Chromatographic band dispersion

Recalling that a separation is achieved by moving the solute bands apart in the column and, at the same time, constraining their dispersion so that they are eluted discretely, it follows that the resolution of a pair of solutes is not successfully accomplished by merely selective retention. In addition, the column must be carefully designed to minimize solute band dispersion. Selective retention will be determined by the interactive nature of the two phases, but band dispersion is determined by the physical properties of the column and the manner in which it is constructed. It is, therefore, necessary to identify those properties that influence peak width and how they are related to other properties of the chromatographic system. This aspect of chromatography theory will be discussed in detail in Part 2 of this book. At this time, the theoretical development will be limited to obtaining a measure of the peak width, so that eventually the width can then be related both theoretically and experimentally to the pertinent column parameters. 

The dispersion effect of the sample volume was discussed in Chapter 4 and little more needs to be said about it. It will be seen later that the sample volume controls both the concentration and the mass sensitivity of the chromatographic system and thus, should be made as large as possible. This means that all other sources of band dispersion must be kept to an absolute minimum to permit the maximum possible sample volume to be used. A better understanding of the causes of band dispersion has resulted in... 

Band dispersion from sample mass overload is a direct result of the chromatographic process proceeding under conditions, where the adsorption isotherm of the solute on the stationary phase, is no longer linear. The development of an equation that describes the extent, of band spreadinn as a function of mass of sample placed on the column, is complex. This problem has been elegantly approached by 6uiochon and his co-workers (15-18) from the basis of the adsorption isotherm of the solute on the stationary phase. 

The rate theory examines the kinetics of exchange that takes place in a chromatographic system and identifies the factors that control band dispersion. The first explicit height equivalent to a theoretical plate (HETP) equation was developed by Van Deemter et al. in 1956 [1] for a packed gas chromatography (GC) column. Van Deemter et al. considered that four spreading processes were responsible for peak dispersion, namely multi-path dispersion, longitudinal diffusion, resistance to mass transfer in the mobile phase, and resistance to mass transfer in the stationary phase. 

Band dispersion at the elution in GC is shown in the chromatographic peak as its standard deviation cr. Supposing a gaussian distribution for peak shape, we can experimentally estimate cr from peak width measures, as peak width at the baseline Wb) or at half peak height (wh), the latter being the parameter most commonly used in order to decrease the influence of baseline noise. In GCxGC, peak shapes are approximately elliptical, and the values of their and axis correspond to peak width in the and columns Cwh and which can be measured from recorded data. 

The volume of the cell was kept as small as possible to minimize band dispersion, but at the same time, the geometry of the flow cell had to provide optimum synchronization with the NMR sensing coil. Consequently, the walls of the cell had to be straight and parallel to the axis of the coil. the volume of the cell was about 420 pi (very large for high-performance liquid chromatographic (HPLC) columns) and thus, at a flow rate of 1 ml/min allowed a residence time of about 2 5 sec. To achieve adequate sensitivity, Fourier Transform techniques were employed. The layout of the system used by Bayer et al (1). is shown in Figure 2 and is fairly typical of all LC/NMR combinations. 

The chromatographic column has a dichotomy of purpose. During a separation, two processes ensue in the column, continuously, progressively and virtually independent of one another. Firstly, the individual solutes are moved apart as a result of the differing distribution coefficients of each component with respect to the stationary phase in the manner previously described. Secondly, having moved the individual components apart, the column is designed to constrain the natural dispersion of each solute band (i.e. the band... 

The separation impedance represents the difficulty of achieving a certain performance and should be minimized for optimum performance. The highest performance is achieved by a column which combines low resistance to flow and mlnlnua dispersion of chromatographic solute bands. 

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