Standards spectral responsivity

Spectral Responsivity Standards (for Corrected Spectra). Depending on the conditions, many different organic and inorganic compounds in various solvents have been used as standards for determining the spectral responsivity of instruments. Several measurement proce-... 

When considering light of a certain spectral energy distribution falling on an object with a given spectral reflectance and perceived by an eye with its own spectral response, to obtain the perceived color stimulus it is necessary to multiply these factors together as ia Eigure 6. Standards are clearly required for both the observer and the illuminant. 

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. 

Calibration. In general, standards used for instrument calibration are physical devices (standard lamps, flow meters, etc.) or pure chemical compounds in solution (solid or liquid), although some combined forms could be used (e.g., Tb + Eu in glass for wavelength calibration). Calibrated lnstr iment parameters include wavelength accuracy, detection-system spectral responsivity (to determine corrected excitation and emission spectra), and stability, among others. Fluorescence data such as corrected excitation and emission spectra, quantum yields, decay times, and polarization that are to be compared among laboratories are dependent on these calibrations. The Instrument and fluorescence parameters and various standards, reviewed recently (1,2,11), are discussed briefly below. 

This diagram shows the energy spectrum of a given source, coupled with a filter of defined transmittance, which is established by a detector of known spectral response, as modified by a standard source and modified to that of a Standard Observer. Once an instrument has been set up properly with the proper optical... 

The wheat sample residue level is determined from the relative mass spectral responses of the analytes to the corresponding isotopically labeled internal standards. The sample relative response is compared with the average relative response of a standard solution of analyte and internal standard analyzed before and after the sample (bracketing standards). Both samples and standards receive the same amount, 100 ng, of each internal standard to facilitate the comparison. The calculations to determine the residue level in wheat tissues are outlined in Section 7.3.1. 

A host of mathematical techniques for standardizing calibration models to facilitate their transfer is available. These generally focus on the coefficients of the model, the spectral response or the predicted values. ... 

Standardizing the coefficients of the model entails modifying the calibration equation. This procedure is applicable when the original equipment is replaced (situation 1 above). Forina et al. developed a two-step calibration procedure by which a calibration model is constructed for the master (F-X), its spectral response correlated with that of the slave X-X) and, finally, a global model correlating variable Y with both X and X is obtained. The process is optimized in terms of SEP and SEC for both instruments as it allows the number of PLS factors used to be changed. Smith et al. propose a very simple procedure to match two different spectral responses. 

Standardizing the spectral response is mathematically more complex than standardizing the calibration models but provides better results as it allows slight spectral differences - the most common between very similar instruments - to be corrected via simple calculations. More marked differences can be accommodated with more complex and specific algorithms. This approach compares spectra recorded on different instruments, which are used to derive a mathematical equation, allowing their spectral response to be mutually correlated. The equation is then used to correct the new spectra recorded on the slave, which are thus made more similar to those obtained with the master. The simplest methods used in this context are of the univariate type, which correlate each wavelength in two spectra in a direct, simple manner. These methods, however, are only effective with very simple spectral differences. On the other hand, multivariate methods allow the construction of matrices correlating bodies of spectra recorded on different instruments for the above-described purpose. The most frequent choice in this context is piecewise direct standardization... 

Compounds 1,2,3,5,10,11,12,13,14 were dissolved in EPIP (diethyl ether, petroleum ether, isopropanol 5 5 2)whereas compounds 4,6,7,8,9,15 were dissolved in THF-DE (tetrahydrofurane, diethyl ether 1 1). These solvent mixtures can be frozen as glassy samples at 77 K. The absorption spectra were recorded on a standard spectrophotometer SF-10 or Beckman-5270. The measurements of fluorescence excitation and emission spectra were made with the aid of a spectrofluorometer SLM-4800 with automatic correction of spectral response. Fluorescence lifetimes were measured with the aid of a pulse fluorometer PRA-3000. Magnetic circular dichroism (MCD) measurements were carried out in a 8 kG magnetic field using a JASCO J-20 circular dichrometer. Triplet state formation was observed for investigated compounds at the experimental set up, whose detailed description can be found in our paper (27). The optical experiments were carried out with a porphyrin concentration of 4.10- - 4.10 mol.l". In NMR investigations (Bruker WM-360) we used higher concentrations ( 5.10" raol.l ) and dried solvents (CDCl, C 2 and toluene-d0). 

