Spectrometers continuous-wave

As far as apparatus is concerned, chlorine resonances are more easily detected with superregenerative spectrometers in which induction phenomena yield enhanced signals. Continuous wave spectrometers, which do not have this advantage, have their effective sensitivity limited by the long relaxation times encountered in many of the compounds studied. 

The spectral accumulation approach is also used in Fourier Transform Spectrometers. In this case spectral data are collected far more rapidly and spectra may be obtained in a reasonable period of time fium about 20 g of sample. However, these instruments are much more expensive than continuous wave spectrometers although they may also be used to prepare C spectra. 

In pulsed ESR samples are exposed to a series of short intense microwave pulses, e.g. the 2-pulse sequence shown in Fig. 1.14 in place of the continuous radiation with microwaves at low power in the traditional continuous wave spectrometer. 

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 is usually more expensive than a continuous wave spectrometer, since it must have fairly sophisticated electronics capable of generating precise pulses and accurately receiving the complicated transients. A good C 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. 

Continuous wave spectrometers have been replaced by FT-NMR spectrometers in which the sample is irradiated with a short pulse that covers the entire range of relevant rf frequencies, and a free induction decay (FID) is recorded and then converted into a spectrum. 

EPR measurements were made with a Bruker EMX X-band continuous wave spectrometer equipped with a Bruker ER 4116 DM rectangular cavity operating at 9.658 GHz. Experiments were performed at room temperature (300K) with a hyper frequency power of 1 mW and a modulation amplitude of 0.5 G. The amplitude of the magnetic field modulation and microwave power were adjusted so that no line-shape distortion was observable. The received gain was 63,200 and the sweep time was 42 s. Absolute quantification was obtained by comparison with a TEMPO sample of known concentration after double integration of EPR spectra. 

Continuous wave (CW) lasers such as Ar and He-Ne are employed in conmionplace Raman spectrometers. However laser sources for Raman spectroscopy now extend from the edge of the vacuum UV to the near infrared. Lasers serve as an energetic source which at the same hme can be highly monochromatic, thus effectively supplying the single excitation frequency, v. The beams have a small diameter which may be... 

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... 

It is observed that the infrared spectrum obtained with a continuous wave infrared spectrometer has increasing resolution as the scan speed is decreased. Explain this observation. 

---