Quantifying respirable silica: excellent performance on four reflections of a silica phase in as little as 90 minutes
Airborne silica particles pose a carcinogenic threat, in particular for-high risk occupations such as foundry work, stonecutting, quarry work and tunneling, at concentrations as low as 50 μg/m3. Standard methods such as NIOSH7500, OSHA ID-142, and MDHS 51/2 describe how X-ray diffraction (XRD) can be employed to quantify the mass of SiO2 phases per volume of air, sampled either directly on polymer collection filters, or on samples ashed and deposited on silver membrane filters. The maximum permissible exposure limit (PEL) of respirable silica in the atmosphere at a workplace is under constant review, with lower maxima proposed worldwide. Therefore, there is a continuous push to improve the limits of detection and quantification while keeping a sensible measurement time.
X-ray diffraction delivers the best limit of detection and precision for silica analysis, and can also distinguish between the common polymorphs of silica – quartz, cristobalite and tridymite. PANalytical’s unique combination of high sensitivity and speed of detection with an unrivaled peak-to- background ratio delivered by the Bragg-BrentanoHD optic enables even better performance and offers a robust turnkey solution for the quantification of respirable silica. This example of such a solution is based on a classical straight line calibration accounting for the intensity of four quartz reflections, with a total measurement time per sample as low as 90 minutes.
Powder XRD measurements were performed on a high- performance industrial diffractometer, CubiX3, configured with a Cu LFF HR tube, Bragg-BrentanoHD module, sample spinner with a beam knife and a line detector. Bragg-BrentanoHD is a dedicated module for high-quality powder data with significantly improved background providing better detection limits. Six calibration standards with loads ranging from 10 to 200 μg quartz were utilized to construct the calibration curve. Standards measurements, calibration and setup of an automatic respirable silica quantification method were performed using the Industry software module.
Figure 1. Blasting, cutting, grinding and drilling of silica–containing materials can produce hazardous airborne silica dust
Results and discussion
The main challenge of respirable silica quantification is the small amount of sample. For XRD this results in challenging particle statistics and possible preferred orientation influencing the diffracted intensities and hence the quantification results. Another common problem is the possible peak overlap with other mineralogical phases, which might be present.
To minimize these effects, intensities of multiple quartz reflections are measured and the concentration is derived from the average peak intensities. This approach ensures more accurate quantification results.
In Figure 2 the typical example of XRD patterns measured on the position of four quartz reflections is shown. A total measurement time of 90 minutes per sample is sufficient to achieve a limit of quantification (LOQ) of 8.6 μg for all four quartz reflections. Five minutes measurement of the primary quartz peak is sufficient to achieve a LOQ = 1 μg!
The calibration curve (Figure 3) demonstrates the linear relationship between average intensity (peak area) and weight of quartz on the filter in the studied concentration range, 10 – 200 μg.
Using the Industry software module the described respirable silica quantification method can be easily set up as a fully automatic push-button solution with customizable reporting best suiting your needs.
Figure 2. Profile fit (blue) of raw XRD scans (red) measured on the positions of four quartz reflections from the standard sample containing 10 μg of quartz
Figure 3. Calibration curve for quartz quantification based on four quartz reflections
An optimized optical path in combination with a fast and sensitive detector on an industrial diffractometer, CubiX3, enables fast and reliable analysis for the quantification of respirable silica, and improves respirable silica detection limits to approximately 1 μg load.