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Signal-to-noise ratio of UV-visible absorption spectrophotometry

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Computes the theoretical signal-to-noise ratio for a UV-visible absorption specrophotometer with a continuum source (e.g. tungsten incandescent lamp) modeled as a blackbody, dispersive monochromator, and a photomultiplier detector, given the molar absorptivity of the analyte and the solvent at the measured wavelength. Includes unabsorbed stray light, source flicker, photon, and thermionic emission noise. Note that this simulation predicts only a theoretical maximum signal-to-noise ratio limited by photon and detector noise; in practice, the limits to the measurement of small absorbances are more likely set by low-frequency flicker noise and sample-to-sample baseline variations, caused by light source instability, vibration, sample turbulence, light scattering by suspended particles, dust, bubbles, fingerprints on the cuvettes, etc. Most of those noises can be reduced or eliminated by using double-beam, dual-wavelength, derivative spectroscopy, wavelength modulation and photodiode array instruments, coupled with suitable multicomponent software.

Assumptions: The Beer-Lambert law is observed, except for the presence of unabsorbed stray light. Double-beam design measures both sample and reference beams, and both beams are subject to source flicker, photon, and thermionic emission noise. Source flicker, photon, and thermionic emission noises are random and uncorrelated and therefore add quadratically. Source flicker is only that portion of the source flicker not compensated by the double-beam operation. The detector is linear. Quantum efficiency of photocathode is independent of light level and detector current. Spectral response of the detector is much wider that the spectral bandpass of the monochromator.

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Other related simulations:
U.V.-Visible Spectrophotometer
Dual Wavelength Spectrophotometer
Effect of Slit Width on Signal-to-Noise Ratio in Absorption Spectroscopy
Multiwavelength Spectrometry

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