General Chemistry II

General Chemistry IIFundamentals of Analytical SeparationsModule 8Analytical SeparationsMajority of cases complex mixtures are encountered and the analytical chemist must: separate identify and measure one or more components

Extraction: transfer of a solute from one phase to anotherIsolate or concentrate the desired analyte Separate it from species that would interfere in the analysis.Most common extraction of an aqueous solution with organic solventIn two phase system, one phase is usually water and one is usually an organic solvent

Solvent ExtractionPartition coefficient (K) is equilibrium constant for the reaction

Where As1 and As2 are the activities of the solution in phases 1 and 2, respectively and m is the total number of moles of S, q is the fraction of S remaining in phase 1 at equilibrium, V1 and V2 are the volumes of phases 1 and 2.

Solvent ExtractionFraction Not Extracted

pH effectsImportant if solute is an acid or base (e.g. B/BH+)Usually, neutral is more soluble in organic solutionCalculate a distribution coefficient

Where aB is the fraction of weak base in the neutral form in phase 1

Solvent ExtractionP539 for derivation

Extraction with a Metal ChelatorNeutral complexes are extracted into organic phaseCharged complexes typically stay in aqueous phase

Km and KL are partition coefficientsKa is the acid dissociation constant for HL, b is the metal:ligand equilibrium constant

Solvent Extraction

P541 for derivationChromatography

Chromatography is a PHYSICAL METHOD OF SEPARATION Components to be separated are DISTRIBUTED between TWO PHASESSTATIONARY PHASEMOBILE PHASE8ChromatographySeparation results from REPEATED SORPTION/DESORPTION acts during the movement of the sample components along the stationary bedA useful separation will only result when:KINETIC FACTORS controlling zone broadening are favorableDISTRIBUTION CONSTANTS of the sample components are different9Chromatography

Mobile Phase Selection11Mobile PhaseResponsible for transportMay participate in the distribution mechanismCritical properties are DENSITY, DIFFUSIVITY and VISCOSITY12DensityProvides an indication of the free space between molecules and the likelihood of intermolecular interactions(g / mL)Gas2 x 10-4Almost no interactionsFluid0.1 0.8Liquid1Significant interactions13DiffusivityProvides an indication of the speed with which particles move in a mediumDepends on the mass of the particle, temperature and viscosity(cm2 / s)Gas0.1 1FastFluid10-3 10-4Liquid10-5Slow14ViscosityA measure of the work required to achieve a useful mobile phase velocityDepends on temperature(cP)Gas0.02LowFluid0.02 0.1Liquid1High15Stationary Phase Selection16Types of ChromatographySolid stationary phaseLiquid or gaseous mobile phaseThe more strongly a solute is adsorbed, the slower it travels through the column.

Liquid stationary phase bonded to solid surface (e.g. silica chromatography column in GC)Solute equilibrates between stationary liquid and mobile phase (in GC this is a flowing gas)18

Anions or cations are attached to solid stationary phase, usually a resin.Solute ions of the opposite charge are attracted to the stationary phase by electrostatic force.Mobile phase is a liquid19

Also called gel filtration or gel permeations chromatography

Separates molecules by size

Larger solutes pass through quickly

No attractive action between solute and stationary phase20

Most selective type of chromatography

Employs specific interactions between one kind of solute molecule and a second molecule bound to stationary phase. Mobile phase is liquid.21http://en.wikipedia.org/wiki/Column_chromatographyhttp://en.wikipedia.org/wiki/HPLC

Chromatogram

Contains all the useful information from a chromatographic experiment A record of the detector response to the sample components as a function of the movement of the mobile phase

23Chromatogram

Link to fundamental properties of the sample or separation systemPeak positions provide information about THERMODYNAMIC PROPERTIESPeak widths provide information about KINETIC PROPERTIES 24The chromatogram is a graph showing the detector response as a function of elution time.

