Fiberoptic spectrometer helps clean up environment
John D. Hanby
Fast, reliable, on-site contamination detection in the aftermath of hazardous-material spills can speed up soil and water cleanup efforts and minimize environmental damage. Although they are thorough, detection and evaluation methods requiring laboratory analyses are expensive and often slow down the cleanup process.
The Environmental Protection Agency-approved Hanby Method, a colorimetric field test, is a simple, portable soil and water contamination-analysis technique based on the Friedel-Crafts aromatic alkylation reaction. Organic compounds subjected to the reaction produce precipitates of varying hues and intensities that can be visually compared to a chart of photographic standards. The color of the precipitate reveals the type of compound, while the intensity of the color indicates the concentration level of the contaminant.
The subjective nature of the visual comparison in this last step in the Hanby test has sparked efforts to eliminate variables such as visual acuity, lighting, and other interpretive factors inherent in this process. In particular, Hanby Environmental Laboratory Procedures Inc. (Wimberley, TX) has developed the H.E.L.P.MATE 2000. A lightweight spectrophotometer system in this instrument assesses the reflectance spectrum of the sample under test to provide objective, on-site assessment, adding quantitative and qualitative validity to the existing procedure.
The five-step process consists of acquiring a sample, extracting the sample with a solvent, pouring the extract into a test tube, adding a catalyst that causes a precipitate to form, and inserting the test tube into the well of the spectrophotometer to get the reflectance reading. No longer a procedure that depends solely on visual comparison, this field method takes five to ten minutes to produce a colorimetric indication of the concentration and type of contaminant present.
System design
The instrument incorporates an ultraviolet/visible-wavelength spectrometer (see figure). Manufactured by Ocean Optics Inc. (Dunedin, FL), the PS1000 uses a bundle of six 200-?m-diameter multimode silica fibers to illuminate the sample under test. Light from the 6.5-W tungsten-halogen source is collected into a parallel beam by a collimating-lens/backreflector combination and launched into the fiber for a sample illumination on the order of 7400 foot-candles. A single 200-?m-diameter multimode central fiber collects the return signal and sends it to a collimating mirror. A flat diffraction grating ruled at 600 lines/mm disperses the collimated reflectance signal into its spectral components, and a spherical positioning mirror relays it to a
1024-pixel linear array charge-coupled-device (CCD) detector.
For this application, the spectrometer is optimized for the 360- to 850-nm wavelength range; actual sensitivity range is 200 to 1000 nm. Spectral resolution ranges from 0.3 to 20 nm (full width at half maximum). The silicon CCD array (NEC, Tokyo, Japan) is fitted with a quartz window to enhance short-wavelength detection, and instrument sensitivity is
300 photons per A/D count.
The H.E.L.P.MATE compares the reflectance spectrum signature of the sample under test with reference spectra to determine the type and concentration of the contaminants. Special data-acquisition and processing software allows operators to set and control reflectance measurements. Instrument integration time averages 50 ms. The PS1000`s ribbon connector and 100-kHz, 12-bit A/D PC card allow the instrument to interface easily with a 486 SX notebook computer.
The Hanby field test has accelerated cleanup efforts at such environmental-damage sites as the 1989 Exxon Valdez oil spill. With the addition of the H.E.L.P.MATE 2000, the method is even more dependable for verifying, pinpointing, and quantifying soil and water contamination. n
The 61/8 3 5/8 11/4-in. PS1000 spectrometer (see inset) produces reflectance spectra from chemically processed soil and water samples.