E VALUATION OF S OIL /A IR T RANSFER OF P ESTICIDES

10.2.3 E VALUATION OF S OIL /A IR T RANSFER OF P ESTICIDES

(S PRAY D RIFT AND V OLATILIZATION )

As shown in the precedent paragraph, pesticides in ambient air are commonly sampled by high-volume samplers on filters and adsorbents (PUFs, XAD-2). After sampling, compounds trapped on the adsorbent must be released before determin- ation. For this, a solvent for desorption with Soxhlet or ultrasonic extraction, followed by a concentration step, is commonly used. It is generally time consuming and the different steps (extraction, cleanup, concentration, etc.) induced many losses and subsequently increased detection limits.

Even if the association of high-volume sampling and solvent extraction is accurate for the measurement of ambient level of trace contaminants, this method cannot be applied to assess spray drift and volatilization processes. Indeed, this kind of study required a short sampling periodicity to be close to the variation of atmospheric dissipation processes.

Sampling and Analysis of Pesticides in the Atmosphere 271 As quoted by Majewski, 59 estimation of volatilization rate in the field is classically

carried out using the aerodynamic profile. It gives an estimate of this mass transfer under actual field conditions and its variation with time. This method, based on the measurements of vertical profiles of pesticide concentrations in the atmosphere, needs

a good precision for the estimation of these concentrations. Also, the determination of concentration gradients requires the measurements of concentrations at four heights at least and consequently greatly increases the number of samples to analyze.

Thermal desorption can present a novel approach since it substantially simplifies analyses (no concentration step is needed) and increases sensitivity (a large part of the preconcentrated material may be recovered for determination), and detection limits and background noise are lower because of the disappearance of solvent components. Moreover, this technique is easily automatable. Because of these aspects, it seems to be an interesting alternative to solvent extraction to assess atmospheric transfer of pesticides during and after application. Thermal desorption has often been used for the analysis of VOCs in indoor and outdoor atmospheres. Thermal desorption for the analysis of pesticides has already been described for the volatile and stable pesticides trifluralin and triallate 60,61

62 in field measurements and atrazine in laboratory volatilization experiments.

Thermal desorption was extended to six pesticides in order to evaluate atmos- pheric transfer of pesticides following application (spray drift and volatilization). 63 To

the best of my knowledge, this was the first time that a thermal desorption unit –GC was interfaced with a mass selective detector to provide both pesticide quantification and confirmation. From the first results obtained in this study, it appears that thermal desorption followed by GC –mass spectroscopy (MS) analysis is accurate and sensi- tive but presents some limitations, especially as a result of the physicochemical properties of pesticides such as thermal stability and low volatility.

The principle of thermal desorption is detailed in Figure 10.4. It consists of two steps: (1) primary desorption, which consists of desorption of pesticides adsorbed on

PRIMARY DESORPTION

disadvantages of this (300°C during 15 min)

Thermal desorption

Advantages and

of sampling tube

technique: ADVANTAGES

He • Automatisation Sampling

Cold

tube

samples Cold trap

trap

He • Decrease of manipulations of

• Increase of sensitivity (no step re-condense

Mass

between sampling and analysis) compound

spectrometer

SECONDARY DESORPTION DISADVANTAGES GC • Only one injection

Thermal • Decomposition of compound desorption of cold

possible with temperature trap

Split

column

• Storage stability

Detection and

↓ 40°C ⫺1 .s

FIGURE 10.4 Principle of thermal desorption.

272 Analysis of Pesticides in Food and Environmental Samples the resin of the sampling tube and accumulation on a trap maintained at 308C by

peltier effect and (2) secondary desorption, which consists of the rapid heating of the trap before introduction on the GC column maintained at 508C.

Application of thermal desorption for pesticides presents some difficulties mainly because of the very low volatility of some of them. Briand et al. 64 have extended the method developed by Clément et al. 63 to deethylatrazine (DEA), deisopropylatrazine (DIA), carbofuran, cyprodinil, epoxyconazole, iprodione, 3,5-dichloroaniline, lindane, a-HCH, metolachlor, terbuconazole, and trifluralin.

The main problem dealing with the extraction of pesticides is the memory effects on the thermal desorption system. This problem was located on the cold trap containing glass wool.

