Buyer’s Guide: Choosing the Best GC Columns

A column is the heart of the gas chromatography (GC) system, and proper column choice is vital to efficient separations and healthy peaks. GC columns can come in a range of dimensions, formats, stationary phases and tubing materials, and interpreting what each specification means for your analysis can sometimes feel more complicated than interpreting a chromatogram. This buyer’s guide is designed to help break down the pros, cons and functions of different column qualities and help ease the process of choosing the best column for your application. 

Packed vs. Capillary

You most likely already know the difference between a packed and capillary GC column, but knowing when to use which might be less certain. Capillary GC columns are a go-to for many analyses due to their high resolution, separation efficiency and ability to produce much sharper peaks than packed columns, making them much more popular nowadays than the original packed format. However, there are still some situations where packed columns may be preferred, so consider the following factors before ruling them out:

• Fixed gas analysis - Packed columns, such as molecular sieve columns, are still commonly used for analysis of fixed gases in industries such as petrochemical and environmental science, and can often achieve better separation of these gases than capillary columns.¹ In these applications, packed columns are typically paired with a thermal conductivity detector (TCD).

• Sample size - Packed columns have a much higher sample capacity than capillary columns, reducing the risk of overloading and the need to dilute the sample. A higher capacity may be necessary when diluting the sample would significantly hamper detection of certain components. 

• Stationary phase options - Fewer stationary phase options are available for capillary columns than for packed columns, which can be seen as both a positive and negative. On the one hand, analysts can do “more with less” given the greater separation efficiency of capillary columns, but on the other hand, some analyses may benefit from a specific, selective stationary phase that is only available as a packed column. 

• Cost - In general, packed columns are less expensive than capillary columns, which is always important to consider when balancing your analytical needs with your budget. 

With those items in mind, capillary columns are still generally preferred for most common applications, with up to hundreds of times more theoretical plates than packed columns, translating to superior resolution and sensitivity suitable for use with mass spectrometer (MS) detectors. 

WCOT vs. PLOT vs. SCOT

Over the past few decades, capillary columns have far surpassed packed columns in popularity, and are estimated to be used in more than 80% of GC applications.² Instead of being packed with particles, capillary columns, also called open tubular columns, have a layer of stationary phase coated on the inner walls with open space in the center, where gas can flow throughwith much lower resistance. Capillary columns can be grouped into three main varieties based on the arrangement of this stationary phase layer. When you come across these acronyms in product names and descriptions, Here is some information to keep in mind:

• Wall-coated open tubular (WCOT) columns are the most widely-used variety of capillary column and consist of a liquid stationary phase coated directly onto the inside of the column, usually a fused silica tube with a protective outer coating of polyimide. These columns provide excellent separation efficiency with a high inertness, a high number of theoretical plates per meter and fast analysis speed for a wide range of analytes, depending on the stationary phase and temperature program used. These columns are great for GC-MS, but have a very low sample capacity and higher risk of overloading, which can affect peak shape. 

• Porous layer open tubular (PLOT) columns have a porous, solid absorbent material on the inner walls and do not contain a liquid phase. In some ways similar to packed columns, PLOT columns are mainly used for fixed gas and light hydrocarbon analysis applications, with stationary phases such as molecular sieves, porous polymers and alumina. While they provide better resolution than packed columns, these columns come with the risk of particles coming loose, which can interfere with analysis and even clog or damage the system. Therefore, they should be used with particle traps, especially when combined with MS analysis. 

• Support-coated open tubular (SCOT) columns have an inner coating of solid support particles, such as diatomaceous earth, that are then coated with a liquid stationary phase. The benefit of this type of column is that it provides a higher sample capacity than WCOT columns, but SCOT columns are also less efficient. 

Now that we’ve gone over the basic principles of the most common column types available, let’s explore one of the other main factors that makes the heart of our GC system “tick.”

