IGNOU MCH 2 2018 - List various factors which contribute to broadening of a bond in gas c

Question

List various factors which contribute to broadening of a bond in gas chromatography. Write the expanded form of van Deemter equation and give the significance of various terms appearing in it.

08 Mar 2018

Answer :

Word Count : 804

It is well recognized now that column band broadening originates from three main sources:

multiple path of an analyte through the column packing;
molecular diffusion;
effect of mass transfer between phases.

In 1956 J.J. Van Deemter introduced the equation which combined all three sources and represented them as the dependence of the theoretical plate height (HETP) on the mobile phase linear velocity. Originally, it was introduced for gas chromatography, but it happened that the same physical processes occurs in HPLC, and this equation is perfectly fit for liquid chromatography.

Multiple path

The velocity of mobile phase in the column may vary significantly across the column diameter, depending on the particle shape, porosity, and the whole bed structure. This is schematically shown below.

Image ignouacademy-com--p-ignou-93136

Variation of the zone flows

Band broadening is caused by differing flow velocities through the column, which may be written in form

Image ignouacademy-com--p-factors-29157

where Hp is the HETP arising from the variation in the zone flow velocity, dp

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