Gas chromatography is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition.
2. Introduction
• Separation of gaseous & volatile substances
• Simple & efficient in regard to separation
• A Gas Chromatograph is used to detect the
components based on the selective affinity of
components towards the adsorbent materials.
3. Stationary & Mobile Phase
• 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.
5. Working of GC
• 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
6. Gas Chromatography
• Sample to be separated is converted into vapour And
mixed with gaseous M.P
• Components having high boiling point-> travels slower
• Components having low boiling point -> travels faster
• Components are separated according to their Partition
Co-efficient
• Criteria for compounds to be analyzed by G.C
1. VOLATILITY
2.THERMOSTABILITY
7. Different compounds have different
retention times
• For a particular compound, the retention time
will vary depending on:
• Boiling point of the compound high boiling
point means a long retention time.
• Solubility in the liquid phase High solubility
in the liquid phase means a high retention
time.
• Temperature of the column A high column
temperature shortens retention times
9. CLASSIFICATION OF GC
• When the stationary phase is a solid adsorbent,
the process is termed gas–solid
chromatography (GSC),
• When it is a liquid on an inert support, the
process is termed gas–liquid chromatography
(GLC).
10. INTERACTION IN GC
• GC is the only form of
chromatography that does not utilize a mobile
phase for interacting with the analyte.
11.
12. • The mobile phase i.e gas in gas chromatography is called the carrier gas
and must be chemically inert.
• Helium (He) , Argon (Ar), Nitrogen(N2 ), and hydrogen (H 2 ).
• Flow rates in gas chromatographs were regulated by controlling the gas
inlet pressure.
• Inlet pressures usually range from 10 to 50 psi (lb/in 2 ), yielding flow rates
of 25 to 150 mL/min with packed columns and 1 to 25 mL/min for open
tubular capillary columns.
• Newer chromatographs use electronic pressure controllers both for packed
and for capillary columns.
14. Injector
• The injection port Is a hollow, heated, glass-lined
cylinder
• The injector is heated so that all components in
the sample will be vaporized.
• If the temperature is too low, separation is poor
and broad spectral peaks should result or no peak
develops at all.
• If the injection temperature is too high, the
specimen may decompose or change its structure.
• The temperature of the sample port is usually
about 50°C higher than the boiling point of the
least volatile component of the sample.
16. Column
The column is the most important component of GC. Here
is where the separations take place.
There are four types of columns:
• Wall-coated open tubular (WCOT),
• Support coated open tubular (SCOT), micropacked,
• Fused silica open tubular (FSOT),
• Packed column
The FSOT column is the most flexible. Open tubular is
also capillary. Particle size is important because the
efficiency of GC column increases rapidly with
decreasing particle size of the packing material
18. Packed & Capillary columns
• Gas chromatographic columns are classified into
two major categories, namely,
• Packed columns
1.5 - 10 m in length and have an internal
diameter of 2 - 4 mm.
• Capillary columns
They have very small internal diameter, on
the order of a few tenths of millimeters, and
lengths between 25-60 meters are common
21. Capillary columns are preferred over
packed columns
• Sample size If detector sensitivity is low then packed columns can be considered as these have higher sample load
capacities.
• Cost Packed columns are less expensive than capillary columns.
• Resolution Power Capillary columns provide much better resolution leading to the desired separation between
closely spaced peaks
• Time saving The ability to resolve components easily using capillary columns helps to increase laboratory
throughput thereby increasing the number of samples that can be analysed in the same time.
• Sample Polarity Packed columns are generally made of stainless steel or glass. Stainless steel columns are
generally useful for separation of non polar compounds whereas glass columns are suitable for polar compound
separations
• Ruggedness Metallic columns are rugged in nature and can tolerate all types of handling but care should be taken
not to drop them as this can disturb the packing inside the column and affect separation power. Capillary columns
are fragile in nature and require very careful handling particularly at time of installation and removal inside the
column oven.
22. Detectors
• The part of a gas chromatograph which signals
the change in composition of the mixture
passing through it.
23. The ideal detector for gas chromatography has the
following characteristics
• Adequate sensitivity, (10 -8 ~ 10 -15 g solute/s).
• Good stability and reproducibility.
• A linear response to solutes that extends over
several orders of magnitude.
• A temperature range from room temperature to
at least 400OC
• A short response time that is independent of flow
rate.
• High reliability and ease of use.
24. Classification
• Bulk property and solute property detectors
• Concentration sensitive devices
• Specific or non-specific detectors.
25. Bulk property and solute property
detectors
• The bulk property detector measures some
bulk physical property of the eluent (such as
dielectric constant or refractive index).
• The solvent property detector, measures
some physical or chemical property that is
unique to the solute (such as heat of
combustion or fluorescence).
26. Concentration sensitive devices
• A Specific detector would be the nitrogen
phosphorous detector (NPD), which as its
name implies detects only those substances
that contain nitrogen or phosphorous.
• A non-specific detector would be the
katharometer detector which senses all vapors
that have specific heats or thermal
conductivities different from those of the
carrier gas.
27. Specific or non-specific detectors
• Detectors can also be classified
as concentration sensitive devices
• Such as the katharometer or mass sensitive
devices such as the flame ionization detector
(FID)
28. Advantages
• This method has a high resolution power
compared to other methods.
• This method has high sensitivity when used
with thermal detectors.
• This technique gives relatively good accuracy
and precision.
• Separation and analysis of sample very
quickly.
• Sample with less quantity is also separated.
29. Disadvantages
• Only volatile samples or the sample which can
be made volatile are separated by this
method.
• During injection of the gaseous
sample proper attention is required.
• The sample of gas which is about to inject
must be thermally stable so that it does not
get degraded when heated.
