LIBRARY May 19iS>TATE PLANT BOARD ET-267 United States Department of Agriculture Agricultural Eesearch Administration Bureau of Entomology and Plant (^ae.rantine DIBECTIONS FOR DBTBBMINING PAETICLB SIZE OF AEBOSOLS AND FINE SPEAYS By A. H. Yeomans, Division of Control Investigations The "best method that has "been found for determining the particle size of insecticidal aerosols and fine sprays is to deposit a sample on a glass slide and measure the particles Txnder a high-power microscope. This method shows the complete range of particle sizes involved. Goodhue ejb al . (_2.» ]+) used It for measuring particle sizes of aerosols deposited "by settling. This paper describes la detail the technifae as it is now used. Other, less satisfactory, methods have "been devised, G-ihhs (l) used the rate of fall of the particles, and constructed a special instrument for timing their fall. Goodhue et al, (,2) used a dye in the solution and "by employing a photoelectric photometer determined the amount of deposit per unit of time. Other workers determined "by chemical analysis the rela- tive deposition on wires of different sizes, hut- their results did not clearly show the range In sizes. Instruments that pass a light heaa through an aerosol cloud and measure the polarization of light scattering at right angles, or the number of spectra formed in the scattered light, are s\iita'ble only for measuring particles smaller than 2 microns in diameter. Preparation of Slides Particles of relatively nonvolatile materials can "be measured before they evaporate. To prevent excessive spreading, filming, or coalescence, the slide must "be coated with an oleophohlc substance that will cause the individual droplets to maintain their convexity to some degree* Two of the »ost satisfactory materials for this purpose proved to be a 1-percent alco- holic solution of mannltan monolaurate, and a silicon product marketed under the trade name Dri-f ilm 9987. The slides are first immersed in a cleaning solution, dried, then immersed in the oleophobic coating solution, and re- dried. When dry the slides should be lightly polished with a soft cloth. They may be stored In ordinary slide boxes for several days before they are used. Particles of volatile materials, which evaporate rapidly, cannot be measured directly, but their size can be estimated by measuring the craters they leave at the points of contact on slides coated with magnesium oxide or carbon soot. It is important to apply the right thickness of coating for the range of particle sizes anticipated. The relation between the actual particle diameter and the central circular spot (centrum) of the crater is Illustrated in the excerpt from the report of the University of Chicago Toxicity Laboratory, which is appended, A dye coating of polyvinyl acetate suggested by workers in England for the same purpose proved less satisfactory than the magnesium oxide or carbon soot. - 2 Deposition of Particles on Slides A sample of an aerosol or spray cloud can "be deposited on a slide "by impiE^enent or "by settling. Deposition "hj impingement may "be accomplished "by moving the slide through an aerosol or spray cloud, or hy moving the aerosol or spray cloud past a slide in fixed position. The velocity of movement in either case must he adjusted to the particle-size range expected. The velocity must he increased as the average particle size decreases. Since the deposition is in proportion to the particle size, compensation must he made for this factor. The Cascade and Micro Impactors developed in England, wherein an aerosol cloud is drawn through a series of orifices* to vary the velocity, and impinged on a different slide at each orifice, is useful only with very small particles, mostly out of the range of insecticidal aerosols and sprays. Deposition hy settling should he limited to particle-size ranges helow 20 microns in diameter. It may he accoiaplished hy two means. An aerosol or spray cloud is released in an enclosure and allowed to settle onto slides placed on the floor or hottom. The cloud must he mixed to he uniform, and the aerosol or spray released in such a way as to prevent impingement on the sides or ceiling of the enclosure. The amount released should he small enough to prevent coalescing in the air or too heavy a deposit on the slide. Adequate time mra.st he allowed for all the smaller particles to settle a distance equal to the height of the enclosure. Convection currents should he prevented as much as possihle. A second and more rapid method is to draw the aerosol or spray cloud through an electrical precipitator in which slides have heen placed. When the machine is turned on, all particles in the field are precipitated in a matter of seconds. Deposition hy settling results in a slide representative of the entire range of particle sizes in the ssasple, v/ith each size present in true proportion so that no adjustment or weighting is necessary. Determination of Particle Size After the sample of aerosol or spray has heen deposited on a slide, it is placed under a microscope and the individual particles are measured with an eyepiece micrometer. A mechanical stage on the microscope is necessary. The diameter as measured on the slide is then corrected for the amount of spread that has taken place, and the diameter of the original sphere is determined. At least 200 particles should he measured, according to DalaValle (^) , The more homogeneous the aerosol or spray, the fewer particles need he counted. All particles should he counted as they are seen in the field. An accurate method is to measure all particles from one edge of a slide to the other thr.t pass through the micrometer scale as the slide is moved hy the mechanical stage. Under some conditions of impingement, particles of the smaller size groups are congregated along the margin of the slide. Measurements in such areas should he avoided. 3 - It is sometimes useful to photogrcph the particles or to project them on a screen through a microscope. Better results have "been o"btained, hov:- ever, hy measuring the particles directly as seen in the microscope. It is often more convenient to measure in terms of the divisions of an eyepiece micrometer, and convert these divisions into microns after the median has "been determined. Impinged Slides . — Samples may "be collected hy impingement on a coated slide "by waving the slide through an aerosol or spray cloud, or hy drawing the aerosol or spray past a slide in fixed position, such as in a wind tunnel. The slide should he nearly perpendicular to the movement of the aerosol or spray. In either case the rate of deposition has "been demonstrated to he in ratio to the sq\xare of the diameter. This rate of deposition was suggested "by the Central Aerosol Laboratory of Col-uunhia University and was based on Sell*s law. To compensate for the decrease in the rate of deposition as the particle decreases in size, each diameter is •multiplied by the number of particles of that size, l/ and expressed as the percent of the total of such products. Eepresentative data illustrating this method are given in table 1, Table 1. -Representative coimt of aerosol particles impinged on microscope slides Diameter (scale divisions; Number of paxticles Diameter times, number " Percent of total of column 3 |Accuinalative ' percentage 0.5 2 1.0 26 1.5 33 2.0 82 2.5 3^ 3.0 17 ?-5 k 5 U.5 1 Total 201^ 1 0.2 26 6.3 ^9.5 11.9 i6k 39.5 85 20.5 a 12.3 20 U.5 1.1 Ui5,0 0.2 6.p 18.U 57.9 78.U 90.7 9U.I 98,9 99.9 ^ The diajneter is used in the first power only, since the particles icrpinge in ratio to Ir, but the mass median diameter is computed on the basis of their volume, which is in ratio to d3; therefore, the number of particles is multiplied by D^/D^, or by D, -k- Tlie accurrulatlve percentages from the last coliamn are plotted on the arithradtic probability ac&le In figure 1. The 50-percent point of the line so plotted is taken as the median of the particles as they appear on the slide. In this example the 50~percent point has a value of 2 scale divisions, or J,0 microns, as each division was predetermined to equal I5 microns. A correction factor nnist be determined for each slide. The original spherical droplet as it is impinged on the slide becomes a convex lens, and the extent of its spread from its original shape can be calculated by determining the focal length of the lens so foraed. This method is des- cribed in Port on Beport 2U63 (May 6), a digest of v/hich is appended. In the example cited the correction factor is O.UO; therefore 30 microns X O.UO gives a median particle dijmeter of 12.0 microns. Settled Slides , — The median diameter of the particles collected on a slide by gravitational settling or by electric precipitation is determined by calculating the volume of each particle. The diameter of the particles is measured in microns. The volume is determined by multiplying the cube of the corrected diameter by 77/6, or 0,5236. The volume of the particles of each diameter is expressed as a percentage of the total volume, Eepre- sentative data illustrating this method are given in table 2. The accumu- lative percentages from table 2 are also plotted on the arithmetic proba- bility scale in figure 1, and the median particle diameter is determined to be U,05 microns. Table 2. — A representative cofont of aerosol particles settled oa a microscope slide. (Volxoaa =tr/6 D^ = 0.52361)^) Diameter (macrons) Number of particles Volume (microns3) Percent of total volume Accumulative percentage 1,U 1 1 0,01 0.01 2.1 2.8 55 101 267 1161 ^u 3.31 17. SI li 50 1119 iU,o 31. SI 57 22U 27.7 59.51 k.S 11 677 8.5 6g,0i 5.6 20 I838 23.0 91,01 6.3 k 32h e.e 97.61 7.0 1 ISO 2.3 99.91 Totals 300 797s - 5 - 7 a: 6 UJ 1- UJ 1 > ^ ^ ^ ,^ >^ y ^ Settled sample (Microns) 1^ o UJ 4 _i o £3 x^ X ^ ^ r^ ' ^ iT ^ ^ ^ / Xx .