Polarimetry
Optical & specific rotation angle
A polarimeter is a device for determining the direction of polarization of light or the rotational power of an optically active substance. The measurement of the optical activity of a substance results in the angle of rotation – also known as the rotation value. A difference is made between the optical rotation and the specific angle of rotation. The optical rotation determines the measured value of the polarimeter, without taking specific physical influences into consideration.
The specific angle of rotation considers the sample concentration and the length of the measuring tube. A higher sample concentration and a longer measuring tube will increase the optical activity. This ultimately results in a higher specific angle of rotation. The specific angle of rotation can be used to determine the sample and the purity of the samples.
Chirality
Polarimetry utilizes the optical activity of chiral substances. Chiral substances are structured in such a way that the reflection of the spatial arrangement of their molecules can never be mapped onto themselves by a rotation. Chirality can be vividly described using your own two hands. The left hand represents the substance to be observed. The right hand corresponds to the reflection of the spatial arrangement. It is not possible to make the two hands congruent by arbitrary rotation. Many chemical compounds show the same behavior. Due to the lack of symmetry, these chemical compounds have an optical activity that can be measured with the polarimeter.
Optical activity
The ability of a substance to rotate the plane of oscillation of linearly polarized light is referred to the optical activity. Examples of optically active media are sugar, lactic acid and quartz. A distinction is made according to vibration plane of the polarized light rotated into a clockwise or counterclockwise direction. Clockwise rotation is referred to as dextrorotatory and has a positive sign (+). Counterclockwise rotation is referred to as counterclockwise rotation and has a negative sign (-). For example, sucrose shows a clockwise rotation, while fructose shows an anticlockwise rotation.
Enantiomers
Enantiomers are also exemplified by our two hands: Both hands, despite not being identical, but have the same elements (hand and five fingers). This also applies to chemical compounds that are chiral. Chemical compounds that have the same molecules (molecular formula) and the same bonds, but are mirrored to each other, are called enantiomers. Different enantiomers of a substance can have significantly different properties. For example, one enantiomer of a substance can taste sweet, while another tastes bitter. Or one enantiomer can be harmless, while another is harmful to health. This is why measurement with polarimeters is an important analysis.
How does a polarimeter work?
How do polarimeters measure?
Polarimeters measure the change of the plane of polarization of linearly polarized light after it has interaction with an optically active (chiral) substance. The interaction is indicated by a rotation of the polarization direction of the light wave. This rotation, which is called the angle of rotation, is measured by a polarimeter. This allows information about the molecular structure, purity and concentration of an optically active substance to be determined.
Example: If the tube is filled with granulated sugar (sucrose), the angle of rotation is dextrorotatory (clockwise).
What affects the polarimeter measurement?
The optical activity – and thus the measured value – is also influenced by the temperature, the wavelength of the light and the length of the optical path. The length of the optical path is specified by the length of the used polarimeter tube. A longer optical path results in a higher angle of rotation. The angle of rotation also increases with rising concentration. In the case of temperature and wavelength, it depends on the substance under investigation whether a larger or smaller angle of rotation is measured by increasing these variables.
Example: While the angle of rotation decreases with rising temperature for sucrose, it increases with rising temperature for quartz.
Open & see how polarimeters measure
Polarising filter and the plane of oscillation of light
Light as it surrounds us is unpolarised. This means that each light wave oscillates randomly in one spatial direction. To convert unpolarised light into polarised light, polarising filters are used. So do polarimeters.
A polarising filter transmits only light in a certain polarisation direction and suppresses the light of other polarisation directions. Light which has passed through a polarising filter is called polarised.
When polarised light passes through a second filter placed parallel to the first filter, the plane of oscillation is maintained and light reaches the screen If the second filter is rotated by 90°, it suppresses the polarised light beam; the screen behind it remains dark.
Design of the polarimeter
A polarimeter measures the direction and extent of the polarisation plane rotation. A polarimeter is the entirety of light source, polariser, polarimeter tube, analyser and detector.
If a tube with no optically active substance is placed between two parallel polarisation filters, the oscillation plane is retained. Light will be visible on the screen at the end. If the tube is filled with an optically active substance, e.g. B. a solution of granulated sugar a change becomes visible….
… The operation of an optically active substance has the following effect: the light at the back of the screen becomes much less visible. This happens because the oscillation plane of the light has been changed. An optically active substance (sugar solution), as shown in the example, has reversed the direction of polarization.
