Lasers and tubing improve Raman signal when measuring biological fluids

April 6, 2007
April 6, 2007, Rochester, NY--University of Rochester researchers announce in the current issue of Applied Optics a Raman spectroscopy technique that in 60 sec or less measures multiple chemicals in body fluids, using a laser, white light, and a reflective tube.

April 6, 2007, Rochester, NY--University of Rochester researchers announce in the current issue of Applied Optics a Raman spectroscopy technique that in 60 sec or less measures multiple chemicals in body fluids, using a laser, white light, and a reflective tube. The technique tests urine and blood serum for common chemicals important to monitoring and treatment of diabetes and cardiovascular, kidney, urinary, and other diseases, and lends itself to the development of fast batch testing in hospitals and other clinical settings.

Co-researchers Andrew Berger, associate professor of optics, and Dahu Qi, doctoral candidate, used low-refractive-index tubes instead of cuvettes or other bulky containers for holding biological specimens. And, to get more information from the fluids, they used white light along with the laser.

Because Raman signals are notoriously weak, using Raman spectroscopy to test biofluids, which have lighter chemical concentrations than many fluids, is not a natural choice. Berger and Qi injected fluid samples into a thin transparent tube specially made to contain the light, and the tube's long path length of interaction let the scientists collect more Raman scattering.

"The tubes have a refractive index lower than water, so the light bounces along inside the liquid core, just as in solid optical fibers for telecommunications," said Berger. "Other groups had used these fibers to strengthen their Raman signals, so we wanted to see if we could translate that advantage to use with biofluids."

They did get the stronger signal they were looking for, but the increase threw off measurements when samples of urine or blood serum varied in color. In previous experiments, Berger and his team had explored how a concentration of each chemical relates to the strength of Raman signal. It turned out the relationship is not a simple linear one. They were able to use that information for dealing with differences in sample color.

"We can't neglect that body fluid samples absorb light," said Berger. "We'd have two different samples with the same amount of protein and not get the same strength of signal. If we had two samples of blood serum, maybe one sample would be a little pinker due to a few ruptured red blood cells. Then we wouldn't get the same signal strength."

The team measured 11 chemicals in blood serum, including total protein, cholesterol, LDL and HDL levels, glucose, triglyceride, albumin, bilirubin, blood urea nitrogen, globulin, and CO2. In urine, they identified urea nitrogen and creatinine. The technique does not measure ions such as calcium or sodium, or other chemicals present at concentrations below about 0.01 mg/mL.

Robert Mooney, professor of pathology and laboratory medicine, collaborated in planning the experiments and arranged for serum and urine specimens from the University of Rochester Medical Center. The Whitaker Foundation provided funding for the research.

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