Transcript phg 431 (5th lecture..

Gas Chromatography
GC
Introduction
 GC is a sensitive & rapid technique for analysis of volatile substances at a
temperature sufficient to keep them in the vapor state.
 Volatile mixtures are carried by a mobile phase consisting of an inert gas
(carrier gas) & pass through a stationary phase (solid or liquid) kept in a
column.
 The components migrate at different rates between the two phases (mobile
& stationary) according to their distribution coefficients.
 GC can be performed over a wide range of temperatures extending from -
196° to +500°C.
GAS LIQUID CHROMATOGRAPHY (GLC)
It consists of:
1- A pressurized tank to supply the carrier gas
2- A heated chamber (injector) into which is introduced the
sample.
3- The column to which the sample components are transferred by
the carrier gas
4- A detector fixed at the end of the column to convert changes in the effluent
into electric signals which are sent to a recorder.
5- A thermo stated oven used to heat the column, injector, & detector.
Mobile Phase - Carrier Gas
Helium, nitrogen, hydrogen, & argon are used. At ordinary pressures
& temperatures, these gases are considered as chemically inert
They are relatively inexpensive, readily available, & safe (except the
flammability of hydrogen).
The distribution coefficient of a component is determined
entirely by its volatility from the stationary phase.
Selection of the carrier gas
It should be of high grade of purity
It should be selected according to the type of detector (e.g. thermal
conductivity detectors work best with hydrogen or helium).
Liquid Phase
The versatility of GLC is in large part due to the wide variety of
liquid phases available.
Requirements of a good liquid phase
It should be:
Non-volatile.
Thermally stable.
Yielding appropriate K values (distribution coefficients) for the analyzed
components
Inert toward the solutes.
N.B.
No single liquid meets all requirements for all possible solutes.
Common Detectors
Hydrogen Flame Detector.
Flame Ionization Detector (FID): non-selective
Thermal conductivity detector (TCD): non- selective but not
used if the carrier gas is Nitrogen.
Electron Capture Detector (ECD): specific for halogenated
compounds as pesticides
Qualitative Analysis: (Retention parameters)
Retention Index (Kovats index, I)
In this system the relative retention is referred to the series of
normal paraffin hydrocarbons as standard substances.
Advantages:
A series of well-defined, readily available standards having a wide boiling
range is used.
The Kovats indices values (I) are relatively insensitive to temperature.
Temperature effects
There are three important parts of the chromatograph in which the
temperature must be controlled.
InjectorTemperature
The injector is kept at a relatively high temperature about 50oC above the
component boiling points to ensure the fastest rate of vaporization of the sample.
Detector Temperature
An optimum temperature is selected just above the column temperature in
order to prevent condensation of the constituents in the detector
ColumnTemperature
This is an important factor in determining retention & resolution.
Isothermal temperature
Programmed Temperature
GAS-SOLID CHROMATOGRAPHY (GSC)
GSC was used many years before gas-liquid GLC e.g. charcoal
filters to purify air.
GSC differs from GLC in that the partition within the column
is caused by partial & selective adsorption on a solid
surface rather than solubility in a liquid phase.
Common adsorbents in GSC
 Carbon
 Molecular Sieves
 Porous Polymers
 Inorganic Salts
Applications of GC
GC is used for analysis of:
Any compound sufficiently stable when heated to a
temperature at which it is volatile.
All non-volatile substances that may be converted to
volatile ones by chemical or thermal reactions.
Derivatization of non-volatile substances
(Conversion to volatile ones)
This is carried by: silylation, esterification (acids),
acylation (alcohols, sterols & sugars), ether formation
(sugars) & pyrolysis (polymers & complex compounds).
Transformation may be carried out
column.
pre-column or on
Silylation
The trimethylsilyl derivatives have allowed substances having -OH, -SH,
COOH, -NH2, & -NH functional groups to be easily gas
chromatographed.
Example: silylation of carbohydrates using a mixture of hexamethyl
disilazane (HMDS) & trimethyl chlorosilane (TMCS).
Reaction involves replacement of active hydrogen by an alkylsilyl group to
produce derivatives which are less polar, more volatile & more
thermally stable
Major areas of applications of GC
Analysis of natural products
1- Fats (triglycerides)
Nonvolatile cannot be chromatographed directly.
Must be saponified by ethanolic KOH or NaOH to the
corresponding fatty acid salts & glycerol.
Fatty acids are extracted, methylated & injected in GC.
2- carbohydrates 3- glycosides
And other compounds.
4- alkaloids
5- amines
Industrial applications
1- Fuel gases
Fuel gas manufactured from sewage & other wastes is analyzed by a
combination of 2 columns containing Porapak Q & Linde Molecular
Sieve 5 A.
2- Auto Exhaust Gas
GC analysis of exhaust gases of cars helped in understanding the
combustion processes & improvement of all parts of the fuel system to
minimize air pollution.
3- Detection of metal purity
4- Volatile complexes can be prepared for almost all metals in trifluoro& hexafluoroacetyl-acetone.
5- Electron-capture detector (ECD) is used due to its high sensitivity
for fluoro compounds e.g. (trace amounts of 0.1 ppm of Cr & Al are
easily determined in uranium metal).
Biomedical applications
GC is a reliable method for chemical analysis of body fluids e.g.
over 280 substances were identified in human urine and 250
substances in human breath.
Detection of pesticides
According to their chemical origin pesticides are monitored by
particular detectors as follows:
Organo-chlorine origin, e.g. Lindane, Heptachlor, Eldrin, DDE,
& DDT by EC detector.
Organo-phosphorous origin, e.g. Ethion, by dual FI or FP
detectors.
Organo-nitrogen origin, e.g. Carbamate & substituted urea
derivatives, by FID.
Organo-sulfur origin e.g.Thiocarbamate, by FP detector.
Others like Pyrethrins, Piperonyl Butoxide, etc……, by FI detector.