Philipp Hönicke

The physical and chemical analysis of aerosols using reliable and physically traceable methods is important for the thorough investigation of airborne particles to support a better understanding of their origin as well as their health and climate impacts. Within the European Metrology Research AeroMet project, the aim of Physikalisch–Technische Bundesanstalt’s (PTB) X-ray spectrometry group is to develop and establish traceable and reliable X-ray methods to measure the elemental mass deposition per unit area, the elemental composition, and the chemical binding state of particulate matter supported by a flat substrate. This approach can substantially contribute to support quantitative analytical methods during on-site measurement campaigns where portable instrumentation is employed by qualifying suitable calibration samples for commercial analytical X-ray instruments and by investigating samples collected during the field campaign.

he legal definition and enforcement of air quality regulations require an accurate and reliable monitoring of airborne particulate matter (PM). The monitoring needs to be evaluated at different levels to correctly assign the origin or source of airborne PM and pinpoint the possible adverse effect on health while assessing the impact of anthropogenic sources on atmospheric processes. Analytical techniques are therefore required to deliver quantitative information on the size of the PM, along with the chemical composition in terms of each element’s mass. For example, this applies to the PM collected during outdoor field campaigns aimed at making adequate and correct correlations with human activities and environmental processes. The analytical techniques need to be able to cope with collection times of airborne PM of only a few hours to deduce or assign changes in PM load resulting from industrial activities and changing weather conditions. This means that sufficient sensitivity is required in order to enable sufficient time resolutions and to provide accurate data on the mass of each element collected, even for fine and ultrafine particles, which are in the size range below 1 μm. Only with this information can appropriate preventive measures to protect health and climate be realized and controlled.

Different analytical techniques with suitable sensitivity are available for the detection of trace level amounts of airborne PM collected during outdoor campaigns (1). Among them, the combination of cascade impactors using quartz or silicon substrates and total reflection X-ray fluorescence (TXRF) has emerged as a promising contributor to the field. TXRF is routinely used for the detection of trace-level contaminants on semiconductor samples (2). The technique presents several advantages: First, sample preparation and handling requirements are minimal because the substrates used for the collection of airborne PM can be directly used for analytical purposes. Second, the nonconsumptive character of the analysis leaves intact samples for cross-comparative analytical studies by means of different techniques as long as validation studies are feasible. This aspect is important within the European Metrology Research AeroMet project where the aim of PTB’s X-ray spectrometry group is to develop and establish traceable and reliable X-ray methods to measure the elemental mass deposition per unit area, the elemental composition, and the chemical binding state of PM collected on the top of a flat substrate. This approach can substantially contribute to support quantitative analytical methods during onsite measurement campaigns where portable instrumentation is employed by qualifying suitable calibration samples for commercial analytical X-ray instruments and by investigating a posteriori samples collected during field campaigns.

In one study, airborne PM was sampled during field campaigns in a time and size-dependent partitioning using two different cascade impactors and substrates suitable for TXRF analysis. Some of the results using TXRF instrumentation are reported elsewhere (3). The cascade impactors used were a commercial Dekati (DLPI10) and a May-type instrument. A selection of the samples collected has been reinvestigated using reference-free X-ray spectrometry (XRS). In reference-free XRS, radiometrically calibrated instrumentation and knowledge of fundamental parameters involved in the production of the detected XRF are used in a forward calculation to extract the mass deposition (number of atoms or mass of a specific element per unit area) from the experimental data (4).

The reference-free XRS investigation on the samples from the Dekati impactor were realized at 10 keV incident photon energy and at a shallow incidence angle of 10°. This 

 means that standard excitation conditions are used without the creation of an X-ray standing wave (XSW) technique and that the quantification scheme can be realized on the basis of Sherman’s equation (5). The size of the synchrotron radiation beam used required a mapping of the collected PM to probe the whole sample surface and to provide the correct mass for each element collected (Figure 1). Due to possible beam overlap in neighboring measurement positions, a deconvolution of each elemental mass deposition map, considering the spatial incident beam profile, is required before a two-dimensional integration provides the total elemental mass collected on the substrate analyzed. The investigation described makes it possible to verify that the deposition patterns, in terms of spatial distribution of the collected PM, match the expectation, which is the pattern of the nozzles used in the corresponding impactor stage in order to exclude possible contamination contributions to the measurement. More importantly, this procedure allows for an independent validation of the quantitative results reported during field campaigns. This aspect is important for a systematic monitoring of the air quality.

In another study, the impact of the collected PM mass on the TXRF results was investigated. In TXRF, the detected intensity depends on the composition of the matrix on the top of the substrate. The mass of each element within the area illuminated by the incident X-ray beam is often considered to be in a linear relationship from the detected count rate. This approach is, however, compromised when an XSW is involved in the experiment and the total mass of the deposited PM is unknown. The modulation of the XSW influences the excitation probability for XRF radiation, and an accurate quantification can only be deduced when the XSW is correctly modeled.

