S AMPLING AND E XTRACTION OF P ESTICIDES IN R AINWATER S AMPLES

10.2.2 S AMPLING AND E XTRACTION OF P ESTICIDES IN R AINWATER S AMPLES

10.2.2.1 Sampling of Rainwater Rainwater samples are collected using different systems depending on studies and

47 authors. Asman et al. 24 and Epple et al. used for their study on pesticides in rainwater in Denmark and Germany, respectively, a cooled wet-only collector of

the type NSA 181=KE made by G.K. Walter Eigenbrodt Environmental Measure- ments Systems (Konigsmoor, Germany). It consists of a glass 2(Duran) funnel of ~500 cm diameter connected to a glass bottle that is kept in a dark refrigerator below the funnel at a constant temperature of 48C –88C. A conductivity sensor is activated when it starts to rain and then the lid on top of the funnel is removed. At the end of the rain period the lid is again moved back onto the funnel. With this system, no dry deposit to the funnel during dry periods is collected. Millet

et al. 48 and Scheyer et al. 49,50 used also a wet-only rainwater sampler built by Précis Mécanique (France). This collector is agreed by the French Meteorological Society (Figure 10.2). It consists of a PVC funnel of 250 mm diameter connected to

a glass bottle kept in the dark. No freezing of the bottle was installed and the stability of the sample was checked for one week in warm months. This collector is equipped with a moisture sensor which promotes the opening of the lid when rain occurs.

Quaghebeur et al. 27 used for their study in Belgium, a bulk collector made in stainless steel by the FEA (Flemish Environmental Agency, Ghent, Belgium).

The sampler consists of a funnel (D ~ 0.5 m) the sides of which meet at an angle of 1208. The outlet of the funnel is equipped with a perforated plate (D ~ 0.05 m). The holes have a diameter of 0.002 m. The funnel is connected with

a collecting flask. 22 26

Haraguchi et al. and Grynkiewicz et al. used a very simple bulk sampler which consists of a stainless steel funnel (40 cm or 0.5 m 2 diameter, respectively)

inserted in a glass bottle for their study of pesticides in rainwater in Japan and Poland, respectively.

Sampling and Analysis of Pesticides in the Atmosphere 265

Protection cover

Rain sensor

Collection cone

Sampling bottle

FIGURE 10.2 Wet-only rainwater collector. (From Scheyer, A., PhD thesis, University of Strasbourg, 2004.)

10.2.2.2 Extraction of Pesticides from Rainwater Extraction of pesticides was made using the conventional method used for water;

liquid –liquid extraction (LLE), solid-phase extraction (SPE), and solid-phase microextraction (SPME).

10.2.2.2.1 Liquid –liquid extraction This method was used by many authors. Chevreuil et al. 51 extracted pesticides from

rainwater by LLE three times with a mixture of 85% n-hexane=15% methylene chloride. Recoveries obtained were higher than 95% except for atrazine degradation metabolites (>75%). Depending on the chemical nature of the pesticide, Quaghe-

beur et al. 27 used different LLE extraction methods. Organochlorine pesticides, polychlorinated biphenyls, and trifluralin were extracted from the rainwater sample

using petroleum ether (extraction yield > 80%) while organophosphorous and organonitrogen compounds (i.e., atrazine) were extracted with dichloromethane (extraction yield > 80%).

266 Analysis of Pesticides in Food and Environmental Samples Kumari et al. 52 for their study of pesticides in rainwater in India used the

following procedure to extract pesticides from rainwater. Representative (500 mL) sample of water was taken in 1 L separatory funnel and 15 –20 g of sodium chloride was added. Liquid –liquid extraction (LLE) with 3 3 50 mL of 15% dichloromethane in hexane was performed. The combined organic phases were filtered through anhydrous sodium sulphate and this filtered extract was concentrated to near dryness on rotary vacuum evaporator. Complete removal of dichloromethane traces was ensured by adding 5 mL fractions of hexane twice and concentrating on gas manifold evaporator since electron capture detection (ECD) was used for the analysis of some pesticides.

All these authors do not use a cleanup procedure after LLE of rainwater samples mainly since they used very specific methods such as GC –ECD, GC–NPD, and GC –MS.

10.2.2.2.2 Solid-phase extraction

22 Solid-phase extraction (SPE) was used by Haraguchi et al., 46 Millet et al., Coupe

21 26 53 et al., 28 Grynkiewicz et al., Bossi et al., and Asman et al. These authors used XAD-2 resin or C 18 cartridges and they follow the classical

procedure of SPE extraction consisting of conditioning of the cartridge, loading of the sample, and elution of pesticides by different solvents. Haraguchi et al. 22 used

dichloromethane for the elution of pesticides trapped on XAD-2 cartridge while Asman et al. 28 used 5 mL of ethylacetate=hexane mixture (99:1 v=v) for the elution of pesticides from Oasis HLB 1000 mg cartridges (Waters) before GC –MS analysis.

