Precise Mass Determination

Qualitatively, the spark source mass spectrum is relatively simple and easy to interpret. Most instrumentation has been designed to operate with a mass resolution Al/dM of about 1500. For example, at mass M= 60 a difference of 0.04 amu can be resolved. This is sufficient for the separation of most hydrocarbons from metals of the same nominal mass and for precise mass determinations to identify most species. Each exposure, as described earlier and shown in Figure 2, covers the mass range from Be to U, with the elemental isotopic patterns clearly resolved for positive identification. 

The isotopic molar masses are precise to five or more significant figures, so we are tempted to express the result with five significant figures. The mass defect is determined by addition and subtraction, however, and two of the isotopic molar masses are known to just three decimal places, so the mass defect is precise to three decimal places, and the... 

This type of mass spectrometer, which is not widely used, allows mass determination with a high precision. An ion cyclotron resonance spectrometer is basically an ion trap ions formed by electron impact, for example, are subjected to the orthogonal magnetic field B, which induces cyclotronic movement in the. rv plane (Fig. 16.8). The radius of the circular movement, which depends on kinetic energy, is given by equation (16.2). If the velocity v is small and the magnetic field B is intense, the radius of the trajectory will be small and the ions will be trapped in the ionisation... 

Support Protocol Density measurement The required time depends strongly on the equipment being used. Modern analytical equipment can correctly measure density within minutes. If the density measurement is conducted using the specific gravity bottle method, then an initial calibration is required that can take up to several hours (due to the fact that the volume of the bottle has to be determined as a function of temperature). After the initial calibration curve has been obtained, tests can be conducted within 15 to 20 min since only a precise mass determination is necessary. 

Several recommendations arose from the interlaboratory smdy to minimize analytical challenges and to ensure data quality. As discussed above, it is recommended that mass labelled PFCs be employed as internal standards [93, 97]. It should be noted, however, that some electrospray ionization suppression may still occur if these internal standards are used at high concentrations [97]. Matrix effects can also be minimized by employing matrix-matched calibration standards in lieu of solvent-based calibration standards [97]. Unfortunately, matrix-matched standards can be impractical when an appropriate clean matrix cannot be found [94]. Other quality assurance and quality control measures, such as spike and recovery analyses of an analyte added to the sample matrix, repetitive analysis of samples to determine precision and comparison of internal standard quantitation to quantitation via standard additions, are also useful in determining data quality [94]. 

Mass spectrometer precision was determined by making repeated measurements on a gas sample prepared by combustion of carbon isotope reference material NBS-22. The standard deviation of the mean derived from 10 consecutive measurements of this gas was 0.02 %o The error associated with the combustion and purification procedure was measured by replicate combustions of the NBS-22 reference material, which resulted in a standard deviation of 0 12 %o for five samples ITius, the overall precision associated with the mass spectrometric measurement of vs PDB was 0.12 %o, or in absolute terms, 1.3 ppm. Most of the error clearly was associated with the combustion and sample handling process. Since sealed-tube combustions have been shown to produce theoretical recoveries of carbon (33). these small errors most likely arise from handling the CO2 after it is released from the sample tube ... 

Profiling should be conducted in high resolution mode in order to precisely determine mass and verify precursor isotopic distributions of M + 1 and M + 2 isotopic patterns where appropriate. Excessively high standard concentrations will interfere with isotopic distribution verification and accurate mass determination if the signal for the molecular ion is saturated. Once the precursor and... 

Techniques such as matrix-assisted laser desorption and ionization (MALDI) and electrospray ionization (ESI) allow the generation of ions of proteins and peptides in the gas phase. The mass of such protein ions can be determined with great accuracy and precision. Masses determined by these techniques act as protein name tags because the mass of... 

As stated earlier, obesity is a predisposing factor for type 2 diabetes. What is the biochemical basis for this relation Although much remains to be determined, recent research suggests that the adipocytes secrete a hormone called resistin resistance to insulin) that renders tissues insensitive to insulin. Moreover, the amount of resistin secreted is directly proportional to fat mass. The precise physiological role of resistin remains to be determined. 

Stepwise isothermal analysis This has been described (46,47) as a method whereby reaction temperatures and mass loss steps can be accurately characterized by a complementary combination of simultaneous TG and DTA methods. The sample is heated at a slow, constant rate until a reaction is detected and, on reaching a specified (low) rate of mass loss, the temperature is maintained constant. When this rate process has been completed, indicated by diminution to a second specified (relatively much lower) rate, the slow rate of temperature increase is recommenced. Thus, each successive rate process is completed isothermally and both the mass loss (evidence of stoichiometry) and the temperature of that step are determined precisely. 

When applied precisely, the gas density method may be used for atomic mass determinations, particularly for the lighter elements. Even crude gas density experiments may be used, in conjunction with chemical composition data and known atomic masses to establish the molecular mass, and hence the molecular formula of a gaseous compound. 

One method of obtaining an empirical formula is from a precise mass determination of the molecular ion. No atom has a mass equal to a whole number (except by definition, carbon, which has a mass of exactly 12 Dalton), e.g., = 1.0078, >60=15.9949, >4n=14.0031. Therefore, the... 

Although the infrared measurement of methane production leads to qualitative agreement with the direct mass determination, quantitative agreement is not good. This is most probably a result of axial dispersion in the gas product stream which results in a loss of kinetic information, difficulties in precisely regulating the product stream flow rate which would lead to cumulative errors, and the formation of small amounts of higher molecular weight hydrocarbons. 

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