GAS CHROMATOGRAPHYPHAM VAN HUNG, PhD Function Separation of volatile organic compounds Volatile –when heated, VOCsundergo a phase transition into intact gas-phase species Separation o
Trang 1GAS CHROMATOGRAPHY
PHAM VAN HUNG, PhD
Function
Separation of volatile organic compounds
Volatile –when heated, VOCsundergo a phase transition into intact gas-phase species
Separation occurs as a result of unique equilibria established between the solutes and the stationary phase (the GC column)
An inert carrier gas carries the solutes through the column
Application
Uses
Separation and analysis of organic compounds
Testing purity of compounds
Determine relative amounts of components in mixture
Compound identification
Isolation of pure compounds (microscale work)
Similar to column chromatography, but differs in 3 ways:
Partitioning process carried out between Moving Gas Phase
and Stationary Liquid Phase
Temperature of gas can be controlled
Concentration of compound in gas phase is a function of
the vapor pressure only.
GC also known as Vapor-Phase Chromatography (VPC) and
Gas-Liquid Partition Chromatography (GLPC).
Filters/Traps
Ai H
ier Column
Components
gas system
inlet
column
detector
data system
Data system
Syringe/Sampler Inlets
Detectors Regulators
H
RESET
Schematic Diagram of Gas Chromatography Carrier Gas-Supply
Carrier gases, which must be chemically inert, include helium, nitrogen, and hydrogen Associated with the gas supply are pressure regulators, gauges, and flow meters In addition, the carrier gas system often contains a molecular sieve to remove water or other impurities
Must be at a constant flow rate so that retention times & retention volumes may be equated
Trang 2A GC syringe penetrates a septum to inject
sample into the vaporization camber
Instant vaporization of the sample, 280 °C
Carrier gas transports the sample into the head
of the column
Purge valve controls the fraction of sample that
enters the column
Split or splitless
Usually operated in split mode unless sample
limited
Chromatographic resolution depends upon the
width of the sample plug
In splitlessmode the purge valve is close for
30-60 s, which means the sample plug is 30-30-60
seconds
As we will see, refocusing to a more narrow
sample plug is possible with temperature
programming
Column Configurations
Two general types of columns are encountered
in gas chromatography, packedand open
Chromatographic columns vary in length from less than 2 m to 50 m or more They are constructed of stainless steel, glass, fused silica,
or Teflon In order to fit into an oven for thermostating, they are usually formed as coils having diameters of 10 to 30 cm
Columns
• Packed
•Capillary
0.32 mm ID
Liquid Stationary phase
Mobile phase (Helium) flowing at 1 mL/min
Open Tubular Capillary Column
15-60 m in length
0.1-5 μm
Trang 3Polar vs nonpolar
Separation is based on the vapor pressure and
polarity of the components
Within a homologous series (alkanes, alcohol,
olefins, fatty acids) retention time increases with
chain length (or molecular weight)
Polar columns retain polar compounds to a
greater extent than non-polar
C18 saturated vs C18 saturated methyl ester
C16:0
C18:0 C18:1
C16:1
C16:0
C18:0 C18:1 C18:2
C16:1
RT (min)
RT (min) Polar column
Non-polar column
Oven
Programmable
Isothermal-run at one constant temperature
Temperature programming -Start at low
temperature and gradually ramp to higher
temperature
More constant peak width
Better sensitivity for components that are retained
longer
Much better chromatographic resolution
Peak refocusing at head of column
Typical Temperature Program
Time (min)
50°C
220°C
160°C
Detection Systems
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
Trang 4Flame Ionization Detectors (FID)
Electron Capture Detectors (ECD)
Electron impact/chemical ionization (EI/CI)
Mass spectrometry
FIDs
Effluent exits column and enters an air/hydrogen flame
The gas-phase solute is pyrolized to form electrons and ions
All carbon species are reduced to CH2+ions
These ions collected at an electrode held above the flame
The current reaching the electrode is amplified
to give the signal
FID
A general detector for organic compounds
Very sensitive (10-13g/s)
Linear response (107)
Rugged
Disadvantage: specificity
ECD
Ultra-sensitive detection of halogen-containing species
Pesticide analysis
Other detectors besides MS
IR
AE
SEMI- QUANTITATIVE ANALYSIS OF FATTY ACIDS
C
C
C Detector Response
Retention Time
14
16
18 Peak Area (cm )
Sample Concentration (mg/ml) 2
4 6 8 10
0.5 1.0 1.5 2.0 2.5 3.0
2
T h e c o n te n t % o f C f a tt y a c id s = C
C + C + C 1 4 ∗ 10 0
1 8
1 6
1 4
1 4
TENTATIVE IDENTIFICATION OF UNKNOWN COMPOUNDS
Response
GC Retention Time on Carbowax-20 (min)
Mixture of known compounds
Hexane Octane Decane 1.6 min = RT
Response
Unknown compound may be Hexane
1.6 min = RT
Trang 5GC Retention Time on SE-30
Unknown compound RT= 4 min on SE-30
Response
GC Retention Time on SE-30
Hexane RT= 4.0 min on SE-30
1 Very good separation
2 Time (analysis is short)
3 Small sample is needed - μl
4 Good detection system
5 Quantitatively analyzed
DISADVANTAGES OF GAS CHROMATOGRAPHY
Material has to be volatilized at 250C without decomposition
R C OH CH 3 OH H 2 SO 4
O
R C O CH 3
O
CH 2 O C R
CH O C R
CH 2 O C R
O
O
O
CH 3 OH
O
R C O CH 3
CH 3 ONa
Fatty Acids Methylester
Reflux
Volatile in Gas Chromatography
Volatile in Gas Chromatography
Gas Chromatogram of Methyl Esters of Fatty Acids