The development and optimisation of a large-volume-injection method on a GC-MS/MS instrument for selected organophosphate pesticide analysis in selected juice samples.

Abstract

During agricultural production, organophosphate pesticides are used to control, destroy and prevent pests. However, excessive application of the chemical pesticides on fruits and vegetables has severe effects on the environment and poses harm to humans due to their high acute toxicity. Food safety is ensured by strict monitoring of residue levels, as organophosphate pesticides and carbamates inhibit the enzyme acetylcholinesterase (AChE) that results in accumulation of acetylcholine (ACh) at cholinergic receptor sites, leading to paralysis. Various standard methods including Liquid chromatography (LC) and Gas chromatography (GC) are used for the detection of multiple classes of pesticides in various sample matrices including juice samples. There are continuous efforts to improve trace analysis and sensitivity of current methods. This study investigated the applicability a large-volume injection method (LVI) for the determination of organophosphate pesticides in juice samples. The study aimed to evaluate the developed and optimised LVI in comparison to a currently used hot splitless injection method (HSI). Investigations were conducted to identify which method was most suitable. Representative pesticides selected from the organophosphate class was malaoxon (MLXN), malathion (MLTN), chlorpyrifos (CPFS), bromophos-methyl (BRMP), bromophos-ethyl (BRPE), methidathion (MTDN) and profenofos (PRFF). These pesticides present in fruit juice samples were extracted using the QuEChERS extraction protocol and analysis performed using a Gas Chromatography Tandem Mass Spectrometry instrument (GC-MS/MS) instrument. Method development of LVI involved an investigation of various injection techniques, HSI, CSI and LVI at volume of 5, 10 and 25 μL. The injection method selected for further method optimization was the 5 μL LVI method. Parameters investigated for optimal conditions was vent flow, vent pressure, vent time, inlet and oven temperature. Additionally, comparison between the HSI and optimised 5 μL LVI method was conducted by assessing specificity, limit of detection (LOD), limit of quantification (LOQ), precision (repeatability) and trueness (recovery). Proficiency test samples from schemes such as the National Metrology Institute of South Africa (NMISA) and the Food Analysis Performance Assessment Scheme (FAPAS) were analysed for expanded insight. The LVI method produced greater sensitivity, with larger peak areas and improved peak shapes. Chromatograms generated from the LVI analysis necessitated little to no manual integration during the analysis as better peak shapes and resolution was achieved. The evaluation of LVI and HSI for specificity, LOD, LOQ, repeatability, and trueness in apple and orange juice samples resulted in all pesticides meeting the acceptable criteria of repeatability below 20% and trueness within the 70%-120% range. Both methods produced satisfactory z-scores from the analysis of proficiency test samples. This study concluded by confirming the proposed hypothesis, that the LVI method would demonstrate enhanced analytical performance compared to the HSI method for the analysis of juice samples. This implied more accurate determinations of pesticides due to significant improvements in detection limits, sensitivity and recovery.