LaMotte SMART Spectro Spectrophotometer - Manual (Page 12)

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AN INTRODUCTION TO COLORIMETRIC ANALYSIS
& SPECTROSCOPY
Most test substances in water are colorless and undetectable to the human eye.
To test for their presence we must find a way to “see” them. The LaMotte
SMART Spectro can be used to measure any test substance that is itself colored
or can be reacted to produce a color. In fact a simple definition of colorimetry is
“the measurement of color” and a colorimetric method is “any technique used to
evaluate an unknown color in reference to known colors”. In a colorimetric
chemical test the intensity of the color from the reaction must be proportional
to the concentration of the substance being tested. Some reactions have
limitations or variances inherent to them that may give misleading results. Many
such interferences are discussed with each particular test instruction. In the
most basic colorimetric method the reacted test sample is visually compared to a
known color standard. However, accurate and reproducible results are limited by
the eyesight of the analyst, inconsistencies in the light sources, and the fading of
color standards.
To avoid these sources of error, a colorimeter or spectrophotometer can be used
to photoelectrically measure the amount of colored light absorbed by a colored
sample in reference to a colorless sample (blank).
White light is made up of many different colors or wavelengths of light. A
colored sample typically absorbs only one color or one band of wavelengths from
the white light. Only a small difference would be measured between white light
before it passes through a colored sample versus after it passes through a colored
sample. The reason for this is that the one color absorbed by the sample is only a
small portion of the total amount of light passing through the sample. However,
if we could select only that one color or band of wavelengths of light to which
the test sample is most sensitive, we would see a large difference between the
light before it passes through the sample and after it passes through the sample.
The SMART Spectro uses a quartz halogen lamp as the source of white light.
The white light passes through an entrance slit and is focused on a ruled grating
consisting of 1200 lines/mm. The grating causes the light to be dispersed into its
various component wavelengths. The monochromator design allows the user to
select which specific wavelength of interest will be passed through the exit slit
and into the sample. The use of mirrors and additional filters prevents light of
undesired wavelengths (overtones, stray light) from making it to the sample. A
photodetector measures the amount of light which passes through the sample.
The difference in the amount of monochromatic light transmitted through a
colorless sample (blank) and the amount of monochromatic light transmitted
through a test sample is a measurement of the amount of monochromatic light
absorbed by the sample. In most colorimetric tests the amount of
monochromatic light absorbed is directly proportional to the concentration of
the test factor producing the color and the path length through the sample.
However, for a few tests the relationship is reversed and the amount of
monochromatic light absorbed is inversely proportional to the concentration of
the test factor.
The choice of the correct wavelength for testing is important. It is interesting to
note that the wavelength that gives the most sensitivity (lower detection limit)
12 SMART SPECTRO SPECTROPHOTOMETER 05.04