Determination of Trace Amounts of Organic Lead in Air Composite Sample Method Louis J. Snyder Research and Dmelopment Department, Ethyl Corp., Baton Rouge, La. A composite sampling method has been developed for measuring trace quantities of organic lead in ambient air. The mean error of the method i s +0.001 pg/m3 and the precision (standard deviation of the difference between the quantities found and present in known samples) i s +0.004 pg/m3 of air. The method consists of collecting a large air sample (100 m 3 to 200 m3) by passing it through an activated carbon scrubber for a period of two to four days. Organic lead collected in the activated' carbon is decomposed, extracted from carbon, and colorimetrically measured by a single extraction dithiizone method. Interference from particulate lead is eliminated by effective filtration prior to collecting organic lead. The average quantity of organic lead found in ambient air in Los Angeles for a six-week period of time was 0.078 pg/m3, or approximately one one,.thousandth of the maximum allowable limit of 75 pg/nn3 of air for an 8-hour exposure. The method is of sufficient accuracy to detect changes in the organic lead content of the atmosphere at this low level.
THEDETERMINATION of inorganic lead in air is a well known subject. Numerous methods are available for both the collection and analysis of samples. The determination of organic lead (tetraethyllead or tetramethyllead) in ambient air, however, is of recent interest only. Prior to 1962 no method was described in the literature for measuring less than 0.1 pg of organic lead/m3 of air although methods were available for measuring higher concentrations (75 pg13m) associated with permissible occupational exposures ( I , 2, 3). In January 19612we attempted to measure the organic lead content of select urban areas by modifying a field method for measuring occupational exposure to organic lead (3) and improving its precision (twice the standard deviation of the difference between the quantities found and present in known samples) to *0.3 pg of organic lead/m3 of air by collecting 2.5 M 3 air samples through 20 grams of crystalline iodine in 1 hour. Using this modified method we measured the organic lead content of 36 air samples collected in Baton Rouge, La., and Los Angeles, Calif. Results from these analyses demonskated that the average organic lead content of the atmosphexe in the above two cities is well below the sensitivity of the improved method (&0.3 pg/m3). Therefore, to measure changes in the organic lead content of ambient air over urban areas it became necessary to develop an analytical method of improved accuracy. The development of such a method, proof of its accuracy, and typical organic lead-in-air analyses are described below. EXPERIMENTAL
Reagents. Niiric-Perchloric Acid Solutions. Mix 200 ml of perchloric acid (72x7,)with 300 ml of concentrated nitric acid. (1) J. P. h i k e and I. J. Bloomfield, U.S. Public Health Service Bull., 163 (1962). (2) L. J. Snyder, Vr'. R. Barnes, and J. V. Tokos. ANAL.CHEM., 20,
772 (i948). (3) L. J. Snyder and S. R. Henderson, !bid., 33, 1175 (1961).
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ACTIVATED CARBON (10.0 GRAMS
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G L S S WOOL SUPPORTS
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Figure 1. Organic lead-in-air scrubber for composite sampling Reagent Solution. Weigh 20 grams of potassium cyanide, 40 grams of dibasic ammonium citrate, 200 grams of anhydrous sodium sulfite, and dilute to 1 liter with distilled water. Add 600 ml of concentrated ammonium hydroxide. Prepare this solution in a well ventilated hood. Nitric Acid (1 :4), Dilute 200 ml of concentrated nitric acid to 1 liter with distilled water. Buffer Solution. Dilute 400 grams of dibasic ammonium citrate, and 40 grams of potassium cyanide to 1 liter with distilled water. Mix the I liter of citrate-yanide solution with 2 liters of concentrated ammonium hydroxide (28 Dithizone Solution. Dissolve 60 mg of dithizone in 1 liter of chloroform. Activated Carbon. MSA No. 24207 or activated carbon from MSA organic vapor cannister No. 2306. Grind the carbon particles in a coffee mill and screen in a stainless steel sieve. Collect particles from 30 to SO mesh only. Store in tightly covered glass container. Sodium Diethyldithiocarbamate Solution. Dissolve 1 gram of the reagent in 500 ml of water. Apparatus. See Figure 1 for details of the activated carbon scrubber. Use a millipore membrane filter, Type HA, and filter holder. Use any suitable gas meter with capacity to measure up to 1 cubic foot of air per minute, and any vacuum pump with a capacity of 1 cubic foot of air per minute. Use a Lindberg hot-plate, Type H-2, or steam bath. See reference (4) for details of the spectrophotometer (Beckman Model DU) and modified absorption cell and cell compartment cover. Purchase No. 30 and No. 50 stainless steel sieves from W. S. Tyler Co., Cleveland 14, Ohio. Grind
x).
