Plate height capillary columns

Table 3 Minimum plate height and maximum theoretical plates for capillary columns of various inner diameters...

Another approach to improving resolution is to use thin films of stationary phase. Capillary columns used in gas chromatography and the bonded phases commonly used in HPLC provide a significant decrease in plate height due to the reduction of the Hs term in equation 12.27. 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

Hydrophobic monolithic methacrylate capillary columns have been introduced by copolymerization of butyl methacrylate and EDMA as cross-linking agent. The polymerization, however, was not thermally or photochemically but chemically initiated ammonium peroxodisulfate [154]. The resulting monolithic columns were applied to RP separation of small analytes like uracil, phenol, or alkylbenzenes. Reasonable results have been obtained under isocratic conditions, delivering typical values for theoretical plate height ranging between 40 and 50 pm. 

In packed columns, all three terms contribute to band broadening. For open tubular columns, the multiple path term, A, is 0, so bandwidth decreases and resolution increases. In capillary electrophoresis (Chapter 26), both A and C go to 0, thereby reducing plate height to submicron values and providing extraordinary separation powers. 

Capillary electrophoresis provides unprecedented resolution. When we conduct chromatography in a packed column, peaks are broadened by three mechanisms in the van Deemter equation (23-33) multiple flow paths, longitudinal diffusion, and finite rate of mass transfer. An open tubular column eliminates multiple paths and thereby reduces plate height and improves resolution. Capillary electrophoresis reduces plate height further by knocking out the mass transfer term that comes from the finite time needed for solute to equilibrate... 

Capillary supercritical fluid chromatography (SFC) columns are 0.1-0.025 mm ID and 3-20 m in length. Good reviews of the technique of SFC have been recently published [52-55]. It was reported that the optimum inner diameter for capillary SFC based on plate height, linear velocity, analysis time, and column length was around 0.050 mm. 

There is much interest in high-efficiency- and high-speed separation media for liquid chromatography. The plate numbers available in practice have been in the range of 10,000-30,000 in HPLC for 20 years or so, but these are low compared to well over 100,000 theoretical plates in capillary gas chromatography or in capillary electrophoresis. This is caused by the limitation in the use of small-sized particles for HPLC, where a particle-packed column is commonly used under a pressure-drop of up to 40 MPa. An increase in column efficiency by using small particles, which is the approach taken in the past, is accompanied by an increase in the pressure-drop, as expected from Eqns. 5.2 and 5.3, below. Eqns. 5.1-3 describe the efficiency (plate height) and flow resistance of a column packed with particles [1-3], where N stands for the... 

Direct comparison of column efficiencies of the continuous separation medium prepared in 4.5 cm long both capillary and linear channel as measured using non retained marker acetone did not reveal any difference and an identical plate height of 4-5 pm was observed for both at flow velocities of 0.5-1.5 mm/s. This indicates that there is again no substantial difference between beds formed in both formats. Obviously, the channel behaves as a capillary although it does not have the circular cross section. 

If the required number of plates is moderate (say several thousands), then short capillary columns may be used to provide fast analysis of the sample. The required column length and retention time can easily be calculated from eqns.(7.3) and (7.6). For example, if we operate a capillary column with a diameter of 200 pm at a reduced velocity of 5 with a reduced plate height of 1.5, then 2000 theoretical plates require a column with... 

For capillary columns, of course, y = l. The plate height, H = cr2IL, becomes... 

Calculate the plate height contributed by sorption-desorption mass transfer (nonequilibrium) through a uniform liquid layer (configuration factor q = 2/3) of thickness 1.0 x 10 3 cm coated on the inside of an open tubular (capillary) column. The gas velocity v is 10 cm/s. The solute retention ratio is 0.10 and its diffusion coefficient Ds through the stationary liquid is 1.0 x 10 5 cm2/s. 

For both packed and ideal capillary columns, we have simplified the plate height expressions by using coefficients A, A B> Cm, and Cs to replace groups of more basic parameters. In Table 12.1 we summarize the expressions for these coefficients. 

TABLE 12.1 Summary of Plate Height Coefficients for Packed and Ideal Capillary Columns a... 

Figure 9 Plot of the plate height against temperature. Column, 50 pm x 21/29-cm capillary packed with 5-pm Spherisorb ODS 300 A eluent, 10 mM Na-phosphate in 2 3 water ACN mixture (v/v) samples, ( ) benzaldehyde, (A) naphthalene, (A) biphenyl, (O) fluorene, and ( ) m-terphenyl. (Reprinted from Ref. 42, copyright 1999, with permission from Elsevier Science.)...

A1 Rifai et al. studied the chromatographic behavior of macroporous particles in reversed-phase electrochromatography and compared the results with pressure-driven LC with capillary columns as well as with standard-bore 4.6-mm-i.d. columns [23]. Using 400-nm-pore particles they obtained 650,000 plates/m in the CEC mode, corresponding to a reduced plate height of only 0.2. These high efficiencies were obtained for low retained solutes and provided a five-fold improvement of the optimal efficiency obtained in pressure-driven LC on the same column, and a 10-fold improvement compared to the 4.6-mm-i.d. column. 

Figures 24-8 and 24-9 and Example 24-5 illustrate why capillary columns, lightly loaded columns, and columns operated under conditions where the solutes have low partition ratios have never entirely lived up to the promise of their sometimes enormous plate numbers or small plate heights. In fairness, it should be pointed out that capillary and lightly loaded columns are frequently used under conditions where high specific retention volumes partly compensate for the loss of separating efficiency resulting from the large dead space in these columns.

---