AOAC Official 995.14 Methomyl in Insecticidal ulations.pdf

7.3.17 AOAC Official Method 995.14 Methomyl in Insecticidal Formulations Reversed-Phase Liquid Chromatographic Method First Action 1995 Final Action 2001 (Applicable to technical, solid, and liquid formulations.) See Table 995.14 for the results of the interlaboratory study sup- porting the acceptance of the method. Caution:Methomyl formulations are highly toxic. Formulations are poisonous if inhaled and may be fatal if swallowed. A. Principle Testportionisdissolvedinmethanol.Methomylandinternalstan- dard are separated on reversed-phase C18column using acetonitrileH2O mobile phase. Methomyl is measured by UV absorbance at 254 nm. B. Apparatus (a) Liquid chromatograph (LC).With 254 nm UV detector and, preferably, heated column compartment. Autoinjector or loop-type valve injector is recommended for injections. Operating conditions: flow rate, 2.0 mL/min; column temperature, 45C; de- tector sensitivity, 0.8 AUFS (or appropriate setting to give largest possible on-scale peaks); injection volume, 10 L. (b) LC column.C18, 150 4.6 mm id, retention times: 5.7 min for benzamide (internal standard) and 7.8 min for methomyl. De- pending on previous history of LC column, composition of mobile phasemayneedtobemodifiedtoobtainrequiredretentiontimes.In- creasing concentration of organic modifier (acetonitrile) in mobile phase decreases retention times and vice versa. (c) Filters.For test solutions; 0.45 m porosity, disposable. C. Reagents Use distilled deionized H2O throughout. (a) Methanol. (b) Mobile phase.8% acetonitrileH2O (v/v). Add 80 mL acetonitrile to clean 1 L volumetric flask and dilute to volume with H2O. Shake well and degas before use. (c) Internal standard solution.Weigh 1.2 0.1 g benzamide into1Lvolumetricflaskandadd100mLacetonitrile.Dilutetovol- ume with H2O and shake well. (d) Methomylstandardsolutions.(1)Methomylstandardstock solution.Accurately weigh ca 150 0.1 mg methomyl analytical standardinto50mLvolumetricflask.Dilutetovolumewithmetha- nolandsonicatetodissolvemethomyl.Storestandardstocksolution in refrigerator if solution is to be used for more than one day. (2) Methomyl working standard solutions.Equilibrate methomyl standard stock solution to room temperature. Accurately pipet 6.0, 8.0,and10.0mLintoseparate,labeled25mLvolumetricflasksand thenaccuratelypipetintoeachflask15.0mLinternalstandardsolu- tion, (c). Dilute contents of each flask to volume with methanol, if necessary, and shake well. D. Preparation of Test Solutions Thoroughly grind solids and shake liquid formulations well. Ac- curately weigh test portion containing ca 240 mg methomyl into 100 mL volumetric flask. For LannateL and LV, the appropriate amountsare950and790mg,respectively.Fortechnicalmethomyl, Lannate 90, and Lannate 25, the appropriate amounts are 265, 260, and 870 mg, respectively. Dilute to volume with methanol, C(c), sonicate 10 min, and let equilibrate to room temperature. Pipet10.0mLanalytedilutioninto25mLvolumetricflaskandadd bypipet15.0mLinternalstandardsolution,C(c).Shakewelltomix. E. Determination Filter all solutions through 0.45 m filter before injecting. After LC column has equilibrated and stable baseline has been obtained, make duplicate injections of 10 L methomyl working standard so- lutions,C(d)(2),andtestsolution.Alternatesolutionsduringcourse of analyses to determine if any significant change in detector re- sponse occurs. Chromatographicconditionsareadjustedtoprovidegoodresolu- tion between internal standard, methomyl, and oxime of methomyl that can be present. It is important that these 3 compounds are sepa- ratedbecauseinterferencefromoximecanaffectaccuracyofanalyt- ical result for methomyl. Retention time of oxime will probably be lessthanthoseofmethomylandinternalstandard.Totestoximere- tention time, dissolve analytical standard methomyl in 5% Na2CO3 solution. Methomyl will hydrolyze in a few hours. Neutralize aliquotofthissolution,injectontoLCcolumn,andrecordoximere- tentiontime.(Alternatively,bothstandardmethomylandoximecan be obtained from Du Pont Co., Agriculture Products, Rte 141 Ws= weight of test portion, mg. (b) Least-squares linear regression. 2002 AOAC INTERNATIONAL Table 995.14Interlaboratory study results for determina- tion of methomyl in insecticidal formulations by liquid chro- matographic method MethomylNaMean, %sRRSDR, %Rb Technical11c98.120.690.701.93 Lannate 90 WP/SP1293.571.571.684.40 Lannate 25L11c25.741.355.233.78 Lannate L (24%)1223.670.502.111.4 Lannate L (24%)1224.420.311.250.87 Lannate LV (29%)1229.830.270.910.76 Lannate LV (29%)1229.080.551.901.54 a N= No. of laboratories. b R = 2.8 sR. c One laboratory identified as outlier by Grubbs test. Methomyl, mg = RI S where R = average methomyl:benzamide area ratio