Gas Chromatography Dr. Haibin Wan or Tel: (home) Gas Chromatography Columns

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1 Dr. Haibin Wan mail: or Tl: (hom) ost widly usd instrumnt for organic analysis. Suitabl for volatil or smi volatil analyts. Not suitabl for non volatil compounds. Dos not lik polar compounds. Inlt Dtctor Data systm Basd on th proprtis of th stationary phas, columns can b classifid as non polar column and polar column. Typical non polar stationary phas: dimthylsiloxan (.g. HP-1, DB-1,OV-1, CP-l5, SPB-1). Stationary phas of mid polarity:diphnyldimthylsiloxan (HP-5+, OV-17, DB-17, RTX-5). Typical polar stationary phas: polythyln glycol (HP-Innowax, DB-WAX, FFAP, Carbowax ). On a non polar column, th sparation is basd on boiling points of th analyts. On a polar column th sparation is basd on both boiling point and polarity. Carrir gas Column Ovn Control panl Dr. Wan Hai Bin 1 CH3 CH3 O O CH3 CH3 dimthylsiloxan m CH3 Ph O O [ CH2-CH2-O ] n m n CH3 Ph diphnyldimthylsiloxan Polythyln glycol Dr. Wan Hai Bin 3 Basd on dimnsion, can b classifid as packd column and opn tubular wall coatd column or capillary column). Packing for packd column Support Stationary phas Fusd silica polyimid Exampl 1. Bnzn (8 dgrs), hptan (98 dgrs), tolun (11 dgrs). On a non polar column, th rtntion tim will b: bnzn<hptan<tolun On a polar column, th ordr may chang to: hptan<bnzn<tolun Exampl 2. O-xyln (144 dgrs), m-xyln (139 dgrs), and p- xyln (138 dgrs). Th sparation on a non poalr column will b vry clos. A polar column is mor promising for th sparation. Slction of capillary column: 1) if th componnts hav diffrnt boiling points, us non polar column. If th componnts hav similar boiling point, start with a polar column. 2) If th idntitis of componnts ar unknown, start with a non polar column as it is also most stabl and of high tmpratur limit If th sparation is not satisfactory, thn try a polar column Dr. Wan Hai Bin 2 Dr. Wan Hai Bin 4

2 Inlt (injction port) Normal typs of inlts: Inlt for packd column; split/slitlss inlt for capillary columns Cool on column inlt fro capillary columns Inlt for larg volum injction A significant wak point in GC analysis ost dcomposition ractions and adsorption occur in this plac du to th facts: ost important inlt - split/splitlss inlt. Split mod 9-99% of th analyts gos to th vnt, only 1-1% of analyts ntr th column. Easy to us (+), giv narrow paks (+), thrmal dgradation is lss significant (+), suitabl for high concntration analyts. Splitlss mod All th injctd sampl ntrs th column. As th sampl stays in th inlt for longr tim, thrmal dcomposition is mor srious (-) and th paks bcom broadr (-). Paks nd to b focusd. th tmpratur is highr (- dgrs), mtal or glass wool may catalys th dcomposition; Coxtractivs from sampl can stay in th inlt and bcom a adsorbnt or catalyst. A good inlt must b inrt and allow smooth and unbiasd transfr of analyts to th column Dr. Wan Hai Bin 5 Dr. Wan Hai Bin 7 ost important inlt - split/splitlss inlt Split/splitlss inlt Carrir gas Column flow 1.5 ml/min Split vnt flow ml/min Split ratio :1 To gt a split ration of 5: 1, simply incras th split vnt flow to 75 ml/min. linr Purg valv (opn) rgulator Sptum Purg vnt Split vnt Focusing by solvnt ffct Us th thick film of solvnt to catch th analyt. or ffctiv to analyts of similar volatility as th solvnt. Th initial ovn tmpratur must b - dgrs lowr than th boiling point of th solvnt to form a thick film. Focusing by stationary ffct Us th stationary phas to rtard movmnt of analyts by using lowr initial ovn tmpratur. or ffctiv to analyts of boiling point highr than th solvnt. Dr. Wan Hai Bin 6 Dr. Wan Hai Bin 8

3 Dtctors Can b classifid as slctiv and non slctiv dtctors. As non slctiv dtctors will giv similar rspons to xtractivs from sampl matrix, it is difficult to s th analyts. Slctiv dtctors ar snsitiv only to crtain typ of compounds. Thy ar mor usful in trac analysis. Chromatogram from a FID Chromatogram from a slctiv dtctor Elctron captur dtctor - most snsitiv dtctor chanism: 1) 63 Ni gnrat high nrgy lctrons (bta particls); 2) thy collid with carrir gas molculs to gnrat mor lctrons; 3) th lctrons gnrat a currnt btwn th anod and th Ni surfac whn a voltag is applid; 4) analyts catch som of th lctrons and rduc th currnt; 5) ltromtr gnrat a pak signal. Dr. Wan Hai Bin 9 Dr. Wan Hai Bin 11 Commonly usd dtctors Typ of dtctor Suitabl analyts Snsitivity Rang Flam ionization (FID) Hydrocarbons 1-1 pg Thrmal conductivity (TCD) Univrsal 5-1 ng Elctron captur (ECD) Organohalogns.5-1 pg Nitrogn phosphorus (NPD) Organo N and P.1-1 pg Flam photomtric (FPD) Sulfur 1-1 pg Phosphorus 1-1 pg SD Univrsal/spcific 1-1 pg (scan).5-1 pg (SI) Nitrogn phosphorus dtctor chanism: 1) Activ lmnt (Rubidium salt) is hatd to gnrat thrmal ions; 2) Th nitrogn and phosphorus compounds ar convrtd into ions 3) Th ions mov to th collctor undr a voltag, gnrating a signal. Air inlt H2 inlt collctor Activ lmnt jt Dr. Wan Hai Bin 1 Dr. Wan Hai Bin 12

