Partition Chromatography
Partition chromatography is defined as a differential migration separation technique that employs the distribution of a migrating component between two or more different states whose relative velocities are not all equal to zero.
Example No. 1: Gas-Liquid Partition Chromatography As an example, consider gas-liquid partition chromatography, where a pulse containing a mixture of voltatile components is injected at the inlet of a long, hollow, stainless-steel or glass capillary column coated on the inside with a thin film of a non-volatile liquid. The column is long, perhaps 10 meters or more.
A continuous, steady flow of helium gas passes through the column. After injection, the components later appear, in sequence, as a series of separated, broadened, Gaussian pulses at the exit of the column (Figure 1).
Why is the injected mixture separated into its components by this apparatus? Because each component has a unique value of its solubility in the non-volatile liquid. A component that is completely insoluble in the liquid phase passes quickly through the capillary column with a residence time equal to the residence time of the flowing helium gas.
A component that is highly soluble in the liquid phase takes considerably longer to elute from the column. Components with solubilities that are intermediate between "insoluble" and "highly soluble" elute at intermediate times. What is the Gimmick Associated with Ideal Partition Chromatography?
The basic "gimmick" behind ideal partition chromatography is the existence of two phases -- one stationary and one mobile -- between which the components of a mixture rapidly equilibrate. In the ideal case, the equilibration of each component is ideal, in the sense that there exist no mass-transfer limitations to equilibration and the quantity of each component is sufficiently small such that linear partition coefficients apply.
Ideal partition chromatography can be considered to be, in effect, an example of two-dimensional thermodynamics. The three vector directions in Example No. 1 include a single, axial coordinate direction (the length of the capillary tubing; see Figure 1) and two lateral coordinate directions (one being the radial direction) over which thermodynamic equilibrium exists at every point z and at all times t within the chromatographic column.
Typical Chromatographic Apparatus: A Tubular Flow System Figure 1. Non-steady-state diffusion and convection in a tubular flow system. A mixture of components is injected rapidly into the column at z=0 and t =0 and elutes later in time as a series of separated Gaussian peaks at the exit, z = L .
What is the Theoretical Basis for Partition Chromatography? The teaching of the basic principle behind partition chromatography has not been a part of chemical engineering curriculum for decades, with the significant exception of Howard Saltzman when he was a faculty member at the University of Rochester. For example, the author needed to understand a basic derivation for partition chromatography while at the Monsanto Corporation several years after he received his Ph.D. degree in 1965. He found it difficult to find a satisfactory answer in textbooks (most of which focused on either the discrete Craig countercurrent apparatus or on HETP), most of which were oriented towards chemists.
At the time, the author discovered the key gimmick to chromatography in an unusual place, namely, the classic textbook, "The Mathematics of Diffusion", by J. Crank [1]. Crank's Chapter VIII treats simultaneous diffusion and chemical reaction. Section 8.2 in Crank's book [1] describes instantaneous reeaction, in which "In the simplest case, the concentration, S , of immobilized substance is directly proportional to the concentration C of a substance free to diffuse," i.e,
(1) The result is given by Crank as: (2) S RC = 2 2 1 C D C t R x ? ? = ? + ? Page 3 3 Crank stated [1]: "Clearly the effect of the instantaneous reaction is to slow down the diffusion process. Thus, if R + 1 = 100, the overall process of diffusion with reaction is slower than the simple diffusion process by a hundredfold. In fact, if the linear relationship (1) holds, solutions of the diffusion-with- reaction problem for given initial and boundary conditions are the same as for the corresponding problem in simple diffusion, except that the modified diffusion coefficient D/(R + 1) is to be used. This is true irrespectively of whether the diffusion-with-reaction occurs in a plane sheet, cylinder, or sphere, or any other geometric shape, and whether diffusion occurs in one dimension or more." [1] In the author's opinion, Equation (1) describes the partitioning of a component between immobilized (S) and freely diffusing (C) states. This so-called "instantaneous reaction" appears to be a reversible equilibrium rather than an irreversible reaction of C to S. Equations (1) and (2) provided the key clue to the author as to why a partition chromatographic separation yields an individual elution peak for each component.
