Continuous wave spectrometer sensitivity

As far as apparatus is concerned, chlorine resonances are more easily detected with superregenerative spectrometers in which induction phenomena yield enhanced signals. Continuous wave spectrometers, which do not have this advantage, have their effective sensitivity limited by the long relaxation times encountered in many of the compounds studied. 

One of the main advantages of FT spectrometers is that, since the FID is in digital form, we can repeat the excitation/detection process a number of times and all the resulting scans can be added and the FT performed on the resultant FID. In this way, we can improve the signal-to-noise ratio and can detect nuclei which are not very abundant (e.g. C) or have low sensitivity to NMR (see Section 4.2). These nuclei could not have been detected on the older continuous wave instruments, as the spectrum was the result of a single scan, obtained as one of the frequency or magnetic held were varied while keeping the other constant. 

Proton NMR was the first type of NMR spectroscopy to become available to the researcher. The high natural abundance of the NMR-active H-nucleus (-100%) and its relatively high sensitivity allowed early low-field continuous-wave (CW) spectrometers, with little in the way of signal-averaging capabilities, to successfully produce spectra from milligram quantities of small organic molecules. Such instruments,... 

Presaturation Traditionally, the solvent signal is irradiated for a period of time with a continuous wave rf field Can be easily set up Useful for eliminating single solvent signal. Extremely sensitive to spectrometer stability and shimming. Not easy to suppress multiple solvent peaks simultaneously. Also, suppresses exchange peaks Suppresses NMR signals from the compound that overlaps with the solvent. 

Because C NMR is less sensitive than H NMR, special techniques are needed to obtain a spectrum. If we simply operate the spectrometer in a normal (called continuous wave or CW) manner, the desired signals are very weak and become lost in the noise. When many spectra are averaged, however, the random noise tends to cancel while the desired signals are reinforced. If several spectra are taken and stored in a computer, they can be averaged and the accumulated spectrum plotted by the computer. Since the NMR technique is much less sensitive than the H NMR technique, hundreds of spectra are commonly averaged to produce a usable result. Several minutes are required to scan each CW spectrum, and this averaging procedure is long and tedious. Fortunately, there is a better way. 

Experimental techniques available to investigate sulfoxyl radicals have not changed significantly since the publication of the aforementioned reviews. ESR spectroscopy still remains the most informative and probably the most sensitive detection method. Unfortunately, a low time resolution of continuous wave (CW) ESR spectrometers with 100 kHz field modulation makes them incapable of providing significant kinetic information except for, perhaps, recombination rates. More advanced time resolved ESR techniques still have a limited application in kinetic studies and we are not aware of such techniques having been employed to study the reactions of sulfoxyl radicals. 

In continuous-wave electron spin resonance, extended multifrequency capabilities from 0.3 to over 100 GHz have been developed based on loop-gap and other types of resonators. The lower frequencies seem particularly useful for some biological applications. Very high frequency spectrometers have also been developed up to 700 GHz, with commercial instrumentation available at 95 GHz. The higher frequencies are based on Fabry-Perot resonators and give superior g-anisotropy resolution, suppression of second-order effects, and better sensitivity for small samples. 

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