Dummy scans

Dummy Scans and Relaxation Delays between Successive Pulses... 

To reach a steady state before data acquisition, a certain number of dummy scans are usually required. If the relaxation delay between the... 

What are dummy scans, and why are a number of these scans acquired before actual data acquisition ... 

Dummy scans are the preparatory scans with the complete time course of the experiment (pulses, evolution, delays, acquisition time). A certain number of these dummy scans are generally acquired before each FID in order to attain a stable steady state. Though time-consuming, they are extremely useful for suppressing artifact peaks. 

If the number of dummy scans in a 2D experiments is set too low, data acquisition may be started before a steady-state e.g. thermal equilibrium, has been established. Consequently the first few increments (data points in tl) may be distorted. With backward LP these wrong data points in tl can be recalculated and replaced using the information gained from the residual increments. 

For HSQC use at least 16 dummy scans, some authors additionally recommend DS = 2 NS. 

Steady-state, or dummy, scans are used to allow a sample to come to equilibrium before data collection begins. As in a regular experiment, a number of scans are taken, but data are not collected during what would be the normal acquisition time. Steady-state scans are usually performed before the start of an experiment, but, for certain experiments on older instruments, may be acquired before the start of each incremented time value. This technique is not necessary in typical one-dimensional NMR experiments, but is employed in onedimensional methods that involve spectral subtraction (e.g., DEPT Section 7-2b) and virtually all two-dimensional experiments. 

A disadvantage of DEPT is that it is a subtraction experiment and, therefore, much more sensitive to certain problems than are typical one-dimensional techniques. One remedy that is used in many experiments that suffer fi om stability problems is the employment of steady-state, or dummy, scans (parameter 2 in the foregoing list Section 2-4i). Poor signal cancelation is generally the result of difficulties in one or more of the following areas ... 

Check it 5.2.1.7 demonstrates the effect of dummy scans on the efficiency of the phase cycling in suppressing the unwanted coherence of a quaternary carbon in the DEPT45-l3C lH experiment. Because Tj acquisition time, the initial magnetization before each scan depends upon the number of dummy scans and the relaxation delay dl. With no dummy scans or relaxation delay the initial magnetization is changing before each scan so that the destructive accumulation of the quaternary carbon coherences during the phase cycle fails. 

All spectra were acquired using a relaxation delay of 2 s, a spectral width of 8278 Hz, an acquisition time of 3.96 s and a sample temperature of 90 °C. All acquisitions were initiated with two dummy scans followed by 16 scans. All zg spectra were recorded with one receiver gain and all water-suppressed spectra with another (4.5 times hi er). 

The DEPT experiment, or distortionless enhanced polarization transfer, is a carbon selectivity experiment. ° ° ° Depending on the pulse length selected, one can selectively observe different types of carbon entities. We recommend setting the DEPT proton pulse length to 135°. In this case, quarternary carbons are suppressed, methylenes are inverted, and methine and methyl carbons appear upright. Methines and methyls are distinguished based on chemical shift and 2D proton correlations. Methines usually appear downfield of methyls. Alternatively, if time permits, the entire series of DEPT experiments can be performed to conclusively distinguish methine from methyl resonances. One second is a reasonable default value for the recycle time. The spectrum should be set to capture the resonances of interest with 32-64 K data points and four dummy scans. While this is an easily interpreted data set, the multiplicity edited HSQC provides much more robust information, both multiplicity as well and the one-... 

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