Description 8
4.3. PRECISION & ACCURACY
Precision is how closely repeated measurements are to one another, 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.
Precise, accurate
Precise, not accurate
Not precise, accurate Not precise, not accurate
Figure 1:
Precision versus Accuracy
4.4. 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.
– log
I/I
o
=
e
l
c d
or
A =
e
l
c d
I
o
= intensity of incident light beam
I
= intensity of light beam after absorption
e
l
= molar extinction coefficient at wavelength
l
c = molar concentration of the substance
d = optical path through the substance
Figure 2:
Lambert-Beer law
4.5. OPTICAL SYSTEM
LED
Filter
Beamsplitter
Reference Detector
Microprocessor
Cuvette
Focusing Lens
Light Detector
Figure 3:
Instrument Block Diagram