Measurement of Crystalline Silica Aerosol Using Quantum Cascade Laser-Based Infrared Spectroscopy

Compact, high-flow, water-based, turbulent-mixing, condensation aerosol concentrator for collection of spot samples

A new high-flow, compact aerosol concentrator, using rapid, turbulent mixing to grow aerosol particles into droplets for dry spot sample collection, has been designed and tested. The "TCAC (Turbulent-mixing, Condensation Aerosol Concentrator)" is composed of a saturator for generating hot vapor, a mixing section where the hot vapor mixes with the cold aerosol flow, a growth tube where condensational droplet growth primarily occurs, and a converging nozzle that focuses the droplets into a beam. The prototype concentrator utilizes an aerosol sample flow rate of 4 L min-1. The TCAC was optimized by varying the operating conditions, such as relative humidity of the aerosol flow, mixing flow ratio, vapor temperature, and impaction characteristics. The results showed that particles with a diameter ≥ 25 nm can be grown to a droplet diameter > 1400 nm with near 100% efficiency. Complete activation and growth were observed at relative humidity ≥ 25% of the aerosol sample flow. A consistent spot sample with a diameter ofD 90 = 1.4 mm (the diameter of a circle containing 90% of the deposited particles) was obtained regardless of the aerosol particle diameter (d p = 20 - 1900 nm ). For fiber counting applications using phase contrast microscopy, the TCAC can reduce the sampling time, or counting uncertainty, by two to three orders of magnitude, compared to the 25-mm-filter collection. The study shows that the proposed mixing-flow scheme enables a compact spot sample collector suitable for handheld or portable applications, while still allowing for high flow rates.

NanoSpot™ collector for aerosol sample collection for direct microscopy and spectroscopy analysis

We describe design and characterization of an aerosol NanoSpot^™ collector, designed for collection of airborne particles on a microscopy substrate for direct electron and optical microscopy, and laser spectroscopy analysis. The collector implements a water-based, laminar-flow, condensation growth technique, followed by impaction onto an optical/electron microscopy substrate or a transmission electron microscopy grid for direct analysis. The compact design employs three parallel growth tubes allowing a sampling flow rate of 1.2 L min^-1. Each growth tube consists of three-temperature regions, for controlling the vapor saturation profile and exit dew point. Following the droplet growth, the three streams merge into one flow and a converging nozzle enhances focusing of grown droplets into a tight beam, prior to their final impaction on the warm surface of the collection substrate. Experiments were conducted for the acquisition of the size-dependent collection efficiency and the aerosol concentration effect on the NanoSpot^™ collector. Particles as small as 7 nm were activated and collected on the electron microscopy stub. The collected particle samples were analyzed using electron microscopy and Raman spectroscopy for the acquisition of the particle spatial distribution, the spot sample uniformity, and the analyte concentration. A spot deposit of approximately 0.7-mm diameter is formed for particles over a broad particle diameter range, for effective coupling with microscopic and spectroscopic analysis. Finally, the NanoSpot^™ collector's analytical measurement sensitivity for laser Raman analysis and counting statistics for fiber count measurement using optical microscopy were calculated and were compared with those of the conventional aerosol sampling methods.

Benefits and limitations of field-based monitoring approaches for respirable dust and crystalline silica applied in a sandstone quarry

With the advent of new sensing technologies and robust field-deployable analyzers, monitoring approaches can now generate valuable hazard information directly in the workplace. This is the case for monitoring respirable dust and respirable crystalline silica concentration levels. Estimating the quartz amount of a respirable dust sample by nondestructive analysis can be carried out using portable Fourier transform infrared spectroscopy (FTIR) units. Real-time respirable dust monitors, combined with small video cameras, allow advanced assessments using the Helmet-CAM methodology. These two field-based monitoring approaches, developed by the National Institute for Occupational Safety and Health (NIOSH), have been trialed in a sandstone quarry. Twenty-six Helmet-CAM sessions were conducted, and forty-one dust samples were collected around the quarry and analyzed on-site during two events. The generated data generated were used to characterize concentration levels for the monitored areas and workers, to identify good practices, and to illustrate activities that could be improved with additional engineered control technologies. Laboratory analysis of the collected samples complemented the field finding and provided an assessment of the performance of the field-based techniques. Only a fraction of the real-time respirable dust monitoring sessions data could be corrected with laboratory analysis. The average correction factor ratio was 5.0. Nevertheless, Helmet-CAM results provided valuable information for each session. The field-based quartz monitoring approach overestimated the concentration by a factor of 1.8, but it successfully assessed the quartz concentration trends in the quarry. The data collected could be used for the determination of a quarry calibration factor for future events. The quartz content in the dust was found to vary from 14% to 100%, and this indicates the need for multiple techniques in the characterization of respirable dust and quartz concentration and exposure. Overall, this study reports the importance of the adoption of field-based monitoring techniques when combined with a proper understanding and knowledge of the capabilities and limitations of each technique.

