Name: total internal reflection absorption spectroscopy (tiras) for the detection of solvated electrons at a plasma-liquid interface
Uploaded: 2018-01-24T14:30:00
Description: Watch this Scientific Journal Video about Total Internal Reflection Absorption Spectroscopy TIRAS for the Detection of Solvated Electrons at a Plasma-liquid Interface at JoVE.com

This work was supported by the U.S. Army Research Office under Award Numbers W911NF-14-1-0241 and W911NF-17-1-0119. DMB is supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award Number DE-FC02- 04ER1553.

1. Constructing the Experimental Setup
Note: To run this experiment, assemble a system consisting of a plasma reactor where the reaction will take place, optical components for absorbance measurements, and the electronic lock-in amplification system to process the signal.
<ol>
	<li>Construct the plasma electrochemical cell.
	<ol>
		<li>Manufacture a reactor cell consisting of a transparent glass vessel, 50.8 mm (2 in) in diameter, with two optical windows at angles of approx...

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Chem. 102, 16-22 (2014).</li><li>Kai, T., Yokoya, A., Ukai, M., Fujii, K., Watanabe, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermal+equilibrium+and+prehydration+processes+of+electrons+injected+into+liquid+water+calculated+by+dynamic+Monte+Carlo+method.">Thermal equilibrium and prehydration processes of electrons injected into liquid water calculated by dynamic Monte Carlo method.</a> Radiat. Phys. Chem. 115, 1-5 (2015).</li><li>Hart, E. J., Anbar, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> The hydrated electron. , (1970).</li><li>Hart, E. J., Boag, J. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Absorption+spectrum+of+the+hydrated+electron+in+water+and+in+aqueous+solutions.">Absorption spectrum of the hydrated electron in water and in aqueous solutions.</a> J. Am. Chem. Soc. 84 (21), 4090-4095 (1962).</li><li>Boag, J. W., Hart, E. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Absorption+spectra+in+irradiated+water+and+some+solutions.">Absorption spectra in irradiated water and some solutions.</a> Nature. 197 (4862), 45-47 (1963).</li><li>Matheson, M. S., Mulac, W. A., Rabani, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Formation+of+the+hydrated+electron+in+the+flash+photolysis+of+aqueous+solutions.">Formation of the hydrated electron in the flash photolysis of aqueous solutions.</a> J. Phys. Chem. 67 (12), 2613-2617 (1963).</li><li>Rumbach, P., Bartels, D. M., Sankaran, R. M., Go, D. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+solvation+of+electrons+by+an+atmospheric-pressure+plasma.">The solvation of electrons by an atmospheric-pressure plasma.</a> Nat. Commun. 6 (7248), (2015).</li><li>Anbar, M., Hart, E. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+effect+of+solvent+and+of+solutes+on+the+absorption+spectrum+of+solvated+electrons.">The effect of solvent and of solutes on the absorption spectrum of solvated electrons.</a> J. Phys. Chem. 69 (4), 1244-1247 (1965).</li><li>Buxton, G. V., Greenstock, C. L., Helman, W. P., Ross, A. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Critical+review+of+rate+constants+for+reactions+of+hydrated+electrons,+hydrogen+atoms+and+hydroxyl+radicals+(&#183;OH/&#183;O-)+in+aqueous+solution.">Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (&#183;OH/&#183;O-) in aqueous solution.</a> J. Phys. Chem. Ref. Data. 17 (2), 513-886 (1988).</li><li>Rumbach, P., Xu, R., Go, D. 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The results show that the measurement of absorbance of light at the plasma-liquid interface is an effective method to detect and measure the concentration of plasma-solvated electrons in an aqueous solution. The subsequent measurement at different wavelengths results in the measurement of the absorption spectrum. Though this experiment was done in an aqueous NaClO4 solution, the methodology should be valid for a great variety of other liquids, provided that electrons can solvate in the liquid.
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Plasma-liquid interactions represent an area of growing interest in the plasma science and engineering community. The complex interface between plasmas and liquids, which contains a variety of highly reactive free radicals, has found applications in many areas including analytical chemistry, plasma medicine, water and wastewater treatment, and nanomaterial synthesis1,2,3,4,5,6. While there are various configurations that can be used to bring a plasma in contact with a liquid7, perhaps the simplest is the plasma analog of an electrolytic cell, where one of the standard metal electrodes is replaced with a plasma or gas discharge8. The plasma electrochemical cell consists of a reactor vessel, a submerged metal electrode, and a plasma discharge, which can function as either the cathode or anode (or both). When the plasma discharge is used as a cathode, gas-phase electrons generated in the plasma are injected into the solution. After the electrons enter the solution, their kinetic energy dissipates on the timescale of femtoseconds9,10,11 primarily through inelastic scattering off the solvent molecules. Once the electrons have reached a near-thermal kinetic energy, they trap and solvate in a cavity formed by surrounding solvent molecules. Depending on the solvent and temperature, these &#34;solvated&#34; electrons may be stable until they react with some reducible species in the solution or with another solvated electron. In aqueous solution, solvated electrons are also referred to as hydrated electrons12.
