Page 30 - Instrumental Analysis - Pharm D Clinical- 07-PA403
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UV-visible Spectrophotometry
2- Instrumental deviations
II- Stray light effect
• Stray light is defined as radiation from the instrument other than the
selected wavelength.
• It may originate from:
1- Scattering and unwanted reflections within the optical
components.
2- Imperfections or damage in the dispersing system or other optical
surfaces.
• Stray light adds an additional contribution (Istray) to the radiant power
reaching the detector, thus:
I0 + Istray
A = log ──────
It + Istray
• This leads to negative deviation in absorbance readings especially at
high concentrations.
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To avoid stray light effect: 56
• The interiors of spectrophotometers are
painted with non-reflective black paint
to absorb the stray light.
• Maintenance of the instrument and
proper choice of monochromators.
• Measure A values between 0.3 and 1.0
Absorbance levels above 2.0 are rarely
measured.
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