127 An evaluation of validation schemes for the direct-on-filter X-ray diffraction method for analysis of respirable crystalline silica

Abstract Direct-on-filter X-ray diffraction analysis (DOF-XRD) of respirable crystalline silica RCS) is a commonly used method for quantifying the different polymorphs of respirable crystalline silica in aerosol samples. XRD has the advantage of enhanced selectivity with respect to other methods like Fourier-transformed spectrometry or Raman spectrometry. The direct-on-filter XRD measurement is less cumbersome and has lower overall uncertainty than the alternative indirect method, because it does not involve destruction of the sample substrate and redeposition of the collected aerosol on another substrate for measurement. In this contribution, a guideline for optimizing the XRD measurement will be presented, based on data collected at a wide range of instrumental conditions. The effect of measurement parameters and several commonly used optical elements in XRD systems will be explained and demonstrated with actual RCS measurements. The preparation of adequate calibration series for DOF-XRD will be addressed. In addition, the outcomes of different validation schemes (limits of detection and quantification, expanded uncertainty) described in the ISO and ASTM standards, as well as in alternative schemes, obtained from the same set of calibration measurements, will be discussed. The last part of this contribution will deal with the potential of external standards, single reflection, multi-reflection, and full pattern methods for the quantification of respirable crystalline silica.