Acquisition parameters

A signal ratio of 1 0.68 in the lower spectrum and 1 1.04 in the upper one is found. One would not suppose this from the upper spectrum, while the height of the peaks may suppose a ratio of 1 3. But as the half width for PPhj is 2.3 and 7.8 Hz for PPhjO, the area under the peaks is more or less identical. The estimated Tj values are 5.05 s for PPhj and 1.73 s for PPhjO. Therefore, PPhj is saturated more and more with every scan in the lower spectrum, and the area under the peak is not quantitative any more. PPhjO wiU be fuUy relaxed after 12 s, but PPhj will need 35 s for full relaxation. The difference of the half width of the peaks is based on another relaxation parameter Tj. T2 is always smaller or equal to Tj. 

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A number of parameters have to be chosen when recording 2D NMR spectra (a) the pulse sequence to be used, which depends on the experiment required to be conducted, (b) the pulse lengths and the delays in the pulse sequence, (c) the spectral widths SW, and SW2 to be used for Fj and Fi, (d) the number of data points or time increments that define t, and t-i, (e) the number of transients for each value of t, (f) the relaxation delay between each set of pulses that allows an equilibrium state to be reached, and (g) the number of preparatory dummy transients (DS) per FID required for the establishment of the steady state for each FID. Table 3.1 summarizes some important acquisition parameters for 2D NMR experiments. 

Phase cycling The process of repeating a pulse sequence with identical acquisition parameters but with varying r.f. phase. This allows real NMR signals to add coherently whilst artifacts and unwanted NMR transitions cancel. 

One-dimensional proton NMR spectroscopy is the most straightforward method for process validation and development. It can be used as a limit test, i.e., to demonstrate that a particular analyte is below the detection limit. It can also be used to accurately quantify an analyte by comparing the NMR peak area from a test sample against a standard curve. To get accurate quantitation, it is important to keep the acquisition parameters and conditions constant for both standard and test samples. For example, the receiver gain, power level, and duration of all pulses must stay the same within an assay. In addition, the probe should remain tuned for all samples. 

The quantitation limit is the lowest concentration of analyte in the standard curve with a signal-to-noise ratio of at least 10. Typically, the quantitation limit for most small organic molecules is 10 pg/ml. If necessary, the quantitation limit can be lowered, as long as the acquisition parameters are adjusted to yield sufficient sensitivity. 

Because sensitivity depends on so many different experimental factors, NMR spectroscopists generally use the signal-to-noise ratio, SIN, as a figure of merit for sensitivity comparisons. For example, in a comparison between NMR probes or spectrometers from two vendors, the spectral SIN measured for a standard sample acquired with specified acquisition parameters and probe geometry would provide a direct indication of relative sensitivity. The SIN is calculated for an NMR experiment as the peak signal divided by the root mean square (RMS) noise, given by Equation 7.6, and is directly related to the performance of the radiofrequency coil [3,6]... 

However, a fast and simple mono-exponential on-line evaluation procedure included in the control software of an EEC relaxometer is not only possible but, in reality, is a must since it provides the operator with relaxation-rate data estimates essential for correct setting of acquisition parameters. The fact that the mono-exponential hypothesis may be inaccurate does not really change the fact that some kind of a preliminary estimate is essential for correct data acquisition. 

Change probe temperature, or other acquisition parameters. 

Samples are analysed using gas chromatography coupled with mass spectrometry (GC-MS). This instmment separates the organic hydrocarbons in the extract with a very high resolution fused silica capillary column prior to mass spectrometric detection. Acquisition parameters have been customized to detect the target list of 162 hydrocarbons with the sensitivity required to obtain 1 part-per-trillion reporting limits which represents an innovation in these analytics. 

Reynolds FR, Enriquez RG, Choosing the best pulse sequences, acquisition parameters, postacquisition processing strategies, probes for natural product structure by NMR spectroscopy, JNat Prod 65 221—244, 2002. 

Acquisition parameters—Parameters can be restricted to the ranges allowed by a particular scanner manufacturer, but often a common set of variables can be changed in order to optimize for desired scan endpoints (see Note 9). 

Figure 1 In vivo spectra acquired from the human occipital lobe at 4 T (top)and 7 T (bottom) using the same experimental setup and NMR acquisition parameters (TR=3 s and 128 signal averages). PE, phosphoethanolamine PC, phosphocholine Pi, inorganic phosphate GPE, glycerophosphoethanolamine GPC, glycerophosphocholine ...

Light-up, ion lens tuning, mass calibration, optimization of data acquisition parameters, determination of Cd in water by isotope dilution... 

To optimize the dwell time, set up a data acquisition procedure using the dwell times shown in Table B.4, aspirate the optimization solution and acquire count rate data for the ° Cd and Cd isotopes. The method for setting the data acquisition parameters will vary between instruments, but a similar procedure should be possible for all makes of instrument. Record the data in Table B.4, calculate the mean and RSD for the lo Cdri Cd ratio for each dwell time and hence determine the best precision. 

Table 3.1.7 Acquisition parameters of the GC-MS TIC method. GC Gas chromatograph, MS mass spectrometer ... 

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