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How to Optimize Key Variables in GC Analysis - GC Columns and Detectors

Columns

Selecting a GC column can seem like a daunting task, it may seem like there are a never ending number of phase chemistries, (however, there are really only five), or an inordinate number of column geometry options. Hopefully this short article can give you some quick rules to help select the correct column for your application.

In terms of selecting an appropriate stationary phase, there are four primary analyte/stationary phase interactions which need to be considered:

Dispersive interactions (<<1 kJ/mol, Figure 1) are lower energy van der Waals forces which occur in all molecules and are always present even when other types of interactions dominate. These interactions will dominate when using any silica based stationary phase as the majority of the phase polymeric backbone (polydimethylsiloxane) is non-polar in nature.

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Figure 1: Representation of dispersive interactions.


Dipole-dipole and dipole-induced dipole interactions (3 and 1 kJ/mol respectively, Figure 2) are predominant whenever unsaturated, aromatic, or more polar functional groups (i.e. C-Cl or C-N bonds) are present in the stationary phase or analyte molecule.  Stationary phases containing phenyl, cyano, or trifluoro functional groups are more polar than PDMS and the more of these functional groups there are, the greater their influence on the separation. 

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Figure 2: Representation of dipole-dipole and dipole-induced dipole interactions.

Hydrogen bonding interactions (19 kJ/mol, Figure 3) are the strongest intermolecular forces in capillary GC and occur whenever the stationary phase contains cyano, trifluoro, or hydroxyl functional groups. This type of force is in play when analyzing alcohols using a polyethylene glycol or ‘wax’ type phase.

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Figure 3: Representation of hydrogen bonding interactions.

When selecting a column for a new application (or to improve an existing one) there are some simple quick rules which can be followed

  • Use the principle of ‘like dissolves like’ wherever possible and match the polarity of the analyte to the polarity of the stationary phase
  • Remember that there are only five ‘chemistries’ that need to be considered (PDMS, phenyl, cyanopropyl, trifluoro, and polyethylene glycol (PEG)).  To increase retention or selectivity based on a particular interaction, increase the amount of the functional group within the phase (i.e. move from a 14% to a 35% cyanopropyl phase)
  • Use the least polar phase possible as more polar phases bleed more (it’s 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 wax (PEG) column cover the widest range of possible interactions (stationary phase polarities) in the fewest number of columns

Once an appropriate phase chemistry has been selected the physical parameters of the column have to be decided upon - length, internal diameter and film thickness.  Some quick rules to help selection are:

  • Select column length according to the number of species which need to be separated in the sample.  Two components - 10 m column, hundreds of components - 60 m or 120 m column.
  • The phase ratio (β) is the column radius (mm) divided by 2 x the film thickness (μm).  Keep this constant between columns and the retention time will be approximately constant. 
  • Use smaller internal diameter columns when the separation is dependent upon the stationary phase selectivity, i.e. when sample components are very similar or when multiple components need to be separated in shorter timeframes.  Use phase ratios <100 for highly volatile (low molecular weight analytes).  Use phase ratios >400 for high molecular weight analytes or for trace analysis.
  • Use thin films (0.1-0.25μm) when increased signal to noise ratio is required or when analytes are relatively involatile.  Use thicker films (1-5μm) when dealing with volatile analytes, analytes at high concentration or when peak shape is poor. 

Detectors

Selecting a detector based on your analytes and analysis type can afford more specificity.  The flame ionization detector (FID) is the workhorse of GC detectors (Figure 4).  This detector responds to compounds which contain carbon and hydrogen (so most compounds that will be analyzed).  One area of optimization that is required with the FID is to ensure that the three gases required to operate the detector are in the correct ratio - 1 (carrier) : 1 (H2) : 10 (air). 

In fact gas stoichiometry is the most important feature for optimum operation of an FID - higher H2 and air flows allow better handling of solvent peaks and high concentration components, while lower gas flow rates result in lower background noise and better detection limits - no matter what the flow rate it is imperative that the ratio of H2 to air is constant. 

