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The CHROMacademy Essential Guide Tutorial to Column Choice for Capillary GC
Tony Taylor In this month’s Resolver, CHROMacademy Technical Director Tony Taylor discusses key factors in GC Column Choice, including Stationary Phase Chemistry and the Rules we can use to choose the optimum phase for our separations.
Stationary Phase Interactions:   jump to section »
Stationary Phase Chemistry :   jump to section »
Golden Rules of Column Selection :   jump to section »
Stationary Phase Interactions:    
 
Mechanisms of Analyte – Stationary Phase Interaction

In order to understand the basis of the choice of stationary phase for capillary GC columns, you should know something about the intermolecular forces which govern the analyte – stationary phase interactions (i.e. how well retained an analyte is by the stationary phase).

The intermolecular forces responsible for adsorption interactions between analyte molecules and GC stationary phases can be broadly categorized as: - (Table 1)
Interaction
Energy (kJ/mol)
Dispersive
<<1
Dipole-Induced Dipole
1
Dipole-Dipole
3.3
Hydrogen bonding
19

Table 1 – Typical energies for various intermolecular interactions

 


 

The Concept of Polarity

All covalently bonded molecules will share electrons between the bonded atoms. A ‘non-polar’ covalent bond has a uniform distribution of electron charges between the atoms –the simplest non-polar covalent bonds exist in homonuclear diatomic species such as Cl2 or H2. In this type of molecule there is no permanent localised electrical charge build up, electrons are shared uniformly within the molecule.

Alternatively, a ‘polar’ bond displays a non-uniform electron distribution cloud. This typically occurs when two non-metal atoms which are more than two positions apart in the periodic table are involved in the bond –some examples are shown below. Here an electrical ‘dipole’ is established where the atoms within the bond are permanently charged due to the delocalization of the electron cloud.

The blue arrow beneath the HCl molecule indicates a permanent charge associated with the bond called a ‘Dipole Moment’. The chlorine atom carries a small negative charge and the hydrogen a small positive charge

Figure 1: Graphic representation of a Dipole Moment

 
Such dipole moments occur when the electronegativity of the atoms (essentially their power to attract electrons), differ substantially. You can find a great link to more information and an excellent table of values at this link: //en.wikipedia.org/wiki/Electronegativity
 

 
One can predict the strength of a dipole within a molecule by comparing the relative electronegativity values of the atoms involved in the bond.
 
C-S = 2.5 – 2.5 = 0.0
C-H = 2.5 – 2.1 = 0.4
C-N = 3.0 - 2.5 = 0.5
C-O = 3.2 - 2.5 = 1.0
O-H = 3.5 – 2.1 = 1.4
C-F = 4.0 - 2.5 = 1.5
EN units difference
EN units difference
EN units difference
EN units difference
EN units difference
EN units difference
Increasing electronegativity

 

Table 2: Electronegativity differences for some typical covalent bonds

 

 

The C-F bond has a dipole moment so high that it borders on becoming an ionic bond.

You will notice above that some of the functional groups typical of our own analytes have relatively high dipole moments.

When assessing the polarity of a molecule one should look out for common functional groups such as: Acids, Alcohols, Amines, Amides, Thiols etc.

Figure 2: Dipole moments within common organic molecules

 

 

Dispersive Interactions

All substances contain small dipoles (because electronegativities of all atoms differ). These dipoles fluctuate throughout the molecule and as two molecules approach each other their dipoles can induce the opposite dipole in the other molecule and a small attractive effect is seen – this is often called a dispersive interaction and occurs between compounds which are predominantly non-polar. Dispersive interactions occur with all substances, regardless if there is another overriding interaction (such as a dipole interaction).

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Dipole Interactions

Dipole interactions come in two sorts – Dipole-Dipole and Dipole-Induced Dipole. Dipole interactions occur between substances whose permanent dipoles come into close contact with each other. Dipole-Induced dipole interactions occur when a polar substance meets a less polar or non-polar compound and the stronger dipole induces a more permanent dipole in the other substance and an intermolecular attraction occurs.

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Hydrogen Bonding Interactions

Hydrogen bonding is a special case of a dipole-dipole interaction in which the dipoles associated with the functional groups of the two molecules come into close proximity. Hydrogen bonding interactions are very strong compared to dispersive interactions and in the extreme (e.g., the association of water with methanol) the dipole-dipole interaction energy can approach that of a chemical bond.

It should be noted that even when molecules are undergoing hydrogen bonding –there is still an underlying weak dispersive interaction occurring simultaneously.

Figure 3: Typical Hydrogen Bonding Interactions

 
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Stationary Phase Chemistry :   back to top »
 

We tend to chose GC stationary phases on the basis of the three interaction types outlined above. For example, a non-polar analyte will undergo predominantly dispersive types of interaction. For good retention, we would therefore choose a dispersive type stationary phase.

