Tubular columns

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. 

An open tubular column in which the stationary phase is coated on the column s walls. 

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. 

Kovat s retention index (p. 575) liquid-solid adsorption chromatography (p. 590) longitudinal diffusion (p. 560) loop injector (p. 584) mass spectrum (p. 571) mass transfer (p. 561) micellar electrokinetic capillary chromatography (p. 606) micelle (p. 606) mobile phase (p. 546) normal-phase chromatography (p. 580) on-column injection (p. 568) open tubular column (p. 564) packed column (p. 564) peak capacity (p. 554)... 

The Liquid Phase. The stationary phase in an open tubular column is generally coated or chemically bonded to the wall of the capillary column in the same way the phase is attached to the support of a packed column. These are called nonbonded and bonded phases, respectively. In capillary columns there is no support material or column packing. 

M. Lee, E. Yang, and K. D. Bartie, Open Tubular Column Gas Chromatography Wiley-Interscience, John Wiley Sons, Inc., New York, 1984. 

In general, (Q) and ( ) will be equal, but the general case is assumed, where they are not. Equation (37) gives an explicit and accurate expression for the retention volume of a solute. The importance of each function in the expression will depend on the physical properties of the chromatographic system. At one extreme, using an open tubular column in GC, then... 

Consequently, as the inlet pressure increases, the mean flow rate will be reduced according to the pressure correction function and the expected decrease in elution rate will not be realized. Consider an open tubular column. 

Figure 2. The Relationship between the Inlet/Outlet Pressure Ratio and Exit Flow Rate for an Open Tubular Column...

For example, consider an open tubular column with the dimensions previously defined, operated at constant mass flow rate of helium (which is normal for temperature programming purposes), then, from Poiseuille s equation, after time (t),... 

The idea of the effective plate number was introduced and employed by Purnell [4], Desty [5] and others in the late 1950s. Its conception was evoked as a direct result of the introduction of the capillary column or open tubular column. Even in 1960, the open tubular column could be constructed to produce efficiencies of up to a million theoretical plates [6]. However, it became immediately apparent that these high efficiencies were only obtained for solutes eluted at very low (k ) values and, consequently, very close to the column dead volume. More importantly, on the basis of the performance realized from packed columns, the high efficiencies did not... 

Having established that a finite volume of sample causes peak dispersion and that it is highly desirable to limit that dispersion to a level that does not impair the performance of the column, the maximum sample volume that can be tolerated can be evaluated by employing the principle of the summation of variances. Let a volume (Vi) be injected onto a column. This sample volume (Vi) will be dispersed on the front of the column in the form of a rectangular distribution. The eluted peak will have an overall variance that consists of that produced by the column and other parts of the mobile phase conduit system plus that due to the dispersion from the finite sample volume. For convenience, the dispersion contributed by parts of the mobile phase system, other than the column (except for that from the finite sample volume), will be considered negligible. In most well-designed chromatographic systems, this will be true, particularly for well-packed GC and LC columns. However, for open tubular columns in GC, and possibly microbore columns in LC, where peak volumes can be extremely small, this may not necessarily be true, and other extra-column dispersion sources may need to be taken into account. It is now possible to apply the principle of the summation of variances to the effect of sample volume. 

The Golay equation [9] for open tubular columns has been discussed in the previous chapter. It differs from the other equations by the absence of a multi-path term that can only be present in packed columns. The Golay equation can also be used to examine the dispersion that takes place in connecting tubes, detector cells and other sources of extra-column dispersion. Extra-column dispersion will be considered in another chapter but the use of the Golay equation for this purpose will be briefly considered here. Reiterating the Golay equation from the previous chapter. 

As the open tubular column is geometrically simple, the respective functions of (k ) have been explicitly developed and... 

If u Dm/r, which will be true for all conditions except if open tubular columns are... 

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 amount of gas employed in a GC analysis is not usually important, particularly where open tubular columns are used. In LC, however, solvent use presupposes a solvent disposal difficulty if not a toxicity problem and, thus, solvent consumption can be extremely important. 

In a similar manner to the design process for packed columns, the physical characteristics and the performance specifications can be calculated theoretically for open tubular columns. The same protocol will be observed and again, the procedure involves the use of a number of equations that have been previously derived and/or discussed. However, it will be seen that as a result of the geometric simplicity of the open tubular column, there are no packing factors and no multi-path term and so the equations that result are far less complex and easier to manipulate and to understand. 

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. 

Equation (13) is the first important equation for open tubular column design. It is seen that the optimum radius, with which the column will operate at the optimum velocity for the given inlet pressure, increases rapidly as an inverse function of the separation ratio (cc-1) and inversely as the square root of the inlet pressure. Again it must be remembered that, when calculating (ropt)5 the dimensions of the applied pressure (P) must be appropriate for the dimensions in which the viscosity (r)) is measured. 

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. 

The design process for open tubular columns is similar to that for packed columns, and the physical characteristics and performance specifications can be calculated... 

In the previous two chapters, equations were developed to provide the optimum column dimensions and operating conditions to achieve a particular separation in the minimum time for both packed columns and open tubular columns. In practice, the vast majority of LC separations are carried out on packed columns, whereas in GC, the greater part of all analyses are performed with open tubular columns. As a consequence, in this chapter the equations for packed LC columns will first be examined and the factors that have the major impact of each optimized parameter discussed. Subsequently open tubular GC columns will be considered in a similar manner. 

In a similar manner to the optimization of an LC column, in order to obtain numeric values for the optimized parameters, it is necessary to define a given separation and the equipment and materials by which the sample is to be analyzed. The data given in Table 2 are for a general GC separation using an open tubular column. 

From the optimization calculations, it is clear that the open tubular column can have a performance that is far better than that accepted by contemporary GC operators. There are two main reasons for this. Firstly, the remarkable level of performance that can be obtained form the open tubular column in GC is largely not known or appreciated. Consequently, a grossly inferior performance is tolerated in the majority of GC analyses. This problem can be solved only by education. Secondly,... 

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