Rapid detection of pesticides in honey and juice by electrospray ionization mass spectrometry based on functionalized solid substrates

Abstract

Detection of pesticide residues in food samples is important to safeguard food quality and safety. Conventional approaches for detection of pesticides in food samples typically involves sample pre-treatment and chromatographic separation before mass spectrometric analysis, and can be time-consuming, labour and material intensive. Electrospray ionization-mass spectrometry (ESI-MS) with solid substrates is a technique that allows direct analysis of raw samples. In this study, we aim to couple ESI-MS with C18 pipette tips (i.e., C18 pipette-tip ESI-MS) and solid-phase microextraction (i.e., SPME-ESI-MS) for rapid detection and quantitation of pesticides in honey and juice. Honey and juice were chosen as the food samples for the study as they are popularly consumed and pesticide contaminations in honey and juice have been public concerns. Moreover, the large content of sugars in honey and juice, the viscous texture of honey and flesh pulp in juice represent analytical challenges and can serve as interference models for detection of target analytes. Pesticides used for the analysis include atrazine, benalaxyl, carbofuran, dimethoate, imidacloprid, malathion and pirimicarb. C18 pipette tips and SPME tips were used to extract and enrich pesticides in honey and juice samples, and were directly connected to mass spectrometer for qualitative and quantitative analysis. The analytical performance of C18 pipette-tip ESI-MS and SPME-ESI-MS for analysis of pesticides in honey and juice was evaluated. It was found that the linear ranges varied among pesticides but generally were in the range of 1 - 800 ng mL⁻¹. Linear calibration curves were obtained with R² coefficients of 0.943 - 1.000 for C18 pipette-tip ESI-MS and of 0.951 - 1.000 for SPME-ESI-MS, among which excellent R ² coefficients (R² = 0.993 - 1.000) were obtained for both C18 pipette-tip ESI-MS and SPME-ESI-MS analysis of atrazine and pirimicarb in all food samples. The limits of detection (LODs) and limits of quantitation (LOQs) were 0.3 and 1 ng mL⁻¹ respectively for C18 pipette-tip ESI-MS analysis of carbofuran and pirimicarb in honey, and were 0.3 and 0.5 ng mL⁻¹ respectively for SPME-ESI-MS analysis of atrazine, benalaxyl, carbofuran, pirimicarb in apple juice. The LODs and LOQs of most pesticides fulfilled the cut-off values (50 ng mL⁻¹) in the international standards. Compared to C18 pipette-tip ESI-MS, SPME-ESI-MS enabled more precise and accurate results, especially for analysis of pirimicarb. The precision and accuracy for analysis of pirimicarb by SPME-ESI-MS were determined to be 3 - 8% and 98 - 100% in honey; 7 - 27% and 91 - 111% in apple juice and 4 - 14% and 100 - 102% in orange juice. These results demonstrated that C18 pipette-tip ESI-MS and SPME-ESI-MS could be used for rapid detection of pesticides in honey and juice, particularly for those pesticides with good responses under ESI conditions. C18 pipette-tip ESI-MS and SPME-ESI-MS are easy to set up. Analysis of samples by these two techniques requires minimum sample preparation and no chromatographic separation, and results could be obtained within minutes per sample. These techniques could be further extended for analysis of other analytes and sample systems.