Page 1 of 10 Determination of 6-Chloropicolinic Acid (6-CPA) in Crops by Liquid Chromatography with Tandem Mass Spectrometry Detection EPL-BAS Method No. 205G881B Method Summary: Residues of 6-CPA are extracted from a 2.5 g crop test portion with 25 ml of aqueous 0.1 N sodium hydroxide solution. A 2.0 ml aliquot of the extract is acidified with 5.0 ml of 1.0 N hydrochloric acid. The sample is centrifuged and the supernatant is purified using C18 and alumina solid phase extraction (SPE) followed by addition of an isotopically labeled internal standard. The sample solutions are then analyzed by liquid chromatography with tandem mass spectrometry detection (LC-MS/MS). The method limit of quantitation is 0.01 mg/kg (ppm).
Page 2 of 10 I. Analytical Standards Common Name 6-Chloropicolinic acid (6-CPA) CAS Number 4684-94-0 Purity (%) >95 Storage Conditions Ambient Common Name Storage Conditions 15 N, 13 C3-6-Chloropicolinic acid (6-CPA) Internal Standard (6-CPA-IS) Ambient Volumes given may be adjusted as necessary as long as proportionality is maintained. Additional working and calibration standard concentrations may be used as needed. 6-CPA Stock Solution, 1,000 µg/ml Weigh 0.100 g of the neat reference substance into a 100 ml volumetric flask and dilute to volume with methanol. Maintain frozen up to 365 days when not needed in the laboratory. 6-CPA IS Stock Standard Solution, 100 µg/ml (100,000 ng/ml) Weigh 0.0010 g of the neat 6-CPA IS into a 10 ml volumetric flask and dilute to volume with methanol. Maintain frozen up to 365 days when not needed in the laboratory. 6-CPA Working Solution, 10 µg/ml (10,000 ng/ml) Pipet 1.0 ml of the 1,000 µg/ml stock solution into a 100 ml volumetric flask and bring to volume with methanol. Maintain frozen up to 365 days when not needed in the laboratory. 6-CPA Working Solution, 1 µg/ml (1,000 ng/ml) Pipet 0.10 ml of the 1,000 µg/ml stock solution into a 100 ml volumetric flask and bring to volume with methanol. Maintain frozen up to 365 days when not needed in the laboratory.
Page 3 of 10 6-CPA IS Working Solution, 1 µg/ml (1,000 ng/ml) Pipet 1.0 ml of the 100 µg/ml stock solution into a 100 ml volumetric flask and bring to volume with methanol. Maintain frozen up to 365 days when not needed in the laboratory. Calibration Solution, 300 ng/ml 6-CPA, 20 ng/ml 6-CPA IS Pipet 3.0 ml of the 10,000 ng/ml 6-CPA working solution into a 100 ml volumetric flask. Pipet 0.020 ml of the 100,000 ng/ml 6-CPA IS stock solution into the same 100 ml volumetric flask.. Bring to volume with 25/75 (v/v) methanol/di water. Maintain refrigerated up to 365 days when not needed in the laboratory. Calibration Solution, 100 ng/ml 6-CPA, 20 ng/ml 6-CPA IS Pipet 1.0 ml of the 10,000 ng/ml 6-CPA working solution into a 100 ml volumetric flask. Pipet 0.020 ml of the 100,000 ng/ml 6-CPA IS stock solution into the same 100 ml volumetric flask. Bring to volume with 25/75 (v/v) methanol/di water. Maintain refrigerated up to 365 days when not needed in the laboratory. Calibration Solution, 50 ng/ml 6-CPA, 20 ng/ml 6-CPA IS Pipet 0.50 ml of the 10,000 ng/ml 6-CPA working solution into a 100 ml volumetric flask. Pipet 0.020 ml of the 100,000 ng/ml 6-CPA IS stock solution into the same 100 ml volumetric flask. Bring to volume with 25/75 (v/v) methanol/di water. Maintain refrigerated up to 365 days when not needed in the laboratory. Calibration Solution, 10 ng/ml 6-CPA, 20 ng/ml 6-CPA IS Pipet 0.10 ml of the 10,000 ng/ml 6-CPA working solution into a 100 ml volumetric flask. Pipet 0.020 ml of the 100,000 ng/ml 6-CPA IS stock solution into the same 100 ml volumetric flask. Bring to volume with 25/75 (v/v) methanol/di water. Maintain refrigerated up to 365 days when not needed in the laboratory. Calibration Solution, 5 ng/ml 6-CPA, 20 ng/ml 6-CPA IS Pipet 0.050 ml of the 10,000 ng/ml 6-CPA working solution into a 100 ml volumetric flask. Pipet 0.020 ml of the 100,000 ng/ml 6-CPA IS stock solution into the same 100 ml volumetric flask. Bring to volume with 25/75 (v/v) methanol/di water. Maintain refrigerated up to 365 days when not needed in the laboratory.
