Final Report: Environmental Applications of Novel Instrumentation for Measurement of Lead Isotope Ratios In Atmospheric Pollution Source Apportionment StudiesEPA Grant Number: R826177
Title: Environmental Applications of Novel Instrumentation for Measurement of Lead Isotope Ratios In Atmospheric Pollution Source Apportionment Studies
Investigators: Keeler, Gerald J. , Graney, Joseph R.
Institution: University of Michigan , The State University of New York at Binghamton
EPA Project Officer: Hahn, Intaek
Project Period: December 1, 1997 through November 30, 1998
Project Amount: $99,776
RFA: Exploratory Research - Environmental Chemistry (1997) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Engineering and Environmental Chemistry
Objective:The measurement of lead isotope ratios coupled with other trace element determinations on a spatial and temporal basis can potentially be used to differentiate between, and trace, the local and regional movement of aerosols from different sources of pollution. In most of the early studies employing this tracer technique with aerosols, thermal ionization mass spectrometry (TIMS) was used to measure the Pb isotope ratios in aerosols, today less precise Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) techniques are more commonly used to trace pollutant sources. The measurement of Pb isotope ratios in precipitation has been less common than in aerosols. However, wet deposition processes often dominate pollutant transfer of heavy metals from the atmosphere, and the incorporation of Pb from aerosols into precipitation has the potential to be traced using Pb isotope ratios. The low concentrations of lead in precipitation and ambient aerosols, the potential for contamination during sampling, and the time and expense involved in measuring isotope ratios to high precision using TIMS are reasons why this tracer approach has not been used extensively to date. The development of magnetic sector-ICPMS techniques has the potential to make precise Pb isotope ratio measurements from precipitation and aerosol samples more routine. The goal of this exploratory research project is to demonstrate how precise measurements of lead isotope ratios from samples with low concentrations can be made in a time and cost efficient manner using magnetic sector ICP-MS instruments including the VG Elemental Plasma54 and the FinniganMat ELEMENT. Once a cost-effective method is developed for measuring precise lead isotope ratios, the measurements can then be coupled with Hg and other trace element determinations for a better understanding of source/receptor relationships.
Summary/Accomplishments (Outputs/Outcomes):To test and contrast the capabilities of magnetic sector ICP-MS and conventional ICP-MS instruments for pollutant tracing, Pb isotope ratios were measured from samples of precipitation and aerosols previously collected to study Hg and trace element transport and deposition in the south Florida/Everglades and Great Lakes regions. In south Florida, event precipitation and aerosol samples were collected from locations proximal to several distinct point sources (oil fired power plants and municipal incinerators) as well as from background sites (offshore) and downwind in the Everglades. In the Great Lakes area, event precipitation and aerosol samples were collected from five sites including a downwind site in central Illinois, an urban site in Chicago, and three rural sites surrounding Lake Michigan.
A brief synopsis of results of the exploratory research are presented in chronological order:
1. First, we successfully developed methods to obtain precise Pb isotope ratio measurements using the Plasma54 multiple-collector magnetic sector ICP-MS; and then contrasted the results from the Plasma54 with those obtained using conventional ICP-MS on "as is" samples of precipitation. "As is" refers to samples of precipitation from which Pb isotope ratios were obtained without separation and pre-concentration of the Pb from other elements prior to analysis. In addition, we also tested another type of instrument that can be used for Pb isotope ratio measurements, the ELEMENT single-collector magnetic sector ICP-MS, and found precision and accuracy of Pb isotopic ratio measurements obtained with the ELEMENT to be similar to that of the Plasma54. The precision of the Pb isotope ratio measurements using the magnetic sector instruments was far superior to those obtained using conventional ICP-MS technology.
2. We then tested if precision of the Pb isotope ratio measurements could be further improved by increasing the Pb concentrations in the solutions to be analyzed using the magnetic sector instruments. This work involved testing several methods to pre-concentrate Pb from precipitation samples including Pb selective resin ion chromatography as well as evaporation techniques.
