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23 Cards in this Set
- Front
- Back
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Van Deemter model equation |
H = A + B/u + u [CM + CS] A= random movement through stationary phase B= diffusion in mobile phase C= interaction with stationary phase H = plate height u = average linear velocity |
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Van Deemter model |
relates the variance per unit length of a separation column to the linear mobile phase velocity by considering physical, kinetic, and thermodynamic properties of a separation |
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The rate theory of chromatography |
Describes the process of peak dispersion |
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What does a Van Deemter plot show? |
Plate height vs average linear velocity of mobile phase- used to determine the optimum mobile phase flow rate |
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What is term A in VD model? |
-Molecules may travel unequal distances -independent of u -depends on size of stationary particles or coating (TLC) |
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Eddy diffusion |
Solute molecules take different paths through stationary phase at random, causing broadening of solute band because different paths are of different lengths. |
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Term B of VD model |
Longitudinal diffusion B=2γDM γ= Resistance factor due to packing DM = molecular diffusion coefficient |
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In what situation does the B term dominate? |
at low u B term dominates and is more important in GC than LC as diffusion coefficient is much bigger in gas than liquid |
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DM |
Molecular diffusion coefficient: Measures the ratio at which a substance moves randomly from a region of high conc to low conc |
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Term C of VD model |
C=resistance to mass transfer Cs: stationary phase mass transfer CM: mobile phase - mass transfer |
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Cs equation |
Cs= [df^2]/Ds df = stationary phase film thickness Ds= diffusion coefficient of analyte in SP |
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Cm equation |
[dp]^2 /Dm dp: particle diameter DM: diffusion in MP |
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How does the rate theory compare to the plate model? |
Takes account of time taken to equilibrate between stationary phase and mobile phase (plate model assumes eq to be infinitely fast) |
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Limitations of van Deemter |
Assumes all columns and chromatographic techniques are the same. Doesn't consider viscosity changes Doesn't consider pressure changes |
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Paper chromatography liquid-liquid partition |
The mobile phase is a solution that travels up the stationary phase, due to capillary action. The mobile phase is generally an alcohol solvent mixture, while the stationary phase is a strip of chromatography paper. |
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What is the SP and MP for paper chromatography? |
SP = H2O bound to cellulose MP = organic solvents |
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TLC (Thin layer chromatography) |
After the sample has been applied on the plate, a solvent is drawn up the plate via capillary action, because different analytes ascend the TLC plate at different rates, separation is achieved. |
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What is the SP and MP for TLC? |
SP = alumina or silica gel coated onto glass MP = Solvents |
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What does varying the polarity of the mobile phase do in TLC? |
varies the time polar solutes spend on SP resulting in separation. |
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Adsorption solvent strength parameter, ε0 |
measure of adsorption energy/ unit area of solvent |
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Rf |
=distance of travel of substance/ distance of travel of solvent Reports elution characteristics |
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Column chromatography |
Preparative technique rather than analytical (i.e. used for seperation) Usually silica used (range pH:2-8) particle diameter 40-60 μm |
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How does increasing the polarity of functional group on silica effect speed? |
Increased polarity is slower e.g. Carboxylic acid is slow, Alkanes are fast |
