Standards decay time

Decay Time. Many organic and Inorganic materials In various solvents have been suggested as decay-time standards. Selected standards listed In Table V cover the decay-time range from nanoseconds to... 

The need foi Ufeliine standuds for time-domain measurements has been recognized for some time. A number of labontories have suggested samples as single-decay-time standards. The data ate typically repotted ouly in tables Clhbles n.3-n.S). so representative figures are not... 

Requirements for standards used In macro- and microspectrofluorometry differ, depending on whether they are used for Instrument calibration, standardization, or assessment of method accuracy. Specific examples are given of standards for quantum yield, number of quanta, and decay time, and for calibration of Instrument parameters. Including wavelength, spectral responslvlty (determining correction factors for luminescence spectra), stability, and linearity. Differences In requirements for macro- and micro-standards are considered, and specific materials used for each are compared. Pure compounds and matrix-matched standards are listed for standardization and assessment of method accuracy, and existing Standard Reference Materials are discussed. 

Definition and Uses of Standards. In the context of this paper, the term "standard" denotes a well-characterized material for which a physical parameter or concentration of chemical constituent has been determined with a known precision and accuracy. These standards can be used to check or determine (a) instrumental parameters such as wavelength accuracy, detection-system spectral responsivity, and stability (b) the instrument response to specific fluorescent species and (c) the accuracy of measurements made by specific Instruments or measurement procedures (assess whether the analytical measurement process is in statistical control and whether it exhibits bias). Once the luminescence instrumentation has been calibrated, it can be used to measure the luminescence characteristics of chemical systems, including corrected excitation and emission spectra, quantum yields, decay times, emission anisotropies, energy transfer, and, with appropriate standards, the concentrations of chemical constituents in complex S2unples. 

Calibration. In general, standards used for instrument calibration are physical devices (standard lamps, flow meters, etc.) or pure chemical compounds in solution (solid or liquid), although some combined forms could be used (e.g., Tb + Eu in glass for wavelength calibration). Calibrated lnstr iment parameters include wavelength accuracy, detection-system spectral responsivity (to determine corrected excitation and emission spectra), and stability, among others. Fluorescence data such as corrected excitation and emission spectra, quantum yields, decay times, and polarization that are to be compared among laboratories are dependent on these calibrations. The Instrument and fluorescence parameters and various standards, reviewed recently (1,2,11), are discussed briefly below. 

Table V. Materials Used as Standards for Fluorescence Decay Time, r...

Because the reproducibility of a-particle counting with the Pylon sources was quite good, this study was conducted as follows. Four identical samples, i.e., same material, were placed in the decay product standard sources for a period of 24 h at a time. The four samples were then removed and measured. Gross a-particle count and a-spectrometry analysis of the samples were done. 

R. A. Lampert, L. A. Chewter, D. Phillips, D. V. O Connor, A. J. Roberts, andS. R. Meech, Standards for nanoseconds fluorescence decay time measurements, Anal. Chem. 55, 68-73 (1983). 

Element standards were grouped into four standard libraries, corresponding to the four decay counting times. Decay time boundaries for each standard library are shown in Table II. In each library, at least two elements were calculated from different standards. These two standards represented different concentrations, counting geometries, dead times, decay times, and sample matrices. Visual inspection of the computer listing provided a rapid spot check for computer program malfunctions. 

As stated above (see Chapter II.B), LII signals also contain information about the size distribution. To compare the influence of different plasma powers on primary particle diameters, different ways of size evaluation have been accomplished. It could be shown by assuming a monodisperse distribution that the mean primary particle diameter is 31 nm for 30 kW and 33 nm for 70 kW. In contrast, under the assumption of a log-normal distribution and by applying the two-decay time evaluation, the determination yields a different result which can be seen in Figure 15. Size distributions with median sizes of 17nm and 28 nm and standard deviations of 0.39 and 0.18 for 30 kW and 70 kW were observed, respectively. This indicates that in practical production systems, the evaluation of a mondisperse distribution is not sufficient. Unfortunately, the reconstruction of particle size distributions is relatively sensitive on... 

Figure 7. Phosphorescence decay times (t) for hair samples from eight blondhaired donors. The error bars represent 1 standard deviation from the mean value of t for 10 hair samples from each donor.

The second facility was used in the case of isotopes of a half-life more than one week. Again there are two groups of elements, namely, Ba, Br, La, Lu, Mo, Rb, Sb, U and W (irradiation time of 25 hr, decay time of 1 week, counting time of 30 min) and Ba, Ce, Co, Cr, Eu, Fe, Hf, Rb, Sb, Sc, Se, Sr, Ta, Th and Zn (irradiation time of 25 hr, decay time of 1 month, counting time of 3 hr). In both cases the neutron flux is 2.6 x lO n cm s with a Cd ratio Rcd = 3. Different y-spectrometric measuring equipments by EG G ORTEC were used. The Certified Reference Materials (CRMs) used for the calibration of the measurement systems were Urban Particulate and laboratory standards, as detailed in Table 14.3. 

Under routine experimental conditions, it is not necessary to use A q (sample) and A o (standard). Indeed, after irradiation the induced radioactivity decreases exponentially according to the ti/2 of the radioisotopic species the decay factor (D) is given by td being the time between the end of the irradiation and the moment of the measurement. Thus, since At = Ao-e". the comparator equation (3) can also be written for any stated decay time, td, after the end of the irradiation ... 

---