Forensic determination of pesticides in human serum using metal ferrites nanoparticles and SALDI-MS

Pesticides can have harmful effects on the environment and on humans, resulting in acute, or in some cases, fatal poisoning. Pesticides are inexpensive, easily accessible, and commonly linked to forensic investigations involving suicide or attempted suicide. Pesticide exposure is monitored by determining the presence of the pesticide (or its metabolites) in biological samples, such as blood serum. Traditional methods require the use of a large sample volume and extensive sample preparation to confirm the presence of such harmful materials. Thus, owing to their unique physico-chemical properties, metal ferrites nanoparticles (NPs) were developed as assisting agents in surface-assisted laser desorption/ionization mass-spectrometry (SALDI-MS) for detecting pesticides in human blood serum. Specifically, ZnFe₂O₄, MnFe₂O₄, and CuFe₂O₄ NPs were synthesized using co-precipitation method and were characterized using different analytical techniques, including X-ray diffraction, UV-vis spectroscopy, X-ray photo-electron spectroscopy, transmission electron microscopy, and the Brunauer-Emmett-Teller method. First, it was shown that the integration of metal ferrites NPs in the SALDI technique enabled sensitive detection towards fatty acids used as model analytes with the limit-of-detection (LOD) in the range of 10 μg mL^-1 to 1 fg mL^-1. Additionally, the extent of internal energy transfer was evaluated for the ZnFe₂O₄, MnFe₂O₄, and CuFe₂O₄ NPs using the survival yield (SY) method with benzylpyridinium (BP) as the chemical thermometer. The obtained SY values were 0.95, 0.94, and 0.88 for ZnFe₂O₄, MnFe₂O₄, and CuFe₂O₄ NPs, respectively; this indicated the higher degree of internal energy transfer of CuFe₂O₄ NPs. Finally, CuFe₂O₄ NPs were utilized to probe human serum spiked with different pesticides using a small sample volume and minimal sample pretreatment. The findings confirmed the successful detection of napropamide, metalaxyl, and pestanal with satisfactory reproducibility and LODs of 10 μg mL^-1, 10 ng mL^-1, and 100 pg mL^-1, respectively. Thus, the development of the high-efficiency SALDI technique will enable its use as an analytical tool in forensic investigation using minute volumes of sample and substrate and with minimum sample handling.