Moving Toward a Handheld “Plasma” Spectrometer for Elemental Analysis, Putting the Power of the Atom (Ion) in the Palm of Your Hand

Many of the current innovations in instrument design have been focused on making them smaller, more rugged, and eventually field transportable. The ultimate application is obvious, carrying the instrument to the field for real time sample analysis without the need for a support laboratory. Real time data are priceless when screening either biological or environmental samples, as mitigation strategies can be initiated immediately upon the discovery that contaminant metals are present in a location they were not intended to be. Additionally, smaller “handheld” instruments generally require less sample for analysis, possibly increasing sensitivity, another advantage to instrument miniaturization. While many other instruments can be made smaller just by using available micro-technologies (e.g., eNose), shrinking an ICP-MS or AES to something someone might carry in a backpack or pocket is now closer to reality than in the past, and can be traced to its origins based on a component-by-component evaluation. While the optical and mass spectrometers continue to shrink in size, the ion/excitation source remains a challenge as a tradeoff exists between excitation capabilities and the power requirements for the plasma’s generation. Other supporting elements have only recently become small enough for transport. A systematic review of both where the plasma spectrometer started and the evolution of technologies currently available may provide the roadmap necessary to miniaturize the spectrometer. We identify criteria on a component-by-component basis that need to be addressed in designing a miniaturized device and recognize components (e.g., source) that probably require further optimization. For example, the excitation/ionization source must be energetic enough to take a metal from a solid state to its ionic state. Previously, a plasma required a radio frequency generator or high-power DC source, but excitation can now be accomplished with non-thermal (cold) plasma sources. Sample introduction, for solids, liquids, and gasses, presents challenges for all sources in a field instrument. Next, the interface between source and a mass detector usually requires pressure reduction techniques to get an ion from plasma to the spectrometer. Currently, plasma mass spectrometers are field ready but not necessarily handheld. Optical emission spectrometers are already capable of getting photons to the detector but could eventually be connected to your phone. Inert plasma gas generation is close to field ready if nitrogen generators can be miniaturized. Many of these components are already commercially available or at least have been reported in the literature. Comparisons to other “handheld” elemental analysis devices that employ XRF, LIBS, and electrochemical methods (and their limitations) demonstrate that a “cold” plasma-based spectrometer can be more than competitive. Migrating the cold plasma from an emission only source to a mass spectrometer source, would allow both analyte identification and potentially source apportionment through isotopic fingerprinting, and may be the last major hurdle to overcome. Finally, we offer a possible design to aid in making the cold plasma source more applicable to a field deployment.

Studying Cassini’s photoelectrons during a series of solar eclipses in Saturn, using data from Cassini's Langmuir Probe

<p>The Langmuir Probe (LP) onboard Cassini was one of the three experiments that could measure the cold inner magnetospheric plasma, along with the Radio and Plasma Waves Science (RPWS) and the Cassini Plasma Spectrometer (CAPS). While the century-old LP theory looks quite straight-forward, in reality things are much more complicated.</p> <p>The operation of the LP is quite simple: by applying positive bias voltages, the probe attracts the electrons and repels the ions of the surrounding plasma. From the resulting current-voltage curve characteristics of the ambient electrons can be estimated, i.e. density and temperature. When negative bias voltages are applied to the probe the characteristics of the ambient ions can be estimated, i.e. density, temperature, and mass.</p> <p>Though the LP operation and interpretation are quite simple and straightforward, there are assumptions made and therefore the theoretical models may not always reflect the actual plasma conditions in Saturn’s magnetosphere. For this study we are focused on the effect of the photoelectrons, i.e. electrons that are generated by the incident sunlight on Cassini’s surfaces, which are difficult to be observed and corrected for in a laboratory plasma.</p> <p>We developed a robust algorithm that identifies the transitions of the LP in and out of shadow caused by the Saturn and its rings. The LP data inside and outside the eclipses are compared using the algorithm developed. In this presentation we will discuss the impact of the photoelectron generation from the spacecraft surfaces to the LP current-voltage curves, and understand the variations of the measured plasma density connected with the photoelectrons.</p>

