CIRCULAR DICHROISM METER BASED ON POLARIZATION DIFFRACTIVE WAVEPLATE 1

. Circular dichroism (CD) is an important parameter, characterizing optically active medium. The devices for CD measurements presented on today market measures CD in wide spectral range. The high-precision standard optics, electronics and optomechanics are used in these devices, and data processing requires the application of a complex mathematical apparatus. These devices are quite expensive, have large dimensions, require specially trained staff, and measurement duration is tens of minutes. In this work we present a device for CD measurement on only one wavelength, the key element of which is polarization diffractive waveplate, the optical element of new generation. Device has a number of advantages: easy to use, compact, working in real time, and low cost. These advantages make it ideal for educational programs.


Introduction
Optical activity and circular dichroism Circular dichroism (CD) is an important parameter, characterizing optically active medium. Th e vast majority of biological substances are optically active, i.e. they rotate the plane of polarized light. Optical activity is conditioned by asymmetric structure of molecules -left and right rotations, which are optical isomers. Th eir physiological and biochemical eff ects on human body are oft en quite diff erent. For example, organism does not accept proteins artifi cially synthesized from D-amino acids: bacteria mix only one of the isomers, absolutely not interacting with each other.
Th e subject of optical activity impacts on various areas of science, ranging from classical optics to elementary particle physics, cosmology and origin of life. Th e main physical parameters which characterized the optically active substances are optical rotation dispersion (ORD) and CD. ORD spectroscopy is especially useful for assigning the conformation of sugars. CD spectroscopy is highly important means for study of biological molecules, their structure and interactions with other molecules. CD measurement is widely used in physics, chemistry, biology, food and pharmaceutical industries; therefore, this topic is included in university curriculum.
Devices for CD measuring CD measurement in wide spectral range is realized by spectropolarimeters [3; 5; 7; 10−13]. Th e basic functional element of modern spectropolarimeters [www.jascoinc.com, www.photophysics.com] is photoelastic modulator, by means of which the left -and right-circularly polarized components of light passed through the sample, are separated in time. Such method is accompanied by high losses of light intensity. Th at is why the powerful light sources (hundreds of Watts) and high sensitive detectors are used in these devices. In addition, to ensure high accuracy of measurement and high values of signal/noise ratio the complex mathematical approximations and long time data accumulation and processing are required. Th ese devices are high-precision and quite expensive, have large dimensions and require specially trained staff .
Only aft er synthesis of new liquid crystal (LC) polymers [1] a few years ago, and creation of optical elements of a new generation [8; 9] on their base, it has become possible to review the basis of optical instrument engineering. One of such elements is a polarization diff ractive waveplate (PDW), which allows the registration of circular dichroism in real time. Th e important feature of PDW is the fact that at the its output the spatialy separated left -and right-circularly polarized components are formed, and the diff raction effi ciency can be up to 100%. Th e usage of PDW allows developing small-size, operating in real time, spectropolarimeters with simplifi ed registration part, functioning in real time [2; 4; 6]. However, as our laboratory testing shows the sufficient measurement accuracy in these devices is provided either for large values of CD (hundreds of millidegrees) or in a narrow spectral range.
In this work we present a device for CD measurement on only one wavelength, the key element of which is polarization diff ractive waveplate [8].

Results and Discussion
Th e schematic of developed device is shown in fi g. 1. Th e monochromatic light beam passing through a cuvette, fi lled with the sample with CD is being spatially separated by the PDW into the left -and right-hand circularly polarized components. Th e intensities of those components are being recorded by the two single-active area photodetectors. Th e photocurrents from photodetectors are recorded by a diff erential logarithmic impedance amplifi er, and through the controller are sent to the computer, where further processing of the recorded data takes place using specially designed soft ware. А quartz cuvette is used for a sample placement. If the front and back walls of the cuvette are not perfectly parallel, the additional systematic error of CD measurement Fig. 1. Schematic of developed CD meter will be added. To avoid this error the intensities of the left -and right-circularly polarized components of the beam passed through an empty cuvette and spatially separated by PDW are measured in advance. Th is systematic error is being accounted in soft ware during data collection and processing.
A narrow spectral bandwidth LED is used as the light source. Alternatively, a monochromatic light beam may be formed from a wide spectral bandwidth light source, for example a xenon lamp, using a monochromator or from white light source coupled to narrow spectral bandwidth fi lters. A laser operating at the required light wavelengths may be used as a light source also.
Th e external view of device is presented on the fi g. 2. To test the developed CD meter the CD values of aqueous solution of vitamin B12 of diff erent concentrations have been measured at 420 nm wavelength (fi g. 3).  Th e laboratory testing of developed device was carried out by the authors, the results of which are given in fi g. 3. Th e aqueous solution of Vitamin B-12 with 0.4 mg/ml concentration was chosen as a sample.
As it seen from the fi g. 3, the CD value proportionally growths with the increasing of vitamin B12 concentration in aqueous solution. Th ere is a good coincidence of measurement results with the literature data. For example, CD of aqueous solution of vitamin B12 with 0.2 mg/ml concentration at 420 nm wavelength is about 20 mdeg (www.photophysics.com).

Conclusion
Advantages and fi eld of application Methods based on optical activity measurement, are widely used in physical, chemical, biological researches and in industry. Purely technical diffi culties in CD measuring have been overcome only recently, so CD method can be considered as new. Devices for CD measuring, presented in today market, are high-precision, expensive and are intended only for scientifi c research. A complex mathematical apparatus for data processing and high-precision optics are used in them. CD measurement is realized in wide spectral range and lasts tens of minutes.
Specifi city of proposed technology is the usage of new generation optical elementpolarization diff raction waveplate, realized on the base of novel materials (liquid crystal polymers) by using innovative optical recording methods. PDW allows simultaneously spatially separating of linearly polarized beam to left and right circularly polarized components. Th is makes possible to simplify the registration part of the device, making it easy to use, compact, inexpensive, operating in real time. Th ese advantages make the device very attractive for the universities for teaching and laboratory works on biology, biophysics and biochemistry, optics, pharmacology and organic chemistry. Such a device may be widly used in pharmaceutics also for intermediate control of production.