Narrow extra-column dispersion

The standard deviation of the extra-column dispersion is given as opposed to the variance because, as it represents one-quarter of the peak width, it is easier to visualize from a practical point of view. It is seen the values vary widely with the type of column that is used, (ag) values for GC capillary columns range from about 12 pi for a relatively short, wide, macrobore column to 1.1 pi for a long, narrow, high efficiency column. 

The system dead volume must be reduced to an absolute minimum, particularly when using very efficient narrow-bore SEC columns. Extra column dispersion becomes a greater consideration as the column volume is reduced, and dead volume should be minimized in all parts of the system, including injection valves, connecting tubing, and detectors, if the column performance is to be realized. 

It is clear from table 7.2 that in terms of extra-column dispersion a wide-bore capillary column requires instrumentation similar to that used for the packed column. However, the capillary column provides eight times as many plates (in a fifteen-fold analysis time). Conventional capillary columns require a reduction in the dispersion by about an order of magnitude, whereas narrow-bore columns require a further reduction by a factor of about 100. This, combined with the high pressures required, puts narrow-bore columns out of reach for current GC instruments. 

In GC the use of wide-bore" capillary columns allows the use of instruments designed to accommodate packed columns (in terms of extra-column dispersion). For capillary columns of conventional diameter a reduction of the extra-column dispersion by a factor of /0, and for narrow bore columns a reduction by a factor of 1000, are required. 

Now for the whole of the chromatogram to be effectively useful, the most narrow peaks (i.e., those peaks eluted close to the column dead volume) must not be broadened by the extra column dispersion. That is to say, in any chromatographic separation peaks eluted close to the dead volume should be considered equally important as those eluted elsewhere in the chromatogram. 

Although the above calculation is somewhat oversimplified because the effects of the compressibility of the gas have been neglected, it serves to illustrate that a reduction of the column diameter cannot be fully compensated by an increase in the column length to keep the column dispersion constant. Therefore, when narrow-bore capillary columns are to be used in GC, the extra-column contribution to band broadening will need to be reduced. 

As previously indicated, this discussion is organized for chromatograms from very narrow polymer standards for which we can consider that the effect of molecular weight distribution is negligible and for which the unique separation process is size exclusion. With these limitations, the contribution to band broadening is conveniently separated into extra column effects, eddy dispersion, static dispersion, and mass transfer. In the most classical chromatographic interpretation, extra-column effects are not discussed and the three other contributions are considered as Gaussian, so there is simply the addition of their variances. The number of theoretical plates is defined as N = VJaY and the influence of v, the linear velocity of the eluent, is summarized by the so-called Van Deemter equation ... 

Low-dispersion HPLC systems are necessitated by the increasing trend of using shorter and narrower HPLC columns, which are more susceptible to the deleterious effects of extra-column band-broadening. HPLC manufacturers are designing newer analytical HPLC systems with improved instrumental bandwidths compatible with 2-mm i.d. columns by using micro injectors, smaller i.d. connection tubing, and detector flow cells. A new generation of ultra-low dispersion systems dedicated for micro and nano LC is also available. 

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