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Speakers

John V HinshawGC Dept. DeanCHROMacademy

Tony TaylorTechnical DirectorCHROMacademy

Moderator

Dave WalshEditor In ChiefLCGC Magazine

An Introduction to Column Selection for Capillary GC

1. Important analyte / stationary phase interactions

2. GC stationary phase polymer types

3. Selecting an appropriate stationary phase

4. Stationary Phase Selectivity Effects –tuning the chemistry

5. Choosing column dimensions

6. Effects of column dimensions on selectivity, retention, efficiency and resolution

7. Bringing it all together from a practical standpoint

Aims & Objectives

Capillary GC column selection - what's important?

Stationary Phase

Length (m) x Internal Diameter (mm) x Film Thickness (mm)

L rc df

Efficiency (N) carrier gas / L / rc

Retention (k) oC / rc / df

Selectivity (a) oC / phase

5% Diphenyl dimethylpolysiloxane

30m x 0.25mm x 0.25mm

Stationary phase selection:Major analyte / stationary phase interactions

Major Analyte / Stationary Phase Interactions

Non-polar

Polar - Dipole

Electronegativity / Dipole Moments / Polarity

Stationary phase selection:Dispersive interactions

1. All substances contain small dipoles (small electronegativity differences)

2. Instantaneous dipoles fluctuate throughout the molecule (electron / nuclei vibration)

3. As two molecules approach – transient dipoles can induce the opposite dipole in the other molecule and a small attractive effect is seen

4. Often called a ‘dispersive interaction’ and occurs between compounds which are predominantly non-polar

5. Dispersive interactions occur with all substances, regardless if there is another overriding interaction

Van der Waals Forces

p8.flv

Stationary phase selection:Dipole interactions

1. Dipole interactions come in two sorts:

Dipole-Dipole / Dipole-Induced Dipole

1. Dipole interactions occur between substances whose permanent dipoles come into close contact with each other

2. Dipole-Induced dipole interactions occur when a polar substance meets a polarisable compound (typically containing pi-electrons)

3. The stronger dipole induces a more permanent dipole in the other substance and an intermolecular attraction occurs

Dipole – DipoleInteraction

p10.flv

p11.flv

Stationary phase selection:Hydrogen Bonding Interactions

1. A special case of a dipole-dipole interaction

2. Dipoles associated with the functional groups of two molecules come into close proximity

3. Hydrogen bonding interactions are very strong compared to dispersive interactions

4. In the extreme (e.g. the association of water with methanol) the dipole-dipole interaction energy can approach that of a chemical bond

5. Still an underlying weak dispersive interaction occurring simultaneously

Hydrogen BondingInteractions

Modern Stationary Phase Chemistry

‘Polysiloxane’ Phases

1. Immobilized Polymeric Liquids bonded to the inner surface of a silica capillary via silyl-ether linkages

2. Deactivation treatments applied before and/or after bonding

Higher percentage of functional monomer indicate higher degree of that interaction

50:50 phase shows stronger Induced Dipole Interactions with Aromatics

Typical Ratio of monomers (X:Y):5:9535:6550:50

Modern Stationary Phase Chemistry (III)

‘Polysiloxane’ Phases

Typical Ratio of monomers (X:Y):35:6550:50

Typical Ratio of monomers (X:Y):6:9414:8650:50

Modern Stationary Phase Chemistry (III)

‘Glycol / Wax’ Phases

Stationary Phase Interaction Summary

Lower polarity phases bleed less!

