Gas chromatography is a technique used to separate components in a mixture based on how they partition between a mobile gas phase and a stationary liquid phase. Key aspects of the document include: - The main types are gas-solid and gas-liquid chromatography depending on whether the stationary phase is a solid or liquid. - Separation is based on the partition coefficient, which is related to how soluble components are in the stationary phase. Components with higher solubility elute later. - Key instrumentation includes the carrier gas, injection port, columns (packed or capillary), and detectors such as FID, TCD, ECD. - Factors that affect separation include the polarity of the stationary phase versus components

1. GAS CHROMATOGRAPHY
DR SHILPA HARAK

2. Contents
 Introduction
 Principle
 Instrumentation
 Applications

3. INTRODUCTION
 Gas chromatography – “It is a process of separating component(s) from the
given crude drug by using a gaseous mobile phase.”
 It involves a sample being vaporized and injected onto the head of the
chromatographic column.
 The sample is transported through the column by the flow of inert, gaseous
mobile phase.
 The column itself contains a liquid stationary phase which is adsorbed onto the
surface of an inert solid

4. Types
Two major types:
 Gas-solid chromatography:
 Here, the mobile phase is a gas while the stationary phase is a solid.
 Used for separation of low molecular gases, e.g., air components, H2 S, CS2 ,CO2 ,rare
gases, CO and oxides of nitrogen
 Gas-liquid chromatography:
 The mobile phase is a gas while the stationary phase is a liquid retained on the surface as
an inert solid by adsorption or chemical bonding

5. Principles
 The principle of separation in GC is “partition.”
 The mixture of component to be separated is converted to vapour and mixed with gaseous
mobile phase.
 The component which is more soluble in stationary phase travel slower and eluted later.
 The component which is less soluble in stationary phase travels faster and eluted out first.
 No two components has same partition coefficient conditions. So the components are
separated according to their partition coefficient.
 Partition coefficient is “the ratio of solubility of a substance distributed between two
immiscible liquids at a constant temperature.’

6. Instrumentation
 Carrier gas - He (common), N2, H2, Argon
 Sample injection port - micro syringe
 Columns
 Detectors
 Thermal conductivity (TCD)
 Electron capture detector(ECD)
 Flame Ionization detector (FID)
 Flame photometric (FPD)

7. Carrier gas
 The cylinder/ gas tank is fitted with a pressure controller to control the pressure of gas, a
pressure gauge that indicates the pressure, a molecular sieve to transfer filtered dry gas and a
flow regulator to ensure a constant rate of flow of mobile phase to the column.
 It should meet the following criteria:
 Should be chemically inert
 Should be cheap and readily available
 Should be of high quality and not cause any fire accidents
 Should give best possible results
 Should be suitable for the sample to be analyzed and for the detector

8. Carrier gas
 Hydrogen, helium, nitrogen and carbon dioxide are commonly used.
 Hydrogen has low density and better thermal conductivity. However, it reacts
with unsaturated compounds and is inflammable and explosive in nature.
 Nitrogen is inexpensive but it gives reduced sensitivity.
 He is the most preferred gas.
 Inlet pressure ranges from: 10-50 psi -Flow rate : 25-150 mL/min for packed
columns -Flow rate: 2-25 mL/min for open tubular column

9. Sampling unit
 Sampling unit or injection port is attached to the column head.
 Since the sample should be in vapourized state, the injection port is provided
with an oven that helps to maintain its temperature at about 20-500 C above the
boiling point of the sample.
 Gaseous samples may be introduced by use of a gas tight hypodermic needle of
0.5-10 ml capacity.
 For Liquid samples , micro syringes of 0.1-100µL capacity may be used.
 Microsyringe

10. Injections of samples into capillary columns
 a. Split injections- it splits the volume of
sample stream into two unequal flows by
means of a needle valve , and allow the
smaller flow to pass on to the columns
and the bigger part is allowed to be
vented to the atmosphere.
 This technique is not suitable when
highest sensitivity is required.

11. Injections of samples into capillary columns
b. Splitless injectors-
They allow all of the sample to pass
through the column for loading.
Sample should be very dilute to avoid
overloading of the column and a high
capacity column such as SCOT or heavily
coated WCOT columns should be used.

