Mass spectrometry , laboratory experiments

Because use of mass spectrometry by chemists has increased greatly, most U.S. chemists have access to mass spectrometry facilities at their own institutions to confirm synthesis and support structure elucidations. Heavily used national centers provide more expensive instrumentation and more complex experiments. Most notably, a section of the National High Magnetic Field Laboratory at Florida State University provides state-of-the-art Fourier transform ion cyclotron resonance mass spectrometry. The NSF Arizona Accelerator Mass Spectrometry Laboratory is used primarily to provide radiocarbon measurements. NIH funds a number of national mass spectrometry centers to support biomedical research, including those at Boston University and the Pacific Northwest National Laboratory. 

National Institutes of Health, United States Public Health Service (Research Grants GM 19978 and GM 20441), by the United States Army Research Office (Research Grant GM DAHC04-75-6-0179) and the gas chromatography-mass spectrometry laboratory supported by the University of Minnesota Agricultural Experiment Station, Scientific Journal Series No. 10,738, Agricultural Experiment Station, University of Minnesota, St. Paul, Minn. 55108. 

The advent of the atmospheric pressure ionization (API) source in the early 1990s allowed direct coupling of LC to MS. By the mid-1990s, this technology was a common in drug metabolism laboratories. The enhanced selectivity of tandem mass spectrometry (MS/MS) experiments reduced the need for exhaustive chromatographic separations prior to detection and this feature was exploited to significantly reduce analysis times. 

L. Paulson, R. Persson, G. Karlsson, J. Silberring, A. Bierczynska-Krzysik, R. Ekman, and A. Westman-Brinkmalm. Proteomics and Peptidomics in Neuroscience. Experience of Capabilities and Limitations in a Neurochemical Laboratory. J. Mass Spectrometry, 40(2005) 202-213. 

The extensive research on nitro compounds confirms a rich gas-phase ion chemistry. It is, however, noteworthy that Porter, Beynon and Ast in the classical review, The Modern Mass Spectrometer—A Complete Chemical Laboratory, were able to demonstrate the capabilities of mass spectrometry with no less than thirty different experiments involving a single compound, i.e. nitrobenzene1. 

CAI s that were once molten (type B and compact type A) apparently crystallized under conditions where both partial pressures and total pressures were low because they exhibit marked fractionation of Mg isotopes relative to chondritic isotope ratios. But much remains to be learned from the distribution of this fractionation. Models and laboratory experiments indicate that Mg, O, and Si should fractionate to different degrees in a CAI (Davis et al. 1990 Richter et al. 2002) commensurate with the different equilibrium vapor pressures of Mg, SiO and other O-bearing species. Only now, with the advent of more precise mass spectrometry and sampling techniques, is it possible to search for these differences. Also, models prediet that there should be variations in isotope ratios with growth direction and Mg/Al content in minerals like melilite. Identification of such trends would verify the validity of the theory. Conversely, if no correlations between position, mineral composition, and Mg, Si, and O isotopic composition are found in once molten CAIs, it implies that the objects acquired their isotopic signals prior to final crystallization. Evidence of this nature could be used to determine which objects were melted more than once. 

Hyphenated analytical techniques such as LC-MS, which combines liquid chromatography and mass spectrometry, are well-developed laboratory tools that are widely used in the pharmaceutical industry. Eor some compounds, mass spectrometry alone is insufficient for complete structural elucidation of unknown compounds nuclear magnetic resonance spectroscopy (NMR) can help elucidate the structure of these compounds (see Chapter 20). Traditionally, NMR experiments are performed on more or less pure samples, in which the signals of a single component dominate. Therefore, the structural analysis of individual components of complex mixtures is normally time-consuming and less cost-effective. The... 

Two homogeneous metal complex water-gas shift catalyst systems have recently appeared 98, 99). The more active of these comes from our Rochester laboratory (99, 99a). It is composed of rhodium carbonyl iodide under CO in an acetic acid solution of hydriodic acid and water. The catalyst system is active at less than 95°C and less than 1 atm CO pressure. Catalysis of the water-gas shift reaction has been unequivocally established by monitoring the CO reactant and the H2 and C02 products by gas chromatography The amount of CO consumed matches closely with the amounts of H2 and C02 product evolved throughout the reaction (99). Mass spectrometry confirms the identity of the C02 and H2 products. The reaction conditions have not yet been optimized, but efficiencies of 9 cycles/day have been recorded at 90°C under 0.5 atm of CO. Appropriate control experiments have been carried out, and have established the necessity of both strong acid and iodide. In addition, a reaction carried out with labeled 13CO yielded the same amount of label in the C02 product, ruling out any possible contribution of acetic acid decomposition to C02 production (99). 

In the paper that introduced FPTRMS [1], as well as early work from other laboratories, it is amply recorded that the experiments were hampered by low sensitivity, and it is apparent from reading those works that the amount of useful information was limited. Modern instrumentation and techniques of data aquisition and analysis have largely overcome the sensitivity problem, so that today mass spectrometry is a versatile and reliable technique for accurate studies of kinetics and mechanism. The sensitivity has improved to the point where free radicals can be detected at low enough concentrations that their reactions can be studied in the absence of radical-radical interactions that would otherwise complicate the kinetic analysis. Among modern methods for experimental chemical kinetics of gas reactions, FPTRMS has much to offer and should be seriously considered when evaluating alternative methods for kinetics investigations. 

The theory and instrumentation of Fourier transform mass spectrometry (FTMS) have been discussed extensively in this book and elsewhere [21-23]. All experiments were performed on a Nicolet prototype FTMS-1000 Fourier transform mass spectrometer previously described in detail [24] and equipped with a 5.2 cm cubic trapping cell situated between the poles of a Varian 15 in. electromagnet maintained at 0.85 T. The cell was constructed in our laboratory and utilizes two 80 transmittance stainless steel screens as the transmitter plates. This permits irradiation with a 2.5 kW Hg-Xe arc lamp, used in conjunction with a Schoeffel 0.25 m monochromator set for 10 nm resolution. Metal ions are generated by focusing the beam of a Quanta Ray Nd YAG laser (either the fundamental line at 1064 nm or the frequency doubled line at 532 nm) into the center-drilled hole (1 mm) of a high-purity rod of the appropriate metal supported on the transmitter screen nearest to the laser. The laser ionization technique for generating metal ions has been outlined elsewhere [25]-... 

Gas phase molecular aggregates that contain acid molecules have been produced with free jet expansion techniques and detected by using electron impact ionization mass spectrometry. The clusters of aqueous nitric acid paralleled many properties of the condensed phase. Multiple nitric acid molecules were found in the clusters that were sufficiently dilute. The acid molecule was absent in the ionized clusters involving HC1 and only water was evident. Experiments also demonstrated the reactivity of ammonia with aqueous nitric acid and sulfur dioxide clusters and of sulfur trioxide with water clusters. The natural occurrence of acid cluster negative ions offers a means to probe the gas phase acid loading of the atmosphere through laboratory and field studies of the ion chemistry. 

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