SAMPLING AND ANALYTIC METHODS FOR ZINC CADMIUM SULFIDE
THE CONCENTRATIONS OF airborne fluorescent particles of ZnCdS in the Army atmospheric-dispersion studies were measured with impingement and filtration methods. Two of those methods are thoroughly described by Leighton and others (1965), but the methods used at specific locations are not described in detail in Army risk-assessment documents.
Leighton and others (1965) describes the use of membrane filters for collection of ZnCdS samples in atmospheric-dispersion experiments. The cellulose acetate-nitrate membrane filters are mounted in an open-faced holder with a cowl. The filter is dyed to provide a dark background for counting particles under ultraviolet illumination, and most of the collected particles are deposited on the smooth upper surface of the filter. Both 25-mm- and 50-mm-diameter filters are used; they have deposition areas of 2 and 13 cm2, respectively. The flow rate is typically about 5 L/min per square centimeter of deposition area. Collec-
tion efficiency of the filters is virtually 100%. (Collection efficiency is the number of particles deposited on the collection medium from an air sample divided by the number of particles in an equal volume of ambient air at the sample location averaged over the same time that the sample is taken.)
The Rotorod consists of 2 thin metal rods coated with silicone grease that are attached to the shaft of a small electric motor by a cross arm (Leighton and others 1965). As the rods move through the air, particles touch their surfaces and are retained by the silicone grease. The sampler described by Leighton and others (1965) had a pair of 0.38 x 60-mm collecting surfaces, a rotation radius of 60 mm, and a rotation speed of 2,400 rpm.
The sampling rate of the Rotorod sampler is the volume of air swept out by the sampling rods per unit time. Thus, it is equal to the rod cross-sectional area times its tangential speed—41 L/min for the sampler described by Leighton and others. Correction of Rotorod particle counts for collection efficiency is essential because the collection efficiency of this sampler is generally lower than that of filter sampling and is a strong function of particle size. The collection efficiency can be determined by sampling ZnCdS aerosols with both a Rotorod sampler and a sampler with known collection efficiency. The Rotorod collection efficiency is the uncorrected concentration measured with the Rotorod divided by the efficiency-corrected concentration measured by the other method. For several lots of ZnCdS 2266, collection efficiency ranged from 28% for a lot with a mass median diameter (MMD) of 1.8 µm and 7.9 x 1010 particles per gram to 73% for a lot with an MMD of 3.1 µm and 1.6 x 1010 particles per gram (Leighton and others 1965).
Gelman paper tape samplers were used for some of the Army experiments. The air sample is drawn through a paper filter tape. Periodically, the tape advances, collecting a series of sequential particle samples. Particles in each sampling ''spot'' on the tape are counted.
Collection media are illuminated with ultraviolet light, usually from a mercury-arc lamp, providing high-intensity light at about 366-nm wavelength. For counting, a microscope with 100 x magnification is usually used. Counting fields are established with an eyepiece reticule, and the number of particles is counted separately for each field.
The size of the smallest particle discernible with this method is a function of the illumination, the background characteristics, the visual acuity of the counter, and the microscope configuration. A typical lower limit of particle diameter detectable by the ultraviolet-illumination and fluorescent-counting method described by Leighton and others (1965) is 0.5 µm. Thus, no particles smaller than that size were counted in these experiments. But particles smaller than 0.5 µm make up only a small fraction of the ZnCdS mass, and on the basis of the particle size distribution of the ZnCdS used at Corpus Christi (Smith and Wolf 1963), this should not create more than a 1% negative bias in the estimation of particle concentration.
Leighton, P.A., W.A. Perkins, S.W. Grinnell, and F.X. Webster. 1965. The fluorescent particle atmospheric tracer. J. Appl. Meteorol. 4:334-348.
Smith, T.B., and M.A. Wolf. 1963. Vertical Diffusion from an Elevated Line Source over a Variety of Terrains. Part A. Final Report. Contract DA-42-007-CML-545. Prepared by Meteorology Research, 2420 North