12. Injections of samples into capillary columns
c. Cold On-Column Injection (OCI)
Method or On column injectors:
 A syringe with a very fine quartz needle
is used.
 Air cooled to -20degC below the b.p. of
the sample.
 After then the warmer air is circulated to
vaporize the sample.

13. Injections of samples into capillary columns
 d. Automatic injectors: For improving the reproducibility and if a large
number of samples are to be analyzed or operation is required without an
attendant, automatic injectors are used.
 The solid samples are introduced as a solution or in a sealed glass
ampoule, crushed in the gas stream with the help of a gas tight plunger,
and the sample gets vapourized and flows into column under the
influence of carrier gas.

14. d. Automatic injectors/
Programmed Temperature Vaporization (PTV)
 In this injection method, when the
sample is injected, the injection port is
set to below the boiling point of the
injection sample solvent.
 After the sample is injected, the
injection port is heated rapidly, causing
the injected sample to vaporize.
 Changes in composition
(discrimination) due to heating of
components remaining in the syringe
needle tip are minimal, so this is
suitable for the analysis of compounds
that are thermally unstable (prone to
degradation).

15. d. Automatic injectors/
Programmed Temperature Vaporization (PTV)
 Unlike OCI analysis, a glass insert is
used, and the method can be used for
both split and splitless analysis, enabling
support for low and high-concentration
samples.
 In this analysis method, the column does
not become very dirty even when
analyzing samples containing many
relatively nonvolatile components.
 Large volume injections (LVI) can be
performed by using a GC unit equipped
with an electronic flow controller (AFC)
to control the carrier gas flowrate.

16. Column unit
 Columns are of different shapes and sizes that includes: “U” tube type or coiled helix type.
 They are mainly made of copper, stainless steel, aluminium, Glass, nylon and other synthetic
plastics.
 Support material:-
 it’s main function is to provide mechanical support to the liquid phase.
 An ideal support should have a large surface area, chemically inert, should get uniformly wet
with liquid phase, should be thermostable.
 Commonly used solid phases are: diatomaceous earth or kieselguhr, glass beads, porous
polymers, sand, etc

17. Liquid phase
 It should have the following requirements:
 It should be non-volatile
 Should have high decomposition temperature
 Should be chemically inert
 Should posses low vapour pressure at column temperature
 Should be chemically and structurally similar to that of the solute i.e., polar for polar solute.

18. Examples of different liquid phases
 CATEGORY EXAMPLES
 Non-polar hydrocarbon phases Paraffin oil (nujol), silicon oil, silicon
rubber gum (used for high temp of about 4000
 Polar compounds (having polar groups like -CN, -CO and –OH)
 Polyglycols (carbowaxes) Liquids having hydrogen bonding Glycol,
glycerol, hydroxy acids

19. Types of columns
 There are two general types of columns:
 1. Packed columns:- In GLC, they are densely packed with finely divided, inert, solid support
material ( diatomaceous earth) coated with liquid stationary phase.
 In GSC, the columns are packed with adsorbents or porous polymers.
 Length- 1.5 - 10m
 internal diameter- 2 - 4mm.
 1. Capillary columns-
 length ranges from 10-100m
 inner diameter is usually 0.1-0.5mm

20. Capillary columns
 It is mainly of two types:
 Wall-coated columns - consist of a capillary tube whose walls are coated with
liquid stationary phase.
  Support-coated columns- the inner wall of the capillary is lined with a thin
layer of support material such as diatomaceous earth, onto which the stationary
phase has been adsorbed. It is also known as PLOT (porous-layer open tubular
columns).
 SCOT columns are generally less efficient than WCOT columns. Both types of
capillary column are more efficient than packed columns.

21. Capillary Columns

22. Equilibrium of the column
 The packed columns are equilibrated before introduction of the sample.
 This is done by allowing continuous flow of heated carrier gas through the columns for a
specific duration of time (24hrs) at prescribed temperature.
 Ideally prepared and conditioned columns show a zero base line on the recorder upon passage
of carrier gas alone.
 Column temperature:-
 This can be controlled by jackets equipped with vapours of a boiling liquid, electrically heated
metal blocks or circulating air baths.
 Compounds of low B.P- eluted at lower temperature
 Compounds of high B.P- boils at higher temperature resulting in broader and shallower peaks,
require temperature programming.

