Number of Scans

For H NMR spectra, the number of scans should be a multiple of 4, since this is the length of the CYCLOPS phase cycle used to minimize imperfections associated with quadrature signal detection (Section 5-8). Anywhere from 4 to 128 scans are usually sufficient to obtain a good spectrum with a relatively flat baseline. This number, however, depends heavily on the concentration of the sample. Nevertheless, accumulation times of over 1 hour are relatively rare. 

In this regard, a useful feature of many spectrometers is the block-size parameter. For NMR, and in general, block sizes are set to 4 or a multiple of 4, again depending on the concentration of the sample. The number of scans is set to an accumulation time that might correspond to the total time that the operator has reserved. (Modern spectrometers also have programs that calculate the total experimental time from the number of scans, and any delay times.) At the end of each block, the summed FIDs (see earlier) are written into the computer memory, where they can be Fourier transformed. When the spectrum displays a sufficient S/N ratio, the acquisition is halted. Remember, from the square-root relationship just discussed, in order to double the S/N ratio, we must quadruple ns. 

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Time constraints ate an important factor in selecting nmr experiments. There are four parameters that affect the amount of instmment time requited for an experiment, A preparation delay of 1—3 times should be used. Too short a delay results in artifacts showing up in the 2-D spectmm whereas too long a delay wastes instmment time. The number of evolution times can be adjusted. This affects the F resolution. The acquisition time or number of data points in can be adjusted. This affects resolution in F. EinaHy, the number of scans per EID can be altered. This determines the SNR for the 2-D matrix. In general, a lower SNR is acceptable for 2-D than for 1-D studies. 

The ease of reduction of the Pu 4+ ions apparently can be affected by inclusion in the glass of additional additives. Whereas the addition of Ti02 apparently does not affect in-situ reduction, the addition of CaO, ZnO, or B2O3 appreciably accelerated reduction of the Pu 4+ ions. This effect is demonstrated with 4f core level spectra of glasses F, G, H, and I shown in Fig. 3. Each sample was subjected to the same X-ray exposure, i.e., the same number of scans at the same flux. 

It is better to maintain the same number of data points and reduce the spectral width as far as possible. In the alternative case, the improved digital resolution will be at the cost of sensitivity, since it will produce a corresponding increase in acquisition time, AT. Either a greater time period would then be required or a lesser number of scans would be accumulated in the same time period, with a corresponding deterioration in the signal-to-noise ratio. 

In figure 5 spectra are presented for a Pt-Re catalyst that has just been dried at 120°C (curve 1) and the same catalyst after reducing at 400°C for 18 hours (curve 2). The potassium containing alumina also shows the weak, interferring peak (curve 3, noisy because of a limited number of scans). 

FT-IR spectra were recorded at RT on a Perkin-Elmer 1760-X spectrophotometer equipped with a cryodetector, at a resolution of 2 cm-" (number of scans -100). In the 1070-960 cm- region, band integration and curve fitting were carried out by Curve fit, in Spectra Calc. (Galactic Industries Co.). Powdered materials were pelleted in self-supporting discs of 25-50 mg cm-2 and 0.1-0.2 mm thick, placed in an IR cell allowing thermal treatments in vacuo or in a controlled atmosphere. 

Fig. 11. Time-of-flight spectra for YH2 and YOCH3 products at indicated lab angles for the Y+CH3OH reaction at con = 28.1 kcal/mol. TOFs have been scaled to the same number of scans. Solid-line fits generated using the CM distributions shown in Fig. 12.

Fig. 40. Lab angular distributions for YC3H4 (open circles), YH2 (open triangles), and YCH2 (open squares) products from the Y +propene reaction at con = 28.8 kcal/mol. Corresponding product yields are given in the upper right corner of each graph. Note that each distribution is scaled to the same number of scans (2).

Most recently, we have been able to obtain the in situ surface EXAFS spectrum of a half-monolayer of underpotentially deposited copper on a bulk Pt(lll) single crystal pretreated with iodine. The spectrum shown in Fig. 23 is a bit noisy (due to limited number of scans) but at least five well-defined oscillations can be observed. Preliminary data analysis indicates that the copper adatoms sit on threefold hollow sites with copper neighbors at 2.80 0.03 A. This distance is very close to the Pt—Pt distance in the (111) direction and indicates the presence of a commensurate... 

