Determination of organophosphorus pesticides in soil by headspace solid-phase microextraction

Headspace solid-phase microextraction (SPME) has been developed for the analysis of common organophosphorus pesticides in soil. Factors such as adsorption-time, sampling temperature and matrix modification by addition of water were carefully considered to optimize the extraction efficiency. This technique could achieve limits of detection of 143 ng/g for Malathion and Parathion, and 28.6 ng/g for Phorate, Diazinon and Disulfoton in humic soil when the extracted sample was analyzed by gas chromatography-flame ionization detector (GC-FID). Lower limits of detection of 28.6 ng/g for Malathion and Parathion, and 14.3 ng/g for Phorate, Diazinon and Disulfoton can be achieved by analyzing the extracted sample with gas chromatography/mass spectrometric detector (GC/MS). As the extraction efficiency was generally better when analyzing sandy soil, the limits of detection are envisaged to be even better for such a matrix. The technique was found to be reliable with good precision of about 6.5% RSD for the sandy soil and about 15% for the humic material. The poorer precision of extraction from the humic material is probably related to the poorer homogeneity of this material. The linearity of extraction was good with linear calibration in the range of 0.143 to 28.6 μg/g. Finally, headspace SPME was compared to aqueous extraction of soil followed by SPME (LE-SPME). The recoveries obtained by headspace SPME were comparable to those from liquid-liquid extraction of soil followed by SPME. However, the analysis of headspace SPME has less background interference. Perhaps, the greatest advantage of this technique is its non-destructive nature so that it is possible to perform further laboratory analysis of the samples after headspace SPME has been carried out.     

Near-Optimal Controls of a Class of Volterra Integral Systems

In a recent paper by Zhou (Ref. 1), the concept of near-optimal controls was introduced for a class of optimal control problems involving ordinary differential equations. Necessary and sufficient conditions for near-optimal controls were derived. This paper extends the results obtained by Zhou to a class of optimal control problems involving Volterra integral equations. The results are applied to study near-optimal controls obtained by the control parametrization method.