Extension of the Gelatin Method for the Detection of Micron-Sized Particles JAMES P. LODGE, JR., and HANNY M. FANZOI Department of Meteorology, University of Chicago, Chicago,
Products, 3100 South ashland Ave., Chicago, Ill. h mixture of 2 grams of gelatin and 5 ml. of’ water was allowed to stand for I d minutes, then heated in an oven at 85” until the gelatin had di*solved. Then 5 ml. of glycerol were added, and the mixture was heated for an additional 15 minutes. Finally 1 ml. of a saturated solution of barium chloride in 20% hydrochloric acid was st’irred in, and the gelatin was returned to t’he oven for 10 minutei longer. Slides were prepared as for detection of calcium.
Application of the gelatin technique has been extended to the detection of calcium, nitrate, and sulfate ions in air-borne fine particles.
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I.\IULTANEOUSLY with the work reported by Pidgeon ( 2 ) ,a limited amount of work was done using similar techniques to tirvc.lop tests for other ions in air-borne fine particles. The critrri:i of L: successful test ryere that every particle gave a reaction spot, and that other ions either gave no test or gave a spot readily distinguished from that of the test substance.
The sulfate reaction gives a rat,her grainy white circular spot which vanishes under crossed polaroids. The individual crystals are roughly the same size as the calcium ammonium ferrocyanide crystals in the preceding test. Several hours’ standing is necessary before all the particles in a sample react to give suctli spots. S o naturally occurring substance int,erferes.
EXPERIMENTAL PROCEDURES
Calcium Ion. -4 mixture of 2.5 grams of purified pigskin gelatin (Eastman Kodak Co., Rochester, S . Y.)>8 ml. of glycerol, and 5 ml. of 30% aqueous ammonium ferrocyanide ( 1 ) was heated in an oven at 85” C. until the gelatin was dissolved. Slides were prepared as described by Pidgeon ( 2 ) .
Nitrate Ion. Purified pigskin gelatin was prepared in the s a i i i ~ manner as for sulfate detection. The “sensitizing agent” was 1 ml. of a 4070 solution of Sitron in 1Oyo acetic acid. The characteristic reaction gives circular groups of fine ratliates, the individual q - s t a l s of which are birefringent. Thrw appear to be no interferences.
The calcium reaction sp0t.s consist of circular areas filled with tiny discrete crystals. These spots differ from the chloride “halos” pictured by Pidgeon ( 2 ) in that the individual crystals are larger, and there is no center “embryo.” It is doubtful if other substances giving insoluble or highly colored ferrocyanides clsist in soluble form except as localized industrial dusts. However, several were checked and all were found to differ markedly from the calcium spots-for example, ferric ion gives transparent areas of Prussian blue, and silver, copper, and zinc form areas of 1:rrge crystals only after several days’ standing. Sulfate Ion. Because commercial purified pigskin gelatin reacted strong1 with sulfate reagents, i t was necessary to use gelatin in w h i c i the final pH adjustment had been made with Iiydrochloric, rather than sulfuric acid. A suitable gelatin is UCop-Co 12XPF, obtainable from United Chemical and Organic
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ACKNOWLEDGMENT
The authors wish to thank Anthony Trozzolo, who did some of the early portions of the work reported here. LITERATURE CITED (1) Feigl, F., “Qualitative Analysis by Spot Tests,” 3rd English ed.. p. 169, New York, Elsevier Publishing Co., 1947. (2) Pidgeon, F. D., A x ~ L .CHEM.,26, 1832 (1954).
RECEIVED for review March 12, 1954. Arcepted April 8, 1954. Research sponsored by the Geophysics Research Directorate of the A i r Force Cambridge Research Center, Air Research and Development Command, under Contracts fiF 33(038)-23017 and .A.F lR;GO4)-G18.
Analysis of Micron-Sized Particles JAMES P. LODGE Cloud Physics Project, University of Chicago, Chicago 37, 111.
The commercial filter material Jlillipore w-as utilized as a medium for sampling and analyzing particles in the micron-size range. Studies of ammonium, calcium, and magnesium ions, halides, sulfates, and nitrates indicate that the technique is suited f o r identification of atmospheric articulate matter.
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S MANY fields of research, attention is becoming centcretl
upon the identification of airborne particles in the micronsize range. The experimental met~eorologistis especially concerned with those soluble particles which may play a role in condensation and precipitation phenomena in the atmosphere. Rlicromanipulative methods have been developed to a high degree of perfection ( 2 ) but inevitably require much skill and patience in their use. A highly simplified method of spot-testing in this size range was developed by Seely (6), u-ho used the method of impaction on gelatin previously impregnated with the specific reagent for some one test.
The author has made use of the recently perfected Millipore filter material as the medium both for sampling and for carrying out the tests. Following the philosophy of Seely’s method, the entire filter is treated with one specific. reagent, and the typical reaction is observed wherever it occurs. Millipore (1) is a commercial product (available from the Love11 Chemical Co., FVatertown, Mass.) derived from the “membrane filters” of Sanarelli, Zsigmondy, and others (6). As available, it retains particles down to ca. 0.2 micron, quantitatively. I t provides a remarkably uniform surface for the reactions, and becomes transparent when treated with immersion oil, which permits ready microscopic examination of the reaction sites. The individual tests, as developed, have been checked to assure that a reaction site is obtained from every particle. In order to do this, individual filters, after sampling the test material, were cut in two; half was treated chemically and the other half n a s examined directly. In all cases the number of reaction sites 1829
. ANALYTICAL CHEMISTRY
1830 was equal, within statistical error, to the number of particles in
an equal area of the untreated filter. GENERAL METHOD
I n these initial studies, the Millipare was held in the universal aerosol-type filter holder supplied by the manufacturer. Air was drawn through the filter by a small vacuum pump. The salts were sprayed as 10 to 20% solutions from a nebulizer into 8. jet of dry air directed at the filter, and about 5 feet from it. The particles arriving a t the filter were assumed to be dry, since, in a number of tests, the spraying of dry powders gave the same re-" .,1+.. "."I.
The filter was then removed from the holder with forceps and either floated OQ the reagent solution, or placed on a pad of blotting paper saturated with the reagent. The proper "development" times ranged from a few seconds to 20 minutes. A Petri dish of 60-mm. diameter is a,convenient container for the reagent. For solutions which must be more carefully protected from the air-cg., Nessler's reagent-low-form weighing bottles of 55-mm. internal diameter are useful. Following development, the filter is in most cases washed by agitating in successive 10-ml. portions of distilled water in B weighing bottle or Petri dish. Depending on the type of reagent and the solubility of the product, from one to three washings are used. In some case8 the excess reagent is removed by placing the filter on successive pieces of blotting paper saturated with distilled water. The filter is finally dried in vacuo on the blotter supplied with it in D - ~ - 'x-I.
F i g u r e 2. Calcium Chloride w i t h A m m o n i u m Ferrocyanide Crossed Polsmids
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cedure for magnesium. AB only one test may be run on a @ample, the test must be specific.
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The individual tests described here must, of course, be applied with some caution. They are intended primarily for the identification of particles of natural, rather than industrial, origin. Hence, in some cases no consideration has been given t o interfering substances introduced into the air by industrial processes. This, however, in no way vitiates the basic method. It is probable that in most special cases such interferences can he eliminated by suitable modifications.
F i g u r e 1. Reaction of Large A m m o n i u m Sulfate Particle w i t h Nessler's Reagent Crossed Polaroids The line on this and all other photomicrographsis in0 lone
For examination, the filter, or a convenient portion of it, is placed on a microscope slide coated with immersion oil, covered with a cover glass and examined under drtrk-field illumination for the characteristic reaction sites. I n some cmes, viewing by polarized light will improve the contrast. Where the particle concentration is high, grid-marked Millipore is a convenience for counting aliquots of the full sample. Portions of the filter not used in microscopic examination may be saved dry far future reference, or mounted permanently with Canada. balsam. Disposable plastic Petri dishes with tight-fitting covers are available especially for use with Millipore. and are convenient for drying and dry storage of the filters. The choice of specific or selective r a g e n t s must be governed hy the following considerations: The reagent must not attack Millipore during the normal time of treatment. This excludes only a.few classes of substances, most of which are unimportant in this type of test--such 8.6 ketones, esters, concentrated acids and bases, and a few mixtures such as ethanolic sodium hydroxide. The reaction product must precipitate rapidly. If the reaction is too slow, the substance being tested may leach off the filter and into the reagent solution, m that no test can be obtained. From similar considerations, the product must be given by treatment with a single reagent. For example, preliminary treatment with acid is excluded, since the resulting soluble salts will be extracted into the solution. An exception is treatment with gases such as hydrogen chloride, as in the procedure for aeidsoluble calcium salts. However, the work to dabe has been limited largely to soluble materials.
