Chapter 26: An Introduction Chromatographic Separations

Column ConceptsIn GC since mobile phase is under pressure & weoperate at various temperaturesgiven that P V is proportional to TSometimes use retention volumes (VR, VM)VR tR F for retained species tR retentiontimeVM tM F for unretainedtM retentiontimeF flow rateProblem ‐ pressure drop across a columnPressure at head of column may be 5 atm & atend of column may be 1 atmNeed a correction factor3[(Pi/P)2 ‐1]j �‐2[(Pi/P)3 ‐ 1]Where Pi inlet pressure &P outlet pressure (atmospheric)

Detectors – dozens of detectors availableCharacteristics of an ideal detector:1) Adequate sensitivity for desired analysis(typical 10‐8 to 10‐15 g analyte/sec)2) Stable – background constant with time3) Reproducible – good precision4) Linear response over several orders ofmagnitude5) Temperature range – room temp ‐ 400 oCCharacteristics of ideal detector: (continued)6) Rapid response time7) Independent of flow rate8) Reliable9) Easy to Use – inexperienced operators10) Either selective or universal response11) NondestructiveNo detector exhibits all these characteristics

Flame IonizationDetector (FID)‐ one of most widelyused GC detectors‐ good sensitivity toalmost all organiccompoundsFID Basics‐ column effluent mixed with air and burned inH2 flame producing ions & electrons thatconduct electricity‐ a few hundred volts applied between burnertip & a collector electrode above the flameproducing currents on the order of 10‐12 amps‐ amplify & measure‐ signal approximately proportional to numberof reduced carbon atoms in flame

FID Basics (continued)‐ mass sensitive rather than concentration‐ insensitive to non combustible gases – H2O,CO2, SO2, NOxFID exhibits‐ High sensitivity (as low as 10‐13 g/s)‐ Large linear response range (107)‐ Easy to use‐ Rugged‐ DESTRUCTIVEThermal ConductivityDetector (TCD)‐ One of earliest GCdetectors‐ Not popular today‐ Low sensitivity‐ Several designs‐ Use heated wire orsemiconductor‐ Resistance of wirechanges withanalyte vs carrier

TCD uses bridge circuit withSample & Reference CellsTCD‐ New TCDs use pulsed current to increasesensitivity & reduce drift‐ Thermal conductivity of He & H2 are about 6to 10 times greater than most organiccompounds (must use these carrier gases)‐ Other carrier gases (N2, Ar, etc) have thermalconductivities too close to organics

Advantages of TCD‐ Simple Æ Reliable & Easy to use‐ Universal response (organic & inorganic)‐ Large linear dynamic range 105‐ Nondestructive, can use in tandem‐ Older instruments have built‐in TCDDisadvantages‐ Low sensitivity‐ Often can’t use with capillary columnsbecause amount of analyte is smallECD

Electron Capture Detector‐ Sample passes over β emitter (radioactive) like63Ni foil or 3H adsorbed on Pt or Ti foil2‐ β particles (i.e. electrons) hit carrier gas(usually N2) causing a burst of e‐ to be released& measured by electrode standing current orconstant signal‐ When analyte molecule that absorbs e‐ passesthrough, current is reduced signal‐ Response is non‐linear unless pulsedECD Advantages‐ Responds well to molecules withelectronegative atoms like halogens (F, Cl, Br,I), peroxides, quinones, & nitro groups‐ Insensitive to amines, alcohols, hydrocarbons‐ Chlorinated pesticides are big application‐ Highly sensitive‐ Easy to use‐ Pretty reliable, although foil can get coated‐ Selective

ECD Disadvantages‐ Narrow linear range‐ Radioactive‐ Regular wipe test‐ Bake out contaminants‐ Some limits to applicability because highlyselectiveOther Conventional DetectorsThermionic Detector (TID)‐ Selective for N & P compounds‐ 500 x more sensitive than FID for P‐ 50 x more sensitive than FID for N‐ Bad for C‐ Design similar to FID with rubidium silicatebead at 180 V vs collector Æ get hot plasma600 ‐ 800 oC‐ Produces large number of ions with N & P

Flame Photometric Detector (FPD)‐ Selective for P & S compounds‐ Again sample goes through H2/air flame‐ Observe optical emission of HPO at 510 nm &526 nm & S2 at 394 nm‐ Use optical filters to isolate signal‐ Can also measure halogens, N, some metals(e.g. Cr, Ge, Se)Photoionization Detector (PID)‐ Column effluent irradiated with intense UVlight source‐ Ionizes molecules‐ Measure ions with electrodes in detector cell

