Different mass spectrometric approaches have been used to study the composition of ice and dust particles in space. Mass spectrometers onboard spacecraft such as Cassini, Rosetta and Giotto at Saturn, comet Churyumov-Gerasimenko and comet Halley respectively, have expanded our understanding of these planetary bodies by probing the composition of their dusty environments (Grün et al. 2019). The outstanding results from different space missions prove in situ mass spectrometry to be a reliable analytical tool with which to investigate the Solar System. Many of the mass spectrometers flown through the Solar System have used the impact ionisation technique, and an impact ionisation mass spectrometer, the Surface Dust Analyzer (SUDA), will be a part of NASA’s upcoming Europa-Clipper mission to Jupiter’s moon Europa. During the impact ionisation process, ice and dust grains impinging onto a detector’s solid target plate, at speeds above ≈ 1 km/s, generate a cloud of ions, neutrals and electrons (Srama et al. 2004; Auer & Sitte (1968)). The ions (cations in this work) are accelerated through a drift or reflectron region, towards a detector, from which time of flight mass spectra are generated (Friichtenicht et al. 1971).
Electrostatic accelerators are used to calibrate (Mocker et al. 2012) impact ionization mass spectrometers on Earth, accelerating micron and submicron cosmic dust analogue grains, typically metallic or with a conductive coating, to suitable velocities. As the controlled acceleration of µm-sized ice grains to these hypervelocities is not yet possible in the laboratory, an analogue experiment using Laser Induced Liquid Beam Ion Desorption (LILBID), is employed to simulate the impact ionisation process of ice grains in space (Klenner et al. 2019). In the LILBID process, a thin stream of an aqueous solution of, for example, organic compounds is exposed to laser irradiation in a vacuum. The dissolved compounds, together with the solvent matrix, are ionized and fragment when the matrix, in which charges are stochastically distributed, is dispersed (e.g. Wiederschein et al. 2015). By varying the laser power, as well the ion collection efficiency, it is possible to simulate the spectra which result from impact ionisation at varying impact speeds (Wiederschein et al. 2015, Klenner et al. 2019). The resulting cation time of flight mass spectra then contain mass spectral features corresponding to organic fragments and water cluster species. This technique has been successfully used (Postberg et al. 2009, 2018; Khawaja et al. 2019) to simulate the mass spectra produced when Cassini’s Cosmic Dust Analyzer (CDA) sampled salt- and organic-bearing ice grains emitted into space by Saturn’s moon Enceladus.
In this work we present a comparison between the mass spectral features of organic molecules generated by LILBID simulation of the impact ionisation mass spectra of organic-bearing ice grains, and the widely used standard technique of electron ionization (EI), the results of which can be found in existing databases, such as the National Institute of Standards and Technology (NIST) Chemistry WebBook. In electron-ionisation (EI), the sample is directly exposed to a beam of high-energy electrons (≈ 70 eV) which converts neutral molecules into molecular ions (M+) with high internal energies. Owing to the generated energies, the structures of the molecular ions become unstable and break up, producing a characteristic population of fragment ions. We have compared the cationic fragments of isomeric carbonyl compounds measured with LILBID and with those from EI obtained from the NIST WebBook database. The tested compounds are divided into three pairs of aldehydes and ketones i.e., propanal & acetone, butanal & 2-butanone, hexanal & 2-hexanone, with 3, 4 and 6 carbon atoms, respectively. To compare the spectral features related to organic fragments, water-cluster features are excluded from the LILBID spectra. In most cases, both the LILBID and EI spectra of the compounds show typical cleavage patterns characteristic of carbonyls.
In ketones and aldehydes, for both LILBID and EI, we observe a correlation between the number of parent molecule carbon atoms and the relative intensities of particular cation fragments. We will present an in-depth comparison of the spectral features obtained from these techniques is currently, using LILBID spectra obtained for a mass spectral database applicable to the SUDA instrument on NASA’s Europa Clipper mission. The mass spectrometers (MASPEX & PEP-NIM) onboard Europa-Clipper, and ESA’s Jupiter Icy Moon Explorer (JUICE) spacecraft, respectively, will employ an EI (70 eV) technique, and the work presented here provides a valuable comparison and cross-calibration between the well-known EI and the more esoteric impact ionization techniques. Understanding the differences between the EI and impact-induced fragmentation and ionization behaviour of organics may also allow large existing libraries of EI spectra to be used to interpret the in situ impact ionization mass spectra of ice-free grains, such as those expected to be returned by JAXA’s Destiny+ mission to the unusual active asteroid 3200 Phaethon, progenitor of the Geminid meteor shower.
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