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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Polarimeter | 3/4 | https://en.wikipedia.org/wiki/Polarimeter | reference | science, encyclopedia | 2026-05-05T09:43:28.106070+00:00 | kb-cron |
=== Manual === The earliest polarimeters, which date back to the 1830s, required the user to physically rotate one polarizing element (the analyzer) whilst viewing through another static element (the detector). The detector was positioned at the opposite end of a tube containing the optically active sample, and the user used his/her eye to judge the "alignment" when least light was observed. The angle of rotation was then read from a simple fixed to the moving polariser to within a degree or so. Although most manual polarimeters produced today still adopt this basic principle, the many developments applied to the original opto-mechanical design over the years have significantly improved measurement performance. The introduction of a half-wave plate increased "distinction sensitivity", whilst a precision glass scale with vernier drum facilitated the final reading to within ca. ±0.05º. Most modern manual polarimeters also incorporate a long-life yellow LED in place of the more costly sodium arc lamp as a light source.
=== Semi-automatic === Today, semi-automatic polarimeters are available. The operator views the image via a digital display adjusts the analyzer angle with electronic controls.
=== Fully automatic ===
Fully automatic polarimeters are now widely used and simply require the user to press a button and wait for a digital readout. Fast automatic digital polarimeters yield an accurate result within a few seconds, regardless of the rotation angle of the sample. In addition, they provide continuous measurement, facilitating high-performance liquid chromatography and other kinetic investigations. Another feature of modern polarimeters is the Faraday modulator. The Faraday modulator creates an alternating current magnetic field. It oscillates the plane of polarization to enhance the detection accuracy by allowing the point of maximal darkness to be passed through again and again and thus be determined with even more accuracy. As the temperature of the sample has a significant influence on the optical rotation of the sample, modern polarimeters have already included Peltier elements to actively control the temperature. Special techniques as temperature controlled sample tubes reduce measuring errors and ease operation. Results can directly be transferred to computers or networks for automatic processing. Historically, accurate filling of the sample cell had to be checked outside the instrument, as an appropriate control from within the device was not possible. Nowadays a camera system can help to monitor the sample and accurate filling conditions in the sample cell. Furthermore, features for automatic filling introduced by few companies are available on the market. When working with caustic chemicals, acids, and bases it can be beneficial to not load the polarimeter cell by hand. Both of these options help to avoid potential errors caused by bubbles or particles.
== Sources of error == The angle of rotation of an optically active substance can be affected by:
Concentration of the sample Wavelength of light passing through the sample (generally, angle of rotation and wavelength tend to be inversely proportional) Temperature of the sample (generally the two are directly proportional) Length of the sample cell (input by the user into most automatic polarimeters to ensure better accuracy) Filling conditions (bubbles, temperature and concentration gradients) Most modern polarimeters have methods for compensating or/and controlling these errors.
== Calibration == Historically, a sucrose solution with a defined concentration was used to calibrate polarimeters relating the amount of sugar molecules to the light polarization rotation. The International Commission for Uniform Methods of Sugar Analysis (ICUMSA) played a key role in unifying analytical methods for the sugar industry, set standards for the International Sugar Scale (ISS) and the specifications for polarimeters in sugar industry. However, sugar solutions are prone to contamination and evaporation. Moreover, the optical rotation of a substance is very sensitive to temperature. A more reliable and stable standard was found: crystalline quartz which is oriented and cut in a way that it matches the optical rotation of a normal sugar solution, but without showing the disadvantages mentioned above. Quartz (silicon dioxide, SiO2) is a common mineral, a trigonal chemical compound of silicon and oxygen. Nowadays, quartz plates or quartz control plates of different thickness serve as standards to calibrate polarimeters and saccharimeters. In order to ensure reliable and comparable results, quartz plates can be calibrated and certified by metrology institutes. Alternatively, calibration may be checked using a Polarization Reference Standard, which consists of a plate of quartz mounted in a holder perpendicular to the light path. These standards are available, traceable to NIST, by contacting Rudolph Research Analytical, located at 55 Newburgh Road, Hackettstown, NJ 07840, USA. A calibration first consists of a preliminary test in which the fundamental calibration capability is checked. The quartz control plates must meet the required minimum requirements with respect to their dimensions, optical pureness, flatness, parallelism of the faces and optical axis errors. After that, the actual measurement value - the optical rotation - is measured with the precision polarimeter. The measurement uncertainty of the polarimeter amounts to 0.001° (k=2).
== Applications == Because many optically active chemicals such as tartaric acid, are stereoisomers, a polarimeter can be used to identify which isomer is present in a sample – if it rotates polarized light to the left, it is a levo-isomer, and to the right, a dextro-isomer. It can also be used to measure the ratio of enantiomers in solutions. The optical rotation is proportional to the concentration of the optically active substances in solution. Polarimetry may therefore be applied for concentration measurements of enantiomer-pure samples. With a known concentration of a sample, polarimetry may also be applied to determine the specific rotation (a physical property) when characterizing a new substance.