Partition Chromatography as a Linear Multistate System The author has rederived Equation (2) based upon the following linear partition coefficient between states 1 and 2 for an eluting component i ,
(3) which has units of concentration/concentration. The resulting conservation-of-species equation -- involving diffusion, convection, and irreversible first-order reaction along the axial coordinate direction z -- is,
(4) In the absence of axial convection and first-order reaction, Equation (4) simplifies to,
(5) which is identical to Equation (2). What is a State? Partition chromatography basically is the superposition of two-dimensional thermodynamics -- a mobile phase and a stationary phase -- upon non-steady-state diffusion, reactopm. and convection in a third dimension. For simplicity, the two-dimensional equilibrium and third-dimension dynamics can be characterized by a pair of states for each eluting component in a chromatographic column (see Figure 2). 2 2 1 i i i c c ? = 2 2 0 is is is is ieff ieff ieff c c c c t z z D v k ? ? ? ? + + = ? ? ? 2 2 0 is is ieff c c t z D ? ? ? = ? ? Page 4 4 Figure 2. Box representation of the two states (for each eluting component i ) in gas-liquid partition chromatography (in a tubular column system). In Figure 2, the subscript i represents an eluting component; the subscript numbers 1 and 2 represent the mobile and stationary phases, respectively; D is the diffusion coefficient in a phase; and v is the convective velocity of phase 1
. At the bottom of each state box, the value 0 indicates that no irreversible first-order reaction is occurring in each phase.
Richard Laurence Millington Synge was born at Liverpool on October 28th, 1914, as the son of Laurence Millington Synge, of Liverpool Stock Exchange, and Katharine Charlotte Swan. In 1928 he went to Winchester College, where he studied mainly classics until 1931, thereafter natural science. In 1933 he entered Trinity College, University of Cambridge and studied physics, chemistry and physiology for Part I of the Natural Sciences Tripos (1935) and biochemistry for Part II (1936). During 1936-1939 he was a research student under supervision of Mr. N.W. Pirie in the University Biochemical Laboratory headed by Sir Frederick G. Hopkins, and during 1939-1941 at the Wool Industries Research Association at Leeds. He obtained his Ph.D. degree at Cambridge in 1941. In the same year, he joined the staff of the Wool Industries Research Association at Leeds and in 1943 that of the Lister Institute of Preventive Medicine, London, in the Biochemistry Department under W.T.J. Morgan. Since 1948, he has been Head of the Department of Protein Chemistry at the Rowett Research Institute at Bucksburn, Aberdeen.
The circumstances of his work up to 1945, including the collaborative work on partition chromatography and related topics, are described in the Nobel Lectures by A.J.P. Martin and himself. They gave the first demonstration of partition chromatography to the Biochemical Society at its meeting at the National Institute for Medical Research, London, on June 7th, 1941 [Chem.Ind.(Lond.), 19 (1941) 487], the first published description appearing in the Biochemical Journal, 35 (1941) 1358.
Since 1945 Dr. Synge has been mainly interested in analytical problems concerning the larger peptide molecules, as antibiotics and as intermediates in protein metabolism. From 1942 to 1948 he worked almost exclusively with the antibiotic peptides of the gramicidin group. In 1946-1947 he spent eight months with Professor Tiselius at Uppsala, studying the application of his adsorption methods to these compounds.
At the Rowett Research Institute, directed by D.P. Cuthbertson, he has been particularly concerned with the digestion of proteins by the ruminant animal and its associated micro-organisms, with peptides, proteins and other components of plant material, and with physico-chemical methods for the purification of intermediates in the metabolism of proteins. Work begun about 1950 with D.L. Mould and A. Tiselius on electrokinetic ultrafiltraltion of various polysaccharides has been developed in a number of directions to take advantage of molecular-sieve effects, especially in the presence of hydrogen-bond breaking solvents.
In 1958-1959, he spent a year at Ruakura Animal Research Station, Hamilton, New Zealand, working with E.P. White on isolation of the toxic fungal component sporidesmin.
Dr. Synge was made a Fellow of the Royal Society in 1950 and of the Royal Institute of Chemistry in 1952. He is an honorary member of the American Society of Biological Chemists.
In 1943 he married Ann Stephen, daughter of the late Adrian and Karin Stephen, psychoanalysts. They have four daughters and three sons, in order of decreasing age: Jane, Elizabeth, Matthew Millington, Patrick Millington, Alexander Millington, Charlotte, and Mary.
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