ACEPHATE 338
ACEPHATE
338
OCH3
CH3S
P
NH
O
C
CH3
O
ISO common name
Chemical name
Empirical formula
RMM
m.p.
v.p.
Solubility
Description
Stability
Formulations
Acephate
O,S-Dimethylacetylphosphoramidothioate
(IUPAC);
N-[methoxy(methylthio)phosphinoyl]acetamide (CA; 30560-19-1)
C4H10NO3PS
183.2
88 - 90 °C
2.3 × 10-4 Pa at 24 °C
In water: 790 g/l at 20 °C; acetone: 151 g/l;
ethanol: more than 100 g/l; ethyl acetate:
35 g/l: benzene: 16 g/l; hexane: 0.1 g/l, all at
20 °C
Colourless crystals
Relatively stable to hydrolysis
Soluble powders
5
ACEPHATE 338
ACEPHATE TECHNICAL
*
338/TC/(M)/1 Sampling. Take at least 500 ml.
2 Identity tests
2.1 GLC. Use the GLC method below. The relative retention time of
acephate with respect to the internal standard in the sample solution should
not deviate by more than 1% from that of the calibration solution.
2.2 Infrared. Prepare potassium bromide discs from the sample and from
pure acephate using 1.5 mg of compound and 300 mg KBr. Scan the discs
from 4000 to 600 cm-1. The spectrum obtained from the sample should not
differ significantly from that of the standard.
3 Acephate
OUTLINE OF METHOD Acephate is dissolved in dichloromethane
containing diisobutyl phthalate as internal standard, separated by gas
chromatography with flame ionisation detection, and quantitated using peak
area ratios.
REAGENTS
Dichloromethane
Acephate standard of known purity; purity better than 990 g/kg (available
through Tomen Agro, Inc., 100 First St., San Francisco, CA 94105,
USA).
Diisobutyl phthalate
Internal standard solution. Dissolve 1.2 ± 0.1 g diisobutyl phthalate (1.2 ±
0.1 g) into dichloromethane (4 l). Store at room temperature.
Calibration solution. Weigh (to the nearest 0.1 mg) acephate of known
purity (100 mg, s mg) into a volumetric flask (100 ml). Dilute to volume
with internal standard solution, close the flask with the cap and shake
until the contents are dissolved. Prepare fresh just before analysis.
APPARATUS
Gas chromatograph equipped with a flame ionisation detector
Column glass, 500 × 2 (i.d.) mm, packed with 10 % trifluorpropyl methyl
silicone on 100 - 120 mesh inert diatomite (SP-2401 or equivalent).
Condition freshly prepared columns 24 h at 250 °C with a nitrogen flow
of 30 ml/min. It is critical that the column specified is used. The 10
% stationary phase loading is important for obtaining optimum
performance of the method.
*
Provisional AOAC- CIPAC method 1995.
6
ACEPHATE 338
Electronic integrator or data system
Microsyringe 10 µl
Syringe 5 ml with Luer slip tips; disposable
Syringe filter 25 mm, 0.45 µm porosity; disposable
PROCEDURE
(a) Operating conditions (typical):
Oven temperature
155 °C
Injector temperature
170 °C
Detector temperature
250 °C
Gas flow rates
Nitrogen (carrier)
30 ml/min.
Hydrogen
)
as specified for the detector
Air
)
Retention time
acephate: 2 to 4 min
Adjust the flow rate or the column temperature to maintain 2-4 min retention
time for acephate.
(b) Preparation of sample. Weigh (to the nearest 0.1 mg) enough sample to
contain about 100 mg acephate (w mg) into a volumetric flask (100 ml).
Dilute to volume with internal standard solution, close the flask with the cap
and shake until the contents are dissolved.
(c) Determination. Adjust the operating parameters of the gas
chromatograph, so that acephate and diisobutyl phthalate elute as close as
possible, still maintaining clear baseline separation. Unacceptable resolution
between analytes is most likely caused by the analytical column. If baseline
separation of acephate and diisobutyl phthalate cannot be obtained, replace
the GC column.
Make repetitive injections of calibration solution onto the GC column until
the peak area ratios (i.e. diisobutyl phthalate/acephate) of sequential
injections are within 2%.
Make duplicate injections of the sample solution. Peak area ratios of sample
solution injections should agree within 2%. Otherwise, repeat the analysis
starting with the injection of the calibration solution. Re-inject the calibration
solution. Peak area ratios of the calibration solution injections immediately
before and after the sample solution injections should agree within 2%.
Otherwise, repeat the analysis sequence starting with injection of acephate
standard solution. Minimise the loss of dichloromethane from the sample
solution by replacing the cap on the sample vial immediately after removing
the aliquot of the sample.
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ACEPHATE 338
Average the two peak area ratios (diisobutyl phthalate/acephate) of the
sample solution injections (R) and also the two peak area ratios (R’) of the
calibration solution injections obtained immediately before and after sample
injections.
(d) Calculation
Content of acephate =
R ×s × P
1g/kg
R′ × w
where:
R = average peak area ratio of internal standard to acephate in the sample
solution
R’ = average peak area ratio of internal standard to acephate in the
calibration
solution
s = mass of acephate in the calibration solution (mg)
w = mass of sample taken (mg)
P = purity of acephate reference substance (g/kg)
Reproducibility R = 29 g/kg at 998 g/kg active ingredient content
68 g/kg at 964 g/kg active ingredient content
57 g/kg at 960 g/kg active ingredient content
ACEPHATE SOLUBLE POWDERS
*
338/SL/(M)/1 Sampling. Take at least 500 g.
2 Identity tests
2.1 GLC. As for acephate technical 338/TC/(M)/2.1.
2.2 Infrared. Extract a suitable portion of the sample with ethyl acetate, dry
the solution with anhydrous sodium sulphate and filter. Evaporate the solvent
with a stream of clean, dry air and continue as for acephate technical
338/TC/(M)/2.2.
3 Acephate. As for acephate technical 338/TC/(M)/3, except:
Preparation of sample. Remove insoluble inert material from acephate
formulated product prior to analysis as follows: (Inert material may clog
*
Provisional AOAC- CIPAC method 1995.
8
ACEPHATE 338
injection syringes used by manual or automatic samplers.) Weigh (to the
nearest 0.1 mg) enough sample to contain 100 mg of acephate into a
volumetric flask (100 ml). Dilute to volume with internal standard solution.
Close the flask with cap and shake for 30 s to ensure complete solution of
acephate in dichloromethane. Before analysis, filter an aliquot of the sample
solution through a syringe filter as follows: using a Luer tip syringe (5 ml)
transfer an aliquot of sample to a glass vial (20 ml), attach a 0.45 µm syringe
filter, and expel a portion into the autosampler or another vial (20 ml). To
prevent loss of dichloromethane from the sample, close the flasks or the
autosampler vials immediately after adding diisobutyl phthalate internal
standard solution. Continue as for acephate technical 338/TC/(M)/3(b).
Reproducibility R = 43 g/kg at 707 g/kg active ingredient content
27 g/kg at 723 g/kg active ingredient content
56 g/kg at 740 g/kg active ingredient content
9