The advantages of representing experimental plate height data as a plot of Kv/u0(2) or H2/Kv versus Kv/(Hu0) instead of as H versus u0 are discussed (Kv=column permeability). Multiplying the values on both axes by the ratio of a reference pressure drop and mobile-phase viscosity, the obtained plots directly yield the kinetic performance limits of the tested support structure, without any need for further numerical optimization. Directly showing the range of plate numbers or analysis times wherein the tested support geometry can yield faster separations or produce more plates than another support type, such kinetic plots are ideally suited to compare the performance of differently shaped or sized LC supports. The approach hence obviates the need for a common reference length, which is a clear problem if it is attempted to compare differently shaped supports on the basis of their flow resistance phi and reduced plate height h. It is also shown how an MS Excel template file, only requiring the user to paste the column permeability Kv and a series of experimental (u0, H) data, can be used to automatically establish a series of so-called kinetic performance (KP) numbers, which can be used to completely describe the performance characteristics of the considered support. The advantages of the proposed data representation methods are demonstrated by applying them to several recent literature plate height data sets, showing that the obtained kinetic plots directly visualize the range of plate numbers where new approaches such as ultra-high-pressure HPLC or the use of open-porous silica monoliths can be expected to provide a substantial gain and where not. The data analysis also showed that the most generally relevant KP numbers are N(opt) (the plate number for which the support achieves its best analysis time/pressure cost ratio), t(opt) (the time needed to obtain N(opt) plates), and t(1K) (the time needed to generate 1000 or 1 kilo of theoretical plates). These KP numbers are much more informative than the H(min), u(0,opt), and Kv data traditionally employed to quantify the performance of LC supports.
Building upon the micromachined column idea proposed by the group of Regnier in 1998, we report on the first high-resolution reversed-phase separations in micromachined pillar array columns under pressure-driven LC conditions. A three component mixture could be separated in 3 s using arrays of nonporous silicon pillars with a diameter of approximately 4.3 microm and an external porosity of 55%. Under slightly retained component conditions (retention factor k' = 0.65-1.2), plate heights of about H = 4 microm were obtained at a mobile phase velocity around u = 0.5 mm/s. In reduced terms, such plate heights are as low as hmin = 1. Also, since the flow resistance of the column is much smaller than in a packed column (mainly because of the higher external porosity of the pillar array), the separation impedance of the array was as small as E = 150, i.e., of the same order as the best currently existing monolithic columns. At pH = 3, yielding very low retention factors (k' = 0.13 and 0.23), plate heights as low as H = 2 microm were realized, yielding a separation of the three component mixture with an efficiency of N = 4000-5000 plates over a column length of 1 cm. At higher retention factors, significantly larger plate heights were obtained. More experimental work is needed to investigate this more in depth. The study is completed with a discussion of the performance limits of the pillar array column concept in the frame of the current state-of-the-art in microfabrication precision.
A new method for producing zeolitic monoliths is used to produce a ZSM-5 based monolith for gas separation. The new method involves the 3DFD (Three Dimensional Fiber Deposition) printing of several layers of zeolite fibers on top of each other in a well-defined way,resulting in an open monolithic structure with open and inter-connected channels. The monolithic structure, consisting of ZSM-5 zeolite, was characterized with SEM, Ar and Hg porosimetry. Single component isotherms of CO 2 , CH 4 and N 2 were recorded on the 3DFDprinted ZSM-5 monolith, at different temperatures (283K, 291K, 302K and 309K) using a gravimetric method. Isosteric heats of adsorption show that CO 2 is the most strongly adsorbing component, in order followed by CH 4 and N 2 . The monolithic structure was subjected to breakthrough separation experiments with CO 2 /N 2 and CO 2 /CH 4 gas mixtures.Excellent separation performance is achieved. Moreover, the ZSM-5 monolith can be easily regenerated in isothermal conditions.
Metal-organic framework materials (MOFs) are a class of microporous and crystalline materials with great potential for adsorption-based separations. Harnessing the separation ability of these materials in high-performance liquid chromatography (HPLC) requires the use of small and uniform particles in order to achieve good column packing. The well-known MOF material [Cu 3 (BTC) 2 ] is, however, typically synthesized as a polydisperse mixture due to its nucleation
We report on the possibility to achieve ultra high efficiencies (order of 1 million theoretical plates) in liquid chromatography in a relatively short time of 20 min (elution time of unretained marker). This was achieved using a micropillar array column with optimized pillar diameter (5 μm) and interpillar distance (2.5 μm) to operate close to the Knox and Saleem limit of micropillar array columns in the region of the 1 million theoretical plate mark under the prevailing pressure restriction (350 bar in the present study). The obtained efficiency was slightly affected (some 15 to 20% around the optimal flow rate) by the turns that were inevitably needed to arrange a 3 m long column on a 4 in. silicon wafer.
The present paper describes a method for measuring the molecular diffusion coefficient of fluorescent molecules in microfluidic systems. The proposed static shear-driven flow method allows one to perform diffusion measurements in a fast and accurate manner. The method also allows one to work in very thin (i.e. submicron) channels, hence allowing the investigation of diffusion in highly confined spaces. In the deepest investigated channels, the obtained results were comparable to the existing literature values, but when the channel size dropped below the micrometer range, a significant decrease (more than 30%) in molecular diffusivity was observed. The reduction of the diffusivity was most significant for the largest considered molecules (ssDNA oligomers with a size ranging between 25 to 100 bases), but the decrease was also observed for smaller tracer molecules (FITC). This decrease can be attributed to the interactions of the analyte molecules with the channel walls, which can no longer be neglected when the depth of the channel reaches a critical value. The change in diffusivity seems to become more explicit as the molecular weight of the analytes increases.
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