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High-precision chemical analysis: an optical reference procedure

A novel combination of surface-enhanced Raman scattering and isotope-labelled standards allows a precise and sensitive measurement of substance concentration to be performed by optical spectroscopy. This method makes, for instance, a molecule-specific concentration determination of diagnostic markers in biological substances possible, which are, for example, as complex as blood serum.

Optical spectroscopy methods are widespread in the field of organic and biochemical analysis. Molecule-specific methods (IR, Raman) even provide a ÓfingerprintÓ of the sample investigated but are often not sufficiently sensitive. An outstanding signal amplification is, however, possible by means of surface-enhanced Raman scattering (SERS). It is achieved by adding metallic nanoparticles to the analyte. Up to now, however, it has not been possible to make metrological use of this advantage since the measurements were not sufficiently reproducible. This problem could now be solved by a method based on the principle of the so-called isotope dilution.

To precisely determine the analyte concentration, a known quantity of a compound that is chemically identical to the analyte but isotopically labelled (isotopomer) is added to the sample. Due to the mass difference, the optical selectivity of the Raman effect provides separate signals for the two substances so that the quantity of the unknown can be determined from the quantity of the known. The evaluation of the spectra is therefore reduced to the determination of intensity ratios, which is of special interest for metrological purposes. Potential disturbing effects such as material losses or matrix effects, which equally affect both isotopomers of the analyte, have no influence on the result.

This approach of a primary ratio method is well known from mass spectrometry and was developed and validated here by means of a practically relevant example: the determination of the creatinine concentration in blood serum, a value that provides information about the functional efficiency of the kidneys. Compared to the optical measurement methods used up to now, results could be obtained whose detection limit has been improved by as much as three orders of magnitude. In this way, the measurement accuracy is increased to the extent that even the highest metrological requirements can be met.

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