How to Buy GC Columns



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The 'How to Buy GC Columns' eBook provides you with all the information you need to help you to make the right decision on a GC column, based on your requirements. Learn about key factors and application considerations when buying new columns, and view videos from scientists and application specialists. Plus, read impartial user reviews to help you to purchase with confidence and get one step closer to reproducible data.

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Introduction to gas chromatography

Gas chromatography is an analytical separating technique used to separate volatile and semivolatile substances from complex mixtures. The method consists of a gaseous mobile phase, usually an inert carrier gas, such as helium, and a liquid stationary phase adsorbed onto an inert cylindrical solid support called a column. Once the sample to be analyzed is injected into the GC system, an isothermal oven temperature or temperature gradient is applied to the column. The compounds in the mixture interact with the stationary phase. Depending on the type of interaction and the boiling points of the compounds, separation occurs.

GC columns are broadly classified into two types: packed and capillary (Figure 1). The packed columns are filled with inert solid material coated with the liquid stationary phase, while the capillary columns maintain a hollow interior with the stationary phase coated along its inner walls. Packed columns are preferred for analysis of gas samples, but for most analytical separations, capillary columns are more efficient and provide good peak separation and consistent results.

Figure 1: Packed GC columns (left) and capillary GC columns (right). Stationary phase appears in blue.

The cross section of a capillary GC column
There are three distinct layers that make up the cross section of a capillary GC column (Figure 2):

Figure 2: The cross section of a capillary GC column. Image courtesy of Phenomenex.

  1. Polyimide coating: This is a protective coating applied to the outer surface of the column. Polyimide is most commonly used as a coating. It not only gives the column its distinctive brownish appearance but also makes the column flexible and resistant to temperature.
  2. Fused silica: The main material with which the column is built needs to be inert, with the compounds being separated. Fused silica, which is synthetic quartz of high purity, has been a reliable and routinely used material for GC columns. It is supported with an outer layer of polyimide coating to add strength and an inner coating of stationary phase that performs the separation.
  3. Stationary phase: A thin film of stationary phase coated on the inner wall of the capillary column serves as the most important factor in selecting a GC column. The fundamental rule in choosing the stationary phase for your application is ‘like dissolves like’. Analytes interact better with stationary phases of similar chemical nature, yielding a better separation.

The structural characteristics of the stationary phase divide them into three categories (Figure 3): (i) Wall-coated open tubular column (WCOT), (ii) Porous-layer open tubular column (PLOT), and (iii) Support-coated open tubular column (SCOT). A wall-coated open tubular column (WCOT) consists of a thin film coated on the inner wall. The porous-layer open tubular column (PLOT) is a porous solid layer on the capillary’s inner wall, while the support-coated open tubular column (SCOT) includes a liquid stationary phase in addition to a porous support.

Figure 3: There are three categories of GC columns depending on the structure of the stationary phase, (i) Wall-coated open tubular column (WCOT), (ii) Porous-layer open tubular column (PLOT), and (iii) Support-coated open tubular column (SCOT).

Compound separation in gas chromatography
In gas chromatography, compounds are separated primarily based on their boiling points, although intermolecular interactions also have an influence on resolution. Elution of solutes generally follows the boiling point of the compounds; the higher the boiling point, the more the retention time in the column. Intermolecular interactions between the stationary phase and the solutes can also aid compound separation. For example, for two analytes with similar boiling points but different chemical structure, the strength of their dipole-dipole interactions with the column can help achieve separation.

How to choose a capillary GC column

Once you’ve identified the analytes that you want to separate, there are a number of additional considerations in a GC column to ensure a high-quality separation and a clean chromatogram.

