Microcolumns efficiency

Microcolumns use less solvent and, because the sample is diluted to a lesser extent, produce larger signals at the detector. These columns are made from fused silica capillaries with internal diameters of 44—200 pm and lengths of up to several meters. Microcolumns packed with 3-5-pm particles have been prepared with column efficiencies of up to 250,000 theoretical plates. 

Open tubular microcolumns also have been developed, with internal diameters of 1-50 pm and lengths of approximately 1 m. These columns, which contain no packing material, may be capable of obtaining column efficiencies of up to 1 million theoretical plates.The development of open tubular columns, however, has been limited by the difficulty of preparing columns with internal diameters less than 10 pm. 

E. M. Ean as and S. R. Rissato, Influence of temperature, pressure, modifier and collection mode on superaitical CO2 extraction efficiencies of diuron from sugar cane and orange samples , J. Microcolumn Sep. 10 473-478 (1998). 

H. Daimon and Y. Hirata, Trapping efficiency and solute focusing in on-line supercritical fluid exti action/capillai y supercritical fluid chi omatography , 7. Microcolumn Sep. 5 531-535 (1993). 

A six-port valve was first used to interface the SEC microcolumn to the CZE capillary in a valve-loop design. UV-VIS detection was employed in this experiment. The overall run time was 2 h, with the CZE runs requiring 9 min. As in the reverse phase HPLC-CZE technique, runs were overlapped in the second dimension to reduce the apparent run time. The main disadvantage of this yu-SEC-CZE method was the valve that was used for interfacing. The six-port valve contributed a substantial extracolumn volume, and required a fixed volume of 900 nL of effluent from the chromatographic column for each CZE run. The large fixed volume imposed restrictions on the operating conditions of both of the separation methods. Specifically, to fill the 900 nL volume, the SEC flow rate had to be far above the optimum level and therefore the SEC efficiency was decreased (22). 

Efforts toward integrating SPE onto a lab-on-a-chip device are currently being investigated by the Collins group. Two complementary approaches are being pursued. One approach is to use small-diameter, Cl8 functionalized silica beads that are packed into a microchannel to form an extraction bed [46], A sample solution containing trace levels of explosives is electrokinetically directed across the microcolumn bed, causing the hydrophobic explosive molecules to adsorb onto the stationary phase with nearly 100% efficiency. Subsequently,... 

Karlsson, K. E. and Novotny, M., Separation efficiency of slurry-packed hquid-chromatography microcolumns with very small iimer diameters. Analytical Chemistry 60(17), 1662-1665, 1988. 

The low flow rate in the microbore column ensures sample volumes compatible with the secondary conventional column and permits the injection of a small volume onto the secondary column, making the transfer of incompatible solvents possible without peak shape deterioration or resolution losses [63], The possible disadvantage could be the lower sample capacity of microbore LC columns. However, in LCxLC, a sensitivity enhancement can be obtained if the formation of compressed solute bands at the head of the secondary column is achieved during the transfer from the first to the second dimension. Moreover, a larger volume can be injected into the first-dimension microcolumn, used as a highly efficient pre-separation step, and a limited decrease in efficiency due to a large injection volume can be tolerated. 

Karlsson and Novotny [12] introduced the concept of nanoliquid chromatography in 1988. The authors reported that the separation efficiency of slurry packed liquid chromatography microcolumns (44 xm, id) was very high. Since then, many advance have been reported in this modality of chromatography and it has been used as a complementary and/or competitive separation method to conventional chromatography. Unfortunately, to date no correct and specific definition of this technique has been proposed, probably due to the use of varied column sizes (10 to 140 xm). Some definitions of nanoliquid chromatography are found in the literature based on column diameter and mobile... 

The quest for improved efficiency provides the continuing impetus to the study of reduced diameter columns, and though still in their early stage of development, these new column technologies are having considerable influence on the practice of HPLC [104,105]. The incentive for the development of microcolumns for HPLC lies in the various practical advantages they have over standard analytical columns [106] ... 

In conclusion, the higher column efficiencies available with microcolumn HPLC could prove to be an absolute requirement for the analysis of complex biological samples. Technical advances in the... 

Injection effects as a result of mass and volume overloading or the injection technique can detract from column efficiency. As with other extracolumn effects, injection effects become more critical with smaller bore columns, which require smaller injection volumes and low flow rates refer to Reference 110 for a discussion of extracolumn effects in microcolumn systems. 

Chen et al. reported the enantioseparation of dansyl amino acids on 100 pm i.d. ligand exchange-chiral monolithic microcolumns. Using continuous beds modified with chiral selectors such as L-phenylalanin-amide, L-alaninamide, L-prolinamide, he achieved efficiencies of 1000 plates per meter. 

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