effect of packing particle size on plate height. resolution between 2 adjacent peaks
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The General Elution Problem
1. Poor separation of compounds with smallcapacity factors.
2. Excessive separation time and poordetectability of compounds with large capacityfactors.
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)
Tc = 130oC
Ti = 100oC Tf = 200oC
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GAS CHROMATOGRAPHY INSTRUMENTATION
1. Mobile Phase2. Sample Injector3. Column (Stationary Phase)4. Detector
GC Stationary Phases
PDMS: PolyDiMethylSiloxane
Organic stationary phases must have sufficient molecular weight to achieve low volatility
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
Advantage of Bonded Phases:
1. High Reproducibility2. Low Bleed3. Fast Equilibration4. Temperature Stability5. Easy to Back Flush6. Long Life
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 Columns
tr dc [η/(Po Dm)]1/2
where:
η mobile phase viscosityPo column outlet pressure
Dm Diffusion coefficient of analyte in gas
dc Internal column diameter
Dm MW-1/2