Liquid Chromatographic Determination of Phenolic Impurities in Technical MCPA Using Electrochemical Detection (Coulometric Mode)

Abstract

Abstract Fifteen samples of technical 4-chloro-2-methylphenoxy acetic acid (MCPA) from 6 manufacturers were analyzed for the presence of 13 different phenolic impurities. Reverse-phase liquid chromatography with an electrochemical (coulometric mode) detector was used for qualitative and quantitative determinations. The phenols were separated using a 40-70% methanol and 60-30% 0.02M KH2P04 (pH 4.0) 35 min gradient system on a Spheri-5-RP-18 column. Confirmation of the phenols present was done by retention time comparison with the corresponding standards, using 2 other isocratic mobile phases, methanol-0.2M acetic acid (60 + 40) at a flow rate of 1.3 mL/min and acetonitrile-0.02M KH2P04 (45 + 55) (pH 3.0) at a flow rate of 0.90 mL/min. The similarity of results on 3 different solvent systems demonstrates the absence of any interfering responses. 2-Methyl-4-chlorophenol (average 0.168%) and 2-methyl- 4,6-dichlorophenol (average 0.004%) were detected in all 15 samples. 3-Methylphenol (0.002%) and 2,6-dimethyl-4-chlorophenol (0.002%) were detected in one sample only. The minimum detectable amount ranged from 0.1 to 0.6 ng, depending on the phenol. This corresponds to less than 0.002% when expressed relative to the weight of sample. The coefficient of variation for multiple analyses of the same sample (n = 6) is 1% for 2-methyl-4-chlorophenol and 3% for 2-methy 1-4,6- dichlorophenol.