Provides a detailed coverage on Gas Chromatography.

1. Gas Chromatography
ER. FARUK BIN POYEN, Asst. Professor
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
faruk.poyen@gmail.com

2. Contents
 Introduction
 Chromatographic Classifications
 Definition
 Working Principle
 Basic Terms
 GC Instrumentation
 Main Components
 GC Modes of Separation
 Detectors
 Qualitative Analysis
 Quantitative Analysis
2

3. Introduction:
 Chromatography is a physical method of separation of the components
of a mixture by distribution between two phases, of which one is a
stationary bed of a large surface area and the other is a fluid phase that
penetrates through or along the stationary phase.
 The procedure of chromatographic separation embroils the transport of a
sample of the mixture through a column.
 The stationary phase may be a solid adsorbent or a liquid partitioning
agent.
 The mobile phase is usually a gas or a liquid and it transforms the
constituents of the mixture through the column.
3

4. Chromatographic Classifications:
 Gas Chromatography
 Gas / Liquid (partition)
 Gas / Solid (adsorbent)
 Liquid Chromatography
 Paper
 Column
 Liquid / Liquid (partition)
 Liquid / Solid (adsorption)
 Gel permeation
 Ion exchange
 Thin layer
4

5. Adsorption Chromatography Principle:
 Adsorption chromatography definition:
 It is a process of separation of components in a mixture introduced into
chromatography system based on the relative differences in adsorption
of components to the stationary phase present in the chromatography
column.
 This adsorption chromatography applies to only solid-liquid or solid-gas
chromatography. Because the adsorption phenomenon is inherent
property of solids and hence it is used with only solid stationary phase
chromtographys’.
5

6. Partition (coefficient) Chromatography:
 Partition chromatography is process of separation whereby the components of
the mixture get distributed into two liquid phases due to differences in partition
coefficients during the flow of mobile phase in the chromatography column.
 Here the molecules get preferential separation in between two phases. i.e. both
stationary phase and mobile phase are liquid in nature. So molecules
get dispersed into either phases preferentially.
 Polar molecules get partitioned into polar phase and vice-verse.
 This mode of partition chromatography applies to Liquid-liquid, liquid-gas
chromatography and not to solid-gas chromatography because partition is the
phenomenon in between a liquid and liquid or liquid and gas or gas and gas.
But not in solid involvement.
 In high performance liquid chromatography (HPLC), Paper
chromatography, gas chromatography, high performance thin layer
chromatography (HPTLC), partition chromatography is the principle of
separation.
6

7. Definition:
 Gas Chromatography (GC) is an analytical procedure employed for
separating compounds based primarily on their volatilities. GC offers
both qualitative and quantitative information for individual compounds
present in a sample. The differential partitioning into the stationary
phase allows the compounds to be separated both in time and space. Gas
chromatography (GC) is a common type of chromatography used
in analytical methods for separating and analyzing compounds that can
be vaporized without decomposition.
 Basic Principle of GC – Sample vaporized by injection into a heated
system, eluted through a column by inert gaseous mobile phase and
detected.
7

8. Working Principle of GC:
 A gas chromatograph uses a flow-through narrow tube known as
the column, through which different chemical constituents of a sample
pass in a gas stream (carrier gas, mobile phase) at different rates
depending on their various chemical and physical properties and their
interaction with a specific column filling, called the stationary phase.
 As the chemicals exit the end of the column, they are detected and
identified electronically.
 The function of the stationary phase in the column is to separate
different components, causing each one to exit the column at a different
time (retention time).
8

9. Working Principle of GC:
 Other parameters that can be used to alter the order or time of retention
are the carrier gas flow rate, column length and the temperature.
 In a GC analysis, a known volume of gaseous or liquid analyte is
injected into the "entrance" (head) of the column, usually using a micro
syringe (or, solid phase micro extraction fibers, or a gas source
switching system).
 As the carrier gas sweeps the analyte molecules through the column, this
motion is inhibited by the adsorption of the analyte molecules either
onto the column walls or onto packing materials in the column.
 The rate at which the molecules progress along the column depends on
the strength of adsorption, which in turn depends on the type of
molecule and on the stationary phase materials.
9