This process of solvation has long been known, and the detection of hydrated electrons generated by procedures such as pulse radiolysis or flash photolysis has been studied since the 1960s13,14,15. In traditional radiolysis and photolysis, the solvated electrons are produced via ionization of the solvent molecules; however, electrons solvated at the plasma-liquid interface are injected from the gaseous plasma16. Previous experiments have determined that hydrated electrons absorb red light near 700 nm13,14,17, which allows them to be experimentally studied via optical absorption spectroscopy. Other experiments have measured their diffusion constants, their reaction rates with hundreds of chemical species, their radius of gyration, and their charge mobility, among other properties of interest12,18.
Within the literature, several methods to detect solvated electrons have been reported, which can be separated mainly into two types: bulk dosimetry, where solvated electron presence is inferred from the bulk chemical analysis of their reaction products, and direct transient absorption spectroscopy, where the electrons&#39; absorbance is measured as the reaction takes place. The latter category, upon which the methodology presented here is based, has the advantage of direct and instant evidence, as well as the ability to monitor intermediate reactions.
The rationale behind the development of the total internal reflection absorption spectroscopy (TIRAS) methodology was to directly study the role of solvated electrons at the plasma-liquid interface. The reflection geometry was chosen, because the production of solvated electrons using a plasma discharge, as opposed to methods like radiolysis or photolysis, occurs at the interface between the plasma and the liquid. When a probe laser grazes the surface at a shallow angle of incidence, it is totally reflected back into the solution and out into a detector, less the small amount of light absorbed by the electrons. With no light escaping into the plasma, the experimental technique only measures free radicals in the liquid phase, just beneath the interface, and is thus a highly sensitive interfacial measurement technique. Additionally, the total internal reflection phenomenon has the advantage of eliminating noise from the changing of partial reflections due to surface fluctuations, which could otherwise dominate the signal.
The TIRAS protocol outlined in this article has three essential features. The first is a plasma electrochemical cell, which consists of a transparent glass beaker with two optical windows at angles of approximately 20&#176; facing downward and a controlled headspace of argon gas. The second feature is the optical measurement system, which includes a diode laser, an optical cage, and a photodiode detector. The laser provides the light that is absorbed by the solvated electrons, and is mounted in line with an adjustable iris and a 50 mm lens in an optical cage. This arrangement is mounted on a goniometer, which allows it to be rotated to a desired angle of incidence. The intensity of the transmitted light is then measured by the photodetector, which consists of a large area photodiode wired in a reverse-bias leakage circuit. Finally, because of their high reactivity, solvated electrons only penetrate ~10 nm into the solution, which yields an extremely small optical absorption signal of ~10-5 optical density. To ensure a sufficiently high signal-to-noise ratio, the third essential component is a lock-in amplification system, which consists of a plasma switching circuit and a lock-in amplifier. In the switching circuit, a solid-state relay circuit modulates the plasma current between a high and a low value at a carrier frequency of 20 kHz set by a function generator. This, in turn, also modulates the solvated electron concentration at the interface and their optical absorbance. The lock-in amplifier then takes the signal from the photodetector and filters all noise outside the carrier frequency.
The TIRAS method has great potential to reveal important chemical processes in plasma-liquid experiments, particularly in plasma electrochemistry. The reduction and oxidation pathways are primarily driven by a variety of short-lived radicals at the plasma-liquid interface, and the detection of the species is extremely important for understanding the interfacial chemistry. The in situ monitoring capabilities of TIRAS will help establish a greater understanding of the important electron-driven reactions involved at the plasma-liquid interface. TIRAS, for example, makes the measurement of reaction rates possible in the presence of electron scavengers. Previous studies have focused on the reduction of NO2-(aq), NO3-(aq), and H2O2(aq) scavengers dissolved in the aqueous solution16, as well as the reduction of dissolved CO2(aq)19. Other studies have focused on the effect of the plasma carrier gas on plasma-solvated electron chemistry20.