We should begin then to classify stationary phase types according to their predominant interactions with analytes to enable us to make good stationary phase choice for retention, and hopefully resolution!

Most GC stationary phases are highly viscous liquid polymers whose chemical composition depends upon the monomeric units used to derive the polymeric form. The careful choice of these monomers and their ratio in the polymeric form will dictate the chemical nature of the stationary phase and its application area.

Figure 4 shows the broad classification of phases currently available and their predominant interactions and Table 3 summarises the information

Figure 4: Various Stationary Phase Chemistries

   

Name 100% Polydimethylsiloxane (Methyl)
Predominant Interactions Dispersive
Primary Applications Boiling point separations / Hydrocarbon Analysis

Typical Ratio of monomers (X:Y):
5:95
35:65
50:50

Name Phenyl Dimethylpolysiloxane (Phenyl)
Predominant Interactions Dispersive / Induced Dipole
Primary Applications Aromatic / Aliphatic mixtures
Notes The ratio of X:Y will dictate the relative polarity of the phase with the ‘5% Phenyl’ phase being highly popular as a generic starting point for method development

Typical Ratio of monomers (X:Y):
6:94
14:86
50:50

Name Cyanopropylphenyl Dimethylpolysiloxane
Predominant Interactions Dispersive / Dipole / Hydrogen Bonding
Primary Applications Functionalised Molecules
Notes This column can be used to separate analytes whose basic structure is similar but whose functional group chemistry differs

Typical Ratio of monomers (X:Y):
35:65
50:50


Name Trifluoropropyl Dimethylpolysiloxane
Predominant Interactions Dispersive / Dipole / Hydrogen Bonding
Primary Applications Similar compounds / Isomers
Notes This column can be used to separate compounds whose boiling point differs very little but whose functional or stereochemistry can be differentiated

Name Polyethylene glycol (PEG)
Predominant Interactions Dispersive / Dipole / Hydrogen Bonding
Primary Applications Polar compounds

 
 
Functional Group   Dispersion   Dipole   Hydrogen Bonding
Methyl   Strong   None   None
Phenyl   Very Strong   Weak   None
Cyanopropyl   Strong   Very Strong   Moderate
Triflouropropyl   Strong   Moderate   Weak
PEG   Strong   Strong   Moderate
 
 

Table 3: Summary of Various Stationary Phase Types and their predominant interactions

 
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Golden Rules of Column Selection :   back to top »
 

Dispersive Interactions

Using the ‘like dissolves analogy’ dispersive phases are typically used to analyse non-polar analytes. When a series of n-alkanes is chromatographed with a diemthylpolysiloxane column, they elute in boiling point order with the lowest boiling analyte eluting first.

However, the boiling point analogy is lost when a series of aromatic compounds are also chromatographed at the same time. Whilst they remain in boiling point order the relative spacing of the analyte bands is not as predicted and this is a reflection that the two ‘species’ of analyte undergo different degrees of dispersive interaction with the stationary phase.

 

Figure 5: Boiling point separation of an n-alkane and aromatic series on a 100% polydimethylsiloxane stationay phase


As a general rule of thumb, if the boiling point of two compounds differs by 30oC or more, then they may be separated by most stationary phases. This is due to the fact that dispersion is the dominant interaction for a wide range of stationary phase types. If the compounds boiling point differs by less than 10oC (and the compounds do not belong to a homologous series), then the boiling point analogy is more likely to be in error.

When dealing with analytes with mixed functionality –the boiling point analogy is often confounded as can be seen. Note that s-butylbenzene and n-decane have the same boiling point and are separated via the differing strengths of their dispersive interaction.

Column: DB1 (30m x 0.25mm, 0.25μm) : Carrier: Helium @ 32 cm/sec. : Oven: 45 – 115 oC @ 5 oC/min.

Figure 6: Failure of the boiling point analogy when analytes of mixed functional groups are chromatographed simultaneously

 

 

Dispersive Interactions / Polarity

Another stationary phase characteristic that may affect retention in a predictable manner is the phenyl content. The higher the phenyl content of the stationary phase, the higher the retention of aromatic solutes RELATIVE to aliphatic solutes. This does not necessarily mean that the aromatics are more retained, but that they shift relative to aliphatic solutes. This is demonstrated in the top example opposite. Increasing the phenyl content of the phase causes earlier elution of the n-alkanes and later elution of the alkyl-benzenes.

