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Gas Chromatography
ER. FARUK BIN POYEN, Asst. Professor
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
faruk.poyen@gmail.com
Contents
 Introduction
 Chromatographic Classifications
 Definition
 Working Principle
 Basic Terms
 GC Instrumentat...
Introduction:
 Chromatography is a physical method of separation of the components
of a mixture by distribution between t...
Chromatographic Classifications:
 Gas Chromatography
 Gas / Liquid (partition)
 Gas / Solid (adsorbent)
 Liquid Chroma...
Adsorption Chromatography Principle:
 Adsorption chromatography definition:
 It is a process of separation of components...
Partition (coefficient) Chromatography:
 Partition chromatography is process of separation whereby the components of
the ...
Definition:
 Gas Chromatography (GC) is an analytical procedure employed for
separating compounds based primarily on thei...
Working Principle of GC:
 A gas chromatograph uses a flow-through narrow tube known as
the column, through which differen...
Working Principle of GC:
 Other parameters that can be used to alter the order or time of retention
are the carrier gas f...
Working Principle of GC:
 Since each type of molecule has a different rate of progression, the
various components of the ...
Working Principle of GC: 11
Basic Terms:
 Retention Time (tR): The total time that a compound spends in both the
mobile phase and the stationary phas...
Basic Terms:
 Capacity Factor (or Partition Ratio) (k’): The ratio of the mass of the
compound in the stationary phase re...
Basic Terms:
 Selectivity (or Separation Factor) (α): It is a ratio of the capacity factors
of two peaks. It is always gr...
Basic Terms:
 Pressure drop factor (j): Is used to calculate average pressure from inlet
pressure Pinlet and outlet press...
Basic Terms:
Separation factor (S)
Ratio of partition co-efficient of the two components to be separated.
 If more differ...
Theoretical Plate
 An imaginary unit of the column where equilibrium has been established
between S.P & M.P
 It can also...
Theoretical Plate
L = Length of the column
N = No. of theoretical plates
HETP is given by Van Deemter equation
HETP=
A = E...
Efficiency ( No. of Theoretical plates)
It can be determined by using the formula
𝑁 =
𝑅𝑡2
𝑊2
N = no. of theoretical plates...
GC Instrumentation: Block Diagram 20
Main Components:
 Carrier Gas:
 Injector:
 Column:
 Oven:
 Detectors:
21
Quantification in Chromatography:
 Area or height of the peak is proportional to the concentration of the
analyte.
 The ...
Three major types:
 Gas - Solid chromatography (stationary phase: solid)
 Gas - Liquid chromatography (stationary phase:...
Carrier Gas:
 He (common),
 Others: N2, H2, Ar and Air.
 Safety, availability, non-flammability, cost and efficiency ar...
Requirements for a carrier gas
 Inertness
 Suitable for the detector
 High purity
 Easily available
 Cheap
 Should n...
Flow Regulators
 Deliver the gas with uniform pressure/flow rate
 Flow meters:- Rota meter & Soap bubble flow meter
 Ro...
Flow Regulators 27
Rotameter Soap Bubble Meter
Injector:
 Transfers the analyte into the column.
 It provides the means to introduce a sample into a continuous flow of...
Split/ Splitless Injector
 Usually consists of heated liner (a glass sleeve, prior to the column
(200–300 °C).
 A sample...
Split/ Splitless Injector
 In splitless mode the split valve opens after a pre-set amount of time to
purge heavier elemen...
Split/ Splitless Injector 31
Split Injector 32
Splitless Injector 33
On-Column Injector
 The sample is here introduced directly into the column in its entirety
without heating or at a temper...
On-Column Injector 35
PTV Injector
 Temperature-programmed sample introduction was first described by Vogt in
1979.
 Originally Vogt developed...
PTV Injector 37
P/T (Purge-and-Trap) System
 An inert gas is bubbled through an aqueous sample causing insoluble
volatile chemicals to be...
Sample Injection:
 The real chromatographic analysis starts with the introduction of the sample
onto the column.
 The te...
