Acquisition time

The acquisition time (z ) is related to the two previous parameters by the formula 

This parameter is not normally set directly but is a function of the values that you set for spectral width and number of points. The narrower the spectral width, the longer will be the acquisition time and the greater the number of points, the longer the acquisition time. 

Of course, it is quite easy to solve the bandwidth needs of proton spectra - they only have a spread over about 20 ppm (8 kHz at 400 MHz). Things get a bit more difficult with nuclei such as 13C where we need to cover up to 250 ppm (25 kHz) spread of signals and we do notice some falloff of signal intensity at the edge of the spectrum. This is not normally a problem as we seldom quantify by 13C NMR. However, it can be a problem for some pulse sequences that require all nuclei to experience 90° 

One last comment about pulse widths it is important that we know what the 90° pulse width is for the nuclei that we observe as accurate pulse widths are required for many pulse sequences (as mentioned previously). Failure to set these correctly may give rise to poor signal to noise or even generate artifacts in the spectrum. When instruments are serviced, these pulse widths are measured and entered into a table to ensure that the experiments continue to work in the future. 

For 2-D experiments, not only will you need to set the number of points for your direct detection dimension, you will also need to set the number of experiments in the second dimension as this will determine what resolution you have in that dimension. There is no simple answer to help here - it 

Manual shimming is not yet a thing of the past but it is certainly less of a badge of honour for budding spectroscopists. 

The FID must be sampled at a rate adequate to cover the spectral width, but how long should the data be acquired That depends on both the desired resolution and considerations of signal/noise ratio. The FID for a given spectral component decays with a time constant T2, which depends on the natural transverse relaxation time of the component and the inhomogeneity in the magnetic field. Thus, even acquisition of the FID for an unlimited time leads, on Fourier 

FIGURE 3.7 (a) Depiction of magnetization M precessing in the rotating frame at the frequencies 

FIGURE 3.8 (a) Schematic representation of the exponential decay of the FID from a single 

NMR line truncated at time T. (b) Fourier transform of truncated signal. 

FT theory shows that the effect of abrupt truncation of the FID is to convolve the normal Lorentzian line shape with a function of the form 

---

Fig. 5. A 2-D nmr experiment of 2-methyl-5-bromopentane [626-88-0] where and correspond to evolution and acquisition time, respectively.

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. 

Multidimensional or hyphenated instmments employ two or more analytical instmmental techniques, either sequentially, or in parallel. Hence, one can have multidimensional separations, eg, hplc/gc, identifications, ms/ms, or separations/identifications, such as gc/ms (see CHROMATOGRAPHY Mass spectrometry). The purpose of interfacing two or more analytical instmments is to increase the analytical information while reducing data acquisition time. For example, in tandem-mass spectrometry (ms/ms) (17,18), the first mass spectrometer appHes soft ionization to separate the mixture of choice into molecular ions the second mass spectrometer obtains the mass spectmm of each ion. 

Figure 4 Chemical images of a nickei TEM grid. Fieid of view is approximateiy 25 x 15 im, 50 X 50 pixeis. Analyticai conditions Ga sputtering, spot size about 0.2 pm, 24S-nm radiation, acquisition time 33 minutes.

To date, the usual way of recording the LEED pattern is a light-sensitive video camera with a suitable image-processing system. In older systems movable Earaday cups (EC) were used which detected the electron current directly. Because of long data acquisition times and the problems of transferring motion into UHV, these systems are mostly out of use nowadays. 

Dozens of compounds have been used in in vivo fluonne NMR and MRI studies, chosen more for their commercial availability and established biochemistry than for ease of fluonne signal detection [244] Among the more common of these are halothane and other fluormated anesthetics [245, 246], fluorodeoxyglucose [242 243], and perfluormated synthetic blood substitutes, such as Fluosol [246], a mixture of perfluorotnpropylamine and perfluorodecahn Results have been Imut-ed by chemical shift effects (multiple signals spread over a wide spectral range) and long acquisition times... 

A benefit of the use of accelerometers is that they do not require a calibration program to ensure accuracy. However, they are susceptible to thermal damage. If sufficient heat radiates into the piezoelectric crystal, it can be damaged or destroyed. However, thermal damage is rare since data acquisition time is relatively short (i.e. less than thirty seconds) using temporary mounting techniques. 

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

According to what has been stated above, good results have been obtained as a result of a spectrophotometric technique that entails a colored tracer. Two measuring probes are set up one at the inlet and the other at the outlet of the device. The acquisition time is set to 0.12 s. The operating protocol adopted during RTD experiments is as follows ... 

The most popular, and also a very accurate, experimental method for measuring nonselective spin-lattice relaxation-rates is the inversion recovery (180°-r-90°-AT-PD)NT pulse sequence. Here, t is the variable parameter, the little t between pulses, AT is the acquisition time, PD is the pulse delay, set such that AT-I- PD s 5 x T, and NT is the total number of transients required for an acceptable signal-to-noise ratio. Sequential application of a series of two-pulse sequences, each using a different pulsespacing, t, gives a series of partially relaxed spectra. Values of Rj can... 

In the example given in the preceding secdon, the number of words of data storage was 15,000 and the spectral width was 7500 Hz, so an acquisition time of 15,000/(2 X 7500) = 1.0 s was required after each pulse. 

We should decide in advance the digital resolution at which we wish to acquire a spectrum and then set the acquisition time accordingly. The acquisition time AT (that is, the product of the number of data points to be collected and the dwell time between the data points) is calculated as simply the reciprocal of the digital resolution ... 

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. 

---