To visualize an eventual memory effect for the 10 pesticides and metabolites under study, first experiments were performed as follows: A 400 ng amount of each compound was deposited at the end of a tube which was placed in the thermal desorption unit followed by four empty tubes (tubes without adsorbent) which were analyzed following the spiked tube.

To check the influence of the amount of pesticides accumulated on the cold trap, two parameters can be modified in the ATD system, the inlet-split flow rate (initially 1 at 0 mL min ), situated between the tube and the trap and the outlet-split flow rate

(initially at 20 mL min 1 ), located after the cold trap just before injection into the analytical column. This last flow rate imposed the gas velocity in the cold trap.

To evaluate the memory effect, two kinds of experiments were performed: one

1 modifying inlet-split flow rate of 10 mL min 1 (outlet-split flow rate 20 mL min ) and second modifying outlet-split flow rate (inlet-split flow rate 0 mL min 1 ). From the first experiment (inlet-split flow rate of 10 mL min 1 ), a strong decrease of the memory effect in all empty tubes, analyzed after sample tube, was observed since it remained only for cyprodinyl (0.93%) and tebuconazole (1.70%). Thus, the amount of pesticides reaching the cold trap seems to be the reason for the observed memory effect. However, a strong loss of sensitivity (20% –60%) especially for the most volatile compounds (DIA, DEA, a-HCH, trifluralin, carbofuran, lindane, atrazine, and alachlor) was observed. Thus, increasing the inlet-split flow rate cannot be used to resolve the memory effect problem. Experiments conducted with increasing outlet-split flow rate (30 and 35 mL min 1 ) induced a strong decrease of

the memory effect: 0.90% for cyprodynil with 30 mL min 1 , 1% for iprodione with

35 mL min 1 in the first empty tube. Percentages obtained in the second tube were not significant and can be neglected.

From these experiments, it appeared that increasing the outlet-split flow rate 1 from 20 to 30 mL min limit the memory effect. These outlet splits correspond to 5% and 3.3%, respectively, of the total amount of spiked compound in the tube actually injected into the GC-column. Increasing the outlet-split flow rate to 35 mL 1 min will not be accurate, since a too great loss of sensitivity was observed.

Loss of sensitivity when increasing outlet-split can be compared to the principle of GC split=splitless injector, in which more volatile compounds (especially solvent) are preferentially removed before entering in the column.

Experiments were conducted without glass wool in the cold trap to remove the memory effect completely. These experiments showed that the memory effect was

Sampling and Analysis of Pesticides in the Atmosphere 273 very low (maximum 0.10% for epoxyconazole) and disappeared completely in the

second empty tube. The resolution and sensitivity of each pesticide and metabolite under study were not affected by this removal since no significant decrease of areas was observed.

An experiment was performed by changing desorption rate of the cold trap from >40 to 58C s 1 . This change greatly improved the peak resolution. From the

different tests performed, it appeared that the memory effect was located in the cold trap, and that it could be partially removed by using an empty trap. Following these observations, complementary tests were performed with decreased outlet-split flow rates (25 and 20 mL min 1 ) to increase the method sensitivity. Tests performed with spiked tube at 400 ng showed recurrence of the memory effect with an outlet-

split flow rate under 25 mL min 1 ; therefore, no more tests were conducted. Decreasing the outlet-split flow rate could be envisaged for very low amounts of

pesticides to improve method performances. ATD optimal conditions for the quantitative desorption of the 10 pesticides and metabolites under study are presented in Table 10.2. This study used an ATD 400 from Perkin-Elmer Corp. (Norwalk, CT, USA) where some temperature ranges are limited (transfer line, valve). With Turbomatrix new systems, temperature can be increased and can improve the efficiency of thermal desorption for pesticides analysis.

10.2.3.1 Method Performances 10.2.3.1.1 ATD –GC=MS repeatability and calibration range

For repeatability experiments, five assays were conducted successively with condi- tions defined in Table 10.2. From this experiment, it appears that repeatability (determined by five replications) was good for each compound, with a relative standard deviation of 9% –12% (deviation due to the manual tube spiking step is included in this result).

