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 1.00 mg/L chlorine and a
representative lot of reagent, an operator obtained
with a single instrument a standard deviation of
0.02 mg/L.
PRECISION AND ACCURACY
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.
PRINCIPLE OF OPERA
TION