3. Based on the results from the instrument comparison and pre-concentration studies, a standardized cost-effective method was developed to measure batches of samples for Pb isotope ratios using the ELEMENT. We are now using this batch measurement technique to assess the concept of using lead isotopes as a tracer on several scales. We analyzed the Pb isotope ratios from precipitation and aerosol samples collected over the same time frame from two areas. One included the south Florida/Everglades region where several well- defined emission sources of atmospheric pollutants are located, and the second a larger geographic area with numerous emission sources, the Great Lakes region.
Precision of Lead Isotope Ratio Measurements?Comparisons between Instruments. All of the precipitation samples on which Pb isotope ratios were determined had previously been analyzed for other trace element concentrations, including mercury (from co-located samples). We measured the isotopic ratios of lead using a Perkin Elmer Elan 5000 ICP-MS (PE 5000) prior to analysis of the same samples with the VG Elemental Plasma54 (Plasma54) in Year One and in Year Two tested another instrument, the FinniganMAT ELEMENT (ELEMENT). The Plasma54 and the ELEMENT differ from conventional ICP-MS technology in that the Plasma54 incorporates ICP sample introduction with magnetic sector multiple-collector technology to measure isotope ratios, while the ELEMENT uses magnetic sector single-collector technology. Therefore the Plasma54 and the ELEMENT should be able to achieve high precision analyses and fast, cost efficient throughput, because by design both instruments combine the speed of ICP-MS analyses with high precision measurement of isotope ratios through use of magnetic sector technology. Comparisons of the precision and accuracy using the PE5000, Plasma54 and the ELEMENT were assessed through replicate analyses of a solution of NIST SRM-981 as well as through analyses of the same precipitation samples. Following are examples of the results from comparisons of the PE5000 and the Plasma54 using NIST SRM-981 and precipitation samples reported in the Year One annual report for this project.
The isotope ratios from individual samples measured by the PE5000 and the Plasma54 are similar (within analytical error). Note the wide range in the isotope ratios among samples collected from different locations as well as different rain events at the same location. Therefore, in this sample set, results using the PE5000 to measure differences in isotope ratios might be adequate to suggest differences in lead sources. Note the increase in precision of the ratios obtained using the Plasma54, especially when Pb concentrations are greater than 0.50 ppb. The precision of the Pb isotope ratios obtained using the Plasma54 from precipitation samples with Pb concentrations below 0.50 ppb is probably not adequate for purposes of source apportionment. Pre-concentration of the Pb will be needed from such samples in order to increase the precision of the isotope ratio measurements as discussed below. Nonetheless, the precision of Pb isotope ratios obtained from precipitation samples containing ppb levels of Pb with the Plasma54 (without pre-concentration) is remarkable.
The isotope ratios obtained from individual samples measured by the PE5000 and the Plasma54 are similar (within analytical error). Unfortunately, the precision of the measurements obtained using the PE5000 temper our ability to fingerprint Pb emission sources in south Florida because most of the Pb isotope ratios overlap when measurement uncertainty is accounted for. However, the increase in precision obtained using the Plasma54 indicates that differences in Pb isotope ratios are present. The next step will be to determine if the differences in isotope ratios are sufficient for emission source differentiation and apportionment.
Comparisons Between the ELEMENT and the Plasma54. In order to compare precision and accuracy between the Plasma54 and the ELEMENT, representative results from measuring lead isotope ratios from the same samples of precipitation are presented below:
The isotope ratios from individual samples measured by the ELEMENT and the Plasma54 are very similar (within analytical error). The precision of Pb isotope ratios obtained from precipitation samples containing ppb levels of Pb with the ELEMENT and Plasma54 (without pre-concentration) is remarkable, and a substantial improvement over routine precision obtainable with a conventional ICP-MS. For the purpose of this study, which was to obtain cost effective measurements of Pb isotope ratios, comments about overall instrument cost and capabilities are needed. Due to its multiple-collector design, the relatively expensive Plasma54 is preferred for studies in which very high precision isotopic ratios are needed or required. The single-collector design of the ELEMENT is less costly than the Plasma54 design, and we have found the precision of the Pb isotope ratios obtained with the ELEMENT to be more than sufficient for source apportionment studies utilizing rain and aerosol samples. Therefore the single-collector instrument was found to be more cost effective in this study, because analytical recharge rates were directly related to instrument cost.
