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The CHROMacademy Essential Guide Webcast:
Principles of GC Column Selection

Thursday 20th February 2014, 16:00 GMT

Whilst the choice of GC columns may seem myriad, the number of chemistries is actually rather limited, and in this webcast you will learn how to select the best stationary phase composition for your application.  We also consider the factors which should influence your selection of column dimensions, and highlight how to avoid wasting time and money, as well as maximizing your chances of a successful separation!  The Essential Guide from LCGC’s CHROMacademy presents the definitive guide in GC column selection.  In this session, Dr. John Hinshaw (Senior Scientist , BPL Global Ltd) and Dr. Dawn Watson (Technical Specialist, Crawford Scientific) explore the various factors associated with GC stationary phase selection. 

  • GC column selection is critical to obtaining the correct separation selectivity – what factors influence stationary phase choice?
  • Whilst choosing a ‘generic’ phase can bring satisfactory results, there are often better choices which will improve specificity and sensitivity
  • GC column dimensions directly influence the quality of a separation - learn how to choose the best column length, internal diameter, and film thickness for your application
  • Learn how to optimize your separation through smart column selection
  • Appreciate when to stop optimizing method conditions and start choosing a different column!
 

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Who Should Attend:

  • Anyone using or developing Capillary Gas Chromatography methods!
   


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The CHROMacademy Essential Guide Tutorial

Whilst the choice of GC columns may seem myriad, the number of chemistries is actually rather limited, and in this webcast you will learn how to select the best stationary phase composition for your application.  We also consider the factors which should influence your selection of column dimensions, and highlight how to avoid wasting time and money, as well as maximizing your chances of a successful separation!  The Essential Guide from LCGC’s CHROMacademy presents the definitive guide in GC column selection.  In this session, Dr. John Hinshaw (Senior Scientist , BPL Global Ltd) and Dr. Dawn Watson (Technical Specialist, Crawford Scientific) explore the various factors associated with GC stationary phase selection. 

  • Discover the primary physico-chemical interactions between analytes and the stationary phase in Capillary GC
  • Learn about the stationary phase chemistries and how to choose between them for a particular application
  • Discover how to optimize the monomer ratios for each phase
  • Learn how to choose the right column length, internal diameter, and film thickness for your application
  • Identify when a column is not optimized for a separation and what can be done
  • Learn to recognize symptoms of a poorly performing column

 

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GC Column Selection

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In order to understand the basis of the choice of stationary phase for capillary GC columns, a knowledge of the intermolecular forces which govern the analyte/stationary phase interactions (i.e. how well retained an analyte is by the stationary phase) is required.

The intermolecular forces responsible for adsorption interactions between analyte molecules and GC stationary phases can be broadly categorized as dispersive, dipole-induced dipole, dipole-dipole, and hydrogen bonding (Table 1).

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Interaction Energy (kJ/mol) Comments
Dispersive << 1 Ubiquitous and arise in all intermolecular interactions
Dipole-Induced Dipole 1 Occur between a molecule with a permanent dipole and a polarizable molecule
Dipole-Dipole 3.3 Very strong and occur between molecules with permanent dipoles
Hydrogen Bonding 19 Very strong and occur between moieties capable of hydrogen bonding i.e. hydroxyl groups

Table 1: Typical energies for various intermolecular interactions.

 
 

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 localized 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 (Figure 1). 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: Graphical representation of non-polar and polar bonds.
 

Such dipole moments occur when the electronegativity of the atoms (essentially their power to attract electrons), differ substantially.
One can predict the strength of a dipole within a molecule by comparing the relative electronegativity values of the atoms involved in the bond (Figure 2).

 

Figure 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. It should be noted that some of the common functional groups contained in analytes have relatively high dipole moments (Figure 3). When assessing the polarity of a molecule one should look out for common functional groups such as: acids, alcohols, amines, amides, thiols etc.

 
Figure 3: Dipole moments within common organic molecules.
 
 

Dispersive Interactions

Dispersive interactions are ubiquitous and arise in all intermolecular interactions as a result of small charge fluctuations which occur throughout a molecule due to electron/nuclei vibrations (Figure 4).  The fluctuations are random in nature and are basically a statistical effect.  Every molecule will have a number of arrangements of nuclei and electrons which have a dipole moment that fluctuates resulting in an overall charge of zero.

