Spectrometers stability

NOE experiments typically require significant spectrometer time because the enhancements being sought are rather small. The NOE difference experiment represents a stringent test of both short- and long-term spectrometer stability, since changes in rf phase or frequency or in magnetic field contribute to... 

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. 

A typical x-ray spectrometer stability test requires repeated measurements of a single intensity without deliberate changes in any other variable. This is basically a statistical time series problem where the independent variable X is time. Generally, the intensity is counted for a fixed time increment and sufficient counts are accumulated for each measurement so that the counting variance will be significantly lower than any other source of variance. It has been shown that for photon counting with a stable source intensity, the population variance is given by... 

Early attempts to define phase contact by use of tritiated water and tritium-labeled organics were unsuccessful due to a variety of problems ranging from variations between vials to sample matrix, absolute efficiency measurement problems cind lack of recognition that phase contact variations Ccin occur over surprizingly small concentration ranges. In the method presented here, all of these common problems, with the exception of statistics and 48 hours spectrometer stability, are eliminated. 

The precision of the method described here depends primarily on counting statistics and spectrometer stability. 

Assuming perfect spectrometer stability, which is monitored by the values for the 1% concentration samples, the most likely source of error is in counting the samples for the second round before they are completely equilibrated. 

CRAMPS is an extremely demanding technique requiring a spatially homogeneous irradiation field and a very high degree of spectrometer stability, both in timing and phase. For a recent survey of the specific experimental requirements and some of the measures to overcome them see the article by Maciel et al. [130]. In common with other solid-state techniques, development of pulse sequences that are more tolerant of spectrometer imperfections [131]... 

A considerable amount of time is necessary to reach the point at which sample analyses can commence, and it is essential that the stability and reliability of the mass spectrometer be high to ensure maximum sample throughput during the limited time available between calibration checta. 

Aliphatic amines undergo a characteristic a cleavage in the mass spectrometer, similar to that observed for alcohols. A C-C bond nearest the nitrogen atom is broken, yielding an alkyl radical and a resonance-stabilized, nitrogen-containing cation. 

Markovnikov s rule. 191-193 alkene additions and, 191-193 alkyne additions and. 263 carbocation stability and. 192-193 Hammond postulate and, 198-199 hydroboration and. 224-225 oxymercu ration and, 222 Mass number (A), 4 Mass spectrometer, double-focusing, 411... 

Fe which have full width 2r at 0.2 mm s . Other isotopes are less demanding, e.g., Au, for which the lines are ten times wider. Most spectrometers are equipped with electromechanical Mossbauer velocity transducers of the loudspeaker type. This technique is suitable for velocity variations ranging from less than 1 mm s full scale up to several cm s and covers the whole reach of hyperfine splitting for most of the common isotopes. Kalvius, Kankeleit, Cranshaw, and others [1-5] have been pioneers in the field, who laid foundations for the development of high-precision drives with feedback amplifiers for proper linear velocity scales with high stability and low hum. Other techniques for Doppler modulation have been developed for isotopes with extremely narrow hyperfine lines, e.g., Zn. For such isotopes, piezoelectric transducers are mostly used [6, 7], more details of which are found in Sect. 7.2.1. 

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