unknowns into element concentrations, together with appropriate statistical computations. The use of either of these methods results in a significant saving in time for the analyst. The machine time required for processing of the data depends critically on the particular computer system. The time required for the analytical chemist depends primarily on the ease of entering data. This time can be made very small if the spectrometer is equipped with a digital readout on cards or tape for direct input to the computer or if the spectrometer and computer are directly interfaced. There are other advantages to the use of a computer for fitting analytical functions. In Procedure 11, all significant decisions are incorporated in the computer, so that operator judgment will not influence the analytical results. Similarly,
the effects of human error are minimized. An additional advantage is that the computer can be piogrammed t o perform further computations and thus to provide in the output a n indication of both the precision and accuracy of the analyses. A computation procedure is described which bases the estimate of the accuracy on the differences between the concentrations given for the standards and those found for those standards from the analytical function and the instrument readings. The validity of this method of estimating accuracy depends on assumptions which are described. It is felt that even a limited estimate of accuracy of the analyses is preferable to basing the estimated uncertainty of the analyses entirely on the precision of replicate instrument readings.
RECEIVED for review April 9, 1969. Accepted May 21,1969.
Direct Determination of Silver in Air by Atomic Absorption Spectrometry Harry W. Edwards Department of Mechanical Engineering, Colorado State University, Ft. Collins, Colo. 80521 Silver-containing air streams are introduced in the primary air supply to the burner of an atomic absorption spectrometer for direct silver determination. The analytical method is calibrated by a filtration technique in which continuous silver-containing air streams are produced by passing air over heated silver iodide. The amounts of silver on the filters are determined by conventional methods of extraction and AAS solution analysis. The use of an integrating digital voltmeter to measure the spectrometer signal significantly enhances the readability of the relatively noisy absorptions associated with direct measurement. The detection limit is approximately 3 gg/m3 which is applicable to the direct determination of silver in air in the concentration range typical of certain weather modification activities.
THE QUANTITATIVE determination of silver added to the atmosphere is of considerable interest in weather modification. Rigorous theoretical and economic evaluations of the effectiveness of silver iodide as a cloud seeding agent require the determination of silver in air. Previously reported meteorological methods for the determination of silver in the atmosphele rely primarily on the ice-nucleating properties of silver iodide ( I ) . Criticisms of cloud chamber methods for airborne silver analysis focus upon the following characteristics of such methods : nonspecificity, lack of calibration, and excessive time between sampling and readout. Methods for the determination of elements dispersed as particulates in air are usually characterized by concentration and extraction steps prior to analysis. Direct determinations-e.g., methods which do not involve concentration and extraction steps prior to analysis-appear comparatively infrequently in the literature. The relatively severe sensitivity requirement imposed by the pg/m3 concentration level of solid material in a typical aerosol requires manipulation of inconveniently large volumes of air for conventional analysis. Atomic absorption spectrometry therefore offers “Clouds, Rain, and Rainmaking,” University Press, Cambridge, 1962.
(1) B. J. Mason,
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considerable promise for the direct determination of metallic elements dispersed as particulates in air. A predecessor of AAS has long been used for the detection of mercury vapor in air (2), and AAS has recently been employed by White (3) for the measurement of lead and cadmium fumes in foundries and by Thilliez ( 4 ) for the determination of tetraethyl- and tetramethyllead in the environment of a manufacturing plant. Reported in this paper is a method for the direct determination of silver in air by AAS. Calibration is accomplished by a filtration technique in which the absorbances associated with air streams containing varying amounts of finely divided silver iodide are measured. Application of the method to the direct determination of silver in air for meteorological studies is discussed. EXPERIMENTAL
Apparatus. The apparatus used is shown in Figure 1. Air enters the oil-free diaphragm pump, A , at the rate of 3.96 l./min as measured by the rotometer, B. Silver iodide is dispersed in finely divided form in the air stream by the silver iodide smoke generator, C. By means of pinchcocks located at D and F, the air stream is either passed directly into the Perkin-Elmer 303 spectrometer burner, H , or diverted through the Millipore filter apparatus, E. The primary air supply for the burner, which is filtered by means of the standard cartridge unit furnished by the spectrometer manufacturer, enters the system at G. The instrumental settings were asofollows : silver lamp current, 12 mA ; wavelength, 3280.7 A ; slit width, 0.3 mm; primary air pressure, 28 psi. With the diaphragm pump in operation, it was found necessary to reduce the primary air flow by approximately 15% from that recommended by the spectrometer manufacturer for flame stability. The diaphragm pump was selected to provide a system which could be readily adapted to field conditions. The primary advantage of this pump is that (2) T. T. Woodson, Rev. Sci. Iustrum., 10,308 (1939). (3) R. A. White, J. Sci. Instrum., 44, 678 (1967). (4) G. Thilliez, ANAL.CHEM., 39,427 (1967).
