Limiting noises

Minimum water velocities should be maintained in the upper floors of high-rise buildings where air may tend to come out of solution because of reduced pressures. High velocities should be used in down-return mains feeding into air-separation units located at a low level in the system. Table 27.10 can be taken as a guide. Water velocities shown in Tables 27.9 and 27.10 are indicative parameters only on the one hand, to limit noise problems and... 

In most infrared measurements situations, the detector constitutes the limiting noise source. Because the resulting fluctuations have the same effect as a fixed uncertainty in the signal readout, they appear as a constant error in the transmittance . [4]... 

Tossing a mental coin, the decision was to analyze the case of noise proportional to the square root of the signal. This, as you will recall, is Poisson-distributed noise, characteristic of the noise encountered when the limiting noise source is the shot noise that occurs when individual photons are detected and represent the ultimate sensitivity of the measurement. This is a situation that is fairly commonly encountered, since it occurs, as mentioned previously, in UV-Vis instrumentation as well as in X-ray and gamma-ray measurements. This noise source may also enter into readings made in mass spectrometers, if the detection method includes counting individual ions. We have, in... 

Electrode noise may not be the limiting noise anymore, but electronically generated noise. This noise may be amplified by the cell impedance, which means that the electronics have to be developed very carefully. 

High noise levels may also be generated when high flows are encountered. In these circumstances, special noise reducing fittings are available to limit noise impacts from the system to the surrounding area. 

Limiting noise levels is not restricted to the SRV alone (Figure 8.5). Design of piping and, in particular, the isolation downstream and pipe supports are generally greater contributors to noise than the SRV itself. 

In a theoretical study of the advantages to be gained from nondispersive systems, Chester and Winefordner (28) have studied frequency-modulated sources in nondispersive atomic fluorescence, and have demonstrated a multiplex disadvantage for nondispersive atomic fluorescence systems. These studies have indicated that the limiting noise is flame background noise. 

According to Kaiser [43], the limit of detection of an analytical procedure is the concentration at which the analytical signal can still be distinguished from a noise level with a specific degree of uncertainty. In the case of a 99.86% uncertainty and provided the signal fluctuations of the limiting noise source can be described by a normal distribution, the lowest detectable net signal Yl is three times the relevant standard deviation ... 

As the sample introduction into the plasma is very stable, RSDs in ICP-AES are about 1% and the limiting noise is proportional to this, including flicker noise and nebulizer noise. 

Noise can be divided into three categories (1) radiofrequency interference, microphonics, unstable main power lines and other disturbances in the environment (2) oscillator source noise, both AM and FM (3) the overall receiver noise figure. In well-designed spectrometer systems, the third category is always the limiting noise source. 

Finally, it should be noted that with our present system and certain samples, background or analyte photon signal fluxes are large enough that measurements are flicker noise limited (noise proportional to the signal). In this case, the IDA already provides detection limits equivalent to that obtained with PMT detection. 

Noise measurements results (1) background noise, (2) ID fan and purging noise (no combustion), (3) combustion noise at nominal firing capacity, and (4) upper limit noise spectrum. 

Figure 3. Broadband spectrum of a conventional 2000 Globar IR source (short dashed line), and the spectrum of the NSLS synchrotron source (solid line) limited by an experimental throughput of 4.4><1 O 4 mm2sr. This is the etendue for a 1 pm by 1 pm sample measured with an infrared microscope. The measured, background limited Noise Equivalent Power (NEP) of a Mercury Cadmium Telluride (MCT) (long dashed line) detector is shown. This detector is operated at liquid nitrogen temperatures.

All of the flicker noises can be effectively eliminated by the use of double-beam optics in conjunction with a background correction system such as Zeeman splitting or a well-aligned (or wavelength-modulated) continuum source. Thus the ultimate limiting noise in atomic absorption is source shot noise, which can be reduced (relative to total source intensity or I, ) by increasing the source intensity, up to the point of optical saturation. 

Table III presents some examples of limiting noises in different atomic absorption determinations. These measurements are a compilation of information from several sources, but primarily from the work of Ingle [20,21] using a very simple, single-beam...

If the burner head Is rotated to reduce sensitivity, we find that the limiting noise Is no longer flame transmission flicker, but source shot noise, since the absorption path has been reduced by a factor of 20. Although the sensitivity Is decreased by a factor of 20, the detection limit Is decreased by only a factor of 10, since the flame transmission noise Is no longer limiting. Thus, referring back to a statement made earlier, sensitivity, or more correctly characteristic concentration [18] cannot be used as an accurate measure of detection limit in AAS. Unlike the case of SBR in emission, because of the complexity of noises In atomic absorption, a general and simple relationship cannot be derived to relate characteristic concentration and detection limit. 

If we look at a few cases In the literature, we see a wide variation In the reported limiting noises [2A-27]. Table IV lists noises and detection limits for several laser systems. A flashlamp-pumped system, because of the relatively low Intensity and wide temporal pulse width, about 1 /is. Is limited by flame emission shot and flicker, which are temporally continuous noises. The... 

TABLE IV. Limiting Noises and Detection Limits in Laser-Excited Atomic Fluorescence Spectrometry using the 296.7/373.5 nm Transition of Iron... 

The relatively low throughput of reverse optic/polychroma tor designs utilized In photodiode array UV absorbance detectors and an electronic readout noise Incurred In scanning an array currently limit noise In this detector to approximately 5x10 au. 

Gold in collected atmospheric particular matter was determined by digestion, LLE and two-color laser-induced fluorescence (LIF) with atomization in a graphite furnace (GF-LIF) absolute instrumental LOD 1 fg, corresponding to an atmospheric concentration of 50 fgm . These LOD values increase by a 20-fold factor due to blank limited noise . 

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