23. Comparison
Packed Colum
 Short, thick columns made of glass or
stainless steel tubes, packed columns
have been used since the early stages of
gas chromatography.
 Packed columns produce broad peak
shapes and have low separation
performance, but can also handle large
sample volumes and are not susceptible
to contamination.
 They are still used today in official
analytical methods and for gas analysis.
Capillary Column
 Long, thin columns with its stationary
phase being coated on its inner surface.
 Capillary columns produce sharp peak
shapes, achieve excellent separation
performance, and are suited to high-
sensitivity analysis.
 Currently they are prevailing column type

24. Comparison
Packed Colum
• Internal Diameter: 2 to 4 mm
• Length: 0.5 to 5 m (most
commonly 2 m)
• Packing: Support material with
0.5 to 25 % liquid phase
(partition material) or no liquid
phase (adsorbent material)
• Liquid Phase: Multiple types
available
Capillary Column
• Internal Diameter: 0.1, 0.25,
0.32, 0.53 mm
• Length: 5 to 100 m (most
commonly 30 m)
• Material: Fused silica glass
• Liquid Phase: Good separation
but less variety than packed
columns

25. Comparison
Packed Colum
Capillary Column
PLOT column
(contains immobilized porous
polymer/alumina, etc.)
WCOT or chemical bonding column
(lined with liquid phase or a chemical
bonding layer)

26. Column Type and Effect on Separation
 Packed columns produce broad
peaks and capillary columns produce
sharp peaks.
 In addition, capillary columns produce
taller peaks, which allows the
detection of lower concentrations
(high detection sensitivity).
 This is the advantage of capillary
columns.

27. Column Type and Effect on Separation
 Sharper peaks provide better separation but also shorter analysis
times.

28. Component separation is affected by the following elements

29. Vapor pressure
 The boiling point of a compound is often related to its polarity.
 The lower the boiling point is, the higher the vapor pressure of the compound and the shorter
retention time usually is because the compound will spent more time in the gas phase.
 That is one of the main reasons why low boiling solvents (i.e., diethyl ether, dichloromethane)
are used as solvents to dissolve the sample.
 The temperature of the column does not have to be above the boiling point because every
compound has a non-zero vapor pressure at any given temperature, even solids.
 That is the reason why we can smell compounds like camphor (0.065 mmHg/25 oC),
isoborneol (0.0035 mmHg/25 oC), naphthalene (0.084 mmHg/25 oC), etc.
 However, their vapor pressures are low compared to liquids (i.e., water (24 mmHg/25 oC),
ethyl acetate (95 mmHg/25 oC), diethyl ether (520 mmHg/25 oC)).

30. The polarity of components versus the polarity of stationary phase on
column
 If the polarity of the stationary phase and compound are similar, the retention time
increases because the compound interacts stronger with the stationary phase.
 As a result, polar compounds have long retention times on polar stationary phases and
shorter retention times on non-polar columns using the same temperature.
 Chiral stationary phases that are based on amino acid derivatives, cyclodextrins and
chiral silanes are capable of separating enantiomers because one enantiomer interacts
slightly stronger than the other one with the stationary phase, often due to steric
effects or other very specific interactions.
 For instance, a modified -cyclodextrin column is used in the determination of the
enantiomeric excess in the chiral epoxidation experiment (Chem 30CL).

31. Column temperature
 A excessively high column temperature results in very short retention time but also in a
very poor separation because all components mainly stay in the gas phase.
 However, in order for the separation to occur the components need to be able to interact
with the stationary phase.
 If the compound does not interact with the stationary phase, the retention time will
decrease.
 At the same time, the quality of the separation deteriorates, because the differences in
retention times are not as pronounced anymore.
 The best separations are usually observed for temperature gradients, because the
differences in polarity and in boiling points are used here.

32. Column temperature
 As a rule of thumb, for every 15 °C higher or lower, the retention of a column
decreases or increases by a factor of 2.
 That means if the last peak elutes at 100 °C after 10 minutes, it will elute at 5 minutes at
115 °C and at 20 minutes at 85 °C.
 Resolution Equation: Retention Factor k is primarily impacted by temperature

34. Carrier gas flow rate
 A high flow rate reduces retention times, but a poor separation would be
observed as well.
 Like above, the components have very little time to interact with the
stationary phase and are just being pushed through the column.

