Chapter 24 GC Gas Chromatography 1

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 2

Volatility  Boiling points     H 2 S H 2 Se H 2 Te H 2 O -60 C -45 C -15 C 100 C (why is this different) 3

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

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

Combustion Result (CO 2 ) Mass Spec Detection 6

Block Diagram of GC System 7

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 8

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

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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

18

Nonpolar Column Polar Column 19

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

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22

23

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 t r ’ 25

26

Retention Index

I

 100 [

n

 (

N



n

) log

t r

' (

unknown

) log

t r

' (

N

)   log

t

log '

r

(

t n

)

r

' (

n

) 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 27

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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29

30

31

Carrier Gas Considerations 32

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

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Split Injector 34

Split less Injection 35

36

37

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) 38

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 39

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

A x

[

X

] 

F

  

A s

[

S

]    40

Thermal Conductivity Detector 41

42

Flame Ionization Detection 43

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 44

ECD of Atmosphere 45

GC of Natural Gas 46

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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48

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Sample Preparation    Derivatization Solid Phase Micro extraction Purge and trap 50

51

52

Example of Solid Phase Micro extraction 53

54

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

56

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

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

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 59