Terminologies used in mass spectrometry - adam-amara/Metabolomics-tutorial GitHub Wiki

Common terminologies used in mass spectrometry:

Isomers: isobars: Isobaric compounds: Compounds with the same nominal mass but with a different molecular formula.

Experimentally measured masses are rarely exactly the same as a proposed molecular formula, so a formula will be assigned by software which has a mass deviation from the exact mass measured in ppm (ppm = (deviation of measured mass in atomic mass units/exact molecular weight)x106).

Resolution: A term which defines the performance of an analyser. This parameter describes the degree to which two ions of adjacent m/z can be distinguished from each other with less than a defined amount of overlap. The latter is often quoted as 10% for sector instruments and 50% for quadrupoles. For isolated peaks, the same term can be calculated from the width of the peak at defined heights. (see FWHM).

FWHM (Full Width at Half Height): The resolution for a 50% valley overlap between peaks of adjacent m/z.

Isotopes: Most elements are composed of a mixture of isotopes. These will be separated in a mass spectrometer. Atoms or molecules containing such elements will display a cluster of ions reflecting the isotopic composition.

Monoisotopic Ion: The ion containing only the most abundant isotopes. (also called the principal ion).

Terms realted to mass

Nominal Mass: Mass of a molecular ion or molecule calculated using the isotope mass of the most abundant constituent element isotope of each element rounded to the nearest integer value and multiplied by the number of atoms of each element.

Monoisotopic Ion Mass: Exact mass of an ion or molecule calculated using the mass of the most abundant isotope of each element.

Average Mass: The mass of a particle or molecule of given empirical formula calculated using atomic weights for each element.

Accurate Mass: Experimentally determined mass of an ion of known charge. Isotopes have unique precise masses, a consequence of which is that the elemental composition of any molecule, or fragment of one, can be calculated from its mass if this is sufficiently accurately determined. Note 1: Can be used to determine elemental composition to within limits defined by both the accuracy and precision of the measurement. Note 2: Accurate mass and exact mass are not synonymous. Accurate mass refers to a measured mass, and exact mass refers to a calculated mass.

Unified atomic mass unit, u dalton: Non-SI unit of mass defined as one-twelfth of the mass of one atom of 12C at rest in its ground state and equal to 1.660 538 921 (73) × 10–27 kg where the digits in parentheses indicate the estimated uncertainty in the final two digits of the value. Equivalent to the dalton (Da) unit. Note: The abbreviation amu for atomic mass unit is deprecated

The term product ion is recommended in place of daughter ion and precursor ion in place of parent ion.


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