Mass spectra procedures

Nd in samples. Unfortunately, mass spectrometry is not a selective technique. A mass spectrum provides information about the abundance of ions with a given mass. It cannot distinguish, however, between different ions with the same mass. Consequently, the choice of TIMS required developing a procedure for separating the tracer from the aerosol particulates. 

Abscisin II is a plant hormone which accelerates (in interaction with other factors) the abscission of young fruit of cotton. It can accelerate leaf senescence and abscission, inhibit flowering, and induce dormancy. It has no activity as an auxin or a gibberellin but counteracts the action of these hormones. Abscisin II was isolated from the acid fraction of an acetone extract by chromatographic procedures guided by an abscission bioassay. Its structure was determined from elemental analysis, mass spectrum, and infrared, ultraviolet, and nuclear magnetic resonance spectra. Comparisons of these with relevant spectra of isophorone and sorbic acid derivatives confirmed that abscisin II is 3-methyl-5-(1-hydroxy-4-oxo-2, 6, 6-trimethyl-2-cyclohexen-l-yl)-c s, trans-2, 4-pen-tadienoic acid. This carbon skeleton is shown to be unique among the known sesquiterpenes. 

Schmitt [17] in his book on the analysis of surfactants includes details of a number of HPLC-based procedures. LC-MS can be used for positive identification. Figure 29 shows the molecular ion mass spectrum for the surfactant lauryl hydrogen sulfate detectable as its (M—H) ion by positive ESI. 

A rapid technique for the identification of surfactants in consumer products by ESI-MS was proposed by Ogura and co-workers [6], After a simple preparation procedure, infusion of the sample, which was prepared in a water/methanol mixture (50 50) containing 10 mM ammonium acetate, allowed assignment of the [M + NH4]+ ions of Cio- and Ci2-mono- and -diglucoside in the mass spectrum (ion masses as in Table 2.7.1). The approach even permitted quantitative analysis when deuterated internal standards were used. 

The general procedure is to use reconstructed ion chromatograms at appropriate m/z values in an attempt to locate compounds of interest and then look at the mass spectrum of the unknown to determine its molecular weight. MS-MS can then be employed to obtain spectra from this and related compounds to find ions that are common to both and which may therefore contain common structural features. Having the same m/z value does not necessarily mean the ions are identical and further MS-MS data or the elemental composition may be required. If these data do not allow unequivocal structure identification, then further MS" information may be required. 

Note The preferred procedure to reveal the presence of S and Si in a mass spectrum is to examine the Xh-2 intensity carefully this signal s intensity will be too high to be caused by the contribution of alone, even if the number of carbons has been obtained from X-rl without prior subtraction of the S or Si contribution. 

Any mass spectrometer requires mass calibration before use. However, the procedures to perform it properly and the number of calibration points needed may largely differ between different types of mass analyzers. Typically, several peaks of well-known m/z values evenly distributed over the mass range of interest are necessary. These are supplied from a well-known mass calibration compound or mass reference compound. Calibration is then performed by recording a mass spectrum of the calibration compound and subsequent correlation of experimental m/z values to the mass reference list. Usually, this conversion of the mass reference list to a calibration is accomplished by the mass spectrometer s data system. Thereby, the mass spectrum is recalibrated by interpolation of the m/z scale between the assigned calibration peaks to obtain the best match. The mass calibration obtained may then be stored in a calibration file and used for future measurements without the presence of a calibration compound. This procedure is termed external mass calibration. 

In a mass spectrum, the observed mass divided by the difference between two masses that can be separated m/Am. The procedure by which Am was obtained and the mass at which the measurement was made should be reported. 

An alternative to quantitative analysis by ICP-MS is semiquantitative analysis, which is generally considered as a rapid multielement survey tool with accuracies in the range 30-50%. Semiquantitative analysis is based on the use of a predefined response table for all the elements and a computer program that can interpret the mass spectrum and correct spectral Interferences. This approach has been successfully applied to different types of samples. The software developed to perform semiquantitative analysis has evolved in parallel with the instrumentation and, today, accuracy values better than 10% have been reported by several authors, even competing with typical ones obtained by quantitative analysis. The development of a semiquantitative procedure for multielemental analysis with ICP-MS requires the evaluation of the molar response curve in the ICP-MS system (variation of sensitivity as a function of the mass of the measured isotope) [17]. Additionally, in the development of a reliable semiquantitative method, some mathematical approaches should be employed in order to estimate the ionisation conditions in the plasma, its use to correct for ionisation degrees and the correction of mass-dependent matrix interferences. 

Dimethyl bromosuccinate was prepared from bromosuccinic acid by the diazomethane method (26) using the procedure of Eisenbraun, Morris, and Adolphen (27). It was distilled under vacuum (0.08-0.1 Torr) at 45°-49°C to yield a clear colorless oil. Thin layer chromatography with benzene as the solvent on SiC>2 yielded a symmetrical single spot, indicating either a pure compound or no separation with this particular solvent. Its mass spectrum had a very small peak corresponding to the parent compound, but none to a dibromo compound. The mass spectrum for dimethyl bromosuccinate was not found in the literature, but that for dimethyl succinate also has a small peak corresponding to the parent compound (25). 

No precautions to exclude air are necessary as the product is completely stable in air. The mass spectrum of the product showed that no complexed phosphine oxides are present. The tricarbonyl hydride produced by this simple procedure is the starting material for the preparation of the ionic tetracarbonyl salt described below in the next procedure as well as for preparation of the covalent tosylate obtained by treatment of the hydride with p-toluenesulfonic acid. 

When this method, in a slightly modified form, was applied to Salmonella free lipid A, mainly two compounds were liberated which were detected by gas-liquid chromatography and characterized by mass spectrometry. The spectrum of the first compound was identical with that of authentic 3-hydroxytetradecanoic acid, 3-0-acylated by dodecanoic acid (3-0-dodecanoyl-tetradecanoic acid methyl ester, 3-0(12 0)-14 0) (27). The mass spectrum of the second peak showed, inter alia, characteristic fragments at 496 (M ), 465 (M-31), 239, 240 and 241, thus corresponding to 3-0-hexadecanoyl-tetradecanoic acid methyl ester (3—0(16 0)—14 0). Therefore, the two components present in Salmonella lipid A in amide linkage and released by the above described procedure... 

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