RetentionRetention =Column+ AdjustedTimeHold-up Retention Time TimetR=tM+tRtM = time sample spends in the mobile phase (same for all compounds)tR = time sample spends in the stationary phase (different for all separated compounds)26Column Hold-up TimeTime required by the mobile phase entering the column to reach the detectorIt is equivalent to the volume of streaming mobile phase contained in the columnIt should be corrected for extracolumn volumes27Retention Factor (Capacity Factor)

More fundamentally important than the absolute retention timeRepresents the ratio of the time spent by the solute in the stationary phase to the time it spends in the mobile phasek = tR, / tM = (tR tM) / tM

28Separation (selectivity) factorRelative retention of two adjacent peaks in a chromatogram = tR2, / tR1 , = k2 / k1Provides a measure of the selectivity of the chromatographic systemConstrained to have values >1

29Distribution constantDistribution = Retention X PhaseConstant Factor Ratio K = k = volume of mobile phase / volume (or surface area) of stationary phase30General Elution Problem

Sample components have a narrow range of distribution constantsPreferred separation approachIsothermal (constant temperature) in GCIsocratic (constant mobile phase composition) in LCIsopycnic (constant density) in SFC31General Elution Problem

Sample components have a wide range of distribution constantsFixed separation conditionsLong separation timesPoor separation of early eluting peaksPoor detectability of late eluting peaks due to band broadening

32General Elution Problem

Sample components have a wide range of distribution constantsPreferred separation modesGCTemperature FlowLCMobile phase composition FlowTemperature SFCDensityComposition Temperature

33Band BroadeningFor Gaussian peaks the PLATE NUMBER is given by N = (tR / st)2 st is the band variance

34Band Broadening

Peak width measurements are normally used for convenience N = a (tR / w) 2a = 4 w = wiwi = inflection pointa = 5.54w = wh wh = half height a = 16w = wb wb = at base35Band Broadening

The PLATE HEIGHT (H) is related to the plate number by

H = L / N

where L is the column length

36Rate TheoriesFactors Contributing to band broadening:Flow Anisotropy (Eddy Diffusion)Longitudinal DiffusionResistance to Mass TransferExtracolumn Sources37Flow AnisotropyResults from flow inequalities in a packed bedLocal velocities vary considerably within interparticle channels in pressure driven systems due to parabolic flowBecause of radial diffusion molecules experience many streamlines during their passage down the column acting to reduce flow anisotropy

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CHAPTER 22: Figure 22.20

Flow AnisotropyUncompensated flow inequalities result in a contribution to band broadening that would not exist for an ideal columnHE = 2dpUse particles of the smallest practical sizeUse particles with a narrow size distributionPack column homogenouslyDoes not contribute to band broadening in open-tubular columns40Flux , the number of moles of solute crossing each square meter per minute is proportional to the concentration gradient.

Diffusion coefficient (D) measures the rate at which a substance moves randomly from a region of high concentration to one of lower concentration.

Longitudinal DiffusionResults from the tendency of a solute band to diffuse away from regions of high concentration to regions of lower concentrationHL= 2DM / uAlways important for GasesMay be negligible for liquidsHigh values of u minimize HLbut adversely affect resistance to mass transfer42Resistance to Mass TransferResults from the limitations of diffusion in the mobile and stationary phases as a transport mechanism to move analyte molecules to the boundary region between phases

43Resistance to Mass TransferCHAPTER 22: Figure 22.18

Resistance to Mass TransferImportant experimental variablesRetention factorStationary phase film thickness (GC)Diffusion coefficients in the mobile and stationary phasesMobile phase velocityColumn internal diameter (GC)Particle size (LC)45Mobile Phase VelocityThe mobile phase linear velocity is more important than flow rate for fundamental studiesFor compressible mobile phases the local velocity at any position in the column depends on:Flow resistance of the columnRatio of the column inlet to outlet pressure46Van Deemter EquationH= A + B/u + CuA = flow anisotropyB = longitudinal diffusionC = resistance to mass transferMinimum value for the plate height exists at an optimum velocity

47Resolution

48ResolutionRS = 1 corresponds to 94% peak separationRS = 1.5 corresponds to baseline separation

49ResolutionRS = 1 corresponds to 94% peak separationRS = 1.5 corresponds to baseline separation

50Resolution And Column PropertiesResolution is proportional to LSeparation time is proportional to LN increases with decreasing column diameter for GCN increases with decreasing particle size for LC

51Resolution And Column Properties

52Resolution And Column Properties

53Resolution And Column Properties

54Resolution And Column Properties

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