Pulsed extraction techniques

The short pulse of ions produced by laser desorption is ideally suited to mass analysis by time-of-flight (ToF), which makes it possible to record a complete mass spectrum for a single laser shot. Actually, MALDI has greatly contributed to the revival of ToF-MS. The detection efficiency decreases as the ion mass increases. Key instrumental parameters of MALDI-MS are mass resolution and mass accuracy. Mass resolution of MALDI-ToFMS has increased considerably by pulsed extraction techniques. Resolutions of 30,000 or better and mass accuracy at low ppm level are achieved, which allow separation of single oligomers. The sensitivity of ToF is in the sub-fmol range. Recently also the... 

Mercury was determined after suitable digestion by the cold vapour atomic absorption method [40]. Lead was determined after digestion by a stable isotope dilution technique [41-43]. Copper, lead, cadmium, nickel, and cobalt were determined by differential pulse polarography following concentration by Chelex 100 ion-exchange resin [44,45], and also by the Freon TF extraction technique [46]. Manganese was determined by flameless atomic absorption spectrometry (FAA). 

Figure 10 shows a spectrum of butyl rubber gum stock obtained on the solid at 80°C using normal pulsed FT techniques. Clearly it could be identified as a component in fabricated materials by direct nmr spectral analysis. Figure 11 shows spectra obtained from various portions of typical rubber products. These samples were cut from the rubber product, placed in an nmr tube without solvent, and spectra obtained at an elevated temperature. The data show how polyisoprene, a polyisoprene/polybutadiene blend and a polyisobutylene/polyisoprene/polybutadiene rubber blend are quickly identified in the materials. Figure 11a shows processing oil was present, and which was confirmed by solvent extraction. 

Pulsed-current techniques can furnish electrochemical kinetic information and have been used at the RDE. With a pulse duration of 10-4 s and a cycle time of 10-3 s, good agreement was found with steady-state results [144] for the kinetic determination of the ferri-ferrocyanide system [260, 261], Reduction of the pulse duration and cycle time would allow the measurement of larger rate constants. Kinetic parameter extraction has also been discussed for first-order irreversible reactions with two-step cathodic current pulses [262], A generalised theory describing the effect of pulsed current electrolysis on current—potential relations has appeared [263],... 

Under hypoxic conditions, cellular enzymes reduce the benzotriazine di-N-oxide [(reaction (68) P450 reductase Cahill and White 1990 and NADPH may be involved Walton et al. 1992 Wang et al. 1993]. Upon microsomal reduction of tirapazamine the radical formed in reaction (68) has been identified by EPR (Lloyd et al. 1991). Using the pulse radiolysis technique, it has been shown that this radical has a pKd of 6 (Laderoute et al. 1988), and it is the protonated form that undergoes the DNA damaging reaction (Wardman et al. 2003). The rate constants of the bimolecular decay of the radical [reaction (70)] has been found to be 2.7 x 107 dm3 mol-1 s 1. The reaction with its anion is somewhat faster (8.0 x 108 dm3 mol-1 s 1), while the deprotonated radicals do not react with one another at an appreciable rate. From another set of pulse radiolysis data, a first-order process has been extracted (k = 112 s 1) that has been attributed to the water elimination reaction (72), and the tirapazamine action on DNA [reaction (74)] has been considered to be due to the resulting radical (Anderson et al. 2003). 

If a pulsed sources such as MALDI seems to be well suited to the TOF analyser, the quality of its pulsed ion beam is insufficient to obtain the high resolution and high mass accuracy. This situation is substantially improved with the development of two techniques delayed pulsed extraction and the reflectron. 

The Fourier transform ion cyclotron resonance (FT-ICR) trapping of mass-selected cluster ions is an important emerging technique for the study of ion cluster reactivity. " This technique offers very high resolution and sensitivity. An alternative approach has been used by Brucat et al. who demonstrated that the reactivity of cluster ions can be studied in an experimental configuration identical to that used for the study of neutrals, except that ions are detected directly by pulsed extraction in the time-of-flight mass spectrometer. Other experiments " are exploring the reactions of mass-selected cluster ions in beam-gas-cell or drift-tube type configurations. This approach avoids the problems of mass overlap and offers a direct probe of cluster and cluster-adduct stabilities. For further experimental details, the reader is referred to the references. 

The strength and sensitivity of the -H NMR signals of oil shales are sufficient such that pulsed NMR techniques afford a means of dynamically observing thermal transformation processes. Parameters can be extracted from the NMR data that relate to the hydrogen content, phase structure, molecular mobility and free radical content of the specimen. 

As is evident, the power of the pulsed-extraction ion TOF-MS techniques derives from their generality, a result of the fact that REMPI may be applied to a large number of molecules. But velocity measurements of low-energy ions is a difficult business. Stray fields are never truly eliminated even in the hands of the cleverest experimentalist. Space charge is another potential problem for which one must be on the look out. In order to carry out the highest resolution measurements of velocity, one must work with neutrals. 

It has been found that nanosized carbons, such as single-waUed carbon nanotubes and graphenes, can be used as SAs for ML of solid-state lasers, as long as these carbon materials are semiconductive. For example, semiconducting SWCNTs with a high content of 80 % have been produced by using a polymer extraction technique. These SWCNTs exhibited a saturation intensity of 1.7 MW cm, modulation depth of 6 %, and nonresidual loss of 5.5 %, which ensured to generate stable pulses of 8.3 ps, with a repetition rate of 90 MHz, when combined with a 0.5 at.% Nd YAG ceramic laser pumped with a CW 808-nm diode [164]. 

Betso and McLean [11] have described a differential pulse polarographic method for the determination of acrylamide and acrylic acid in polyacrylamide. A measurement of the acrylamide electrochemical reduction peak current is used to quantify the acrylamide concentration. The differential pulse polarographic technique also yields a well-defined acrylamide reduction peak at 2.0 V versus SCA (reduction potential), suitable for qualitatively detecting the presence of acrylamide. The procedure involves extraction of the acrylamide monomer from the polyacrylamide, treatment of the extracted solution on mixed resin to remove interfering cationic and anionic species, and polarographic reduction in an 80/20 v/v) methanol/water solvent with tetra-n-butylammonium hydroxide as the supporting electrolyte. The detection limit of acrylamide monomer by this technique is less than 1 ppm. 

Slow dissociation rates (10 -10 s ) have been measured in Dunbar s laboratory by time-resolved photodissociation, which consists of trapping ions in an ICR cell during a variable delay time after a phot-odissociating photon pulse. The technique called time-resolved photoionization mass spectrometry , developed by Lifshitz, consists of trapping photoions in a cylindrical trap at very low pressure to avoid bimolecular collisions, and then ejecting them into a mass filter after a variable delay covering the microsecond to millisecond range. When the dissociation rate constant becomes lower than ca. 10 s competition with infrared fluorescence takes place and limits the lifetime of the decomposition process. This has to be taken into account to extract the dissociation rate constant from the experimental data. 

On the other hand, there are some ionization techniques that are very useful, particularly at very high mass, but produce ions only in pulses. For these sources, the ion extraction field can be left on continuously. Two prominent examples are Californium radionuclide and laser desorption ionization. In the former, nuclear disintegration occurs within a very short time frame to give a... 

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