Table 111. The retention losses to the acid-insoluble residue were negligible for all analytical elements with the exception of titanium. Table IV lists the relative standard deviation and the concentration a t which it was determined. The relative standard deviation was 13% or less, values which are generally acceptable for most air pollution analysis requirements.
ACKNOWLEDGMENT The author wishes to thank the analysts working in Daihatsu Kogyo, Ltd., Environmental Pollution Control
Center, 'Osaka Pref.; Fujitsu, Ltd.; Government Industrial Research Institute, Osaka; Nippon Steel Corp.; Osaka Prefectural Industrial Research Institute; Kobe Steel, Ltd.; Matsushita Electric Industries, Ltd.; Sumitomo Electric Industries, Ltd.; and Sumitomo Metal Industries, Ltd. for the atomic absorption analysis of the airborne particulate matter. They are members of the Kansai Bunseki Kenkyukai (Chairman: Shigero Ikeda, Osaka University; Vice-chairman: Takanobu Hamaguchi, Osaka Prefectural Industrial Research Institute). Received for review November 7, 1973. Accepted January 11, 1974.
Analysis of Micrometer-Size Particles of Magnesium Oxide and Manganese Dioxide in Low Rise-Velocity, Laminar Flames Richard D. Sacks' and John R. Rentner2 Department of Chemistry, University of Michigan, Ann Arbor, Mich. 48704
Current interest in the analysis of airborne particulate material emphasizes the need for rapid, inexpensive, and in situ methods for the analysis of individual micrometersize particles. This may provide information regarding the abundance and meteorological dispersion of particles of specific size and elemental composition. In addition, detailed analysis of individual particles may be used to fingerprint the point of origin as well as the specific natural or industrial process responsible for the production of these particles. Since micrometer-size particles are in the picogram range, high sensitivity is of prime importance. Few analytical methods have the requisite sensitivity. McCrone ( 1 ) has discussed the optical microscope for single particle analysis. Limited sensitivity and specificity greatly restrict its application. In addition, microscopy is not satisfactory for in situ analysis of large numbers of particles. Riggs et al. ( 2 ) used the electron microprobe to detect Ge, Zn, Cu, Ni, Co, Fe, Mn, and Cr in particles collected at high altitudes. Landstrom and Kohler (3) analyzed particles from test aerosols, automobile exhausts, and the atmosphere with the electron microprobe. While particles as small as 0.1 pm may be studied, the method is semiquantitative at best and is very time-consuming. Again, in situ analysis is not practical. Recently, McHugh and Stevens ( 4 ) used an ion-microprobe mass spectrometer for the analysis of micrometersize oil-soot particles. They found greater sensitivity, particularly for light elements, than is typical with the electron microprobe. Emission spectroscopic methods can be implemented at a cost comparable to morphological methods and have the advantage of being highly element-specific. Flame excitaTo whom correspondence should be directed.
* Present address, Department of Chemistry, University ofWis-
consin, Madison, Wis. 53706. ( 1 ) W. C. McCrone, "Morphological Analysis of Particulate Pollutants," in "Air Pollution,'' Vol. I I , A . C. Stern, Ed., Academic Press, New York, N.Y., 1968, Chap. 22. ( 2 ) F. 6 . Riggs, Jr., F. W . Wright, and P. W . Hodge, NASA Doc. N o . N62-16007, 46 pp (1962). ( 3 ) D. K . Landstrom and D. Kohler, U.S. Clearinghouse F e d . Sci. Tech. Inform., P 6 Rep. N o . 1897282, 25 pp (1969). (4) J. A . McHugh and J . F. Stevens, Ana/. Chem.. 44, 2187 (1972).
