Pulse Fourier transform spectrometers

Perhaps the widest application is that of conventional high-resolution spectroscopy in solution for the purpose of learning in detail about polymer chain structure. In this field, proton NMR, formerly dominant, has given way to carbon-13 NMR with the development of pulse Fourier transform spectrometers with spectrum accumulation. Carbon spectroscopy is capable of giving very detailed and often quite sophisticated information. For example, a very complete accounting can be provided of comonomer sequences in vinyl copolymers and branches can be identified and counted, even at very low levels, in polyethylenes. 

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. 

Figure 4 shows the frequency domain spectra obtained with our pulsed Fourier transform spectrometer for c(13)h20 in natural abundance displayed on an oscilloscope. The formaldehyde pressure was approximately 1 mTorr. The spectra cover 25 MHz and each frequency point corresponds to 100 KHz. The displayed line corresponds to the 111 llO t otational transition of c(13)h20 at 4593.3 MHz. The carrier frequency, ff/ o, was kept at 4 MHz off-resonance in the upper spectrum and 21 MHz off-resonance in the lower spectrum. Both spectra were obtained after an averaging time of 15 sec and an optimum exponential filter was used in the digital conversion. The line has an absorption coefficient of 6 x 10 8 cm"l. The obtained signal-to-noise ratio (peak signal amplitude to rms noise amplitude) is approximately 50 1. 

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. 

It is intuitively easy to understand that the pulse Fourier transform spectrometer leads to an improvement in the signal-to-noise ratio as compared to the conventional steady-state experiment. In the steady-state experiment, the frequency is swept over a frequency range, and the spectrum is recorded as a function of the frequency. 

Due largely to the very low natural isotopic abundance of carbon-13, spectra can be recorded only by a pulsed Fourier transform spectrometer. Chemical shifts are much greater than in proton spectra and are of particular value in establishing the skeletal structures of organic molecules. 

Like NMR spectrometers some IR spectrometers oper ate in a continuous sweep mode whereas others em ploy pulse Fourier transform (FT IR) technology All the IR spectra in this text were obtained on an FT IR instrument... 

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... 

Broadband proton-decoupled pulse Fourier transform C n.m.r. were recorded in deuterochloroform at 20 MHz using a Varian CFT-20 spectrometer. 

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 NMR spectrum of calcitriol, recorded on a Varian XL-100/Nicolet TT-100 pulsed Fourier Transform NMR spectrometer, with internal deuterium lock, is shown in Figure 2 (2). The spectrum was recorded using a solution of 0.84 mg of sample dissolved in 50 microliters of CD OD (100%D) containing 1% v/v tetramethylsilane in a 1.7 mm capillary tube. The spectral assignments are given in Table I. 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

The 500-MHz, H-n.m.r. spectra were recorded with a Bruker WM-500 spectrometer operating in the pulsed, Fourier-transform mode and equipped with a Bruker Aspect2000 computer having an 80k memory-capacity. The D resonance of D20 was used as the field-frequency lock-signal. The spectra were obtained by using a 90° pulse-width, and accumulated into 16k addresses with an acquisition time of... 

Figure 10.36. Schematic diagram of the laser vapourisation source and pulsed Fourier transform microwave spectrometer developed to study rare earth oxides [88].

NMR was for many years restricted to a few nuclei of high natural abundance and high magnetic moment ( H, F and P). Less receptive nuclei, such as C and Si, required too long observation times to be applicable. With modern pulsed Fourier-Transform (FT) spectrometers, which have largely replaced the old continuous wave (CW) technique, individual spectra can be collected much more rapidly, so that 13C NMR has become a routine. 

A Fourier transform is the mathematical technique used to compute the spectrum from the free induction decay, and this technique of using pulses and collecting transients is called Fourier transform spectroscopy. A Fourier transform spectrometer requires sophisticated electronics capable of generating precise pulses and accurately receiving the complicated transients. A good 13C NMR instrument usually has the capability to do H NMR spectra as well. When used with proton spectroscopy, the Fourier transform technique produces good spectra with very small amounts (less than a milligram) of sample. 

Further improvements in superconducting materials50 permitted the development in 1970 of a 300-MHz spectrometer that is equipped with a low-loss Dewar vessel, an improved sweep-system that allows both field sweeps and frequency sweeps up to a maximum width of 20 kHz, and accessories for a variety of nuclei and pulse-Fourier-transform techniques. 

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