High-Resolution, Accurate Mass Measurement Elemental Compositions

High-Resolution, Accurate Mass Measurement Elemental Compositions 

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Chapter 38 High-Resolution, Accurate Mass Measurement Elemental Compositions... 

In addition to MS/MS with CID to obtain structural information, it is also useful to use high-resolution exact mass measurements to confirm the elemental compositions of ions. Essentially, exact mass measurements permit the unambiguous composition analysis of low-molecular-weight compounds (mol. wt. <500) through precise and accurate mlz measurements. The types of mass spectrometers capable of exact mass measurements include magnetic sector mass spectrometers, QTOF hybrid... 

In general terms, the main function of the magnetic/electric-sector section of the hybrid is to be able to resolve m/z values differing by only a few parts per million. Such accuracy allows highly accurate measurement of m/z values and therefore affords excellent elemental compositions of ions if these are molecular ions, the resulting compositions are in fact molecular formulae, which is the usual MS mode. Apart from accurate mass measurement, full mass spectra can also be obtained. The high-resolution separation of ions also allows ions having only small mass differences to be carefully selected for MS/MS studies. 

One can get an enormous amount of information from studying the region of the molecular ion in a mass spectrum. The mass of M+ is the molecular mass of the analyte. The ratio of the isotopic peaks (see below) allows one to roughly establish the elemental composition, while accurate mass measurements using high resolution mass spectrometry give exact elemental composition. The relative intensity of the M+ peak... 

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. 

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

Systematic Errors in Accurate Mass Measurements. 1. Problem Definitions The value of high resolution mass spectrometry is diminished if the mass measurements do not give unambiguous elemental compositions. Accurate mass measurements in FTMS require a precise measurement of ion frequencies and an accurate calibration law for converting ions frequencies to mass. The ion frequencies can be measured to nine significant figures with modern electronics however, the relationship between ion frequencies in the cubic cell and mass still requires further development. 

The major application of high resolution mass spectrometry is for obtaining accurate mass measurements in order to determine elemental compositions. The present FTMS calibration equation, derived by Ledford, et al. [14] shows that the mass-to-charge ratio m/z of a given ion is related to the magnetic field, B, the electric field, E, and the measured frequency, F, by a relation having the form ... 

The elemental composition of the 70-Da post-translational modification group was determined by accurate mass measurement using high-resolution... 

An instrument is generally considered high resolution if it can separate two ions differing in mass by at least 1 part in 5,000 to 15,000 (R = 5,000- 15,000). An instrument with 10,000 resolution can separate an ion of mass 200.00 from one of mass 200.02 (R = 200/0.02 = 10,000). This important class of mass spectrometers allows the measurement of the exact masses of ions, which in turn leads to elemental compositions of ions. Chapter 3 deals with high-resolution mass spectrometry and accurate mass measurement in detail. 

Chapter 2 of this book describes the mass spectra produced by El ionization and how the spectral peak pattern relates to molecular structure. Chapter 3 describes in detail the use of high-resolution and accurate mass measurement to determine elemental compositions of ions as an aid to identifying unknown substances. Chapter 4 provides an introduction to some of the more modern techniques used to extend the usefulness of mass spectrometry into high molecular weight and more polar substances. 

The measurement of the masses of ions with accuracies to three or four digits beyond the decimal point allows the determination of possible elemental compositions of the ions. The basis of such determinations is that elemental masses (except carbon) and, consequently, the ionic masses of analytes, do not have integer values. An important application of high-resolution MS, with accurate mass measurement, has been the identification of unknown analytes based on the determination of their elemental... 

The very high sensitivity inherent to MS (10-100 pg) and compatibihty with GC makes a GC-MS combination extremely valuable. Mass spectrometers may be classed as low-resolution (LR) or high-resolution (HR) instruments. The LR instruments provide mass measurements to the closest whole unit mass. Since many combinations of atoms may give the same unit mass, LR MS may provide the molecular weight of a compound but does not provide elemental composition. HR instruments provide sufficiently accurate mass measurements to permit determination... 

By high-resolution mass spectrometry, ions of known mass from a standard substance can be separated from ions of unknown mass derived from a sample substance. By measuring the unknown mass relative to the known ones through interpolation or peak matching, the unknown can be measured. An accurate mass can be used to obtain an elemental composition for an ion. If the latter is the molecular ion, the composition is the molecular formula. 

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... 

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