MT 188

2

MISCELLANEOUS TECHNIQUES *MT 188 DETERMINATION OF FREE PARATHION-METHYL

IN CS FORMULATIONS

SCOPE

The method is suitable for determining free parathion-methyl (up to 100 % free active ingredient relative to the total active ingredient content) in CS formulations up to 400 g/kg.

OUTLINE OF METHOD

A known amount of capsule suspension is mixed with a defined volume of an aqueous solution containing a non-ionic surfactant. The mixture is stirred for a defined period at a defined temperature. After two centrifugation steps the active ingredient content is determined in the supernatant.

REAGENTS

POE-(20)-sorbitan monolaurate (PSM), e.g. Tween 20, Uniqema, Belgium (CAS No. 9005-64-5)

Water, HPLC grade

Extraction solution PSM-water, 10 + 90 (v/v)

APPARATUS

Measuring cylinder 500 ml

Centrifuge tubes 30 ml

Beaker 1000 ml with a diameter of 102 ± 2 mm (low form)

Stirrer motor with speed control

Stainless steel stirrer propeller type with four fixed stirrer blades set at an angle of 45°. Use a stirrer with dimensions conforming to those in Fig 56 (MT 174). Do not use a magnetic stirrer.

*CIPAC method 2004. Prepared by the German Formulation Panel (DAPA). Based on a method supplied by Cheminova A/S, Denmark.

Constant temperature bath capable of maintaining the specified temperature within ± 1°C

Centrifuge capable of a speed of 3500 rpm, and suitable for at least 30 ml tubes PROCEDURE

(1) Extraction of free methyl-parathion

Weigh about 10.0 g of sample (w g) into a beaker and add extraction solution (300 ml) from a measuring cylinder (500 ml). Place the beaker in the temperature bath at 30 °C. The stirrer should be centrally located in the beaker and positioned in such a way that there is a clearance of 15 mm between the stirrer blades and the bottom of the beaker. The pitch of the stirrer blades and the direction of rotation must be such that the propeller pushes the water upwards. Switch on the stirrer with the speed set at 170 rpm. Continue stirring for 60 min. Then switch off the stirrer and transfer 20 ml to a 30 ml tube. Centrifuge the sample at 3500 rpm for 10 min. Transfer 10 ml of the supernatant to another tube and centrifuge again for 10 min. at 3500 rpm to obtain a clear liquid. Take the upper 5 ml for chemical analysis.

(2)Determination of parathion-methyl in the aqueous phase

OUTLINE OF METHOD The concentration of parathion-methyl is determined by HPLC using UV-detection and external standardisation.

REAGENTS

Acetonitrile HPLC grade

Water HPLC grade

Parathion-methyl standard of known purity

Extraction solution PSM-water, 10 + 90 (v/v)

Calibration solutions. Weigh (to the nearest 0.1 mg) into a volumetric flask (10 ml) parathion-methyl standard (0.1 g, s g). Fill to the mark with acetonitrile and mix thoroughly (stock solution). Transfer by pipette 1.0 ml of this solution to a volumetric flask (200 ml), dilute to volume with extraction solution and mix thoroughly (solution C A). Prepare dilutions of solution C A: e.g. 1 volume of C A + 1 volume of extraction solution (solution C B) and 1 volume of C A + 2 volumes of extraction solution (solution C C).

APPARATUS

Liquid chromatograph equipped with a binary eluent delivery system, autosampler, a UV-detector and an electronic integration system

Column 150 × 3.2 (i.d.) mm packed with Prodigy 5 μm ODS (3) 100R (from Phenomenex) or equivalent

Guard column 4 × 3.0 (i.d.) mm, ODS security guard (from Phenomenex) or equivalent

PROCEDURE

(a) Operating conditions (typical):

Eluent water – acetonitrile, 50 + 50 (v/v)

Flow rate 1.0 ml/min (isocratic)

°C

Column temperature 40

Injection volume 5

μl

Detector wavelength 275 nm (bandwidth: 10 nm)

Reference wavelength500 nm (bandwidth: 100 nm)

Retention time parathion-methyl: about 5 min

(b)System and linearity check. Equilibrate the column by pumping the mobile phase through the column until a stable baseline is obtained.Inject a 5 μl portion of the extraction solution in order to assess the absence of interfering peaks. Then check the linearity of the detector response by injecting twice 5 μl portions of the calibration solutions. Calculate the linear regression coefficient. The regression coefficient should be better than 0.98.

(c) Determination. Inject 5μl portions of the calibration and sample solutions in the following order:

C A, S1, S1, S2, S2, C A, S3, S3, S4, S4, C A, etc.

If the of parathion-methyl peak area of the samples exceeds the peak area of the strongest calibration solution, make an appropriate dilution.

Measure the of peak areas from the calibration and the sample solutions. Average the values of the duplicate sample injections. Calculate the mean value of the response factors of the calibration solution bracketing two sample solutions and use this value to calculate the parathion-methyl concentration of the bracketed samples.

(d) Calculation

s

H 50P s f .××=

v f H Q w ××=

Percentage of free a.i. =

w 030Q .× % (w/w) where:

f =

response factor H s =

peak area of parathion-methyl in the calibration solution H w =

peak area of parathion-methyl in the sample solution s =

mass of parathion-methyl taken for the preparation of the calibration solution (g) w

= mass of sample taken (g) Q

= concentration of parathion-methyl in the sample (mg/l) v

= dilution factor (= 1 without dilution; = 10 upon 10 fold dilution) P = purity of parathion-methyl standard (g/kg)

Fig. 46 Standard solution

Fig. 47 CS formulation test sample