Lecture 33. Chromatography Pt2

Lecture 33. Chromatography Pt2 - Explain why chromatograms...

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Unformatted text preview: Explain why chromatograms 2 to 5 are not symmetrical! Time • Tailing • Leading/Fronting Peak Shape Peak Ideal: • Sample distribution between stationary and mobile phase does not depend on concentration. depend • Symmetrical peak (Gaussian shape). Symmetrical • Retention time does not depend on sample concentration. Retention Normal: • Sample concentration high; stationary phase saturated. Sample • What is the expected peak shape? What • How does the retention time depend on the sample concentration? How Poor Chromatogram: • Sample concentration high; sample condenses in stationary phase. Sample • What is the expected peak shape? What • How does the retention time depend on the sample concentration? How In real life there are no real physically distinct sections (plates) separated In from one another. However, the analogy is helpful. from Analogy Mobile Phase Mobile Stationary Phase Mobile Phase Real Life Types of Types Chromatography Xm Xs Distribution Coefficient: [X] s Kc = [ X] m Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Chromatograms Chromatograms tM = Elution time of an unretained solute. tR = Retention time tR’ = Adjusted retention time wb = Peak width at baseline w1/2 = Peak width at half height Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. 2004. Theoretical Plates Theoretical # of theoretical plates: æ t r ö2 5.55t 2 r N = 16 ç ÷ = 2 w1/2 èw b ø H= L N plate height: L: length of column Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Theoretical Plates Theoretical Effect of theoretical Effect plates on the analysis of a mixture: of Theoretical Plates Theoretical Typical number of Typical theoretical plates per m: per • TLC (thin layer chromatography) 1500 chromatography) • HPLC >100,000 HPLC • GC (capillary column) 4000 4000 Peak Broadening Peak Bands get wider over time Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Eddy Diffusion (A) Eddy Independent of flow rate (u) Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Eddy Diffusion (A) Eddy How should the ideal particles How in a packed column look like? in Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Liquid Phase Diffusion from a Plane Source Concentration vs Distance D = 1e-5 cm^2/s 1000 t = 0. 01s t = 0. 03s 500 Concentration 0 0. 000 Di sta nce (cm ) t = 0. 1s t = 0. 3s t = 1s -0. 010 -0. 005 0. 005 0. 010 Longitudinal Diffusion (B) Longitudinal 1 broadening ∝ u Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Mass Transfer (C) Mass Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Mass Transfer (C) Mass Diffusion Coefficients Diffusion H2 in N2 (105 Pa, 200 K) H2 in N2 (105 Pa, 400 K) O2 in H2O (105 Pa, 20 ºC) 0.01 M NaCl in H2O (25 ºC) 0.01 Phenol in rubber (20 ºC) Valinomycin in ISE membrane (20 ºC) Li+ in glass (electrode glass, 25 ºC) 0.4 cm2 s-1 1.27 cm2 s-1 1 1.8 x 10-5 cm2 s--1 1.55 x 10-5 cm2 s-1 1 x 10-8 cm2 s-1 1.8 x 10-8 cm2 s-1 5 x 10-20 cm2 s-1 Diffusion Coefficients Diffusion H2 in N2 (105 Pa, 200 K) O2 in H2O (105 Pa, 20 ºC) Phenol in rubber (20 ºC) Phenol 0.4 cm2 s-1 1 1.8 x 10-5 cm2 s--1 1 x 10-8 cm2 s-1 Compare two GC columns with the same thickness of the Compare stationary phase but diameter differing by 20%. Do you expect any significant differences in mass transfer? any Diffusion Coefficients Diffusion H2 in N2 (105 Pa, 200 K) O2 in H2O (105 Pa, 20 ºC) Phenol in rubber (20 ºC) Phenol 0.4 cm2 s-1 1 1.8 x 10-5 cm2 s--1 1 x 10-8 cm2 s-1 Compare two GC columns with a thin and a thick stationary Compare phase but the same diameter: phase •What is the advantage of the thin layer? •What is the advantage of the thick layer? Mass Transfer (C) Mass broadening ∝ u Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Van Deemter Equation Van Optimizing flow rate Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. 2004. Van Deemter Equation Van Optimizing flow rate Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Packed Columns: Packed Reducing Particle Size Increases N Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. 2004. ...
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This note was uploaded on 04/06/2011 for the course CHEM 2011 taught by Professor Buhlman during the Spring '10 term at Minnesota.

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