ANALYTICAL CHEMISTRY CHEM 3811

CHAPTER 22

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 22

GAS AND LIQUIDCHROMATOGRAPHY

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, N2, H2) carries vaporized sample to a detector

- Detector temperature is higher than column temperatureto keep solutes in the gaseous state

GAS CHROMATOGRAPHY

Liquid Sample Size0.1 to 2 µL for analytical chromatography

20 to 1000 µL for perspective chromatography

Gas Sample Size0.5 to 10 mL (gas tight syringe should be used)

GAS CHROMATOGRAPHY

Open Tubular Column

- Usually made of fused silica (SiO2)

- Liquid or solid stationary phase is coated on the inner wall

- Stationary phase may be porous carbon

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

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

GAS CHROMATOGRAPHY

Molecular Sieves

- Included in the stationary phase

- Has cavities and made of inorganic materials

- Used to dry gaseous solutes

- Strongly retains H2O

- Separates other small molecules (CH4, H2, O2, N2, CO2)

GAS CHROMATOGRAPHY

Guard Column

- Collects nonvolatile solutes that are not eluted

- Attached to the front of a chromatography column5 to 10 meters long

- Has no stationary phase and is silanized

- Ends are cut off with time to discard nonvolatile solute buildup

GAS CHROMATOGRAPHY

Comparing Open Tubular Column to Packed Column

- Gives better separation

- Narrower peaks

- Handles smaller samples (analytical chromatography)

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

GAS CHROMATOGRAPHY

Sample Injection

- Sample is injected through a thin rubber disc (septum)heated glass port where it is vaporized

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)

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

GAS CHROMATOGRAPHY

Solvent Trapping

- Sample is initially injected at about 40o below boiling point of sample

- A thin band of solute is trapped

- Column temperature is later raised

GAS CHROMATOGRAPHY

Cold Trapping

- Used for high-boiling solutes

- Sample is initially injected at about 150o 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

GAS CHROMATOGRAPHY

On-column Injection

- Sample is injected directly into the column

- Used for compounds that decompose at temperaturesabove 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

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)

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

LIQUID CHROMATOGRAPHY

Stationary phase

- Usually silica (SiO2 · xH2O)- Alumina (Al2O3 · xH2O)

- Adsorption of water slowly deactivates adsorption sites of silica

- May be reactivated by heating to about 200oC in an oven

LIQUID CHROMATOGRAPHY

Eluent Strength (εo)

- A measure of adsorption energy of solvent

- εo of pentane is 0

- More polar solvents have greater eluent strengths

- Solutes elute more rapidly when eluent strength is greater

LIQUID CHROMATOGRAPHY

Eluent Strength (εo)

- Weakly retained solutes are first eluted with alow 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

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

HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)

Stationary Phase

- Microporous particles of silica

- Octadecyl (C18)

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

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

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

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

HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)

Detectors

- Ultraviolet (most common)

- Electrochemical

- Refractive index

- Evaporative light-scattering

- Charged aerosol (most sensitive)