(4) S. R. Henderson and I.. J. Snyder, ANAL.CHEM.,31, 2113 (1959). VOL 39, NO. 6, MAY 1967
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activated carbon in a Colonial Coffee Mill, No. 1147, rnanufactured by Wrightsville Hardware Co., or the equivalent. Procedure. Add 10 grams of activated carbon (30 X 50 mesh) and a small wad of lead-free glass wool t o the scrubber shown in Figure 1. Cover the dry glass joint with electrical tape to avoid air leaks. Use no stopcock grease on the glass joint. Connect the top of the carbon scrubber to the millipore filtering device and the bottom of the scrubber to a vacuum pump and gas meter. Collect approximately 200 cubic meters of air a t a mte of about 0.7 ft3/minute. Disconnect the scrubber from the sampling train and remove the tape from the 24/40 glass joint. Remove the male glass joint from the scrubber and carefully pour the activated carbon from the scrubber into a large mouth 500-ml Erlenmeyer flask. While the scrLbber is inverted force the glass wool into the Erlenmeyer flask. With a freshly prepared mixture of concentrated hydrochloric acid (75 ml) and concentrated nitric acid (25 ml) rinse all carbon from the scrubber into the Erlenmeyer and wash down the sides of the Erlenmeyer flask. Heat overnight on a low temperature hot plate (100" C) or steam bath. Add 30 ml of concentrated H N 0 3 acid to the nearly dry residue, heat (100" C) for 1 hour, add approximately 100 ml of H 2 0 , and mix well. Allow the mixture to stand a t room temperature for 2 to 3 hours. Decant off the supernatant liquid into a lead-free 500-ml Erlenmeyer flask. Filter the remaining carbon from The acid extract through a Whatman No. 41-H filter paper and collect the filtrate in the 500-ml flask. Rinse the residual carbon with three portions of distilled water, Discard the carbon and filter paper. To the acid extract in the 500-ml Erlenmeyer flask, add 20 mi of nitric-perchloric acid solution (2:3) and heat on a hot plate (1 50' C) to fumes of perchioric acid. If all carbon is not oxidized, add another 10-ml portion of nitric-perchloric acid solution and again heat to fumes of perchloric acid and cool. Add 20 ml of nitric acid (1 :4) and approximately 25 mi of distilled water to the perchloric acid in the 500-ml flask. Aliow 30 minutes for complete solution of sample a t 30" to 40" C and transfer the mixturt: to a 250-ml modified absorption cell. Add 50 mi of buffer solution and 20 ml of reagent solution, mix, and allow 20 minutes for complete reductiqn of the sample. Add 5.0 ml of dithizone solution and shake the mixture vigorously for 30 seconds. Insert the modified absorption cell into the Beckman spectrophotometer and measure the absorbance of the lower layer at 510-mp wavelength, using air as a reference (10Oz transmittance). Add 5 mi of sodium diethyldithiocarbamate solution to the modified absorption cell, shake it vigorously for 15 seconds and immediately measure the absorbance of the lower layer. The difference between the two absorbance readings represents the quantity of lead dithizonate present in the sample. Sodium diethyldithiocarbamare quantitatively decomposes lead dithizonate. Subtract a biank on all reagents carried through the complete procedure. From a standardization curve (micrograms of lead per unit of absorbancej, calculate the amount of lead present in tne sample and report the result as micrograms of lead per cubic meter of air. I f a largequantity of lead dithizonate is present in the chioruform solution (absorbance reading over 2.0) add a n additional 5.0-ml portion of dlthizonc solution to the sample and measure the lead R S previously described for 5.0 ml of dithizone solution. precautions. The extraction of lead from activated carbon is approximately 98 to 100% when employing the resommended method; short digestion periods (1 to 4 hours) may produce low results. When adding sodium diethyldithiocarbamate t o lead dithizonate solution, measurements must be made within 30 t o 45 seconds after shaking t o ensure a n accuracy of measurement to i 0 . 0 1 absorbance unit. Upon long standing, sodium diethyldithiocarbamate apparently decomposes some of the 592
ANALYTICAL CHEMISTRY
