Method for simultaneous determination of monocrotophos and dimethyl phosphate in human biofluids

An NMR method was developed for the simultaneous determination of the organophosphate insecticide monocrotophos and dimethyl phosphate (DMP)- in human plasma and urine. Following lyophilization and solvent exchange into deuterium oxide (D₂O), samples were spiked with known concentrations of target analytes and ammonium formate as an internal standard. Dimethyl phosphate was synthesized via acid-catalyzed hydrolysis of trimethyl phosphate and characterized by 1H NMR, confirming 96.5% purity. Key acquisition parameters-including a 90° single-pulse sequence, 1 s relaxation delay, digital resolution of ~0.2 Hz/point, and 16 scans-were optimized on a Bruker Avance Neo 500 MHz instrument to achieve signal-to-noise ratios ≥250:1. The method demonstrated excellent linearity across 10-250 ppm, with correlation coefficients (R²) exceeding 0.99 in both matrices. The limit of detection (LOD) and limit of quantification (LOQ) were established at 0.25 ppm (S/N ≥3) and 0.5 ppm (S/N ≥10), respectively. Precision was verified via six replicates at 50 ppm, yielding intra-assay relative standard deviations below 2.1%. Recovery in spiked samples ranged from 98.0-102.0% across low (10 ppm), medium (100 ppm), and high (250 ppm) levels. Specificity, as confirmed by the absence of interfering signals in blank matrices. This NMR approach provides a rapid, accurate, and non-destructive method for forensic and clinical toxicology. However, applying the same method to real-time patient samples could not be carried out due to the limitation arising from the non-availability of actual patient samples.