Philip W. West
and Sham L. Sachdev Louisiana State University Baton Rouge, Louisiana 70803
I
I
Air Pollution Studies The r i n g oven technique
T h e analysis of airborne particulates may involve both qualitative and quantitative studies of organic as well as inorganic materials. The criteria for met,hods used in studying such mat.erialsarenot much different from those for trace analytical methods in general. Individual samples of airborne particulates collected for air pollution studies may consist of material collected from the atmosphere by means of high volume samplers over extended periods of time. I n other cases, it is desirable t o collect smaller samples during brief sampling periods. This is often done by means of automatic sequential samplers that collect individual spots of particulates on moving filter tapes. I n some cases, electrostatic precipitabors are used to collect the samples and in other cases impact,ors may he used. Regardless of the method of collection, it is usually desirable t o have analytical methods capable of isolating, concentrating, identifying, and determining small amounts of various materials present in complex mixtures of dusts or mist,s. Knowledge of the nature of airborne particulates may be of great assistance in judging their potential hazard to health, their nuisance level, and their corrosiveness. Also, analytical studies of the pollutants help in tracing and controlling sources. There are certain special problems that require consideration; but, generally the ideal method must be reliable, sensitive, simple, fast, and as inexpensive as possible. The ring oven technique meets all of these requirements. Reliability of an analytical method is the most import,ant criterion. All too often analytical procedures are used which simply measure a general property. Such general measurements may be valuable, but t,hey should not he accepted in hlind faith because they may be misleading. The ring oven procedures are usually very selective and the results obtained can be accepted with confidence. The analytical problems of air pollution studies are generally of microchemical nature, therefore, highly sensitive analytical methods are required. The ring oven methods generally apply in microgram to nanogram range. Theprocedures are so simple that no special training is required of the analyst and only 10-30 min are required per analysis. The ring oven is such a simple piece of equipment that it can he shop made or a commercial unit can be obtained for less than $150, or even two aluminum wanhers of different inner diameters when placed on a hot plate can serve as a ring oven.
centration, and analysis of both inorganic and organic airborne pnrticulat,es. The ring oven methods are based on chemical reactions that are carried out on filter paper. Non-volatile solute materials are concentrated in a sharply defined circle by means of a heated ring. A disc of filter paper is placed on the heated surface of the ring oven, the sample is placed a t the center of t,he paper and 510 microdrops of an appropriat,e solvent are then added. The solute sample is transported through the pores of the fiker paper and is deposited in the form of a circle when the carrier solvent is evaporated as it approaches the heated ring zone. Thc sample is thus transported and evenly distributed in the form of a sharply defined ring which permits t,he final identification or determination of as many constituents as may be of interest. For example, the filter paper can be cut into quadrants and any of the quadrants may be used for the determination of iron, a second quadrant may be used for copper, the third for nickel, and the fourth for sulfate. If a larger number of determinations are to be made, it is only necessary to cut the ring into more segments. Experimental
Appa~atus. A small vacuum cleaner unit, an aluminum screen, a funnel, a small anemometer, capillary tubes, lambda pipets (1, 2, and 5-p1 capacity), a hair drier (or a hot air blower), a few petri dishes, some reagent bottles with eyedroppers, and a ring oven are all the equipment that is needed. The ring oven may he commercially obtained (Kational Appliance Co., Portland, Ore.) or two thick aluminum washers of 2.5 and 4.0 cm id., respectively, when placed on a hot plate, can serve as a ring oven. The inner edges of the washers must be sharply defined. The washer with the
Principle
The ring oven technique was first introduced by Weisz in 1954 (1). West, et al. (2-4), pointed out the unique usefulness of the method in the trausfer, con96
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Pisvre 1. plate.
