GAS CHROMATOGRAPHY (GC)

1. GAS CHROMATOGRAPHY (GC)

2. GAS CHROMATOGRAPHY Sample is injected (using a syringe) into the injection port. Sample vaporizes and is forced into the column by the carrier gas ( = mobile phase which in GC is usually helium) Components of the sample mixture interact with the stationary phase so that different substances take different amounts of timeto elute from the column. The separated components pass through a detector. Electronic signals, collected over time, are sent to the GC software, and a chromatogram is generated.

3. GAS CHROMATOGRAPHY Compounds A and B interact with the stationary phase through intermolecular forces: (van der Waals or dipole-dipole forces, including hydrogen bonding). A interacts more strongly with the stationary liquid phase and is retained relative to B, which interacts weakly with the stationary phase. Thus B spends more time in the gas phase and advances more rapidly through the column and has a shorter retention time than A. Typically, components withsimilar polarity elute in order of volatility. Thus alkanes elute in order of increasing boiling points; lower boiling alkanes will have shorter retention times than higher boiling alkanes.

4. GC - Alkane Standards Pentane Isooctane Gas Chromatograph of alkane standard mixture containing equimolar amounts of: n-hexane, 2,3-dimethylbutane, 3-methyl-pentane, n-heptane, 3-methylhexane, n-octane, 3-methylheptane, 2,2,4-trimethyl-pentane (= isooctane), all dissolved in pentane.

5. GC of Alkane Standards vs. distillation fraction #1 Standards Distillation Fraction #1

6. GC – Peak Areas and Resolution

7. GC – Isothermal vs Temperature Programming

8. GC – Example Chromatograms

9. GC – Packed vs. Capillary Columns Alltech Chromatography catalog, 550

10. GC – Packed vs. Capillary Columns

11. GC – Stationary Phase Experimental Organic Chemistry D. R. Palleros, Wiley, NY, 2000

12. GC – Stationary Phase Alltech Chromatography catalog, 550

13. GC – Elution order vs Stationary Phase Alltech Chromatography catalog, 550

14. GC - Derivatization • Why is chemical derivatization needed? • GC is best for separation of volatile compounds which are thermally stable. • Not always applicable for compounds of high molecular weight or containing polar functional groups. These groups are difficult to analyze by GC either because they are not sufficiently volatile, tail badly, are too strongly attracted to the stationary phase, thermally unstable or even decomposed. • Chemical derivatization prior to analysis is generally done to: • increase the volatility and decrease the polarity of compounds; • reduce thermal degradation of samples by increasing their thermal stability; • increase detector response • improve separation and reduce tailing • Derivatizing ReagentsCommon derivatization methods can be classified into 4 groups depending on the type of reaction applied: • Silylation • Acylation • Alkylation • Esterification

15. GC - Derivatization • Derivatizing ReagentsCommon derivatization methods can be classified into 4 groups depending on the type of reaction applied: • Silylation • Acylation • Alkylation • Esterification

16. Alltech.com

17. GC – Resolution and Efficiency Skoog and Leary: Principals of Instrumental Analysis, 4th ed. Suanders, 1992

18. GC – Resolution vs Column Efficiency (N, H) H = L / N van Deemter Equation H = A + B/u +(Cs + Cm)u Skoog and Leary: Principals of Instrumental Analysis, 5th ed. Suanders, 1998

19. CHROMATOGRAPHY Preparative vs Resolution vs Speed vs Expense