Graphene in Titan

<p>Benzene (C<sub>6</sub>H<sub>6</sub>) ice has been observed in the Titan’s stratosphere [1], and benzonitrile (C<sub>6</sub>H<sub>5</sub>CN) is a possible constituent in the benzene and nitrogen-rich environment of Titan’s atmosphere [2]. The energetic processing of such aromatic molecules can synthesize large and complex aromatic molecules such as the Polycyclic Aromatic Hydrocarbons (PAHs). To-date a number of laboratory experiments have reported the formation of complex organics from the energetic processing of aromatic molecules [3-6]. In particular, Scanning Electron Microscopy (SEM) micrographs of the residues resulting from irradiated benzene ices are found to contain geometrically shaped particles [6]. Therefore, by employing electron microscopes, we can understand the physical nature of the dust leftover from the aromatic molecule irradiation.</p> <p>In the present investigation, we subjected benzonitrile ice made at 4 K to vacuum ultraviolet (9 eV) radiation at two beamlines, BL03 and BL21A2 of Taiwan Light Source at NSRRC, Taiwan. After irradiation, the ice was warmed to room temperature, which left a brownish residue on the Potassium Bromide (KBr) substrate. The VUV spectrum of the residue is observed to have characteristic aromatic signatures. The residue is then transferred to a quantifoil grid for High-Resolution Transmission Electron Microscope (HR- TEM) imaging. HR-TEM micrographs revealed the presence of graphene in the residue. This result suggests that N-graphene could be present in benzene and nitrogen-rich icy clouds of Titan. The high masses observed by the Cassini plasma spectrometer in Titan’s atmosphere could then be attributed to the presence of N-graphene along with the more common tholins [7].</p> <p><strong>References</strong></p> <p>[1] Vinatier S. et al. (2018) <em>Icarus, 310,</em> 89.</p> <p>[2] Loison J. C. et al. (2019) <em>Icarus 329,</em> 55.</p> <p>[3] Strazzulla G. et al. (1991) <em>A&A, 241</em>, 310.</p> <p>[4] Callahan M. P. et al. (2013) <em>Icarus, 226</em>, 1201.</p> <p>[5] James R. et al. (2019) <em>RSC Adv. 9</em> (10), 5453.</p> <p>[6] Rahul K. K. et al. (2020) <em>Spectrochim. Acta A, 231, </em>117797.</p> <p>[7] Rahul K. K. et al. (2020) <em>arXiv:2008.10011</em>.</p>

Comparison of Plasma and Magnetic Overshoots of Interplanetary Shocks

<p>The structure of quasiperpendicular interplanetary (IP) shock fronts was studied based on the data from the BMSW plasma spectrometer, installed onboard the SPEKTR-R spacecraft, supplemented by magnetic field measurements on the WIND. Special attention was paid to periodic growths (overshoots) in the value of the ion flux relative to their mean values outside the ramp. A comparison of plasma overshoot was performed with the overshoot in the magnetic field, with the Mach number, and with the β parameter. Based on the analysis of 26 crossings of IP shocks, in which the overshoots in the ion flux and magnetic field value were observed, it was shown that the value of the magnetic field overshoot is, on the average, less than a similar value in the solar wind’s ion flux, which is associated with different time resolution of measurements.</p><p>The ion flux overshoot value is found to grow with the growth of the Mach number. It is shown that overshoots are formed not only in the supercritical shocks, but also in those with Mach numbers that are less than the value of the first critical Mach number. It is also found that the estimates of the coherent downstream oscillations of the ion flux and magnetic field good correlate with the convected ion gyroradius.</p><p>This work was supported by the Russian Foundation for Basic Research, grant no. 19-02-00177.</p>

Ion properties of Mercury’s northern cusp under extreme solar wind observed by MESSENGER

<p>Mercury’s magnetic cusp allows solar wind plasma to precipitate into the magnetosphere, exosphere, and directly to the surface. This precipitation of solar wind leads to the production of neutrals in the exosphere and/or ions in the magnetosphere and thus it has an important role in shaping Mercury’s space environment. Characterizing the ion properties in the cusp region is important for obtaining a better understanding of the Sun-planet interactions and assessing the solar wind penetration in Mercury’s magnetosphere.</p><p>The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has observed the northern cusp regularly during its orbital phase. We have analyzed plasma data obtained by the Fast Imaging Plasma Spectrometer (FIPS) onboard MESSENGER under extreme solar wind events and compared the resulting ion properties in the northern cusp with those under non-extreme solar wind events for the first time. <span>We found that (1) flux enhancement is confirmed under the extreme solar wind, and (2) the ion distribution in the cusp has a smaller kappa value than in the magnetosheath, suggesting ion acceleration occurs in the magnetosphere.</span></p>

Heavy positive ion groups in Titan's ionosphere: Cassini Plasma Spectrometer IBS observations

