Why are Restek capillary columns tested with different components? What can I, as a user, do with this information?

We have all bought a new capillary GC column, but only a few of us have probably reviewed the QC test report. For those of you who may have reviewed a report, you might have wondered about the data presented. In this post, I hope to clarify some of the information in regards to the column’s quality/performance.

Capillary GC columns are typically tested using a mix of compounds. Since there is not one mix that will work for every column, the components of each mix vary, but generally speaking, each mix contains the same classes of compounds. The compound classes provide a quality control mechanism for the column. For demonstration purposes, excerpts from an Rtx-1 column are below.

Rtx1

The alkanes – Tridecane and Pentadecane – are very stable and will elute through a column without a significant loss of response. For that reason, other compounds contained in the mix can use the alkanes for comparison purposes. On the Rtx-1, all other compounds have their responses compared to Tridecane. Pentadecane is used to measure the column’s efficiency in plates per meter. Columns with high theoretical plates will exhibit peaks with short retention times and narrow widths. Pentadecane is also used to measure the capacity factor, which can confirm the film thickness of the column. Additionally, a fatty acid methyl ester, such as methyl nonoate can be used for these purposes.

Some labs may use a test mix for system suitability purposes, to evaluate a used column, or for troubleshooting purposes. If you do use a test mix in your lab and the alkanes do not chromatograph, you should check the instrument setup before blaming the column. Improper column installation or injection port issues are the likely culprits.

Acenaphthylene, which is a polycyclic aromatic hydrocarbon, is used to determine the column-to-column reproducibility via retention index. This helps ensure that all Rtx-1 columns of the same dimensions are identical. The retention time index verifies that the correct polymer was used to manufacture the column.

The alcohols – 1,6-Hexanediol and 1-Undecanol – are used to determine the presence of hydrogen bonding. Silanol sites occur on bare tubing. The deactivation process and polymer phase coating act to shield these sites. Tailing peaks and decreased alcohol response indicate the column coating was not uniform creating unwanted activity. As a column is used over time, tailing alcohols can show that the phase has been stripped exposing these active sites. Additionally, the retention index of 1-Undecanol is used in a similar manner to Acenaphthylene to measure column reproducibility.

An acid, 4-Chlorophenol, and a base, 4-Propylaniline, are used to determine the suitability of the column for these types of compounds. Acids will interact with basic active sites in a column, and bases with acidic sites. A peak’s asymmetry, which can be measured for peak tailing, is indicative of these interactions. As a note, poor performance for the acid or base compounds does not always mean the column is bad. Some columns are specifically deactivated for the analysis of basic or acidic compounds.

Column bleed is also measured. This is normally done at the maximum isothermal temperature for the column. Low bleed values lead to better signal-to-noise ratios (lower detection limits).

As mentioned, not all column test mixes are the same. There are some that contain additional fatty acid methyl ester, which are used to monitor column efficiency. Some mixes contain an aldehyde to look for non-hydrogen bonding activity. There are also mixes that are method specific because the column was designed for that particular methodology.

I hope that provides a little insight in regards to a column’s test report. Next time you purchase a column, check it out.

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