Mass spectrometric method

The use of mass spectrometry for the measurement of the fluxes of reactants and products has the very great advantage of directness. In this application, it is essential that the ioniser of the mass spectrometer allow unobstructed passage of the neutral beam to avoid surface recombination of atoms. A further requirement [41] is that the electron-emitting filament should be at a temperature below that at which atomisation can occur, but this condition has not always been met. 

Smith and Fite used modulation techniques to differentiate material which had interacted with the tungsten surface from background material 

Cassuto and co-workers [47] have studied the atomisation of oxygen over polycrystalline platinum by the method of mass spectrometry applied to stationary conditions. They demonstrated that the desorption of 02 molecules was second order with an activation energy which decreased with increasing coverage. At zero coverage, their relation for the rate coefficient is 

Cassuto and co-workers [51—53] investigated the interaction of oxygen with rhenium in a manner which combined the methods used by Schissel 

The activation energy decreases linearly with coverage, viz. 

MS has been used as a means of obtaining accnrate information regarding breakdown products produced upon pyrolysis of polymers. This includes applications to PS [152, 153], PVC [154], polyethers [155], PVC-polycarboxy piperadine polyurethanes [156], phenolics [157], PTFE [158], polybenzimidazole epoxies [159], ethylene-vinyl acetate copolymers [160], ethylene-vinyl alcohol copolymers [161], polybenzoxazines [162], polyxylyene sulfides [163], trimethoxysiloxy-snbstitnted polyoxadisilpentanylenes [164], chlorinated natural rubber [165], and polyacrylonitrile [166]. 

For qualitative analysis of polymers, it is not necessary to know the product of the reaction, since identification can be based on temperatures and relative heights at the maximum of several of the more prominent fragments once these have been established for known materials. Additional information about the degradation chemistry can be secured if one chooses peaks characteristic of specific products. 

Since a Knudsen cell inlet system and MS are being evacuated continuously, the pressure in the vicinity of the ionising filament is dependent on the rate of production 

Polymer samples were placed in a tungsten crucible in a Knudsen cell inlet system of a time-of-flight MS. The system was evacuated. Samples were then heated at a linear rate. Spectra were determined at 1 minute intervals. The thermocouple was welded to a support rod 3 mm below the crucible and had been calibrated against a thermocouple in the bottom of the crucible at each heating rate. Details are summarised in Tables 9.4 and 9.5 for three polymers. 

Product Yield, % of volatiles Activation energy, kcal/mole 

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APPLICATION OF LASER BASED MASS-SPECTROMETRIC METHODS FOR TRACE ANALYSIS OF SYNTHETIC AND NATURAL CRYSTALS... 

Laser based mass spectrometric methods, such as laser ionization (LIMS) and laser ablation in combination with inductively coupled plasma mass spectrometry (LA-ICP-MS) are powerful analytical techniques for survey analysis of solid substances. To realize the analytical performances methods for the direct trace analysis of synthetic and natural crystals modification of a traditional analytical technique was necessary and suitable standard reference materials (SRM) were required. Recent developments allowed extending the range of analytical applications of LIMS and LA-ICP-MS will be presented and discussed. For example ... 

Useful yield provides an overall measure of the extent to which the sputtered material is used for analysis. It is a quantity employed to estimate the sensitivity of the mass spectrometric method. Values of Y (X (A)) for elements typically range from 10 to 10 in TOF SIMS. The number of sputtered particles A per incident primary ion (sputtering yield) can be measured from elemental and multielemental standards under different operational conditions and can, therefore, by judicious interpolation between standards, be estimated with reasonable accuracy for the material being analyzed. 

All four monohalides of gold have been prepared but the fluoride only by mass spectrometric methods. AuCl and AuBr are formed by heating the trihalides to no more than 150°C and Aul by heating the metal and iodine. At higher temperatures they dissociate into the elements. Aul is a chain polymer which features linear 2-coordinate Au with Au-I 262 pm and the angle Au-I-Au 72°. 

Origin of Ions in Hydrocarbon Flames. Many ions, both positive and negative, are observed in hydrocarbon flames studied by mass spectrometric methods (9, 14, 26). Most of these are produced by ion-molecule reactions following the formation of primary ions from neutral species. 

Do of 1.40 eV for AI2 is within the error bounds of the experimental value of 1.55 0.15 eV determined by Stearns and Kohl (46) using a Knudsen cell mass spectrometric method and assuming a ground state. 

MAZUR W, FOTSIS T, WAHALA K, OJALA S, SALAKKA A and ADLERCREUTZ H (1996) Isotope dilution gas chromatographic-mass spectrometric method for the determination of isoflavonoids, coumestrol and lignans in food samples. Biochem. 233 (2) 169-80. 

When [ H]-labeled precursors are employed the resulting compounds can be used as internal standards for analysis, especially by utilization of mass spectrometric methods. Appropriate deuterated standards are shown in Fig. 7. The introduction of deuterium into the A9-THC precursors can be done with Grignard reagents such as C[ H3]MgI or reducing substances such as LiAl[ H4]. The general procedures for the synthesis with these [ Hj-labeled precursors are the same as described above for the unlabeled compounds [76,78]. 

Studies of the Thermochemical Properties of Reactive Intermediates by Mass Spectrometric Methods... 

In most alpha and mass spectrometric methods for which sample preparation is extensive and chemical recoveries can vary considerably from sample to sample, precise elemental concentrations are determined by isotope dilution methods (e.g., Faure 1977). This method is based on the determination of the isotopic composition of an element in a mixture of a known quantity of a tracer with an unknown quantity of the normal element. The tracer is a solution containing a known concentration of a particular element or elements for which isotopic composition has been changed by enrichment of one or more of its isotopes. 

There are methods available to quantify the total mass of americium in environmental samples. Mass spectrometric methods provide total mass measurements of americium isotopes (Dacheux and Aupiais 1997, 1998 Halverson 1984 Harvey et al. 1993) however, these detection methods have not gained the same popularity as is found for the radiochemical detection methods. This may relate to the higher purchase price of a MS system, the increased knowledge required to operate the equipment, and the selection by EPA of a-spectrometry for use in its standard analytical methods. Fluorimetric methods, which are commonly used to determine the total mass of uranium and curium in environmental samples, have limited utility to quantify americium, due to the low quantum yield of fluorescence for americium (Thouvenout et al. 1993). 

Nowadays, MS is often no longer the analytical bottleneck, but rather what precedes it (sample preparation) and follows it (data handling, searching). Direct mass-spectrometric methods have to compete with the separation techniques such as GC, HPLC and SFC that are commonly used for quantitative analysis of polymer additives. Extract analysis has the general advantage that higher-molecular-weight (less-volatile) additives can be detected more readily than by direct analysis of the polymer compound. 

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