Dispersion, column Peak dispersion, (or peak spreading) occurs in all types of chromatographic columns. There are three main sources of peak dispersion in a packed column and two sources of peak dispersion in a capillary column: they have been given the terms, multi-path dispersion, longitudinal diffusion and dispersion due to resistance to mass transfer in the two phases. Resistance to mass transfer in the two phases are usually treated as two similar, but separate, processes and have been termed, resistance to mass transfer in the mobile phase and resistance to mass transfer in the stationary phase. Multi-path dispersion only occurs in packed columns and results from the different paths taken by the solute molecules as they wind their way through the intersticies of the packing. Those molecules taking shorter paths than the average pass through the column more quickly and elute in the front of the peak. Those molecules that take longer paths than the average, pass through the column more slowly and are eluted in the tail of the peak. The effect of the different path lengths causes the peak to be dispersed and instead of eluting as a sharp spike it is eluted as Gaussian or Error Function Curve. During the period of residence of a solute peak in a column while passing through the column, diffusion takes place and, as diffusion transfers solute from positions of high concentration to those of low concentration, the peak spreads from the peak center towards its edges. This peak dispersion process is called longitudinal diffusion and occurs in all types of columns. During passage through the column, the solute continually transfers from the mobile phase to the stationary phase and from the stationary back to the mobile phase. This process is not instantaneous because a finite time is required for the solute to transfer by diffusion through the mobile phase in order to enter the stationary phase. Thus, the molecules close to the stationary phase will enter it immediately, whereas those some distance from the stationary phase will enter it sometime later. However, as the mobile phase is moving, during the time the molecules are diffusing towards the stationary phase they will be swept along the column and, thus, away from those molecules that were close and entered it rapidly. This causes the solute peak to be dispersed and the process is called resistance to mass transfer in the mobile phase.Dispersion due to resistance to mass transfer in the stationary phase is exactly analogous to that in the mobile phase. Those molecules close to the surface of the stationary phase will leave the surface and enter the mobile before those that have diffused further into the stationary phase and requires a longer period to diffuse back to the surface. Thus, those molecules that quickly enter the mobile phase because they were close to the surface will be swept away from those molecules still diffusing to the surface. This process also causes the solute peak to be dispersed and the process is called resistance to mass transfer in the stationary phase phase. The total variance of a solute peak is obtained by adding the individual variances resulting from all the dispersion processes that are taking place in the column.

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Author: RPW Scott Book:Extra Column Dispersion
Section:EC-Dispersion   Small-Bore-Columns

10% to the column variance, is determined by the type of column, dimensions of the column and the chromatographic characteristics of the solute. In practice, the majority of the permitted extra-column dispersion should be allotted to the sample volume, as a large sample volume may be necessary to handle a particular sample type for successful analysis. In any event, the chromatograph should always be designed so that the dispersion from other parts of the system is kept to the absolute minimum. Dispersion in sample valves can be minimized by mechanical design (internal loop valves tend to provide the minimum dispersion). Dispersion from unions can be minimized by using drilled-out unions or low dead volume unions. Stainless steel frits provide very little dispersion and can be employed without great concern for their contribution to the overall dispersion of the system. Connecting tubes are one of the major sources of extra-column dispersion and should be kept as short as possible

EC-Dispersion   Small-Bore-Columns

Author: RPW Scott Book:Extra Column Dispersion
Section:EC-Dispersion   Sources   Sensor-Volume

none of these sources of extra column dispersion can be completely eliminated but, by careful design, they can be reduced to a level where they no longer significantly impair column performance. Before extra column dispersion can be considered in detail, however, it is necessary to determine the volume of the peaks produced in contemporary high efficiency columns by the normal column dispersive processes. This is necessary in order to place a specific limit on the acceptable level of extra column dispersion. It will be seen that extra column dispersion can be far more serious in liquid chromatography than in gas chromatography and so the two types of column system will be considered separately. Extra column dispersion in LC systems will be considered first

EC-Dispersion   Sources   Sensor-Volume

Author: RPW Scott Book:Extra Column Dispersion
Section:EC-Dispersion   Maximum-Permissible

and () is the variance due to column dispersion. It has been established that the variance of a rectangular distribution of sample volume (Vi) will be . It can again be assumed that the peak variance can be increased by 10% as a result of extra column dispersion without seriously denigrating column performance. Unfortunately, all the permitted extra column dispersion can not be assigned to the effect of a finite sample volume as some must be allocated to other dispersion sources. As an arbitrary judgment half of the permissible extension of peak width will be allotted to the effect of the sample volume (i.e., the variance can be increased by 5 %). This is a variable proportion as it will depend on the magnitude of other sources of extra column dispersion. If the chromatographic apparatus is carefully designed to minimize all other dispersion sources then significantly more than half the permissible extra column dispersion could be allotted to the

EC-Dispersion   Maximum-Permissible

Author: RPW Scott Book:Dispersion in Chromatography Columns
Section:Dispersion   Rate-Theory

of a column may be changed or, what causes peak dispersion in a column in the first place. It does not tell us how dispersion is related to column geometry, properties of the packing, mobile phase flow-rate, or the physical properties of the distribution system. Nevertheless, it was not so much the limitations of the Plate Theory that provoked Van Deemter et al  (2) (who were chemical engineers and mathematicians) to develop, what is now termed the Rate Theory for chromatographic dispersion, but more to explore an alternative mathematical approach to explain the chromatographic process. Virtually all basic chromatography theory evolved over the twenty five years between 1940 and 1965 and it was in the middle of this period that Van Deemter and his colleagues presented their Rate Theory concept in (1956). Since that time, other Rate Theories have been presented, together with accompanying dispersion equations and in due course each will be discussed, but most were very

Dispersion   Rate-Theory

Author: RPW Scott Book:Extra Column Dispersion
Section:EC-Dispersion   Sensor-Volume

from a multipath type of dispersion, similar to that which occurs in a packed column (see Dispersion in Chromatography Columns). Multipath dispersion in a packed column increases with the particle size of the packing and so will increase with the porosity of the frit. However, the net contribution will still be small unless the pores are exceedingly large. As a consequence, in analytical chromatography, the contribution by the frit need not be considered a significant source of extra column dispersion in any chromatographic system.   Dispersion in the Detector Sensor Volume Irrespective of the type of detector or whether it is for LC or GC, if it is concentration sensitive, the device that actually senses the solute contained in the mobile phase must address a finite volume of the column eluent in order to measure the concentration of the solute and sense its presence. The caveat, that the sensor is concentration sensitive is important as, if the detector is mass

EC-Dispersion   Sensor-Volume

Author: RPW Scott Book:Extra Column Dispersion
Section:EC-Dispersion   HPLC-Sample-Valves

volume and the column, and will provide the minimum extra column dispersion. There will. however, still remain some small contribution to dispersion from the Newtonian flow through the sample volume itself and the small aperture acting as a conduit to the column.   Many sample valves are connected to the column by a sequence of unioncapillary tubeunioncolumn. Most contemporary unions, used for this purpose, are designed to have low dead volumes which has largely eliminated union-dispersion. Consequently unions no longer contribute significantly to extra column dispersion in most chromatographic systems. However, it should be pointed out that they are still not completely dispersion free.   If an external tube sample valve is employed, then the vast majority of the dispersion that takes place in it will be the same as that, which would occur in a connecting tube of equivalent length (this will be discussed below). Dispersion resulting from the internal sample

EC-Dispersion   HPLC-Sample-Valves