GAS CHROMATOGRAPHY

Parameters for column efficiency:
Kinetic effects cause band broadening and is expressed by Square root of N or
H/μ
Thermodynamic effect of solute being separated
The Ka and Kb values and volume of mobile and stationary phase

Resolution is good with good N = L/H
For the same Length of column is increased or H is reduced .
Column length cannot be changed by large so H is reduced by
• Decrease in particle size
• Decreasing solvent viscosity increase the distribution coefficient In MP

In gas chromatography, the sample is vaporised and analyte components
separated based on distribution coefficient between a mobile gaseous
phase and a liquid or a solid stationary phase held in a column.

Elution is brought about by the flow of an inert gaseous mobile phase

Two types of gas chromatography

Gas-liquid chromatography (GLC) and

gas-solid chromatography (GSC).

Gas-liquid chromatography finds widespread use in all fields of
science

Gas-solid chromatography is based on a solid stationary phase in which
retention of analytes occurs because of physical adsorption.

Gas-solid chromatography has limited application because of semipermanent
retention of active or polar molecules and severe tailing of elution peaks.

The tailing is due to the nonlinear character of adsorption process

Many changes and improvements in gas chromatographic instruments have appeared
in the marketplace since their commercial introduction.

In the 1970s, electronic integrators and computer-based data-processing
equipment became common.

The 1980s saw computers being used for automatic control of such instrument
parameters as column temperature, flow rates, and sample injection.

development of very high-performance instruments

open tubular columns that are capable of separating components of complex
mixtures in relatively short times.

150 different models of gas chromatographic equipment at costs that vary from
$1000 to over $50,000.

Carrier Gas System:
The mobile phase gas in gas chromatography is called the carrier gas
and must be chemically inert.
• Helium, argon, nitrogen, and hydrogen are also used.
• These gases are available in pressurized tanks.
• Pressure regulators, gauges, and flow meters are required to control

the flow rate of the gas.
• A two-stage pressure regulator at the gas cylinder and some sort of

pressure regulator or flow regulator mounted in the chromatograph
were used.

• Inlet pressures usually range from 10 to 50 psi (lb/in2 ) above room
pressure

• This maintain flow rates of 25 to 150 mL/min with packed columns
• 1 to 25 mL/min for open tubular capillary columns.
• With pressure-controlled devices, it is assumed that flow rates are

constant if the inlet pressure remains constant. Newer
chromatographs use electronic pressure controllers both for packed
and for capillary columns.

Sample injection system:

• For high column efficiency, sample of optimum quantity need to be
introduced as a “plug” of vapor.

• Slow injection or excess samples lead to band spreading and poor
resolution.

Calibrated microsyringes, such as those shown, are used to inject liquid
samples through a rubber or silicone diaphragm, or septum, into a heated
sample port located at the head of the column.

The sample port is usually kept at about 50C greater than the boiling point
of the least volatile component of the sample.

For ordinary packed analytical columns, sample sizes range from a few
tenths of a microliter to 20 mL.

Capillary columns require samples that are smaller by a factor of 100 or
more.

For these columns, a sample splitter is often needed to
deliver a small known fraction (1:100 to 1:500) of the injected
sample, with the remainder going to waste.
splitters allow for splitless injection for packed columns as
well

A rotary sample valve. Valve position
(a) is for filling the sample loop ACB; position
(b) is for introduction of sample into the column

The sample is picked up through a septum on
the vial and is injected through a septum on
the chromatograph.
150 sample vial on the turntable.
Injection volumes :0.1 mL with a 10-mL syringe
to 200 mL with a 200-ml syringe. For
introducing gases, a sample valve, is often used
instead of a syringe.
reproduced to better than 0.5%
Liquid samples can also be introduced through
a sampling valve.
Solid samples are introduced as solutions or
alternatively are sealed into thin-walled vials
that can be inserted at the head of the column
and punctured or crushed from the outside.

Column Configurations and Column Ovens :

The columns in gas chromatography are of two general types:

packed columns or capillary columns.

Majority of gas chromatographic analyses used packed columns.

For most current applications, packed columns have been replaced by more efficient
and faster capillary column

Chromatographic columns length from less than 2 m to 60 m or more.
They are constructed of stainless steel, glass, fused silica, or Teflon.
they are usually formed as coils having diameters of 10 to 30 cm
Column temperature is an important variable that must be controlled to a
few tenths of a degree for precise work.
Thus, the column is placed in a thermostated oven.
The optimum column temperature depends on the boiling point of the
sample and the degree of separation required.
Temperature equal to or slightly above the average boiling point of a sample
results in a reasonable elution time (2 to 30 min).
.

For samples with a broad boiling range, it is often desirable to use temperature
programming whereby the column temperature is increased either continuously or in
steps as the separation proceeds. shows the improvement in a chromatogram brought
about by
temperature programming

In general, optimum resolution is associated
with minimal temperature. The cost of lowered
temperature, however, is an increase in elution
time and, therefore, the time required to
complete an analysis. Figures a and b illustrate
this principle. Analytes of limited volatility can
sometimes be determined by forming derivatives
that are more volatile. Likewise, derivatization is
used at times to enhance detection or improve
chromatographic performance

Effect of temperature
on gas chromatograms.
(a) Isothermal at 45C. (b)

Isothermal at 145C.
(c) Programmed at 30 to 180C.

Characteristics of the Ideal Detector :

The ideal detector for gas chromatography has the following
characteristics:
1. Adequate sensitivity: Sensitivity range of 10–8 to 10–15 g

solute/s. (ng to pg)
2. Good stability and reproducibility, short response time.
3. A linear response to solutes from low to high conc. Range.
4. A temperature range from room temperature to at least

400C. Nondestructive nature
5. High reliability and ease of use.
6. Response toward one or more classes of solutes in similar

pattern.