10. Working Principle of GC:
 Since each type of molecule has a different rate of progression, the
various components of the analyte mixture are separated as they
progress along the column and reach the end of the column at different
times (retention time).
 A detector is used to monitor the outlet stream from the column; thus,
the time at which each component reaches the outlet and the amount of
that component can be determined.
 Generally, substances are identified (qualitatively) by the order in which
they emerge (elute) from the column and by the retention time of the
analyte in the column.
10

11. Working Principle of GC: 11

12. Basic Terms:
 Retention Time (tR): The total time that a compound spends in both the
mobile phase and the stationary phase i.e. the time between sample
injection and an analytical peak reaching a detector at the end of the
column. The time taken for the mobile phase to pass through the column
is called tR.
 Dead Time (tm): The time that a non-retained compound spends in the
mobile phase, which also is the amount of time the non-retained
compound spends in the column.
 Adjusted Retention Time (TR
’): The time that a compound spends in the
stationary phase. It is the difference between the dead time and the
retention time for a compound
Tr
’ = tr - tm
12

13. Basic Terms:
 Capacity Factor (or Partition Ratio) (k’): The ratio of the mass of the
compound in the stationary phase relative to the mass of the compound
in the mobile phase.
 Phase Ratio (b): The phase ratio relates the column diameter and the
film thickness of the stationary phase. The phase ratio is unitless and
constant for a particular column and represents the volume ratio β.
 Distribution Constant (KD): The distribution constant is a ratio of the
concentration of a compound in the stationary phase relative to the
concentration of the compound in the mobile phase.
13

14. Basic Terms:
 Selectivity (or Separation Factor) (α): It is a ratio of the capacity factors
of two peaks. It is always greater than or equal to one. The higher the
selectivity, the more will be the separation between two compounds or
peaks.
 Linear Velocity (u): It is the speed at which the carrier gas or mobile
phase travels through the column.
 Efficiency: It is related to the number of compounds that can be
separated by the column.
 Retention Volume: VR = tR*F (retained) VM = tM*F (non- retained)
 F = average volumetric flow rate (mL/min)
 VR and VM both depend on pressure inside the column and temperature
of the column.
14

15. Basic Terms:
 Pressure drop factor (j): Is used to calculate average pressure from inlet
pressure Pinlet and outlet pressure Poutlet .
j = 3[(Pinlet / Poutlet )2 -1]/ 2 [(Pinlet / Poutlet )3 -1]
 Corrected Retention Volume:
VR
0 = j*tR*F (reatined) VM
0 = j* tM*F (non- retained)
 Specific Retention Volume:
Vg = [(VR
0- VM
0)/ MS]*[273/Tcolumn];
MS = mass of stationary phase.
Vg = [K/ ρstationary]*[273/T column];
K = partition ratio; ρ stationary = density of stationary phase.
15

16. Basic Terms:
Separation factor (S)
Ratio of partition co-efficient of the two components to be separated.
 If more difference in partition co-efficient b/w two compounds, the
peaks are far apart & S
 Is more. If partition co-efficient of two compounds are similar, then
peaks are closer.
16

17. Theoretical Plate
 An imaginary unit of the column where equilibrium has been established
between S.P & M.P
 It can also be called as a functional unit of the column
HETP – Height Equivalent to a Theoretical Plate
 Efficiency of a column is expressed by the number of theoretical plates
in the column or HETP
 If HETP is less, the column is more efficient.
 If HETP is more, the column is less efficient
17

18. Theoretical Plate
L = Length of the column
N = No. of theoretical plates
HETP is given by Van Deemter equation
HETP=
A = Eddy diffusion term or multiple path diffusion which arises due to
packing of the column
B = Molecular diffusion, depends on flow rate
C = Effect of mass transfer, depends on flow rate
u = Flow rate
18

19. Efficiency ( No. of Theoretical plates)
It can be determined by using the formula
𝑁 =
𝑅𝑡2
𝑊2
N = no. of theoretical plates
Rt = retention time
W = peak width at base
 The no. of theoretical plates is high, the column is highly efficient
 For G.C the value of 600/ meter
19

