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Chapter 24

GCGas 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

)(log)(log

)(log)(log)([100

''

''

ntNt

ntunknowntnNnI

rr

rr

Where N is the number of carbons in the higher alkanen is the number of carbons in the lower alkanetr

’ 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 progressesAdvantage 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) = 1.064*peak ht*w1/2

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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.

][][ S

AF

X

A sx

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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