Indirect laser-induced

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. 

Wallenborg, S. R. and C. G. Bailey. Separation and detection of explosives on a microchip using micellar electrokinetic chromatography and indirect laser-induced fluorescence. Anal. Chem. 72, 1872-1878 (2000). 

An alternative to derivatization of nonfluorescent compounds is to perform indirect fluorescence detection. The procedure is performed on-column, by incorporating a fluorescent ion into the electrolyte. When ionic analytes interact with the fluorophore, the result is either displacement of the fluorophore or ion pairing with it.47 Kuhr and Yeung51 explored indirect laser-induced fluorescence detection using 1.0 mM salicylate as the fluorophore in the electrolyte they analyzed 10 amino acids and obtained detection limits on the order of 10 5 M. 

J.E. Melanson, C.A. Boulet and C.A. Lucy, Indirect laser-induced fluorescence detection for capillary electrophoresis using a violet diode laser, Anal. Chem., 73, 1809-1813 (2001). 

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. 

FIGURE 4 Indirect laser-induced fluorescence detection of 19 amino acids by microchip electrophoresis. Separation buffer 1.0 mM sodium carbonate, 0.5 mM fluorescein, and 0.2 mM CTAOH at pH 10.3. Separation condition /eff 5.5 cm, 15 s injection at 417 V/cm (reversed polarity), 183 V/cm separation voltage, sample amino acid concentrations of 0.4 mM in 1.0 mM sodium carbonate and 0.2 mM CTAOH. Reprinted with permission from [66]. Copyright 2000, The American Chemical Society. 

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. 

Indirect UV detection is a common technique which has been applied to the analysis of cations and anions. A UV-absorbing anion plus an electroos-motic flow modifier is added to the electrolyte. The displacement chromophore permits indirect photometric detection. Optimum separations can be achieved by choosing an electrolyte anion which has a mobility similar to the ions of interest. Some investigators have used conductivity detection, while others have used indirect laser-induced fluorescence for anion detection. Detection of the nonfluorescence analyte is obtained by charge displacement of the fluorophore (339,340). 

ILIT Indirect laser-induced temperature jump (technique)... 

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. 

Liu X, Liu X, Liang A, Shen Z, Zhang Y, Dai Z, Xiong B, Lin B. Studying protein-drug interaction by microfluidic chip affinity capillary electrophoresis with indirect laser-induced fluorescence detection. Electrophoresis 2006 27 3125-8. 

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),... 

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