Echelle monochromator

One optical arrangement in an echelle-based monochromator. In this case a Schmidt cross-disperser is used to disperse the UV light instead of The prism which is used for visible light only. This results in better transmission in the UV (reproduced with permission from the Perkin Elmer Corporation). 

Q. Why is an echelle monochromator ideal for use with an array detector ... 

There is a commercially available instrument for HR-CS AAS in which a flame, a graphite furnace, or a CVG system are used to carry out atomization. The instrument has a double monochromator with a prism premonochromator and a high-resolution echelle monochromator, which allows a wavelength from 189 to 900 nm to be used in a sequential measurement mode.19... 

As explained in Chapter 1, section 7, unless a very high resolution monochromator, e.g. an echelle monochromator, is used to isolate a very narrow (< ca. 0.005 nm) band of light from a continuum spectrum prior to absorbance measurement, the sensitivity will be very poor.1,2 Although there are occasional reports of analysis by flame AAS using continuum sources such as xenon arc lamps, these are invariably from research laboratories. The vast majority of reported applications use single element line sources, and more than 99% of these applications use hollow cathode lamps. 

Heitmann et al. [11] designed a very compact double monochromator, consisting of a 300 mm prism pre-monochromator and a 400 mm echelle grating monochromator, both in Littrow mounting, which is shown schematically in Figure 4.3. The prism monochromator selects the part of the spectrum that is of interest, and the echelle monochromator provides the high dispersion of the selected spectral interval, which is better than 2 pm per pixel at 200 nm (see Welz et al. [10]). 

Obviously, such a high-resolution monochromator requires active wavelength stabilization in order to avoid drift problems. This has been accomplished through an internal neon lamp, mounted on an adjustable stand in front of the intermediate slit between the pre- and echelle-monochromator, so that it can be moved into the beam automatically if necessary. The neon lamp emits many relatively narrow lines in the 580-720 nm range, and, in the absence of any pre-selection, these are separated by the echelle grating into various superimposed orders. This means that without pre-dispersion at least two neon lines for every grating position surely fall on the detector, and can be used for stabilization. The precision of this stabilization is only limited by the stepper motor for grating adjustment, and is better than one-tenth of a pixel width (see Welz et al. [10]). 

However, any spectrometer that uses CS and a double monochromator with an echelle grating makes it possible to reach any line within an extremely short period of time of much less than 1 s, as both the grating and the prism are stepper-motor controlled. This feature allows a fast sequential multi-element determination to be performed with the great advantage that flame conditions and burner... 

Several systems for automation of spectrometers have been discussed. A computer-controlled Echelle monochromator allowed wavelength increments of 0.01 nm. A wavelength-scan and lamp-intensity control scheme for the popular Bausch and Lomb high-intensity monochromator has been described. The accurate synchronization of monochromator wavelength-scan and chart-recorder speed, and the possibility of rapid scanning allowing spectra to be displayed in real time on an oscilloscope, has also been discussed. Details have been provided for the modification of a commercially available mirror mount (Oriel model 1450) for use as a stepper-motor controlled grating mount. 

Kanda, V., Taira, M., 1988. Sequential multi-element analysis of sediments and soils by inductively coupled plasma atomic emission spectroscopy with a computer controlled rapid scanning echelle monochromator. Anal. Chem. 207, 269-281. 

Most commercial AAS systems have the monochromator, optics, and detector designed for the measurement of one wavelength at a time they are single-element instruments. There are a few systems available that do perform multielement determinations simultaneously, using an Echelle spectrometer (discussed in Chapter 2) and a bank of HCLs all focused on the atomizer. The limitation to this approach is not the sources or the spectrometer or the detector, but the atomizer. The atomizer can only be at one set of conditions, and those conditions will not necessarily be optimum for all of the elements being measured. There will be a tradeoff in detection limits for some of the elements. 

Echelle monochromators. In the past 10 years, Echelle monochromators have become increasingly common in multielement emission systems. These are often... 

Describe the components and operation of an Echelle monochromator. What are its advantages over a Rowland circle design ... 

Possibilities in continuum AAS include the use of a Fourier transform spectrometer, television-like detectors with an echelle monochromator, a resonance monochromator, and an instrument based on resonance schlieren (Hook) spectrometry. 

The spectrometers used are adapted either for sequential or simultaneous multi-element measurements. Commonly used grating spectrometers in plasma AES include (i) spectrometers with the Paschen-Runge mount, (ii) echelle spectrometers, (iii) spectrometers with Ebert and Czerny-Turner mounts, (iv) spectrometers with Seya-Namioka mounts, and (v) double monochromators. Also Fourier transform spectrometers may be used in plasma AES. 

Figure 117 Wavelength scan which demonstrates the high resolving power of an echelle monochromator (ARL)...

Figure 119 Echelle monochromator. (Adapted from P. N. Kelihar and C. C. Wohlers, Anal. Chem., 1976, 48, 333A)...

Stray light can be reduced and higher resolution obtained by using double monochromators. An echelle spectrometer equipped with a predisperser has been used. As the two instruments are operated in tandem, the exit slit of the predisperser is the entrance slit of the monochromator. 

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