Access Agilent eNewsletter, September 2013
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Understanding GC columns – the heart of gas chromatography – Part 1
By Allen K. Vickers, Daron Decker, Ronald E. Majors
Agilent GC Applications
Gas chromatography is popular and its prevalence is ever increasing. Today, there are more gas chromatographs in use than any other analytical instrument and the market is still growing. The GC market is expected to reach US $1.2 billion by 2015. The column is at the heart of every GC system. In this first of a two-part overview, we will cover some of the aspects of GC column development, manufacture, and testing.
Improvements in columns provide more choices and better results
The major shift in the widespread application of practical GC occurred in 1979 with the development of the fused silica capillary column [1]. Prior to this, GC made use of low-efficiency packed columns, fragile borosilicate glass capillary columns, and active metal capillary columns. Table 1 lists the main column types in order of popularity.
Type |
Support/phase |
Column material |
Uses |
Analyte separation mechanism |
Dimensions |
|---|---|---|---|---|---|
Open tubular capillary |
Stationary phase is coated and usually chemically bonded and cross-linked throughout the polymer matrix |
Stainless steel, fused silica |
General |
Partitioning via boiling points and other chemical or physical parameters |
0.05 to 0.53 mm id, 0.01 µm to 10 µm film thickness, 5 to 150 m long |
Wall-coated open tubular |
Thin liquid film layer (0.1 to 3.0 µm) coated or bonded onto inner walls |
||||
Support-coated open tubular |
Thin layer of support material such as diatomaceous earth onto which a stationary phase has been adsorbed |
0.25 to 0.53 mm id, 5 to 60 m long |
|||
Porous-layer open tabular |
Thin layer (5 to 50 µm) of porous solid on inner walls |
Gases at room temperature |
|||
Packed |
Gas-solid chromatography – silica, alumina, active granular support material |
Borosilicate glass, stainless steel polymer tubing |
Classical hydrocarbon separations, regulated environmental methods, preparative applications where the sample capacity of capillary columns is too low to provide adequate yields |
Adsorption |
2 to 4 mm id, 1.5 to 10 m long |
Gas-liquid chromatography (GLC) – usually a high surface area inert substrate such as Celite, Chromosorb W, or firebrick, coated with a non-volatile liquid stationary phase, usually 3-10% by weight. |
Partitioning via boiling points and other chemical or physical parameters |
Table 1. GC columns, in order of popularity
As you can see from Table 1, Open-tubular GC columns are the most popular. In an open-tubular GC column, the column material is a long inert tube. The center of this tube is open so that carrier gas can freely flow without resistance from the packing material. In most modern open-tubular columns, the stationary phase is coated and chemically bonded onto the inner walls of the tube, and cross-linked throughout the polymer matrix. There are still a few “classic” columns where the stationary phase is not bonded but simply coated with a viscous liquid that is considered to be immobile, according to the relatively low vapor pressure of the stationary phase. Almost any analytical separation that currently uses a capillary column with a non-bonded, non-cross-linked phase can be improved by using the equivalent bonded, cross-linked phase [2].
1. |
Work order/production planning |
2. |
Fused silica tubing selection |
3. |
Winding onto column cage |
4. |
Fused silica tubing preparation |
5. |
Leaching |
6. |
Deactivation |
7. |
Fill with dissolved stationary phase |
8. |
Coating |
9. |
Crosslinking |
10. |
Rinse |
11. |
Cure |
12. |
Special treatments |
13. |
Cure |
14. |
Quality control |
Table 2. Steps in the production of a GC capillary column
It takes time and care to make a fused silica capillary column
It takes about 10 days to make a fused silica capillary column. Table 2 outlines the production steps. Sometimes steps 12 and 13 can be skipped because they include “enhancements” to a standard capillary column. For example, these enhancements would make a standard column, such as a 50% diphenyl-50% dimethylpolysiloxane, better suited for performing pesticide analysis.
The most significant parameters of any column are the inner diameter and length, so it is important to know these dimensions precisely when making a column selection. In addition, the inner diameter is the basis for phase ratio (β) that governs retention (k), and so knowledge of the tolerance of tubing id is imperative and must be measured.
Proper tubing preparation and deactivation – the key to analyzing demanding samples
The deactivation step is extremely important when you analyze demanding samples. For polar compounds – such as acidic phenols, carboxylic acids, or basic amines – the underlying silanols or other surface impurities can cause irreversible adsorption, strong tailing, or other undesirable features.
This is usually done with organochlorosilanes, alkoxysilanes, hexamethyldisilazine, or other reactive compounds. Finally, this additional deactivation ensures that the surface accepts a nonpolar or polar stationary phase. Thus, we must be certain of its “wettability” for the stationary phase. This treatment ensures that the stationary phase film will spread evenly. This is important because an uneven film application will adversely affect column efficiency. Before coating, the final step is usually a wash with a pure solvent, followed by drying in an inert atmosphere.
Learn more about GC columns next month
We’ll continue our exploration of GC columns next month, when we look at stationary phase coating, quality control, and testing specialty columns. In the meanwhile, you can explore Agilent’s full range of Ultra Inert, capillary, packed, low-bleed GC/MS, polysiloxane, PEG, specialty, and PLOT GC columns by visiting the Agilent J&W GC Column Selection Tool.
Acknowledgement
This article is adapted from Vickers et al. The Art and Science of GC Capillary Column Production. LC.GC. July 1, 2007.
References
- R. D. Dandeneau, E. H. Zerenner. Chromatogr. Commun. 1, 351 (1979).
- K. Grob, G. Grob, J. Chromatogr. 347, 351 (1985).
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