However, at any instant in time the dipoles are capable of interacting with other instantaneous dipoles of other molecules.  Dispersive interactions can occur in isolation; however, they will always be present in molecular interactions even when other types of interaction dominate (i.e. hydrogen bonding).  An example of molecules which contain purely dispersive interactions are hydrocarbons; the lower molecular weight hydrocarbons are liquids and not gases due to the dispersion forces which act between the hydrocarbon molecules.

 

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Figure 4: Dispersive interactions.
 
 

Dipole Interactions
There are two distinct types of dipole interaction (Figure 5):

  1. Dipole-dipole – the interaction between two species which contain a permanent dipole (i.e. between alcohols, esters, ethers, amines, amides, nitriles etc.).
  2. Dipole-induced dipole – the interaction between a molecule possessing a permanent dipole and a polarizable molecule.  Polarizable molecules will commonly contain π-electron systems (i.e. aromatic or unsaturated compounds).

Dipole interactions are relatively strong and will be the predominant contribution to intermolecular forces when they are present; however, dispersive forces will also be present.

 

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Figure 5: Dipole interaction.
 
 

Hydrogen Bonding

Hydrogen bonding is a special case of dipole-dipole interaction in which dipoles associated with, most commonly, hydroxyl groups of the two molecules come into close proximity (Figure 6).  Hydrogen bonding interactions are very strong in comparison to dispersive interactions and in the extreme (e.g. the association of water and methanol) the dipole-dipole interaction energy can approach that of a chemical bond.  

 

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Figure 6: Hydrogen bonding interactions.
 
 

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Stationary phases tend to be chosen on the basis of the three main interactions between the analyte and stationary phase; dispersion, dipole, and hydrogen bonding (Table 2).  For example, a non-polar analyte will undergo predominantly dispersive type interactions, therefore, for good retention a dispersive type stationary phase should be chosen.

An excellent rule of thumb to help with the choice of stationary phase is ‘like dissolves like’, i.e. the analyte polarity and predominant interaction type should be matched to the stationary phase polarity (Figure 7) and the predominant interaction.

 

Figure 7: GC capillary column stationary phase polarity.

 
 

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.
The broad classification of phases currently available and their predominant interactions are shown below and summarized in Table 2.

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

Table 2: Summary of GC stationary phases and their predominant interactions.

 
 

Polysiloxane Stationary Phases

Dimethyl polysiloxane

 
Name 100% Dimethyl polysiloxane (Methyl)
Predominant Interaction Dispersive
Primary Applications Boiling point separations, hydrocarbon analysis
 
 
 

Typical Monomer
Ratios (X:Y)
5:95
35:65
50:50

 
Name Phenyl dimethyl polysiloxane (Phenyl)
Predominant Interaction Dispersive, induced dipole
Primary Applications Aromatic/aliphatic mixtures
Comments 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.
Higher percentage of functional monomer indicates a higher degree of that interaction. 50:50 phase shows stronger induced dipole interactions with aromatics.
 
 
 

Typical Monomer
Ratios (X:Y)
6:94
14:86
50:50

 
Name Cyanopropylphenyl dimethyl polysiloxane (Cyano)
Predominant Interaction Dispersive, dipole, hydrogen bonding
Primary Applications Functionalized molecules
Comments This column can be used to separate analytes whose basic structure is similar but whose functional groups chemistry differs.
 
 
 

Typical Monomer
Ratios (X:Y)
35:65
50:50

 
Name Trifluoropropyl dimethyl polysiloxane (Fluoro)
Predominant Interaction Dispersive, dipole, hydrogen bonding
Primary Applications Similar compounds, isomers
Comments This column can be used to separate compounds whose boiling points differ very little but whose functional or stereochemistry can be differentiated.
 
 
 

 

 
Name Polyethylene glycol (PEG)
Predominant Interaction Dispersive, dipole, hydrogen bonding
Primary Applications Polar compounds
 
 

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From the fundamental resolution equation (Figure 8) it can be seen that selectivity has the largest impact on resolution.  GC capillary column selectivity is dependent on the nature of the stationary phase, the nature of the components, and the oven temperature.

When developing a GC method stationary phase selectivity should be optimized first; stationary phase selection is the most important parameter in GC, however, it can be ambiguous especially when analyzing mixtures of analytes.  

 

Figure 8: Parameters affecting resolution.

