjusted to 12 by adding a small amount of NaOH solution (1.0 mol.L⁻¹), and then heated to forty degrees for four minutes [10]. Fluorescence measurements are then performed on the Cary Eclipse commercial spectrofluorometer.

Solid phase extractions are performed with C18 cartridge (LiChrolutRP-18E200mg40–63 mm). Statistical computations are done with Statgraphics 18 Centurion software.

## 2.3. Methods

Stock standard solutions of the pesticides (about 300 mg.L⁻¹) were prepared by dissolving the compounds in methanol. Serial dilutions were carried out to obtain the working standard solutions in water. To obtain natural water matrix samples free from organic contamination, traces of organic compounds were removed by preparative chromatography using SPE C18 cartridge. The cartridge was preconditioned with 5mL of methanol followed by 5mL of ultrapure water; 50mL of water samples were passed through the cartridge before being spiked.
<table><caption>**Table1:** Chemical properties of deltamethrin and cyhalothrin.</caption><thead><tr><th>Pesticides</th><th>Chemical properties</th><th>Formula</th></tr></thead><tbody><tr><td rowspan="8">Deltamethrin</td><td>Formula</td><td>C<sub>23</sub>H<sub>19</sub>Br<sub>2</sub>NO<sub>3</sub></td></tr><tr><td>Molecular</td><td>505.2 g mol<sup>-1</sup></td></tr><tr><td>Water solubility (25°C)</td><td>&lt; 2x10<sup>-4</sup> mg L<sup>-1</sup></td></tr><tr><td>Melting point</td><td>90°C</td></tr><tr><td>Boiling point</td><td>300°C</td></tr><tr><td>Vapor pressure (25 °C)</td><td>1.5x10<sup>-8</sup> mmHg</td></tr><tr><td>Log Kow</td><td>6.1</td></tr><tr><td>Henry's constant</td><td>1.2x10<sup>-4</sup> atm·m<sup>3</sup>/mol</td></tr><tr><td rowspan="8">Cyhalothrin</td><td>Formula</td><td>C<sub>23</sub>H<sub>19</sub>ClF<sub>3</sub>NO<sub>3</sub></td></tr><tr><td>Molecular</td><td>449.85 g mol<sup>-1</sup></td></tr><tr><td>Water solubility (20°C)</td><td>5x10<sup>-3</sup> mg L<sup>-1</sup></td></tr><tr><td>Melting point</td><td>49.2°C</td></tr><tr><td>Boiling point</td><td>Decomposes before boiling</td></tr><tr><td>Vapor pressure (25 °C)</td><td>1.1x10<sup>-6</sup> mmHg</td></tr><tr><td>Log Kow (25 °C)</td><td>7</td></tr><tr><td>Henry's constant</td><td>1.4x10<sup>-5</sup> atm·m<sup>3</sup>/mol</td></tr></tbody></table>

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

K<sub>ow</sub>: octanol-water partition coefficient

# 3. Results and Discussion

## 3.1. Thermo-induced and photo-induced fluorescence properties and degradation process

Deltamethrin and cyhalothrin show no native fluorescence, whereas elevated temperature in alkaline media, or UV irradiation in aqueous media, results in the formation of strongly fluorescent byproducts.

For photo-induced fluorescence, by classical PIF an optimal irradiation time of 3 mn has been found for both deltamethrin and cyhalothrin (Figure 1) and consequently selected for all the study; for AUTOPIF we have found the same optimal irradiation time of 3 mn for both pesticides which is selected by adjusting the flow. For thermo-induced fluorescence, an optimal heating time of 4 mn at an optimal pH of 12 was chosen for deltamethrin and cyhalothrin by reference to a previous study [10].
<figure><img src="image_4.png" /><figcaption>**Fig 1:** Formation of the PIF photoproducts of cyhalothrin and deltamethrin (1 µg.mL⁻¹) in pure water as a function of the UV irradiation time</figcaption></figure>

The Figure 2 compares the thermo-induced and photo-induced excitation and emission fluorescence spectra for deltamethrin (left panel) and for cyhalothrin (right panel). As AUTOPIF and PIF uses the same irradiation source, we obtain quite similar spectra, and we have chosen to present only the classical PIF spectra.

By comparing first the excitation spectra, for deltamethrin the TIF excitation spectrum has only one band, while the PIF one present three bands (Table2). The most intense band of the PIF (220 nm) is close to the TIF one (222 nm); while the two others are red-shifted at 250 and 280 nm. Approximately the same observations can be made about the cyhalothrin excitation spectrum: the main PIF band is at 220 nm and two less intense bands are at 250 and 290 nm, while the TIF band at 222 nm is close to the PIF one.

