Absorption noise sources limiting noises

Ideal Performance and Cooling Requirements. Eree carriers can be excited by the thermal motion of the crystal lattice (phonons) as well as by photon absorption. These thermally excited carriers determine the magnitude of the dark current,/ and constitute a source of noise that defines the limit of the minimum radiation flux that can be detected. The dark carrier concentration is temperature dependent and decreases exponentially with reciprocal temperature at a rate that is determined by the magnitude of or E for intrinsic or extrinsic material, respectively. Therefore, usually it is necessary to operate infrared photon detectors at reduced temperatures to achieve high sensitivity. The smaller the value of E or E, the lower the temperature must be. 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

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. 

Laser Fluorescence Noise Sources. Finally, let us examine a technique with very complex noise characteristics, laser excited flame atomic fluorescence spectrometry (LEAFS). In this technique, not only are we dealing with a radiation source as well as an atomic vapor cell, as In atomic absorption, but the source Is pulsed with pulse widths of nanoseconds to microseconds, so that we must deal with very large Incident source photon fluxes which may result in optical saturation, and very small average signals from the atomic vapor cell at the detection limit [22]. Detection schemes involve gated amplifiers, which are synchronized to the laser pulse incident on the flame and which average the analyte fluorescence pulses [23]. 

Stability may not be as much of a problem as with a diode source. However, there are problems with this method as well. The range of tunability is limited by the absorption properties of the nonlinear crystal which generates the difference frequency. At present, tunability is limited to wavenumbers >2500 cm-1 and conversion efficiencies are low. Typical laser powers in the CH2 experiments (82) were 20 n W (compared to the power of the CO lasers, 10 mW-1 W). This produces a situation where IR detectors, particularly fast ones, may be close to or background noise limited. However, it is clear that more applications of this technique will appear in the future. 

Detecting single nano-objects in a far-held laser spot requires carefully optimized setups, discussed in several reviews and books [3, 19]. Hereafter, we assume ideal conditions, perfect optical elements (detectors, sources, hlters, etc.), to concentrate on the fundamental limitations to signal-to-noise ratio in optical single-molecule studies. We consider three main detection techniques applied to individual small absorbers, emitting or not photoluminescence direct absorption, huorescence and photothermal contrast. 

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