Stationary Phase Selection

1. Critical – Phase and Temperature directly effect selectivity

2. Principle of ‘like dissolves like’ holds well

3. Separate polar analytes using a more polar phase and vice versa

4. The skill is knowing the degree of polarity required to avoid overly long retention times whilst still obtaining a satisfactory separation

5. Separating compounds of intermediate polarity or mixed polarity & functionality requires knowledge of the retentivity and selectivity of each phase

6. May require ‘fine tuning’ of the phase chemistry using the monomeric ratios

Like Dissolves Like

Stationary Phase Selection:Test Probe Chemistry

http://upload.wikimedia.org/wikipedia/commons/2/2e/Toluol.svg

http://upload.wikimedia.org/wikipedia/commons/b/bf/1-hexanol.PNG

http://upload.wikimedia.org/wikipedia/commons/8/8b/Phenol2.svg

http://upload.wikimedia.org/wikipedia/commons/c/ca/Decane-2D-Skeletal.svg

http://upload.wikimedia.org/wikipedia/commons/4/4f/Naphthalene-2D-Skeletal.svg

http://en.wikipedia.org/wiki/File:Dodecane-2D-Skeletal.svg

Stationary Phase Selection:Dispersive Interactions

100% Methyl Polysiloxane

Boiling Point Column?

12

3

4

5

6

1. Toluene2. Hexanol3. Phenol4. Decane (C10)5. Naphthalene6. Dodecane (C12)

110oC156oC182oC174oC218oC216oC

Strong DispersionNo DipoleNo H Bonding

Stationary Phase Selection:Dipole (Induced Dipole) Phases5% Phenyl Phase

12

34

5,6



Strong DispersionWeak (Induced) DipoleNo H Bonding

1 2 34

56

?

5% Phenyl

100%Methyl

Stationary Phase Selection:Dipole (Induced Dipole) Phases50% Phenyl Phase



Strong DispersionWeak (Induced) DipoleNo H Bonding

50% Phenyl

100% Methyl

1 234

5

1 2 34

56

?

6

Stationary Phase Selection:Dipole & Hydrogen Bonding Phases14% Cyanopropylphenyl Phase



Strong DispersionNone/Strong Dipole (Ph/CNPr)Weak/Moderate H Bonding (Ph/CNPr)

14% Cyano-propylphenyl

100% Methyl

1 23

4

5

12

34 5

6

?

6

Stationary Phase Selection:Dipole & Hydrogen Bonding Phases50% CyanopropylphenylPhase



Strong DispersionStrong DipoleModerate H Bonding

50% Cyano-propylphenyl

100% Methyl

1 23

45

1 23

4 56

?

6

Stationary Phase Selection:Dipole & Hydrogen Bonding Phases35% Trifluoropropyl Phase

1. p-Xylene, 2. m-Xylene, 3. o-XyleneColumn: DB1 / DB200 (30m x 0.25mm, 0.25µm)Carrier: Helium @ 32 cm/sec.Oven: 45 – 115oC @ 5oC/min.

Strong DispersionModerate DipoleWeak H Bonding

35% Trifluoropropyl

100% PDMS

DB-1 DB-200

1

2

3

1,2

3

Stationary Phase Selection:Dipole (& H Bonding) Phases100% Polyethylene Glycol



Strong DispersionStrong DipoleModerate H Bonding

100% PEG

100% Methyl

12

3

4

5

1 23

4 5

6

?

6

Stationary Phase Selection:PLOT columns

1. Porous Layer Open Tubular (PLOT)for Gas Solid Chromatographic (GSC) applications

2. PLOT columns are coated with small, solid porous particles using a binder. Particles are Alumina or Molecular sieve

3. Solutes are separated on differences in their adsorption properties, size and shape

4. PLOT columns used to separate highly volatile liquids and permanent gases without the need for cryogenic or subambient cooling of the GC oven

Stationary Phase Selection:PLOT columns

5. Alumina columns are wellsuited to the analysis of C1 – C10

hydrocarbons and small aromatics

6. KCl derivatised columnsproduce altered selectivity

7. The Q designated columnsshow better selectivity for C1-C3

hydrocarbons (not good for >C6)

8. Q columns are also able to separate sulphur gases and most light hydrocarbons.

9. Molecular sieve columns used for noble and permanent gas samples - also good for the separation of solvents.

50m, 0.53mm IDRt-Alumina™ PLOT

Practically Speaking!

