Indirect laser-induced method

Valproic acid has been determined in human serum using capillary electrophoresis and indirect laser induced fluorescence detection [26], The extract is injected at 75 mbar for 0.05 min onto a capillary column (74.4 cm x 50 pm i.d., effective length 56.2 cm). The optimized buffer 2.5 mM borate/phosphate of pH 8.4 with 6 pL fluorescein to generate the background signal. Separation was carried out at 30 kV and indirect fluorescence detection was achieved at 488/529 nm. A linear calibration was found in the range 4.5 144 pg/mL (0 = 0.9947) and detection and quantitation limits were 0.9 and 3.0 pg/mL. Polonski et al. [27] described a capillary isotache-phoresis method for sodium valproate in blood. The sample was injected into a column of an EKI 02 instrument for separation. The instrument incorporated a conductimetric detector. The mobile phase was 0.01 M histidine containing 0.1% methylhydroxycellulose at pH 5.5. The detection limit was 2 pg/mL. 

The time range of the electrochemical measurements has been decreased considerably by using more powerful -> potentiostats, circuitry, -> microelectrodes, etc. by pulse techniques, fast -> cyclic voltammetry, -> scanning electrochemical microscopy the 10-6-10-1° s range has become available [iv,v]. The electrochemical techniques have been combined with spectroscopic ones (see -> spectroelectrochemistry) which have successfully been applied for relaxation studies [vi]. For the study of the rate of heterogeneous -> electron transfer processes the ILIT (Indirect Laser Induced Temperature) method has been developed [vi]. It applies a small temperature perturbation, e.g., of 5 K, and the change of the open-circuit potential is followed during the relaxation period. By this method a response function of the order of 1-10 ns has been achieved. 

Transient technique — A technique whose response is time dependent and whose time dependence is of primary interest, e.g., -> chronoamperometry, -> cyclic voltammetry (where current is the transient), -> chronopotentiometry and -> coulostatic techniques (where voltage is the transient). A transient technique contrasts with steady-state techniques where the response is time independent [i]. Some good examples are cyclic voltammetry [i, ii] (fast scan cyclic voltammetry), the indirect-laser-induced-temperature-jump (ILIT) method [iii], coulostatics [i]. The faster the transient technique, the more susceptible it is to distortion by -> adsorption of the redox moiety. 

Refs. [i] Bard AJ, FaulknerLR (2001) Electrochemical methods, 2nd edn. Wiley, New York, pp 487-516 [ii] Amatore C, Maisonhaute E (2005) Anal Chem 77-.303A [iii] FeldbergSW, Newton MD, Smalley JF (2003) The indirect laser-induced temperature jump method for characterizing fast interfacial electron transfer concept, application, and results. In Bard AJ, Rubinstein I (eds) Electroanalytical chemistry, vol. 22. Marcel Dekker, New York, pp 101-180... 

An adaptation of the temperature-jump method, named indirect laser-induced temperature jump [29], was used in studies of distance dependence of electron transfer at electrodes. A pulsed Nd YAG laser was used to cause a sudden (<5 ns) change in temperature (<5 K) at an electrode/electrolyte interface. The increase in temperature causes a change in the open-circuit potential. The relaxation step is a function of the dissipation of thermal energy and the rate of electron transfer between the electrode and its redox partners. 

NACE with indirect detection has been applied to the determination of fatty acids (FAs) and ascorbic acid (AA), respectively. C2-C18 FAs have been separated in less than 12 min using 8-hydroxy-7-iodoquinoline sulfonic acid as chromophores in NACE with indirect absorbance. The dissociation constant (p/linear plot -log[(mu 0/mu) - 1] versus pH, using 20% isopropanol and 40% ACN as the organic modifier in NACE, are all above about two units than those obtained in aqueous solution. NACE with indirect laser-induced fluorescence, using merocyanine 540 MC540) asfluorophores, has been performed for the analysis of AA and its stereoisomer, isoascorbic acid (lAA), and the limits of detection of AA and lAA are 0.30 and 0.17 pM, respectively. This method has been applied to the determination of AA in a lemon juice spiked with lAAasthe internal standard in less than three minutes, and its concentration is 76.7 + 0.4 mM. 

Iqbal, J., Scapozza, L., Eolkers, G, and Muller CE., Development and validation of a capillary electrophoresis method for the characterization of herpes simplex virus type 1 (HSV-1) thymidine kinase substrates and inhibitors. J. Chromatogr. B-Anal. Technol. Biomed. Life Sci., 846, 281-290, 2007. Wang, W, Tang, J., Wang, S., Zhou, L., and Hu, Z., Method development for the determination of coumarin compounds by capillary electrophoresis with indirect laser-induced fluorescence detection, J. Chromatogr. A, 2007. Vol 1148(1) 108-114. 

Capillary electrophoresis has been applied to the analysis of enzymically glucosylated flavonoids and of monosaccharides from glycosaminoglycans, using UV detection and indirect laser-induced fluorescence detection, respectively, and the separation of the cyanogenic glycosides amygdalin and prunasin from their isomers has been achieved by a micellar capillary electrophoresis method. ... 

