Spectrometer signal gain

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. 

It is well known that UV detectors used in liquid chromatographs are concentration-sensitive devices. Injection of the same mass of a particular compound onto two columns with identical plate number and length but different inner diameters, will result in a higher response from the column with the smaller inner diameter. The gain in the signal is inversely proportional to the square of the ratio of the inner diameters of the two columns. The situation is different for a mass spectrometer, which is a mass-flow sensitive detector. Under constant flow conditions,... 

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. 

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 problem is quite different with pulse spectrometers, for which the prospects are good free precession and echo signals are rather long as compared to those of protons in solids and they can readily be fed into a standard digital memory oscilloscope with an access time of about 40 microseconds. The gain is considerable 15> 161 provided good sample temperature and spectrometer frequency stabilizations are achieved. 

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