Open tubular columns conditioning

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. 

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. 

Equation (I.IS) Is valid for open tubular columns under all normal conditions and for packed columns at low mobile phase velocities. The average carrier gas velocity is calculated from the outlet velocity by correcting the latter for the pressure drop across the column, and is simply given by u - ju, where j is the gas compressibility correction factor, defined In equation (1.2). 

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). 

Figure 1.2 Plot of theoretical plate nunber (n), effective plate nunber (N) and separation nuober (SN) against the capacity factor for an open tubular column operated under isothermal conditions. (Reproduced with permission from ref. 41. Copyright Friedr. Vieweg Sohn).

FIGURE 4.5 Chromatograms of a-phenylethanol enantiomers nsing (a) SFC and (b) open tubular column GC. Conditions (a) 12 cmx250 p.m ID capillary packed with 5-p.m porous (300 A) silica particles encapsulated with fS-CD polymethylsiloxane (10% w/w) and end-capped with HMDS, 30°C, 140 atm, CO2, FID, 10 cmxl2 p.m ID restrictor, (b) 25 mx250 p.m ID cyano-deactivated capillary cross-linked with fi-CD polymethylsiloxane (0.25 xm df) 130°C He FID. (Reprinted from Wu, N. et al. 2000. J. Microcol. Sep. 12 454-461. With permission.)... 

As in the chapter on packed column design, the characteristics of many of the equations discussed in this chapter will be examined employing realistic chromatographic conditions and the typical conditions chosen for an open tubular column are given in table 1. 

Equation (13) gives the minimum analysis time that can be obtained from an open tubular column, when separating a mixture of defined difficulty, under given chromatographic conditions. It is seen that, in a similar manner to the packed column, the analysis time is inversely proportional to the fourth power of the function (a-1) and inversely proportional to the inlet pressure. The contribution of the function of (k1), to the analysis time is not clear and can be best seen by calculation. It is also seen (perhaps a little surprisingly) that the analysis time is completely independent of the diffusivity of the solute in the mobile phase but is directly proportional to the viscosity of the mobile phase. 

Figure 3 shows that, there might indeed, be a limited practical range of column dimensions and operating conditions tnat would make tne open tubular column a possible alternative to the packed column in LC. To separate a solute mixture with the separation ratio for the critical pair of 1.01 and an inlet pressure of 1 p.s.i would require an analysis time of 6,5... 

The properties of open tubular columns shown in figures (I) to (6) indicate that the areas where such columns would have practical use is very restricted. At pressures in excess of 10 ps.i., and whatever the nature of the separation, whether simple or difficult, the optimum column diameters are so small that they would be exceedingly difficult to fabricate or coat with stationary phase. The maximum sample volumes and extra column dispersion that couid be tolerated would also be well below that physically possible at this time. At relatively low pressures, that Is at pressures less than 10 p.s.l. the diameter of the optimum column is large enough to fabricate and coat with stationary phase providing the separations required are difficult i.c. the separation ratio of the critical pair must be less than 1.03. However, even under these conditions the sample volume will be extremely small, the extra column dispersion restricted to an almost impossibly low limit and the analysis time would be very long Nevertheless, open tubular columns used for very difficult separations... 

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.]...

Figure 24-6 Effect of stationary phase thickness on open tubular column performance. Increasing thickness increases retention time and Increases resolution of early-eluting peaks. Conditions DB-I stationary phase in 15-m-long x 0.32-mm-diameter wall-coated column operated at 40°C with He linear velocity of 38 cm/S. [Courtesy J6W Scientific. Folsom. CA]...

Figure 24-15 Representative injection conditions for split, splitless, and on-column injection into an open tubular column.

FLOW. The rate at which zones migrate down the column is dependent upon equilibrium conditions and mobile phase velocity on the other hand, how the zone broadens depends upon flow conditions in the column, longitudinal diffusion, and the rate of mass transfer. Since there are various types of columns used in gas chromatography, namely, open tubular columns, support coated open tubular columns, packed capillary columns, and analytical packed columns, we should look at the conditions of flow in a gas chromatographic column. Our discussion of flow will be restricted to Newtonian fluids, that is, those in which the viscosity remains constant at a given temperature. 

Inertial forces of the fluid increase with density and the square of velocity (pv2) while viscous forces decrease with increasing diameter of tube (nv/d) and increase with viscosity and velocity. High Reynolds numbers (Re>4000) result in turbulent flow with low Reynolds number (Re<2000) the flow is laminar. Laminar flow results from formation of layers of fluid with different velocities after a certain flow distance, as illustrated in Figure 2.10A. Flow at the walls is zero and increases approaching the center of the tubes. The laminar flow pattern results from layers of mobile phase with different velocities travelling parallel to each other. The maximum flow at the center is twice the average flow velocity of the fluid. Molecules in the fluid can exchange between fluid layers by molecular diffusion. Most open tubular columns operate under laminar flow conditions. 

The United States Pharmacopeia (USP) test (467) describes three different approaches to measuring organic volatile impurities in pharmaceuticals. Method I uses a wide-bore coated open tubular column (G-27, 5% phenyl-95 % methylpolysiloxane) with a silica guard column deactivated with phe-nylmethyl siloxane and a flame-ionization detector. The samples are dissolved in water and about 1 p is injected. Limits are set for benzene, chloroform, 1,4-dioxane, methylene chloride, and trichloroethylene. Methods V and VI are nearly identical to method I except for varying the chromatographic conditions. For the measurement of methylene chloride in coated tablets, the headspace techniques described above are recommended. 

---