Spark Discharge: Application Multielement Spectrochemical Analysis

The performance of an inexpensive spark-induced breakdown spectroscopy instrument for near real-time analysis of toxic metal particles

To meet the demand for identifying and controlling toxic air contaminants in environmental justice communities, we have recently developed a cost-effective spark-induced breakdown spectroscopy (SIBS) instrument for detecting and quantifying toxic metal air pollutants. We characterized the detection limit and linearity of this SIBS instrument by analyzing nebulized elemental standard solutions. The experimental parameters affecting SIBS performance were optimized, including the time delay to observation, the distance between electrodes, and the ablation voltage. The instrument successfully detected Cr, Cu, Mn, Fe, Zn, Co, and Ni, with limits of detection ranged from 0.05 μg m-3 to 0.81 μg m-3 at a flow rate of 15 l min-1 and a 30 min sampling duration. Similar to other investigations using ion breakdown spectroscopy, we did not observe strong emissions lines for As, Sb, Se, Hg, Pb, and Cd, which were likely due to spectral overlap, matrix effects, and the limited detection range of the optical components. Overall, SIBS is a promising technique for field measurements of toxic metals for environmental justice, industrial and urban applications.

Extraction of high-contrast diffraction patterns of fine-structured electrical sparks from laser shadowgrams

The fine-structured electrical spark is a complex gas discharge phenomenon, which appears as a cluster involving dozens of closely-packed thin plasma filaments that can be revealed by laser shadowgraphy. However, the immense complexity of the spark, together with the features of laser imaging, challenges the spark image processing. Herein, we developed an image processing procedure, providing outstanding shadowgram denoising while preserving the spark image capacity. By employing this procedure, we show that the passage of laser radiation through the spark is accompanied by complicated diffraction, entailing pronounced changes in the radiation intensity distribution in the zones with strong filament overlapping.

Quantification of toxic metals using machine learning techniques and spark emission spectroscopy

The United States Environmental Protection Agency (US EPA) list of hazardous air pollutants (HAPs) includes toxic metal suspected or associated with development of cancer. Traditional techniques for detecting and quantifying toxic metals in the atmosphere are either not real time, hindering identification of sources, or limited by instrument costs. Spark emission spectroscopy is a promising and cost-effective technique that can be used for analyzing toxic metals in real time. Here, we have developed a cost-effective spark emission spectroscopy system to quantify the concentration of toxic metals targeted by the US EPA. Specifically, Cr, Cu, Ni, and Pb solutions were diluted and deposited on the ground electrode of the spark emission system. The least absolute shrinkage and selection operator (LASSO) was optimized and employed to detect useful features from the spark-generated plasma emissions. The optimized model was able to detect atomic emission lines along with other features to build a regression model that predicts the concentration of toxic metals from the observed spectra. The limits of detections (LODs) were estimated using the detected features and compared to the traditional single-feature approach. LASSO is capable of detecting highly sensitive features in the input spectrum; however, for some toxic metals the single-feature LOD marginally outperforms LASSO LOD. The combination of low-cost instruments with advanced machine learning techniques for data analysis could pave the path forward for data-driven solutions to costly measurements.

A Portable Spark-Induced Breakdown Spectroscopic (SIBS) Instrument and its Analytical Performance

A compact spark-induced plasma spectroscopic device was developed to detect elements used in a variety of applications. The system consists of a spark generator connected to tungsten electrodes, a custom-built delay generator, and two spectrometers that together cover the ultraviolet visible (UV-Vis) range (214-631 nm). The system was evaluated by qualitatively and quantitatively sampling copper standards. Prominent spectral peaks were identified using the NIST database for atomic emissions. The effectiveness of the proposed system was also tested with a lanthanide sample (gadolinium) and provided qualitative identification of the characteristic peaks. A semi-quantitative measurement for silicon and gold was performed using variable amounts of each particulate. Silica microbeads in solution were applied to paper wafers, while gold nanoparticles were sputter-coated onto silicon wafers. Results showed a positive correlation between the intensity of the signal and the concentration of each type of particulate. The variation of signal intensity was investigated to determine the repeatability, and the coefficient of variation was lowered from 60% to 25% after averaging measurements of multiple ablations per observation.

Ab Initio Study of SOF2 and SO2F2 Adsorption on Co-MoS2

The detection of partial discharge by analyzing the decomposition components of SF6 gas in gas-insulated switchgears plays an important role in the diagnosis and assessment of the operational state of power equipment. Recently, the application of transition metal-modified MoS2 monolayer dioxide in gas detection has received wide attention. In this paper, first-principle density functional theory calculations were adopted to study the gas-sensitive response of Co-MoS2 monolayer to SOF2 and SO2F2. It is found that the conductivity of the Co-MoS2 monolayer has been effectively enhanced after Co atom doping on the MoS2 monolayer. After gas adsorption, electrons transfer from the Co-MoS2 monolayer to the gas molecules, resulting in significant reduction of conductivity of the adsorption system. The calculation results reveal that the Co-MoS2 monolayer is sensitive and selective to SOF2 and SO2F2 gases. This study provides the theoretical possibility of using Co-MoS2 as a gas sensor for SOF2 and SO2F2 gas detection.

