Range Ion Abundance

The positive-ion electron-ionization spectra of BFB and DFTPP must exhibit molecular and specified fragment ions, the relative abundances of which must fall within a predefined range. Ion abundance criteria for BFB and DFTPP are shown in Table 41.1. 

The major advantage of array detectors over point ion detectors lies in their ability to measure a range of m/z values and the corresponding ion abundances all at one time, rather than sequentially. For example, suppose it takes 10 msec to measure one m/z value and the associated number of ions (abundance). To measure 100 such ions sequentially with a point ion detector would necessitate 1000 msec (1 sec) for the array detector, the time is still 10 msec because all ions arrive at the same time. Therefore, when it is important to be able to measure a range of ion m/z values in a short space of time, the array detector is advantageous. 

Figure 7. Simple model for predicting the ">0 " 0 ratio in observed secondary ions as a function of bonding configuration, (a) Condensation allows for a range of abundances depending on the number of bonds between the silane molecule and the surface. Electron donation (b). and protonation (c). both predict that no mixing of O O should be observed.

It is evident from the spectra that absolute ion abundances cannot be used for quantitation. However, the ion abundance ratio of the two compounds remained constant over the selected concentration range. Therefore, internal standards must be used for quantitation by SIMS. This has been found to be true for fast-atom bombardment mass spectrometry (FABMS), also [124]. 

Taken as a whole, the literature on isotope effects in mass spectrometry exhibits two salient features. The values of isotope effects for different molecules and different experimental conditions vary greatly and the isotope effects for some decompositions are very large (>100). These isotope effects are based on ion abundances, as has already been emphasised, but the kinetic isotope effects if measured would show not dissimilar variety and magnitudes. Both features arise because the range of internal energies encountered in reactive ions is very wide and the isotope effects are dependent upon internal energy, usually increasing... 

A conventional mass spectrometer was used to measure ion abtmdance ratios of the diligand fragments [Fe(6511702)2] which were formed during electron-impact ionization. Sample isotopic enrichment levels were obtained from standard curves that related ion abundance ratios to enrichment levels. Tracer concentration was calculated from the values for total iron content and enrichment level. The relative standard deviation for the ion abundance measurement was less than 2%. Recovery of tracers from spiked fecal samples ranged from 90% to 104%. The method was used to analyze samples collected from a human study. Iron availability from breakfast meals was determined in 6 yo mg women by giving 7 mg of in apple juice on one... 

CFgCg) + 282(g) by mass spectrometry. The various molecular species were found to be formed as products of the reaction of gaseous SFg with graphite at temperatures in the range 1436-1611 K. This study employed three different effusion cell configurations which were used to optimize the reaction conditions, and ion abundances for each species were measured at 2 eV above their appearance potentials in order to eliminate fragmentation effects. We analyze the reported equilibrium data by the 2nd and 3rd law methods with the results being presented below. 

The formation of molecular ions takes place with a range of internal energies, and more than one fragmentation path is possible for a given molecule. The mass spectrum is given as a chart showing the ion abundance (normalized to the most abundant ion) versus m/z of the fragments. For the interpretation of the mass spectra, two main questions should be answered, namely ... 

The primary objective of data processing is to obtain the routine mass spectrum, viz., to produce a chart showing the number of ions (ion current or ion abundance) at each m/z value within-a— given preset range of m/z values. To this end, the computer controls electrical or magnetic fields, so scanning of one or the other starts at some predetermined value and changes uniforaily until... 

Another positive aspect was the explicit concern for confirmation of analyte identity that used the identification points (IPs) approach (European Contunission 2002, see discussion of Tables 9.1-9.3 in Section 9.3.3b). For low resolution MS/MS detection using an ion trap as was done here, one IP is awarded for the m/z selected precursor ion and 1.5 IPs for each MRM transition monitored, with three IPs required (European Commission 2002) for acceptable confirmation of analyte identity. To qualify for the IPs, at least one ion abundance ratio for two MRM product ions must be measured and fall within tolerance intervals of contemporary values measured for an authentic analytical standard, defined (European Commission 2002) as 20 % for a relative abundance of >50 %, to 25 % for 10-50 % and to 50 % for < 10 %, although the US FDA recommends (FDA 2003) that these tolerances should be within 10% regardless of the absolute values of the relative abundances. The present work (Yang 2006) satisfied both tolerance criteria over the calibration range, although it was considered unlikely that they could be satisfied at the MDL. 

Using SIM, the Cl (methane) mode however, did not appear to be suitable for detecting low quantities in the nanogram range, probably because the ionization efficiency in this mode is low. It should be stressed that relative ion abundance in a mass spectrum, is not the determining factor for sensitivity of SIM in that particular ionization mode. The important criteria are the absolute ion currents, which can be compared by measuring a fixed amount of the compound of interest in the different modes of ionization. 

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