tion is suggested because of its freedom from any timevarying electrode matrix and its inherent long-term stability. Crider (5) and Crider et al. (6) used 02-H2 and airH2 flames, respectively for the in situ analysis of a number of elements in micrometer-size particles. While sensitivity is high for Li, Na, and Ca, the minimum detectable diameter for MgO particles is 1.22 pm, and for CdO and PbSO4 particles it is about 4 pm. Woodcock and Spencer (7) used an air-propane flame to detect sea-salt particles. Hogan and Rich (8) detected lithium stearate particles in an air-Hz flame. Recently, Sacks and Brewer (9) discussed flame methods in a review of the analysis of airborne particulates using emission spectroscopy. The principal limitation in these flame methods is that they use low-temperature, high-rise-velocity flames. The high-rise velocity results in low particle residence times in the flame. This, coupled with low flame temperature, results in very inefficient vaporization of refractory materials such as MgO. The use of low-rise-velocity flames such as the nitrous oxide-acetylene and oxycyanogen systems is suggested because of the greatly increased particle residence times. Oxycyanogen is particularly attractive because of its high temperature. The specific objective of this research was to design a burner system and calibration procedure for the analysis of individual micrometer and submicrometer diameter particles in low-rise-velocity, high-temperature flames.
EXPERIMENTAL Fuel and oxidant gases are transmitted t o the burner via a gashandling system. Premixing is used to obtain uniform combustion. An aerosol carrier gas flows into a 150-ml brass container where particles of a known size distribution (heated to keep them dry to avoid agglomeration) are agitated, thus causing them to be carried into the burner by the aerosol carrier gas. Nitrogen is used (5) W . L. Crider, Rev. Sci. lnstrum.. 39, 212 (1968). (6) W. L. Crider, A. A . Strong, and N . P. Barkley, Appl. Spectrosc.. 22, 542 ( 1 968), (7) A . H. Woodcock and A . T . Spencer, J . Meteorol.. 1 4 , 4 3 7 (1957) (8) A. W. Hogan and T. A. Rich, in Proc. l n t . Conf. Cloud Phys.. 1968, p 45, International Ass. Meteor. and Atmos. Phys., Toronto, Cana-
da. (9) R . D. Sacks and S. W . Brewer. Jr.. Appl. Spectrosc. Rev.. 6 , 313 (1972),
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
1125
sample lube
90s 10 pilot tlame
water wtIU
@a to pilot l l o m e
water Inlei
1
Moo
2000
u
',ampic
wovelength
inlet
Figure 1. Section drawing of t h e water-cooled, laminar-flow
as a carrier gas because it can be obtained in pure form a t low cost. Radiation from the flame is focused onto the entrance slit of an f/5.3 McPherson Model 218, 0.3-m scanning monochromator with a 1.5-inch diameter, 4-inch focal length quartz lens. A magnification of 0.2 is used so that the entire flame length is imaged on the entrance slit of the monochromator. Both entrance and exit slits are 150 fim wide and 20 mm high. Radiation is detected with a 1P28 photomultiplier tube operated a t 1200 V and using a 10-kR load resistor. Capacitive shunts are used to stabilize the last six dynodes. The photomultiplier is capacitively coupled to a Hewlett-Packard type 1200A, dual trace, differential input oscilloscope. Burner Design. Production of laminar flames necessitates premixing the fuel and oxidant. This can be accomplished in mixing chambers filled with glass wool. The low diffusion rate of cyanogen into other gases requires that the chambers be fairly large. The other requirement for a laminar flame is that there exists a laminar gas flow of the unburned flame gases from the burner exit port. Figure 1 shows a section drawing of the burner. Sample aerosols enter the sample inlet and then pass into the main flame which burns concentrically about the top end of the sample inlet tube. A low-burning-velocity pilot burner also is provided to allow a greater range of main-flame flow rates without the main flame blowing off the top of the burner. With the gas flow rates used here, the pilot burner is not required. The burner body is about 2 inches tall and made of brass. A water cooling coil of 3hs-in. 0.d. copper tubing is soldered to the body. This maintains a burner-body temperature of less than 70 "C. Three 0.05-in. diameter holes are drilled 0.15 in. from the top of the burner body to allow an inlet for air which becomes entrained in and stabilizes the main