Ni-CNT Chemical Sensor for SF₆ Decomposition Components Detection: A Combined Experimental and Theoretical Study

SF₆ decomposition components detection is a key technology to evaluate and diagnose the insulation status of SF₆-insulated equipment online, especially when insulation defects-induced discharge occurs in equipment. In order to detect the type and concentration of SF₆ decomposition components, a Ni-modified carbon nanotube (Ni-CNT) gas sensor has been prepared to analyze its gas sensitivity and selectivity to SF₆ decomposition components based on an experimental and density functional theory (DFT) theoretical study. Experimental results show that a Ni-CNT gas sensor presents an outstanding gas sensing property according to the significant change of conductivity during the gas molecule adsorption. The conductivity increases in the following order: H₂S > SOF₂ > SO₂ > SO₂F₂. The limit of detection of the Ni-CNT gas sensor reaches 1 ppm. In addition, the excellent recovery property of the Ni-CNT gas sensor makes it easy to be widely used. A DFT theoretical study was applied to analyze the influence mechanism of Ni modification on SF₆ decomposition components detection. In summary, the Ni-CNT gas sensor prepared in this study can be an effective way to evaluate and diagnose the insulation status of SF₆-insulated equipment online.

Near-Real Time Measurement of Carbonaceous Aerosol Using Microplasma Spectroscopy: Application to Measurement of Carbon Nanomaterials

A sensitive, field-portable microplasma spectroscopy method has been developed for real-time measurement of carbon nanomaterials. The method involves microconcentration of aerosol on a microelectrode tip for subsequent analysis for atomic carbon using laser-induced breakdown spectroscopy (LIBS) or spark emission spectroscopy (SES). The spark-induced microplasma was characterized by measuring the excitation temperature (15,000 - 35,000 K), electron density (1.0 × 1017 - 2.2 × 1017 cm-3), and spectral responses as functions of time and interelectrode distance. The system was calibrated and detection limits were determined for total atomic carbon (TAC) using a carbon emission line at 247.856 nm (C I) for various carbonaceous materials including sucrose, EDTA, caffeine, sodium carbonate, carbon black, and carbon nanotubes. The limit of detection for total atomic carbon was 1.61 ng, equivalent to 238 ng m-3 when sampling at 1.5 L min-1 for 5 min. To improve the selectivity for carbon nanomaterials, which consist of elemental carbon (EC), the cathode was heated to 300 °C to reduce the contribution of organic carbon to the total atomic carbon. Measurements of carbon nanotube aerosol at elevated electrode temperature showed improved selectivity to elemental carbon and compared well with the measurements from thermal optical method (NIOSH Method 5040). The study shows that the SES method to be an excellent candidate for development as a low-cost, hand-portable, real-time instrument for measurement of carbonaceous aerosols and nanomaterials.

Measurement of elemental concentration of aerosols using spark emission spectroscopy

A coaxial microelectrode system has been used to collect and analyse the elemental composition of aerosol particles in near real-time using spark emission spectroscopy. The technique involves focused electrostatic deposition of charged aerosol particles onto the flat tip of a microelectrode, followed by introduction of spark discharge. A pulsed spark discharge was generated across the electrodes with input energy ranging from 50 to 300 mJ per pulse, resulting in the formation of controlled pulsed plasma. The particulate matter on the cathode tip is ablated and atomized by the spark plasma, resulting in atomic emissions which are subsequently recorded using a broadband optical spectrometer for element identification and quantification. The plasma characteristics were found to be very consistent and reproducible even after several thousands of spark discharges using the same electrode system. The spark plasma was characterized by measuring the excitation temperature (~7000 to 10 000 K), electron density (~10¹⁶ cm^-3), and evolution of spectral responses as a function of time. The system was calibrated using particles containing Pb, Si, Na and Cr. Absolute mass detection limits in the range 11 pg to 1.75 ng were obtained. Repeatability of spectral measurements varied from 2 to 15%. The technique offers key advantages over similar microplasma-based techniques such as laser-induced breakdown spectroscopy, as: (i) it does not require any laser beam optics and eliminates any need for beam alignment, (ii) pulse energy from dc power supply in SIBS system can be much higher compared to that from laser source of the same physical size, and (iii) it is quite conducive to compact, field-portable instrumentation.

Design and characterization of a theta-pinch imploding thin film plasma source for atomic emission spectrochemical analysis

A new atomization device for direct atomic spectrochemical analysis has been developed that uses the theta-pinch configuration to generate a pulsed, high-energy-density plasma at atmospheric pressure. Energy from a 20-kV, 6.05-μF capacitive electrical discharge was inductively coupled to a sacrificial aluminum thin film to produce a cylindrical plasma. Current waveform analysis indicates an average power dissipation of 0.5 MW in the plasma. Electromagnetic modeling studies were used to identify theta-pinch designs possessing characteristics favorable to both plasma initiation and plasma heating. The discharge was most robust when the induced current and rate of magnetic field change were maximized. Minimizing the ratio of the coil's width to its radius was also critical. Counter to intuition, a larger diameter was found to be more successful. Spectroscopic studies indicate that the discharge forms a heterogeneous plasma with a dense, cylindrical plasma sheet confined by the walls of the discharge tube surrounding a less energetic plasma in the center. Al(II) emission in the outer plasma cylinder was temporally aligned with the induced current whereas in the center it aligns with the magnetic field. Ionization of support gas species (Ar, He, and air) was not observed, although the identity of the gas had a significant influence on the plasma reproducibility. The optimized design utilized a 5.5-turn, 19-mm-diameter theta coil with argon as the support gas. Sb(I) emission from an antimony oxide solid powder sample deposited on the thin film was observed primarily in the outer part of the plasma. Analyte emission shows contributions from magnetic compression early in the discharge and from the induced current late in the discharge. The discharge produced analytically useful signals from solid antimony oxide samples. Using spatially and temporally resolved detection, the line-to-background ratio for Sb(I) was found to be greater than 4 for emission integrated from 55 to 120 μs.