{"id":98,"date":"2023-02-23T01:25:33","date_gmt":"2023-02-23T06:25:33","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/instanchem1\/?post_type=chapter&p=98"},"modified":"2025-03-09T17:37:17","modified_gmt":"2025-03-09T21:37:17","slug":"mass-spectrometry-and-hyphenated-methods","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/instanchem1\/chapter\/mass-spectrometry-and-hyphenated-methods\/","title":{"raw":"Mass Spectrometry and Hyphenated Methods*","rendered":"Mass Spectrometry and Hyphenated Methods*"},"content":{"raw":" \r\n

Mass spectrometry (MS) measures the mass-to-charge ratio (m<\/em>\/z<\/em>) of gas-phase ions. These ions come from a sample of interest. The quantitative amount of each ion is measured. The ions may be molecular ions<\/em>, representing the full (but ionized) structure of an analyte, or may be fragment ions<\/em> from the breakdown of the molecule during ionization. A mass spectrum is a plot of signal intensity versus m<\/em>\/z<\/em>. When the m<\/em>\/z<\/em> of a molecular ion is not sufficient to unambiguously identify an analyte, the fragmentation pattern (i.e.<\/em> molecular weights of different ionized pieces of the analyte) observed in a mass spectrum may enable unique identification.<\/p>\r\n

Mass spectrometers measure the exact mass of an ion. As such, mass spectrometers are sensitive to isotopes (e.g.\u00a0<\/em>methane, CH4<\/sub>, is 16.034 amu with 12<\/sup>C and 17.035 amu with 13<\/sup>C) and mass spectra will have isotopic peaks. Molecular weights that differ by less than one atomic mass unit can be resolved.<\/p>\r\n\r\n

Instrument Design<\/h2>\r\n

The main components of a mass spectrometer are its ionization source, which converts a condensed-phase sample to gaseous ions; its mass analyzer (e.g.<\/em> quadrupole, time-of-flight), which separates ions based on their m<\/em>\/z<\/em>; and its detector, which quantifies the ions for each m<\/em>\/z<\/em> ratio.<\/p>\r\n

Ionization sources and analyzers are discussed more in Chapters 26<\/a> and 27<\/a>, respectively.<\/p>\r\n

Detectors.<\/strong>\u00a0The ion detectors used for MS are typically electron multipliers. For example, a positive ion from the analyzer strikes a conversion cathode and ejects electrons from the impact. An applied potential gradient then accelerates these electrons between discrete dynodes, along a continuous horn-shaped dynode, or within a plate-like array of microchannels \u00a0to produce more secondary electrons with each impact. The amplification is up to 106<\/sup>-108<\/sup>-fold and produces a measurable electrical current at an anode for an ion strike.\u00a0Alternatively, a microchannel plate may be coupled to a phosphorescent screen, where light is produced when the secondary electrons hit the screen. The light is imaged on a CCD camera.<\/p>\r\n \r\n\r\n\"\"\r\n

Hyphenated Techniques<\/h2>\r\n

In addition to its utility as a standalone technique, mass spectrometry (MS) is frequently combined with other analytical techniques. These methods are called hyphenated techniques<\/em> and include GC-MS<\/strong> and LC-MS<\/strong>, among others. The MS serves as a detector for the chromatography. It is the largest and most costly chromatography detector, but provides by far the most information about analyte identities, as well as the best sensitivity for a wide scope of analytes.<\/p>\r\n

The technical challenge with hyphenated methods is to go from the relatively high pressure of the LC or GC to the vacuum required to separate ions based on m<\/em>\/z<\/em>. The interface between the ionization source and analyzer must solve this challenge. This high pressure-to-vacuum transition is usually accomplished through a series of skimmers between chambers of progressively higher vacuum. Ions are electrostatically directed through small apertures while neutral molecules are removed by vacuum pumps.<\/p>\r\n

MS can also be combined with itself in the form of tandem mass spectrometry<\/em><\/strong> (abbreviated MS\/MS or MS2). The first MS analyzer selects for an ion of particular m<\/em>\/z<\/em>, which is then fragmented, with the fragment m<\/em>\/z<\/em> to be detected by the second MS analyzer.<\/p>\r\n\r\n

Molecular and Elemental Analysis<\/h2>\r\n

Although the above discussion has focused on molecular analysis, MS can also be used for elemental analysis in the form of ICP-MS<\/strong>. The ICP produces atoms that the MS then analyzes based on m<\/em>\/z<\/em>. Detection limits for elements are lower than those for ICP-OES or ET-AAS in most cases.<\/p>\r\n \r\n\r\n

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Connections<\/h3>\r\n