Phase cycling routines

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

Fig. 2. Left Experimental profiles of the conventional DANTE sequence (top) and of the DANTE-Z sequence (bottom). The sample used was 5% H2O in D2O with a tiny amount of copper sulfate added (leading to a T of approximately 3 s). The different traces were obtained by shifting the carrier frequency in 50 Hz steps without readjustment of the spectrometer phase. For each experiment, four scans were acquired in order to perform the complete phase cycling of DANTE-Z. Right (a) The conventional H spectrum of a small protein (toxin 7 60 residues) in D2O at 318 K (b) selection of the aromatic region by the conventional DANTE sequence (c) same as (b) using the DANTE-Z procedure. Experiments were performed at 200 MHz using a routine AC200 Bruker spectrometer.

Identify security requirements and attributes during the requirements phase, establish access control guidehnes during the design phase, challenge security features during the test phase, and routine audit security measures during the operation and maintenance phase of the system life cycle. Use procedures for maintenance of accounts and access authorization after system acceptance and as described in the operational system support procedures. 

Prior to the advent of the above methods that allowed the presentation of phase-sensitive displays, 2D data sets were collected that were phase-modulated as a function of ti rather than amplitude-modulated. Phase-modulation arises when the sine and cosine modulated data sets collected for each ti increment are combined (added or subtracted) by the steps of the phase cycle, meaning each FID per tj increment contains a mixture of both parts. Here it is the sense of phase precession that allows the differentiation of positive and negative frequencies. This method is inferior to the phase-sensitive approach because of the unavoidable mixing of absorptive and dispersive lineshapes, so is generally only suitable for routine, low-resolution work. 

---