Instrument standardization, luminescence

Instrument calibration standards - physical sources such as line and tungsten lamp emissions, known instrument independent luminescence standards, and so forth... 

Many current multidimensional methods are based on instruments that combine measurements of several luminescence variables and present a multiparameter data set. The challenge of analyzing such complex data has stimulated the application of special mathematical methods (80-85) that are made practical only with the aid of computers. It is to be expected that future analytical strategies will rely heavily on computerized pattern recognition methods (79, 86) applied to libraries of standardized multidimensional spectra, a development that will require that published luminescence spectra be routinely corrected for instrumental artifacts. Warner et al, (84) have discussed the multiparameter nature of luminescence measurements in detail and list fourteen different parameters that can be combined in various combinations for simultaneous measurement, thereby maximizing luminescence selectivity with multidimensional measurements. Table II is adapted from their paper with the inclusion of a few additional parameters. 

Requirements for standards used In macro- and microspectrofluorometry differ, depending on whether they are used for Instrument calibration, standardization, or assessment of method accuracy. Specific examples are given of standards for quantum yield, number of quanta, and decay time, and for calibration of Instrument parameters. Including wavelength, spectral responslvlty (determining correction factors for luminescence spectra), stability, and linearity. Differences In requirements for macro- and micro-standards are considered, and specific materials used for each are compared. Pure compounds and matrix-matched standards are listed for standardization and assessment of method accuracy, and existing Standard Reference Materials are discussed. 

In general, luminescence measurements are relative rather than absolute, since the Instrument characteristics and sample properties that determine the fluorescence Intensities are often not well defined. Absolute luminescence measurements are difficult to perform and require time and Instrumentation not available In most laboratories. Thus, luminescence measurements rely heavily on standards to determine Instrument responses and parameters, the chemical composition of samples, and the characteristics of chemical systems. To... 

Definition and Uses of Standards. In the context of this paper, the term "standard" denotes a well-characterized material for which a physical parameter or concentration of chemical constituent has been determined with a known precision and accuracy. These standards can be used to check or determine (a) instrumental parameters such as wavelength accuracy, detection-system spectral responsivity, and stability (b) the instrument response to specific fluorescent species and (c) the accuracy of measurements made by specific Instruments or measurement procedures (assess whether the analytical measurement process is in statistical control and whether it exhibits bias). Once the luminescence instrumentation has been calibrated, it can be used to measure the luminescence characteristics of chemical systems, including corrected excitation and emission spectra, quantum yields, decay times, emission anisotropies, energy transfer, and, with appropriate standards, the concentrations of chemical constituents in complex S2unples. 

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. 

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. 

The basic principles of dc and ac voltage measurements are discussed in Chapter XVI and in standard textbooks on electronics. In many apphcations these measnrements are carried out by complex electronic instruments designed to produce a visual record of the detected signal, either as a trace on a luminescent screen, a plot on paper, or a numerical... 

Luminescent standards have been established for use in calibrating fluorescence spectrometers and have been suggested for Raman spectroscopy in the past (18). The standard is a luminescent material, usually a solid or liquid, that emits a broad reproducible luminescence spectrum when excited by a laser. Once the standard is calibrated for a particular laser wavelength, its emission spectrum is known, and it can provide the real standard output , d)i(AF) depicted in Figure 10.8. In practice, a spectrum of the standard is acquired with the same conditions as an unknown then the unknown spectrum is corrected for instrument response function using the known standard... 

Recalibration of the instrument response function reduces or eliminates most of the instrumental factors that lead to relative intensity variations over time. For example, a luminescent standard could be used at the beginning of each session as described in Section 10.3.3. Use of the same standard and correction procedure during qualification could establish the true value of one or more peak ratios for future reference. Table 10.9 shows results for this approach applied to the example of calcium ascorbate. The ratio of the 767- and 1587 cm" peak intensities was monitored after calibration of the response function with a luminescent standard. The standard deviations listed in Table 10.9 for the 767/1582 peak height ratio provide indications of the reproducibility of the response correction and sample spectra. 

Products/technologies Porvair sells the Microlute Solid Phase Extraction in a Microplate system that provides 96 solid phase extractions in one compact unit (using any brand of sorbent). It can be automated using most standard liquid handling and robotic systems. Porvair s 384-well plate is compatible with most automated liquid handling instruments, readers for EIA, fluorescence, luminescence, and scintillation assays as well as robotic-handling devices. 

In contrast to the ultraviolet and visible absorption methods described earlier, details of the methodology of luminescence spectroscopy are not widely known and few standard methods have evolved. Books on theory and techniques are helpful (44) as are memoranda on applications from instrument manufacturers. ASTM Committee E-13.06 on Molecular Luminescence has had large task forces working for several years on practices for instrument testing, nomenclature, and analytical procedures. Recent symposia sponsored by that committee are the basis for two new books (45, 6). 

Unless otherwise noted, the aerogels were interrogated using standard spectroscopic instrumentation. The sensor respraises are recorded using a number of different types of spectroscopic measurements, including absorpti(Mi spectroscopy, luminescence emission and lifetime measurements, and Raman scattering techniques. 

Hanssen, L. M. New instrument development at National Institute of Standards and Technology for spectral diffuse reffectance and transmittance measurements,in Spectrophotometry, Luminescence and Colour Science and Compliance, Elsevier, New York (1995). 

KG Ray, RL McCreery. Simplified calibration of instrument response function for Raman spectrometers based on luminescent intensity standards. Appl Spectrosc 51 108-116, 1997. 

KJ Frost, RL McCreery. Calibration of Raman spectrometer instrument response function with luminescence standards An update. Appl Spectrosc 52 1614-1618, 1998. 

Measurements. Absorption spectra measurements on the film and solutions of the metal complexes were measured on a modified Cary 14 spectrometer. Luminescence spectra were recorded with a custom photon counting spectrometer or a PTI (Deer Park Drive, South Brunswick, NJ 08852) luminescence spectrometer system. The FT-IR spectra were recorded with a PE-1600 spectrometer or a Bio-Rad FTS-40 spectrometer (Professor R. Crooks, Chemistry Department, University of New Mexico). RA spectra for LB films were measured on the FTS-40 with a Hanick gra2dng angle attachment and MCT detection system. Typically 256 scans were requir to obtain adequate intensity for the monolayer films. The scanning tunneling microscopy measurements employed a Nanoscope II system (Digital Instruments, Inc.). Etched Pt-Ir tips were used with the standard, 0.6 micron head. The system can be operated in either the constant current or constant height mode but the film contours were similar for either. 

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