Mass spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It is most generally used to find the composition of a physical sample by generating a mass spectrum representing the masses of sample components. The technique has several applications, including:

- identifying unknown compounds by the mass of the compound and/or fragments thereof.
- determining the isotopic composition of one or more elements in a compound.
- determining the structure of compounds by observing the fragmentation of the compound.
- quantitating the amount of a compound in a sample using carefully designed methods (mass spectrometry is not inherently quantitative).
- studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in vacuum).
- determining other physical, chemical or even biological properties of compounds with a variety of other approaches.

A mass spectrometer is a device used for mass spectrometry, and produces a mass spectrum of a sample to find its composition. This is normally achieved by ionizing the sample and separating ions of differing masses and recording their relative abundance by measuring intensities of ion flux. A typical mass spectrometer comprises three parts: an ion source, a mass analyzer, and a detector.

How it works: A simple example

Different chemicals have different masses, and this fact is used in a mass spectrometer to determine what chemicals are present in a sample. For example, table salt (NaCl), is vaporized (turned into gas) and ionized (broken down) into electrically charged particles, called ions, in the first part of the mass spectrometer. The sodium ions and chloride ions have specific atomic weights. They also have a charge, which means that they can be moved under the influence of an electric field or magnetic field. These ions are then sent into an ion acceleration chamber and passed through a slit in a metal sheet. A magnetic field is applied to the chamber, which pulls on each ion equally and deflects them (makes them curve instead of travelling straight) onto a detector. The lighter ions deflect farther than the heavy ions because the force on each ion is equal but their masses are not (this is derived from the equation F = ma which states that if the force remains the same, the mass and acceleration are inversely proportional). The detector measures exactly how far each ion has been deflected, and from this measurement, the ion's 'mass to charge ratio' can be worked out. From this information it is possible to determine with a high level of certainty what the chemical composition of the original sample was.

This example was of a sector instrument, however there are many types of mass spectrometers that not only analyze the ions differently but produce different types of ions; however they all use electric and magnetic fields to change the path of ions in some way.


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