When Should I Replace My Volatiles Column?

“When should I replace my volatiles column?” This is a question I have been asked several times, most recently this past Friday. The easy answer to this question is when you can no longer meet method specific requirements, such as response factors for system performance compounds, or resolve critical separations. I’ve heard it suggested more than once that a volatiles column should last forever. As long as you maintain a leak free setup, your volatiles column should last quite a while. The purge and trap system prevents the majority of the non volatile matrix components that wreak havoc via direct on column and splitless injection from reaching the inlet and head of the column.

carbon tet and 111-trichloroethane

EIC of carbon tetrachloride and 1,1,1-trichloroethane on a 30m x 0.25mm x 1.4µm Rtx-VMS run under the suggested conditions published in EPA Method 524.3 (5 mL sample spiked at 5 µg/L)

The customer I spoke with on Friday was wondering why his software was incorrectly reporting small hits of carbon tetrachloride when his samples had large hits of 1,1,1-trichloroethane. As you can see in the figure above, carbon tetrachloride and 1,1,1-trichloroethane not only elute fairly close together, but they also share m/z=117, the recommended quant ion for carbon tetrachloride. The EI mass spectra for both compounds are shown below (these spectra are from the NIST library).

111-trichloroethane EI spectra

EI Mass Spectrum for 1,1,1-Trichloroethane

carbon tetrachloride EI spectra

EI Mass Spectrum for Carbon Tetrachloride

There is always the risk of the software reporting a false positive when there is a large response for a compound that shares ions with a close neighbor. This is why it is important to manually verify every integration for every peak when dealing with client samples (o-xylene and styrene is another example of this type of critical separation on the Rtx-VMS). In this case, the retention time for the falsely identified carbon tetrachloride should be wrong (it would be the same as the retention time for 1,1,1-trichloroethane).

This was only part of the customer’s problem, though. When he runs a standard, carbon tetrachloride and 1,1,1-trichloroethane are eluting so close together that they have nearly the same retention time, and the minor m/z=117 peak from 1,1,1-trichloroethane looks like a shoulder defect on the right side of the carbon tetrachloride peak. The customer also said that the peak shapes of many compounds had deteriorated over time. How much time, you ask? Nearly 10 years; the column was installed in his 5972 in 2003.

Normally, I would attribute chromatographic deterioration to the trap. They tend to pack/clog over time, and the desorb cycle becomes less and less efficient as the flow is obstructed. This is why it is so important to monitor the flows on a purge and trap instrument.

That being said, in this case, I would attribute the loss of resolution between carbon tetrachloride and 1,1,1-trichloroethane to the loss of stationary phase over time (10 years) due to bleed. It’s also a testament to the quality of the Rtx-VMS.

 

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