Separation Techniques
Extraction and Chromatography
Extraction
 Simple
liquid-liquid extraction in analytical
work is generally done for one of two
reasons:


 If
Purification, i.e., separating the analyte from
interferents, or
Concentration, i.e., getting the analyte into a
smaller volume
the analyte is a metal ion, the extracting
medium will often include a complexing
agent.
Partition Coefficient, or
Distribution Ratio
 When
a solute distributes itself between
two immiscible phases, typically aqueous
and organic, equilibrium may be
established
Saq ↔ Sorg
and the resulting equilibrium expression is
KD = [Sorg]/[Saq]
where KD is the partition coefficient.
Distribution Ratio
 Some
solutes may appear in more than
one form in a solvent, e.g., when an acid
dissociates, HA ↔ H+ + A-.
 If such dissociation occurs in the aqueous
phase but not the organic phase, then we
can define the distribution ratio as
D
= [HA]org / {[HA]aq + [A-]aq}
Extraction Efficiency
 If
we define the fraction of solute extracted
in a single step extraction, Ei, and the
fraction remaining behind unextracted as,
Ui, it can be shown (see your handout)
that
E1 = KD/ [(V1/V2)+KD], and
U1 = V1/ [V1 + V2KD] *
Further, Ui = (U1)i
*(same as equation 7.24 in your text with different symbols)
Chromatography

Chromatography can be thought of as a method
involving continuous extraction in very small
increments of clean extracting solvent.
 Chromatography always has two phases, just as
in extraction, but they are called the mobile
phase and the stationary phase.
 The movement of the solute through the column
is called elution.
 The phase distribution of the solute(s) in
chromatography can occur by any of several
equilibria, not just solubility, as in extraction.
Types of Equilibria Applied to Chromatography
Adsorption – the stationary phase is a solid on
which the solutes adsorb.
 Solubility – the stationary phase is a liquid into
which the solutes dissolve.
 Ion-exchange – the stationary phase is
composed of ion exchange beads which attract
cations or anions
 Size-exclusion – the stationary phase is porous
beads which temporarily trap solutes based on
their particle size.

A way of visualizing what
happens in chromatography
by thinking of it as taking
place in a series of many
small steps.
This diagram simulates what
happens when two solutes
are separated. As the caption
says, the A component has a
KD of 1 and the B component
has a KD of 3.
General Chromatogram
Chromatographic Terms
 Retention
time, tr,A, the time it takes component A to exit the column past the
detector.
 Void time, tm, the time it takes a nonretained component (air in GC) to exit.
 Adjusted retention time, tr ’ = tr – tm.
 Baseline width, Wb, measured at intersection of tangents with sides and
baseline.
Resolution
Different Resolutions
The Chromatographic Challenge

Looking at the resolution curves, it can be seen
that resolution can be increased by increasing
the time between the peaks or by decreasing the
width of the peaks.
 The challenge for the chromatographer is to
accomplish optimal resolution within a
reasonable amount of time. Thus, simply
increasing the time by some means is not
necessarily the best way to go.
 Let us consider what causes band broadening
and see if we can figure ways to decrease it.
Capacity Factor
factor, k’ (also known as retention
factor), takes into account the relative
volumes of the mobile and stationary
phases,
k’ = D (Vs/Vm)
 It turns out that it can also be related
rather simply to retention times,
k’ = tr’/tm
 Capacity
Column Selectivity Factor
selectivity factor, α, relates to the
ability of a column to separate two solutes,
α = kB’ / kA’
 The
 It
will be equal to one when the two
components have the same retention
times.
Plate Theory of Band Broadening
Early on, theorists drew parallels between the
behavior of chromatographic columns and
fractional distillation columns with plates
separating temperature regions.
 Chromatography can be thought of as
behaving like a fractional distillation column
with many plates.
 Each time the solute enters the stationary
phase essentially constitutes a ‘theoretical
plate.’

Calculating Column ‘Plates’
 The
distance a solute travels, on
average, between stops in or on the
stationary phase determines, in part, how
wide a peak gets.
 Further, the longer the solute stays in the
column, the wider the peak gets.
 Thus the height equivalent to a
‘theoretical plate,’ HETP or just H, can be
related to peak width and retention time
H = Lw2/16tr2
Calculating Column ‘Plates’
 The
number of times on average the
solute stops in or on the stationary phase
is essentially the number ‘theoretical
plates’ a column behaves as though it has.
N = (4tr/w)2
 It is also equal to the column length
divided by H.
Band Broadening
 Van
Deemter gave an equation which
relates the number of theoretical plates in
a column to the flow rate, u, of the mobile
phase and three parameters
H = A + B/u + Cu
Van Deemter Parameters
 A,
eddy diffusion,is related to the size and
uniformity of the stationary phase
particles.
 B, longitudinal diffusion, is the inevitable
diffusion of a fluid in all directions.
 C, mass transfer, is complexly related to
the geometry of the stationary phase, the
distribution coefficient, and diffusion rates
in the mobile and stationary phases
So, with all that, how do I increase
resolution?
 Increase Δtr by



Increasing L
Increase amount of stationary phase
Get a better selectivity factor, α
• Decrease temperature
• Get a better stationary phase
• Get a better liquid phase (in liquid chromatograpy)
(more)
More resolution improvement
 Decrease







band width, w, by
Using more uniform packing
Using smaller packing
Use no packing, use SCOT column (for GC)
Optimize flow rate
Reduce sample size
Reduce dead space in column
Decrease diameter of column