Reflectometric measurement

In the following, the application of these concepts is shown both for reffactometric and reflectometric measurements using Mach-Zehnder interferometers and reflectometric interference spectroscopy. 

The lowermost, last layer is the carrier layer made from a transparent polyester plastic (e.g. polyethylene terephthalate). Reflectometric measurement can be effected from this side and through this layer. 

The reflectometric evaluation procedure of the Ektachem system differs considerably from that of other dry chemistry systems. Ulbricht s sphere is not required in this reflectometric measurement. Measurement is effected from the underside of the slides. The arrangement of the measurement makes it possible to eliminate interfering substances that can influence the reflectometric measurement signal by their colour only. 

During the 12-second cycle the incubation rotor rotates continuously for 10 seconds and then stops for 2 seconds to remove slides from the rotor that have been assayed. During the 10 seconds the rotor performs 2 complete rotations and travels one position (dwell position) further. This last position is located before the pre-heating position and the rotor stops to eliminate the assayed slide and to fit in a new one. The reflectometric measurement is effected as the incubation rotor rotates. A total of 54 measurements has been performed by the time the incubation phase of a slide of approx. 5.5 minutes is completed. 

Reflectometric measurements are technically easy to perform and can yield useful information about thin films and their interfaces. The partial reflection of incident light from two interfaces of a thin film with a thickness of about 1 pm can function as a Fabry-Perot interferometer with low reflectance. In the situation depicted in Fig. 14.45a, the reflected light will show an interference pattern as a function of wavelength, according to ... 

Figure 17. Structure affinity relationship measurement using reflectometric interference spectroscopy of atrazine versus an antibody in water. A variety of derivatives of triazines are measured at a number of concentrations to obtain affinity constants.

The possibilities of measuring time-resolved hybridization in this parallel setup are shown in Figure 23, and it even allows to obtain kinetic information for all spots in parallel. Since reflectometric interference... 

In this section an overview of the numerous methods and principles for the discrimination of enantiomers is given. First, the interaction principles of the polymer-based methods adapted from chromatographic procedures are illustrated. The discrimination of enantiomers was achieved some decades ago by using different types of stationary materials, like cyclodextrins or polymer-bonded amide selectors. These stationary-phase materials have successfully been appointed for label-free optical sensing methods like surface plasmon resonance (SPR) or reflectometric interference spectroscopy (RIfS). Furthermore, various successful applications to optical spectroscopy of the well-established method of fluorescence measurements for the discrimination of enantiomers are described. 

Approximately 10-11 al of the sample or specimen to be assayed (serum or plasma, and for some chemistries urine or cerebrospinal fluid) are applied to the reagent carrier slide. The specimen will diffuse into the slide within a short time, where it reacts with a reagent that is present in a gelatine or agarose matrix. The dye that forms can be measured reflectometrically. The electrolytes (sodium, potassium, chloride and carbon dioxide) are assayed by means of single use ion-selective electrodes. 

After the dye has formed in the reagent layer the slide can be assessed reflectometrically. Since the construction of the reflectometer is not the same with all Kodak instruments, the following is a description in principal only. For details of the technique of measurement see the sections describing the individual instruments. 

Fig. 9. Reflectometric immunosensor. (a) Construction scheme (b) dependence of the measuring signal on layer thickness upon reaction between adsorbed immunoglobulin G (IgG) and anti-IgG. (Gelatin is added to suppress nonspecific adsorption on the free silicon surface.) (Redrawn from Welin et al., 1984).

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... 

Figure 17-2 Multi-point measurements by optical time domain reflectometric detection.

An increase in optical thickness of the thin film, caused by e.g. ligand adsorption, will shift the interference spectrum to a higher wavelength and widen the distance between the minima and maxima in the inteference spectra as illustrated in Fig. 14.45b. This is the principle behind reflectometric interference spectroscopy, or Rifs [315]. Due to the high sensitivivity of the detection (ppm levels of phase shifts can be measured [316]), the RIfS device has been successfully used for the study of various biological interactions at surfaces, such as mouse anti-atrazine/ atrazine [317] and DNA-ligand interactions [318]. The principle of RIfS also allows the construction of low-cost devices. 

Advanced analytical techniques as well as special accessories evolve from custom-made laboratory units into commercially available devices. Toward a more comprehensive characterisation of swelling and switching layer systems, different analytical techniques are combined in a single set-up. Examples closely related to the topic of this chapter are a unit for simultaneous measurements by QCM-D and ellipsometry (see application notes at www.q-sense.com) as well as a combined set-up for microslit electrokinetic measurements and reflectometric interference spectroscopy or Fourier transform infrared spectroscopy, respectively (see www.zetascience.info). 

Bener, M., M. Ozyurek, K. Guclu et al. 2013. Novel optical fiber reflectometric CUPRAC sensor for total antioxidant capacity measurement of food extracts and biological samples. J. Agric. Food Chem. 61(35) 8381-8388. 

This test, which is important for automotive leathers in particular, determines the condensation on cooled glass panes of volatiles from the leather or the finish coat It is described in DIN 75201. There are two different methods of measurement (a) the reflectometric method and (b) the gravimetric method. While the gravimetric method indicates the condensed mass, for example after 16 h at 100 °C, the reflectometric method describes the clouding of the cooled glass plate after 3 h at 100 °C. 

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