Concerning secondly the emission spectra, both pesticides presents only one band significantly different for PIF and TIF. For deltamethrin the PIF band is the most intense at 320 nm and the TIF one is blue shifted at 295 nm. For cyhalothrin the PIF band is also the most intense at 310 nm and the TIF one is found at 292 nm.
<figure><img src="image_5.png" /><figcaption>**Figure 2:** Thermo-induced and photo-induced excitation and emission spectra of deltamethrin (left panel) and cyhalothrin (right panel) (1 µg.mL⁻¹ for both pesticides), after 4mn of thermo-hydrolysis in alkaline medium for TIF and 3 mn of UV irradiation in pure water for PIF.</figcaption></figure>

<table><caption>**Table 2:** Comparison of the maximal excitation and emission wavelength for deltamethrin and cyhalothrin and for thermo and photo induced by-products.</caption><thead><tr><th rowspan="2">Pesticide</th><th colspan="2">Maximal excitation wavelength (nm)</th><th colspan="2">Maximal emission wavelength (nm)</th></tr><tr><th>TIF</th><th>PIF</th><th>TIF</th><th>PIF</th></tr></thead><tbody><tr><td>Deltamethrin</td><td>224</td><td><b>220</b>; 250; 280</td><td>295</td><td>320</td></tr><tr><td>Cyhalothrin</td><td>222</td><td><b>200</b>; 250; 290</td><td>292</td><td>310</td></tr></tbody></table>

The emission spectra of the two pesticides are very different, confirming that photo-degradation and thermo-degradation processes are different, as some authors have pointed out.

Regarding photo-degradation, Liu et al. [29], and Nahri-Niknafs et al. [30], worked on the process of fenvalerate and deltamethrin pyrethroids under simulated natural conditions, and identified the by-products by gas chromatography-mass spectrometry. They showed that photodegradation of two pyrethroids is mainly based on four main reactions, namely, ester cleavage, decyanation, photoisomerization and photooxidation. They identified three identical byproducts to both pesticides, namely the 3-phenoxy benzaldehyde, *p*-chlorobenzaldehyde, and 4-chlorobenzoic acid ethyl ester (Table 3). As well as the work of Xie et al. [31], on the influence of coexisting humic acids or Cu²⁺ on the photodegradation pyrethroid lambda-cyhalothrin and cypermethrin in aqueous solution. The different by-products of the photodegradation were determined using gas chromatography/mass spectrometry. The ester bond cleavage, debromination, dechlorination, decarboxylation and phenyl group removal were the possible photodegradation pathways, which made it possible to identify eleven by-products (Table 3). Recently, Thiaré et al. [22] have studied
the photodegradation of pyrethroid fenvalerate and identified eighteen by-products by NMR and chromatography/mass spectrometry. They showed that the photo-degradation of fenvalerate based on decyanation, decarboxylation, photooxidation, ester cleavage, photoisomerization and dechlorination obeys the same mechanism as deltamethrin and cyhalothrin.

Regarding thermo-degradation, two by-products results of the basic hydrolysis reaction of deltamethrin namely the 1-R,cis-decamethrinic acid and 3-phenoxybenzaldehyde were identified by Aiello et al. [32], (Table 4). These two by-products were identified by combining NMR, HPLC, GC-MS, and ESI-MS analyses. Summooguma et al. [33], studied the thermal decomposition of pyrethroid alpha-cypermethrin, and identified eleven by-products by GC-MS among which we can note the 3-Phenoxybenzaldehyde (Table 4).