Page 4 of 10 Calibration Solution, 2 ng/ml 6-CPA, 20 ng/ml 6-CPA IS Pipet 0.20 ml of the 1,000 ng/ml 6-CPA working solution into a 100 ml volumetric flask. Pipet 0.020 ml of the 100,000 ng/ml 6-CPA IS stock solution into the same 100 ml volumetric flask. Bring to volume with 25/75 (v/v) methanol/di water. Maintain refrigerated up to 365 days when not needed in the laboratory. Calibration Solution, 1 ng/ml 6-CPA, 20 ng/ml 6-CPA IS Pipet 0.10 ml of the 1,000 ng/ml working solution into a 100 ml volumetric flask. Pipet 0.020 ml of the 100,000 ng/ml 6-CPA IS stock solution into the same 100 ml volumetric flask. Bring to volume with 25/75 (v/v) methanol/di water. Maintain refrigerated up to 365 days when not needed in the laboratory. Calibration Solution, 0.5 ng/ml 6-CPA, 20 ng/ml 6-CPA IS Pipet 0.050 ml of the 1,000 ng/ml 6-CPA working solution into a 100 ml volumetric flask. Pipet 0.020 ml of the 100,000 ng/ml 6-CPA IS stock solution into the same 100 ml volumetric flask. Bring to volume with 25/75 (v/v) methanol/di water. Maintain refrigerated up to 365 days when not needed in the laboratory. II. Reagents and Reagent Solutions A. Reagents Acetic acid, glacial Acetonitrile, HPLC grade Ammonium hydroxide, concentrated Methanol, HPLC grade Deionized (DI) water 6 N Hydrochloric acid (aqueous) 0.1 N Hydrochloric acid (aqueous) Sodium hydroxide, 0.1 N (aqueous) B. Reagent Solutions 1 N Hydrochloric Acid: Add 400 ml of 6 N HCl to 2000 ml of DI water and mix well. The solution is stored ambient for up to 365 days. 25/75 (v/v) Methanol/DI Water Combine 250 ml of methanol with 750 ml of DI water. The solution is stored ambient for up to 365 days.
Page 5 of 10 0.01% (v/v) Acetic Acid Pipet 0.1 ml of glacial acetic acid into 1,000 ml of DI water and mix. The solution is stored ambient for up to 180 days. 85/15 (v/v) Methanol (MeOH)/Ammonium Hydroxide (AmOH) For every liter of solution prepared, pipet 150 ml of concentrated AmOH into a 1000 ml class A volumetric flask containing approximately 500 ml of methanol. Swirl the flask and allow the solution to equilibrate to room temperature before diluting to volume with methanol. The solution is stored ambient for up to 365 days. 99/1 (v/v) MeOH/AmOH For every liter of solution prepared, pipet 10 ml of concentrated AmOH into a 1000 ml class A volumetric flask containing approximately 500 ml of methanol. Swirl the flask and allow the solution to equilibrate to room temperature before diluting to volume with methanol. The solution is stored ambient for up to 365 days. 99/1 (v/v) Acetonitrile (ACN)/1 N HCl Pipet 10.0 ml of 1 N HCl into a 1,000 ml volumetric flask. Dilute to volume with acetonitrile. The solution is stored ambient for up to 365 days. Internal Standard Dilution Solution (Contains 20 ng/ml 6-CPA IS) Pipet 0.020 ml of the 100,000 ng/ml 6-CPA stock solution into a 100 ml volumetric flask and bring to volume with 25/75 (v/v) methanol/di water. Maintain refrigerated up to 365 days when not needed in the laboratory. III. Laboratory Equipment Balance, analytical, capable of weighing to the nearest 0.1 mg Balance, top-loading, capable of weighing to the nearest 1.0 mg. Centrifuge tubes, 50 ml, polypropylene Centrifuge tubes, 15 ml, polypropylene Centrifuge Culture tubes, 16 x 15 mm, glass with PTFE-lined screw caps SPE extraction manifold SPE Columns, Bakerbond Octadecyl (C18), 6 ml/1 g, cat. no. 7020-07 SPE Columns, Agela Technologies, Cleanert Alumina-N-SPE, 6 ml/ 1 g, cat. no. AL0006-N Pipettes, Air-displacement, with disposable tips Nitrogen evaporation apparatus (N-Evap) with heating block