General criteria for injection system selection 40
Injectors 41
True Cold
On Column
Split/ Splitless PTV Packed and
Wide born Column
Sample Injection:
 Some general requirements which a good injection technique should
fulfil are:
1) It should be possible...
Sample Injection: 43
Sample Injection: Features
 Volume Injected is typically 0.1-3μL (liquid)
 The injected volume is limited by the volume ...
On Column Injection:
 On column injection for samples which would decompose at higher
temperatures
 Injects the sample d...
Features
 Split injection - A fraction of a solute (solvent) is injected, therefore
peaks are sharp.
 Splitless injectio...
Injectors: 47
Columns:
 Gas chromatography columns are of two designs: packed and capillary.
 Packed columns are typically a glass or ...
Columns:
 Columns: Separate the analytes. 2-50 m coiled stainless
steel/glass/Teflon.
 The main chemical attribute regar...
Columns:
 Packed
1. Solid particles either porous or non-porous coated with thin (1 μm) film of
liquid
2. 3 - 6 mm ID; 1 ...
Columns: 51
Capillary Column
Packed Column
Equilibration of the column
 Before introduction of the sample
 Column is attached to instrument & desired flow rate by ...
Temperature Control Devices
 Preheaters: convert sample into its vapour form, present along with
injecting devices
 Ther...
Columns: Stationary Phases
 The most common stationary phases in gas-chromatography columns are
polysiloxanes, which cont...
Columns: Stationary Phases
 Stationary Phases: Must have:
1. Low volatility
2. Thermal stability
3. Chemical inertness
4....
Column Stationary Phases:
 Packed
• liquid coated silica particles (<100-300 mm diameter) in glass tube
• best for large ...
Columns: 57
Immobilized Liquid Stationary Phases:
 Low volatility
 High decomposition temperature
 Chemically inert (reversible int...
Some Common Stationary Phases – GLC 59
Stationary Phase Compound Selection:
 The polarity of the solute is crucial for the choice of stationary phase
compound, ...
GC - Modes of Separation:
1. Isothermal GC
2. Programmed temperature GC
3. Programmed pressure GC
 Temperature Effect
 I...
Oven:
 0-400 °C ~ average boiling point of sample.
 Accurate to <1 °C.
 The column(s) in a GC is/are contained in an ov...
Oven:
 A method which holds the column at the same temperature for the entire
analysis is called "isothermal".
 Most met...
Selecting Temperature Conditions:
 Temperature of injector: ensures evaporation of sample, but do not
decompose it (200 –...
Detectors:
 The detector is placed at the exit of the column.
 It is employed to detect and provide a quantitative measu...
Name of few detectors
1. Kathorometer or Thermal Conductivity Detector (TCD)
2. Flame Ionisation Detector (FID)
3. Flame P...
Detectors Classification 67
Commonly Used Detectors:
 Among all the GC detectors available, the most commonly used types
are
1. Thermal Conductivity ...
Thermal Conductivity Detector TCD
 This common detector relies on the thermal conductivity of matter
passing around a tun...
Thermal Conductivity Detector TCD
 Detector sensitivity is proportional to filament current while it's
inversely proporti...
Thermal Conductivity Detector
 When a separated compound elutes from the column , the thermal
conductivity of the mixture...
Merits & Demerits of TCD
Advantages
 Linearity is good
 Applicable to most compounds
 Non destructive
 Simple & inexpe...
Flame Ionization Detector FID
 In this common detector, electrodes are placed adjacent to a flame fueled
by hydrogen / ai...
Flame Ionization Detector FID
 FID compatible carrier gasses include nitrogen, helium, and argon.
 These are rugged, sen...
Merits & Demerits of FID
 Merits
 µg quantities of the solute can be detected
 Stable
 Responds to most of the organic...
Electron Capture Detector ECD
 It uses a radioactive beta particle (electron) source to measure the degree
of electron ca...
Electron Capture Detector ECD
 The radioactive foil emits a beta particle (electron) which collides with
and ionizes the ...