Pb Preconcentration Method Development. A technique for efficient extraction and pre-concentration of lead from precipitation has been developed and tested. In most Pb isotope studies, the Pb is separated from other elements and concentrated using sequential HCl-HBr column chromatography with AG 1X8 anion exchange resin. However, this procedure involves numerous time and reagent consuming steps. Because the goal of this study was to develop a time and cost efficient method for measuring Pb isotope ratios, a different pre-concentration approach was used. Specifically, a Pb selective resin called PbSpecTM was used (available from Eichrom Industries). Briefly the method entails passing a 50 ml aliquot of a precipitation sample over 0.1-0.3 grams of the pre-cleaned resin, and then eluting the Pb from the resin using dilute sulfuric acid. Pb recoveries are greater than 90 percent of the expected value, Pb concentration in the collected eluant are 10-20x that present in the original sample, and Pb contribution from the process blanks average less than 0.25 percent of the total Pb measured, provided the resin is pre-cleaned prior to use. In Year 2, we measured the Pb isotopic ratios obtained from the pre-concentrated samples using the Plasma54. We found that a substantial increase in analytical precision accompanied increasing Pb concentrations in solution. However, we also found that the Pb isotopic composition of the blank from the ion chromatography pre-concentration efforts may need to be subtracted from Pb isotope ratios determined from samples that originally contained Pb concentrations less than 0.5 ppb. Rather than having to correct for blank contributions, another simpler pre-concentration method was tested, evaporation of the sample within a cleanhood. By comparison of results between the ELEMENT and the Plasma54, an optimal value for Pb concentrations in solution for the measurement of Pb isotope ratios was determined to be 1 ppb. This 1 ppb concentration is high enough to insure that sufficient Pb is in solution to obtain precise and accurate Pb isotope ratio measurements, and low enough that the evaporation pre-concentration method did not produce levels of total dissolved solids that would interfere with instrument performance.
Further Analysis of Precipitation and Aerosol Samples from the South Florida and Great Lakes Regions. Based on the results from the instrument comparison and pre-concentration method results, a standardized, cost efficient method was used in the next phase of the study. This method involved use of the ELEMENT and analytical solutions with a constant Pb concentration of 1 ppb for determination of Pb isotope ratios. The constant Pb concentration was achieved by either pre-concentrating the samples using evaporation, or diluting the samples with ultra-pure water.
The goal of this phase of the study was to assess the concept of using lead isotopes as a tracer on several scales. Therefore, we analyzed the Pb isotope ratios from precipitation and aerosol samples collected over the same time frame from two areas. One included the south Florida/Everglades region where several well defined emission sources of atmospheric pollutants are located within a small area, and the second involved a larger geographic area with numerous emission sources, the Great Lakes region. In the Great Lakes region, aerosol and rain samples were collected at five locations over a one-year period of time as part of the Lake Michigan Mass Balance Study in 1994 and 1995. Aerosols were collected every six days, and precipitation on an event basis. A subset of the samples were analyzed for their lead isotope ratios; the criterion for sample selection included that from at least two of the sites, aerosols and precipitation had been collected on the same day. Eleven such periods of time were selected, from which 29 measurements of Pb isotopes ratios from aerosols and 29 measurements of Pb isotopes ratios from precipitation were made. In the south Florida region, as a part of the SoFAAMS study, aerosols and precipitation were collected at seven sites from August 6-September 6, 1995. Samples of precipitation and aerosols were chosen from periods of time when all sites registered a rain event. Four periods of time were selected, and a total of 28 samples of aerosols and 28 samples of precipitation were analyzed for their Pb isotope ratios.
Both of these efforts were a success from developing a cost effective, standardized batch method for measuring Pb isotope ratios standpoint. Therefore, we have achieved the overall purpose of this exploratory research grant which was to develop, test, and contrast analytical and instrumental methods to measure Pb isotope ratios accurately, precisely, and cost effectively. However, our long-term goal is to couple the results from measurement of Pb isotope ratios with those of Hg and other trace element concentrations and fluxes for a better understanding of source/receptor relations. This goal will be accomplished by combining lead isotope ratios and other trace element measurements from aerosol and precipitation samples with meteorological data and 3-D trajectory calculations using Regional Atmospheric Modeling (RAMS) and lagrangian particle diffusion (LPDM) and HY-SPLIT transport models (NOAA-ARL).