High Resolution Accurate Mass Measurements

The Most Intense Peaks. It is not so easy to extract valuable information dealing with the most intense peaks in mass spectra. In contrast to other physicochemical methods (IR, NMR, UV), registration of an ion peak of an integer m/z value does not provide an unequivocal identification of its structure or even composition. Even accurate mass measurements (high resolution mass spectrometry) defining the elemental composition of an ion does not establish its structure, as it could be formed directly from the M+, with minimal distortion of the authentic structure, or as a result of numerous rearrangement processes. 

High-Resolution, Accurate Mass Measurement Elemental Compositions... 

Notwithstanding the efforts of groups attempting to precisely define acceptable practices and requirements, [20] confusion still exists on the differences between definition of the terms high resolution and accurate mass. [21] High resolution, as in any context, implies the ability to make confident measurements of small differences. In mass spectrometry, high resolution implies the ability to measure the small differences deriving from different elemental compositions, which have the... 

Fig. 10 High-resolution accurate mass measured positive ion ESI mass spectrum of nitrosodehydronifidipine acquired on a TOE LC/MS system.

Orthogonal extraction, improvements in TOF/MS resolution, and improvements in electronics have created a new application for TOF/MS in accurate mass measurements [70,71]. Accurate mass applications have traditionally been associated with high-resolution magnetic sector and FTMS systems. The ability to provide accurate mass measurements (+5 ppm) with more cost-effective instrumentation has created potential opportunities in the pharmaceutical industry. Such applications can offer improved selectivity for single-stage instrumentation and metabolite or impurity/degradation product identification, particularly when standard compounds are not available, such as in early drug discovery. 

Lim et al. [93] investigated the use of LTQ-Orbitrap to identify human liver microsomal metabolites of carvedilol, using parent mass list triggered data-dependent multiple stage accurate mass analysis. In this study, LTQ-Orbitrap was used for accurate mass measurements at a resolving power of 60,000 to obtain mass accuracy <2 ppm for all full-scan and MS" spectra in the external calibration mode. The chemical formulas obtained from high-resolution accurate mass measurements were used to confirm structures of the product ions proposed by Mass Frontier. 

The application of an Orbitrap platform was also described for this purpose. In this case, one can use the excellent full scan sensitivity of the high-resolution accurate mass measurement in both the survey and data-dependent scan modes in order to identify the reactive intermediates and then provide data for their structure assignment [23], A labeled form of GSH 2 1 ratio of 5mM glutathione and I3( V5N Gly glutathione was used. The high mass accuracy in the survey scan can be used to detect the GSH adduct. The method was successfully applied for the detection and characterization of GSH conjugates of acetaminophen, tienilic acid, clozapine, ticlo-pidine, and mifepristone. 

Simple cleavage with the loss of 43 mass units may be indicative of an acetyl or a propyl unit in the molecule. Isotope peak intensities may help to identify the correct composition, but of greater value in this respect is high-resolution mass measurement. From the accurate masses of the molecular and fragment ions, the elanental composition of a neutral fragment can be determined. More about this important technique is discussed in Chapter 3. 

In more complex or less well-characterized systems, the FI/FD molecular weights alone may not be sufficient to elucidate the proper chemical structures. In these cases, the FI/FD results can suggest appropriate methods (spectroscopic and/or chromatographic) for further study. For example, other ionization methods can be used to obtain additional mass spectra. Tandem mass spectrometry (MS/MS) can be used to obtain fragmentation patterns, and high resolution accurate mass measurements can be used to obtain atomic compositions (AC-MS) for various components. 

Ballistreri and co-workers [59] examined the primary thermal decomposition mechanism of this polymer by Py-MS. Several MS techniques were used to identify compounds present in the pyrolysis mixture comparison of electron impact and chemical ionisation spectra, high resolution accurate mass measurements and tandem mass spectroscopy (daughter and parent ion spectra). The results obtained indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation processes yielding cyclic oligomers up to tetramer. No other pyrolysis products were detectable. 

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