Acquisition time, irradiated

Acquisition time, irradiated PE, 247 Acyclic terpenes 33C NMR of natural... 

Reaction rate experiments were conducted in NMR tubes sealed with Teflon valves. In an inert atmospere glovebox, catalysts and internal standard, TMS4C, were weighed into the tube, followed by addition of solvents and reactants. The tube was immediately inserted in the preheated (50 °C) probe of a 500 MHz Varian Unitylnova spectrometer. To acquire spectra the sample was irradiated twice with a 30° pulse, 5 sec acquisition time, and 120 sec delay. 

The degree of activation of the sample is measured by post-irradiation spectroscopy, usually performed with high-purity semiconductors. The time-resolved intensity measurements of one of the several spectral lines enables to get the half-life of the radioactive element and the total number of nuclear reactions occurred. In fact, the intensity of a given spectral line associated with the decay of the radioactive elements decreases with time as Aft) = Aoexp[—t/r], where Aq indicates the initial number of nuclei (at t = 0) and r is the decay time constant related to the element half-life (r = In2/ /2), which can be measured. Integrating this relation from t = 0 to the total acquisition time, and weighting it with the detector efficiency and natural abundance lines, the total number of reactions N can be derived. Then, if one compares this number with the value obtained from the convolution of... 

As with the NOE experiment, one proton or a group of protons i.s selected for selective perturbation prior to the acquisition time. Among several variations the simplest is the one using an initial selective 90 pulse and a of series of experiments may be performed with the selected target spin(s) to be irradiated varied from experiment to experiment. In contrast to the ID NOE experiment ID ROE spectra are directly obtained after Fourier transformation of the corresponding FIDs and no difference spectra need to be calculated with this simple variant. However problems with quantitation occur which may be partially circumvented by using alternative experimental schemes. 

How does a spectrometer deliver this RF irradiation to the probe Compared to normal excitation pulses, which are very high-power and short ( 10 p,s) duration, decoupling requires low-power irradiation for the entire acquisition time (1-2 s). This is usually accomplished by having two separate sources of RF power, a broadband transmitter that can be... 

Since ]H nuclei normally exhibit 7) values on the order of 1 s, we need essentially no additional delay after 4 s acquisition time. For l3C and other slow-to-relax nuclei, however, substantial delay times are sometimes required. Following the pulse delay, the sample is irradiated with another pulse, and the data acquisition sequence begins anew. 

The Raman spectra on solid metal substrates were measured using a Renishaw RM2000 micro-Raman apparatus, coupled with a diode laser source emitting at 785 nm. Sample irradiation was accomplished using the 50x microscope objective of a Leica microscope DMLM. The beam power was 3 mW, the laser spot diameter was adjusted between 1 and 3 pm. Raman scattering was filtered by a double holographic notch filter system and collected by an air-cooled CCD detector. The acquisition time for each measurement was 10 s. All spectra were calibrated with respect to a silicon wafer at 520 cm. ... 

Fig. 2. Human thyroid biopsies. 1D >H MR spectra (8.5 T, 37°C) residual water suppressed by selective gated irradiation, sweep width 3957 Hz, using 8192 data points, 128 accumulations, and acquisition time of 1.14 s and relaxation delay of 2 s. (a) Normal thyroid tissue, (b) Known follicular carcinoma, (c) Thyroid follicular neoplasm predicted by MRS to be normal—confirmed by histopathology. (d) Thyroid follicular neoplasm predicted by MRS to be cancer and confirmed by histopathology—the presence of capsular invasion. Reprinted with permission from the National Research Council, Canada.

A method of reducing the acquisition time to avoid photo-damage of living cells during irradiation with the near-UV light and tracking cell responses in real time. 

Fluorine-19 NMR data were acquired at a frequency of 188.22 MHz with a Varian XL-200 spectrometer. Typically, 100 transients were accumulated from a 5% polymer solution by volume in dimethylformamide-d7 placed in a 5 mm sample tube at 120 C with internal hexafluorobenzene as a reference ( = 163 ppm). A sweep width of 8000 Hz was used with 8 K computer locations (acquisition time 0.5s) and a 5.0 s delay between 90 pulses (9.0 s duration). Proton heteronuclear coupling was removed by broad-band irradiation centered at 200 MHz. A modified Bruker WH-90 spectrometer allowed carbon-13 NMR spectra to be obtained with simultaneous proton and fluorine-19 broadband decoupling (13). 

In practice, a number of obvious experimental tricks are available to keep irradiation effects under control, such as focusing on one area, and moving to a contiguous area to acquire a spectrum using very short acquisition times from fresh areas to acquire noisy spectra, and then superimposing them to reconstitute an acceptable spectrum - whose spatial resolution is now governed by how close together in space the individual acquisitions can be made [3.34]. 

Prominent advantages of these methods include multi-element simultaneous analysis via commonly well-spaced spectral lines, and chemical-state information accessible via smaU, but characteristic and measurable shifts or shape changes in the lines. The stability of surface layers under photon or electron irradiation limits the possible duration of data acquisition time. Seah [20] has described a system for the intensity/energy calibration of electron spectrometers used in AES and XPS, necessary for quantitative analysis. Both AES and XPS may be made quantitative with reasonably good precision, although a great deal of care is necessary. Riviere [21] has compared AES and XPS to other methods of surface analysis (SIMS, ISS, EPMA, RAIRS). 

How much light goes into a sample for a single exposure in the two setups For this, an identical laser source and identical frame rates are assumed for both systems. In single beam scanning, the beam moves with finite speed over all regions of the sample. The total irradiation R of the sample is thus the product of the intensity of the laser beam I and the total acquisition time t for a frame. 

The final limitation to multiple contact operation is the power-handling capability of the spectrometer, the sample probe, and associated electronics in particular. The acquisition time for a FID is determined by the required or achievable spectral resolution. Even for modest resolution of the order of 10 Hz, t q would be 0.1 s, so ten contacts would require of the order of 1 s continuous irradiation of 60 kHz amplitude, say, and a Tip( H) of the order of 1 s or greater. 

Figure 1 Dissipation of fluorescence in wool on extended exposure to laser irradiation. (A) 0, (B) 30 and (C) 210 s. (633 nm, 5 s acquisition time).

In the case of homonuclear NOE measurements (most commonly H- H), single frequency irradiation of one proton resonance is performed to produce the spectmm with NOE, and irradiation in a position away from all of the resonances is performed to suppress the NOE (control spectmm). In both experiments (NOE and control), the decoupler is turned off during the acquisition time (blue periods in Figure 16) so that normal coupled proton spectra are obtained. If the extreme narrowing condition is satisfied, the theoretical maximum NOE for each signal is... 

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