Absorbance-time profile

The laser flash photolysis technique relies on the use of a pulsed UV laser for the rapid synthesis of the reactive intermediate of interest by photochemical decomposition of a suitable stable precursor, and (most commonly) fast time-resolved UV/VIS spectrophotometry to detect the species and monitor its decay19. The absorbance-time profile so... 

Fig. 12. (A) Differential pulse polarogram of a dye waste water before electrolysis at a percolated graphite electrode. (B) after electrolysis. (C) Absorbance - time - profile of the dye wastewater measured on-line during the electrochemical reduction at a percolated graphite electrode using fibre optic and guided-wave spectrometer. ...

Half-Life Dependence of Conventional Pseudo-First-Order Rate Constants. The procedure involves converting absorbance—time profiles (Abs—t) to —In (1 — E.R.)—time profiles which is the traditional way to obtain a first-order rate constant. The —In (1 — E.R.)-time profiles are then subjected to linear least squares analysis on five different segments of the data over time ranges corresponding to 0—0.5 half-Hves (HL, when E.R. =0.5), 0-1 HL, 0-2 HL, 0-3 HL, and 0-4HL. The response of the single-step mechanism to this procedure is that the five apparent pseudo-first-order rate constants are equal within experimental error. This is a very simple, but effective method to differentiate single-step and complex reaction kinetics. 

In our study of the proton transfer reactions of three simple nitroal-kanes (nitromethane, nitroethane and 2-nitropropane), absorbance—time profiles due to the formation of reactive intermediates were observed at the isosbestic points where changes in reactant and product absorbance... 

Figure 1.15 Absorbance-time profiles (a, for the reactions of nitroalkanes (a, a NM ...

The time profiles of the absorbance due to MV+ at 600 nm are illustrated in Figures 18. Note that they depend on the MV2+ concentration. The curves for the poly(A/St/Phen)-MV2+ systems are biphasic and can be explained in terms of a simple mechanism illustrated in Scheme 2. Here, D A, A represents a compartmentalized Phen moiety (D) and MV2+ dications (A) bound to the hydrophobic microdomain. 

Fig. 18. Time profiles of transient absorbance at 602 nm due to MV+ for the poly (A/St/Phen)-MV2+ system [Phen](residue) = 0.66 mM [MV2+] = 5mM ( ), 10mM (a). The solid lines represent the best-fit curves calculated from Eq. 10 with the use of the parameters given in Table 6 [120]...

Time profile of the MV absorption at 610 nm as a function of time for various laser flash intensities (expressed as moles of photons absorbed per liter)... 

A typical time profile of the excited PMMA-Phe fluorescence intensity decay is shown in Figure 2. The MEK permeation commences at 24 sec. The SPR increases during the plasticization period until it becomes constant, the onset of the steady state. It is characterized by a linear relationship between the amount of solvent absorbed and time. It was determined from a linear regression analysis that the PMMA-Phe fluorescence intensity starts to deviate from linearity at 197 sec. This indicates a decrease in the SPR and/or the unquenched PMMA-Phe. The decrease in SPR is unexpected at this film thickness since the SPR in thicker PMMA-Phe films show no anomaly at 1 /tm. A more plausible explanation is the reduction in available PMMA-Phe, which is expected when the front end of the SCP reaches the substrate. 

Rostami-Hodjegan A, Shiran MR, Ayesh R, Grattan TJ, Burnett I, Darby-Dowman A, Tucker GT. A new rapidly absorbed paracetamol tablet containing sodium bicarbonate. I. A four way crossover study to compare the concentration-time profile of paracetamol from the new paracetamol/sodium bicarbonate tablet and a conventional paracetamol tablet in fed and fasted volunteers. Drug Dev Ind Pharm 2002 28 523-531. 

Time profiles in vitro and in vivo represent distribution functions in a mathematical and statistical sense. For example, a release profile Fj)(t) in vitro expresses the distribution of drug released at time t the corresponding probability distribution function (PDF) profile fo(t) characterizes the rate of release. Similarly, a plasma concentration profile fp(t) represents the distribution of drug in the plasma at any time t, i.e., absorbed but not yet eliminated its cumulative distribution function (CDF) equivalent FP(t) represents the drug absorbed and already eliminated. 

