Spectral multiplexing

This comparison of the spectroscopic properties of the different types of fluorescent reporters underlines that semiconductor QDs and upconverting nanoparticles have no analogs in the field of organic dyes. Therefore, their unique features are unrivaled. The different molecular labels detailed here each display unique advantages that can compete with some of the favorable features of QDs and upconverting phosphors such as long lifetimes in the case of MLC systems and lanthanide chelates or very narrow emission bands for lanthanide chelates beneficial for spectral multiplexing. 

Spectral multiplexing or multicolor detection is typically performed at a single excitation wavelength, and relies on the discrimination between different fluorescent labels by their emission wavelength. Desirable optical properties of suitable fluoro-phores are a tunable Stokes shift and very narrow, preferably well-separated emission bands of simple shape. 

Nafie LA (2000) Dual polarization modulation a real-time, spectral-multiplex separation of circular dichroism from linear birefringence spectral intensities. Appl Spectrosc 54 1634-1645... 

Roseler et al., 1993). However the spectrally multiplexing procedure requires a photometric technique to be applied. To circumvent an absolute calibration of the intensity scale and to avoid experimental errors due to different optical paths and particularly, different irradiated areas, the evaluation should be based on quotients of intensities which were obtained under identical conditions except the polarization states. 

Figure 2.3 Spectra of QDs and organic dyes, (a-f) Absorption (lines) and emission (symbols) spectra of representative QDs (a-c) and organic dyes (d-f) color coded by size (blue < green < black < red). MegaStokes dyes were designed for spectral multiplexing in dimethylformamide (DMF). (Reproduced by permission from Macmillan Publishers Ltd. JVat. Meth-ods, copyright (2008).)...

Clearly, if one rasters the sample under the focused beam, it is possible to acquire a two-dimensional array of spectra from which a confocal Raman map can be reconstructed. With a two-dimensional detector, this process can be doubly multiplexed that is, in addition to spectral multiplexing along the long dimension of the detector, it is, in principle, possible to multiplex in one spatial direction along the short dimension of the detector. This is achieved by illuminating a line that is projected onto the entrance slit of the spectrograph. 

More recently, and predictably, Fourier transform infrared spectrometers have been used to measure dynamic linear dichroic spectra. However, substantial modifications to the conventional type of FTIR spectrometer have been necessary in order to overcome a basic problem. This arises from the fact that it is necessary to separate the time dependence of the sample response from that of the spectral multiplexing, because the interferogram itself is a cosine function of time. For example, if the moving mirror has a velocity of 0.5 mm per second at 3000 cm the radiation is frequehcy modulated at 300 Hz. 

Because there is no motion of the mirror at the retardation points, Fourier frequencies are eliminated and do not contribute to the measurement. In this manner, spectral multiplexing is decoupled from the time domain. Thus the entire spectral range is modulated at a single frequency. 

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