Acousto-optic spectroscopy

IR spectroscopy is not a very sensitive analytical tool and is, therefore, not well suited to the detection of small amounts of material. If, however, intermediates have intense and well-resolved IR absorptions, the progress of their chemical transformation can be followed by IR spectroscopy [83,88,91-93], Near-infrared spectroscopy, in combination with an acousto-optic tunable filter, can be sufficiently sensitive to enable the on-bead identification of polystyrene-bound di- and tripeptides, even if the peptides have very similar structures (e.g., Leu-Ala-Gly-PS and Val-Ala-Gly-PS) or differ only in their amino acid sequence (e.g., Leu-Val-Gly-PS and Val-Leu-Gly-PS) [94]. Special resins displaying an IR and Raman barcode have been developed, which may facilitate the deconvolution of combinatorial compound libraries prepared by the mix-and-split method [48]. 

Abbreviations AOD, Acousto-optical deflection BCB, bisbenzyocyclobutadiene CCD, indirect contact conductivity detection CL, chemiluminescence ECD, electron capture detector FCS, fluorescence correlation spectroscopy FRET, fluorescence resonance energy transfer ICCD, integrated contact conductivity detection GMR, giant magnetoresistive LED-CFD, light emitting diode confocal fluorescence detector LIF, laser-induced fluorescence LOD, limit of detection MALDI, matrix-assisted laser desorption ionization PDMS, poly(dimethylsiloxane) PMMA, poly(methylmetha-crylate) SPR, surface plasmon resonance SVD, sinusoidal voltammetric detection TLS, thermal lens spectroscopy. 

Treado, P. J., Levin, I. W. and Lewis, E. N. (1992) Near-infrared acousto-optic filtered spectroscopy a solid state approach to chemical imaging, Appl. Spectrosc. 46, 553-9. 

Figure 3.19 Intensity-modulated photocurrent spectroscopy, showing (a) the layout of a typical spectrometer, and (b) the response obtained AOM, acousto-optic modulator RE, reference electrode WE, working electrode CE, counter electrode FRA, frequency response analyzer...

A modification of the Debye-Sears effect, i.e., the modulation of the refractive index by acoustical waves functioning as tunable grating, is now applied to fast scanning NIR spectroscopy The Acousto-Optic Tunable Filter (AOTF), (Harris and Wallace, 1969 Chang, 1981). In combination with array detectors, chemical imaging is possible (Treado et ah," 1992). 

Beside the purely spectroscopic part there has also been important development of active optical components such as electro-optic and acousto-optic modulators. Such components allowed the use of modulation schemes earlier used in the microwave region to be used in optical spectroscopy. In the early 80 s the FM-sideband technique both for stabilisation to molecular lines as well as to passive cavities were demonstrated [45,46]. The developing field of non-linear optics and the wave-mixing picture established a framework in which line shapes of different modulation schemes in saturation spectroscopy could be understood [47, 45, 46, 48,49]. 

Precision Laser Spectroscopy Using Acousto-Optic Modulators, W. A. fhn Wijingaarden Highly Parallel Computational Techniques for Electron-Molecule Collisions, Carl Winstead and Vincent McKoy... 

This is the basic idea of heterodyne polarization spectroscopy [275, 276], where the pump wave at the frequency cop passes through an acousto-optical modulator, driven at the modulation frequency /, which generates sidebands at co = cop 2nf (Fig. 2.47). The sideband at coi = cop-h 2nf is sent as a pump beam through the sample cell, while the probe beam at frequency cop is split off the laser beam in front of the modulator. Otherwise the setup is similar to that of Fig. 2.22. 

Instead of two different lasers, a single laser can be employed when its output is amplitude-modulated at the variable frequency /. Besides the carrier frequency Vo two tunable sidebands vq / appear. If the carrier frequency vq is stabilized onto a selected molecular line, the sidebands can be tuned across other molecular transitions (sideband spectroscopy) [924, 925]. The experimental expenditure is smaller since only one laser has to be stabilized. The sideband tuning can be achieved with acousto-optical modulators at relatively small RF powers. The difference frequency Aco is, however, restricted to achievable modulation frequencies (Aco < 27r/iiiax). 

E. W. Ciurczak, A novel spectrophotometric system based on an acousto-optic tunable filter. Spectroscopy 5(1), 10 (1990). 

P. J. Treado, I. W. Levin, E. N. Lewis. High fidelity Raman imaging spectroscopy a rapid method using an acousto-optic tunable filter. Appl Spectrosc 46 1211-1216,1992. 

---