TABLE 10.2 ATD Conditions Parameter

Optimal Conditions Oven temperature for tube

Initial Conditions

3508C Desorb flow and time for tube

3508C

60 mL min 1 Inlet-split

60 mL min 1 ; 15 min

0 mL min 1 0 mL min 1 Temperature of cold trap

308C Temperature of desorption for the trap

308C

3908C Desorb time for the trap

3908C

15 min Trap fast ( 308C to 3908C)

15 min

No Outlet-split

Yes (>408C s 1 )

20 mL min 1 25 mL min 1 Temperature of the transfer valve

2508C Temperature of the transfer line

2508C

2258C Source: From Briand, O. et al., Anal. Bioanal. Chem., 374, 848, 2002.

2258C

274 Analysis of Pesticides in Food and Environmental Samples

A calibration range was performed between 1 and 100 ng deposited on tubes. Linear range was observed as listed below:

1 –100 ng for carbofuran and epoxyconazole

2 –100 ng for alachlor and cyprodinyl and HCH and trifluralin

4 –100 ng for atrazine, iprodione, metolachlor, and tebuconazole

10 –100 ng for desethylatrazine, disopropylatrazine, and 3,5-dichloroaniline

Detection limits were determined as two times lower than values of the quantification limit. No memory effect was observed in these range of concentrations.

10.2.3.1.2 Pesticides recoveries from Tenax The optimal temperature for sampling tube desorption was 3508C. No trace of compounds had been observed during the second desorption of the tube. Recovery efficiencies obtained from Equation 10.1 equal 100%.

A i ,1 A Bi

R :Ei (%) ¼ ( 10:1)

(A i ,1 þA i ,2 ) A Bi

where R.E i is the recovery efficiency for the analyte i

A i,1 is the peak area of analyte i for the first desorption of the spiked tube

A i,2 is the peak area of analyte i for the second desorption

A Bi is the count of analyte i from the adsorbent blank (if any) No additional peak was observed in GC=MS, which seems to indicate that no

thermal degradation occurs during tube desorption. Recovery efficiencies obtained at the other temperatures were lower than those at 3508C and were directly correlated to desorption temperature. Recovery efficiencies ranged from 17%, 22%, and 35% at 2258C for low volatile pesticides (iprodione, epoxyconazole, and tebuconazole, respectively) to more than 90% at 3008C. The other compounds gave recovery efficiencies of 60% –95% at 2258C–3008C.

10.2.3.1.3 Resin efficiency Performance of Tenax TA to retain pesticides under study was tested by an experi- ment with three tubes in series and a GC oven. This technique offers some advan- tages such as simplicity and low cost, or the possibility to investigate two parameters at the same time, to evaluate adsorbent performances or reliabilities, retention efficiency, and breakthrough percentage.

For this, three tubes were connected in series. A heating system was combined with a stream of gas to sweep volatile pesticides from solid (125 mg of Tenax 1 enclosed in tube 1) into the vapor phase. Pesticides were then adsorbed on sample

tubes (tubes 2 and 3), also packed with 125 mg of Tenax 1 . Tube number 1, located in GC oven, spiked with a known amount of pesticides, was connected with Teflon tubes to a pump at one extremity and to two precondi- tioned Tenax 1 tubes (kept at room temperature) on the other. The tube 1 was then

Sampling and Analysis of Pesticides in the Atmosphere 275 heated in GC oven at the same temperature than the first step of the ATD (3508C).

After 15 min, the temperature of GC oven was brought down to and maintained at 608C for 2 h 45 min. During all experiments, a stream of clean air was continuously passed through the first tube to carry volatiles in subsequent tubes 2 and 3. In total, 300 L of air were passed through tubes for 3 h to simulate field conditions. Tubes 2 and 3 were maintained at ambient temperature (208C –258C) with a stream of compressed air on their surface.

At the end of the experiment, tubes were separated and analyzed by ATD – GC=MS. For each compound, peak areas were then compared to a reference value (achieved by direct injection on the top of Tenax 1 tube just before analysis). With this experiment, it was possible to calculate the actual quantity of pesticides which was volatilized (Equation 10.2, Tenax 1 retention efficiency Equation 10.3) and to collect nonretained pesticides with the third tube in order to estimate break- through percentage (Equation 10.4).

