The advent of the 785nm laser has revolutionised Raman. Excitation with visible, green and red wavelengths typically results in fluorescent spectral contamination. The new NIR laser wavelength offers significant fluorescence rejection because fluorescence is not common in the NIR part of the spectrum.
However, there can still be some residual fluorescence contamination in the Raman spectrum. One “cure” for this is photo-bleaching. Photo-bleaching involves irradiating the sample of interest with intense light for a period of time; this irradiation can often induce photolytic decomposition, breaking down the fluorescent molecules and reducing the fluorescent background. The light source for the photo-bleaching is simply the Raman spectrometer’s laser.
It could be said that photo-bleaching “modifies” the sample, and that is correct, but it effectively removes a low level contaminant which is seldom, if ever, of interest while leaving the species of interest unchanged. A photo-bleaching option is built into the software on some modern Raman instruments.
|
|
|
| Fluorescence before photo-bleaching. | Fluorescence after photo-bleaching. | Fluorescence after photo-bleaching and auto-baseline. |
Contributed by PerkinElmer Life and Analytical Sciences (www.perkinelmer.com)