 
 

Dispersive Interactions

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

However, the boiling point analogy is lost when a mixture of aliphatic, aromatic, and alcohols are chromatographed at the same time (Table 3).  It can be seen that although a PDMS column is often considered as a ‘boiling point column’ the elution order does not hold with the boiling points of these analytes, indicating that the different ‘species’ of analyte undergo different degrees of dispersive interaction with the stationary phase (Figure 9).  It can be seen that the boiling point analogy is often confounded when dealing with analytes with mixed functionality.

 
Compound Boiling Point (°C) Primary Interaction
Polar Aromatic Hydrogen Bonding Dipole
1. Toluene 111.0 No Yes No Induced
2. Hexanol 158.0 Yes No Yes Yes
3. Phenol 181.7 Yes Yes Yes Yes
4. Decane 174.1 No No No No
5. Naphthalene 218.0 No Yes No Induced
6. Dodecane 216.2 No No No No

Table 3: Boiling points and primary interaction types of a mixture of GC test compounds.

 
 
 

 

1. Toluene, 2. Hexanol, 3. Phenol, 4. Decane, 5. Naphthalene, 6. Dodecane

 

Figure 9: Separation of a mixture of aliphatic, aromatic, and phenolic compounds on a 100% dimethylpolysiloxane stationary phase. 

Column:   DB-1 30 m x 0.25 mm x 0.25 μm
Carrier gas:   32 cm/sec, He
Oven:   45-115 oC at 5 oC/min
 

Any homologous series of compounds, that is, analytes from the same chemical class (e.g. all alcohols, all ketones, or all aldehydes, etc.) will elute in boiling point order on any stationary phase. However, when different compound classes are mixed together in one sample, intermolecular forces between the analytes and the stationary phase are the dominant separation mechanism, not boiling point.

As a general rule of thumb, if the boiling point of two compounds differs by 30 oC 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 10 oC (and the compounds do not belong to a homologous series), then the boiling point analogy is more likely to be in error.

 
 

Dipole-Induced Dipole Interactions

The predominant mechanism of stationary phase analyte interaction for Phenyl dimethyl polysiloxane columns are dispersive and induced dipole.

The phenyl content of the stationary phase may affect retention in a predictable manner.  The higher the phenyl content of the stationary phase, the higher the retention  of aromatic solutes relative to aliphatic solutes – this can be seen in Figure 10 where an increase in phenyl content from 5 to 50% results in greater retention of naphthalene and phenol in comparison to hexanol, decane, and dodecane.  This does not necessarily mean that the aromatics are more retained but that they shift relative to aliphatic analytes.  Increasing the phenyl content of the phase will result in earlier elution of n-alkanes.

Polar compounds are more strongly retained by polar stationary phases and vice versa.  An increase in phenyl content of the stationary phase increases the polarity of the column due to the increase in pi electrons.

 
   

1. Toluene, 2. Hexanol, 3. Phenol, 4. Decane, 5. Naphthalene, 6. Dodecane

 
 

Figure 10: Separation of a mixture of aliphatic, aromatic, and phenolic compounds on a 5% (top) and 50% (bottom) diphenyl dimethylpolysiloxane stationary phase. 

Column:   DB-5 or DB-17 30 m x 0.25 mm x 0.25 μm
Carrier gas:   32 cm/sec, He
Oven:   45-115 oC at 5 oC/min
 
 

Dipole Interactions/Hydrogen Bonding

Similarly the use of a cyanopropylphenyl stationary phase which exhibits dipole and hydrogen bonding interactions will produce separations where compounds which are capable of these types of interactions are more highly retained relative to their aliphatic counterparts (Figure 11). 

 
   

1. Toluene, 2. Hexanol, 3. Phenol, 4. Decane, 5. Naphthalene, 6. Dodecane

 

Figure 11: Separation of a mixture of aliphatic, aromatic, and phenolic compounds on a 14% (top) and 50% (bottom) cyanopropylphenyl dimethylpolysiloxane stationary phase. 

Column:   DB-1701 or DB-225 30 m x 0.25 mm x 0.25 μm
Carrier gas:   32 cm/sec, He
Oven:   45-115 oC at 5 oC/min
 
 

If the stationary phase is capable of dipole interaction, it enhances its power to separate solutes whose dipole moments are different. Cyanopropyl, 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 these types 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 by the separation of orth-, meta-, and para-xylene shown in Figure 12. It could be argued that by using a 50% trifluoropropyl phase the para and meta substituted forms could be even further resolved.

 
 

 

1. p-Xylene, 2. m-xylene, 3. o-xylene

 

Figure 12: Comparison of the separation of o-, m-, and p-xylene using a 100% PDMS (left) and 35% trifluoropropyl dimethylpolysiloxane (right) column.

