Flame Flicker noise

For easily atomized elements, hydrogen flames are generally used because of their low flame-emission and flame-flicker noise. For the more refractory elements, however, a fuel-rich nitrous oxide-acetylene flame is required. 

Finally, zinc Is an example of an element whose absorption wavelength Is lower than 230 nm, the range where flame transmission noise dominates. Let us look a bit more closely at the case of zinc. The Instrument performance can be characterized using a precision plot, shown In Figure 2, where the relative standard deviation of concentration Is plotted on the vertical axis and concentration on the horizontal axis. The detection limit Is defined by the Intersection of the precision curve with the RSD which represents the criterion used to define detection, about 30% RSD for k - 3. Note that flame transmission flicker noise limits detection. 

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. 

The lamp, electronics and flicker of the flame may all contribute to short-term, irregular fluctuations in the signal, i.e. noise. The meter will follow this to some extent. It is usual to include some electronic damping to reduce the noise, i.e. to average it out to some extent. 

Minimal emission. In an a.c. system, emission from the burner will not produce a photometric error. However, high emission will contribute to the flicker of the output, because the noise current from the photomultiplier detector increases as a function of the total light faUing on it. A very bright flame will therefore tend to produce a fluctuating output. 

The design of the premix burner shown in Figure 10 also presents a number of other advantages and disadvantages as compared with the total consumption type. The flame is not very luminous, and flicker and turbulence are quite low, so that for many elements the flame contributes no apparent noise to the output (Figure 8). Furthermore, there is rather little dependence of absorption upon sample flow rate. This is of benefit in two ways. First, the length of sample capillary, and its depth of immersion in the solution, are not very critical, so that samples can be aspirated from any vessel. For total consumption burners, by contrast, Petri dishes or very small sample containers are often recommended. Second, viscosity interferences caused by variations in sample concentration are minimized, though not eliminated. In the Perkin-Elmer burner, when the sample flow rate is cut by a factor of 2, absorption is reduced by approximately 4%. 

Radiation source flicker Flame background emission noise... 

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... 

The indirect premixed nebulizer-burner combination is inherently quieter, more stable, and gives less trouble with chemical interferences. The residual flicker or flame noise may often be the principal obstacle to improving detection limits. The dropwise nature of the aerosol also causes statistical fluctuation of the signal, similar to the shot noise of photoelectric measurements, in both flame emission and atomic absorption. 

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