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Theory for gas chromatography

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- 1. Presented by: Anvita Jadhav M.Pharm (IP)
- 2. Gas Chromatography Gas chromatography is a common type of chromatography used in analytical chemistry for separating and analyzing the compounds that can be vaporized without decomposition. It has two types • Mobile Phase : Gas • Stationary Phase: SolidGSC • Mobile Phase : Gas • Stationary Phase : LiquidGLC
- 3. The distribution of an analyte between stationary and mobile phase is expressed by the distribution constant K. K = Cs/Cm Cs = concentration of a component in the stationary phase Cm = concentration of a component in the mobile phase In case of GSC, the interaction of solutes with the stationary phase is in the form of their adsorption on it & this adsorption is non-linear. This does not keep the ratio of the concentration of a solute in the stationary phase (Cs) to that in the mobile phase (Cm) constant. In case of the GLC ratio of the concentration of a solute in the stationary phase (Cs) to that in the mobile phase (Cm) constant.
- 4. Isotherm for Linear-Nonideal GLC Isotherm for Nonlinear-Nonideal GSC The isotherm is a graphical representation of the distribution constant K CS = concentration in stationary phase; CG = concentration in mobile phase at equilibrium.
- 5. Plate Theory The theory assumes that the column is divided into a number of zones called theoretical plates. At each plate equilibrium of the solute between the mobile phase & the stationary is assumed to take place. The partitioning of a solute between the phases takes plate at each theoretical plate. Thus, the number theoretical plates in the column is used as a measure of efficiency of the column to separate the components from each other
- 6. The number of theoretical plates can be determined by where, n = no. of theoretical plates VR = retention time W = base width of the peak
- 7. HETP value can be determined by, Plate theory disregards the kinetics of mass transfer; therefore, it reveals little about the factors influencing HETP values. The resulting behavior of the plate column is calculated on the assumption that the distribution coefficient remains unaffected by the presence of other solutes and that the distribution isotherm is linear. The diffusion of solute in the mobile phase from one plate to another is also neglected.
- 8. Plate Theory Discrete Flow Model Continuous Flow Model
- 9. Discrete-Flow Model The assumptions in this model are (a) • All the mobile phase moves from one segment to the next segment at the end of a discrete interval (b) • The sample molecules are always in equilibrium with the mobile and stationary phases
- 10. Continuous-Flow Model The assumptions in this model are (a) • The mobile and stationary phases remain in equilibrium throughout the separation (b) • The mobile phase flows from one segment to the next segment at a constant rate (c) • Perfect mixing takes place in all segments
- 11. Rate Theory It was introduced by Van Deemter. It describes the effect of an elution band as well as its time of elution. Van Deemter equation describes the relation of the height of a theoretical plate H and the average linear velocity of the mobile phase.
- 12. Van Deemter Equation H = height of a theoretical plate u = average linear velocity of the mobile phase A = eddy diffusion term B = longitudinal or ordinary diffusion term C = nonequilibrium or resistance to mass transfer term
- 13. Eddy Diffusion The A term refers to band broadening caused by dispersion (multi-pathway) effects (Eddy diffusion) A = 2λdp λ = correction factor for the irregularity of the column packing dp = average particle diameter.
- 14. In this case the spaces along the column are not uniform. When a sample migrates down the column, each molecule “sees” different paths and each path is of a different length. Some molecules take the longer paths and others take the shorter paths. There are also variations in the velocities of the mobile phase within these pathways. The overall result is that some molecules lag behind the center of the zone, whereas others move ahead of the zone.
- 15. Longitudinal Diffusion The B term represents band broadening by longitudinal diffusion, the molecular diffusion both in and against the flow direction: B = 2γDG γ = labyrinth factor of the pore channels (0<γ <1) DG = diffusion coefficient of the analyte in the gas phase
- 16. This process results when there exists a region of high concentration and a region of low concentration. The migration is from the higher to the lower concentration region in the axial direction of the column. Diffusion occurs on the molecular level, resulting from movement of molecules after collision The diffusion is about 100–1,000-fold faster in gases than in liquids, therefore B terms shows higher impact in GC than in LC.
- 17. Mass transfer under non equilibrium The C terms refers to the mass transfer between stationary and mobile phase. As the zone of solute continues to migrate down the column, it is constantly bringing an ever-changing concentration profile in contact with the next part of the column. This effect results in different rates of equilibration along the column. Thus theoretical plate in the column is constantly attempting to equilibrate with a variable concentration zone in the mobile phase. At one time the zone attempts to equilibrate with a low concentration in the mobile phase, and then at another time with a high concentration. These overall processes result in nonequilibrium at each theoretical plate.
- 18. The rapid mass transfer depends on the factors originating from the stationary phase as well as the mobile phase The term ‘C’ in Van Deemter equation is therefore, the sum of Cs & CM. The stationary phase contribution (Cs) to the plate height H, due to the mass transfer under nonequilibrium condition, is given by, q = configuration factor r = a constant dependent upon the relative rate of migration of a solute & the mobile phase, d = thickness of the stationary phase Ds = diffusion coefficient of a solute in the stationary phase.
- 19. The mobile phase contribution (CM) to the plate height H, due to the mass transfer under nonequilibrium conditions, is given by, DG = diffusion coefficient in the gas phase dp = average particle diameter ω = obstruction factor for packed bed
- 20. Van Deemter Plot The term ‘A’ is independent of flow rate of the mobile phase The term B/u decreases drastically in the beginning with increase in the flow rate of mobile phase. Increase in the flow rate beyond particular value, leads to slow decrease in the value of B/u. The term Cu increases with increse in the flow rate

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