1. If no information or ideas about which stationary phase to use is available, start with a DB-1 or DB-5

2. Low bleed ("ms") columns are usually more inert and have higher temperature limits

3. Use the least polar stationary phase that provides satisfactory resolution and analysis times. Non-polar stationary phases have superior lifetimes to polar phases

4. Use a stationary phase with a polarity similar to that of the solutes. This approach works more times than not; however, the best stationary phase is not always found using this technique


5. If poorly separated solutes possess different dipoles or hydrogen bonding strengths, change to a stationary phase with a different amount (not necessarily more) of the dipole or hydrogen bonding interaction

6. If possible, avoid using a stationary phase that contains a functionality that generates a large response with a selective detector

7. 100% Methyl or 5% Phenyl, 50% Phenyl, 14% Cyanopropylphenyl and WAX (PEG) cover the widest range of selectivities with the smallest number of columns

8. Use PLOT columns for the analysis of gaseous samples at above ambient column temperatures

Column Dimensions - Column Length



Selectivity (a) oC / Phase

1. Doubling column length doubles efficiency

2. Doubles analysis time (or by 1.5 – 1.75x for temperature gradient)

3. Increases column costs

4. Improves resolution by a factor of 1.4 x

p30.flv

Column Internal Diameter (or rc)

1. Affects efficiency, retention, carrier flow rate, capacity and pressure drop across the column

2. Inversely proportional to column efficiency – halve diameter, double efficiency, increase resolution by factor of 1.4




Column Internal Diameter (or rc) (II)

3. Inversely proportional to analyte retention for isothermalbut NOT GRADIENT

4. Consider gradient temperature programming in conjunctionwith pressure programming for constant flow




Column Internal Diameter (or rc) (III)

5. Column head pressure an inverse square function of column radius6. Column capacity increases with column internal diameter7. Capacity also depends on the stationary phase type, film thickness

and the nature of the analytes




p34.flv

Phase Film Thickness (df)Efficiency (N) carrier gas / L / rc



1. Affects retention, inertness,

capacity, resolution and bleed

2. Film thickness is directlyproportional to retentiontime (1.5:1 for gradient)

3. Thick stationary phase filmsgive retention for highly volatile analytes

4. Increasing film thickness allows retention of volatile analytes at temperatures at or above ambient

5. Doubling df gives an increase of around 20oC in elution temperature

Phase Film Thickness (df) (II)

6. Retention of late eluting (high boiling point) analytes is reduced using thinner film columns

7. Early eluting analytes (k<2) are better resolved using thicker film columns

8. Resolution may DECREASE for analytes with k values between 5 with INCREASING film thickness

9. Thicker films bleed more - upper temperature limits of thick film columns will be lower

10. Thicker film columns are more inert as the film shields the analyte from active sites on the silica tubing

11. Thicker film columns have higher analyte capacity, and so may reduce peak fronting




p37.flv

Phase Ratio (b)

1. Stationary phase to mobile phase ratio

2. Increasing the phase ratio will result in decreased analyte retention (increasing the column radius or decreasing the film thickness)(reduction in capacity?)

Phase Ratio (b)

3. Use to keep retention time approximately constant whilst increasing efficiency (reducing the column internal diameter) and reducing film thickness to keep b constant

4. Net result is a more efficient separation within the same timescale as the original separation!

Phase Ratio (b) (II)


1. Capillary GC columns are typically 10, 15, 20, 25, 30, 50, 60,120 (m)

2. Extending the column length is the least favoured option for increasing resolution and should be avoided if possible

3. Cost and analysis time are proportional to column length

4. Use the shortest column that will give you the required resolution (begin with 25-30 m columns if the number/ nature of samples is unknown).

5. To increase resolution try changing the stationary phase or column internal diameter first

6. Narrow internal diameter columns are capable of separating multiple analytes in a single analysis


7. Increase film thickness when volatile analytes are involved or reduce film thickness to decrease retention of highly adsorbed analytes

8. Use phase ratio to increase separation efficiency in the same timeframe as the original separation

9. Column head pressure and bleed increase with column length.

10. 10-15 m columns are well suited to samples containing well separated analytes or where the number of analytes is low

11. 50-60m columns should be used only where very large numbers of components need to be separated and as a last resort when reducing the column internal diameter and changing the stationary phase and temperature program have failed!

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