Spurious fluorescence

The signal is not observed in the absence of adsorbed molecules (MB or otherwise). Note that a thorough cleaning (plasma cleaning in this case) is necessary to ensure that no spurious fluorescent species remain on the sample. 

Spurious fluorescence spectra may sometimes be seen with samples that are too concentrated. This is due to the iiiiier filter effect, in which a significant proportion of the excitation beam is absorbed by the sample before it reaches the centre of the cuvette (where the collection optics are focused), or the emitted light is similarly re-absorbed before it gets out of the cuvette. A useful rule of thumb is that the absorbance (A) of the sample at the excitation/emission wavelengths should be less than 0.2. 

The materials used are spectra grade and further purified by recrystallization Or passing through a column of activated alumina to eliminate impurities to the level necessary to avoid solvent or spurious fluorescence. The picosecond data are recorded either via a streak camera, emission, or by means of imaging devices for absorption. The data are analyzed and plotted by a microvax computer [3]. Typical time-resolved emission data are shown in Fig. 2 for bromoaryls which have been excited by a 266-nm,... 

The accuracy with which a system can measure lifetimes depends on a number of different factors including calibration of the instrument, the number of detected photons and also the efficiency of the analysis routines. In addition, sources of background and scattered light should be eliminated. Emission filters should be chosen with great care to make sure that no scattered laser light reaches the detector. Detection of scattered excitation light results in a spurious fast component in the decay and complicates the interpretation of the data. The choice of emission filters is much more critical in FLIM than in conventional fluorescence intensity imaging methods. 

Collection of multiple data sets for each time span, with frequent alternation of the polarization, is an essential feature of our protocol. This provides some protection against the effects of drifts in laser power, photomultiplier quantum yield, and absolute calibration of the TAC, photochemical decomposition of the dye, and any other long-term processes that may alter the measured fluorescence response curves. Separate analysis of each data set is necessary to provide an indication of the uncertainty in run-to-run reproducibility and to detect and delete the rare spurious data set. 

Laser-induced electronic fluorescence. Two devices reported recently look very promising for continuous atmospheric monitoring. Sensitivities of 0.6 ppb for nitrogen dioxide and ppb for formaldehyde are claimed. Careful attention to possible interference from other species is necessary. Detection of the hydroxyl radical in air ( 10 molecules/cm ) has been claimed for this technique, but it has been pointed out that this concentration seems much too high, especially because the air had been removed fix>m the sunlight 6 s before analysis spurious effects, such as photolysis of the ozone in the air by the laser beam and two-photon absorption by water vapor, might have been responsible for the hydroxyl radical that was observed. 

The answer is From harmful substances, i.e. elements or compounds which can interfere with the processes to be studied. One may encounter compounds which neutralise a catalyst or which generate spurious catalysts, compounds whose intense fluorescence in minute concentrations may frustrate a photochemical experiment, or others whose adsorption as a monolayer may alter the behaviour of an electrode or some other surface. 

His experiments were primarily upon a single soda-lime silicate, which was chosen because it had been studied extensively in the literature and can be made in small amounts with fair optical quality. The decay for this glass with 0.8 per cent Nd203 added is shown in Fig. 33. It is quite clearly not a single exponential. One finds that the slope changes by a factor of three over the range examined and that the curvature increases with neodymium concentration. The measurements made at long times were done with a mechanical shutter that shielded the photomultiplier tube from the initial intense fluorescence. When this was not done, hysteresis in the photomultiplier tube led to spurious results. 

Partial saturation of the HO absorption and use of excitation line widths wider than the HO absorption line width are in general undesirable, because they may produce spurious HO while exciting HO fluorescence with less than 100% efficiency (103). However, the issue is not as simple as matching the HO absorption profile exactly and avoiding all saturation. For instance, a matched laser excitation line width is more difficult to achieve and to keep tuned to the HO absorption, and the... 

Substitution of isobutane for a small fraction of the airstream increases the quenching of spurious HO fluorescence in the background channel by about 1%, so exact cancellation is not achieved in subtraction and a false positive signal is introduced (80, 103, 107). This situation applies not only to ozone photolysis but also to all other photolytic HO sources. 

This method was dependent on product scans of m/z 157. a characteristic fragment ion for the dansyl moiety. The added specificity and chromatographic resolution was shown to remove ambiguity in cases where spurious peaks in the HPLC fluorescence masked the retention time window for a code. Fig. 5.21 shows a comparison of the HPLC fluorescence trace and the CEC/MS/MS result for such a case. 

Fig. 5.21. Comparison of HPLC/fluorescence result with CEC/MS/MS. The spurious interfering peak co-reteniive with Ebu in the HPLC/fluorescence trace is absent from the more specific CEC/MS/MS trace clearly showing Ebu not to be present. (Reprinted with permission from I-34. )...

For the past 4 years, in connection with multidisciplinary studies of remote sensing, we have measured the spectra of light attenuation and fluorescence of particulate matter collected from the world s oceans (5). The number of fluorescent cells for many of the open ocean areas is so low that technical problems quickly occurred in the attempt to obtain monochromatic spectra of light emission and absorption. Furthermore, when the cells are placed in cuvettes they tend to settle out therefore, spurious results are obtained. To overcome these problems we elected to concentrate the cellular particles on membrane filters, which in turn were mounted in the light path of a conventional spectrofluorometer. (For details of this method see Reference 1.)... 

---