Grantee Research Project Results

Final Report: A Portable Spectrometer for the Accurate Determination of Arsenic in Waters

EPA Contract Number: 68D02025
Title: A Portable Spectrometer for the Accurate Determination of Arsenic in Waters
Investigators: Gurleyuk, Hakan
Small Business: Frontier Geosciences Inc.
EPA Contact: Richards, April
Phase: I
Project Period: April 1, 2002 through September 1, 2002
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2002) RFA Text |  Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)

Description:

In this Phase I research project, Frontier Geosciences Inc., investigated the feasibility of a portable hydride-generation atomic fluorescence spectrometer (AFS) for the determination of low levels of arsenic in waters. Considering the new regulations, a detection limit of 0.1 µg/L was targeted. The final product will be operated using portable power generators and controlled with a laptop computer, eliminating packaging and shipping of samples to distant laboratories and yielding real-time information at a lower cost.

Currently, atomic spectrometric detectors are used in U.S. Environmental Protection Agency (EPA) methods for the determination of arsenic and other contaminants; however, they cannot be operated in the field. The field test kits currently available for arsenic analysis have various disadvantages. These kits produce only semiquantitative results and are prone to false positives. They have long analysis times (more than 30 minutes per sample), high detection limits (10 µg/L), and most importantly, could be hazardous because of worker exposure to arsine.

Frontier Geosciences, Inc., collaborates closely with various water utilities, industrial concerns, and their regulatory counterparts in the areas of trace metal analysis, speciation, and geochemistry. The firm has experienced an increasing demand for sensitive and accurate arsenic speciation in the last few years. With the announcement of a new arsenic standard by EPA (10 µg/L), even more interest in arsenic is predicted. Therefore, a portable instrument that can accurately detect arsenic at concentrations below 1 µg/L and produce laboratory-quality data in the field for a fraction of the cost could be invaluable, especially when time is a concern.

To prevent arsine exposure, various different chemistries that use and generate less toxic products have been investigated. A unique hydride-generation vessel also has been designed. The hydride-generation reaction occurs in a closed system, preventing any arsine exposure. The reaction is controlled by the instrument for reproducible signals. The arsines formed in the vessel then can be swept into the atomization cell, where they are decomposed and atomic fluorescence detection occurs.

Summary/Accomplishments (Outputs/Outcomes):

Frontier Geosciences, Inc., investigated the possibility of replacing NaBH4 with other reagents that also could form volatile As species, but with boiling points higher than 50ºC, allowing them to be trapped onto a chromatographic column at room temperature. Two alternatives, sodium methyl thioglycolate (TGM) and an HCl/NaCl mixture that are both known to react with As to form volatile species, were studied. Using these reagents, it was expected to enhance signals by preconcentrating them on a column at room temperature without the need for liquid nitrogen. This allows for the separation of the analyte from the burst of H2 gas, and opens the door for the determination of organo arsenic species in addition to total inorganic arsenic. In addition, these volatile arsenic species are not as dangerous and lethal compared to arsine gas.

The TGM was found to work well in these experiments. The As-TGM complex was trapped on a glass wool packed column and then released by heating it using a nichrome wire wrapped around the column. Unfortunately, the TGM has a very strong odor that requires handling under the hood. Therefore, it was decided not to proceed with this species. The formation of AsCl3 species also was studied using a mixture of NaCl and HCl. It was found that formation of this species is not efficient enough to allow its analytical use.

Frontier Geosciences, Inc., continued studies with NaBH4 using a bubbler and no preconcentration. Using a commercial AFS system and a 15 mL sample volume, a detection limit of 12 ppt was obtained. It was decided to continue these studies using conventional hydride-generation reaction, and the same hydride-generation conditions with the new vessel design were used. With only 2 mL of sample, a detection limit of 60 ppt, which is below the targets for this study, was obtained. The analysis takes approximately 6 minutes, but this probably can be improved in later stages by changing a few parameters. With this vessel, the two reagents required for hydride generation do not come in contact with each other until the analyst starts the reaction. Because the reaction does not start until the vessel is closed and gas-tight, exposure to arsine gas was completely prevented. Even though the reaction was controlled manually for this feasibility study, very precise results (less than 5 percent relative stability deviation at 5 ppb) were obtained. In the final product, the hydride-generation reaction will be controlled via a simple motor, and therefore, reproducibility can be improved. When the same vessel was used for consecutive samples with minimal cleaning, no carryover was observed. This means that the vessel also can be manufactured as a nondisposable part of the instrument as well as a disposable unit ready for analysis.

Conclusions:

Originally, it was proposed to develop a micro-plasma source for atomization and atomic fluorescence for detection. Therefore, an He plasma on a quartz chip with a channel of a square cross section with dimensions of 1.0 mm x 1.0 mm was sustained. The length of the plasma was approximately 3-4 mm extending out of the chip. The plasma was operated at 10 W, which can be provided easily by portable power generators. The gas consumption was minimal due to its miniature size. Even though the vessel design has not been coupled with this plasma, mercury vapor has been used, and emission spectra for mercury have been obtained. Even though satisfactory results from a commercial flame AFS instrument have been obtained, Frontier Geosciences, Inc., will continue development of this miniature plasma source for AFS analysis.

Supplemental Keywords:

arsenic, drinking water, atomic spectrometer detector, arsine, atomic flourescence, sodium methyl thioglycolate, SBIR, RFA, Scientific Discipline, Toxics, Water, Ecosystem Protection/Environmental Exposure & Risk, National Recommended Water Quality, Chemical Engineering, Environmental Chemistry, Arsenic, Chemistry, Monitoring/Modeling, Analytical Chemistry, Environmental Monitoring, Drinking Water, Engineering, Chemistry, & Physics, Environmental Engineering, Mercury, monitoring, detection, field portable systems, fluoresence spectroscopy, environmental measurement, field portable monitoring, drinking water regulations, contaminated waters, risk management, community water system, chemical detection techniques, field monitoring, Selenium, analytical methods, environmental contaminants, fluorescence detection, measurement, field detection, drinking water contaminants, field analysis, arsenic exposure, drinking water system, flourescene spectral analysis, mercury concentrations