GC

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Chapter 24

• GC
• Gas Chromatography

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GC

• Mechanism of separation is primarily volatility.
• Difference in boiling point, vapor pressure etc.
• What controls this?
• Molecule to molecule bonds
• Van der Waals, dipole dipole for example.
• Molecular Weight

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Volatility

• Boiling points
• H2S -60 C
• H2Se -45 C
• H2Te -15 C
• H2O 100 C (why is this different)

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Molecular Weight

• Methane -164 C
• Butane -0.5 C
• Pentane 36 C
• Hexane 69 C
• Octane 125 C
• Decane 174 C

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GC ExampleCholesterol and other lipids in bone
(trimethylsilane)
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Combustion Result (CO2) Mass Spec Detection
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Block Diagram of GC System
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Block Items

• Carrier gas - He, N2, or H2
• Injector - usually septum introduction
• Column with Stationary phase a nonvolatile
  liquid carbowax is a common example
• Detector converts chemical to electrical
  information.
• Last three items are held at elevated
  temperatures, usually

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Column

• Where separation takes place.
• Open tube
• Packed

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Side View of Column
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Open Tube Types
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Low Temperature Separation of an Alcohol Mix -
Packed Column Carbowax FID
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Open Tube Separation of the Headspace of a Can of
Beer Carbon Column
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Chromatogram
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Stationary Phases
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After volatility we can work with polarity
differences.

• Simple rule is that likes dissolve likes. We
  could determine log P or just use our chemical
  intuition.
• There is not a big effect here so a short list of
  columns will usually get the job done.

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Nonpolar Column Polar Column
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Specialized Stationary Phases

• Zeolites (Molecular Sieves)
• Alumina
• Chiral stationary phases

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Packed Columns

• Still find their uses.
• Can handle larger samples.
• Have a support coated with stationary phase
• Support often diatomite.
• Issues with active sites.

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Retention Index.

• A measure of retention compared to the n-alkanes.
• The alkanes are assigned a number that is 100
  times the number of carbons. There is related in
  a linear way to the log tr

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Retention Index
Where N is the number of carbons in the higher
alkane n is the number of carbons in the lower
alkane tr is the adjusted retention time
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What if an analysis is too slow?

• Temperature programming
• Increase temp as the run progresses
• Pressure programming
• Increase pressure as the run progresses
• Advantage is that pressure can be quickly
  returned to original value where it takes time to
  reduce the heat.

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Carrier Gas Considerations
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Sample Injection

• Manual syringe through the septum port
• Automatic syringe through the septum port.

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Split Injector
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Split less Injection
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Detectors

• A transducer converts chemical information to
  electrical signal. Most tell us no additional
  information other than there is a detector
  response.
• TCD
• FID
• ECD
• Others (Mass spec)

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Peak Identification / Quantification

• Co-injection.
• Run on multiple columns of different polarity.
• Area of peak is proportional to amount of sample.
  Different samples can have different responses.
• Area (Gaussian peak) 1.064peak htw1/2

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Internal Standard

• A compound added that is close in nature to the
  compound being analyzed. Gets around a variety
  of problems.

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Thermal Conductivity Detector
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Flame Ionization Detection
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Less common detectors

• Nitrogen Phosphorus - burner heats a glass bead
  that contains Rb2SO4 - 104 to 106 greater
  response to N and P over C.
• Flame Photometric - P, S, Pb, Sn
• Photo ionization detector. Aromatics,
  unsaturated compounds
• Sulfur (nitrogen) chemiluminescence detector
• SO mixed with O3 from flame 107 over carbon
• Atomic emission

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ECD of Atmosphere
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GC of Natural Gas
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Mass Spectroscopy

• Since full spectra are collected at each time
  point then we can selectively look for our
  analyte of interest.

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Sample Preparation

• Derivatization
• Solid Phase Micro extraction
• Purge and trap

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Example of Solid Phase Micro extraction
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Method Development

• Goal of Analysis
• Sample preparation
• Detector
• Column
• Injection

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Resolution Improvement

• Longer Column
• Narrower Column
• Thinner stationary phase
• Different Stationary phase

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Injection Comparison

• Split Injection
• Concentrated sample
• High resolution
• Dirty samples
• Thermal decomposition issues

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Injection Comparisons

• Splitless
• Dilute samples
• High resolution
• Requires solvent trapping or cold trapping
• On-column injection
• Best for quantification of analytes
• For thermally sensitive compounds
• Has lower resolution