Diode array, grating monochromator

Figura 3 Grating spectrometers commonly used for ICP-OES (a) monochromator, in which wavelength is scanned by rotating the grating while using a singie photomultiplier tube (PMT) detector (b) polychromator, in which each photomultiplier observes emission from a different wavelength (40 or more exit slits and PMTs can be arranged along the focal plane) and (c) spectrally segmented diode-array spectrometer.

Filter-photometer or spectrophotometer incorporating prism or grating monochromator, phototube photomultiplier or diode array, glass, quartz or plastic cells. 

Near-infrared Spectroscopy for Process Analytical Technology 119 5.3.2 The scanning grating monochromator and polychromator diode-array... 

The tail of the plasma formed at the tip of the torch is the spectroscopic source, where the analyte atoms and their ions are thermally ionized and produce emission spectra. The spectra of various elements are detected either sequentially or simultaneously. The optical system of a sequential instrument consists of a single grating spectrometer with a scanning monochromator that provides the sequential detection of the emission spectra lines. Simultaneous optical systems use multichannel detectors and diode arrays that allow the monitoring of multiple emission lines. Sequential instruments have a greater wavelength selection, while simultaneous ones have a better sample throughput. The intensities of each element s characteristic spectral lines, which are proportional to the number of element s atoms, are recorded, and the concentrations are calculated with reference to a calibration standard. 

Extremely quiet and rapid monochromators are now available from numerous vendors. As covered in detail in Chapter 2, top-notch monochromators (grating and interferometer), diode arrays, accousto-optic tunable filters, as well as modern interference-filter instruments now exist. Fiber optic probes, multiple detector modules, and transmission attachments all lend themselves to superior sample handling of powders and solid dosage forms. 

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

In another configuration of instmmentation, irradiation of white light onto the electrode surface and detection of reflected light by the use of a multi-channel detector (such as CCD or photo-diode array) through a grating monochromator makes multi-channel measurement possible [21]. 

There are four basically different instrument designs, based on how incident energy is selected (McClure, 1994). These are grating instruments (monochromators), Fourier transform instruments, filter instruments and Diode Array-based instruments. Figure4 illustrates the basic design of a scanning NIR spectrophotometer. 

If needed, spectral analysis of light from the output beam can be achieved using a simple monochromator with a multiwavelength detector (CCD or diode array). The spectral resolution is determined by the size of the monochromator, the width of the entrance slit, the density and order of the grating, and the distance between individual elements in the detector array. Fast temporal analysis may be achieved using a fast detector such as a streak camera with picosecond or subpicosecond temporal resolution. 

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