Dispersive spectrometers, optical

In the infrared spectral range in general Fourier transform (FT) interferometers are used. In comparison with dispersive spectrometers FTIR enables higher optical throughput and the multiplex advantage at equivalent high spectral resolution. In... 

The initial single beam dispersive spectrometers that did not, at the time, produce digitised spectra (this would have allowed for baseline correction) were soon replaced by double beam spectrometers. This more complex arrangement can directly yield the spectrum corrected for background absorption. The use of two distinct but similar optical paths, one as a reference and the other for measurement, allows the alternate measurement of the transmitted intensity ratios at each wavelength. 

An atomic fluorescence spectrometric determination of selenium was first reported by Dagnall et al. [185] using a dispersive spectrometer equipped with an air-propane flame, giving a detection limit of 0.25 xg/ml of selenium on aspiration of aqueous solutions using a pneumatic nebuliser. Fluorescence from the 204 nm selenium resonance line was observed when the flame was irradiated by radiation from a selenium electrodeless discharge lamp, the optical axis of which was aligned at 90 °C to the optical axis of the monochromator. 

Owing to the line broadening mechanisms, the physical widths of spectral lines in most radiation sources used in optical atomic spectrometry are between 1 and 20 pm. This applies both for atomic emission and atomic absorption line profiles. In reality the spectral bandwidth of dispersive spectrometers is much larger than the physical widths of the atomic spectral lines. 

Atomic spectrometric methods of analysis essentially make use of equipment for spectral dispersion so as to isolate the signals of the elements to be determined and to make the full selectivity of the methodology available. In optical atomic spectrometry, this involves the use of dispersive as well as of non-dispersive spectrometers. The radiation from the spectrochemical radiation sources or the radiation which has passed through the atom reservoir is then imaged into an optical spectrometer. In the case of atomic spectrometry, when using a plasma as an ion source, mass spectrometric equipment is required so as to separate the ions of the different analytes according to their mass to charge ratio. In both cases suitable data acquisition and data treatment systems need to be provided with the instruments as well. 

Dispersive spectrometers. For a long time, mid-IR spectrometers were constructed on the principle of the double beam design. This fairly complex optical assembly, which remains in use for the UV/Vis, yields progressively over real time the... 

The wavelength dispersive spectrometer is the conventional optical instrument in which the photons are spatially dispersed by diffraction by means of a crystal goniometer. For each position of the goniometer, the detector sees only a narrow wavelength band. These spectrometers have good resolution, but can determine only one element at a time. 

Conventional IR spectrometers follow the same principles as described for UV/vis spectrometers, but, bearing in mind that IR radiation is basically radiant heat, the detectors are usually sensitive thermocouples and the monochromator/spectrometer optics must be adapted to transmit in this spectral region. Samples are frequently prepared as thin films or solid dispersions between IR transparent optical surfaces (KBr discs, etc.). 

In the double-beam system, the source radiation is split into two beams of equal intensity. The two beams traverse two light paths identical in length a reference cell is put in one path and the sample cell in the other. The intensities of the two beams after passing through the cells are then compared. Variation in radiation intensity due to power fluctuations, radiation lost to the optical system (e.g., cell surfaces, mirrors, etc.), radiation absorbed by the solvent, and so on should be equal for both beams, correcting for these sources of error. A dispersive spectrometer used for absorption spectroscopy that has one or more exit slits and photoelectric detectors that ratio the intensity of two light beams as a function of wavelength is called a spectrophotometer. 

The X-ray analysis system for the EMA is a wavelength dispersive spectrometer with gas proportional counter detectors. In the SEM, an energy dispersive X-ray spectrometer with a Si(Li) detector is used. The entire electron and X-ray optical systems are operated under a vacuum of about 10 torr. Modem systems are completely automated with computer control of the instmment parameters, specimen stage movement, data collection and data processing. 

The main advantages of the Fourier transform spectrometer over conventional dispersive spectrometers are (i) higher energy throughput, because no slits are required, (ii) higher optical resolution, and (iii) ability for simultaneous monitoring of all spectral information for an extended period. 

The dispersive spectrometers suffer from greater wavenumber errors, of a less predictable form, owing to their general mechanical and thermal instability and can also be affected by non-uniform illumination across the monochromator entrance slit [26]. FT-spectrometers typically use a He-Ne laser as a reference beam to monitor the displacement of the moving optical element, so providing an active internal absolute wavelength calibration... 

By changing the throughput of the spectrometer since the optical conductance of the interferometer is at least one order of magnitude higher than that of a dispersive spectrometer. 

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