Refractive optical detection method

Sensitive Optical Detectors. More sensitive optical techniques that have been used with CE include fluorescence, refractive index, chemiluminescence, Raman spectrophotometry, and circular dichroism. The most sensitive optical detection method used in CE is laser-induced fluorescence (LIE), which is capable of detection limits in the 10 to 10" mol (or better) range. This detection mode is easily accomplished with analytes that are either easily labeled with a fluorescent substrate (e.g., intercalators for double-stranded DNA) or are naturally fluorescent (e.g., proteins or peptides containing tryptophan). CE systems have also been interfaced with mass spectrometers, and electrochemical detection methods have been developed, although such detectors must be isolated electrically from the electrophoretic voltages. 

Cyclohexapeptide monolayers on quartz microbalances are able to discriminate between different analytes in the liquid phase (see Section 10.5.1). On the basis of these results, we also have immobilized cyclopeptides (Fig. 10.14) on glass transducers. In this case, the interaction between cyclopeptides and analytes was monitored by reflectometric interference spectroscopy (RIfS) [98], RIfS is an optical detection method in which the phenomenon of reflection and interference of light at phase boundaries is used to measure changes in optical thickness (refractive index x layer thickness) of transparent films... 

The CPAC group at the University of Washington has developed a refractive index optical detection method that is very sensitive and tailor-made for microchip applications. The advantage of such a technique is that it is a universal detection method, and does not depend on analyte molecules containing chromophores. 

In TLC the detection process is static (sepaurations achieved in space rather than time) and free from time constraints, or from interference by the mobile phase, which is removed between the development and detection process. Freedom from time constraints permits the utilization of any variety of techniques to enhance detection sensitivity, which if the methods are nondestructive, nay be applied sequentially. Thus, the detection process in TLC is nore flexible and variable than for HPLC. For optical detection the minimum detectable quantities are similar for both technlqpies with, perhaps, a slight advantage for HPLC. Direct comparisons are difficult because of the differences in detection variables and how these are optimized. Detection in TLC, however, is generally limited to optical detection without the equivalent of refractive... 

A chemical sensor is a device that transforms chemical information into an analytically useful signal. Chemical sensors contain two basic functional units a receptor part and a transducer part. The receptor part is usually a sensitive layer, therefore a well founded knowledge about the mechanism of interaction of the analytes of interest and the selected sensitive layer has to be achieved. Various optical methods have been exploited in chemical sensors to transform the spectral information into useful signals which can be interpreted as chemical information about the analytes [1]. These are either reflectometric or refractometric methods. Optical sensors based on reflectometry are reflectometric interference spectroscopy (RIfS) [2] and ellipsometry [3,4], Evanescent field techniques, which are sensitive to changes in the refractive index, open a wide variety of optical detection principles [5] such as surface plasmon resonance spectroscopy (SPR) [6—8], Mach-Zehnder interferometer [9], Young interferometer [10], grating coupler [11] or resonant mirror [12] devices. All these optical... 

With CE, separated analytes are detected and measured as they migrate past an optical detector without prior staining. Optical detection is based on classical methods, such as photometric absorbance, refractive index, and fluorescence (see Chapter 3). These techniques miniaturize and simplify the process of measurement by eliminating several of the steps required in conventional gel electrophoresis. Because the amount of protein measured and the measurement path are both very small, however, special conditions are often used to improve sensitivity. 

The most widely used detection methods are based upon the optical properties of the analytes absorption, fluorescence and refractive index (see also ELSD detector. Section 7.4). 

Detection methods applied in ion chromatography (IC) can be divided into electrochemical and spectrometric methods. Electrochemical detection methods include conductometric, amperometric, and potentiometric methods, while spectroscopic methods include molecular techniques (UVA is, chemiluminescence, fluorescence, and refractive index methods), and spectroscopic techniques such as atomic absorption spectrometry (AAS), atomic emission spectrometry (AES), inductively coupled plasma-optical emission spectrometry (ICP-OES), inductively coupled plasma-mass spectrometry (ICP-MS), and mass spectrometry (MS). ... 

The schlieren microscope is able to detect refractive index variations to six decimal places. Any small difference in optical path (index difference, film thickness, etc) is very precisely detected by the schlieren microscope, especially in the Dodd modification. It is, in effect, a darkfield method. The specimen is illuminated with light in a portion of the illuminating cone and that direct light is masked in the conjugate back focal plane of the objective (Fig. 3). The only light to pass through this plane is refracted, reflected, or diffracted by the specimen. 

This temperature rise can be detected directly (laser calorimetry and optical calorimetry), or indirectly by measuring the change in either the refractive index (thermal lensing, beam deflection or refraction and thermal grating) or the volume (photo- or optoacoustic methods). This review will focus primarily on photoacoustic methods because they have been the most widely used to obtain thermodynamic and kinetic information about reactive intermediates. Other calorimetric methods are discussed in more detail in a recent review.7... 

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