The OCM cylinders were verified to be within size tolerances at the time of manufacture at HP Technical Ceramics, Ltd in Sheffield (65+0.5/−0 mm×57+0.1/−0.2 mm). At JPL, the bricks were subsequently machined so that the top rim was chamfered and an alignment slot created in the base into which the coupling plate would secure the brick against rotation (Fig. 2).
The bricks were baked at 550 °C for four hours under gentle flow of Earth atmospheric gas to combust any residual material. While fused silica will tolerate a higher temperature bakeout to 1000 °C without risking the formation of cristobalite (high temperature SiO2 crystals), it is important not to dehydrate the silica too much, which diminishes its capability to adsorb the dopant fluorocarbon markers. Cleaned bricks were then wrapped in double sheets of baked ultra high vacuum foil until installation into the containment canisters.
All hardware components were precision cleaned by a multi-step procedure. Note that all solvents were HPLC grade to minimize the potential for contamination by traces of other compounds. First the parts were immersed in a clean beaker of HPLC grade acetone and sonicated for 2 to 5 minutes. Then they were immersed in HPLC grade hexanes under sonication for another 2 to 5 minutes. A third sonication in HPLC grade isopropyl alcohol (IPA) for 2 to 5 minutes followed this step. A final rinse was then performed on a class 100 laminar flow bench using HPLC grade hexanes, wetting all hardware surfaces. The rinse solvent was collected in a cleaned vessel and labeled for later verification analysis. The cleaning procedure was completed by forcing the hardware dry with ultra high purity N2 gas.
Fluorocarbon Marker Compounds
The fundamental requirement for the marker compound was that it be detectable by SAM, so we targeted nanomolar concentrations that are readily detectable by SAM (Table 6). Preliminary experiments concluded that the minimum 1 FN/3 FP ratio that could be achieved was 2.046 μL/mg (4.437 nmol/nmol). The nominal expected sample size to be delivered to SAM is 0.05 cm3 of <150 μm powdered FS-120 brick. To achieve 10-nmol detections in a 0.05 cm3 sample volume, 2.3 μL of 1 FN and 4.14 mg of 3 FP per single brick are needed; giving a 1 FN/3 FP ratio is 0.56 (Tables 4 and 5). This ratio is ∼4 times less than the achievable minimum ratio determined experimentally. Thus, the experimental minimal 1 FN/3 FP ratio was applied in making the dopant solution. Furthermore, a target concentration of 10-nmol 3 FP was chosen since larger quantities of 1 FN may make up for the expected evaporative loss of 1 FN once exposed to 10-mbar atmospheric conditions on Mars.
Assembly and Doping of OCM Units
Hardware and Bricks
After the bricks, canisters, doping port/gland assemblies and brick retention hardware were cleaned, the OCM units were assembled in a class 1000 clean room at the Jet Propulsion Laboratory. All personnel donned full clean-room garb: “bunny suits”, masks and double gloves. Work surfaces were covered with clean sheets of ultra high vacuum (UHV) foil for each assembly.
The flow of canister assembly was designed to minimize the chance of contamination and units were fully assembled one at a time. First, the doping ports were installed and their leak-tight seal checked with a helium leak detector. Because the seals were tested prior to installation of the bricks and welded foil lids, a special leak-test cover and viton O-ring were used and the actual gland port assemblies closed off at the distal ends of the nickel tubes by the Swagelok valves. All hardware was handled with precision-cleaned tools (cleaned by the multi-step process described in Sect. 6.2). Following successful installation of the doping ports (two per can as shown in Fig. 6) The coupling plate was installed in the bottom of the can, then the brick, by peeling away one layer of foil and grasping the brick by the remaining layer whilst lowering it into the can and pressing it onto the coupling plate. The retaining hardware was then installed by pressing the conical face of a clamp ring over the chamfered edge of the brick and then a wave spring and spiral retaining ring. Shims were employed beneath the wave spring to maintain a consistent spring height, and shim thickness recorded for each unit.
The units were then ready for closure with the welded lid. The welding procedure included a custom fixture on a rotational stage. The foil disks were positioned using the welder operated by the laser-welding technician. Helium flow was established for five minutes prior to the welding operation, and then the welding was executed. Following the welding procedure, a helium leak test was conducted by using one of the Swagelok valves on the gland ports to establish test pressure on the leak tester, and then flowing helium at the weld site. If the leak rate was less than 1×10−9 atm-cm3 s−1 He, the weld was deemed successful, and the can gently purged with nitrogen and valves closed. A protective aluminum cover was installed over the foil lid.
Once the units were assembled, they were double-wrapped in baked UHV foil, enclosed in two dry nitrogen-purged Amerstat bags and labeled for shipment. Fifteen units (five for flight, five for the SAM testbed, two flight spares, two manufacturing contingency units and one qualification unit) were hand carried to NASA Goddard Space Flight Center for the doping operations.
The OCM units were received into the ISO class 6 clean room at GSFC and visually inspected. Again all personnel were in full bunny suit, masks and double nitrile gloves. The process required construction of a special manifold for doping (Fig. 8) and execution of the process in a chemical fume hood because of the use of volatile fluoro-hydrocarbons.
