Luminescence techniques, compared

Table 5.12 shows the main features of luminescence spectroscopy. The much higher sensitivity and specificity of luminescence techniques compared to absorption techniques is an obvious advantage for excitation spectra. In solution studies, pg ml. 1 levels can often be determined, as compared to p,gmL-1 levels in absorption spectroscopy. The greater sensitivity of luminescence techniques stems from the fact that the... 

Comparing this value to the typical sensitivity provided by a spectrophotometer, (OT>)niin = 5 X 10 (see Section 1.4), we see that the luminescence technique is much more sensitive than the absorption technique (about 10 times for this experiment). Although this large sensitivity is an advantage of photoluminescence, care must be taken, as signals from undesired trace luminescent elements (not related to our luminescent center) can overlap with our luminescent signal. 

One example demonstrates the advantage of the time-resolved technique compared to the steady-state technique. The time-integrated cathodolumines-cence spectrum of apatite enables us to detect only two dominant luminescence... 

Commercial spectrometers, such as the Perkin-Elmer MPF-43A fluorescence spectrometer, that allow interlocking of excitation and emission monochromators lately have become available for utilizing this underexploited analytical technique. The synchronous luminescence technique reduces the complexity of the luminescence spectrum of a compound compared with a conventionally obtained luminescence spectrum. One can, therefore, better tackle the analysis of fairly complex mixtures without resorting to techniques that are expensive or excessively time consuming. 

Environmental pollution requirements have been responsible for the interest in analysis of polyaromatic hydrocarbons (PAH). John and Soutar have reviewed the problems of using luminescence techniques for examination of oil spill. Various methods have been devised to deal with the analysis of complex mixtures involved. Synchronous fluorescence is a useful method using comparatively simple equipment. Rank annihilation methods applied to data acquired as an... 

Fluorescent chemical sensors occupy nowadays a prominent place among the optical devices due to its superb sensitivity (just a single photon sometimes suffices for quantifying luminescence compared to detecting the intensity difference between two beams of light in absorption techniques), combined with the required selectivity that photo- or chemi-luminescence impart to the electronic excitation. This is due to the fact that the excitation and emission wavelengths can be selected from those of the absorption and luminescence bands of the luminophore molecule in addition, the emission kinetics and anisotropy features of the latter add specificity to luminescent measurements8 10. 

Fluorescence spectroscopy forms the majority of luminescence analyses. However, the recent developments in instrumentation and room-temperature phosphorescence techniques have given rise to practical and fundamental advances which should increase the use of phosphorescence spectroscopy. The sensitivity of phosphorescence is comparable to that of fluorescence and complements the latter by offering a wider range of molecules for study. 

Current photochemical research is strongly linked with the study of photophysical behavior of excited particles. Data on photophysical processes (such as luminescence, internal conversion, intersystem crossing, intramolecular energy dissipation) assist photochemists in the identification and interpretation of chemical deactivation modes. Most of the data related to the elementary steps within deactivation of excited particles have been obtained by fast flash techniques in nano-, pico-, and femtosecond time domains. Photophysics is, in general, as rich a branch of science as photochemistry, and both the parts of excited-state research deserve comparable attention and extent. In the present review, some results on photophysics will be mentioned where suitable and necessary. We will restrict our discussion, however, predominantly to photochemical behavior of metallotetrapyrroles. 

The use of europium chelates, with their unusually long fluorescence decay times, as labels for proteins and antibodies has provided techniques that are referred to as time-resolved fluoroimmunoassays (TRFIA). Fluorophores as labels for biomolecules will be the topic of Sect. 3. Nevertheless, TRFIAs always have to compete with ELISA (enzyme-linked immunosorbent assays) techniques, which are characterized by their great versatility and sensitivity through an enzyme-driven signal amplification. Numerous studies have been published over the past two decades which compare both analytical methods, e.g., with respect to the detection of influenza viruses or HIV-1 specific IgA antibodies [117,118]. Lanthanide luminescence detection is another new development, and Tb(III) complexes have been applied, for instance, as indicators for peroxidase-catalyzed dimerization products in ELISAs [119]. 

A diagnostic three-step luminescence measurement was designed to compare oxidation and crosslinking in a series of irradiated samples [86]. The validity of this measurement was tested by applying it to two series of samples. Established techniques, such as tensile testing, gel permeation chromatography, IR spectroscopy and thermal analysis, were used to corroborate the conclusions drawn from the three-step luminescence measurement. 

Shin, H. S., Rhee, S. W., Lee, B. H., and Moon, C. H. (1996). Metal binding sites and partial structures of soil fulvic and humic acids compared Aided by Eu(III) luminescence spectroscopy and DEPT/QUAT 13C NMR pulse techniques. Org. Geochem. 24, 523-529. 

In the specific case of the determination of trace amounts of actinides, it is interesting to compare the results obtained by TRES to those obtained by other techniques. This very brief presentation is based on a very detailed and comprehensive lecture on radioactive ultra-trace determination in the environment (Aupiais, 2001, in French). In order to detect radioactive traces in environmental samples, various techniques are available (a and ft liquid scintillation, y spectrometry, mass spectrometry,. ..), which most of the time are coupled to a preconcentration of the sample. Such methods allow isotope discrimination, which is impossible with TRES. Another restriction of TRES as compared to the other techniques available is that TRES is strictly limited to luminescent elements. On the other hand, liquid scintillation is a rather time-consuming method as compared to TRES. For example, detection limits with a liquid scintillation are equal to 2 x 10-10 mol for 238U and 9 x 10-19 mol for 244Cm but the acquisition time is on the order of a few days, to be compared with TRES acquisition times of a few minutes. In the case of Cm, the advantage of a liquid scintillation is clear but TRES appears to be competitive in the case of U, if no isotopic discrimination is required. 

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