Internal lock

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. 

January Maryland becomes the first state to require that new handguns be equipped with internal locking mechanisms. 

The reference nuclei may be part of the sample (internal, field-frequency stabilization, or internal lock) or contained in a separate ampule within the homogeneous volume of magnetic field (external, field-frequency stabilization, or external lock). Also, the reference nuclei may be of the same species as those whose spectra are being observed (homonuclear lock), or of a different species (heteronuclear lock). Each of these methods has advantages and disadvantages. 

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)... 

The relatively small changes in chemical shift with pressure can be qualitatively explained in terms of changes in bulk susceptibility as most spectra in SCFs are obtained without internal lock. Large chemical shift ranges are a characteristic of metal NMR spectroscopy, and the changes observed on changing temperature, pressure or solvent are no more exaggerated in supercritical solvents than those observed with conventional solvents. 

Fig. 2. Expanded view-of the fuUerene region in the NMR spectrum of 1.6 mg of Ci -C g in CS2, taken at 12S.6 MHz, with acetone as an internal lock. In addition to the fuUerene resonances, the fiiU spectrum from 0 tt> 2M ppm only shows the acetone peaks near 30 and 6 ppm and the solvent peak near 190 ppm.

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. 

In undeuterated dioxane, Hgq and coincidentally have the same chemical shift (at the field studied), so they cannot be differentiated at low temperatures. (See Sections 1-8 and 5-2.) In l,4-dioxane- 7 (an impurity in commercial l,4-dioxane- 5 g), both and Hgq exhibit isotope shifts to a lower frequency, but H x is shifted somewhat farther. As a result, the axial and equatorial protons give separate resonances at low temperatures, in contrast to the undeuterated material. Because of a chlorine isotope effect, chloroform is a poor substance for an internal lock or a resolution standard at fields above about 9.4 T. At high resolution, the chloroform proton resonance shows up as several closely spaced peaks, due to CH( 5c1)( C1)2, CH( C1)2( C1), CH( C1)3, and CH( C1)3. 

Carbon 13 NMR spectra were obtained using a JEOL GX-500 spectrometer with a quadruture detection operating at a frequency of 125.77 MHz in a pulse-Fourier transform mode. FID s were acquired with a 16 bit A/D converter and stored on 32 or 64 K memory locations with 32 bit word length. The chemical shift was measured from an internal standard, hexamethyldisiloxane, which was taken as 2.03 ppm from tetramethylsilane(TMS). Internal lock was provided by an addition of benzene-dg. 

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... 

ESR spectra were taken on a Bruker Scientific 418s X-band spectrometer equipped with Hall-probe stabilized magnetic field sweep. The 100 MHz proton NMR spectra were obtained on a standard Varian HA-100 spectrometer operating with internal lock using a solvent peak. All samples contained a trace of tetramethylsilane (TMS) as an internal reference. 7Li NMR spectra were obtained at 23.3 MHz on a JEOL C60H NMR spectrometer equipped with a JNM-NS-100 Nuclear Single Sideband Unit. Samples for 7Li NMR contained a capillary of 5.0M LiBr in methanol as external reference. 

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... 

The A-60 was the instrument of change. Built in Palo Alto, California, by Varian Associates, it was a hands-on, routine instrument that graduate students in chemistry could operate. Precalibrated charts, together with an internal lock on a water sample tube, allowed for a single sweep to record a spectrum (Anon. 1961). The A-60 brought NMR to the masses. Chemists took to it like fish to water (Jackman 1996). 

A field-frequency internal lock provides corresponding changes in the irradiating frequency for minor variations in field strength to furnish a constant field/frequency ratio. The frequencies are locked to a master oscillator. 

The most common internal locking scheme in high resolution solution NMR is to lock on the deuterium in deuterated solvents and many spectrometer locks are set up only for deuterium. What if the sample does not contain any deuterium (The following argument is true for any nuclei chosen for locking.) An instance of this would be with an air sensitive sample well sealed in a tube and not containing any deuterium. 

The amount of energy released by the incident beam is sufficiently high in the vicinity of the point of impact that sample molecules are ionized by proton transfer from a protonated matrix. Typically, the spectrum obtained shows cluster ions formed from the matrix. In particular, the positive ion spectrum of glycerol (mw 92 u) shows a series of ions 92 +l where n can be 1 up to 12 or more. This spectrum can be used to calibrate the mass spectrometer and also provide internal lock masses for accurate mass measurement, described in applications below. 

To conclude, each spectrum consisted of 64 scans of 32K data points with a spectral width of 6.000 Hz and an acquisition time of 5.3 s, a recycle delay of 25 second per scan and a pulse angle of 90°. The analysis of each solid extract was performed using D2O as an internal lock. Spectra were acquired under an automation procedure (automatic shimming and automatic sample loading) requiring about 33 min per sample. 

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