Description of gas chtomatography

1. Gas chromatography
Gas chromatography is a chromatographic technique that
can be used to separate volatile organic compounds.
To be suitable for GC analysis, a compound must have
sufficient volatility and thermal stability.
In gas chromatography (GC), the sample is vaporized and
injected onto chromatographic columns and then separated
into many components.
GC is of two types :
Gas-Liquid Chromatography (GLC) in which the stationary
phase is liquid that is immobilized on the surface of a solid support
by adsorption or by chemical bonding.
Gas-Solid chromatography (GSC) in which the stationary phase
is a solid that has a large surface area at which adsorption of the
analyte species (solutes) take place.
Most GC instruments are of the type of (GLC).

2. Gas Chromatography
In gas chromatography (GC) the sample, which may be a gas or
liquid, is injected into a stream of an inert gaseous mobile phase
(often called the carrier gas).
The sample is carried through a packed or capillary column where
the sample’s components separate based on their ability to
distribute themselves between the mobile and stationary phases
Integrator/Plotter
Injector Detector
Pressure
regulator
Carrier gas
Column
Oven
Valve Work Station
GC Instrument
A schematic diagram of a capillary gas chromatograph.

3. Components of GC:
The basic components of instruments for GC are:
carrier gas supply, sample introduction system, separating tool
and a detection system .

4. 1- Carrier gas :” the mobile phase”
The carrier gas must be chemically inert dry and free of oxygen.
Commonly used gases include nitrogen, helium, argon, and carbon
dioxide. The choice of carrier gas is often dependent upon the type of
detector used.
Soap-bubble flow meter
and digital flow meter.
Small variations in the carrier gas flow-rate
will affect column performance and retention
times. Therefore, to achieve reproducible
separations, it is necessary to maintain a
constant flow-rate. Recently pressure
regulator and flow-rate controller are built
into the carrier gas lines at the cylinder and
in the instrument.
Before gas chromatographic analysis,
you should set the flow rate of carrier
gas that is adequate for your analysis.
Remember this preparation is the basic
starting point for gas chromatography

5. The introduction of a sample into GC is the first stage in the
chromatographic process and its efficiency is reflected in the
overall efficiency of the separation procedure and the accuracy
and precision of the qualitative and quantitative results.
For optimum column efficiency, the sample should not be too
large, and should be introduced onto the column as a "plug" of
vapor - slow injection of large samples causes band broadening
and loss of resolution. The most common injection method is
where a microsyringe is used to inject sample through a rubber
septum into a flash vaporizer port at the head of the column.
2-Sample injection port :
The temperature of the sample port is
usually about 50oC higher than the boiling
point of the least volatile component of the
sample.

6. For packed columns, sample size ranges from tenths of a microliter up to
20 microliters. Capillary columns, on the other hand, need much less
sample, typically around 1 microlitrs. For capillary GC, split/splitless
injection is used.
The injector can be used in one of two modes; split or splitless.
The injector contains a heated chamber containing a glass liner into
which the sample is injected through the septum.

7. a) Packed column
A wide-bore column containing a particulate packing material
It is constructed from glass, stainless steel, copper
or aluminum and is typically 2–6 m in length, with
an internal diameter of 2–4 mm. The column is
filled with a particulate solid support, coated with
liquid stationary phase.
To minimize the multiple path contributions to plate
height, the packing material should be of as small
a diameter as is practical and loaded with a thin
film of stationary phase.
3- Chromatographic Columns
It is Heart of the chromatographic system, it’s construction influences
the amount of sample that can be handled, the efficiency of the separation,
the number of analytes that can be easily separated, and the amount of
time required for the separation.
There are two general types of column, packed and capillary
“also known as open tubular”.

8. Capillary, are constructed from fused silica coated with a protective
polymer. Columns may be up to 100 m in length with an internal
diameter of approximately 150–300 μm
Capillary columns are of two principal types:
Wall-coated open tubular columns (WCOT)
contain a thin layer of stationary phase, typically 0.25 μm thick,
coated on the capillary’s inner wall
Support-coated open tubular columns (SCOT)
a thin layer of a solid support coated with a liquid stationary phase is
attached to the capillary’s inner wall
b) Capillary, or open tubular columns
A column that does not contain a particulate packing material
capillary columns are not packed with any solid
support,the van Deemter equation becomes

9. Capillary Columns
Capillary columns provide a significant improvement in separation
efficiency
The pressure needed to move the mobile phase through a packed
column limits its length
The absence of packing material allows a capillary column to be longer
than a packed column
Most capillary columns contain more theoretical plates per meter than a
packed column, the more important contribution to their greater efficiency
is the ability to fashion longer columns
Packed columns can handle larger samples.
Due to its smaller diameter, capillary columns require smaller samples.
Capillary column

10. Capillary columns advantages compared to packed columns
1. higher resolution
2. shorter analysis times
3. greater sensitivity
4. have less pressure inside the column.
5. High efficiency.
Capillary columns disadvantage compared to packed columns
1. smaller sample capacity
2. need better experience
3. more expensive
Similarities Between Packed Column and Capillary Column:
They are two types of stationary phase.
Both can be used in (GC).
The main function of both is to retain the components of the
mixture that are to be separated in the column.