A 200 mL volume of isooctane was added to the extract as a keeper to avoid losses of more volatile compounds during evaporation. For LC –MS–MS analysis, these authors used Oasis HLB 200 mg cartridges (Waters) and pesticides were eluted with 8 mL methanol. The extracts were evaporated to dryness and then redissolved in

1 mL of a Millipore water=methanol mixture (90:10 v=v) before LC –MS–MS in ESI mode analysis. Grynkiewicz et al. 26 used Lichrolut EN 200 mg cartridges (Merck) for the extraction of pesticides in rainwater. Pesticides were eluted with 6 mL of a mixture of methanol and acetonitrile (1:1). After it, a gentle evaporation to dryness under nitrogen was performed before analysis by GC –ECD (organochlorine pesticides) and

GC –NPD (organophosphorous and organonitrogen). Epple et al. 24 have compared two kinds of SPE cartridges for the extraction of

pesticides in rainwater samples and their analysis by GC –NPD: Bakerbond C 18 solid-phase extraction cartridges (Baker, Phillipsburg, NJ, USA) and Chromabond HR-P SDB (styrene –divinyl–benzene copolymer) cartridges 200 mg (Macherey- Nagel, Duren, Germany). The latter one is more efficient for polar compounds, such as the triazine metabolites. Prior to SPE extraction, rainwater samples were filtered by a glass fiber prefilter followed by a nylon membrane filter 0.45 nm. After that, filtered rainwater was filled with 5% of tetrahydrofuran (THF).

Elution was carried out with 5 mL of THF, the solvent evaporated, and the residue dried with a gentle stream of nitrogen and then dissolved in 750 mL of ethyl acetate. The sample was then cleaned by small silica-gel columns to remove polar components from precipitation samples. For this, 3 mL silica-gel columns

Sampling and Analysis of Pesticides in the Atmosphere 267 (5 3 0.9 cm boro silicate glass) with Teflon frits were used. The silica-gel type (60,

70 –230 mesh, Merck) was dried overnight at 1308C, mixed with 5% by weight of water, and transferred into glass tubes as a mixture with ethyl acetate, so that each column contained 0.8 g of silica gel. The sample (750 mL) was transferred to the column and eluted with 4 mL of ethyl acetate before GC –NPD analysis.

Recoveries of the method for all the pesticides studied are summarized in Table 10.1.

TABLE 10.1 Relative Standard Deviations, RSD, Recoveries, Rec., and Determination Limits, DL, (n ¼ 10, P ¼ 95%) for Determination of Pesticides in Wet-deposition Samples

Bakerbond C 18 Chromabond HR-P SDB

RSD Rec. DL Pesticide

(%) (%) (ng L 1 ) Desethyl atrazine 2

1.64 31 15 1.39 102 13 Desethyl terbuthylazine 2

Flusilazol 3 — 1.70 97 40 2.52 81 60 Propiconazol 3

— — — Source: From Epple, J. et al., Geoderma, 105, 327, 2002. With permission.

Concentration ranges: (1) 5 –50 ng L 1 ; (2) 20 –200 ng L 1 ; (3) 100 –1000 ng L 1 ; (4) 250 –2500 ng L 1 . Enantiomeric pairs numbered in the order of their elution times.

268 Analysis of Pesticides in Food and Environmental Samples Millet et al. 48 used also SPE extraction on Sep-Pak C 18 cartridges (Waters) and

elution with methanol for the analysis of pesticides in rainwater. Before analysis, they performed a HPLC fractionation as described earlier. 46

10.2.2.2.3 Solid-phase microextraction Among studies on pesticides in precipitation, extraction of pesticides was performed using classical developed methods for surface water. No special development was specifically done for atmospheric water. More recently, Scheyer et al. 49,50 used SPME for the analysis of pesticides in rainwater by GC –MS–MS. They used direct extraction for stable pesticides and a derivatization step coupled to SPME extraction for highly polar pesticides or thermo labile pesticides. These developments were derived from studies in water. SPME is a very interesting method for a fast and inexpensive determination of organic pollutants in water, including rainwater. The main advantage of SPME techniques is that it integrates sampling, extraction, and concentration in one step. This method is actually poorly used for the extraction of organic pollutants in atmospheric water probably because of low levels commonly found in precipitation.

For the evaluation of the spatial and temporal variations of pesticides’ concentra- tions in rainwater between urban (Strasbourg, East of France) and rural (Erstein, East of France) areas, Scheyer et al. 49 have developed a method using SPME and ion trap GC

MS – methyl, captan, chlorfenvinphos, dichlorvos, diflufenican, a and b-endosulfan, –MS for the analysis of 20 pesticides (alachlor, atrazine, azinphos-ethyl, azinphos-

iprodione, lindane, metolachlor, mevinphos, parathion-methyl, phosalone, phosmet, tebuconazole, triadimefon, and trifluralin) easily analyzable by gas chromatography (GC). For some seven other pesticides (bromoxynil, chlorotoluron, diuron, isopro-

turon, 2,4-MCPA, MCPP, and 2,4-D), Scheyer et al. 50 used SPME and GC –MS–MS but they add, prior to GC analysis, a derivatization step. SPME was chosen because it permits with accuracy a rapid extraction and analysis of a great number of samples and MS –MS enables the analysis of pesticides at trace level in the presence of interfering compounds without losing identification capability because of a drastic reduction of the background noise.

