Question

Please answer
What are some factors that could affect the selectivity and the resolution in Gas Chromatography (GC) & Liquid Chromatography (LC), and How do we affect the separation in (GC)&(LC) ?

Answer 1

While doing chromatography, peaks are obtained with their base as time, so optimum resolution in minimum time is desired. resolution is the separation between 2 peaks. resolutions depends on : selectivity, efficiency , retention.

In liquid chromatography mobile phase is liquid whereas it is gaseous in GC. so resolution can be altered by

LIQUID CHROMATOGRAPHY

1. retention factor can be altered by: i) adjusting solvent strength(more polarity of mobile phase more retention)

ii) increasing organic content leads to reductions of retention factor

iii) with more retention, chromatographic peak height will decrease, so retention factor should range from 2 to 10

v)quality of separation is poor when retention factor is high.

2.selectivity factor alteration: it is ratio of retention factors of two peaks.

i)changing solvent

ii)mobile phase change

iii)stationary phase change

iv)temperature

v) resolution increases exponentially after values of selectivity factor greater than 1.1.

3. efficiency altered:   where R is resolution and N is efficiency

i)column length

ii) quality of column packing

iii)injection volume

iv)flow rate

GAS CHROMATOGRAPHY

temperature(higher temerature, lower retention time, lower resolution) and flow rate(higher flow rate lower resolution) can change resolution. but lowering the temerature or flow rate will broaden the peaks. also attenuation can be adjusted to make peaks longer.

column length(longer column better resolution) and the type of mobile phase also influence resolution.

Separation is the selectivity factor. so separation is affected by affecting selectivity factor as above