Chem. 133 – 5/7 Lecture

Announcements I Exam 3 on Tuesday (will give summary of material to know later) Format will be similar to other exams I will allow use of a 3” x 5” notecard Covers Ch. 20 (Emission Instrumentation and interferences), NMR, Ch. 21, Ch. 22 (just through retention and relative retention) Last Homework (+bonus) and Quiz (replacement) today

Announcements II Final Exam: Thurs. May 21 (12:45 – 2:45) Will be ~ 50% multiple choice You will be allowed x 11” sheet with notes on it Besides Exam 1 to 3 material, will also most recent chromatography topics Term Project Should finish up by next Tuesday with check out on next Thursday

Announcements III Term Project – cont. I will give you your poster reviewing assignments on Thursday Poster Presentation on Friday of Finals Week (5/22) Today’s Lecture Review for Exam 3 Chromatography Band Broadening (last topic on Exam 3) Next Thursday, we will complete Ch. 22 (resolution and optimization I expect Ch. 22 will be last chapter covered for final exam

Exam 3 Topics (cont.) A.Chapter 20 (Atomic Spectroscopy) 1.Methods for Elemental Analysis (solid + liquid samples) 2.Basic theory of atomic transitions 3.Types of interferences and means of dealing with them 4.How to use Standard Addition to calculate concentrations* 5.Block diagrams of AAS, AES, and ICP-MS instruments including specific components used

Exam 3 Topics B. NMR Topics (Rubinson and Rubinson Ch. 11) 1.How to relate resonance frequency to magnetic field strength* 2.Types of nuclei that are NMR active and number of and spins of states based on I value of nucleus* 3.Origin of sensitivity problem and factors that affect senstivity in NMR* 4.Causes and effects of relaxation (spin-lattice and spin-spin) 5.Causes of deshielding 6.Relative position of protons in spectra due to amount of deshielding 7.Magnetic anisotropy (know electron circulation in aromatics)

Exam 3 Topics B. NMR Topics (cont.) 8.Relationship between ppm and Hz scales* 9.Source of splitting and predicting splitting patterns from 1 set of neighboring equivalent nuclei 10.General Proton Interpretation (# equivalent nuclei, relative # of each, relative position, # neighboring nuclei) 11.Understand magnetic field requirements 12.Understand main spectrometer components in NMR instruments (e.g. light source)

Exam 3 Topics C.Mass Spectrometry Topics 1.Main Instrument Components 2.Methods of ionization in mass spectrometry: phase used for, ions produced, and amounts of fragmentation 3.Methods to separate ions and practical uses 4.Resolution* and Resolution Needs (nominal vs. high resolution) 5.Isotope effect calculations* 6.Multiple charging calculations to determine M and z* 7.MS-MS use

Exam 3 Topics D.Chromatography 1.Liquid – liquid partitioning equation* 2.How reactions in water (e.g. acid/base) affect distribution between two phases 3.Partitioning Equation in Chromatography* 4.Qualitative understanding of effect of partition coefficient on retention on a chromatographic column 5.Determination* and meaning of retention factor 6.Determination* and meaning of relative retention 7.Practical ways to change retention in GC and HPLC 8.What factors affect relative retention

Exam 3 Topics D.Chromatography – Cont. 9.Relate partition coefficient and capacity factor (including calculation of volumes)* 10.Be able to calculate N and H from chromatograms* 11.Main components of chromatographs 12.Understand meaning of N and H

Chromatography Column Efficiency – The Bad Original theory developed from number of simple separation steps (e.g. from fractional distillation columns) N = number of theoretical plates (or now plate number) = best absolute measure N = 16(t r /w) 2 or = 5.55(t r /w 1/2 ) 2 –w = peak width at baseline –w 1/2 = peak width at half height

Chromatography Shape of Chromatographic Peak Gaussian Distribution Normal Distribution Area = 1 Widths –σ (std deviation) –w = 4σ –w 1/2 = 2.35σ –w’ = Area/y max = 2.51σ (commonly given by integrators) Gaussian Shape (Supposedly) 2σ2σ Inflection lines w Height Half Height w 1/2

Chromatography Column Efficiency Good efficiency means: –Large N value –Late eluting peaks still have narrow peak widths Relative measure of efficiency = H = Plate height = L/N where L = column length H = length of column needed to get a plate number of 1 Smaller H means greater efficiency Note: H is independent of L, N depends on L large N Value low N value

Chromatography Measurement of Efficiency Measuring N and H is valid under isocratic conditions Later eluting peaks normally used to avoid effects from extra-column broadening Example: N = 16(14.6/0.9) 2 = 4200 (vs. ~3000 for pk 3) H = L/N = 250 mm/4200 = 0.06 mm W ~ 0.9 min

Chromatography Causes of Band Broadening There are three major causes of band broadening (according to theory) These depend on the linear velocity (u = L/t m ) Given by van Deemter Equation: –where H = Plate Height, and A, B, and C are “ constants ”

Chromatography Band Broadening u H Most efficient velocity A term B term C term

Chromatography Band Broadening “ Constant ” Terms –A term: This is due to “ eddy diffusion ” or multiple paths –Independent of u –Smaller A term for: a) small particles, or b) no particles (best) X X X dispersion

Chromatography Band Broadening B Term – Molecular Diffusion –Molecular diffusion is caused by random motions of molecules –Larger for smaller molecules –Much larger for gases –Dispersion increases with time spent in mobile phase –Slower flow means more time in mobile phase X X X Band broadening

Chromatography Band Broadening C term – Mass transfer to and within the stationary phase –Analyte molecules in stationary phase are not moving and get left behind –The greater u, the more dispersion occurs –Less dispersion for smaller particles and thinner films of stationary phase X X dispersion Column particle

Chromatography Some Questions 1.Column A is 100 mm long with H = mm. Column B is 250 mm long with H = mm. Which column will give more efficient separations (under conditions for determining H)? 2.Which van Deemter term is negligible in open tubular GC? 3.How can columns in HPLC be designed to decrease H? In open tubular GC? 4.Why is it usually more difficult to improve the relative retention (  ) when there are a larger number of analytes/contaminants? 5.Both using a longer column or using a column of smaller H will improve resolutions? Which method will generally lead to a better chromatogram? Why?