Chromatography Flashcards
What is the A term in chromatography?
Multipath term
Not affected by the mobile phase velocity
Proportional to the particle size of packing
What is the B term in chromatography?
Longitudinal diffusion
Molecules spread off - diffuse
Very high diffusion in gases - important in gas chromatography
In liquid chromatography this term is close to 0
Faster movement = less time to spread out, faster is better
What is the C term in chromatography?
Mass transfer
Molecules move into and out of the stationary phase at different rates
Molecules spending too much time in stationary phase are left behind, the faster the mobile phase velocity the farther these molecules are left behind
Fast mobile phase is not good
What is plate theory?
The plate model supposes that a chromatographic column contains a large number of separate layers called theoretical plates
Each plate is a length of column sufficient to effect one equilibrium distribution between the two phases (like a pit stop)
More theoretical plates = better separation
What is band broadening?
Bands separate and broaden when they are swept through the stationary phase. Band broadening is bad because excessively broad bands will overlap even though the band centres are on average separated
Can be minimised by careful selection of stationary/mobile phase and particle size
Equation for HETP?
HETP = A +B/u + Cu
What is chromatography?
The sample is partitioned between 2 phases, one moving (mobile) and one not moving (stationary). When molecules are in the stationary phase they are not moving - the more time a molecule spends in stationary phase the longer it will take - this achieves separation.
