INSTRUMENTAL ANALYSIS CHEM 4811

CHAPTER 12

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 12

GAS CHROMATOGRAPHY (GC)

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)

INSTRUMENTATION

- Injectors (syringes, autosamplers)

- The column

- Detectors

SAMPLE INJECTION

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

- Injection is commonly done with syringes

- Syringe needle must be very narrow

- Wide needle could cut out a plug of the septum (coring)

Autosamplers- Can analyze 100 or more liquid samples without manual injection

- Samples are put in autosampler vials which are capped with septa

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)

SAMPLE INJECTION

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

SAMPLE INJECTION

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

SAMPLE INJECTION

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

SAMPLE INJECTION

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

SAMPLE INJECTION

OPEN TUBULAR COLUMN

- Stationary phase is usually made of fused silica (SiO2)(silicone polymers)

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

- Stationary phase may be porous carbon

- Another type is porous layer open tubular (PLOT) for relatively small molecules

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)

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

OPEN TUBULAR COLUMN

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

OPEN TUBULAR COLUMN

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

OPEN TUBULAR COLUMN VS PACKED COLUMN

- Gives better separation

- Narrower peaks

- Handles smaller samples (analytical chromatography)

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

ELUTION AND RESOLUTION

Elution and resolution behavior depends on

- The composition of the stationary phase and mobile phase gas

- Column dimensions (length)

- Column phase ratio (internal diameter and film thickness)

- Column gas flow (pressure)

- Column temperature

DETECTORS

Mass Spectrometer (GC-MS)- Very sensitive and the most versatile

- Major problem is interfacing

Flame Ionization Detector (FID)- Destructive, mass-flow detector

- For almost all organic compounds (not for CO, CO2, HCN)- Low detection limits

Thermal Conductivity Detectors (TCD)- Nondestructive, concentration detector

- Not sensitive to narrow columns (diameter < 0.53 mm)

Electron Capture Detector (ECD)- Very sensitive to halogen-containing compounds

- Insensitive to ketones, alcohols, and HCs- Nondestructive, mass-flow detector

Electrolytic Conductivity Detector (ELCD)- For organic compounds with halogen, N, or S substituent

- Destructive, Mass-flow detector

Sulfur Chemiluminescence Detector (SCD)- For organic compounds with S atoms only

- Destructive, mass-flow detector

DETECTORS

Sulfur – Phosphorus Flame Photometric Detector (SP-FPD)- For organic compounds with S or P atoms

- Destructive, mass-flow detector

Nitrogen – Phosphorus Detector (NPD)- Selectively sensitive to nitrogen and phosphorus

- Used for analysis of drugs- Destructive, mass-flow detector

Photoionization Detector (PID)- For organic compounds with more easily ionizable π-electrons

- Nondestructive, mass-flow detector

DETECTORS

Helium Ionization Detector (HID)- Universal detector for everything except neon

- Nondestructive, mass-flow detector

Atomic Emission Detector (AED)- Selective for compounds containing many atoms

- Tunable- Destructive, mass-flow detector

DETECTORS

GC-MS

GC-IR

GC-GC or GC2 (2D-Gas Chromatography)

HYPHENATED TECHNIQUES

- Environmental pollution monitoring

- Analysis of contaminants

- Biochemical, medical, and pharmaceutical research

APPLICATIONS

- Complex molecules (DNA, RNA) are too large to be volatilized and may decompose

- HPLC is preferred for analyzing such molecules

- Not suitable for solutions in aqueous media

LIMITATIONS