Excitation/detection methods

This delimits three aspects of FFC signal detection (detector hardware, excitation detection method, and data-reduction algorithm). The following paragraphs explain briefly the most popular choices we have made to handle these aspects. 

Excitation/Detection Methods The Road to Four-Color Multicapillary... 

Luminescence has been used in conjunction with flow cells to detect electro-generated intennediates downstream of the electrode. The teclmique lends itself especially to the investigation of photoelectrochemical processes, since it can yield mfonnation about excited states of reactive species and their lifetimes. It has become an attractive detection method for various organic and inorganic compounds, and highly sensitive assays for several clinically important analytes such as oxalate, NADH, amino acids and various aliphatic and cyclic amines have been developed. It has also found use in microelectrode fundamental studies in low-dielectric-constant organic solvents. 

Atomic Absorption/Emission Spectrometry. Atomic absorption or emission spectrometric methods are commonly used for inorganic elements in a variety of matrices. The general principles and appHcations have been reviewed (43). Flame-emission spectrometry allows detection at low levels (10 g). It has been claimed that flame methods give better reproducibiHty than electrical excitation methods, owing to better control of several variables involved in flame excitation. Detection limits for selected elements by flame-emission spectrometry given in Table 4. Inductively coupled plasma emission spectrometry may also be employed. 

Fluorescence. The fluorescence detection technique is often used in clinical chemistry analyzers for analyte concentrations that are too low for the simpler absorbance method to be appHed. Fluorescence measurements can be categorized into steady-state and dynamic techniques. Included in the former are the conventional simultaneous excitation-emission method and fluorescence polarization. 

In the CL detection method, the excitation of a molecule is achieved via a chemical reaction that is generally an oxidation process. That is, an exciting light source is not required thus, the CL is not accompanied by any scattering light and source instability. This permits a large signal-to-noise ratio (S/N), which finally provides an increase in sensitivity. 

Fig. 1.14. Multiphoton excitation scheme in Hj showing the laser-induced fluorescence detection method...

Recent developments in laser technology and fast detection methods now allow the kinetic behaviour of the excited state species arising from absorption of radiation by polymers to be studied on time-scales down to the picosecond region ( ). An example of a time-resolved fluorescence spectrometer which can be used to study such ultrafast phenomena is illustrated in Figure 5 Q). 

The use of high-speed modulated excitation (f> kr + knr) combined with coherent detection methods has resulted in the popular techniques of frequency domain fluorometry, also known as phase-modulation fluorometry. These techniques can be used to determine the temporal characteristics of both fluorescence and phosphorescence and will also be addressed later in this chapter. 

Most conventional techniques for the determination of biological molecules or other species with similar properties use their ability to absorb ultraviolet or visible light, their fluorescence after excitation with light of the appropriate wavelength, or their electrochemical behaviour. It possible to enhance the detectability of some species by making them react with UV-visible absorbing or fluorescent compounds. Applied to complex matrices, these detection methods are at best only selective, because a wide variety of chromophores will give a response. 

Two analytical methods for priority pollutants specified by the USEPA (38) use HPLC separation and fluorescence or electrochemical detection. Method 605, 40 CFR Part 136, determines benzidine and 3,3-dichlorobenzidine by amperometric detection at +0.80 V, versus a silver/silver chloride reference electrode, at a glassy carbon electrode. Separation is achieved with a 1 1 (v/v) mixture of acetonitrile and a pH 4.7 acetate buffer (1 M) under isocratic conditions on an ethyl-bonded reversed-phase column. Lower limits of detection are reported to be 0.05 /xg/L for benzidine and 0.1 /xg/L for 3,3-dichlorobenzidine. Method 610, 40 CFR Part 136, determines 16 PAHs by either GC or HPLC. The HPLC method is required when all 16 PAHs need to be individually determined. The GC method, which uses a packed column, cannot adequately individually resolve all 16 PAHs. The method specifies gradient elution of the PAHs from a reversed-phase analytical column and fluorescence detection with an excitation wavelength of 280 nm and an emission wavelength of 389 nm for all but three PAHs naphthalene, acenaphthylene, and acenaphthene. As a result of weak fluorescence, these three PAHs are detected with greater sensitivity by UV-absorption detection at 254 nm. Thus, the method requires that fluores-... 

In this chapter we will consider the techniques developed to detect and quantitatively measure how much ionization and/or excitation is caused by different nuclear radiations. As all radiation creates ionization and/or excitation, we will separate the discussion of detection methods according to the general techniques used to collect and amplify the results of the interaction of the primary radiation with matter rather than by the type of radiation. These detection methods can be classified as (a) collection of the ionization produced in a gas or solid, (b) detection of secondary electronic excitation in a solid or liquid scintillator, or (c) detection of specific chemical changes induced in sensitive emulsions. 

Dornhofer, Hack, and Langel (180), in a detailed study of the fluorescence that is induced by an ArF laser, have been able to show that an intense ArF laser can distort the observed vibrational distribution by photodissociating CS radicals with v" > 5. The ArF laser absorption by CS will also produce electronically excited CS which, when it emits, will redistribute the vibrational populations. Probing the CS quantum state population under these conditions could distort the CS ground state populations. The LIF measurements will underestimate the amount of CS radicals that are produced, while the direct detection methods will overestimate the amount of S(3p) atoms because of the secondary photolysis of CS. The vibrational distribution of Lu et al. (178) will be less prone to this secondary photolysis because very low laser powers (< 1 mj) were used. Dornhofer, et al. concluded from their results that the S(3p)/S(J-D) ratio was 3, which is in reasonable agreement with the LIF measurements of Lu et al. 

The fast isomerization of the spiropyran to the merocyanine provides a possibility of generating an interfacial shock wave. The methods used so far in studying the transmission of waves in mono-layers and the adjacent bulk phases require mechanical (16) or electrocapillary (17) excitation of the interface which involves the displacement of the aqueous bulk phase. In addition, the range of frequencies accessible to the investigation of interfacial waves by the conventional techniques is very limited. The fast photochemical generation of an interfacial shock wave is strictly occurring in the monolayer and provides a larger spectrum of frequencies which can be fully explored only after the development of appropriate detection methods. 

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