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Van deemter equation
- 1. VAN-DEEMTER EQUATION Dr. TambeV.S. PES Modern College of Pharmacy (for Ladies), Moshi
- 2. Band Broadening Processes InitialTime, t1 Time, t2 N= 𝐿 𝐻 Number of theoretical plates = 𝐿𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑐𝑜𝑙𝑢𝑚𝑛/𝐻𝑒𝑖𝑔ℎ𝑡 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑡𝑜 𝑜𝑛𝑒 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑝𝑙𝑎𝑡𝑒 𝐻𝐸𝑇𝑃
- 3. Types of BandBroadening Processes Non column broadening/ extra column band broadening Dispersion of analyte in- • Dead volumes of system (injector, connection between injector and column, connection between column and detector) • Can be minimized by reducing dead volume On column broadening (Explained by Van Deemter equation)
- 5. A: Eddys Diffusion/Multiple path effect
- 7. Inhomogeneity's of flowvelocities and pathlengths around packed particles A = λ dp λ is packing factor, function of packing uniformity and the column geometry Column should be packed uniformly dp: particle size, should be small and uniform with narrow particle size distribution
- 8. B: Molecular orLongitudinal diffusion Tendency of molecules to migrate from the concentrated part of the band to the dilute region on either side. B dominates at low velocity, as diffusion increases with time, solute gets enough time to diffuse in both directions It can occur in MP and SP D gas ≥ 104 D liquid More important for gas as compared to liquid
- 9. B: Molecular orLongitudinal diffusion ϒ is obstruction factor, hindrance to free molecular diffusion offered by particles or bed structure, Offered by packed column (0.7 in packed column, 1 for open column) In solution, molecule has equal probability of diffusion in any direction. In packed column, the solid packing material may restrict the ability of solute to diffuse in one particular direction, thereby hindering longitudinal diffusion. If the SP is packed uniformly and tightly, there is more obstruction provided to the diffusing particles, reducing band width and increasing column efficiency Tm is interparticle tortuosity factor (How tortuous path MP takes) 𝐵 = 2ϒ 𝐷𝑚/𝑇𝑚
- 10. C : ResistanceTo Mass Transfer Under Non Equilibrium Conditions Equilibrium of analyte between stationary and mobile phase is not instantaneous, it takes finite time, which is barely given in chromatography
- 11. Cs: Resistance tomass transfer at solute to stationary phase interface 𝐶𝑠 = 𝐷𝑓 𝐷𝑠 Df: In partition chromatography, solid support is coated with liquid SP. Lesser is the thickness of SP, lesser will be resistance to mass transfer at SP. (but also reduces capacity of column) Ds: Diffusion coefficient of solute in stationary phase, it should be more to achieve fast equilibrium To increase the Diffusion coefficient • Non-viscous liquid stationary phases should to used • Increase in column temperature to increase diffusion Resistance to mass transfer should be low
- 12. At low flowrate enough time is available for mass transfer of solute under equilibrium conditions
- 13. Cm: Resistance tomass transfer of solute between adjacent stream lines of mobile phase 𝐶𝑚 = 𝑑𝑃2 𝐷m dp: Particle Size for packed column, and column diameter for open tubular column (smaller should be diameter of column) Smaller is the particle size of the stationary phase, lesser will be resistance to mass transfer. Dm: Diffusion coefficient of solute in mobile phase, it should be more to achieve fast equilibrium To increase the Diffusion coefficient- • Non-viscous liquid mobile phases should to used • Increase in column temperature to increase diffusion Resistance to mass transfer should be low
- 14. GC: CM <CS HPLC: CM = CS
- 15. Optimum Mobile PhaseVelocity
- 16. Effect of particlesize on H
- 17. SELECTION OF CARRIERGAS AND OPTIMUM FLOW RATE
- 18. Van-Deemter equation isused to compare performance of various stationary phases and mobile phases Comparison of performance of SP/ Columns • Run the chromatograms using a single SP for a particular analyte by varying flow rates • Calculate the number of theoretical plates using the equation • Calculate H using the formula: H = L/N • Plot H versus Flow rate • Repeat for other Columns
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