Electron Capture Detector ECD 78
Selectivity: Halogens, Nitrates and conjugated carbonyls
Sensitivity: 0.1 – 10 pg
Tempera...
Electron Capture Detector ECD
 The detector consists of a cavity that contains two electrodes and a radiation
source that...
Atomic-Emission Detector AED
 As capillary column based gas chromatography takes its place as the
major, highest resoluti...
Atomic-Emission Detector AED
 AED detector, while quite expensive compared to other commercially
available GC detectors, ...
Atomic-Emission Detector AED
 The strength of the AED lies in the detector's ability to simultaneously
determine the atom...
Atomic-Emission Detector AED
 The components of the AED include
1. An interface for the incoming capillary GC column to t...
Atomic-Emission Detector AED 84
Flame Photometric GC Detector FPD
 The reason to use more than one kind of detector for gas
chromatography is to achieve ...
Flame Photometric GC Detector FPD
 The lambda max for emission of excited S2 is approximately 394 nm.
 The emitter for p...
Flame Photometric GC Detector FPD 87
Flame Photometric GC Detector FPD 88
Qualitative analysis:
 Generally chromatographic data is presented as a graph of detector
response (y-axis) against reten...
Quantitative analysis:
 The area under a peak is proportional to the amount of analyte present in
the chromatogram.
 By ...
Advantages of GC
 High Resolution
 Very high sensitivity, detect down to 100 ppm.
 Very good precision and accuracy.
 ...
Applications of GC
 G.C is capable of separating, detecting & partially characterizing
the organic compounds , particular...
References
 Analytical Instrumentation, Bela G. Liptak
 Principles of Instrumental Analysis 6E, Skoog, Holler and Crouch...
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Gas Chromatography

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Gas Chromatography

  1. 1. Gas Chromatography ER. FARUK BIN POYEN, Asst. Professor DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA faruk.poyen@gmail.com
  2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 11. Working Principle of GC: 11
  12. 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. 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. 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. 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. 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. 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. 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. 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. 20. GC Instrumentation: Block Diagram 20
  21. 21. Main Components:  Carrier Gas:  Injector:  Column:  Oven:  Detectors: 21
  22. 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. 23. Three major types:  Gas - Solid chromatography (stationary phase: solid)  Gas - Liquid chromatography (stationary phase: immobilized liquid)  Gas - Bonded phase (relatively new) 23
  24. 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. 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. 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. 27. Flow Regulators 27 Rotameter Soap Bubble Meter
  28. 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. 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. 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. 31. Split/ Splitless Injector 31
  32. 32. Split Injector 32
  33. 33. Splitless Injector 33
  34. 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. 35. On-Column Injector 35
  36. 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. 37. PTV Injector 37
  38. 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. 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. 40. General criteria for injection system selection 40
  41. 41. Injectors 41 True Cold On Column Split/ Splitless PTV Packed and Wide born Column
  42. 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. 43. Sample Injection: 43
  44. 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. 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. 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. 47. Injectors: 47
  48. 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. 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. 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. 51. Columns: 51 Capillary Column Packed Column
  52. 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. 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. 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. 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. 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. 57. Columns: 57
  58. 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. 59. Some Common Stationary Phases – GLC 59
  60. 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. 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. 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. 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. 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. 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. 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. 67. Detectors Classification 67
  68. 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. 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. 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. 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. 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. 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. 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. 74
  75. 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 75
  76. 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. 76
  77. 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. 77
  78. 78. Electron Capture Detector ECD 78 Selectivity: Halogens, Nitrates and conjugated carbonyls Sensitivity: 0.1 – 10 pg Temperature: 300 – 400 °
  79. 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 79
  80. 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. 80
  81. 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. 81
  82. 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. 82
  83. 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. 84. Atomic-Emission Detector AED 84
  85. 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. 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. 87. Flame Photometric GC Detector FPD 87
  88. 88. Flame Photometric GC Detector FPD 88
  89. 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. 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. 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. 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 92
  93. 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- phpapp02 93

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Provides a detailed coverage on Gas Chromatography.

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