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Chapter 24

Chapter 24. GC Gas Chromatography. 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 . Volatility . Boiling points H 2 S -60 C

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Chapter 24

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  1. Chapter 24 GC Gas Chromatography

  2. 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

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

  4. Molecular Weight • Methane -164 C • Butane -0.5 C • Pentane 36 C • Hexane 69 C • Octane 125 C • Decane 174 C

  5. GC ExampleCholesterol and other lipids in bone (trimethylsilane)

  6. Combustion Result (CO2) Mass Spec Detection

  7. Block Diagram of GC System

  8. 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

  9. Column • Where separation takes place. • Open tube • Packed

  10. Side View of Column

  11. Open Tube Types

  12. Low Temperature Separation of an Alcohol Mix - Packed Column – Carbowax – FID

  13. Open Tube Separation of the Headspace of a Can of Beer – Carbon Column

  14. Chromatogram

  15. Stationary Phases

  16. 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.

  17. Nonpolar Column Polar Column

  18. Specialized Stationary Phases • Zeolites (Molecular Sieves) • Alumina • Chiral stationary phases

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

  20. 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’

  21. 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

  22. 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.

  23. Carrier Gas Considerations

  24. Sample Injection • Manual – syringe through the septum port • Automatic – syringe through the septum port.

  25. Split Injector

  26. Split less Injection

  27. 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)

  28. 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) = 1.064*peak ht*w1/2

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

  30. Thermal Conductivity Detector

  31. Flame Ionization Detection

  32. 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

  33. ECD of Atmosphere

  34. GC of Natural Gas

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

  36. Sample Preparation • Derivatization • Solid Phase Micro extraction • Purge and trap

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