OPA

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

These limitations have recently been eliminated using solid-state sources of femtosecond pulses. Most of the femtosecond dye laser teclmology that was in wide use in the late 1980s [11] has been rendered obsolete by tliree teclmical developments the self-mode-locked Ti-sapphire oscillator [23, 24, 25, 26 and 27], the chirped-pulse, solid-state amplifier (CPA) [28, 29, 30 and 31], and the non-collinearly pumped optical parametric amplifier (OPA) [32, 33 and 34]- Moreover, although a number of investigators still construct home-built systems with narrowly chosen capabilities, it is now possible to obtain versatile, nearly state-of-the-art apparatus of the type described below Ifom commercial sources. Just as home-built NMR spectrometers capable of multidimensional or solid-state spectroscopies were still being home built in the late 1970s and now are almost exclusively based on commercially prepared apparatus, it is reasonable to expect that ultrafast spectroscopy in the next decade will be conducted almost exclusively with apparatus ifom conmiercial sources based around entirely solid-state systems. 

Perhaps the ultimate femtosecond light source, the OPA exploits a nonlinear parametric process to amplify a portion of... 

The OPA should not be confiised with an optical parametric oscillator (OPO), a resonant-cavity parametric device that is syncln-onously pumped by a femtosecond, mode-locked oscillator. 14 fs pulses, tunable over much of the visible regime, have been obtained by Hache and co-workers [49, with a BBO OPO pumped by a self-mode-locked Ti-sapphire oscillator. 

Schematic diagrams of modem experimental apparatus used for IR pump-probe by Payer and co-workers [50] and for IR-Raman experiments by Dlott and co-workers [39] are shown in figure C3.5.3. Ultrafast mid-IR pulse generation by optical parametric amplification (OPA) [71] will not discussed here. Single-colour IR pump-probe or vibrational echo experiments have been perfonned with OP As or free-electron lasers. Free-electron lasers use...

Por IR-Raman experiments, a mid-IR pump pulse from an OPA and a visible Raman probe pulse are used. The Raman probe is generated either by frequency doubling a solid-state laser which pumps the OPA [16], or by a two-colour OPA [39]. Transient anti-Stokes emission is detected with a monocliromator and photomultiplier [39], or a spectrograph and optical multichannel analyser [40]. 

Opa.nte. There are two methods used at various plants in Russia for loparite concentrate processing (12). The chlorination technique is carried out using gaseous chlorine at 800°C in the presence of carbon. The volatile chlorides are then separated from the calcium—sodium—rare-earth fused chloride, and the resultant cake dissolved in water. Alternatively, sulfuric acid digestion may be carried out using 85% sulfuric acid at 150—200°C in the presence of ammonium sulfate. The ensuing product is leached with water, while the double sulfates of the rare earths remain in the residue. The titanium, tantalum, and niobium sulfates transfer into the solution. The residue is converted to rare-earth carbonate, and then dissolved into nitric acid. 

Several methods were reported for the analysis of histamine, but the fluorimetric determination with o-phthaldialdehyde (OPA) the most widely used. It was shown that adducts, formed in the reaction of histamine with OPA in the presence of reducing agent, is more stable and gives high relative fluorescence intensity. The influences of different tiols on the fluorimeric determination histamine with OPA have been investigated. 

Seven Cardinal Rules of Risk Communication, OPA-87-020, U.S. Environmental Protection Agency, Washington, DC, March 1996. 

A Citizen s Guide to Radon What It Is and What to Do About It. U.S. Environmental Protection Agency and U.S. Department of Health and Human Services, OPA-8-004. Government Printing Office, Washington, DC, 1986. 

Dipping solution Make 0.1 g o-phthalaldehyde (phthaldialdehyde, OPA) and 0.1 ml 2-mercaptoethanol (2-hydroxy-l-ethanethiol) up to 100 ml with acetone. 

FIGURE 4.22 HPLC chromatogram of amino acids employing precolumn derivatiza-tion with OPA. Chromatography was carried out on an Ultrasphere ODS column using a complex tetrahydrofuran methanol 0.05 M sodium acetate (pH 5.9) 1 19 80 to methanol 0.05 M sodium acetate (pH 5.9) 4 1 gradient at a flow rate of 1.7 mL/min. 

Separation of Amino Acid Enantiomers after Derivatization with Or/ho-Phthaldialdehyde (OPA) and a Unichiral Tliiol Compound... 

The mixture of free amino acids is reacted with OPA (Fig. 7-8) and a thiol compound. When an achiral thiol compound is used, a racemic isoindole derivative results. These derivatives from different amino acids can be used to enhance the sensitivity of fluorescence detection. Figure 7-9 shows the separation of 15 amino acids after derivatization with OPA and mercaptothiol the racemic amino acids may be separated on a reversed-phase column. If the thiol compound is unichiral, the amino acid enantiomers may be separated as the resultant diastereomeric isoindole compound in the same system. Figure 7-10 shows the separation of the same set of amino acids after derivatization with the unichiral thiol compound Wisobutyryl-L-cysteine (IBLC). 

Fig. 7-8. Derivatization of amino acids with OPA and a thiol compound.

The advantages of this method are a short reaction time and the nonfluorescence of the OPA reagent. Therefore, excess reagent must not be removed before the chromatography stage. Using this method, it is possible to measure tryptophan, but not secondary amino acids such as proline or hydroxyproline. Cysteine and cystine can be measured, but because of the low fluorescence of their derivatives, they must be detected using an UV system, or alternatively oxidized to cysteic acid before reaction. 

Fig. 7-9. Separation of amino acids after derivatization 5 with OPA and mercaptoethanol. Column Superspher 100 RP-18 (4 pm) LiChroCART 250-4, mobile phase 50 mM sodium acetate buffer pH 7.0/methanol, flowrate 1.0 ml min temperature 40 °C detection fluorescence, excitation 340 nm/emission 445 nm. Sample amino acid standard sample (Merck KGaA Application note W219180).

A number of drawbacks in the application of the 0PA/2-ME reagent system include the instability of the fluorescent isoindole derivative (5-7) the use of the noisome reagent 2-mercaptoethanol the low and solvent-dependent fluorescence efficiencies (8,9) of the isoindole and—perhaps the most limiting—the effective restriction of the OPA assay to primary aliphatic amines and to amino acids. 

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