Modulated detector, small-volume

In order to utilize fully the column efficiency, the dead volume from the injection point to the detector should be as small as possible. Unfortunately, some manufacturers do not consider this factor in the construction of connections between different modules. The coupling of subunits of different manufacturers is often difficult since different kinds of connection unions are used. 

An array of commercial instruments is also available to perform conductivity detection. Among others, it is worth mentioning the ICS-5000-1- (Dionex), model 432 (Waters), model 650 (Chomtech), and a series of detectors and modules from eDAQ (http //www.edaq.com). Most of these systems feature small cell volumes (<0.1 pi) and large dynamic ranges (up to 15 000 pS/cm). 

As the average of the intensity of a modulated light beam is nonzero, the heat energy in the illuminated volume will rise continuously. Therefore, the temperature will slowly increase and the density will decrease until the heat deposition rate is equal to the loss rate due to heat conduction. This process is also governed by the heat-diffusion equation. For a closed cell the average density is constant therefore a pressure rise will occur. This DC component of the released heat power density changes the thermodynamic state of the material, in particular in very small PA detectors ( cm ). 

With modern slow-scan CCD detectors, read-out noise in a well-designed ROA spectrometer is negligible in comparison with photon shot noise. Yet, flicker, or 1/f noise, clearly plays its role. Unfortunately, to date no serious attempt to identify and reduce its influence has been made. What is well known since the first recording of entire ROA spectra in the mid 1970 s is that dust can have a deleterious effect. In particular in microcapillaries, where sample volumes are small and convection essentially absent, single grains of dust sometimes appear to get trapped and gently oscillate in the modulated focused laser beam. Though not understood at that time, it probably was one of the first observations of the optical tweezer effect. Thus, careful sample preparation is important. 

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