Investigation of the Surface Composition by Laser Ablation/Ionization Mass Spectrometry

We present a Laser Ablation Ionization Mass Spectrometer (LIMS) for the sensitive, chemical analysis of solid matter as an analytical instrument on a landed spacecraft on the Moon, as a possible application in the ARTEMIS program of NASA. Our LIMS system is compact, features simple and robust operation, and is based on current measurement capabilities of a real-size prototype system. Our LIMS instrument can be part of the payload of a rover, can be portable for field excursions of astronauts, or be part of an instrument suite at the lunar base for detailed investigations of samples collected in the field. LIMS measurements provide chemical analysis within seconds, detection of trace elements at the ppm level and below, without sample preparation to support investigations from pure scientific interest all the way to in situ resource utilization (ISRU) related tasks. The LIMS instrument is a reflectron-type time-of-flight mass spectrometer coupled to a femtosecond laser for ablation and ionization of sample material for mass spectrometric analysis of solid samples. The LIMS was originally designed for the application on Mercury surface and since then was continuously developed further for in situ research on planetary surfaces. We operated a fully functional LIMS prototype that has been operated for many years in our laboratory. It has a mass resolution up to 900, with an accuracy of the mass scale of 500 ppm, a detection limit around 10 ppb depending on mass, and a dynamic range of 8 decades, which allows for quantitative measurements of almost all elements in laser ablation mode. Furthermore, it enables the detection of complex molecules in laser desorption mode. With every laser pulse a small amount of sample material is removed, thus, when staying at the same spot the sequence of mass spectra resulting from these laser pulses can be used to derive a depth profile of the atomic composition at the sampled location, allowing, for example, to penetrate through the space weathered layer on grains and rocks to access the true chemical composition of the material. We combined our LIMS system with a high-resolution microscope camera system (MCS), which has its own multicolor sample illumination. MCS has an optical resolution of 2 µmfor detailed optical investigation of the sample. In this way, mass spectrometric analysis and imaging information are available from the exact same location on the sample. Microscope images provide characterization of a sample for context analysis and supporting mineralogical classifications. For the demonstration of performance of our miniature LIMS system, we will present exemplary studies conducted on suitable sample materials that are relevant for in situ space exploration, that includes e.g., the identification of the mineralogy of heterogeneous samples, as well as element and isotope studies conducted on lunar and chondritic meteorites, which allows studies of in situ radio-isotope geochronology.