CURRENT RESEARCH Atomic Absorption and Optical Emission Analysis of NASN Atmospheric Particulate Samples for Lead Donald R. Scott", David C. Hemphill, Larry E. Holboke, Sharon J. Long, Warren A. Loseke, Louis J. Pranger, and Richard J. Thompson Environmental Monitoring and Support LaboratoryiRTP, Environmental Protection Agency, Research Triangle Park, N.C. 277 11
Lead analyses were performed on 945 quarterly composited particulate samples collected in 1970 by the National Air Surveillance Network by atomic absorption and spark excited optical emission spectroscopy. Duplicate analyses by atomic absorption (283.3-nm line) of 203 samples gave a pooled precision of 3.4%, RSD, with a detection limit of 0.50 pg/ml (0.10 pg/m3). Duplicate optical emission (220.4-nm line) analyses of 800 samples gave a pooled precision of 11%,RSD, with a 9.6-pg/ml (0.15 pg/m3) detection limit. Spike recovery data were 93 f 9% for atomic absorption and 102 f 8% for optical emission data. Analysis of different portions of the same filters gave no significant difference by either method. The lead blank value for glass fiber filters was approximately 1pg/in.2. For 795 samples run by both methods, the most frequently occurring difference in concentration was 8%which was statistically significant at the 99?h confidence level. The emission data were higher than the atomic absorption data. No evidence was found for interferences on the 283.3-nm atomic absorption line. Times required for sample preparation and analysis were 1.2-1.4 man-hours per sample. The determination of lead in atmospheric particulate samples by atomic absorption spectroscopy was apparently first accomplished by Chakrabarti et al. ( I ) in 1965. West and coworkers suggested the use of a hydrogen-oxygen flame and the 217-nm analysis line with the addition of EDTA to the liquid sample to reduce interferences. Further investigations with the 217-nm line by Burnham et al. ( 2 ) using 38 samples collected in the Chicago and Cook County area indicated the necessity of using the standard addition technique to overcome matrix effects. Later work by Burnham et al. ( 3 )showed that the 283.3-nm line gave less noise and baseline shift than the 217-nm line. However, differences of up to 66% were obtained with either line when comparing standard addition and regular calibration data. A solvent extraction procedure was suggested by Sachdev and West ( 4 ) in 1970 to concentrate trace quantities of lead, making an accurate determination possible when using the 217-nm lead line and an air-acetylene flame. In 1970 particulate matter samples collected by the National Air Surveillance Network (NASN) were analyzed with an air-acetylene flame at the 217-nm line by Thompson et al. ( 5 ) .They found that the matrix effect could be overcome by keeping the dissolved solids content less than 0.5% and did not use the standard addition technique. Hwang (6) in 1971 investigated particulate interferences on the 217-nm lead line and concluded that the standard addition technique was required to reduce sulfate and silicate interferences in the air-acetylene Present address, Environmental Monitoring and Support Laboratory, Environmental Protection Agency, Las Vegas, Nev. 89114.
flame. Zdrojewski et al. ( 7 ) reported a very small interference on the 283.3-nm line due to the glass fiber filter extract. To our knowledge, no comparison of optical emission and atomic absorption data for lead in particulate matter for large numbers of samples has appeared in the literature. Because lead is recognized as a toxic atmospheric pollutant, it is necessary to have a rapid, precise, and accurate method for its analysis. Whether or not the standard addition technique is actually required for atomic absorption analysis needs to be determined because of the additional amount of time imposed by the use of this method when handling large numbers of samples. The National Air Surveillance Network (NASN) consists of stations operating on a cooperative basis with local agencies at some 250 sites across the United States (8).This network serves in part to collect samples of air particulate pollutants on glass fiber filters. These samples are then analyzed to provide data on long-range trends of various air pollutants. The NASN particulate samples are routinely analyzed for some 24 trace elements including lead by optical emission spectroscopy. The 1000 NASN samples collected in 1970 were also analyzed by atomic absorption spectroscopy using the 283.3-nm line and an air-acetylene flame. The data obtained from the 1970 samples will be presented in this paper. Precision, accuracy from spiked samples, time requirements, and possible interferences involved with the atomic absorption analysis will be discussed. This study is only concerned with a comparison of the optical emission and atomic absorption techniques. No attempt is made to evaluate the efficiency of the sampling medium. Experimental
Sample Collection and Preparation. Particulate samples were collected at the NASN sites on previously weighed 8 X 10 in. glass fiber filters mounted on standard Hi Vol samplers. The sampling time was 24 h a t an average flow rate of 100 m3/h. The soiled filters, along with flow rate and sampling time data, were returned to EPA in North Carolina. They were reweighed to determine total particulate collected and then partially sectioned to distribute to the various analytical groups. The total exposed filter area was 7 X 9 in. of the 8 X 10 in. filter. A pizza cutter and plastic template were used to cut 1-in. wide strips of appropriate lengths from the exposed filters to total 17.5 i n 2 of exposed filter area per calendar quarter for each site. Each quarterly composite consisted of from five to seven strips, each taken from a different filter. The composited filter strips were ashed for 1 h a t about 150 "C in a lowtemperature asher (Tracerlab LTA 600) under an oxygen atmosphere. The asher rf power was 250 W, the chamber pressure was about 1 mm Hg, and the oxygen flow rate was 50 cm3/min. After being ashed, the composite filter strips were placed in a glass extraction thimble (23 mm id X 62 mm length) which Volume 10, Number 9 , September 1976
