Background noise, optical

Liquid scintillation counters are highly efficient for low CL intensities and consist of two photon-counting channels provided with a variable discriminator. The sample is placed between these two detectors to ensure a high optical efficiency. The discriminator is adjusted to allow photon impacts to be transmitted and small background noise pulses to be rejected. As disadvantages they suffer from saturation errors and provide nonlinear relationships between the CL intensity and the total counts. 

For good sensitivity, proper care of the PM tube, its optics, and its housing is essential. The background noise is the primary limiting factor in PM tube... 

In contrast, liquid chromatography lends itself to chiral separations and there are two basic procedures for separating optically active solutes. Firstly, reversed phase chromatography can be employed and a chiral substance can be added at low concentrations in the mobile phase. The chiral additive will be absorbed onto the surface of the reversed phase and act as an adsorbed stationary phase having chiral activity. This approach makes chiral detection more difficult, as it provides a background of optical activity in the mobile phase that will be many orders greater than that from the chirally active solutes. This is inevitably accompanied by a high noise level and consequent poor sensitivity. The second approach is to employ specific chirally active materials that are bonded to a silica or polymer surface to provide chirally specific interactions with the solutes. 

The sensitivity of photon detectors and the level of their background noise are paramount for dynamic analysis of single molecules. We have analyzed a new development of APDs produced in the CMOS form [63]. For a 2 x 2 APD array, the detected photons have been measured in all four detectors using illumination by diffractive optics ([64]) and simultaneous analysis with four correlators [65] (Fig. 4.28). 

The optical background noise was also measured at 3000 m depth in Capo Passero. Data collected in Spring 2002 and 2003, for several months, show that optical background induces on 10 PMTs (0.5 s.p.e.) a constant rate of 20 4- 30 kHz (compatible with the one expected from 40K decay), with negligible contribution of bioluminescence bursts. These results were confirmed by biological analysis that show, at depth> 2500, extremely small concentration of dissolved bioluminescent organisms [39]. 

Fluorescence detection, another common HPLC detection method, has also been used in capillary SFC (2,12,16). Fluorescence detection offers several advantages. First, sensitivity is much greater than in UV-absorption, on the order of 500 fg s with a fiber optics arrangement (16). The very low background noise in... 

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