35. Column length
 A longer column generally improves the separation.
 The trade-off is that the retention time increases proportionally to the column length
and a significant peak broadening will be observed as well because of increased
longitudinal diffusion inside the column.
 One has to keep in mind that the gas molecules are not only traveling in one direction
but also sideways and backwards.
 This broadening is inversely proportional to the flow rate.
 Broadening is also observed because of the finite rate of mass transfer between the
phases and because the molecules are taking different paths through the column.

36. COLUMN LENGTH
 Generally, a 30 meter column provides the best balance of resolution, analysis
time, and required column head pressure.
Column Length
(m)
Inlet Pressure
(psi)
Peak 1Retention
(min)
Peak 1/2
Resolution
(R)
Efficiency:
Total Plates (N)
15 5.9 8.33 0.8 43,875
30 12.0 16.68 1.2 87,750
60 24.9 33.37 1.7 175,500
Note: Theoretical values for 0.25 mm I.D. columns with 85% coating efficiency, 145 Â °C isothermal analyses, helium at 21 cm/sec, k (peak 1) = 6.00

37. Amount of material injected
 Ideally, the peaks in the chromatogram display a symmetric shape (Gaussian curve).
 If too much of the sample is injected, the peaks show a significant tailing, which causes a
poorer separation.
 Most detectors are relatively sensitive and do not need a lot of material in order to produce a
detectable signal.
 Under standard conditions only 1-2 % of the compound injected into the injection port passes
through the column because most GC instruments are operated in split-mode to prevent
overloading of the column and the detector.
 The splitless mode will only be used if the sample is extremely low in concentration in terms
of the analyte.

38. Conclusion
 High temperatures and high flow rates decrease the
retention time, but also deteriorate the quality of the
separation.

39. Detectors
 The eluted solute particles along with the carrier gas exit from the
column and enter the detector.
 The detector then produces electrical signals proportional to the
concentration of the components of solute.
 The signals are amplified and recorded as peaks at intervals on the
chromatograph.

40. Properties of an ideal detector
 Sensitive
 Operate at high T (0-400 °C)
 Stable and reproducible
 Linear response
 Wide dynamic range
 Fast response
 Simple (reliable)
 Nondestructive
 Uniform response to all analytes

41. General-Purpose Detectors in GC
Detector Detectable Compound Detection Limit
＊
Flame ionization detector
(FID)
Organic compounds (other than
formaldehyde and formic acid)
0.1 ppm (0.1
ng)
Thermal conductivity
detector (TCD)
All compounds other than the carrier gas 10 ppm (10 ng)
Barrier discharge ionization
detector (BID)
All compounds other than He and Ne 0.05 ppm (0.05
ng)
The detection limits are approximations. Actual values will vary depending on the compound structure and analytical conditions.

42. Selective, High-Sensitivity GC Detectors
Detector Detectable Compound Detection Limit＊
Electron capture detector
(ECD)
Organic halogen compounds
Organic metal compounds
0.1 ppb (0.1 pg)
Flame thermionic detector
(FTD)
Organic nitrogen compounds
Inorganic and organic phosphorus
compounds
1 ppb (1 pg)
0.1 ppb (0.1 pg)
Flame photometric detector
(FPD)
Inorganic and organic sulfur
compounds
Inorganic and organic phosphorus
compounds
Organic tin compounds
10 ppb (10 pg)
Sulfur chemiluminescence
detector (SCD)
Inorganic and organic sulfur
compounds
1ppb（0.1pg）
The detection limits are approximations. Actual values will vary depending on the compound structure and analytical conditions.

43. Detector Gas and Makeup Gas
 Each detector requires gas, called the detector gas, based on its
principle of detection.
 For example, the flame ionization detector (FID) uses a hydrogen flame
so it requires hydrogen and air.
 Analysis using a capillary column can also require a makeup gas added
just before the detector to act as an auxiliary gas and ensure the
detector receives a rapid supply of compounds.
 Makeup gas reduces the effects of increasing and decreasing column
flowrates on detector sensitivity by increasing the sample transfer speed
inside the detector and preventing peak broadening.