4 Flam photomtric dtctor chanism: 1) sampl burns in a hydrogn-rich flam; 2)som spcis ar rducd and xcitd; 3) Th gas flow movs th xcitd spcis to a coolr ara (mission zon) whr thy dcay and mit light; 4) Th wavlngth filtr allows light of a spcific wavlngth to ntr th PT 5) Photomultiplir tub convrts th light to lctric signal. Emission zon air H2 akup gas column Wavlngth filtr Flam photomtric dtctor Photomultiplir tub (PT) ass spctromtry dtctor (SD) --- ost vrsatil dtctor Working principl 1) Analyts from GC column ntr th ion sourc; 2) Elctrons gnratd by a hot filamnt collid with th molculs of th analyts; 3) olculs los lctrons to bcom cation radicals (molcular ion); som of th molcular ions bcom smallr ions by furthr fragmntation; 4) Th rpllr and othr focusing parts push th positivly chargd ions to th mass analyzr; 5) ass analyzr sparats th ions basd on thir mass-to-charg ratio; 6) Th ions rach th dtctor and gnrator signals Elctron impactd ionization Filamnt Rpllr + + Dr. Wan Hai Bin 13 Dr. Wan Hai Bin 15 ass spctromtry dtctor (SD) --- ost vrsatil dtctor ajor componnts GC-S intrfac (inlt) Lad th fflunt from th GC column to SD at prst tmpratur. Ion sourc Produc ions and focus thm into mass analyzr. ass analyzr Sparat ions according to thir mass/charg ratio. Dtctor dtct th ions and amplify th signal. Vacuum systm Rduc th chanc of collision btwn gas molculs and producd ions. Dtctor ass analyzr Ion sourc To vacuum systm A GC-S systm GC-S intrfac Rlativ Abundanc (%) Information providd by a SD: quantitativ and qualitativ (84) Bas pak ost abundant pak olcular ion (Parnt pak) Ion of th sam mass as th molcular ass/charg ratio (m/z) commonly rfrrd to as mass. Rlativ Abundanc (%) 1 + (128) Cyclohxan Naphthaln Dr. Wan Hai Bin 14 Dr. Wan Hai Bin 16

5 Slctd ion monitoring (SI) for trac analysis SI mod St th mass filtr to allow only crtain ions to rach th dtctor. It can incras th snsitivity by ovr 1 tims du to rducd nois and highr rspons. Scan mod Allow all th ions to rach th ion dtctor. Suitabl for qualitativ analysis or pr- scrning. Scan mod cyclohxan SI mod using m/z 84 Rlativ Abundanc (%) (84) Qualitativ confirmation Rtntion tim alon is not sufficint for positiv confirmation In cas of GC with a slctiv dtctor, anothr diffrnt column should b usd for th confirmation. If th dtctor is not slctiv (.g. a FID), a diffrnt analytical mthod should b usd (such as TLC). In cas of GC-SD, th mass spctrum should b charactristic nough (at last 3 ions). Dr. Wan Hai Bin 17 Dr. Wan Hai Bin 19 Chmical Ionization Advantags Rduc fragmntation to giv molcular wight information and incras snsitivity chanism Introduc ragnt gas (mthan or ammonia) to th ion sourc; ioniz th ragnt gas; ragnt gas ions ract with sampl molculs to gnrat molcul ions Disadvantags Rducd structur information or complicatd hardwar Charg xchang H + + H + + Elctron captur + - Acid/bas raction CH 5+ + CH 4 + H + CH 3+ + CH 4 + (-H) + OH - + H 2 O + (-H) - Summary 1. Split/splitlss inlt is most widly usd for capillary GC; 2. Sampls injctd into a GC using splitlss injction nd to b focusd to gt narrow paks; 3. Sparation of non polar column is basd on boiling point whil sparation of polar column is basd on both boiling point and polarity; 4. Slctiv dtctors ar mor suitabl for trac analysis; 5. Among th commonly usd slctiv dtctors ar ECD, NPD, FPD and SD; 6. Although SD is a vrsatil dtctor, th cost 1s 5-1 tims highr than othr dtctors; 7. If th numbr of sampl is larg, it is mor cost ffctiv to us an ECD, NPD, or FPD for pr scrning and us SD for confirmation Dr. Wan Hai Bin 18 Dr. Wan Hai Bin