Search

Food samples are usually analyzed with a preliminary step of sample preparation, because pesticide residues are present in low concentrations and embedded in complex matrixes. Solid phase microextraction (SPME) is a solvent-free extraction technique that is fast, relatively inexpensive, easily automated, and versatile with high sample throughput. Analyte extraction, pre-concentration, and introduction into the analytical instrument are achieved in a single uninterrupted process. SPME uses fibers coated with a polymeric stationary phase that can be either a solid, liquid, or a combination of both. The fiber coating extracts the target analytes from a complex food matrix by absorption in the case of liquid coatings or adsorption in the case of solid coatings. The SPME fiber is then transferred into the analytical instrument for desorption and analysis of the target analytes. The use of sol-gel technology in the production SPME coated fiber has helped to increase the range of pesticide residues that can be extracted with the SPME technique. [ABSTRACT FROM AUTHOR]

Stress and couple stress distributions are determined analytically for a coated fiber in an infinite micropolar matrix, loaded remotely by classical symmetric stresses. The determined stresses are then compared with those predicted by Cauchy theory, a size dependence is predicted in the framework of micropolar theory, and when the fiber’s diameter is much larger than the characteristic length of the matrix, the classical prediction can be recovered. The exact average stress in a fiber for a fiber-matrix system (without interphase) is also compared with that approximately derived by the average equivalent inclusion method (AEIM), based on the micropolar Eshelby tensor given by Cheng and He [Int. J. Eng. Sci. 35 (7) (1997) 659], the results show that the approximate AEIM method can give an accurate prediction for the size dependence of the average stress in a fiber. A micro–macro transition method is also proposed to determine effective in plane moduli of a heterogeneous micropolar material, the effective shear and in plane bulk moduli of a micropolar composite with coated fibers are derived analytically by extending Mori–Tanaka’s method to a micropolar composite, and the effective shear and in plane bulk moduli for a two-phase fiber composite (without interphase) are also obtained as a special case. The results show that the effective in plane bulk modulus is identical to that obtained by Cauchy theory, however the effective shear modulus depends on fiber’s diameter, it increases with decreasing fiber’s diameter at the same fiber volume fraction, and the classical prediction is recovered when the fiber’s diameter becomes large compared to the matrix characteristic length. [Copyright &y& Elsevier]