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Design and Background Correction in Spectrophotometry: Understanding Detector Mechanisms

This lecture delves into the principles and designs of spectrophotometers, focusing on detectors, background correction methods, and spectral peak characteristics. Key topics include the Harris Monochromator, various light sources, phototubes, and photomultiplier setups, including their efficiency and response to light at different wavelengths. It discusses the significance of signal processing techniques, the impact of noise on spectral data, and methodologies for eliminating background interference. A comprehensive overview of quantum states, molecular versus atomic spectra, and the Doppler effect in wavelength measurements is also presented.

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Design and Background Correction in Spectrophotometry: Understanding Detector Mechanisms

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  1. Lecture 8 Detectors Spectrophotometer design Background correction Width of the spectral peak Ch. 20 Harris

  2. Monochromator (filter, wavelength selector) Light Source Detector Sample Spectrometer Data Processing

  3. A phototube K/Cs/Sb GaAs - + Ag/O/Cs 90 V Wavelength, nm

  4. Photomultiplier Several electrons for each incident electron Several electrons for each photon Quarz envelope Light, h Photoemissive cathode Anode

  5. Photoelectromultiplier dynode dynode electron photon

  6. Single beam Double beam

  7. Double beam with the beams separated in time

  8. Single beam design with 210-element area detector

  9. Flat background Ipeak I Ipeak – I0 Intensities at two different wavelengthes Peak I0 Baseline wavelength

  10. Flat background Ipeak I constant Peak First derivative eliminates flat background I0 Baseline Wavelength (l)

  11. Linear Background Ipeak I Ipeak – (I1+I2) / 2 I1 Background I2   Wavelength (l)

  12. Linear Background I Ibackground = Kl+b First: Background Second: Second derivative eliminates linear background Wavelength (l)

  13. Original Second 4th First

  14. Well defined peaks and valleys High noise Some peaks visible Almost featureless: little information Low noise

  15. Only photons of one specified wavelength are absorbed Excited state Ground state

  16. Width Velocity of a particle >1000 m/s Doppler Speed of light 300,000,000 m/s collisions Approximately as Doppler Both effects increase with the temperature and decrease with the size of the molecule/atom Width is around 0.01-0.001 nm in visible area Multiple energy levels

  17. Energy levels of excited state Energy sublevels Energy levels of ground state

  18. A primitive rule: Molecular spectra - widebands Atomic spectra - narrowlines

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