Final Report: Spark Induced Breakdown Spectroscopy for Detection of Metals in Groundwater and SoilEPA Contract Number: 68D98130
Title: Spark Induced Breakdown Spectroscopy for Detection of Metals in Groundwater and Soil
Investigators: Hunter, Amy J.R.
Small Business: Physical Sciences Inc.
EPA Contact: Manager, SBIR Program
Project Period: September 1, 1998 through March 1, 1999
Project Amount: $69,990
RFA: Small Business Innovation Research (SBIR) - Phase I (1998) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)
Summary/Accomplishments (Outputs/Outcomes):The goal of this EPA Phase I program has been the application of Spark-Induced Breakdown Spectroscopy (SIBS) to the measurement of lead in water and soil. Traditionally, site characterization is accomplished by the collection of samples and the submission of these samples to analytical laboratories. This method is reliable and accurate, but can be very slow. The net effect of the sample/analysis iteration loop is the slowing of characterization and remediation. This translates directly into higher labor costs. When portable real-time field screening instrumentation becomes available, swifter decision-making during remediation will be enabled, saving time and resources.
Prior to our award of the Phase I contract, Physical Sciences Inc. (PSI) had been developing a SIBS-based monitor for airborne metal-containing particulate. This highly sensitive device has been shown to detect lead and chromium near 10 µg/m3 levels, and has been successfully field-tested as a CEM, in a firing range, and used to monitor fugitive chromium emissions over a plating bath. Based upon our understanding of SIBS as a robust and simple methodology to measure metals in real-time, the Phase I program proceeded through logical pathways to demonstrate that SIBS can be used to measure lead in both water and soil.
The SIBS technique is essentially atomic fluorescence spectroscopy of metal-containing samples excited with a powerful electric spark. SIBS is quite different than conventional spark spectroscopy in that the spark takes place with the sample between two electrodes, and does not require a conductive sample. Because this discharge takes place between two electrodes, the first task associated with the Phase I program was the construction of a sampling head for each matrix to provide the sample to the gap. The soil sampling head is a cylindrical device that holds the electrodes fixed on the surface to be sampled and contains a small optical system rigidly held with respect to the electrode gap. The water sampling head likewise provides a defined orientation between the electrode gap and the optical collection system, but allows fluid to be delivered between the electrodes for analysis.
Soils analysis was performed by making sparks that incorporated a small amount of sample material and observing the atomic fluorescence of the elements of interest. Detection is performed after a short delay to allow the initial bright continuum of the discharge to cool; this simplifies the spectroscopy greatly. To analyze the lead content in soils, we developed a standard addition methodology in which known quantities of lead were added to weighed portions of dried soil. This formed a curve (signal intensity versus concentration) that could be readily extrapolated back to the y-axis to determine the lead content of the original sample.
Data analysis involved comparing the intensity of the lead line to nearby iron features which allowed normalization of the signals to amount of material ablated with each spark. The detection limits of lead in soils were typically between 10 and 25 µg Pb/g soil. These detection limits were obtained with the use of one sample and three standards additions on dried (not sieved or pulverized) samples. Actual time needed for measurement is less than 15 min per sample. These detection limits are comparable to or better than those obtainable by portable XRF units.
We used this technique to analyze three very distinct types of soils and to standard reference materials. We cross-correlated the values that we obtained with SIBS with total lead analyses performed at an independent analytical laboratory and in all cases, this correlation was very good. Table 1 confirms this correlation.
Table 1. Comparison of Lead Determinations by SIBS Analysis, by an Independent, Outside Laboratory, and, for the SRM Soils, the NIST Certified Values (all in µg/g)
The standard additions technique is important to quantitatively evaluate the level of lead in soils. This is because the signal intensities vary with the composition of the matrix. We have discovered a correlation between the amount of very small particles in the soil with overall signal intensity. In actual usage, it will likely not be necessary to apply the standard addition technique to each sample at a site. It will likely suffice to use the technique for a few samples, and then use the resultant curve as a calibration curve. This could be done by adding small, known quantities of lead (a lead salt adsorbed on cellulose, for example) to a sample quickly dried and weighed on a portable balance.
Our testing methodology for lead in water evolved somewhat differently. Sample introduction was accomplished with a small jet into the electrode gap so that the character of the plasma would not be governed by the conductivity of the solution, which would be the case with submerged electrodes. Using this strategy, we carefully optimized the geometry of the electrode and sample introduction stream.
In order to evaluate the effect of commonly encountered environmental matrices upon the analysis, we constructed calibration curves of lead nitrate in solutions having widely varying ionic strengths (0 to 4.4 M NaCl and one made up in nitric acid). This served to prove that the presence of large amounts of sodium causes no spectral interferences to our analysis. We also discovered that large amounts of dissociated material in solution actually helps to couple the spark's energy into the water stream. Organic contamination was added in the form of ethylene glycol; because the ionization potential of this compound is lower than that of water, it also enables more efficient coupling of the spark into the stream. Addition of dissolved solids to the sample stream pre-analysis in the field presents us not only a way to increase our signal levels, but also a way to perform preliminary matrix matching.
We also undertook a study to evaluate possible compound specific effects. This was accomplished by constructing two additional calibration curves with lead chloride and lead acetate in water. The slopes of these calibration curves are very similar to that of lead nitrate in an identical aqueous matrix, and seems to indicate that we are insensitive to species effects in water.
Our studies of water yielded a detection limit on the order of 3 to 4 mg/l. This is largely driven by the large amount of variability in the measurements and is discussed at some length within the report. While this can be useful for TCLP work, in Phase II of this project we will propose changes in hardware that will result in significant increases in the sensitivity. The major rationale for our focus on water in Phase II program is the fact that aqueous matrices are an environment problem of much greater magnitude than are solid ones. Further, there exist no sensitive field screening measurements for many metals in water. Therefore, the market may be more receptive to a new instrument than in the case of soils analysis.
During the course of the program, we have been in contact several times with representatives from EPA Region 1 and the Massachusetts Department of Environmental Protection to enable continuing focus on the EPA mission. This has also allowed us to keep the potential end-user and customer in mind. In order for the instrument to be of interest to licensed site providers, it must be simple, rugged and inexpensive. We are tailoring our Phase II instrument design to these criteria. Additionally, at this time we are contacting potential commercial partners for our Phase II work.
SIBS toxic metal monitoring instruments represent a substantial business opportunity, and PSI is currently seeking partnerships with established environmental and occupational safety and health companies to commercialize a family of SIBS instrument products. Our strategy is to team with established instrument companies to take advantage of existing marketing, sales and distribution networks, with manufacturing to be provided by PSI or by our partners under license from PSI, to be negotiated. We have prepared a comprehensive business plan for commercialization of SIBS instruments and have introduced the plan to over a dozen medium to large instrument companies. Our preferred strategy is to commercialize the technology through licensing.