Echelle spectrometers

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. 

Figure 3.4 shows (i) a line spectrum (one-dimensional dispersive spec-trographic record), (ii) a spectrometric record, (iii) an interferogram obtained by a Fourier transform spectrometer, and (iv, v) two- and three-dimensional double dispersive spectra recorded e.g. by Echelle spectrometers. In principle, all forms may be obtained by OES. 

The LIBS spectral signal is detected using an Echelle spectrometer + iCCD camera, which provides the whole time-resolved NUV-NIR spectrum in a single laser shot. 

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... 

This figure represents only a fraction of the total spectrum and is intended for illustrative purposes only. The echelle spectrometer used in this work covers the range from below 2000 A to above 7000 A so that a representation of the complete spectrum analogous to the segment represented in Figure 1 would have the Hg(1849.5oA) line near the bottom and the K(7698.98A) line near the top. 

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. 

Figure 1. Relative locations of the most sensitive absorption lines for selected elements in the two-dimensional echelle spectrometer display (30).

The wavelength location prediction accuracy of the program used to calculate the locations of specified wavelengths in the normal focal plane of the echelle spectrometer is rated by the supplier (Spectrametrics, Inc., Andover, MA 01810) to be 0.050 mm. The modified routine is expected to have a wavelength position uncertainty of approximately 0.009 mm in the vidicon focal plane... 

H. Becker-Ross, S. Florek, U. Heitmann, M. Schiitz, An echelle spectrometer with external prism order separation designed for continuum source AAS, Poster 575, FACSS, 1997, Book of Abstracts, p. 208. 

Since boron produces weak flame absorption signals, an echelle spectrometer equipped with a d-c plasma excitation source similar to that procurable from Spectrametrics, Inc., 204 Andover Street, Andover, Massachusetts-01810, can be used to determine this element by AES. 

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. 

Image dissector tubes (Fig. 26) make use of an entrance aperture behind the photocathode, by which the photoelectrons stemming from different locations of the photocathode can be scanned and measured after amplification in the dynode train, as in a conventional photomultiplier. Although used in combination with an echelle spectrometer with crossed dispersion for flexible rapid sequential analyses [59], these systems have not had any commercial breakthrough. This might be due to the limited cathode dimensions but also to stability problems. 

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. 

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