Listed below are the factors to consider, in the order of importance, when choosing a GC column:

  • Stationary phase
  • Column internal diameter
  • Film thickness
  • Column length

Selecting the stationary phase

Your choice of a stationary phase in the GC column has the highest impact on the separation, and eventually, your data. Following the principle of ‘like dissolves like,’ the interaction between the analytes of interest with the stationary phase forms the core of chromatographic separations. The physical and chemical properties of the analyte and the stationary phase govern the analyte-phase interactions. In separating two compounds, if the interaction between one analyte and the stationary phase is stronger, it is retained longer in the column, and helps achieve a good separation. Alternatively, with a certain stationary phase, when two analytes do not separate (i.e. they coelute), changing the stationary phase by altering its chemistry can help achieve a separation. GC column manufacturers provide a wide range of capillary column phases to choose from, with each phase providing a specific chemical property.

The first step in selecting a stationary phase is to review the large volume of scientific publications in your area of application. Chances are that you’ll find a stationary phase well-suited for your experiment. Additionally, GC column manufacturers provide phase selection charts, a result of compiling the numerous GC successes in different industries and applications, that can aid your selection.

If, however, your application is unique and has no reference, it is important to know the physical and chemical properties of the analyte of interest. Listed below are some factors that can help with your method development.

A stationary phase is selected on the following criteria:

  • Phase selectivity
  • Phase polarity

Phase selectivity is the ability of the stationary phase to differentiate between two analytes by identifying the differences in their chemical or physical properties. The master resolution equation that defines the best resolution obtained for two solutes in a GC separation comprises of three terms: efficiency, selectivity and retention. Of these, the biggest impact on resolution is made by selectivity (α) which, in turn, directly relates to the stationary phase.

 

Retention mechanisms: Stationary phase-analyte interactions 
Analytes are retained in the stationary phase using the following interactions:

  • Dispersive forces (Van der Waals interactions)
  • Dipole-dipole interactions
  • Hydrogen bonding
Table 1. Polarity of columns and types of interactive forces.
Polarity of column Type of interaction with analyte
Non-polar Primarily dispersive forces
Polar/intermediate polar Stongly dispersive forces. Phases with phenyl functional groups can also have dipole-dipole, and dipole-induced dipole interactions
Highly polar/extremely polar Strongly dispersive, very strongly dipole-dipole, abd very strongly dipole-induced dipole interactions

Dispersive forces are the weakest of all interactions and increase with the size of the analyte i.e. larger compounds with dispersive interactions with the stationary phase have longer retention times. Dipole-dipole interactions, on the other hand, are stronger and these unique chemical interactions with analytes can aid the elution of two compounds with very similar boiling points. Hydrogen bonding between an analyte and the stationary phase causes a poor peak shape or irreversible binding to the column.

Polarity is determined by the structure of the stationary phase. The polarity of the substituted groups in the stationary phase and their abundance affects the overall polarity of the column. Phase polarity is often mistaken to be the most influential factor in solute resolution, which, in fact, is phase selectivity. Polarity is, however, one of the many factors that affects retention and, therefore, peak separation. Phase polarity follows the ‘like dissolves like’ principle. Non-polar compounds are better retained and separated by non-polar columns, and vice versa.

Table 2. Polarity of solute and recommended stationary phases.
Polarity of solute Examples Recommended stationary phases
Non-polar
C and H atoms only, C-C bonds
Alkanes Polymethylphenylsiloxane (PMPS), polydimethylsiloxane (PDMS)
Polar
C and H atoms, plus Br, Cl, F, N, O, P, and/or S
Alcohols, acids, ethers, esters, amines, thiols High % cyano, 100% Polyethyleneglycol (PEG)
Polarizable Alkenes, alkynes, aromatics Cyano, Phenyl Phase

Selecting column length

The efficiency of a column is directly proportional to the column length. Longer columns, therefore, are meant to yield a higher resolution. However, column resolution is related to the square root of the column length. This means increasing the length will only have a limited improvement in terms of resolution. Column length is directly related to run time. Increasing the column length will, therefore, also increase the time taken for analyte separation.

√L ~ R  Length of a column (L) has a square root relationship with resolution (R). Increasing the resolution by increasing column length is not a customary practice due to the inconvenience of very long columns and due to increase run times.
L ~ tR Length of a column (L) is directly related to the run time (tR). Increasing column length will increase the time for analysis.

