Basic Gas Chromatography

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Basic Gas Chromatography
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History

• 1850 - Separation of dyes by Runge
• 1906 - Separation of plant pigments by Tswett
• 1941 - Theoretical gc (Martin Synge)
• 1952 - First gc
• 1954 - TC detector

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Process

• Sample is vaporized (if it is not already a
  vapor)
• Passes through a column where interaction occurs
  - does analyte move with gas phase or stay with
  stationary phase (column coating)
• Separation occurs
• Detection - many types of detectors

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• High purity!
• Source of mobile phase - He or H
• Detector gases - none or air/H (Flame ionization
  detector)

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Gas flow regulators

• Pressure regulators - stainless steel parts - not
  welding quality!
• Flow regulators - Determine gas flow rates
  through system (sensitive precision instruments)

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Injection port

• Introduce sample
• Vaporize sample
• Split sample (?)

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Injection ports - many versions

• Split - only a portion of injection goes on
  column
• Splitless - all material injected goes on
  column
• On-Column - cold injection (sensitive materials)
• Programmed temperature - sensitive materials
  (more durable method than OC)
• Large volume - Can inject 1 ml - solvent removal

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Columns

• Packed (hard to find)
• Capillary (generally open tubular but can be a
  wall coated PLOT type)

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Columns

• Generally fused silica - strong and inert
• Inner diameters - 0.10 - 0.53 mm
• Length - 1 - 60 m
• Coatings - several - range in thickness from 0.1
  - 5 um

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Common Stationary Phase Coatings
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Phase selection

• PUBLISHED INFORMATION
• Kovats indices compilations
• Journal articles
• Internal work
• INTUITION
• like structures
• NO IDEA?
• Sample information
• Nonpolar column
• Change to polar if needed

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Separation theory

• 1. Adsorption
• 2. Molecular exclusion
• 3. Partition
• 4. Vapor pressure

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Adsorption chromatography

• Interaction with a granular support e.g. Tenax,
  charcoal, silica gel,

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Molecular exclusion

• Used for the separation of permanent gases e.g.
  Zeolites, Linde molecular sieves

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Partition chromatography

• Partitioning between mobile phase and carrier gas
  vapor pressure
• SEPARATION BASED ON THE BOILING PT

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Column coatings (stationary phases)

• Polar to nonpolar
• Polar - Carbowax
• Non Polar - silicone based phases

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Column ovens

• Usually heat ovens to help in separations
• Ovens can be controlled from about -60 - 400C

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Detectors

• Many types varying in sensitivity and selectivity
• Discuss most common types

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Thermal conductivity detector
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Characteristics of TC detector

• Specificity - very little - will detect almost
  anything including H2O - called the universal
  detector.
• Sensitivity to 10-7 grams/sec - this is poor -
  varies with thermal condition of the compound.
• Linear dynamic range 104 - this is poor -
  response easily becomes nonlinear.

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Flame ionization detector
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Characteristics of a Flame Ionization Detector
(FID)

• Specificity - most organics.
• Sensitivity - 10-12 g/sec for most organics --
  this is quite good.
• Linear range 106 - 107 -- this is good.
• A special type of FID is called an alkali
  flame (AFID). Rubidium sulfate is burned in the
  flame and the detector becomes specific for N and
  P. Organics are not detected. Used for amines and
  nitrosoamines. (more commonly called the NPD)

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Electron Capture Detector
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Characteristics of an ECD

• Specificity - sensitive to halogens, conjugated
  carbonyls, nitriles, and a few others - no
  response with ordinary organics or H2O.
• Sensitivity 5 x 10-14 g/sec - excellent
• Linear range 104
• The radioactive detectors have definite
  temperature limits.

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Separation - terms
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RESOLUTION
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• SELECTIVITY relative interaction of column
  stationary phase with both compounds to be
  separated
• ? tr2
• tr1
• CAPACITY retention time of compounds to be
  separated
• k tr - tm tr
• tm tm
• THEORETICAL PLATES column EFFICIENCY
• n 5.545 (tr/Wh)2

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Optimizing Gas Chromatography
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Key factors influencing efficiency in gas
chromatography are column phase (nonpolar are
most efficient) and column diameter.
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Carrier gas type and velocity
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Phase thickness

• Capacity and Efficiency influenced by column
  diameter and phase thickness
• Thick phase capacity
• Thin Phase less capacity

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Column length

• Longer means better separations but longer
  analysis times
• Time proportional to length
• Separation proportional to sq root of length
• Poor means of getting separation costs too much
  in time. Use diameter, phase thickness or phase
  type

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What do you need?
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THANK YOU