Scanning monochromator

Multielemental Analysis Atomic emission spectroscopy is ideally suited for multi-elemental analysis because all analytes in a sample are excited simultaneously. A scanning monochromator can be programmed to move rapidly to an analyte s desired wavelength, pausing to record its emission intensity before moving to the next analyte s wavelength. Proceeding in this fashion, it is possible to analyze three or four analytes per minute. 

Three different types of grating spectrometer detection sterns are used (Figure 3) sequential (slew-scan) monochromators, simultaneous direct-reading polychroma-... 

Direct-reading polychromators (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrument is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and software are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. 

The LS-3B is a fluorescence spectrometer with separate scanning monochromators for excitation and emission, and digital displays of both monochromator wavelengths and signal intensity. The LS-5B is a ratioing luminescence spectrometer with the capability of measuring fluorescence, phosphorescence and bio- and chemiluminescence. Delay time (t) and gate width (t) are variable via the keypad in lOps intervals. It corrects excitation and emission spectra. 

The pyroelectric DTGS detector is a very useful low-cost, general purpose, wideband NIR detector well suited for use in FT-based analyzers. It is not normally used in scanning monochromators where higher sensitivity detectors are needed to match the lower optical throughput and discrete wavelength scanning requirements. 

In an analyzer based on scanning monochromator technology, one should be aware of a number of critical design issues ... 

Traditional optical spectrometers for both mid-IR and NIR were based on a scanning monochromator. This design features a single source and detector, and a mechanically scanned dispersion element in combination... 

Figure 1. A series of rapid scan spectra obtained by the OLIS RSM-1000 Rapid-Scanning Monochromator (Courtesy of Olis Instruments, Inc., www.olisweb.com).

Fourier-transform infrared (FTIR) spectrometers encode infrared wavenumbers by moving a mirror in a Michelson interferometer which results in a unique, path-dependent pattern of interference for each light wavelength in the IR beam. FTIRs have come to totally dominate the IR market and are the means by which most of the work described in this review was accomplished. Only for some special applications (modulation spectra and time-dependence studies) are dispersive-based (scanning monochromator or tuned laser) spectrometers still used. The advantages of the FTIR approach are that the entire spectral region of interest can... 

Other desirable features of a monochromator are stability and multi-element capabUity. Initially, direct reading spectrometers, based on a polychromator, were used for simultaneous multi-element analysis, although these were expensive, bulky and generally limited to specific elements. The development of rapid-scanning monochromators under... 

More sophisticated Instruments (such as the Technicon InfraAlyzer 500 system) employ a scanning monochromator so that data can be collected at up to 700 different wavelengths. In most instruments data are collected between 1100 and 2500 nm, although provision is made for observations which begin in the visible region when useful information is there. 

Fluorescence from labeled adsorbed protein has also been excited with the evanescent surface wave created by integrated optics. Both optical fiber I60) and flat rectangular waveguides193) have been used. Interesting use of optical fiber as a remote protein sensor was demonstated the excitation light was sent down the fiber whose tip was immersed in protein solution, evanescently excited fluorescence was collected by the same fiber and delivered to a scanning monochromator 160). 

In order to study absorbance change as a function of wavelength, we are currently modifying the flow apparatus to include a Perkin-Elmer Model 108 Rapid-Scan Monochromator. This will allow us to study absorbance as a function of both wavelength and time and should help in the assigning of decay rates of the various bands. It should also allow us to detect absorbing intermediates with lifetimes of a few milliseconds or longer. 

What follows is a detailed discussion of the principles of operation and practical limitations of analysers based on scanning monochromators. Included in this discussion will also be the background information on NIR source and detector technologies, which are in fact common to all of the NIR spectroscopic technologies listed above. 

One further point to mention about the design and use of concave holographic gratings concerns the exit focal plane. For a scanning monochromator it is of no great concern that... 

---