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Transcript
Chapter 27 Gas Chromatography
1. Principles
Two types
- Gas-solid chromatography (limited because of semipermanent retention of polar
molecules)
Gas-liquid chromatography
Retention volume
VR  t R  F

retained
VM  t M  F

non retained
F: average volumetric flow rate mL/min, can be determined
by measuring flow rate exiting the column using a soap bubble meter
Tc ( P  Pwater )
F  Fm  
T
P
Fig. 27-2 (p.790) A soap-bubble
flow meter
But VR and VM depend on pressure inside column, and column has resistance to flow
- At inlet, P high , F low
- At outlet, P low, F high  PF = constant
Pressure drop correction factor j
- to calculate average pressure from inlet pressure Pi and outlet pressure P
3[( Pi / P) 2  1]
j
2[( Pi / P) 3  1]
corrected retention volume
VR0  j  t R  F
VM0  j  t M  F
Specific retention volume Vg
- semi-useful parameter for identifying species eluting
Vg 
VR0  VM0
ms

mass of stationaryphase

273
Tc


jF (t R  t M ) 273

ms
Tc
column temperat ure
Relationship between Vg and k
(k 
tR  tM
)
tM
VM0 k 273
Vg 

mS
Tc
(k 
KVS
)
VM
Vg 
KVS 273


mS
Tc
K

S
densityof liquid
on stationaryphase

273
Tc
2. Instrumentation
Fig. 27-1 (p.790) Block diagram
of a typical GS
2.1 Carrier gas (mobile-phase gas)
He (common), N2, H2, Pi 10-50 psi above atom, F = 25-150 mL/min for packed column,
1-25 mL/min for open tubular capillary
2.2 Sample injection
- Direct injection into heated port to promote
fast vaporization
-
Sample volume
1-20 L for packed column
10-3 L for capillary column, a sample
splitter is needed (1/50-1/500 to column,
rest to waste)
or use purge valve
Fig. 27-4 (p.791) Cross-section view
of a microflash vaporizer direct injector
2.3 Column
-
Packed column: 1- 5m
-
Open tubular (capillary): 1m to 100m
Both constructed of coiled stainless steel/glass/Teflon, with coil diameter of 10-30 cm
2.4 Oven
accurate to < 1C
equal or slight high than average boiling point of sample
For broad boiling range--temperature programming
2.3 Column
-
Packed column: 1- 5m
-
Open tubular (capillary): 1m to 100m
Both constructed of coiled stainless steel/glass/Teflon, with diameter of 10-30 cm
2.4 Oven:
accurate to < 1C
equal or slight high than average boiling point of sample
For broad boiling range--temperature programming
2. 5 Detectors
Requirement and desire:
1. Sensitive (10-8 -10-15 g solutes/s)
2. Stability and reproducibility
3. Linear response
4. Operate at high T (RT-400 C)
5. fast response wide dynamic range
6. simple (reliable)
7. uniform response to all analytes
8. nondestructive
Flame Ionization Detector (FID)
Sample pyrolyzed and produce
current in electrical field.
Signal depends on #C atoms in
organic analyte –mass sensitive not
concentration sensitive
Weakly sensitive to carbonyl, amine,
alcohol, amine groups
Insensitive to non-combustibles –
H2O, CO2, SO2, NOX
Advantages
Rugged
Sensitive (10-13 g/s)
Wide dynamic range (107)
Disadvantage
Destructive
Fig. 27-8 (p.794) FID
Thermal Conductivity Detector (TCD)
-
Thermal conductivities of He and H2
much larger than organics
Organics cause T rise in detector
Advantages
Simplicity
Wide linear dynamic range (105)
Responds to both organic and
inorganic
Nondestructive
Disadvantage
Low sensitive (10-8 g/mL carrier gas)
Fig. 27-9 (p.794) TCD
Electron Capture Detector (ECD)
- electron from -source ionize carrier
gas
organic molecules capture electron
and decrease current
Advantages
Simple and reliable
Sensitive to electronegative groups
(halogens, peroxides)
Largely non-destructive
Disadvantages
Insensitive to amines, alcohols, and
hydrocarbons
Limited dynamic range (102)
Other detectors:
AES, AAS, chemiluminescence
reaction, MS, FTIR
Fig. 27-10 (p.795) ECD
3. Column and Stationary Phases
3.1 Open tubular columns (Capillary)
-
Wall coated (WCOT) <1 m thick liquid coating on inside of silica tubing
support-coated (SCOT) 30 m thick coating of liquid-coated support on inside of silica
tubing
Best for speed and efficiency but only small samples
3.2 Packed columns
-
Liquid coated silica particles (<100-300 m diameter) in glass tubing
Best for large scale but slow and inefficient
3.3 Immobilized liquid stationary phases
-
Low volatility
High decomposition temperature
Chemically inert (reversible interaction with solvent)
Chemically attached to support (prevent “bleeding”)
Appropriate k and  for good resolution
Many based on polysiloxanes or polyethylene glycol (PEG)
-
[Check Table 27-2 for common
stationary phases for GLC]
Stationary phases usually bonded and/or cross-linked for longer-lasting
- Bonding – covalent linking of stationary phase to support
- Cross-linking – polymerization reactions after bonding to join individual stationary
phase molecules
Film thickness (0.5-1 M) affects retention and resolution
- thicker films for volatile analytes
Non-polar stationary phases best for non-polar analytes
- non-polar analytes retained preferentially
Polar stationary phases best for polar analytes
- polar analytes retained preferentially
Chiral phases being developed for enantiomer separation (pharmaceutical)