Extracolumn band spreading

SO that a 60-cm-long and 0.2-mm-i.d. connecting tube can be used without appreciable contribution to extracolumn band spreading. 

The first approach to interfacing CE with MS was reported by Smith et al.48 when they incorporated the electrospray ionization (ESI) technique introduced by Dole et al.66 This development was based on the recognition that it is not necessary for the detection end of the CE capillary to be immersed in the buffer reservoir as conventionally practiced, as long as it is biased negative of the cathode potential (assuming a cathodic detector end). The ESI was created directly at the terminus of the CE capillary, avoiding any postcolumn region that would contribute to extracolumn band spread or analyte adsorption. A quadrupole mass filter was combined with ESI to produce the first on-line MS detection with capillary electrophoresis. 

Equipment for recycle operations differs from conventional HPLC equipment. For a recycle system to be useful, the extracolumn band spreading must be small relative to the band spreading of the column. This involves the solvent delivery system, transport tubing, and detector(s). Also, because a recycle system is a closed system with a finite volume, the operator must be aware that fast-moving materials could eventually overtake slower-moving materials and remix. To prevent peak overlap, a means must be provided to allow the operator to remove a portion of the sample components before overlap can occur. 

To get a handle on the relative contributions of the different contributions to extracolumn band spreading, we need to make some estimates of the individual variances. Let us first estimate the contribution of the injection... 

This means that the contribution of a 30-cm section of 0.2S inni-Ld. tubing contributes a standard deviation of about 8.5/iL to the extracolumn band spreading. One can see that it pays to reduce the length of connection tubing. A length of only 30 on is actu y quite good for a typical HPLC instrument. 

Figare 3.8 Extracolumn effects apparent plate count obtained on columns with different internal diameters as a function of the retention factor. The extracolumn band spreading was assumed to be 15 iL. A significant deterioration of the performance is observed for the smaller-diameter columns. 

The time>based contributions to the extracolumn band spreading play a role only for fast chromatography or early-eluting peaks in normal chromatog-... 

In some injector types, worn seals or partially clogged tubing can cause excessive extracolumn band spreading. In other injectors, the material of construction can adsorb the sam[ffe. A measurement of the extracolumn band spreading and a comparison to previous results should help in the diagnosis of this cause. Consult tte manual of the manufacturer of the injector for specific effects and troubleshooting recommendations. 

AUFS. The retention of the sample should be adjusted to minimize extracolumn influences on the measurement. If we would like less than a 5% influence of extracolumn band spreading on the plate-count measurement, the standard deviation of the peak should be about 5 times lar r than the standard deviation of the extracolumn band spreading (see Section 3.2.2). 

The influence of large injection volumes can be treated like any other extracolumn band-spreading effects. The band spreading inside the column is independent of the band spreading outside the column. Thus we can simply add the variance of the injection volume to the variance of the band contributed by the column ... 

Fignrc Measurement of extracolumn band spreading. A sample is injected without a column in place. The width of the peak is measured at 4.4% of the height of the peak. This width w is then equal to 5 standard deviations. 

Extracolumn dispersion is that contribution to the total band dispersion that arises from spreading processes taking place outside the column and for convenience will be taken as. ... 

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