Figure 1. Relative spectral responses of different broadband detectors in the UV spectral region. The dashed line corresponds to the CIE action spectrum. The numbers in the legend correspond to the weighted integral (Warm 2) of a standard solar spectrum (30° SZA, 330 DU).

In principle, there are two ways to achieve the radiometric calibration of an instrument measuring solar radiation. The first is by comparison to a standard radiation source of known output and the second by comparison to a prototype standard instrument that is capable in measuring the same radiometric quantity. The fist can be applied to broadband detectors only if their spectral response over the whole range of the radiation source is known with sufficient accuracy. The second method requires that the standard instrument has exactly the same spectral response, which is rather unlikely to occur. 

Mid-IR has also been demonstrated for real-time concentration monitoring of a fermentation using a standard transmission cell, and the spectral response was similar, regardless of whether the broth was filtered or not.38 A PLS regression was used to perform quantitation of the substrate, the lactic acid bacterium, and the major metabolites. In another article describing quantitative mid-IR for fermentation studies, a model system was investigated under various fermentation conditions.39 The mid-IR provided insight into the relative concentrations of carbohydrates, nucleic acids, proteins and lipids in the host cells. Mid-IR has also been demonstrated for multi-component quantitation... 

Figure 6 shows a typical chromatogram obtained from such a procedure. In this chromatogram, the elution of the thio-tetrazole, the R and S isomers of moxalactam, and the moxa-lactam decarboxylated product are shown. In this test for other related substances, only those substances which are not dealt with by specific tests are considered. Quantitative information is obtained by summing the response for all the extraneous peaks and comparing them to the sum of the response for the R and S isomers of a diluted moxalactam reference standard. The assumption is made that all of these materials have similar spectral response characteristics at 254 nm. 

Either PLS or PCR can be used to compute b, at less than full rank by discarding factors associated with noise. Because of the banded diagonal structure of the transformation matrix used by PDS, localized multivariate differences in spectral response between the primary and secondary instrument can be accommodated, including intensity differences, wavelength shifts, and changes in spectral bandwidth. The flexibility and power of the PDS method has made it one of the most popular instrument standardization methods. 

According to Malyj and Griffiths (1983), determining the equilibrium rotational or vibrational temperature by the Stokes/anti-Stokes ratio is not as simple and straightforward as the equations imply. The authors discuss the problems which evolve as a result of using standard lamps and show how to meet these difficulties by using reference materials to measure the temperature as well as to determine the instrumental spectral response function. The list of suitable materials includes vitreous silica and liquid cyclohexane, which are easy to handle and available in most laboratories. The publication includes a detailed statistical analysis of systematic errors and also describes tests with a number of transparent materials. 

The instruments used for color measurements are nowadays spectrophotometers determining the reflectance of a sample. Three-filter colorimeters, trying to mimic the spectral response of the human eye are now next to obsolete. For applications in the field of uni-pigments photometer illuminating/viewing geometries are standardized as methods A and B, and are designed to suit the individual application (see Section 1.4.2). For standards, see Table 1.1 ( Color Differences, Conditions/ Fvaluation... ). 

The threshold curve is a plot of retention time versus a similarity factor threshold, below which the presence of an impurity cannot be distinguished from spectral noise. The threshold trace may be computed automatically from the standard deviation of a number of user-selected pure noise spectra. Alternatively, the threshold may be set at a fixed value. Similarity and threshold curves tend to rise at the extremities of the eluted peak, even when no impurity is present. As signal strength decreases, a larger proportion of the spectral response is caused by noise. If an impurity is present at a detectable concentration, the similarity curve will intersect the threshold (Fig. 5). 

Color Meter. The Chroma Meter CR-300 (Minolta, Osaka, Japan) provides both accurate and precise color data (Post et al., 1993). This color meter uses a pulsed xenon light for stable and uniform illumination of the sample. Three photocells measure the photons diifusely reflected by the sample through filters matching the CIE standard observer spectral response. A standard white plate is used for calibration. For analysis, the hand-held detector unit which is protected by a glass window may be placed on top of a small pile of a powdered sample (300 mg), thus standardizing the sample surface. This way, up to one hundred samples can be precisely measured within one hour. Further advantages are the mobility of the unit, and the output of the three color parameters in several color systems (see below). 

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