Extraction and analysis of pesticide residues in basmati rice using handheld-SERS

Prior to spiking, basmati rice samples were cleaned thoroughly by rinsing in tap water, dH₂O and air dried. For extraction 1 (E1) and extraction 2 (E2) basmati rice samples were spiked by weighing 5 g of dried basmati rice into a clean plastic weigh boat and mixing with 1 mL of pesticide solution at concentrations ranging from 1-100 ppm. After mixing several times with a spatula over the course of 30 min, the samples were left to completely dry at room temperature overnight (~12 h). For extraction 3 (E3) and extraction 4 (E4), 5 g of dried basmati rice was weighed into a 50 mL centrifuge tube and spiked with 2 mL of pesticide solution at concentrations ranging from 10 ppb-100 ppm. The samples were placed on a roller for 30 min and left to completely dry at room temperature overnight (~12 h). The four extraction procedures were conducted as follows:

Swab extraction (E1): Swab sticks pre-soaked in ethanol (EtOH) were drawn through the rice samples evenly for ~90 s and immersed in 1 mL of the extraction solvent (EtOH). The swab tip was removed and vortexed in EtOH for ~30 s to release pesticide residues. Subsequently, 50 µL of the extracted residues were removed for SERS analysis.

Solvent extraction (E2): 1 g of spiked basmati rice was placed directly into a 2 mL plastic Eppendorf tube with 1 mL of EtOH. The sample was vortexed for ~30 s to release pesticide residues and 50 µL was removed for SERS analysis.

QuEChERs acetate (E3) and original QuEChERs (E4): QuEChERs extractions were conducted with minor adaptions to a previous report⁵⁰. Due to the low water content of rice, water was first added (1:1, 5 mL) to hydrate the sample, followed by 15 mL of extraction solvent (MeCN, with the addition of 1% HAc for E3) for single-phase extraction of the sample and vortexed for 1 min. Liquid-liquid partitioning was performed with the addition of salts; 7 g of MgSO₄ and 1.8 g NaOAc (E3) or 4 g of MgSO₄ and 1 g of NaCl (E4) and vortexed for 1 min followed by a 5 min centrifugation at 1970 xg (RCF). Subsequently, 10 mL of the supernatant was removed and analysed using the handheld spectrometer and benchtop microscope.

For all extractions, SERS analysis was conducted using the method described in the section ‘analysis of pesticide standard solutions using handheld-SERS’. Due to the hazardous nature of the chemicals used during the procedures, all waste was collected and disposed of accordingly.