One way of doing spectrometry is by a Quadrupole Mass Analyzer. Like many spectrometers, the QMS is actually a filtration device that only allows the ions with the right mass-to charge ratio (m/e) to pass. A beam of ions, which is a bunch of different isotopes, passes through a tunnel surrounded by four rods. The four rods act as a quadrupole and can thus filter out the right ions. More info can be found in the book by de Hoffmann and Stroobant (2003), Mass spectrometry: principles and applications. Toronto: John Wiley & Sons, Ltd.