We report an experimental study of separation efficiency in microchip high-performance liquid chromatography (HPLC). For this study, prototype HPLC microchips were developed that are characterized by minimal dead volume, a separation channel with trapezoidal cross section, and on-chip UV detection. A custom-built stainless steel holder enabled microchip packing under pressures of up to 400 bar and ultrasonication. Bed densities were investigated with respect to the packing conditions and consistently related to pressure drop over the packed microchannels and separation efficiency under isocratic elution conditions. The derived plate height curves show a decrease of mobile phase mass transfer resistance with increasing bed density. High bed densities are critical to separation performance in noncylindrical packed beds, because only at low bed porosities does hydrodynamic dispersion in noncylindrical packings come close to that of cylindrical packings. At higher bed porosities, the presence of fluid channels of advanced flow velocity in the corners of noncylindrical packings affects hydrodynamic dispersion strongly. We demonstrate that the separation channels of HPLC microchips can be packed as densely as the cylindrical fused-silica capillaries used in nano-HPLC and that consequently microchip-HPLC separation efficiencies comparable to those of nano-HPLC can be achieved.

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This work investigates the impact of conduit geometry on the chromatographic performance of typical particulate microchip packings. For this purpose, high-performance liquid chromatography (HPLC)/UV-microchips with separation channels of quadratic, trapezoidal, or Gaussian cross section were fabricated by direct laser ablation and lamination of multiple polyimide layers and then slurry-packed with either 3 or 5 microm spherical porous C8-silica particles under optimized packing conditions. Experimentally determined plate height curves for the empty microchannels are compared with dispersion coefficients from theoretical calculations. Packing densities and plate height curves for the various microchip packings are presented and conclusively explained. The 3 microm packings display a high packing density irrespective of their conduit geometries, and their performance reflects the dispersion behavior of the empty channels. Dispersion in 5 microm packings correlates with the achieved packing densities, which are limited by the number and accessibility of corners in a given conduit shape.

Journal of Liquid ChromatographyPublication details, including instructions for authors and subscription information:ABSTRACT Solid phase extraction has been combined with High Perfornance Liquid Chromatography using a Photodiode Array Detector to isolate, determine, quantify and recover trace concentrations of aldicarb and its toxic metabolites, aldicarb sulfoxide and aldicarb sulfone, in soils and potatoes. A method is proposed which eliminates matrix interferences. The procedure involved extracting with acetonitrile or dichloromethane, initial separation and clean-up with a Sep-Pack Florisil or Silica cartridges. The extract can be successfully analyzed by HPLC-DAD and recoveries for pesticides were not influence by the use of this extraction method. The results have been applied to the systematic study of these compounds in growns of patatoes. The lowest detectable concentration for each pesticide is 40 pg kg-1 in soil and 15 pg kg-1 in potatoes.