10-72 Chicago sample, the 5-28-70 London sample, and the 12-61-71 Ankara sample. In most cases, however, a minimum C:H molar ratio was observed a t about 0.5-rm diameter, a level of apparent maximum saturation of carbon compounds. Again, more comprehensive data are needed to clarify the C:H association.
CONCLUSIONS Application of an automated Pregl-Dumas technique for determining C, H, and N in size-fractionated ambient aerosols is a relatively simple analysis which can provide considerable information on the nature of these constituents in airborne particles. In samples collected in Chicago, London, and Ankara, the mass median diameter of C and H associated particles was generally larger than for total suspended particulate matter, thereby reflecting less respirability. The N-containing particles generally exhib-
ited MMD values lower than for TSP and may have had a predominantly biological origin. Fuel combustion appears to be a substantial factor in the percent of carbon in airborne particles with the highest levels observed in Ankara where lignite coal is the predominant fuel. An estimate of the degree of saturation of hydrocarbon compounds in suspended particles as a function of size can be made by calculating the C:H molar ratio although the contribution of carbonates is not taken into account. The automated Pregl-Dumas technique described here is well suited for developing definitive relationships among these constituents. Received for review September 24, 1973. Accepted January 15, 1974. Mention of commercial products does not constitute endorsement by the Environmental Protection Agency.
Pyrolysis Generation of Dilute Concentration of Sulfur Dioxide Douglas Cornell' and Wing Tsang Inst/tute for M a t e r d s Research. National Bureau of Standards. Washington. D .C. 20234
Improvement in the reliability of analytical determinations of dilute pollutant gases in the atmosphere is a pressing and many-faceted problem. The wet chemical techniques ( 1 ) currently employed are time-consuming and the accuracy of these methods is critically dependent on the skill and care of the analyst. Complicating factors with these methods are identifiable as 1) decomposition of chemicals over periods of time, 2) variability of collection efficiency with pollutant concentration and flow rate, 3) lack of' complete specificity for the particular pollutant of interest, 4) need for high purity chemicals, and 5 ) lack of calibration standards. Fluorescence and chemiluminescence detectors show promise as providing direct sampling instrumental techniques making less severe requirements of the analyst. Such methods, however, still require calibration with accurately known dilute concentrations of pollutant gas. Small concentrations of pollutants have been produced with precision flow dilution systems (2, 3) and with permeation tubes ( 4 ) . Our approach has been to produce the pollutant a t low concentrations in a carrier gas stream by pyrolyzing a parent molecule which decomposes solely by unimolecular reaction into equimolar amounts of pollutant molecule and a nonpollutant molecule. We report here on a useful SO:! source. Similar methods for NO, NOz, and CO sources are under development. Equation 1 gives the chemical reaction utilized for SO2 production.
On sabbatical leave from Fairleigh Dickinson University, Rutherford, N .J. "Methods of Air Sampling and Analysis," American Public Health Association, Washington, D C . E. E . Saltzman, A n a / . Chem. 3 3 , 1100 (1961). E . E. Saltzman and A F . Wartburg, Anal. Chem , 3 7 , 1261 (1965) A . E. O'Keeffe and G. C Ortman, A n a / Chem.. 3 8 , 760 (1966)
CH2-
CH,
SO2
I I CH2-CH2
-SOz+
/ \ CH2-CH,
4
C,H,(Propylene)
Note the requirement that the stoichiometry be obeyed under all reaction conditions restricts the number of suitable compounds. Obviously, a reaction that proceeds as written rather than, for example, through a free radical pathway will have a greater chance of fulfilling this requirement.