Finally, these works elucidated that the photo-degradation and thermo-degradation process have, among all the by-products founded, three identical by-products for pyrethroids, namely 3-Phenoxybenzaldehyde, 3-phenoxyphenyl acetonitrile and 3-phenoxybenzoyl cyanide. These three by-products can be the source of the TIF and PIF fluorescence signal.
<table><caption>**Table 3:** Deltamethrin and cyhalothrin photodegradation products reported in the literature</caption><thead><tr><th>Byproducts</th><th>Structure relating to:<br>Deltamethrin (D);<br>Cyhalothrin (C);<br>Fenvalerate (F)</th><th>Reference</th></tr></thead><tbody><tr><td>3-Phenoxybenzaldehyde</td><td>D; C; F</td><td>Liu et al. 2010;<br>Nahri-Niknafs et al. 2013;<br>Thiaré et al. 2015;<br>Xie et al. 2011</td></tr><tr><td>3-Phenoxybenzoic acid</td><td>D; C; F</td><td>Liu et al. 2010;<br>Nahri-Niknafs et al. 2013;<br>Thiaré et al. 2015;<br>Xie et al. 2011</td></tr><tr><td>4-chlorobenzoic acid ethyl<br>ester</td><td>D; C</td><td>Liu et al. 2010;<br>Xie et al. 2011</td></tr><tr><td>1-methyl-3-<br>phenoxybenzene</td><td>C</td><td>Xie et al. 2011;</td></tr><tr><td>3-phenoxyphenyl<br>acetonitrile</td><td>D; C; F</td><td>Liu et al. 2010;<br>Thiaré et al. 2015;<br>Xie et al. 2011</td></tr><tr><td>3-methylphenol</td><td>F</td><td>Xie et al. 2011</td></tr><tr><td>α-cyano-3-hydroxybenzyl-<br>3-(3, 3, 3-trifluoroprop-1-<br>en-1-yl)2, 2-dimethylcyclopropanecarb<br>oxylate</td><td>F</td><td>Xie et al. 2011</td></tr><tr><td>α-cyano-3-phenoxybenzyl<br>alcohol</td><td>D; C; F</td><td>Liu et al. 2010;<br>Nahri-Niknafs et al. 2013;<br>Thiaré et al. 2015;<br>Xie et al. 2011</td></tr><tr><td>[3-(2-chloro-3, 3, 3-<br>trifluoroprop-1-en-1-yl)-<br>2,2-dimethylcyclopropyl]<br>(3-phenoxyphenyl)<br>acetonitrile</td><td>C</td><td>Xie et al. 2011</td></tr><tr><td>3-(2-chloro-3, 3, 3-<br>trifluoroprop-1-en-1-yl)-2,<br>2-dimethylcyclopropanecarb<br>oxylic acid</td><td>C</td><td>Xie et al. 2011</td></tr></tbody></table>
<table><caption>**Table 4:** Deltamethrin and cyhalothrin thermodegradation products reported in the literature.</caption><thead><tr><th>Byproducts</th><th>Structure</th><th>Relating to:<br>Deltamethrin (D);<br>Cypermethrin (C)</th><th>Reference</th></tr></thead><tbody><tr><td>(1R,3R)-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropanecarbonylic acid</td><td><img src="" alt="Chemical structure of (1R,3R)-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropanecarbonylcyanide"></td><td>D</td><td>Aiello et al. 2021</td></tr><tr><td>3-Phenoxybenzaldehyde</td><td><img src="" alt="Chemical structure of 3-Phenoxybenzaldehyde"></td><td>D; C</td><td>Aiello et al. 2021; Summoogum et al. 2015</td></tr><tr><td>1-methyl-3-phenoxybenzene</td><td><img src="" alt="Chemical structure of 1-methyl-3-phenoxybenzene"></td><td>C</td><td>Summoogum et al. 2015</td></tr><tr><td>3-phenoxyphenyl acetonitrile</td><td><img src="" alt="Chemical structure of 3-phenoxyphenyl acetonitrile"></td><td>D; C</td><td>Aiello et al. 2021; Summoogum et al. 2015</td></tr><tr><td>3-phenoxybenzoyl cyanide</td><td><img src="" alt="Chemical structure of 3-phenoxybenzoyl cyanide"></td><td>D; C</td><td>Aiello et al. 2021; Summoogum et al. 2015</td></tr></tbody></table>

## 3.2. Analytical applications

Analytical applications of the thermo-induced, photo-induced and AUTOPIF methods were performed with the following protocol.

Calibration curves were constructed by preparing triplicate samples containing increasing concentrations of each pesticide. The study was performed with concentration ranges of 0.01–2 µg.mL⁻¹ for deltamethrin and cyhalothrin. The linearity of the calibration and standard addition curves was evaluated by a variance analysis. The regression variance (V<sub>REG</sub>) is significantly greater than the residual variance (V<sub>RES</sub>) if the *p*-value is less or equal to 5%, meaning that the regression is significant. In addition, the lack of fit variance is not significantly greater than the pure error variance if the *p*-value is greater than 5%, thus validating the linear model. A Student's *t* is then used to check if intercepts of calibration and addition curves are not significantly different from zero (*p*-value > 5%).

The method was tested by recovery studies performed on natural tap and well water samples collected in the Niayes district. Samples were analysed by the standard addition method and initially fortified at 0.2 µg.mL⁻¹ for both pesticides. Increasing concentrations were added to the samples, and the resulting standard addition curves were compared to the calibration curves. The linearity of the standard addition curves is also tested by variance analysis. The parallelism between the standard addition curves and the reference calibration curve is then evaluated by a student's *t* test on the slope
of the curves. If the tabulated Student's *t* value (5 % confidence level) is greater than the *t* calculated value of the difference of the two slopes, this shows that there is no significant difference between them, attesting the absence of a significant matrix effect. Recovery values for each addition and the mean recovery are also calculated along with its relative standard deviation.