11. Stationary Phases
The main criteria for selecting a stationary phase are that:
. it should be chemically inert
. thermally stable
. low volatility
. an appropriate polarity for the solutes being separated
Elution order in GLC is determined:
. primarily by the solute’s boiling point and,
. to a lesser degree, by the solute’s interaction with the stationary phase
Solutes with significantly different boiling points are easily separated ,
two solutes with similar boiling points can be separated only if the
stationary phase selectively interacts with one of the solutes
In general, nonpolar solutes are more easily separated with a
nonpolar stationary phase, and polar solutes are easier to separate
using a polar stationary phase

12. Sample Introduction
Three considerations determine how samples are introduced to the gas
chromatograph:
. First, all constituents injected into the GC must be volatile
. Second, the analytes must be present at an appropriate concentration
. Finally, the injecting process must not degrade the separation
Not every sample that can potentially be analyzed by GC:
. To move through the column, the sample’s constituents must be volatile
. Solutes of low volatility may be retained by the column and continue to
elute during the analysis of subsequent samples
. So nonvolatile analytes must be chemically converted to a volatile
derivative before analysis
Adjusting the Analyte Concentration
Analytes present at concentrations too small to give an adequate
signal need to be concentrated before analyzing.
When an analyte is too concentrated, it is easy to overload the column,
thereby seriously degrading the separation.

13. Injecting the Sample
To avoid any loss in resolution due to band broadening, a sample of
sufficient size must be introduced in a small volume of mobile phase.
The injector block is heated to a temperature that is at least 50 °C
above the sample component with the highest boiling point. In this
way rapid vaporization of the entire sample is ensured
Capillary columns require the use of a special injector to avoid
overloading the column with sample
Several capillary injectors are available, the most common:
. a split injection only about 0.1–1% of the sample enters the column,
with the remainder carried off as waste
. a splitless injection, which is useful for trace analysis
The column temperature is held 20–25 °C below the solvent’s boiling
point
As the solvent enters the column, it condenses, forming a barrier that
traps the solutes
After allowing time for the solutes to concentrate, the column’s
temperature is increased, and the separation begins

14. Temperature Control
Control of the column’s temperature is critical to attaining a good
separation in gas chromatography. For this reason the column is
located inside a thermostated oven.
Normally, the temperature is set slightly below that for the lowest
boiling solute so as to increase the solute’s interaction with the
stationary phase.
One difficulty with an isothermal separation is that a temperature
favoring the separation of low-boiling solutes may cause
unacceptably long retention times for higher boiling solutes.
Ovens capable of temperature programming provide a solution to
this problem.

15. Temperature Programming
A- Isothemal: at constant temperature
B- Temperature Programming
Temperature gradient
improves resolution while
also decreasing retention
time
Temperature is raised during the separation

16. 4- Detectors for Gas Chromatography
The final part of a gas chromatograph is the detector.
The Three most wildly used detectors for gas chromatography are:
Thermal conductivity detector (TCD)
Flame-ionization detector (FID)
Electron-capture detector (ECD)
After the components of a mixture are separated, they must be
detected as they exit the GC column.
The purpose of a detector is
to monitor the carrier gas as it emerges from the column and
to generate a signal in response to variation in its composition
due to eluted components.

17. 1. Adequate sensitivity
2. Good stability and reproducibility.
3. A linear response to analyses that extends over several orders
of magnitude.
4. A temperature range from room temperature to at least 400 C.
5. A short response time that is independent of flow rate.
6. High reliability and ease of use.
7. Similarity in response toward all analyses.
8. Nondestructive of sample.
The ideal detector has several desirable features:

18. Thermal Conductivity Detector: TCD
One of the earliest gas chromatography detectors, which is still
widely used
is based on the mobile phase’s thermal conductivity
Because of its high thermal conductivity, helium is the mobile phase
of choice when using a thermal conductivity detector (TCD)
The advantages of TCD detector:
. universality, since it gives a signal for any
solute whose thermal conductivity differs from
that of helium
. The detector also is nondestructive
The disadvantage of TCD detector:
TCD’s detection limit is poor in comparison
with other popular detectors

19. Flame Ionization Detector: FID
Combustion of an organic compound in an H2/air flame results in a flame
rich in electrons and ions
If a potential of approximately 300 V is applied across the flame, a small
current of roughly 10–9–10–12 A develops When amplified, this current
provides a useful analytical signal
This is the basis of the popular flame ionization detector (FID)
Most carbon atoms generate a signal, making
the FID an almost universal detector for organic
compounds
Advantages of the FID include a detection limit
that is approximately two to three orders of
magnitude smaller than that for aTCD and a
linear response over 106–107 orders of
magnitude in the amount of analyte injected
The sample, of course, is destroyed when using FID

20. Electron Capture Detector: ECD
The electron capture detector is an example of a selective detector
The detector consists of a beta emitter (a beta particle is an electron) 63Ni
The emitted electrons ionize the mobile phase, which is usually N2, resulting
in the production of additional electrons that give rise to an electric current
between a pair of electrodes
When a solute with a high cross section for the capture of electrons elutes
from the column, the electric current decreases
This decrease in electric current serves as the signal
Although its detection limit is excellent, its
linear range is small.
The ECD is extremely sensitive to molecules
containing highly electronegative
functional groups such as halogens,
peroxides, quinones, and nitro groups.
It is therefore a popular detector for trace
level determinations of chlorinated
insecticides and halocarbon residues in
environmental samples.