Multichannel emission spectrometers

Both sequential and multichannel emission spectrometers are of iwo general types, one using a classical grating spectrometer and ihe other an echelle spectrometer, such as that shown in Figure 7-23. 

A Charge-lnjectioii Device Instrumeat. A number of companies offer multichannel simultaneous spectrometers based on echelle spectrometers and two-dimensional array devices. This type of instrument has replaced other types of multichannel emission spectrometers in many applications. 

M. Grotti, C. Lagomarsino, F. Soggia and R. Frache, Multivariate optimisation of an axially viewed inductively coupled plasma multichannel-based emission spectrometer for the analysis of environmental samples, Ann. Chim. (Rome), 95(1-2), 2005, 37-51. 

PMTs and linear PDA detectors are discussed in detail in Chapter 5. This section will cover the 2D array detectors used in arc/spark and plasma emission spectrometers. In order to take advantage of the 2D dispersion of wavelengths from an echelle spectrometer, a 2D detector is required. The detector should consist of multiple individual detectors positioned (arrayed) so that different wavelengths fall on each individual detector. Such an array detector is called a multichannel detector. 

Optical emision spectra nowadays are simply measured using a fiber optic cable that directs the plasma light to a monochromator, which is coupled to a photodetector. By rotating the prism in the monochromator a wavelength scan of the emitted light can be obtained. Alternatively, an optical multichannel analyzer can be used to record (parts of) an emission spectrum simultaneously, allowing for much faster acquisition. A spectrometer resolution of about 0.1 nm is needed to identify species. 

By far the most common lamps used in AAS emit narrow-line spectra of the element of interest. They are the hollow-cathode lamp (HCL) and the electrodeless discharge lamp (EDL). The HCL is a bright and stable line emission source commercially available for most elements. However, for some volatile elements such as As, Hg and Se, where low emission intensity and short lamp lifetimes are commonplace, EDLs are used. Boosted HCLs aimed at increasing the output from the HCL are also commercially available. Emerging alternative sources, such as diode lasers [1] or the combination of a high-intensity source emitting a continuum (a xenon short-arc lamp) and a high-resolution spectrometer with a multichannel detector [2], are also of interest. 

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. 

This report will discuss the results of a study in which an optical multichannel analyzer (OMA) was coupled to standard spectrometers to record both the UV/VIS absorption and fluorescence emission spectra of complex mixtures of PAH s separated by HPLC techniques "on-the-fly" (i.e., one second spectral scans of the HPLC effluent stream) and stored on a floppy disc for subsequent retrieval and data analysis. The system described has the capability of storing 250 (500 point) spectra and can readily be used to increase the effectiveness of HPLC analysis by allowing both quantitative and qualitative data to be obtained. 

A fluorescence spectrometer (Fig. 11.27) typically uses an input monochromator (with prism or grating) to select one particular frequency for the input light beam, and then it uses an output monochromator (also with prism or grating) to monitor the emission wavelength. If one uses a multichannel analyzer instead of a simple diode detector, then the output monochromator is not needed. 

MIO. Mavrodineanu, R., and Hughes, R. C., A multichannel spectrometer for simultaneous atomic absorption and flame emission analysis. Appl. Opt. 7, 1281-1285 (1968). 

Multichannel spectrometers which would have a large number of measurement channels and allow the simultaneous determination of a large number of elements, as is done in atomic emission spectrometry, have as yet not found a way into AAS. However, work over a number of years with high-intensity continuous sources and... 

In Fig. 4.31 an example is given controlling a dye laser by one microprocessor and a UVA is-spectrometer by another. These two microprocessors are interconnected via data and control lines. Their programs are synchronised. The first microprocessor controls the spectrometer, the second an optical multichannel analyser, which rapidly takes emission spectra during the time the laser pumps the dye solution. Both have to be synchronised with the spectrometer which takes the absorbance spectra. The laser has to be triggered by the optical multichannel analyser. 

A multichannel plate (MCP) is a type of CDEM in which a series of microchannels on a disk-shaped device are coated with an electron-emissive material to generate 10 to 10" amplification as the electrons cascade through the microchannels. MCPs can be stacked to increase amplification or focused onto a fluorescent surface for ion-beam imaging. Because of the short electron pulse widths ( 1 ns) obtained with MCPs, they are the ion detector of choice for time-of-flight mass spectrometers. 

Because of their instabilities, it is necessary to integrate the emission signals from arc and spark sources for at least 20 s and often for a minute or more to obtain reproducible analytical data. This requirement makes the use of sequential spectrometers, such as those described in Section lOA-3, impractical for most applications and demands the use of a simultaneous multichannel instrument. Two types of multichannel instruments have been applied to arc and spark spectroscopy (1) spectrographs, which are considered briefly in the section that follows, and (2) multichannel spectrometers, such as those described in Section lOA-3. 

Currently, the primary use of spark source emission spectroscopy is for the identification and analysis of metals and other conducting materials. Detection is often carried out with a polychromator equipped with photomultiplier tubes, but a number of vendors offer spectrometers with array detectors as well. In addition, several modern multichannel instruments arc now equipped with interchangeable sources that permit excitation by plasmas, arcs, sparks, glow discharge, and lasers. High-voltage sparks have also become... 

In order to be able to measure many electron lines simultaneously a multichannel detector is frequently placed in the detector plane instead of using a detector behind an exit slit. The electrons then impinge on a micro-channel plate (see also Fig.6.38), in which electron multiplication occurs due to secondary emission by the inner wall material in the densely packed tubes in the plate. The original electron line image in the focal plane of the spectrometer is amplified and, by using two channel plates in series an electron multiplication of 10 can be obtained. The electron showers are converted into optical signals on a phosphor screen which is viewed by a diode array or vidicon (TV camera) (Fig.6.38). 

Fluorescence. We have also measured fluorescence produced by two-photon excitation for thick films of polysilane. For this experiment, the laser was a Spectra-Physics sub-picosecond dye laser system, focussed onto the polymer films to produce intensities of =440 MW/cm. Emission was focussed into a 0.5 m spectrometer and spectra were collected using an optical multichannel analyzer and analyzed on an IBM PC. For poly(di-n-hexylsilane), the two-photon induced emission is broadband (AXpwHM -10 nm at room temperature), with line center at =380 nm, as shown in figure 10. The emission spectrum is identical to that observed for this compound by UV excitation, and the average degree of fluorescence anisotropy (=0.2) produced at the two-photon resonance (579 nm) is quite similar to that oteerved for on-resonance UV excitations in polysilanes [26]. 

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