Shift method

One advantage of the photon counting teclmique over the phase-shift method is that any non-exponential decay is readily seen and studied. It is possible to detect non-exponential decay in the phase-shift method too by making measurements as a fiinction of tlie modulation frequency, but it is more cumbersome. 

Jablonski (48-49) developed a theory in 1935 in which he presented the now standard Jablonski diagram" of singlet and triplet state energy levels that is used to explain excitation and emission processes in luminescence. He also related the fluorescence lifetimes of the perpendicular and parallel polarization components of emission to the fluorophore emission lifetime and rate of rotation. In the same year, Szymanowski (50) measured apparent lifetimes for the perpendicular and parallel polarization components of fluorescein in viscous solutions with a phase fluorometer. It was shown later by Spencer and Weber (51) that phase shift methods do not give correct values for polarized lifetimes because the theory does not include the dependence on modulation frequency. 

This chapter considered ionizable drug-like molecules. Absorption properties that are influenced by the pKj were explored. The impact of the pKj-absorption relationship on key physicochemical profiling underlying absorption (solubihty, per-meabihty and ionization) was examined in detail and several simpUfying equations were discussed. The various diff relationships considered in the chapter are systematized in Table 3.2. Table 3.3 summarizes the apparent pfQ shift method for detecting aggregates in solubility profiles, when the apparent pff value derived from Henderson-Hasselbalch analysis of log S pH profile does not agree with the... 

Both the subtractive and the phase shift method are valid as long as the RF waveform remains sinusoidal [184]. For asymmetrical discharges the integral method is to be used. 

If the spin-spin information was being transmitted by the normal through-bond mechanism the upfield three proton signal would be expected to occur as a doublet because these protons are the only ones which can assume the required planar zig-zag conformation 77>78h Preliminary results, using the change in chemical shift method 79>, indicates that the energy barrier to rotation is of the order of 20 k.cal.mole O. As expected the silicon compound (39) shows a nine proton doublet... 

Fluorescence Lifetimes. Fluorescence lifetimes were determined by the phase shift method, utilizing a previously-described phase fluorimeter. The emission from an argon laser was frequency doubled to provide a 257 nm band for excitation. Fluorescence lifetimes of anisole and polymer 1 in dichloro-methane solution were 2.2 and 1.4 nsec, respectively. Fluorescence lifetimes of polymer films decreased monotonically with increasing DHB concentration from 1.8 (0) to 0.7 nsec (9.2 x 10 3 MDHB). Since fluorescence lifetimes (in contrast to fluorescence intensities) are unaffected by absorption effects of the stabilizer, these results provide direct evidence in support of the intensity measurements for RET from polymer to stabilizer. 

The determination of electron concentration by the frequency shift method is limited to time resolution greater than a few hundred nanoseconds and is therefore not applicable to liquids. The microwave absorption method can be used virtually down to the pulse width resolution. Under conditions of low dose and no electron loss, and assuming Maxwellian distribution at all times, Warman and deHaas (1975) show that the fractional power loss is related to the mean electron energy (E) by... 

Furuyama, S., Atkinson, R., Colussi, A.J., Cvetanovic, R.J. (1974) Determination by the phase shift method of the absolute rate constants of reactions of oxygen ( ,P) atoms with olefins at 25°C. Int 7. J. Chem. Kinet. 6, 741. 

Ti measurements were performed at 250, 275 and 300 K by inversion-recovery (7r-T-7r/2-5T i) sequences on a JE0L-FX-100 and a Bruker WP-80 spectrometers. On this latter the "repetitive frequency shift" method of Brevard et al. (18) was used, where two systematic instrumental errors (drift, round off errors in FT processing.. .) are uniformly distributed through all data points. The NOE measurements are reproducible within 10-20 %, while the average standard error on the Tj values is of about 5 %. 

It is common practice, when similar materials are being compared, to ignore the shape of the curve and to take the time for the property to reach some percentage, say 50%, of its initial value. This may be expedient but is clearly less satisfactory than modelling the curve and could be extremely misleading if materials with substantially different curves are compared. If a time/temperature shift method is used to model the effect of temperature (see below) no function to describe the change of property with time need be assumed. 