Column:   DB-1 or DB-200 30 m x 0.25 mm x 0.25 μm
Carrier gas:   32 cm/sec, He
Oven:   45-115 oC at 5 oC/min
 
 

Pragmatic Phase Selection Rules

When making the initial choice of stationary phase for a GC separation some pragmatic selection rules can prove useful.

  • Use the principles of ‘like dissolves like’ wherever possible and match the polarity of the analyte to the polarity of the stationary phase.
  • Remember that really there are only five ‘chemistries’ we need to consider.  To increase retention or selectivity based on a particular interaction, increase the amount of the functional group with in the phase (i.e. move from 14% to a 35% cyanopropyl phase).
  • Use the least polar phase possible as more polar phases bleed more (it is inherent in the chemistry).
  • A 5% phenyl column should be used to screen unknown samples – analyte retention and selectivity can then be assessed and a more appropriate phase chosen if necessary.
  • A 5% phenyl, 50% phenyl, 14% cyanopropyl, and a WAX (PEG) column cover the widest range of possible interactions (stationary phase polarities) in the fewest number of columns.
 
 

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Length

The ultimate goal of any chromatographic method is to produce robust chromatography in the minimum time.  When choosing a capillary GC column, as well as the stationary phase type, the physical dimensions of the column must also be specified, usually length, internal diameter, and stationary phase film thickness.

All of these dimensions are critical to the performance of any separation.  A useful equation that describes the contributing factors to GC retention time and the ways in which analysis speed can be optimized are shown in Equation 1.

 

Efficiency (N)

carrier gas / L / rc

Retention (k)

oC / rc / df

Selectivity (α)

oC / phase

 
 

Length (L)

Column efficiency is proportional to column length.  Doubling column length will double the number of theoretical plates, and hence, double efficiency (Figure 13).  However, resolution is proportional to the square root of efficiency, therefore, doubling column length will only provide a theoretical increase in resolution of a factor of 1.4 (i.e. the square root of 2). 

 

Figure 13: Effect of column length on the efficiency of the separation of 5 phenol compounds.

 

Doubling column length will double analysis time for isothermal operation and increase analysis time by 1.5-1.75 times for gradient temperature programmed analysis.  This therefore, reduces the importance of increasing column length to obtain or improve separation.  In practice column length is only increased when peak separation is very small and high efficiency is required, or when the sample contains many analytes, all of which need to be separated.  Increasing the column length also increases the pressure required to achieve a set flow rate which is not a problem practically unless very narrow columns are used.  Furthermore, column costs will be increased. 

 
 

It can be seen in Figure 14 that the separation of 16 polycyclic aromatic hydrocarbons (PAH) on a 30 m x 0.25 mm x 0.25 μm column is achieved in approximately 16 minutes.  Reduction of the column length (all other column parameters remain the same) reduces the analysis time to approximately 12 minutes with improved resolution for the highlighted peak pairs.

 

1. Naphthalene, 2. acenaphthaylene, 3. acenaphthene, 4. fluorene, 5. phenanthrene, 6. anthracene, 7. fluoranthene, 8. pyrene, 9. benzo[a]anthracene, 10. chrysene, 11. benzo[b]fluoranthene, 12. benzo[k]fluoranthene, 13. benzo[a]pyrene, 14. indenol[1,2,3-cd]pyrene, 15. dibenzo[a,h]anthracene,
16. benzo[g,h,i]perylene.

 

Figure 14: Column length effect on analysis time and resolution for the separation of 16 polycylcic aromatic hydrocarbons (PAH) compounds.1

Carrier gas:   1.2 mL/min., He, constant flow
Injection:   Split injection 30:1, 1 μL
Gradient temperature program:   90 oC (1 min), 25 oC/min to 280 oC, 4 oC/min to 32 oC (5 min)
Detector:   FID at 280 oC
 
 

Internal Diameter (rc)

Altering the column diameter affects five operational parameters – efficiency, retention, required carrier gas flow rate to achieve optimum linear velocity, capacity, and pressure drop across the column.  The column internal diameter is inversely proportional to column efficiency.  Therefore, halving the column internal diameter doubles the efficiency and improves resolution by a theoretical factor of 1.41 (Table 4).  Increases in efficiency arise due to the increase in analyte/stationary phase interactions in the smaller diameter columns.