Unconventional Detectors(Hyphenated Techniques)Atomic Emission Detector (AED)‐ Fairly new‐ Very powerful‐ Sample eluent introduced to He microwaveplasma atomizing all atoms in sample‐ Uses diode array detector measuring opticalemission over wide spectral range (170 ‐ 780nm)‐ Measure many elements simultaneouslyGC‐AED

GC‐AED‐ Potentially can measure70 or more elements‐ If look at C signal fromAED get chromatogramwith hundreds of peaks‐ If look at O signal getvery simplechromatogram with onlya few peaksGC – Mass Spectrometry (GC‐MS)‐ Already covered Mass Spec‐ Interfacing GC & MS normally difficult‐ GC at pressure above atmospheric while MSunder high vacuum‐ Need special interfaces for packed columns‐ Jet separator – discussed below‐ Membrane separator – a membrane sandwichbetween spiral channels, column efluent on oneside under pressure, MS on other side undervacuum – relies on differential permeability ofcarrier gas vs analyte molecules

GC‐MS SchematicInterface less critical for capillary columns

Jet Separator‐ Purpose is to get more analyte into MS thancarrier gas‐ Usually an all glass device‐ Principle is that heavier atoms have greatermomentum and travel a fairly straight pathinto the MS, lighter carrier gas molecules aredeflected outward by vacuum & pumpedawaySeveral types of Mass Specs available‐ Rarely magnetic sector or time of flight‐ Usually quadrapole or ion trap for GC‐MS‐ Less expensive‐ Less maintenance‐ Easy to use‐ Normally use electron multiplier as detector‐ All MS systems need ion source, either electronimpact or chemical ionization

Three modes of operation for GC‐MS1) Spectral mode – look at mass spectrumevery second or so during chromatogram ‐gives most information for research ormethod development2) Total ion current – sum signal for all ions asone large signal – highest sensitivity3) Selective ion monitoring (SIM) – look atcertain mass/charge ratios for compounds ofinterest – routine analysisGC‐MS‐ sensitive‐ can be very selective in SIM mode‐ powerful for qualitatively & quantitativelyThere is also one other kind of Mass SpecIon Cyclotron MS which is a very high resolution,Fourier transform instrument not used for GC

GC‐FTIR‐ Powerful technique for identifying compounds‐ Use heated light pipe 1 to 3 mm diaGC‐FTIR‐ Powerful technique for identifying compounds‐ Use heated light pipe 1 to 3 mm dia and 10 to 40cm long‐ Heat to prevent condensation of sample‐ Cool detector for sensitivity‐ Gives structural information from spectrum‐ Not very common

GC Columns & Stationary Phases‐ Historically used packed columns‐ Stationary phase coated as a thinfilm ona high surface area solid support‐ Theoretical studies showed that unpackedcolumns with narrow diameters were better‐ Open tubular columns first developed‐ Capillary columns came later because‐ Very fragile, difficult to construct, hard to connectto GCs, small samples hard to detect, difficult tocoat column walls, etc.Packed Columns‐ Tubing of metal, glass, Teflon, etc.‐ 2 to 3 m long and 2 to 4 mm in dia‐ Packed with diatomaceous earth (SiO2), clay,carbon particles, glass microbeads, polymer‐ Diameter 150‐250 µm (60‐100 mesh) 1 m2/g‐ Thin coating of liquid stationary phase

Stationary Phase CoatinggoodbadSilicaParticleSilicaParticleDon’t want stationary phase liquid coating to bleed orpuddle in column – gives zone broadening & poorresolutionOpen Tubular Columns Æ Capillary ColumnsColumn evolutionThree typesWall Coated Open Tubular (WCOT) – open glasstube with coating on wall – duhSupport Coated Open Tubular (SCOT) – opentube with particles of support material stuck tothe wallsFused Silica Open Tubular (FSOT) – WCOT madeof fused silica

Surface chemistry – glass & silicawith ‐OH at surfaceOH OH OH –O–Si–O–Si–O–Si–O‐are SiO2SilanolgroupSilicacoreOH is a problem because it can adsorb polarsubstances with strong affinity causing peaktailing – must deactivate by reacting

React Si‐OH groups with silaneSometimes still have –OH groupsIf silica not pure may have metal impurities M‐OHtypically use high purity silica – acid washSame chemistry to making specialty bonded phaseLiquid coatings on stationaryphase should exhibit:1) Chemical inertness2) Low volatility (b.p. 100 oC max temp)3) Thermal stability4) Good solvent characteristics (i.e. k’ & αsuitable)Many different liquid coatings have been usedor attempted for GC, only about 10 havewithstood the test of time

Retention time of a solute depends on K (partitioncoefficient) which is dependent on stationaryphase – must have different K’s for differentanalytesHowever, if K’s too large Æ long retention timeif K’s too small Æ short retention timeresulting in incomplete separationIn choosing a stationary phase use generalprinciples such as “like dissolves like”, polargroups interact with polar groups, non polar withnon polar, etc.