44. Detector Gas and Makeup Gas
Detector Detector Gas Makeup Gas (Capillary)
FID H2 and Air He or N2
TCD Unnecessary He or Ar or N2 or H2 ,etc.
BID He None
ECD Mainly N2 (The combination of gases varies by equipment model.)
FTD H2 and Air He
FPD H2 and Air None (required in some models)
SCD H2 and O2 N2

45. Flame Ionization Detectors (FID)
Main Applications - Organic compound analysis
 The FID is the most common detector used in gas chromatography.
 The FID is sensitive to, and capable of detecting, compounds that
contain carbon atoms (C), which accounts for almost all organic
compounds.
 However, the FID is not sensitive to carbon atoms with a double bond to
oxygen, such as in carbonyl groups and carboxyl groups (CO, CO2,
HCHO, HCOOH, CS2, CCl4, etc.).

46. Flame Ionization Detectors (FID)

47. Schematic Diagram of the FID
 The FID creates a hydrogen flame by burning air and hydrogen
supplied from below.
 The carbon in a sample carried into the detector on carrier gas is
oxidized by the hydrogen flame, which causes an ionization
reaction.
 The ions formed are attracted by a collector electrode to an
electrostatic field, where the components are detected.

48. FID schematic

49. Thermal conductivity detector
 Main Applications - Water, formaldehyde, formic acid, etc.
Analysis of compounds not detectable by the FID
 The TCD can detect all compounds other than the carrier gas.
 The TCD is mainly used to detect inorganic gas and components
that the FID is not sensitive to.
Helium is commonly used as a carrier gas.
 N2 and Ar are used to analyze He and H2.

50. Thermal conductivity detector
 “TCD is based upon the fact that the heat lost from a filament depends
upon the thermal conductivity of the stream of surrounding gas as well as
its specific heat.”

51. Thermal conductivity detector
 The principle of detection used by the TCD is as follows.
 The TCD detects target components by reading the change in filament
temperature caused by the difference in thermal conductivity between the
carrier gas and target components.
 When the thermal conductivity of the analytical target component is lower than
the carrier gas, the TCD reads an elevation in filament temperature.
 Conversely, when the thermal conductivity of the analytical target component is
higher than the carrier gas, the TCD reads a decrease in filament temperature.

52. Thermal conductivity detector
A direct voltage is applied between A and B.
 When only the carrier gas is flowing at a constant
flowrate
-Each filament maintains a constant temperature
and a constant voltage is produced between C
and D.
 Components are eluted from an analysis-side
column.
-A change in filament temperature occurs, which
-Changes the resistance value, and
-Changes the voltage between C and D

53. Thermal Conductivity
Coefficients
(10-6 cal/s ·cm ·°C)
Component Thermal
Conductivity
H2 547 (extremely
high)
He 408 (extremely
high)
Ethane 77
O2 76
N2 73
H2O 60
Ar，methanol 52
Methanol 40
Chloroform 24

54. When the Thermal Conductivity of the Analytical Target Component is Lower
than the Carrier Gas

55. Barrier Discharge Ionization Detectors (BID)
 Main Applications - Organic compound analysis
Trace gas analysis
 The BID can detect all inorganic and organic compounds other
than He and Ne.
 The BID is also capable of detecting trace amounts of impurities at
the ppm level that the TCD failed to detect during an inorganic gas
analysis.

56.  The principle of detection used by the BID is as
follows.
 The BID generates a stable He plasma, uses
the energy emitted by the excited He to ionize
compounds, then attracts these ions to a
collector.
 The He plasma energy emitted is extremely high
and capable of ionizing all compounds other
than He, which is used to create the plasma,
and Ne, which has extremely high ionization
energy.
 As a result, the BID can detect any compound,
in principle, other than He and Ne.

57. Principle of Ionization
 Compounds eluted from the column are
ionized by light energy from the plasma.
-Ions are attracted to the collection
electrode and output as peaks.
 The light energy from the He plasma is
17.7 eV (electron volts), which is
extremely high.
-The BID is capable of high-sensitivity
detection of all compounds other than the
plasma gas He, and Ne, which has a
higher ionization energy than He.

