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liquid chromatography column - Scott R.P.W.

Scott R.P.W. liquid chromatography column - John Wiley & Sons, 2001. - 144 p.
Download (direct link): liquidchromatographycolumntheory2001.djvu
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The Column Environment
It has already been stated that the retention of a solute depends on the magnitude of the distribution coefficient of the solute between the mobile and stationary phases. Furthermore, according to Vant Hoff's Law, the distribution coefficient will vary according to the exponent of the reciprocal of the absolute temperature. In addition, the dispersion of a solute band in a column will be shown to depend on the diffusivity of the solute in both phases, the viscosity of the mobile phase and also on the distribution coefficient of the solute, all of which vary with temperature. It follows that, for consistent results, the column must be carefully thermostated. The column and its contents have a significant heat capacity and, consequently, it is of little use trying to thermostat the column in an air bath; for satisfactory temperature control, the thermostating medium
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must be a liquid. The thermostat must also bring the temperature of the mobile phase precisely to the column temperature before entering the injector and column.
It will be assumed in all subsequent theoretical arguments, that the column is properly thermostated and, unless otherwise stated, all chromatographic separations are carried out isothermal ly.
The purpose of studying the theory of liquid chromatography columns is not merely to understand the function of the column and how a separation can be achieved, but to provide sufficient knowledge to be able to design a column that will provide a given separation, and consequently an analysis, in the minimum time.
The 'technical cost' of a separation is paid in units of time and pressure-both of which are limited in practice. It follows, that there is a limit to the maximum time that can be tolerated before an analysis is completed. Conversely, there will also be a limit to the complexity of a mixture that can be separated In an acceptable time. Column theory must allow these limits to be identified. Although, as already stated, only packed columns are presently in general use, it may be possible that eventually chromatographic apparatus, particularly the detector and injection system, will be improved to the point where capillary columns become a viable alternative. Column theory must, therefore, also aid in capillary column design and be able to define the specifications of the ancillary apparatus that will permit the efficient use of such columns.
Chromatography Nomenclature
Before dealing with the theory of LC columns, however, it would be wise to define some of the terms used to describe the different parts of a chromatogram. Figure (3) depicts a chromatogram showing the resolution of
two solutes.
The envelope of each eluted solute is called a peak. On injecting a sample onto the column, a mark is usually made on the chart, sometimes automatically, and this is called the injection point. A substance that is not
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retained on the column , if not naturally present in the mixture, is often added to the sample prior to injection as it may aid in component identification. The position on the chromatogram where this unretained substance is eluted is called the dead point. The volume of mobile phase that has passed through the column between the injection point and the dead point is called the dead volume which, in fact, includes the total volume between the injection valve and the the detector that is not occupied by the stationary phase or the support. (It should be emphasized that the dead volume is not merely the total volume of mobile phase in the column) However, this does assume that the two Phases are in equilibrium and that there is no mobile phase trapped in the stationary phase and as a consequence isolated from the bulk of the mobile phase. The elapsed time between the injection point and the dead point is called the dead time and
Figure 3
The Nomenclature of a Chromatogram
thus, the dead volume is obtained from the product of the dead time and the column tlow-rate. The base line is that portion of the chromatogram
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recorded when only mobile phase Is emerging from the column. The point at the maximum concentration of any eluted peak is the peak maximum . The distance between the peak maximum and a line joining the base of the peak by the extrapolation of the base line is called the peak height. There are various measurements used for the peak width. The width that has a fundamental theoretical significance, and which should be used wherever possible, is the peak width at 0.6065 of the peak height. The importance of this particular peak width will be discussed later. Throughout the book, if the peak width is not otherwise defined, then it will refer to width at 0.6065 of the peak height. The product of the peak height and peak width will always give 79.8% of the total peak area for true error function or Gaussian curves.
For convenience, in quantitative analysis, the peak area is often taken as the product of the peak height and the peak width at half peak height, obviously this has been given the term peak width at haif height. Another measure of the peak width that is sometimes used, involves constructing tangents to the points of inflection of the elution curve and measuring the distance between their points of intersection with the base line. This is termed the peak width at the base and is, in fact, equivalent to twice the peak width at 0.6065 of the peak height.
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