1. 1 Chapter 24 GC Gas Chromatography

2. 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. 3 Volatility Boiling points H 2 S-60 C H 2 Se-45 C H 2 Te-15 C H 2 O100 C (why is this different)

4. 4 Molecular Weight Methane-164 C Butane-0.5 C Pentane36 C Hexane69 C Octane125 C Decane174 C

5. 5 GC Example Cholesterol and other lipids in bone (trimethylsilane)

6. 6 Combustion Result (CO 2 ) Mass Spec Detection

7. 7 Block Diagram of GC System

8. 8 Block Items Carrier gas - He, N 2, or H 2 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. 9 Column Where separation takes place. Open tube Packed

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11. 11 Side View of Column

12. 12 Open Tube Types

13. 13 Low Temperature Separation of an Alcohol Mix - Packed Column – Carbowax – FID

14. 14 Open Tube Separation of the Headspace of a Can of Beer – Carbon Column

15. 15 Chromatogram

16. 16 Stationary Phases

17. 17 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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19. 19 Nonpolar Column Polar Column

20. 20 Specialized Stationary Phases Zeolites (Molecular Sieves) Alumina Chiral stationary phases

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24. 24 Packed Columns Still find their uses. Can handle larger samples. Have a support coated with stationary phase Support often diatomite. Issues with active sites.

25. 25 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 t r ’

26. 26

27. 27 Retention Index Where N is the number of carbons in the higher alkane n is the number of carbons in the lower alkane t r ’ is the adjusted retention time

28. 28 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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32. 32 Carrier Gas Considerations

33. 33 Sample Injection Manual – syringe through the septum port Automatic – syringe through the septum port.

34. 34 Split Injector

35. 35 Split less Injection

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37. 37

38. 38 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)

39. 39 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*w 1/2

40. 40 Internal Standard A compound added that is close in nature to the compound being analyzed. Gets around a variety of problems.

41. 41 Thermal Conductivity Detector

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43. 43 Flame Ionization Detection

44. 44 Less common detectors Nitrogen Phosphorus - burner heats a glass bead that contains Rb 2 SO 4 - 10 4 to 10 6 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 O 3 from flame 10 7 over carbon Atomic emission

45. 45 ECD of Atmosphere

46. 46 GC of Natural Gas

47. 47 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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50. 50 Sample Preparation Derivatization Solid Phase Micro extraction Purge and trap

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53. 53 Example of Solid Phase Micro extraction

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55. 55 Method Development Goal of Analysis Sample preparation Detector Column Injection

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57. 57 Resolution Improvement Longer Column Narrower Column Thinner stationary phase Different Stationary phase

58. 58 Injection Comparison Split Injection Concentrated sample High resolution Dirty samples Thermal decomposition issues

59. 59 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