High open tubular columns

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. 

Figure 6.13 Flow restrictors of different design A, linear B, tapered C, integral and D, frit. On the right side is shown a modified high pressure cell for UV detection using open tubular columns.

Svensson, L. M. and Markides, K. E., Fiber optic-based UV-absorption detector cell for high-temperature open tubular column liquid chromatography,... 

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. 

Chiral SFC can be performed in open tubular [41,42], and packed column [43,44] modes. Packed column SFC can be further categorized into analytical, semipreparative, and preparative SFC [7, 8], Packed column SFC is more suitable for fast separations than open tubular column SFC, since a packed column generally provides low mass transfer resistance and high selectivity [45, 46], Packed column SFC also provides high sample loading capacity [27,47], which can increase sensitivity. Only packed column SFC is suitable for preparative-scale enantioseparation. This chapter will focus on chiral separation using packed column SFC in the analytical scale. 

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