Scientists who study the ocean often want to know about the organic molecules it contains, to learn about water pollution, say, or to study the effects of fossil-fuel burning on atmospheric carbon. Doing such studies usually involves collecting vials of water and analyzing them in the laboratory. But researchers at the University of Massachusetts, Boston, are using a different technique—a portable fiberoptic spectrofluorometer.
"We actually make the measurements in seawater as we tow the device behind the boat," said Robert F. Chen, assistant professor in the university`s environmental, coastal, and ocean sciences department. "No one else has solved the problem of doing this without actually taking samples back to the lab." He said the system is modeled on another used by Stephen Lieberman at the Space and Naval Warfare Systems Command (San Diego, CA), but Lieberman`s device is designed for detection in soil.
Chen`s device measures dissolved pyrene, one of a large group of polycyclic aromatic hydrocarbons (PAHs). Many PAHs have been shown to cause cancer in animals and may also be dangerous to humans. Conventional fluorescence spectroscopy instruments can measure PAHs in water, but the sea is also full of natural organic compounds, some of which fluoresce strongly, masking the PAH signal. So Chen's group uses time-resolved fluorescence spectroscopy.
The natural substances, when excited by light, fluoresce within about 1 to 6 ns. Pyrene, however, has an excitation lifetime of 128 ns. So by sending a laser pulse into the water and waiting 100 ns to turn on the camera, Chen and his group know they are picking up photons emitted by the pyrene. The chemical also has what Chen calls a "wonderful absorption spectrum" at 337 nm, so he uses a sensor fairly specific for pyrene.
The spectrofluorometer consists of a nitrogen laser producing 1.4-mJ pulses at 337 nm. The laser is connected to about 30 meters of fiberoptic cable, in which one 400-µm fiber—the emission fiber—is surrounded by nine 200-µm fibers that serve as detectors. The light is returned to an intensified charge-coupled-device camera (Princeton Instruments; Trenton, NJ) with a resolution of 256 × 1024 pixels. Computer software allows the researchers to assess their samples in near real time. If they get a high reading, they take a physical sample of the seawater to perform gas chromatography/mass spectrometry.
The instrument can detect pyrene at levels of 5 ng/liter of water. Most of the signal it detects comes from within 2 or 3 mm of the probe tip, depending on the optical density of the water being sampled. The researchers normalize their signal to the Raman signal of water. "That way we can normalize for laser pulse differences and optical density differences," Chen said. The system can take a measurement every 10 s, or approximately every 20 or 30 m.
Chen said a survey that takes 10 or 15 samples—based on estimates of where pollution might be found—is considered a big survey, and its success cannot be known until a laboratory tests the samples. His system eliminates guesswork and saves time and money while taking hundreds of measurements, he said. "We can bring back 100 measurements that actually mean something, as opposed to 10 samples that might not be representative of the true distribution," Chen said.
Nature of pyrene
Pyrene itself is not one of the more carcinogenic PAHs and is only one of several found in seawater. But there is a strong correlation between it and other forms of PAH, so researchers can use it as a proxy for overall pollution. Pyrene can be petrogenic, coming from oil sources through sewers or as runoff from streets, or pyrogenic, coming from the combustion of gasoline in diesel engines or from wood-burning fires. The pyrogenic PAH can either be particulate matter or dissolved. The dissolved form is more important to researchers, and it is that form that the fiberoptic spectrofluorometer measures. A University of Massachusetts doctoral student, Steven Rudnick, is investigating whether the signals he sees are from pyrogenic or petrogenic PAH.
Chen, Rudnick, and research associate Bernie Gardner recently completed a study to map distributions of pyrene in Boston Harbor, funded by the National Oceanic and Atmospheric Administration (Washington, DC) Sea Grant program through the Massachusetts Institute of Technology (Cambridge, MA). No previous good measurements of dissolved pyrene in Boston Harbor existed, Chen said. Towing the spectrofluorometer behind their boat the researchers were able to track plumes of pollutants to their source. "As we tow it up toward the Charles River, we can see the signal increase," Chen said.