Quant. Anal. Chp. 23 Separatio

Flashcard maker : Larry Charles
Extraction:

– The transfer of a solute from one phase to another.

 

  • Carried out to: (1) isolate or concentrate the desired analyte; (2) Separate it from species that would interfere in the analysis.

1. Partition Coefficient:

 

2. Fraction Remaining:

1. K = [S]2/[S]1

 

2. K  = [S]2/[S]1 = ((1-q)m/V2)/(qm/V1)

 

  • q = V1/(V1 + KV2
  • After n extractions with volume V2; qn= (V1/(V1 + KV2))2

 

pH Effects on Extraction:

  • If a solute is an acid or base, its charge changes as the pH is changed.
  • Usually a neutral species is more soluble in an organic solvent and a charged species is more soluble in aqueous solution.

Extraction with Metal Chelator:

  • Most complexes that can be extracted into organic solvents must be neutral.
  • Charged complexes, such as Fe(EDTA) or Fe(1,10-phenanthroline)32+, are not very soluble in organic solvents
  • Selectively complex one ion with an organic ligand and then extract it into an organic solvent.
  • Three ligands commonly employed are: Dithizone, 8-hydroxyquinoline, cupferron.

What is Chromatography?

  • Most powerful tool in an analytical chemist’s arsenal for separating and measure the components of a complex mixture.
  • Process in which compounds areseparated from one another by passing them all through a column that retains some compounds longer than others.
  • Operates on same principle as extraction, but one phase is held in place while the other moves past it.

Mobile phase:

  • Either a liquid or a gas.
  • In gas chromtography, is a gas.
  • In liquid chromatography, is a liquid.

Stationary phase-
-Either a solid or a viscous liquid coated onto solid particles or onto the inside wall of a hollow capillary column.
Eluent-
-Fluid going in column.
Eluate-
-Fluid exiting column
Packed column-
Filled with particles containing stationary phase.
Open tubular column:

  • Narrow, hollow capillary with stationary phase coated on the inside walls.

[Types of Chromatography]

Adsorption Chromatography-

-Uses a solid stationary phase and a liquid or gaseous mobile phase: Solute is adsorbed on the surface of the solid particles.

[Types of Chromatography]

Partition Chromatography-

– Involves a thin liquid stationary phase coated on the surface of a solid support: solute equilibrates between the stationary liquid and the mobile phase.

[Types of Chromatography]

Ion Exchange Chromatography-

– Features ionic gropus such as -SO3- or -N(CH3)3+ covalently attached to the stationary solid phase, which is usually a resin: solute ions are attracted to the stationary phase by electrostatic forces; the mobile phase is liquid.

[Types of Chromatography]

Molecular exclusion chromatography (Gel filtration)-

-Separates molecules by size, with larger molecules passing through most quickly

[Types of Chromatography]

Affinity Chromatography-

Most selective kind of chromatography, employs specific interactions between one kind of solute molecule and a second molecule that is covalently attached to the stationary phase.
Retention time, tr
– For each component is the time needed after injection of the mixture onto the column until that component reaches the detector.
Retention volume, Vr

– The volume of mobile phase required to elute a particular solute from the column.

;

– Vr = tr * uv

Adjusted retention time, t’r

-For a solute is the time requried for solute to travel the length of the column beyond the time required by unretained solvent, tm.

 

t’r = tr – tm

Relative retention

  • α = t’r2/t’r1
  • α will be > 1
  • greater relative retention, the greater the separation b/w two components.

Capacity factor:

  • k’ = (tr – tm)/tm
  • The longer a component is retained by the column, the greater the capacity factor.

  • To monitor performance of a column, measure the:

  1. capacity factor for a standard
  2. number of plates
  3. peak symmetry
  • Changes above reflect degradation of the column.

What two factors contribut to how well compounds are separated by chromatography?

  1. Difference in elution times between peaks; the further apart the better.
  2. How broad the peaks are; the narrower the better.

Resolution:

  • Res. = Δtr/wav = ΔVr/wav
  • In quant. anal. a resolution of > 1.5 is desirable.

Plate Height:

  • More theoretical plates, the narrower the bandwidth when the compound emerges.
  • The smaller the plate height, the narrower each eluted peak, and the better the separation.

Factors Affecting Resolution:

  • Doubling column length increases res. by √2
  • Res. increases as α; change the stationary phase in GC or either stationary phase or mobile phase in LC.
  • Res. increases as capacity factor increases. Increasing capacity factor is equivalent to increasing the fraction of time spent by the solute in the stationary phase.

Advantages of Open Tubular Columns:

  1. Higher resolution
  2. Shorter analysis time
  3. Increased sensitivity
  4. Lower sample capacity
  • Particles in packek column resist flow of the mobile phase, so flow rate cant be very fast
  • Open tubular column has higher linearflow rate for same length column
  • Allows for longer column that provides 100 times more theoretical plates.

Van deemter Equation:

H ≈ A + B/ux + Cux

 

  • A- multiple paths
  • ux – longitudnal diffusion
  • C- Equilibrium time
  • A, B, and C are constant for given column and stationary phase. 

Packed column: A, B, C ≠ 0

Open Tubular Column: A = 0

Capillary electrophoresis: A  = C = 0

Get instant access to
all materials

Become a Member