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5.3. PRECISION & ACCURACY
Precision is how closely repeated measurements are to one another. Precision is usually
expressed as standard deviation (SD).
Accuracy is defined as the closeness of a test result to the true value and is method
specific.
Although good precision suggests good accuracy, precise results can be inaccurate. The
figure explains these definitions.
5.4. PRINCIPLE OF OPERATION
Absorption of light is a typical phenomenon of interaction between electromagnetic
radiation and matter. When a light beam crosses a substance, some of the radiation
may be absorbed by atoms, molecules or crystal lattices. Photometric chemical analysis
is based on specific chemical reactions between a sample and reagent to produce a
light-absorbing compound.
If pure absorption occurs, the fraction of light absorbed depends both on the optical
path length through the matter and on the physical-chemical characteristics of the
substance according to the Lambert-Beer Law. If all other factors are constant, the
concentration "c" can be calculated form the absorbance of the substance.
Lambert-Beer Law:
= intensity of incident light beam
I
o
= intensity of light beam after absorption
I
= molar extinction coefficient at wavelength l
e
l
c
= molar concentration of the substance
d
= optical path through the substance
-log I/I
= e
c d
o
l
or
A = e
c d
l
11
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