Transcript gcx - ichapps.com

GAS
CHROMATOGRAPH
Principle:
Capillary Tube
Liquid Stationary Phase
A ABA
A
A
A A
BA B B
B
He Carrier gas
AA B
A BA
B
He Carrier gas
A
A
A A
B
B
B B
B
B
B
B B
0
Immediately after injection
After several minutes
Which type of compounds can analyse?
•Volatile
•Thermostable
A
Time
Resulting chromatogram
•Carrier gas supply: Hydrogen, Helium, Nitrogen,
Argon
•Requirements of a carrier gas:
Inert
High purity
Easy available
Cheap
Less risk of explosion
Best column performance
Compatible with the stationary phase
and the sample and detector used
Carrier gas cylinder
Carrier gas flow regulator
• Sample injection devices:
Manual - Direct Injection
Automated - Auto sampler
- Purge and Trap
Commercial GC- Split &
Splitless inj.& On column inj.
 Liquids
 Gases
 Solids
Gases
Position -I
Position -II
On-Column Injector
• Columns:
 Column selection
 Column length
 Column efficiency
 Column head pressure
 Column cost
 Column diameter
GC Columns
Packed columns
•Typically a glass or stainless steel
coil.
•1-5 total length and inner
diameter:2 & 4mm
• Packing particles diameter: 100 to
250 micrometers
Capillary columns/Open tubular
wall coated
open tubular
column (WCOT)
support-coated
open
tubular(SCOT)
column
porous-layer
open tubular
(PLOT) column
Packed
Capillary
List of common stationary phases and their polarities
Column Stationary Phase
Polarity
Diatomaceous Earth (SiO2, Al2O3, Fe2O3, CaO,
MgO, Na2O, and K2O)
relatively non-polar
Clay
relatively non-polar
Celite
relatively non-polar
Squalane (C30H62) (purified with charcoal and
alumina)
non-polar
Hexadecane
non-polar
Apolane-87 (C87H176)
non-polar
Apiexon Greases (purified with charcoal and
alumina)
non-polar
Dialkyl Phthalates
moderately polar
Tetrachlorophthalates
moderately polar
Polyethylene Glycol (Carbowax 20M)
moderately polar
Polysiloxanes
non-polar-polar (depending on the R group
attached)
List of functional groups attached to the stationary phase and their polarities
Stationary Phase Functional Group
Polarity
Polyester Phases
Highly Polar
-OH
Polar
-CN
Polar
-CO
Polar
Hydrocarbons
Non-polar
Dialkyl Siloxanes
Non-polar
Analyte Functional Groups (Polar to Non-polar)
Water
Acids
Alcohols
Amines
Amides
Aldehydes
Ketones
Esters
Ethers
Aromatics
Hydrocarbons
Column Oven
A gas chromatography oven, open to show a
capillary column
The oven can be operated in two manners:
•isothermal programming
•temperature programming
The effect of column temperature on the shape of the peaks
Detection Systems
Characteristics of the Ideal Detector: The ideal
detector for gas chromatography has the
following characteristics:
1. Adequate sensitivity
2. Good stability and reproducibility.
3. A linear response to solutes that extends
over several orders of magnitude.
4. A temperature range from room
temperature to at least 400oC.
Characteristics of the Ideal Detector
5. A short response time that is independent of
flow rate.
6. High reliability and ease of use.
7. Similarity in response toward all solutes or a
highly selective response toward one or more
classes of solutes.
8. Nondestructive of sample.
Detection Systems
Type of Detector
Applicable Samples
Flame Ionization (FID)
Hydrocarbons
Thermal Conductivity (TCD)
Universal
Electron-Capture (ECD)
Halogenated hydrocarbons
Argon ionisation detector
Organic compounds
Nitrogen Phosphorous detector
Molecules containing N2/P
Flame Ionization Detectors
Advantageous :
•the detector is unaffected by flow rate, noncombustible gases and water.
These properties allow FID high sensitivity and low noise.
•relatively easy to use.
Disadvantages:
• this technique does require flammable gas and also destroys the sample.
The Electron Capture Detector
•used for detecting environmental samples as the device selectively detects organic
compounds with moieties such as halogens, peroxides, quinones and nitro groups .
•best suited in applications where traces quantities of chemicals such as pesticides are to
be detected
•high selectivity and sensitivity towards certain organic species with electronegative
functional groups.
Thermal conductivity detector
advantages :
•ease and simplicity of use
•application to inorganic and organic compounds, and the ability of the analyte to be
collected after separation and detection.
drawback :
•TCD is the low sensitivity of the instrument in relation to other detection methods, in
addition to flow rate and concentration dependency
Nitrogen Phosphorus Detector