Cuvette
A cuvette ( French cuvette , small vessel ) is a vessel with plane-parallel side surfaces that is used for optical examinations ( UV / VIS spectroscopy ), for example as a resonator in dye lasers . Cuvettes are available in different qualities, depending on the intended use. Usually they are made of glass or plastic . For wavelengths below 200 nm, quartz glass cuvettes are required. A special type of cuvette is used in the hobby Fotoküvette . The simplest cuvette imaginable consists of just 2 panes of glass - without spacers -, enables layer thicknesses in the micrometer range, without dilution and is suitable for the visual comparison of liquids such as blood, cosmetics or heavy fuel oil that absorb light.
Classification
Cuvettes are divided into:
- the glass (depending on the desired transmission and resistance)
- the path length (e.g. in normal and micro cuvettes with and without spacers)
- the construction (in separable, compact and disposable cuvettes)
- the physical state of the sample (in liquid and gas cuvettes)
- the sample feed (in flow cells and cuvettes for manual sample feed)
- the measuring accuracy (in routine and precision cuvettes)
Cuvette test
The cuvette test is the term used to describe analytical methods with which chemical parameters of solutions are measured photometrically . For this purpose, the solution is reacted with reagents specific to the parameter of interest. This reaction causes a change in the color or other optical properties of the solution. These changes can then be measured photometrically. They are directly related to the concentration of the examined substance in the solution and therefore enable quantitative statements.
So-called single-use cuvettes, which already contain the necessary reagents in the correct quantity, are often used. Only a defined amount of the liquid to be examined (e.g. drinking water) is then poured into the cuvette and the reaction begins after mixing. Depending on the parameters examined, the reaction time ranges from a few minutes to several hours, sometimes heating (e.g. with COD ) or the addition of further reagents during the course of the reaction is necessary. After the reaction is complete, the photometric measurement can be carried out. Commercial providers of cuvette tests offer photometer devices with microprocessors that convert the measured absorbance immediately into the desired concentration value and display it digitally.
Cell tests are due to their speed and easy handling u. a. used in water analysis. Examples are environmental and food processing companies such as sewage treatment plants , waterworks, breweries or dairies. Another area is the HB rapid test for blood donations .
Flow-through cells
In the case of flow-through cells, the sample is not pipetted and removed again after the measurement. Rather, the sample is fed into the cuvette via a tube, through the measuring chamber and out of the cuvette again. As long as the connected pump is running, the sample flows continuously through the cuvette, hence the name flow cuvette. These are very often used for tablet dissolution tests. These tablet dissolution tests examine when the active ingredient in a tablet is released in the stomach as it dissolves.
Blistering
Air bubbles that are in the measuring chamber during the measurement can lead to incorrect measurements. This can mainly be observed in cuvettes with a large path length and a round aperture .
Flow velocity (flow rate)
The flow rate depends on the cross-section of the feed tubes, the smallest cross-section in the cuvette, the performance of the connected pump and, last but not least, the nature of the sample itself ( viscosity ).
Light beam cross section
Particularly with very small apertures , it is important to ensure that the light beam cross-section is in the center of the measuring chamber . It should be smaller than the measuring chamber so that neither the floor nor the side walls are touched.
Measuring chamber
The cross-section of the measuring chamber is either rectangular or round in flow-through cells. The use of black glass or black quartz glass in the area of the measuring chamber ensures that no extraneous or false light affects the measurement. The black frame around the measuring chamber acts as a screen .
cleaning
Flow cells should only be cleaned while they are in flow. For this purpose, the inlet of the cuvette is connected to a storage container with a special cuvette cleaning medium via a pump. The hose from the drain is also led into the storage container. You have to advise against cleaning with ultrasound , as the ultrasound waves can destroy the high-precision window or even cause the entire cuvette to shatter.
Individual evidence
- ↑ W. Meinicke: DD 282079 and DD 282078 - measuring method for a disc cuvette . 1989.