Selection of the chemical adsorbents and operating conditions for the injection traps onboard the Dragonfly Mass Spectrometer Gas Chromatograph

Dragonfly is a relocatable lander that will explore Titan in the mid-2030’s [1]. It is equipped with the Dragonfly Mass Spectrometer (DraMS) instrument to investigate Titan chemistry at geologically diverse locations. DraMS’ gas chromatography-mass spectrometry (GCMS) mode will investigate organic molecule diversity and look for potential molecular biosignatures in surface samples. In this mode, solid samples are thermally volatized or chemically derivatized in a pyrolysis oven. The evolved components are concentrated on a chemical injection trap during the whole duration of the thermal or chemical treatment of the sample. The adsorbed compounds are then desorbed by flash-heating the trap for a rapid injection into the chromatographic column. The role of the column is to separate the different components so that they can be detected and identified with the mass spectrometer.

DraMS-GC is composed of two independent injection traps. At least one of them is necessarily composed of Tenax for its performances and heritage, but the chemical adsorbent in the other trap may be different. Despite its overall performance, Tenax has shown some contamination that challenges the interpretation of the origin of the molecules. This has been widely documented on the Sample Analysis at Mars (SAM) instrument onboard Mars Science Laboratory (MSL) mission [2],[3],[4],[5]. Both Carbograph and Carbotrap adsorbents have been considered as an alternative, but the former was abandoned due to its low mechanical resistance to vibration.

Desorption performance was evaluated for various chemical compounds mimicking the ones expected in future Titan samples, such as linear alkanes, fatty acid methyl esters, amines, amides, amino acids and nucleobases. Some of these were derivatized beforehand using N,N-dimethylformamide dimethyl acetal (DMF-DMA), as they will be on DraMS-GC.

The desorption temperature and the flash-heat duration have to be optimized for each adsorbent to ensure the best efficiency within the mission constraints. While the optimal desorption temperature for Tenax is 280°C, Carbotrap requires at least 300°C to significantly desorb most compounds. At the highest temperature tested (350°C), alkanes up to C₂₆ can be desorbed from Carbotrap. Results also showed a greater increase in desorption efficiency by extending the flash-heat duration from 10 to 40 seconds rather than by increasing its temperature alone (for example from 280 to 300°C).

Moreover, DraMS-GC must be able to detect a potential enantiomeric excess in the samples since this could be a bioindicator. Thus, some homochiral compounds are studied using a chiral chromatographic separation. Preliminary results show adsorption and desorption processes on Carbotrap do not induce a significant racemization of those compounds.

The final choice for the nature of the adsorbent and the operating conditions will consider those results along with the strong constraints on the power available to reach and maintain the optimal desorption temperature.

 

[1]          J.W. Barnes et al., 2021, Planet. Sci. J.

[2]          D.P. Glavin et al., 2013, J. Geophys. Res. Planets

[3]          C. Freissinet et al., 2015, J. Geophys. Res. Planets

[4]          A. Buch et al., 2019, J. Geophys. Res. Planets

[5]          K.E. Miller et al., 2015, J. Geophys. Res. Planets