Exact Mass Measurements High Resolution

Resolution does not affect the accuracy of the individual accurate mass measurements when no separation problem exists. When performing accurate mass measurements on a given component in a mixture, it may be necessary to raise the resolution of the mass spectrometer wherever possible. Atomic composition mass spectrometry (AC-MS) is a powerful technique for chemical structure identification or confirmation, which requires double-focusing magnetic, Fourier-transform ion-cyclotron resonance (FTICR) or else ToF-MS spectrometers, and use of a suitable reference material. The most common reference materials for accurate mass measurements are perfluorokerosene (PFK), perfluorotetrabutylamine (PFTBA) and decafluorotriph-enylphosphine (DFTPP). One of the difficulties of high-mass MS is the lack of suitable calibration standards. Reference inlets to the ion source facilitate exact mass measurement. When appropriately calibrated, ToF mass... 

Haas, M.J. Fully Automated Exact Mass Measurements by High-Resolution ESI on a Sector Instrament. Rapid Commun. Mass Spectrom. 1999, 73,381-383. 

Exact mass measurement at high resolution is an important tool along with other spectroscopic methods to help confirm the structure of novel flavonoids. It is used as structural proof when elemental analysis is not possible, e.g., when studying minor components. When EI-MS can be used, 1 to 10 pmol samples are required for one measurement however, when FAB-MS is used, 0.1 to 1 nmol is normally required. The use of ESI on a double focusing mass spectrometers and MALDI-TOF-MS requires smaller amounts of sample, and subpi-comole amounts of flavonoids may be adequate. 

During the 1960s, high-resolution, double-focusing magnetic sector instruments became available from multiple manufacturers and were widely used in organic chemistry for exact mass measurements and elemental composition analysis. El was used for generating struc-... 

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... 

High-resolution mass speedometers which can measure m/e values to four decimal places are capable of confirming the molecular formula of die molecular ions. These so-called exact mass measurements can be used because die atomic weights of die elements are not exactly whole numbers (except for 12C, which is die standard at 12.0000 amu). The exact masses of some elements and their most abundant isotopes are given in Table 11.2. To find the exact mass of a molecule, die atomic mass of the most abundant isotope for each element is used to calculate the exact mass of the compound. This is compared to die exact mass measured on a high-resolution... 

Exact Mass Measurements and High-Resolution Mass Spectrometry 165... 

EXACT MASS MEASUREMENTS AND HIGH-RESOLUTION MASS SPECTROMETRY 165... 

MS/MS modes of operation. Additionally, when looking at unknown samples such as the case of metabolite identification, having exact mass measurements and high resolution helps to decipher simple and complicated biotransformations. 

Certain software algorithms such as the one mentioned previously uses the high-quality exact mass data from the high-resolution TOF systems to report calculated elemental compositions within the results browser. Good scientific practice mandates the use of an internal reference or lock mass to obtain valid exact mass measurements (sub-5-ppm). Exact mass measurement gives greater confidence in the confirmation of expected metabolites and allows the prediction of the elemental composition of unknowns. The Q-TOF mass spectrometer can be set up to automatically acquire exact mass LC-MS and exact mass MS-MS data to within the recommended mass accuracy guidelines of 5 ppm. 

Fourier transform mass spectrometry (FTMS) is a rapidly growing technique of increasing analytical importance. Foremost among its many attributes are its high mass resolution and wide mass range capabilities, as well as its ability to store ions. This relatively new technique has been employed in a wide variety of applications, ranging from the exact mass measurement of stable nuclides to the determination of peptide sequences. The future holds considerable promise both for the expanded use of FTMS in a diverse range of chemical problems, as well as advances in the capabilities of the technique itself. 

The approach to MS/MS is thus quite different from that for high resolution mass spectrometry. There the exact mass measurement which provides the enpirical formula of the ion is a physical measurorient. In metastable ion studies, the focus is on the nature of the individual chemical reactions. Each metastable ion represents an individually defined transition for which masses and abundances of both products and reactants can be specified. 

QTOF (high resolution and exact mass measurement) MS3... 

Double-focusing magnetic sector mass spectrometers and Fourier transform (FT) mass spectrometers (described in Chapter 9) are capable of mass measurements with high resolution. The most important use of high-resolution MS is the direct determination of molecular formulas by exact mass measurements. 

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