Signal-limited detection, ideal

The ideal of signal-limited detection, in which the measurement accuracy is independent of the size and fluctuations of the background, has not yet been achieved in any spectroscopic measurement of ambient HO. However, because the signal-limited condition may be achieved in the future, this case is included in Table V. 

Optical heterodyne detection is based on interferometric mixing of the weak signal beam to be detected with a frequency offset local oscillator beam on the surface of the detector. It can be shown that, in principal, ideal (quantum limited) detection can be achieved however, it requires a complex optoelectronic system with rigorous mechanical tolerances. 

In the limit of ideal detection, the 2D 3PPE signal is obtained by a double Fourier transformation of the nonlinear polarization Pypif, t, 7) with respect to the coherence time X (delay between the first two pulses) and the detection time t. 

While both populations are equivalent in principle, being related by a unitary transformation, one of them may be more clo.sely related to experiment than the other. For example, if there are dipole. selection rules forbidding the optical transition to or from a subset of the interacting electronic states, these selection rules are usually obeyed to a much larger extent in the diabatic basis than in the adiabatic ba.si.s. Then the diabatic electronic populations are monitored via the intensities of spontaneous and induced emission (the adiabatic populations may be more relevant if the optical transition takes place within the interacting manifold). More specifically, in the limit of ideally short pump and probe pulses the time-resolved pump-probe signal as a function of the delay time has been shown to be proportional to the diabatic population, equation (51). For the more realistic case of finite pulse durations the situation is more complex. In the present article we leave these problems aside and focus on the purely intramolecular aspects of the vibronic dynamics. The various aspects associated with their detection in real time have been surveyed in a recent review article. ... 

Spike recoveries for samples are used to detect systematic errors due to the sample matrix or the stability of the sample after its collection. Ideally, samples should be spiked in the field at a concentration between 1 and 10 times the expected concentration of the analyte or 5 to 50 times the method s detection limit, whichever is larger. If the recovery for a field spike is unacceptable, then a sample is spiked in the laboratory and analyzed immediately. If the recovery for the laboratory spike is acceptable, then the poor recovery for the field spike may be due to the sample s deterioration during storage. When the recovery for the laboratory spike also is unacceptable, the most probable cause is a matrix-dependent relationship between the analytical signal and the concentration of the analyte. In this case the samples should be analyzed by the method of standard additions. Typical limits for acceptable spike recoveries for the analysis of waters and wastewaters are shown in Table 15.1. ... 

Optical fiber detectors (OFD) are devices that measure electromagnetic radiation transmitted through optical fibers to produce a quantitative signal in response to the chemical or biochemical recognition of a specific analyte. Ideally, an OFD should produce a specific and accurate measurement, continuously and reversibly, of the presence of a particular molecular species in a given sample medium. Additionally, OFD should pro vide maximum sensitivity and minimal interferences fromsuperfluous ions or molecules to obtain low detection limits. Other attractive features include the miniaturization of the fiber s tip to accommodate single-cell analysis and portable instrumentation to allow in situ analysis. 

Greenfield ef. ai.l l) observed a reduction of signal intensity that correlates with sample intake effects from the modified solution viscosity and/or surface tension of mineral acids. This, coupled with peristaltic pumping of solutions into the nebulizer, considerably reduces physical interferences. Increased salt concentration also has an effect on solution physical properties. In the experience of these authors, the high levels of salt in the matrix also increases the noise from the nebulizer system. This degradation of nebulizer performance, which is not necessarily accompanied by a proportional reduction in sensitivity, is the cause of the observed deterioration of detection limits in real samples as opposed to ideal solutions. 

The principles of diode-array detection in HPLC and their application and limitations to peak purity are described in the literature [25-27], Examples of pure and impure HPLC peaks are shown in Figure 1. While the chromatographic signal indicates no impurities in either peak, the spectral evaluation identifies the peak on the left as impure. The level of impurities that can be detected with this method depends on the spectral difference, on the detector s performance, and on the software algorithm. Under ideal conditions, peak impurities of 0.05-0.1% can be detected. 

Typically, the relationship in the assay between the detection signal and the concentration of the mAb is nonlinear. A simple regression analysis is, therefore, not possible. Usually, a third-to-fifth degree polynomial is used to relate the detection signal to the concentration. Because of this relationship, a typical sample dilution is not possible and careful evaluations are necessary if the concentration of the antibody is above the upper limit of quantification of the assay. As a consequence, the assay should ideally cover the whole concentration range of all samples measured. 

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