20. GC Instrumentation: Block Diagram 20

21. Main Components:
 Carrier Gas:
 Injector:
 Column:
 Oven:
 Detectors:
21

22. Quantification in Chromatography:
 Area or height of the peak is proportional to the concentration of the
analyte.
 The area is a more precise measure.
 Nevertheless when peaks co-elute (are not separated on the baseline),
the height may be used.
 Both the external and internal methods of calibration are employed
 Area=height×width at half-height=h×w (1/2)
 Mole % A=area of Peak Atotal area×100%
22

23. Three major types:
 Gas - Solid chromatography (stationary phase: solid)
 Gas - Liquid chromatography (stationary phase: immobilized
liquid)
 Gas - Bonded phase (relatively new)
23

24. Carrier Gas:
 He (common),
 Others: N2, H2, Ar and Air.
 Safety, availability, non-flammability, cost and efficiency are factors for
gas selection.
 Purity of 99.995 % and higher is considered for selection as well.
 Pinlet = 10-50 psig
 F = 25-150 mL/min for packed column
 F = 1-25 mL/min for open tubular column
24

25. Requirements for a carrier gas
 Inertness
 Suitable for the detector
 High purity
 Easily available
 Cheap
 Should not cause the risk of fire
 Should give best column performance
25

26. Flow Regulators
 Deliver the gas with uniform pressure/flow rate
 Flow meters:- Rota meter & Soap bubble flow meter
 Rota meter
Placed before column inlet
It has a glass tube with a float held on to a spring.
The level of the float is determined by the flow rate of carrier gas
 Soap Bubble Meter
Similar to Rota meter & instead of a float, soap bubble formed indicates
the flow rate
26

27. Flow Regulators 27
Rotameter Soap Bubble Meter

28. Injector:
 Transfers the analyte into the column.
 It provides the means to introduce a sample into a continuous flow of
carrier gas.
 Injectors are usually heated to ensure analyte’s transfer to a gas phase.
 Volatile liquid or gaseous sample is injected through a septum.
 Vapor is swept through column.
 Types:
1. Split/ Splitless
2. On – Column
3. PTV Injector
4. P/T (Purge and Trap) System
28

29. Split/ Splitless Injector
 Usually consists of heated liner (a glass sleeve, prior to the column
(200–300 °C).
 A sample is introduced into a heated small chamber via a syringe
through a septum – the heat facilitates volatilization of the sample and
sample matrix.
 The carrier gas then either sweeps the entirety (splitless mode) or a
portion (split mode) of the sample into the column.
 In split mode, a part of the sample/carrier gas mixture in the injection
chamber is exhausted through the split vent.
 Split injection is preferred when working with samples with high
analyte concentrations (>0.1%) whereas splitless injection is best suited
for trace analysis with low amount of analytes (<0.01%).
29

30. Split/ Splitless Injector
 In splitless mode the split valve opens after a pre-set amount of time to
purge heavier elements that would otherwise contaminate the system.
 This pre-set (splitless) time should be optimized, the shorter time (e.g.,
0.2 min) ensures less tailing but loss in response, the longer time (2 min)
increases tailing against signal.
 – Split (dilution) only part of sample is introduced to the column 1:25 -
1:600
 – Splitless – all the sample is introduced (but only for limited time
period)
30

31. Split/ Splitless Injector 31

32. Split Injector 32

33. Splitless Injector 33

34. On-Column Injector
 The sample is here introduced directly into the column in its entirety
without heating or at a temperature below the boiling point of the
solvent.
 The low temperature condenses the sample into a narrow zone.
 The column and inlet can then be heated, releasing the sample into the
gas phase.
 This ensures the lowest possible temperature for chromatography and
keeps samples from decomposing above their boiling point.
 Analytes are injected directly on the column.
 This technique is suitable for thermally unstable compounds.
34

35. On-Column Injector 35

36. PTV Injector
 Temperature-programmed sample introduction was first described by Vogt in
1979.
 Originally Vogt developed the technique as a method for the introduction of
large sample volumes (up to 250 µL) in capillary GC.
 Vogt introduced the sample into the liner at a controlled injection rate.
 The temperature of the liner was chosen slightly below the boiling point of the
solvent.
 The low-boiling solvent was continuously evaporated and vented through the
split line.
 Based on this technique, Poy developed the Programmed Temperature
Vaporizing injector; PTV.
 By introducing the sample at a low initial liner temperature many of the
disadvantages of the classic hot injection techniques could be circumvented.
36