Implementing Silica Nanoparticles in the Study of the Airborne Transmission of SARS-CoV-2

Aerosol transmission constitutes one of the major transmission routes of the SARS-CoV-2 pathogen. Due to the pathogen's properties, research on its airborne transmission has some limitations. This paper focuses on silica nanoparticles (SiO2) of 40 and 200 nm sizes as the physicochemical markers of a single SARS-CoV-2 particle enabling experiments on the transmission of bioaerosols in public spaces. Mixtures of a determined silica concentration were sprayed on as an aerosol, whose particles, sedimented on dedicated matrices, were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Since it was not possible to quantitatively identify the markers based on the obtained images, the filters exposed with the AirSampler aspirator were analyzed based on inductively coupled plasma optical emission spectroscopy (ICP-OES). The ICP-OES method enabled us to determine the concentration of silica after extracting the marker from the filter, and consequently to estimate the number of markers. The developed procedure opens up the possibility of the quantitative estimation of the spread of the coronavirus, for example in studies on the aerosol transmission of the pathogen in an open environment where biological markers-surrogates included-cannot be used.

Mid-Infrared Reflectance Spectroscopy Based on External Cavity Quantum Cascade Lasers for Mineral Characterization

Mineral characterization using tunable external cavity quantum cascade lasers for mid-infrared reflectance spectroscopy is presented. Reflection spectra taken on common minerals are shown along with their spectral variations. Measurements at different incident angles were also obtained to mimic in the field applications. Finally, spectra were obtained on mineral mixtures and their analysis assesses the quality and usability of the technique for mineral characterization.

Aerosol Analysis Using Handheld Raman Spectrometer: On-site Quantification of Trace Crystalline Silica in Workplace Atmospheres

A method for aerosol chemical analysis using handheld Raman spectrometer has been developed and its application to measurement of crystalline silica concentration in workplace atmosphere is described. The approach involves collecting aerosol as a spot sample using a wearable optical aerosol monitor, followed by direct-on-filter quantitative analysis of the spot sample for crystalline silica using handheld Raman spectrometer. The filter cassette of a commercially available optical aerosol monitor (designed to collect aerosol for post-shift analysis) was modified to collect 1.5-mm-diameter spot sample, which provided adequate detection limits for short-term measurements over a few tens of minutes or hours. The method was calibrated using aerosolized α-quartz standard reference material in the laboratory. Two Raman spectrometers were evaluated, one a handheld unit (weighing less than 410 g) and the other a larger probe-based field-portable unit (weighing about 5 kg). The lowest limit of quantification for α-quartz of 16.6 μg m-3 was obtained using the handheld Raman unit at a sample collection time of 1 h at 0.4 l min-1. Short-term measurement capability and sensitivity of the Raman method were demonstrated using a transient simulated workplace aerosol. Workplace air and personal breathing zone concentrations of crystalline silica of workers at a hydraulic fracturing worksite were measured using the Raman method. The measurements showed good agreement with the co-located samples analyzed using the standard X-ray powder diffraction (XRD) method, agreeing within 0.15-23.2% of each other. This magnitude of difference was comparable to the inter- and intra-laboratory analytical precision of established XRD and infrared methods. The pilot study shows that for silica-containing materials studied in this work it is possible to obtain quantitative measurements with good analytical figures of merit using handheld or portable Raman spectrometers. Further studies will be needed to assess matrix interferences and measurement uncertainty for several other types of particle matrices to assess the broader applicability of the method.

An indirect Raman spectroscopy method for the quantitative measurement of respirable crystalline silica collected on filters inside respiratory equipment

This article describes the development of an analytical method to measure respirable crystalline silica (RCS) collected on filters by a miniature sampler placed behind respirators worn by workers to evaluate their 'true' exposure. Test samples were prepared by aerosolising a calibration powder (Quin B) and by pipetting aliquots from suspensions of bulk material (NIST 1878a and Quin B) onto filters. Samples of aerosolised RCS collected onto polyvinyl chloride PVC filters were ashed and their residue was suspended in isopropanol and filtered into a 10 mm diameter area onto silver filters. Samples were also collected by the Health and Safety Executive's (HSE) miniature sampler from within the facepiece of a respirator on a breathing manikin during a simulated work activity. Results obtained using Raman spectroscopy were compared with X-ray diffraction (XRD) measurements, which was used as a reference method and a linear relationship was obtained. Raman has similar estimates of uncertainty when compared with the XRD methods over the measurement range from 5 to 50 μg and obtained the lowest limit of detection (LOD) of 0.26 μg when compared with XRD and Fourier Transform Infrared FTIR methods. A significant intercept and slope coefficient greatly influenced the higher LOD for indirect XRD method. The level of precision and low LOD for Raman spectroscopy will potentially enable workplace measurements at lower concentrations below the Workplace Exposure Limit (WEL) than are achieved using current analytical instrumentation. Different inward leakage ratio (ILR) measurement approaches were compared using six aerosolised sandstone dust tests. For the three highest inward leakage ratios the Portacount® obtained higher values than the RCS mass or the miniWRAS ratios, the latter of which reporting both particle number and quartz mass concentration. However, these limited ILR data were insufficient to establish statistical correlations between the measurement methods.