58. Electron Capture Detectors (ECD)
 Main Applications - Environment analysis
Residual chlorinated pesticides and residual PCBs
Chlorinated VOCs in discharge water
Environmental organic mercury
 The ECD is a selective, high-sensitivity detector for electrophilic compounds.
 The ECD is capable of detecting organic halogen compounds, organic metal compounds,
diketone compounds, etc.
 Because the ECD is fitted with a radioactive isotope, installation requires a notice of use be
sent to the Japanese Ministry of Education, Culture, Sports, Science and Technology

59. Schematic Diagram of the ECD
 The principle of detection used by the ECD is as follows.
 The ECD detects ions by reading the change in voltage value that
maintains a constant ion current gathered at the collector.
 N2, which is used as the carrier gas, is ionized by β waves emitted
from the 63Ni radiation source.
 N2 → N2+ + e-
A current flows when the ions gather in the collector.
 When an electrophilic compound is placed in this equation,
PCB + e- → PCB-
 PCB- is much larger and heavier than e- and so takes more time to
reach the collector.
-A higher voltage is needed for a constant ion current to flow.

60. Flame Thermionic Detectors (FTD)
 Main Applications - Drug analysis
Analysis of nitrogen and phosphorus pesticides
 The FTD is a selective, high-sensitivity detector for organic nitrogen
compounds and inorganic and organic phosphorus compounds.
 (The selectivity of the FTD for phosphorus compounds is not as good as the
FPD.)
 The FTD does not react to inorganic nitrogen compounds.


61. Schematic Diagram of the FTD
The principle of detection used by the FTD is as follows.
 The FTD detects ions by reading the change in ion current
gathered at the collector.
 When a current is passed through the platinum coil with
an alkali source attached to the coil (rubidium salt), the
coil increases in temperature, which creates plasma
around the alkali source.
 Rubidium radicals (Rb*) are generated within this plasma.
-Capable of oxidizing CN and organic phosphorus
compounds
-PO2 reacts with Rb* as shown below, creating ions.
 CN + Rb* → CN- + Rb+
PO2 + Rb* → PO2- + Rb+
A current flows when ions gather in the collector.

62. Flame Photometric Detectors (FPD)
 Main Applications - Analysis of phosphorus pesticides
Analysis of sulfur-based malodors & food odor
components
Analysis of organic tin in marine products
 The FPD is a selective, high-sensitivity detector for phosphorus (P)
compounds, sulfur (S) compounds, and organic tin (Sn) compounds.
 The FPD is highly selective as it detects element-specific light emitted
within a hydrogen flame.

63. Flame Photometric Detectors (FPD)
The principle of detection used by the FPD is
as follows.
 Sulfur compounds, phosphorus
compounds, and organic tin compounds
each emit light at unique wavelengths
when burned.
 By passing light through a filter, only light
of these unique wavelengths reaches a
photomultiplier tube.
 The photomultiplier tube then converts the
detected light intensity into an electrical
signal.

64. Sulfur Chemiluminescence Detectors (SCD)
 Main Applications - Detection of infinitesimal amounts of sulfur compounds in petroleum oil and gas
Measurement of sulfur compounds in gasoline
Analysis of food odor components
Measurement of volatile sulfur compounds in beverages
 The SCD is a selective, high-sensitivity detector for sulfur (S) compounds.
 The SCD is highly sensitive and capable of detecting infinitesimal amounts of sulfur compounds.
 Compared to the FPD, which is similarly capable of selective detection of sulfur compounds, the SCD is around one order of
magnitude more sensitive and exhibits a proportional linear relationship between the SCD sensitivity and the sample
concentration.
 The SCD also exhibits equimolar sensitivity and measures sulfur compounds with the same relative sensitivity regardless of
compound structure.
This characteristic of the SCD allows the use of calibration curves for other compounds to determine an approximate
concentration of a target compound, even when no standard sample is available.
 The SCD also differs substantially from other detectors in that a low-pressure environment is maintained inside the SCD.

65. Sulfur Chemiluminescence Detectors (SCD)
The principle of detection used by the SCD is as
follows.
 The sulfur chemiluminescence detector (SCD)
uses the chemiluminescence reaction caused by
ozone oxidation.
 Sulfur compounds are converted to an X-S
chemical species (mainly SO) that is capable of
exhibiting chemiluminescence inside an
extremely high temperature (around 1000 °C)
oxidative-reductive furnace.
 The X-S chemical species is carried to the
detector area where ozone converts it into an
excited-state SO2* (radical).
 The SO2* emits light upon returning to its base
state, and the SCD detects the sulfur component
by measuring this light with a photomultiplier
tube.

66. Analysis Results
 The retention time is time taken by the injected sample to reach the
detector. It is a characteristic value of each component.