37. PTV Injector 37

38. P/T (Purge-and-Trap) System
 An inert gas is bubbled through an aqueous sample causing insoluble
volatile chemicals to be purged from the matrix.
 The volatiles are 'trapped' on an absorbent column (known as a trap or
concentrator) at ambient temperature.
 The trap is then heated and the volatiles are directed into the carrier gas
stream.
 Samples requiring pre concentration or purification can be introduced
via such a system, usually hooked up to the S/SL port.
38

39. Sample Injection:
 The real chromatographic analysis starts with the introduction of the sample
onto the column.
 The technique of on-column injection, often used with packed columns, is
usually not possible with capillary columns.
 The injection system in the capillary gas chromatograph should fulfill the
following two requirements:
 The amount injected should not overload the column.
 The width of the injected plug should be small compared to the spreading due
to the chromatographic process.
 Failure to comply with this requirement will reduce the separation capability of
the column.
 As a general rule, the volume injected, Vinj and the volume of the detector cell,
Vdet should be about 1/10 of the volume occupied by the portion of sample
containing the molecules of interest (analytes) when they exit the column.
39

40. General criteria for injection system selection 40

41. Injectors 41
True Cold
On Column
Split/ Splitless PTV Packed and
Wide born Column

42. Sample Injection:
 Some general requirements which a good injection technique should
fulfil are:
1) It should be possible to obtain the column’s optimum separation
efficiency.
2) It should allow accurate and reproducible injections of small amounts
of representative samples.
3) It should induce no change in sample composition.
4) It should not exhibit discrimination based on differences in boiling
point, polarity, concentration or thermal/catalytic stability.
5) It should be applicable for trace analysis as well as for undiluted
samples.
42

43. Sample Injection: 43

44. Sample Injection: Features
 Volume Injected is typically 0.1-3μL (liquid)
 The injected volume is limited by the volume of solvent as a vapour phase.
 At 200°C and pressure on column 100 kPa
 1 μL of hexane (l) forms 222 μL (g)
 1 μL of methylene chloride (l) forms 310 μL (g)
 1 μL of water (l) form 1111 μL (g)
 Volume of vapour > then volume of injector = backflash (system
contamination)
 Concentration
 Is defined by column retaining capacity
 Columns with a thicker film thickness (a stationary phase) retain more of the
analyte.
44

45. On Column Injection:
 On column injection for samples which would decompose at higher
temperatures
 Injects the sample directly on the column or the guard column.
 All the sample is introduced on the column.
 Also all interfering components are injected.
 In past, the column has to be ca. 0.53 mm I.D. so the syringe needle can
fit in.
45

46. Features
 Split injection - A fraction of a solute (solvent) is injected, therefore
peaks are sharp.
 Splitless injection – for trace analysis. The split valve is closed and
most of the sample is introduced on the column. The volume of the gas
going through the injector is only ca. 1 ml/min. Thus, sample
components are transferred to the column for long time. Thus peak is
tailing.
 Splitless time - If the split valve is opened after certain time 20 - 120 s,
the transfer of sample is stopped. Still the transfer can be prolonged,
causing an increased peak width.
 Solvent trapping - Injecting the sample to the column at temperature
bellow boiling point of a solvent <20°C, after 30s (splitless time) a fast
increase in the temperature to 20°C above solvent’s boiling point. Fast
transfer from gas to liquid and again to the gas phase sharpens the
elution band.
46

47. Injectors: 47

48. Columns:
 Gas chromatography columns are of two designs: packed and capillary.
 Packed columns are typically a glass or stainless steel coil (typically 1-5
m total length and 5 mm inner diameter) that is filled with the stationary
phase, or a packing coated with the stationary phase.
 Capillary columns are a thin fused-silica (purified silicate glass)
capillary (typically 10-100 m in length and 250 mm inner diameter) that
has the stationary phase coated on the inner surface.
 Capillary columns provide much higher separation efficiency than
packed columns but are more easily overloaded by too much sample.
48