Page 6 of 10 Homogenizer, Polytron or equivalent Vortex mixer Volumetric flasks and pipets Autosampler vials with caps, low volume recovery Shaker, variable speed, reciprocating IV. Methods A. Sample Extraction 1. Using a top-loading balance capable of weighing to the nearest 1.0 mg, weigh 2.5 g (2.450 2.549 g) of sample into a 50 ml plastic centrifuge tube. 2. Fortify laboratory QC samples with the appropriate spiking solution and allow the solvent to evaporate on the bench top for at least 20 minutes. LOD Fortification: Pipet 0.0075 ml (7.5 µl) of the 1 µg/ml 6-CPA working solution directly onto the sample matrix. LOQ Fortification: Pipet 0.025 ml (25.0 µl) of the 1 µg/ml 6-CPA working standard solution directly onto the sample matrix. 10x LOQ Fortification: Pipet 0.25 ml (250 µl) of the 1 µg/ml 6-CPA working solution directly onto the sample matrix. 100x LOQ Fortification: Pipet 0.25 ml of the 10 µg/ml 6-CPA working solution directly onto the sample matrix. 3. Add 25 ml of 0.1 N sodium hydroxide. Vortex mix to thoroughly wet and mix the sample matrix with the solvent. 4. Place the tubes horizontally on a platform shaker and shake for at least two hours at a setting of 350 rpm. 5. Centrifuge at 4000 rpm for 10 minutes as controlled by the centrifuge settings. 6. Transfer 2.0 ml of the sample extract into a clean 15 ml plastic centrifuge tube. Store the remaining extract in the 50 ml centrifuge tube refrigerated in case additional aliquots are needed. 7. Add 5.0 ml of 1.0 N hydrochloric acid and vortex mix thoroughly. 8. Centrifuge at 2000 rpm for 5 minutes as controlled by the centrifuge settings. B. C18 SPE Purification 1. Place a C18 SPE column on a multi-port SPE manifold and rinse the column with 5.0 ml of acetonitrile.
Page 7 of 10 2. Condition the SPE column with 5.0 ml of 0.1 N HCl. Do not allow the column bed to dry. Discard the column effluent. 3. Transfer the entire supernatant from Step IV.A.8 to the SPE column taking care not to transfer any of the matrix pellet. Draw the sample solution through using gravity flow. Slight vacuum may be applied if flow is restricted. Discard the column effluent. 4. Add 2.0 ml of 0.1 N HCl to sample tube and re-suspend the pellet by vortex mixing. 5. Centrifuge at 1800 g for 5 minutes as controlled by the centrifuge settings. 6. Add the supernatant to the SPE column and discard the effluent. Discard the sample tube with pellet. 7. Use vacuum (-10 inches Hg) to dry the SPE column for 45 minutes. Do not use pressures below -10 inches Hg or exceed the 45 minute drying time or loss of analyte will occur. 8. Elute the 6-CPA into a clean 15 ml glass culture tube with 5.0 ml of 99/1 (v/v) ACN/1 N HCl. Discard the C18 SPE column. C. Alumina-N SPE Purification 1. Place an Alumina-N SPE column on the manifold and rinse with 5.0 ml of methanol. Discard the rinse. 2. Condition the SPE column with 5.0 ml of ACN and discard. 3. Transfer the entire sample solution from Step IV.B.8 to the SPE column and use gravity to pull the sample through the column. Discard the column effluent. 4. Rinse the sample vial with 2.0 ml of 99/1 (v/v) ACN/1 N HCl solution and transfer the rinse to the SPE column and use gravity flow to draw the solution through the column. Discard the column effluent. Discard the sample tube. 5. Dry the Alumina SPE column under vacuum (-10 inches Hg) for at least 1 minute. Do not use pressures below -10 inches Hg or loss of analyte will occur. 6. Rinse the SPE column with 4.0 ml of 99/1 (v/v) MeOH/AmOH solution and discard. 7. Using gravity flow, elute the 6-CPA, collecting eluate in a 16 x 150 mm glass culture tube, with 16.0 ml of 85/15 (v/v) MeOH/AmOH. 8. Pipet 0.020 ml of the 1,000 ng/ml 6-CPA IS working solution into the tube. 9. Evaporate the eluate just to dryness under a stream of nitrogen (N-Evap). Use a heating block temperature setting of 35ºC. 10. Redissolve the dried sample with 1.0 ml 25/75 (v/v) methanol/di water. 11. Vortex mix for at least 10 seconds at a speed adequate to rinse the sides of the culture tube. 12. Transfer the solution to a low volume recovery autosampler vial for LC- MS/MS analysis. Note: If sample dilution is necessary (peak area of undiluted sample exceeds that of the highest calibrant * 1.05), dilute with the internal standard dilution
