Quantitative XRD, respirable silica dust analysis

Introduction Airborne silica particles pose a carcinogenic threat at concentrations as low as 50 µg/m3. Standard methods such as NIOSH7500, OSHA ID-142, and MDHS 51/2 use XRD to quantify the mass of SiO2 phases per volume of air sampled directly on the polymer

collection filter or on samples ashed and deposited on silver membranes. The maximum allowed concentration of respirable silica in the atmosphere at a workplace is under constant review, with lower maxima proposed worldwide, so low limits of detection are of utmost importance. PANalytical’s diffraction systems, equipped with the X’Celerator detector, offer advances in both speed of measurement and sensitivity compared to conventional point detectors.

SiO2 loaded silver membrane mounted on a standard sample holder

Discussion Industry software greatly simplifies the practice of quantitative X-ray diffraction because all programming and measurement commands, calibration mathematics, and customizable reporting are conveniently available in one software package.

Calibration standards with loads ranging from 3 to 210 µg SiO2 were utilized to construct the calibration curve below. The calibration curve demonstrates the linear relationship between intensity (peak area) and weight of quartz on the filter, up to about 200 µg SiO2. These specimens were prepared at the INSHT in Spain compliant with their standard methods - MTA/MA-036/A00 and MTA/ MA-056/A05.

The deconvolution of interfering peaks from other minerals with reflections near quartz (101) at 26.6 degrees can be perfectly done by using the Industry software from Malvern Panalytical.

Typical instrument setup

Calibration

Sensitivity The X’Celerator detector brings increased sensitivity and accuracy to the application of respirable silica analysis by two mechanisms

First, the X’Celerator detector is a solid-state linear array of more than 100 detector elements measuring more than 2 degrees simultaneously. Crystallites slightly inclined from being parallel to the surface can also participate in the diffraction signal with the X’Celerator detector, improving the particle statistics as compared to a point detector that only receives diffracted X-rays from parallel crystal planes. This leads to greater sensitivity for low levels of respirable silica phases. Secondly, the design of the X’Celerator also leads to an increase in the speed of a measurement by up to 100 times over a conventional point detector. This speed of measurement can allow for a reduction in the counting statistical error, thus in the same amount of time as a conventional XRD measurement, the counting statistical error can be improved by a factor of 10. An X’Celerator scan in a quarter of the time of a conventional XRD measurement will have 5 times lower counting statistical error in the measurement, and so on.

The scans demonstrate the X’Celerator’s ability to detect 3, 6, and 12 µg respirable quartz samples measured directly on a PVC membrane in 18 minute scans measuring the primary and secondary quartz peak. Similar sensitivity is achievable for samples prepared on silver membranes.

Quartz (101) for loads of 12, 6 and 3 µg respectively. Blue corresponds to 12 µg, green to 6 µg and red to 3 µg

Precision The precision of quantitative X-ray analysis is generally governed by the time spent measuring the sample. For a factor of 2 improvement in precision, one must measure four times longer. The standard deviation is an approximation of the limit of detection.

A precision test, in which a 10 µg quartz sample prepared on a silver membrane was measured repeatedly, demonstrated both the precision and reproducibility of the X’Celerator for respirable silica measurement. Using the X’Celerator detector provided data not only faster, but also better and more precise because it is counting many seconds per point in these short scans.

Malvern Panalytical’s Industry software is ideally suited for the respirable silica analysis application. The analysis program can be customized to perform whichever standard analysis method you choose. Logic can be built into the program, such as measuring confirmation peaks, testing for sample displacement errors or any other sample effects on your diffraction pattern. Industry also has statistical process control (SPC) charting capability, to help you determine how long to count to achieve a certain level of precision in your analysis.

Reproducibility measurements with primary and tertiary quartz peaks

10 replicate scans of 10 µg quartz on silver mebrane

Conclusion In this application note we have shown that the X’Celerator is an excellent tool for measuring the amount of quartz dust both directly on a cellulose filter and after treatment on an Ag membrane. By properly using the X’Celerator, detection limits can be reduced to approximately 3 µg load.

Connexion