Lock signal internal

FIG. 9. Li chemical shifts in organolithium compounds, measured in three different solvents. The shieldings were originally determined relative to the proton frequency of tetramethylsilane which served as an internal lock signal, and subsequently referenced to external LiBr (0-7g/ml). (80)... 

Choice of Solvent. The most appropriate solvent for NMR studies of polymers would allow a range of polymer concentrations to be investigated, be free of overlap problems and hopefully provide a signal for internal lock. Not all of these conditions can usually be met as many high molecular weight polymers pose solubility problems and can be examined in only a limited number of solvents. Deuterium resonance is the typical choice for an internal lock signal on most modern NMR spectrometers. Unfortunately, the majority of available deuterated solvents are poor solvents for many addition polymers such as the polyolefins while it is generally possible to find a number of appropriate deuterated solvents for many of the condensation polymers. The... 

C NMR Measurements. The NMR spectra were observed with a Varian XL-100 spectrometer modified for pulse Fourier transform spectroscopy and interfaced with a Nicolet model 1080 computer. The protons were decoupled from the carbon nuclei using a random noise decoupling field. Free induction decays were stored in 8K computer locations using a dwell time of 200 / sec, i.e., a spectral window of 2500 Hz. The pulse width was 23/xsec (for a 90° pulse), and the pulse interval was 3.0 sec. Hexamethyldisiloxane was used as an internal reference (2.0 ppm vs. TMS), and the internal deuterium lock signal was... 

NMR spectra were recorded at 60 and 100 MHz with Varian A-60 and HA-100 spectrometers. Proton shifts in five arenes and complexes were measured in 5% w/w deuterochloroform solutions with tetramethyl-silane as internal reference and locking signal. The probe temperature was calculated using methanol in the temperature range -60 to +40°C. 

The close /lock signal lecids to a wrong interned state. The close /lock signal is disabled. 

Deuterated solvents are utilized with FT NMR spectrometers to provide an internal lock signal to... 

MEASUREMENTS The H-NMR spectra were obtained at 60 C with a JEOL JNM-GX500 spectrometer at 500MHz. The sample concentration was 5%(w/v) in perdeuterated dimethyl sulfoxide (dg-DMSO), which provides the internal lock signal. Approximately 256-10000 free induction decays were accumulated using a pulse width of 4ps, a pulse interval of 5.0s, and spectral width of lOkHz. 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

In this scheme, analog Fourier decomposition of the signal using lock-in amplifiers becomes impractical and a digital fast Fourier transformation is preferred. Such an analysis would lead to 22 linear combinations of the 16 Mueller matrix components of the sample. This is an overspecified system, and the extra information can be used as either an internal check of consistency or discarded. It is important to make proper choices of the angular velocities, and D2. Clearly, these frequencies cannot be simple multiplies of one... 

In the former case, an internal lock is established at the deuterium frequency of the solvent by adjusting the frequency of the lock transmitter until it matches that frequency. The operator typically observes a decreasing number of interference-pattern sine waves as the lock transmitter frequency approaches that of the deuterium nuclei in the solvent. A null appears when the two frequencies are identical the operator then turns the lock control to On. On most modem spectrometers, autolocking procedures are also available that search for the deuterium resonance and automatically lock the spectrometer when the signal is found. 

The deuterium nucleus, used normally for the lock channel, can be employed as an internal NMR thermometer." Using two deuterated compounds, a power pseudo-FID can be constructed from either a conventional continuous wave (CW) deuterium sweep or a pulsed deuterium FID. The Fourier transform of this signal gives a Lorentzian line with double the natural... 

Materials Photoacoustic measurements were made on a component-assembled PAS spectrometer consisting of a 9W argon ion laser (Spectra Physics), a 0.5 cc internal volume PAS cell equipped with a sensitive electret microphone (Radio Shack, 3.2 mV/Pa). Circular polarization modulation was achieved with a special low frequency (220 Hz) photoelastic modulator (15) (Hinds International). Signals were detected and processed with a vector tracking lock-in amplifier (PAR model 5204), and intensity modulation was done with a 30-slot blade mechanical chopper (Ortec). Syntheses of all compounds were by well established literature methods. 

There are many reasons behind the need for solvent suppression. The most obvious is the occlusion of containing peaks of interest (solute) in the background of a massive solvent signal such as 90% H20 containing 10% H20 (also shown as D2O) for an internal lock reference. We also always add... 

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