Detectors nonlinear responses

In addition to ELS, charged aerosol (CA) or corona detector has more recently been introduced as a very promising HPLC detection system [105] while the sensitivity of the two systems is quite close, CA detector offers the advantage of a nearly linear response factor, particularly crucial for the assessment of enantiomeric purities, whereas ELS provides a nonlinear response at very low or high levels of analytes, resulting from several light scattering mechanisms and particle size distribution. 

Linear Range The concentration range where increasing concentrations of an analyte have a proportional increase in LC-MS response. Overall QqQ-type mass spectrometers (triple quadmpoles, Q-TRAPS) are superior in terms of linearity. Most common causes for nonlinear response include MS detector saturation, dimmer/adduct formation, API droplet/vapor saturation at high concentrations, and space charge effects. 

In addition, highly intense laser light sources with an energy output greater than 5 to 10 mW that are used for flow cytometry, fluorescence microscopy, and laser-induced fluorescence measurements will rapidly photodecompose some fluorescence analytes. This decomposition introduces nonlinear response curves and loss of the majority of the sample fluorescence. Fluorescence-based assays for analytes at ultralow concentrations require optimization of laser intensity and the use of a sensitive detector. 

Whatever the nonlinear process involved, the metal/dielectric nanocomposites have been often inquired into for their optical limiting (OL) properties [120, 123, 124, 140-150], owing to the important stake that such a functionality represents for civil and military applications in human eye or detector protection. Indeed, as for telecom applications, metal nanoparticles present the advantages of both intense and fast nonlinear response. 

Because the luminol detection system is also sensitive to NO2, chemical amplification methods have been attempted to further decrease detection limits for PANs below the pptv range for trace-level measurements. With this approach, the PANs are thermally decomposed to NO2 in the presence of large amounts of NO (6 ppm) and CO (8%). Thermal decomposition of the PANs yields peroxy radicals which initiate a free-radical chain oxidation of NO to NO2, producing several NO2 molecules (approximately 180 (20) for each PAN decomposed. This technique has been used as a gas chromatography detector to achieve ultratrace detection limits without sample preconcentration. The detector exhibits a slightly nonlinear response relative to conventional BCD, attributed to the nonlinear response of the luminol reaction in the presence of NO at 6 ppm. 

Instrumental Factors. Unsatisfactory performance of an instrument may be caused by fluctuations in the power-supply voltage, an unstable light-source, or a nonlinear response of the detector-amplifier system. A double-beam system helps to minimize deviations due to these factors. In addition, the following instrumental sources of possible deviations should be understood ... 

The flat-fielding correction is generally based on an assumption that the detector responds linearly to radiation exposure. If some or all of the dels have nonlinear response, then a two-constant (slope and intercept) correction will only operate properly under exactiy the exposure conditions used to obtain the calibration constants. Under other signal levels, different nonlinearities of the del responses will result in image nonuniformities (Fig. 2.7d). 

The nonlinear crystal can be omitted, if the detector itself has a nonlinear response. This is, for instance, the case for a semiconductor detector with a band gap A >hv, where only two-photon absorption contributes to the signal. 

Figure 8. An example of a gas detector s response ou ut. The solid line indicates a hnear response output characteristic. The dotted line indicates a nonlinear response output characteristic.

The basic behavior of the electrochemical detector is shown in Fig. 9. Here the current response as a function of voltage applied is displayed. The rather nonlinear response is typical of electrochemical systems. Throughout the most of the range of applied voltages, positive or... 

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