Page 8 of 10 solution (containing 20 ng/ml 6-CPA IS) to bring the 6-CPA sample concentration within the concentration range of the calibration curve. D. LC-MS/MS Analysis The following LC-MS/MS parameters are used to determine 6-CPA concentrations. The parameters may be modified to achieve adequate chromatographic resolution or detector sensitivity and selectivity. The actual parameters used are documented with each analysis sequence. LC System: Acquity UPLC, Waters Corporation MS Detector: TQ, Waters Corporation Column: HSS T3, 1.8 µm, 2.1 x 100 mm, Waters Corporation Column Temperature: 40ºC Injection Volume: 5 µl Mobile Phase: A: 0.01% (v/v) Acetic Acid B: Methanol Gradient Program Time (minutes) %A %B Curve Flow (ml/minute) 0.00 100 0 1 0.300 0.10 100 0 1 0.300 6.00 5 95 6 0.300 6.50 5 95 1 0.300 6.51 100 0 1 0.300 7.50 100 0 1 0.300 Divert flow to waste from 0.00 to 3.00 minutes. Direct flow to the MS detector from 3.00 to 5.00 minutes. Divert flow to waste again after 5.00 minutes. MS Source: Electrospray Ionization Source Temperature: 130ºC Vaporizer Temperature: 400ºC Nebulizer Gas (N2) Flow: 550 L/hour Cone Gas (N2) Flow: 30 L/hour Spray Voltage: 2.8 kv Scan Type: Multiple Reaction Monitoring (MRM)
Page 9 of 10 MRM Program: The following MRM transitions are collected from 3.2 to 5.00 minutes following injection. Precursor Ion (m/z) Product Ion (m/z) Dwell (s) Cone (V) Collision Energy 156 (Q) 112 0.200 20 10 158 (C) 114 0.200 20 10 160 (Q IS) 115 0.100 20 10 162 (C IS) 117 0.100 20 10 (Q) = Quantitation (C) = Confirmation (Q IS)= Quantitation Internal Standard (C IS) = Confirmation Internal Standard Expected Retention Time: approximately 3.95 minutes Calibration standard solutions ranging in concentration from 0.5 100 ng/ml are injected at least once over the course of the analysis sequence, interspersed with sample solutions. The instrument data system is used to construct a linear regression internal standard calibration curve of the calibrant peak area ratios (yaxis) vs. the standard concentration ratios in ng/ml (x-axis). A weighting factor of 1/x is used. The curve origin is not included in the linear regression equation. V. Calculations The amount found (ng/ml) of analyte is calculated from the linear regression equation: y = mx + b Where: y = (Peak area ratio) m = Slope of linear regression equation x = (Concentration ratio in ng/ml) b = y-intercept of linear regression equation The linear regression equation is used to solve for x, which is correlated to amount found in ng/ml. ppm Found = Amt Found (ng/ml) * Final Volume (1.0 ml) * Extract Volume (25.0 ml) * Dilution Factor Sample Weight (g) * Aliquot Volume (2.0 ml) * 1000 ng/µg
Page 10 of 10 For matrices with high water content (>20 g/100 g), a correction to the extract volume is necessary. The water content of the analytical test portion is calculated using USDA National Nutrient Database values for water content of a given commodity: Water Content (g) = Sample Weight (g) * (g water/100 g of commodity/100) The total volume of the sample extract is then calculated: Total Extract Volume (ml) = Water Content (g) + Extract Solution Volume (25.0 ml). This value is substituted for the Extract Volume (25.0 ml) in the formula above for ppm found. Fortification Level (ppm) = Volume Spiking Soln. (ml) * Spiking Soln. Concn. (µg/ml) Sample Weight (g) Recovery (%) = (ppm Found in Sample ppm found in Control) / Fortification Level (ppm) * 100 Limit of Quantitation (LOQ): The method LOQ is 0.01 ppm. Limit of Detection (LOD): The method LOD is 0.003 ppm.