A candidate for "dark matter", predicted and now apparently detected to have both a higher thermal conductivity and higher diffusion rate than H2 is purported in this PowerPoint Presentation file. From that presentation, this output of a Thermal Conductivity Detector (run at 60C) of a HP 5890 Series II gas chromatograph using an Agilent molecular sieve column with Helium carrier gas is puzzling. The Hydrogen peak was establish by a control run. However, in the experimental run there appears a mysterious feature annotated with "???" indicating some sort of gas (that is not Helium) with a higher diffusion rate than Hydrogen.
A TCD temperature of 60C was chosen to ensure hydrogen would not show up as a negative peak as indicated by Figure 2 of "Thermal Conductivity Detector Analysis of Hydrogen Using Helium Carrier Gas and HayeSep® D Columns", by K Snavely and B Subramaniam.
What are plausible candidates for this feature?
For context, here is a control run:
Here is the corresponding experimental run from which the above detail screenshot is taken:
After the reaction, the sealed vessel stood for over 24 hours. This TCD graph obtained. Although a small peak associated with Nitrogen appeared, due to atmospheric ingress to the vessel, the Hydrogen peak no longer exhibited the "???" features: