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Instrumental Lecture 14

Instrumental Lecture 14 - Rate Theory of Chromatography H =...

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Rate Theory of Chromatography H = H L + H S + H M + H SM H = height equivalent to theoretical plate (as in Plate Theory) H L = contribution due to longitudinal diffusion H S = stationary phase mass transfer contribution H M = diffusion associated with mobile phase effects H SM = diffusion into or mass transfer across a stagnant layer of mobile phase (neglect) H = B/μ + Cμ +A van Deemter Equation A, B & C are coefficients, μ = velocity
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1) Longitudinal Diffusion H L = (B/μ) t = 0 0 < t < t R t R σ L 2 = 0 σ L 2 = 2 D M t M Variance due to longitudinal diffusion = 0 at start Variance increases with time & diffusion coefficient D
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2) Mass transfer in & out of stationary phase t 1 t 2 t 3 Mobile Phase Stationary Phase Resulting Peaks Broadening of peaks is a function of mobile phase velocity (moving molecules faster than those in stationary phase) Not the same as longitudinal diffusion H S = Cμ In Plate Theory condition at t 1 assumed to hold throughout
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3) Uneven Flow or Eddy Diffusion Path 1 is shorter than path 2 H M = A
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