Quadrupolar spectrometer

Since the second-order quadrupolar broadening is inversely proportional to the Larmor frequency [(35) and (36)], one obvious means for improving the resolution is to employ the highest available magnetic field strength [94], The use of several spectrometers operated at different magnetic field strengths can be helpful in... 

The NMR experiments were performed using the quadrupolar echo pulse sequence 7i/2x—Ti—7i/2y—T2—acquisition with phase-cycling and quadrature detection. A Bruker MSL 400 spectrometer was used for the high pressure studies operating at a resonance frequency of 61.4 MHz. In the liquid-crystalline phase, perdeuterated lipids display NMR spectra, which are superpositions of axially symmetric quadrupolar powder patterns of all C-D bonds.From the sharp edges, the quadrupolar splittings... 

The procedures for recording spectra of heteronuclei often differ considerably from those for H and (which would today be considered routine ) since it is necessary, even for routine measurements, to adjust the experimental conditions to suit the special properties of the nuclei to be observed. For example, the spin-lattice relaxation times for some nuclides, such as N, are very long, whereas for others (especially those with an electric quadrupole moment, such as N) they are very short. Also, the spectra observed for some nuclides contain interfering signals caused by other materials present, for example the glass of the sample tube ("B, Si), the spectrometer probe unit ( Al) or the transmitter/receiver coil. For many nuclides the sample temperature and its constancy are important factors for example, quadrupolar nuclides such as O give narrower signals when the temperature is increased. 

The syntheses of the precursors were carried out following procedures reported in the literature. All methods used distilled solvents, an argon filled glove box or Schlenk type glassware connected to a vacuum line. Thermoanalytical studies were carried out on a Setaram TG-DTA 92 analyzer in line with a Leybold Heraeus QX 2000 quadrupolar mass spectrometer. 

Scheme of a hybrid mass spectrometer including a quadrupole analyser, a quadrupolar collision cell and an orthogonal acceleration time-of-flight analyser. 

The faithful representation of the shape of lines broadened greatly by dipolar and, especially, quadrupolar interactions often requires special experimental techniques. Because the FID lasts for only a very short time, a significant portion may be distorted as the spectrometer recovers from the short, powerful rf pulse. We saw in Section 2.9 that in liquids a 90°, t, 180° pulse sequence essentially recreates the FID in a spin echo, which is removed by 2r from the pulse. As we saw, such a pulse sequence refocuses the dephasing that results from magnetic field inhomogeneity but it does not refocus dephasing from natural relaxation processes such as dipolar interactions. However, a somewhat different pulse sequence can be used to create an echo in a solid—a dipolar echo or a quadrupolar echo—and this method is widely employed in obtaining solid state line shapes (for example, that in Fig. 7.10).The formation of these echoes cannot readily be explained in terms of the vector picture, but we use the formation of a dipolar echo as an example of the use of the product operator formalism in Section 11.6. 

In order to remove the anisotropic effects of quadrupolar interactions which can obscure MAS spectra other techniques such as Double Rotation (DOR) [18, 19], Dynamic Angle Spinning (DAS) [18] or Multiple Quantum Magic Angle Spinning (MQMAS) have to be employed. Of these the most simple to apply practically is MQMAS, which can be conducted on a standard MAS-equipped NMR spectrometer. A number of texts and review papers discuss MQMAS in significant technical detail [20, 21]. 

N is one of the few quadrupolar nuelei with integer spin. Since these nuelei have no ( 2, — 2) state, all the transitions suffer first-order quadrupolar broadening, with all the intensity spread over a range of about 2vq. Thus, the experimental conditions for observing such broad lines make similar demands on the spectrometer as those used to detect the satellite transitions of nuclei with non-integer spins. The values of Xq for compounds fall in the range 0-8 MHz, with typical values 3-4 MHz (Yesinowski and Hill 1999). 

GC can be combined with all kinds of mass spectrometers, but, for practical and economic reasons, only quadrupolar and magnetic/electric-sector instruments are in wide use. Ion traps can also be used. 

Temperature-programmed surface reaction (TPSR). Measurements were performed in a flow apparatus with a BALZERS quadrupolar mass spectrometer to monitor effluent products versus temperature. Samples of 300 mg were used in each measurement. The gases employed were He (99.9%) and mixtures 4% NH3/He, 1% NO/He and 5% 02/He. TPSR measurements were obtained after NH3 adsorption at 70°C under continuous flow until base line stabilization. The reactor was heated to 500°C at 20°C/min and kept at this... 

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