Detectors chiral

There are two basic types of chiral detectors for LC, those that measure optical rotation and those that measure circular dichroism. At the time of writing this book, the only commercially available chiral detectors are those that measure optical rotation. Nevertheless, a detector that measures circular dichroism and utilizes a diode array sensor system is thought to be in the design stage and will be briefly described later. 

The successful development of a chiral detector based on optical rotation measurement hinges on the use of the Faraday effect. If a plane polarized beam of light passes through a medium that is... 

A Chiral Detector Monitoring Optical Rotation Courtsey of JM Science Inc. 

The Separation of Some Carbohydrates Monitored by a Chiral Detector Courtesy of JM Science... 

The Separation of Three Optically Active Fragrance Compounds Monitored by a Chiral Detector... 

The peak represented 3 pm of material and, taking the concentration at the peak maximum as twice the peak average concentration, the peak maximum concentration was about 8 pm/ml. The peak height appears to be about three times the noise and so the sensitivity (that concentration that will give a signal equivalent to twice the noise) is about 5.3 pm/ml or in more standard terms 5.3 x 10 g/ml. As the chiral detector is a bulk property detector, a sensitivity of 5.3 x 10 g/ml seems more realistic. Nevertheless, this sensitivity is a great improvement on many chiral detectors previously described. 

The value of a chiral detector in the analysis of physiologically active materials is clear, but the methods so far used have been found somewhat insensitive. A more encouraging procedure would be the measurement of circular dichroism and such instrumentation employing diode array detection is presently under development. Details of the device are difficult to obtain due to patent applications pending and particulars are not available. The basic arrangement, however, is thought to be similar to that depicted in figure 11. 

The Production and Properties of Polarized Light Piezo-Optical Modulators Practical Chiral Detectors... 

A JASCO model OR-990 chiral detector was tested as a means for assessing racemization in the reference samples. This instrument was equipped with a 44 pi flow cell (25 mm path length), a 150 W Hg-Xe lamp as light source, and an open-loop polarizer/analyzer. Samples were analyzed at 350 nm, with a lOx gain, 0.1 sec response, and 16 mdegree deflection. Separations were performed on a Hewlett Packard HP 1090 HPLC with a diode array detector. 

When a molecule contains more than one chiral center, there are pairs of molecules not related by mirror symmetry. These molecules are called diastereomers. They differ in the relative chiralities at pairs of chiral centers (see Figure 14.3) and their chemical and physical properties are not identical. In this they differ from enantiomers, which have identical chemical and physical properties except those connected with their interaction with chiral detectors — plane-polarized light or enzymes, for example. Members of a pair of diastereoisomers that differ only in the configuration at one carbon atom are called epimers. 

Furthermore, the chiral discrimination of monoterpenes has been recognized as one of the most important analytical techniques in flavor chemistry and pharmacology because the optically active stereoisomers have different sensory qualities and biological activities. HPLC offers powerful techniques for separation and quantification of enantiomers because of the progressive improvement of chiral chromatographic materials and chiral detectors such as optical rotatory dispersion (ORD) and circular dichroism (CD) detectors. In contrast, determination of chiral compounds by GC typically requires coinjection of the reference compound with known stereochemistry. An HPLC system equipped with a chiral detector, on the other hand, allows direct determination of the configuration of chiral compounds.84... 

The chromatograms of the racemic hydroperoxide which was partly decomposed into the corresponding carbinol were recorded by UV and a chiral detector (Fig. 1). First the carbinol was separated and then a better optical resolution and longer retenton time was observed for the hydroperoxide. A comparison of both chromatograms indicates that the optical rotation of the enantiomer of the carbinol, which was eluted first, was plus. This method was successfully used for the enantiomeric separation of other chiral hydroperoxides on an analytical scale [25]. Thus for the first time a practical method is available for the determination of enantiomers of hydroperoxides and related alcohols in the same solution. 

