Signal averaging, effect

The main advantage of multivariate calibration based on CLS with respect to univariate calibration is that CLS does not require selective measurements. Selectivity is obtained mathematically by solving a system of equations, without the requirement for chemical or instrumental separations that are so often needed in univariate calibration. In addition, the model can use a large number of sensors to obtain a signal-averaging effect [4], which is beneficial for the precision of the predicted concentration, making it less susceptible to the noise in the data. Finally, for the case of spectroscopic data, the Lambert Bouguer Beer s law provides a sound foundation for the predictive model. 

In the multichannel scanning mode (fig. 19c), the mass analyzer scans continuously over the mass region of interest. The scans are done repetitively (typically 100-200 times in 1-2 min) and individual scans are added for a signal averaging effect. With either scan mode, at any instant in time, only ions of one miz value are transmitted through the rods the other ions are lost. Also, the widths of the peaks shown in fig. 19 are realistic for a quadrupole mass analyzer. Unit mass... 

Effect of signal averaging on a spectrum s signal-to-noise ratio (a) spectrum for a single scan ... 

Figure 20-30 Effect of signal averaging on a simulated noisy spectrum. Labels refer to number of scans averaged. [From R. Q. Thompson, Experiments in Software Data Handling," J. Chem. 

Figure 8.2.16 COSY spectra acquired with the four-coil probe, where the compounds and concentrations were the same as those of the one-dimensional spectra. Data acquisition parameters spectral width, 2000 Hz data matrix, 512 x 128 (complex) 16 signal averages delay between successive coil excitations, 400 ms effective recycle delay for each sample, ca. 1.7 s. Data were processed by using shifted sine-bell multiplication in both dimensions and displayed in magnitude mode. Reprinted with permission from Li, Y., Walters, A., Malaway, P., Sweedlar, J. V. and Webb, A. G., Anal. Chem., 71, 4815M820 (1999). Copyright (1999) American Chemical Society...

Figure 14. The effect of signal averaging on the SNR of Au 4f spectra (obtained with a McPherson ESC A 36 spectrometer) (21). Spectra were obtained with an MCP intensifier optically coupled to the SIT (OMA) detector. A full (SIT) target scan is completed within 0.032 sec.

The equilibrium state is characterized by a complete lack of coherence (random phase), a slight excess of population in the a state (M2 + 5), and a deficit in the p state (N/2 - 5). Anything that perturbs this equilibrium (e.g., an RF pulse) will be followed immediately by a process of relaxation back to the equilibrium state that can take as long as seconds to reestablish. Relaxation is extremely important in NMR because it not only determines how long we have to wait to repeat the data acquisition for signal averaging, but it also determines how quickly the FID decays and how narrow our NMR fines will be in the spectrum. Relaxation is also the basis of the nuclear Overhauser effect (NOE), which can be used to measure distances between nuclei one of the most important pieces of molecular information we can obtain from NMR. 

Although the peak power of the pump laser must be high, the power of the probe laser should be kept low to avoid nonlinear effects (multiphoton absorption, stimulated Raman scattering see Section 3.9.2) and dielectric breakdown (ionization of molecules) that damage the sample. Thus, signal averaging of many pulses (high repetition rate) is made to obtain acceptable S/N ratios. Multichannel detectors such as an intensified silicon photoiode... 

The application of refractive index and differential viscometer detection in SEC has been discussed by a number of authors [66-68]. Lew et al. presented the quantitative analysis of polyolefins by high-temperature SEC and dual refractive index-viscosity detection [69]. They applied a systematic approach for multidetector operation, assessed the effect of branching on the SEC calibration curve, and used a signal averaging procedure to better define intrinsic viscosity as a function of retention volume. The combination of SEC with refractive index, UV, and viscosity detectors was used to determine molar mass and functionality of polytetrahydrofuran simultaneously [70]. Long chain branching in EPDM copolymers by SEC-viscometry was analyzed by Chiantore et al. [71]. 

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