Stationary Phase Selection

For capillary gas chromatography, stationary phase selection typically begins with analyte polarity and the principle of “like dissolves like.”³ With this focus on polarity, many GC column catalogs present each option on a scale from nonpolar/low polar to polar/high polar. This can be a helpful way to visualize stationary phase options without needing to memorize the names of every available chemistry, especially as many stationary phases are marketed with manufacturer-specific names. Still, most capillary column stationary phases are based on a handful of common chemistries, with different combinations and modifications producing the precise polarity and applicability of any given column.⁴ Here is an explanation of the five most common stationary phase components:

1. Dimethylpolysiloxane - 100% dimethylpolysiloxane columns are nonpolar, generally have a maximum temperature of up to 350°C, and typically have applications in analyzing compounds such as hydrocarbons, pesticides, drugs of abuse, solvent impurities and polychlorinated biphenyls (PCBs).

2. Diphenyl dimethylpolysiloxane - Diphenyl dimethylpolysiloxane columns (ex. 5% diphenyl/95% dimethylpolysiloxane) generally fall in the slightly polar to mid-polar range, have a slightly lower maximum temperature than 100% dimethylpolysiloxane columns and more useful for analyzing aromatic compounds in addition to semivolatiles, phthalate esters, sterols and rosin acids. 

3. Cyanopropylphenyl dimethylpolysiloxane - Cyanopropylphenyl dimethylpolysiloxane columns (ex. 50% cyanopropylphenyl/50% dimethylpolysiloxane) are in the mid to high polarity range depending on the percentage, have lower maximum temperature (around 240-270° maximum), and are especially useful for analyzing fatty acid methyl esters (FAMEs), volatile organic compounds (VOCs) and oxygenated compounds.

4. Trifluoropropyl dimethylpolysiloxane - Trifluoropropyl dimethylpolysiloxane columns (ex. 35% trifluoropropyl/65% dimethylpolysiloxane) are also in the mid to high polarity range, may have a maximum temperature as high as 340° depending on the exact formulation, and are especially preferred for halogenated compounds, fluorocarbons, silanes, glycols, alcohols and ketones. 

5. Polyethylene glycol (PEG) - PEG columns, sometimes referred to as wax columns, are high-polarity columns with a typical maximum temperature around 250°C and are excellent for retaining polar compounds that are difficult to analyze using less polar dimethylpolysiloxane-containing columns. Some examples of polar analytes that can be analyzed with PEG columns are perfumes/fragrances, essential oils, food additives, solvents, alcohols and aldehydes, as well as FAMEs. 

These are just a few examples of the applications of these various column types, and temperature ranges may vary widely due to extended temperature technologies available for some columns. Many manufacturers also apply proprietary chemistries to their stationary phases in order to fit specific parameters and applications, so noting the precise polarity, temperature range and application recommendations for different columns is key before making any final purchasing decisions. 

Column Dimensions

The parameters of length, inner diameter and stationary phase film thickness can also make a significant difference in analysis time and separation efficiency, so this is certainly a specification you don’t want to overlook when selecting your column. Here are some general principles to keep in mind when determining whether to choose a short or long, narrow-bore or wide-bore, and thin or thick film coating column:

Column Length

Choosing column length involves selecting a balance between resolution and analysis time, as well as taking into consideration inlet pressure. Longer columns provide more theoretical plates, but increase analysis time and inlet pressure. For example, analysis with a 15-meter column will take a fraction of the time it takes to run an analysis of a 60-meter column, which could take half an hour or longer depending on all the parameters, but the much lower efficiency of the shorter column could result in poorer quality peaks (ex. split peaks).^3,5 

A 30-meter column is often considered ideal to balance resolution, speed and pressure, but if high resolution isn’t crucial, a shorter column can save time, while a longer column may be needed for more complex and challenging separations. Also keep in mind that double the length does not mean double the resolution, and that as length increases, so does cost. 

