INSTRUMENTAL ANALYSIS CHEM 4811 CHAPTER 12 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of...

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