TPPI method

Figure 13 Pulse sequence for the solid-state 31P—1H frequency-switched Lee-Goldburg (FSLG) experiment. Proton-proton homonuclear decoupling was performed by using the FSLG decoupling sequence. Quadrature detection in Wi was achieved by using the TPPI method. During the acquisition period, TPPM heteronuclear decoupling was applied.

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. 

An application of the TPPI method to the eight proton system of an alkyl-deuter-ated cyano biphenyl liquid crystal is shown in fig. 4. All eight orders are observed. Resolution is limited by truncation. Actual linewidths are <100 Hz in a spectral width of a 40 kHz for each order. 

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. 

Illustration of the TPPI method. The normal spectrum is shown in a, with peaks in the range -SWI2 to +SW/2. Adding a frequency of SW2 to all the peaks gives them all positive offsets, but some, shown dotted) will then fall outside the spectral window - spectrum b. If the spectral width is doubled all peaks are represented correctly - spectrum c. 

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. 

Illustration of the TPPI method. Each time that is... 

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. 

The StateS TPPI method [2.44] This method combines elements from both the States et al. and TPPI method to obtain phase sensitive spectra. 

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. 

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