Chapter 5b Gas Chromatography

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Chapter 5bGas Chromatography
Department of Chemistry, Faculty of Science
Universiti Teknologi Malaysia ____________________
__________ Analytical Chemistry Course SSC 1293
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Outline

• Introduction to GC
• Instrumentation
• Injector
• Oven
• Columns
• Applications

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GC

• A chromatographic technique that uses a gas as
  the mobile phase and either a liquid or solid as
  the stationary phase.
• The analytes are adsorbed or dissolved in the
  stationary phase due to an equilibrium based on
  the vapor pressure and other additional
  interactive forces.
• The mobile phase in GC is referred to as the
  carrier gas because there is little interaction
  between the analyte and the gas phase.
• Gas-solid chromatography (GSC) uses a solid
  stationary phase while gas-liquid chromatography
  (GLC) uses a liquid stationary phase that is
  bonded or coated onto a solid support.

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GC equipment
Data handling
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Schematic diagram of a capillary gas chromatograph
Column
oven
 .

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GC Instrument

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Gases for GC

• Gas type Flow rate (mL/min)
• Carrier gas
• N2 or He
• Capillary column 1-2
• Packed Column 40
• Detector gases
• Hidrogen 30
• Air 300

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Split/splitless injector for GC
Septum closure
Septum purge
Carrier gas in
Valve
Heater
Exit
Graphite ferrule
Capillary column
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Split Injection

• Simplest method, ideal if only a limited number
  of components are present or trace analysis not
  required.
• Injection port fitted with 2 valves,
• one acting as a septum purge, allowing a small
  flow of carrier gas from just below the septum
  and
• The second taking carrier gas from the bottom of
  the injection port near the column inlet.
• In split mode, lower valve is used to adjust the
  ratio of carrier gas going to waste in the
  atmosphere compared to flow onto the column.
• Typical ratios 101 to 501 used so that only a
  small proportion of the sample is transferred
  onto the column.

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Splitless Injection

• Suitable for samples containing trace compounds
  in low conc. necessary to transfer as much of
  the sample as possible onto the column to
  increase sensitivity.
• Lower valve is closed at time of injection and
  only a small septum purge flow is used.
• Injection takes place into heated zone over a
  period of 20 s, so that all sample is
  transferred to the column.
• After a predetermined period (40-60 s), the lower
  valve is opened to purge the injection port and
  to prevent residual sample from causing tailing
  of the peaks.

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Column Oven

• For simple samples, separation may be carried out
  under isothermal conditions of constant
  temperature.
• For samples with wide range of volatility, need
  to use temperature programming.
• Enable volatile compound to be resolved at low
  temperature and elution of later less-volatile
  compounds to be speeded up as the temperature
  increases.
• Profile include a series of time events initial
  isothermal period, pauses at intermediate fixed
  temperature, different ramp rates, a final
  isothermal period and automated cooling-down of
  column to initial temperature at end of run,
  ready for the next cycle.

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Oven Temperature Programming
Final
Cool-down
Ramp 2
Temp
Hold
Inject
Inject
Ramp 1
Initial
Ready
Time
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Isothermal vs temperature-programmed GC
GC separation of n-alkanes using HP-101
(methylpolysiloxane) column, 50 m x 0.32 mm I.D.,
0.3 ?m thickness. (a) Isothermal GC at 140 oC.
(b) Temperature programmed GC 50 - 230 oC at 4
oC min?1.
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GC Columns and Stationary Phases

• Heart of the chromatographic system
• Determine efficiency and selectivity

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GC columns packed vs open tubular
Silica Column
Polyamide layer
Packed column
Liquid phase layer
Open tubular column
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Packed columns

• Three components
• Column tubing
• Support material
• Liquid stationary phase

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Column tubing

• Criteria
• Inert, thermally stable, coil up
• Types
• Copper, stainless steel, glass
• Typical sizes
• 1-3 m long, 1/16, 1/8,1/4 inch OD, 2-3 mm ID
• Inner surface silylated
• To reduce interaction with polar analytes

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Packing materials
Liquid phase
Solid support
Active site
Schematic diagram showing cross section of
packed column comprising of solid support coated
with liquid phase.
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Support materials

• Criteria
• Unreactive towards analyte and liquid phase,
  uniform particles and pore size
• Diatomaceous earths Chromosorb
• Particle sizes
• Analytical column 80-100, 100-120 mesh
• Preparatory column 40-60, 60-80 mesh
• Chemical treatment
• Acid wash (AW) removes metallic impurities
• Acid wash and dimethyl dichlorosilane-treated
  (AW-DMCS) remove silanol groups

