7
Precision
is how closely repeated measurements
agree with each other. Precision is usually
expressed as standard deviation (SD).
Accuracy
is defined as the nearness of a test
result to the true value.
Although good precision suggests good accuracy,
precise results can be inaccurate. The figure
explains these definitions.
In a laboratory using a standard solution of
50 mg/L nitrate and a representative lot of
reagent, an operator obtained with a single
instrument a standard deviation of 5 mg/L.
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.
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:
‑log
I
/
I
o
=
ε
λ
c d
or
A =
ε
λ
c d
Where:
‑log
I
/
I
o
= Absorbance (A)
I
o
= intensity of incident light beam
I
= intensity of light beam after absorption
ε
λ
= molar extinction coefficient at wavelength
λ
c
= molar concentration of the substance
d
= optical path through the substance
Therefore, the concentration “c” can be calculated from the absorbance of the substance as the
other factors are known.
Photometric chemical analysis is based on the possibility to develop an absorbing compound from a
specific chemical reaction between sample and reagents. Given that the absorption of a compound
strictly depends on the wavelength of the incident light beam, a narrow spectral bandwidth should
be selected as well as a proper central wavelength to optimize measurements.
PRINCIPLE OF OPERA
TION
PRECISION AND ACCURACY