flame. The pilot burner, which is soldered to the top of this portion, is 0.6 in. in diameter and made entirely of stainless steel. Located along a 0.394-in. radius are 24 holes, 0.014 in. in diameter, which comprise the pilot burner jets. The central opening in the pilot burner for the main burner is 0.197 in. in diameter. Two stainless steel tubes enter the pilot burner from opposite sides and carry the combustion mixture to the pilot burner. The main burner jet holder is brass and threads into the bottom of the main body. The main burner jet is a 0.07-in. i.d., 0.02-in. wall stainless steel tube which is silver soldered into the top of the main -burner jet holder. The entire assembly can be screwed up or down to allow positioning of the main jet at different heights in the pilot flame. The sample inlet is a 0.04-in. i.d., 0.01-in. wall stainless steel tube which is soldered into a threaded collar and screws into the rn
so00
I
do00
(8)
Figure 2. Flame background spectra
burner
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ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
( a ) oxycyanogen flame, inner cone; ( b ) oxycyanogen flame, outer cone; (c) nitrous oxide-acetylene flame, inner cone: ( d ) nitrous oxide-acety-
lene flame. outer cone bottom of the main burner jet holder. The sample tube fits concentrically inside the main burner jet and is centered with three set screws in the main jet holder. The sample tube can be adjusted to different heights in the main flame. When the sample tube is inserted in the main burner jet, the main flame jet becomes a circular slot with an inside diameter of 0.06 in. and a n outside diameter of 0.07 in. The Reynolds numbers for this burner with the gas delivery rates discussed below are about 480 for the 02-(CN)2 flame and 820 for the NzO-CzHz flame. G a s Handling System. The gas handling system is basically two identical flow control systems, one supplying a fuel-oxidant mixture to the main burner and one supplying a fuel-oxidant mixture to the pilot burner. The fuel and oxidant pass through the tank regulators and through Matheson rotameter-type flow meters equipped with needle valves which have been repositioned from the inlet side of the flow meters to the outlet side. This allows the rotameter tubes to operate at constant pressure (10). Next, the gases flow to chambers for pre-mixing. The mixing chambers are 1-in. diameter, 4-in. long stainless steel cylinders filled with glass wool. A hole in the side of each chamber is sealed by a steel washer pressing an aluminum foil diaphragm against an O-ring. The foil diaphragm acts as a safety pressure release in case of a flashback. The connecting lines of the gas handling system are all yd-in. 0.d. copper tubing connected with brass Swagelok fittings. All tubing joints are wrapped with filter paper impregnated with a 3% ethyl alcohol solution of gum guajac and a 0.1% solution of copper sulfate. In the event of a cyanogen leak, the SchoenbeinPagenstecher reaction occurs, resulting in an intense blue color which ultimately turns brownish red (11).
RESULTS AND DISCUSSION Stable operation is o b t a i n e d from t h e b u r n e r w i t h e i t h e r the N ~ O - C ~ HorZ the 02-(CN)2 f l a m e s y s t e m as long as the total g a s flow t h r o u g h t h e burner including carrier gas exceeds about 350 c m 3 / m i n . Below t h i s v a l u e , flashback occasionally occurs. Stoichiometric g a s m i x t u r e s a r e used to obtain m a x i m u m f l a m e t e m p e r a t u r e . T a b l e I lists the gas flow rates used here. T h e r a t h e r high carrier g a s flow rate is required to p r e v e n t settling of larger particles. T h i s has n o apparent effect on t h e s t r u c t u r e of e i t h e r f l a m e , but reduction i n f l a m e t e m p e r a t u r e m a y occur from volu m e dilution. Both g a s m i x t u r e s produce l a m i n a r - a p (10) G . K . Kirkbright and M . Sargent, Analyst ( L o n d o n ) ,93,558 (1968) ( 1 1 ) L. M . Dennis, "Gas Analysis,"Macmillan, New York. N . Y . , 1913.
IO-
Table I. Flow R a t e s of Flame Gases (cm3/min)
N2 carrier gas
279 160 160 92 460
(CWZ
08o c
r
0
(a)
1
06-
-
-
2 04-
-0 p
-
02-
0-
Table 11. Properties of Particulate Test Materials
Boiling point, O K Density, g/cm3 Size range, pmo Analysis line, A a
M go
MnO,
0.3-3.0 Mg(1) 2852.13
2370a 5.026 0.5-7.5 Mn(1) 4030.76
3873 3.58
At 808 OK, MnOr decomposes to Mn which boils at 2370 OK.
