Isotope pattern filter

Zhu P, Tong W, Alton K, Chowdhury S. An accurate-mass-based spectral-averaging isotope-pattern-filtering algorithm for extraction of drug metabolites possessing a distinct isotope pattern from LC-MS data. Anal Chem 2009b 81 5910—5917. 

Isotope pattern filter (IPF) [170,172] Predicted isotope pattern Sensitive detection of unexpected metabolites Not suited for metabolites that have no nnique isotope pattern... 

Different isotopic filters can be applied to minimize the nnmber of possible elemental compositions for a certain mass window. As a resnlt of the advances in HRMS technologies discussed above, several accurate-mass based data mining tools, including isotope pattern filter (Park et al. 2011), background snbtraction and noise reduction (Zhang and Zhang 2008 Zhu et al. 2009) and mass defect filter (Zhang et al. 2003,2009) were developed to facilitate data analysis. 

The algorithm isotopic pattern filter is based on the accurate m/z of ions, where the m/z differences of selected isotopic ion-pairs (e.g. M-t 1 M and M+2 M, where M is the monoisotopic peak and M -h 1 and M+2 are the first and the second isotopic peak) must fall into the pre-assigned accurate mass tolerance window (e.g. 5 ppm), so satisfying the predefined relative abundance criteria. These filters are based on the isotopic pattern deviation between the empirically measured spectrum and the theoretical spectrum. The application of carbon, chlorine, bromine or sulfur filters also allows reduction of the number of proposed elemental compositions that would fit for a certain mass-accuracy window, as their presence in the molecule produces a characteristic isotopic distribution. The additional information obtained from the isotopic signature reduces dramatically the number of proposed calculated empirical formulae to a number typically below 5. The reduced number of elemental compositions can be searched in databases using elemental composition as the search criterion (e.g. Merck Index, Chemindex, catalogues of standards manufacturer, and Google). 

In the analyses, isotope-dependent MS/MS acquisition methods with or without polarity switching are employed to record MS/MS spectra of analytes that display isotope patterns similar to those predefined. Then, NL filtering of 129 and 307 Da in the positive ion mode, or product ion filtering of m/z 272 in the negative ion mode, are carried out to identify GSH adducts [161,162],... 

An interesting feature in order to generate molecular formulas and to facihtate an extra confident identification is the use of different isotopic filters. These filters work based on the isotopic pattern deviation between the empirically measured and the theoretical spectrum. The presence of an abundant isotopic pattern in the analyte molecule helps to confirm the presence of that compound in the sample. Thus, the presence of atoms such as carbon, chlorine, bromine, or sulfur in the molecule gives a characteristic isotopic pattern that allows reducing the number of possible elemental compositions fora certain mass-accuracy window. The match between empirical and theoretical data is given by the isotope fit (i-FTT) or sigmaFlT values. These values are calculated, taking into account not only the isotopic distribution but also the accurate masses. The lower the value, the more plausible the elemental composition (Ojanpera et al., 2006 Ibanez et al., 2008). 

Mass Spectra. Whereas In the depth profiling mode the mass spectrometer Is used as a filter to obtain only one or a few masses for detection In the mass spectral mode the mass spectrometer Is used for the Identification and comparison of a wide range of masses. A survey mass spectrum Is often performed as the first step In any SIMS analysis and determines the presence of various elements and species on or near the surface and the relative Intensity of their respective signals. Further work would be required to relate signal Intensities to absolute or even relative concentrations. However the relative Intensities observed In Isotopic distributions for elements or In cracking patterns for molecules are useful In species Identification. 

The term radiocolloidal was applied in the early days of radiochemistry to describe certain behavior patterns of radionuclides in aqueous solution that did not conform to expectations based on the normally higher concentrations of the chemical forms assigned to them. Because many of these deviations were related to failure to pass through filters or to remain uniformly distributed in solution, colloidal behavior was inferred. This behavior was observed to affect in different ways many of the steps in the radioanalytical chemistry process itemized above, with the potential of invalidating analytical results because the radioisotope did not consistently follow the known path of its stable isotopes. 

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