EXTRACTION PROCEDURE

Transcription

1 SPE Application Note for Multiresidue Exraction and Clean Up from Fruit and Vegetables This note outlines solid phase extraction (SPE) methodology for the multiresidue extraction and clean up of fruits and vegetables, as described by the Florida Department of Agriculture and Consumer Services (Cook et al, Journal of AOAC International Vol 82, No 6, 1999, pp ). For details of sample selection, collection and storage procedures, and details of the analytical methodology used, please refer the paper above. In developing this approach, factors such elimination of hazardous chlorinated solvent waste, reduction of sample size and solvent consumption, efficiency in analysis time and ruggedness of the methodology were considered and optimized. The method can be used for the extraction of a range of commodities (fresh fruits and vegetables) and is suitable for analysis of organophosphorus, organochlorine and carbamate pesticide types, using a variety of appropriate analytical techniques. The method uses initial acetonitrile extraction of the commodity, followed by gravity fed C18 SPE clean-up. The extract is concentrated, and the solvent exchanged to acetone for analysis of phosphorus, sulphur and nitrogen containing pesticides. Further fractions are cleaned up using Florisil and aminopropyl SPE columns for the analysis of halogenated and carbamate pesticides respectively. ISOLUTE SPE Column: EXTRACTION PROCEDURE ISOLUTE C18(EC) 500 mg/6 ml, part # C ISOLUTE FL 500 mg/10 ml, part # H ISOLUTE NH2 500 mg/10 ml, part # H See General Comments for accessories Pre-treatment: Acetonitrile extraction Weigh homogenized sample (50 g +/- 0.5 g) into a 250 ml polyethylene bottle, and where applicable, add standard spiking solution. Add acetonitrile (100 ml) and cap the bottle. Shake for 3 minutes to extract pesticides. Solvation: C18 SPE clean-up Place the ISOLUTE C18(EC) column onto a vacuum manifold using a connecting PTFE stopcock (part # ). All solvents flow under gravity unless stated otherwise. Condition the column with methanol (1 column volume) Equilibration: Rinse the column with 2.5:1 acetonitrile: water v/v (reagent A, 1 column volume), followed by deionized water (1 column volume). Close the stopcock when approximately 1 cm water remains above the top frit. Remove the stopcock the column will retain a liquid Page 1 of 5

2 head. Sample application: Stack a 70 ml reservoir above the preconditioned ISOLUTE C18(EC) column by means of a PTFE stopcock and adaptor (part # ). Insert a funnel containing a fluted filter paper into the top of the 70 ml reservoir. Weigh NaCl (10 g +/- 0.2 g) into a clean 250 ml polyethylene bottle, and place underneath the column stack. Pour the sample through the filter paper into the 70 ml reservoir, until the sample reaches approximately 1 cm from the top. The remainder of the sample can be discarded. Open the stopcock, and using vacuum or positive pressure (connect gas line using 70 ml column adaptor, part # ), load the sample at a flow rate of about 5 ml/min (fast drip). Collect the eluent in the bottle. Cap the bottle, shake for 2 mins, then centrifuge for 2 minutes at 1000 rpm. The sample will separate into aqueous (bottom) and acetonitrile (top) layers. N.B. Samples can be refrigerated or frozen overnight (no more than 24 hours) at this stage if the remainder of the procedure is not to be completed on the same day. Allow to reach room temperature prior to the next stage of sample preparation. Interference elution: Analyte elution: N/A To elute analytes, apply first volume of elution solvent to extraction cartridge. Soak for two minutes. Add second volume of elution solvent to extraction cartridge and collect. Fractionation Decant the sample into a 70 ml reservoir fitted with a closed stopcock, and allow to stand for at least 5 minutes. Drain the lower aqueous layer to waste using the stopcock. Stand for 3 minutes. Repeat, and include a small amount of the acetonitrile layer to ensure all water is removed. Collect a 15 ml aliquot of the extract in a beaker and retain as Fraction P (nitrogen, phosphorus and sulphur containing pesticides). Collect a further 15 ml aliquot of the extract in a beaker and retain as Fraction C (halogenated pesticides). Collect the remainder of the extract and seal in a test tube (carbamate pesticides). Place both the beakers on a steam bath and evaporate until approximately 2 ml remains. Remove from heat and gently evaporate off acetonitrile just to dryness using airflow in a fume cupboard. Do not over dry as losses of volatile pesticides will occur. Page 2 of 5