A
U
w H
Figure 1. Apparatus for dispersal and direct determination of silver in air A . Diaphragm pump B. Rotometer C . Silver iodide smoke generator D. Clamp E. Millipore filter holder F. Clamp G . Primary air supply H. Spectrometer burner
the silver iodide aerosols used in this study passed through the pump without significant concentration diminution. In Figure 2, details of the silver iodide smoke generator are shown. Silver iodide smoke is produced by heating silver iodide powder in a manner similar to that reported by Vonnegut (5). The body of the apparatus is constructed of borosilicate glass, and connecting tubing is 3/16-in~h i.d. Tygon laboratory tubing. The heating element, A , is a 100-W quartz-fiber immersion heater supplied with line current through a variable transformer. Silver iodide powder, B, is placed in the well of the apparatus in contact with the heating element. The chromel-alumel thermocouple junction, C, was used to estimate the bulk temperature of the heated silver iodide. A small piece of borosilicate glass wool placed in the apparatus at D prevented clogging caused by passage of large particles of silver iodide into the system. The spectrometer output signal was monitored concurrently with a potentiometric recorder and a Model DY-2401A Hewlett-Packard integrating digital voltmeter. The latter instrument was set for integration periods of 1 second for which the manufacturer specifies a maximum resolution of 1 pV/digit (five decade display units) for the 0.1-V scale employed. Dead-time between 1-second integrations was set at approximately 5 seconds. All absorbances were calculated from integrating digital voltmeter data. Both IO and I were taken respectively as the averages of 20 consecutive integrations for each data point. The resulting standard deviation in I (or lo)for a single data point was typically 0.05 %. Reproducible absorbances were thus obtained in the region in which I and IO are indistinguishable when measured by the potentiometric recorder alone. Calibration Procedure. Calibration was accomplished by measuring the absorbance produced by a given silver-containing air stream as a function of the amount of silver found on the corresponding filter. The amount of silver entrained in the air stream was varied by changing the variable transformer setting. I was measured both before and after the 5 minute filtration period and the average taken. I,, was taken as the signal produced by the filtered air stream. The filters employed, Glass Fiber Type A filters supplied by Gelman Instrument Company, are specified by the manufacturer to be 99.97 effective for particles in the range of 0.3 p. This figure was qualitatively borne out by the atomic absorption data; the blank absorbance for air containing no added silver and absorbances for filtered silver-containing air streams are identical. The glass fiber filters were found sufficiently resistant to solution disintegration for use in the subsequent silver extractions. It was established experimentally that background silver in our laboratory atmosphere is virtually insignificant with ( 5 ) B. Vonnegut, J. Appl. Phys., 18, 593 (1947).
Figure 2.
Silver iodide smoke generator A . Quartz-fiber heater B. Silver iodide powder C . Thermocouple junction D. Glass wool
respect to the detection limit of the method reported. Significant background silver can readily be detected by pumping the unfiltered air supply in question into the system. A comparison of two absorbances produced by the air supply is then made with no current passing through the generator heating element. The first measurement is taken with the air flowing directly into the burner, and the second measurement is taken with the air flowing through filter E represented in Figure 1. If the two absorbances thus obtained are identical, it may be concluded that background silver is inconsequential. Filter Extraction. Each filter was subjected to two extractions. The first extraction, consisting of successive extractions with three separate 10-ml portions of 1 g KI/I. acetone solutions served to remove silver iodide. The second extraction, consisting of a single extraction with 1.OM HN03, served to remove silver reduced to metallic silver. The extractions were carried out with a standard Millipore filter extraction apparatus. Suction was not required. Determination of Extracted Silver. Silver (as AgI) was determined by AAS analysis for each of the three KI-acetone extractions. Each portion is made up to 10 ml in a 10-ml volumetric flask and aspirated into the spectrometer burner for absorption measurement. Calibration was accomplished by measuring the absorbances for a series of standard solutions. Calibration curves of absorbance as a function of silver concentration are linear in the range 0.01-0.7 pg Ag/ml. The standard deviation for a single point on the calibration curves is 0.0028 pg Ag/ml. The sum of the amounts of silver found gives the amount of silver present in the air stream as silver iodide. The average first extraction was found to contain approximately 96% of the total silver found as a silver iodide, and the average percentages for the second and third extractions were 4 % and 0 Z, respectively. Silver (as AgN03) was determined by a modification of the method for the determination of silver in water reported by West et al. (6). The sample is prepared by evaporating the liquid from the nitric acid extraction to dryness on a hot plate. The residue is then dissolved in 10 ml of 0.01M Na2EDTA. Silver is extracted from this solution as the dithizonate into 10 ml 0.5 gram dithizonell. ethyl propionate. The procedure reported by West et al. is then followed without further modification. The standard deviation for a single point on the calibration curve is 0.0024 pg Ag/ml in the range 0.01-0.1 pg Ag/ml. The use of the integrating digital voltmeter thus facilitates extension of the West method by approximately one order of magnitude. (6) F. K. West, P. W. West, and T. V. Rarnakrishna, Emiron. Sci. TecAmd., 1, 717 (1967). VOL. 41, NO. 10,AUGUST 1969
