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Picking Out Faces in a Chemical Crowd
May 7, 2015   
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Elements and their compounds will no longer be able to “hide” in chemical mixtures, even if such mixtures are made up of many components. Researchers at the Polish Academy of Sciences’ Institute of Physical Chemistry in Warsaw have developed a new, more accurate method for identifying the “fingerprints” of chemical substances using spectral analysis.

Elements and chemical compounds exist in their pure form only in laboratories. The world of nature is a world of multi-component mixtures. Can individual components be pinpointed in the crowd of chemicals that make up a mixture? Can this be done purely by examining the light scattered by the mixture? Existing methods of spectral analysis have not been able to determine all such components. Sometimes they even suggested the presence of compounds that were simply not there. However, now detection of the components of a chemical mixture will be much more reliable—thanks to the method developed by researchers at the Institute of Physical Chemistry. Their research, financed with the “Quantum semiconductor nanostructures” grant under the European Union’s Innovative Economy Operational Programme for 2007-2013, has resulted in the development of a new algorithm for spectral analysis, with much greater accuracy than in the case of previous methods.

Light carries information about the structure of matter—about the atoms and molecules with which it has interacted. Analysis of the light emitted or absorbed by the tested substance is an excellent way of determining its chemical composition. All researchers need to know is how the desired compound interacts with light, and to detect a similar trace in the analyzed light. In theory, this is simple, but the real world is filled with complex mixtures of various chemical compounds. Signals from all of them overlap and the detection of specific elements and compounds becomes extremely difficult.

“When you talk to someone face to face, you have no trouble understanding the message,” says the Institute of Physical Chemistry’s Sylwester Gawinkowski, Ph.D. “But when you try to pick out the voice of a friend in a crowd of fans during a rock concert, the task is not so simple. The situation is similar for scientists dealing with spectroscopy: they are trying to pick out the light equivalent of ‘shouts’ of their ‘friends’ from the ‘screams’ of a crowd of chemical compounds. As if that were not enough, some friends can quite unexpectedly start speaking more quietly, or even go hoarse.”

If the only problem was the overlapping of “light screams,” singling out the signature of a specific chemical substance would not be especially difficult. Matters, however, become extremely complicated when it comes to the actual measurements. Inevitable measurement errors—including those arising from the physics of interaction of light with matter itself—mean that the light signature registered, even if it comes from only one substance, always has a slightly different structure than the theoretical pattern. In addition, it does not have to be one compound that is being sought, but several from a known pool. Moreover, the contribution of each of the compounds in the structure of the analyzed light may be different—weaker in the case of one substance and stronger in the case of another.

The institute’s Tomasz Roliński, D.Sc., explains, “When examining the structure of the registered light—that is in the course of spectral analysis—the trick is to pick out only the most important features characteristic of a given substance from the spectrum of the mixture. This approach is somewhat similar to the automatic face detection method used, for example, in security systems at airports. This does not involve a comparison of the appearance of every detail of the face, but a search for similarities in simple relationships, such as eye spacing, the position of the mouth or the end of the nose. Then it no longer matters if the wanted person is wearing a hat or not, or whether he has a suntan or has shaved off his mustache.”

With this idea in mind, physicists at the Institute of Physical Chemistry have developed an analysis method that is sensitive not to the intensity of the spectral lines of the light passing through a mixture, but to their relative positions and shapes. To test the efficacy of the method, a series of tests was conducted. One of the researchers prepared mixtures of several amino acids randomly selected from a known pool of 20 compounds. The number of components of each mixture ranged from two to eight, but the volumes of the components were similar (which does not mean that all the substances reacted equally strongly with the light). The mixture prepared in this way was analyzed by another researcher.

The tested mixtures first underwent spectral analysis using the so-called least squares method widely used in the field. It turned out that in seven of 20 cases one component was not found; in one case two components were not found. Moreover, in two mixtures the analysis detected the presence of two amino acids that were not even there.

“In the case of our method the results of analyses of the same spectra were qualitatively better,” says Roliński, “In 20 of the tested mixtures we correctly identified the composition in 18 cases. In the remaining two mixtures, one made up of five components, the other of eight, we did not detect one component. In no case did we identify a compound that was not present in the mixture.”

The new spectral analysis method can be applied in sophisticated research techniques such as Surface Enhanced Raman Spectroscopy (SERS). The specific nature of SERS spectroscopy results from the fact that signals emitted by chemical molecules can be enhanced hundreds of thousands or even millions of times. Such a level of enhancement has encouraged researchers to consider designing detectors capable of detecting single chemical molecules, which would mark a major breakthrough in modern chemistry.
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