### 3.3. Thermo-induced fluorescence analytical applications

The TIF calibrations curves (Figure 3) are linear with an intercept not significantly different from zero (Table 5) using an optimal heating time of 4 mn [10]. The standard addition curves are parallel showing the absence of matrix effect (Table 6). Even if deltamethrin seems to be more sensitive, the detection limits obtained are very close with 5.6 for deltamethrin and 5.4 for cyhalothrin. In tap water and well water, the mean recovery values are comprised between 95% and 105% (Table 7).

<figure><img src="image_6.png" /><figcaption>Fig 3: Calibration and standard addition curves of cyhalothrin and deltamethrin (TIF method)</figcaption></figure>

<table><caption>**Table 5:** Evaluation parameters of the linear functions by variance analysis at a confidence level of 5% (TIF method).</caption><thead><tr><th></th><th></th><th>Deltaméthrin</th><th>Cyhalothrin</th></tr></thead><tbody><tr><td rowspan="3">ANOVA 1</td><td>V<sub>REG</sub></td><td>8.73 10<sup>4</sup></td><td>5.13 10<sup>4</sup></td></tr><tr><td>V<sub>RES</sub></td><td>2.83 10<sup>2</sup></td><td>8.74</td></tr><tr><td>p value</td><td>0.00</td><td>0.00</td></tr><tr><td rowspan="3">ANOVA 2</td><td>V<sub>LOF</sub></td><td>342.35</td><td>10.89</td></tr><tr><td>V<sub>PE</sub></td><td>224.01</td><td>6.94</td></tr><tr><td>p value</td><td>0.31</td><td>0.29</td></tr></tbody></table>
<table><caption>**Table 6.** Comparison of the standard addition curve and calibration curve slopes by Student *t* tests.</caption><thead><tr><th rowspan="2"></th><th colspan="3">Deltamethrin</th><th colspan="3">Cyhalothrin</th></tr><tr><th>Ultrapure water</th><th>Tap water (Brest)</th><th>Tap water (Mboro)</th><th>Ultrapure water</th><th>Tap water (Brest)</th><th>Tap water (Mboro)</th></tr></thead><tbody><tr><td>Slope</td><td>146.45</td><td>161.65</td><td>161.98</td><td>92.78</td><td>98.35</td><td>96.60</td></tr><tr><td>Intercept</td><td>15.89</td><td>59.47</td><td>37.77</td><td>3.10</td><td>40.63</td><td>37.91</td></tr><tr><td>Intercept STD</td><td>8.64</td><td>7.62</td><td>5.52</td><td>2.08</td><td>4.65</td><td>3.24</td></tr><tr><td>SD to 0</td><td>No</td><td></td><td></td><td>No</td><td></td><td></td></tr><tr><td>DL (ng.mL<sup>-1</sup>)</td><td>5.6</td><td></td><td></td><td>5.4</td><td></td><td></td></tr><tr><td>t<sub>D</sub></td><td></td><td>0.273</td><td>0.298</td><td></td><td>0.181</td><td>0.288</td></tr><tr><td>ts (5%)</td><td></td><td>2.260</td><td>2.260</td><td></td><td>2.260</td><td>2.260</td></tr><tr><td>d.o.f</td><td></td><td>4</td><td>4</td><td></td><td>4</td><td>4</td></tr><tr><td>SD to calibration</td><td></td><td>No</td><td>No</td><td></td><td>No</td><td>No</td></tr></tbody></table>

STD: standard deviation of the slope of the standard addition curve; d.o.f.: degrees of freedom; t<sub>D</sub>: calculated Student value of the difference between the two slopes; t: tabulated Student *t* value; SD significant difference; DL: detection limit, defined as the concentration of analyte giving a signal-to-noise ratio of 3.

