Chapter 23. Analytical Separations. We have looked briefly at distillation and more fully at extraction. How does this apply to chromatography? Both separations were based on multiple equilibria. For Distillation this was a evaporation / condensation step. (in a vertical column)
Both separations were based on multiple equilibria.
For Distillation this was a evaporation / condensation step. (in a vertical column)
For extraction this was a solvent extraction step (in a piece of glassware).What is Chromatography
To improve this equilibrium step in distillation we force interaction between the vapor and liquid. This is done a variety of ways but one common way is to place plates in the column to collect the liquid.
This has become a key term in separations.
It now means a separation step.Chromatography
1850-1900 Runge, Schoenbein, and Goeppelsroeder: Studied capillary analysis on paper.
1876 Lemberg: Illustrated the reversibility and stoichiometry of ion exchange in aluminum silicate minerals.
1892 Reed: First recorded column separation: tubes of kaolin used for separation of FeCI3 from CuSO4.
1903-1906 Tswett: Invented chromatography with use of pure solvent to develop the chromatogram; devised nomenclature; used mild adsorbents to resolve chloroplast pigments.History
1935 Holmes and Adams: Synthesized synthetic organic ion exchange resins.
1938 Reichstein: Introduced the liquid or flowing chromatogram, thus extending application of chromatography to colorless substances.
1938 Izmailov and Schraiber: Discussed the use of a thin layer of unbound alumina spread on a glass plate.
1939 Brown: First use of circular paper chromatography.History
1941 Martin and Synge: Introduced column partition chromatography.
1944 Consden, Gordon, and Martin: First described paper partition chromatography.
1947-1950 Boyd, Tompkins, et al: Ion-exchange chromatography applied to various analytical problems.
1948 M. Lederer and Linstead: Applied paper chromatography to inorganic compounds.History
1952 James and Martin: Developed gas chromatography.
1956 Sober and Peterson: Prepared first ion-exchange celluloses.
1956 Lathe and Ruthvan: Used natural and modified starch molecular sieves for molecular weight estimation.
1959 Porath and Flodin: Introduced cross-linked dextran for molecular sieving.
1964 J. C. Moore: Gel permeation chromatography developed as a practical method.History
Mobile phase – The part of system that moves
Elution – Eluent (in), eluate (out)
Open tube column.Terms
Retention Time - How long a compound stays in the column. (tr) or could be expressed in terms of volume (Vr)
Dead volume Vm or could be expressed as a time(tm)
Volume to get through the system even without any interaction. A constant for a given column.
Adjusted retention time tr’
tr’ = tr - tmTerms of Chromatography
This can be converted to moles. Thus the capacity factor is molesstat / molesmobile
This allows us to write k’ the following way
k’ = CsVs / CmVmRetention time and partition coefficient
The system can be scaled up to separate at the gram scale.
Develop an analytical scale separation and then scale it up.Scale Up
This comes directly from the Craig Model.
We know certain facts about Gaussian peaks.The Peak
How do we quantify Resolution.Resolution
Narrow peaks will allow us to resolve peaks coming out at nearly the same time.
Different compounds passing through a column at different times might have different plate heights since they will generally have different diffusion coefficients.
Plate theory calls for constant plate height since diffusion is ignored in this model.Plate Height is a Measure of Column Efficiency
Where the H value is minimum
How do we find this point.
Run about 20 or more experiments at different flow rates, find H and then plot the resulting curve. Pick Hopt from this plot.Optimum Flow Rate
For open tubular column this means that we can get
Increased linear flow rate and/or a longer column
Decreased Plate height, which means improved better resolution