A ring oven constructed f m m two olvminum warhen and o hot
smaller diameter is placed on a hot plate, a disc of filter paper (5.5 cm diam) is ccntercd on it and held in place using the second washer (4 cm i d . ) which is placed on t.he filter paper in such a manner that the center of the filter paper coincides with the center of the two washers. The asscmbly, as shown in Figure 1, works as a ring oven. Standard Solutions. Standard solutions(100 pg/ml) of ions such as Fe3+, Cu'+, Ni2+, and S O P , etc. should be prepared in stock form by dissolving either pure metal in acid and then diluting with distilled water or by dissolving appropriate amounts of analytical grade salt. The stock solutions should be stored in polyethylene or Vycor bottles and final standards should he made by appropriate dilutions of the stock solutions. Reagents. Most of the reagents used are organic reagents. A number of these reagents are very selective and many others can be made selective or ever1 specific by adjusting the conditions of the reaction. Almost all of the reagents used in the detection and estimation of the pollutants by ring oven technique are precipitants that produce highly colored insoluble products that are precipitated in the pores of the filter paper and remain fixcd in position throughout any subsequent operations. Ordinarily, it is only necessary to add thc reagent to the ring zone and this is done either by spraying the reagent, adding a drop of solution, or by dipping the ring sector in the reagent solution. The following reagents are required to demonstrate the examples described later: potassium ferrocyanide (I%), malonic acid (20%), dimethylglyoxime (1% in ethanol), dithiooxamide (saturated solution in ethanol), barium chloride (27, in 2% solution of Iii\ln04), and oxalic acid (27,). Techniques
Sampling. Airborne particulates or aerosols, which consist of either liquids or solid particles ranging in diameter from 0.01 p or less up to about 100 p, can he collected by any of the standard methods used such as dust tapes, electrostat,ic precipitators, high volume samplers, membrane filters, or dust fall jars. The use of the dust tapes is particularly attractive because usuwlly all operations can be performed directly on the sample spot on the collecting tape itself. A simple unit for collecting airborne particulates can be very easily assembled by the use of a small vacuum cleaner and a small anemometer which measures the velocity of air passing through the sampling unit. An analytical grade filter paper (Whatman 41 or 42) i s placed on an aluminum screcn which is placed on a small glass funnel (diameter 2.0 cm) t,he stem of which is connected to the hose of the vacuum cleaner by means of a rubber stopper. The components are assembled as shown in Figure 2. The assembly is placed in air from which particulates are to be collected. The ANEMOMETER
ALUMINUM
TO VACUUM
vacuum cleaner is switched on a t a known time and the velocity of the air passing through the filtcr paper is noted from the anemometer. After sufficient dust is deposited on the filter paper, which may take anywhere from a few minutes t o an hour, depending upon the quality of the air being sampled, the vacuum cleaner is switched off. The sampling time is noted and the volume of air sampled is calculated as follows V
where V is the volume of air sampled (m3),a is the area of the sample spot (m2),u is the velocity of the air passing through the filter paper (m/min), and t is the time for sampling (min). The dust samples are analyzed and the concentration of the pollutant is expressed as pg/cuhic meter of air. Analytzcal P~ocedu~e.The dust spot (as obtained by the sampling procedure described) should be smaller in diametcr than the inner edge of the heating ring (Fig. 2). Place the filter paper ou the heating ring in such a manner that the center of the dust spot is a t the center of the heating ring. Adjust the temperature of the heated ring surface between 110-130°C by controlling the temperature of the hotplate. If the dust spot is obtained by some other sampling procedure and the spot is larger than the inner diameter of the heating ring, punch out a circular disc of the dust spot by means of a cork borer of size 10 or 12. Place this little disc on a disc of filter paper so that its center coincides with that of the filter paper. Place three tiny drops of Duco cement along the edge of the small sample disc and let dry while the papers are pressed together so as to insure that the sample disc will be held in proper position. Now, place the filter paper with the sample disc up on the heating ring such that the center of the spot is a t the center of the heating ring. Place the support ringonthe filter paper and wash the dust spot by slowly adding 3 W HC1 a t thc center of the spot. Only 2-3 p1 should be added a t a time by means of a capillary tube. Tbc acid will slowly wash out inorganic (and some organic) materials from the dust spot and as it reaches the inner edge of the heating ring, the solvents evaporate, depositing the nonvolatile solutes along the circular edgc of the heating ring. About 20 washings (each of 2-3 p1) should be carried out. The solvent should he added in sufficient amounts and a t proper intervals to insure that solute is transported completely to the heating zone. At the same time it is important that the solSTEP I Sompie
, Figure 2.