<table><caption>**Table 7:** Recovery results obtained by TIF method for deltamethrin and cyhalothrin in tap water of Brest (France) and well water of Niayes area (Senegal).</caption><thead><tr><th>Matrix</th><th>Pesticide</th><th>Added (µg mL<sup>-1</sup>)</th><th>Found (µg mL<sup>-1</sup>)</th><th>Recovery %</th><th>Mean recovery %</th></tr></thead><tbody><tr><td rowspan="10">Tap water (Brest)</td><td rowspan="5">DTM</td><td>0.50</td><td>0.45 ± 0.04</td><td>91.14 ± 9.73</td><td rowspan="5">94.16 ± 2.39</td></tr><tr><td>0.70</td><td>0.64 ± 0.05</td><td>92.59 ± 8.00</td></tr><tr><td>1.00</td><td>0.94 ± 0.05</td><td>94.92 ± 5.35</td></tr><tr><td>1.20</td><td>1.17 ± 0.03</td><td>97.37 ± 2.70</td></tr><tr><td>1.50</td><td>1.42 ± 0.09</td><td>94.79 ± 5.49</td></tr><tr><td rowspan="5">CLT</td><td>0.70</td><td>0.76 ± 0.06</td><td>108.79 ± 8.08</td><td rowspan="5">105.83 ± 2.92</td></tr><tr><td>1.00</td><td>1.07 ± 0.07</td><td>107.75 ± 7.20</td></tr><tr><td>1.30</td><td>1.33 ± 0.03</td><td>102.63 ± 2.56</td></tr><tr><td>1.60</td><td>1.66 ± 0.06</td><td>104.15 ± 3.98</td></tr><tr><td>1.90</td><td>2.01 ± 0.11</td><td>106.10 ± 5.75</td></tr><tr><td rowspan="10">Tap water (Mboro)</td><td rowspan="5">DTM</td><td>1.00</td><td>0.80 ± 0.06</td><td>80.12 ± 11.07</td><td rowspan="5">85.02 ± 3.91</td></tr><tr><td>1.2</td><td>1.00 ± 0.08</td><td>83.64 ± 9.92</td></tr><tr><td>1.50</td><td>1.32 ± 0.12</td><td>88.00 ± 11.10</td></tr><tr><td>1.70</td><td>1.50 ± 0.11</td><td>88.34 ± 8.35</td></tr><tr><td>1.90</td><td>1.71 ± 0.15</td><td>90.16 ± 10.15</td></tr><tr><td rowspan="5">CLT</td><td>0.50</td><td>0.51 ± 0.01</td><td>101.21 ± 1.20</td><td rowspan="5">102.01 ± 3.63</td></tr><tr><td>0.70</td><td>0.67 ± 0.02</td><td>96.62 ± 3.49</td></tr><tr><td>1.00</td><td>1.02 ± 0.02</td><td>102.06 ± 2.02</td></tr><tr><td>1.30</td><td>1.38 ± 0.08</td><td>106.54 ± 6.13</td></tr><tr><td>1.60</td><td>1.65 ± 0.06</td><td>103.64 ± 3.51</td></tr></tbody></table>
### 3.4. Photo-induced fluorescence analytical applications

Concerning PIF results, the irradiation time has been set to its optimum, the calibrations curves obtained (Figure 4) are also linear with an intercept not significantly different from zero (Table 8). Like for TIF, the standard addition curves are parallel to the calibration showing the absence of matrix effect (Table 9 and 10). The sensitivities are very close as the detection limits i.e. obtained are very close with 3.9 and 3.7 for deltamethrin and cyhalothrin respectively. These detection limits are lower than those obtained by the TIF method.

<figure><img src="image_7.png" /><figcaption>**Fig 4:** Calibration and standard addition curves of cyhalothrin and deltamethrin in tap water of Brest (France) and well water of Mboro (Senegal).</figcaption></figure>

<table><caption>**Table 8.** Evaluation parameters of the linear functions by variance analysis at a confidence level of 5%.</caption><thead><tr><th></th><th></th><th>Deltamethrin</th><th>Cyhalothrin</th></tr></thead><tbody><tr><td rowspan="3">ANOVA 1</td><td>V<sub>REG</sub></td><td>1.38 10<sup>5</sup></td><td>2.2910<sup>5</sup></td></tr><tr><td>V<sub>RES</sub></td><td>90.81</td><td>10.82 10<sup>2</sup></td></tr><tr><td><i>p</i> value</td><td>0.00</td><td>0.00</td></tr><tr><td rowspan="3">ANOVA 2</td><td>V<sub>LOF</sub></td><td>62.88</td><td>822.87</td></tr><tr><td>V<sub>PE</sub></td><td>118,73</td><td>1343.04</td></tr><tr><td><i>p</i> value</td><td>0,77</td><td>0.72</td></tr></tbody></table>
<table><caption>**Table 9.** Comparison of the standard addition curve and calibration curve slopes by Student *t* tests in tap water of Brest (France) and well water of Mboro (Senegal).</caption><thead><tr><th rowspan="2"></th><th colspan="3">Deltamethrin</th><th colspan="3">Cyhalothrin</th></tr><tr><th>Ultrapure water</th><th>Tap water</th><th>Well water</th><th>Ultrapure water</th><th>Tap water</th><th>Well water</th></tr></thead><tbody><tr><td>Slope</td><td>265.21</td><td>270.47</td><td>254.83</td><td>333.55</td><td>375.31</td><td>379.86</td></tr><tr><td>Intercept</td><td>6.56</td><td>45.53</td><td>86.32</td><td>22.68</td><td>120.73</td><td>109.62</td></tr><tr><td>Intercept STD</td><td>5.08</td><td>2.91</td><td>3.88</td><td>17.09</td><td>2.167</td><td>2.18</td></tr><tr><td>SD to 0</td><td>No</td><td></td><td></td><td>No</td><td></td><td></td></tr><tr><td>DL (ng.mL<sup>-1</sup>)</td><td>3.9</td><td></td><td></td><td>3.7</td><td></td><td></td></tr><tr><td>t<sub>D</sub></td><td></td><td>0.305</td><td>0.213</td><td></td><td>0.395</td><td>0.463</td></tr><tr><td>ts (5%)</td><td></td><td>2.260</td><td>2.260</td><td></td><td>2.260</td><td>2.260</td></tr><tr><td>d.o.f</td><td></td><td>4</td><td>4</td><td></td><td>4</td><td>4</td></tr><tr><td>SD to calibration</td><td></td><td>No</td><td>No</td><td></td><td>No</td><td>No</td></tr></tbody></table>