FID is a mass sensitive detector, which means that the response is proportional to the mass of carbon (stated in terms of picograms carbon per sec).  This unit carbon response means that the FID responds linearly to the mass of carbon, independent of compound structure, allowing one to quantify mixtures without having calibration standards for every component.  Amounts of components in a sample relate to their relative peak areas.

So, a simple area per cent report will fairly closely reflect the mass per cent of each component in a mixture.  Hydrocarbons are oxidized in the flame eventually producing
H2O and CO2, with the charged intermediate (primarily +CHO) giving rise to the FID response - molecules which contain oxygen are further along the oxidation process, hence, yield a lower proportion of ions and a lower response per molecule or mass.  This means that the response for oxygenated compounds generally decreases in the following order alcohols, ethers, aldehydes, ketones > esters > acids.

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Figure 4: Flame ionization detector (FID).

Another universal detector, which is primarily used for gas analysis, is the thermal conductivity detector (TCD, Figure 5).  This detector will respond to any analyte that has a difference in thermal conductivity from the carrier gas.  The TCD is susceptible to variations in detector body temperature and carrier gas flow, therefore, constant temperature supply and carrier operating in constant flow mode is required.

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Figure 5: Thermal conductivity detector (TCD).

For greater specificity detectors such as the nitrogen phosphorous detector (NPD, Figure 6) and the electron capture detector (ECD, Figure 7) can be utilized.   As the name would suggest, this detector has enhanced sensitivity towards nitrogen and phosphorous containing compounds which means that these types of compounds can be detected without interference from C and H containing compounds. 

The NPD is an ionizing detector like the FID but operates using a different principle, mainly that there is no flame and instead a rubidium silicate bead is resistively heated to emit thermionic electrons which migrate to the collector electrode and form the background current.  

Operating at higher temperatures (260-350 °C), at the lowest acceptable bead power (start at 2 V and increase in 10 mV steps if unsure), and interrupting the hydrogen flow when the solvent peak elutes and during oven cool down can help to extend bead lifetime. 

Hydrogen flow must be low enough; otherwise nitrogen response will be zero.  The NPD is mass/flow sensitive and it is recommended that constant flow operation is used to avoid drifting baselines.  The NPD is now rare due to cheaper MS devices.

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Figure 6: Nitrogen phosphorous detector (NPD).

The ECD is particularly sensitive to electron capturing species such as halogens.  When operating an ECD the detector gases should be clean (99.999%+) and dry as both oxygen and water are electronegative and contribute to noisy baselines. 

Chromatographic peaks obtained from a dirty ECD have a distinctive negative dip either before or after each peak.  The sensitivity of the detector can often increase as detector performance deteriorates as well as exhibiting a deviation away from the expected linear range, especially at higher analyte concentrations. 

A contaminated ECD can be cleaned using a hydrogen bake out, however, more intensive maintenance may have to be carried out by the manufacturer due to the radioactive 63Ni source in the detector. 

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Figure 7: Electron capture detector (ECD).
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Dr. Dawn Watson
 

This article was written by Dr. Dawn Watson.

Dawn received her PhD in synthetic inorganic chemistry from the University of Strathclyde, Glasgow. The focus of her PhD thesis was the synthesis and application of soft scorpionate ligands. As well as synthetic skills, this work relied on the use of a wide variety of analytical techniques, such as, NMR, mass spectrometry (MS), Raman spectroscopy, infrared spectroscopy (IR), UV-visible spectroscopy, electrochemistry, and thermogravimetric analysis.

Following her PhD she spent two years as a postdoctoral research fellow at Princeton University studying the reaction kinetics of small molecule oxidation by catalysts based on Cytochrome P450. In order to monitor these reactions stopped-flow kinetics, NMR, HPLC, GC-MS, and LC-MS techniques were utilized.

Prior to joining the Crawford Scientific and CHROMacademy technical team she worked for Gilson providing sales and support for the entire product range including, HPLC (both analytical and preparative), solid phase extraction, automated liquid handling, mass spec, pipettes, and laboratory consumables.

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