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 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

0 025 12

Gradient Elution:Substituted Benzenes

Water

Organic

GAS CHROMATOGRAPHY

GAS CHROMATOGRAPHY INSTRUMENTATION

1. Mobile Phase2. Sample Injector3. Column (Stationary Phase)4. Detector

GC MOBILE PHASES

1. He2. H2

3. N2

4. 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. High Reproducibility2. Low Bleed3. Fast Equilibration4. Temperature Stability5. Easy to Back Flush6. 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 ≈ 400,000 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

Packed

Open Tubular

Packed GC Columns

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

Open Tubular Columns

So: tr [η MW1/2]1/2

Gas η MW1/2

Ar 1138CO2 995

N2 741

He 440H2 90

Open Tubular Columns with

Different Gases