Signal enhancement

During normal operation, the entire ISS spectrum, covering all elements, is scanned in about 1 second. A number of these scans are then added for signal enhancement and to control the predetermined depth to which sputtering is... 

The problems involved in quantitative analysis using NMR spectroscopy, have been discussed by several authors and it is evident that it still causes a lot of problems as especially pointed out by Hays55 in his excellent review on the subject. Thus in liquid state NMR spectroscopy the quantitative estimation of atoms and groups involves the use of normal analytical method. In the case of solid state NMR spectroscopy, however, the application of the cross-polarization technique results in signal enhancements and allows repetition rates faster than those allowed by the carbon C-13 Tl. Therefore, the distortion of relative spectral intensities must always be considered a possibility, and hence quantitative spectra will not always be obtained. 

There are two notable features of the quantitative performance of this type of interface. It has been found that non-linear responses are often obtained at low analyte concentrations. This has been attributed to the formation of smaller particles than at higher concentrations and their more easy removal by the jet separator. Signal enhancement has been observed due to the presence of (a) coeluting compounds (including any isotopically labelled internal standard that may be used), and (b) mobile-phase additives such as ammonium acetate. It has been suggested that ion-molecule aggregates are formed and these cause larger particles to be produced in the desolvation chamber. Such particles are transferred to the mass spectrometer more efficiently. It was found, however, that the particle size distribution after addition of ammonium acetate, when enhancement was observed, was little different to that in the absence of ammonium acetate when no enhancement was observed. 

Signal enhancement The increase in analyte signal intensity brought about by the presence of extraneous materials in the sample. 

The nuclear Overhauser effect resulting from the broad-band decoupling during the decoupled INEPT experiment also contributes to the signal enhancement of the C lines. 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

In this chapter we discuss techniques both for signal enhancement and signal restoration. Techniques to model or to reconstruct the deterministic part of a digital signal in the presence of noise are discussed in Chapter 41. 

As said before, there are two main applications of Fourier transforms the enhancement of signals and the restoration of the deterministic part of a signal. Signal enhancement is an operation for the reduction of the noise leading to an improved signal-to-noise ratio. By signal restoration deformations of the signal introduced by imperfections in the measurement device are corrected. These two operations can be executed in both domains, the time and frequency domain. 

Ideally, any procedure for signal enhancement should be preceded by a characterization of the noise and the deterministic part of the signal. Spectrum (a) in Fig. 40.18 is the power spectrum of white noise which contains all frequencies with approximately the same power. Examples of white noise are shot noise in photomultiplier tubes and thermal noise occurring in resistors. In spectrum (b), the power (and thus the magnitude of the Fourier coefficients) is inversely proportional to the frequency (amplitude 1/v). This type of noise is often called 1//... 

We first discuss signal enhancement in the time domain, which does not require a transform to the frequency domain. It is noted that all discrete signals should be sampled at uniform intervals. 

Triethylamine (TEA) is sometimes used as an additive for signal enhancement. However, in the positive ESI mode, TEA readily ionizes to give an intense [M + H]+ ion at miz 102. This then suppresses the ionization of the less basic compounds in the positive ESI mode. In the negative mode, TEA can enhance ionization for certain compounds because of its basic properties. 

HPLC/MS and HPLC/MS/MS analyses are susceptible to matrix effects, either signal enhancement or suppression, and are often encountered when the cleanup process is not sufficient. To assess whether matrix effects influence the recovery of analytes, a post-extraction fortified sample (fortified extract of control sample that is purified and prepared in the same manner as with the other samples) should be included in each analytical set. The response of the post-extraction fortified sample is assessed against that of standards and samples. Matrix effects can be reduced or corrected for by dilution of samples, additional cleanup, or using calibration standards in the sample matrix for quantitation. 

Fig. 5.5.13 Spatially resolved 13C DEPT pulse sequence. This provides signal enhancement for 13C observation without need for using isotropically enriched materials. The signal is acquired under conditions of ]H decoupling.

Signal enhancement in liquids (NOE) and solids (H-C cross-polarisation). 

As shown later, the same signal enhancement of C-4 is seen when... 

A) Recorded with 256 increments, 4 transients, and a recovery delay of 1 s. (B) Recorded with 256 increments, 8 transients, and a recovery delay of 0.2 s. The flip angle a was set to 90°. The typical F2 rows (at the 13C chemical shift of C-18, indicated by the horizontal arrows and depicted on the top of the spectra) show the signal enhancements. 

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