INDUSTRIAL HYGIENE MICROSCOPY 



accurate information by use of a calibrated 

 ej^epiece micrometer. As in counting, the 

 use of phase microscopy should increase the 

 accuracy in determination of particle size. 

 According to Richards (8), more precise 

 measurements can be made with phase mi- 

 croscopy owing to the sharp edges of speci- 

 mens free from indefinite diffraction pat- 

 terns. Also, because of their definite edges, 

 one can more readily determine whether a 

 particle is single or aggregate. 



A limitation of the usual particle size dis- 

 tribution curve obtained by use of the opti- 

 cal microscope, is the fact that particle size 

 below its resolution is not included. Also the 

 fact that little or no information is usually 

 obtained as to the size of various constituents 

 of a mixture. For example, particle size infor- 

 mation on dust consisting mainly of clays 

 and quartz sand in a ceramic plant may 

 indicate that a large percentage of the dust 

 is below 3 microns in size. It is important 

 to know whether the small particle size dust 

 is mainly quartz or clay. If present as quartz, 

 the degree of hazard is considerably greater 

 than in the case of small particle size clay. 



Simultaneous measurement of particle size 

 and identification of the particles measured 

 can be made in many instances by use of the 

 dispersion staining dark-field microscope. 

 Unfortunately, samples collected on mo- 

 lecular filters cannot be used for this deter- 

 mination due to the fact that the index liquid 

 employed must be selected for the purpose 

 of rendering the filter transparent rather 

 than for the purpose of identification of the 

 particulate material collected. An exception 

 is the rare case when the dust on the filter 

 has an index equal to or very close to the 

 index of the clearing liquid. Samples col- 

 lected by other methods can be measured 



by use of the dispersion staining dark-field 

 microscope. For example, in the case of dust 

 from the cerami(^ plant containing primarily 

 quartz sand and clay we are mainly inter- 

 ested in the size of the quartz particles. If 

 examined in styi'ene, the quartz particles 

 measured will appear blue with a trace of 

 red changing in color on rotation of the cap 

 analyzer to blue with a large amount of red 

 or homogeneously red dependent on their 

 particle size. Clay particles such as kaolinite 

 present in the preparation, having different 

 indices, appear in other colors of the spec- 

 trum or white and are thus readily distin- 

 guished from quartz. 



REFERENCES 



1. Crossmon, G. C, "New Developments in Dis- 



persion Staining Microscopy as Applied to 

 Industrial Hygiene", Am. Ind. Hyg. Assoc. 

 Quart., 18, 341 (1957). 



2. Chamot, E. M. and Mason, C. W., "Handbook 



of Chemical Microscopy", 3rd Ed., John 

 Wiley & Sons, Inc., New York, (1958). 



3. Ross, H. L. AND Sehl, F. W., "Determination 



of Free Silica-Modified Petrographic Immer- 

 sion Method", Ind. Eng. Chem., Anal. Ed., 

 1, 30 (1935). 



4. Crossmon, G. C, "Counting of Dust Particles 



b}^ Phase Microscopy", A. M. A. Arch. Ind. 

 Hyg. Occup. Med., 6, 416 (1952). 



5. First, M. W. and Silverman, L., "Air Sam- 



pling with Membrane Filters", A. M. A. 

 Arch. Ind. Hyg. Occup Med., 7, 1 (1953). 



6. Drinker, P. and Hatch, T., "Industrial 



Dust", 2nd Ed., McGraw-Hill Book Co., New 

 York, N. Y. (1954). 



7. Paulus, H. J., Talvitie, N. A., Eraser, D. A., 



AND Keenan, R. G., "Use of Membrane Fil- 

 ters in Air Sampling," Am. Ind. Hyg. Assoc. 

 Quart., 18, 267 (1957). 



8. Richards, O., "Photomicrography with the 



Phase Microscope," Photographic Soc. Amer- 

 ica J. Sec. B., 16, 94 (1950). 



Germain Crossmon 



410 



