Gas Chromatography (G.C)

Gas Chromatography (G.C)

Gas Chromatography (G.C)

Principal of G.C:

Gas chromatography is a separation technique based on solid stationery phase and gas mobile phase. Separation is achieved by adsorption partition process.

Type of Detector For G.C: Mostly three type of detector used for gas chromatography.

    1. F.I.D. (Flame Ionization detector) For Typical sample and Hydrocarbons.
    2. T.C.D. (Thermal Connectivity Detector or Universal Detector) For General use.
    3. E.C.D. (Electron capture Detector) For Organohalogens, Chlorinated solvent and pesticide.
Some of other detector which is used in gas chromatography
  • N.P.D. (Nitrogen phosphorous Detector) for Organonitrogen & Organ phosphorous compound.
  • F.P.D. (Flame Photometric Detector 393nm) For Sulfur compound.
  • F.P.D. (Flame Photometric Detector 526nm) For Phosphorous compound.
  • P.I.D. (Photo Ionization Detector) Compound Ionized by U.V.
  • E.L.C.D. (Electrolytic Conductivity Detector) For Halogens, Nitrogen and Sulfur. 
  • F.T.I.R.D. (Fourier Transform Infrared Detector) For Organic compounds.
  • A.E.D. (Atomic Emissions Detector) Tunable for any species.
  • M.S.D. (Mass Spectrometric Detector) Tunable for any species.

F.I.D. (Flame Ionization detector):

The flame ionization detector is almost universally employed where the flame commonly is generated with hydrogen and air. The flame ionization detector is based on the detection of ions formed during combustion of organic compounds in a flame which generated by hydrogen and air to detect these ions. It has two electrodes which is used for potential difference when flame is produced,positive electron increase at the nozzle head. other negative electrode is situate above the flame when an organic compound is mixed within the hydrogen flame mainly carbon ions are generated and a current is produced between the electrodes proportionally to the amount of organic compound present this current is measured with a elector meter amplified into proper voltage and the final data is displayed on the computer in software the number of Peaks present can indicate how many components are in the mixture usually the x-axis of the gas chromatogram shows the amount of time taken for the analytes to pass through the column and reach the FID detector typically the y-axis or the area of the peak is a reflection of the amount of a specific analyte that's present you.

T.C.D.(Thermal conductivity detector):

Each gas enters the gas chromatograph instrument separately. The sample goes into one column while the pure carrier gas goes into another column. Electrically heated resistance wires are located in chambers inside of the TCD. The power supply provides a current to the resistance wires which causes the wires to heat up. The electrical circuitry shown here that is characteristic of thermal conductivity detectors are known as a Wheatstone bridge. The gas flows to the TCD the physical properties of the reference and sample gases. Different rates this change in temperature will result in a change in resistance from both the reference gas and the sample gas which produces an electrical signal unique to the compounds being analyzed this signal is proportional to the concentration of the sample components providing a direct means of measuring component concentrations in a particular sample and from this you receive a chromatogram that represents the components found in a sample analyzed. TCD chemically active compound like acids halogenated compounds and oxidizing substances can corrode the wires of the TCD and lead to permanent damage. TCD work best when there is a large difference in thermal conductivity between the carrier gas which is usually helium and the sample analyzed thus analyses of samples possessing a thermal conductivity close to the carrier gas will not result in accurate measurements additionally. TCD have a very low sensitivity for organic compounds compared to flame ionization detectors.

E.C.D.(Electron capture detector):

The electron capture detector is one of the most powerful tools. Chromatography has for determining housing containing analytes such as pesticides or halocarbons. The signal of the electron capture detector is derived from a decrease in a standing current which is present when the detector is on and carrier gas is flowing that decrease is caused by the presence of analytes entering the detector cell from the chromatographic column but what exactly does happen to the gaseous analyte as it passes the region near the radioactive source the analyte accepts or captures electrons created by the radioactive sources interaction with the carrier gas this affects the standing current that is being measured between the detectors electrodes. The intensity of that drop in current is also a measure of the quantity of analyte passing through the electron capture detector.

