Mass Defect Filter MDF

Mass defect is defined as the difference between the exact molecular weight and nominal molecular weight of an element. The atomic mass scale defines carbon-12 with a mass of exactly 12.0000 Da, therefore all other elements will have a uniquely different mass defect. For example, the mass defects of hydrogen and oxygen are 0.007825 and —0.005085 Da, respectively. Therefore, oxidation will introduce a mass defect of—5.1 mDa, while glucuronidation will introduce a mass defect of -i- 32 mDa. The mass defects of common nonsynthetic (Phase 1) and synthetic (Phase II) metabolites typically fall within 50 mDa. Therefore with LC/MS instruments capable of high mass accuracy, it is possible to filter out matrix-related interference ions whose mass 

FIGURE 11.5 Chromatographic profiles of omeprazole samples analyzed by a Q-TOF LC/MS system and after examination by the MDF approach, (a) and (b) are TIC profiles of plasma spiked with omeprazole metabolites obtained without and with MDF processing, respectively. Reprinted from Zhu et al. (2006) with permission of the American Society for Pharmacology and Experimental Therapeutics. 

Mass defect filter is solely dependent on the availability of accurate mass data. Therefore, it can detect unusual metabolites resulting from novel biotransformation pathways and is especially useful in metabolite profiling studies when radiolabeled drug is not available. 

Although mass spectrometry (LC/MS and LC/MS/MS) is by far the most widely used technique for metabolite profiling and identification, it does not always provide sufficient information for unequivocal structural elucidation of metabolites. NMR spectroscopy is an excellent complementary tool for 

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Zhang, H., Ray, K., Ma, L., Zhang, D., Drexler, D., and Sanders, M. (2005a). Applicability of mass defect filters (MDF) to drug metabolite detection in biological matrices. In Proceedings of the 53rd ASMS Conference on Mass Spectrometry and Allied Topics, San Antonio, TX. 

Mass defect filter (MDF) [159,163] Predicted mass defect window Suitable for all types of metabolites Sensitivity and selectivity depend on matrix... 

In addition, as previously illustrated in Fig. 6.1, multiple filters may be established for a single data set, each corresponding to a different mass range, to provide for the examination of a wide range of mass defects resulting from diverse components. (The use of multiple filters may be necessary for such samples because of the metabolic transformations of the molecular species to be investigated.) The parameters for the MDF can be defined prior to examination of the acquired data, based solely on the mass defect of the chug and the expected variation in the mass defects of its related modifications. The MDF parameters can also be defined after examination of the acquired data based on observed trends in the actual acquired data (e.g., the likelihood that interferences will have mass defects similar to that of the metabolites of interest). 

The clusters from urine extend toward lower mass and mass defect values than those of plasma, bile, or feces. As a result, the mass defect clusters of urine are overlapped with most of the 115 marketed dmgs. Given such a profile, one can expect some of the interference ions to remain with a typical MDF setting for urine. Of course, with MDF processing, any output data are always simplified since interferences outside of the filter range are discarded (as illustrated in Fig. 6.1). However, depending on the amounts of urine injected, the levels of the remaining interference ions may still impact metabolite peak identification if based on a TIC profile. 

The profiles of biological interference ions in Fig. 6.11 can serve as a guide to predict the effectiveness of MDF for a drug metabolite sample or to assess whether a stringent filter parameter should be used to facilitate detection of drug metabolite ions. The mass defect window of a MDF and its chosen mass range are set based on the relationship of a... 

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