Equation 16 provides a practical means for measuring the standard deviation of jitter, uJ. If the aperture time and sampling interval are approximately equal, Equation 16 shows that the noise spectral density due to jitter increases from zero at zero frequency to a maximum at the Nyquist sampling frequency. Therefore, by digitizing a constant signal, u 2can be determined from the component of the powerdensity spectrum of the signal that both increases with the
mean of the signal and varies with respect to frequency according to Equation 16 (13).
RECEIVED for review June 5, 1972. Accepted November 8, 1972. Financial support by the National Research Council of Canada and the University of Alberta is gratefully acknowledged.
Determination of Organic and Total Lead in the Atmosphere by Atomic Absorption Spectrometry Larry J. Purdue, Richard E. Enrione, Richard J. Thompson, and Barbara A. Bonfield Dicision of Atmospheric Surceillance, Encironmental Protection Agency, National Encironmental Research Center, Research Triangle Park, N.C. Concentrations of total lead in the atmosphere are determined by passing air through a membrane filter for collecting particulate lead and then through an iodine monochloride solution for collecting material which contains lead, probably organic lead species. The lead collected on the membrane filter is determined directly by atomic absorption analysis of acid extracts of the filters. The lead collected in the iodine monochloride solution is determined by extraction with ammonium pyrrolidine-dithiocarbamate and methyl isobutyl ketone followed by analysis on the atomic absorption spectrophotometer. The method is applicable to lead analyses when the organic lead concentrations range from 0.1 to >4.0 pg of organic lead/m3 of air. An atmospheric sampling study for organic lead indicates that the average level of organic lead at these sites i s about 0.2 pg/m3.
METHODSI N CURRENT USE for determining trace amounts of total lead in the atmosphere involve passing the atmospheric samples through a membrane or glass fiber filter for collecting particulate lead and then through a suitable absorbing reagent for collecting any lead compounds that pass through the filter. For the purposes of this paper, any lead compounds that pass through a membrane or glass fiber filter of 0.45-micron pore size are defined as organic lead. This includes volatile organic lead compounds such as tetraethyl and tetramethyl lead. Particulate lead collected by this procedure can be determined by direct analysis of acid extracts of the filters by either atomic absorption or emission spectrometry or by colorimetric dithizone procedures. The organic lead can be collected by use of solid scrubbers such as iodine crystals ( I ) or activated carbon (2), or by absorption in a 0.1M solution of iodine monochloride (ICI) (3). Organic lead collected on these absorbers has been determined by colorimetric dithizone procedures, which are time-consuming, technically complex, and often lack the sensitivity required for atmospheric sampling. This paper describes a method developed for use in the Division of Atmospheric Surveillance of the Environmental Protection Agency (EPA) for analysis of total lead in the atmosphere. The organic lead collected in IC1 solution is chelated -
with ammonium pyrrolidine-dithiocarbamate (APDC) and extracted into an organic solvent for analysis by atomic absorption spectrophotometry. Iodine crystals were not evaluated as a collection medium in this study because the repetitive shaking required is not practicable because of 24hour unattended sampling procedure used in the National Air Surveillance Network ( 4 ) . Activated carbon, a suitable collecting medium for 24-hour unattended sampling, could possibly be used as the collecting medium for this method with appropriate adjustments to the extraction procedure. EXTRACTION OF LEAD FROM ICL SOLUTION
Determination of the amount of lead present in the IC1 absorbing solutions involves reducing the iodine with sodium sulfite and hydroxylamine hydrochloride, and adjusting the pH to 3.6 in order to transfer the APDC-lead complex quantitatively to an organic solvent. APDC was chosen as the complexing agent and methyl isobutyl ketone (MIBK) as the organic solvent because these compounds are widely used in organic solvent methods for atomic absorption spectrometry (S). MIBK is considerably soluble in reduced IC1 solutions (approximately 3z VjV). Methyl N-amyl ketone, which is less volatile and less soluble in reduced IC1 solutions (approximately 0 . 6 z VjV) may also be used as the organic solvent for this method. EXPERIMENTAL
Reagents. IODINEMONOCHLORIDE (1.OM). T o 800 ml of 25% (WjV) potassium iodide solution: add 800 ml of concentrated hydrochloric acid and cool to room temperature. Slowly add 135 grams of potassium iodate while stirring the solution vigorously and continue stirring until all free iodine has dissolved to give a clear orange-red solution. Cool to room temperature and dilute to 1800 ml. IODINEMONOCHLORIDE (0.1M). Dilute 100 ml of 1.OM iodine monochloride to 1 liter. This reagent is stable for an indefinite period under ambient laboratory conditions. HYDROCHLORIC ACID-NITRICACID MIXTURE.Add one volume of distilled constant boiling (about 19 hydrochloric acid to four parts 40% distilled nitric acid.