A i ,ref A i ,T1

i (%) ¼

A i ,ref

where

i is the volatilization efficiency for the analyte i

A i,ref is the peak area of analyte i for the reference desorption (20 ng injected)

A i,T1 is the peak area of analyte i for the tube 1 analyzed

A i ,T2

T R i (%) ¼ ( 10:3)

A i ,ref

where

T R i is the Tenax 1 retention efficiency for the analyte i

A i,T2 is the peak area of analyte i for the tube 2

A i,ref 3 i represents the actual volatilized quantity

A i ,ref

where

i is the breakthrough percentage for the analyte i

A i,T3 is the peak area of analyte i for the tube 3

10.2.3.1.4 Tenax 1 TA retention efficiency Testing the capacity of an adsorbent to quantitatively retain all molecules present in the air during the sampling duration is fundamental in terms of accuracy and precision of the method.

Determining the maximum quantity of air passed through the adsorbent with 100% retention of molecules or breakthrough volume is required when air sampling is performed. Generally, breakthrough is determined by using two

276 Analysis of Pesticides in Food and Environmental Samples sampling tubes in series. Molecules going to the second indicate the limit of

the sampling method. To test the Tenax 1 TA retention efficiency, the same device as the one used for the resin efficiency was used. This experiment refers to the physical interaction between a molecule of gas, coming from the tube 1, and a solid surface, the porous polymer of the adsorbent. The sorption capacity was determined by passing a known amount M i of analyte i through the sorbent bed and then analyzing the tube and measuring the amount of retained pesticides.

Vapor pesticide mixture comes from tube 1, where compounds that were first in adsorbed form were volatilized by heating action, and transferred to tube 2. In lack of suitable standard gaseous mixtures, this test was an alternative from a direct liquid injection on the cartridge, and must be more representative of field experiments where pesticides are in vapor phase or coming as an aerosol. Values of efficiency obtained ranged between 68.4% and 99.1%. Two phenomenon could explain this variability: a competitive adsorption (the molecules with the highest affinity for Tenax 1 displace those of lowest affinity previously adsorbed and produce a migration in the sorbent bed) or kinetics of capture (which are different for each compound).

Presence of pesticides in the third tube indicated that some of them had pene- trated through the front section. Thus, in the first tube, adsorption capacity was exceeded so that some layers of the sorbent bed must be partially or completely

saturated and breakthrough occurs. However, breakthrough percentage gives an indicative value of nonretained pesticides for a known volume of gas passed through the tube but cannot replace breakthrough volume or breakthrough time measurements using stable standard atmosphere and a continuous effluent monitoring with an appropriate detector. These conditions are rarely obtained for pesticides studies.

Breakthrough percentage was never more than 0.75%, whatever the compound, for about 300 L passed through the tubes. This appeared to be very low and have a direct application on field experiment since this volume covers greatly all field- sampling volumes.

Nevertheless, an increase to 10% of the breakthrough in relation with increasing ambient temperature from 208C to 608C was observed.

10.2.3.1.5 Recoveries and method detection limits From the previous results described (resin retention efficiency and recoveries from

Tenax 1 ), no corrections of the atmospheric concentrations were needed. According to the type of studies, determination of spray drift or characterization of postapplication transfers, or determination of volatilization fluxes, sampling periods can be very different and conduct variable detection limits of the method.

For spray drift, sampling periods are short, about a few minutes. Detection limits ranged from 50 to 500 ng m 3 (carbofuran, epoxyconazole, and metabolites,

respectively) based on a 20 L air volume sampled. In postapplication, on account of night –day cycles, sampling periods are longer; generally a few hours. For this study, they were fixed at 4 h so that detection limits ranged from 2 to 20 ng m 3 , based on a 500 L air volume sampled.

Sampling and Analysis of Pesticides in the Atmosphere 277 These results illustrate the effectiveness of this present method to assess atmos-

pheric pesticide concentrations. Performances could be compared to conventional method (liquid extraction). For example, Demel et al. 65 have obtained detection limits between 1 and 9 mg m 3 based on 1 m 3 air volume sampled (trapping on Tenax 1 of propiconazole, deltamethrine, etc.). These differences confirm the interest of thermodesorption to analyze atmospheric pesticides in exposed area.