Reading assignment: section 26E(p781) Chapter 26 # 2, 3, 14, 15, 16 Chapter 27 # 7(a,d,f), 22, 23, 24, 25

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 Fig (p.790) A soap-bubble flow meter

But V R and V M 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 P i and outlet pressure P

Specific retention volume V g -semi-useful parameter for identifying species eluting Relationship between V g and k

2.1 Carrier gas (mobile-phase gas) He (common), N 2, H 2, P i psi above atom, F = mL/min for packed column, 1-25 mL/min for open tubular capillary Fig (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  L for capillary column, a sample splitter is needed (1/50-1/500 to column, rest to waste) or use a purge valve Fig (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 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 ( g solutes/s) 5. fast response wide dynamic range 2. Stability and reproducibility 6. simple (reliable) 3. Linear response 7. uniform response to all analytes 4. Operate at high T (RT  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, halogen, alcohol, amine groups - Insensitive to non-combustibles – H 2 O, CO 2, SO 2, NO X Advantages - Rugged, general applicability, ease of use - Sensitive ( g/s) - Wide dynamic range (10 7 ), low noise Disadvantage - Destructive Fig (p.794) FID

Thermal Conductivity Detector (TCD) -Thermal conductivities of He and H 2 much larger than organics - Organics cause T rise in detector Advantages - Simplicity, general applicability - Wide linear dynamic range (10 5 ) - Responds to both organic and inorganic - Nondestructive Disadvantage - Low sensitivity (10 -8 g/mL carrier gas) Fig (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 (10 2 ) Other detectors: AES, AAS, chemiluminescence reaction, MS, FTIR Fig (p.795) ECD

GC-MS Fig (p.799) GC/MS

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 (<  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 GC]

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)

separation + analysis (retention time, retention volumes) 1. Limited for qualitative analysis 2. The area under the peak is proportional to extent of components 3. Retention index Retention index = 100 x number of carbon for normal alkane. Within a homologous series, a plot of log(t R ’)=log(t R -t M ) versus the number of C atoms is linear.