Shift Happens: Fixing Spectral Searching Errors Caused by X-Axis Shift

About five years ago, I was visiting a customer who asked me to look at a Raman spectrum.  It was a spectrum of what should have been an easily identifiable plastic compound, yet they were not able to get a good hit when searching my Division’s comprehensive spectral databases using our KnowItAll software.  Looking into the problem more, it was clear that the spectrum was shifted on the X-axis by over 10 wavenumbers (they said that it had been a while since the instrument was calibrated), and the sharp peaks of the Raman query spectrum no longer matched the equally sharp peaks of the reference spectra, resulting in a poor match as judged by the software’s algorithms.

Manually offsetting the X-axis shift to correct for the calibration error and repeating the search in KnowItAll gave a spot-on match.  The take-home lesson from this visit was to always calibrate your instrument.  Unfortunately, as this customer visit clearly indicated, not everybody does.  Upon my return to Bio-Rad, we decided to investigate the prevalence of X-axis shift in a broader context.  To do so, we compared a set of over 1,300 infrared spectra from our high quality KnowItAll IR Spectral Library to the corresponding IR spectra from the free NIST Chemistry WebBook.  The results were shocking:  the NIST spectra were shifted  anywhere from positive 20 to negative 20 wavenumbers.  Shift REALLY happens!  

This observation got us thinking about other problems in searching spectra of unknowns that we had seen time and time again, mostly caused by human error.  Our contemplation lead to the development of Optimized Corrections,  Bio-Rad’s patent-pending technology to maximize spectral search results by iteratively optimizing both the query and reference spectra.  By automatically applying multiple corrections to compensate for differences between the spectra, Optimized Corrections solves many of the issues surrounding spectral search that cannot be adequately addressed by traditional algorithms or manual methods.  For the X-axis shift described in the example above, Optimized Corrections fixed the offset and gave the correct result as the top “hit”.  

So, even though shift does happen, Bio-Rad can help fix it with Optimized Corrections.  You can check out our YouTube video to learn more (https://youtu.be/gIHwWMm1vWU) or sign up for a free trial of our KnowItAll software and spectral databases (https://info.bio-rad.com/KnowItAll-Trial.html).