Steady-state scans

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. 

Steady-state scan numbers given in the experimental sections of this chapter and Chapter 7 are performed at the beginning of the experiment described, unless stated otherwise. 

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Steady-state scans (Section 2-4i) are used before the start of essentially all 2D experiments. They are particularly important in a number of pulse sequences in order to compensate for spin-lock (Sections 7-7b and 7-10b) and decoupler (Section 7-8) heating effects. Larger numbers of steady-state scans are employed in experiments that have either particularly long spin-lock times or X-nucleus decoupling over especially wide spectral widths. 

If computer speed and memory permit, 2D NMR experiments generally are planned so that at least a 2K (2,048-point) FT is carried out in each dimension. In this approach, np2 should be 1,024 and zero filled by one level, to 2,048, prior to the FT2. In addition, ni should then be linear predicted two- to fourfold, to 1,024, and zero filled by one level, to 2,048, before FTl. RT s of 0.8-1.2 s are generally sufficient, and most experiments call for the use of steady-state scans. 

Figure 6-9 Cyclic voltammograms on a 4, 4 -dithiodipyridine-modified gold electrode of a 200 xm P. denitrificans CCP solution in lOOmM bis-tris-propane, lOOmM KNO3, pH 6.3 and ImM Ca " (calcium loaded CCP). Scan rate, 20mVs , temperature, 22 + 1 °C. The potential axis is defined versus the NHE and only covers the positive potential range. Panel A First and steady-state scan. Panel B Potentials restricted to values between -1-270 and -70 mV. Panel C Potentials restricted to values between (1) -1-620 and -l-220mV (2) -1-620 and - -140mV (3) -1-620 and -t-80mV (adapted from [22]).

Figure 12. Convolution, as shown by the detection of a sample spectrum from a conventional steady-state scanning spectrometer (a). The true spectrum (b) is scanned through a slit of finite width (c). The detected spectrum (d) is derived by multiplying (b) with (c) and integrating for each slit position as the slit is scanned across the spectrum (i.e., adding all the light that passed through the slit at each slit position). Note the broadening effect of the "convolution" process (compare (d) to (b)).

Figure 23 Steady-state scan after repetitive cycling of Mb-DDAB electrodes in pH 5.5 buffer containing 30 mM trichloroethylene. (Adapted from Ref. 34 with permission. Copyright 1995 American Chemical Society.)...

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