Multiple echo testing

APT Attached proton test, a modification of the J-modulated spin-echo experiment to determine C//multiplicities, a less sensitive alternative to DEPT... 

Fig. 6. Top 2D MAT sequence for correlating isotopic chemical shift and CSA with two separate experiments P+ and P . All pulses following CP are 90°. A four-step phase cycling is used with 6 = —y, x, —y, x. and 62 = —y, x, x, -y. The receiver phases are x, -x, — y, -y for the P+ pulse sequence and x, —x,y, y for the P pulse sequence. (The sign of receiver phases with an asterisk depends on the relation between the pulse phase and the receiver phase of the particular spectrometer in use. These receiver phases must be changed in sign when the quadrature phase cycle (x,y, —x, -y) of the excitation pulse and the receiver phase in a single-pulse test experiment result in a null signal.) Phase alternation of the first H 90° pulse and quadrature phase cycling of the last 13C 90° pulse can be added to the above phase cycle. The time period T can be any multiple of a rotor period except for multiples of 3. Bottom 2D isotropic chemical shift versus CSA spectrum of calcium formate powder with a three-fold MAT echo extension. (Taken from Gan and Ernst178 with permission.)...

Figure 10. Reduction in floor echoes obtained using multiple coating treatment. Performance is for one particular downward-looking test geometry and includes some multiple reflections.

APT (attached proton test) Spin echo based selection of multiplicities of signals. Quaternary C and CH2 carbon atoms give positive signals (Peak UP), while CH and CH3 give negative signals (Peak DOWN). 

There are multiple objectives associated with each follow-up evaluation. These may vary from one session to another and will determine the relative complexity of the particular session whether ancillary testing such as an echo-Doppler evaluation or chest x-ray is indicated or if the sole test that is required is a simple ECG rhythm strip. 

If more sophisticated experiments are aimed, such as those involving multiple pulse sequences, spin echo detection, rotor synchronization, or 2D methods, additional calibration steps, trial tests, and optimization procedures are required. The reader is referred to the specific literature dealing with each particular method for a proper description of the experimental setup. A general account of these methods can also be found in the monographs by MacKenzie and Smith [4] and Duer [15]. 

Difficulties are encountered in establishing carbon multiplicities with the SFORD technique when signal overlap becomes severe, or when only small quan-titites of sample are available, thus dividing the signal intensity between the lines of the multiplet. New techniques that overcome the limitations of SFORD utilizing the spin echo are the J-modulated spin-echo (231) or the Attached Proton Test (312), the Insensitive Nuclei Enhanced by Polarization Transfer (56, 79, 104), and the Distortionless Enhancement by Polarization Transfer (80). 

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