Copyright for NIST Standard Reference Data is governed by A few examples of these rearrangement mechanisms may be seen by clicking the following button. The five peaks in this spectrum demonstrate clearly that natural bromine consists of a nearly 50:50 mixture of isotopes having atomic masses of 79 and 81 amu respectively. Relevance of long range substituent parameters, InChI=1S/C12H26/c1-3-5-7-9-11-12-10-8-6-4-2/h3-12H2,1-2H3. The calculator on the right may be used to calculate the isotope contributions to ion abundances 1 and 2 amu greater than the molecular ion (M). This is manifested most dramatically for compounds containing bromine and chlorine, as illustrated by the following examples. Useful tables of common fragment ions and neutral species may be viewed by clicking the right button. Copyright © 2020 by John Wiley & Sons, Inc., or related companies. Use or mention of technologies or programs in this web site is not The following diagram displays the mass spectra of three simple gaseous compounds, carbon dioxide, propane and cyclopropane. When non-bonded electron pairs are present in a molecule (e.g. View image of digitized Some of these ions fragment into smaller cations and neutral fragments. (accessed Nov 13, 2020). Select a region with data to zoom. Plausible assignments may be seen by clicking on the spectrum, and it should be noted that all are even-electron ions. jcamp-plot.js. uses its best efforts to deliver a high quality copy of the Of the three cleavage reactions described here, the alpha-cleavage is generally favored for nitrogen, oxygen and sulfur compounds. The positive charge commonly resides on the smaller fragment, so we see a homologous series of hexyl (m/z = 85), pentyl (m/z = 71), butyl (m/z = 57), propyl (m/z = 43), ethyl (m/z = 29) and methyl (m/z = 15) cations. (e.g.. NIST Mass Spectrometry Data Center, 1998. All Rights Reserved. Mass Spectral Interpretation of dodecane Interpretation of Mass Spectra using "bond breaking" routines and bond information from the "mol" file. The three essential functions of a mass spectrometer, and the associated components, are: 1. Further examples of functional group influence on fragmentation are provided by a selection of compounds that may be examined by clicking the left button below. When a high energy electron collides with a molecule it often ionizes it by knocking away one of the molecular electrons (either bonding or non-bonding). Most of the ions formed in a mass spectrometer have a single charge, so the m/z value is equivalent to mass itself. In the case of methylene chloride, the molecular ion consists of three peaks at m/z=84, 86 & 88 amu, and their diminishing intensities may be calculated from the natural abundances given above. Compound Dodecane with free spectra: 7 NMR and 7 FTIR. Only the mass of the most abundant isotope, relative to C (12.0000), is used for these calculations. However, NIST makes no warranties to that effect, and NIST Copyright © 2016-2020 W. Robien, Inst. This MassBank Record with Accession JP005756 contains the MS mass spectrum of 'DODECANE'. Simply enter an appropriate subscript number to the right of each symbol, leaving those elements not present blank, and press the "Calculate" button. 3. The separated ions are then measured, and the results displayed on a chart. The purpose of the fee is to recover costs associated This is called an EI (electron-impact) source. The most intense ion is assigned an abundance of 100, and it is referred to as the base peak. NIST subscription sites provide data under the Fragment ions themselves may fragment further. The molecular ion is a radical cation, but the fragment ions may either be radical cations (pink) or carbocations (orange), depending on the nature of the neutral fragment. Standard Reference Data Act. Two other common elements having useful isotope signatures are carbon, 13C is 1.1% natural abundance, and sulfur, 33S and 34S are 0.76% and 4.22% natural abundance respectively. A similar bond cleavage in cyclopropane does not give two fragments, so the molecular ion is stronger than in propane, and is in fact responsible for the the base peak. on N or O), fragmentation pathways may sometimes be explained by assuming the missing electron is partially localized on that atom. on behalf of the United States of America. Both distributions are observed, but the larger ethyl cation (m/z=29) is the most abundant, possibly because its size affords greater charge dispersal. Consequently, the radical cation character of the molecular ion (m/z = 170) is delocalized over all the covalent bonds. The nature of the fragments often provides a clue to the molecular structure, but if the molecular ion has a lifetime of less than a few microseconds it will not survive long enough to be observed. NIST Standard Reference m/z = 55, 41 &27) formed by loss of 2 H. All of the significant fragment ions in this spectrum are even-electron ions. Extremely small samples of an unknown substance (a microgram or less) are sufficient for such analysis. shall not be liable for any damage that may result from Phillips Petroleum Company, Bartlesville, Oklahoma, Chloroform-d; Reference=TMS Spectrometer= Varian HA-100/Digilab FT-NMR-3. 