Spectrometer deadtime

Another problem in many NMR spectrometers is that the start of the FID is corrupted due to various instrumental deadtimes that lead to intensity problems in the spectrum. The spectrometer deadtime is made up of a number of sources that can be apportioned to either the probe or the electronics. The loss of the initial part of the FID is manifest in a spectrum as a rolling baseline and the preferential loss of broad components of... 

Legrand et al. [100] reported F wideline NMR measurements on a 70/30 mol% VDF-TrFE random copolymer, made in order to study molecular motion both below and above the ferroelectric transition temperature, Tc. The samples consisted of semicrystalline copolymer films of 0.51 mm thickness, with biaxial orientation of the crystalline axis. The samples were rolled (without poling) at 70°C, with a draw ratio of 300%. The F resonance was chosen, rather than the proton resonance, because the abundance ratio of F to nuclei is 1.4. In addition, the F free-induction decay (FID) lasts longer than that of the proton, which decreases the influence of spectrometer deadtime. FID analyses were made assuming a simple superposition of two... 

The main shortcoming of the two-pulse experiment is that the primary echo decays within the phase memory time, 7m, which is often very short. This can prevent the observation of low-frequency modulations, and thus the estimation of the magnetic parameters can become uncertain. Another important limitation arises from the spectrometer deadtime u (typically 100-150 ns at X-band frequencies), which restricts the observation of the signal to times t > ti. The loss of the initial part of the time trace can cause severe distortions in the frequency-domain spectrum, especially in disordered systems where destructive interference from differ-... 

When T is varied the echo envelope is modulated only by the two basic frequencies CDa and (Up, the sum and difference frequencies do not appear, in contrast to the two-pulse ESEEM experiment. This is usually advantageous, as it simplifies spectra, but it may also be a disadvantage for disordered systems where the sum-combination line is often the only narrow feature in the ESEEM spectrum. Another important difference is the dependence of the three-pulse ESEEM amplitudes on r, as is apparent from Eq. (17) by the factors 1 - cos(copr) and 1 - cos(cOcir). Due to this suppression effect, individual peaks in the spectrum can disappear completely. These blind spots occur for the a(P) peak when r = 2n /(Up(a) (k = 1, 2,. ..). In principle they can be avoided by using r < Inlco, where (Umax is the maximum nuclear frequency however, this is usually precluded by the spectrometer deadtime. Consequently, the three-pulse ESEEM experiment has to be performed at several r values to avoid misinterpretation of the spectra due to blind-spot artifacts. 

The use of a remote-echo detector allows r values shorter flian the spectrometer deadtime to be employed [55]. This is important in two-pulse ESEEM experiments where the deadtime prevents the signal for times r < from being recorded. Also in the deadtime-free four-pulse experiments described in 3.3, a small T value is often needed to avoid blind spots. Bhnd spots are a particular concern for flie measurement of proton spectra at X-band, where flie signals typically extend from 5 to 25 MHz, and with a r = 100 ns blind spots occur at nh = 0, 10, 20,... MHz. 

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

A similar result has been observed with methylmalonyl-CoA mutase (Padmakumar and Banerjee, 1997). When this enzyme was reacted with prateo-methylmalonyl-CoA, homolysis of AdoCbl was almost complete within the deadtime of the stopped flow spectrometer. However, with (ds-methyl)-methylmalonyl-CoA homolysis is much slower and the isotope effeet is estimated to be at least 20. This suggests that coupling between CooC bond eleavage and substrate hydrogen abstraction is likely to be a general phenomenon. 

This preamplifier unit adds only a negligible amount of time to the total deadtime of the spectrometer, which is short enough to allow the observation of the NMR signal between two 90° pulses delayed by not more than... 

Although FT NMR can become a major tool for wide line NMR spectroscopy, cw spectrometers will still be necessary to record spectra which are so broad that the deadtime is comparable to or greater than T2 from samples which do not give rise to echoes. In the frequency domain, this means that the (sinx)/x wiggles shown in the last figure will have frequencies and amplitudes comparable or greater than the peak itself, thus obscuring the desired spectrum. See IV. B. 3. and VI.D.4. for other pulse techniques to overcome the deadtime problem. 

The key feature of ReMims (Doan) ENDOR is that Ti can be less than the deadtime of the spectrometer. This is because, in contrast to Mims ENDOR, the stimulated spin echo (which would be distorted by cavity ringdown) is not detected. Instead, an additional tt pulse is applied after time tz, which leads to a standard spin echo at time tz (Hahn echo, since it results from the original Hahn sequence here from the third tt/2 pulse and the following tt pulse). More important, there are two additional spin echoes formed one at time (tz + x ), which is observable for all values of x and xz, and is denoted the RME, which is detected, and one at time (xz — x ), which is observable only for values of t2 > ti, and is denoted the refocused stimulated echo (RSE). The deadtime for the ReMims (Doan) sequence is thus the minimum feasible value of (t2 + Ti), rather than the minimum value of x in a Mims sequence (Figure 6). As mentioned earlier (Section 3.3.4), the maximum undistorted Aiso (MHz) is - 1/(2ti (jls)). The deadtime in the X-band pulsed spectrometer is /d 0.1 J,s... 

Since deadtimes in this type of spectrometer are quite long ( 60 fis), the system must normally operate with deadtime losses in the 10 to 60% range. Consequently, most multichannel analyzers are equipped with an electronic means of deadtime correction, such that the observed spectrum represents the true number of photons arriving at the detector during the period of data accumulation. In addition to the ability to display the spectrum on a cathode-ray tube or television monitor, the analyzer can usually drive an X-Y plotter to produce a permanent copy. Alternatively, the contents of the analyzer memory can be printed as the number of counts in each channel, listed by channel number. Most quantitative fluorescence spectrometers include a personal computer with approximately 2-6 megabytes of memory plus some form of mass storage. In such a system the computer may control specimen presentation, the excitation conditions, and data accumulation in the multichannel analyzer. At the end of data acquisition for each specimen the computer analyzes the spectrum in the multichannel analyzer, computes the raw element intensities, corrects for interelement effects, and computes the concentration of each element. 

The purpose of this section is to outline the factors that limit the accuracy of the analysis at high counting rates with the electronics in energy-dispersive x-ray spectrometers. The limitations can be grouped into three categories spectral distortions, loss of throughput efficiency, and systematic errors in the deadtime correction. 

Most energy-dispersive spectrometers include a means for automatically correcting for deadtime losses. The residual error in the deadtime loss correction is generally an increasing function of counting rate. Thus, systematic errors in the deadtime correction scheme can limit the counting rates that can be employed. 

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