<p><strong>Introduction</strong></p><p>Titan is the largest moon of Saturn and has a thick extended atmosphere along with a large ionosphere. Titan's ionosphere contains a plethora of hydrocarbons and nitrile cations and anions as measured by the Ion Neutral Mass Spectrometer and Cassini Plasma Spectrometer (CAPS) onboard the Cassini spacecraft<sup>1</sup>.</p><p>Previous ion composition studies in Titan’s ionosphere by Cassini instruments revealed "families" of ions around particular mass values and a regular spacing of 12 to 14 u/q between mass groups<sup> 2</sup>. These are thought to be related to a carbon or nitrogen backbone that dominates the ion chemistry<sup>2</sup>. Previous studies also identified possible heavy ions such as naphthalene, anthracene derivatives and an anthracene dimer at 130, 170 and 335 u/q respectively<sup>1</sup>. </p><p> </p><p><strong>Methodology</strong></p><p>               The CAPS Ion Beam Spectrometer<sup>3</sup> is an electrostatic analyser that measures energy/charge ratios of ions. During the Titan flybys Cassini had a high velocity (~6 km/s) relative to the low ion velocities (< 230 m/s) observed in the ionosphere. The ions were also cold, having ion temperatures around 150K. The combination of these factors meant that the ions appeared as a highly-directed supersonic beam in the spacecraft frame. This means the ions appear at kinetic energies associated with the spacecraft velocity and the ion mass, therefore the measured energy spectra (eV/q) can be converted to mass spectra (u/q).</p><p> </p><p><strong>Results and Conclusions</strong></p><p>Positive ion masses between 170 and 310 u/q are examined with ion mass groups identified between 170 and 275 u/q containing between 14 and 21 heavy (carbon/nitrogen/oxygen) atoms<sup>4</sup>. These groups are the heaviest positive ion groups reported so far from the available in situ ion data at Titan.</p><p>The ion group peaks are found to be consistent with masses associated with Polycyclic Aromatic Compounds, including Polycyclic Aromatic Hydrocarbon (PAH) and nitrogen-bearing polycyclic aromatic molecular ions. The ion group peak identifications are compared with previously proposed neutral PAHs<sup>5 </sup>and are found to be at similar masses, supporting a PAH interpretation. The spacing between the ion group peaks is also investigated, finding a spacing of 12 or 13 u/q indicating the addition of C or CH. Lastly, the occurrence of several ion groups is seen to vary across the five flybys studied, possibly relating to the varying solar radiation conditions observed across the flybys.</p><p>The discovery of these groups will aid future atmospheric chemical models of Titan through identification of prominent heavy positive ions and further the understanding between the low mass ions and the high mass negative ions, as well as the process of aerosol formation in Titan's atmosphere.</p><p><strong>References</strong></p><p>1. Waite et al., The Process of Tholin Formation in Titan’s Upper Atmosphere, Sci., 2007, doi:10.1126/science.1139727</p><p>2. Crary et al., Heavy ions, temperatures and winds in Titan's ionosphere: Combined Cassini CAPS and INMS observations, P&SS, 2009, doi:10.1016/j.pss.2009.09.006.</p><p>3. Young et al., Cassini Plasma Spectrometer Investigation. Space Sci. Rev., 2004, doi:10.1007/s11214-004-1406-4</p><p>4. Haythornthwaite et al., Heavy Positive Ion Groups in Titan's Ionosphere from Cassini Plasma Spectrometer IBS Observations, eprint arXiv:2009.08749</p><p>5. López-Puertas et al., Large Abundances of Polycyclic Aromatic Hydrocarbons in Titan's Upper Atmosphere, ApJ, 2013, doi:10.1088/0004-637X/770/2/132</p>

Observations of Short Large Amplitude Magnetic Structures at the Kronian bow shock

<p>We observed Short Large Amplitude Magnetic Structures (SLAMS) at Saturn upstream of the quasi-parallel bow shock. Cassini surveyed the quasi-parallel regime mainly during 2004 and 2005, and we present a few detailed case studies from this time interval. For our analysis we used the measurements of the Cassini Plasma Spectrometer and the Magnetometer.<br />Locally the SLAMS act as fast mode shock waves, and we observed ion beam reflection, multiple beams, deceleration and plasma heating of the solar wind protons. These features are in agreement with the near Earth observations of SLAMS. We also detected whistler precursor waves multiple times, which was also documented in studies of the Earth's foreshock region. Since the frequency of the upstream ULF waves observed at Saturn is lower than it is at Earth, it also has an effect on the spatial extension of the SLAM structures, which arise from these waves. With only one spacecraft's measurements it is not possible to study the SLAMS with the same efficiency as with the four-point measurements of the CLUSTER probes, but the basic observational features and the description of their evolutional characteristics are summarized. </p>

A transient enhancement of Mercury’s exosphere at extremely high altitudes inferred from pickup ions

Mercury has a global dayside exosphere, with measured densities of 10−2 cm−3 at ~1500 km. Here we report on the inferred enhancement of neutral densities (<102 cm−3) at high altitudes (~5300 km) by the MESSENGER spacecraft. Such high-altitude densities cannot be accounted for by the typical exosphere. This event was observed by the Fast-Imaging Plasma Spectrometer (FIPS), which detected heavy ions of planetary origin that were recently ionized, and “picked up” by the solar wind. We estimate that the neutral density required to produce the observed pickup ion fluxes is similar to typical exospheric densities found at ~700 km altitudes. We suggest that this event was most likely caused by a meteroid impact. Understanding meteoroid impacts is critical to understanding the source processes of the exosphere at Mercury, and the use of plasma spectrometers will be crucial for future observations with the Bepi-Colombo mission.