Units in Mass Spectrometry Kenneth L. Busch t has been estimated that a billion mass spectra are recorded daily on this planet alone. Even so, mass spectrometrists always seem to find time to argue about units and nomenclature, and they display an incredible energy and enthusiasm in performance of their neologistic duties. Standards for terms have been promulgated and published (1–3). However, in teaching mass spectrometry (MS) and in reading the published I literature, it seems common that too little attention is given to units. This column is therefore dedicated to compensating for such a shortfall. The focus will be on the more common units, with commentary on history and usage, and on debate, when appropriate. We begin with the units for the x axis in the mass spectrum, which have evolved over the years. The current accepted unit is m/z, said aloud as “m over z.”(I have heard Brits use “m upon zed,”but then they say “spanner” instead of “wrench,”too.) In this nomenclature, m Kenneth L. Busch’s represents the mass of the ion, and z represents the charge. It has interest in units dates back to his been argued that m/z is a symbol, and should therefore be set in ital- tinkerings with stereo equipment during his gawky teen years ics (3). However, this is not a uniform typographical practice, and (1962–1987). The stereo there are those that would debate at length the difference between a equipment at that time had panel symbol and a unit. More to the point, although the unit m/z is uni- indicators for signal strength versally accepted, it is awkward in that a lowercase m is the SI unit called VU meters (now mostly for meter. The correct unit for mass is u, the unified atomic mass replaced by red LEDs). Being retentive in nature, Busch unit. A more correct unit for use on the x axis of a mass spectrum investigated the origin of the would therefore be u/z. Mass spectrometrists simply choose not to name VU (see www.sound.au. use it. The usage “M/z” has appeared intermittently, where M is pre- ϩ. com/project55.htm). As described sumably derived from the usage of M to represent the molecular in this article, MS has its own ion of a compound of molecular mass M. A unit in common use be- collection of (commonly used and sometimes misused) units. Busch fore about 1980 was m/e, and this unit appears as the x-axis unit is Spectroscopy’s ”Mass label in many of the earlier classic books on MS. The lower case e, Spectrometry Forum“ columnist however, is the term for the charge on an electron (1.602 ϫ 10Ϫ19 C) and an employee of the National rather than the number of such charges, which is designated z. Science Foundation in Arlington, The u in u/z is the abbreviation for mass in the unified atomic VA. As always, the opinions and views in this article are strictly his mass scale and is derived from the unified atomic mass unit. The his- own and not those of the National tory of this unit establishes its relationship with amu, the abbrevia- Science Foundation. He can be tion (sometimes seen as a.m.u.) for atomic mass unit. The relative reached at buschken@hotmail. masses of the various chemical elements were first established by com. careful study of the stoichiometry of their reactions. Hydrogen, the lightest element, was assigned a relative mass of 1 amu (a suggestion S32 Current Trends in Mass Spectrometry 18(5S) May 2003 www.spectroscopyonline.com Units originating with John Dalton). Relative Table I. Standard units of mass and charge used in mass spectrometry. masses of other elements seemed to be simple multiples of that value. For ex- Mass unit u 1.66054 ϫ 10Ϫ27 kg ample, oxygen seemed to have a relative Electron charge e 1.60210 ϫ 10Ϫ19 C mass of about 16. As MS was devel- ϫ Ϫ27 Proton rest mass mp 1.67252 10 kg oped, Aston worked to formulate the 1.00727663 u whole-number rule. According to this ϫ Ϫ27 Neutron rest mass mn 1.67482 10 kg rule, atomic masses are approximately 1.008665 u integral masses of the mass of hydro- ϫ Ϫ31 Electron rest mass mc 9.10908 10 kg gen (a codification of the stoichiomet- 5.48597 ϫ 10Ϫ4 u rically based conclusions), and devia- Velocity of light c 2.997925 ϫ 108 m sϪ1 tions from the integral values are due in a vacuum to the presence of isotopes of the ele- ments. Aston showed experimentally, in his discovery of the isotopes of chlo- 1.000275 between them (physical scale Equivalently, the mass of 12C is defined rine, that the measured relative weight Ͼ chemical scale). as 12.000000 