z)
( 1 ) “Tentative Method of Test for Lead in the Atmosphere.”
ASTM Book of Standards, Volume 23, American Society for Testing and Materials, Philadelphia, Pa., 1970. (2) L. J. Snyder, ANAL.CHEX,39, 591 (1967). (3) R . Moss and E. V. Browett, Aiinlyst (Loiido/i),9, 428 (1966).
(4) G. B. Morgan, C. Golden, and E. C . Tabor, J A P C A , 17, 300 (1 967). (5) W. Slavin, “Atomic Absorption Spectroscopy,” Interscience Publishers, New York, N. Y . . 1968. p 75.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
527
~
Table 111. Effects of'organic Lead Absorption on Loaded Prefilters Sample IQ Sample IIb Organic Particulate Organic Particulate lead, lead, lead, lead, Dayc m/m3 4 m 3 pdm3 pg/m3
Table I. Analyses of Five Replicate Samples Taken in an Underground Parking Garage Organic Organic ParticuTotal lead, lead, late lead, lead, Sample No. IJg/m3 llg/m3 &/m3
z
1 2 3 4 5
15.7 17.1 17.6 14.9 18.0 16.7
1.9 1.9 1.9 1.8 2.2 1.9
Average (8) Rel. std. devia7.9z tion (Srel). 7.9 Standard deviation a Srel = x 100. Average
10.3 9.2 8.9 10.3 10.0 9.7
12.2 11.1 10.8 12.1 12.2 11.7
2.0 2.0 1.9 1.9
5.2
0.0 0.0
0.0 0.0
BUFFERSOLUTION (3.4 pH). Dissolve 17.0 grams of potassium acid phthalate in water. Add 1.4 ml of concentrated hydrochloric acid and dilute to 1 liter with distilled water. STANDARD INORGANIC LEADSOLUTIONS (1000 bg Pb/ml). Dissolve 1.598 grams of dried lead nitrate in 1 liter of 0.5z nitric acid. Prepare working solutions of 1, 5, and 10 pg of lead per ml by appropriate dilution of stock solution with 0.5 nitric acid. Prepare working standards daily. STANDARDORGANIC LEAD SOLUTION(1000 pg Pb/ml). Dissolve 0.2437 gram of lead N,N-diethyl dithiocarbamate in 100 ml of MIBK and store in brown reagent bottle. Prepare working solutions of 0.5 and 1 pg of lead per ml by appropriate dilution of stock solution with MIBK; prepare working solutions daily; prepare a fresh stock solution weekly. Apparatus. GASSAMPLER.A sampling device developed by EPA ( 4 ) and manufactured by the Research Appliance Corporation (RAC), Allison Park, Pa., was used for collecting air samples for evaluating this method. The gas sampler consists of a glass inlet manifold, membrane prefilters, tubes containing appropriate collecting solutions through which a sample can be passed, a n air trap, membrane exhaust filters (to protect critical orifices), a n outlet manifold, and an external vacuum pump. Number 20 gauge hypodermic needles were used as critical orifices in order to obtain a 2.5-m3 air sample at a constant flow rate of about 2 liters per minute for a 24-hour period. The traps for the gas sampler were filled with activated carbon (10-20 mesh) in order to protect the critical orifices from IC1 vapors. Glass tubing with a 1-2 mm i.d. opening was used as dispersers for the air stream. This sampling device, currently in use by EPA, allows collection of up to five samples simultaneously. Any sampling train consisting of a prefilter, a n air scrubber, a trap, an air measuring device, and a vacuum pump is suitable for this method. Millipore type H A (0.45-micron pore MEMBRANE FILTERS. size) were used. SEPARATORY FUNNELS. These were 125-ml capacity with Teflon (Du Pont) stopcocks and stoppers. Procedure. SAMPLECOLLECTION. Add 50 ml of 0.1M IC1 to each of the sample tubes in the gas sampler. Connect a membrane filter to the inlet of each tube and connect the vacuum pump to the outlet manifold of the gas sampler. Determine the floa rate through each tube with a wet test meter.
