Instruments and Procedures
Powder X-Ray Diffraction
- Technique: Powder X-Ray Diffraction.
- Instrument Name: Rigaku MiniFlex 6G benchtop.
- ITSS Instrument Name: Rigaku.
- Location: ARES, NASA Johnson Space Center, Houston, TX
- Procedure: Powder XRD patterns were acquired under ambient laboratory conditions and were normally collected between 3 º2θ and 70 º2θ at a scan rate of 1.0 º2θ/min using a Co Kα source operated at 40 kV and 15 mA. Silicon metal sample holders with 0.2 mm diameter wells were used to accommodate the small sample volumes available for analysis. Sample holders were not rotated during analysis because the combination of centrifugal force .2θ scanning (relative to the fixed location for the x-ray beam) resulted in tendency for samples to slide off the sample holder. The scan rate employed (1.0 º2θ/min) was selected to minimize pattern acquisition time at the potential expense of minor shifts in shape and maxima of diffraction peaks, because of concerns of temporal phase stability. The instrument is periodically calibrated against a lanthanum boride (LaB6) standard. Diffraction patterns were analyzed for phase identification by JADE 9.6 software (Materials Data, Inc.) and JADE 10 software (ICDD) coupled with PDF-4/Minerals database (ICDD).
Concerns for phase stability arise, for example, when H2O-bearing samples are equilibrated at relative humidities different from measurement relative humidity. Samples characterized by a measure of kinetic stability with respect to hydration state change were loaded within the desiccation chamber into Rigaku sample holders and an air-tight transfer container and taken to the instrument with analysis in ambient start in ambient air within <4 m. A second scan was immediately acquired to verify (or not) hydration state stability on the timescale of the procedure. When the kinetic-based procedure was not adequate, a procedure that did not expose samples to air during measurement was used. The circular edge of the sample holder was coated with high-vacuum grease and placed in the desiccation chamber along with a pre-cut piece of thin mylar film large enough cover the sample holder. Within the sample chamber, sample is loaded into sample well, and mylar film placed over the holder and pressed onto the samples with a flat metal disc having a diameter larger than the sample holder, resulting in sample isolated from external environment by the vacuum-grease sealant between sample holder and mylar film. This process is mechanically difficult inside the desiccation chamber and can result in samples not perfectly placed in the sample holder resulting, in particular, z-axis offsets.
Deep UV Raman Spectroscopy
- Technique: Deep UV Raman Spectroscopy.
- Instrument Name: ACRONM (Built in house NASA JSC).
- ITSS Instrument Name: ACRONMR.
- Location: ARES, NASA Johnson Space Center, Houston, TX
- Procedure: DUV Raman spectra were normally collected in air at room temperature using the JSC Analogue Complementary Raman for Operations oN Mars (ACRONM) instrument. The instrument was built in-house and was designed to mimic the Mars 2020 SHERLOC flight instrument. ACRONM uses a NeCu70-248 hollow cathode laser (Photon Systems Inc.) with an excitation wavelength of ~248.6 nm in the form of an annulus beam. The incident excitation was focused using a 5x objective (ThorLabs, LMU-5X-UVB) to an 100 μm diameter beam on sample surfaces with an annulus ring thickness of ~7.5 μm. Laser excitation was pulsed at a repetition rate of 20 Hz and a pulse width of 40 μs. Raman and signal from the sample was collected in backscatter geometry and focused into the slit of Horiba Scientific iHR 320 spectrometer equipped with a kinematic triple grating turret dispersed the light. A 2400 g/mm grating (530-13-120) was used, producing a spectrum with a resolution of 2.16 cm‑1/pixel and a Raman shift range of 50-4500 cm‑1. Spectra were detected using a Horiba Scientific Synapse Plus CCD (SYN-PLUS-2048X512-BU) thermoelectrically cooled to -75 C. Differences between ACRONM and SHERLOC include (1) spectral acquisition at laboratory versus Martian environmental conditions, (2) manual versus automatic laser beam focusing, (3) rejection versus retention of the ~654 cm‑1 plasma emission line from the Cu-Ne laser, and (4) Rayleigh rejection filter with 50% transmittance at 403.3 cm‑1 versus 50% transmittance at 731 cm‑1 (Bhartia et al., 2021). Sample cups for ambient measurements are either polyethylene vial caps or an aluminum metal slide with a sample well, although the latter is now used exclusively.
Nominal ACRONMR instrument settings were 4.7 μW laser power (30 Amperes at 20 Hz repetition rate), 150 μm slit width, 10 s acquisition time, and average of 6 integrations per recorded spectrum. Anomalous spectral pixels occurring with random positions, intensities, and times are interpreted as cosmic ray hits on detector pixels. Their intensities are replaced manually by the average values of adjacent pixels. The instrument was calibrated against cyclohexane (ASTM E1840-96, 2014) and then linearly shifted along the x-axis so that the peak from atmospheric N2 occurs at the literature value (2330 cm‑1; e.g., Petrov et al., 2018). The final step is removal of baseline contributions determined as the average multiple spectra acquired with no sample present but with retention of the atmospheric peaks from O2 and N2. The intensity (y‑axis) is normalized using Teflon tape and a powdered sample (<150 μm) of enstatite (pyroxene from Bamble, NO).
Samples prepared in desiccation chambers are susceptible to phase changes (e.g., in hydration state) before and during analyses by ACRONM in laboratory air. For samples where testing showed such changes occurred slowly, aerial XRD and high- analyses of aliquot samples were concurrently analyzed to monitor by XRD for the nature of phases present in Ranam spectra. For samples hydration kinetics are fast, a procedure involving sealing sample at the bottom of a high-purity and thin-walled SiO2 glass tubes (Suprasil: 5 mm OD x 0.38 mm walls) before removal from the desiccation chamber. This procedure effectively excludes sample-air contact but does introduce Raman peaks for SiO2 glass into Raman spectra as Raman measurements are made through glass walls.