Polarimeters are optical instruments that measure the rotation of plane-polarised light. They are used throughout the food, beverage, chemical and pharmaceutical industries to characterise optically active samples, where samples of a known constituency can be compared to its published data (specific rotation).
The published specific rotation relies on the measured optical activity of the sample, the length the light passes through, the temperature and importantly the wavelength of light used for analysis.
Calculation of Specific Rotation (optically active liquids/solutions)
The sample under test is applied to a polarimeter in a polarimeter tube so that light passes through its length unencumbered. As the polarimeter tube is an integral part of the overall measurement, it must be manufactured to highest precision and maintained in good condition at all times. The windows (or cover glasses) at each end are removable for cleaning and may be replaced in the event of loss or damage. They provide a clear path through which the light can travel.
For most samples, the wavelength cited for polarimetry is 589nm (Sodium D, yellow), that is, a visible wavelength.
However, in some circumstances it is desirable to use an alternative wavelength. For example, samples of very low concentration may be measured in the ultraviolet region (e.g. 365 nm) so that the sensitivity of the reading is increased to a measurable value or for short path lengths, the reading is amplified enough to achieve reliable results. Alternatively, the use of near infrared light may be desirable, where dark samples (<4.0 OD) are required to be measured, such as within the sugar industry.
No matter what wavelength is adopted for measurement, light must be able to pass through the sample and importantly, through the windows of the polarimeter tube. Crown glass is commonly used for polarimeter tube windows; however, this material blocks ultraviolet light and so is not suitable for use with wavelengths less than 400 nm. Instead, silica windows having a wide UV, Vis and NIR band-pass must be used. It can be seen in the graph below that silica is suitable for use right across the UV/Vis/NIR spectrum and so is ideal for use with all types of polarimeter such as the single wavelength ADP450 polarimeter or ADP600 Series multiple wavelength polarimeters, no matter what wavelength they are operating.
From January 2023, all Bellingham + Stanley polarimeter tubes and their spare windows are now manufactured from silica, eliminating the need to consider special windows for operation in the ultraviolet region.
- XPC Tube Windows (diameter 10mm)
- Standard Tube windows (diameter 15.5mm)
- Low Volume Tube windows (diameter 20mm)
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