Background noise measurement

Received Signal Level = Background Noise + Measurement Requirement. (5.1)... 

Background noise in a meter obtained by measuring signal over time in the absence of analyte. 

The accuracy with which a wavefront sensor measures phase errors will be limited by noise in the measurement. The main sources of noise are photon noise, readout noise (see Ch. 11) and background noise. The general form of the phase measurement error (in square radians) on an aperture of size d due to photon noise is... 

In principle, atomic fluorescence is a simpler and more versatile technique than atomic absorption, but suffers from a susceptibility to quenching effects and to background noise arising from the scattering of radiation by particles in the flame. The latter is particularly serious for refractory materials and in high-temperature flames. Detection limits for some elements are lower than by atomic absorption or flame emission measurements, e.g. elements with resonance lines around 200 nm or below, such as As, Se,... 

There have also been recent improvements to reduce background noise, which is caused by random thermal motions of electrons in the detection coil and in the first amplifier range.15 Cryoprobes in which the coil and first amplifier are cooled to 30 K and the size of sample coil reduced lower electrical resistance and noise. A further benefit noted by Grivet et al.15 is that "the smaller the probe, the closer the conductors are to the sample and the higher the measure of the coil s [efficiency], as felt by the nuclei."... 

A small addition to a logarithmic curve will be very evident at the lower end of the curve and virtually invisible at the top. For good matches with experimental curves plotted on a logarithmic scale (to see small peaks) it is necessary to add a background noise. With a well set up system, this is essentially the detector background, which may be measured at an angle remote from the substrate peak. 

For instrumental procedures that exhibit background noise, it is common to compare measured signals from samples with known low concentrations of analyte with those of the blank samples. The minimum concentration at which the analyte can reliably be detected is established using an acceptable signal-to-noise ratio of 2 1 or 3 1. Presentation of relevant chromatograms is sufficient for justification of the DL. 

Quantitation Limit. When the quantity of heavy metals is determined from the calibration curve, it is recommended to estimate the lowest value of heavy metals concentration as the quantitation limit. The methods to estimate the quantitation limit are described in JP and ICH Q2B Guidelines [2], and an appropriate method should be selected from among these methods. An estimation of the quantitation limit can be obtained from the standard deviation of measured values of the low-concentration test solution. The standard deviation of background noise will be used to estimate a value for the standard signal-to-noise ratio (10 1). 

Using this procedure, the estimated quantitation limit is obtained from the variation of actual measurement values of low-concentration test solutions instead of background noise. However, the actual value of quantitation limit needs to be verified by experiment. 

Tests. Air is scanned in the absorbance mode for 10 min. peak-to-peak noise is recorded at 500 nm. The root mean square (RMS) noise is then calculated. The RMS noise measurement is a measure of the standard deviation of the background signals. Modem spectrophotometers are usually equipped with the noise estimation function. For older spectrophotometers, the RMS noise can be estimated by multiplying the highest peak-to-peak noise level by a factor of 0.7 (Figure 10.8). 

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