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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

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Presentation Transcript
slide3

The General Elution Problem

Small k′

Large k′

slide4

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.

slide5

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.

slide6

Temperature Programming:

Separation of Normal Alkanes (C6 – C12)

Ti = 100oC Tf = 200oC

Tc = 130oC

25

12

0

0

slide7

Gradient Elution:

Substituted Benzenes

Organic

Water

slide9

GAS CHROMATOGRAPHY INSTRUMENTATION

1. Mobile Phase

2. Sample Injector

3. Column (Stationary Phase)

4. Detector

slide10

GC MOBILE PHASES

1. He

2. H2

3. N2

4. CO2

slide16

GC Stationary Phases

PDMS: PolyDiMethylSiloxane

Organic stationary phases must have sufficient molecular weight to achieve low volatility

slide17

GC Stationary Phases

Increase Polarity by adding phenyl rings or CN groups…

slide18

GC Stationary Phases

Or by de-shielding the oxygen and forming a wax.

(Carbowax)

slide19

GC Stationary Phases

Decrease Polarity by replacing methyl groups with longer chain alkanes.

slide20

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

slide21

GC Stationary Phases Types

Preparation of Bonded Phases:

slide22

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

slide23

GC Stationary Phases Types

What about Packed GC Columns??

Open Tubular Column

Packed Column

slide25

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

slide26

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

slide29

Open Tubular Columns

tr dc [η/(Po Dm)]1/2

where:

η mobile phase viscosity

Po column outlet pressure

Dm Diffusion coefficient of analyte in gas

dc Internal column diameter

Dm  MW-1/2

slide30

Open Tubular Columns

So: tr [η MW1/2]1/2

Gas η MW1/2

Ar 1138

CO2 995

N2 741

He 440

H2 90