STD: standard deviation of the slope of the standard addition curve; d.o.f.: degrees of freedom; t<sub>D</sub>: calculated Student value of the difference between the two slopes; t: tabulated Student *t* value; SD significant difference; DL: detection limit, defined as the concentration of analyte giving a signal-to-noise ratio of 3.

<table><caption>**Table 10.** Recovery results obtained by PIF method for deltamethrin and cyhalothrin in tap water of Brest (France) and well water of Mboro (Senegal).</caption><thead><tr><th>Matrix</th><th>Pesticide</th><th>Added (µg mL<sup>-1</sup>)</th><th>Found (µg mL<sup>-1</sup>)</th><th>Recovery %</th><th>Mean recovery %</th></tr></thead><tbody><tr><td rowspan="6">Tap water</td><td rowspan="5">DTM</td><td>0.50</td><td>0.42 ± 0.08</td><td>84.09 ± 18.92</td><td rowspan="5">88.04 ± 3.03</td></tr><tr><td>0.80</td><td>0.70 ± 0.09</td><td>88.22 ± 13.36</td></tr><tr><td>1.00</td><td>0.88 ± 0.11</td><td>88.36 ± 13.18</td></tr><tr><td>1.20</td><td>1.09 ± 0.10</td><td>91.48 ± 09.32</td></tr><tr><td>1.40</td><td>1.36 ± 0.04</td><td>97.14 ± 02.94</td></tr><tr><td rowspan="5">CLT</td><td>0.60</td><td>0.48 ± 0.11</td><td>80.37 ± 24.42</td><td rowspan="5">84.78 ± 5.17</td></tr><tr><td>0.80</td><td>0.65 ± 0.14</td><td>81.73 ± 22.36</td></tr><tr><td>1.00</td><td>0.81 ± 0.18</td><td>81.15 ± 23.23</td></tr><tr><td>1.20</td><td>1.06 ± 0.13</td><td>89.04 ± 12.31</td></tr><tr><td>1.40</td><td>1.28 ± 0.11</td><td>91.62 ± 9.14</td></tr><tr><td rowspan="6">Well water</td><td rowspan="5">DTM</td><td>0.50</td><td>0.56 ± 0.06</td><td>112.45 ± 11.07</td><td rowspan="5">111.27 ± 1.38</td></tr><tr><td>0.80</td><td>0.88 ± 0.08</td><td>111.01 ± 9.92</td></tr><tr><td>1.00</td><td>1.12 ± 0.12</td><td>112.49 ± 11.10</td></tr><tr><td>1.20</td><td>1.30 ± 0.11</td><td>109.11 ± 8.35</td></tr><tr><td>1.40</td><td>1.55 ± 0.15</td><td>111.30 ± 10.15</td></tr><tr><td rowspan="5">CLT</td><td>0.80</td><td>0.70 ± 0.09</td><td>88.19 ± 13.39</td><td rowspan="5">87.24 ± 3.03</td></tr><tr><td>1.00</td><td>0.82 ± 0.17</td><td>82.76 ± 20.83</td></tr><tr><td>1.20</td><td>1.06 ± 0.13</td><td>88.60 ± 12.87</td></tr><tr><td>1.40</td><td>1.25 ± 0.15</td><td>89.42 ± 11.83</td></tr><tr><td>1.60</td><td>1.45 ± 0.14</td><td>91.13 ± 09.74</td></tr></tbody></table>
### 3.5. AUTOPIF analytical applications

For AUTOPIF results, the flow has been adjusted to obtain the optimum irradiation time, the calibrations curves obtained (Figure 5) are also found linear with an intercept not significantly different from zero (Table 11). Like for TIF and PIF, the standard addition curves are parallel to the calibration showing the absence of matrix effect (Table 12 and 13). The detection limits obtained for deltamethrin and cyhalothrin, 1.65 and 0.29 respectively, are lower than those obtained by TIF and PIF methods.