The doping manifold was designed to not only enable a uniform injection process for each unit, but to minimize the potential for contamination and allow for verification of the cleanliness of the injection pathway as well as verification of the fluorocarbon marker compounds downstream of the injected canister.
The use of a large oven (Lindbergh/Blue M mechanical oven) enabled the heating of several OCM units at one time, and the oven was under gentle purge of N2 gas to prevent discoloration of the hardware. Injection of the dopant into the canisters required heating to evenly distribute the marker compounds and then cooling prior to establishing a vacuum to roughly 10−5 Torr. The concluding step was pinching off the doping ports and capping them (Fig. 9c) to minimize later risk of mechanical damage. The doping operation could only be executed on one canister at a time and the procedure is summarized as:
Assemble a precision-cleaned injection adaptor into doping manifold.
Attach a bypass tube from canister input and output manifold lines and pump down the manifold with N2, then take a residual gas analysis (RGA) measurement of manifold background. Also collect a tenax trap sample to record manifold background. Then bypass the trap.
Connect the OCM unit into doping manifold (see Fig. 8) and open inlet valve.
Wrap the manifold with heating tape and thermocouples and heat to ∼75 °C±15 °C to encourage migration of the dopant toward the cooler brick
Inject dopant through the syringe adaptor into the cans.
Cool until manifold temperature is down to 40 °C and remove OCM can from manifold and place in tray for baking at 250 °C for four hours.
Pinch-off and Sealing of Dopant Ports
After the cans had been doped and baked, the OCM units were pumped down until no decrease in pressure was registered for fifteen minutes—to around 10−5 Torr, and the dopant tubes were mechanically sealed by a mechanical crimp and pinch-off tool, leaving a leak-tight seal on both ends of each pinched-off tube (Fig. 9a, b). The helium leak test could only be conducted on the tube end left in the Swagelok valve; the pinch-off on the canister port could not be verified by He test. However post pump-down concavity of the foil lid was measured with a tool developed for that purpose and then measured again following pinch-off of the gland/port assemblies (Fig. 10). Had there been a leak, the foil would have been deflected up to some degree, and in fact the measurements were corroborative of a good seal. It should be noted that the concavity of the seals was measured once again after delivery to JPL for installation on the rover and again after helium bomb pressurization and prior to rover close-out before launch, and the measurements were consistent with a good seal. Note that Helium “bomb” pressurization to 1.2 atm for 1 minute was performed after receiving canisters at JPL and canisters were subsequently tested for helium leaking out. This verified structural integrity of the foil seal. A qualification-level vibration test and 3 protoflight-level thermal cycles were performed on an EM assembly with one flight-type manufacturing contingency canister and the helium bomb pressurization was repeated. A flight acceptance-level vibration test and 3 protoflight-level thermal cycles were performed on the flight unit with only seal-deflection measurement verification to avoid overstressing the flight units.
The brick canisters were always wrapped in a layer of UHV foil during doping and heating so that handling would be minimized.
Verification of Dopant Distribution
The OCM verification canister was opened in the clean room fume hood where it had been doped. The foil lid was sliced with a precision cleaned blade and the brick sampled in place while it was in the canister to minimize surface area for potential contamination exposure. The locations of the verification sample collection are mapped in Fig. 11.
OCM bricks were sampled with a solvent cleaned steel ice pick and hammer and directly collected in UHV foil. About 250 milligrams of each OCM brick sample were placed in an ashed glass tube immediately upon removal from the brick. All samples were collected in duplicate. Blank material (unused FS120 brick ashed at 550 °C for 3 hours) placed in a glass tube at the same time as the other samples was interspersed in the analytical sequence.
Samples were analyzed via automated thermal desorption-gas chromatography mass spectrometry (TD-GCMS) using a Water Quattro Micro GC fitted with a Gerstel thermal desorption unit attached to a pressure-temperature volatilization (PTV) inlet (Gerstel CIS4). The Quattro Micro GC was tuned to optimize detection for 50–550 m/z scans (on MS2) at 5000 scans/second and enhance signal to noise. Samples were thermally desorbed at 300 °C for 3 minutes and condensed in the inlet at 50 °C before being transferred to the column at 300 °C. The analytes were separated on a RTX-5ms column (30 meter length, 250 micrometer i.d., 25 micrometer film thickness) that was held for 1 min at 50 °C then ramped at 6 °C/minute to 320 °C. Dopants were detected based on matching GC retention time and mass spectra of the original dopant solution.
The results of the doping verification showed that both 3 FP and 1 FN were detectable in all samples from the brick and absent from the blanks (Table 7 and Fig. 12). The more mobile 1 FN was present at a higher concentration, as was expected. The SAM flight instrument is even more sensitive than the benchtop GCMS used to verify that the dopants perfused the OCM brick (see Mahaffy et al. 2012, this issue), which provides confidence that the marker compounds will be detectable by SAM on Mars. Note that access to the brick was not flight like, however there are verification canisters remaining at the JPL test bed for future verification with the SA-SPaH test unit.