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			<td height="37" style="height:37px;width:144px;">Lock-in Amplifier</td>
			<td style="width:144px;">Stanford Research Systems</td>
			<td style="width:144px;">SR830</td>
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			<td height="37" style="height:37px;width:144px;">High-Voltage Power Supply</td>
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			<td height="21" style="height:21px;width:144px;">Photodetector</td>
			<td style="width:144px;">Self-built</td>
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			<td style="width:144px;"></td>
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			<td height="21" style="height:21px;width:144px;">Flowmeter</td>
			<td style="width:144px;">Key Instruments</td>
			<td style="width:144px;">60310 R5</td>
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			<td height="21" style="height:21px;width:144px;">Flow controller</td>
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			<td height="21" style="height:21px;width:144px;">Camera</td>
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			<td height="21" style="height:21px;width:144px;">Voltmeter</td>
			<td style="width:144px;">Amprobe</td>
			<td style="width:144px;">AM-510</td>
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			<td height="21" style="height:21px;width:144px;">Optical Cage System</td>
			<td style="width:144px;">Thorlabs</td>
			<td style="width:144px;"></td>
			<td style="width:144px;">30 mm cage system</td>
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			<td height="37" style="height:37px;width:144px;">Goniometers</td>
			<td style="width:144px;">Thorlabs</td>
			<td style="width:144px;">RP01 - &#216;2</td>
			<td style="width:144px;">Manual rotation stage</td>
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			<td height="21" style="height:21px;width:144px;">Diode lasers</td>
			<td style="width:144px;">Thorlabs</td>
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			<td height="37" style="height:37px;width:144px;">Electrochemical cell</td>
			<td style="width:144px;">Adams &#38; Chittenden Scientific Glass</td>
			<td style="width:144px;">Custom-made product</td>
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			<td height="37" style="height:37px;width:144px;">Stainless steel capillary</td>
			<td style="width:144px;">Restek</td>
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			<td style="width:144px;">0.007 in. ID</td>
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			<td height="37" style="height:37px;width:144px;">SHV Coax Cable</td>
			<td style="width:144px;">SRS</td>
			<td style="width:144px;">Custom-made product</td>
			<td style="width:144px;"></td>
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			<td height="37" style="height:37px;width:144px;">Sodium Perchlorate</td>
			<td style="width:144px;">Sigma-Aldrich</td>
			<td style="width:144px;"></td>
			<td style="width:144px;">ACS reagent, &#8805;98.0%</td>
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			<td height="21" style="height:21px;width:144px;">Argon</td>
			<td style="width:144px;">Airgas</td>
			<td style="width:144px;">AR UHP300</td>
			<td style="width:144px;">Ultra-high purity</td>
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			<td height="73" style="height:73px;width:144px;">LabVIEW</td>
			<td style="width:144px;">National Instruments</td>
			<td style="width:144px;"></td>
			<td style="width:144px;">Software used to generate in-house program used to collect data</td>
		</tr>
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total internal reflection absorption spectroscopy (tiras) for the detection of solvated electrons at a plasma-liquid interface

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated electrons produced by a low-temperature plasma in contact with an aqueous solution. Solvated electrons are powerful reducing agents, and it has been postulated that they play an important role in the interfacial chemistry between a gaseous plasma or discharge and a conductive liquid. However, due to the high local concentrations of reactive species at the interface, they have a short average lifetime (~1 &#181;s), which makes them extremely difficult to detect. The TIRAS technique uses a unique total internal reflection geometry combined with amplitude-modulated lock-in amplification to distinguish solvated electrons&#39; absorbance signal from other spurious noise sources. This enables the in situ detection of short-lived intermediates in the interfacial region, as opposed to the bulk measurement of stable products in the solution. This approach is especially attractive for the field of plasma electrochemistry, where much of the important chemistry is driven by short-lived free radicals. This experimental method has been used to analyze the reduction of nitrite (NO2-(aq)), nitrate (NO3-(aq)), hydrogen peroxide (H2O2(aq)), and dissolved carbon dioxide (CO2(aq)) by plasma-solvated electrons and deduce effective rate constants. Limitations of the method may arise in the presence of unintended parallel reactions, such as air contamination in the plasma, and absorbance measurements may also be hindered by the precipitation of reduced electrochemical products. Overall, the TIRAS method can be a powerful tool for studying the plasma-liquid interface, but its effectiveness depends on the particular system and reaction chemistry under study.

