Plate height, chromatography

Elution volume, exclusion chromatography Flow rate, column Gas/liquid volume ratio Inner column volume Interstitial (outer) volume Kovats retention indices Matrix volume Net retention volume Obstruction factor Packing uniformity factor Particle diameter Partition coefficient Partition ratio Peak asymmetry factor Peak resolution Plate height Plate number Porosity, column Pressure, column inlet Presure, column outlet Pressure drop... 

High-Performance liquid Chromatography. Typical performances for various experimental conditions are given in Table 11.15. The data assume these reduced parameters h = 3, V = 4.5. The reduced plate height is... 

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. 

COLUMN EFFICIENCY AND PLATE HEIGHTS IN UNIFIED CHROMATOGRAPHY... 

The plate height in chromatography, H, is a useful indicator of the rate of peak broadening. The local plate height at any point on a column is given by the following ... 

There have been a few reports of column efficiency and reduced plate height measurements in several unified chromatography techniques. These have been based on the apparent plate height observed at the column outlet. In the notation used by Giddings (32) the apparent plate height, H, is given by the following ... 

The point of all this is simply that we must not use the apparent plate height or the apparent plate number as performance criteria in the unified chromatography techniques on the justification that they already work well for LC and that they work well for GC when a pressure correction is applied. A considerable expansion of theory and an effective means for evaluating equations (7.4) or (7.5) are required first. Likewise, as we consider multidimensional chromatography involving techniques existing between the extremes of LC and GC, we must not build judgments of the multidimensional system on unsound measures of the individual techniques involved. 

The band broadening may be characterised by a plate height, and its causes provide a basis for understanding why modem chromatography is such an efficient separation technique. 

In gas chromatography the B and C terms are usually larger than the A term. A plot of plate height against velocity shows a minimum. 

Diffusion rates in liquids (LC) are typically three to four orders of magnitude less than in gases (GC). The lower mobile-phase diffusivity Dm affects two of the plate-height terms in liquid chromatography given in Table 19.1. First, the B/u term is small. Secondly, the Cmu term is large. The Csu term is small in many LC applications where the stationary phase is only a monolayer of liquid bonded to the surface of a solid... 

A second reason why very high resolution can be obtained in chromatography is that very large numbers of theoretical plates are readily achieved. If the column is well packed with particles having a narrow spread of sizes, the plate height is about twice the particle diameter 9, n A typical large-scale GC or LC column will contain of 103-104 plates. 

Figure 4.3 Effect of particle diameter on plate height. (Reproduced from Lichrospher Lichroprep Sorbents Tailored for Cost Effective Chromatography, EM Separations, Gibbstown. With permission from Merck kGaA, Darmstadt, Germany, and EMD Chemicals, Inc.)...

In liquid chromatography (LC) the plate height H is related to the various band broadening terms as described by the van Deemter equation ... 

As in chromatography, the most common variable to describe dispersion in FFF is the plate height (H), defined as the ratio between the spatial variance (Oj) of the band and its mean position (s) as it migrates inside the separation medium of length (L) ... 

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