Figure 11 Observed data (amount absorbed in vivo vs. amount released in vitro) for the three ER formulations whose dissolution data are shown in Figure 3 and absorption—time profiles in Figure 10. The fitted IVIVC equation is shown as well as the corresponding equation and predicted line. The dotted line represents a 1 1 relationship.

Within this approximation, at least half of the photons that contribute to reflectance of a semi-infinite layer have been scattered only once. On (5-irradiation this part of photons is reflected from the illuminated surface with an exponential time profile Jo(t) = 2 xNoSac-exp(-(Sa + K )c-t), where N, is the number of incident photons at t = 0. Considering a weakly absorbing and not too strongly scattering sample (A = 1 cm S 102 cm-1), the decay time of the single-scattered photons is in the order of r = 500 fsec. The second half of photons is multiply scattered and decays... 

Figure 20 Time profiles of induced absorbance at 580 nm after focusing a 30-psec, 532-nm laser pulse at various fluences F onto gold clusters, 30 nm diameter. Dotted line time profile of the pulse. The fast amplitude increase, up to a maximum in less than 1 nsec for fluences >0.9 J cm , is assigned to light scattering by expanded metal clusters. The delayed increase with a maximum at 3 nsec occurs at fluences >0.18 J cm already, and is assigned to light scattering by solvent bubbles. (From Ref. 213.)...

Table 1. Time constants for fits to the absorbance vs time profiles for studied compounds.

Here, A is the reaction s measured IR spectral absorbance, Nt is the number of measurements at different times, N0 is the number of wavelengths, C is the concentration matrix with the concentration-time profiles of each absorbing component in the columns, Nc is the number of chemical components and E is the pure spectra matrix with the spectral absorption at each wave number of each pure absorbing component in the rows. If a chemical component does not absorb, the corresponding spectrum of the pure chemical will be a vector of zeros. 

Immediately upon excitation of an IPCT band with a fs laser at 400 nm, transient absorption was observed for both salts in solutions with a peak at about 600 nm, characteristic of 4,4/-bipyridinium radical cations. Figure 20 shows the transient absorption spectra of PV2+(I )2 in methanol solution. A marked increase in the absorbance of the 4,4/-bipyridinium radical cations took place within 1 ps after excitation. 4,4/-Bipyridinium radical cations were thus formed in a fs time scale by the photoinduced electron transfer from a donor I- to an acceptor 4,4/-bipyridinium upon IPCT excitation [48], The time profiles of transient absorption at 600 nm are shown in Fig. 21 for (a) PV2+(I )2 in a film cast from DME and (b) PV2+(TFPB )2 in DME solutions. Both of them showed a very rapid rise in about 0.3 ps, which was almost the same as the time resolution of our fs Ti sapphire laser measurement system with a regenerative amplifier. Similar extremely rapid formation of 4,4/-bipyridinium radical cations was observed for PV2+(I )2 salts in methanol and dimethylsulfoxide solutions upon IPCT excitation, respectively. These results demonstrated that the charge separated 4,4/-bipyridinium radical cations were formed directly upon IPCT excitation because of the nature of IPCT absorption bands (that the electrons correlated with the IPCT band are transferred partially at the ground state and completely at the excited state). Such a situation is very different from usual photochromism which is caused by various changes of chemical bonds mainly via the excited singlet state. No transient absorption was observed for PV2+(I )2... 

The photocoloration rate of 20a was measured in the crystalline phase, as shown in Figure 4. The time evolution of the absorption at 505 nm indicated that the appearance of the colored isomer 20b is very rapid even in the crystalline phase. The time profile of the absorbance at 505 nm immediately after the excitation is shown in Figure 4b. The solid lines in the figure are simulation curves, taking into account the duration of the excitation and monitoring laser pulses and the time constants of... 

Fig. 1. Time profile of absorbance at 600 nm of E. coli strains during flask cultivation.

Fig. 3. Time profile of absorbance (A600) of recombinant E. coli in malt and soy wastes.

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