Laser flicker noise

Because process mixtures are complex, specialized detectors may substitute for separation efficiency. One specialized detector is the array amperometric detector, which allows selective detection of electrochemically active compounds.23 Electrochemical array detectors are discussed in greater detail in Chapter 5. Many pharmaceutical compounds are chiral, so a detector capable of determining optical purity would be extremely useful in monitoring synthetic reactions. A double-beam circular dichroism detector using a laser as the source was used for the selective detection of chiral cobalt compounds.24 The double-beam, single-source construction reduces the limitations of flicker noise. Chemiluminescence of an ozonized mixture was used as the principle for a sulfur-selective detector used to analyze pesticides, proteins, and blood thiols from rat plasma.25 Chemiluminescence using bis (2,4, 6-trichlorophenyl) oxalate was used for the selective detection of catalytically reduced nitrated polycyclic aromatic hydrocarbons from diesel exhaust.26... 

Flicker noise is commonly defined classically for absorption and emission spectroscopy (see Ref. 1, pp. 144-145) but also applies to Raman spectroscopy. Variation in laser intensity at both low and high frequencies will... 

Flicker noise could indeed contribute, but it is difficult to address theoretically and may vary significantly for different spectrometer configurations and lasers. For a well-designed experiment, flicker noise can usually be reduced to a negligible level. 

For conventional analysis by ICP or DCP, liquid samples are used, which are either easily prepared or commercially available. Interference problems are reduced to a minimum if the cahbration solutions are matched to the samples with respect to their content of acids and easily ionisable elements (see above). Calibration curves obtained with sparks, arcs, and laser ablation systems are usually curved so that 8—15 calibration samples or more are needed to define a suitable calibration. In the case of liquid analysis by DCP and ICP, fewer cahbration samples can be used due to the better linearity and dynamic range and absence of selfabsorption effects. With the introduction of hquids, the spray chamber is the major source of flicker noise due to aerosol formation and transport. While shot noise can easily be compensated by longer integration times, the flicker noise is of multiplicative nature so that any element can be used as an internal standard provided that a true simultaneous measurement of the analyte and internal standard line intensity is possible. 

If we look at a few cases In the literature, we see a wide variation In the reported limiting noises [2A-27]. Table IV lists noises and detection limits for several laser systems. A flashlamp-pumped system, because of the relatively low Intensity and wide temporal pulse width, about 1 /is. Is limited by flame emission shot and flicker, which are temporally continuous noises. The... 

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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