<figure><img src="image_8.png" /><figcaption>**Fig 5:** Calibration and standard addition curves of cyhalothrin and deltamethrin AUTOPIF method in tap water from Brest (France) and well water from Mboro (Senegal).</figcaption></figure>

<table><caption>**Table 11:** Evaluation parameters of the linear functions by variance analysis at a confidence level of 5%. (AUTOPIF method)</caption><thead><tr><th></th><th></th><th>Deltamethrin</th><th>Cyhalothrin</th></tr></thead><tbody><tr><td rowspan="3">ANOVA 1</td><td>V<sub>REG</sub></td><td>3.6710<sup>6</sup></td><td>2.09 10<sup>6</sup></td></tr><tr><td>V<sub>RES</sub></td><td>30.17 10<sup>2</sup></td><td>27.48 10<sup>2</sup></td></tr><tr><td>p value</td><td>0.00</td><td>0.00</td></tr><tr><td rowspan="3">ANOVA 2</td><td>V<sub>LOF</sub></td><td>3279.85</td><td>3821.34</td></tr><tr><td>V<sub>PE</sub></td><td>2711.83</td><td>1676.26</td></tr><tr><td>p value</td><td>0.42</td><td>0.17</td></tr></tbody></table>
<table><caption>**Table 12.** Comparison of the standard addition curve and calibration curve slopes by Student *t* tests in tap water from Brest (France) and well water from Mboro (Senegal).</caption><thead><tr><th rowspan="2"></th><th colspan="3">Deltamethrin</th><th colspan="3">Cyhalothrin</th></tr><tr><th>Ultrapure water</th><th>Tap water</th><th>Well water</th><th>Ultrapure water</th><th>Tap water</th><th>Well water</th></tr></thead><tbody><tr><td>Slope</td><td>601.94</td><td>601.07</td><td>606.88</td><td>516.19</td><td>519.52</td><td>516.9</td></tr><tr><td>Intercept</td><td>53.62</td><td>434.05</td><td>258.27</td><td>23.75</td><td>133.33</td><td>147.05</td></tr><tr><td>Intercept STD</td><td>5.45</td><td>31.13</td><td>31.13</td><td>26.66</td><td>64.73</td><td>61.69</td></tr><tr><td>P value</td><td>No</td><td></td><td></td><td>No</td><td></td><td></td></tr><tr><td>SD to 0</td><td>1.65</td><td></td><td></td><td>0.29</td><td></td><td></td></tr><tr><td>DL (ng.mL<sup>-1</sup>)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>t<sub>D</sub></td><td></td><td>1.055</td><td>1.055</td><td></td><td>0.681</td><td>0.724</td></tr><tr><td>ts (5%)</td><td></td><td>2.260</td><td>2.260</td><td></td><td>2.260</td><td>2.260</td></tr><tr><td>d.o.f</td><td></td><td>4</td><td>4</td><td></td><td>4</td><td>4</td></tr><tr><td>SD to calibration</td><td></td><td>No</td><td>No</td><td></td><td>No</td><td>No</td></tr></tbody></table>

STD: standard deviation of the slope of the standard addition curve; d.o.f.: degrees of freedom; t<sub>D</sub>: calculated Student value of the difference between the two slopes; t: tabulated Student *t* value; SD significant difference; DL: detection limit, defined as the concentration of analyte giving a signal-to-noise ratio of 3.

