Chem201 lec #19 8/24/07 page 1 of 7

Chem 201 lecture #19 Last time: Column efficiency, parameters, R, H, N Van Deemter Equation Today: Van Deemter Equation GC Student survey Van Deemter Equation: plate height

H = A + B/ux + Cux Multiple longitudinal equilibration Paths diffusion time

2nd term: longitudinal diffusion term B/ux = longitudinal diffusion: Minimized by: Keeping time of passage short (ie faster flow). For GC, lower T’s also lowers long. diffusion (comes from: σ2=HDL => HD = σ 2/L = 2 Dmt /uxt , or HD = 2 Dm /ux = B/ux where Dm = diffusion coeff in mobile phase & ux is linear vel of mobile phase. ). Broadening due to molec’s tendency to move parallel to flow C is not concentration 3rd term: mass transfer term

Cux = due to finite equil’b’n time req’d for solute to equilibrate between mobile & stationary phase. C = (Cs + Cm) where: Cs describes stationary phase and Cm describes mobile phase. Broadening due to molec’s tendency to move perpendicular to flow E.g. for GC: Cs α d2 (d=thickness of stationary phase). Cm α r2 (r=column radius) chromatography occurs in nonequilibrium conditions. (to minimize

Chem201 lec #19 8/24/07 page 2 of 7

Actual bands may deviate from Gaussian shape: 3 phenomena are: a) overloading, b) ideal and c) tailing Comparing LC with GC, plot H vs flow rate:

Although H is lower for LC, flow rate in LC is less. More importantly, N is very much greater for GC. Remember that R α √N Gas Chromatography

Chem201 lec #19 8/24/07 page 3 of 7

An eccentric scientist is working on a research project in the Amazon jungle. He sends for a research assistant and a gas chromatograph because he's close to a cure for cancer.

Chem201 lec #19 8/24/07 page 4 of 7

Why not use O2 as a carrier? Detectors: 2 common ones: thermal conductivity detector (TD) and flame ionization detector (FID)

Chem201 lec #19 8/24/07 page 5 of 7

High performance liquid chromatography (HPLC) Effect of particle size Pressures required:

Chem201 lec #19 8/24/07 page 6 of 7

What HPLC looks like in comparison with Regular LC.

Solvents must be pure to avoid degrading expensive HPLC columns. The stationary phase can be bonded to silica packing material.

Chem201 lec #19 8/24/07 page 7 of 7

Normal- and Reversed-phase chromatography: When polar phase attached to silica support, and less polar solvent used as mobile phase, it’s known as "normal-phase chromatography." When nonpolar bonded phase and polar solvent is used: "reverse-phase chromatography". Like dissolves like: Say we have 3 substances: A, B , C Where polarities are: A<B<C A. In normal-phase chromatography: The least polar component is eluted first. (i) low polarity mobile phase first eluted

(ii) medium polarity mobile phase

Reversed-phase chromatography gives excellent separation and eliminates peak tailing associated with adsorption of polar compounds by polar packings. It is also less sensitive to polar impurities (such as water) in the eluent. B. In reversed-phase chromatography The least polar component is eluted first. (i) high polarity mobile phase first eluted

(ii) medium polarity mobile phase

The order of polarities (low to high) can be listed as: hydrocarbons < ethers < esters < ketones < aldehydes < amides < amines < alcohols < water