u. The numerical value of of chlorine (35.45 amu) derives from The implicit difficulty of maintaining the u is 1.66053873 ϫ 10Ϫ27 kg. In terms the fact that the isotopes 35Cl and 37Cl two separate mass scales was apparent, of energy, 1 u equals approximately occur naturally in approximately a 3:1 and explicit suggestions for a new scale 931.494 MeV (see the following section abundance ratio. As knowledge of the appeared in 1957. Meetings of the In- on the unit eV). Petley’s article (4) is masses of isotopes was compiled, Aston ternational Union of Physicists (1960) instructive. made a further valuable contribution in and the International Union of The description of the situation for his observation that the integral nature Chemists (1961) agreed to a new mass units of mass is not yet complete. The of the isotopic masses is only approxi- scale (the unified mass scale) in which unit dalton (Da) has been accepted as mately valid. The exact mass of an iso- the unified atomic mass unit was de- an alternate name for the unified 1 tope (the nuclidic mass) differs slightly fined as ⁄12 the mass of an atom of 12C. atomic mass unit. The unit dalton is from the summed mass value of the component protons, neutrons, and electrons (each in its free unbound state). This difference is known as the mass defect (or mass decrement). The mass defect is equivalent (via E ϭ mc2, where E is energy, m is mass, and c is the speed of light) to the binding en- ergy that holds the assembly together. Aston used 16O as the mass standard from which to derive a packing fraction curve that underlies the development of much of modern nuclear physics. The brief aside into a description of the mass defect mentioned earlier con- veys the historical reason for the use of 16O as the standard against which a mass scale for amu was developed. Two scales were developed, however. In the physical atomic mass scale, 16O was as- signed an exact mass of 16.000000 amu, and all other atomic masses were de- fined relative to this standard (that is, 1 the amu is defined as ⁄16 the mass of 16O). In the chemical atomic mass scale, 1 the amu was defined as ⁄16 of the aver- age atomic mass of oxygen (which in- cluded the contribution of the low- abundance isotopes 17O and 18O). Both scales were internally accurate, but there was a difference factor of Circle 415 May 2003 18(5S) Current Trends in Mass Spectrometry S33 Units most often used by biochemists (and single electron through a potential dif- simple label fails to reflect the fact that therefore has appeared much more fre- ference of 1 V. The accurate numerical the precursor ion is a doubly charged quently in MS literature recently), with value of 1 eV is 1.602 ϫ 10Ϫ19 J. In an ion. The solution to date has been to units of kDa and MDa used to describe electron ionization source, an electron use a unit of volts in the lab frame (sim- the masses of large biomolecules. Even emitted from a hot metal filament is di- ply saying that the instrumental poten- seasoned nomenclators admit that spo- rected to a collector that is held at a po- tial difference is 50 V) and then to spec- ken “kamu” and “ku” are not euphonic tential 70 V more positive than the fila- ify elsewhere the identity and charge of (nor clear). Terms such as “kilodaltons,” ment itself. Each electron therefore the precursor ion. This issue of nomen- therefore, serve a recognized purpose moves through a potential difference of clature and units will surely become a and are accepted — even if not strictly 70 V, and acquires the appropriate ki- topic of future debate. In the interim, required by the rules. For example, the netic energy for that potential differ- careful descriptions of the experiments x-axis label of “(m/z)/1000” recently ap- ence. Remember that the units for e are leading to such MS-MS spectra must be peared in print. The typographical sim- coulombs, and the units for the poten- documented. plicity of kDa would seem to be a tial difference are volts, and therefore The charge on the ion reappears for telling advantage. There is ample room the product is expressed in units of consideration in ion detection at a Fara- for continued confusion and debate, joules (that is, C ϫ V ϭ J). day cup or an electron multiplier. Re- however. The unit Da has been coupled The electronvolt is also a unit of member that the charge on a singly directly with the term m/z, as in “For mass, used in some areas of particle charged ion is 1.602 ϫ 10Ϫ19 C.