z
528
e
1.8 1.6
2.0 1.8
11.1 7.3
Average, Day 1 and Day 2 1.7 8.9 1.9 9.2 Same prefilter used for both days. New prefilter used each day. c Results for each day are an average of three determinations. Q
6.1%
Table 11. Results of Collection Efficiency Study for Organic Lead Organic lead, pg/m3 Sample No. 1st Bubbler 2nd Bubbler 1 2 3 4
1 2
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
Sample for 24 hours, collecting approximately 2.5 m3 of air. Redetermine ihe flow rates with the wet test meter. PARTICULATE LEAD ANALYSIS.Remove the membrane filter with care from the holder and place it in a 100-ml beaker. Add 5 ml of water and 5 ml of the hydrochloric acid-nitric acid mixture and digest o n a hot plate for 1 hour. Allow the solution to cool. Dilute the mixture to 10 ml. Determine the lead concentration of each solution directly by atomic absorption spectrophotometry by comparing the absorbance of each sample extract with the absorbance of standard lead solutions made up in 0.5 % nitric acid. Multiply the number of micrograms of lead per milliliter of sample extract by the appropriate dilution factor and divide by the number of cubic meters of air represented by the sample. Determine the cubic meters of air sampled from the average of the two flow measurements taken before and after sampling. ORGANICLEADANALYSIS. Disconnect each of the tubes containing the IC1 solution from the gas sampler. Empty each sampling solution into a 150-ml beaker and rinse the disperser, tube, and cap with distilled water into each beaker. Make two blank IC1 solutions by pouring 50-ml portions of 0.1M IC1 into each of two 150-ml beakers. Make two 5-pg lead standards by adding 1 ml of 5 pg Pbiml standard to each of two 50-ml portions of IC1 solution. Titrate all blanks, standards, and samples with 20% sodium sulfite to a colorless end point. Add two drops of methyl orange indicator. Titrate with 1 0 % ammonium hydroxide to a yellow end point. Add 10 ml of 3.4 p H buffer to each solution. Add 2 ml of 1 % APDC. Add 1 ml of 1 hydroxylamine hydrochloride. Measure the pH of each solution and adjust to 3.6 0.1 with dilute hydrochloric acid or dilute ammonium hydroxide if necessary. Transfer each sample to a 125-ml separatory funnel, rinsing each beaker with distilled water. Add 5 ml of MIBK. Shake each separatory funnel for 1 minute, then drain off and discard the aqueous layer. Loosely pack the stem of each separatory funnel with a small piece of cotton and drain the remaining MIBK into a small container that can be tightly capped. Analyze each of the MIBK solutions on the atomic absorption spectrophotometer according to the procedure recommended by the instrument manufacturer for analysis of organic solutions. Adjust the nebulizer for optimum response using a 1-pg/ml organic lead standard in MIBK. Determine the amount of lead in the MIBK extracts of each sample by comparing the absorbance of each sample extract with the absorbance of the MIBK extract of the lead standards in IC1 prepared by the same procedure used with the samples. Correct both samples and standards for the average blank absorbance. Calculate concentrations of organic lead by dividing the number of micrograms of lead found in the MIBK extract by the volume of the sample in cubic meters.