The first step in developing a method for SPME is the choice of the type of fiber. To do that, all other parameters are fixed (temperature, pH, ionic strength, etc.). The fiber depth in the injector was set at 3.4 cm and the time of the thermal desorption in the split –splitless injector was 5 min at 2508C, as recommended by

Supelco and confirmed by Scheyer et al. 49 Deeper fiber in the injector gave rise to carryover effects and less deeper fiber caused loss of response. The liner purge

was closed during the desorption of the analytes from the SPME fiber in the split – splitless injector (2 min delay time). A blank must be carried out with the same

fiber to confirm that all the compounds were desorbed within 5 min of thermal desorption.

In the method of Scheyer et al., 49 extractions were performed by immersion of the fiber in 3 mL of sample, with permanent stirring and temperature control at 408C,

during 30 min. Indeed, a headspace coating of the fiber is possible but, in the case of pesticides, this method cannot be used with efficiency because of the general low volatility of pesticides from water (Figure 10.3). However, for some volatile pesticides

Sampling and Analysis of Pesticides in the Atmosphere 269

SPME holder

Needle

Septum

Fiber exposed

Split /Splitless

injector

Sample Stirrer

Heating block

Immersion mode Headspace mode Capillary GC column FIGURE 10.3 Principle of SPME extraction. (From Scheyer, A., PhD thesis, University of

Strasbourg, 2004.) such as some organophosphorous pesticides, headspace coating of the fiber can be

developed. Since the SPME technique depends on an equilibrium process that involves the adsorption of analytes from a liquid sample into the polymeric phase according to their partition coefficient, the determination of the time (duration of extraction) required to reach this equilibrium for each compound is required.

The equilibration rate is limited by the mass transfer rate of the analyte through a thin static aqueous layer at the fiber –solution interface, the distribution constant of

the analyte, and the thickness and the kind of fiber coating 54 Moreover analytes with high molecular masses are expected to need longer equilibrium times because of

their lower diffusion coefficient since the equilibrium time is inversely proportional to the diffusion coefficient. 55

The temperature and the duration of extraction are associated since when increas- ing the temperature, it is possible to reach the equilibrium faster. Temperature can also modify the partition coefficient of the fiber and consequently decrease the amount of 54 extracted compound.

A compromise has to be determined between the temperature and the duration of the extraction in order to obtain a sensitive method for the analysis of pesticides in rainwater.

To increase the extraction efficiencies, it is possible to add some salts which have for effect to modify the ionic strength and to decrease the solubility of the molecules in the water.

SPME of Pesticides in Rainwater with a Derivatisation Step . The SPME tech- nique, firstly developed for GC analysis, integrates sampling, extraction, and con- centration in one step followed by GC analysis, even the use of HPLC is possible.

However, many pesticides such as phenyl ureas (PUHs), phenoxy acids, or carbamates cannot be analyzed directly by GC because of their low volatility or thermal instability. GC analysis of these molecules requires a derivatization step to stabilize or increase their volatilities.

270 Analysis of Pesticides in Food and Environmental Samples The use of SPME with derivatization is not commonly used for pesticides,

especially in the simultaneous determination of many class of pesticides such as phenyl ureas, phenoxy acids, phenolic herbicides, etc.

Derivatization (sylilation, alkylation, acylation) is employed for molecules where properties cannot permit their direct analysis by GC. 56,57 Alkylation with PFBBr is a very common reaction and permits the derivatization of molecules containing NH groups (chlorotoluron, diuron, and isoproturon),

–OH groups on aromatic ring (bromoxynil) and –COOH groups (MCPP, 2,4-D, 2,4-MCPA). The mechanism of reaction on a molecule containing a hydrogen acid is

a bimolecular nucleophile substitution (SN 2 ). 58

After extraction, samples present in organic solvents are derivatized by addition of a small amount of derivatizing agent. In the case of SPME, no solvent is present

and some approaches have been tested for combining derivatization and SPME. 54

Derivatization directly in the aqueous phase followed by SPME extraction

(direct technique). Derivatization on the fiber. This method consists of headspace coating of

PFBBr for 10 0 of the fiber followed by SPME extraction. In this case, extraction and derivatization are made simultaneously. Extraction of the analytes present in water followed by derivatization on the

fiber or onto the GC injector. For the direct technique, it is necessary to adjust the pH of the water below of the

pKa of the molecules to be derivatized (i.e., <2.73, which is the lowest pKa value for 2,4-D) since in this case they are protonated and consequently derivatization becomes possible.

Scheyer et al. 50 clearly showed that the exposure of the fiber to the derivatization reagent followed by extraction gave the better results and this method was used for the analysis of the seven pesticides, which required derivatization before analysis by GC, in rainwater.