Lifetime standard

When normal photomultipliers that exhibit a color effect (see above) are used, the compounds of Table 6.2 that have a short lifetime (e.g. POPOP, PPO) can be used in place of a scattering solution in order to remove this effect (this method is valid for both pulse and phase fluorometries). Such a reference fluorophore must 

The maximum entropy method has been successfully applied to pulse fluorometry and phase-modulation fluorometry3- . Let us first consider pulse fluorometry. For a multi-exponential decay with n components whose fractional amplitudes are a , the d-pulse response is 

The choice of the preferred solution within the feasible set can be achieved by maximizing some function F[a(r)] of the spectrum that introduces the fewest artefacts into the distribution. It has been proved that only the Shannon-Jaynes entropy S will give the least correlated solution 1. All other maximization (or regularization) functions introduce correlations into the solution not demanded by the data. The function S is defined as 

In phase-modulation fluorometry, it is convenient to use the fractional contributions f instead of the fractional amplitudes, as shown in Eqs (6.30) and (6.31). The above equation is thus rewritten as 

The commercially available software (Maximum Entropy Data Consultant Ltd, Cambridge, UK) allows reconstruction of the distribution a.(z) (or f(z)) which has the maximal entropy S subject to the constraint of the chi-squared value. The quantified version of this software has a full Bayesian approach and includes a precise statement of the accuracy of quantities of interest, i.e. position, surface and broadness of peaks in the distribution. The distributions are recovered by using an automatic stopping criterion for successive iterates, which is based on a Gaussian approximation of the likelihood. 

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Morris KJ, Roach MS, Xu WY, Demas JN, DeGraff BA (2007) Luminescence lifetime standards for the nanosecond to microsecond range and oxygen quenching of ruthenium(II) complexes. Anal Chem 79 9310-9314... 

The values measured in these two ways should of course be identical and independent of the modulation frequency. This provides two criteria to check whether an instrument is correctly tuned by using a lifetime standard whose fluorescence decay is known to be a single exponential. 

J. R. Lakowicz, 1. Gryczynski, G. Laczko, and D. Gloyna, Picosecond fluorescence lifetime standards for frequency- and time-domain fluorescence,./. Fluorescence 1, 87-93 (1991). 

Absolute quantum yields measured upon ligand excitation (relative error 10%), lifetimes (standard deviation in parentheses), and associated hydration numbers of the [Ln(H290)]3 podates, 6 x 10 3 M in aqueous solutions at... 

It is valuable to have fluorophores of known lifetimes for use as lifetime standards in time-domain oc frequency-domain measurements. Perhaps more inportaot than the actual lifetime is kno edge that the fluoroi icw disfda single-expcmential decays. Such fluoro c es are useful for testing the time-resolved instruments for systonatic errors. We have summarized the results on lifetime standards from several laboratm ies. Th e is no attempt to compare the values or to evaluate which values are more reliable. Much of the data is from our laboratory because it was readily available with all the exp imen-tal details. 

Rose Bengal can serve as a picosecond lifetime standard at longer wavelengdis (Figuie II.7). Rose Bengal can also be used as a standard for a short rotational correlation time (Figure US). 

Fluorescence Lifetimes. Decay Times. Fluorescence Lifetime Standards in the ns and ps Time Scales... 

Boens N, Qin WW, Basaric N et al (2007) Fluorescence lifetime standards for time and frequency domain fluorescence spectroscopy. Anal Chem 79 2137-2149... 

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