Gas Chromatography

Gas Chromatography

1. Theory

   • Analyte will be in equilibrium with gas
   • Inert MP
   • GSC→ Adsorption
   • GLC→ Solubility , Chambers & Gates
   • ⊥ on BP & polarity (out of order )

2. Sample

   • Volatile + thermal stable+ not ionic+Mwt 2-1000
   • HC/ Ald/ketones/ esters →Volatile no decomposition
   • FA→ methyl esters (BF3 methanol) , AA→ Fluromide ester
   • COOH, OH, NH, SH form intermolecular H Bond
     • silylation (all)
     • Acylation (N,OH, SH)→ acetic anhydride / fluro→ Halogen →ECD
     • Alkylation( acidic , phenols)→ester , diazemethane →toxic, BF3

3. Pros and cons

   • + accurate, sensitive, speed
   • volatile <400 , blood→clean , x identify

4. 1. Gas Supply System

   • Carrier gas
     • Inert , ⊥ on detector , pure , filters to remove impurities
     • Water dry so not to hydrolyze
     • pressure ⊥ on viscosity & column length
     • FID( H,HE, AR, N) Detector (H, air)
     • ECD(N) x for detector
     • TCD ( high conduc, low visc. ). H,HE detector
   • Gas in detector

5. 2. Sample Injection System

   • not too large (Vapor Plug)
   • Band brodening due to slow injection
   • Micro syringe to rubber septum
   • Heated metal block >50 than least volatile
   • all or part using split splitless mode (detector )

6. 3. Columns & thermostating system(oven fundamental)

   • a) Packed (2-3 m length, 2-4 mm, 30-50ml/min)
   • b) Capillary (open tubular) (15-100 m, fraction of mm, 1ml/min)
     • WCOT
       • walls with liquid SP
     • SCOT (less efficient )
       • thin layer of support onto which SP adsorbed
     • FSCOT(most fexible )
   • column thermostating tem⊥ on BP of sample

7. 4. Detectors

   • Properties
     • sensitive, stable, short , linear , temp >400
   • types
     • TCD (katharometer )
       • carrier gas H, HE
       • +universal , x detructive -- low senstivity
     • FID
       • sensitive + linear response wide -- destructive
       • Cathode =flame , current ∝ number of carbons
     • ECD
       • sensitive to electronegative species
       • linear range limited , not sensitive to HC, Amines , alcohols

8. Applications

   • 1. Pesticides (ECD)
   • 2) HSGC ( forensic analysis of blood, urine alcohol levels )
   • 4. drugs (AI, adulterants)
   • 3. food analysis ( most need derivatization)

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