X-Ray Absorption Effect in Aerosol Samples Collected on Filter Media

    Published: Dec 2013

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    Crystalline silica collected on membrane filters is commonly analyzed by X-ray diffraction (XRD). If the dust layer on the filter is very thin, the linearity of X-ray response is not affected. As loading increases, the X-ray beam is attenuated and the linear relationship no longer holds. The basic method for correction is to employ a silver filter to assess and compensate for the reduced X-ray response. No comprehensive study has yet been published that defines the dust loading beyond which the linear relationship no longer holds. As a consequence, a variety of dust loading limits are currently taken for linearity, ranging from 0.2 up to 2 mg. Members of the ISO working group for silica measurement collaborated in an experimental study on absorption for defining the critical sample mass, mcr, above which the measured intensity of a reflection of the analyte deviates from the theoretically correct intensity by more than a set threshold. Quartz, calcite, hematite in the respirable size range, as pure minerals and mixtures, were used to test both the indirect and the direct-on-filter XRD methods. Analyses were carried out in three laboratories using CuKα radiation. The critical sample mass for a weight fraction of quartz, fQtz, depends on the area of deposition on the filter, A, and the mass absorption coefficient of the sample, μsample. For a typical fQtz = 5% value, if a 10 % deviation is accepted to represent the limit of linearity of XRD response, the critical sample mass derived from theoretical calculations is given by the equation: mcr10 = 35.711 ⋅ A ⋅ μsample−1.065, where mcr10 is given in mg, A in cm2, and μsample in cm2/g. Equations were also found for deviation values of 15 % and 20 %. A standard “average” sample of dust was defined, characterized by fQtz = 5% and μsample = 100 cm2/g, allowing the setting of a critical sample mass valid for the general case.


    X-ray diffraction, analysis, absorption, quartz, silica, air, filter

    Author Information:

    Mecchia, Marco
    INAIL, Italian Workers' Compensation Authority, Risk Assessment and Prevention Technical Advisory Office, Rome,

    Pretorius, Cecilia
    CSIR, Council for Scientific and Industrial Research, Brummeria,

    Stacey, Peter
    HSL, Health and Safety Laboratory, Buxton, Derbyshire,

    Mattenklott, Markus
    IFA, Institute for Occupational Safety and Health of the German Social Accident Insurance, Division “Chemical and Biological Hazards,”, Sankt Augustin,

    Incocciati, Emma
    INAIL, Italian Workers' Compensation Authority, Risk Assessment and Prevention Technical Advisory Office, Rome,

    Committee/Subcommittee: D22.04

    DOI: 10.1520/STP156520130035

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