Data Acquisition Parameters

As mentioned in Section 3.4, the success of the pulsed-mode NMR technique depends on the careful selection of data acquisition parameters. These data for the six spectra discussed in this chapter appear in Table 5.2. The same sample was used for all the spectra. 

Notice the following trends. First, as expected, it takes many more scans to generate an adequate C spectrum than a H spectrum because of the lower sensitivity of the carbon nuclei (Table 2.1). On the other hand, the higher the field and operating frequency, the fewer scans needed to produce the desired S/N. Because the carbon spectral width is substantially larger than that of hydrogen at the same field strength, the dwell time is shorter for carbon, and since this fills the 

TABLE 5.2 Data Acquisition Parameters for NMR Spectra of Toluene  

Before we leave the discussion of these C spectra, there is one more qualifier to add. All three C spectra discussed in this chapter (and all subsequent ones we will encounter, unless specifically noted to the contrary) involve simultaneous irradiation of the hydrogens as well as the carbons, a technique known as H-spin decoupling. (It is this simultaneous, or double, irradiation that gives rise to the NOE mentioned in Section 5.4.1.) A H-spin-decoupled C spec-tmm is labeled C H. Spin decoupling will be described more fully in Chapter 12, but for now, suffice it to say that if we had not made use of this decoupling, these spectra would have looked far more complicated, and all intensity differences due to NOE would be absent. 

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Light-up, ion lens tuning, mass calibration, optimization of data acquisition parameters, determination of Cd in water by isotope dilution... 

To optimize the dwell time, set up a data acquisition procedure using the dwell times shown in Table B.4, aspirate the optimization solution and acquire count rate data for the ° Cd and Cd isotopes. The method for setting the data acquisition parameters will vary between instruments, but a similar procedure should be possible for all makes of instrument. Record the data in Table B.4, calculate the mean and RSD for the lo Cdri Cd ratio for each dwell time and hence determine the best precision. 

Data acquisition parameters. Precision and accuracy in the measurement of isotope ratios can be improved if the number of measurements is increased (e.g. if the measurement time is increased). Various measurement protocols can be applied and those whereby the time actually spent on measuring the isotope ratios of interest is maximised are preferable. The data acquisition parameters of an ICP-MS device that can be changed to improve the isotope ratio precision... 

Over the past 40 years fluorine nuclear magnetic resonance (19F-NMR) spectroscopy has become the most prominent instrumental method for structure elucidation of organofluorine compounds. Consequently the amount of spectral data published has grown almost exponentially Unfortunately NMR data for fluonnated compounds are not as well, or as easily, organized as proton data To facilitate retrieval of fluorine NMR information and comparison of data, acquisition parameters should be clearly defined Guidelines for publication of NMR data have been established by the International Union for Pure and Applied Chemistry (IUPAC) [7] The following niles for acquisition and reporting of NMR data should be strictly observed... 

Figure 8.2.16 COSY spectra acquired with the four-coil probe, where the compounds and concentrations were the same as those of the one-dimensional spectra. Data acquisition parameters spectral width, 2000 Hz data matrix, 512 x 128 (complex) 16 signal averages delay between successive coil excitations, 400 ms effective recycle delay for each sample, ca. 1.7 s. Data were processed by using shifted sine-bell multiplication in both dimensions and displayed in magnitude mode. Reprinted with permission from Li, Y., Walters, A., Malaway, P., Sweedlar, J. V. and Webb, A. G., Anal. Chem., 71, 4815M820 (1999). Copyright (1999) American Chemical Society...

Instrumental operating conditions and data acquisition parameters [198]. 

For all of the applications outlined above, and many others besides, it is desirable to use NMR parameters which possess an intrinsic temperature dependence in order to measure directly the sample temperature. These measurements can either be performed as a pre-experiment calibration procedure using identical data acquisition parameters as for the actual experiment, or as an in situ measurement using the actual sample. Temperature-dependent NMR parameters include spin lattice (Ti) and spin-spin T2 relaxation times, chemical shifts, dipolar and scalar couplings, molecular diffusion coefficients and net equilibrium polarization. Dependent upon the particular application, each of these parameters has been utilized as an NMR thermometer . 

Optimal data acquisition parameters were derived by maximizing the S/N of the temperature-dependent phase difference images ... 

TABLE 8.4. Data Acquisition Parameters of MRM Transitions Used for LC/ESI-MS Analysis of Biogenic Amines in Wine"... 

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