To assess the impact of the different masses of PM, grazing incidence XRF (GIXRF) measurements were performed at an incident photon energy of 1622 eV on silicon wafers used for the collection of PM in the May-type cascade impactor. Here, the deposition pattern corresponded to a line, which was aligned prior to the GIXRF measurements along the incidence direction of the synchrotron radiation. The PM was found to be distributed uniformly along the line. In GIXRF, the dependence of the XRF signal on the incidence angle is monitored above and below the critical angle of total external reflection.

As a result, a GIXRF measurement includes more data than TXRF. This additional information can be used to assess the reliability of the quantification performed. The goal of making GIXRF measurements on samples with different amounts of PM was to compare the quantification under TXRF conditions (where the XSW needs to be considered) to XRF under shallow incidence conditions (where no XSW is created). It was found that the XSW field is drastically changed for high particulate load, which makes correct quantification tedious and requires correction factors for the attenuation within the collected deposit (Figure 2). Finally, an accurate quantification relies heavily on the sample loading on the top of the substrates, hence on the collection time of airborne PM, the PM size distribution, the cascade impactor related spatial deposition pattern, and the pollution level. For the reliability of TXRF quantification it will be crucial to establish thresholds on the mass of the collected PM. If no dependence on the grazing incidence angle of the XRF signal is measured, additional information like the reflectivity of the sample investigated or the XRF intensity of the substrate material should be considered and compared to blank substrates to deduce if matrix effects are crucial for the experiment realized. In case of large amounts of PM, accurate quantification can only be performed if absorption effects within the PM are considered, which requires, however, complete knowledge of elemental composition of the matrix.

Finally, reference-free XRS is a suitable means for an independent validation of results obtained by means of laboratory-based TXRF instrumentation, which can be used for a high-throughput investigation of airborne PM collected on substrates. It was shown that additional information can be delivered that allows tracing back possible correlations within the elemental maps collected with the Dekati impactor, respectively assessing limits for PM mass which can reliably quantified by means of TXRF. Means of control can be straightforwardly implemented by means of X-ray reflectivity of angular variations to assess the validity of absolute quantification.

One may conclude that XRF analysis under grazing incidence conditions, also available as commercial TXRF and GIXRF laboratory instruments, provides access to quantitative elemental analysis of size-fractionated air-borne particles allowing to substantially contribute to regulated toxicity assessments and validation of orthogonal analytical techniques such as inductively coupled plasma mass spectrometry.

The project is supported by the EMPIR program cofinanced by the participating states and from the European Union’s Horizon 2020 research and innovation program through grant agreement 16ENV07 AeroMet. A weblink of the European Research project can be found here: 

B. Beckhoff, R. Fliegauf, M. Kolbe, M. Müller, J. Weser, and Gerhard Ulm, 

J. Osán, E. Börcsök, O. Czömpöly, C. Dian, V. Groma, L. Stabile, and S. Török,

 are with Physikalisch-Technische Bundesanstalt (PTB) in Berlin, Germany. 

 is with Bruker Nano GmbH in Berlin, Germany. 

 is in the Environmental Physics Department at the Centre for Energy Research in Budapest, Hungary. 

is with the Federal Institute for Materials Research and Testing (BAM) in Berlin, Germnay, and 

 is with PTB and the Federal Institute for Materials Research and Testing. Direct correspondence to:


Articles

This approach provides traceable and reliable quantitative elemental analysis of airborne particles for on-site environmental measurement with portable instrumentation.

Reliable Chemical Analysis of Aerosols by Reference-Free X-ray Spectrometry for Monitoring Airborne Particulate Matter

	The increasing demand for secondary electrochemical storage devices requires well characterized and working battery systems. Each battery suffers from degradation effects that lead to capacity fading and life cycle reduction. The single mechanisms vary for different types of batteries. With electrochemical methods the decreasing capacity can easily be monitored, but to completely understand the responsible underlying chemical and physical properties, further analytic techniques are needed. The German National Institute for Metrology (PTB) has demonstrated the possibilities offered by X-ray spectrometry (XRS) to investigate aging mechanisms for different high-energy density batteries. XRS, traceable to the International System of Units, enables the quantitative determination of elemental mass depositions on a very large dynamic range that can be used to investigate, for example the transition metal deposition at the solid electrolyte interphase (SEI) in currently used lithium-Ion batteries (LIBs). Moreover, even the oxidation state and the species of an element, which plays a major role regarding ageing mechanisms, can be specified with XRS. In addition, XRS is a non-destructive and contactless analytical technique that can be used not only in post mortem (ex situ) mode, but also in in situ and even operando mode, which is shown in the following for the characterization of fading effects in lithium-sulfur (Li/S) batteries.