It s more likely these days that you will be using a 250 or 400 MHz Fourier transform instrument with multi-nuclei capability. If such an instrument is operating in walk up mode so that it can acquire >60 samples in a working day, then it will probably be limited to about 32 scans per sample (a handy number - traditionally, the number of scans acquired has always been a multiple of eight but we won t go into the reasons here. If you want more information, take a look at the term phase cycling in one of the excellent texts available on the more technical aspects of NMR). This means that for straightforward... 

Probably the most basic parameter that you will be able to set is the number of spectra that will be co-added. This is normally called the number of transients or number of scans . As mentioned elsewhere in the book, the more transients, the better the signal to noise in your spectrum. Unfortunately, this is not a linear improvement and the signal to noise increase is proportional to the square root of the number of transients. As a result, in order to double your signal to noise, you need four times the number of scans. This can be shown graphically in Figure 3.1. 

As an example of the form of the information that may be derived from a pyrolysis-MS, Figure 26 [69] shows the structure of the polycarbonate (PC) and the EI-MS spectra of pyrolysis compounds obtained by DPMS of poly(bisphenol-A-carbonate) at three different probe temperatures corresponding to the three TIC (total ion current) maxima shown in Figure 27(b) Figure 27 compares the MS-TIC curve with those obtained from thermogravimetry. (The TIC trace is the sum of the relative abundances of all the ions in each mass spectrum plotted against the time (or number of scans) in a data collection sequence [70].)... 

In practical applications, it can be a factor that the above approach by virtue of the cycle over A values has a higher minimum number of scans per ti value than the standard experiment and its various accordion versions. For dilute samples, this does not matter but for concentrated samples the instrument time can be longer than required considering the inherent sensitivity. 

Figure 27 Edited broadband HMBC spectrum of cyclosporine using the pulse sequences shown in Figure 26 in an interleaved manner. The two subspectra, CH + CH3 (left) and C + CH2 (right), exemplify the editing properties. The spectrum in the bottom displays the two subspectra, CH + CH3 (black) and C + CH2 (grey) in the same frame. The number of scans was 32 for each of the 128fi increments, the relaxation delay was 1 s, and the range for the third-order low-pass. /-filter was 115 Hz < Vch < 165 Hz. The spectra were processed to maintain the absorptive profiles in F, while a magnitude mode was done in F2.

Corrigan and Weaver employed the PDIR approach to study the potential-dependent adsorption of azide, N , at a silver electrode. The potential was switched between the reference value, —0.97 V vs. SCE (where adsorption is known to be limited) and the working potential every 30-60 scans, i.e. up to a minute per step, to a total of c. 1000 scans. The high number of scans was required in order to obtain the required S/N ratio hence the PDIR technique was employed to minimise instrumental drift. Since the electrochemical process under study was totally reversible on the timescale of the experiment, the PDIR technique was a viable option. 

When performing 2D-NMR experiments one must keep in mind that the second frequency dimension (Fx) is digitized by the number of tx increments. Therefore, it is important to consider the amount of spectral resolution that is needed to resolve the correlations of interest. In the first dimension (F2), the resolution is independent of time relative to F. The only requirement for F2 is that the necessary number of scans is obtained to allow appropriate signal averaging to obtain the desired S/N. These two parameters, the number of scans acquired per tx increment and the total number of tx increments, are what dictate the amount of time required to acquire the full 2D-data matrix. 2D-homo-nuclear spectroscopy can be summarized by three different interactions, namely scalar coupling, dipolar coupling and exchange processes. 

FIGURE 48. Representative 2H NMR spectra (full lines) of dark-adapted bR (90 mg) containing deuteriated retinal, with line shape simulations (dashed lines) superimposed. Both the powder spectrum (a) from randomly oriented PM patches and the tilt series (b) over sample inclinations, a = 0, 45° and 90°, were recorded at — 60 °C (number of scans, 1.7 x 105, for a = 0°). Spectrum (c) was measured at 21 °C with a = 0° (number of scans, 3 x 105). Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

NUMBER OF SCANS Each data point of both the reference and the response signals is memory averaged over several scans to complete a run. 

STATISTICS At the end of the experiment the statistics (average, standard deviation, and chi-square) of each harmonic are computed. The data quality (precision and accuracy) depends on both the numbers of scans and the number of runs. 

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