Pulse Fourier transform spectrometer sensitivity

In the early days there was a sensitivity problem when using CW techniques which has been overcome - at least in part - by indirect multiple-resonance experiments, but with the advent of pulse Fourier Transform spectrometers (ca. 1970) sensitivity is no longer an obstacle. 

The optimum sensitivity of the Fourier transform spectrometer obtained by calculating the signal-to-noise ratio of a pulse Fourier transform spectrometer relative to a conventional absorption spectrometer has also been given. Suppose that a spectral range F has to be investigated in a total time, T. Both experiments will use a superheterodyne detection system with a balanced mixer. The noise is assumed to be white with a power density Pg per spectral unit. The S/N ratio is defined as the ratio of the peak signal amplitude to the rms noise amplitude. 

A considerable improvement in speed and sensitivity can be achieved with a pulsed Fourier transform (FT) spectrometer. Here the sample is subjected to a series of short duration high intensity RF pulses (1-100 us)... 

The development of pulsed Fourier-transform NMR spectrometers has greatly increased the sensitivity of NMR measurements, allowing spectra to be obtained for and other nuclei at natural abundances and low sample concentrations. In this experiment this enhanced capability is utilized in a low-density gas-phase measurement of the equilibrium constant Kp for H-D exchange in the reaction... 

NMR spectrometers of the late 1960s did not permit the detection of higher spin orders for sensitivity reasons, so no new name was coined for them the term used today is "higher multiplet effects". More importantly, with the cw instruments ubiquitous at that time a separation of different spin orders n was principally impossible. The advent of pulsed and Fourier transform spectrometers reduced that to a trivial task Because for a weakly coupled spin system the amplitude of the detectable signal is proportional to sim cos i , one simply has to acquire spectra with different flip angles i9 and form suitable linear combinations (e.g. one- and two-spin orders are separated by adding and subtracting two spectra acquired with = 45° and = 135°). ... 

More recently, we have improved the time-resolution of the system substantially. The present instrument is capable of recording high time- and frequency-resolution spectra of transients having decay times from the nanosecond to the millisecond regime. The minimum time delay between the initiation of the transient and the first spectral observation can be arbitrarily short. (Typically, the first spectrum is recorded just before the transient in order to provide a background observation. A maximum of 128 successive time-delayed spectra of a single transient can be recorded the minimum time delay between each of these is 10 ns. All operational parameters (resolution, sensitivity, etc.) of the commercial Fourier transform spectrometer with which the system is used, are unchanged by time-resolved operation. Variability in die baseline due to amplitude instabilities in the excitation source (usually a pulsed laser) are taken into account, and appropriate corrections are made. 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

The Fourier transform ion cyclotron resonance (FT-ICR) trapping of mass-selected cluster ions is an important emerging technique for the study of ion cluster reactivity. " This technique offers very high resolution and sensitivity. An alternative approach has been used by Brucat et al. who demonstrated that the reactivity of cluster ions can be studied in an experimental configuration identical to that used for the study of neutrals, except that ions are detected directly by pulsed extraction in the time-of-flight mass spectrometer. Other experiments " are exploring the reactions of mass-selected cluster ions in beam-gas-cell or drift-tube type configurations. This approach avoids the problems of mass overlap and offers a direct probe of cluster and cluster-adduct stabilities. For further experimental details, the reader is referred to the references. 

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