Spectrum Estimation, Techniques

Evans et al., 1981] Evans, J., Johnson, J., and Sun, D. (1981). High resolution angular spectrum estimation techniques for terrain scattering analysis and angle of arrival estimation. Proc. IstASSP Workshop on Spectral Estimation, pages 5.3.1-5.3.10. 

A very popular method of estimating antioxidants in polymer extracts is by coupling or oxidising them to form coloured products and measuring the resulting absorbance in the visible region of the spectrum. This technique is not particularly specific for individual antioxidants, but is specific for phenolic antioxidants in general (also amine antioxidants), and hence can often be applied without interference from other types of polymer additives. 

Figure 6 On the accuracy of the Point Estimate technique for calculating the spectrum of the unconditional standard deviation of SDOF displacement. Solid line Point Estimate method. Dashed line Monte Carlo simulation (1,000 samples). The dotted line corresponds to the standard deviation without considering uncertainties in the Kanai-Tajimi model and using the mean values of the parameters.

Figure 6 Spectrum estimation of geraniol using the expanded three-dimensional HOSE code technique. The methyl groups at C-7 can be clearly distinguished, despite the severe overlap when taking into account the total expectation ranges...

The different techniques of NDT were applied to evaluate the method allowing to give an optimal spectrum so that the interpretation can be done easily. In addition, and for the purpose of the defects quantification, we have done an optimization on the magnetic powders, colored and fluorescent, by applying magnetic powders of variable dimensions. This will enable us to estimate defects with a high precision. 

A recent modification of mass spectrometric technique involves the use of a spark source. A spark is struck by means of a high voltage between two rods of the material under examination. Under this drastic treatment many substances decompose completely into their elements and give positive ions. The ion detector is usually a photographic plate which shows a line mass spectrum. A number of exposures are taken for each sample and a quantitative estimate of the presence of an element in the sample can be made from the exposure time. 

Steady-State ESR Spectroscopy. In this method, the ESR spectrum is monitored, usually with constant sample irradiation. Radical concentrations reach a steady state, and the rate constants for termination can be determined from the radical concentrations if the rate constants for radical formation are known. Estimates of the rate of formation of radicals are often a source of large errors. The technique often is applied at low temperatures so that diffusion is slowed and adequate radical concentrations are achieved, and extrapolation of the results to ambient temperatures can give large errors. 

Two final examples of the sensitivity and general applicability of the FTIR gas analysis technique are illustrated in Fig. 8. Trace (A) shows the spectrum obtained from an ultra-air filled 70 liter sampling bag into which had been injected, 18 hours previously, 4.8 microliters of TDI, toluene diisocyanate. On the basis of the single feature at 2273 cm l, it is estimated that 50 ppb TDI could be detected. The lower Trace (B), shows the spectrum of nickel carbonyl. This highly toxic but unstable gas was found to decay rapidly at ppm concentrations in ultra air (50% lifetime 15 minutes). Calibration of its spectrum was established by recording successive spectra at ten minute intervals and by attributing the increase in carbon monoxide concentration (calibration known) to an equivalent but four times slower decrease in nickel carbonyl concentration. The spectrum shown represents 0.6 ppm of the material. Note the extraordinary absorption strength. The detection limit is thus less than 10 ppb. 

The advantage of estimating a model with stepwise MLR rather than with the full-spectrum techniques (e.g., PLS and PCR, Section 5-3-2) is that the MLR model is simple. It does not add variables whose variability is described by previously entered variables or that are not linearly related to the anal te of interest (e.g., have large contributions from interfering species). With the full-spectrum techniques, all sources of variation are implicitly accounted for in the model. This is a more complicated way of dealing with the variation not related to the anahte of interest. 

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