In laboratories in general photometry describes the method of determining the concentration of colored solutions with the aid of light. Here, white light is projected through a prism onto a screen. Depending on which wavelength is used, the color of the test object or of the test solution changes:
- 390 - 435 nm: purple / violet
- 435 - 495 nm: blue
- 495 - 570 nm: green
- 570 - 590 nm: (greenish) yellow
- 590 - 630 nm: orange
- 630 - 770 nm: red
As a light source either infrared or ultraviolet light can be used. Infrared radiation, also called radiant heat is radiated by a body even more the warmer it is. Therefore infrared radiation occurs primarily in medically treating premature infants in incubators, but the energy efficiency of homes can also be determined. UV radiation, however, has the ability to kill bacteria. They are used in the sterilization in the operating room, as well as to kill germs in food. Furthermore the UV light is used for curing composite fillings in dental practices - the majority, however, know the UV light in conjunction with solariums and sun beds. UV light in sun beds is used to build up vital vitamin D.
The principle of the photometry is based on the attenuation of the light as it passes through matter. The weakening of the light is the result of an interaction between light of a specific wavelength and of the test substance that needs to be present as a solution. The amount of light attenuation is a measure of the concentration, under specified conditions.
Standard solutions for photometry
The lab user can choose from two different options when using cuvette tests. One of these are standard solutions which are used as a stock solution for dilution of solutions, e.g. in water analysis.
These freshly prepared control standards from stock solutions must be used in a few parameters (e.g. nitrite) since they are subject to rapid change. When using these standard solutions, the correct amount of dilution must be calculated and carried out by hand. A series of standard solutions which must be freshly prepared on a regular basis:
- free chlorine
- hydrogen peroxide
- anionic surfactants
and some more. Apart from the additional "effort" having to create the solutions again and again, these control standards are generally cheaper than the ready-to-use multi-parameter standards.
Multi-parameter standards for photometry
The second option is therefore the use of solutions that have already been created, the ready-to-use so-called multi-parameter standards. They convince with a simple handling since they already have the right concentration to the test set. Compared to the already named standard solutions there is no need for dilution with calculations for concentration and pipetting volume - thus accidental volume and pipetting errors can be avoided. Furthermore those standards score with pre-increase solutions for the study of matrix effects or interfering ions.