G.C.Column: Generally two types of column are used in gas chromatography.

  • Capillary column
  • Packed column

Capillary column:

Capillary column having three type coating, first coating is Polymide, Second coating is Fused silica and third coating is stationary phase.Its available in two basic forms.
Coated -  Simple coating on the inside of a fused silica tube. 
Bonded - Chemically bound via a silane bond.
Both types are coated on the outside with a polyamide to reduce breakage.

Gas Chromatography Capillary column.
Image Source-Google | Image by -QC/QA Methodology
This all adds up to allow components to remain on the column longer while still retaining good peak shape. 

Capillary column most effective when its have smaller ID, longer length, no packing, smaller sample capacity
  
Packed vs. capillary columns
Property of column
Packed column
Capillary column
Length in meter
0.5 - 5
5 - 100
ID, mm
2 - 4
0.1 - 0.7
Flow, ml/min
10 - 60
0.5 - 15
Head pressure, psig
10 - 40
3 - 40
Total plates
4000
250,000
Capacity
10 µg/peak
100 ng/peak
Film thickness, µm
1 - 10
0.1 - 0.8

Factors influencing separation:  Six major interrelated factors to consider.
  • Column length
  • Column internal diameter
  • Film thickness 
  • Carrier gas type
  • Carrier gas velocity
  • Column temperature
Column selection: On the basis of stationary phase thickness.

Increasing the thickness will allow for a greater sample capacity.It will result in wider peaks and lower resolutions.  They also tend to degrade more rapidly.
Thin film - 0.10 - 0.25 µm 
Thick film - 1 - 5 µm
Column selection: On the basis of internal diameter.As diameter increases, the pressure requirements are reduced.  The sample capacity increases and resolution decreases.
Diameter in mm
Plates/meter
Capacity ng/peak
0.2 - 0.25
4000-5000
5-100
0.32
3000
400-500
0.53
1600
1000-15000
2 (Packed)
2000
20000
Column selection: On the basis of phase polarity.
  • Non-polar: (0-5% pheny)- methylpolysiloxane
  • Intermediate:(20 -50% pheny)- methylpolysiloxane (5-15% cyanopropyl-phenyl)-methylpolysiloxane
  • Polar: Carbowax 20M (50%-trifluoropropyl)-methylpolysiloxane Polyethylene glycol

G.C. Inlet: 

Liners: The liner you use is dependent on the method of injection.It must be replaced at regular intervals.Most crud ends up in the liner.Generally three type of liner used.

  • Split liner
  • Splitless liner
  • Auto-injector liner

Split liner: Key to method is assuring total vaporization and mixing with the carrier gas.  This is accomplished in the split liner.
• Provides for efficient heat transfer 
• Promotes mixing of sample and carrier 
• Surface is used to trap nonvolatile components.

Splitless liner: Split injection is best when you have relatively high levels of the eluents of interest.For trace analysis, splitless injection can be used.
1) Two approaches:  Splitless - total sample enters column
2) Splitless/Split - Grob injection  Attempt to selectively remove   solvent 


Purge off: Initially the purge is off.All flow entering the injection port will go into the column.
The normal ‘split’ flow bypasses the liner.This is done to help maintain the column head pressure.


Purge on: After a fixed period of time, the purge is turned back on - split mode. Any remaining material is essentially flushed from the injection port.


Calculation of the split ratio: Split vent flow can be measured directly at the split vent with a bubble meter.Determining the column flow is a bit more difficult.


                    Split vent flow+Column flow
Split ratio=-------------------------------------
                                Column flow

Cool on-column injection:
The process of initial sample volatilization can degrade some samples. 
• On-column injection is a method to directly place sample on the column under low temperature          conditions.
• A special injector is required as the ‘needle’ is a section of capillary column that actually fits inside    the analytical column.
• The injector is typically kept cool using fan driven air.






 


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