Analysis of fatty acids in fatty acid-based herbicides by gas chromatography

Abstract: Fatty acids-based herbicides are considered to be a green alternative to synthesized weed control products. However, the environmentally sustainable effect of those herbicides needs more investigation to be confirmed. The aim of this work was to develop a chemical analysis method that enables identification of short-chain fatty acids (SCFA´s) and medium-chain fatty acids (MCFA´s) in a commercial fatty acid-based herbicide product, and to set up extraction and gas chromatography (GC) methods that can be used to detect the exposure to the product in environmental matrices. Fatty acids (FA´s) were extracted from the herbicide by liquid-liquid extraction (LLE) using n-heptane as organic solvent. FA´s were extracted from soils spiked with the herbicide; on a supercritical fluid extraction (SFE) system using extraction solvent consisting of 90% supercritical carbon dioxide (SC-CO2) and 10% methanol (MeOH) (v/v). The extracted FA´s were derivatized under acidic conditions into fatty acid methyl esters (FAME´s) to be analysed on gas chromatography-flame ionization detector (GC-FID) system and gas chromatography-mass spectrometry (GC-MS) system. Standard solutions of known FAME´s were used to construct retention index (RI) models and internal calibration models to identify and quantify FAME´s in the samples. The herbicide product was found to contain octanoic acid, pelargonic acid, decanoic acid, and 2-methyoctanoic acid. R2 values of the calibration curves ranged between 0.992 and 0.996. Repeatabilities for the peak areas and retention times had RSD% values varied between 1.32 -7.69 and 0.011-0.783, respectively. Recoveries (R%) of pelargonic acid in the spiked soils were estimated to vary between 8% and 128%; showing that more work could be required to investigate the optimum parameters for SFE extraction method.