Each element has a characteristic number of protons, called the atomic number, which can be found in the periodic table. In atoms the number of electrons is the same as the number of protons. Since they have opposite charges, this ensures that every atom is neutral.

In atoms:
number of protons = atomic number
number of electrons = atomic number
number of neutrons = mass number − atomic number

For charged ions, the number of negatively charged electrons changes. Negative ions will have a higher number of electrons since they have a negative charge that comes from the negative electrons. Thus, positive ions will have the usual number of electrons that belongs to the atoms minus their charge which indicates the number of electrons lost.

Relative atomic mass is a weighted average mass of all the isotopes of an element. Weighted means that it takes into account the natural abundance of each isotope.

To find the relative atomic mass from given atomic masses of isotopes and their abundances, multiply corresponding atomic masses and their abundances in decimals and add the multiplication products together. If you use abundances as percentages, divide your final result by 100%.

A mass spectra shows the proportion of sample ions with each m/z value. Thus, this peak height is proportional to the abundance of compounds or isotopes when determining the relative atomic mass. When analysing m/z graphs, the peak with the highest m/z value in it is the molecular peak. Sometimes there might another small peak after molecular one arising due to various isotopes present in a molecule. This small peak may be ignored for the determination of the molecular peak value due to its small abundance.

In mass spectrometry a sample is vaporised and ionised by bombarding a sample with electrons (electron impact, loss of 1 electron) or adding protons (electrospray ionisation, gain of 1 proton). The electrons break some of the weaker covalent bonds, while the protons attach themselves to the molecules without much splitting. Since the charge of mass spectrometry ions is usually +, the mass of the molecule analysed is usually the same as the m/z value at the molecular peak when using electron impact. If the electronspray ionisation is used instead, because of the additional attached proton the mass of the molecular peak is bigger than the molecular mass by 1 unit. The base peak in an m/z graph is the highest peak.

atomic number = number of protons = 4
Beryllium has an atomic number of 4 and as an atom has 4 electrons.
2 electrons are missing, thus, this is an ion with a charge of 2+.

beryllium, 2+

abundance of ¹⁹²Pt (a₁) = 0.782% = 0.00782
abundance of ¹⁹⁴Pt (a₂) = 32.864% = 0.32864
abundance of ¹⁹⁵Pt (a₃) = 33.787% = 0.33787
abundance of ¹⁹⁶Pt (a₄) = 25.211% = 0.25211
abundance of ¹⁹⁸Pt (a₅) = 7.356% = 0.07356
RAM = m₁a₁ + m₂a₂ + m₃a₃ + m₄a₄ + m₅a₅
RAM = 192 × 0.00782 + 194 × 0.32864 + 195 × 0.33787 + 196 × 0.25211 + 198 × 0.07356 = 195.121

195.121

abundance of ¹⁰B (a₁) = 20% = 0.2
abundance of ¹¹B (a₂) = 80% = 0.8
RAM = m₁a₁ + m₂a₂ = 10 × 0.2 + 11 × 0.8 = 10.8

10.8

abundance of ⁷⁹Br (a₁) = 51% = 0.51
abundance of ⁸¹Br (a₂) = 49% = 0.49
RAM = m₁a₁ + m₂a₂ = 79 × 0.51 + 81 × 0.49 = 79.98

79.98

During electrospray ionisation an additional H⁺ is attached to a molecule.
Thus, the molecular ion peak at 530.1 m/z corresponds to MH⁺.
Since the charge of the ion is +, the mass is 530.1 a. u.
Subtracting the mass of a proton from 530.1 a. u. gives a mass of 529.1 a. u.
531.1 m/z is due to the presence of an isotope (²H or ¹³C).

529.1, H and C isotopes