Download - Chapter 27 Gas Chromatography 1. Principles
Chapter 27 Gas Chromatography
1. Principles
Two types
- Gas-solid chromatography (limited because of semipermanent retention of polar molecules)- Gas-liquid chromatography
Retention volume
F: average volumetric flow rate mL/min, can be determined
by measuring flow rate exiting the column using a soap bubble meter
retainednon
MM
retained
RR
FtV
FtV
P
PP
T
TFF watercm
)(
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 PF = constant
Pressure drop correction factor j
- to calculate average pressure from inlet pressure Pi and outlet pressure P
FtjV
Ftj
corrected
PP
PPj
MM
R
i
i
0
0R
2
V
volumeretention
]1)/[(2
]1)/[(33
Specific retention volume Vg
- semi-useful parameter for identifying species eluting
Relationship between Vg and k
c
density
ScS
Sg
M
S
cS
Mg
M
MR
T
K
Tm
KVV
V
KVk
Tm
kVV
t
ttk
273273
)(
273
)(
phase stationaryon liquid of
0
cs
MR
column
c
mass
s
MRg Tm
ttjF
Tm
VVV
273)(273
ure temperatphase stationary of
00
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. Instrumentation
Fig. 27-1 (p.790) Block diagram of a typical GS
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
2.3Column
- 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 < 1C
equal or slight high than average boiling point of sample
For broad boiling range--temperature programming
2.3Column
- 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 < 1C
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) 5. fast response wide dynamic range2. Stability and reproducibility 6. simple (reliable)3. Linear response 7. uniform response to all analytes4. Operate at high T (RT-400 C) 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)