67. Qualitative Analysis
 The elution time when analyzed under given conditions is a characteristic of each component.
 If the same component is analyzed under the same conditions, a peak is confirmed at the same
retention time.
 For example, imagine an unknown sample known to contain component A and component B.
 The chromatogram obtained from the unknown sample looks as follows.
 It is not possible to know which peak is component A, and which peak is component B.

68. Qualitative Analysis
 However, if standard samples of A and
B are prepared, and are analyzed under
the same conditions, the retention times
for A and B become evident.
 By comparing these chromatograms,
the peaks for A and B in the
chromatogram of the unknown sample
can be determined.
With GC, the retention time is the sole qualitative information.
→ For qualitative analysis, a standard sample is required (in principle).

69. Q: Can GC be used for Qualitative Analysis?
What are its limitations?
 The purity of a sample can be assessed using gas chromatography. The number of peaks present can
indicate how many components are in the mixture. However with GC, the retention time is the sole
qualitative information.
 If a standard sample is not available, it is not possible to determine a unknown substance.
Accordingly, one could say that this method is intended for the analysis of samples for which the
components they contain are reasonably certain.
 Chromatography is used in conjunction with other techniques when purity is determined. It is necessary
to use analysis methods with a higher qualitative capacity such as GCMS.
 Also different components can exist with the same retention times under given analysis conditions. That
is a seemingly single peak could indicate multiple components.
 In this case, cross checks must be performed by changing the column or the temperature conditions.
For this reason, when performing GC analysis, it’s very important to completely separate the peaks.

70. Quantitative Analysis
Peak Area determination
 Mechanical or Electronic Integration
 Triangulation
 Planimetry
 Cut & Weigh
 Retention Time Method

71. Mechanical or Electronic Integration
 Mechanical or Electronic Integration
 Modern electronic integrators are used
 Maximum Precision & accuracy
 Std Deviation of 0.5% or less

72. Triangulation
 It manual methods of
integration

73. Planimetry
 The planimeter is an instrument composed of a lever, a little wheel and a pin.
 The analyst must carefully follow the profile of the peak and the interpolated base line, previously
drawn with a pencil on the chromatogram, with the pin, which moves the lever.
 The movement of the lever makes the wheel turn and a counter records the number of turns and the
fraction of turn the wheel has rotated during the travel around the peak. This number is proportional to
the peak area.
 The proportionality coefficient is obtained by calibration, by measuring the area of a square of known
base. This method is tedious and very slow.
 The precision depends very much on the ability of the operator to carefully follow a thin continuous
line.
 It is comparable to the precision obtained when using the product of the height by the width at half
height.
 https://youtu.be/aLSx1eM27P4

74. Cutting the Peak and Weighing the Piece of Paper
 The peak is cut with scissors, while attempting to follow its contour closely.
 A square of comparable area is also cut and the two pieces of paper are weighed.
 The main inconvenience of the method, besides the time spent in the operation, is that
the chromatogram is destroyed.
 This can be remedied by making photocopies of the chromatograms, using good,
homogeneous, heavy paper, and cutting them.
 This is a very accurate method provided it is used with a highly homogeneous paper
and that great care is taken that the water content of the paper remains constant.
 The cut paper should be kept for a certain time in an oven, at constant temperature and
humidity.

75. Peak Height Determination
 Product of the Peak H
 The peak area is estimated as the product of the peak height by its width
at halfheight.
 the width at half height is given by:

76. Data interpretation
1. Internal Normalization of Peak Areas
 In this method concentration of a component in a mixture is defined as the percentage of the
total peak represented by individual component peak area
 The concentration of component j is given by:
 where A1A2, A3 ……An, are the areas of the peaks of the various components of the mixture.
 This method assumes first that all components of the mixture are eluted off the column
 The method can be applied only
 (i) if all the components of the mixture are eluted from the column,
 (ii) if they are all identified and
 (iii) if their relative response factors have been properly determined.
 Most computer software is designed to apply this procedure when required.

77. Area normalization with response factor correction
 In chromatography, a response factor is defined as the ratio between the concentration of a compound
being analysed and the response of the detector to that compound.
 A chromatogram will show a response from a detector as a peak.
 While there are several ways to quantify the peak, one of the most common is peak area, thus: Ai = Ci
x fi
 Ai = Peak Area Ci = Concentration fi
 =Or Peak Area = Concentration x Response Factor