Development of spectroscopic techniques for the measurement of Respirable Crystalline Silica
Authors:   , Stacey Peter 
Published: July 2023

Inhalation of aerosols containing respirable-sized crystalline silica (RCS) is a significant hazard for hundreds of thousands of workers in Great Britain (GB), which can cause lung cancer and silicosis. There are elevated levels of disease attributed to exposure to RCS in GB. Personal aerosol monitoring provides an indication of a person’s exposure to RCS, which correlates to the potential risk of ill-health. RCS is often measured in a portion of the aerosol collected in the workplace using X-ray diffraction (XRD) or Fourier Transform Infrared (FTIR) instruments. Uncertainty of XRD or FTIR measurements can exceed international requirements for occupational hygiene analyses at RCS concentrations where there remains a significant risk to worker health. This thesis investigates the application of Raman spectroscopy to improve sensitivity and limits of detection (LOD) for RCS measurements and chemometric methods to improve FTIR measurements when interference is present. For Raman spectroscopy, a feasibility study was undertaken employing optimal measurement conditions and artificial dust mixtures. This study was developed into a method to assess in-respirator samples of RCS from a breathing manikin challenged with a simulated work task. RCS inward leakage ratios (ILRs) based on mass concentration were compared with particle mass and number concentration ILRs from particle counting instruments. The performance of Raman spectroscopy was compared with XRD to assess its limitations when measuring sub-fractions of quartz and RCS in 119 samples from stonemasonry activities. For FTIR, the performance of chemometric methods were compared when measuring artificially prepared mixtures with kaolinite interference and the measurement uncertainty of a Principal Component Regression method was evaluated using real mine dusts. This work was the first to demonstrate the potential of Raman spectroscopy for quantitative measurements of RCS at low concentrations (less than 1μg.m-3). When measuring respirable quartz, Raman response is directly proportional to the mass of analyte when the depth of the sample is a couple of layers. For the quartz standard A9950, this depth is less than 5 μm (which equates to about 3.6 layers of particles 1.39 μm in diameter). An increase in Raman response/μg v was observed at mass loadings above this particle density. The change in the Raman band response is coincident with the depth of sample deposit where significant attenuation of the XRD response would occur due to absorption by multilayers of particles. The Raman band integrated area obtained similar mass values to that measured with XRD integrated intensity when analysing sub-fractions of dusts with differing particle size. Measurements of 119 stone dust samples highlighted the advantage of Raman spectroscopy to quantify quartz with zircon. For FTIR, a multicomponent chemometric analysis of aerosol particulate on filters, was demonstrated, which substantially improves the accuracy of FTIR measurements of quartz in mine dusts. Nevertheless, the measurement of RCS remains potentially challenging due to the range of industries and exposures. This Raman spectroscopy work supported other Health and Safety Executive research to investigate differences between emissions of natural and artificial stones. The short sampling times for laboratory-based experiments would have resulted in many of the samples reporting below or at the LOD of current measurement techniques. Therefore, the use of Raman spectroscopy made the findings of the research more comprehensive.