Decoupler, calibration

All P.M.R. spectra were measured with a Varian HA 100 spectrometer operating in the frequency-sweep mode with tetramethylsilane as the reference for the internal lock. The double and triple resonance experiments were performed using a Hewlett Packard 200 CD audio-oscillator and a modified Hewlett Packard 200 AB audio-oscillator (vide infra). Spectra were measured using whichever sweep width was required to ensure adequate resolution of the multiplets under investigation, generally 250 or 100 Hz, and sweep rates were selected as necessary. Extensive use was made of the Difference 1 and Difference 2 calibration modes of the instrument, both for the decoupling experiments and for the calibration of normal spectra. 

Two identical polysilicon temperature sensors with a nominal resistance value of 10 kQ are located in the membrane center. One resistor is connected to the temperature controller, the other sensor is totally decoupled from the circuitry. This second temperature sensor can be directly accessed via bond pads in a four-point configuration. It enables an accurate calibration and a verification of the temperature controller... 

Fig. 11. Single-spin double-resonance experiment for the calibration of irradiation power. The proton line in chloroform is irradiated with the homonuclear decoupler. The strong central feature is at the irradiation frequency, and the separation of the two satellites is...

Solid state 13C NMR measurements were performed on a JEOL GSX-270 spectrometer operating at 67.8 MHz equipped with a CP/MAS accessory. The field strength of the H decoupling was 1.2 mT, contact time 2 ms, repetition time 5s, and spectral width 27.0 kHz. 8K data points were used. Samples was placed in a cylindrical rotor and spun at about 4 kHz. The nC chemical shifts were calibrated indirectly through the adamantane peak(29.5 ppm relative to tetramethylsilane, TMS, (CH3)4Si). 

Facilities for a variety of multiple-resonance experiments, including broad-band, homo- or hetero-nuclear decoupling, are now generally built into the spectrometer, instead of being provided by auxiliary audio-oscillators. Oscilloscopes for optimization of magnetic-field homogeneity, precalibrated recorder-charts, and frequency counters for accurate calibration of charts and frequencies for multiple... 

If delayed extraction increases the mass resolution without degradation of sensitivity compared with continuous extraction, it also has limitations. Indeed, delayed extraction complicates the mass calibration procedure. It can only be optimized for part of the mass range at a time and is less effective at high mass. Delayed extraction partially decouples ion production from the flight time analysis, thus improving the pulsed beam definition. However, calibration, resolution and mass accuracy are still affected by conditions in the source. For instance, in the usual axial MALDI-TOF experiments, optimum focusing conditions depend on laser pulse width and fluence, the type of sample matrix, the sample preparation method, and even the location of the laser spot on the sample. 

In Section 2-4, we encountered a number of spectral acquisition parameters. The selection of these parameters is relatively direct and straightforward, with the exception of the flip angle a and the decoupler modulation frequency (for WALTZ decoupling). Neither a nor the decoupler modulation frequency is entered directly, and both parameters require that the spectrometer be calibrated. 

The standard method for calibrating 7 2 is a technique known as off-resonance decoupling (Section 5-3 and Figure 5-8). In Figure 1-25, we saw that methyl carbons appear as quartets, methylene carbons as triplets, and methine carbons as doublets in fully proton-... 

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