EXPERIMENTAL Chemicals. Trimethylene Sulfone: 3.5 grams of trimethylene sulfide was dissolved in water and refluxed 2 hr with 13 ml of 30% hydrogen peroxide. Needles of trimethylene sulfone were crystallized from water and melted a t 71-74 "C; literature value (76 "C). Apparatus. The SO2 produced by reaction 1 was measured by its fluorescence and the hydrocarbon produced was measured by gas chromatography. T h e apparatus employed is shown in Figure 1. T h e cell containing the trimethylene sulfone was a 20-mm Pyrex tube joined a t both ends t o 4-mm Pyrex tubing. Helium was passed continuously over the trimethylene sulfone which was deposited in the large diameter section of the tube. T h e sample cell and connecting stainless steel tubing were enclosed in a Carle Instruments valve oven. The effluent from the sample cell went to a Chemical D a t a System Pyrolyzer and thence t o a Carle switching valve where a 0.5 cm3 volume of pyrolysis products contained in the coil of t h e valve was injected into a chromatograph using a flame ionization detector. T h e columns employed were 6-ft lengths of Porapaks Q and T . Peak areas were determined using a electronic integrator. Calibration of chromatograph peak areas was made using a mixture of CzH4, C3H6, and i-C4Hg. each a t 45 ppm, in helium. T h e sensitivity of the flame ionization detector t o cyclopropane is assumed to he the same as t h a t for propylene. Sulfur dioxide fluorescence was excited with a Zn l a m p operated with a Spectroline power supply (Spectronic Corp.). T h e radiation from the l a m p was passed through an Optical Coating Laboratory interference filter ( P / N 6307) to isolate the 2138 A line and focused in the center of the fluorescence cell. Fluorescence raA N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 7 , J U N E 1 9 7 4
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Table I. Concentration T r i m e t h y l e n e Sulfone as a Function of T e m p e r a t u r e Vapor presure, trimethylene sulfone, pprn
T 'C
25
11.3 18.6 36 48.8 106
34 38 43 52
Figure 1. Experimental setup I
I
I
I
I
I
1
I
I
1
1
I
.W
fI
1oc
400
iw
bx
e
Figure 2. Plot of photocurrent from SO2 fluorescence vs. C3 hydrocarbon concentration from the decomposition of trimethylene sulfone over temperature range 410-490 " C . Flow rate 20 cm3/min. Sample cell temperature 80 "C. (--) best fit. ( - - -) from SO2 calibration. diation passed through a No. 9863 Corning filter to block scattered radiation and was detected with a photomultiplier. The photocurrent was measured with a Keithley Instruments 610 BR Electrometer. Calibration was made with a SO2 permeation tube certified by the National Bureau of Standards. Further details of the fluorescence instrument may be found in the paper by Okabe, Splitstone, and Ball ( 5 ) . It should be noted that other methods for SO2 detection are equally suitable. The present device has been employed because of its availability.
RESULTS The only hydrocarbon products from trimethylene sulfone decomposition are cyclopropane and propylene. The relative amount of the latter increases with temperature. This is in accord with the expected decyclization of cyclopropane to propylene. Figure 2 shows a plot of the concentration of hydrocarbons as determined from the chromatographic areas us. the photocurrent from the SO2 detector. Also included is the expected variation from the calibration run using the SO2 permeation tube. The difference in slope is slightly more than 10% and is within our experimental error. Some of these are adsorption of the SO2 on surfaces and the fluctuating photocurrent background due to scattered light. At a flow rate of 20 cm3/min through the pyrolyzer, the sample is 3% decomposed a t 400 "C and 99% decomposed a t 490 "C. We have observed no side reactions a t temperatures below 550 "C. Variation of the flow rate of He through the sample cell has no effect on the concentration of pyrolysis products provided that the flow rate is not excessive and that the pyrolysis temperature is high enough to obtain 100% reac( 5 ) Hideo Okabe, Paul L. Splitstone, and Joseph J. Ball, J. Air Poliut Contr. ASS.. 23, 514 (1973).
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A N A L Y T I C A L CHEMISTRY, V O L . 46, N O . 7 , J U N E 1974
lOOOl1
Figure 3. Plot of concentration ( p p m ) of trimethylene sulfone from pyrolysis of trimethylene sulfone at 550 "C. (Flow rate 20 cm3/min.)vs. sample cell temperature tion of the sample entering the pyrolyzer. For example, a t a pyrolysis temperature of 550 "C, the same peak area was obtained with a carrier flow rate of 37 cm3/min as with a flow rate of 19 cm3/min. This demonstrates that, in this range of flow rates a t least, the saturation vapor pressure of trimethylene sulfone is being carried out of the sample cell. Thus, if the pyrolysis is carried out in a manner that assures 100% decomposition, then the concentration of SO2 produced is dependent only on the equilibrium vapor pressure of trimethylene sulfone. As an example of what may be achieved in this direction, the results of several experiments at 20 cm3/min flow rate and a pyrolysis temperature of 550 "C (100% decomposition) but varying sample cell temperatures are summarized in Table I. A plot of log P us. 1/T is shown in Figure 3. From the plot, we estimate a vapor pressure of 400 ppm a t 70 "C and 0.8 ppm a t 0 "C.
