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Gas and Supercritical Fluid Chromatography
Lecture Date: April 7th, 2008
Gas and Supercritical Fluid Chromatography
Outline
– Brief reviewof theory
– Gas Chromatography
– SupercriticalFluid Extraction
– SupercriticalFluid Chromatography
Reading (Skoog et al.)
– Chapter 27, Gas Chromatography
– Chapter 29, Supercritical Fluid Chromatography
Reading (Cazes et al.)
– Chapter 23, Gas Chromatography
– Chapter 24, Supercritical Fluid Chromatography
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GC and SFC: Very Basic Definitions
Gas chromatography – chromatography using a gas as the mobile phase and a solid/liquidas a stationary phase
– In GC, the analytes migrate in the gas phase, so their boilingpoint plays a role
– GC is generally applicable to compounds with masses up to about 500 Da and with ~60 torr vapor pressure at room temp (polar functional groups are trouble)
Supercriticalfluid chromatography – chromatography using a supercriticalfluid as the mobile phase and a solid/liquidas a stationary phase
– In SFC, the analytes are solvated in the supercritical fluid
– SFC is applicableto a much wider range of molecules
Review of Chromatography Important concepts/equations to remember:
Selectivity:
Retention volume:
= KB / KA
V = tF
Column/separation performance:
Plates: N = L/H
Linear velocity of mobile phase:
u = L/tm
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Review of Chromatography
Terminology and equations from Skoog:
GC Theory
Mobile-phase flow rates are much higher in GC (pressure drop is much less for a gas)
The effect of mobile-phase flow rate on the plate height (H) is dramatic
– Lower plate heights yield better chromatography
– However, much longer columns can be used with GC
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GC Instrumentation Basic layout of a GC:
Injector Detector
Carrier Gas
Column
Oven
See pg 703 of Skoog et al. for a similar diagram
GC Instrumentation
Atypical modern GC – the Agilent 6890N:
Diagram from Agilent promotional literature.
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GC Instrumentation
Typical carrier gases (all are chemically inert): helium, nitrogen and hydrogen. The choice of gas affects the detector.
Injectors: most desirable to introduce a small “plug”, volatilizethe sample evenly
– Most samples introduced in solution: microflashinjections “instantly” volatilizethe solvent and analytes and sweep them into the column
Splitters: effectively dilute the sample, by splitting off a portion of it (up to 1:500)
Ovens: Programmable, temperature ranges from 77K
(LN2) up to 250 C.
Detectors: wide variety, to be discussed shortly…
Headspace GC
Avery useful method for analyzing volatiles present in non-volatilesolids and liquids
Sample is equilibratedin a sealed container at elevated temperature
Needle
Headspace
The “headspace” in the container is sampled and introduced into a GC
Liquid/solid
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