Modulation principle

When Walsh started to think about using AAS for analytical purposes back in 1952, one of his key conclusions was that, in order to carry out absorption measurements on luminous atomic vapors, it would be necessary to employ a modulated light source and a synchronously tuned detection system, so that any radiation emitted by the sample would produce no signal at the output of the detection system [2]. This modulation principle, using either an AC-operated radiation source or a chopper in the radiation beam, and a selective amplifier tuned to the same modulation frequency, has ever since been applied in all commercially available atomic absorption spectrometers. It has been considered one of the major advantages of... 

The first report on the concept of thermal modulation was published in 1985 by Phillips s group [26]. By then, the modulation principle was not yet involved in GC xGC but rather in the so-called multiplex gas chromatography concept that was used in the field of headspace sampling without preconcentration for low-level analytes. On-column thermal desorption modulation was then applied in the field of sample introduction as a device capable of narrow injection pulses for high-speed GC [27]. 

In this chapter, we will describe and discuss the fabrication and characterization of metal oxide thick- and thin-film sensors (which are based on the conductivity modulation principle) and SAW devices (which are based on frequency modulation). The sensing properties of thick or thin semiconductor films with various components will be discussed, focusing on sensitivity and stability. In addition, SAW-type CWA sensors will be introduced for different pairs of inteidigitated transducer (IDT) fingers, various wave lengths, and different polymer coatings on the input and output IDTs of the device. 

In terms of modulated thermal analysis techniques , TMDSC clearly dominates the group. However, there is limited literature on temperature modulated thermogravimetic analysis (TMTGA) and temperature modulated thermomechanical analysis (TMTMA). Modulation principles have been applied to some less common thermal analysis techniques such as DMA and thermally stimulated current analysis and these developments will be briefly addressed here. It appears that the major development in thermal analysis in the next decade will be in the temperature modulated domain. 

Some ion trap mass spectrometers use variants of the original axial modulation principle. For example, some instruments use an auxiliary AC potential angular frequency (o/3, apphed to the end cap nearer to the detector. In this case, when Vq is ramped, ion ejection occurs at a fixed working point well removed from the 3 = 1 boundary of the stability diagram. 

FIGURE 12.40 Phase modulator principle and vector diagram. 

This method sank into oblivion until 1953, when Singleton and Collier [2, 3] took advantage of developments in electronics and built the earliest derivative spectrophotometer. They modified an IR spectrophotometer by an analog device to generate second-order spectra and took out a patent for this concept [2]. At the same time, Hammond and Price [4] proposed the wavelength modulation principle (cf. Section 3.3.1), and Giese and French [5] adapted it to study first-order derivative visible spectra of photosynthetic systems. 

Let us consider the potential for wavelength modulation. In 1953 Hamond and Price [4] proposed the wavelength modulation principle based on a vibrating Littrow mirror. One year later the photobiologists French, Church and Eppley [5] adapted it to study first-order derivative spectra of photosynthetic systems in the visible region [6-10]. In the years that followed, various modulation techniques were employed to generate derivatives of different spectra a review is given in Table 3-1. 

The complement-modulating principles of several plants have been identified as flavonoids, e.g. in leaves of Morinda morindoides (Baker) Milne Redh. (Rubiaceae) [17-19]. It was demonstrated that quercetin, quercitrin and rutin inhibited the classical pathway in a dose-dependent manner, but that the effect on the alternative pathway was not dose-dependent [18]. Morindaoside, a 7-0-rhamnosylsophoroside of kaempferol (kaempferol 7-0-[rhamnosyl-(l-6)]-[glucosyl-(l-2)]-glucoside), showed a dose-dependent inhibition of the classical pathway (40.8 8.6% inhibition at 1 mM), and a dose-dependent activation of the alternative pathway (56.1 4.5% activation at 1 mM). Rosmarinic acid, used as a reference compound, inhibited both the classical and the alternative pathway in a dose-dependent way. Kaempferol (19) was about as active as morindaoside as an inhibitor of the classical pathway (33.2 11.5% inhibition at 1 mM), but less active as an activator of the alternative pathway (28.6 3.7% activation at 1 mM) [19]. 

The principles referred to so far are common to all New Approach directives. The individual directives obviously need to be specific on topics such as the scope, the essential requirements, the classification of the products and on the use of modules for conformity assessment. The pressure equipment directive includes some additional sector specific dispositions which relate to recognised third party organisations , the European approval for materials , a committee for pressure equipment and the user inspectorates . 

The axial filter (Oak Ridge National Laboratory) (30) is remarkably similar to the dynamic filter in that both the rotating filter element and the outer shell are also cylindrical. An ultrafiltration module based on the same principle has also been described (31). Unlike the disk-type European dynamic filters described above, the cylindrical element models are not so suitable for scale-up because they utilize the space inside the pressure vessel poorly. 

RAIRS is a non-destructive infrared technique with special versatility - it does not require the vacuum conditions essential for electron spectroscopic methods and is, therefore, in principle, applicable to the study of growth processes [4.270]. By use of a polarization modulation technique surfaces in a gas phase can be investigated. Higher surface sensitivity is achieved by modulation of the polarization between s and p. This method can also be used to discriminate between anisotropic near-sur-face absorption and isotropic absorption in the gas phase [4.271]. 

Figure 7-19. Principles of operation of static mixer modules.

Over the past three decades, there has been a growing industrial interest in using reverse osmosis for several objectives such as water purification and demineralization as well as environmental plications (e.g.. Comb, 1994 Rorech and Bond, 1993, El-Halwagi, 1992). The first step in designing the system is to understand the operating principles and modeling of RO modules. 

Figure 5-43. Principles of operation of static mixer modules. By permission, Kenics Corp., Div. Chemineer, Inc.

Click Chemistry Interactive for the self-study module Le Chatelier s principle a water tank analogy. 

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