Detecting Less Than the Eye Can See

Theresa Hannon
Department of Chemistry
Stanford University
March 2002

You can tell roughly how much smog is in the air or how much food coloring is in a glass of water by seeing how much light can pass through it. In a similar way, chemists identify and accurately measure the amount of a colored chemical in a sample by shining a light through it and detecting how much light comes out the other side; we call this "spectroscopy". My spectroscopy research focuses on developing a new way to look at colored chemicals in concentrations so low you can't see them by eye, particularly in liquids. In the future, this technique may have environmental and biological applications.

Often there is so little fog or smog in the air that you need to look over a distance of several kilometers even to notice it at all. Maybe it takes an entire swimming pool to convince your eye that water really does have a bluish tint. Similarly, chemists often study gases or liquids that seem transparent in small amounts but appear colored when there's a large quantity available. For example, we want to detect contaminants in air or drinking water before they accumulate enough to be obvious to the senses! By looking at a huge sample through a long distance, or pathlength, chemists can find colored chemicals present that might be undetectable otherwise. There are a few problems, however: they can't build laboratories many kilometers long, and they don't usually have access to that much of any single sample!

Like many chemists today, I use mirrors to tackle these problems. Have you ever stood in a room with mirrors on opposite walls and seen ten or twenty reflections of yourself? You were able to see so many images because the rays of light that created them were bouncing back and forth between the mirrors like a ball in a tennis match. Although it may not be realistic to look at a very large chemical sample, I can achieve the same effect by putting a smaller amount between two mirrors that are separated by only a meter. The light simply bounces back and forth through the sample numerous times! Using this setup, called an optical cavity, I can detect smog or water vapor even in a very small quantity of air.

In order for this multi-bounce method to work, I need to have a light source that is lined up so a beam will be able to pass straight from mirror to mirror. I also need to use an electric "eye", or photodetector, to measure how much light energy is left after one, two, or many passes through the sample. I shine a laser beam through the back of the first mirror, and I place the photodetector after the second mirror. A tiny bit of laser light gets through the first mirror and heads toward the other one. Each time the beam bounces off the second mirror, an even smaller amount passes through it to the detector. Just as each reflection of yourself in the room mirror is a little dimmer than the one before it, the beam gets dimmer with each bounce. By measuring the amount of light energy that passes through the second mirror over time, I can calculate the quantity of the colored chemical in the sample. The technique is called cavity ring-down spectroscopy, or CRDS.

While CRDS has been well established for the study of gas samples like air, my research focuses specifically on the development of liquid CRDS. Applying CRDS to liquids is more complex, because the liquid containers used for spectroscopy, called cuvettes, have mirror-like properties. Using anti-reflective coatings and taking advantage of the special properties of light radiation, I work to minimize these undesired extra reflections. I avoid some problems caused by a glass cuvette by placing my samples on top of a special coated prism instead.

By expanding the use of CRDS to the liquid phase, I hope to add another useful tool to the analytical scientist's bag of tricks. Liquid CRDS may aid in preventing environmental crises by allowing ecologists to detect toxins in lakes at an earlier stage of pollution. The technique also may improve medical diagnoses by helping biologists to find a few disease antibodies among a host of more abundant proteins in human blood. Scientists will learn more about such important liquids by being able to detect smaller concentrations of interesting chemicals within those liquids.