49. Columns:
 Columns: Separate the analytes. 2-50 m coiled stainless
steel/glass/Teflon.
 The main chemical attribute regarded when choosing a column is
the polarity of the mixture, but functional groups can play a large part in
column selection.
 The polarity of the sample must closely match the polarity of the column
stationary phase to increase resolution and separation while reducing run
time.
 The separation and run time also depends on the film thickness (of the
stationary phase), the column diameter and the column length.
49

50. Columns:
 Packed
1. Solid particles either porous or non-porous coated with thin (1 μm) film of
liquid
2. 3 - 6 mm ID; 1 - 5 m length
 Capillary (open tubular) silica columns
1. 0.1 - 0.5 mm I.D. (internal diameter); 15 - 100 m length
2. Inner wall modified with thin (0.1-5 μm) film of liquid (stationary phase)
3. Easy to install
4. Well defined stationary phase
5. Optimal flow rate depends on carrier gas, I.D., film thickness
6. As the linear velocity, I.D. and film thickness increases so is the van Deemter
curve steeper.
50

51. Columns: 51
Capillary Column
Packed Column

52. Equilibration of the column
 Before introduction of the sample
 Column is attached to instrument & desired flow rate by flow regulators
 Set desired temperature.
 Conditioning is achieved by passing carrier gas for 24 hours.
52

53. Temperature Control Devices
 Preheaters: convert sample into its vapour form, present along with
injecting devices
 Thermostatically controlled oven: Temperature maintenance in a column
is highly essential for efficient separation.
 Two types of operations
 Isothermal programming:-
 Linear programming:- this method is efficient for separation of
complex mixtures
53

54. Columns: Stationary Phases
 The most common stationary phases in gas-chromatography columns are
polysiloxanes, which contain various substituent groups to change the polarity of the
phase.
 The nonpolar end of the spectrum is polydimethyl siloxane, which can be made more
polar by increasing the percentage of phenyl groups on the polymer.
 For every polar analytes, polyethylene glycol (a.k.a. carbowax) is commonly used as
the stationary phase.
 After the polymer coats the column wall or packing material, it is often cross-linked to
increase the thermal stability of the stationary phase and prevent it from gradually
bleeding out of the column.
 Small gaseous species can be separated by gas-solid chromatography. Gas-solid
chromatography uses packed columns containing high-surface-area inorganic or
polymer packing.
 The gaseous species are separated by their size and retention due to adsorption on the
packing material.
54

55. Columns: Stationary Phases
 Stationary Phases: Must have:
1. Low volatility
2. Thermal stability
3. Chemical inertness
4. Solvation properties giving suitable values for k’, α.
 Stationary phases are usually bonded and/or cross-linked
• bonding - covalent linking of stationary phase to support.
• cross-linking - polymerization reactions after bonding to join individual
stationary phase molecules.
55

56. Column Stationary Phases:
 Packed
• liquid coated silica particles (<100-300 mm diameter) in glass tube
• best for large scale but slow and inefficient.
 Capillary/Open Tubular
• wall-coated (WCOT) <1 mm thick liquid coating on inside of silica tube
• support-coated (SCOT) 30 mm thick coating of liquid, coated support on
inside of silica tube
• best for speed and efficiency but only small samples.
56

57. Columns: 57

58. Immobilized Liquid Stationary Phases:
 Low volatility
 High decomposition temperature
 Chemically inert (reversible interactions with solvent)
 Chemically attached to support (prevent "bleeding")
 Appropriate k' and α for good resolution.
58

59. Some Common Stationary Phases – GLC 59

60. Stationary Phase Compound Selection:
 The polarity of the solute is crucial for the choice of stationary phase
compound, which in an optimal case would have a similar polarity as
the solute.
 Common stationary phase compounds in open tubular columns are
cyanopropylphenyl dimethyl polysiloxane, carbowax polyethylene
glycol, biscyanopropyl cyanopropylphenyl polysiloxane and diphenyl
dimethyl polysiloxane.
 For packed columns more options are available. Solid stationary phase
adsorbents are SiO2 (silica gel), Al2O2 (alumina), charcoal and Na/ Ca
Al Silicates.
60