Polar groups include –CN, –CO, –OHPolar analytes include alcohols, acids, aminesNon polar Æ hydrocarbonsWhere analyte & stationary phase match is goodÆ elution order is determined by boiling pointsBonded Stationary PhasesUse silylation chemistry to covalently attachstationary phase to solid support or columnwall

Bonded Stationary PhasesAdvantages‐ monolayer coverage can be obtained‐ reduced bleeding of stationary phase‐ longer lasting‐ better stability‐ can be solvent washedChiral Stationary Phases – separating stereo‐isomers is the ultimate in chromatography,separate molecules that are mirror imagesPredicting retention (or identifying compoundsI) Selectivity Factorsby retention)If B is a standard compound & we know α, canthen be able to identify compound A even if wechange the the chromatographic conditionsor go to another chromatograph, etc.This is limited to specific applications where adatabase is available, not universally applicable

II) Retention Index (I)Proposed by Kovats in 1958Index based on normal alkanesIf have a mixture of 2 known alkanes & 1unknown compound & the 2 knownsbracket unknown in tR can then determineI for unknown & identify itI 100 x # of carbon atomsRegardless of column packing, temp. or otherconditionsKovats Retention IndexDoesn’t work as well for other typesof compounds (Hc), but useful insome cases e.g. homologous seriesPlot log adjusted retention time (tR’ tR – tM)vs number of carbon atoms is linearUseful in particular fields – petroleum industry,cosmetics, pharmaceuticals, etc. since havetheir own unique “standards”

Note number ofcarbons thatwould becalculated forthese 3compoundsbased on IHomework 27‐4 27‐5 27‐19

Chapter 28: High-Performance LiquidChromatography (HPLC) Scope Instrumentation – eluants, injectors, columns Modes of HPLC– Partition chromatography– Adsorption chromatography– Ion chromatography– Size exclusion chromatographyHPLC Most widely used separation technique Broad applicability – organic & inorganic Can be very sensitive, accurate & precise Suitable for separation of nonvolatile species Has found numerous uses in industry, clinicalsettings, environmental areas, pharmaceuticals,etc.

Instrumentation for HPLC: For reasonable analysis times, moderate flow raterequired but small particles (1-10 μm) Solvent forced through column 1000-5000 psi - moreelaborate instrument than GC Solvents degassed - "sparging“ High purity solventsSingle mobile phase composition - isocratic elutionProgrammed mobile phase composition - gradient elution

Gradient elutiondramaticallyimproves theefficiency ofseparation

Solvents (mobile phase) – are storedin special reservoirs connected to thepumping system – must be free ofparticles that can clog components & free ofbubble forming gases that get trapped incolumn or detectorThree basic ways to degas solvents1) vacuum or suction filter (0.4 or 0.2 µm)2) ultrasonicate (with vacuum)3) He purge (sparge units often built in)Can purchase HPLC solvents & water - stillHPLC pumping systems typically employ tworeciprocating or piston pumpsCheck valves& pump sealsneed to bereplacedPulse‐freeflow is neverreallyachieved Up to 10,000 psi, small internal volumes Produces pulsation

In GC the analyte affinity for the column isinfluenced by tempIn HPLC the solvent strength affects an analytesretention on columnTherefore, analogous to temp programming inGC, do solvent programming in HPLCThis is also referred to as gradient elutionHPLC sample injectors are exclusively 6 port valvesthat are overfilled by syringe giving extreme accuracy& precision – typical volumes are 10 to 50 µL but canbe larger

Rotary Injection ValveCommon for HPLC, rare in GCInjector for HPLC 6 port rotary valve Similar to FIA, GC Introduce small sample (0.1-100 μL) without depressurization Microsyringe/septum system (only 1500 psi)

Columns- usually stainless steel- can be PEEK (poly ether ether ketone)- may cost 200- 1000 packed- Length 10-30 cm, ID 4-10 mm- Packings are 3, 5, or 10 µm particle size- Most common 25 cm, 5 µ, 4.6 mm ID- N 40,000 to 60,000- Normally packed under 6000 psi pressure atfactory as a slurryGuard columns are normally used before theanalytical column to protect & increaselifetime of column – operator usually slurry ordry packs short guard column regularly withsame or similar packing used in analyticalcolumn (old column material) – can purchaseguard systems, cartridges, etc.

