L7 - Lecture 7 Separation Theory 2 Resolution Describes how...

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Lecture 7 Separation Theory 2
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Resolution Describes how well 2 compounds are separated Rs = 1 4 N 1/2 ( -1) k’ 1+k’ ( ) efficiency selectivity retention k’ = t R -t M t M 1 < k’ < 10
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Resolution Describes how well 2 compounds are separated Rs = 1 4 N 1/2 ( -1) k’ 1+k’ ( )
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L - length of column Cannot increase indefinitely Limited by: • Long runs times • Back pressure (LC) / losing pressure Resolution H - height equivalent of a theoretical plate Measure of Efficiency Always want to minimize H • Getting the best performance from system H depends on: • column parameters • mobile phase • flow rate Described by van Deemter: the relationship between H and flow rate N = L H
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van Deemter Equation H B A + + C is flow rate
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van Deemter Equation H (flow rate) H B A + + C A C B H min
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van Deemter Equation A term ‘Multipath Effect’ Also known Eddy diffusion A term only applies to packed column the mobile will have different paths passing along the column this does not apply to the open tubular (i.e. GC) column
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van Deemter Equation A term ‘Multipath Effect’ A C e d p Ce = particle shape dp = diameter of particle A term • Entirely dependent on column • Only important in packed column (i.e. LC) • particle: Uniform in shape and round, smooth, small
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H (flow rate) H A A van Deemter Equation A term ‘Multipath Effect’
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van Deemter Equation B term ‘Longitudinal diffusion’ Diffusion is a process that a species will migrate from a more concentrated part of a medium to a more dilute region.
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van Deemter Equation B term ‘Longitudinal diffusion’ B D MP D MP = diffusivity of mobile phase Diffusivity is inversely to viscosity , (i.e., More diffusive, less viscous; Less diffusive, more viscous) Relatively important in GC because gaseous molecules diffuse at relatively high rates when compared to liquid in LC because diffusion rates are much smaller for liquid Low flow rate limit for GC that will cause peak broadening
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van Deemter Equation B term ‘Longitudinal diffusion’ B D MP D MP = diffusivity of mobile phase B term • Inversely proportional to flow rate (fast)
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This note was uploaded on 01/05/2012 for the course CHM 410 taught by Professor - during the Fall '11 term at University of Toronto- Toronto.

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L7 - Lecture 7 Separation Theory 2 Resolution Describes how...

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