INDUSTRIAL HYGIENE >lICROSCOPY 



molecular filters in preference to the im- 

 pinger method for collection of dust. Molec- 

 ular filters are cellulose ester gels resembling 

 ordinary filter paper in appearance but have 

 a pore size such that they can retain dust 

 particles of small particle size. One of the 

 main advantages of their use, as pointed out 

 by First and Silverman (5), is the fact that 

 there is negligible penetration of the filter. 

 Dust particles are deposited on tin surface 

 in the same state in which they existed when 

 suspended in air. 



In using these filters, the usual procedure 

 is to place the filter in a holder designed for 

 that purpose and collect the dust in the air 

 at 0.1 to 0.5 cubic foot per minute using an 

 elect ricall}^ driven pump, Freon-powered 

 equipment, or hand pump. After collection, 

 the filter or a section of it is placed dust side 

 down on a clean slide or on the ruling of a 

 counting chamber such as a hemacytometer. 

 Since the filter is opaque and transmitted 

 light using the light-field microscope has been 

 the usual method of counting, it is necessary 

 to render the filter transparent by applica- 

 tion of a drop or two of liquid equal to the 

 refractive index of the filter. 



A major objection to the above procedure, 

 as pointed out by Drinker and Hatch (6), 

 and by Paulus, Talvitie, Fraser and Keenan 

 (7), is the fact that toxic dust particles such 

 as quartz and diatomite close to the index 

 of the clearing liquid will not all be included 

 in the count, especially if of small particle 

 size. A solution to the problem, as in the 

 case of the impinger method of collection, is 

 the use of the phase microscope. The superi- 

 ority of phase illumination as compared to 

 the usual light-field method is illustrated in 

 Fig. 7 of diatomaceous earth particles col- 

 lected on a molecular filter (Millipore type 

 AA). The filter was rendered transparent by 

 appHcation of a drop of liquid of 1.507 at 

 25°C and covering with a cover-glass. Em- 

 ploying the phase microscope, the liquid 

 used should be equal or very close to the 

 index of the filter in the third decimal place 



Fig. 6A. Magnesium fluoride. Light-field illu- 

 mination. 



Fig. 6B. Exactly the same area as figure 6A. 

 Phase illumination. 



in order that the texture of the filter will 

 not be revealed. Fig. 8 is a photomicrograph 

 of diatomaceous earth collected on a Milli- 

 pore Filter, tj^pe AA using a liquid of index 

 1.515 as the clearing liquid such as has been 

 suggested for the usual light-field method. 

 Although satisfactory for the light-field 

 technique of counting, it cannot be used 

 with phase illumination. As noted in the 

 photomicrograph, it is difficult or impossible 

 to differentiate small diatom particles from 

 the texture of the filter. 



The liquid used for clearing must have no 

 solvent effect on the filter. IMolecular filters 

 are solubk^ in ('(Mtaiii ketones, esters and 

 methanol. A mixtiu'e of light mineral oil with 

 Ai'oclor 1242 adjusted to an index of 1.507 

 on a refractometer at 25°C has proved 



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