This section will ignore the information you can get from the molecular ion or ions. The fragmentation pattern can be difficult to interpret, but, in the case of branched chain alkanes, the carbon chain is preferentially cleaved at tertiary or quaternary carbons due to the relative stability of the resulting free radicals. Spectroscopyspectroscopy basic theory mass spectroscopy deals with the examination of the characteristics fragments ions arising from the breakdown of organic molecules. The electron impact mass spectrometry of straight chain alkanes c8h18c40h82, squalane, methylstearate, 1chlorohexadecane, 1bromohexadecane, and dioctylphthalate was studied by sampling them with supersonic molecular beams.
This slide contains fragmentation of important functional groups i. Mass spectrometry and free radicals really radical cations when an organic molecule is ionized in an ms instrument, especially by ei, it generates a radical cation. Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. Major fragmentation pattern is alhpa cleavage resulting in loss of sidechain and formation of acylium inon. The mass to charge ratio mz is used to describe ions observed in mass spectrometry. The author starts at a very basic level and slowly works through all aspects of ms, including ionization, fragmentation patterns, gcms, and lcms.
Alkanes the mass spectra of these simple hydrocarbons. Alkanes have a high ionization energy, and the molecular ion is usually weak. In straightchain alkanes, fragmentation occurs through loss of a methyl, leading to. This carbocation can then undergo stepwise cleavage down the alkyl chain, expelling neutral twocarbon units ethene. Mass spectral fragments of common hydrocarbons mass spectral peaks can be identified to have originated from the fragmentation of specific hydrocarbon sources including. Alkane and carbonyl compounds in mass spectrometry. Molecular ion peaks are present, possibly with low intensity. Alkanes for straight chain alkanes molecular ion peak is usually present but weak esp. University mass spectrometry fragmentation pdf molecule science and metabolomics which need to be clarified. Major fragmentation pattern is alhpa cleavage resulting in loss of sidechain and formation of acylium inon also mclafferty rearrangement where possible. Discussions about gcms, lcms, lcftir, and other coupled analytical techniques. In the case of instrumentation chapter 4, there are even summaries covering mass analyzers type by type. In the second part of the chapter, the molecular fragmentation for alkanes.
Study notes when interpreting fragmentation patterns, you may find it helpful to know that, as you might expect, the weakest carboncarbon bonds are the ones most likely to break. Organic lecture series 3 mass spectrometry ms organic lecture series 4 a mass spectrometer a mass spectrometer is designed to do three things convert neutral atoms or molecules into a beam of positive or negative ions separate the ions on the basis of their mass tocharge mz ratio measure the relative abundance of each ion. The differences of 14 mass units are not due to elimination of units of ch 2 an energetically unfavourable process but rather to fragmentation at different places in the molecular and fragment ions scheme 10. The basic aspect of organic mass spectrometry consist of bombarding. In straightchain alkanes, fragmentation occurs through loss of a methyl, leading to fragments.
The positive apci mass spectra in air of linear npentane, nhexane, nheptane, noctane, branched 2,4dimethylpentane, 2,2dimethylpentane and 2,2,4trimethylpentane ioctane, and cyclic cyclohexane alkanes were analyzed at different mixing ratios and temperatures. That is covered in three other pages which you can get at via the mass spectrometry menu. The fragmentation pattern contains clusters of peaks 14 mass units apart. Plausible assignments may be seen by clicking on the spectrum, its molecular mass mz 87 is an odd number. Electric, magnetic or a combination of electric and magnetic fields can be used for the separation. Fragmentation of important functional group alkane and. Lecture notes in eimass spectrometry by torben lund.
