10.1

CHAPTER 10: CHROMATOGRAPHY: A DEEPER LOOK

origin of chromatographyMikhail Tswett - 1903- separated colored plant pigments

chromatography = color writing (in Greek)

type of mobile phaseliquidgassupercritical fluid

10.2

10.1 SCOPE OF CHROMATOGRAPHY

Mechanism of retention1. Adsorption chro.-- GSC, LSC

2. Partition chro. -- GLC, LLCpolarity natureCBP's

3. Size exclusion chro. - GFC, GPC

4. Ion exchange chro.

5. Hydrophobic interaction chro. (HIC)

10.3

incoming solute concentration profile

1. zonal : sample introduced as a narrow zone- follows gaussian

2. frontal analysismixed sample introduced as continuous stream

10.4

10.2 CHROMATOGRAPHIC MIGRATION

mechanismwhen sample enters the head of the columnR’

1-R’

sample zone velocity : V = fraction x <v> or v= R' v = R v

R' = measure of retardationk' = more common in use

10.5

retention time

for nonretatined peak

retention volume

Vr = RVm

equilibrium and retention parametersR or R' : equilibrium fraction of a comp. in mobile ph.

the ratio of solute in st. ph.to solute in mo. ph.

Thus,

10.6

eq. 9.45 gives

R =

when a single staionary phase employed !Vr = Vm + KVs

K = ? chemical potential ∝

K = exp ⎛⎜⎝- RT

∆µo ⎞⎟⎠

10.7

10.4 EFFECT OF GAS COMPRESSIBILITY ON RETENTION IN GC

Pressure drives flow: always ∆P over column* incompressible fluid:

<v> = v uniform

* compressible (gas): gas expands toward exit; v increases

function of x, as above

(eq. 4.30b)

∫∫∑ ===Lt

r xRvdxdtdtt r

00 )(

10.8

∫ −−=L

ir dxLxPP

RPvt

0

2/122 )/)1((1

LxPPy )1( 22 −−= L

Pdxdy 12 −−=

RPvPLt

ir )1( 2 −

−= ∫

12/1

2Pdyy

111

23

3

2

<⎟⎟⎠

⎞⎜⎜⎝

⎛−−

=PPj

James-Martin pressure – gradient correction factor

10.9

Meaning of j ? j is always smaller than 1

00 RvL

jRvLtr >=

because displacementslowed by compression

in column

0/ RvLtr >

1<∴ j

01 ttR r == ,

rr tV ∝

Vr measured at outlet

10.10

10.5 EFFECT OF GRADIENTS ON MIGRATION INTLC AND PC

phase ratio: variable

migration in TLC & PC

10.11

10.12

10.6 NONEQUILIBRIUM MIGRATION

basic assumption in retention

: equil. in solute distribution bet. phaseszone velocity : V = Rv

Gaussian like conc. profile.

systematic departures from eq.

ends up with loss of zone sharpness

10.13

generalized non-equilibrium theory

ahead of zone center:solute-enriched mob. ph. (coming from high conc. reg.) causesa continuous increase in conc.

some excess solutes move toward st. ph. to reach new eq.but needed finite time (teq)

During teq, more fluids come in and upset the balanceactual conc. at A remains higher than eq. conc.

at rear of zone:actual conc. at A remains lower than eq. conc.

10.14

departures from eq. causes zone broadening.

at. eq. solute migrates at a velocity proportional to its fraction

in mobile phaseV = R v at zone center

due to noneq.

ahead of zone center : solute moves at a velo. > R vat rear: solute " " < R v

⇒

zone broadening

10.15

effective diffusion coeff. for non-eq. spreading

thus

band broadening from noneq. when eq. time rapidsmall v narrow zones

eqn tvRRD 21 )( −=

=nH