*
RESULTS AND DISCUSSION
Organic Lead Analysis. CALIBRATION.The response of the atomic absorption spectrophotometer for organic lead
,!hnplea NO.^ 1 2 3 4
5 6 7 8 9 10
Table IV. Results of Atmospheric Sampling for Organic and Particulate Lead Cincinnati Denver Washington, D.C. St. Louis Philadelphia ___Org. Part. Org. Part. Org. Part. Org. Part. Part.c 0rg.b Pb Pb Pb Pb Pb Pb Pb Pb Pb Pb 0.5 1 .3 0 . 5 2 . 0 0.0 1.4 0.2 2.3 0.2 1.5 2.0 1.1* 1.7 1.4 0.2 0.2 0.5* 1.8 0.0 0.9 0.0 0.2 0.4 0.2 0.2 0.4 0.2 0.2 0.2
0.9 1.9 3.7 1.6 1.8 1.3 1.5 2.4
1.4* 0.3 0.2 0.1 0.0 0.2 0.2 0.5* 0.2
1.6 1.7 1 .o 1.8 2.2 1.8 2.0
...
0.1 0.3 0.1 0.1 0.1 0.2 0.4 0.2 0.2
0.4 0.2 0.3 0.2 0.3 0.4 0.4 1.3* 0.3
1.5 1 .o 1.1 0.8 1.7 1.2 0.8 1.6 1.2
1.8 2.5 1.9 1.8 2.2 2.2 2.0 1.2 2.0
0.2 0.4 0.8 0.4 0.0 0.3 0.1 0.1 0.3
2.9 2.4 1.1 1.1 1.7 2.1 1.8 2.3 1.8
Chicago Org. Part. Pb Pb 0.2 0.4 0.2 1.6* 0.2 0.2 0.3 0.1 0.2 0.2 0.2
5.9
5.6 5.3 3.5
4.6 5.5 5.8 4.8
5.0 5.1
Av.~ 1.7 1.8 5.1 Cities were not sampled concurrently. a Sample No. represents order in which samples were taken. b Org. Pb = Organic lead (pg/m3). All values are average of two determinations. c Part. Pb = Particulate lead (pg/rn3)). Determined by NASN method (6). d Organic lead averages do not include starred values because the replicate values for these samples were more than twice the standard deviation from the average.
samples is determined in terms of absolute amounts of lead added to 50 ml of IC1 and extracted with 5 ml of MIBK. The instrumental response is linear to greater than 10 pg of lead, and the minimum amount of lead analytically detectable is 0.5 pg; for a 2.8-m3air sample, this corresponds to a range of 0.2 to >4.0 pg organic lead per m 3 of air. The relative standard deviation (standard deviation divided by average and multiplied by 100) of the absorbance for eight replicate standards of 2 p g of lead added to 50 ml of IC1 was 8.2%;. Blank absorbance is significant and must be corrected for each sample. Blanks and standards must be analyzed with each batch of samples because of difficulties in adjusting parameters to exactly the same response for every analysis. Table I shows the results of five replicate samples taken in a n underground parking garage, where relatively high concentrations of organic and total lead would be expected. Filterable and particulate lead levels are summed to total lead concentrations and the % organic lead is given. The R A C gas sampler, with a prefilter for each sample tube, was used to collect the samples. The relative standard deviation for these five replicate samples, with a n average organic lead value of 1.9 pg/m3, was 7 . 9 x . The relative standard deviation for five replicate samples taken in a n area with a n average organic lead value of 0.4 pg/m3 was 34%. COLLECTION EFFICIENCY.Moss and Browett (3) determined collection efficiencies for tetra-ethyl and tetra-methyl lead in 0.1 IC1 solutions using 100 ml of solution with a Dreschel-type scrubber. Their results indicate that recovery of these compounds is nearly 100% at a flow rate of 1.89 liter/ min (4 ft3/hr). Since in this method only 50 ml of solution was used with an open-tube bubbler (1-2 m m i.d. opening), the collection efficiency was checked under these conditions. Table I1 shows the results of a recovery study made by sampling in the underground parking garage with a sampling train incorporating a prefilter and two bubblers in series. These results indicate that the collection efficiency for this method is essentially quantitative. The effect of organic lead absorption o n the prefilter, which becomes loaded with particulate matter during a 24-hour sampling period, was examined. Two R A C gas samplers with three bubblers in each box were set u p in the underground parking garage to collect air from a common source. One sampler collected two 24-hour samples, the same prefilter being used on both days. The other satnpler also collected