Detectors for HPLC- Ideal characteristics same as GC- Exception is temp range- Low dead volume 1 to 10 µLBulk property detectors - measure property of mobile phase(refractive index, dielectric constant, density)Solute property detectors - measure property of solute not presentin mobile phase (UV absorbance, fluorescence, IR absorbance)Most common detector is UV-vis absorbanceThree types1) Filter instrument – optical filters, Hg lamp2) Variable wavelength – monochromator3) Diode array detector- provide spectraMany HPLC detectors availableFor universal & selective detection

1) Filter based UV-vis detector –Typically set at254 nm using the most prominent band inHg spectrum – can also use 313, 365, 334 nmand other lines as well2) Variable wavelength detectors – usecontinuum source like (D2 or H2) & amonochromator, select any λ, less sensitive3) PDA - D2 or H2 source, disperse & focus ondiode array, get complete spectrum every 1sec, powerful, expensive, less sensitive, lotsof data generatedFrom columnCell for UV‐visdetectorfor HPLC‐ Low vol

sources: single line (arc or hollow cathode lamp, laser) continuum (Xe, D2 lamp) detector: photodiode/photomultiplier tube photodiode arrayCombination of separation and analysis (GC-MS, HPLC-UV-Vis) very powerfulDiode ArrayDetectorFluorescence detector – normally fixed wavelengthfilter fluorometer excitation filter & emissionfilter can be changed for particular λ of interestgives selectivity based on:- ability to exhibit fluorescence- excitation wavelength- emission wavelengthVariable λ monochromator based fluorescencedetectors also availableFilter based detectors usually more sensitive

Refractive index detector (RI) - responds tonearly all solutes but has poor sensitivity –detects changes in refractive index as samplepasses through as long as solute has differentRI than solvent – analogous to TCD in GCElectrochemical Detection Amperometric – fix potential & measurecurrent (i) Conductometric – measure conductivity Coulometric – fix potential & integrate i Voltammetric – vary potential & measure i Potentiometric – measure potentialCan use 2 or 3 electrode design with Pt or carbonelectrodes (glassy C or C paste)Electrochem. detector nearly universal

Other HPLC detectors LC-MS using thermospray – new popularity(pharmaceuticals) Evaporative light scattering - polymers LC-FTIR LC-plasma emission or ICP-MSModes of SeparationPartition Chromatography – most used form ofHPLC primarily for nonionic compounds of varyingpolarity with low MW ( 3000)Most common form is bonded phase chrom. using silicabased packing materials functionalized by silylation(as for GC)Partition Chromatography: Most popular method Low molecular weight (mw 3000) analytes Polar or non-polar Bonded stationary phase column (liquid chemically bonded to support particles)3, 5 or 10 μm hydrolyzed silica particles coated with siloxanesNormal phase HPLC nonpolar solvent/polar columnReversed phase HPLC polar solvent/nonpolar column

Early work with partition chrom. was done with polarstationary phases (like bare silica) & nonpolar solutes normal phase chromatog.Later bonded phases were introduced using C18 groupsÆ very non-polar with polar solvents reversed-phasechromatographyToday almost all partition chrom. done in reversed-phasemode with many different bonded phases (althoughC18 very popular)Normal- (polar column) versus Reversed Phase(nonpolar) elution:Reversed-phase HPLC most common (high polarity solvent, high polarity solutes elutefirst)

‐Si‐CH2‐(CH2)16‐CH318 carbon chainLong chain acts as if it were an alkane coated onsilica Æ analyte molecules partition into it,hence the nameIn chromatogram, most polar compounds elutefirst because they partition into C18 least – likedissolves like – most non‐polar compoundscome out last

Besides C18 can have C8, C4, C3, C2, C1 plusfunctionalities like cyano (-C2H4CN), amino (C2H4NH2), diol (-C3H6O-CH2-CHOHCH2OH)Each has different polarityCan also do Ion Pair Chromatography or Paired-IonChromatography – type of RP-HPLC used to separateionic speciesStill partition chrom. but use a reagent like a quaternaryammonium salt (C4H9)4N to pair with analyte ions toseparate by RPColumn Optimization in HPLC:Can optimize k' and αMore difficult than GC- in GC mobile phase just transported solute- in HPLC mobile phase interacts with soluteAnalyte Polarity:hydrocarbons ethers esters ketones aldehydes amines alcoholsStationary Phase Choice:Choose column with simila

Chapter 26: An Introduction to Chromatographic Separations Column Chromatography Migration Rates – Distribution Contstants – Retention Times – Selectivity Factor Zone Broadening & Column Efficiency Optimizing Performance Resolution Intro to Chromatography