61. GC - Modes of Separation:
1. Isothermal GC
2. Programmed temperature GC
3. Programmed pressure GC
 Temperature Effect
 Increase in temperature
 Decreases retention time
 Sharpens peak
61

62. Oven:
 0-400 °C ~ average boiling point of sample.
 Accurate to <1 °C.
 The column(s) in a GC is/are contained in an oven, the temperature of
which is precisely controlled electronically.
 The rate at which a sample passes through the column is directly
proportional to the temperature of the column.
 The higher the column temperature, the faster the sample moves through
the column.
 However, the faster a sample moves through the column, the less it
interacts with the stationary phase and the less the analytes are
separated.
62

63. Oven:
 A method which holds the column at the same temperature for the entire
analysis is called "isothermal".
 Most methods, however, increase the column temperature during the
analysis, the initial temperature, rate of temperature increase (the
temperature "ramp"), and final temperature manipulations are called the
temperature program.
 A trade-off is maintained between the length of analysis and level of
separation.
63

64. Selecting Temperature Conditions:
 Temperature of injector: ensures evaporation of sample, but do not
decompose it (200 – 300 °C).
 Temperature of the column (GC oven).
 Effect of injection.
 For the split injection– no specific requirements.
 For the splitless and on column injection – solvent trapping technique
 Oven temperature - optimized to improve the separation.
 Temperature of the detector: has to be high enough to prevent
condensation of analytes on the detector.
64

65. Detectors:
 The detector is placed at the exit of the column.
 It is employed to detect and provide a quantitative measurement of the
various constituents of the sample as they emerge from the column in
combination with the carrier gas.
 The choice of a particular type of detector is governed by the following
factors:
 High sensitivity, sufficient enough to provide adequate signal for even small sample
 Response should be linear, unaffected by temperature and flow rate.
 Non distorted shape of peak and non destructive.
 Detector temperature must not condense the eluted vapours in it.
 Simple & Inexpensive
 Applicable to wide range of samples
 Good reproducibility, rapidity and linearity.
65

66. Name of few detectors
1. Kathorometer or Thermal Conductivity Detector (TCD)
2. Flame Ionisation Detector (FID)
3. Flame Photometric Detector (FPD)
4. Photo Ionization Detector (PID)
5. Argon Ionization Detector (AID)
6. Electron Capture Detector (ECD)
7. Cross Section Ionization Detector (CSID)
8. Atomic Emission Detector (AED)
9. Chemiluminescence Spectroscopy- based Detector (CSD)
10. Nitrogen Phosphorus Detector
11. Catalytic Combustion Detector (CCD)
12. Discharge Ionization Detector (DID)
13. Helium Ionization Detector (HID)
14. Pulsed Discharge Ionization Detector (PDD)
15. Thermionic Ionization Detector (TID)
16. Hall Electrolytic Conductivity Detector (HECD)
66

67. Detectors Classification 67

68. Commonly Used Detectors:
 Among all the GC detectors available, the most commonly used types
are
1. Thermal Conductivity Detector TCD
2. Flame Ionization Detector FID
3. Electron Capture Detector ECD
4. Atomic-Emission Detector AED
5. Flame Photometric GC Detector FPD
68

69. Thermal Conductivity Detector TCD
 This common detector relies on the thermal conductivity of matter
passing around a tungsten -rhenium filament with a current traveling
through it.
 In this set up, helium or nitrogen serves as the carrier gas because of
their relatively high thermal conductivity which keeps the filament cool
and maintains uniform resistivity and electrical efficiency of the
filament.
 However, when analyte molecules elute from the column, mixed with
carrier gas, the thermal conductivity decreases and this causes a detector
to loose response.
 The response is due to the decreased thermal conductivity causing an
increase in filament temperature and resistivity resulting in fluctuations
in voltage.
69

70. Thermal Conductivity Detector TCD
 Detector sensitivity is proportional to filament current while it's
inversely proportional to the immediate environmental temperature of
that detector as well as flow rate of the carrier gas.
 It is rugged and has wide range and also it is non – destructive. However
sensitivity is non – uniform.
70
Selectivity: All compounds except for carrier gases.
Sensitivity: 5 – 20 ng
Temperature: 150 – 250 °

