Signal-to-Noise Ratio, Precision and Limit of Detection

5 Signal-to-Noise Ratio, Precision and Limit of Detection 

As the signal , i.e. the sensitivity has been fully treated in Section 4.4, 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 the imprecision of sample introduction, the flame noise, or the inhomogeneity of the sample itself, which obviously also make important contributions. 

The noise contribution of the detector has already been treated in Section 2.2.2, where it has been shown that for a CCD the absorbance noise is independent of the spechal bandwidth (Equation 2.31), but it depends on the number of measurement pixels n-sam and reference pixels nref in such a way that ngam should be as small as possible and n-ref should be larger than nsa. The other component that influences the noise, according to Equation 2.31), is the radiance L of the radiation source, in such a way that the minimal detectable volume concentration of absorbing atoms (Vmin is inversely proportional to the square root of L. As the intensity of the source in CS AAS is some 1-2 orders of magnitude higher than that of typical line sources for conventional AAS, an improvement in the SNR and LOD by factors of 3 -10 could be expected, unless other factors, such as the flame noise 

Besides the intensity of the radiation source, its stability obviously also plays an important role, and arcs are notorious for their instability. However, as this instability results in a wavelength-independent, white noise, which causes an identical noise-over-time distribution on all the pixels of the CCD array, it can easily be cancelled out with the use of reference pixels, as has already been discussed in Section 4.2. As a result of the almost perfect correlation of the spectral intensity values within the small range of observation, the minimum detectable absorbance signal is determined only by statistical variations in the intensity between the neighboring pixels (shot-noise). This means that an increase in illumination time or in radiation intensity by a factor of 4 will reduce the absorbance noise by a factor of 2 (square root of 4). 

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