Fluorescence signal

At low laser powers, the fluorescence signal is Imearly proportional to the power. Flowever, the power available from most tunable laser systems is suflFicient to cause partial saturation of the transition, with the result that the fluorescence intensity is no longer linearly proportional to the probe laser power. While more... 

The fluorescence signal is linearly proportional to the fraction/of molecules excited. The absorption rate and the stimulated emission rate 1 2 are proportional to the laser power. In the limit of low laser power,/is proportional to the laser power, while this is no longer true at high powers 1 2 <42 j). Care must thus be taken in a laser fluorescence experiment to be sure that one is operating in the linear regime, or that proper account of saturation effects is taken, since transitions with different strengdis reach saturation at different laser powers. 

Figure C 1.5.5. Time-dependent fluorescence signals observed from liquid solutions of rhodamine 6G by confocal fluorescence microscopy. Data were obtained with 514.5 mn excitation and detected tlirough a 540-580 nm...

One approach is to prepare a sample blank using urine known to be free of quinine. The fluorescent signal for the sample blank is subtracted from the urine sample s measured fluorescence. 

The strength of a fluorescence signal is directly related to the absorption cross section CJ ... 

Fig. 4.10. Fluorescence signal from small particles or thin films deposited on a silicon substrate used as sample carrier. The intensity was calculated for particles, thin films, or sections ofdiffe-rent thickness but equal mass of analyte, and plotted against the glancing angle f. A Mo-Ka beam was assumed for excitation. Particles or films more than 100 nm thick show double intensity below the critical angle of0.1° [4.21].

Raman scattering is essentially undelayed with respect to the arrival of the incident light, in this technique the detector is activated only during each laser pulse and deactivated at all other times. This allows only Raman signals to be recorded but fluorescence signals and detector noise are gated out (Fig. 19). Improvement in Raman signal to fluorescence ratio has been achieved as illustrated in Fig. 20. The technique, however, at present seems to be restricted by several instrumental limitations [37). 

The fluorescent signal will change with variation in quantum yield of fluorescence and with molar absorptivity. Not only do fluorescence quantum yields vary with the different dansyl derivatives formed, but so do the molar absorptivities (12). Another problem is exemplified by the 30-nm difference in the emission maxima of the dansyl derivatives of phenol and 2,4,5-trichlorophenol (13). 

It would be convenient if the quantum yields and molar absorptivities were constant within a set of derivatives. Finally, the fluorescent signal from dansyl derivatives decreases in acidic solution (13) due to protonation of the dimethylamino group. 

In the equation, the subscripts 1 and 2 refer to the reference compound and the compound of interest, respectively, is the intensity of the fluorescent signal of each compound measured as peak height in centimeters, 8 is the molar absorptivity, c is the concentration in moles per liter, and is the fluorescence quantum yield. In this application, i is set at 1.00. The concentrations of the solutions that were tested ranged from 10 to 10 M. The solutions run at the higher concentrations were all checked for self-quenching, but none was found. All measurements, except the fluorescence-versus-solvent study, were made in 0.1-N phosphate buffer, pH 7.4. Slit settings on the Perkin-Elmer MPF-2A were 10 mp (nm) for both emission and excitation monochromators. 

Fig. 3 Polypeptide vesicle with endocytosis capability, (a) Vesicles formed from poly(L-arginme)6o-h-poly(L-leucme)2o- The poly(L-arginme) block provides an added cell-penetrating feature to the vesicles, (b, c) LCSM images of internalized vesicles (green) containing Texas-Red-labeled dextran (red) in (b) epithelial and (c) endothelial cells. Colocalization of the vesicles and Texas-Red-labeled dextran appears as a yellow fluorescent signal. Adapted from [44] with permission.Copyright 2007 Macmillan Publishers...

Figure 9. Typical fluorescence signals obtained from a suspension of isolated rat cardiac myocytes after the application of maitotoxin (MTX). The arrow indicates the addition of MTX (10 g/mL), a detergent Emulgen 810 (1%), which frees all vesicular Ca , or EGTA (3.5 mM), a chelator that removes all free Ca in the cuvette. The intensity of Quin 2 fluorescence is expressed in arbitrary units. (Reproduced with permission from Ref. 20. Copyright 1987 Elsevier)...

Our results on Ar I2 [57] are particularly noteworthy because of the extensive previous work aimed at characterizing the continuum fluorescence signals that were found to extend above the 12(B) dissociation limit. It was proposed that these signals resulted from a one-atom caging mechanism, in... 

Labelling Na,K-ATPase with ATP analogues provides evidence for contribution from charged residues that are widely separated in the sequence of a subunit of Na,K-ATPase. The first indication came from ATP sensitive covalent insertion of fluorescein-isothiocyanate (FITC) into Lys ° in the a subunit [90], The strong fluorescence signal provides a convenient probe for monitoring conformational transitions in the proteins. Site-directed mutagenesis of Lys reduces the activity of... 

During an XAS experiment, core electrons are excited. This produces empty states called core holes. These can relax by having electrons from outer shells drop into the core holes. This produces fluorescent X-rays that have a somewhat lower energy than the incident X-rays. The fluorescent signal is proportional to the absorption. Detection of this signal is a useful method for measuring absorption by dilute systems such as under potential deposited (UPD) monolayers. 

Fluorescence measurements are fundamentally different to absorption measurements [20,173,180]. The fluorescence intensity depends only on the population of sample molecules and can be calculated in several ways. Independent of "the method chosen at low sample concentrations the fluorescence signal, F, is adequately described by equation (7.26)... 

Concentration At high concentrations fluorescence emission l ecomes non-linear due to self-absorption by the sample itself or complete absorption of the excitation energy before it reaches the cell center. High fluorescence Intensity may overload the photomultiplier tube which returns slowly to its normal operating conditions and misrepresents the actual fluorescence signal until restabilized. 

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