 

In summary, longer columns yield a greater resolution but increase back pressure and run times, while shorter columns are preferred for applications where resolution is not the criteria, for example, when screening for analytes. In general, the column I.D. is often changed along with column length to yield the desired separation (Figure 4).

Figure 4: Considering length of a GC column.

Selecting column internal diameter

The internal diameter (I.D.) of a column directly affects the column efficiency (i.e. the number of theoretical plates) and the sample capacity (i.e. the amount of sample that can be injected into the column without causing an overload). The column efficiency, measured in plates (N), is inversely proportional to column I.D.; it increases as column I.D. decreases. Whereas the sample capacity is directly proportional to column I.D.; it decreases as the column I.D. decreases (Figure 5).

Table 3. The relationship between internal diameter, column efficiency and sample capacity.
Internal diameter (mm) Column efficiency: Total plates (N) Sample capacity (ng)
0.53 39,000 1000-2000
0.32 69,00 400-500
0.25 87,750 50-100
0.20 109,500 <50
0.18 121,800 <50
0.10 219,000 <10

 

Figure 5: Considering the internal diameter of a GC column.

Selecting film thickness

The thickness of the stationary phase on the inner wall of the capillary column determines solute resolution and sample capacity. The thinner the film coating, the higher the resolution, but lower the sample capacity, and vice versa. Decreasing the film thickness yields sharper peaks, reduces column bleeds and increases signal-to-noise ratio. Whereas increasing the film thickness increases sample capacity and reduces maximum operating temperatures but decreases analyte-phase interaction (Figure 6).

Figure 6: Considering the film thickness of a GC column.

 

Additional factors to consider

Column contamination: A contaminated column is one of the most common causes of performance degradation in GC systems. The contaminants typically originate from injected samples. Over time, semivolatile and nonvolatile sample components, upon injection, accumulate in the injector and the column surface. The semivolatile contaminants accumulate in the column, but eventually elute out, however, the nonvolatile ones do not elute and continue accumulating. Loss of peak size and shape, and peak tailing are usual symptoms of a contaminated column. Additionally, the stubborn nonvolatile residues can reflect on the baseline of the chromatogram, causing instability, wander, drift and ghost peaks.

To tackle column contamination, consider heating the column to a high temperature, known as ‘baking out’ at the end of your run. This helps remove high boiling contaminants from the columns. Care should be taken to control the temperature and time of such a bake-out so as to prevent thermal damage. Alternatively, in some GC columns, soluble contaminants can be removed with solvent rinsing, starting with the most polar solvent first. It is advisable to check whether your column’s stationary phase is compatible with a solvent rinse before proceeding as it can cause swelling of the stationary phase. Finally, consider cutting off contaminated parts of the column, usually 1-2 loops from the column’s inlet, to retain the remaining functional parts of the column. Note that each GC column is different. Please read the manufacturer’s instructions carefully before proceeding towards troubleshooting.

Guard Columns and retention gap: The use of guard columns, especially when samples injected are dirty, can increase the life of the analytical column. A short, deactivated fused silica tubing (0.5 – 1 meter) installed between the injection port and the analytical column serves as the guard column. It catches contaminant residues injected with the sample and prevents it from accumulating on the analytical column. The guard column, when longer, between 3 – 10 meters long, can serve as a retention gap, which allows for an improvement in peak shapes for early eluting analytes, stationary phase and GC conditions.

In an interview with a technical specialist at Phenomenex, this article highlights the most common issues with GC and a recommended troubleshooting process for researchers

Always buy from a reliable source: When investing in GC columns, it’s important to choose a reliable source. Manufacturers perform multiple tests on the column before it finds its way to the customer. These tests include resolution of critical pairs, bleed test, retention factor of probe analytes, efficiency of the column and so on. Choose a reliable source that performs testing on individual columns instead of batch testing. To aid your decision, you can read what other scientists think of their GC columns by viewing their reviews under GC products at the SelectScience® product directory.

