Gas Chromatography

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Effect of Packing Particle Size on Plate Height
Resolution
between 2
adjacent
peaks
The General Elution Problem
Small k′
Large k′
The General Elution Problem
1.
Poor separation of compounds with small
capacity factors.
2.
Excessive separation time and poor
detectability of compounds with large capacity
factors.
Solutions to The General Elution Problem
Alter k′ values during the separation:
1.
Temperature Program (GC)
2.
Gradient Elution (LC)
3.
Coupled Columns
(change the stationary phase)
Temperature programming and gradient elution
require “re-equilibration time” after the analysis of
each sample.
Temperature Programming:
Separation of Normal Alkanes (C6 – C12)
Ti = 100oC
Tf = 200oC
Tc = 130oC
0
25 0
12
Gradient Elution:
Substituted Benzenes
Organic
Water
GAS CHROMATOGRAPHY
GAS CHROMATOGRAPHY INSTRUMENTATION
1.
2.
3.
4.
Mobile Phase
Sample Injector
Column (Stationary Phase)
Detector
GC MOBILE PHASES
1.
2.
3.
4.
He
H2
N2
CO2
GC On-Column Injector
GC Glass-lined Injector for a
Packed Column
GC Injector for a
Capillary Column
GC “Split” Injector for a Capillary Column
GC “Split-less” Injector for a Capillary Column
GC Stationary Phases
PDMS: PolyDiMethylSiloxane
Organic stationary phases must have sufficient
molecular weight to achieve low volatility
GC Stationary Phases
Increase Polarity by adding phenyl rings or CN groups…
GC Stationary Phases
Or by de-shielding the oxygen and forming a wax.
(Carbowax)
GC Stationary Phases
Decrease Polarity by replacing methyl groups with
longer chain alkanes.
GC Stationary Phases Types
Non-Bonded: Liquid phase is used to coat the inside
walls of the column to a thickness of about 0.1 – 1.0
μm
Bonded: Silanol groups on the column wall form a
chemical bond with the stationary phase
GC Stationary Phases Types
Preparation of Bonded Phases:
GC Stationary Phases Types
Advantage of Bonded Phases:
1.
2.
3.
4.
5.
6.
High Reproducibility
Low Bleed
Fast Equilibration
Temperature Stability
Easy to Back Flush
Long Life
GC Stationary Phases Types
What about Packed GC Columns??
Open Tubular Column
Packed Column
GC Stationary Phases Types
FSOT: Fused Silica Open Tubular
WCOT: Wall Coated Open Tubular
SCOT: Support Coated Open Tubular
WCOT and SCOT are less flexible and more
fragile (made of glass)
FSOT with bonded phase is most common
N ≈ 400,000 H ≈ 0.1 mm L ≈ 50 m
GC Stationary Phases Types
Packed Column: H = A + B/u + Cu
C dp2 (particle diameter)2
Open Tubular: H = B/u + Cu
C  dc2 (column internal diameter)2
Open Tubular
Packed
Packed GC
Columns
Open Tubular Columns
tr  dc [η/(Po Dm)]1/2
where:
η
Po
Dm
dc
mobile phase viscosity
column outlet pressure
Diffusion coefficient of analyte in gas
Internal column diameter
Dm  MW-1/2
Open Tubular Columns
So: tr  [η MW1/2]1/2
Gas
η MW1/2
Ar
CO2
N2
He
H2
1138
995
741
440
90
Open Tubular
Columns with
Different Gases
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