F i g u r e 3. Chalk Particles T r e a t e d w i t h Hydrogen Chloride Gas, Then Developed w i t h A m m o n i u m Ferrooyanide Crossed Polamids
Implicit in this work is one Itmumption which the author hope6 in the near future either t o substantiate or to disprove: that the majority of normal individual atmospheric particles are relatively pure chemical species. Recent work by Facy (S) appears to support this view. I n other words, i t is assumed that, in general, interference here means similarity between the reaction products of two different substances, rather than the masking of one p r e eipitste by another in the mme area. SPECIFIC TESTS
The Millipore technique has been applied successfully t o tests for the following ions in the author's laboratory. They are given here in brief form. Ammonium Ion. A blotter is saturated with standard Nessler's solution and the filter is placed on it. Development time is ca. 1 minute, and three washings are necessary to remove excess reagent. The reaction sites are circular groups of orange to pale
V O L U M E 26, N O . 11, N O V E M B E R 1 9 5 4 yellow crystals (Figure 1). There are no important interferences. However, the filter must he handled rapidly while the reagent is on it, a8 it picks up ammonia. from the air very readily. The test fades fairly rapidly, and should be examined within a few hours. Calcium Ion. Ammonium ferrocyanide ( 4 ) in 30% aqueous solution has been found the most satisfactory reagent. Develop ment for 15 minutes is recommended. Two washings are sufficient; it is best to wash by placing the filter on thoroughly wet blotting paper. Calcium salts give a rather diffuse circular area of h e crystals (Figure 2) which are more sharply visible under polarized light. Other ions also give precipitates, hut none is likely to he confused with calcium. For example, ferric salts give rather transparent blue areas; zinc salts give small, very definite circles showing slight granular structure; copper is similar to einc, but has s definite red-brown color; silver gives slight, irregular areas, lesa granular than those of zinc. Furthermore, these all vanish under
1831 chloride are shown in Figure 4 . Viewing by polarized light increases the contrast markedly. The principal difficulty is the mutual interference of the different halides. Sulfates. Lead nitrate in warm saturated solution gives, after development for 20 minutes and two washings, characteristic white areas (Figure 5). The reaction is specific as far as normal atmospheric constituents are concerned.
Sodium dihydroxytaitrate en;& may be used a8 a calcium reagent, hut the author has found it Iem generally applicable, especially in the above method far insoluble salts. F i g u r e 6 . N i t r a t e Reaction w i t h Nitron Cmamd Polemids
Nitrates. The reagent is a 4'34 solution of nitron in 10% acetic acid. The development time is 20 minutes; the filter is then dried without washing. The stellar radiates are best seen by Polarieed light; they exhibit birefringence (Figure 6). The res* tion is specific. CONCLUSIONS AND FUTURE WORK
Figure 4.
S o d i u m Chloride Particles w i t h Mercurous Fluosilicate Crossed Polsroids
M~~~~~~~ Ion. Treatment for 15 minutes with a o,3% tion of nitrobenaeneazo-1-naphthol in 7.5% aqueous sodium hydroxic?i(4) followed by three washings, gives blue circles, best observed on the still moist filter by reflected light. These tend t o fade ou standing, and should be examined immediately. This is probably the least technique to date, but exemplifies the many possible modificatiana of the method. The list of posaible interferences has not been fully checked, but i t is dodhted that any would exist as soluble salts in free air. Halides. The reagent is a 5 % aqueous solution of mercurous fluosilicate oontaining a few drops of 30% fluosilicic acid (6). The development time is 3 to 5 minutes, and two washings are sufficient. The reagent has the advantage of forming typical crystals of sodium fluosilicate with sodium salts, and hence constitutes a double-ended test. Tvpical rertetions with sodium
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The use of Millipore affords a general method of extending spo+test techniques to the range from lo-' to below 10-18 gram, a range especially suited to the identification of atmospheric particulate matter. Furthermore, the filtration of a known volume of air provides information as to the Concentration of the species tested for in the air. Seely (6) found that in the reaction of halide particles with gel% tin sensitiaed with mercurous fluosilicate, the sise of the "halo" was approximately nine times that of the original particle. Hence i t was possible to determine not only particle concentration, but also the particle size spectrum. It is prohahle that for a given reagent concentration a similar relationship will hold for Millip o w and work is in progress to determine this growth factor. Another obvious step is the routine application of this method t o actual air samples; this program is already under way. ACKNOWLEDGMENT
The Of TrOzzO1O and Hanny Fanzoi Of this l a h o r a t w in the PrePm-atian of special reagent8 and in checking the results of the technique, of Roscoe R . Rraham, Jr., of this ratory, and John H. Bush, Lovell Chemical Co., in many disons, is gratefully acknowledged. LITERATURE (XTED
h s h , J. H., Sci. Monthlv, 75, 303 (1952). hdle. R. D.. ANAL. CXEM..23. 196 1196ll i a o y , ' ~ .J, . Bci. ~ e t A z . 3, . 6 z , s 6 (1951j: %I, F.. "Spot Tests." p. 16'3, New York, Elsevier Publishing I
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Go., 1947. ,,
h e t u , A., Fist final report 1312, Joint Intelligence Objeotives Agenoy. U. S. Department of Commerce. Washington, D. C..
1947. (6) Seely. B. K., ANAL.CHEM.,24, 576 (1952).
Figure 5. Reaction of A m m o n i u m Sulfate Particles w i t h Lead Nitrate
R e o ~ l v mfor review Ootober 21,'1953. Accepted January 22, 1954. Research sponsored by the Geophysics Rese&rch Directorate of the Air Force Cambridge Research Center, Air Research and Development Command. under Contrhct A F 19(604j-618 and Contract AF 33(0381-25 913. Presented befoie the Division of Analytical Chemistiy &tthe 124th Meeting of the A M E ~ C ACHEMICAL N SOC~ETY, Chicago, Ill. ',
