Phase cycling limitation

In all appHcations involving zirconia, the thermal instabiHty of the tetragonal phase presents limitations especially for prolonged use at temperatures greater than - 1000° C or uses involving thermal cycling. Additionally, the sensitivity of Y—TZP ceramics to aqueous environments at low temperatures has to be taken into account. High raw material costs have precluded some appHcations particularly in the automotive industry. 

It s more likely these days that you will be using a 250 or 400 MHz Fourier transform instrument with multi-nuclei capability. If such an instrument is operating in walk up mode so that it can acquire >60 samples in a working day, then it will probably be limited to about 32 scans per sample (a handy number - traditionally, the number of scans acquired has always been a multiple of eight but we won t go into the reasons here. If you want more information, take a look at the term phase cycling in one of the excellent texts available on the more technical aspects of NMR). This means that for straightforward... 

A combination of SIPS with the stabilising and synthesis-favouring properties of clay minerals was studied by Rode et al. (1999) in experiments involving dry/wet cycles. The simultaneous use of both SIPS and clay minerals as catalytically active surfaces led to peptides up to and including the hexamer (Gly)6. The question as to whether this technique fulfils prebiotic conditions can (within certain limitations) be answered positively, since periodic evaporation phases in limited areas (lagoons, ponds) are conceivable. The container material could have consisted of clay minerals. Further progress in the area of peptide synthesis under conditions which could have been present on the primeval Earth can be expected. 

The full ADEQUATE phase cycle is 16 transients. Practically, the length of the phase cycle will not be a limitation in most instances and most samples will necessitate several complete passes through the phase cycle, constituting an overnight acquisition. 

Artifacts may be roughly categorized into those due to inherent limitations (e.g. pulses cannot excite unlimited bandwidths even if all hardware components work perfectly) and those that result from improper set-up of the experiment or nonideal functioning of the NMR spectrometer system. In this chapter we will mainly focus on the latter two. These artifacts are more likely to appear in multiple-pulse experiments. Quite often, they are avoided by clever programming of the experiments (e.g. interleaved acquisition of data for NOE spectra, use of pulsed-field gradients instead of phase-cycling). 

The cycles of reduction and oxidation of Fe and Mn oxides in intermittently submerged soils provide opportunities for co-precipitation with trace metals. In most natural systems it is the rate of dissolution of the sohd phase that limits solid solntion formation rather than thermodynamics, so conditions in snbmerged soils are highly conducive to formation of solid solntions. 

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. 

Quadrature images in the Fi dimension can be suppressed by expanding the 8-step phase cycle to 32 steps or 16 steps, respectively, using CYCLOPS [20] or 2-step CYCLOPS [21]. In the CYCLOPS scheme, the phases of all pulses are simultaneously incremented by 90°, 180° and 270°. In the 2-step CYCLOPS scheme, the incrementation of the pulse phases is limited to the 90° step. 

In order to determine couplings to nuclei in natural abundance, it is necessary to suppress the signals of protons that are not bonded to a magnetically active heteronucleus. An (iix hetero half-filter that selects for such nuclei in F, via the phase cycle was used for this purpose. Presaturation of the protons bonded to 12C by the BIRD pulse allows a rapid pulse-sequence (2 scans per second). The resulting 2D spectra are TOCSY spectra in which the cross peaks show the desired E. COSY pattern. From the results shown, the only limitation seems to be the resolution obtained, although the authors do not hesitate to use a third heteronuclear frequency domain for improvement. 

To save space and yet enable limited discussion, the pulse sequences are given here in very condensed form in the body of the text for phase cycling, etc. the reader is referred to the source literature. 

If the intensity of the desired signal is the limiting factor, then phase cycle selection is preferred. A phase cycle selection version of HMQC that uses a spin-lock to purge signals from molecules containing NMR silent 5 nuclei has been developed—poor man s gradient-heteronuclear... 

One limitation of the phase-cycling method of selective detection is the expanding length of the required phase cycle as the order N increases. The number of steps in the phase cycle is 4N so for large N, this dictates that the phase cycle becomes prohibitively long and, in practice, this limits the number of increments that can be obtained in f and hence ultimately the digital resolution of the MQ spectra which can be obtained. 

Speed As there is no requirement to complete a phase-cycle, experiment times are dictated by sensitivity and resolution considerations only. When sensitivity is not limiting, 2D experiments can be acquired with a single transient per tj increment, leading to significant time savings. 

N signals is complete and conveniently implemented. Phase-cycling is also not essential because signal selection is achieved solely by gradient refocusing, and in situations where sample quantities are not limiting these experiments may be performed within a matter of minutes. 

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