4. Absorption and scattering

Absorption photometry (4/6)

A double beam photometer for scientific research

The photometer for school's tuition presented on worksheet 4.2 is very compact and clear. The measurements made in absorption photometry by those are easily understandable. This is not the case talking of photometers which are used in highly precise measurements for scientific purposes! All components of those instruments are arranged in a closed case as shown in the following picture. For the user, only the sample compartment with the holders for sample and reference solutions are accessibe.

Laboratory photometer ...marked Lamp compartment

A double beam photometer for scientific laboratories. Those come with a computer (not shown in the picture) for controlling the course of the measurements and for data processing and saving. The marked photo shows the position of the lamp compartment, in what a halogen lamp (on the bottom right in the picture of the lamp compartment) for the visible range is incorporated such as a deuterium lamp (bottom left) for the ultraviolet. The instrument has been used on research ships for 20 years on nearly all the oceans what explaines that it is not in pristine condition nowadays.

View from above into the sample compartment and on the sample and the reference cuvette in their precisely adjusted holders. In the picture on the left you can see two optical windows through what the light (devided into its spectral parts by the monochromator) hits the cuvette. Ensuing to that, the light passes through the windows on the right and hits the photodetectors.

Standard cuvettes are very inconvenient for research trips on the sea. It is hard to fill them with liquids on a rocking ship and on top of that the liquids might splash from the cuvettes as soon as the weather is fitful. Closed cuvettes which can be externally filled over a funnel avoid such problems.

Opened sample compartment of a photometer with cuvettes from stainless steel. With their length of 10 cm, they would fill in nearly the whole space of the sample compartment. They can be filled and emptied while ajusted in the photometer. Therefore the for the measurements important accurate adjustment is maintained.

The cuvettes are optically adjusted and mounted to the same holder. Their windows are made of synthetic quartz which is transparent also in the ultraviolet. The three-way valve allows to fill the cuvettes with the same liquid (purified water) for synchronising the two signals ('Autozero'). Then the reference cuvette will be seperated from the circulation so that the reference sample constantly stays in it. For the follwoing analyses, the sample cuvette will be filled with the different water samples. The steel spring helps to fixate the three-way valve on a plastic mount.