CH EN 5503: Instrumental Analysis of Process Products

CH EN 5503: Instrumental Analysis of Process Products 9.2 Chromatographic Separation Methods

Theoretical Plate model tR: retention time w = 4: Gaussian peakwidth at 4 ; b0.5 = 2.354 :Gaussian peakwidth at ½ height ( 2 2 N = 16(tR/w) = 8 ln 2(tR/b0.5) : number of theoretical stages R = 2(tR - tR)/(w1+w2): resolution H=L/N = A + B/u + C*u : van Deemter equation for H vs. u (velocity) A: eddy diffusion = f(particle size, packing) B: longitudinal diffusion C: mass transfer resistance to equilibrium between stationary & fluid phase n H  dp , n=2 (dp>80 microns)… 1 (dp< 3 microns) Chromatography Adsorption Isotherms Linear: ideal Gaussian peaks Nonlinear: concave down  peak tailing x: adsorbate amount m: adsorbent amount Freundlich: x/m = ac1/n: Langmuir: x/m = a*c/(c+b): finite monolayer adsorption BET: Langmuir  concave upward: multilayer adsorption Elution Isocratic: constant eluent conditions to develop separation Step change in eluent conditions to stronger eluting solvent (ion exchange) Gradient: gradual change in eluent to stronger eluting solvent: reduced separation time Displacement: Displacing agent A adsorbs more strongly than B: overloaded conditions Stationary phases Normal phase: hydrophilic silica stationary phase (surface OH groups – silanol Si-OH) hydrophobic solvent Reversed phase: dichlorosilanes (H2SiCl2) react with SiOH to bond HC chains alkylchlorosilanes(RSiCl3) also used Hydrophobic stationary phase, hydrophilic solvent Mechanisms: partitioning + adsorption Elution series with decreasing retention: Aliphatic Induced & permanent dipoles (chlorinated HC) Lewis bases (ethers, aldehydes, ketones, amines) Lewis acids (alcohols, phenols, carboxylic acids) Chemically bonded phases: less trace water interference than normal phase Terminal cyano (-CN), phenyl, or amine groups Eluents: THF, heptane Ion-pair: inorganic anion pairs w/quaternary ammonium salt dissolved in stationary phase Ion-exchange: - sulfonic acid (-SO3 ) cation exchangers quaternary ammonium (–NR3+) anion exchangers

© D. Keith Roper 2006 CH EN 5503: Instrumental Analysis of Process Products Ion-exclusion: excludes inorganic ions, allows carbonic acids to pass Gel-permeation: molecular size sieving Affinity (biological: Ab-Ag; enzyme-substrate)

Liquid Chromatography LC: Dcolumn > 1 cm; 100 2-6 mm; 33-6 noise from gas, pulsations, drift, temperature Fraction collector

Detector types: Refractive Index Temperature, flowrate sensitive Low SNR Photodiode Array Multiple wavelengths High SNR Amperometric Current intensity at constant potential Ion chromatography Suppressor column neutralizes eluent signal from Na2CO3 & NaHCO3 Suppressor: R-H or H2SO4 Chemical reaction Hexacyanoferrate (III) oxidation of vitamin B1 (thymine) for fluorescence detection

© D. Keith Roper 2006 CH EN 5503: Instrumental Analysis of Process Products Gas Chromatography Equipment Column Quartz/glass tube (ID=3-8mm; L=1-3 m) Qcarrier gas = 30-40 mL/min Nt ~ 1,000 Capillary (ID=0.2-1 mm; L~ m) Support-coated open tubular column (SCOT): adsorption Wall-coated open tubular column (WCOT): thin film of 1-3 microns on ID Nt ~ 100,000 Qcarrier gas = 0.2-2 mL/min

Vinjection = 0.1-1 L Split injection: Choke in parallel with column reduces Vinjection Packing di-/tri-methylsilyl inactivation of silanol groups Detectors flame ionization detector (FID): C-C or C-H containing compounds air-H2 flame burns eluents to from CHO+ ions between anode & cathode current recorded as signal heat conductivity detector (HCD/TCD) resistive Pt or W (tungsten) element in measuring cell Wheatstone bridge circuit detects sample thermal conductivity < H2 carrier gas Thermionic detector (TID): phophorus- and nitrogen-containing compounds Glass bead containing rubidium form low-T plasma on Pt wire Phosphorus & Nitrogen-containing substances pyrolize, form radicals CN + Rb  CN- + Rb+ Electrode detection of ions electron capture detector (ECD): halogen, sulfur, heavy-metal compounds He carrier gas is ionized, releasing electrons Eleuent may absorb electrons, decreasing measurement by pulsed D.C. voltage Temperature elution Isotherm

b0.5  tR

log tS  carbon number temperature programmed (gradient) b0.5 constant

tS and TR  carbon number Effluent analysis Drift: carrier flow changes Temperature gradient increases column bleeding Ghost peaks Septum degradation products Carrier gas impurities Peak asymmetry Symmetry factor = (tR – b0.05,lead) / b0.05 Asymmetric: overloaded Fronted: Temperature too low Tailed polar substance: insufficient inactivation of carrier