Mass Spectral Analysis of Benzoic Acid and its Derivative Compounds using an Analogue Experiment for Space-based Impact Ionization Mass Spectrometers

The Cassini-Huygens space mission investigated Saturn, its ring system and moons for 13 years, with outstanding results [1]. Cassini’s mass spectrometers — the Cosmic Dust Analyzer (CDA) [2] and the Ion & Neutral Mass Spectrometer (INMS) [3] — contributed to many breakthrough discoveries, confirming mass spectrometry to be a reliable analytical tool for investigating planetary bodies and their vicinities [4]. Compositional analysis of micron and sub-micron sized dusty grains was undertaken by the CDA, an impact ionization time of flight mass spectrometer. Particles striking the instrument’s metal target at speeds ≥ ~1 km/s were disrupted, vaporized and partially ionized. The generated cations were accelerated toward the instrument’s detector to produce mass spectra with resolutions of (m/∆m) ~20-50 [2]. With the limited mass resolution, CDA revealed subsurface hydrothermal activity [5] on Enceladus and discovered potential biologically-relevant complex [6] as well as volatile organic [7] in ice grains emerging from Enceladus subsurface ocean.  A more recent, higher performance, spacecraft-borne impact ionization mass spectrometer — the SUrface Dust Analyzer (SUDA) [8] — will be a part of NASA’s Europa Clipper mission to explore the habitability of Jupiter’s moon Europa.

To calibrate these instruments, terrestrial analogue experiments are needed. In contrast to impact ionization, some soft ionization mass spectrometric techniques use a matrix to protect the structure of analyte molecules. These techniques include Matrix-Assisted Laser Desorption Ionization (MALDI) and Laser Induced Liquid Beam Ion Desorption (LILBID), which primarily use 2,5- or 2,3-dihydroxybenzoic acid (DHBA) [9], and water [10] as matrix substances, respectively. Although MALDI is by far the more common laboratory technique used for the analysis and the detection of biomolecules [11], LILBID is proven to accurately simulate impact ionization mass spectra of water ice grains [6][7][10]. In LILBID, a thin micrometer-scale liquid water beam, containing analyte compounds, is injected into a vacuum and irradiated by a pulsed infrared laser. Due to the laser’s energy, the solvent together with the analyte compounds are explosively dispersed, ionized and fragmented, generating a cloud of cations, anions, electrons, and neutrals [12][10].  The cations or anions (and electrons) can then be analyzed in a commercial time-of-flight mass spectrometer. A wide variety of compounds have been measured with the LILBID setup, to assist in the ongoing interpretation of CDA mass spectra, and predict those expected from SUDA in situ (cationic & anionic) mass spectra, thus inferring the composition of ice grains ejected from Enceladus’ or Europa’s oceans respectively. Laboratory campaigns, using the LILBID apparatus to generate mass spectra of a wide range of both inorganic and organic compounds, are ongoing. The data produced forms an expanding database, from which methodologies and rules for LILBID (and therefore impact ionization) mass spectral interpretation are being developed, in preparation for application to future missions [13].

Here, we present a comparison between the LILBID mass spectra of benzoic acid (C₆H₅COOH) and its derivative compounds: 2,5- & 2,3-DHBA (C₇H₆O₄). In this work, we study with our LILBID setup the correlation between fragmentation behaviour of these aromatic acids and the position of hydroxy functional groups (at aromatic ring). These compounds share common structural features including one benzene ring and one carboxylic functional group. However, the dihydroxybenzoic acids have two hydroxyl groups attached at different carbon positions on the aromatic ring (2 and 5 or 2 and 3). We observe the same fragmentation pattern and features in the LILBID spectra of both DHBA compounds, indicating that the exact positions of the hydroxy functional groups have no impact on the mass spectral features. However, when comparing benzoic acid with its derivatives (2,5-DHBA & 2,3-DHBA), we find characteristic differences in their spectral   signatures probably due to the attached hydroxy functional groups to the aromatic ring of these compounds. These data will be applied to the interpretation of CDA mass spectra of organic containing grains emitted from Enceladus, in which hitherto unidentified aromatic compounds have already been detected [6][7]. Understanding the behaviour of aromatic acids (e.g. benzoic acid) in comparison with its hydroxy derivatives (2,5-DHBA & 2,3-DHBA) under conditions which simulate impact ionization also relevant for the interpretation of unique results that will be produced by spaceborne mass spectrometers such as SUDA [8] onboard NASA’s future Europa Clipper mission.

References

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[12] F. Wiederschein et al. (2015) Royal Society of Chemistry 17, 6858

[13] Klenner et al. (2021; in preparation)