Hanna Instruments HI 83208-2008 - Manual (Page 3)

4 5
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.
The optical system of HI 83208 is based on special subminiature tungsten lamps and narrow-band
interference filters to guarantee both high performance and reliable results.
Four measuring channels allow a wide range of tests.
Instrument block diagram (optical layout)
A microprocessor controlled special tungsten lamp emits radiation which is first optically conditioned and beamed
to the sample contained in the cuvette. The optical path is fixed by the diameter of the cuvette. Then the light
is spectrally filtered to a narrow spectral bandwidth, to obtain a light beam of intensity -I
o-
or -I-.
The photoelectric cell collects the radiation -I- that is not absorbed by the sample and converts it into
an electric current, producing a potential in the mV range.
The microprocessor uses this potential to convert the incoming value into the desired measuring unit and to
display it on the LCD.
The measurement process is carried out in two phases: first the meter is zeroed and then the actual
measurement is performed.
The cuvette has a very important role because it is an optical element and thus requires particular attention. It
is important that both the measurement and the calibration (zeroing) cuvette are optically identical to provide the
same measurement conditions. Most of methods use the same cuvette for both, so it is important that
measurements take place in the same optical point. The instrument and the cuvette cap have special marks
that must be aligned in order to obtain better
reproducibility .
The surface of the cuvette must be clean and not scratched. This is to avoid measurement interference due to
unwanted reflection and absorption of light. It is recommended not to touch the cuvette walls with hands.
Furthermore, in order to maintain the same conditions during the zeroing and the measurement phases,
it is necessary to close the cuvette to prevent any contamination.
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.
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 substance according to the Lambert-Beer Law:
-log I/I
o
= ε
λ
c d
or
A = ε
λ
c d
PRECISION AND ACCURACY
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.
For each method, the precision is expressed in the
related measurement section as standard deviation
at a specific concentration value of the analite. The
standard deviation is obtained with a single instrument
using a representative lot of reagents.
SPECIFICATIONS
Light Life Life of the instrument
Light Detector Silicon Photocell
Environment 0 to 50°C (32 to 122°F);
max 90% RH non-condensing
Power Supply external 12 Vdc power adapter
built-in rechargeable battery
Dimensions 235 x 200 x 110 mm (9.2 x 7.87 x 4.33")
Weight 0.9 Kg
For specifications related to each method (e.g. range, precision, etc.) refer to the related measurement
section.