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.

Popular Abstract

Herbicide products are widely used as weed-controlling agents by both professionals and individuals. The formulas of some of those products contain chemicals that may have harmful impacts on the environment, which incentivized governments and health authorities to ban some of those herbicides. Researchers and producers were also encouraged to synthesize and examine weed-controlling agents that are greener and more environmentally sustainable. Some products of the modern generation of herbicides depend mainly on fatty acids and natural oils to achieve their mode of action as weed-controlling agents. Nonanoic acid, also known by its trivial name as pelargonic acid, is widely used as the main ingredient in the so-called bio-herbicides. However, the friendly environmental impact of those bio-herbicides is yet to be investigated. Research and studies that could achieve that goal, implicitly, shall include analytical methods to extract, detect and quantify those products in environmental matrices. The aim of this work was to identify the fatty acid composition in a commercial fatty acid-based herbicide product and to detect it in environmental matrices.
Soils collected from different locations were spiked with different amounts of the herbicide in order to mimic the exposure to the herbicide in nature. The extractions of FA´s from the soil were performed using extraction solvent consisting of SC-CO2 and methanol as a co-solvent to tune the polarity of the extraction solvent in order to create optimal solubility conditions for the extraction of the FA´s from the soils. Qualitative and quantitative analysis’s were performed on GC-FID system and GC-MS system. The free FA´s used to construct the models as well as lipids in the extractions were derivatized under acidic conditions and then analyzed in their FAME´s form. The qualitative analysis was performed by constructing retention index models for the FAME´s to detect and identify the corresponding FA´s in the herbicide and the soils. The quantitative analysis was performed by constructing internal calibration models for the FAME´s.
The herbicide was found to contain octanoic acid, pelargonic acid, 2-methyl octanoic acid, and decanoic acid, with pelargonic acid and 2-methyl octanoic acid having the highest abundance, respectively. The constructed internal calibration models showed acceptable R2 values that ranged between 0.992 and 0.996. The performance of the analysis showed good precision where repeatabilities for the peak areas and retention times with RSD% values ranged from 1.32 to 7.69 and from 0.011 to 0.783, respectively. Validation of the extraction process of FA´s from the spiked soils was estimated quantitatively in terms of recovery percentage for the concentrations of pelargonic acid. Recoveries of pelargonic acid in the spiked soils were below the acceptable criterion in five spiked samples with values: 8%, 22%, 62%, 66 %, and one sample with 129%, while three spiked samples were within the acceptable range, two samples with 79% and one sample with 100%. The low obtained recoveries may indicate that a better extraction method can be considered in future work in order to achieve optimal extraction conditions.

• author: Abd Elwadood, Mohamad
• supervisor: Peter Spégel; Margareta Sandahl
• organization: Department of Chemistry
• course: KEMR10 20221
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Analytical Chemistry, Fatty Acids, Gas Chromatography, Green Herbicides, Retention Index, Flame Ionization Detector, Mass Spectrometry, Supercritical Carbon Dioxide Extraction.
• language: English
• id: 9112319
• date added to LUP: 2023-03-16 13:22:23
• date last changed: 2023-03-16 13:22:23