Steps toward accurate large-area analyses of Genesis solar wind samples: evaluation of surface cleaning methods using total reflection X-ray fluorescence spectrometry

Total reflection X-ray fluorescence spectrometry (TXRF) was used to analyze residual surface contamination on Genesis solar wind samples and to evaluate different cleaning methods. To gauge the suitability of a cleaning method, two samples were analyzed following cleaning by lab-based TXRF. The analysis comprised an overview and a crude manual mapping of the samples by orienting them with respect to the incident X-ray beam in such a way that different regions were covered. The results show that cleaning with concentrated hydrochloric acid and a combination of hydrochloric acid and hydrofluoric acid decreased persistent inorganic contaminants substantially on one sample. The application of CO2 snow for surface cleaning tested on the other sample appears to be effective in removing one persistent Genesis contaminant, namely germanium. Unfortunately, the TXRF analysis results of the second sample were impacted by relatively high background contamination. This was mostly due to the relatively small sample size and that the solar wind collector was already mounted with silver glue for resonance ion mass spectrometry (RIMS) on an aluminium stub. Further studies are planned to eliminate this problem. In an effort to identify the location of very persistent contaminants, selected samples were also subjected to environmental scanning electron microscopy. The results showed excellent agreement with TXRF analysis

Steps Toward Accurate Large Area Analyzes of Genesis Solar Wind Samples: Evaluation of Surface Cleaning Methods Using Total Reflection X-ray Fluorescence Spectrometry

Total reflection X-ray fluorescence spectrometry (TXRF) was used to analyze residual surface contamination on Genesis solar wind samples and to evaluate different cleaning methods. The Genesis mission collected solar wind during a period of 854 days by embedding the charged particles into collectors made of various ultra clean materials such as silicon, sapphire and silicon-on-sapphire. The sample return capsule unexpectedly crashed on return to Earth fracturing the collectors and exposing them to the desert soil of the landing side. The ubiquitous contaminants are separated from the atoms of solar wind by only 5-15 nm, presenting significant challenges for solar wind analysis as well as the development of cleaning techniques. Currently, an ultrapure water and ozone UV radiation treatment is routinely applied to the collectors by the curatorial team at NASA’s Johnson Space Center. Additional cleaning steps involving various forms of acid treatment and/or carbon dioxide snow treatment are being evaluated as well. To gauge the suitability of the cleaning method, two samples were analyzed following cleaning by lab-based TXRF. The analysis comprised of an overview and a crude manual mapping of the samples by orienting them with respect to the incident X-ray beam in such way that different regions were covered. The results showed that cleaning with concentrated hydrochloric acid and a combination of hydrochloric acid and hydrofluoric acid decreased persistent inorganic contaminants substantially on one sample. Application of carbon dioxide snow for surface cleaning tested on the other sample appears to be effective in removing one persistent Genesis contaminant, namely germanium. Unfortunately, the TXRF analysis results of the second sample were impacted by relatively high background contamination. This was mostly due to the relatively small sample size and that the solar wind collector was already mounted with silver glue for resonance ion mass spectrometry (RIMS) on an aluminum stub. Further studies are planned to eliminate this problem. In an effort to identify the location of very persistent contaminants, selected samples were also subjected to environmental scanning electron microscopy. The results showed excellent agreement with TXRF analysis

Differentiation of Surface Contaminants and Implanted Material on Genesis Solar Wind Samples Using Total Reflection X-Ray Fluorescence Spectometry and Grazing Incidence X-Ray Fluorescence

During the Genesis mission solar wind was implanted in collector materials for analysis by various instrumental methods. Unfortunately the space craft crash landed upon return to Earth shattering the collectors into small fragments and exposing them to desert soil and spacecraft debris. Thus only small fragments are available for analysis with each having different degrees of contamination present at and embedded within the surface. Cleaning procedures were developed and applied to remove the contamination. To aid in this process bench top total reflection X-ray fluorescence spectrometry (TXRF) was used to characterize a sample surface before and after various cleaning steps. In contrast to TXRF, synchrotron grazing incidence X-ray fluorescence spectrometry (GI-XRF) is capable of probing at the surface and below the surface thus providing information about surface deposits as well as implanted material. A number of samples were subjected to both, TXRF and GI-XRF analysis and it was observed that some elements detected by TXRF were present not on top of but below the surface of the collector fragment. This suggested the possibility of using laboratory TXRF to distinguish between surface deposits and ion-implanted subsurface material. The feasibility of this approach was tested with a surface deposited and an ion implanted control sample. In addition a careful TXRF angle scan was also executed with one Genesis flight sample and compared to GI-XRF measurements, confirming the ability of bench top TXRF to distinguish between surface and subsurface material

Solid Sampling with a Diode Laser for Portable Ambient Mass Spectrometry

A hand-held diode laser is implemented for solid sampling in portable ambient mass spectrometry (MS). Specifically, a pseudocontinuous wave battery powered surgical laser diode is employed for portable laser diode thermal desorption (LDTD) at 940 nm and compared with nanosecond pulsed laser ablation at 2940 nm. Postionization is achieved in both cases using atmospheric pressure photoionization (APPI). The laser ablation atmospheric pressure photoionization (LAAPPI) and LDTD-APPI mass spectra of sage leaves (Salvia officinalis) using a field-deployable quadrupole ion trap MS display many similar ion peaks, as do the mass spectra of membrane grown biofilms of Pseudomonas aeruginosa: These results indicate that LDTD-APPI method should be useful for in-field sampling of plant and microbial communities, for example, by portable ambient MS. The feasibility of many portable MS applications is facilitated by the availability of relatively low cost, portable, battery-powered diode lasers. LDTD could also be coupled with plasma- or electrospray-based ionization for the analysis of a variety of solid-samples.Peer reviewe

Interfaces and Composition Profiles in Metal–Sulfide Nanolayers Synthesized by Atomic Layer Deposition

The sharpness of interfaces in multilayer metal–sulfide thin films synthesized by atomic layer deposition (ALD) is virtually unexplored. Presented here are some first results that indicate metal–sulfide multilayer thin films deposited by ALD are in an entirely different regime than metal–oxides in terms of their composition profile. We propose a mixing number to characterize interfacial sharpness. The mixing number is the diffusion distance of mobile atomic species during layer deposition divided by the layer thickness in which the mobile species is diffusing. Ultrathin metal–sulfide multilayers with the structure ZnS/SnS₂/Cu₂S/Si substrate were synthesized by ALD because of the relevance of these binaries for formation of the photovoltaic alloy Cu₂ZnSnS₄ (CZTS). The composition profiles were measured by time-of-flight secondary-ion mass spectrometry (TOF SIMS) with high depth resolution, both as deposited and after annealing at different temperatures in argon. Diffuse interfaces between layers containing the intended elemental species were found in the as-deposited case, indicating a mixing number similar to unity at the synthesis temperature of 135 °C for several pairs of adjacent layers. Annealing the metal–sulfide multilayer structure at 425 °C for 60 min was sufficient to fully mix the layers. Composition profiles were also measured for 20 nm ZnO and 22 nm ZnS capping layers on annealed CZTS films. ZnO suppresses diffusion relative to ZnS but does not prevent it completely. This indicates that the real-time mobility of atomic species in the substrate or underlying layer, as synthesis takes place, plays a critical role in determining the composition profile of ALD multilayer films