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Project Progress

Laser ablation (LA) analysis is one of the primary means of in situ sampling that can be used on rover-based analytical platforms, as well as employed in analysis of samples returned from Mars in Earth-based laboratories. A key issue in maximizing precision and decreasing sample volume is increasing the efficiency of aerosol generation during LA, as well as decreasing the size of ablated particles, which in turn can dramatically increase ionization efficiency in the mass spectrometer. As part of efforts in building the next-generation of astrobiology instrumentation, we have been investigating new ablation cells, coupled to ultra-fast (femtosecond) LA systems.

We have tested two commercially available ablation cells to evaluate the aerodynamic size of aerosols produced by femtosecond laser ablation (fs-LA) of iron oxides, sulfides, and carbonates. The two cells that were tested include a Frames cell and a HelEx cell both produced by Photon Machines. The Frames cell is a single volume cell in which He flows over the substrate to remove aerosols produced during laser ablation. The HelEx cell is a two volume cell in which a small inner volume has He flowing down onto the substrate in a helical fashion and the outer volume has He flowing up into the smaller volume and these two gas flows are used to remove the aerosols produced during ablation. The cells were evaluated by characterizing the aerodynamic size of ablated particles using a cascade impactor. And Figure 1 shows the iron mass distribution (IMD)

of aerosols produced for the two cells. There is no difference in the IMD for different substrates, but the IMD for the frames cell is unimodal centered on 0.25μm where as for the HelEx cell there are a higher proportion of smaller sized particles and the IMD is bimodal with peaks at 0.14 μm and 0.025 μm. The washout time (time it takes for an ion signal to decrease to 95% of the ion signal after the laser stops firing) between the two cells is different: ~2 seconds for the Frames cell and ~0.2 seconds for the HelEx cell. The difference in aerodynamic size of particles from the Frames and HelEx cell is believed to be caused by the longer residence of particles in the Frames cell where particle agglomeration can result in larger aerodynamically sized particles. The differences in aerosol size results in a more stable ion signal being produced when using the HelEx cell. It is believed that agglomeration processes in the Frames cells results in non steady state delivery of the aerosol to the ICP torch producing an ion signal that fluctuates more as compared to the HelEx cell. Pulsating ion signals caused by unsteady delivery of the aerosol to the ICP source can result in poorer precision of an isotope ratio measurement as compared to a more stable ion signal produced by the HelEx cell. Use of the HelEx cell allows us to analyze a sample for less time to produce the same internal precision resulting in better spatial resolution where for example we are able to analyze magnetite grains over areas of 40 by 40 μm using the HelEx cell (see Li et al., 2013 for details) as compared the Frames cell that requires an area of 60 by 60 μm to produce the same precision measurement for the ⁵⁶Fe/⁵⁴Fe ratio (see Czaja et al., 2013 for details).

To further improve spatial resolution of iron isotope ratio measurements we have installed next generation Faraday cup pre amplifiers made by Isotopx to decrease noise, resulting in needing less on peak counting time to produce the same precision measurement as compared to using our older (circa 2000) pre amplifiers. These new pre amplifiers have allowed us to further decrease the sampled area (20 by 15 μm) and produce the same precision for the ^56Fe/⁵⁴Fe ratio as compared to use of the older generation pre amplifiers. Overall the combination of the HelEx cell and new generation pre amplifiers has allowed us to make significant improvements in the spatial resolution of Fe isotope ratio measurements. In terms of sampled volume our fs-LA analyses that use 600μm3 are similar to that of ion probe analyses and are largely a factor of 10 or better than previous Fe isotope analyses done by laser ablation (Table 1).