XRD analysis of aerosol on membrane filter: use of collodion as fixation solution

It is well known that the collection of health-related aerosols in workplaces is carried out according to sampling conventions defined for approximating the penetration efficiency for the distinct portion of the respiratory system they can reach. Dust exposure assessment requires the collection of samples and the determination of their masses by weighing, but sometimes subsequent analyses are necessary to determine whether a specific hazardous chemical agents is present in the workplace air. If the chemical agent is a crystalline phase, X-ray diffraction (XRD) is often the best technique for quantitative measurement. In this case, samples should be deposited on membrane filters suitable for analysis. A technique developed to avoid dislodging of material from the filter, before and during analysis, consists in fixing the dust particulate to the membrane by using a collodion fixative solution, that leaves a coating of nitrocellulose on the sample. This study provides quantitative assessment of the effect of the nitrocellulose layer on the diffraction peak intensity of a crystalline mineral present in the sample. The experimentation was carried out using 30 samples made of a respirable α-quartz standard collected on silver membrane filters, directly analyzed by XRD before and after fixation with a collodion solution. A reduction of diffraction intensity for the primary peak of quartz was generally found after fixation. Using an average of 3 drops of collodion solution, the signal attenuation for a dust sample made of a pure mineral corresponds to an underestimation of approximately 8 µg of that mineral, with a wide ±12 µg range of variability. If a diffraction signal loss up to 5% may be considered acceptable, the use of a collodion fixation solution should be allowed for samples of dust with mass greater than 380 µg.