Open tubular columns column diameter

Figure 24-4 Effect of open tubular column inner diameter on resolution. Narrower columns provide higher resolution. Notice the increased resolution of peaks 1 and 2 in the narrow column. Conditions DB-1 stationary phase (0.25 xm thick) in 15-m wall-coated column operated at 95°C with He linear velocity of 34 cm/s. [Courtesy JSW Scientific. Folsom. CA.]...

Capillary column. Synonymous with open tubular column. Small diameter tubing (0.25-1.0 mm i.d.) in which the inner walls are used to support the stationary phase. 

R. Tijssen, J.P.A. Bleumer, A.L.C. Smit, M.E. van Kreveld, Microcapillary LC in open-tubular columns with diameters of 10-50 jum. Potential application to CI-MS detection, J. Chromatogr., 218 (1981) 137. 

For packed columns, the most important variable that affects column efficiency is the diameter of the particles making up the packing. For open tubular columns, the diameter of the column itself is an important variable. The effect of particle diameter is demonstrated by the data shown in Figure 26-11 for GC. A similar plot for LC is shown in Figure 28-2. To take advantage of the effect of column diameter, narrower and narrower columns have been used in recent years. 

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. 

This extreme condition rarely happens but serious peak distortion and loss of resolution can still result. This is particularly so if the sensor volume is of the same order of magnitude as the peak volume. The problem can be particularly severe when open tubular columns and columns of small diameter are being used. Scott and Kucera measured the effective sensor cell volume on peak shape and their results are shown in Figure 13. 

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. 

In a packed column the HETP depends on the particle diameter and is not related to the column radius. As a result, an expression for the optimum particle diameter is independently derived, and then the column radius determined from the extracolumn dispersion. This is not true for the open tubular column, as the HETP is determined by the column radius. It follows that a converse procedure must be employed. Firstly the optimum column radius is determined and then the maximum extra-column dispersion that the column can tolerate calculated. Thus, with open tubular columns, the chromatographic system, in particular the detector dispersion and the maximum sample volume, is dictated by the column design which, in turn, is governed by the nature of the separation. 

Figure 1.4 Variation of the resistance to mass transfer in the mobile phase, C , and stationary phase, Cj, as a function of the capacity factor for open tubular columns of different internal diameter (cm) and film thickness. A, df 1 micrometer and D, 5 x 10 cm /s B, df 5 micrometers and D, 5 x 10 cm /s and C, df - 5 Micrometers and 0, 5 x 10 cm /s.

An open tubular column with an internal diameter of 50 microeeti should be capable of providing about 100-500 times... 

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... 

Upon substitution of the reduced parameters given above the separation time for a packed column and an open tubular column would be Identical if d 1.73 dp given the current limitations of open tubular column technology the column diameter cannot be reduced to the point %diere these columns can compete with packed columns for fast separations. This is illustrated by the practical txanple in Figure 6.3 (57). Ihe separation speed cannot be Increased for an open tubular column by increasing the reduced velocity since the reduced plate height is increased... 

Contrast the packed column and the open-tubular capillary column in terms of design, diameter, length, how the stationary phase is held in place, ability to resolve complex mixtures, and amount of sample injected. 

An open-tubular capillary column is a very long (30 to 300 ft), narrow-diameter tube in which the stationary phase is held in place by adsorption on the inside wall. Such a column is useful because it allows the use of a very long column (for better resolution) with minimal gas pressure required. 

The packed column can be from 2 to 20 ft in length, typically has a diameter of V8 or V4 in., and has small particles, often coated with a thin layer of liquid stationary phase, packed in the tube. The open-tubular capillary column can be up to 300 ft in length, has an extraordinarily small diameter (capillary), and has the liquid stationary adsorbed on the inside surface of the tube. In terms of separation ability, the open-tubular capillary column is better because the mixture components contact more stationary phase (column is longer) while passing through the column. The amount injected for the open-tubular capillary column must be much less (0.1 mL maximum, as opposed to 20 mL for the V8-in. packed column) because the column diameter is much less and a greater volume would overload it. 

Several approaches towards monolithic GC columns based on open pore foams prepared in large diameter glass tubes were reported in the early 1970s [26,27, 110]. However, these columns had poor efficiencies, and the foams possessed only limited sample capacities in the gas-solid GC mode. Subsequent experiments with polymerized polymer layer open tubular (PLOT) columns where the capillary had completely been filled with the polymer were assumed to be failures since the resulting stationary phase did not allow the gaseous mobile phase to flow [111]. 

These have now been superseded by capillary columns, which offer greatly improved separation efficiency. Fused silica capillary tubes are used which have internal diameters ranging from 0.1 mm (small bore) to 0.53 mm (large bore) with typical lengths in excess of 20 m. The wall-coated open tubular (WCOT) columns have the internal surface of the tube coated with the liquid (stationary) phase and no particulate supporting medium is required. An alternative form of column is the porous-layer open tubular (PLOT) column, which has an internal coating of an adsorbent such as alumina (aluminium oxide) and various coatings. Microlitre sample volumes are used with these capillary columns and the injection port usually incorporates a stream splitter. 

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