This criterion was originally established for the fragmentation of alkanes by. Linear alkanes produce fragments that differ by 14 mass. The fragmentation pattern is characterized by peaks separated by 14 mass units a ch 2 group. Fragmentation patterns for associated with functional groups alkanes. The metastable ion mass spectra provide only a limited opportunity to distinguish between isomers. Mass spectrometry introduction a mass spectrum can be thought of as being the record of the damage done to a. Following are examples of compounds listed by functional group, which demonstrate patterns which can be seen in mass spectra of compounds ionized by electron impact ionization. Each chapter ends with a concise summary that is subdivided into compact sections highlighting the basics of the subject, its figures of merit, typical applications, and its role in current mass spectromety. Mass spec fragmentation an extremely useful result of ei ionization in particular is a phenomenon known as fragmentation. The fragments of a molecule cause a unique pattern in the mass spectrum. Imperial chemical industries limited, dyestuffs division, blackley, manchester, england. Electron impact mass spectrometry of alkanes in supersonic. Joseluis jimenez 2 business items last real lecture is today material from today, art ms, protein ms lectures included in exam. Data system mass detector analyzer vacuum system source inlet region more precisely mass spectrometry determines the mass of a molecule.
R1 c o r2 c o r1 c o r2 c o and o r1 x esters, acids, amides. Mass spectrometry fragmentation the molecular ion may possess a large amount of internal energy which causes it to undergo chemical reactions leading to smaller fragments. Fragmentation often splits off simple alkyl groups. Fragmentation mass spectrometry wikimili, the free.
Thomson used a primitive form of ms to prove the existence of isotopes with neon20 and neon22. For the analytical chemist, a mass spectrum is useful for two applications. Mass spectrometry is a destructive method used to measure molecular weight and provide. All books are in clear copy here, and all files are secure so dont worry about it. The radical cation that is produced when an electron is knocked out of a neutral closedshell molecule in eims initially possesses a lot of energy. Mass spectrometry fragmentation patterns chemistry libretexts. Chapter 2 fragmentation and interpretation of spectra 2. Use the back arrow to return to a spectroscopy problem return to mass spectrum home page. Simple alkanes tend to undergo fragmentation by the initial loss of a methyl group to form a m15 species.
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group. In this video, i have explained the general fragmentation pattern of alkanes with the help of example. Mass spectrometry dont call it spectroscopy absorption of electromagnetic radiation electron impact mass spectrometry 70 ev 1614 kcalmol contrast with energy from ir 110 kcalmol or nmr 0. One of the best introductory texts on mass spectrometry in its second edition.
Mass spectrometry fragmentation patterns chemistry. At the same time mass spectrometry, particularly the matrixassisted laser desorptionionization maldi method, has grown to be an indispensible tool for the polymer chemist 2. The pressure in a mass spectrometer is very low which means that collisions are rare. Prediction and simulation of mass spectra a prediction of the isomer structure or substructures from a given mass spectrum the structure is directly deduced from the mass spectrum or generated by a molecular isomer generator or existing structures can be found in a structure database b simulation of a mass spectrum from a given isomer structure. In addition, chemical ionization is less effective with alkanes both in terms of extensive fragmentation and also because re. A flythrough brinktype electron impact ion source was used, utilizing a vacuum background ion filtration technique based on differences. These reactions are well documented over the decades and fragmentation pattern is useful to determine the molar.
Chad presents the common fragmentation patterns for alkanes most substituted fragments and alkenes aromatic compounds resonancestabilized fragments. A mass spectrum is the plot of relative abundance of ions against their mass charge ratio. By convention, m is the numerical value for the mass of the ion and z is the numerical value for the charge of the ion. At least one question from each exam on tue 6nov during class period. During 195080 an important discipline in mass spectrometry research was to investigate the.
In mass spectrometry, fragmentation is the dissociation of energetically unstable molecular ions formed from passing the molecules in the ionization chamber of a mass spectrometer. Protein sequencing and identification with mass spectrometry. Additional information can be found in mass spectrometry. Ion fragmentation of small molecules in mass spectrometry.
These examples do not provide information about the fragmentation mechanisms that cause these patterns. This page looks at how fragmentation patterns are formed when organic molecules are fed into a mass spectrometer, and how you can get information from the. This site is like a library, you could find million book here by using search box in the header. A mass spectrometry study of alkanes in air plasma at. Mass spectrometry instrumentation a mass spectrometer is composed of an inlet system which introduces the sample to the instrument and vaporizes the sample a molecular leak which produces a steady stream of the vapor, an ionization chamber where a beam of high energy electrons bombards the vapor.