The most elaborate use of MM calculations in the LIS analysis was described by DeTar and Luthra (298). Their approach was based on the traditional relative shift method, wherein lanthanide parameters are adjusted to give optimum agreement with the observed relative LIS values. Based on their previous analysis of proline conformations (228), they determined that A-acetylproline methyl ester (71) exists in CDQ3 as a 60 40 mixture of half-chair and envelope conformations by simultaneously adjusting the substrate geometries and the conformer mole fractions, in addition to the lanthanide parameters (298). 

Fig. 6. Experimental arrangement for lifetime measurements by the phase-shift method, using laser excitation. The laser beam is amplitude-modulated by a Pockel cell with analysing Nicol prism and a small part of the beam is reflected by a beam splitter B into a water cell, causing Rayleigh scattering. This Rayleigh-scattered light and the fluorescence light from the absorption cell are both focused onto the multiplier cathode PMl, where the difference in their modulation phases is detected. (From Baumgartner, G., Demtroder, W., Stock, M., ref. 122)).

Compared to the phase-shift method, this technique has the advantage that nonexponential decays can also be investigated. 

Fig. 1 Schematic representation of the experimental setups of the mobility-shift method and the Hummel-Dreyer method (A) the vacancy peak method and the vacancy affinity capillary electrophoresis method (B) the equilibrium-mixture method and the frontal analysis method (C) for drug-protein binding analysis. drug protein gg drug-protein complex Q buffer. (Reprinted with permission from Ref. 38. Copyright 1992 Elsevier Science.)...

The 7-shifting method depends on our ability to identify a unique bottleneck geometry and is particularly well suited to reactions that have a barrier in the entrance channel. For cases where there is no barrier to reaction in the potential energy surface, a capture model [149,150,152] approach has been developed. In this approach the energy of the centrifugal barrier in an effective onedimensional potential is used to define the energy shift needed in Eq. (4.41). For the case of Ai = 0, we define the one-dimensional effective potential as (see Ref. 150 for the case of AT > 0)... 

Useful applications of the Evans proton NMR shift method have permitted the determination of effective magnetic moments peff (see Section 8) and spin multiplicities for several high-spin radical-cation species.This work makes available a simple procedure for such assignments when the spin carriers are stable in solution. 

The nonsteady state methods may be conveniently divided into two categories (A) pulse methods and (B) phase-shift methods. 

B) Phase-shift methods. The phase shift method for determining fluorescence lifetimes is based on the principle that if fluorescence is excited by suitably modulated light source, emitted radiation will also be similarly modulated. With reference to a scattering substance, emission from a fluorescent substance will introduce a time lag due to finite time between absorption and emission. This, by definition is the lifetime of the excited state. The time lag will cause a phase-shift relative to the exciting light. Phase fluorimetry requires a modulated light source and a phase sensitive detector. 

Figure 10.8 A block diagram of phase-shift method for measurement of life-time.

The protein digestibility values for pH-shift methods generally correlate well with in vivo digestibility methods in the sense that both tests yield the same relative ranking for test proteins (Pedersen and Eggum, 1983). The difficulty with an in vitro method is that it does not accurately estimate qualitative differences between samples with low and high protein digestibilities. 

By combining this method with the previously discussed chemical shift method, which is sensitive in the 0.05-20 range for the ionization ratio, the acidity could be measured over more than 5 Hq units with the same indicator. Figure 1.8 shows the complementarity of both methods. 

Asynchronous Pitch Shifting. Asynchronous pitch shifting, the simplest pitch shifting method, simply changes the clock rate of each output digital to analog converter (DAC) to vary the pitch. Each channel requires a separate DAC. Each DAC has its own clock whose rate is determined by the requested frequency for that channel. When a DAC clock occurs, the DAC issues a request to a memory controller that supplies a waveform sample to the DAC. The earliest samplers had a separate memory for each DAC. 

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