Acoustic ringing

Fig.45a-c Platinum-195 spectra, 64.52 MHz.aK2PtCl4inD20,bandcds[Pt(NH3)2(l-methylura-cil-N3)]. Spectrum b was recorded using a normal pulse sequence, with 90° pulses. Spectra a and c were recorded using the ARING pulse sequence for removing acoustic ringing... 

This effect is caused by an RF pulse through a conductor such as the NMR coil in a magnetic field Acoustic ringing increases at lower NMR frequencies and special pulse sequences are used to suppress it. 

Ab initio methods, 147-49 Acetate ion, decomposition, 135 Acetylene, interaction with palladium, tunneling spectroscopy, 435,437f Acid-dealuminated Y zeolites catalytical properties, 183 sorption, 175-78 Acid sites, on zeolites, 254 acidification effects, 266 Acoustic ringing, in NMR, elimination, 386 Active sites, nature, 104 Activity measurements, Co-Mo catalysts, 74 Adsorbed molecules,... 

Acoustic ringing of the probe assembly after an RF pulse is a pesky problem which often limits the measurements of nuclides with low gyromag-netic ratios (it can also strongly interfere with measurements of samples containing piezoelectric components). The disturbance is often misinterpreted as a particularly long dead-time disturbance, until one notices that, unlike normal dead-time components, it disappears when B is set to zero. It is difficult to remove because it follows the phase of the RF pulse and thus cannot be eliminated by any simple RF phase-cycling. 

The classical cure 131,132), apart from special probe construction precautions, is a pulse sequence using a phase device detection cycle in which one exploits the fact that acoustic ringing increases linearly with pulse width while NMR signal follows the sinusoidal nutation-angle curve. In its most elementary form, the cycle is composed of four steps (ideally with null 5) ... 

I. P. Gerothanassis, Methods of avoiding the effects of acoustic ringing in pulsed Fourier transform nuclear magnetic resonance spectroscopy. Prog. Nud. Magn. Reson. Spectrosc., 1987,19, 267-329. 

Backward LP (Fig. 5.21) is usually applied to repair the first few points of an FID, distorted by some spectrometer perturbation or a mis-set acquisition parameter, e.g. incorrect receiver gain. Backward LP is also used to reconstruct an FID back to t=0 in those cases where the start of data acquisition has been delayed, e.g. to exclude unwanted spectrometer noise such as the signals from acoustic ringing, and the first few data points are missing. In this case backward LP cancels or at least suppresses the corresponding spectral artefacts such as baseline roll etc. 

J. B. Miller, A. N. Garroway, and B. H. Suits, Nuclear Quadrupole Resonance (NQR) Method and Probe for Generating RF Magnetic Fields in Different Directions to Distinguish NQR from Acoustic Ringing Induced in the Sample, US Patent No. 6 522 135 (2003). 

As previously outlined, in most cases, 33S NMR spectra are characterized by low S/N, and the first points of the FID are corrupted by acoustic ringing and pulse breakthrough. 

One of the major problems of working with Ge is the baseline roll as a result of the acoustic ringing, which creates difficulties with the expected broader lines for many germanium derivatives. A solution to this problem might be the use of pulse techniques such as RIDE, PHASE or EXSPEC . The first example for the application of such techniques was in solving the behavior of several exchange processes. ... 

Because the application of proton broad band decoupling in solution N NMR causes quantification problems, the inverse-gated decoupling technique often is used. With this technique no enhancement is observed, and the spectra can be quantified. However, a problem that arises in NMR experiments with nuclei with low sensitivity such as N is acoustic ringing. Ringing occurs because the rf pulse causes brief vibrations in the probe which masquerade as a signal. This signal overlaps the entire spectra which results in a distortion of the baselines. Quantification of these distorted spectra is impossible and in many cases the spectra become useless. 

The authors pointed out that use of a higher field (300 MHz or higher) magnet, and a dedicated germanium probe will make the observation of this low resonance frequency more accessible (Table 2). They added, however, that more powerful pulses are needed to maintain reasonable observation windows, and that this aggravates the acoustic ring . 

Recent progress in Ge NMR spectroscopy has been briefly reviewed. The crucial point of heteronuclear NMR spectroscopy is the extent to which it can achieve that which may be carried out by and C NMR spectroscopy. As described in this review, Ge NMR spectroscopy can employ almost all techniques used in H and C NMR spectroscopy. There exists, however, a severe limitation in Ge NMR spectroscopy. Ge is a quadrupolar nucleus and a high symmetry of electric field gradient around germanium is required for proper observation of signals. Acoustic ringing is another problem associated with Ge NMR spectroscopy. 

Rg. 29. A O NMR spectrum of 0.5 mM D- uc( e in H2O at 353 K recorded at 48.8 MHz. (A) Reference spectrum acquired using ttfl pulses. TTie baseline is distorted due to acoustic ringing effects. (B) Combination of the inversion-recovery technique with the RIDE sequence for acoustic ringing elimination. TTie asterisk denotes the water resonance. (From Schulte and Lauterwein with permission.)... 

Observation of low-frequency quadrupolar nuclei with very broad resonances. Sequence suppresses acoustic ringing responses from probehead. 

Figure 435. Sequences for the observation of quadrupolar nuclei with very broad lines for which acoustic ringing is a problem. Sequence (a) eliminates ringing associated with the 90° pulse whilst ACOUSTIC (b) and RIDE (c) further eliminate that associated with the 180° pulses.

Figure 4.36. O spectra of ethyl acetate recorded (a) with and (b) without the RIDE sequence. The severe baseline distortion in (b) arises from acoustic ringing in the probehead. Spectrum (c) was from the same FED of (b) but this had the first 10 data points replaced with backward linear predicted points, computed from 256 uncorrupted points. The spectra are referenced to D2O and processed with 100 Hz line-broadening.

In NMR-SIM the simulation of an NMR experiment is based on the density matrix approach with relaxation phenomena implemented using a simple model based on the Bloch equations. Spectrometer related difficulties such as magnetic field inhomogenity, acoustic ringing, radiation damping or statistical noise cannot be calculated using the present approach. Similarly neither can some spin system effects such as cross-relaxation and spin diffusion can be simulated. 

Key Words NQR, Nitrogen, Multi-pulse sequence. Explosive detection. Magneto-acoustic ringing. Polarization enhancement. 

Combining the MOEF with the method of blocks permits achieving 40 dB of cancelling magneto-acoustic ringing at the PETN resonance frequency of 890 kPIz (not shown). 

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