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A mass spectrum is an intensity vs. mass-to-charge plot representing a chemical analysis. Hence, the mass spectrum of a sample is a pattern representing the distribution of components (. Not all mass spectra are the same. For example some mass spectrometers break the analyte molecules into fragments; others observe the intact molecular masses with little fragmentation. A mass spectrum can represent many different types of information based on the type of mass spectrometer and the specific experiment applied; however, all plots of intensity vs. mass-to-charge are referred to as mass spectra. X-AXIS: MASS-TO-CHARGE RATIO The X-axis of a mass spectrum is a Mass-to-charge Ratio . The mass-to-charge ratio is most often written as the IUPAC standard ''m/z'' to denote the quantity formed by dividing the mass number of an ion by its charge number. For example, for the ion C7H72+, = 45.5 See IUPAC definition of mass-to-charge ratio in mass spectrometry . Although a mass spectrum x-axis represents mass-to-charge it contains mass information that may be extracted by a knowledgable mass spectrometrist. Once this is done many mass spectrometrists use Dalton (Da) as the unit of mass in order to avoid the clumsy "atomic mass units". Alternative x-axis notations There are several alternatives to the standard m/z notation that appear in the literature. ''m/e'' appears in older historical literature. A label more consistent with the SI unit system is ''m/q'' where ''m'' is the symbol for mass and ''q'' the symbol for charge with the units u/e where u is the unit of mass in Atomic Mass Units and e is the unit of Charge in elementary charge units. This sometimes appears in units Da/e. It was also suggested to introduce a new unit Thomson (Th) for the Physical Propery ''m/q'', where 1 Th = 1 u/e. According to this convention, mass spectra x axis should be labled ''m/q'' (Th). History of x-axis notation In 1897 the mass-to-charge ratio of the s with an instrument he called a parabola spectrograph {Link without Title} . Although this data was not represented as a modern mass spectrum, it was similar in meaning. Eventually there was a change to the more physically meaningful mass-to-charge ratio with some early notation as ''m/e'' giving way to the current IUPAC standard of ''m/z''. Early in mass spectrometry research the resolution of mass spectrometers did not allow for accurate mass determination. , IUPAC green book, IUPAP red book). Currently there is an effort to redefine the standard for x-axis notation. Y-AXIS: SIGNAL INTENSITY The Y-axis of a mass spectrum represents signal intensity of the ions. When using counting detectors the intensity is often measured in counts per second (cps). When using analog detection electronics the intensity is typically measured in Volts. In most forms of mass spectrometry, the intensity of ion current measured by the spectrometer does not accurately represent relative abundance, but correlates loosely with it. Therefore it is common to label the y-axis with "arbitrary units". Y-axis and relative abundance Signal intensity may be dependent on many factors, especially the nature of the molecules being analyzed and how they ionize. The efficacy of ionization varies from molecule to molecule and from ion source to ion source. For example, in electrospray sources in positive ion mode a quatenary amine will ionize exceptionally well whereas a large hydrophobic alcohol will most likely not be seen no matter how concentrated. In an EI source these molecules will behave very differently. On the detection side there are many factors that can also affect signal intensity in a non-proportional way. The size of the ion will affect the velocity of impact and with certain detectors the velocity is proportional to the signal output. In other detection systems, such as FTICR , the number of charges on the ion are more important to signal intensity. In order to make conclusions about relative intensity a great deal of knowledge and care is required. Additionally there may be factors that affect ion transmission disproportionally between ionization and detection. A common way to get more quantitative information out of a mass spectrum is to create a standard curve to compare the sample to. This requires knowing what is to be quantitated ahead of time, having a standard available and designing the experiment specifically for this purpose. A more advanced variation on this the use of an internal standard which behaves very similarly to the analyte. This is often an isotopically labeled version of the analyte. There are forms of mass spectrometry, such as Accelerator Mass Spectrometry that are designed from the bottom up to be quantitative. SEE ALSO
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