71. Thermal Conductivity Detector
 When a separated compound elutes from the column , the thermal
conductivity of the mixture of carrier gas and compound gas is lowered.
 The filament in the sample column becomes hotter than the control
column.
 The imbalance between control and sample filament temperature is
measured by a simple gadget and a signal is recorded.
71

72. Merits & Demerits of TCD
Advantages
 Linearity is good
 Applicable to most compounds
 Non destructive
 Simple & inexpensive
Disadvantages
 Low sensitivity
 Affected by fluctuations in temperature and flow rate
 Biological samples cannot be analyzed
72

73. Flame Ionization Detector FID
 In this common detector, electrodes are placed adjacent to a flame fueled
by hydrogen / air near the exit of the column, and when carbon containing
compounds exit the column they are pyrolyzed by the flame.
 This detector works only for organic / hydrocarbon containing compounds
due to the ability of the carbons to form cations and electrons upon
pyrolysis which generates a current between the electrodes.
 The increase in current is translated and appears as a peak in a
chromatogram. FIDs have low detection limits (a few picograms per
second, but they are unable to generate ions from carbonyl containing
carbons.
73
Selectivity: Compounds with C – H bonds.
Sensitivity: 0.1 – 10 ng
Temperature: 250 – 400 °

74. Flame Ionization Detector FID
 FID compatible carrier gasses include nitrogen, helium, and argon.
 These are rugged, sensitive and have wide dynamic range, however they
are destructive and are not sensitive to non - combustibles.
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75. Merits & Demerits of FID
 Merits
 µg quantities of the solute can be detected
 Stable
 Responds to most of the organic compounds
 Linearity is excellent
 Demerits
 FIDs are mass sensitive rather than conc. sensitive
 Destroy the sample
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76. Electron Capture Detector ECD
 It uses a radioactive beta particle (electron) source to measure the degree
of electron capture. ECD is used for the detection of molecules
containing electronegative / withdrawing elements and functional
groups like halogens, carbonyl, nitriles, nitro groups, and organo -
metalics.
 In this type of detector, either nitrogen or 5% methane in Ar is used as
the mobile phase carrier gas.
 The carrier gas passes between two electrodes placed at the end of the
column and adjacent to the anode (negative electrode) that resides in a
radioactive foil such as 63Ni.
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77. Electron Capture Detector ECD
 The radioactive foil emits a beta particle (electron) which collides with
and ionizes the carrier gas to generate more ions resulting in a current.
 When analyte molecules with electronegative / withdrawing elements or
functional group electrons are captured, it results in a decrease in current
generating a detector response.
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78. Electron Capture Detector ECD 78
Selectivity: Halogens, Nitrates and conjugated carbonyls
Sensitivity: 0.1 – 10 pg
Temperature: 300 – 400 °

