# Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Mass (mass spectrometry)

The mass recorded by a mass spectrometer can refer to different physical quantities depending on the characteristics of the instrument and the manner in which the mass spectrum is displayed.

## Accurate mass

The accurate mass (more appropriately, the measured accurate mass) is an experimentally determined mass that allows the elemental composition to be determined. For molecules with mass below 200 u, a 5 ppm accuracy is sufficient to uniquely determine the elemental composition.

## Average mass

The average mass of a molecule is obtained by summing the average atomic masses of the constituent elements. For example, the average mass of natural water with formula H$_2$O is 1.00794 + 1.00794 + 15.9994 = 18.01528.

## Exact mass

The exact mass (more appropriately, the calculated exact mass) is obtained by summing the masses of the individual isotopes of the molecule. For example, the exact mass of water containing two hydrogen-1 ($^1$H) and one oxygen-16 ($^{16}$O) is 1.0078 + 1.0078 + 15.9994 = 18.0106. The exact mass of heavy water, containing two hydrogen-2 (deuterium or $^2$H) and one oxygen-16 ($^{16}$O) is 2.014 + 2.014 + 15.9994 = 20.027.

## Isotopomer

An isotopomers (isotopic isomers) is an isomers having the same number of each isotopic atom but differing in their positions. For example, CH$_3$CHDCH$_3$ and CH$_3$CH$_2$CH$_2$D are a pair of constitutional isotopomers. An isotopomer should not be confused with an isotopologue, which are chemical species that differ only in the isotopic composition of their molecules or ions. An example is water, where three of its hydrogen-related isotopologues are: HOH, HOD and DOD, where D stands for deuterium ($^2$H).

## Mass number

The mass number, also called the nucleon number, is the number of protons and neutrons in an atomic nucleus. The mass number is unique for each isotope of an element and is written either after the element name or as a superscript to the left of an element's symbol. For example, carbon-12 ($^{12}$C) has 6 protons and 6 neutrons.

## Molecular mass

The molecular mass (abbreviated M$_r$) of a substance, formerly also called molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of $^{12}$C). Due to this relativity, the molecular mass of a substance is commonly referred to as the relative molecular mass, and abbreviated to M$_r$.

## Monoisotopic mass

The monoisotopic mass is the sum of the masses of the atoms in a molecule using the unbound, ground-state, rest mass of the principle (most abundant) isotope for each element instead of the isotopic average mass. For typical organic compounds, where the monoisotopic mass is most commonly used, this also results in the lightest isotope being selected. For some heavier atoms such as iron and argon the principle isotope is not the lightest isotope. The term is designed for measurements in mass spectrometry primarily with smaller molecules. It is not typically useful as a concept in physics or general chemistry. Monoisotopic mass is typically expressed in unified atomic mass units (u), also called daltons (Da).s

## Most abundant mass

The mass of the molecule with the most highly represented isotope distribution, based on the natural abundance of the isotopes.

## Nominal mass

The nominal mass of an ion or molecule is calculated using the integer mass (ignoring the mass defect) of the most abundant isotope of each element. This is the equivalent of suming the mass numbers of all constituent atoms. For example H = 1, C = 12, O = 16, etc. The nominal mass of H$_2$O is 18, for example.

## References

* 1 2 O. David Sparkman, Mass Spec Desk Reference 2nd. ed. p.60 ISBN 0966081390
* ↑ Grange AH, Winnik W, Ferguson PL, Sovocool GW (2005). "Using a triple-quadrupole mass spectrometer in accurate mass mode and an ion correlation program to identify compounds". Rapid Commun. Mass Spectrom. 19 (18): 2699–715. doi:10.1002/rcm.2112. PMID 16124033.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ Gross, M. L. (1994), "Accurate Masses for Structure Confirmation", J. Am. Soc. Mass Spectrom, 5: 57, doi:10.1016/1044-0305(94)85036-4
* ↑ International Union of Pure and Applied Chemistry. "isotopomer". Compendium of Chemical Terminology Internet edition.
* ↑ IUPAC definition of monoisotopic mass spectrum
* ↑ Goraczko AJ (2005). "Molecular mass and location of the most abundant peak of the molecular ion isotopomeric cluster". Journal of molecular modeling. 11 (4–5): 271–7. doi:10.1007/s00894-005-0245-x. PMID 15928922.
* ↑ Yergey, James (1983). "Isotopic distributions in mass spectra of large molecules" (PDF). Analytical Chemistry. 55: 353–356. doi:10.1021/ac00253a037. Retrieved 2007-09-04. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)