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Details of Mass spectrometry

Mass spectrometry (previously called mass spectroscopy (deprecated) or informally, "mass-spec" and MS) 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 mass spectrum is measured by a mass spectrometer.

All mass spectrometers consist of three basic parts: an ion source, a mass analyzer, and a detector system. The stages within the mass spectrometer are:

1. Producing ions from the sample
2. Separating ions of differing masses
3. Detecting the number of ions of each mass produced
4. Collecting the data and generating the mass spectrum

The technique has several applications, including:

• identifying unknown compounds by the mass of the compound molecules or their fragments
• determining the isotopic composition of elements in a compound
• determining the structure of a compound by observing its fragmentation
• quantifying 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

Simplified working example

Schematics of a typical mass spectrometer

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), may be vaporized (turned into gas) and ionized (broken down) into electrically charged particles (Na⁺ and Cl^-), called ions, in the first phase of the mass spectrometry. The sodium ions are monoisotopic, with mass 23u. Chloride ions have two isotopes of mass 35u (~75%) and mass 37u (~25%). They also have a charge, which means that their speed and direction may be changed with an electric or magnetic field. The ions are accelerated to a high speed by an electric field after which they are directed into a magnetic field. The magnetic field applies a force to each ion perpendicular to the plane defined by the particles' direction of travel and the magnetic field lines. This force deflects the ions (makes them curve instead of traveling in a straight line) to differing degrees depending on their mass-to-charge ratio. The lighter ions are deflected more than the heavier ions because according to Newton's second law of motion the acceleration of a particle is inversely proportional to its mass. Thus the magnetic field deflects the lighter ions more than the heavier ions. The detector measures the deflection of each resulting ion beam. From this measurement, the mass-to-charge ratios of all the ions produced in the source can be determined. From this information it is possible to determine the chemical composition of the original sample (i.e. that both sodium and chlorine are present in the sample) and the isotopic compositions of its constituents (i.e. whether the ratio of ³⁵Cl to ³⁷Cl has been changed by some process).

This example was of a sector instrument, however there are many types of mass spectrometers. All of these have in common that they possess an Ion Source, that produces ions, an Analyzer that sorts them in some way by their masses, and a Detector that measures the relative intensities of different masses. The underlying principle of all mass spectrometers is that the paths of gas phase ions in electric and magnetic fields are dependent on their mass-to-charge ratios which is used by the analyzer to distinguish the ions from one another.