Mass Spectrometer

Mass Spectrometer When you hear the words mass spectrometer, what do you think of? To some people, these words do not have any meaning; however the mass spectrometer has a great significance. The mass spectrometer has been a universal research tool, mostly involved in comparing the masses of atoms. Since the invention of this tool about 100 years ago, the mass spectrometer has been responsible for the description of molecular structure, the detection of isotopes, the classification of atomic weights and more! The mass spectrometer has been vital to chemistry, and will continue to do so for more years to come. The history of the mass spectrometer starts with a man named Sir Joseph John Thompson. Thompson studied conductivity of gases, which led him to discover the electron in 1897. Soon after Thompson constructed the first mass spectrometer, that’s purpose was to determine the mass to charge ratio of ions. In this instrument, ions in discharge tubes were passed into magnetic and electric fields, which caused the ions to move through paths. Then, the rays were revealed on a photographic plate or fluorescent screen. After Thompson, a man named Francis W. Ashton improved this mass spectrometer. In Ashton’s spectrometer, the ions were scattered specifically by mass, which allowed Ashton to study isotopes. In 1920, a professor named A. J. Dempster created a magnetic deflection instrument focusing on direction. He also developed the first impact source of electrons that ionizes molecules with a beam of electrons. This is still used today in modern mass spectrometers. By the end of the 1930’s mass spectrometry had become a reputable method for the separation of atomic ions by mass. The mass spectrometer has many common uses and applications. For example, Anesthesiologists use mass spectrometry during surgery to measure the metabolic gas exchange of their patients. This allows them to determine the “respiratory quotient” which indicates whether or not the patient’s cells are getting enough oxygen and releasing enough carbon dioxide to stay healthy. Environmental scientists to find toxins in contaminated fish also use mass spectrometry. It can be used to test water quality or food contamination and measures the amount of particles in the atmosphere, which examines climate change. One of the main uses of the mass spectrometer is drug discovery and drug development. The mass spectrometer determines the structures of drugs and can screen for drugs in biological systems, which is used in drug testing. The mass spectrometer has 5 main components or stages when operating Vaporization: The sample that was injected into the vaporization chamber is turned into a gas using an electrical heater. If the sample is a compound, it will be separated into its original components when vaporized. The gas then streams into the ionization chamber. By vaporizing the sample, the atoms can now be more easily analyzed. Ionization: The gas particles are bombarded with high-energy electrons to ionize them. Electrons are knocked off the particles leaving positive ions. When an electron is lost, it weakens the bond while the collision gives it kinetic energy. The purpose of ionization is to make it more likely for the molecular ion to break into fragments as it travels through the mass spectrometer. These positive ions are then spit out to the rest of the machine by another metal carrying a slightly positive charge called an ion repeller. Acceleration: The positive ions are repelled away from the very positive ionization chamber and then accelerated by an electric field. The ions pass through 3 slits, the middle which has medium voltage, and the third which is at 0 volts. Then the ions are accelerated into a focused beam. Deflection: Different ions are deflected by magnetic fields by different amounts. This depends on two factors, the mass of the ion, and the charge of the ion. The lighter ions are deflected more than heavier ones, and ions with more positive charges are deflected more than ions with only one positive charge. These two factors are then combined into the mass to charge ratio. For example if an ion with a mass of 38 and a charge of +1, and an ion with a mass of 66 and a charge of +2, would have the same mass to charge ration of 38. Detection: the controller varies the magnetic field, but ultimately the strength is slowly increased. This changes the mass to charge ratio of ions that can reach the detector. A mass spectrum is produced. Ions with different masses are detected and then recorded on the mass spectrum.

In the stick diagram for molybdenum above, the current produced by ions is shown by their mass to charge ratio. The vertical axis shows the occurrence of ions, which pass the detector, otherwise known as the current of ions. The graph may be labeled “relative abundance” or “relative intensity,” however both mean the same. As you will see from the diagram, the most frequent mass to charge ratio of molybdenum is 98, although molybdenum consists of other ions that have mass to charge ratios of 92, 94, 95, 96, 97, and 100. This in essence means that molybdenum consists of 7 different isotopes. Out of the masses of the 7 isotopes, assuming the ions all have a charge of +1, on a carbon-12 scale, the most frequent is 98.
Sources: http://www.chemguide.co.uk/analysis/masspec/howitworks.html http://masspec.scripps.edu/mshistory/perspectives/sborman.php http://www.piercenet.com/method/overview-mass-spectrometry