Flame noise

It has been shown that a high frequency of modulation of the electrodeless discharge lamp (e g. 10 kHz) is advantageous. This frequency is well away from the low frequency of flame noise. If the amplifier is locked-in to this high frequency via a reference signal, an optimum signal-to-noise ratio is achieved. 

As the signal, that is, the sensitivity, has been fully treated in Section 4.3.5, only the noise component of the above terms has to be discussed here. In addition, only the contribution of instrumental components to the noise will be treated, not the analytical part, such as imprecision in sample introduction, flame noise, or the inhomogeneity of the sample itself, which obviously are important contributions as well. 

For a CCD detector the absorbance noise is independent of the spectral bandwidth, but it depends on the number of measurement pixels sam and reference pixels ref in such a way that sam should be as small as possible and rel should be larger than sam. The other component that influences the noise is the intensity I of the radiation source, in that the absorbance noise is inversely proportional to the square root of I [12]. As the intensity of the radiation source in CS AAS is in some cases up to two orders of magnitude higher than that of a typical LS for conventional AAS, an improvement in the SNR and limits of detection (LoD) by factors of 3-10 could be expected, unless other factors, such as flame noise, become dominant. The values given in Table 4.1 show that this expectation has in fact been realized for the majority of the elements. 

Thomas, A., and Williams, G. T. "Flame Noise Sound Emission from Spark-Ignited Bubbles of Gombustion Gas." Proceedings of the Royal Society, A 294 (1966) 449. 

The diameter of the droplets produced by a pneumatic nebulizer varies from < 5 ixm to about 25 xm. The spray chamber allows droplets to reach the burner which can be vaporized and atomized in the flame. If the spray chamber prevents small droplets (diameter of about 10 tm or less) from entering the flame, sensitivity will be decreased. On the other hand, if large droplets (>10/x-m) reach the flame, the flame noise will increase and the temperature will decrease. From the total mass of the sample nebulized, the maximum useful amount of droplets is about 10%, which gives the limit for the maximum attainable efficiency of the nebulizer. However, the nebuliza-tion efficiency can be improved in various ways by altering the droplet size distribution. A bead or bar placed close to the orifice of the nebulizer, or a counter flow nebulizer can be used for this purpose (Figure 37). 

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. 

Several instrument manufacturers supply flame photometers designed specifically for the determination of sodium, potassium, lithium, and sometimes calcium in blood serum, urine, and other biological fluids. Single-channel and multichannel (two to four channels) instruments are available for these determinations. In the multichannel instruments, each channel can be used to determine a separate element without an internal standard, or one of the channels can be reserved for an internal standard such as lithium. The ratios of the signals from the other channels to the signal of the lithium channel are then taken to compensate for flame noise and noise from fluctuations in reagent flow rate. Flame photometers such as these have been coupled with flow injection systems to automate the sample-introduction process (see Section 33B-3). Typical precisions for flow-injection-analysis-based flame photometric determinations of lithium, sodium, and potassium in serum are on the order of a few percent or less. Automated flow injection procedures require l/KIO the amount of sample and 1/10 the time of batch procedures. -... 

KHn hood (flame noise) Hood to be sealed as effectively as possible. Annular cover to close gap at outlet end seal. Sound insulation of fans near the kiln outlet and firing platform, just as for primary air fan. 

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