3 Fraction P (nitrogen, phosphorus and sulphur containing pesticides): Pipette 2 ml acetone into the beaker, and swirl for 30s to redissolve the pesticide extract. Seal in a test tube and refrigerate to precipitate any salt, prior to GC analysis. The sample is now ready for GC analysis using nitrogen, phosphorus and sulphur selective detectors such as NPD, FPD, or AED. Florisil SPE clean up of Fraction C (halogenated pesticides): Condition the column with acetone/hexane (425:75, v/v, 2/3 column volume), and allow to flow through under gravity. Place the conditioned column over a 15 ml graduated test tube. Pipette 2 ml acetone/hexane (425:75, v/v, reagent B) into the beaker, and swirl to redissolve the extract. Apply immediately to the pre-conditioned Florisil column. NB if water is suspected in the extract, place sodium sulphate into the Florisil column prior to column conditioning. Apply the sample and allow to flow under gravity. Add a further 5 ml reagent B to the beaker, swirl, and apply to the column, and collect. Repeat with a further 5 ml, and collect. Evaporate in a water bath (< 50 Celsius) under a stream of nitrogen until 2-3 ml remains. Remove from water bath, allow to reach room temperature and adjust to 4.0 ml with iso-octane. The sample is now ready for GC analysis using halogen specific detectors such as ECD, ELCD, XSD or AED. Aminopropyl SPE clean-up for carbamate pesticides: Condition column with Reagent C (approx 7 ml). Allow to flow under gravity, until solvent is just above surface of top frit. Stop the flow by closing the stopcock. Place a graduated test tube under the column, and apply 4.0 ml of extract at a flow rate of 2-5 drops / 5 s and collect. Add 4 ml reagent C to the column and collect. Place the test tube in an evaporator that has been allowed to heat to 40 Celsius, then turned off. The residual heat is ample for evaporation. Evaporate under a gentle stream of nitrogen to just dryness. DO NOT OVER DRY. THIS WILL RESULT IN ANALYTE LOSS. Add methanol (1 ml) to redissolve the extract, and vortex mix for 15 s. Filter the solution through a 0.2 um solvent resistant filter. This fraction is now ready for HPLC analysis by fluorescence detection with post column derivitisation, or diode array detection. Page 3 of 5

4 Structure Various Structural considerations Matrix considerations Analytical method Reagents Pesticides of varying polarity. The matrix is complex and contains a variety of potential interferences. Quantitation is by GC or LC, with appropriate detectors, and qualitative identification is by GC-MS. Reagent A: Water/acetonitrile mixture for C18(EC) column conditioning Measure deionized water (360 ml) and acetonitrile (900 ml) into a clean bottle, and mix well by shaking. Store in a sealed bottle. Prepare this mixture at least monthly. This volume is ample for 3 x 24 samples. Reagent B: Hexane/acetone mixture for Florisil SPE Measure hexane (425 ml) and acetone (25 ml) into a clean bottle and mix well by shaking. Store in a sealed bottle. Prepare this reagent freshly for each sample set. This volume is ample for 1 x 24 samples. Reagent C: Acetone/methanol reagent for aminopropyl SPE Measure methanol (30 ml) into a 1 L volumetric flask, and make up to mark with acetone. Mix well by shaking. Store in a sealed bottle. Prepare this mixture fresh each week. This volume is ample for 3 x 24 samples. General comments Accessories ordering infomration PTFE stopcock, part # or PTFE stopcock/needle part # Column adaptor, 1,3,and 6 ml columns, part # Column adaptor, 70 ml columns, part # Vacuum manifolds for sample processing VacMaster-10, 16 mm rack (processes 10 samples simultaneously), part # VacMaster-20, 16 mm rack (processes 20 samples simultaneously), part # Page 4 of 5

5 ISOLUTE column part numbers represent the product configuration of choice for use with a vacuum sample processing station. For 96-well and alternative column configurations compatible with any SPE automation system, please contact Argonaut Technologies, now company. All rights reserved. ISOLUTE is a registered trademark of Argonaut Technologies, now a company. United States and Canada T: Toll-Free: ordermailbox@biotage.com United Kingdon, EIRE T: eurosales@eu.biotage.com Sweden T: order@eu.biotage.com Japan T: order@biotage.co.jp Page 5 of 5