Introduction to Gas Chromatography 31-1 Objectives To know what is chromatography To understand the mechanism of compound separation To know the basic of gas chromatography system 31-2
Chromatography Definition Chromatography, The Science of Separation A physical method of separating sample components from a mixture by selective adsorption or partitioning of the analyte between two phases: a mobile phase & a stationary phase 31-3 Chromatography Phases Mobile Phases: Liquids (methanol, water ) Changing dielectric strength Gases (helium, hydrogen ) Temperature Stationary Phases: Solids (alumina, silica, carbon ) Adsorption chromatography Liquids (siloxanes, polyethylene glycols ) Partition (distribution) chromatography Seminar Focus: GLC (Gas-Liquid Chromatography) 31-4
GC & HPLC Applicable Ranges GC HPLC 31-5 Classification of chromatographic methods Column Chromatography Gas Liquid Packed Open Tubuler GSC GLC WCOT SCOT PLOT 31-6
GLC vs. GSC Gas Liquid Chromatography (Packed column) Carrier Gas Liquid Phase Solid Support Very porous with very high surface area Gas Solid Chromatography (Packed column) Carrier Gas Absorbent packing Porous with large surface area 31-7 WCOT vs. SCOT vs. PLOT Wall Coated Open Tubular (WCOT) Carrier Gas Liquid Phase Support Coated Open Tubular (SCOT) Carrier Gas Liquid Phase Porous Layer Open Tubuler (PLOT) Carrier Gas Solid Phase 31-8
Types of GC Capillary Columns WCOT (wall coated open tubular) Partition chromatography Typical phases: Siloxanes and Carbowaxes 0.10 through 0.53mm internal diameters PLOT (porous layer open tubular) Adsorption chromatography Permanent gas and light hydrocarbon analysis Adsorbents: molecular sieve, porous polymers, alumina... 31-9 The history of Chromatography Term first applied by M.S.Tswett in 1903 Inventor of chromatography Separation of plant pigments on adsorbents L.S. Palmer in 1922 American who revived technique Separation of natural products Martin and Synge in 1941 Used silica gel packing Introduced partition chromatography Awarded the Nobel Prize in chemistry 1952 with Plate theory for chromatography 31-10
Separation process Garrier Gas 31-11 The Principal of Separation Intermolecular interactions between stationary phase and sample compound 31-12
Intermolecular Interactions on GC column Intermolecular Interaction Reactions Dispersive Interaction Dipole-Dipole Interaction Hydrogen Bonding 31-13 Basic Gas Chromatography The walls of a small diameter piece of tubing (column) of lengths from 1 to over 50 meters are coated with a high temperature liquid (usually a silicone oil). A low flow of pure carrier gas (Nitrogen, Helium, or Hydrogen) is passed through the column while the column is maintained at a constant temperature. One end of the column has a high temperature rubber cover that can be penetrated by a small syringe needle. The other end is connected to a detector (e.g. flame ionization detector). 31-14
Gas Chromatograph System Carrier Gas Purifier Inlet Detector Display Data System Pneumatic Controls Column Oven 31-15 Characteristics for Gas Chromatography Advantages Easy to handle Low maintenance cost Fast Analysis High Resolution Easy to hyphen with MS and other detecting tool Requires small samples, typically µl Highly accurate quantitative analysis, typical RSDs of 1-5% Limitations Not easy to recover the sample. Hard to analyze the heat labile sample. Hard to introduce the reproducible amount of sample Limited mobile phase (gas) 31-16
Compounds Amenable to Gas Chromatography Moderately thermally stable Vapor pressure in column (boiling point) allows for partitioning Routinely used below F.W. ~ 1000 amu Permanent gases through F.W. ~ 2000 amu for certain compound classes Unreactive/non-absorptive to chromatographic system 1 amu (atomic mass unit) = 1.660538 10-27 kilograms 31-17 Carrier Gas (Mobile phase for GC) Inert gases like, He, H 2, N 2, Ar Prefer low diffusible gases. Purity : referred to as five 9s or 99999 grade No air, water, hydrocarbons and etc. Adequate to GC detector 31-18
Carrier Gas for each GC detector Detector Carrier Gas Description Popular GC Detectors TCD He H2 Good Provide highest sensitivity N2 For the analysis of hydrogen FID N2 Good H2,He Not bad NPD He Best N2 Provide highest sensitivity ECD N2 Provide highest sensitivity Ar/CH4 Most wide dynamic range FPD N2 Good 31-19 Gas purifying system for GC A : Moisture trap B : Hydrocarbon trap C : Indicating Oxygen trap 31-20
Separation - The Ultimate Goal Many Factors Affect Separation The sample transfer or injection technique Column choice Partitioning Selectivity Resolution The detection method FID, ECD, GC/MS 31-21 Separation Step 1 Sample Transfer Injection how the sample is transferred to the column As a liquid via syringe Non-liquid techniques Purge & trap Headspace Gas sample loop NOTE: It is critical to get the sample into the column in a focused band 31-22
...because analytes Band Broaden in time and space Initial Final Bandwidth due to carrier gas flow profiles and increased diffusion when in the gas phase 0 meters 30 m Bandwidth (peak width) increases as an analyte travels along the column 31-23 Band Broadening Eddy diffusion Molecular diffusion Therefore, producing a narrow initial bandwidth (focusing) is critical to separation! 31-24
Separation Step 1 Sample Transfer How to get a focused initial band: Solvent Focusing Set oven temp. lower than boiling point of all analytes and solvent (approximately 20 C lower) Analyte Focusing If solvent boiling point is much lower than boiling point of firsteluting analyte, set oven temp. lower than boiling point of that analyte 31-25 Separation Step 2 Column choice Partitioning Oven temp. increased until effective vapor pressures are reached and compounds leave the column head Compounds flow with the carrier gas until they partition into the stationary phase Partitioning is a function of: Stationary phase type Column dimensions Oven temperature Carrier gas type and linear velocity 31-26
Separation Step 3 Column choice Selectivity CH 3 CH 3 CH 3 CH 3 O O O O Si Si Si Si CH 3 CH 3 CH 3 CH 3 e.g. DB-1 column 100% Dimethyl Polysiloxane 31-27 Separation Step 3 Column choice Resolution Not Efficient, not Selective Not Efficient, but Selective Efficient, but not Selective Efficient and Selective 31-28
Separation Step 4 Detection Flame ionization detector (FID) Materials that ionize in an air/h2 flame Nitrogen phosphorus detector (NPD) Organic compounds containing nitrogen & phosphorus Electron capture detector (ECD) Poly-halogenated compounds, organometallics, conjugated carbonyls etc. Flame photometric detector (FPD) Sulfur (or P) containing compounds. Etc. 31-29 Real Example Component list: 1. 2,4,5,6-tetrachloro-m-xylene 2. alpha-bhc 3. gamma-bhc 4. beta-bhc 5. delta-bhc 6. heptachlor 7. aldrin 8. heptachlor epoxide 9. gamma-chlordane 10. alpha-chlordane 11. 4,4'-DDE 12. endosulfan I 13. dieldrin 14. endrin 15. 4,4'-DDD 16. endosulfan II 17. 4,4'-DDT 18. endrin aldehyde 19. methoxychlor 20. endosulfan sulfate 21. endrin ketone 22. decachlorobiphenyl Run Conditions: 30m, 0.32mm ID, 0.50 µm DB-1 31-30
Summary History Fundamentals Factors for achieving a GC separation Terms: Selectivity, Partitioning, resolution Focusing and separation 31-31