Polarimeter applications – Importance for the pharmaceutical industry
Many major pharmacological agents are optically active, due to their chirality and are present as enantiomers. Enantiomers (comparable to the left and right hand) generally differ only slightly in their physical properties, such as density or refractive index. They can hardly be distinguished and separated in this way, even using HPLC (liquid chromatography method). Enantiomers often have a differences with their reactivity with enzymes or their sensory properties such as smell and taste. The enantiomers of limonene, for example, are perceived as having an orange or lemon scent. Racemic mixtures (mixture of equal amounts of two enantiomers) often have very different properties in terms of their physiological activity, Mechanism of effectiveness, toxicity and side effects.
An outstanding example here is thalidomide: The substance was in the Contergan sleeping pills present as a racemate (enantiomeric ratio 1:1). One of the enantiomers had a teratogen effect, i.e. when taken, it caused malformations in pregnancy on the embryo. The tragic incidence surrounding the production of thalidomide prompted the introduction of legal guidelines in many countries. We now have standards, that specify the exact rules to be followed during the development of chiral drugs.
Standards and guidelines
Analytical measurement according to norms & international standards
With a polarimeter, you can determine the specific characteristics of substances without chemically altering or destroying them. Polarimeters are used according to normative specifications, especially for sensitive and highly regulated applications. The measuring devices must comply with industry standards (e.g. GMP, GAMP 5) or offer functions required by standards, e.g. in accordance with 21 CFR Part 11 (e.g. audit trail).
Standards in polarimetry regulate the environmental conditions and sample preparation as well as measurement tolerances, properties and equipment of a device or calibration medium under the aspect of “best practice”. Those who use polarimeters in accordance with standard specifications can ensure that measurements are correct and reproducible.
Below is an overview of all standards and guidelines known to us that refer to polarimetry. Note:This is a general overview of the standards. We would be happy to discuss the specifications fulfilled by the various A.KRÜSS polarimeter models in a personal consultation. Please feel free to contact us.
Samples and measured values
Samples and measurement values
Polarimeters are modern, highly selective and offer mainly very fast measurements in many areas of analysis. Polarimeters measures the angle of rotation of optically active substances. Their measurement results are used for quality control in the production of active pharmaceutical ingredients and to check the identity of chiral substances in incoming and outgoing goods laboratories. In the food, starch and sugar industries, they are used for concentration and purity tests. Every industry has different needs and requirements for this type of measuring technology. We meet them with a wide range of different models and measuring tubes.
Which substances can be measured?
In the following you will find a comprehensive overview of samples and their specific angle of rotation. Most of the results are valid for standard measuring conditions (T=20 °C and λ = 589 nm), deviations are indicated. Deviations are indicated.
Note: This overview is intended as a summary. We will be happy to discuss which polarimeter models are best suited to your measurement requirements in a personal consultation. Please feel free to contact us.
Typical areas of application
Polarimeter analyzers for a wide range of applications
Polarimetry is one of the most important quality control methods in the pharmaceutical, chemical, cosmetics, food and beverage industries. The angle of rotation allows you to ascertain the identity and quality of substances as well as their concentration in mixtures. It is also pssible to indicate the progress of reactions and conversions.
The pharmaceutical industry uses polarimeters to ensure perfect enantiometer separation, to determine the concentration of optically active substances or to analyse correlations between toxic and pharmacological characteristics .
In the production of foodstuffs, polarimeters check the purity of raw materials or provide information on product quality. The range of applications for the measuring devices is wide.
For information: Below is an overview of typical areas of application, substances tested, associated standards and the polarimeters we recommend.
Innovation P9000 model
To meet the highest standards, we have designed the P9000 series polarimeter. EIt is an enhanced high-tech measuring instrument with many new features. The P9000 series guarantees measurements with the highest precision and reliably provides you with secure data in the shortest possible time.
Polarimeter with a new design
- Innovative cuvette geometry provides more comfort. The new cuvette enables the fastest temperature control and even temperature distribution across the cuvette cross-section. Maximum ease of use during filling and cleaning is guaranteed. The cuvette is automatically recognized using RFID technology.
- The measuring temperature selected by the user is set automatically by the system. High-precision Peltier elements regulate and maintain the specified temperature.
- A special control function continuously monitors the temperature control with two temperature sensors. Only when temperature control, control function and temperature stabilization show an exactly stable temperature, the control function opens and the measurement begins. The readiness to measure is indicated by an LED signal.
- Optional expansion to up to eight different wavelengths is possible.
How to Use
Depending on the application, we offer various polarimeter solutions. In addition to our new models, many also opt for the proven polarimeters of the P8000 series. They are the fastest polarimeters in the world and conform to global standards and norms (21-CFR Part 11). The P1000 LED, which is mainly used for educational purposes, measures optical rotation according to the half-shade principle. The P3000 model is an economical solution: it is used when a measurement accuracy of ±0.01° is sufficient. With the PTB-certified quartz control plates (OIML, ICUMSA and pharmacopoeia compliant), calibration and adjustment can be carried out precisely and easily.
