Download - ANALYTICAL SEPARATIONS
ANALYTICAL SEPARATIONS
Precipitation
Gravimetry
•Precipitation•Filtration•Washing•Drying or ignition•Measuring•Calculation
Separating species by distillation
Determination of ammonia
Determination of carbon dioxide
Extraction
Distribution between organic and water phase
Separation of metal ions as chelates
Ions are: soluble in water insoluble in non polar-organic phase
O(C6H5 ) NH4
+
NO
O(C6H5 )3 Fe
NO
Cupferron: ionic ferric cupferrate: neutral
Separation of Fe3+ ion
Separating ions by ion exchangeCation exchange:xRSO3
-H+ + Mx+ (RSO3-)xMx+ + xH+
solid soln solid soln
Anion exchange:xRN(CH3)3
+OH- + Ax- [RN(CH3)3]xAx- + xOH-
solid soln solid soln
where: Mx represents a cation and R a part of resin containing sulfonic acid group
where: Ax- represents an anion and R a part of resin containing trimethyl ammonium group
After ion exchange cations or anions are on the resin, it should remove them
Chromatography
Mobile phase
Stationary phasesolid liqiud
Gas chromatographyGC
gas GSC GLC
Supercritical chromatography
SFC
Supercritical fluid
SFC SFC
Liquid chromatographyLC liquid
TLCIC
GPC,SEC
PC
Normal phase
(HPLC-NP)
Reversed phase
(HPLC-RP)
Liquid chromatographyLC
liquidCE
GEL ELFO
Classification of chromatographic methods
Frontal chromatography
Modes of chromatographic separation
Displacement chromatography
Elution chromatography
1. Physical interactions-sorption: adsorption
absorption (solvation, distribution)chemisorption
-hydrofil-interactions
-hydrofob-interactions
-interactions based on size exclusion
2. Chemical interactions-acid-base interactions
-complex formation
-H-bond interactions
3. Biochemical interactions-biochemical affinity
Interactions in chromatography
Consequences:•Analytes are moving with different rates (differential migration)•In the course of chromatographic process band are wider and wider (band broadening)
The chromatographic process
Retention data
Retention time: tR
Dead time: tM (t0)
Reduced retention time: tR’ = tR - tM
Retention volume:
Reduced retention volume:
FtV RR
MRMRRR VVFttFtV
where: F, volumetric flow rate (cm3/min)
The average linear rate of solute migration, (usually cm/s)
RtL
where L is the length of the column, tR retention time
RtLu
The average linear velocity of the mobile phase molecules, u
The relationship between migration rate and distribution constant
The rate as a fraction of the velocity of mobile phase:
phasemobileinspendsoluteoftimeoffractionu
This fraction equals the average numbers of moles of solute in the mobile phase at any instant divided by the total number of moles of solute in the column:
solutesofmolestotalphasemobileinsoluteofmolesu
The total number of moles of solute in the mobile phase is :
nM = CM x VM
in stationary phase: nS = CS x VS
MMSSSSMM
MM
VCVCu
VCVCVC
u/11
M
S
MA
SAC C
Caa
K
Therefore:
Since distribution constant:
therefore:
MSC VVKu
/11
•Time spended of analyte in the stationary phase relating to the mobile phase
The retention factor: k’
k’: relative number of moles of analytes in the stationary and mobile phase
k’ = nS/nM
Other definition of retention factor for analyte A:
M
sAA VV
Kk
AKu
11
AMR ktL
tL
11
where: KA is the distribution constant for analyte A
Substitution to equation earliers:
Rearranging:
M
MRA t
ttk
Selectivity factor:
1
2
R
R
A
B
tt
kk
Always greater than 1.0
Column efficiency and band broadeningThe plate theory of chromatographyOne theoretical plate (N): the part of the column, where quasi-equilibrium takes place between stationary and mobile phase
2L
22
2t
R
σL
σtN
54,5162
wtN R
2
2/1
wtR
Where: standard deviation and 2
Variancew = 4
Gauss equation:
HETP: Height equivivalent to the theoretical plate (H)
NLH
The rate theory of chromatography (van Deemter)
ApedC
Porous silica particleparticle size (diameter): dP
Theory of band broadening1. Eddy diffusion term (A) multiple path effects
uB
uDC Md
2. The longitudal diffusion term (B/u)
uCMM
2pM
DudC
uCSM
2pMS
DudC
3. Mobile phase mass transfer term (CM/u)
4. Stationary phase mass transfer term (CS/u)
uCuu
A S MCBH
H
u
The van Deemter equation of chromatography
The equation has an optimum (Hopt) where the column efficiency is highest.This optimum has been found at a linear velocity: for gas chromatography at. 0.1 – 0.5 cm/sfor liquid chromatography at: 1.0 – 5.0 cm/s
At high linear velocities equation can be estimated as:
uCu
A SBH
)(21
21
12
ww
ttR RRs
Resolution
'k1'k
α1αN
41R
2
22S
Resolution expressed with the terms of plate number, selectivity and retention factors
'k1'k
α1αN
41R
2
22S
Methods to increase resolution
'k1'k
α1αN
41R
2
22S
Effect of increase of retention factor on resolution
How to increase retention factor:•By decreasing eluent strength
Effect of increase of separation factor on resolution
'k1'k
α1αN
41R
2
22S
How to increase separation factor:•By change chemical quality of the mobile phase•By change quality of column
'k1'k
α1αN
41R
2
22S
Effect of increase of plate number on resolution
How to increase theoretical plate number:•Decrease of the flow rate (u)•Increase of the column length (L)