Quadrature phase detection

Figure 14. Contour plot of the 360 MHz H-NMR correlation spectrum of dl-camphor. A 64 x256 data set was accumulated with quadrature phase detection in both dimensions and the data set was zero filled once in the dimension and symmetrized. T was 5 sec and t was incremented by 1.63 msec. Total accumulation time was 24 minutes and data workup and plotting took 15 min.

Figure 17. Contour plot of the 360MHz homonuclear spin correlation mpa of 10 (2 mg, CDCL, high-field expansion) with no delay inserted in the pulse sequence shown at the top of the figure. Assignments of cross peaks indicating coupled spins in the E-ring are shown with tljie dotted lines. The corresponding region of the one-dimensional H NMR spectra is provided on the abscissa. The 2-D correlation map is composed of 128 x 512 data point spectra, each composed of 16 transients. A 4-s delay was allowed between each pulse sequence (T ) and t was incremented by 554s. Data was acquired with quadrature phase detection in both dimensions, zero filled in the t dimension, and the final 256 x 256 data was symmetrized. Total time of the experiment was 2.31 h (17).

A plot of Eq. 3.8 in the complex plane is shown in Fig. 3.3. Because the frequency of the output (a) — 0) is just the frequency at which Mxy precesses in the rotating frame, Fig. 3.3 also provides a visual depiction of this precession as viewed along the z axis. Note that the quadrature phase detection configuration permits us to differentiate between the frequencies (w — w ) and (o)rf — ), whereas such frequencies are indistinguishable with a single phase detector. The phase angle (4> — cf)r,-) can be chosen to provide the pure absorption mode on resonance, and in more complex experiments it can be adjusted as needed, as we shall see in detail later. 

The alternative method for obtaining quadrature phase detection with the use of a single phase-sensitive detector, developed initially by Redfield38 and used in some commercial spectrometers, has certain advantages in two-dimensional NMR. In this approach, the phase of the receiver is advanced by 90° after each measurement. The rationale can be better understood after we discuss digitization rates in Section 3.7. 

We consider now an important example of phase cycling that is used in both ID and 2D NMR, namely the suppression of artifacts resulting from imperfections in the hardware used for quadrature phase detection. We detail the principles and procedures involved in this example as a prototype for many more complex phase cycling procedures that we mention more briefly in later chapters. 

As we saw in Section 3.4, quadrature phase detection discriminates between frequencies higher and lower than the pulse frequency, but it does not prevent foldover from frequencies higher than the Nyquist frequency. For a desired spectral width FT, there are two common methods for carrying out quadrature phase detection, as was indicated in Section 3.4. One method uses two detectors and samples each detector at FT points per second, thus acquiring 2 FT data in the form of FT complex numbers. The other (commonly called the Redfield method ) requires only a single detector and samples at 2 FT points per second while incrementing the phase of the receiver by 90° after each measurement. (In two-dimensional NMR studies, a variant of this method is usually called the rime-proportional phase incrementation, or TPPI, method.) Because these methods result in quite different treatment of folded resonances, we now consider these approaches in more detail. 

The Redfield method of quadrature phase detection operates as shown in Fig. 3.7a. A spectral width from W/2 to — W/7 is defined by 2 IT7 measurements per second with a single detector, the phase of which is augmented by 90° after each measurement. Each signal magnitude, as illustrated in Fig. 3.7[Pg.67]

The large spectral widths required by some of the applications also put more severe demands on pulse power, if uniform excitation is to be achieved across the full width of the spectrum. If both components of the complex magnetization are detected (quadrature phase detection) the carrier can be placed at the centre of the spectrum without any rf carrier folding occurring as in single-channel detection better uniformity of excitation is thus achieved at a given transmitter power. 

When the operator suspects the presence of an aliased signal, the testing procedure for such aliasing also depends on the type of quadrature detection being used. For either type of quadrature phase detection system, the sw parameter can be significantly increased. The position of the putative aliased resonance then changes in relation to the rest of the signals. 

The transmitter offset describes the location of the observation frequency and is closely related to the spectral width. With quadrature phase detection of sample signals (Section 5-8), the frequency of the transmitter is positioned in the middle of the spectral width. In so doing, the operator has the best chance of irradiating, with equal intensity, those nuclei whose resonances are both close to and far from the transmitter frequency. Irradiation is not a problem for protons, with their small chemical shift range, but it can be for nuclei with large chemical shift ranges (Chapter 3). 

A very large image signal appears in CRAMPS spectra because the x-component of the magnetization is different from the y-component. Therefore, half of the spectral region is abandoned. However, some quadrature-phase detection CRAMPS (QD-CRAMPS) methods which cancel the image signal have been reported. ... 

The solid-state H CRAMPS NMR measurements were performed on a Chemagnetics CMX 300 spectrometer equipped with a CRAMPS probe with 5 mm rotor. Quadrature-phase detection was carried out according to the phase-cycling technique proposed by Burum et al. Here, we used the MREV-8 pulse sequence for homonuclear decoupling. The experimental conditions are the same as those described in the previous section. 

Traficante, D. D. Phase-sensitive detection. Part II Quadrature phase detection. Concepts in Magnetic Resonance 2 181-195, 1990. 

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