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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

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

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

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

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

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� union���capillary tube�union�column. 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