Chem 241 - Chromatography

Adsorption chromatograpy

Solid stationary phase

liquid/gaseous mobile phase

solute adsorbed onto surface of stationary phase

more adsorption = slower travel

partition chromatography

liquid stationary phase

gaseous mobile phase

solute equilibriates between stationary and mobile phase

ion-exchange chromatography

solid stationary phase

liquid mobile phase

anions/cations are covalently attached to stationary phase; ions of the opposite charge are attracted

size-exclusion chromatography

gel stationary phase

liquid or gaseous mobile phase

aka molecular exclusion, gel filtration, gel permeable chromatograpy

separates molecules by size; small particles trapped in spaces in gel

Affinity chromatography

most selective

stationary phase contains immobilized molecule that will bond to the target when solute is washed through

Plates and resolution

small plate height = better peaks

larger number of plates = better res

increasing resolution in gas chromatography

narrow columns

longer columns

increasing stationary phase thickness or changing polarity

Thermal conductivity detector

universal

very common

useful for packed columns

non-destructive

not very sensitive

GC

Flame ionization detector

mostly universal

sensitive

destructive

measures current

GC

electron capture

selective for halogens and nitrocompounds

extremely sensitive

mostly non-destructive

GC

mass spec

can be selective or universal

destructive

extremely sensitive

GC or LC

common GC mobile phases

He

N2

H2

Ar

NOT O2 → too reactive

GC separation mechanisms

polarity - different from sp elute first

volatility - more volatile elute first

Van Deemter Equation

H = A + B/Ux + CUx

A term → multiple paths

B term → diffusion term

C term → phase equilibrium

want low H

Normal phase LC

polar sp, nonpolar mobile phase

nonpolar elutes first

Reversed-phase LC

nonpolar sp, polar mp

polar elutes first

refractive index detector

universal

poor sensitivity

does NOT work with gradient

LC

UV-Vis

universal

linear

very common

works with gradient

LC

Flourescence detector

selective - compound must flouresce

100x more sensitive than UV-Vis

LC

Electrochemical detector

very selective

most sensitive

LC

eluent strength

stronger eluent = more like solvent, less like sp