Industry-specific applications

Application search programs: GC column manufacturers offer application-specific searches to aid your selection of GC columns, system considerations and experimental conditions. Listed below are a few application search programs:

…among others.

Examples of GC applications

Drug testing and anti-doping

Chromatographic techniques have long supported the Olympics for anti-doping examinations. Watch the video below to learn about how GC coupled with isotope-ratio mass spectrometry (IRMS) was used for steroid confirmation in the London Olympics Anti-Doping Labs. The technique detects endogenous vs. exogenous testosterone by measuring ratio of C12 to C13 in athlete samples.

Figure 7: Dr. Alan Brailsford, King’s College London, UK, discusses how gas chromatography combustion isotope ratio mass spectrometry (GC-IRMS) was used in the London Olympics Anti-Doping Labs.

An application protocol provided by Phenomenex using the Zebron™ ZB-5MSPlus GC column with 5% Phenyl- Arylene and 95% Dimethylpolysiloxane offers testing for cannabinoids in brownies using GC/MS (Figure 8).

Figure 8: Example of a chromatogram after testing the presence of cannabinoids in brownies using the ZB-5MSPLUS GC column. Chromatogram courtesy of Phenomenex.

Drug testing by employers has also become a common practice so as to reduce absenteeism, accident rates, workers’ compensation expenses, and turnover costs. For those looking to identify employees with repeat or chronic drug abuse, hair follicle testing provides a wider window of detection. In this interview with Lisa Thomas, Thermo Fisher Scientific, learn how GC coupled with mass spectrometry (MS) is used in hair testing for recreational drug use.

In the video below, Mr. Fabien Bévalot, one of the directors at the LAT LUMTOX Forensic Toxicology Laboratory, France, explains the laboratory’s post-mortem forensic toxicology capabilities using Agilent’s GC-MS systems.

Figure 9: Mr. Fabien Bevalot, director at the LAT LUMTOX forensic laboratory in France reviews forensic toxicology applications using the GC/MS system.

Environmental contamination

Analytical methods like GC, often coupled with MS, are essential for studying our environment and for diagnosing, evaluating, and controlling sources of pollution. In a SelectScience video, Inge De Dobbeleer, of Thermo Fisher Scientific, explains how the Trace 1300 Series GC and the TSQ 8000 GC-MS/MS serve environmental applications by providing reproducibility and robustness.

Polybrominated Diphenyl Ethers (PBDEs), considered persistent organic pollutants is toxic and detrimental to health. This application note outlines a fast and accurate analysis of PBDEs using the ZB-Semivolatiles GC column by Phenomenex, Inc. In a lecture filmed by SelectScience, Neville Llewellyn, an application specialist at Thermo Fisher Scientific, explains how liquid chromatography and GC Orbitrap technology can be used for accurate, high-resolution analysis of water contaminants.

A presentation by Anne Hazelden and John Adey of the National Laboratory Service, UK, explains the method development for analysis of low-level contaminants and pesticides in water.

Figure 10: Watch a lecture presentation explaining the analysis of low-level contaminants and pesticides in water.

Additionally, read this application note to learn how the Rxi®-5Sil MS (Fused Silica) columns by Restek Corp. is used to measure volatile organic compounds in contaminated ambient air. Another application note of a GC system from Agilent Technologies coupled with TOF MS (time of flight mass spectrometry), offers a method for fast quantitation of environmental contaminants.

Pesticides

In a SelectScience video interview, Dr. Robert Trengove of Murdoch University, Australia, highlights his team’s crucial work in screening for pesticide residues in human breast milk. By optimizing GC-MS/MS analysis for human breast milk, the team has successfully increased the number of pesticides that can be simultaneously screened to 88. The presence of pesticide residues in breast milk is considered a matter of huge concern for the health of the child.

Figure 11: Dr. Robert Trengove, Murdoch University, Australia, discusses his work on detecting pesticide residues in food, and his overarching goal of healthy food for human consumption.

