H

Fig. 9.4 Schematic view of the newly developed COSAC-instrument onboard ROSETTA Lander Philae. The cometary sample will be filled into pyrolysis ovens mounted on carrousel A. After measurement of an infrared spectrum of the cometary sample, the selected oven will rotate into tapping station B where the oven is closed and heated stepwise. Volatile organic compounds will evaporate and be pressure-controlled transferred via transfer line C into the gas chromatograph D, which consists of eight capillary columns working in parallel. The gas chromatographically separated organic compounds and enantiomers will be detected by thermo-conductivity detectors TCDs E and given into the inlet system F of the linear reflectron time-of-flight mass spectrometer rTOF-MS G, ionized by electron impact, and analyzed after their time-of-flight with a multi-spherical-plate detector H. The obtained data will be sent to Earth. Carrier gas and calibration gas are stored in I. With the help of the COSAC-instrument, enantiomers of organic molecules can be identified, because they will be separated with chiral stationary phases and show identical mass spectra in the TOF mass spectrometer. Credit: Max Planck Institute for Solar System Research, Katlenburg-Lindau

Fig. 9.4 Schematic view of the newly developed COSAC-instrument onboard ROSETTA Lander Philae. The cometary sample will be filled into pyrolysis ovens mounted on carrousel A. After measurement of an infrared spectrum of the cometary sample, the selected oven will rotate into tapping station B where the oven is closed and heated stepwise. Volatile organic compounds will evaporate and be pressure-controlled transferred via transfer line C into the gas chromatograph D, which consists of eight capillary columns working in parallel. The gas chromatographically separated organic compounds and enantiomers will be detected by thermo-conductivity detectors TCDs E and given into the inlet system F of the linear reflectron time-of-flight mass spectrometer rTOF-MS G, ionized by electron impact, and analyzed after their time-of-flight with a multi-spherical-plate detector H. The obtained data will be sent to Earth. Carrier gas and calibration gas are stored in I. With the help of the COSAC-instrument, enantiomers of organic molecules can be identified, because they will be separated with chiral stationary phases and show identical mass spectra in the TOF mass spectrometer. Credit: Max Planck Institute for Solar System Research, Katlenburg-Lindau resolve a wide variety of different classes of chiral compounds. This can be done by gas chromatography, using different stationary phases in which chiral molecules like cyclodextrines are embedded. Chiral stationary phases suitable to perform enantio-selective gas chromatography and applicable to a wide range of chiral organic compounds including alcohols, dioles, amines, amino acids, hydroxy carboxylic acids, and even hydrocarbons were therefore developed and investigated in the laboratory before sending them incorporated in ROSETTA's COSAC instrument toward the comet.

After thorough tests in practice, three types of chiral stationary phases with different polarity and ring size (0- and y-cyclodextrins) seem to give an optimum performance for this kind of problem and were selected to be send to comet 67P/Churyumov-Gerasimenko: A Chirasil-L-Val phase (for the chemical structure confer Chap. 2), a cyclodextrin G-TA phase (Fig. 9.6), and a Chirasil-Dex CB phase (chemical structure in Chap. 2).

Fig. 9.5 COSAC's capillary column with its chiral stationary phase including heater, temperature sensor, and thermo-conductivity detector. Eight such subunits form the set of COSAC's chromatographic columns. Credit: Fred Goesmann, Max Planck Institute for Solar System Research, Katlenburg-Lindau
Fig. 9.6 Chemical structure of the cyclodextrin G-TA stationary GC phase, selected for the COSAC instrument onboard the ROSETTA Lander Philae

Why were these columns chosen for the ROSETTA mission among a wide variety of rival chiral selectors? The Chirasil-L-Val phase was selected for the resolution of chiral amino acids and carboxylic acids, compounds able to build two hydrogen bridge bonds to the stationary polymer phase. Enantiomers with only one functional group cannot be resolved by this phase. Therefore, the Cyclodextrin G-TA phase will be used for the resolution of chiral alcohols and diols. The G-TA phase is less stable, since the y-cyclodextrin molecules are not chemically bonded to the stationary polymer. Finally, the Chirasil-Dex CB phase will be applied for the separation of chiral hydrocarbons. Characteristics of the chosen chiral GC selectors for the ROSETTA mission are summarized in Table 9.1. More detailed information on the resolution of enantiomers including test data can be found in the Annex.

After ROSETTA's Lander Philae touchdown on the cometary surface in November 2014, the gas chromatographic in situ resolution of enantiomers is designed to allow the quantification of specific enantiomers of cometary material and thus the determination of enantiomeric excesses which, should they be found, might be helpful to understand the phenomenon of enantiomeric asymmetry of biomolecules on Earth.

Several enantiomers were selected to be investigated as potential candidates for cometary organic constituents. Chemical compounds in general contain asymmetric centres increasing with the number of carbon atoms. Below a certain carbon number, there are no isomers with stereogenic carbon atoms, i.e. no enantiomers. As for the case of (a) meteorites and (b) laboratory experiments generating hydrocarbons, the absolute (and relative) quantity of each specific isomer decreases exponentially with growing number of carbon atoms. The ability to analyse the individual enantiomer vanishes accordingly above a critical number of carbon atoms, because the quantity of the individual compound would ultimately drop below the detection limit. Consequently a "detection optimum" for chiral cometary molecules is imposed by the intersection of a minimal number of carbon atoms providing a sufficient abundance and a sufficient number of carbon atoms to generate a stereogenic center.

In accordance with the above limitations, the following families of chiral organic molecules, as they are: chiral amino acids, chiral carboxylic acids, chiral macromolecules, chiral hydrocarbons, and chiral amines, alcohols, and diols (see Annex) were selected as candidates for their gas chromatographic resolution. In a first attempt, we tried to achieve this goal without a derivatization step, because performing such a chemical procedure on a cometary nucleus would pose considerable additional problems. Later on, non-volatile carboxylic acids and amino

Table 9.1 Chiral stationary GC phases onboard the ROSETTA Lander Philae. The total set of columns used in the COSAC gas chromatograph including the achiral stationary phases is given in Goesmann et al. (2007b)

Chiral stationary phase

L [m]

ID [mm]

LT [|m]

Targeted chiral analytes

Chirasil-l-Val

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