A method for quantifying the effect of medium composition on the diffusive mass transfer of hydrophobic organic chemicals through thin layers was applied to plant tissue. The method employs two silicone disks, one serving as source and one as sink for a series of PAHs diffusing through thin layers of water, potato tissue, and carrot tissue. Naphthalene, phenanthrene, anthracene, and fluoranthene served as model substances. Their transfer from source to sink disk was measured by HPLC to determine a velocity rate constant proportional to the diffusive conductivity. The diffusive flux through the plant tissue was modeled using Fick's first law of diffusion. Both the experimental results and the model suggest that mass transfer through plant tissue occurs predominantly through pore water and that, therefore, the mass transfer ratio between plant tissue and water is independent of the hydrophobicity of the chemical. The findings of this study provide a convenient method to estimate the diffusion of nonvolatile organic chemicals through various plant materials. The application to a radial diffusion model suggests that “growth dilution” renders the concentration of highly hydrophobic chemicals in potatoes below their equilibrium partitioning level. This is in agreement with field results for the bioconcentration of PAHs in potatoes.
This paper describes the application of liquid chromatography time-of-flight mass spectrometry (LC/TOF-MS), with electrospray ionization, for residue analysis of forchlorfenuron in tomato, zucchini and watermelon. The assessed method includes a sample preparation step based on the buffered QuEChERS approach. The TOF-MS fragmentation pattern of forchlorfenuron was studied at different fragmention voltages in the range of 120-270 V. Analyses were carried out under full-scan conditions by using the extracted ion chromatogram (XIC) of the m/z 248 ion with a 0.2 Da window. The linearity of the analytical response across the studied range of concentrations (10-500 microg/kg) was excellent, obtaining correlation coefficients higher than 0.999, and relative standard deviations of the response factors lower than 14%, for the 15 linear calibration curves of forchlorfenuron evaluated along the complete validation study. No significant matrix effects were observed. The signal-to-noise ratios obtained for the 10 microg/kg forchlorfenuron in matrix matched standards were >70 for all three matrices. Recovery studies were carried out on spiked tomato, zucchini and watermelon blank samples, at three concentration levels (10, 50, and 200 microg/kg) performing five replicates at each level. Forchlorfenuron mean recoveries ranged between 80% and 87% in watermelon and zucchini, and between 65% and 71% in tomato, obtaining in all cases relative standard deviation values lower than 10%. The method readily achieved a lowest validated level of 10 microg/kg, which was fit-for-purpose in residue monitoring applications.
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