Receiver gain

The receiver gain cannot necessarily be set in 2D experiments in the manner described for ID experiments in Section 2-4g. For gradient, H-delected, correlation sequences, 

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The spectrometer supports phase cycling, asynchronous sequence implementation, and parameter-array experiments. Thus, most standard solid-state NMR experiments are feasible, including CPMAS, multiple-pulse H decoupling such as TPPM, 2D experiments, multiple-quantum NMR, and so on. In addition, the focus of development is on its extension of, or modification to, the hardware and/or the software, in the spirit of enabling the users to put their own new ideas into practice. In this paper, several examples of such have been described. They include the compact NMR and MRI systems, active compensation of RF pulse transients, implementation of a network analyzer, dynamic receiver-gain increment,31 and so on. 

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. 

Backward LP (Fig. 5.21) is usually applied to repair the first few points of an FID, distorted by some spectrometer perturbation or a mis-set acquisition parameter, e.g. incorrect receiver gain. Backward LP is also used to reconstruct an FID back to t=0 in those cases where the start of data acquisition has been delayed, e.g. to exclude unwanted spectrometer noise such as the signals from acoustic ringing, and the first few data points are missing. In this case backward LP cancels or at least suppresses the corresponding spectral artefacts such as baseline roll etc. 

Adjust the receiver gain by using the RGA command (for a Bruker instrument). 

Figure 1.13 Increase in receiver gain without solvent signal suppression (a) free induction decay (b) resulting NMR spectrum...

Suppression is mainly used for water in aqueous samples. Such a technique does not require any complicated adjustments of the suppression parameters. In this case, the spectrometer is locked on to the D2O. The signal of the HDO/H2O is found at nearly exactly 4.7 ppm. Adjustment of the receiver gain (RG) is therefore all that is needed before the experiment can be started. 

The aliquots of the solution-state chemical synthesis samples were directly injected into a standard HPLC-NMR probe by using a robotic liquid handler. The NMR software was used to automatically find and suppress the intense NMR signals from any non-deuterated solvents used, typically using the WET sequence [5]. Unlike the characterisation of impurities in organic compounds (see the next section) or drug metabolites (see the appropriate chapter in this volume) where the proportions of the analytes can be very different, combinatorial chemistry samples tend to be all of similar quantity and this simplifies the analysis in that it is not usually necessary to worry overly about carry-over of material from sample to sample, nor it is necessary to readjust the NMR spectrometer receiver gain after every sample, thus saving considerable machine time. 

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