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)