DISCUSSION The generation of dilute mixtures of SO2 in nitrogen or helium by the pyrolysis of trimethylene sulfone has several advantages. They are: 1) Under flow rate and pyrolysis temperature conditions such that the entire sample entering the pyrolyzer is decomposed, the SO2 concentration produced depends only on the vapor pressure of trimethylene sulfone. All that is required for calibration is, therefore, a measurement of the vapor pressure of trimethylene sulfone as a function of temperature and a knowledge of the total gas pressure in the sample cell. This is in contrast to the situation with respect to the SO2 permeation tube method which requires individual calibration. Furthermore, the present method is not crucially dependent on flow rate. Thus, another important variable in the operation of the permeation tube is eliminated. Finally. we
note that in the present mode of operation the pyrolyzer temperature need not be closely controlled. As outlined here, the trimethylene sulfone generator is used as a primary standard. Obviously, much further work is necessary before one can consider this as competitive with the well established permeation tube technique. Of special importance is a comparison of the temperature control necessary for constant and reproducible SO2 generation from the two systems. 2) If the capability of measuring hydrocarbon concentration is built into the calibration system, then this technique of SO2 generation is provided with a key internal standard. In this mode of operation, none of the reaction variables, flow rate, pyrolyzer temperature. or sample cell temperature, are critical. In the use of permeation tubes or. for that matter, the present device in the mode outlined earlier, the latter must be controlled very closely. Thus, it is difficult for technician errors to result in an incorrect value for the calibration factor. When used in this manner. the present device serves as a secondary standard. Xevertheless. it must be emphasized that the primary standard that it derives from is a mixture of a Cs hydrocarbon in NZ or other inert gases. These are simple to prepare, at least in comparison with permeation tubes, and most analytical laboratories already have such mixtures in hand. Overall, we feel that our data justify the consideration of the use of this method for field calibration purposes. Finally, for the quantitative analysis of scores of pollutants, the present approach reduces the calibration problem to a manageable level. That is, by picking suitable parent molecules which yield a simple low molecular weight hydrocarbon and the pollutant of interest, one can ultimately trace all calibrations to a single primary standard of one or several small hydrocarbons in an inert matrix. In an earlier publication, we have demonstrated the possibilities in this direction for the pollutant aldehydes (6).
(6) W . Tsang, J. Res. Nat. Bur. Stand.. in press
3) It offers convenience and economy of operation. No precautions in transport or storage of trimethylene sulfone are required. Its cost is negligible and a small supply is virtually inexhaustible. One can regard this method as simply a means of storing Son, with the added advantage that its concentration levels are regulated by the vapor pressure of trimethylene sulfone. 4) The concentration of SO2 in the effluent stream may be conveniently set by adjusting the pyrolysis temperature (incomplete reaction) or the temperature of the sample cell. In other words, the device has infinite dynamic range. It is known that permeation tubes are not satisfactory for high levels of SO2 concentration. 5 ) The time interval required for the sample to reach its equilibrium vapor pressure when changing sample temperature is negligible. When employing a detector which is not specific for SOZ, it will be necessary, of course, to ascertain the sensitivity of the detector toward cyclopropane and propene relative to the sensitivity toward SOz. If the SO2 generator is operated under conditions such that only part of the trimethylene sulfone is decomposed. use of certain SO2 detectors may require removal of trimethylene sulfone from the flow stream before the stream reaches the detector.
ACKNOWLEDGMENT We are grateful to J. McNesby and project MAQ at the National Bureau of Standards for continued interest and support. We wish to thank H. Okabe and F. Schwartz for assistance with the fluorescence measurements. Received for review October 31, 1973. Accepted January 21, 1974. Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
Nondestructive Analysis for Silicon, Rubidium, and Yttrium in Atmospheric Particulate Material I. Olmez,’ N. K. Aras,2 G. E. Gordon, and W . H. Zoller Department of Chemistry. University
of Maryland. College Park. Md. 20742
In studies of the origin and behavior of atmospheric particulate material, it is advantageous to be able to analyze samples for a wide spectrum of elements. In previous studies, instrumental neutron activation analysis (INAA) has been developed for analysis of up to about 30 elements in air-filter samples ( I , 2). More recently, instrumental Present address, Turkish Atomic Energy Commission, Ankara Nuclear Research Center, Ankara, Turkey. * Present address, Department of Chemistry, Middle East Technical University, Ankara, Turkey. ( 1 ) W H . Zoller and G. E Gordon, A n a l Chem.. 42, 257 (1970). (2) R . Dams, J A Robbins, K . A . Rahn, and J. W. Winchester, Anal. Chem . 42, 861 (1970)
photon activation analysis (IPAA), has been used to analyze filter samples for 14 elements, of which about six are difficult or impossible to observe by INAA (3). These instrumental nuclear methods of analysis have several advantages over competing methods. First, since one need not dissolve the samples, one avoids contamination or loss of trace species that could arise during chemical manipulations. Second, since the incoming nuclear projectiles and outgoing radiations of the active products have long ranges in the sample materials, there are little, if any. matrix effects. Third, the techniques are nondestructive; ( 3 ) N K Aras. W H Zoller G E Gordon, and G J Lutz A n a l Chem 45, 1481 (1973)
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