A detailed description of a commercially available LC chiral detector will be given in the chapter 7. However, some general comments on the properties of chiral detectors would be appropriate here. Contemporary, chiral detectors are relatively insensitive and, consequently, there are no GC chiral detectors commercially available at this time. Capillary columns will only function with very small charges and these types of column must be employed for the great majority of chiral separations, in order to provide the necessary efficiency. Unfortunately, the sensitivity of chiral sensing systems, investigated so far, have been inadequate for use with GC capillary columns. In contrast, after considerable research and development, the sensitivity of LC chiral detectors has been improved to a level where (although still relatively insensitive) they can often be used satisfactorily with contemporary small particle LC chiral columns. 

One commercially available detector is the PDR Chiral detector that is manufactured by PDR-Chiral Inc. The detector has a sensitivity of 25 micro degrees. The flow cell has a path length of 50 mm and a volume of 56 pi. This is very large compared with the normal UV sensor cell, which has a volume of 3 to 8 pi. Such a large volume will cause early peaks, from columns packed with particle 5 pm or less, to merge with consequent loss of resolution. Later peaks, however, will be detected... 

The Separation of the Enantiomers of Dihydrobenzoin at Different Resolution Monitored by the PDR Chiral Detector and the UV Detector Courtesy of PDP-Chiral Inc. 

The separation was carried out on a A Cyclodextrin-1 2000 AC column. The column eluent was methanol/TEA(pH 4.4) separation A, 10/90 v/v, B, 30/70 v/v, C, 50/50 v/v and D pure methanol. The upper set of chromatograms were monitored by the chiral detector and the lower by a UV detector at 254 nm. It is seen that when the enantiomers are will separated (chromatogram (A)) the positive a negative peaks accurately... 

The basic apparatus used in chiral LC (with the exception of the phase system) is sensibly the same as that used in general LC analysis. In chiral LC, occasionally a chiral detector is deemed to be appropriate, but for most analyses, the same type of detectors are also employed. A block diagram of an LC system suitable for chiral analysis is shown in figure 7.1. 

There are a large number of LC detectors available but, of these, a relatively few are in common use. This is largely because some detectors are highly specific and others are very expensive. About 95% of all LC analyses are carried out using one of 5 different types of detectors, the UV adsorption detector, the fluorescence detector, the electrical conductivity detector, the light scattering detector and the refractive index detector. However, in addition to these basic detectors, for obvious reasons an example of a chiral detector will also be included. 

The value of a chiral detector in the analysis of physiologically active materials is clear, but the methods, so far, used have shown the detector to be somewhat insensitive. Nevertheless, development work on chiral... 

The Multiple Angle Laser Light-Scattering (MALS) Detector The Refractive Index Detector Chiral Detectors Data Acquisition and Processing Synopsis References Chapter 8... 

Supported liquid membranes (SLMs) consisting of 5% tri-n-octylphosphine oxide (TOPO) dissolved in di-w-hexylether/n-undecane (1 1) have been used in the simultaneous extraction of a mixture of three stUbene compounds (dienestrol, diethylstilbestrol, and hexestrol) in cow s milk, urine, bovine kidney, and liver tissue matrices [183]. The efficiencies obtained after the enrichment of 1 ng/1 stilbenes in a variety of biological matrices of milk, urine, liver, kidney, and water were 60-70, 71-86, 69-80, 63-74, and 72-93%, respectively. A new method to contribute to the discrimination of polyphenols including resveratrol with synthetic pores was proposed [184]. The work [185] evaluated two types of commonly available chiral detectors for their possible use in chiral method development and screening polarimeters and CD detectors. Linearity, precision, and the limit of detection (LOD) of six compounds (trans-stilbene oxide, ethyl chrysanthemate, propranolol, 1-methyl-2-tetralone, naproxen, and methyl methionine) on four common detectors (three polarimeters and one CD detector) were experimentally determined and the limit of quantitation calculated from the experimental LOD. trans-Stilbene oxide worked well across all the detectors, showing good linearity, precision, and low detection limits. However, the other five compounds proved to be more discriminating and showed that the CD detector performed better as a detector for chiral screens than the polarimeters. 

Linearity, precision, and the limit of detection (LOD) of trans-stilbene oxide and other compounds were investigated [89]. The authors investigated the second factor and evaluated two types of commonly available chiral detectors for their possible use in chiral method development and screening polarimeters and CD detectors. It was shown that frans-stilbene oxide worked well across all the detectors examined, showing good linearity, precision, and low detection limits. 

---