GC/MS Method Development for Separating Lunar Volatile Ice Simulant Headspace Gases

Various investigators propose the lunar surface
contains widely distributed volatiles, especially water-
like species, i.e. OH and H2O. Surface volatiles
are theorized to exist as a hydrated regolith layer,
concentrated in extremely cold polar permanently
shadowed regions (PSR), and/or solar wind
implantation reservoirs in lunar glasses. The
proposed sources of lunar surface volatiles range from
cometary impacts, solar wind, or a supply present
during moon formation.

Future Artemis missions aim to collect and return
the samples containing volatiles collected near lunar
polar craters or PSRs. We, as advanced curation
scientists, are responsible for developing techniques
and methodologies for preserving returned sample
integrity as much as possible. Pristine volatile-bearing
samples are invaluable to the scientific community
seeking to unravel the history of the solar system.
Realistically, a sample will experience alteration
during collection, transportation back to earth, and
storage. The Planetary Exploration and Astromaterials
Research Lab (PEARL) seeks to understand
temperature and pressure effects on high-fidelity
volatile-containing regolith simulants, the foundation
for the future of cold curation. This abstract outlines
the separation, identification, and quantification of
headspace gases over volatile ice feed stock material
using gas chromatography/mass spectrometry
(GC/MS).

Preliminary objectives concentrated on sample
handling, reproducibility, and understanding the
elution characteristics for each analyte. Initial GC/MS
method development experiments utilized diluted
static headspace sample preparation. Diluted samples
were used because sampling headspace gases directly
from a vial containing liquid analyte resulted in
overloading of the column and detector. Overloading
is evident based on chromatogram peak shapes and
instrument contamination, or carry over, between
experiments. A mixture of three alcohols were used for
a majority of the sample handling and reproducibility
studies. Reproducibility was tested via multiple users,
calibration curves, and check standards.

Stock solutions of condensed lunar volatile
analytes included methanol, ammonia in methanol,
hydrogen sulfide in water, and an equal volume
mixture of methanol, ethanol, and isopropanol.
Current samples use room air as the headspace sample
matrix, however future experiments will incorporate
an inert purge gas, such as argon or nitrogen. Three mL of each analyte solution were capped in separate 20 mL
crimp top GC vials. Dilutions were carried out by
removing an aliquot of headspace gases with a
calibrated 1 mL gastight syringe and immediately
transferring to a 20 mL capped crimp top vial.

The GC/MS is a Thermo Fisher Trace 1310/ISQ
7000 with a TriPlus RSH autosampler and
split/splitless injector module. The experiments
outlined in this abstract use the following hardware: a
2.5 mL gastight headspace syringe tool, 1 mm ID x
78.5 mm length ultra-inert straight injection liner, and
a TG-BondQ 30 m × 0.32 mm × 10 μm column.
Various parameters, such as hardware selection and
the temperature, pressure, and split ratio set points,
continue to evolve as the overall experiment is refined.

Diluted headspace chromatograms were collected
for the individual stock solutions. Retention times,
peak shapes, and mass spectra were evaluated and
added to the data processing method for each molecule
of interest.

Figure 1 shows the total ion chromatograms for the
three major lunar volatile simulant stock solutions:
methanol, 7 N ammonia in methanol, and 0.4%
hydrogen sulfide in water. Tailing peak shapes for
ammonia (2.98 min rt) and water (4.06 min rt) indicate
the molecules are not properly eluting from the
selected column with the current separation method.
Additionally, hydrogen sulfide and ammonia have
overlapping peak windows, which could impact
quantification. Ongoing experiments aim to address
the peak shape and overlapping via the separation
method and hardware selection.

Sample preparation reproducibility experiments
used stock solution containing equal volumes of a non-
interactive mixture of methanol, ethanol, and
isopropanol. Mass spectrum ion traces were used to
identify and quantify all three alcohols. Peaks were
automatically detected, identified, and integrated
through the mass spectra detection and processing
parameters. Calibration response curves and check
standards were used to evaluate the validity of the
sample preparation procedure.

Figure 2 shows the methanol chromatogram peak
area versus total headspace dilution volume transferred
from the alcohol mixture vial. The calibration response
curves and check standards validate sample
preparation procedure.

Continuing data analysis efforts are working
towards correlating the peak area and instrument
response factor to the headspace analyte concentration
and condensed phase composition. Static headspace
gas chromatography theory relies on Dalton’s law,
Raoult’s law, Henry’s Law, and the Kolb and Ettre
equation to associate peak area to the analyte
composition in a non-ideal solution. Equation 1 is a
simplified expression derived from the
aforementioned theories.

Future experiments involve liquid injections of
the individual stock solutions, liquid and headspace
analysis of various stock solution combinations, and
the addition of regolith simulants to the mixtures.
Temperature is another variable expected to affect
reaction rates and will be explored.