Mass spectra identify compound

A mass spectrum is a graphic representation of the ions observed by the mass spectrometer over a specified range of m/z values. The output is in the form of an x,y plot in which the x-axis is the mass-to-charge scale and the y-axis is the intensity scale. If an ion is observed at an m/z value, a line is drawn representing the response of the detector to that ionic species. The mass spectrum will contain peaks that represent fragment ions as well as the molecular ion (see Figure 1.3). Interpretation of a mass spectrum identifies, confirms, or determines the quantity of a specific compound. 

The practice of mass spectrometry is carried out with rather sophisticated instruments (mass spectrometers) which produce, separate, and detect both positive and negative gas-phase ions. Since samples are typically neutral in charge, they must be first ionized in the spectrometer. Ionization of molecular substances is often followed by a series of spontaneous competitive decomposition or fragmentation reactions which produce additional ions. The ion masses (more correctly, their mass-to-charge ratios) and their relative abundances are displayed in a mass spectrum. Most compounds produce unique or distinctive patterns, so most substances can be identified by their mass spectra. 

Fig. 6.7 shows the CIMS/MS spectrum for llr,13r-18 2 with an a/ ratio of 1.5. The mass spectrum identifies this isomer as either a cisicis or a trans trans CLA, and the elution profile allows unambiguous identification of this isomer as the trans trans one. Other CLA methyl ester isomers coeluted after this compound in a single peak. Their elution order, relative to the other already identified CLA methyl esters, indicates that they are trans trans isomers. The confirmation of their double... 

The sample solution was subjected to GC-MS, which automatically records the mass spectrum of each compound as it emerges from the column. Modern instruments have a searchable database of mass spectra, which allows the best match between the case sample and known substances to be found very quickly. The results in this case are shown in Figure 3.5. Here the best match with the mass spectrum identified by the computer is with heroin. Visual examination by the human operator (still an essential part of the process ) confirms that the two spectra are virtually identical. There are small differences between the two spectra, but these are accounted for by noise in the sample spectrum (probably due to the very low concentration used) and the fact that the library spectra were probably recorded on a different instrument. The other heroin derivatives found from the database, have obvious differences in their mass spectra and can be ruled out. 

Structures related to ethanediol are in italics Identified by comparison with NIST database Identified by mass spectrum analysis. Compound not present in the NIST database El Electron ionisation NIST National Institute of Science and Technology Reproduced with permission from G. Gallet, B. Erlandsson, A-C. Albertsson and S. Karlsson, Polymer Degradation and Stability, 2002, 77, 55. 2002, Elsevier [47] ... 

MS + LRI, mass spectrum and LRI agree with those of authentic compound ms + Iri, mass spectrum identified using NIST/EPA/NIH Mass Spectral Database and LRI agrees with literature value (23) ms, mass spectrum agrees with spectrum in NIST/EPA/NIH Mass Spectral Database or with other literature spectrum. 

Compound A and compound B are isomers having the molecular formula C10H12O The mass spectrum of each compound contains an abundant peak at m/z 105 The NMR spectra of com pound A (Figure 17 23) and compound B (Figure 17 24) are shown Identify these two isomers... 

Once a mass spectrum from an eluting component has been acquired, the next step is to try to identify the component either through the skill of the mass spectroscopist or by resorting to a library search. Most modem GC/MS systems with an attached data station include a large library of spectra from known compounds (e.g., the NIST library). There may be as many as 50,000 to 60,000 stored spectra covering most of the known simple volatile compounds likely to be met in analytical work. Using special search routines under the control of the computer, one can examine... 

In a process similar to that described in the previous item, the stored data can be used to identify not just a series of compounds but specific ones. For example, any compound containing a chlorine atom is obvious from its mass spectrum, since natural chlorine occurs as two isotopes, Cl and Cl, in a ratio of. 3 1. Thus its mass spectrum will have two molecular ions separated by two mass units (35 -i- 2 = 37) in an abundance ratio of 3 1. It becomes a trivial exercise for the computer to print out only those scans in which two ions are found separated by two mass units in the abundance ratio of 3 1 (Figure 36.10). This selection of only certain ion masses is called selected ion recording (SIR) or, sometimes, selected ion monitoring (SIM, an unfortunate... 

A diagrammatic illustration of the effect of an isotope pattern on a mass spectrum. The two naturally occurring isotopes of chlorine combine with a methyl group to give methyl chloride. Statistically, because their abundance ratio is 3 1, three Cl isotope atoms combine for each Cl atom. Thus, the ratio of the molecular ion peaks at m/z 50, 52 found for methyl chloride in its mass spectrum will also be in the ratio of 3 1. If nothing had been known about the structure of this compound, the appearance in its mass spectrum of two peaks at m/z 50, 52 (two mass units apart) in a ratio of 3 1 would immediately identify the compound as containing chlorine. 

Unfortunately, not every compound shows a molecular ion in its mass spectrum. Although M+ is usually easy to identify if it s abundant, some compounds, such as 2,2-dimelhylpropane, fragment so easily that no molecular ion is observed (Figure 12.3). In such cases, alternative "soft" ionization methods that do not use electron bombardment can prevent or minimize fragmentation. 

