Development of a portable gravimetric dust spectrometer. Final report on CEC Contract 7256-21/015/08

Future research into dust related health effects in coal mines requires more complex measurements of the physical properties of airborne dusts than heretofore. We have defined the ‘ideal’ sampling instrument for this purpose as one which selects and allows determination of the mass distribution as a function of particle aerodynamic diameter of the inhalable fraction of the airborne dust; that is, a gravimetric inhalable dust spectrometer. In addition, it is desirable that information about particle geometric dimensions and composition can also be obtained using the same instrument.A comprehensive review of the prior art in the field of aerosol spectrometry led to the conclusion that some form of cascade impactor would be the most suitable type of device for the proposed range of applications in the coal mining industry. An instrument was designed and built to operate at the sampling flow rate of 10 1 min , comprising a) an inlet system capable of selecting the inhalable fraction (to a first approximation) of airborne dust with particle aerodynamic diameter up to 15 nm (larger particles not being of interest in relation to diseases of the lower respiratory tract), consisting of an ORB entry and an impactor; b) a four-stage cascade impactor, each stage consisting of a long rectangular air jet impinging onto a cylindrical collecting drum; and c) a pumping system capable of delivering the flow rate in question and suitable for underground use, consisting of the fan and battery pack arrangement taken from the German TBP50 sampler. Special features include a) smoothly-profiled internal geometry in order that interstage losses of particles to the walls are minimised; b) slow rotation of the collecting drums (driven by clockwork) so that information about changes in the dust properties with time can be assessed; and c) collection of the dust on polycarbonate membrane films (wrapped around each collecting drum) so that the deposits can subsequently be assessed for mass by means of a (3 particle attenuation system which was specially developed for this purpose. The complete system weighs approximately 8 kg, and can be split into two parts for convenience of carrying by one person.Complete calibration of the new instrument, including experimental determination of the impactor stage collection and the inter stage wall loss characteristics, was carried out in the laboratory.Field trials to demonstrate the practical potential of the new instrument were carried out at coal mines in Scotland and in Wales. The mass data taken from the dust deposits on each of the four impactor stages were analysed to give an approximation to the true aerodynamic size distributions using the mathematical method originally proposed by PICKNETT (1972). The speed of rotation of the impactor drums and the resolution of the mass measurement system were such that aerodynamic size distributions could be obtained for inhalable dust in successive -g-hour intervals within a working shift. The potential for relating changes in aerodynamic size distribution to various mining events was demonstrated. It was also demonstrated how the airborne mass concentrations of given biologically-relevant dust subtractions (e.g. tracheobronchial or respirable) of the inhaled fraction could be obtained. Finally it was demonstrated how the relationship between particle aerodynamic and geometric sizes of inhalable dust particles might be investigated. Although the potential for investigating the relationship between particle aerodynamic size and composition was not specifically demonstrated quantitatively, visual inspection of the dust deposits at the four impactor stages suggested that this is feasible.It is believed that the new instrument possibly in more refined versions will be a valuable tool in future dust related epidemiological research. “”