As mentioned in step 5 of the procedure, this experiment measures the cosine and sine components of the absorbance signal, the phase angle between them, and the magnitude of the signal. A plot of the magnitude of the signal and its two components is shown in Figure 4.
Occasionally, there will be measurements which may be not optimal or even unusable. This may be due to a misalignment of the laser wi...

Watch this Scientific Journal Video about Total Internal Reflection Absorption Spectroscopy TIRAS for the Detection of Solvated Electrons at a Plasma-liquid Interface at JoVE.com

Total Internal Reflection Absorption Spectroscopy TIRAS for the Detection of Solvated Electrons at a Plasma-liquid Interface

This article presents a total internal reflection absorption spectroscopy (TIRAS) method for measuring short-lived free radicals at a plasma-liquid interface. In particular, TIRAS is used to identify solvated electrons based on their optical absorbance of red light near 700 nm.

Total Internal Reflection Absorption Spectroscopy (TIRAS) for the Detection of Solvated Electrons at a Plasma-liquid Interface

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated ...

The overall goal of this experiment is to directly detect solvated electrons in the plasma-liquid interface. This method can help answer important questions regarding the free radical chemistry created by a plasma in contact with an aqueous solution. The main advantage of this technique is that it provides direct measurement of a short lived free radical, the solvated electron, though its absorption of light.The implications of this technique extend toward medicine, as low temperature plasma is now being tested as a form of cancer treatment. This method can also provide insight into chemical synthesis techniques, as low temperature plasmas are used in many indocile processes. For this method, use a custom plasma electrochemical cell, two inches in diameter, with two optical windows, at angles of approximately 20 degrees down from the normal plane.Use a well fitting PTFE cell lid, with four ports for the electrodes and gas lines. Use an anode made of a piece of platinum foil attached to a stainless steel rod and a cathode made of a sharpened stainless steel capillary. Use as the absorption spectroscopy light source a 670 nanometer diode laser in line with an adjustable iris and a 50 millimeter lens, all mounted in a 30 millimeter optical cage system mounted on goniometer to allow adjustment of the angle of incidence.Use as a photodetector a large area photodiode wired in reverse bias leakage circuit with a 670 nanometer bandpass filter in front of the detector. Mount the photo detector on a second goniometer. Ensure that the goniometers are positioned so that the laser will be directed into one of the optical windows of the electrochemical cell and the light will be reflected though the other optical window to the photo detector.Use a solid state relay circuit to modulate the plasma current at a 20 kilohertz carrier frequency, including a lock in amplifier to obtain a sufficiently high signal to noise ratio to detect the extremely small optical absorbent signals of solvated electrons. Lastly, design an automated data collection program to control the instrument, and record signals from the lock in amplifier. To begin the set up, place in the electrochemical cell 60 milliliters of a 0.163 molar solution of sodium perchlorate and deionized water.Insert the platinum foil anode into the electrochemical cell lid, and place the lid on the cell. Insert the platinum foil anode into the electrochemical cell lid. Submerge the anode about one centimeter below the solution surface.Ensure that only the platinum foil contacts the solution. Then, connect the blunt end of the stainless steel plasma electrode capillary to an argon gas line via a mass flow meter. Insert the sharp end of the plasma capillary through the lid so that the point is about one to two millimeters above the surface of the solution.Confirm the distance between the solution and the capillary tip with a camera. Then connect a narrow hose to an argon gas line via another mass flow meter. Insert the hose through one of the ports in the electrochemical cell lid to serve as the purge line.Leave the fourth port unobstructed as a vent. Ensure that the lid will not move during the experiment. Then complete the electrochemical circuit by connecting the capillary to the voltage source and the anode to the ground.Purge the cell with argon at 250 cubic centimeters per minute for at at least five minutes before preceding. Then set the argon gas flow through the plasma capillary to about ten cubic centimeters per minute. Turn on the diode laser, and align the laser so that it will strike the plasma liquid interface as indicated by light being scattered off the dimple by the argon flow.Stop the flow of argon through the capillary and wait for the laser spot to return to its normal size. Leave argon flowing through the main purge line at 250 cubic centimeters per minute. Align the photo detector so that the laser reflection hits the center of the detector.Connect the photo detector output to a volt meter. And measure the voltage produced by the absorbent signal by the laser. Record the voltage to be used later to normalize the absorbance data from the lock in amplifier.The lock in amplifier essentially measures an AC voltage from the photo detector. This voltage must be normalized by the absolute intensity of the laser, measured as the DC offset of the detector. Then, connect the photo detector output to the lock in amplifier input.Connect the transistor transistor logic output of the function generator to the frequency input of the lock in amplifier. Turn on the function generator, lock in amplifier, and high voltage power supply. Ensure that the instruments are communicating with the data collection program and that the lock in amplifier is locked with a signal at the 20 kilohertz carrier frequency.Initiate the experiment and monitor the system as the laser is automatically turned on and the plasma is ignited. Observe the recorded signals, as steady state data is recorded both with and without the laser. We only expect to see the optical absorbency from solvate electrons when the laser and the plasma are both on.Therefore, the signal with only the laser and no plasma, or vice versa, should be relatively small by comparison. Once automated data collection has finished, average the data, subtract noise, and calculate the concentration of solvated electrons in the solution over time. Information about amplitude and phase relative to the switching circuit was obtained from the cosine and sine components of the absorbent signal, or x and y.The signal amplitude could then be used to estimate the concentration of solvated electrons and reaction rate constants of various chemical species. The amplitude of the absorbance was about 10 parts per million, emphasizing the need for a sufficiently large signal to noise ratio for a successful experiment. If unknown noise significantly affects the signal, it is best to discard the data and repeat the experiment.Once mastered, a measurement with this procedure should take no more than 30 minutes. It is critical to provide an inert atmosphere such as argon, and to properly align the laser. Trace amounts of oxygen can quickly quench any free electrons in the plasma and prevent them from entering the solution.It should also be noted that this method does not directly measure concentration of solvated electrons. Rather, it measures the AC amplitude of optical absorbance, which is proportional to the AC amplitude of the 20 kilohertz plasma current. After its development, this technique has yielded important insights into the fast reaction kinetics of solvate electrons at a plasma liquid interface.We would eventually like to extend this to other free radicals produced by plasmas in contact with liquids. Lastly, don't forget that working with high voltages can be hazardous. Precautions such as grounding the optical table and other equipment should be taken before performing this procedure.