Analyte retention is also inversely proportional to the column diameter for isothermal separations, but crucially the retention time change under temperature gradient conditions is 1.25-1.5 times the original retention.  This makes changing the column internal diameter much more attractive in practical terms.
Column head pressure is approximately an inverse square function of column radius – i.e. a 0.25 mm i.d. column requires 1.7 times greater head pressure than a 0.32 mm i.d. column of the same length at the same temperature.

Column capacity increases with column internal diameter (Table 4).  The capacity also depends on the stationary phase type, film thickness and the nature of the analytes.  Larger bore columns and thicker stationary phase films have higher analyte capacity and therefore higher analyte masses can be loaded before peak shape begins to deteriorate.

The optimum linear velocity of the carrier gas also increases when narrower columns are used which allows for shorter analysis times.  The Golay plots (Figure 15) demonstrates that with a 0.15 mm column the optimum linear velocity which provides the greatest efficiency is 32.7 cm/s, compared with 17.7 cm/s for a 0.53 mm column.

 

Figure 15: Influence of column diameter on optimal gas velocity and HETP. 1

 
 
Internal Diameter (mm) Efficiency: Plates/Meter (N/m) Efficiency: Total Plates (N) Capacity Each Analyte (ng)
0.53 1,300 39,000 1000-2000
0.32 2,300 69,000 400-500
0.25 2,925 87,750 50-100
0.20 3,650 109,500 <50
0.18 4,050 121,500 <50
0.10 7,300 219,000 <10

Table 4: Efficiency values for standard GC column diameters.

 
 

Selecting Internal Diameter
Some guidelines for selecting the correct initial column diameter are:

  1. 0.18-0.25 mm I.D. when higher column efficiencies are required.  Smaller diameter columns have the highest head pressure requirements and lowest capacities.
  2. 0.32 mm I.D. when higher sample capacity is needed.  Often provide better resolution for early eluting solutes for splitless or large volume (> 2 μL) injections than 0.25 mm I.D.
  3. 0.45 mm I.D. only for Megabore direct injection and high efficiency is required.  Suited to high carrier gas flow rates as with purge & trap, headspace, and valve injection.
  4. 0.53 mm I.D only when Megabore injection is available.  Well suited to high carrier gas flow rates as with purge & trap, headspace, and valve injection.  Highest sample capacity with constant film thickness (df).
 
 

Film Thickness (df )

Column stationary phase film thickness (df ) affects retention, inertness, capacity, resolution, and column bleed.  Under isothermal conditions, film thickness is directly proportional to retention time (the proportionality is approximately 1.5:1 under temperature gradient conditions).

Thick stationary phase films are used to gain retention for highly volatile analytes such as solvents or some selected permanent gases.  Increasing film thickness allows retention of volatile analytes at temperatures at or above ambient.  Analytes have equal or greater retention at higher column temperatures.  The same principles apply when reducing film thickness, and in this way the retention of highly adsorbed analytes (late eluting, high boiling point, or high molecular weight analytes) may be reduced using thinner film columns.  Doubling film thickness will result in an increase of around 20 oC in elution temperature.
Early eluting analytes (k < 2) are better resolved using thicker film columns.  Resolution will also increase for most analytes with k values between 5 and 10; however, analytes with k > 10 will see no improvement in resolution when a thicker film is used.

For any given stationary phase, thicker films will bleed more and the upper temperature limits of thick film columns will be lower than their thin film counterparts.  Thicker film columns are more inert as the film shields the analyte from active sites on the silica tubing, therefore, increasing film thickness can often improve the peak shape of tailing peaks.  Thicker film columns have a higher capacity and may, therefore, reduce peak broadening; this can be of particular interest when one analyte is present in vast excess compared to the others.  Thicker film columns may prevent co-elution with the larger peak.

Figure 16 demonstrates the decreased analysis times and increased resolution that can be achieved by reducing the column film thickness.  Reducing the column thickness from 0.50 to 0.25 μm (all other dimensions remain constant) results in a reduction in analysis time for eleven phenol compounds from 17.96 minutes to 15.63 minutes, with improved resolution of two critical peak pairs (3,4 and 8,9).

 

1. Phenol, 2. 2-clorophenol, 3. 2-nitrophenol, 4. 2,4-dimethylphenol, 5. 2,4-dichlorophenol, 6.  4-chloro-3-methylphenol, 7. 4,6-trichlorophenol, 8. 2,4-dinitrophenol, 9. 4-nitrophenol, 10. 2-methyl-4,6-dinitrophenol, 11. pentachlorophenol.

