Spectral grating spectrometer

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.

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. 

During the 1960s further improvements made infrared spectroscopy a very useful tool used worldwide in the analytical routine laboratory as well as in many fields of science. Grating spectrometers replaced the prism instruments due to their larger optical conductance (which is explained in Sec. 3 of this book). The even larger optical conductance of interferometers could be employed after computers became available in the laboratory and algorithms which made Fourier transformation of interferograms into spectra a routine. The computers which became a necessary component of the spectrometers made new powerful methods of evaluation possible, such as spectral subtraction and library search. 

As early as 1954, Jacquinot pointed out that interferometers have a considerably higher optical conductance than prism or grating spectrometers. In order to quantify this relation we use Eqs. 3.1-33 and 3.1-39 and add G or / as a superscript to designate the spectral optical conductance of grating instruments and interferometers, respectively ... 

According to this equation, the ratio between the length of the slit h and the focal length of the collimator / is relevant for the spectral optical conductance of a grating spectrometer. It is usually on the order of 0.01 values of up to 0.2 aie only reached in very special cases (the old model 81 Cary Raman spectrometer with an image sheer , made in 1960 ). 

In conclusion it can be stated that the spectral optical conductance for a prism spectrometer, a grating spectrometer, and a Michelson interferometer are approximately as 1 10 1000. 

This is the well-known formula that states that the resolution of a diffraction grating increases (and Jv decreases) with increasing order n of diffraction and number N of lines in the grating >. For the case of three lines, or for any other spectrum, the intensity is measured with a grating spectrometer as fimction of yd (for several orders of diffraction) (see Fig. 5). The data obtained in this way are then easily converted to / (A) or I ( ) and the problem of determining the spectral distribution I ( ) is solved. It should be noted that the linewidth obtained (see Fig. 5) is influenced by the limited resolution of the instrument and that the line-widths of the three laser lines are assumed to be actually much smaller. In other words, we have been discussing the properties of the instrument line-shape function of a diffraction grating. 

Fig. 8. Continuous spectrum I versus wavelength A as recorded with a grating spectrometer (----) and the line-shape function (spectral window) for two orders of diffraction. For comparison, a spectrum with insufficient filtering is shown (-)...

The physical meaning of Eq. (3.7) is that the true spectrum I (i> ) is scanned with a line-shape function or spectral window 5 (i> — i> ) as in the case of the diffraction grating (see Fig. 10). As mentioned already, in contrast to the grating spectrometer, the spectral window can be varied according to the choice of the apodization function. The advantage of apodization is easily seen for a narrow laser line (cf. Fig. 6). 

SPD. The spectral resolution of an SPD is governed by the same factors discussed for the SIT, i.e., it depends on the grating/spectrometer combination. The spatial resolution is degraded by a degree of diode-to-diode cross-talk, i.e., it is... 

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