excess dithizone. Upon further shaking of the mixture, excess dithizone redistributes from the aqueous to the chloroform solution. Large quantities of sodium diethyldithiocarbamate accelerate decomposition of excess dithizone. Disodium ethylenediaminetetraacetate (2 grams/500 ml water) may be substituted for sodium diethyldithiocarbamate solution. Both reagents quantitatively decompose lead dithizonate in chloroform solution. The disodium salt, of EDTA decomposes lead dithizonate a t a much slower rate than sodium diethyldithiocarbamate. Therefore, a longer shaking period (90 to 120 seconds) is required. The resulting solution is considerably more stable than when using sodium diethyldithiocarbamate; therefore, more time is permissible for absorbance measurements. In this respect it is a better reagent. Use of sodium diethyldithiocarbamate or the sodium salt of EDTA is possible only if the final lead dithizonate color is developed in a modified absorption cell (4). Also, if the analyst is interrupted and can't make an absorbance measurement within 45 seconds after decomposing lead dithizonate with diethyldithiocarbamate, nothing is lost. The analyst may shake the mixture another 10 to 15 seconds and read the absorbance immediately, or a t least within the required 45 seconds. Accurate absorbance measurements can be made over an extended period of time by successively shaking the mixture for 10 to 15 seconds and immediately reading the absorbance after each shaking period. The number of times the mixture is shaken is unimportant, but the time required to make the absorbance measurement after shaking is important. The shaking period and absorbance measurement may be repeated several times to ensure equilibrium and accurate photometric measurements. Use of sodium diethyldithiocarbamate or disodium ethyl., enediaminetetraacetate is optional. If the analyst prefets, he may heat the nitric-perchioric acid solution containing all the lead to almost dryness (1 ml). Then dissolve the cooied residue in 20 ml of nitric acid (1 :4) and approximately 25 m: of water and continue from this point on as described in the procedure, Use of these masking reagents eliminates thc need for careful control of pH when developing the lead dithizonate color The absolute quantity of lead in 10 grams of MSA carbo:i (No. 24207) is less than 1 pg. Deleading of solutions is unnecessary because the total lead blank including carboil and glass wool is approximately 2.5 pg. The weight of glass wool used in all scrubbers should be uniform. The long acid digestion period extracts some iead from the borosilicateglass-wool and contributes to reagent blanks. ACCURACY OF METHOD
An air sample containing 0.100 pg of organic Iead/m3 is equivalent t o approximately 11 parts per trillion of organic lead. Therefore, to measure the accuracy of a n analyticai method designed to remove quantitatively parts per trillion of organic lead from air, it was necessary to prepare and analyze known samples of this concentration, To accomplish this the organic lead evaporato; shown in Figure 2 was fabricated. It consists of a 1-dram vial placea within a conventional lead-in-air scrubber ( 5 ) . Into the 1-dram vial is added microgram quantities of tetramethyllead (boiiing point 110" C ) and a sufficient volume (2 to 3 ml) of toiuene (boiling point 110" C) to vaporize completely whiie drawing approximately 200 m 3 of air through the evaporator. The relatively constant evaporation rate of the toluene-tetramethyllead mixture provides a nearly homogeneous organic lead-in-air sample during the period of time in which air i s (5) R. D. MacPhee, M. G. Eye, and E. E. Parkinson, APCD, Los
Angela County, Sept. 1962.
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1- KNOWN TML-IN-AIR SAMPLE
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RUBBER STOPPER
AIR INLET
Figure 3. Sampling train for preparation of known tetramethylead-in-air samples I
(20 MICROGRAMS OF TML IN 2 - 3 ml OF TOLUENE)
Figure 2. Evaporator for preparation of known TML-in-air s:amples
drawn through the entire assembly. Larger samples may be Drepared by simply passing greater quantities of air through the evaporator containing a larger volume of toiuene diluent. Employing this sam Jle-preparation techniquz, we established the accuracy of the recommended analytical method, including sampling and measurement of collected lead, a s follows. Atmospheric air wa; purified by passing it through a 16-inch bed ai activated carbor. and then through a millipore filter (Type HA). The purified air was next passed through the tetramethyliead evaporator (Figure 2) which ensured the deiivery of a known and homogeneous organic lead-in-air sampic of iarge volume :lo0 to 250 m 9 . The known sample was then passed through the recommended scrubber (Figure 1) and iinally through a vacuum pump and gas meter. The knowr: slimpies were ;oliei:ted over approximately a 4- to 7-da) i7eriod of time and anaiyzed by the recommended method described above. A blank was Larried through the entire procedure, bur with IO tetramethyllead added, and subtracted f :om the known sam Ae. A diagram of the complete sampling train. including blank sample, is shown in Figure 3. Results i'rom thebe experimxm (Table l j demonstrate the organic iead can be measur-d with a precision (twice the standard deviation) of I O.Oi)l! pg:m3 of air. APPI.ICATIOh' OF METHOD
.+\iter standardizir g the organic iead-in-air method, i t wz'