J.9 You are asked to identify compound X, which was extracted from a plant seized by customs inspectors. You run a number of tests and collect the following data. Compound X is a white, crystalline solid. An aqueous solution of X turns litmus red and conducts electricity poorly, even when X is present at appreciable concentrations. When you add sodium hydroxide to the solution a reaction takes place. A solution of the products of the reaction conducts electricity well. An elemental analysis of X shows that the mass percentage composition of the compound is 26.68% C and 2.239% H, with the remainder being oxygen. A mass spectrum of X yields a molar mass of 90.0 g-moF. (a) Write the empirical formula of X. (b) Write... 

Some detectors can give additional information about the elutes (the eluted solutes). One example is the gas chromatograph—mass spectrometer (GC-MS), which produces a mass spectrum of each compound as well as its mass and location in the chromatogram. This powerful means of detection can be used when standard samples are not available to help determine the identities of the solutes. A beam of ions bombards each compound as it emerges from the chromatograph. The compound breaks up into ions of different masses, providing a spread of narrow peaks instead of one peak for each compound. The relative amount of each fragment is determined and used to help identify the compound. 

The occurrence of isotopes of some elements allows the presence of these elements to be readily identified in a mass spectrum of a compound. For example, Br has two naturally occurring isotopes with masses 79mu and 81 mu, in the relative... 

The identification of a molecular structure from a mass spectrum requires good chemical detective work. Let s see how that is done by trying to identify a simple compound,... 

More complex detective work is required to analyze large biomolecules and drugs. However, fragmentation generally follows predictable patterns, and one compound can be identified by comparing its mass spectrum with those of other known compounds with similar structures. In Fig. 2, we see the spectrum of a sample of blood from a newborn infant. The blood is being analyzed to determine whether the child has phenylketonuria. The presence of the compound phenylalanine is a positive indication of the condition. Some... 

The MS detector is a particularly suitable tool for confirming the identity of an analyte which has been detected with ECD or NPD. Confirmation of identity should be performed particularly in those cases in which the MRL appears to have been exceeded or in which a compound seems to be present which is not expected in the sample being analyzed. In this case, the scan mode is used in order to identify the compound by means of its mass spectrum. 

The A5-phosphorin 164 and the bicyclic compound 165 are precursors of isopropyl metaphosphate, 16). Thermal fragmentation of 165 leads via [2 -I- 2]cycloreversion to triphenyltoluene 166 and isopropyl metaphosphate 167. The latter is identified as isopropyl phosphate after reaction with water. The mass spectrum of 165 is also dominated by this fragmentation picture (m/e 442 (16%) = M + m/e 320 (100%) = M + — 167). 

Leurosine (75) (Scheme 20) is the most abundant of the dimeric antitumor alkaloids isolated from Catharanthus roseus G. Don. Several species of Strep-tomyces produced a common metabolite of the alkaloid, and S. griseus (UI1158) was incubated with 400 mg of leurosine sulfate to obtain 28 mg of pure metabolite (180). The metabolite was identified as 76 primarily on the basis of its H-NMR spectrum. The mass spectrum indicated that the metabolite contained one oxygen atom more than 75. The H-NMR spectrum contained all of the aromatic proton signals of the vindoline portion of the molecule, and aromatic proton signals for the Iboga portion of the compound occurred as a doublet of doublets... 

Unknown 1. Try to identify a compound with the spectrum represented in Fig. 5.1. The exact molecular mass of the compound is 60.0211 Da, which defines its elemental composition as C2H4O2. At this stage pay attention only to the most abundant peaks in the spectrum m/z 60 (molecular ion) and primary fragment ions of m/z 45, m/z 43, m/z 28, and m/z 15. Use the masses of elements from the periodic table of chemical elements. 

Maximum reliability may be achieved by recording a mass spectrum under the same conditions of the identified compound taken from another source or specially obtained. 

Formation of a new compound PSeF3 has been claimed in the reaction of PF3 with elemental selenium, at 300-400°C, identified from a mass spectrum analysis, showing the fragments (PSeF2)+ with the expected selenium isotopes (34). However, since no further data have been obtained, it appears very likely that the compound actually was... 

All analytical techniques are designed to provide the answer to one or both of the two important questions what is it and how much is there Mass spectrometry possesses attributes that allow it to contribute answers to both of these questions. The nominal (integral) m/z ratio of the molecular ion can sometimes be sufficient to identify a chemical compound, particularly if there is additional information available (either from the mass spectrum itself or another analytical technique). The presence of other signals in a mass spectrum attributable to... 

Mass spectroscopy is useful for qualitative analysis because the signals due to the different mass-to-charge ratios, as seen in the mass spectrum, represent molecular fragments that can be pieced together such that the compound can be identified. 

Fig. 2.6.6. APCI-LC-MS-MS(+) (CID) daughter ion mass spectrum of [M + NH4]+ ion at m/z 678 generated from Cig-SPE of foam sample. Compound could be identified as non-ionic surfactant NPEO (CgHi9-C6H4-0-(CH2-CH2-0)m-H) (inset)... 

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