Table V. Per Cent of Particulate Lead of Total Lead Found in Urban Air Organic PW (~gim9
Particulate Pba
z
Particulate Pbb
City ( Pg i r n .9 0.2 1.7 Cincinnati 0.2 1.8 Denver 0.2 1.2 Washington, D.C. 0.3 2.0 St. Louis 0.3 I .8 Philadelphia 0.2 5.1 Chicago All values are averages of ten determinations. Particulate Pb X 100 b Per cent particulate Pb = Organic Pb + Particulate
89 90 86 87 86 96
Q
Pb
two 24-hour samples on the same two days but was equipped with a new prefilter each day. The results of this study, shown in Table 111, indicate that prefilters loaded with particulate matter d o not appreciably absorb organic lead. In other studies, samplers were operated concurrently, some equipped with new prefilters and others with used, heavily loaded prefilters; such studies also indicate that there is no detectable absorption of the organic lead o n the loaded prefilter a t lead levels measured in the underground parking garage. Particulate Lead Analysis. Analysis of the membrane filters for particulate lead is a relatively simple and direct application of atomic absorption spectrometry. The detection limit for particulate lead in the procedure described is 0.4 pg lead/m3. The relative standard deviation for five replicate samples with an average lead value of 9.7 pg/m3 was 6.7 %. Particulate lead can be determined independently from the organic lead, as in the atmospheric sampling study to be described ; however, for most applications, separate determinations are convenient or necessary. Atmospheric Sampling Study. A six-city lead-in-air survey was conducted in six cities at EPA’s Continuous Air Monitoring stations, which are located in downtown areas close to busy streets or intersections. The amount of organic lead in (6) R . J. Thompson, G . B. Morgan, and L. J. Purdue, A t . Absorprio/i Newslerr., 9, 153 (1970). ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
529
the air a t these cities was determined by use of R A C gas samplers with two sampling tubes in each box and a common membrane prefilter for both sampling tubes. The particulate lead collected on the membrane prefilters was not determined for this study. Two replicate organic lead samples were collected in each 24-hour sampling period at each of the six cities. The organic lead analyses for each city (Table IV) indicate that the average level of organic lead a t these cities is about 0.2 pg/m3. Although this value is close to the detection limit of the method, it appears to be relatively constant for the six cities sampled. Five of the samples listed in Table IV showed organic leas values greater than 0.5 pg/m3; however, two replicate values for each of these samples almost invariably did not agree, a n indication of error in the collection or determination of these samples. This number of questionable samples was not unexpected, considering that this study was the first field evaluation of the method. Samples for which replicate values differed from their average by more than twice the standard deviation (34 relative standard de-
viation a t 0.4 pg/m3) were not included in calculations of the average organic lead value for each city. I n order to determine the total airborne lead and percentage lead in the particulate form, organic lead was sampled with bubbler boxes side-by-side and concomitantly with collection of suspended particulate matter on glass fiber filters using a Hi-Vol sampler. The results of this study are given in Table V ; although almost all ( 8 9 z ) of the lead found in the atmosphere in this study was in the form of particulate matter. Although the organic lead levels were found to be comparable in all cities, the particulate lead concentrations varied by as much as a factor of 3 from city to city. This suggests that organic lead in urban air approaches a steady state. RECEIVED for review August 8, 1972. Accepted November 6, 1972. Mention of product or company name does not constitide endorsement by the Environmental Protection Agency.
