Atoms consist of protons, neutrons and electrons. Each element has a characteristic number of protons, called the atomic number. This can be found in the periodic table. Neutral atoms have the same number of protons and electrons. For charged ions, the number of negatively charged electrons changes. The number of neutrons in an atom can be determined from the atomic mass, also found in the periodic table. Atomic mass - Atomic number = No. of neutrons. As protons and neutrons are a lot heavier than electrons, the centre of mass of atoms is located at the nucleus.
Isotopes are atoms of the same element, which have the same number of protons but a different number of neutrons. They are present in different levels of abundance. Some isotopes are radioactive.
Relative Atomic Mass is the average mass of all the isotopes of an element. It takes into account the natural abundance of each isotope.
Mass spectrometry is an analytical technique which separates gaseous ions by their mass-to-charge ratio (m/z). As the ions travel through the electromagnetic field, they are deflected by a specific radius of curvature, which is dependent on their m/z. Only ions with the target m/z will reach the detector. Another method is time-of-flight (TOF) mass spectrometry. This technique fires ions into a vacuum column and measures how long it takes to reach the detector at the other end. Lighter ions, with a smaller m/z, travel faster. Both these methods produce spectra, showing the proportion of sample ions with each m/z value.