 

Figure 16: Effect of film thickness on analysis time and resolution of eleven phenol compounds. 1

Carrier gas:   1.2 mL/min., He, constant flow
Injection:   Split injection 80:1, 1 μL
Gradient temperature program:   60 oC (1 min), 10 oC/min to 240 oC
Detector:   FID at 280 oC
 
 

Phase Ratio (β)

The phase ratio of a column is a measure of the stationary phase to mobile phase ratio at any point in the column and is calculated using Equation 2.

Where:

r = column radius (μm)

df = film thickness (μm)

 

Increasing the phase ratio will result in decreased analyte retention.  The phase ratio can be increased by increasing the column radius or decreasing the film thickness (Table 5).  The opposite is also true; if the phase ratio is decreased analyte retention will increase.

The phase ratio can be decreased by reducing the column internal diameter or increasing the film thickness.
Note that decreasing the phase ratio will result in an increase in column capacity.

 
Film Thickness
df (μm)
Column Diameter (mm)
0.1 0.18 0.2 0.25 0.32 0.45 0.53
0.1 250 450 500 625 800 1125 1325
0.18 139 250 278 347 444 625 736
0.25 100 180 200 250 320 450 530
0.5   90 100 125 160 225 265
1.0     50 63 80 113 133
1.5       42 53 75 88
3.0       21 27 38 44
5.0       13 16 23 27

Table 5: Phase ratio values for selected GC column dimensions.

 
 

The real elegance of using phase ratio is that it can be used to keep retention time approximately constant whilst altering other aspects of the chromatography.  For example, if one wanted to increase the efficiency of a separation this can be achieved by reducing the column internal diameter, however, this leads to increased analysis time at constant pressure and temperature.  However, by choosing a thinner film the phase ratio can be kept approximately constant; the net result is a more efficient separation within the same timescale as the original separation (Figure 17).

 

Figure 17: Effect of phase ratio on analysis time and efficiency.

Column:   DB-624
Carrier gas:   40 cm/s, He
Injection:   Split injection
Oven:   65 oC
Detector:   FID
 

In general the following rules can be used when making an initial decision on column phase ratio.

  • Volatile samples β < 100
  • General samples β ≈ 250
  • High molecular weight samples β > 400
 
 

Pragmatic Column Dimension Selection Rules

  • Capillary GC columns are typically 10, 15, 20, 25, 30, 50, 60, 120 m
  • Extending the column length is the least favored option for increasing resolution and should be avoided if possible
  • Cost and analysis time are proportional to column length
  • 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).
  • To increase resolution, try changing column internal diameter first; then the stationary phase
  • Narrow internal diameter columns are capable of separating more analytes in a single analysis
  • Increase film thickness when volatile analytes are involved or reduce film thickness to decrease retention of highly adsorbed analytes
  • If you need to increase separation efficiency in the same timeframe use smaller diameter columns withy the same phase-ratio
  • Column head pressure and bleed increase with column length
  • 10-15 m columns are well suited to samples where the number of analytes is low
  • 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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  1. Anila I. Khan, Technical Note 20743, Optimizing GC Parameters for Faster Separations with Conventional Instrumentation, Thermo Fisher Scientific, Runcorn, Cheshire, UK.
 
 

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Whilst the choice of GC columns may seem myriad, the number of chemistries is actually rather limited, and in this webcast you will learn how to select the best stationary phase composition for your application. We also consider the factors which should influence your selection of column dimensions, and highlight how to avoid wasting time and money, as well as maximizing your chances of a successful separation! The Essential Guide from LCGC’s CHROMacademy presents the definitive guide in GC column selection. In this session, Dr. John Hinshaw (Senior Scientist , BPL Global Ltd) and Dr. Dawn Watson (Technical Specialist, Crawford Scientific) explore the various factors associated with GC stationary phase selection.

John V Hinshaw
Senior Scientist
BPL Global Ltd

Dr. Dawn Watson
Technical Specialist
Crawford Scientific

Key Learning Objectives:

  • Discover the primary physico-chemical interactions between analytes and the stationary phase in Capillary GC
  • Learn about the stationary phase chemistries and how to choose between them for a particular application
  • Discover how to optimize the monomer ratios for each phase
  • Learn how to choose the right column length, internal diameter, and film thickness for your application
  • Identify when a column is not optimized for a separation and what can be done
  • Learn to recognize symptoms of a poorly performing column