Phase-sensitive display

The measurement of coupling constants is possible in multiple-quantum COSY spectra with phase-sensitive display. The recording of spec-... 

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. 

Phase errors Chemical shifts also evolve during the application of a gradient pulse due to the static Bq field. Therefore, gradient pulses are generally applied within a spin-echo to refocus this evolution and thus remove phase errors when phase-sensitive displays are required. 

The delays Aj and A2 are chosen to be of the order of l/2JrH and l/3JcH respectively, in order to prevent the mutual cancellation of antiphase signals. If a phase-sensitive display of the entire spectrin is desired, then it is useful to insert 180q,180h pulse pairs into the midpoints of the Aj and A2 delays, but such a display mode is rarely helpful. 

Absolute value versus phase sensitive display The... 

The final 2D spectrum has a projection onto the F2 axis which is the normal H-decoupled "C spectrum with the quaternary carbon.s missing, and a projection onto the Fl axi,s which is the normal H spectrum, including "J, coupling. Thi.s experiment is not phase-sensitive, and must be displayed in magnitude mode. 

In contrast to the basic "C detected experiment, and as a consequence of the final H detection, the 2D spectra obtained with HMQC or HSQC have a projection onto the F2 axis which corresponds to the normal H spectrum and includes all chemical shifts and all Jfi, couplings. The latter may give rise to rather broad cross peaks for extensively coupled protons. The projection onto the Fl axis corresponds to a normal C spectrum but with the quaternary carbons missing. With HMQC, but not with HSQC, cross peaks are additionally split in Fl by "J couplings. The HMQC and the HSQC experiment are usually performed in phase-sensitive mode, which, after proper phasing in both dimensions, allow peaks to be displayed in pure absorption. 

Derivative Detection of EPR Transition. The EPR spectrum is usually displayed as the first derivative of the absorption y"(H), because the nonresonant low-frequency and low-amplitude RF modulation (co1/ 2% = typically 100 kHz) applied to the coils near the magnet is detected by a rectifier in addition to the drop in microwave power level due to the RF resonant absorption (typically co0/27c = 9.1 GHz if H0 = 0.34T) The signal is processed by a phase-sensitive circuit, which detects a back-and-forth sweep across resonance in small magnetic field increments (relative to the DC field and to the width of the measured spectrum), thus generating a response df /dH (see Fig. 11.60). 

FIGURE 3.6 (Continued) (b) Example of foldover of frequencies above the Nyquist frequency with two phase-sensitive detectors. The lower spectrum (spectral width 5000 Hz) faithfully reproduces the true spectrum, but the upper spectrum (spectral width 3600 Hz) shows that peaks near + 2000 Hz now display an aliased frequency near — 1800 Hz and appear near the right-hand end of the spectrum. Sample D-glucorono-6,3-lactone-l,2-acetonide in DMSO-<4 at 500 MHz. Spectra courtesy of Joseph J. Barchi (National Institutes of Health). 

If the phase-sensitive detectors are adjusted to give a phase angle (Eq. 3.8) ( — 4>r ) = 0, the real part of the FT spectrum corresponds to pure absorption at the pulse frequency, but off-resonance lines display phase angles proportional to their off-resonance frequency as a consequence of limited rf power and nonzero pulse width (Eq. 2.55). However, acquisition of data as complex numbers from the two phase-sensitive detectors and subsequent processing with a complex Fourier transform permit us to obtain a spectrum that represents a pure absorption mode. 

In NMR spectroscopy, absorption parts A(phase-sensitive NMR spectrum. The linewidth Ao)i/2 at half height of A a>) is determined by Ti,... 

Phase-sensitive COSY-90 High-resolution display due to absorptive lineshapes. Crosspeak fine structure apparent J measurement possible. Diagonal peaks have dispersive lineshapes which may interfere with neighbouring crosspeaks. Requires high digital resolution to reveal multiplet structures. 

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