Michael D. Wheeler
Raman Spectroscopy Detects Herbicides in Water
ANN ARBOR, Mich. -- Researchers at the University of Michigan have employed Raman spectroscopy to measure two herbicides in water.
The herbicides, paraquat and diquat, contain chemicals that cause plant cell damage.
Detecting the herbicides is important because both reach into the soil, contaminating nearby water supplies.
The US Environmental Protection Agency has set the acceptable level for diquat at 20 parts per billion and paraquat at 60 parts per billion in drinking water.
To monitor contamination, scientists have traditionally employed two methods: gas chromatography mass spectrometry and immunoassay.
Gas chromatography involves taking a syringe full of suspect water and inserting it into a chamber where it is heated to more than 400 °F. A helium jet then separates the molecules and transfers them into a mass spectrometer, which determines molecular weight. One drawback is the high cost ($200,000 to $300,000) of mass spectrometers.
An immunoassay involves developing antigens to herbicides in lab animals and introducing them to a sample of suspect water. Both methods, however, are time-consuming and prone to systematic error.
More reliable, faster
The Michigan team, led by Michael D. Morris, sought a more reliable and faster testing method. The researchers took water samples and used a type of capillary electrophoresis known as isotachophoresis. This increased concentrations by more than 1000 times the original presence to above 0.01 moles, a concentration easily detectable by normal Raman spectroscopy.
In Raman spectroscopy, laser light excites the bonds within molecules, causing vibrations. Depending on a molecule's structure, light shifts at different frequencies enabling identification of the molecule.
Morris' team tested the sample with a Raman microprobe built around an Olympus microscope and a lens, which focuses laser light to a small point on the capillary. The microscope's output travels to a spectrograph fitted with a holographic transmission grating and a charge-coupled device camera. An arc lamp pumps a frequency-doubled Nd:YAG laser, operating at 532 nm, which excites Raman scatter in the water sample. The scientists used the intensities of the Raman spectra to determine the concentration of each herbicide.