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Examples of GC support materials
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Non-diatomite support materials
Porous Polymers - Porapak Polymers
Chromosorb 101 (PSDVB), 103
(PS) Tenax Polymers - 2,6-diphenyl-p-pheny
lene oxide Carbopacks support - Inertness can be
manipulated Adsorbents - Molecular
sieve Silica gel - inertness can be
manipulated Carbon molecular sieves
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Open tubular columns

• No support material
• Liquid phase coated on wall of column (WCOT)
• Flexible fused silica
• Coated with polyimide layer
• Temp. lt 350oC or else coating pyrolysed
• ID 0.1 0.75 mm
• Film thickness 0.1 5 ?m
• Column length 5-50 m
• As ID and film thickness ?, sample capacity ?,
• but efficiency ?
• Typical analytical column 25 m x 0.22 mm x 0.25
  ?m

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Non-polar liquid phases in GLC

• Hydrocarbon phases Squalane (C30H62), Apolene
  (C87 hydrocarbon), Apiezon L(-(CH2)n-)
• - Separation of non-polar molecules
  n-alkanes
• Alkylsilicone liquid phases SE-30, OV-1,
  OV-101
• Dimethylsilicone (-(-Si(Me)2-O-)- polymer)
• BP-1, Ultra-1, DB-1

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GC on non-polar liquid phases
230 C
2 C/min
50 C
Hydrocarbons
Essential oil (Cymbopogon nardus)
Column Ultra 1, 30 m x 0.25 mm x 0.25 mm
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Polar liquid phases in GLC

• Substituted silicone liquid phases
• methylphenyl silicone
• - OV-105, CP-Sil 58
• Ester liquid phases
• - Poly(diethylene glycol adipate) DEGA
• - Poly(diethylene glycol succinate) DEGS
• Polyether liquid phases
• Carbowax 200 to Carbowax 20M
• (Polyethylene glycol, PEG)
• - HP20-M, BP-Wax, BP20

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GC on polar liquid phase
230 C
Hydrocarbons
4 C/min
50 C
Essential oil (Cymbopogon nardus)
Column HP-20M (Carbowax 20M)
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Comparison of Manufacturers Phases
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Factors in selecting stationary phase

• Nature of analyte
• Stationary phase type
• Column internal diameter
• Film thickness
• Column length

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Packed vs Open tubular columns
Factor Packed Column Open Tubular
Efficiency low to moderate high
Sensitivity low high
Operation easy less easy
Sample amount large small
Price low high
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Effect of column internal diameter (ID)
Open Tubular
Packed Column
Characteristics Column ID 2 mm 0.20 mm 0.32 mm 0.75 mm
Sample capacity(each component) 20,000 ng 5.30 ng 400 500 ng 10,000 15,000 ng
EfficiencyTheoretical plates, n 2000 5000 3000 1170
Optimum flow rate(mL/min) 20 0.4 1.4 5.0
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Column conditioning

• Condition at
• 20 oC higher than analysis temp
• at least 10-20 oC less than stated max.
  operational temp of phase
• Never condition at columns max temp
• Program temp slowly to conditioning temp (2-4
  oC/min)
• Cool down slowly (nonbonded phase)
• Purge column with carrier gas for 1/2 hr before
  heating over
• Very high carrier gas flows can be used for
  conditioning
• Conditioning time varies with your need

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GC Applications

• Petrochemical
• Environmental
• Forensic
• Pharmaceutical
• Oleochemical
• Others

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GC Profile of Diesel

• ..

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GC-ECD SEPARATION OF PCB
. GC Conditions HP-5MS capillary column,
Injection port temperature at 280oC and detector
temperature at 290 oC. Temperature programming
from 200oC (held for 3 minutes) then ramped to
230 oC (held for 3 minutes) at a rate of 5 oC/min
and increased to 260 oC (held for 3 minutes) at a
rate of 2 oC/min. Peak identity (1) iso-octane
(2) PCB 28, (3) PCB 52, (4) PCB 35, (5) PCB 101,
(6) PCB 118, (7) PCB 153, (8) PCB 138 and (9)
PCB 180.
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GC-ECD SEPARATION OF NITROEXPLOSIVES
Initial oven temperature at 100 oC, held for 2
min then ramped to 250 oC at 15 oC/min and held
for 1 minute. The injector temperature and
detector temperature at 230 oC and 300 oC. GC
column was Ultra 2 (25 m x 0.20 mm ID x 0.11 µm
film thickness). helium as carrier gas (1 mL/min)
and nitrogen as make-up gas (30 mL/min).