The Modulation Principle

This means that HR-CS AAS, due to its special features, does not need any modulation of the source or any selective amplifier. This also means that a potential source of noise has been eliminated, as both AC operation of hollow cathode lamps and mechanical choppers contribute to noise in LS AAS. In addition, other problems that are associated with strong emission of the atomizer source in LS AAS, such as the emission noise caused by the nitrous oxide/ acetylene flame in the determination of barium and calcium due to the CN band emission, are equally absent in CS AAS for the same reasons, i.e. the much higher intensity of the primary radiation source, and the high resolution. 

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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]. 

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. 

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. 

The modules are computer-based laboratory simulations with engaging activities that emphasize experimental design and visualization of structures and processes at the molecular level. The modules are designed to help students connect chemical principles from lecture with their practical applications in the lab. Every module has a built-in accountability feature that records section completion for use in setting grades and a workbook for students to record and interpret their work. 

Klymchenko AS, Demchenko AP (2002) Electrochromic modulation of excited-state intramolecular proton transfer the new principle in design of fluorescence sensors. J Am Chem Soc 124 12372-12379... 

To improve the S/N ratio, the modulation signal is processed by amplification with a tuned amplifier using phase-sensitive detection. This means that the detected signal must not only be at the modulation frequency, but must also be in phase with the modulation. Since the amplifier itself can introduce a bit of phase shift, there is a phase control which, in principle, should be adjusted to maximize the signal amplitude. In practice, this control needs to be adjusted only rarely and in most cases the best approach is to leave it alone. 

The few homeopathic products which were tested against GIN in sheep and cattle were based on plants and were not associated with a decrease in the worm burdens or faecal egg counts. However, considering the basic principles of homeopathy, it is likely that this approach reacts via modulation of the host s immune system (Cabaret et al., 2002). The use of non-classical parameters for assessing an antiparasitic effect should therefore be considered in future studies of homeopathy. 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

Without discussing in detail the principle of transversal electric and transversal magnetic modes, the modulation depends according to... 

Fig. 9. Peptidomimetic design principle of SH2 antagonists derived from a fragmentation of the native peptide ligand. The lead finding efforts discussed in Sect. 3 are classified according to the modules given above the peptide lead sequence...

Considerable effort has gone into solving the difficult problem of deconvolution and curve fitting to a theoretical decay that is often a sum of exponentials. Many methods have been examined (O Connor et al., 1979) methods of least squares, moments, Fourier transforms, Laplace transforms, phase-plane plot, modulating functions, and more recently maximum entropy. The most widely used method is based on nonlinear least squares. The basic principle of this method is to minimize a quantity that expresses the mismatch between data and fitted function. This quantity /2 is defined as the weighted sum of the squares of the deviations of the experimental response R(ti) from the calculated ones Rc(ti) ... 

There are two widely used methods for measuring fluorescence lifetimes, the time-domain and frequency-domain or phase-modulation methods. The basic principles of time-domain fluorometry are described in Chapter 1, Vol.l of this series(34) and those of frequency-domain in Chapter 5, Vol. 1 of this series.<35) Good accounts of time-resolved measurements using these methods are also given elsewhere/36,37) It is common to represent intensity decays of varying complexity in terms of the multiexponential model... 

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