79. Electron Capture Detector ECD
 The detector consists of a cavity that contains two electrodes and a radiation
source that emits - radiation (e.g.63Ni, 3H)
 The collision between electrons and the carrier gas (methane plus an inert gas)
produces a plasma containing electrons and positive ions.
 If a compound is present that contains electronegative atoms, those electrons
are captured and negative ions are formed, and rate of electron collection
decreases
 The detector selective for compounds with atoms of high electron affinity.
ADVANTAGE
 Highly sensitive
DISADVANTAGE
 Used only for compounds with electron affinity
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80. Atomic-Emission Detector AED
 As capillary column based gas chromatography takes its place as the
major, highest resolution separation technique available for volatile,
thermally stable compounds, the requirements for the sensitive and
selective detection of these compounds increase.
 Since more and more complex mixtures can be successfully separated,
subsequent chromatograms (output of a chromatographic separation) are
increasingly more complex.
 Therefore, the need to differentiate between the sample components
using the GC detector as a means of compounds discriminating is more
and more common.
 In addition, each detector has its own characteristics (selectivity,
sensitivity, linear range, stability, cost etc.) that helps in a decision about
which detector to use.
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81. Atomic-Emission Detector AED
 AED detector, while quite expensive compared to other commercially
available GC detectors, is an extremely powerful alternative.
 Instead of measuring simple gas phase (carbon containing) ions created
in a flame as with the flame ionization detector, or the change in
background current because of electronegative element capture of
thermal electrons as with the electron capture detector, the AED has a
much wider applicability because it is based on the detection of atomic
emissions.
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82. Atomic-Emission Detector AED
 The strength of the AED lies in the detector's ability to simultaneously
determine the atomic emissions of many of the elements in analytes that
elute from a GC capillary column (called eluants or solutes in some
books).
 As eluants come off the capillary column they are fed into a microwave
powered plasma (or discharge) cavity where the compounds are
destroyed and their atoms are excited by the energy of the plasma.
 The light that is emitted by the excited particles is separated into
individual lines via a photodiode array.
 The associated computer then sorts out the individual emission lines and
can produce chromatograms made up of peaks from eluants that contain
only a specific element.
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83. Atomic-Emission Detector AED
 The components of the AED include
1. An interface for the incoming capillary GC column to the microwave
induced plasma chamber,
2. The microwave chamber itself,
3. A cooling system for that chamber,
4. A diffraction grating and associated optics to focus then disperse the
spectral atomic lines, and
5. A position adjustable photodiode array interfaced to a computer.
 The microwave cavity cooling is required because much of the energy
focused into the cavity is converted to heat.
83

84. Atomic-Emission Detector AED 84

85. Flame Photometric GC Detector FPD
 The reason to use more than one kind of detector for gas
chromatography is to achieve selective and/or highly sensitive detection
of specific compounds encountered in particular chromatographic
analyses.
 The determination of sulfur or phosphorus containing compounds is the
job of the flame photometric detector (FPD).
 This device uses the chemiluminescent reactions of these compounds in
a hydrogen/air flame as a source of analytical information that is
relatively specific for substances containing these two kinds of atoms.
 The emitting species for sulfur compounds is excited S2.
85

86. Flame Photometric GC Detector FPD
 The lambda max for emission of excited S2 is approximately 394 nm.
 The emitter for phosphorus compounds in the flame is excited HPO
(lambda max = doublet 510-526 nm).
 In order to selectively detect one or the other family of compounds as it
elutes from the GC column, an interference filter is used between the
flame and the photomultiplier tube (PMT) to isolate the appropriate
emission band.
 The drawback here being that the filter must be exchanged between
chromatographic runs if the other family of compounds is to be detected.
86

87. Flame Photometric GC Detector FPD 87

88. Flame Photometric GC Detector FPD 88

89. Qualitative analysis:
 Generally chromatographic data is presented as a graph of detector
response (y-axis) against retention time (x-axis), which is called a
chromatogram.
 This provides a spectrum of peaks for a sample representing
the analytes present in a sample eluting from the column at different
times.
 The number of components in a sample is determined by the number of
peaks.
 The amount of a given component in a sample is determined by the area
under the peaks.
 The identity of components can be determined by the given retention
times.
89

90. Quantitative analysis:
 The area under a peak is proportional to the amount of analyte present in
the chromatogram.
 By calculating the area of the peak using the mathematical function of
integration, the concentration of an analyte in the original sample can be
determined.
90

91. Advantages of GC
 High Resolution
 Very high sensitivity, detect down to 100 ppm.
 Very good precision and accuracy.
 Very good separation
 Time (analysis is short), fast analysis is possible.
 Small sample is needed - ml
 Good detection system
 Quantitatively analyzed
91

92. Applications of GC
 G.C is capable of separating, detecting & partially characterizing
the organic compounds , particularly when present in small
quantities.
1, Qualitative analysis
Rt & RV are used for the identification & separation
2, Checking the purity of a compound
Compare the chromatogram of the std. & that of the sample
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93. References
 Analytical Instrumentation, Bela G. Liptak
 Principles of Instrumental Analysis 6E, Skoog, Holler and Crouch
 Handbook of Analytical Instruments, 2nd Edition, R S Khandpur
 InterScience: Gas Chromatography & CC-MS Principles IS 2008-04-18
 http://analysciences.com/apat2013gcworkshop-2-130703192516-
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