A downloadable method from Restek Corp. summarizes new method developments for pesticide testing of dietary supplements using the Rxi®-5Sil MS (Fused Silica) columns. Another application method from Phenomenex outlines fast separation of chlorinated pesticides using their ZB-MultiResidue GC columns.

Figure 12: The GC/MS Pesticides Analyzer by Agilent Technologies.

Organochlorines from pesticides enter the ecosystems and food chains, and remain in it for years. There is a need for robust methods in their detection. An application note from Thermo Fisher Scientific highlights the detection of organochlorine pesticides by GC using an electrochemical detector, employing the TRACE™ 1310 Gas Chromatograph. In another application note by Restek Corp., pesticide analysis in strawberries and spinach grown by commercial and Amish growers are compared.

Watch the lecture below to learn about the GC-MS/MS system for multiresidue analysis of pesticides in fruits and vegetables as performed by Professor Amadeo Fernández-Alba, University of Almeria.

Figure 13: Prof. Amadeo Fernández-Alba, University of Almeria, explains pesticide detection in fruits and vegetables at the 53rd Annual North American Chemical Residue Workshop.

Food industry

Chromatographic techniques are employed when testing whether food and beverages are safe for consumption and analyzing their components.

In this video, Yuri Belov, Senior Scientist at Phenomenex, describes how he assesses cis and trans fats in foodstuffs using gas chromatography. His new 30-meter column, the Zebron™ ZB-FAME GC column, has very good baseline separation of the most challenging standard test mixture, the 37 Food Industry FAME (fatty acid methyl ester). Learn about how they enable fast separations in this video interview with Phenomenex’s GC technical manager, Dr. Ramkumar Dhandapani.

The Zebron™ ZB-5MSplus GC column by Phenomenex Inc., designed with a rigorous fused silica deactivation process for improved inertness, is recommended for polycyclic aromatic hydrocarbons (PAHs) testing in drinking water.

A technical application guide by Restek Corp. lists methods for the analysis of alcohols and aldehydes in alcoholic beverages, as well as flavor compounds in distilled liquor products.

Figure 14: Yuri Belov, a scientist who develops GC columns at Phenomenex, explains the separation of 37 food industry FAME mixture.

Figure 15: A chromatogram of FAMEs testing using GC columns from Phenomenex.

Important questions to ask yourself

  • What are my analytes of interest in the GC separation?
  • Are there published findings for this analyte?
    • If yes, can I replicate the chromatogram and optimize the method for my application with standards, and then, with samples?
    • If no, is there a GC column chart on a manufacturer’s website that lists a protocol and recommends a column?
  • In selecting a GC column for my application:
    • What is the polarity of my analytes of interest? Make a stationary phase choice based on analyte polarity.
    • Am I getting optimum peak separation between two analytes? If not, consider changing the column internal diameter or film thickness first, and finally, the column length.
    • Is my GC column performance deteriorating? Consider adding a ‘bake out’ step at the end of your run or a solvent rinsing step, depending on the characteristics of your column’s stationary phase. You can also amputate a few meters of the contaminated column. Additionally, consider a guard column to be installed before your analytic column.

For each new application, start from analytes of interest and proceed to column selection, followed by optimizing other performance criteria like sample injection port, flow rate, temperature gradient, run time, detector type, and coupling considerations as with MS or IRMS.

Summary

Many factors, ranging from dimensions to analyte properties, can influence your choice of a GC column. By reviewing the library of published findings and applying a few simple considerations, it’s easier to select the correct column and get reproducible data.

With the growth in application areas for GC, there will be a rise in ready-to-use GC columns for your experiments. Visit the SelectScience product library to find out about the latest GC systems and columns from leading manufacturers. When making a decision to purchase, be sure to explore our user reviews and request more information about a product of interest. Stay up to date with GC methods and free application notes by visiting the SelectScience application note library.

 

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Editor ImageKerry Parker
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