gas chromatography · pdf file 1.packed column: columns are available in a packed manner s.p...

Gas Chromatography

Gas Chromatography Presented By -

Dr. Sohail Ayub Department of Civil Engineering AMU Aligarh India Email – sohailayub@rediffmail.com

What is Gas Chromatography?

• It is also known as… – Gas-Liquid Chromatography (GLC)

GAS CHROMATOGRAPHY

 Separation of gaseous & volatile substances  Simple & efficient in regard to separation

GC consists of GSC (gas solid chromatography) GLC (gas liquid chromatography

Gas → M.P Solid / Liquid → S.P GSC not used because of limited no. of S.P GSC principle is ADSORPTION GLC principle is PARTITION

Sample to be separated is converted into vapour And mixed with gaseous M.P Component more soluble in the S.P → travels slower Component less soluble in the S.P → travels faster Components are separated according to their

Partition Co-efficient Criteria for compounds to be analyzed by G.C 1.VOLATILITY: 2.THERMOSTABILITY:

What is Gas Chromatography?

• The father of modern gas chromatography is Nobel Prize winner John Porter Martin, who also developed the first liquid-gas chromatograph. (1950)

The Next Generation in Gas Chromatography

How a Gas Chromatography Machine Works

– First, a vaporized sample is injected onto the chromatographic column.

– Second, the sample moves through the column through the flow of inert gas.

– Third, the components are recorded as a sequence of peaks as they leave the column.

Chromatographic Separation

– Deals with both the stationary phase and

the mobile phase. • Mobile – inert gas used as carrier. • Stationary – liquid coated on a solid or a solid

within a column.

Chromatographic Separation • Chromatographic Separation

– In the mobile phase, components of the sample are uniquely drawn to the stationary phase and thus, enter this phase at different times.

– The parts of the sample are separated within the column.

– Compounds used at the stationary phase reach the detector at unique times and produce a series of peaks along a time sequence.

Chromatographic Separation (continued) – The peaks can then be read and analyzed by a

forensic scientist to determine the exact components of the mixture.

– Retention time is determined by each component reaching the detector at a characteristic time.

Chromatographic Analysis

– The number of components in a sample is determined by the number of peaks.

– The amount of a given component in a sample is determined by the area under the peaks.

– The identity of components can be determined by the given retention times.

Peaks and Data

PRACTICAL REQUIREMENTS

• Carrier gas • Flow regulators & Flow meters • Injection devices • Columns • Temperature control devices • Detectors • Recorders & Integrators

CARRIER GAS » Hydrogen better thermal conductivity disadvantage: it reacts with unsaturated

compounds & inflammable » Helium excellent thermal conductivity it is expensive » Nitrogen reduced sensitivity it is inexpensive

Requirements of a carrier gas

 Inertness Suitable for the detector High purity Easily available Cheap Should not cause the risk of fire Should give best column performance

Flow regulators & Flow meters  deliver the gas with uniform pressure/flow rate  flow meters:- Rota meter & Soap bubble

flow meter Rota meter placed before column inlet it has a glass tube with a float held on to a

spring. the level of the float is determined by the

flow rate of carrier gas

Soap Bubble Meter ◊ Similar to Rota meter & instead of a float,

soap bubble formed indicates the flow rate

Injection Devices

Gases can be introduced into the column by valve devices

 liquids can be injected through loop or septum devices

COLUMNS • Important part of GC • Made up of glass or stainless steel • Glass column- inert , highly fragile

COLUMNS can be classified  Depending on its use 1. Analytical column 1-1.5 meters length & 3-6 mm d.m 2. Preparative column 3-6 meters length, 6-9mm d.m

Depending on its nature 1.Packed column: columns are available in

a packed manner S.P for GLC: polyethylene glycol, esters,

amides, hydrocarbons, polysiloxanes… 2.Open tubular or Capillary column or

Golay column  Long capillary tubing 30-90 M in length Uniform & narrow d.m of 0.025 - 0.075 cm Made up of stainless steel & form of a coil Disadvantage: more sample cannot loaded

3.SCOT columns (Support coated open tubular column  Improved version of Golay / Capillary

columns, have small sample capacity  Made by depositing a micron size

porous layer of supporting material on the inner wall of the capillary column  Then coated with a thin film of liquid

phase

Columns

• Packed

• Capillary

Equilibration of the column  Before introduction of the sample  Column is attached to instrument &

desired flow rate by flow regulators  Set desired temp.  Conditioning is achieved by passing

carrier gas for 24 hours

Temperature Control Devices Preheaters: convert sample into its vapour

form, present along with injecting devices Thermostatically controlled oven: temperature maintenance in a column is

highly essential for efficient separation. Two types of operations Isothermal programming:- Linear programming:- this method is

efficient for separation of complex mixtures

Temperature Control

• Isothermal • Gradient

0

40

80

120

160

200

240

0 10 20 30 40 50 60

Time (min)

Te m

p (d

eg C

)

Instrumentation - Oven

DETECTORS Heart of the apparatus The requirements of an ideal detector are-  Applicability to wide range of samples  Rapidity  High sensitivity  Linearity  Response should be unaffected by

temperature, flow rate…  Non destructive  Simple & inexpensive

Measures the changes of thermal conductivity due to the sample (g). Sample can be recovered.

1.Thermal Conductivity Detector (Katharometer, Hot Wire Detector)

Thermal Conductivity Basics

When the carrier gas is contaminated by sample , the cooling effect of the gas changes. The difference in cooling is used to generate the detector signal.

The TCD is a nondestructive, concentration sensing detector. A heated filament is cooled by the flow of carrier gas.

Fl ow

Fl ow

When a separated compound elutes from the column , the thermal conductivity of the mixture of carrier gas and compound gas is lowered. The filament in the sample column becomes hotter than the control column.

The imbalance between control and sample filament temperature is measured by a simple gadget and a signal is recorded

Thermal Conductivity Detector

� Measures heat loss from a hot filament – � filament heated to const T • when only carrier gas flows heat loss to

metal block is constant, filament T remains constant.

• when an analyte species flows past the filament generally thermal conductivity goes

down, T of filament will rise. (resistance of the filament will rise).

Relative Thermal Conductivity

Compound Relative Thermal Conductivity Carbon Tetrachloride 0.05 Benzene 0.11 Hexane 0.12 Argon 0.12 Methanol 0.13 Nitrogen 0.17 Helium 1.00 Hydrogen 1.28

Advantages of Katharometer Linearity is good Applicable to most compounds Non destructive Simple & inexpensive Disadvantages  Low sensitivity Affected by fluctuations in temperature and

flow rate Biological samples cannot be analyzed

Flame Ionization Detector  Destructive detector  The effluent from the column is mixed with H

& air, and ignited.  Organic compounds burning in the flame

produce ions and electrons, which can conduct electricity through the flame.

 A large electrical potential is applied at the burner tip

 The ions collected on collector or electrode and were recorded on recorder due to electric current.

FIDs are mass sensitive rather than conc. sensitive

ADVANTAGES: • µg quantities of the solute can be

detected • Stable • Responds to most of the organic

compounds • Linearity is excellent • DA: destroy the sample