Detectors can be classified into two general
types:

Universal Detectors for detection of a wide range of
analytes:
Flame ionisation detector (FID) (destructive).
Thermal conductivity detector (TCD) (non-destructive).

Selective/Specific Detectors for detection of particular
types of analytes, and include:

Electro capture detector (ECD).
Photo ionisation detector (PID).
Flame photometric detector (FPD).

Cathode
electrodes

Detection involves monitoring
the current produced by
collecting this charge carriers.
A few hundred volts applied
between the burner tip and a
collector electrode located
above the flame causes the ions
and electrons to move towards
the collector.
The resulting current is then
measured with high-impedance air/ hydrogen flame
picoammeter. Pyrolyze analyte

• The number of ions produced is
roughly proportional to the
number of reduced carbon atoms
in flame.

• Detector is insensitive towards
non-combustible gases such as
H2O, CO2, SO2, CO.

Entry of effluent from column

Because the flame ionization detector responds to the number of carbon atoms
entering the detector per unit of time, it is a mass sensitive rather than a
concentration-sensitive device.

Functional groups, such as carbonyl, alcohol, halogen, and amine, yield fewer ions or
none at all in a flame.

Flame ionization detector is a general detector for the analysis of most organic
samples even in presence of water and the oxides of nitrogen and sulfur. The FID
exhibits a high sensitivity (,10–13 g/s),
large linear response range and low noise.
It is generally rugged and easy to use.
Disadvantages of the flame ionization detector are that it destroys the sample

during the combustion step and requires additional gases and controllers.

Thermal conductivity detector (TCD) Katherameter
One of the earliest detectors for gas chromatography, has wide
application. Consists of an electrically heated plate a fine platinum,
gold, or tungsten wire or, alternatively, a small thermistor.
whose temperature at constant electric power depends on the
thermal conductivity of the surrounding gas.
The electrical resistance of this element depends on the thermal
conductivity of the gas.

a) A cross-sectional view of one of the temperature-
sensitive elements in a TCD
https://www.youtube.com/watch?v=0ujaS0jWuV8
b) Four thermally sensitive resistive elements are
often used. A reference pair is located above the sample
injection chamber
• sample pair immediately beyond the column.
Alternatively, the gas stream can be split.
The detectors are incorporated in two arms of a simple bridge
circuit, such that the thermal conductivity of the carrier gas is
canceled.

The thermal conductivities of helium and hydrogen are roughly six to ten times greater than
those of most organic compounds.

Small amounts of organic species cause relatively large decreases in the thermal conductivity
of the column effluent, resulting in a marked rise in the temperature of the detector.

Carrier gases whose conductivities resemble that of sample components cannot be used .

The advantages of the TCD are its simplicity, its large linear dynamic range (about five orders of
magnitude),

its general response to both organic and inorganic species, and its nondestructive character,
which permits collection of solutes after detection.

The chief limitation of this detector is its relatively low sensitivity.

Cannot be used with capillary columns where sample amounts are very small.

Electron Capture Detectors
https://www.youtube.com/watch?v=iSWxuFtxDH8

The electron capture detector (ECD) has
become one of the most widely used
detectors for environmental samples because
this detector selectively responds to halogen-
containing organic compounds, such as
pesticides and polychlorinated biphenyls.
In this detector, the sample elute from a
column is passed over a radioactive Beta
emitter, nickel-63.
An electron from the emitter causes ionization
of the carrier gas (often nitrogen) and the
production of a burst of electrons.
In the absence of organic species, a constant
standing current between a pair of electrodes
results from this ionization process.

The current decreases markedly, however, in the presence of organic
molecules containing electronegative functional groups that tend to
capture electrons.
Compounds, such as halogens, peroxides, quinones, and nitro
groups, are detected with high sensitivity.
The detector is insensitive to functional groups such as amines,
alcohols, and hydrocarbons.
Electron capture detectors are highly sensitive and have the advantage
of not altering the sample significantly (in contrast to the flame
ionization detector, which consumes the sample). The linear response
of the detector, however, is limited to about two orders of magnitude.

Mass Spectrometry Detectors
One of the most powerful detectors for GC is the mass spectrometer.
The combination of gas chromatography and mass spectrometry is known as

GC/MS.
Mass spectrometer measures the mass-to-charge ratio (m/z) of ions that have

been produced from the sample.
Ions produced are singly charged (z = 1) so that mass Spectro metrists measure
the mass of ions when mass-to-charge ratio is one.
The flow rate from capillary columns is usually low enough that the column
output can be fed directly into the ionization chamber of the mass spectrometer.
In packed column large volume of carrier gas eluting from the GC had to be
minimised. Various jet, membrane, and effusion separators were used for this
purpose. Presently, capillary columns are invariably used in GC/MS instruments,
and such separators are no longer needed.
The most common ion sources for GC/MS are electron impact and chemical

ionization. The most common mass analyzers are quadrupole and ion-trap
analyzers.

In GC/MS, the mass spectrometer scans the masses repetitively during a
chromatographic experiment.
A computer data system is needed to process the large amount of data obtained for

large sample size . The data can be analyzed in several ways.
First, the ion abundance in each spectrum can be summed and plotted as a function of

time to give a total-ion chromatogram. This plot is similar to a conventional
chromatogram.
Second, one can also display the mass spectrum at a particular time during the
chromatogram to identify the species eluting at that time.
Finally, a single mass-to-charge (m/z) value can be selected and monitored throughout
the chromatographic experiment, a technique known as selected-ion monitoring.
Mass spectra of selected ions during a chromatographic experiment are known as mass
chromatograms. GC/MS instruments have been used for the identification of thousands
of components that are present in natural and biological systems.