Investigations into the Use of a Pulse Ultrasonic Nebulizer-Burner System for Atomic Absorption Spectrometry N. E. Korte and J. L. Moyers Atmospheric Analysis Laboratory, University of Arizona, Tucson, Ariz. 85721
M. B. Denton’ Department of Chemistry, Unicersity of Arizona, Tucson, Ariz. 85721 An integral pulse ultrasonic nebulizer-burner system is described which com bines the desirable characteristics of flame atomization with an ultrasonic nebulizer system capable of reproducibly nebulizing small volumes of solution. Data are presented comparing this sytem with a conventional pneumatic slot burner system, showing improved sensitivity and a reduction in the required sample volume.
ATOMICABSORPTION SPECTROMETRY has been widely accepted as a sensitive and practical analytical technique for trace metals. Most commercial systems employ indirect pneumatic nebulization into a long-path flame. Such pneumatic systems generate a rather large distribution of drop sizes (1-3). Generally, the larger droplets are selectively removed by baffling systems within the burner before the aerosol reaches the flame. Eliminating the larger droplets reduces light scattering problems such as those observed in total consumption burners and has been shown to reduce certain types of interferences ( 4 ) . However, such systems d o not make effiAuthor to whom correspondence should be addressed. (1) J. Stupar and J. B. Dawson, Appl. Opt., 7, 1351 (1968). (2) John A. Dean and William J. Carnes, AXAL.CHEM.,34, 192
(1962). (3) John A. Dean and Theodore C. Rains, Ed., “Flame Emission and Atomic Absorption Spectrometry-Volume I, Theory,” Marcel Dekker, New York, N.Y., 1969. (4) William B. Barnett, ANAL.CHEM.,44, 695 (1972). 530
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
cient use of the sample since only 3 to 12% of the solution aerosol is actually introduced into the flame (3,5). Through the use of high frequency ultrasonic nebulization, an aerosol can be generated which is composed of only small droplets which can be efficiently introduced into the flame with negligible loss of sample. A previously described ultrasonic nebulizer-burner system ( 6 ) showed an average improvement of more than an order of magnitude in sensitivity; however, this type of system requires 20 to 50 milliliters of sample solution. This requirement is a severe limitation in analyses where sufficient quantities of sample are not available. A number of flame-related “sampling boat” (7, 8) and flameless techniques-filaments (9-11), ribbons (22, 13), and (5) R. Herrmann and C. T. J. Alkemade, “Chemical Analysis by Flame Photometry,” Wiley-Interscience, New York, N.Y., 1963.
(6) M. B. Denton and H. V. Malmstadt, ANAL.CHEM.,44, 241 (1972). (7) H . L. Kahn, G. E. Peterson, and J. E. Schallis, At. Absorption New’slett, 7 (2), 35 (1968). (8) J. D. Kerber and F. J. Fernandez, !bid.,10 (3), 78 (1971). (9) R. G. Anderson, I. S. Maines, and T. S. West, Atial. Chim. Acra, 51, 355 (1970). (10) R. Woodriff. B. R. Culver, and K. W. Olson. Appl. Spectrosc., ‘ 25, 328 (1971).’ (11) R. Woodriff and D. Shrader, ANAL.CHEM., 43, 1918 (1971). (12) J. Y . Hwang, P. A. Ullucci, S. B. Smith, and A. L. Malenfant, ibid., p 1319. (13) H. M. Donega and T. E. Burgess, ibid., 42, 1521 (1970) ~~