Air ramyjling device.
FUNNEL
2
STEP 3 Tests
8
Ring
Oven
I
)
Ring Zone Dust s p o t (hot)
FILTER PAPER
STEP Transfer
Concentrotion
CLEANER A
= a X v X 1
1(F&;2i ;;:;;;
tes
Wash Solutions (5-10 Successive Micro Drops) Figure 3. Steps in the preparation and analysis of ring oven technique.
on air sample by the
Volume 46, Number 2, February 1969
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vent not be added in too great an amount or too rapidly because this would cause flooding and overrunning of the heated area. The solutions must approach the heated surface a t such a rate that the solvent has time to evaporate and deposit the solute as a uniformly and sharply defined ring. Usually an hour of practice is sufficient to consistently obtain good rings. After the solute has been washed to the ring zone, remove the filter paper, dry it by holding in a gentle stream of hot air and cut it into quadrants. Each of these quadrants can be used for detection and determination of different ions. Three steps involved in the procedure, as described above, are shown in Figure 3. A few simple examples are described below. Many more ions can be determined in a similar fashion (5). Test for Iron. Add one drop of K6Fe(CN)esol~itionalong the ring zone of a sector. A blue line denotes Fe. Limit Identification 0.015 pg. Test for Copper. Add a drop of malonio acid followed by a drop of dithiooxmide solution to the ring none on a second sector. A black line indicates Cu. Limit Identificat,ion0.015 pg. Test for Nickel. Expose a sector to NHa fnmes, then add a drop of dimethylglyoxime solution. A red line indicates Ni. Limit Identification 0.075 fig. Test for Sulfate. Add a.drop of BaCL solnt,ion (in 2% KMnO.), dry and then wash in 27, axalic acid until brown stain disappears. A pink line denotes SobP-. Limit Identification 0.1 pg.
Estimation. For estimation of the amount of pollutant present in the ring sector the unknown ring is matched by visual means with a series of standard rings. The standard series can he easily prepared by adding varying amounts of standard solution containing 0.02 to 1.0 pg of the respective ion to a number of filter papers. The standards are washed to the ring zone and the rings are developed according to the procedures described for Fe3+, CuZ+, Nix+, and S O P . The intensity of the ring colors is directly proportional to the amount of respective ion present. By matching the color of the unknown ring with that of the standard scale, one can determine the amount of the pollutant with reasonable accuracy (80-90%). Greater accuracy can he obtained by the technique of Iinodel and Weisz
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(6). I n this approach, three different concentrations of t,he unknown are treated by the ring oven technique and the three rings so produced are matched against standard rings. Using the statistical approach, accuracies of 92-95% can he obtained which is quite remarkable for the determination of microgram to nanogram amounts of material. Summary
The ring oven methods are based on chemical reactions that are carried out on filter paper. The technique is useful for separation, concentration, detection, and determination of submicrogram quantities of various materials. The method is particularly attractive for studies of airborne particulates. A very simple and inexpensive method is described for the collection of airborne particulates and a method based on the ring oven technique is described for the identification and determination of the airborne particulates. The equipment used is inexpensive and can be assembled with simple household appliances. The ring oven methods are usually selective or even specific, examples are listed. The method applies to the microgram to nanogram range. Acknowledgment
This investigation was supported by U.S. Public Health Service Rescarch Grant No. AP 00117, National Center for Air Pollution Control, Bureau of Disease Prevention and Environmental Cont,rol. Literature Cited (1) Wersz, IT., Xikrochim. Acta, 140 (1954). (2) WIST, P. W., WKISZ,H., GAISKK JR., G. C., AND LYLI:S,G., Anal. Chem., 32, 1943 (1960). (3) WEST,P.W., J . Air Pollution Conlr. Assoc., 16, 601 (1966). (4) WEST,P. W., A N D ORDOVFZA, F., Anal. Chim. Aela, 30, 227 ,l"C*, I'VU-,.
( 5 ) WEST, P. W., "Air Pollution" (Editor: Stern, A. C.) Academic Press, New York, 1967. (6) KNODEL, W., .4ND WEISZ,H., Mik~oehim.Ada, 417 (1957).
