A mass spectrometer is a device where atoms and molecules can be forced to acquire a positive charge, even those than don't normally form one. The device then separates these ions by mass, giving peaks for the mass of the positive ions. Even a pure element usually gives more than one peak, so even different atoms of the same element can have different masses. Two atoms of the same element with different mass numbers are called isotopes of that element. Most elements have more than one naturally occuring isotope.
All elements have more than one known isotope.
The average mass or atomic weight of a naturally occuring set of isotopes is the weighed average of the different isotopes, the weighing coming from the natural abundance of the isotopes. This field oscillates both in time and in space. Notice that the larger the wavelength is, the smaller the frequency is. The visible part of the spectrum is only a tiny portion of the entire spectrum. Light to this time had been regarded as exclusively a wave phenomenom.