Sensing lifetime

The economic lifetime was introduced in Section 13.3, and was defined as the point at which the annual cashflow turned permanently negative. This is the time at which income from production no longer exceeds the costs of production, and marks the point when decommissioning should occur, since it does not make economic sense to continue to run a loss-making venture. Technically, the production of hydrocarbons could continue beyond this point but only by accepting financial losses. There are two ways to defer decommissioning ... 

A nice example of this teclmique is the detennination of vibrational predissociation lifetimes of (HF)2 [55]. The HF dimer has a nonlinear hydrogen bonded structure, with nonequivalent FIF subunits. There is one free FIF stretch (v ), and one bound FIF stretch (V2), which rapidly interconvert. The vibrational predissociation lifetime was measured to be 24 ns when excitmg the free FIF stretch, but only 1 ns when exciting the bound FIF stretch. This makes sense, as one would expect the bound FIF vibration to be most strongly coupled to the weak intenuolecular bond. 

Molecular Interaction. The examples of gas lasers described above involve the formation of chemical compounds in their excited states, produced by reaction between positive and negative ions. However, molecules can also interact in a formally nonbonding sense to give complexes of very short lifetimes, as when atoms or molecules collide with each other. If these sticky collisions take place with one of the molecules in an electronically excited state and the other in its ground state, then an excited-state complex (an exciplex) is formed, in which energy can be transferred from the excited-state molecule to the ground-state molecule. The process is illustrated in Figure 18.12. 

Quantitative risk analyses usually produces single-number estimates. These may be used to compare one risk with another in a quantitative sense or occasionally employed in an absolute sense. One of the most popular risk policies employed by industry is tlie FAR Concept (Fatal Accident Rate). FAR represents tlie number of fatal accidents per 1,000 workers in a working lifetime (10 lir), where a working lifetime is assumed to be approximately lO lirs. An acceptable FAR (by industries standards) is 4.0. 

One may gain insight to this matter by referring to the time scale of a given step, defined as its lifetime, l/k. The time scale for the first step is (ki + fc-i)-1. For the second it is lA2. Since k2 k-1, the label fast, applied to the second step, is understandable in this sense. 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

To be detected, the presence of target should provide significant change of % recorded within the time resolution of the method. Application of lifetime detection in sensing is based on several principles ... 

Using the long-lifetime emission as a reference in intensity sensing by shortlifetime dye. This approach known as dual luminophore referencing (DLR) will be considered in the next section. 

The lifetime detection techniques are self-referenced in a sense that fluorescence decay is one of the characteristics of the emitter and of its environment and does not depend upon its concentration. Moreover, the results are not sensitive to optical parameters of the instrument, so that the attenuation of the signal in the optical path does not distort it. The light scattering produces also much lesser problems, since the scattered light decays on a very fast time scale and does not interfere with fluorescence decay observed at longer times. 

Simultaneous application of two emitting reporters allows providing the self-referenced reporter signal based on simple intensity measurements, without application of anisotropy or lifetime sensing that impose stringent requirements on fluorescence reporters. Usually, the two dyes are excited at a single wavelength with the absence or in the presence of interaction between them. 

The calibration may not be needed in anisotropy and lifetime sensing. In lifetime sensing, the single-channel response allows obtaining the signal that does not need calibration. In anisotropy sensing, the two (vertical and horizontal) polarizations provide the necessary two channels, and in FRET to fluorescent acceptor, these two channels are selected as the intensities at two wavelengths. 

Borisov SM, Neurauter G, Schroeder C, Klimant I, Wolfbeis OS (2006) Modified dual lifetime referencing method for simultaneous optical determination and sensing of two analytes. Appl Spectrosc 60 1167-1173... 

A major advantage of fluorescence as a sensing property stems from the sensitivity to the precise local environment of the intensity, i.e., quantum yield (excited state lifetime (xf), and peak wavelength (Xmax). In particular, it is the local electric field strength and direction that determine whether the fluorescence will be red or blue shifted and whether an electron acceptor will or will not quench the fluorescence. An equivalent statement, but more practical, is that these quantities depend primarily on the change in average electrostatic potential (volts) experienced by the electrons during an electronic transition (See Appendix for a brief tutorial on electric fields and potentials as pertains to electrochromism). The reason this is more practical is that even at the molecular scale, the instantaneous electric... 

Szmacinski, H. Lacowicz, J. R. Lifetime-based Sensing Using Phase-Modulation Fluorometry. In Fluorescent Chemosensor for Ion and Molecule Recognition. ACS Symposium Series 538, 1993. 

Knowing the values of the two lifetimes, the fractions/) can be recovered in each pixel, by solving Eq. (2.19) in a least squares sense ... 

Esposito, A., Oggier, T., Gerritsen, H. C., Lustenberger, F. and Wouters, F. S. (2005). All-solid-state lock-in imaging for wide-held fluorescence lifetime sensing. Opt. Express 13, 9812-21. 

Stepping outside of the subject of biochemical protein dynamics there is also a healthy literature on the use of sensing using fluorophores. Spectral and lifetime characteristics of fluorophores are dependent on their environment, for example, pH, O2, and Ca2+, these features are a useful tool, particularly in the study of the basic biology of the cell (see for instance Chapter 4). 

Charged particle tracks in liquids are formally similar to cloud chamber or bubble chamber tracks. In detail, there are great differences in track lifetime and observability. Tracks in the radiation chemistry of condensed media are extremely short-lived and are not amenable to direct observation. Also, it must be remembered that in the cloud or bubble chamber, the track is actually seen at a time that is many orders of magnitude longer than the formation time of the track. The manifestation occurs through processes extraneous to track formation, such as condensation, formation of bubbles, and so forth. In a real sense, therefore, charged particle tracks in radiation chemistry are metaphysical constructs. 

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