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was inserted into an extraction tube. The extraction tube was fitted into the top of a 125-ml Erlenmeyer flask through a 24/40 ground glass joint, and a reflux condenser was fitted to the top of the tube through a 34/45 joint. (Details of this apparatus are shown in ref. 5 . ) The Erlenmeyer flask contained 40 ml of extracting acid containing indium and yttrium at levels so that subsequent concentration of the extract would yield final indium and yttrium levels of 200 kg/ml. The indium and yttrium were used as internal standards for the emission analysis. The extraction acid was a mixture of redistilled reagent grade 70% nitric acid (800 ml) and reagent grade hydrochloric acid (380 ml) made up to a total volume of 2 1. Both of the acids were tested for impurities by emission spectroscopy before use. The extraction unit was placed on a hot plate, and the acid refluxed over the filters for 3 h after the acid mixture attained its boiling point. An apparatus was used to support 32 extraction units per large hot plate. After being refluxed, the extract was carefully concentrated to a volume of 1-2 ml and quantitatively transferred to a calibrated 15-ml glass centrifuge tube. Nitric acid, 70% (containing no indium or yttrium), was added up to a volume of 2.4 ml, and the solution was then made up to a volume of 11.1 ml with 1 : l O diluted 70% nitric acid. The final solution contained about 20% nitric acid and 200 pg/ml indium and yttrium. This solution was centrifuged at 2000 rpm for 30 min to remove suspended material, and then a 5-ml aliquot was transferred to a nitric acid-cleaned polypropylene bottle for storage. The remaining portion of the sample extract was diluted by 1:12.5 with deionized water and retained for atomic absorption analysis. By use of a calibrated automatic dispensing pipet, 0.263 ml of 40% (w/V) lithium chloride solution containing 20% nitric acid and 200 gg/ml of indium and yttrium was added to the 5-ml aliquot before emission analysis. Atomic Absorption Analysis. All of the 1970 NASN sample extracts were analyzed for lead on a Perkin-Elmer 403 atomic absorption spectrophotometer utilizing an automatic sample changer with teletype control. The background corrector was not used during these analyses. This arrangement furnished a punched paper tape output as well as providing a written record of the analyses. The 283.3-nm lead line was used with an oxidizing air-acetylene flame and 1-nm slitwidth which corresponds to a 0.7-nm effective bandpass. The samples were run in blocks of nine followed by a standard. Prior to sample analysis and after every 90 samples, the complete standard series covering the range 0.1-20 gg/ml was run. Working standards were prepared by dilution of certified standards with redistilled nitric acid so that each contained approximately 2% nitric acid and the appropriate lead level. As part of our quality control, 16 blank filters, 16 spiked filters, and 16 replicates (cut from different positions on the same filter) of composites were integrated with the regular samples. Some 203 solutions were reanalyzed at different times in the series of analyses to confirm that they contained 7 fig/ml or more of lead, which corresponds to a relatively high 1.4 pg/m3 for an average air volume of 2500 m3. All initial data tapes were edited on a data handling system composed of a PDP-8/1 digital computer, ASR-35 teletype, and high-speed paper tape reader and punch to remove tape entry errors such as mistyped sample numbers. These corrected tapes were used both to construct linear least-squares calibration curves and to process sample data. Blocks of standards and samples were grouped to form a “floating” calibration curve which was used to process only the samples among which the standards were distributed. This updating served to minimize any instrument instability. The calibration data fit a linear curve to within 1.6-5.5%, relative standard error of estimate, with a mean of 2.6%. These same data were later fitted to a least-squares quadratic curve with no improvement in fit. 878
Environmental Science & Technology
Optical Emission Analysis. The spectrometer was a 2-m ARL Model 9500 Production Control quantometer equipped with a 990 lines/mm grating with 5.2 A/mm reciprocal linear dispersion in the first order. The photomultipliers were either RCA or Hamamatsu 1P28,931A,or equivalent and were distributed over three separate banks covering the spectral range of 2138-7700 A. The ARL source unit was operated under the conditions specified in Table I to produce a high-voltage spark. A Spex 9010 Arc/Spark Stand fitted with a synchronous 20-rpm motor was used with Metbay rotrodes to excite the sample extract which was contained in a porcelain sample boat. The sample extract was 20% in nitric acid and contained 200 pg/ml indium and yttrium as internal standards and 2% lithium chloride as a spectroscopic buffer. The lead line a t 220.4 nm, which was interference free, was used for the analysis line. The indium 451.1-nm and yttrium 324.2-nm lines were used for internal standards. The spectrometer readout system integrated the photomultiplier voltage signal for a fixed time after rotrode wetting and initial spark intervals. Both punched paper tape and typewriter records of the voltage signals for each channel were produced. All samples were analyzed twice. Spiked samples, replicates, blanks, and standards run as samples also were routinely analyzed with the samples. The spectrometer paper tape output was batch processed on the PDP-8 minicomputer with special programs which corrected for background, determined least-squares calibration curves, corrected for interferences, subtracted blank corrections, updated both calibration curves and interferences, calculated both chemical and aerometric concentrations, and kept records of the parameters and analytical data involved in these procedures. Further details concerning the optical emission analysis procedure and data reduction will be published elsewhere (9). Results and Discussion Atomic Absorption Results. During a two-week period, all available 1970 NASN particulate samples collected on glass fiber filters were cut, extracted, and analyzed by atomic absorption. A total of 995 samples were analyzed including quality control samples. The results are listed in Table 11. The mean quarterly composite level for all sampling sites was 1.1 1.1g/m3with a range of
