Mass spectrometry conditions, optimization

Usually 1,2-dioxetanes are thermally too labile to permit observation of the parent ion by mass spectrometry. Under optimized conditions the parent ions have been detected for the dioxetanes lb.12 lz,38 and laa.39... 

During the optimization of mass spectrometry conditions both positive and negative ionization, ESI (-I-) and ESI (-), were investigated. In Fig. 4, the TIC obtained in positive and negative mode are compared for a test mixture. Even though compound no. 2 (4-nitrophenol) shows a weak ionization in ESI (-I-), it is still recognizable, whereas compounds 11, 3, 6, 8, and 10 are visible only in the positive mode. As this example illustrates, the positive ionization mode is preferable, and it was chosen as default. 

Evans, E. H., and Ebdon, L. (1991). Comparison of normal and low-flow torches for inductively coupled plasma mass spectrometry using optimized operating conditions. J. A al. At. Spectrom. 6(6), 421. 

Laly, S., Nakagawa, K., Arimura.T, and Kimijima,T. (1996). Optimization of electrothermal vaporization, inductively coupled plasma mass spectrometry conditions for the determination of iron, copper, nickel and zinc in semiconductor grade acids. Spectrochim. Acta, Part B 51(11), 1393. 

To obtain the maximum concentration of MX2 species, the conditions for these reactions were first optimized by the pyrolytic mass spectrometry method (Kagramanov et al., 1983a). The contents of the mixture during the electron diffraction experiments were controlled by a quadrupole mass spectrometer. 

Dendrimers are regarded as macromolecules with a structural precision comparable to proteins or organic compounds. Accurate analysis and quantitative identification of side products are required to optimize and adjust the reaction conditions for the synthesis of DAB-dendr-(NH2)n and DAB-dendr-(CN)n. Therefore, it is a prerequisite to characterize the products obtained unambiguously. To achieve complete molecular characterization of the polypropylene imine) dendrimers and the possible side-products, NMR- and IR-spectroscopy, HPLC, GPC and electrospray mass spectrometry are used. 

Wang, L. Pan, H. Smith, D.L. Hydrogen exchange-mass spectrometry. Optimization of digestion conditions. Mol. Cell. Proteomics 2002, 1, 132-138. 

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). 

Solutions of triethylamine (Et3N) 14 (1.0M), premixed carboxylic acid/alkyl chloroformate (1.0 M respectively), and 4-dimethylaminopyri-dine 15 (0.5 M) in MeCN were introduced into the reactor from separate inlets and the reaction products collected at the outlet in MeCN, prior to analysis by gas chromatography-mass spectrometry (GC-MS). Under optimized reaction conditions, the authors were able to synthesize the methyl 16, ethyl 17, and benzyl 18 esters in quantitative conversion, with no anhydride or deprotection by-products detected (as observed in conventional batch reactions). In addition to the Boc-glycine derivatives illustrated in Scheme 4, the authors also esterified a series of aromatic carboxylic acids with yields ranging from 91 to 100%, depending on the additional functional groups present. 

Method development for mass spectrometry portion of LC-MS/MS assay is relatively simple and straightforward. For an experienced method development (MD) scientist, the optimization typically takes only hours to complete. In contrast, method development for optimal liquid chromatography conditions can be one of the most challenging tasks. Chromatography development can be very time-consuming. The task is further complicated by the nearly infinite choices in chromatography options such as vendor, sorbent, solvent selection, particle size, and column dimensions. 

The instrumental LoDs set forth in Table 3.1 shows that inductively coupled plasma-mass spectrometry (ICP-MS) is the technique that yields the best values. It should be considered that these limits are based on pure solutions analyzed under optimal conditions. Dealing with real foodstuffs will dramatically change the picture, owing to the complexity of the matrices and contamination phenomena in the laboratory. This means that in food laboratories without clean-room facilities (which is the vast majority) the practical difference in LoDs for ET-AAS and ICP-MS will be of minor importance. The relatively poor LoDs for inductively coupled plasma-atomic emission spectrometry (ICP-AES) when compared to those of ET-AAS implies that this technique is not fit for low-level determination of many elements, for example, Cd and Pb. 

After optimization of the method, the following analytical conditions were used silica capillary with stationary phase HP-5MS (30 m x 0.25 mm internal diameter, 0.25 pm film thickness) column gradient temperature started at 80 °C and increased at a rate of 10 °C/min to 280 °C injection temperature was fixed at 250 °C and helium was used as a carrier gas with constant flow rate of 1 mL/min. A mass spectrometry detector with single quadrupole was used as analyzer, operated at 150 °C with an acquisition scan rate of 50-800/n/z. 

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