Signal/noise ratio background

Expose wetted membrane to X-ray film (e.g., Kodak XAR) immediately (or incubate at 37°C for 30 min to obtain steady-state light emission). Light emission remains constant for about 24 h. Multiple exposures from a single blot may thus be made to obtain an optimal signal/noise ratio. Background in substrate should be less than 25 TLU (Turner light units) over 1 h from 100 jxl of substrate, whereas 10 mol of APase should yield >10 TLU. 

Experimental methods are described in Chapter 3 ( 3.5.5). Here we refer to certain aspects of INS experimental methods which require special attention in measurements on catalysts the mass of sample required if we are to obtain an acceptable signal-to-noise ratio, background subtraction, and the pre-treatment of the catalyst. 

The Raman scattering of water serves as a sensitivity test for fluorometers. This consists of measuring the signal/noise ratio of the Raman peak with a cell filled with water, for example at 397 nm if the excitation wavelength is fixed at 350 nm, as a result of the specific shift of 3380 cm for this solvent, and to compare with the background signal. 

Electrothermal Atomizers. In principle, electrothermal atomization is a very suitable method for AFS since, when an inert gas atmosphere is used, the fluorescence quenching will be minimized without losing atomization efficiency. On the other hand, the strong background emission of the hot graphite tube will decrease the signal/noise ratio. 

Consider the error as a signal and the background as noise. Improve detectabUity by having a high signal/noise ratio—a good contrast. Amplify the signal or reduce the noise. 

Under background of low signal noise ratio, by means of this method, the identification of the parameters can be much more precise. 

Detection limit. The detection limit of an instrument should be differentiated from its sensitivity. The detection limit reflects the smallest flow of sample or the lowest partial pressure that gives a signal that can be distinguished from the background noise. One must specify the experimental conditions used and give the value of signal-to-noise ratio corresponding to the detection limit. 

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