Luminescence instrumentation sources

If the researcher has commercial molecular luminescence instrumentation (e.g., a spectrofluorometer) available, then solid-state luminescence data should not be difficult to obtain. Many good references are available discussing the basic theory of luminescence, " so the focus herein will be on its use in solid-state applications. Instrumentation normally consists of an excitation source, excitation wavelength selector, sample compartment, emission wavelength selector, and detector. The largest issue for conducting measurements on... 

The basic components of luminescence instrumentation are generally arranged as shown in Fig. 1. The sample is placed in a sample cell and excited by either ultraviolet or visible light from a source. A filter or monochromator may select a particular excitation... 

T he luminescence instruments shown in I igures 15-10 and 15-11 both monitor the source intensity via a ret-crence photomultiplier. Most commonly, the ratio of the sample luminescence signal lo the signal from the reference detector is continuously obtained. This can compensate for source intensity fluctuations and drift. Both doubic-bcam-in space and doiihlc beam-in time designs are employed. 

Errors from instrumental sources, c/uenching. and luminescence can be kept very low (ca. 

Luminescence lifetimes are measured by analyzing the rate of emission decay after pulsed excitation or by analyzing the phase shift and demodulation of emission from chromophores excited by an amplitude-modulated light source. Improvements in this type of instrumentation now allow luminescence lifetimes to be routinely measured accurately to nanosecond resolution, and there are increasing reports of picosecond resolution. In addition, several individual lifetimes can be resolved from a mixture of chromophores, allowing identification of different components that might have almost identical absorption and emission features. 

The book starts with a short introduction to the fundamentals of optical spectroscopy, (Chapter 1) describing the basic standard equipment needed to measure optical spectra and the main optical magnitudes (the absorption coefficient, transmittance, reflectance, and luminescence efficiency) that can be measured with this equipment. The next two chapters (Chapters 2 and 3) are devoted to the main characteristics and the basic working principles of the general instrumentation used in optical spectroscopy. These include the light sources (lamp and lasers) used to excite the crystals, as well as the instrumentation used to detect and analyze the reflected, transmitted, scattered, or emitted light. 

In recent years luminescence nomenclature has become confusing within the literature and in practice. Luminescence involves both phosphorescence and fluorescence phenomena. While luminescence is the appropriate term when the specific photochemical mechanism is unknown, fluorescence is far more prevalent in practice. Moreover, the acronym LIE has historically inferred laser -induced fluorescence however, in recent years it has evolved to the more general term light -induced fluorescence due to the various light sources found within laboratory and real-time instruments. Within this chapter fluorescence and LIE are interchangeable terms. 

The sample is purified by distillation to separate the tritium-containing water from both non-radioactive and radioactive impurities. Various substances can cause scintillations by means other than radionuclide emission - by chemical fluorescence or luminescence - or interfere with ( quench ) detection of scintillations due to radionuclides. Even after purification, both processes are inevitable, but to a limited extent. Luminescence due to visible light will decay when the sample is stored in a darkened region of the LS system before the sample is counted. The degree of quenching, notably due to water in the sample, is determined instrumentally by reference to comparison sources and recorded, so that any deviation from the quenching observed for the tritium standard can be taken into account. 

Near-infrared surface-enhanced Raman spectroscopy Some of the major irritants in Raman measurements are sample fluorescence and photochemistry. However, with the help of Fourier transform (FT) Raman instruments, near-infrared (near-IR) Raman spectroscopy has become an excellent technique for eliminating sample fluorescence and photochemistry in Raman measurements. As demonstrated recently, the range of near-IR Raman techniques can be extended to include near-IR SERS. Near-IR SERS reduces the magnitude of the fluorescence problem because near-IR excitation eliminates most sources of luminescence. Potential applications of near-IR SERS are in environmental monitoring and ultrasensitive detection of highly luminescent molecules [11]. 

Recent developments of pulsed light sources, optical components, fast and sensitive detectors and electronic equipment for data collection and analysis have permitted the construction of numerous instruments, often commercially available, for the collection of luminescence data with excellent resolution in time, spectral distribution and space. The sensitivity has reached the ultimate level that allows the characterization of such properties for single molecules (see Section 3.13). Only an overview of some of these techniques is given here. 

The correction for the refractive indices of the sample and reference solutions, (s) and ra(r), allows for the variation in the angles of the luminescence emerging from the solution to air. The correction may be substantial, for example D2(benzene)/ D2(water) 1.27, and may depend somewhat on optical parameters of the instrument. Moreover, internal reflections within the cell also depend on the refractive index. It is therefore preferable to dissolve both sample and reference in the same solvent to avoid errors from these sources. 

Laser excitation for fluorescence detection has received much research interest, but as of yet there is no commercially available instrument. Fluorescence intensity increases with excitation intensity, and it is generally assumed that laser excitation would then offer improved limits of detection. However, as Yeung and Synovec have shown, various types of light scattering, luminescence from the flow cell walls, and emission from impurities in the solvent all increase with source intensity as well, yielding no net improvement in signal-to-noise ratio (53). Where laser excited fluorescence may prove useful is for the design of fluorescence detectors for microbore packed and open tubular LC columns, where the laser source can be focused to a small illuminated volume for on-column detection. 

These approaches differ from fluorescence and other luminescent techniques such as phosphofluorescence in that the excitation event is caused by a chemical reaction rather than photolumination. There are a number of reviews that detail the development and applications of luminescent technology (52,53,54,55,56, 57, 58) and instrumentation (59, 60). Particularly outstanding is the recent detailed review by Stanley (61) where more than 90 luminometers (manual, automatic, microtiter plate, HPLC, LC, GLC, imaging, and others) from more than 60 sources are described. 

Both a- and jS-emitters are used in luminescent paint. The fluoresc t material is usually ZnS. T and are preferred sources since their j3-energies are low, but Kr, Sr, and Pm are also used. The amount or radioactivity varies, depending on the need (watches, aircraft instruments, etc.) but it is usually < 400 MBq (< 10 mCi), although larger light panels may require > 50 GBq (several Curies). For such high activities only T or Kr are acceptable because of their relatively low radiotoxicity. 

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