Download UV-Vis Absorption Spectroscopy

UV-Vis Absorption
Spectroscopy
Lecture 23
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Instrumental Noise as a Function
in Transmittance
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Therefore, an absorbance between 0.2-0.7 may
be advantageous in terms of a lower
uncertainty in concentration measurements.
At higher or lower absorbances, an increase
in uncertainty is encountered. It is therefore
advised that the test solution be in the
concentration range which gives an
absorbance value in the range from 0.2-0.7
for best precision.
However, it should also be remembered that we
ended up with this conclusion provided that
sT is constant. Unfortunately, sT is not always
constant which complicates the conclusions
above.
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EFFECT OF bandwidth
Effect of bandwidth on spectral detail for a sample of
benzene vapor. Note that as the spectral bandwidth
increases, the fine structure in the spectrum is lost. At a
bandwidth of 10 nm, only a broad absorption band is
observed.
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Effect of slit width (spectral bandwidth) on peak heights. Here, the
sample was s solution of praseodymium chloride. Note that as the
spectral bandwidth decreases by decreasing the slit width from 1.0
mm to 0.1 mm, the peak heights increase.
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Effect of Scattered Radiation at
Wavelength Extremes of an Instrument
Wavelength extremes of an instrument are dependent
on type of source, detector and optical components
used in the manufacture of the instrument. Outside
the working range of the instrument, it is not
possible to use it for accurate determinations.
However, the extremes of the instrument are very
close to the region of invalid instrumental
performance and would thus be not very accurate.
An example may be a visible photometer which, in
principle, can be used in the range from 340-780 nm.
It may be obvious that glass windows, cells and
prism will start to absorb significantly below 380 nm
and thus a decrease in the incident radiant power is
significant.
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What defines the instrumental wavelength
extremes?
Two Factors:
1. Source
2. Detector
Measurements at wavelength extremes should
be avoided since errors are very possible
due to:
1. Source limitations
2. Detector limitations
3. Sample cell limitations
4. Scattered radiation
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B: UV-VIS spectrophotometer
A: VIS spectrophotometer
EFFECT OF SCATTERED
RADIATION
Spectrum of cerium
(IV) obtained with a
spectrophotometer
having glass optics
(A) and quartz
optics (B). The false
peak in A arises
from transmission of
stray radiation of
longer wavelengths.
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The output from the source at the low wavelength
range is minimal. Also, the detector has best
sensitivities around 550 nm which means that away
up and down this value, the sensitivity significantly
decrease. However, scattered radiation, and stray
radiation in general, will reach the detector without
passing through these surfaces as well as these
radiation are constituted from wavelengths for which
the detector is highly sensitive. In some cases, stray
and scattered radiation reaching the detector can be
far more intense than the monochromatic beam from
the source. False peaks may appear in such cases
and one should be aware of this cause of such
peaks.
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