Fluorescence excitation-emission matrix system

There are two competing and equivalent nomenclature systems encountered in the chemical literature. The description of data in terms of ways is derived from the statistical literature. Here a way is constituted by each independent, nontrivial factor that is manipulated with the data collection system. To continue with the example of excitation-emission matrix fluorescence spectra, the three-way data is constructed by manipulating the excitation-way, emission-way, and the sample-way for multiple samples. Implicit in this definition is a fully blocked experimental design where the collected data forms a cube with no missing values. Equivalently, hyphenated data is often referred to in terms of orders as derived from the mathematical literature. In tensor notation, a scalar is a zeroth-order tensor, a vector is first order, a matrix is second order, a cube is third order, etc. Hence, the collection of excitation-emission data discussed previously would form a third-order tensor. However, it should be mentioned that the way-based and order-based nomenclature are not directly interchangeable. By convention, order notation is based on the structure of the data collected from each sample. Analysis of collected excitation-emission fluorescence, forming a second-order tensor of data per sample, is referred to as second-order analysis, as compared with the three-way analysis just described. In this chapter, the way-based notation will be arbitrarily adopted to be consistent with previous work. 

We developed two kinds of multidimensional fluorescence spectroscopic systems the time-gated excitation-emission matrix spectroscopic system and the time- and spectrally resolved fluorescence microscopic system. The former acquires the fluorescence intensities as a function of excitation wavelength (Ex), emission wavelength (Em), and delay time (x) after impulsive photoexcitation, while the latter acquires the fluorescence intensities as a function of Em, x, and spatial localization (%-, y-positions). In both methods, efficient acquisition of a whole data set is achieved based on line illumination by the laser beam and detection of the fluorescence image by a 2D image sensor, that is, a charge-coupled device (CCD) camera. 

Summarizing this section, we developed the time-gated excitation-emission matrix spectroscopic system and applied it to the decomposition of a mixed solution of a number of fluorescent dyes. We demonstrated that our approach, which was based on unique optical configuration, efficient acquisition of a multidimensional data set, and decomposition of unknown fluorescent components by using the PARAFAC model, was effective for the analysis ofunknown multi-component targets. 

The first intravascular sensor for simultaneous and continuous monitoring of the pH, pC>2, and pCC>2 was developed by CDI-3M Health Care (Tustin CA)14 based on a system designed and tested by Gehrich et al.15. Three optical fibres (core diameter = 125 pm) are encapsulated in a polymer enclosure, along with a thermocouple embedded for temperature monitoring (Figure 3). pH measurement is carried out by means of a fluorophore, hydroxypyrene trisulfonic acid (HTPS), covalently bonded to a matrix of cellulose, attached to the fibre tip. Both the acidic ( eXc=410 nm) and alkaline ( exc=460 nm) excitation bands of the fluorophore are used, since their emission bands are centred on the same wavelength (/-cm 520 nm). The ratio of the fluorescence intensity for the two excitations is measured, to render the sensor relatively insensitive to fluctuations of optical intensity. 

The Raman scattering (which is called resonance fluorescence when the final molecular state is identical to the initial one g)) is not, however, the only process resulting in spontaneous photon emission. If one repeats the above treatment in a density matrix formalism and allows for intermediate state dephasing, one obtains, for resonant excitation, a fluorescence contribution. In practice, in this case the doorway state is really (not virtually) excited and becomes populated for a significant time interval, as pointed out by Lee and Heller. The system becomes then sensitive to any phase-disturbing perturbation. As a consequence, due to dephasing, the scattering is no more a purely coherent two-photon process, and the Raman emission competes with a relaxed component which is usually called fluorescence. The fluorescence is then simply the spontaneous emission from populated excited states, which have completely lost the memory of the... 

---