This sensor unit is designed to determine the level of activation of lunar dust or simulant particles using a fluorescent technique. Activation of the surface of a lunar soil sample (for instance, through grinding) should produce a freshly fractured surface. When these reactive surfaces interact with oxygen and water, they produce hydroxyl radicals. These radicals will react with a terephthalate diluted in the aqueous medium to form 2-hydroxyterephthalate. The fluorescence produced by 2-hydroxyterephthalate provides qualitative proof of the activation of the sample. Using a calibration curve produced by synthesized 2-hydroxyterephthalate, the amount of hydroxyl radicals produced as a function of sample concentration can also be determined.
There are five main components to the sensor unit: cuvette holder, quartz cuvette, power/control electronics board, software, and a data acquisition board. The quartz cuvette holder will be made of an optically opaque material in order to decrease the possibility of scattered light. An ultraviolet LED producing light in the range of 310–330 nm, and a Si photodiode detector, will be mounted to the walls of the flow cell holder directly opposite one another to form the optical axis.
There are two possible configurations for the sample cuvette. First, test solution could be introduced into a standard, capped quartz cuvette. After testing, the cuvette could be removed and cleaned prior to further testing. Alternatively, a flow-through quartz cuvette could be used. After monitoring the fluorescence intensity of the test solution, the solution can be removed by pumping or other methods.
The power/control electronics board will provide power to the LED and the photodiode, control the LED, and amplify the analog output of the photodiode. Power will be provided by a DC–DC converter, filtered through an LC circuit, and fed to linear voltage regulators to generate clean, stable, positive and negative voltages.
The software is a virtual instrument written in LabVIEW v.6i. From the virtual instrument, the time of illumination can be controlled, as well as data acquisition parameters such as scan rate. In addition, the virtual instrument will apply a user-set calibration curve to the data to obtain the hydroxyl radical concentration. All of the data collected, as well as the calculated hydroxyl radical concentration, will be plotted on the virtual instrument and stored automatically in a text file with a time and date stamp.
A National Instruments data acquisition board will be installed in a personal computer running Microsoft Windows XP. The analog output from the sensor will be fed to the data acquisition board, where it will be digitized. The data will be collected using the virtual instrument running LabVIEW.
For terrestrial activation studies, a small amount of sample will be placed into a mortar and ground using a pestle for 10 minutes. At approximately 2- minute intervals, the sides of the mortar should be scraped in order to ensure that all material is experiencing consistent grinding. At the completion of grinding, or during testing in the lunar environment, a portion of the activated material will be added to a solution consisting of disodium terephthalate diluted in phosphate-buffered saline (PBS) at a concentration of 10 mM. The concentration of the sample in solution should be at least 1 mg/mL in order to provide sufficient fluorescence intensity. After allowing the sample to interact with the solution for 30 minutes, the mixture will be filtered using a 0.2-micron filter. The filtered solution will be placed in the quartz cuvette, and emission spectra will be obtained using an excitation wavelength of approximately 324 nm. The emission spectra will be compared to the calibration curve made using pure 2- hydroxyterephthalate.
This work was done by William T. Wallace of Universities Space Research Association and Antony S. Jeevarajan of Johnson Space Center. MSC-24446-1