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 (C 6 – C 12 ) T c = 130 o C T i = 100 o CT f = 200 o C

Gradient Elution: Substituted Benzenes Water Organic

GAS CHROMATOGRAPHY

GAS CHROMATOGRAPHY INSTRUMENTATION 1. Mobile Phase 2. Sample Injector 3. Column (Stationary Phase) 4. Detector

GC MOBILE PHASES 1. He 2. H 2 3. N 2 4. CO 2

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. High Reproducibility 2. Low Bleed 3. Fast Equilibration 4. Temperature Stability 5. Easy to Back Flush 6. Long Life

GC Stationary Phases Types What about Packed GC Columns?? Packed Column Open Tubular 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 ≈ 400,000H ≈ 0.1 mmL ≈ 50 m

GC Stationary Phases Types Packed Column: H = A + B/u + Cu C  d p 2 (particle diameter) 2 Open Tubular: H = B/u + Cu C  d c 2 (column internal diameter) 2

Packed Open Tubular

Packed GC Columns

Open Tubular Columns t r  d c [η/(P o D m )] 1/2 where: η mobile phase viscosity P o column outlet pressure D m Diffusion coefficient of analyte in gas d c Internal column diameter D m  MW -1/2

Open Tubular Columns So: t r  [η MW 1/2 ] 1/2 Gas η MW 1/2 Ar1138 CO N He440 H 2 90

Open Tubular Columns with Different Gases