<table><caption>**Table 13:** Recovery results obtained by AUTOPIF method for deltamethrin and cyhalothrin in tap water from Brest (France) and well water from Mboro (Senegal).</caption><thead><tr><th>Matrix</th><th>Pesticide</th><th>Added<br>(µg mL<sup>-1</sup>)</th><th>Found<br>(µg mL<sup>-1</sup>)</th><th>Recovery %</th><th>Mean<br>recovery %</th></tr></thead><tbody><tr><td rowspan="10">Well water</td><td rowspan="5">DTM</td><td>0.70</td><td>0.66 ± 0.11</td><td>95.53 ± 04.68</td><td rowspan="5">92.01 ± 5.35</td></tr><tr><td>1.00</td><td>0.87 ± 0.13</td><td>87.23 ± 14.64</td></tr><tr><td>1.60</td><td>1.40 ± 0.19</td><td>87.66 ± 14.08</td></tr><tr><td>1.90</td><td>1.85 ± 0.04</td><td>97.63 ± 02.43</td></tr><tr><td>2.20</td><td>2.18 ± 0.02</td><td>99.09 ± 00.92</td></tr><tr><td rowspan="5">CLT</td><td>1.00</td><td>0.87 ± 0.13</td><td>87.05 ± 14.88</td><td rowspan="5">89.72 ± 4.15</td></tr><tr><td>1.30</td><td>1.11 ± 0.19</td><td>85.47 ± 17.00</td></tr><tr><td>1.50</td><td>1.41 ± 0.09</td><td>94.28 ± 06.07</td></tr><tr><td>1.90</td><td>1.75 ± 0.15</td><td>92.10 ± 08.58</td></tr><tr><td>2.50</td><td>2.32 ± 0.18</td><td>92.94 ± 07.59</td></tr><tr><td rowspan="10">Tap water</td><td rowspan="5">DTM</td><td>1.30</td><td>1.48 ± 0.18</td><td>114.27 ± 12.49</td><td rowspan="5">102.85 ± 11.13</td></tr><tr><td>1.60</td><td>1.48 ± 0.11</td><td>92.84 ± 07.71</td></tr><tr><td>1.90</td><td>1.78 ± 0.12</td><td>93.78 ± 06.63</td></tr><tr><td>2.20</td><td>2.43 ± 0.23</td><td>110.53 ± 09.52</td></tr><tr><td>2.50</td><td>2.61 ± 0.12</td><td>104.77 ± 04.55</td></tr><tr><td rowspan="5">CLT</td><td>1.00</td><td>0.84 ± 0.16</td><td>83.90 ± 19.19</td><td rowspan="5">86.14 ± 4.41</td></tr><tr><td>1.30</td><td>1.07 ± 0.23</td><td>82.35 ± 21.43</td></tr><tr><td>1.60</td><td>1.37 ± 0.22</td><td>85.91 ± 16.40</td></tr><tr><td>1.90</td><td>1.75 ± 0.14</td><td>92.38 ± 08.25</td></tr><tr><td>2.20</td><td>2.01 ± 0.19</td><td>91.39 ± 09.42</td></tr></tbody></table>
4. Discussion and conclusion

We analysed two insecticides deltamethrin and cyhalothrin by three different methods using photo-induced fluorescence or thermo-induced fluorescence. The three methods gave linear calibrations without significant matrix effects and with good recovery rates.

- By TIF, we obtained a detection limit 5.6 and 5.4 ng.mL⁻¹ for deltamethrin and cyhalothrin in water, Mendy et al. obtained better results of 0,8 and 3.0 ng.mL⁻¹ for metolachlor and buprofezin respectively [11] but in micellar medium.

- By PIF we obtained a detection limit 3.9 and 3.7 ng.mL⁻¹ for deltamethrin and cyhalothrin, which is better than the detection limit of 12 ng.ml⁻¹ obtained by Thiare et al. for the determination of deltamethrin in river water [22].

- AUTOPIF method is more sensitive than the classical PIF and TIF methods and we achieved very good detection limits of 0.29 and 1.65 ng.mL⁻¹ for deltamethrin and cyhalothrin respectively. Comparing our results with those of other monitoring systems, Bakhoum et al. obtained for example upper detection limits of 12 ng.mL⁻¹ of fluometuron in river water [24] and of 8.43 ng.mL⁻¹ of acetamipride in tap water [25].

It is also useful to compare the technical advantages and disadvantages of the three methods.

- The TIF method is the simplest, except for the need of a commercial spectrofluorometer, it does not require any specific equipment apart from some basic chemicals and can be used for a reasonable cost.

- The PIF method additionally requires a UV lamp to create the PIF compounds, which is inexpensive but requires some protection for the experimenters. Despite this, the method is not much more expensive and has the advantage of not using chemicals.

- AUTOPIF is perhaps the cheapest method because as it does not require a commercial spectrofluorometer, but only a less expensive miniature spectrometer, UV LEDs and a peristaltic pump. However, it is the most technological method as it requires a good expertise of the experimental system and of the spectrometer control software.

Nevertheless, after evaluating all the methods developed in this work, we can conclude that the AUTOPIF method is analytically useful, as it is sensitive and can be used on a wide range of pesticides.
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