^ ^ •^ -^ ^1 2 2 1 ■^ Impinged sample (Divisions of Scale) x- ' ► 0.1 0.2 0.5 1 2 5 10 20 30 405060 70 80 90 95 VOLUME (PERCENT) 98 99 995 99.9 Figure 1, — Percentage of the total volume of aerosol samples below each stated particle diameter impinged and settled on microscope slides. The mass median diameter is determined from the 50-percent point. The correc- tion factor for spread has been applied to the data for the settled slide (from table 2) but not the data for the impinged slide (table 1), (1) (2) (3) (^) (5) (6) Literature Cited Gibbs, W, E. 192A, Clouds and smokes. 2^0 pp. Philadelphia, Goodhue, Lyle D., and Sullivan, W. N. 19/^3. Making and testing aerosols, Pub. 20: 157-162. Amer. Assoc. Adv. Sci, ., Diamond, P. T., and Riley, R. L, 19i^5. Determination of particle size of liquefied gas aerosols, U. S. Dept. Com., 0. T. S., PB 76015. 19^6. ., and Riley, R. L, Particle-size distribution in liquefied-gas aerosols, Econ. Ent. 39: 223-226. Jour, DalaValle, J. M. 19i;3. MicromeriticB. i^8 pp. New York. May, K. R. 1945. The Cascade Impactor — an apparatus for sampling solid and liquid parti ciaate clouds. Brit. Commonwealth Sci. Off. Porton Rpt. 2^63, 7 pp. - 6 - APPHJDIX TMn-coated Carton Slides (Hxcerpt "by W. E. Schmltz from University of Cliica^o Toxicity La'boratory, Informal Progress Eeport N. S. 2, May 15, 19^) JL method has "been described for the use of carhon-coated slides in estimation of drop size from sprays. In order to calihrate this procedure, which uses a thin film of carhon instead of nagnesiusi oxide, we coated one half of the slide and left the other half plain. The slides were washed carefully, rinsed in ifi e^rosol in water, and dried with a clean towel, A second slide was used to cover one half of the slide to he coated) cjid the two were passed tlirough a small sooty gas flame. When the carbon deposit was thick enough to obscure rather heavy black type it was considered suitable. (Later compfjcison of slides coated by Lt. Wilson at MSI indicate that our slides were somewhat thinner.) Slides prepared in this fashion have a carbon coating on one half and one half is clean. They were eacposed to a graded series of clouds of the non- volatile dibutyl phthalate. The true drop diameters were determined from the lens diameters on the clean surface by determination of the focal length. The sizes of the central circula-r spots produced on the coated surface were also measured. The median diameters obtained on each type of surface were determined^ and the results are given in Table XVII. Table ZVII Slide No, True mass jj^^io ^OP diameter median diameter diameter of ce? 1 hM 0.71 2 I+.63 0.89 3 9.09 O.gl 4 10.5 0.95 5 17.9 0.70 6 18. s 0.71+ 7 20. g 0.67 8 J+9.0 0,67 9 51.5 0.5U There is ajnple evidence of marked variation in the ratio of drop dianeter to centrum diameter of different slides, and the relation of this ratio to increasing diameter (MUD) is not regular. We have found no simple method, applicable to field operations, which will ensure uniformly thick coatings on all slides. Furthermore the best optical definition occurs at a thickness of carbon coating which is a function of the size of the drop. If the coating ia very thin it is possible to detect drops 2 microns la dlaneter but larger drops are poorly defined; conversely, small drops •re invisible on thick coatinga. The diameter and appearajice of the ann\alus surrounding the centrum appear to be very sensitive to variations in thick- ness of coating. - 7 - Procedure for. Measuring Spread Factor of Oil Droplets on Oleopho'bic Slides (A digest of Port on Report ITo. 2}-\-Sj,) 1. Use a compound high-power microscope, 2. Use a, flat mirror, 3. Bemove condenser, k. Use outside light, 5, Focus on particle, and iiieas\ire and record e:::act dianeter. 6, Set reading on fine focus adjustment at zero, 7, I.Ianipulate coarse focus adjustment and mirror until some distant ohject (window frame) is in as sharp a focus as possiMe, using the drop as a lens, S, 2hen focus downward with fine focus adjustment until the drop is in clear focus, 9, 0?he difference between the Ho. 6 reading of the fine focus adjustment (zero) and the No. S reaMng is the focal-length chsjige, 10, Compute fj|_ (focal -length change) 2A (diameter of particle) ib:ajnple: The diameter of a particle covering U divisions in an eyepiece micrometer (l division = 15.^ microns) would lie U X 15, H microns, or 6l,6 microns. T^ith a focal -length change of 206 microns, the spread factor of the .particle vould "be 206/6l,^ or 3.3. TiTith this factor of 3,3, the correction factor ( f »/2A ) for this drop would "be O.UO (see "below for spread-factor ratios), S-ore ad-Fact or Eatios f Correction 2A Factor l.^S O.GO 1.50 0.5s 1.55 0.55 l.SO O.5U 1.65 0.53 1,70 0.52 1.75 0.51 l.SO 0.50 1.95 0.1+9 f« Correction 2A Factor 2.0 OM 2,1 o.hj 2,2 0.1+6 2.3 o.k^ 2.6 0.UI+ 2,65 0.U3 2.g 0.1+2 3.1 0,1+1 3.3 0.1+0 f Correction 2A Factor i+.o 0.375 U.S 0.35 0.34 5.0 5.5 0.33 6.0 0.32 6.S 0.31 7.0 0.30 8.0 0.29 9.0 0.2s 10.0 0,27 UNIVERSITY OF FLORIDA 3 1262 09240 9746 \