Capillary GC Column Killers – Part 1

Most analysts already know that at elevated temperatures, oxygen damages the stationary phase inside a capillary gas chromatography column, and so does operating a column above the recommended maximum temperature. However, there are also several lesser known capillary column killers including the pH of the sample, derivatizing reagents, samples containing elemental sulfur, and aqueous (water) injections. Over the next month or so, I will discuss these column killers in a four-part series.

Although quantitative data to support these recommendations is unavailable, the following information is based upon personal experience and the feedback provided by experienced analysts. By no means are these topics all-inclusive, but I am hoping that before you inject your next sample, you will pause to think about what exactly you are injecting into your capillary column.

Please note that these recommendations are specific to bonded (cross-linked) stationary phases. Extra care must be taken with non-bonded or partially bonded columns, as these columns are much more susceptible to damage, and generally have shorter column lifetimes and higher bleed.

Extremes in Sample pH

The pH of the sample/extract that is introduced into a GC capillary column can potentially cause damage to the stationary phase and/or the deactivation layer on the inside of the fused silica tubing if it falls outside of the pH range of 5 to 9. Levels outside this range can result in a column quickly developing high bleed and/or poor chromatography. The further a sample/extract pH level is outside this “safe” range, the faster damage will likely occur.

If you suspect that your sample/extract may fall outside this safe pH range, test the sample/extract by putting some of it onto a strip of pH paper (shown below).

Photo courtesy of Avantor/VWR

If you discover that the sample/extract pH is outside of this range, you can still analyze your sample, but you should adjust it so that the pH falls within the range of 5-9. This can be accomplished several different ways, but by far the best way is to dilute the sample/extract until its pH is between 5 and 9. We do not recommend the use of buffer salts, as these may become deposited inside the column and cause damage. In addition, keep in mind that “buffering” your sample/extract may have negative effects (via chemical reactions) with your compound(s) of interest.

For analysis of basic compounds, columns like the Rtx-Volatile Amine, Rtx-5 Amine, Rtx-35 Amine, and the Stabilwax-DB have a suggested pH range of pH 7-9.

For analysis of acidic compounds, a column like the Stabilwax-DA has a suggested pH range of 5-7.

Hopefully this information will help save someone’s capillary column. Please stay tuned for parts 2-4 of my series on this topic.