I
or
080
06-
(b)
-"
04-
; c
-
02-
pearing flames of about 10 cm in height. Inner cones of both flames are about 0.5 cm high. The inner cone of the 02-(CN)2 flame is such an intense blue-white color that eye protection is required. General Spectral Features. Figure 2 shows the flame background spectra obtained in the inner and outer cones of each flame system. Inner-cone spectra are obtained by imaging the flame with unit magnification onto the entrance slit of the monochromator using a quartz lens. A vertical mask a t the monochromator slit is used to isolate the inner cone. A similar procedure, but masking the inner cone, is used to obtain the outer-cone spectra. General features of the flame background are quite similar in the two flames except that the strong NH and OH bands of the N20-CzHz flame are weak or absent in the 02-(CN)2 flame. The outer cone of both flames is relatively free of strong band systems except around 3100 A and 3900 A. In addition, the 3660-A NH band in the N2OC2H2 flame persists in the outer cone. Masking the inner cone would be useful if an analysis line fell in a region of heavy inner-cone background. The intensity of any of these flame-background features is reproducible to within 5% on a day-to-day basis, simply by setting gas delivery rates to the values in Table I. Calibration Procedure. MgO and MnOz were chosen as test materials because of their good sensitivity in the oxycyanogen flame and because of the large differences in their thermodynamic properties. MgO is one of the most refractory metal compounds. In addition, the Mn(1) 4030.76-A and Mg(1) 2852.13-A analysis lines are in regions of low flame background and adequate photomultiplier sensitivity. Relevant properties of these compounds are listed in Table 11. Particles are presized with an Anderson seven-stage cascade impactor. Reagent grade MnO2 and MgO are ground to micrometer-size particles in a dry nitrogen atmosphere. A 5-gram sample of unsorted powder is placed in a sidearm flask and agitated with a magnetic stirrer. Dry air enters the flask through the top, and the resulting aerosol is carried through the side arm to the Anderson sampler. The aerosol is drawn through the sampler at a rate of 1.0 ft3/min. The sampler is operated for about 5 min before removing the collection plates. The plates are examined to determine if sample overloading and re-entrainment has taken place. If the plates appear satisfactory, the contents of the plates are placed in sample bottles. Since the fraction produced at each impactor stage has a fairly wide particle-size distribution and since it is not practical to introduce specific measured particles into the flame, a statistical correlation technique is used. The residual powder on each plate is wiped off using micropore
. . - .... . .... ... . .. . .. . .. e
.
-e.-
.N
0
0
I
a e. a*.-
e
I
. e
. e . .
e .
0 . .
.
0.
.b
0
0 .
0.1
0.2
03
I 0.4
05
0.6
PM Tube Outwl (mV)
Figure 3. ( a ) Particle size distribution, and ( b ) photomultiplier output signal distribution for MnOz particles collected on plate 2 of the Anderson cascade impactor filter paper. The filters are placed on microscope slides, dissolved with diethyl ketone and covered with cover glasses. About 100 particles from each slide are sized using a microscope with a calibrated phylar eyepiece and an oil-immersion objective. Particles are selected at random by scanning diagonally across the microscope slide, and all particles within 50 Mm of the scan axis are measured. Since all particles of interest are much smaller than 50 Fm in diameter, this procedure prevents significant biasing toward larger particles. Most of the particles appear nearly spherical, and here diameters are assigned directly. Particles which do not appear nearly spherical are assigned a diameter equal to one-half the sum of their longest and shortest dimensions. This process is used for both compounds, but a phase-contrast microscope is required for MgO since the particles are clear. Another group of particles from each impactor stage is introduced into each flame system by placing about 1 mg of pre-sized powder in the brass sample container and agitating the container to suspend the particles in the carrier gas. As each particle passes through the flame, a nearly triangular voltage pulse is observed on the oscilloscope. Peak heights are measured from photographically recorded oscilloscope traces of between 100 and 200 particles from each impactor stage. The computer procedure used for data reduction converts the microscopic measurements of particle diameters to particle masses using the bulk density of the material. Next, the number of occurrences of each particle size is determined as well as the average and median particle diameters and masses. Finally, plots are obtained of particle diameter and particle mass as functions of occurrence or relative abundance. Figure 3a shows an example of the particle diameter distribution for Mn02 collected on plate 2 of the Anderson impactor. Data from oscilloscope traces of the flame signal are processed using a similar procedure. The number of occurrences of each signal magnitude is determined as well as the average and median signal values. Finally, distribution plots are obtained of photomultiplier tube output in mV as a function of occurrence or relative abundance. An example for MnO2 particles collected on plate 2 of the impactor is shown in Figure 3b. Analytical curves are preANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8 , J U L Y 1974
1127
0.06,-
-
00
0.5
1.0
I5
-
0.02t
0 0
10
20
0 0
IO
20
?a
40
50
40
30
60
f
30
60
Particle Mars (PO)
Figure 5. Analytical curves for MnOz particles in ( a ) the nitrous oxide-acetylene flame, and ( b ) the oxycyanogen flame