For SAMs with attached redox molecules, k (units of s ) can be measured by cychc voltammetry, chronoamperome-try (CA), alternating current impedance spectroscopy (ACIS), alternating current voltammetry (AGV), AG electroreflectance spectroscopy, and an indirect laser-induced temperature (ILIT) jump method. 

THE INDIRECT LASER-INDUCED TEMPERATURE-JUMP METHOD FOR CHARACTERIZING FAST INTERFACIAL ELECTRON TRANSFER CONCEPT, APPLICATION, AND RESULTS... 

The objective of this chapter is to describe the indirect laser induced temperature method (ILIT) - an approach for studying heterogeneous electron transfer ILIT is particularly useful for studying systems where the redox species are attached to the electrode. Our focus will be on the fundamentals of the ILIT methodology and some of the questions that might be answered with its application. 

A fundamental reason for measuring rates of electron transfer (ET) is to identify the physical-chemical factors that control those rates and to learn how to control those factors. For the past several years we have focused our experimental efforts on the measurement of the heterogeneous electron-transfer rate constant, k° (units s ), for systems where the redox species are covalently attached to the electrode by any of a variety of molecular tethers [1-3]. The indirect laser-induced temperature method, which we developed [4-6], has the capability of measuring k° values as large as 10 s and has proven to be ideally suited for these types of studies. We will show some recent development that we anticipate will allow the measurement of k° values greater than 10 s . ... 

Ratios Lred /kmd and Lq J/k [see Eq. (27)] Coupling constant for electron transfer Indirect laser-induced temperature-jump method Indices indicating ILIT medium 1 (quartz),... 

The Indirect Laser-Induced Temperature Jump Method for Characterizing Fast Interfacial Electron Transfer Concept, Application, and Resnlts,... 

Refs. [i] Eigen M (1954) Discuss Faraday Soc 17 194 [ii] Norrish RGW, Porter G (1949) Nature 164 658 (1952) Proc RSocA 210 439 [Hi] Dian EW-G, Herek JL, Kim ZH, Zewail AH (1999) Science 279 847 [iv] Bard A] (1992) Pure Appl Chem 64 185 [v] Bard A], Fan F-R, Mirkin MV (1994) Scanning electrochemical microscopy. In Bard A] (ed) Electroanalytical chemistry, vol. 18. Marcel Dekker, New York [vi] Neudeck A, Marken F, Compton RG (2002) UV/Vis/NIR spectroelectrochemistry. In Scholz F (ed) Electroanalytical methods. Springer, Berlin, pp 167-189 [vii] FeldbergSW, Newton MD, Smalley JF (2004) The indirect laser-induced temperature jump methodefor characterizing fast interfacial electron tranter concept, application, and results. In Bard A], Rubinstein I (eds) Electroanalytical chemistry, vol. 22. Marcel Dekker, New York, pp 101-180... 

Electrochemical (amperometric) detection is also the method of choice for the analysis of nitroaromatic explosives on microchip devices since it offers up to three orders of magnitude higher sensitivity than indirect laser-induced fluorescence. ° Microfluidic device for detection of five TNT-related explosive compounds with exchangeable carbon thick-film screen-printed amperometric detector can be mentioned in this context. ... 

The Sq->S2 absorption spectrum of nitrosobenzene in a supersonic jet has been measured indirectly by monitoring yields of photoeliminated NO." Photolysis of the furoxan (63) (Scheme 5) generates two molecules of NO and the acetylene (64)." At 17%, the yield is low, but this is the first example of the photoelimination of NO from a furoxan to have been reported. A method for detecting nitrobenzene has been developed in which laser photolysis is combined with laser-induced fluorescence of the resulting NO fragments." A similar approach to detecting 2,4,6-trinitrotoluene in soil and groundwater has also been described." ... 

Experimental Analysis Despite the large number of works devoted to the investigation of the radical R" reactivity, the determination of the experimental values of the radical addition rate constants to a monomer double bond remains difficult. Only a small part of these values is based on a direct detection of the produced radical (flash photolysis, ESR). In fact, radicals are very often difficult to observe because of their absorption band below or close to 300 nm. Moreover, the classical overlap of the absorption spectra in the UV (due to the presence of different transient and ground-state species inherent to the production route when using laser flash photolysis) often prevents the direct observation. Indirect methods were tentatively used but they involve a complex set of reactions and are rather difficult to carry out. The same holds true for laser-induced photocalorimetry [265]. 

Several techniques have been developed for CE detection, such as indirect and direct ultravi-olet/visible (UVA is) optical methods. Laser-induced fluorescence OLIF) is widely used. Direct photometric detection based on UVA is light absorbance accounts for over half of all CE application. Absorbance detection operates at wavelength ranging from 190 to 800 nm. It covers both ultraviolet and visible regions. The usually used wavelengths for detection are 190 and 200 nm for... 

Atomic spectrometry Laser-induced atomic emission spectroscopy (AES) is a fast technique to determine directly elemental sulfur. Practically no matrix effects occur and the method is virtually nondestructive and easy to use. A disadvantage is the rather poor sensitivity, for example, a typical detection limit for sulfur in steel is 70pgperg. With indirect atomic absorption spectrometry clearly better... 

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