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RESULTS AND DISCUSSION The method employed to disperse silver iodide in air resulted in a noisy but relatively drift-free signal as seen in Figure 3. Recorder traces produced by silver-containing air streams of 16.1 and 185 pg Ag/m3 are shown in Figure 3 (a) and (b), respectively. The periodic fluctuations in the signals are produced by the discontinuous nature of the diaphragm pump operation and the occurrence of the element sought in particulate form. The recorder trace shown in Figure 3 (c) is the record of a calibration datum at 72.1 pg Ag/m3 which depicts the signal before, during, and after the approximately 5-minute period in which filtration occurs. The signal corresponding to 100% transmission for the filtered air sample is identical to that obtained with no current through the generator heater. A plot of absorbance as a function of the amount of silver (as AgI) found on the filters is linear over the range 2-200 pg Ag/m3. The standard deviation for a single point on the curve is 2.6 pg/m3. Reduced silver, determined from the nitric acid extractions, does not appear until approximately 70 pg Ag (as AgI)/m3 whereupon the plot of absorbance versus total silver becomes highly curved toward the concentration coordinate. Although the concentration of silver iodide produced by a given set of generator operating conditions was highly reproducible, it was found that the amount of reduced silver produced by the same set of operating conditions varied widely. At no time was the characteristic violet color of iodine vapor observed, however. The explanation for the varying amount of silver produced by the device undoubtedly involves a knowledge of the mechanism by which silver iodide is entrained in the air stream. It was found that the calibration procedure used by White (3), in which the concentration of metal in air is calculated by equating for a given absorbance the mass-flow-rate of metal from air to the mass-flow-rate of metal from solution, leads to values of the concentration of silver in air which are consistently high by a factor of approximately seven. For example, the concentration of silver in 1 gram/l. KI-acetone solution producing an absorbance of 0.01 is 0.028 pg/ml. The solution flow rate is 10 ml/min. Calculation of the concentration of silver in air flowing at 3.96 I./min required to produce an identical absorbance yields 70.7 pg/m3. The measured silver-in-air concentration for this absorbance, however, is 10.5 pg/ml. Thus the importance of calibration for the direct determination of metals in air by atomic absorption spectrometry is emphasized. Additionally, conversion of liquid samples to aerosols prior to aspiration may offer a general sensitivity enhancement for atomic absorption analysis of solutions. The detection limit of the method was calculated by a technique similar to that discussed by Fassel and Golightly for elements in solution (7). The detection limit is the concentration (pg/m3) of silver in air which, upon aspiration into the flame, gives rise to a line signal equal to twice the standard deviation in background fluctuations. The standard deviation was obtained from 160 separate background-signal measurements with the integrating digital voltmeter. The detection limit was then calculated from the least-squares calibration equation and found by this method to be 3.26 pg/m3. It is emphasized that the integrating digital voltmeter served as a convenient and effective noise-suppression device by
(7) V. A. Fassel and D. W. Golightly, ANAL.CHEM., 39, 466
(1967). 1174
ANALYTICAL CHEMISTRY
80
60
I8
40
Source :3 2 8 0 . 7 A, I2 ma Sample Flow Rote: 3.96 I/min
20
time, I min.3 d i v
Source: 3 2 8 0 . 7 A , 12,ma SomPle Flow Rate: 3.96 I/min Conc. Ag 1 I85 pg/ms
time,
I min = 3 div
8C
60
c
s 40
Source:3280.7A, 12 m a Somplr Flow R a t e : 3.96 Vmin Conc. A p t 72.1 pg/m3
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time, I m i n i 3 div
Figure 3. Strip-chart recorder traces for direct determination of silver in air 1 6 . 1 pg/ma (b). 185 pg/m3 (c). 7 1 . 1 pg/m3 (a).
providing true signal integration on a real-time, digitalreadout basis. Noise-levels of signals shown in Figure 3, which may be typical for direct measurements involving airborne particulates, would otherwise tend to offset the advantage of direct measurement. An estimate of the sensitivity of the method for meteorological investigations can be obtained by noting that silver iodide particles active as ice nuclei are typically the order of 1000 A in diameter (8). The calculated detection limit of the (8) L. R. Koenig, ibid., 31, 1732 (1959).
