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Analysis for Pesticides

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Analysis for Pesticides LAG - Genevac Article 18/9/06 4:39 pm Page 1 Laboratory Products Focus IMPROVING ANALYSIS OF PESTICIDES A NEW METHOD DEVELOPMENT PROTOCOL TO INCREASE RECOVERY OF VOLATILE COMPOUNDS Anna Maria Marsico, Environmental Protection Agency of Tuscany (ARPAT) Determination of pesticide contamination EXPERIMENTAL SAMPLE CONCENTRATION of fruit and vegetables usually follows Analytical method development involves the METHOD several steps, including, extraction of careful evaluation of instruments with regard to analytes from the sample, concentrating The evaporation method may be set to run for a issues such as ease of use, efficiency, and most fixed time in order to stop the system when we the extract, post extraction clean-up, importantly analyte recovery and integrity. In estimate that we have reached a certain residual solvent exchange, and finally, this study the performance of a new centrifugal volume in each tube. This method was found to be determination of the analytes present. evaporator manufactured by Genevac, U.K., unreliable, therefore a “solvent keeping” approach is evaluated. During sample preparation, evaporation was trialed where a 1ml aliquot of toluene was added to each sample. Toluene (boiling point becomes a critical step when the pesticides The stages of the research methodology 110°C) is less volatile than ethyl acetate (boiling are semi-volatile compounds, because are as follows: point 77°C) in which the samples were dissolved. some of the compound may be lost during • Determination of which analytes are suitable for the study, in terms of volatility and The object of the study was to verify if pesticides the concentration and evaporation stages. recovery, to be used as probes. could be successfully concentrated into the residual Analyte loss is detrimental to accurate toluene by a fractional distillation method, thereby • Evaporation tests on the solvents used analysis, and the official directives(1) must removing only the ethyl acetate. Toluene was for the extraction of pesticides from fruit selected not only for the marked difference in the be satisfied with regard to minimum and vegetables. analyte recovery. Typically a minimum boiling point to ethyl acetate, but also for it’s • Development of an optimal evaporation recovery of 70% must be achieved. excellent solvatation properties of most pesticide method using the EZ-2 ENVI evaporator, chemical structures. In order to evaluate new instruments which avoids unacceptable loss of pesticide and delivers a concentrated sample, rather The task was to build an evaporation program which and improve productivity, which is also than a dry sample. avoids complete drying of the sample, which would the remit of ARPAT, this study was done • Validation of sample preparation method be detrimental, but which allows automatic reduction to determine how a new generation using a test matrix spiked with pesticides, to a fixed solvent volume with no user intervention. All centrifugal evaporator may simplify relative to the existing method previously the evaporation methods were tested with samples the sample preparation process and validated and adopted by ARPAT laboratories. contained in 12 flat bottom ASE collection vials, 27.5 improve analyte recovery. mm diameter. Six were placed into each of two DESCRIPTION OF THE EZ-2 ENVI aluminium sample holders from the machine. Two The study falls in two halves, first, an pre-programmed methods were used in succession, evaluation of the a new evaporator to The EZ-2 ENVI (shown in Figure 1) is a state first the ‘Enviro’ program which reduces the determine the optimal concentration of the art centrifugal evaporation system, which pressure gradually with two vacuum gradients to process; then, the study was addressed allows highly controlled evaporation and prevent bumping, then the very low boiling point towards real matrices spiked with concentration of samples. Vials, tubes, microtitre solvent program, which is designed for safe removal plates and flasks can be accommodated in of very low boiling point solvents. The temperature pesticides to validate the new methods. the system. During evaporation, samples are of the sample holders was programmed to be 30°C. The new processes and methods were centrifuged in order to avoid “bumping” effects The combined method profile is as follows: then compared to the original method which may result from boiling under vacuum. 