Length open tubular column

Capillary Columns Capillary, or open tubular columns are constructed from fused silica coated with a protective polymer. Columns may be up to 100 m in length with an internal diameter of approximately 150-300 )J,m (Figure 12.17). Larger bore columns of 530 )J,m, called megabore columns, also are available. 

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. 

The efficiency obtained from an open tubular column can be increased by reducing the column radius, which, in turn will allow the column length to be decreased and, thus, a shorter analysis time can be realized. However, the smaller diameter column will require more pressure to achieve the optimum velocity and thus the reduction of column diameter can only be continued until the maximum available inlet pressure is needed to achieve the optimum mobile phase velocity. 

The separation nuaber is the only column efficiency par2uaeter that can be deterained under teaperature progr2uued conditions [45,46]. The critical parameters that aust be standardized to obtain reproducible SM values for coluans of different length are the carrier gas flow rate and the temperature program. The SN is widely used as part of a standardized test method to evaluate the quality of open tubular columns for gas chromatography (section 2.4.3). 

Virtually all current research in SFC utilizes either small bore packed columns with particles of 5-10 micrometers in diameter optimized for use in liquid chromatography or narrow bore, fused silica open tubular columns with Immobilized phases similar to those used in gas chromatography. In the latter case columns of saaller internal diameter, 10-100 micrometers, shorter lengths (generally less than 20 m with 1-10 m being the most common length), and more firmly crosslinked stationary phases are used by coaparison with standard columns for gas chromatography. In all... 

Compared to packed columns, open-tubular columns have no bubble formation problems because end-frits are not needed, small internal diameter columns are used, and the stationary phase is homogeneous. The column length can also be easily shortened. Excellent mass sensitivity can be achieved by using capillaries with smaller inner diameters. The EOF in an open-tubular column is higher than that in a packed column because a greater... 

It is seen that the optimum column radius for an open tubular column varies widely with inlet pressure arid the difficulty of the separation. Considering a separation of some difficulty, for example ( a = 1.02), it is seen that at an inlet pressure of lOOOp.s.f, the optimum column diameter would be about 4 micron whereas, at an inlet pressure of only 1 psi, it would be about 43 micron. The former would be quite difficult to coat with stationary phase and would demand detectors and injection systems of almost impossibly low dispersion A column of 43 micron in diameter, on the other hand, would be piactical from the point of view of both ease of coating and an acceptable system extra column dispersion. However, the lengths of such columns arid the resulting analysis times remains to be determined and may preclude their- use. 

Particles in a packed column resist flow of the mobile phase, so the linear flow rate cannot be very fast. For the same length of column and applied pressure, the linear flow rate in an open tubular column is much higher than that of a packed column. Therefore, the open tubular column can be made 100 times longer than the packed column, to give a similar pressure drop and linear flow rate. If plate height is the same, the longer column provides 100 times more theoretical plates, yielding VTOO = 10 times more resolution. 

Consider the peaks for pentafluorobenzene and benzene in the chromatogram shown above. The elution time for unretained solute is 1.06 min. The open tubular column is 30.0 m in length and 0.530 mm in diameter, with a layer of stationary phase 3.0 p,m thick on the inner wall. 

Reduced plate height is h = H/dp with packed columns h = H/dt. with open tubular columns Reduced length of column is Z = L/dp or L/dc. 

Lysozyme, angiotensins Cl8, etched and modified column diol open tubular column 30 mMphosphate, pH 2.14, 30 mMcitric acid, pH 3-25 mM p-alanine, 30 mM acetic acid, pH 4.14 450 mm x 50 pm i.d. 250 mm packed length 102... 

Cloxazolam, nitrazepam, clotiazepam, diazepam Etched cholesterol modified open tubular column Acetonitrile-10 mMTris-HCl, pH 7.3 (0-20 100-80) 700 mm x 50-75 pm i.d. 550 mm packed length 141... 

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