Ion abundance ratio

It is generally assumed that similar product ion abundances indicate that the metastable ions exhibit identical structures. It should, however, be emphasized that the abundance ratios can be highly sensitive to variations in internal energies. Hence, based on variation in the product ion abundance ratios only, it cannot unambiguously be concluded that the parent ion structures are different3-5. 

Table 8 Fragment-ion abundance ratios from CID of proton-bound [S2-M-H]" " clusters... 

Table 10 Fragment ion abundance ratios in the MS spectra of Cu(II)-bound trimer complexes...

Chiral resolution factor, ScMraU as the ratio between the homochiral and the heterochiral ion abundance ratios. 

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]. 

Apart from the problem of nonlinearity, the calibration curve approach has another pitfall measured ion abundance ratios can change with time, leading to the possibility of significant errors since the calibration and sample measurements cannot be simultaneous (Schoeller, 1980). In order to minimize the effect of instrumental drift and to optimize precision, the National Bureau of Standards (NBS) proposed a bracketing protocol for the development of definitive (i.e., essentially bias-free and precise) IDMS methods (Cohen et al., 1980 White et al., 1982 Yap et al., 1983). It involves the measurement of each sample between measurements of calibration standards whose ion abundances most closely surround the ion abundance ratio of the sample. Measurements are made according to a strict protocol, used with samples prepared under restrictive conditions ... 

The qualitative identification criteria (see Section 11) include requirements for retention times, simultaneous detection of three ions per compound, and limits on the ratio of the abundances of the two most intense ions produced by each compound. In instances where a signal is detected that meets all of the qualitative identification criteria except the ion abundance ratio, the method requires calculation of an Estimated Maximum Possible Concentration (EMPC). The presence of interferences that coelute with the compounds of interest may cause the ion abundance ratio to fall outside the limits for qualitative identification and would also affect the quantitative results. The EMPC is a worst case estimate of the sample concentration that the signal would represent if it did meet all the identification criteria (see Seccion 15.7). Because of the quantitative uncertainty associated with the EMPC values, they are not included in the TEF calculations performed in the method. 

The identification of all other PCDD/PCDF isomers is based on their retention times falling within their respective PCDD/PCDF retention time windows as established by a window defining mix. Confirmation of all PCDDs/PCDFs is based on a comparison of the ratio of the integrated ion abundance of the molecular ion species to the theoretical ion abundance ratio. 

The Contractor shall not proceed with the sample analysis until an acceptable initial calibration has been performed and documented according to the following criteria GC resolution, ion abundance ratios, retention times, and instrument sensitivity. 

The relative ion abundance criteria for PCDDs/PCDFs listed in Table 6 must be met for all PCDD/PCDF peaks, including the labeled internal and recovery standards in all solutions. The lower and upper limits of the ion abundance ratios represent a 15 percent window around the theoretical abundance ratio for each pair of selected ions. The 13Ci2-2,3,7,8-TCDD cleanup standard contains no 35C1, thus the ion abundance ratio criterion does not apply to this compound. 

All the analytes in the CC1 solution must meet the ion abundance ratio criteria in Table 6. 

If the analysis of the CC1 solution at the end of the 12-hour period fails either the ion abundance ratio or S/N criteria above, the Contractor must ... 

These reanalyses are necessary because poor S/N ratios indicate a loss of sensitivity that could lead to false negative results, underestimation of concentrations, or could cause ion abundance ratios to fall outsite the QC limits. 

If interferences are present and ion abundance ratios are not met using peaks areas, but all other qualitative identification criteria are met (RT, S/N, presence of all three ions), then die Contractor may use peak heights to evaluate the ion ratio. 

The identification of a GC peak as a PCDF cannot be made if a signal having S/N greater than 2.5 is detected at the same retention time (2 seconds) in the corresponding PCDPE channel (see Table 5). If a PCDPE is detected, it shall be documented in the SDG Narrative, and an Estimated Maximum Possible Concentration (EMPC) shall be calculated for this GC peak according to Section 15.7, regardless of the ion abundance ratio, and reported on Form I. 

NOTE In instances where peak heights are used to evaluate ion abundance ratios due to interferences (see Section 11.4) substitute peak heights for areas in the formulae above. 

If any of the internal standard ion abundance ratios as specified in Table 6 are outside the contract specified control limits, the Contractor must reanalyze the sample extract on a second GC column with different elution characteristics, as discussed in Section 16. No reextraction is required for such an analysis. This reanalysis is only billable if the same internal standard ion abundance ratios are outside the control limits on the second column, indicating matrix effects may have occurred. 

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... 

Table III. Ion abundance ratios of natural Fe (0511702)2 values were calculated from measured peak heights or peak areas.

Figure 4 Standard curves for the ion abundance ratio measurements of 54-Fe and...

Finally, ortho effects involving the radical cations of various nitro-substituted alkylphe-nols are mentioned here. In these cases, two sequential ortho effects have been observed. For example, the El mass spectra of 2-ethyl-4,6-dinitrophenol and its 2-cyclohexyl analogue exhibit pronounced peaks for the formation of [M — H20] + and [M — H2O — OH]+ ions and the spectrum of 2-isopropyl-4,6-dinitrophenol even indicates that the secondary fragmentation step is faster than the primary one, because an ion abundance ratio [M — H2O - OH]+/[M - H20] + = 25 was found . 

The ion abundance ratio determined by mass spectrometry NH2+/NH3+ -— 0.4 (21, 22), but the relative importance of Reactions 1 and 2 under radiolytic conditions is not known. The ion-molecule Reactions 3, 4, and 5... 

In general, to achieve an isotopic ion abundance ratio measurement with the best precision, the ratio of analyte concentration to isotopic analogue concentration... 

In order to achieve the highest level of accuracy, special care must be taken in setting up mass spectrometric instrumentation prior to measurement of isotopic ion abundance ratios, as a long stabilisation period may be necessary in order to achieve the highest precision. A major source of imprecision is the instability of the instrument and the electronic signal level must be correctly set and not be subject to drift. In the case of the latest computer controlled instruments such checks can be carried out automatically. 

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