Research

JoVE Journal

Chemistry

Isolation and Chemical Characterization of Lipid A from Gram-negative Bacteria

Lipopolysaccharide (LPS) is the major cell surface molecule of gram-negative bacteria, deposited on the outer leaflet of the outer membrane bilayer. LPS ...

1,3,5-Triphenylbenzene and Corannulene as Electron Receptors for Lithium Solvated Electron Solutions

The authors report on conductivity studies carried out on lithium solvated electron solutions (LiSES) prepared using two types of polyaromatic ...

Insights into the Interactions of Amino Acids and Peptides with Inorganic Materials Using Single-Molecule Force Spectroscopy

The interactions between proteins or peptides and inorganic materials lead to several interesting processes. For example, combining proteins with minerals ...

NMR Spectroscopy as a Robust Tool for the Rapid Evaluation of the Lipid Profile of Fish Oil Supplements

The western diet is poor in n-3 fatty acids, therefore the consumption of fish oil supplements is recommended to increase the intake of these essential ...

Electrochemical Impedance Spectroscopy as a Tool for Electrochemical Rate Constant Estimation

Electrochemical impedance spectroscopy (EIS) was used for advanced characterization of organic electroactive compounds along with cyclic voltammetry (CV). ...

Detection and Quantification of Plasmodium falciparum in Aqueous Red Blood Cells by Attenuated Total Reflection Infrared Spectroscopy and Multivariate Data Analysis

Detection and Quantification of Plasmodium falciparum in Aqueous Red Blood Cells by Attenuated Total Reflection Infrared Spectroscopy and Multivariate Data Analysis

We demonstrate a method of quantification and detection of parasites in aqueous red blood cells (RBCs) by using a simple benchtop Attenuated Total ...

Elemental-sensitive Detection of the Chemistry in Batteries through Soft X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering

Energy storage has become more and more a limiting factor of today's sustainable energy applications, including electric vehicles and green electric grid ...

Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry

Corrosion developed at the paint and aluminum (Al) metal-paint interface of an aluminum alloy is analyzed using time-of-flight secondary ion mass ...

Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid

The accurate molecular simulation prediction of vibrational spectra, and other structural, energetic and spectral characteristics, of photo-active ...

Biological Samples Preparation for Speciation at Cryogenic Temperature using High-Resolution  X-Ray Absorption Spectroscopy

The study of elements with X-ray absorption spectroscopy (XAS) is of particular interest when studying the role of metals in biological systems. Sample ...