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Raman spectroscopy

Raman spectroscopy. Acquisition, preprocessing and analysis of spectra. Raman spectrometer scheme. Credits: http:// www.doitpoms.ac.uk / tlplib / raman / method.php. Raman spectrometer. Raman spectrometer. Probes and fibers. Non contact probe. Acquisition of Raman spectra.

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Raman spectroscopy

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  1. Raman spectroscopy Acquisition, preprocessing and analysis of spectra

  2. Raman spectrometerscheme Credits: http://www.doitpoms.ac.uk/tlplib/raman/method.php Acquisition, preprocessing and analysis of Raman spectra

  3. Raman spectrometer Acquisition, preprocessing and analysis of Raman spectra

  4. Raman spectrometer Acquisition, preprocessing and analysis of Raman spectra

  5. Probes and fibers Acquisition, preprocessing and analysis of Raman spectra

  6. Non contactprobe Acquisition, preprocessing and analysis of Raman spectra

  7. Acquisition of Raman spectra Acquisition, preprocessing and analysis of Raman spectra

  8. Raman signal is weak Only around 1 in every 30 million photons is Raman scattered

  9. Acquisition of Raman spectra Issues • Cosmic rays  • Noise • Detection limits • Fluorescence Parameters • Laser frequency  • Laser power • Exposure time • Number of scans  Preprocessing • Spectral truncation • Noise reduction  • Baseline correction • Derivatives Acquisition, preprocessing and analysis of Raman spectra

  10. Preprocessing Preprocessing – a way to improve signal for further analysis Whatcanbeimproved • Noise reduction • Correction of baseline • Resolvingmergedmeaks • Removing physicaleffects How it works: • X’ = F(X) • xij = fj(xij) 6. Data preprocessing

  11. Cosmic spikes Noise and detection limits Fluorescence and background correction

  12. Cosmicspikes • occasionally appears in spectra as very narrow peaks • caused by high energy cosmic rays • typical issue for CCD based instruments • most of the acquisition software include algorithms to remove the effect Credits: Confocal Raman Microscopy. ed. Thomas Dieing, et al. Acquisition, preprocessing and analysis of Raman spectra

  13. Cosmic spikes Noise and detection limits Fluorescence and background correction

  14. Noise and detection limits CCD detectors have photon noise, dark noise and read noise Raman signal is weak To get a good signal/noise ratio • cool CCD • higher concentration • longer exposure time • more scans for the same sample • de-noising preprocessing Acquisition, preprocessing and analysis of Raman spectra

  15. Noise and detection limits CCD detectors have photon noise, dark noise and read noise Raman signal is weak To get a good signal/noise ratio • cool CCD • higher concentration • longer exposure time • more scans for the same sample • de-noising preprocessing Acquisition, preprocessing and analysis of Raman spectra

  16. Acquisition parameters and concentration 25% ethanol t = 5s t = 3s t = 1s Acquisition, preprocessing and analysis of Raman spectra

  17. Acquisition parameters and concentration 10% ethanol t = 3s t = 1s t = 1s Acquisition, preprocessing and analysis of Raman spectra

  18. Acquisition parameters and concentration 10% ethanol t = 3s t = 1s Acquisition, preprocessing and analysis of Raman spectra

  19. Acquisition parameters and concentration 1% ethanol t = 5s t = 3s t = 1s Acquisition, preprocessing and analysis of Raman spectra

  20. Trancatingspectra 1% ethanol t = 5s t = 3s t = 1s Acquisition, preprocessing and analysis of Raman spectra

  21. Acquisition parameters and concentration 1% ethanol t = 5s t = 3s t = 1s Acquisition, preprocessing and analysis of Raman spectra

  22. Acquisition parameters and concentration Butter t = 1s, 5 scans t = 1s, 3 scans t = 1s, 1 scan Acquisition, preprocessing and analysis of Raman spectra

  23. Acquisition parameters and concentration Butter t = 1s, 5 scans t = 1s, 3 scans t = 1s, 1 scan Acquisition, preprocessing and analysis of Raman spectra

  24. Playing with acquisition parameters Butter t = 3s, 5 scans t = 1s, 1 scan Acquisition, preprocessing and analysis of Raman spectra

  25. Using filters for noise removal • Linear filters: moving average, gaussian • Wavelet decomposition • Savitzky-Golay smoothing d = 1 w = 5 Acquisition, preprocessing and analysis of Raman spectra

  26. Using filters for noise removal SG filtered noised original Acquisition, preprocessing and analysis of Raman spectra

  27. Cosmic spikes Noise and detection limits Fluorescence and background correction

  28. Fluorescence Mechanism • appears if molecules can absorb the laser radiation at particular wavelength • the absorbed light excites electrons to higher energy levels • electrons return to the ground state by emitting light of longer wavelength Acquisition, preprocessing and analysis of Raman spectra

  29. Fluorescence How decrease/get rid of fluorescence: • remove impurities from solid samples • using microprobes or confocal Raman microscopy (for solid samples) • using lasers with wavelength in NIR range • proper preprocessing (baseline correction) • Features • very common for colored (especially dark) samples • several orders of magnitude stronger than Raman scattering • has a broad emission Acquisition, preprocessing and analysis of Raman spectra

  30. Fluorescence Color of samples Acquisition, preprocessing and analysis of Raman spectra

  31. Laser wavelength • Visible — higher energy, stronger signal, deeper penetration, better resolution, fluorescence (good for inorganic materials) • NIR — lower energy, weaker signal, worse resolution, smaller fluorescence effect (suitable for organic materials) Credits: http://www.horiba,com Acquisition, preprocessing and analysis of Raman spectra

  32. Baseline correction Baseline shift and curvature caused by noise, fluorescence, CCD background, interference, etc. Automatic baseline correction Acquisition, preprocessing and analysis of Raman spectra

  33. Baseline correction Automatic baseline correction d = 4 Acquisition, preprocessing and analysis of Raman spectra

  34. Baseline correction Automatic baseline correction d = 6 Acquisition, preprocessing and analysis of Raman spectra

  35. Baseline correction Semi-automatic baseline correction Acquisition, preprocessing and analysis of Raman spectra

  36. Conclusions Issues • Cosmic rays  • Noise • Detection limits • Fluorescence Parameters • Laser frequency  • Laser power • Exposure time • Number of scans  Preprocessing • Spectral truncation • Noise reduction  • Baseline correction • Derivatives Acquisition, preprocessing and analysis of Raman spectra

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