Continuous-wave instruments

Fourier transform spectroscopy technology is widely used in infrared spectroscopy. A spectrum that formerly required 15 min to obtain on a continuous wave instrument can be obtained in a few seconds on an FT-IR. This greatly increases research and analytical productivity. In addition to increased productivity, the FT-IR instrument can use a concept called Fleggetts Advantage where the entire spectrum is determined in the same time it takes a continuous wave (CW) device to measure a small fraction of the spectrum. Therefore many spectra can be obtained in the same time as one CW spectrum. If these spectra are summed, the signal-to-noise ratio, S/N can be greatly increased. Finally, because of the inherent computer-based nature of the FT-IR system, databases of infrared spectra are easily searched for matching or similar compounds. 

One of the main advantages of FT spectrometers is that, since the FID is in digital form, we can repeat the excitation/detection process a number of times and all the resulting scans can be added and the FT performed on the resultant FID. In this way, we can improve the signal-to-noise ratio and can detect nuclei which are not very abundant (e.g. C) or have low sensitivity to NMR (see Section 4.2). These nuclei could not have been detected on the older continuous wave instruments, as the spectrum was the result of a single scan, obtained as one of the frequency or magnetic held were varied while keeping the other constant. 

As the screening effect increases, the nuclei are said to be shielded on a continuous wave instrument operating at fixed frequency, the intensity of the field B0 has to be increased in order to obtain resonance. Signals to the right of the spectrum are said to be resonant at high field. Signals observed to the left of the spectrum correspond to deshielded nuclei and are said to be resonant at low field (Fig. 9.11). 

Quantitative measurements in NMR are based on the area of the signals present in the spectrum. Signal areas can be produced as numerical values proportional to the area or, on less modern instruments, from the integration plots that are superimposed on the spectrum (Fig. 9.1). For the proton, the precision obtained in area measurements does not exceed l % even if continuous wave instruments are used at slow scanning speeds. In l3C NMR, it is preferable to add a relaxation reagent in order to avoid saturation related to relaxation times that alter the intensity of the signal. Using the molar ratios that are easily accessible from the spectrum, it is possible to deduce concentrations. 

Continuous wave instruments involve a considerable waste of time. A solution to the inefficiency of single - frequency observation is to excite all of the nuclei in a sample simultaneously and to observe the total response of the sample. This is done by periodical, intense, short RF pulses. A RF pulse excites a finite band width of frequencies. The detector observes a pattern called a free induction decay (FID). An example is presented in figure C.2. Fourier Transforming this FID, yields the classical NMR spectrum. 

The proton NMR spectmm shown in Fig. 4.3 has been recorded with a Varian continuous wave instrument at 60 MHz on a dcuterized chloroform solution of PBO. 

MHz continuous-wave instrument (i = 10 — 5). JHD for HD gas is 43 Hz, the maximum value (dHd = 0.74 A). A lower value represents a proportionately shorter dHD. hd determined in solution correlates with d in the solid state via the... 

This is the mathematical basis for the interconversion of the FID, or time domain function g(t), and the absorption spectrum or frequency domain function g (o). This interconversion is known as a Fourier transform (FT). Although mathematically g(t) and g (o) are easily interconvertible, in practice obtaining an FID takes a much shorter time. For instance, a single sweep of a spectrum of width 10 Hz, aiming at a 1-Hz resolution, takes about 10 s with a continuous-wave instrument with the pulse method, an FID with the same degree of resolution is obtained in less than 1 s. 

Proton-NMR is usually performed in continuous-wave instruments on relatively concentrated solution... 

The infrared spectrophotometer was a Perkin Elmer 467 grating instrument. The FT-NMR was JEOL FX-60Q with a C/ H switchable microprobe (1.7 mm diameter tubes). The proton spectra were obtained on a JEOL C60HL continuous wave instrument. 

The advances in the field of Fourier transform infrared (FT-IR) spectrometry in the past 20 years have been quite remarkable. FT-IR spectrometers are installed in just about every analytical chemistry laboratory in the developed world. Actually, we sometimes wonder why so many people still refer to these instruments as FT-IR spectrometers, or more colloquially simply as FTIRs, rather than simply as infrared spectrometers, since almost all mid-infrared spectra are measured with these instruments. We note that scientists who use nuclear magnetic resonance, the other technique that has been revolutionized by the introduction of Fourier transform techniques, no longer talk about FT-NMR, as continuous-wave instruments (e.g., grating monochromators) are a distant memory. Nonetheless, practitioners of infrared spectrometry seem to want to recall the era of grating monochromators, even though the vast majority has never seen one ... 

It can thus be seen that the signal-to-noise ratio, S/N, will grow at a rate proportional to the square root of the number of scans, and after addition of successive scans, the peaks gradually become more intense and emerge out of the noise on accumulation of a sufficiently large number of scans. While this improves the spectrum to some extent, the process is inefficient since it is limited by the long scan time involved in the recording of each spectrum on continuous-wave instruments. 

Nuclear Magnetic Resonance Spectrometer —A low-resolution continuous-wave instrument capable of measuring a nuclear magnetic resonance of hydrogen atoms, and fitted with ... 

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