Signal-to-Noise Ratio Comparisons

An assessment of spectral noise was made for data collected at both the AS-IRM and the SRC IRENI beamlines. The details of the measurements are provided in Table 15.1. root mean squared (RMS) noise was determined from 2450 to 2550 cm for individual spectra, after converting the data from absorbance to percentage transmission. Three spectra were selected in each case. In sum, the two facilities produce similar quality spectral data for 4 X 4 pm sample-projected pixels (aperture based on RS spectromicroscopy and binned WF spectromicroscopy) measuring eight co-added scans for the AS-IRM data, and 64 co-added scans for IRENI data, the latter to overcome the noisier characteristics of the FPA detector. When collecting 64 scans at the AS-IRM beamline, the RMS noise level at 4 X 4 pm aperture and 6 cm resolution was reduced to 0.022% as expected. 

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Arnder, L., M. D. Shattuck. et al. 1996. Signal-to-noise ratio comparison between surface coils and implanted coUs. Magn Reson Med 35 727-Ti i. 

The combination of contrast and granularity produces a signal to noise ratio which allows for direct comparison of various films. The classes have minimum values for eontrast and maximum values for graininess. The ASTM classification system employs the same parameters as the European Standard EN584-1 and ISO CD (see Table 1). 

Williams, R. R., Fundamental Limitations on the Use and Comparison of Signal-to-Noise Ratios, Anal. Chem. 63, 1991, 1638-1643. 

Suppose one wanted to compare the behavior of two polymers and their blends. Let us define the signal as the difference between the logarithims of the viscoelastic quantities and the noise as the error calculated for a specific set of viscoelastic properties associated with a specific composition. The signal to noise ratio would have the appearance of the three curves shown in Figure 2 for a PMMA/Hytrel blend >3/1. Selection of the optimum conditions for comparison is apparent in that figure. Emphasis should be placed at those temperatures with high signal/noise ratios. 

The /3-polymorphic form of anhydrous carbamazepine is official in the USP [3], The USP stipulates that, The X-ray diffraction pattern conforms to that of USP Carbamazepine Reference Standard, similarly determined. No limits have been set in the USP for the other polymorphs of anhydrous carbamazepine. Although several polymorphic forms of anhydrous carbamazepine have been reported, only the a- and /3-forms have been extensively studied and characterized [49]. A comparison of the powder x-ray diffraction patterns of these two forms revealed that the 10.1 A line (peak at 8.80° 26) was unique to a-carbamazepine, and so this line was used for the analysis (Fig. 5). It was possible to detect a-carbamazepine in a mixture where the weight fraction of a-carbamazepine was 0.02 at a signal-to-noise ratio of 2. Much greater sensitivity of this technique has been achieved in other systems. While studying the polymorphism of l,2-dihydro-6-neopentyl-2-oxonicotinic acid, Chao and Vail [50] used x-ray diffractometry to quantify form I in mixtures of forms I and II. They estimated that form I levels as low as 0.5% w/w can be determined by this technique. Similarly the a-inosine content in a mixture consisting of a- and /3-inosine was achieved with a detection limit of 0.4% w/w for a-inosine [51]. 

To calculate the total amount of volatiles included in the hair samples from different body regions the area beneath all peaks was calculated using the GCM-SPostrunAnalysis software including all peaks above a signal-to-noise ratio of 3 1 (exceeding an area of 200.000 units). From those, 20 distinct peaks were selected for peak comparison and the area beneath the peak was used to evaluate the differences between sex and season. 

Because sensitivity depends on so many different experimental factors, NMR spectroscopists generally use the signal-to-noise ratio, SIN, as a figure of merit for sensitivity comparisons. For example, in a comparison between NMR probes or spectrometers from two vendors, the spectral SIN measured for a standard sample acquired with specified acquisition parameters and probe geometry would provide a direct indication of relative sensitivity. The SIN is calculated for an NMR experiment as the peak signal divided by the root mean square (RMS) noise, given by Equation 7.6, and is directly related to the performance of the radiofrequency coil [3,6]... 

FIGURE 7 Comparison of two peaks with signal-to-noise ratio (S/N) of 50 and 10. The random distribution of noise in the noisy peak controls the variation of the peak area measurement. 

More generally the test statistic is constructed as the signal/noise (signal-to-noise) ratio or something akin to this. We will develop this methodology in relation the comparison of two independent means for a between-patient design. The resulting test is known as the unpaired t-test or the two-sample t-test. 

Method. A diagram of the apparatus is shown in Fig.4.29. Any suitable liquid chromatograph may be used. The AutoAnalyzer is modified such that the liquid sampler is fitted to the end of the chromatographic column. The proportioning pump is by-passed. The set-up of the AutoAnalyzer is the same as that for normal measurements of cholinesterase. The application of this technique to the determination of CGA 18809 in plum-leaf extract is shown in Fig.4.30. A comparison is made with UV analysis of the same extract. The limit of detection for CGA 18809 is c . 20 ng at a 3 1 signal to noise ratio. The relative inhibitions of several organophosphates and carbamates are compared in Table 4.9. Diazoxon may be detected in low picogram quantities. 

Griffiths et al. (9) have compared the signal to noise ratios of interferometers and monochrometers under the assumption that the source temperature and resolution for both types of instrument are equivalent. Their analysis involves a comparison of the factors appearing in equation 8 and a representation of the advantage of an interferometer over a monochrometer as the ratio of the factors associated with each instrument. A summary of Griffith et al. s (9) analysis is presented in the balance of this section. 

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