986 KNOWLEDGE Type of equipment Filter Units With a filter photometer, the required wavelength is selected by inserting filters. The filters are activated by a mechanical drive in the beam path. Its use is mostly limited to visible wavelengths, and for those filters which are available the band width is relatively large. However these units are adequate for many routine quantitative measuring tasks. Spectrophotometers Spectrophotometers select a defined section of the spectrum using a grating. Measurements can be obtained at any wavelength within the measuring range of the instrument. These units are typically used for analysis work. Diode Array Units Diode Array Units use a converse optical system to obtain simultaneous measurement over the whole of the unit's spectral range. The advantage of very fast non-mechanical spectral measurements has to be weighed against the generally more limited sensitivity. Wavelength Range There are Vis units which measure in visual range only (approx. 330-1000 nm), as opposed to UV-VIS units, which typically cover a wavelength range of 200-1000 nm. When used in the UV range, special lamps (Deuterium or Xenon) are required, making these units significantly more expensive to purchase and operate. Spectral Wavelength The wavelength of the units is an important factor for scanning equipment. The smaller the wavelength, the more precise is the selection of small maxima. Size of beam path The normal cuvette with an optical path length of 10 mm and a width of 10 mm should only be used with large sample quantities. With only very small sample quantities and tiny sample concentrations, it is important to focus the beam path onto a small point in the cuvette. Thus samples of less than 50 µl can be analyzed, even with layers of 10 mm thickness. Software As well as with the optical features of the units, the data processing options differ significantly from product to product. Special units have been developed for various special operations such as quantification in environmental analysis or in life sciences. These include special programs which facilitate the use of test kits or incorporate special analytical methods. Very little introduction is needed before essential analysis can be carried out. Accessories Typical accessories are: tempered sample holder (enzyme kinetics), and automatic sample exchanger or sipper pumps with flow-through cuvettes to draw the sample into the unit. Adding a printer, recorder or pc connection for data processing is also possible. Cuvettes The sample cuvette is an important part of the analysis system. As well as a wide range of hard-wearing cuvettes made from optical glass or quartz glass 1, single-use cuvettes are available which can also be used for UV purposes 2. Devices which can operate without a cuvette of classic design have been developed for use in biotechnology. These devices such as the Nano-Genova only require the user to pipette the sample (2 µl) on a surface 3. Besides the quicker sample change, the otherwise necessary thinning process can be omitted. Guidelines for calibration A regular check on the unit's calibration will maintain the accuracy of your measurements. It is necessary to use spectrophotometers to check whether the selected wavelength is set correctly. For this, filters are inserted with characteristic peak maxima. Some units are able to carry out automatic wavelength calibration using lamp peaks, when powering on. Author: Dr. Axel Landgraf PHOTOMETERS 1 2 3
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