TPPI method incrementation

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. 

In the TPPI method the carrier can still be set in the middle of the spectrum, but it is made to appear that all the frequencies are positive by phase shifting systematically some of the pulses in the sequence in concert with the incrementation of tx. 

In words, the effect of incrementing the phase in concert with tx is to add a frequency > iddilion il to all of the offsets in the spectrum. The TPPI method utilizes this option of shifting all the frequencies in the following way. 

The States-TPPI method does not suppress these axial peaks, but moves them to the edge of the spectrum so that they are less likely to obscure wanted peaks. All that is involved is that, each time td is incremented, both the phase of the pulse which precedes q and the receiver phase are advanced by 180° i.e. the... 

The TPPI-method or Marion/Wuthrich inethod [2.41] In the TPPI method the transmitter frequency (Otrans is positioned at the centre of the detected frequency range SW. Frequency discrimination is achieved by shifting the apparent sweepwidth from 1/2SW to 0 to SW. This apparent frequency shift is obtained by incrementing the phase of the selection pulse by 90° for each successive tl increments and sampling the data faster than normal by modifying the value of tl by the phc-factor (see section 3.3.1). The TPPI method guaranties both frequency discrimination and pure absorption lineshapes. 

In the TPPI method a single data set with 512 increments is collected. In each successive increment the phase of the 90° pulse at the end of the period is incremented by 90° with respect to the phase of the corresponding pulse in the previous increment. (An equivalent experiment can be performed in which the phases of the pulses before the ti period are shifted by 90°). This is equivalent to changing the reference frame in so that the transmitter in the dimension appears to be shifted to one edge of the spectrum. After performing a real Fourier transformation, all peaks will appear to be shifted to one side of the transmitter in /. The main disadvantage of this technique is that phase distortions can appear for resonances in strongly coupled spin systems. 

Proton-proton homonuclear decoupling has been performed by the ESLG decoupling sequence [46]. Quadrature detection in coj was achieved by using the time proportional phase increment method (TPPI) [47]. During the acquisition period, two pulse phase modulation (TPPM) heteronuclear decouphng ]48] was applied (Figure 7.6). 

We will see that in 2D NMR, the sampling in the second dimension can also be done either way, except that this choice is up to the user and is not hard wired. The alternate ( Bruker-like ) sampling method is called TPPI (time proportional phase incrementation), and the simultaneous ( Varian-like ) method is called States or States-Haberkorn (after the originators of the technique). The consequences for processing and interpretation of the data are the same in the second dimension of 2D spectra as they are in ID NMR. 

Recording and processing the two signals of Eq. 10.14, which are in quadrature, is the analog in the h dimension of the use of two phase-sensitive detectors in the t2 dimension. As we showed in Section 3.7, there is an alternative to use of two detectors. The Redfield method uses one detector but increments the receiver phase by 90°, as illustrated in Fig. 3.8. An analogous technique is available to treat tj data—time-proportional phase incrementation,TPPI.38 108... 

X, X, X, x), 2(x, -X, -X, x). The phases tjj and < 4 were adjusted for optimum coherence transfer. While ip was constant, was inverted every eight scans. Phase 2 was incremented using the States-TPPi (Bax et al., 1991) method. (Adapted from Schmidt et al., 1993, courtesy of Academic Press.)... 

The idea behind the TPPI (time proportional phase incrementation) or Marion-Wuthrich (MW) method is to arrange things so that all of the peaks have positive offsets. Then, frequency discrimination would not be required as there would be no ambiguity. 

The quadrature detection mode is given as a suffix to the main sequence name me magnitude calculated, TPPI time proportional phase increment, E/A Echo / Antiecho The term "selective" is reserved exclusively for sequences using selective pulses. If selectivity is achieved using other methods this is defined using a different term. 

In 2D NMR, it is not possible to use a second detector in the fi dimension. There are alternatives which provide the equivalent of phase sensitive detection in the dimension two of these are the States method and time proportional phase incrementation (TPPI). 

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