Inner Diameter (ID)

A smaller ID is one of the benefits of capillary columns over packed columns, as smaller IDs translate to better efficiency and resolution. However, the smaller the ID, the lower the sample capacity. For example, one of the smallest ID column sizes available, 0.10 mm ID microbore columns, might provide thousands more theoretical plates per meter than a 0.53 mm ID megabore column, but can only accommodate a sample capacity per analyte of less than 10 ng, compared with more than 1,000 ng for the megabore.³ 

A common ID selection for general capillary column analysis is 0.25 mm, which is considered narrow-bore. These columns will typically provide both sufficient separation efficiency and sample capacity for most applications.

Film Thickness 

A decision on the desired thickness of the stationary phase coating inside the column may require more thought than the factors of length and ID due to the fact that film thickness can affect many aspects of the analysis, including retention time, resolution, sample capacity and risk of column bleeding. Additionally, a thicker or thinner film could either increase or decrease resolution depending on the retention factor of an analyte (k), which can also be affected by the temperature. A thicker film will retain early eluting compounds longer, potentially increasing resolution for those analytes, but can also retain late eluting compounds for too long, decreasing resolution. 

A lower thickness can be desirable for analytes with a high boiling point, such as FAMEs, PCBs and semivolatile pesticides, while a higher thickness is preferable for low boiling point VOCs, gases and other earlier-eluting compounds. However, an effective temperature program is important to helping to prevent peak problems on both ends of the chromatogram.

In terms of sample capacity, thicker films will naturally accommodate more sample. Additionally, thicker films will provide better inertness by reducing interactions with active sites.⁶ Disadvantages of a thick film coating include increased risk of column bleed and reduced maximum operating temperature. These are all factors you will need to balance when considering film thickness for your column.

Calculating the phase ratio (β), which is the relationship between ID and film thickness (β = column radius/film thickness x 2), of a column can also help you make a decision on these dimensions; in general, columns with β<100 are better for highly volatile, low molecular weight compounds while columns with β>400 are preferred for high molecular weight compounds and trace analysis. Columns with β between 100-400 are suitable for general use. Most 0.25 mm ID columns will have a film thickness of 0.25 or 0.50 µM, and a phase ratio of 250 or 125, respectively. 

In summary, here are the main specifications you’ll always want to keep your eye on when shopping for GC columns:

• Stationary phase polarity
• Temperature range 
Column length
• Column ID
• Film thickness
• Recommended applications

GC Column Vendors to Consider

• Thermo Fisher Scientific
• Restek
• Phenomenex
• Agilent
• MilliporeSigma
• Shimadzu
• PerkinElmer

References

1. "Packed Column information for the beginner," Restek. https://www.restek.com/row/chromablography/chromablography/packed-column-information-for-the-beginner/
2. Rahman, M.M., Abd El-Aty, A., Choi, J.-H., Shin, H.-C., Shin, S.C. and Shim, J.-H. (2015). Basic Overview on Gas Chromatography Columns. In Analytical Separation Science (eds V. Pino, J.L. Anderson, A. Berthod and A.M. Stalcup). https://doi.org/10.1002/9783527678129.assep024
3. "How to Choose a Capillary GC Column," MilliporeSigma. https://www.sigmaaldrich.com/US/en/technical-documents/technical-article/analytical-chemistry/gas-chromatography/column-selection
4. "Pragmatic Rules for GC Column Selection," LCGC North America. https://www.chromatographyonline.com/view/pragmatic-rules-gc-column-selection
5. "Secrets of GC Column Dimensions," Agilent Technologies. https://www.agilent.com/cs/library/slidepresentation/Public/Secrets_GC_Column_Dimensions.pdf
6. "Impact of GC Parameters on The Separation, Part 4: Choice of Film Thickness," Restek Corporation. https://www.restek.com/globalassets/pdfs/literature/Impact-of-GC-Parameters_Part4.pdf