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 (-CN), 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
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 CN + 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
© D. Keith Roper 2006