Dispersion Correction

Even with micro-equipment and enhanced resolution, selectivity generally appears low. Results must be corrected for band spreading, in order to get individual peaks and true distribution.

There are many approaches for solving the dispersion problem based on Tung's equation, which expresses the experimental chromatogram, F(v), as the result of the true chromatogram, W(y), times a dispersion function, G(v — y):

This equation has received integral and numerical solutions. It has been shown that skewed instrumental spreading functions derived from the plug flow dispersion model fit data for particle separations by HDC, where the spreading function is:

G(v, y) = {4(nPe~1(v/y))}~0's x exp{ — (v — y)2/4Pe_1(v/y)} [8]

(where the dispersion term or Peclet number, Pe = UL/D') when U = superficial velocity, L = length of the packed bed, D' = empiric dispersion coefficient; v = elution volume and at the maximum of peak = y). Increasing Pe leads to narrower peaks.

Pe = 100 corresponds to no peak correction. With packed column, whatever the value of n, Dn increases with Pe, the effect is stronger for higher exponent, n. On the other hand, Dw increases or decreases with Pe, depending on n. A quasi-constant value corresponds to n = 3-4 (Figure 4).

In another representation of results for different Pe values, Dw versus n curves for all samples have a common intercept between n = 3 and 4. A rapid change of D is noted up to Pe = 500, then Dn and Dw tend to converge slowly, independently of n.

Practically, considering the whole set of results with UV detection, n must be chosen as 3-4 and Pe 500, to get the best average and distribution values, as well as narrow peaks, which means enhanced resolution.

Solar Panel Basics

Solar Panel Basics

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