 

1. Toluene, 2. n-Octane, 3. Ethylbenzene,
4. n-Nonane, 5. Propylbenzene,
6. sec-Butylbenzene, 7. n-Decane,
8. 1,2-Dichlorobenzene, 9. Butylbenzene,
10. n-Undecance, 11. n-Dodecane

Figure 7: Predictable retention pattern change when moving to a more aromatic phase –
aliphatic compounds move to shorter retention time relative to the aromatics

 

 
Altering the phenyl content of the stationary phase affects the polarity of the column. By increasing the phenyl content the polarity increases (due to the presence of p -electrons in the aromatic groups) which renders the stationary phase ‘less non-polar’. This is well described in Figure 8 where the retention of the polar analytes (alcohols) is increased relative to the non-polar analytes (n-alkanes) where the phenyl content of the phase (and hence the polarity) is increased.
 

Figure 8: Polar compounds better retained when increasing the phenyl content of the phase – increasing an increase in the overall stationary phase polarity

 

 

Dipole Interactions / Hydrogen Bonding

If the stationary phase is capable of dipole interaction, it enhances its power to separate solutes whose dipole moments are different. Cynopropyl, trifluoropropyl and Poly Ethylene Glycol (PEG) phases all show good dipole interaction properties.

The degree of peak separation for solutes with different dipoles often changes if a stationary phase with a different amount of the dipole interaction is used. If the dipole difference between compounds is small, a greater amount of the appropriate phase is required (i.e. 50% cyanopropylphenyl-methyl instead of 14% cyanopropylphenyl-methyl). It is difficult to be predictive about the magnitude of the separation change for all peaks. Empirical studies show that this type of phase are well suited for compounds which have a base or central structure to which different groups are attached in various positions. Examples include substituted aromatics, halocarbons, pesticides and drugs.

The same stationary phases that undergo dipole interactions also undergo hydrogen bonding interactions –with the PEG phases showing the strongest interaction. Again where the analyte hydrogen bonding potential differs only slightly, a stationary phase with a greater amount of the appropriate group is required. This is demonstrated in Figure 9. It could be argued that by using a 50% Trifluoropropyl phase the para, meta substituted forms could be even further resolved.

 

Figure 9: Increased isomeric separation using a TFP phase which shows strong dipole interactions

 

 
Hydrogen bonding interactions also show similar trends, again relying on increased amounts of the hydrogen bonding monomer within the phase when the dipole moment differences between analytes is small.
 

Figure 10: Increased Creol isomar separation due to hydrogen bonding with the TFP phase

 

 

Figure 11: Hydrogen Bonding interaction between m-Cresol and the trifluoropropyl stationary phase

Strength
Compounds
Strong
Alcohols, carboxylic acids, amines.
Moderate
Aldehydes, esters, ketones
Weak to None
Hydrocarbons, halocarbons, ethers

Table 4 – Hydrogen bonding capability of various analyte species

 

 

Porous Layer Open Tubular (PLOT) Columns

Porous Layer Open Tubular (PLOT) columns are intended for Gas Solid Chromatographic (GSC) applications. PLOT columns are capillaries in the conventional sense but the inner wall of the capillary is coated with small, solid porous particles using a binder. The particles are usually Alumina or Molecular sieve and solutes are separated on differences in their adsorption properties, with size and shape differentiation also occurring.

PLOT columns are used primarily for the separation of highly volatile liquids and permanent gases without the need for cryogenic or subambient cooling of the GC oven. Separations that would require column temperatures well below ambient temperatures, even with thick film capillary columns, can be obtained at ambient temperatures or above using PLOT column technology.

Alumina columns are well suited to the analysis of C1 – C10 hydrocarbons and small aromatics, whilst the KCl derivatised version of the column produces altered selectivity for the same compound group.

The Q designated columns show better selectivity for C1-C3 hydrocarbons, but give very long retention and broadened peaks for anything heavier than C6. These columns are also able to separate sulphur gases and most light hydrocarbons.

Molecular sieve columns are used to separate many noble and permanent gas samples and are also good for the separation of solvents.

Figure 12: Permanent gas analysis using a Molecular Sieve coated PLOT column

 

 

Stationary Phase Selection Summary

1.   If no information or ideas about which stationary phase to use is available, start with a 100% polydimethylsiloxane or 5% phenlmethyl polysiloxane phase.
2.   Low bleed ("MS type") 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. Other co-elutions may occur upon changing the stationary phase, thus the new stationary phase may not provide better overall resolution.
6.   If possible, avoid using a stationary phase that contains a functionality that generates a large response with a selective detector. For example, cyanopropyl containing stationary phases exhibit a disproportionatly large baseline rise (due to column bleed) with Nitrogen Phosphorous detectors (NPD).
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.   PLOT columns are best used for the analysis of gaseous samples at above ambient column temperatures.
 
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