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What Is a Time of Flight Mass Spectrometer?

Christian Petersen
Christian Petersen

A time of flight mass spectrometer determines the molecular composition of a substance by breaking it down into its component ions. By finding the mass to charge ratio of a molecule, it is possible to ascertain, within a range of possibilities, the chemical make-up of the various substances contained within a test sample. The device ionizes, separates, and propels molecules at a detector, and by measuring the time it takes each ion to reach the detector, can determine the ratio of the ion's mass to its charge. From that value, its mass can be calculated, which allows determination of its chemical structure.

Mass spectrometry is a useful tool for researchers as well as for law enforcement, laboratory testing, and analysis. The spectrometer can determine what kinds of materials are in a substance by breaking down a sample into its components and providing data on possible chemical formulas, relative amounts as a percentage of the whole, and molecular weights of each substance present. This is very useful for researchers or technicians for many reasons. It allows analysis of all kinds of samples for pharmaceutical research, forensics work in law enforcement, and product development in many industrial fields.

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Several kinds of mass spectrometer have been developed over the decades since early work on separating ions according to mass began in the late 19th century. The time of flight mass spectrometer is only one type of spectrometer. In general, any time of flight mass spectrometer works according to a few basic principles and has certain components. They may vary in certain aspects of their design, but all work based on the principle that ions will move from the ionization source to the detector with a speed that is dependent on their mass.

The time of flight mass spectrometer ionizes the sample to be tested with an ion generator. This component is most often a laser beam, which quickly vaporizes the material, causing it to break down into ions, which are molecules with an electrical charge. The ions are then separated and propelled by an electric field through a drift, or flight, tube. They move at different rates according to their mass to charge ratio. Larger, more massive ions, move more slowly than smaller, less massive ions.

A component called a reflectron, which directs the incoming ions at the ion detector, is often incorporated into the mass spectrometer. When the ions strike the detector, it records the event as well as the charge of the ion and the elapsed time in flight between the ion generator and the detector. By analyzing this data, it is possible to determine the mass to charge ratio and then the mass of individual ions in the sample. The mass and charge of the individual ions can be used to determine the exact chemical make up of the individual components of a sample and to detect extremely small amounts of particular substances, such as contaminants, poisons, or drugs in a blood sample. These are just a few of the many possible uses for a time of flight spectrometer.

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