Pulsed Fourier transform spectrometry

The result is a so-called free induction decay (FID), which may be described as a decaying interferogram (see Section 5.1 for examples). The signals collected represent the difference between the applied frequency I t and the Larmor frequency vL of each proton. The FIDs are then Fourier transformed by computer into a conventional NMR spectrum. Since relaxation times for protons are usually on the order of a few seconds or fractions of a second, rapid repetitive pulsing with signal accumulation is possible. Some, 3C nuclei—those that have no attached protons to provide T, relaxation— require much longer intervals between pulses to allow for relaxation lack of adequate intervals results in weak signals and inaccurate peak areas (see Section 5.1). 

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The newer instruments (Figure 2.4c) utilize a radiofrequency pulse in place of the scan. The pulse brings all of the cycloidal frequencies into resonance simultaneously to yield a signal as an interferogram (a time-domain spectrum). This is converted by Fourier Transform to a frequency-domain spectrum, which then yields the conventional m/z spectrum. Pulsed Fourier transform spectrometry applied to nuclear magnetic resonance spectrometry is explained in Chapters 4 and 5. 

As described in Section 4.4 for pulsed Fourier transform spectrometry of protons, a short, powerful, rf pulse (on the order of a few microseconds) excites all of the 13C nuclei simultaneously. At the same time, the broadband decoupler is turned on in order to remove the 13C— H coupling. Since the pulse frequencies are slightly off resonance for all of the nuclei, each nucleus shows a free induction decay (FID), which is an exponentially decaying sine wave with a frequency equal to the difference between the applied frequency and the resonance frequency for that nucleus. Figure 5.2a shows the result for a single-carbon compound. 

Depending on how the secondary magnetic field is applied, there are two fundamentally different types of spectrometers, namely, continuous wave (CW) and pulse Fourier transform (PFT) spectrometers. The older continuous wave NMR spectrometers (the equivalent of dispersive spectrometry) were operated in one of two modes (i) fixed magnetic field strength and frequency (vi) sweeping of Bi irradiation or (ii) fixed irradiation frequency and variable field strength. In this way, when the resonance condition is reached for a particular type of nuclei (vi = vo), the energy is absorbed and... 

OES Optical emission spectrometry PFT-NMR Pulse Fourier-transform NMR... 

Instrumentation. The NMR Process. Chemical Shift. Spin-spin Coupling. Carbon-13 NMR. Pulsed Fourier transform NMR (FT-NMR). Qualitative Analysis - The Identification of Structural Features. Quantitative Analysis. Applications of NMR Spectrometry. 

The earlier, continuous-wave, slow-scan procedure requires a large sample and a prohibitively long time to obtain a, 3C spectrum, but the availability of pulsed Fourier transform (FT) instrumentation, which permits simultaneous irradiation of all 13C nuclei, has resulted in an increased activity in 13C spectrometry, beginning in the early 1970s, comparable to the burst of activity in spectrometry that began in the late 1950s. 

In Fourier transform spectrometry, the source supplies a wide range of frequencies. For infrared spectrometry, a polychromatic source is used, while for nuclear magnetic resonance (NMR) a powerful microsecond pulse of radiofrequency energy provides the range of frequencies required. 

The laser desorption experiments which we describe here utilize pulsed laser radiation, which is partially absorbed by the metal substrate, to generate a temperature jump in the surface region of the sample. The neutral species desorbed are ionized and detected by Fourier transform mass spectrometry (FTMS). This technique has... 

To overcome this, instrumental techniques such as pulsed high-pressure mass spectrometry (PHPMS), the flowing afterglow (FA) and allied techniques like the selected-ion flow tube (SIFT), and ion cyclotron resonance (ICR) spectrometry and its modem variant, Fourier transform mass spectrometry (FTMS), have been developed. These extend either the reaction time (ICR) or the concentration of species (PHPMS, FA), so that bimolecular chemistry occurs. The difference in the effect of increasing the pressure versus increasing the time, in order to achieve bimolecular reactivity, results in some variation in the chemistry observed with the techniques, and these will be addressed in this review as needed. 

MS, Mass spectrometry El, electron impact Cl, chemical ionization MID, multiple ion detection PICI, positive-ion chemical ionization NICI, negative-ion chemical ionization SIM, selected ion nmonitoring TSP, thermospray PPINICI, pulsed positive ion-negative ion chemical ionization ECD, electron-capture detector NPD, nitrogen/phosphorous detector NSTD, nitrogen-selective thermionic detector FT-IR, Fourier transform infrared spectrometry. 

Attempts have been made to observe and experimentally determine the structure of CH5+ in the gas phase and study it in the condensed state using IR spectroscopy,764 765 pulse electron-beam mass spectrometry,766 and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).767 However, an unambiguous structure determination was unsuccessful. Retardation of the degenerate rearrangement was achieved by trapping the ion in clusters with H2, CH4, Ar, or N2. 

Another exciting possibility for high sensitivity molecular surface mass spectrometry is the use of laser-excited ion desorption in a pulsed ion cyclotron resonance experiment using Fourier transform techniques. In an ideal situation, this scheme could include all those attributes which are desirable for solid-surface molecular characterization ... 

T. J. Carlin and B. S. Freiser, "Pulsed Valve Addition of Collision and Reagent Gases in Fourier Transform Mass Spectrometry," Anal. Chem., , 571-574 (1983). 

Selected topics in Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry instrumentation are discussed in depth, and numerous analytical application examples are given. In particular, optimization ofthe single-cell FTMS design and some of its analytical applications, like pulsed-valve Cl and CID, static SIMS, and ion clustering reactions are described. Magnet requirements and the software used in advanced FTICR mass spectrometers are considered. Implementation and advantages of an external differentially-pumped ion source for LD, GC/MS, liquid SIMS, FAB and LC/MS are discussed in detail, and an attempt is made to anticipate future developments in FTMS instrumentation. 

Laser Desorption Ionization. A pulsed laser beam can be used to ionize samples for mass spectrometry. Because this method of ionization is pulsed, it must be used with either a time of flight or a Fourier transform mass spectrometer (Section 1.4.5). Two types of lasers have found widespread use A COz laser, which emits radiation in the far infrared region, and a frequency-quadrupled neodymium/yttriumaluminum-garnet (Nd/YAG) laser, which emits radiation in the UV region at 266 nm. Without matrix assistance, the method is limited to low molecular weight molecules (<2 kDa). 

See for example R. R. Ernst, G. Bodenhausen and A. Wokaun, Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Oxford University Press, Oxford, 1988. A. G. Marshall and F. R. Verdun, Fourier Transforms in NMR, Optical, and Mass Spectrometry — A User s Handbook, Elsevier, Amsterdam, 1990. E. Fukushima and S. B. W. Roeder, Experimental Pulse NMR. A Nuts and Bolts Approach, Addison-Wesley Publishing Company, Reading, MA, 1981. 

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