Extra Column Dispersion - Maximum Permissible Extra Column Dispersion > Page 8
The extra column dispersion, however, must be shared between all the possible dispersion sources, one of which, the dispersion due to a finite sample volume, may be critical for a satisfactory analysis. The critical nature of the sample volume arises from the fact that the sample volume controls the maximum amount of the sample that can be placed on the column and thus, the ultimate sensitivity of the analysis.
Now, summing the variances
|where ()||is the overall variance of the eluted peak,|
|()||is the variance of the sample volume,|
|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 effect of sample volume.