0.5 PartIc1.
1.0
1.5
2.0
wars Lpp)
Analytical curves for MgO particles in ( a ) the nitrous oxide-acetylene flame, and ( b ) the oxycyanogen flame Figure 4.
pared by directly correlating the peaks in the flame data and the microscopic data distributions for each impactor stage. The very low burning velocity of the N20-CzH2 and 02-(CN)2 systems, about 160 cm/sec and 140 cm/sec, respectively, results in sufficiently low gas rise velocity to obtain particle residence times of nearly 50 msec for the laminar burner described here when operated a t the gas flow rates listed in Table I. Analytical curves for MgO in the nitrous oxide-acetylene and oxycyanogen flames are shown in Figures 4a and 46, respectively. The linearity of the curves suggests complete vaporization of these sub-micrometer diameter particles. The sensitivity of the two flames is comparable. This is somewhat surprising, considering the large difference in flame temperature. The range of the working curves is limited by the size range of the starting material and the particle-size dispersion of the Anderson impactor. The points on these analytical curves cover a particle diameter range from 0.60 to 1.00 pm. I t is significant to note that this entire size range is considerably less than the 1.22-pm diameter detection limit reported by Crider (5) for MgO particles in an air-Hz flame. From these plots and the measured rms noise in the detector circuit, the minimum detectable particle diameter a t a signal-to-noise ratio of two is about 0.46 pm in both flame systems. This corresponds to a particle mass of 0.19 pg. Figures 5a and 56 show analytical curves for MnO2 particles in the nitrous oxide-acetylene and oxycyanogen flames, respectively. Because of the particle size range of the starting material and the greater density of the MnO2, these analytical curves cover a much larger particle mass range than the curves for MgO. The difference in particle size range, however, is not as extreme going from 0.91-pm to 2.77-pm diameter for the data points in Figure 5 . Sensitivity for these larger but less refractory particles
1128
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20
-
-
t
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8 , JULY 1974
decreases rapidly with increasing particle mass in both flame systems. This suggests incomplete vaporization of these larger particles. The sensitivity of the oxycyanogen flame is considerably greater than that of the nitrous oxide-acetylene system. This is particularly apparent for larger particles (greater than about 20 pg) where the photomultiplier response in the 02-(CN)2 flame is nearly twice that in the N20-C2H2 flame. Detection limits a t a signal to noise ratio of two are 0.82 pg and 0.31 pg in the N Z O - C ~ H and ~ 02-(CN)2 flames, respectively. These values correspond to particle diameters of 0.68 pm and 0.49 pm, respectively. While adequate stability and sensitivity for the analysis of micrometer and sub-micrometer diameter particles of even very refractory materials is readily obtained with these low burning-velocity, laminar flames, the calibration procedure using microscopic, particle sizing is very laborious and time-consuming. However, the use of monodisperse aerosol generators (12) should simplify greatly the calibration procedure. In addition, the use of electronic pulse-height analysis rather than measurements of oscilloscope traces clearly is indicated in future work. It should be pointed out that the oxycyanogen flame appears to be quite safe with only modest safety precautions required. The low cyanogen consumption for the burner reported here also permits relatively economic operation, about $6/hour for continuous operation, in spite of the relatively high cost of cyanogen gas.
ACKNOWLEDGMENT The authors wish to thank Lawrence Hecker for his helpful suggestions and the equipment used in particle sizing, and Denise Ackerman and Elizabeth Stone for their aid in data reduction. Received for review August 13, 1973. Accepted March 28, 1974. This work was supported in part by the University of Michigan Office of Research Administration. (12) R N Berglund and B Y H Liu, Envfron S o Techno/ 7, 147 (1973)