method in terms of the number of silver iodide particles per unit volume is 2 X 109/m3 assuming spherical particles of normal density. A simplified version of the method in which the integrating digital voltmeter was not required has been used to measure directly the rate of decay of contained silver iodide aerosols as a function of container volume (9). It was found that introduction of the silvercontaining air through the liquid nebulizer port at the rate of 0.24 l./min provided adequate sensitivity for air samples reported to contain initially 1O1O nuclei/m3 based on cloud chamber data at -20 “C. The measured initial silver iodide content of the same air samples, however, was found to be approximately 2 mg/m3 which corresponds to 7 x 1013 particles/m3 calculated on the basis of the previously described particle properties. This rough calculation indicates that only one silver iodide particle in approximately Seven thousand is active as an ice nucleus under the conditions of (9) H. W. Edwards, J. Atmospheric Sci., 26, 327 (1969).
the experiment. Thus the detection limit reported represents an upper limit for meteorological investigations because the fraction of silver iodide particles active as ice nuclei is considerably less than unity. The method reported may be useful in obtaining quantified relationships between silver iodide content and nucleation activity for a silver iodide aerosol. ACKNOWLEDGMENT
The author acknowledges the laboratory assistance of Thomas E. Adams. RECEIVED
for review December 20, 1968. Accepted May 15,
1969. This research was supported by the Atmospheric Sciences Section, National Science Foundation, NSF Grant GA-910. The study was presented in part at the Southwest Regional Meeting, American Chemical Society, December 4-6, 1968, Austin, Texas.
Volatility of High Boiling Organic Materials by a Flame Ionization Detection Method F. T. Eggertsen, E. E. Seibert, and F. H. Stross Shell DeGelopmen t Co., Emery uille, Calix A method is described for the convenient and rapid estimation of the volatility of high boiling organic materials. The apparatus consists basically of a sample furnace connected directly to a hydrogen flame ionization detector. Vapor pressure curves can be obtained over the range 0.1 to 5000 millitorr using either isothermal or programmed heating. These measurements are made with a small amount of material contained in a flow-through sample probe designed to allow saturation of nitrogen carrier gas with sample vapors. The results for cetane agreed with literature values within 5%. The equipment is versatile, also permitting volatilization curves to be obtained by heating in a small sample pan to yield information on relative volatility, thermal decomposition, and carbonaceous residue.
THISREPORT describes a sensitive method for determining the volatility of organic materials utilizing a flame ionization detector (FID) in a continuous flow system. It is similar in principle to that of Bell and Groszek ( I ) but embodies a number of improvements to permit the measurements to be made more rapidly and conveniently. The apparatus employed is a modification of a thermal analyzer, described earlier ( 2 ) , for measuring the trace volatiles content and thermal stability of polymers. It consists basically of a small sample furnace coupled directly to a high-temperature flame ionization detector. For measuring vapor pressure, the sample pan ordinarily used for thermal analysis is replaced by a flow-through sample probe designed to achieve saturation of the carrier gas (nitrogen). Suitable flow controls are provided to permit a selection of flow rates (1) G. H. Bell and A. J. Groszek, J. bist. Petrol., 48, 325 (1962). (2) F. T. Eggertsen, H. M. Joki, and F. H. Stross, Proceedings of the Second International Conference on Thermal Analysis,” Worcester, Mass. August 1968.
through the sample while maintaining a constant flow to the detector. The choice of flow rates allows a rapid check to be made on completeness of saturation of the carrier gas, and also extends the pressure range over which accurate measurements can be made. Other desirable features are provision for temperature programming, a simple means for determining detector response factors for various compounds, and a procedure for cleaning the system between tests by air oxidation. As an alternative to evaporation in the flow-through probe, useful volatilization curves can be obtained also by programmed heating in a sample pan. EXPERIMENTAL Apparatus. The equipment used was that described earlier ( 2 ) except for a special gas saturation probe and additional flow controls required for making vapor pressure measurements. A schematic diagram of the modified apparatus is shown in Figure 1. The sample furnace-FID jet assembly, of Vycor tubing, is heated by resistance wire, with separate windings and temperature controls for the furnace and flame jet sections. The pyrolyzer furnace has an inlet for carrier gas or burn-out air and a standard taper joint to receive the sample probe. Hydrogen for the FID enters at the bottom of the jet. Temperatures are measured with iron-constantan thermocouples placed on the outside walls of the furnace tube and FID jet. The resistance wire heaters on the sample furnace and FID jet are controlled by autotransformers. Alternatively, the furnace tube is heated by means of a temperature programmer. The power for the FID jet is supplied through an isolation transformer and a polarizing voltage for the detector is applied as a bias voltage to the FID heater winding. Such an arrangement is necessary to prevent loss of signal when the detector is heated in this way. The FID detector indicated in Figure 1 was constructed using a shop-made stainless-steel base surrounding the flame VOL. 41,NO. 10,AUGUST 1969
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