1. ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 22 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2. CHAPTER 22 GAS AND LIQUID CHROMATOGRAPHY

3. GAS CHROMATOGRAPHY - Mobile phase is a gas - Column is open tubular - Sample is injected through a thin rubber disc (septum) - Sample is vaporized upon entering a heated glass port - A carrier gas (He, N 2, H 2 ) carries vaporized sample to a detector - Detector temperature is higher than column temperature to keep solutes in the gaseous state

4. GAS CHROMATOGRAPHY Liquid Sample Size 0.1 to 2 µL for analytical chromatography 20 to 1000 µL for perspective chromatography Gas Sample Size 0.5 to 10 mL (gas tight syringe should be used)

5. GAS CHROMATOGRAPHY Open Tubular Column - Usually made of fused silica (SiO 2 ) - Liquid or solid stationary phase is coated on the inner wall - Stationary phase may be porous carbon

6. GAS CHROMATOGRAPHY Open Tubular Column - Polar column is usually used for polar solutes - Nonpolar column is usually used for nonpolar solutes - Tailing is seen when stationary phase bakes and SiOH groups (silanol) forms on the silica surface

7. GAS CHROMATOGRAPHY Nonpolar Stationary Phase - Solutes are eluted in order of increasing boiling point - Solutes with higher vapor pressure are eluted faster Polar Stationary Phase - Solutes are separated based on polarity - Less polar solutes are eluted faster than strongly polar solutes

8. GAS CHROMATOGRAPHY Molecular Sieves - Included in the stationary phase - Has cavities and made of inorganic materials - Used to dry gaseous solutes - Strongly retains H 2 O - Separates other small molecules (CH 4, H 2, O 2, N 2, CO 2 )

9. GAS CHROMATOGRAPHY Guard Column - Collects nonvolatile solutes that are not eluted - Attached to the front of a chromatography column 5 to 10 meters long - Has no stationary phase and is silanized - Ends are cut off with time to discard nonvolatile solute buildup

10. GAS CHROMATOGRAPHY Comparing Open Tubular Column to Packed Column - Gives better separation - Narrower peaks - Handles smaller samples (analytical chromatography)

11. GAS CHROMATOGRAPHY Effect of Column Temperature Increase in column temperature - Increases solute vapor pressure - Decreases retention time - Results in sharp peaks Temperature Programming - Used to separate compounds with a wide range of boiling points and polarities

12. GAS CHROMATOGRAPHY Sample Injection - Sample is injected through a thin rubber disc (septum) heated glass port where it is vaporized

13. GAS CHROMATOGRAPHY Split Injection - Good for open tubular columns - Complete injection may be too much for an open tubular column - 0.1 to 10% of the injected sample reaches the column - Not good for quantitative analysis (higher boiling point components may not be vaporized)

14. GAS CHROMATOGRAPHY Splitless Injection - Suitable for quantitative analysis - Suitable for analysis of low concentrations of solutes (trace componenets) - Dilute solution with low-boiling solvent is used - About 80% of the injected sample reaches the column

15. GAS CHROMATOGRAPHY Solvent Trapping - Sample is initially injected at about 40 o below boiling point of sample - A thin band of solute is trapped - Column temperature is later raised

16. GAS CHROMATOGRAPHY Cold Trapping - Used for high-boiling solutes - Sample is initially injected at about 150 o below boiling point of solutes of interest - Solvent and low-boiling solutes are eluted - High-boiling solutes are trapped in a narrow band - Column temperature is later raised

17. GAS CHROMATOGRAPHY On-column Injection - Sample is injected directly into the column - Used for compounds that decompose at temperatures above their boiling points - Solvent trapping or cold trapping is employed to trap a narrow band of analyte - Column temperature is increased afterwards to initiate chromatography

18. GAS CHROMATOGRAPHY Detectors Mass Spectrometer - Very sensitive and the most versatile Flame Ionization - Low detection limits Thermal Conductivity - Not sensitive to narrow columns (diameter < 0.53 mm)

19. GAS CHROMATOGRAPHY Detectors Electron Capture - Very sensitive to halogen-containing compounds - Insensitive to ketones, alcohols, and HCs Sulfur Chemiluminescence Alkali Flame - Selectively sensitive to nitrogen and phosphorus - Used for analysis of drugs Flame Photometric

20. LIQUID CHROMATOGRAPHY Stationary phase - Usually silica (SiO 2 · xH 2 O) - Alumina (Al 2 O 3 · xH 2 O) - Adsorption of water slowly deactivates adsorption sites of silica - May be reactivated by heating to about 200 o C in an oven

21. LIQUID CHROMATOGRAPHY Eluent Strength (ε o ) - A measure of adsorption energy of solvent - ε o of pentane is 0 - More polar solvents have greater eluent strengths - Solutes elute more rapidly when eluent strength is greater

22. LIQUID CHROMATOGRAPHY Eluent Strength (ε o ) - Weakly retained solutes are first eluted with a low eluent strength solvent - Eluent strength is increased by adding a polar solvent to elute strongly retained solutes - Eluent strength is increased by making mobile phase more like the stationary phase

23. HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC) - Most common LC - Uses closed columns under high pressure - Resolution increases with decreasing particle size of stationary phase - Decreasing particle size decreases plate height - Small particle size reduces the terms A and C in the van Deemter equation

24. HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC) Stationary Phase - Microporous particles of silica - Octadecyl (C 18 )

25. HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC) Norman-Phase Chromatrography - Stationary phase is polar - Solvent is less polar - Eluent strength is increased by adding a more polar solvent

26. HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC) Reversed-Phase Chromatrography - More common - Stationary phase is weakly polar or nonpolar - Solvent is more polar - Eluent strength is increased by adding a less polar solvent - Insensitive to polar impurities and eliminates tailing

27. HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC) Column - Irreversible adsorption to the columns is very common - Disposable guard column containing the same stationary phase is attached to the column entrance

28. HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC) Isocratic Elution - Elution with a single solvent - Elution with a constant solvent mixture Gradient Elution - Solvent is changed continuously from weak to strong eluent strength - Used when the solvent does not efficiently elute all components

29. HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC) Detectors - Ultraviolet (most common) - Electrochemical - Refractive index - Evaporative light-scattering - Charged aerosol (most sensitive)