1. Vacuum gradient from 600mbar to 200mbar validated in ARPAT laboratories. Sample temperature, vacuum level, and in 10 minutes, at ambient heating controls are all precisely monitored and controlled throughout the evaporation or 2. Vacuum gradient from 200mbar to 150mbar concentration process. in 15 minutes, at ambient The software also 3. Hold pressure at 75mbar for 65 minutes, controls start up and holder temperature 30°C. can determine the The method was developed based on the end of evaporation hypothesis that toluene will not evaporate under automatically. these conditions, but ethyl acetate will. At Author Details: 75mbar ethyl acetate’s boiling point it is 16°C Anna Maria Marsico, Environmental Protection Agency while the boiling point of toluene is 40°C. of Tuscany (ARPAT), Florence, Italy. Keeping the holder temperature at 30°C will For further information, please contact: mean that there is sufficient energy to boil the Figure 1. EZ-2 ENVI, ethyl acetate, but not the toluene. In this way, Rob Darrington of Genevac Ltd. The State of the the large volume of ethyl acetate is safely on telephone +44-1473-240000 art Centrifugal or email [email protected] Evaporation System removed leaving the analyte in the toluene. LAG - Genevac Article 18/9/06 4:39 pm Page 2 TECHNICAL ARTICLE - GUIDE 2006 0 In order to investigate this further, another A NEW MORE EFFICIENT SAMPLE 100 stability test of pesticides in toluene was 96.0 96.8 95.9 PREPARATION METHOD 90 94.1 performed. This time, instead of referring to the 89.3 80 90.4 Taken together, this information has helped us 82.4 internal standard, endosulfanlattone, vials 70 to develop a new sample preparation and 60 containing pesticides in 1ml toluene were 50 weighed before and after the evaporation step. evaporation method suitable for EZ-2 ENVI, without toluene % Recovery 48.4 40 with toluene Vials were exposed to the same conditions as the effective for fast solvent removal without 30 previous test, 30 minutes at 30°C and at 75mbar. significant loss of pesticides. 20 10 The aim of the test was to validate the The new method is detailed below. The method 0 concentration against the weight and correlate Esachlorobenzene fractionally distils the ethyl acetate/toluene Lindane Aldrin recovery loss to evaporation of toluene. 4,4' DDE mixtures using a vacuum ramp, and maintaining The results are shown in Figure 4. We can see sample temperatures below 30°C. Figure 2. Pesticide recovery following that after 30 minutes there was no significant At the end of the method the pressure is concentration from ethyl acetate, with and reduction of toluene volume (less then 1%), and increased to 150mbar to offset any increase in without addition of toluene an acceptable recovery of pesticides. ambient temperature which may contribute to loss of sample. Figure 2 shows recovery data obtained using this 120 98.7 1. Extract samples in ethyl acetate 50ml method. A Pesticide mix containing very volatile 101.2 100 87.3 compounds (Hexachlorobenzene, Lindane, 95.7 93.4 2. Add 1ml toluene to each sample 80 82.1 96.1 Heptachlor, Aldrin 4,4’ DDE, Endrin) was spiked into 83.4 % Recovery 60 3. Add anhydrous sodium sulphate 50ml ethyl acetate samples in ASE collection vials, to each sample with and without the toluene addition. The tubes 40 were then dried according to the method outlined 20 4. Concentrate using the following method: above and the recoveries determined using GC-ECD. 0 Vacuum gradient from 125 mbar to 15 time (min) 75mbar in 25 minutes to evaporate gently. The results confirm the hypothesis: samples in 30 Lindane Aldrin Evaporation continues at 75mbar Esachlorobenzene ethyl acetate were completely dried and recovery 4,4' DDE was reduced, where as under the same at 30°C for up to 60 minutes. conditions, samples containing toluene were Figure 4. Recovery of pesticides in toluene Pressure rises to 150mbar to prevent further following exposure to vacuum reduced to a small volume - about 100µl – and sample loss for up to 30 minutes. had very high recovery rates. Based on this information, we have further To further investigate the suitability of this developed the EZ-2 ENVI to automatically end the approach, the stability of pesticide in the residual evaporation process within 30 minutes. From this VALIDATION OF STUDY USING toluene at 30°C and 75mbar was evaluated. 2ml data we can see that 30 minutes exposure to the LETTUCE MATRIX conditions of 30°C at 75mbar, is the maximum for Vials containing 1ml of toluene were left for The new approach was tested using samples of different period of times at 30°C at pressure the 4 pesticides tested. These 30 minutes are useful lettuce spiked with a the same mix of volatiles of 75mbar. To facilitate analysis, a molecule with to ensure the complete evaporation of ethyl acetate pesticides (Hexachlorobenzene, Lindane, Heptachclor a high molecular weight was also spiked into from all samples so as to leave them only in the Aldrin 4,4’ DDE, Endrin). the samples, endosulfanlattone, which will toluene. Another benefit of the addition of toluene not evaporate. Pesticide concentrations were is to reduce variations in evaporation speed caused The full process is detailed in figure 5. The original determined by GC-ECD and measured relative to by differing numbers of samples in different runs, sample preparation and testing method is shown the concentration of endosulfanlattone. where one sample may dry before another. to the left, and the steps required with the new method are shown to the right.
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