In a mass spectrometer, mass is selected by having a particle with a certain mass on a particular trajectory. The forces that determine the dynamics and thus the trajectory (I oversimplify a bit, but basically it is correct)  depend on the charge. Obviously for example a particle with charge, q,  in an electric field, E, is subject to a force  F=q E. Similarly, a moving charged particle in a magnetic field is subject to the Lorentz force F=q (v xB). The point is that these forces depend on the charge, q. The dynamics of a particle with mass m is determined by   ma=qE+q(v xB). You see that the acceleration depends on q/m and not on m alone. It makes a difference in the trajectory if your molecule is charged with one or two or more charges. 

Yes, Christian's answer is correct and as a consequence an ion with a single charge appears at a mass equivalent to the molecular weight M, ( say 200) but if an ion of the same mass is doubly charged it appears at half at an apparent mass of half M (ie 100) because q = 2 and so M/2 = 100.  If you are using an EI source you will seldom observe these doubly charged ions but in an Electrospray source they are very common and, depending on the molecule much higher charged states will be observed.

charge is unit charge. m/1= m

As Christian said, charge is required to move the molecules in mass-spec.

Remember, you can manipulate ions under an electric or magnetic field, but not neutral molecules. So neutral molecules are of no use for mass spec analysis. You have to have ions.

There are different ways of separating ions, for instance, magnetic sector, ion traps, TOF, quadrupoles etc. But all of these methods separate ions based upon m/z and not by m (one example is given above by Christian). Of-course, you can then convert m/z into m. But the raw data in mass-spec will always be in m/z.

Best,

Jogender

I would NEVER interpret mass to charge ratio as molecular/atomic weight directly, unless I know the charge state. Many ion sources pre-dominantly (but hardly exclusively) produce singly charged ions. But I would assume that multiply charged ions exist in almost any source to some extent. So inferring the molecular/atomic mass just by the position of the peak in a mass to charge scale can be dangerous.

Best regards

Bodo

Incidentally, you can determine the charge  state by looking at the isotopic cluster around the molecular ion.  The mass difference between the all carbon 12 version of the molecular ion and that containing one carbon 13 is of course 1 Dalton.  However if  the charge state is +2 , for example, this difference will be measured as 1/2 Dalton.  If the charge state is +3, it will be 1/3 Dalton and so on.

Another thing to consider - how do you introduce the necessary charge? Depending on molecule and ionization technique you could be looking at quasimolecular ions such as [M+H]+ or [M+2H]2+ (from e.g. ESI), or M+. (from Electron Impact, EI). I recommend a basic textbook, e.g. Budzikiewicz.

Note that electron impact is obsolete, electron ionization is right. Dalton is a mass unit, not m/z unit, I prefer tio state always m/z and not u, Dalton, etc (thompson has been proposed for m/z unit).  If you do not get the "exact mass" (precise m/z) on a high resolution instrument, the isoptope pattern may be useful to help deciding the molecular composition. Small freewares are available to simulate isotope pattern.

The main attribute in mass spectrometry is the mass to charge ratio. Although this term is frequently used, the definition of the term and the interpretation of its use is not strictly unambiguous. The mass to charge ratio is usually denoted as 'm/z', and the quantity unit for the 'm/z' variable is dimensionless. In the following a brief discussion on the definitions is provided

i) The mass ('m' in 'm/z') is defined as the ratio of the actual mass of a single isotopically specified molecule (in kilograms (kg)) over the mass (in kilograms) of one atom of the most abundant isotope of carbon 12C / 12. Thus, the mass of 12C is defined as 12 units. The 'm' is thus a dimensionless quantity. However, the 'm' is often expressed in unified atomic mass unit (u), which is also recommended by IUPAC.

Sometimes also amu is used, which is a unit based on the mass of 16O. This unit is, however, not recommended. Another unit frequently used is Daltons (Da). Unfortunately, two definitions of this unit have been used. The modern interpretation use isotopic specification. The older interpretation used the average isotopic distribution of each element, since many elements have a number of isotopes. The natural abundance of an isotope of an element is

the percent of that isotope as it occurs in a sample on earth. The average atomic mass is simply a weighted average of the masses of all the isotopes.

The unified atomic mass unit and the Dalton unit are not part of the SI units, but they are recognized by the CGPM.

ii) The charge ('z' in 'm/z') is defined as the ratio of magnitude of charge on the ion (in Coulombs (C)) over the elementary charge (of a single proton) in Coulombs. Thus, this is also a dimensionless quantity. However, the charge is often expressed in elementary charge e, meaning how many elementary charges are present. As shown above, the mass to charge ratio 'm/z' is thus the ratio of two dimensionless quantities. But these quantities have several options: 'm/e', 'm/Q' or 'm/q' for the variable sign and u/e or Da/e for the unit sign. In 1991, Cooks and Rockwood suggested to adopt a unit called the thomson (Th), to describe the dimensionless quantity of the mass to charge ratio (u/e). The Th is not an SI unit and it has not been accepted by IUPAC.

Speaking of IUPAC, it is useful to refer to the mass spectrometry recommendations:

Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013); Pure Appl. Chem., Vol. 85, No. 7, pp. 1515–1609, 2013.

In particular is stated: "Labeling the x-axis of a mass spectrum with any unit of mass such as dalton (Da), atomic mass unit (amu), or unified atomic mass unit (u) is strongly discouraged due to the confusion that would result when reporting spectra of multiply charged ions. The quantity plotted on the x-axis of a mass spectrum is a function of both the mass and charge of the ion. Furthermore, the use of amu in place of u is strongly discouraged in all cases; it has been used to denote atomic masses measured relative to the mass of a single atom of 16O, or to the isotope-averaged mass of an oxygen atom, or to the mass of a single atom of 12C."

Additionally, for the perfectionists, I would like to add that in calculating the exact mass of an ion to the 4th decimal, using atomic masses, a singly charged positive ion lacks one electron mass, m(e) = 0.00055 u (and conversely m(e) to be added for a singly charged negative ion.)

in GPC/SEC is Mz unitless or is it reported in Daltons?

Mz is one of the tree ways to average the molecular weight of a polymer. Molecular weight (or molecular mass) can be given un Da or daltons.

thank you Jean-Francois!

m/z is the correct terminology to use for all ions in a mass spectrometer but it is not unitless since m is mass and z is charge (which is unitless) However since z may not be 1 the term Dalton has been adopted to describe this quantity. Generally the ion at highrst mass is called the "molecular ion" and corresponds to the molar mass although even this is not quite correct since it differs from that value by the mass of the electron and in high resolution mass spectromtery this must be taken into account