Echelle spectrometer detector

The analytical capabilities of LIBS and LA-MIP-OES were recently noticeably improved by use of an advanced detection scheme based on an Echelle spectrometer combined with a high-sensitivity ICCD (intensified charge-coupled device) detector. 

CCD detector consists of 224 linear photodetector arrays on a silicon chip with a surface area of 13 x 18 mm (Fig. 4.16). The array segments detect three or four analytical lines of high analytical sensitivity and large dynamic range and which are free from spectral interferences. Each subarray is comprised of pixels. The pixels are photosensitive areas of silicon and are positioned on the detector atx -y locations that correspond to the locations of the desired emission lines generated by an echelle spectrometer. The emission lines are detected by means of their location on the chip and more than one line may be measured simultaneously. The detector can then be electronically wiped clean and the next sample analysed. The advantages of such detectors are that they make available as many as ten lines per element, so lines which suffer from interferences can be identified and eliminated from the analysis. Compared with many PMTs, a CCD detector offers an improvement in quantum efficiency and a lower dark current. 

S. Euan, H. Pang and R. S. Houk, Application of generalized standard additions method to inductively coupled plasma atomic emission spectroscopy with an echelle spectrometer and segmented-array charge-coupled detectors, Spectrochim. Acta, Part B, 50(8), 1995, 791-801. 

New developments in solid-state array detectors and CCDs, as well as powerful, specially designed echelle spectrometers and improvements in CSs, have led to a fresh concept for AAS, which allows the simultaneous determination of several elements based on atomic absorption measurements.9... 

Simultaneous Multielement Determinations by Atomic Absorption and Atomic Emission with a Computerized Echelle Spectrometer/Imaging Detector System... 

In the work reported here, the two-dimensional spectrum from the echelle spectrometer is displayed onto the active surface of a two-dimensional imaging detector that can monitor the different lines independently so that emission or absorption lines for multiple elements can be monitored simultaneously. 

It should also be noted at this point that a multichannel detector can have multiple detector elements along two axes, one parallel to the direction of wavelength dispersion, and one perpendicular. The latter is parallel to the entrance slit in most dispersive instruments. For example, a CCD may have 1024 pixels along the wavelength axis and 256 along the vertical axis, for a total of 262,144 independent elements. This second dimension of the detector may be used in a variety of applications involving Raman imaging, multiple detection tracks, or echelle spectrometers. 

These two-dimensional detectors [63] are ideally suited for coupling with an echelle spectrometer, which is state of the art in modem spectrometers for ICP atomic emission spectrometry as well as for atomic absorption spectrometers. As for CCDs the sensitivity is high and along with the signal-to-noise ratios achievable, they have become real alternatives to photomultipliers for optical atomic spectrometry (Table 3) and will replace them more and more. 

The spectrometer used is a modified Spectrametrics Spectraspan III echelle grating spectrometer with a quartz prism cross disperser. An echelle spectrometer was chosen because of its two dimensional format display. This format allows efficient simultaneous examination of a much wider spectral range than with a linear dispersion spectrometer when a two dimensional television camera type detector is used. 

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. 

PMTs and linear photodiode array 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... 

What is the advantage to using a CCD or CID detector for an Echelle spectrometer instead of a PMT or multiple PMTs ... 

As described in Chapter 2, spectral resolution determines the amount of detail that can be seen in the spectrum. If the resolution is too low, it will be impossible to distinguish between spectra of closely related compounds if the resolution is too high, noise increases without any increase in useful information. Spectral resolution is determined by the diffraction grating and by the optical design of the spectrometer. With a fixed detector size, there is a resolution beyond which not all of the Raman wavelengths fall on the detector in one exposure. Ideally, gratings should be matched specifically to each laser used. A dispersive Raman echelle spectrometer from PerkinElmer Instruments covers the spectral range 3500-230 cm with a resolution better than 4 cm . ... 

The glow discharge (GD) is a reduced-pressure gas discharge generated between two electrodes in a tube filled with an inert gas such as argon. The sputtered atom cloud in a GD source consists of excited atoms, neutral atoms, and ions. The emission spectrum can be used for emission spectrometry in the technique of GD-OES, but the GD source can also be used for AAS, AFS, and MS. The source can be used with any of the types of spectrometers discussed for plasma emission sequential monochromator, Rowland circle polychromator, echelle spectrometer, or combination sequential-simultaneous designs. The detectors used are the same as described for plasma emission spectrometry PMTs, CCDs, or CIDs. 

Echelle spectrometers [36] are also often used. By a combination of an order-sorter and an echelle grating, in either parallel or crossed-dispersion mode, high practical resolution (up to 300(XX)) can be realized with an instrument of rather low focal length (down to 0.5 m). Therefore, stability and luminosity are high. By using an exit slit mask with a high number of preadjusted slits or a solid-state, two-dimensional array detector highly flexible and rapid multielement determinations are possible. 

These workers used a prototype spectraspan III dc plasma echelle spectrometer, 510-512, (Spectrametrics Inc., Andover, Mais.). They adapted a Varian 1200 gas chromatograph for on-column injection onto a 6 ft X g in. o.d. stainless steel column packed with 2% Dexsil 300 GC on Chromosorb 750, 100 120 mesh (Johns-Manvilie Corp., Denver, Col.). Column effluent was split by an approximately 1 1 ratio between the flame ionization detector of the gas chromatograph and a heated, thermal, and electrically insulated 1/16-in. o.d. stainless steel transfer line to the dc plasma. Preheated argon sheath gas was required in addition to the argon supplied to sustain the plasma, in order to optimize spectral sensitivity. The column and injection port temperature were set at 130 and 160 C, respectively, and the interface temperature was 170 0. Helium carrier gas flow rate was 25 ml/min. 

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