A lighter approach to spectroscopy

US researchers at the University of Rochester have developed a technique that in 60 seconds or less measures multiple chemicals in body fluids, using a laser, white light, and a reflective tube.

The method 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.

The researchers used low-refractive-index tubes instead of cuvettes or other bulky containers for holding biological specimens. To get more information from the fluids, they used white light from a standard light bulb along with the laser.

The method improves on Raman spectroscopy, a laser technique for finding chemicals that overlap and mix in fluid. However, the Raman signal is notoriously weak and not the first choice for use in biofluids.

The researchers 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. 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 colour.

In previous experiments, the team had established that the relationship between the concentration of a chemical and the strength of Raman signal is not a simple linear one. They were able to use that information for dealing with differences in sample colour.

The scientists sent a beam of white light through each sample to see how much light was absorbed at various wavelengths, and then they calculated corrections. They injected light using the end of the tube opposite the laser. The resulting corrections made chemical predictions significantly more accurate.

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.

Spectral tests use no chemical reagents and therefore offer the advantage of being non-destructive to fluid samples, unlike many lab tests. After analysis, practitioners could use undamaged samples for other kinds of tests.