Determination of Dissolved Polysulfides In Lithium-Sulfur Batteries By Coupling Operando NEXAFS Studies with Reference-Free Quantitative XRS

	Li/S cells are promising candidates for batteries in electric vehicles and as stationary energy storage device because of their high theoretical specific energy density, which is five times higher than for LIBs. The Li/S battery is not yet widely commercially used, because the currently accessible energy density is much lower than the theoretical value, and, therefore, the underlying degradation mechanisms are a recent research topic. The mechanisms have not yet been completely unraveled, but it is known that the formation of the polysulfides soluble in the electrolyte is, besides the cathodic volume change and the growth of lithium dendrites, the main ageing mechanism for this type of battery because it leads to the loss of sulfur available as cathodic active material. To understand and consequently to avoid this fading, the evolution and the behavior of the dissolved polysulfides have to be studied.

	Given that the ageing processes in the battery are time dependent, reliable analytical operando methods are needed that allow for the correlation of analytical information with the electrochemical performance. Moreover, the dissolved polysulfides can travel in the electrolyte solution through the cell and the movement between the electrodes is called polysulfide shuttle. Reaching the anode, further damaging effects can occur because of polysulfides undergoing reactions in the negative potentials. Therefore, polysulfides have to be studied not only on the cathode side but also in the near-anode region.

	We developed an operando coin cell that can be used to monitor the formation of dissolved polysulfides, as well as the related shuttle phenomena. We used a small probing hole of 3 mm diameter in the center of the electrode of interest and in the coin cell body. To enable working in a ultra-high vacuum (UHV) environment, the hole in the coin cell case is sealed with a thin window that has sufficient transmission for incident as well as fluorescence radiation around 2.4 keV. The operando cell for investigations of dissolved polysulfides on the anode side is shown in Figure 1.

	When tuning the photon energy of monochromatized synchrotron radiation across the K absorption edge of sulfur, the binding state can be revealed for speciation purposes. Performing such a near-edge x-ray absorption fine structure (NEXAFS) investigation under operando conditions and recording the element-specific fluorescence radiation with calibrated instrumentation in a traceable X-ray fluorescence (XRF) analysis approach, the absolute quantification of the mass deposition of sulfur in dissolved polysulfides became feasible for the first time during three full charge-discharge cycles of a Li/S battery. With PTB’s reference-free XRF quantification method, no calibration samples or certified reference materials are needed, which is important because every aged battery cell is unique. A (dis-)charge current of (C/10) C/5 is applied and each NEXAFS measurement around the sulfur K-edge took around 10 min, which is compared to the charge/discharge current rate and allows for the detection of even fast changes in the polysulfide amount. With the quantified sulfur mass in dissolved polysulfides and the known initial cathodic sulfur mass, the loss of active material could be determined. The results for the polysulfides in the near-anode region is shown in Figure 2.

	With our measurement we identified that the reaction equilibrium for polysulfides is forced to the anode side because the average amount of polysulfides increases over cycles. That can be explained by an inhibited movement of polysulfides from the cathode to the anode side during the recharge process possibly caused by Li ions moving from the cathode back to the anode. This progress corresponds in the electrochemistry curve to the dip at 2.4 V. In addition, by means of appropriate experimental and NEXAFS evaluation procedures, access to the change of the average polysulfide chain length could be achieved which enables a deeper understanding of the capacity fading processes.

	The current operando measurement cells as well as the quantification method can be adopted for other kind of cell chemistries, like LIBs and all solid-state batteries, and also for catalytic processes. Moreover, the cell is suitable for different analytical techniques. We additionally performed cyclic voltammetry, operando Raman spectroscopy, and operando X-ray radiography, which lead to additional and complementary information about the aging mechanism.

	This research was funded by German Science Foundation (DFG Project ISIBAT Sta 324/54). We would like to thank Manfred Stamm at Leibniz Institut für Polymerforschung Dresden for the materials provided and the Raman spectroscopy experiments as well as Sebastian Risse of the Helmholtz-Zentrum Berlin for performing the X-ray Radiography measurements for investigation of our Li/S cells.

 are scientists in the X-ray spectrometry group at Physikalisch-Technische Bundesanstalt (PTB) in Berlin, Germany. 

 is the head of PTB’s X-ray spectrometry group. Direct correspondence to 

The increasing demand for secondary electrochemical storage devices requires well characterized and working battery systems.

Philipp Hönicke

Articles

Reliable Chemical Analysis of Aerosols by Reference-Free X-ray Spectrometry for Monitoring Airborne Particulate Matter

Investigations of Ageing Mechanisms for New High-Energy- Density Battery Materials with X-Ray Spectrometry