1. Repeated clicks will cycle the display. The simplest and most common fragmentations are bond cleavages producing a neutral radical (odd number of electrons) and a cation having an even number of electrons. click the mouse on the plot to revert to the orginal display. Also, the structure of most fragment ions is seldom known with certainty. Ions with no nitrogenor an even # N atoms. The Mass Analyzer 3. 2. Consequently, the radical cation character of the molecular ion (m/z = … This page is the property of William Reusch. All rights reserved. A less common fragmentation, in which an even-electron neutral fragment is lost, produces an odd-electron radical cation fragment ion. These pages are provided to the IOCD to assist in capacity building in chemical education. Most of the fragment ions have odd-numbered masses, and therefore are even-electron cations. The three essential functions of a mass spectrometer, and the associated components, are: The ions are then detected electronically and the resulting information is stored and analyzed in a computer. Modern mass spectrometers easily distinguish (resolve) ions differing by only a single atomic mass unit (amu), and thus provide completely accurate values for the molecular mass of a compound. Most stable organic compounds have an even number of total electrons, reflecting the fact that electrons occupy atomic and molecular orbitals in pairs. All mass spectra in this site (plus many more) are Thus, relative to 12C at 12.0000, the isotopic mass of 16O is 15.9949 amu (not 16) and 14N is 14.0031 amu (not 14). Fragmentation of C-C bonds occurs because they are usually weaker than C-H bonds, and this produces a mixture of alkyl radicals and alkyl carbocations. Gases and volatile liquid samples are allowed to leak into the ion source from a reservoir (as shown). A small sample is ionized, usually to cations by loss of an electron. By continuing to browse the site you are agreeing to our use of cookies. The molecular ion is the strongest ion in the spectra of CO2 and C3H6, and it is moderately strong in propane. Data from NIST Standard Reference Database 69: The National Institute of Standards and Technology (NIST) Alcohols, ethers and highly branched alkanes generally show the greatest tendency toward fragmentation. This distinction is illustrated nicely by the following two examples. of Org. Institute of Standards and Technology, nor is it intended to imply 4. Most of the questions concern the identification of unknown compounds from their mass spectra. A mass spectrometer operating in this fashion is outlined in the following diagram. The mass spectrum of dodecane illustrates the behavior of an unbranched alkane. Chem Service, Inc., West Chester, Pennsylvania. The masses of molecular and fragment ions also reflect the electron count, depending on the number of nitrogen atoms in the species. In most alkane spectra the propyl and butyl ions are the most abundant. and Informatics, NIST / TRC Web Thermo Tables, "lite" edition (thermophysical and thermochemical data), NIST / TRC Web Thermo Tables, professional edition (thermophysical and thermochemical data), NIST Mass Spectrometry Data Center, William E. Wallace, director, Modified by NIST for use in this application. Atmospheric pressure is around 760 torr (mm of mercury). Since a molecule of carbon dioxide is composed of only three atoms, its mass spectrum is very simple. spectrum (can be printed in landscape orientation). A perpendicular magnetic field deflects the ion beam in an arc whose radius is proportional to the mass of each ion. available from the NIST/EPA/NIH Mass Spectral Library. of Vienna. jcamp-dx.js and By localizing the reactive moiety, certain fragmentation processes will be favored. Wiley SpectraBase; Fluorine and iodine, by contrast, are monoisotopic, having masses of 19 amu and 127 amu respectively. Thus, the bromine molecule may be composed of two 79Br atoms (mass 158 amu), two 81Br atoms (mass 162 amu) or the more probable combination of 79Br-81Br (mass 160 amu). The mass spectrum of dodecane on the right illustrates the behavior of an unbranched alkane. Mechanisms for some of these rearrangements have been identified by following the course of isotopically labeled molecular ions. A majority of the fragment ions have even-numbered masses (ions at m/z = 30, 42, 56 & 58 are not labeled), and are even-electron nitrogen cations. High Resolution Mass Spectrometry In assigning mass values to atoms and molecules, we have assumed integral values for isotopic masses.