Luminescence instrumentation detectors

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... 

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. 

Luminescence molecular detectors have also been used for online monitoring of dissolution tests and the characterization of toxic residues using bioluminescence assays. Atomic (atomic absorption spectroscopy, inductively coupled plasma-atomic emission spectroscopy (ICP-AES)) detectors have been coupled to robotic stations either through a continuous system acting as interface or by direct aspiration into an instrument from a sample vial following treatment by the robot. Mass spectrometric and nuclear magnetic resonance (NMR) detectors... 

Room-temperature fluorescence (RTF) has been used to determine the emission characteristics of a wide variety of materials relative to the wavelengths of selected Fraunhofer lines in support of the Fraunhofer luminescence detector remote-sensing instrument. RTF techniques are now used in the compilation of excitation-emission-matrix (EEM) fluorescence "signatures" of materials. The spectral data are collected with a Perkin-Elraer MPF-44B Fluorescence Spectrometer interfaced to an Apple 11+ personal computer. EEM fluorescence data can be displayed as 3-D perspective plots, contour plots, or "color-contour" images. The integrated intensity for selected Fraunhofer lines can also be directly extracted from the EEM data rather than being collected with a separate procedure. Fluorescence, chemical, and mineralogical data will be statistically analyzed to determine the probable physical and/or chemical causes of the fluorescence. 

In recent years no truly new basic principles have been introduced for the detection of luminescence. However, the technical evolution in the field of microelectronics and optoelectronics, charge coupled device (CCD) detectors, fiberoptics, assembly techniques, and robotics resulted in the introduction on the market of new generations of instruments with increased performance, speed, and ease of handling. In this chapter, some of their typical features will be reviewed. To keep this presentation at a concrete level and to illustrate some specific item, instruments of different makes will be referred to. However, this does not imply they are better than those not cited. It is more a matter of availability of recent documentation at the time of writing. Note that numerical values cited typically relate what can be done today and may vary from one instrument to another from the same company. 

This paper describes the luminoscope, a simple laboratory-constructed, portable luminescence detector designed specifically for monitoring occupational skin contamination. The instrument design is based upon a fiberoptics waveguide. The instrument is suitable for detecting trace amounts of various coal tars and has recently been field tested at a coal conversion facility. 

When a luminescence spectrum is obtained on an instrument such as that used to produce the spectra in Figure 7.23, it will depend on the characteristics of the emission monochromator and the detector. The transmission of the monochromator and the quantum efficiency of the detector are both wavelength dependent and these would yield only an instrumental spectrum. Correction is made by reference to some absolute spectra. Comparison of the absolute and instrumental spectra then yields the correction function which is stored in a computer memory and can be used to multiply automatically new instrumental spectra to obtain the corrected spectra. The calibration must of course be repeated if the monochromator or the detector is changed. 

This is a technical question I noticed you had a chemi-luminescent detector in the kit on your flight instruments for detection of NO. At one time you were using resonance fluorescence I was curious what happened in the volution of that system. 

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. 

Figure 11.15 Functioning principle of an analyser hosed upon nitrogen monoxide luminescence. This apparatus is able to find all compounds containing nitrogen by a specific emission of fight. Installed at the outlet of a GC column, the instrument becomes a selective detector.

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. 

However, if size, portability, and ease of use are important criteria for a specific application, then the new generation of solid-state photodiode detectors may be ideal. One such instrument has recently been described for measuring ATP down to 0.5 pmole, using the power from four AA alkaline batteries (M3). Detection on instant film in photocassettes (B30, M33), or in so-called camera luminometers (B32, K30), is rapidly expanding the scope of luminescence measurement and can be exemplified by an assay for hepatitis B viral DNA (see Section 3.3.6.1) (B26). 

Another type of luminescence spectrum is shown in Figure I5 h. The total luminescence spectrum is cither a three-dimensional representation or a contour plot. Both simultaneously show the luminescence signal as a function of excitation and emission wavelengths. Such data aie often called an excitation-emission matrix. Although the total luniinescence spectrum can be obtained on u normal coni])U(cri/ed instrument, it can be acquired more rapidly with array-detector-based systems (see next seel ion). 

The excitation wavelength selector can be either a filter or a monochromator. Filters offer better detection Hmits, but do not provide spectral scanning capabilities. Often, a filter is used in the excitation beam along with a monochromator in the emission beam to allow emission spectra to be acquired. FuU emission and excitation spectral information can be acquired only if monochromators are used in both the excitation and emission beams. In modern instruments with array detectors, a polychromator is used in the emission beam instead of a monochromator. Recent research instraments are able to scan both wavelengths automatically and combine all data into a 2D excitation—emission spectrum. In lifetime spectrometers, a pulsed light source and a gated detector are synchronized in order to measure the time dependence of the luminescence emission. 

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