Capillary columns plate numbers

For enantioseparation on CSPs in CEC, nonstereospecific interactions, expressed as 4>K, contribute only to the denominator as shown in Eq. (1), indicating that any nonstereospecific interaction with the stationary phase is detrimental to the chiral separation. This conclusion is identical to that obtained from most theoretical models in HPLC. However, for separation with a chiral mobile phase, (pK appears in both the numerator and denominator [Eq. (2)]. A suitable (f)K is advantageous to the improvement of enantioselectivity in this separation mode. It is interesting to compare the enantioselectivity in conventional capillary electrophoresis with that in CEC. For the chiral separation of salsolinols using /3-CyD as a chiral selector in conventional capillary electrophoresis, a plate number of 178,464 is required for a resolution of 1.5. With CEC (i.e., 4>K = 10), the required plate number is only 5976 for the same resolution [10]. For PD-CEC, the column plate number is sacrificed due to the introduction of hydrodynamic flow, but the increased selectivity markedly reduces the requirement for the column efficiency. 

Used in virtually all organic chemistry analytical laboratories, gas chromatography has a powerful separation capacity. Using distillation as an analogy, the number of theoretical plates would vary from 100 for packed columns to 10 for 100-meter capillary columns as shown in Figure 2.1. 

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... 

The contribution of the equipment between injection unit and detector cell should be negligable in relation to the column for a sufficient column characterization short connections with narrow capillaries and zero dead volume unions are the precondition for reliable plate numbers. Every end fitting of a column causes additional band broadening. In the past a column type was offered that could be directly combined without any capillary links unfortunately, it has disappeared from the market. 

The selection of the column type is mainly determined by the composition of the sample. In general open-tubular (capillary) columns are preferred for low-density (gas-like) SFC, whereas packed columns are most useful for high-density (liquid-like) SFC. Open-tubular columns can provide a much larger number of theoretical plates than packed columns for the same pressure drop. Volumetric flow-rates are much higher in packed column SFC (pSFC) than in open-tubular column SFC (cSFC), which makes injection and flow control less problematic. 

Packed columns are still used extensively, especially in routine analysis. They are essential when sample components have high partition coefficients and/or high concentrations. Capillary columns provide a high number of theoretical plates, hence a very high resolution, but they cannot be used in all applications because there are not many types of chemically bonded capillary columns. Combined use of packed columns of different polarities often provides better separation than with a capillary column. It sometimes happens that a capillary column is used as a supplement in the packed-column gas chromatography. It is best, therefore, to house the capillary and packed columns in the same column oven and use them selectively. In the screening of some types of samples, the packed column is used routinely and the capillary column is used when more detailed information is required. 

These enantioselective capillary columns showed extremely good performance in the CEC mode. Plate numbers in excess of 100,000 m could be easily achieved for a variety of amino acid derivatives (with chromophoric and fluorophoric labels) (Eigure 1.34a) as well as other chiral acids such as 2-aryloxycarboxylic acids. 

The concept of the effective plate number was introduced and employed in the late nineteen fifties by Purnell (7), Desty (8) and others. Its introduction arose directly as a result of the development of the capillary column, which, even in 1960, could be made to produce efficiencies of up to a million theoretical plates (9). It was noted, however, that these high efficiencies were were only realized for solutes eluted close to the column dead volume, that is, at very low k values. Furthermore, they in no way reflected the increase in resolving power that would be expected from such high efficiencies on the basis of the performance of packed columns. This poor performance, relative to the high efficiencies produced, can be shown theoretically ( and Indeed will be, later in this book) to result from the high phase ratio of capillary columns made at that time. That is the ratio of the mobile phase to the stationary phase in the column. The high phase ratio was... 

The inside of a monolithic electrochromatography column contains silicate fingers" coated with stationary phase. Aromatic compounds were separated with a mean plate number of 80 000 in a 50-cm column with 15-kV applied voltage. [From J. D. Hayes and A. Malik, Sol-Gel Monolithic Columns with Reversed Eleclroosmotic How for Capillary Eleclrochromolography" Anal. Chem. 2000, 72.4090. Photo courtesy A. Malik, University of South Honda.]... 

High-speed columns yield 5000-10,000 plates per meter, whereas most high-resolution columns give 3000-5000 plates per meter. Most applications of capillary columns are with column lengths of 10-50 m and are somewhat dependent on the complexity of the sample and the number of components of interest. A general rule is that samples with 20-50 components are best handled on columns of 20-30 m, whereas samples of 50 or more components will require 30-50 m of column. 

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... 

These capillary columns possessing positively charged surface functionalities were used for the reversed-phase separations of basic and acidic peptides. Figure 6.17 shows the separation of three angiotensins and insulin with plate numbers as high as... 

Figure 1.5c illustrates the variation of the resolution with the plate number N. The square root function initially rises steeply from a value of zero at N= 0. The rate of increase in Vn becomes quickly less with increasing N. At N= 2500 Yn equals 50. A value of 100 is reached for JV= 10,000, and 200 for IV=40,000. These three values of N may be seen as typical for three different kinds of chromatography packed column GC, packed column LC and capillary column GC. Within the constraints of any of these techniques, another factor of 2 in resolution by increasing N by a factor of 4 will be hard to achieve. 

Consequently, if we compare a packed column with an open column for which dp=dc, we find a 30 times lower pressure drop over a capillary column of the same length. Conversely, if we keep the pressure drop constant, much longer capillary columns may be used, yielding a much higher number of plates. 

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