Molecular ion detected

Due to the high mass, low volatility, and thermal instability of chlorophylls and derivatives, molecular weight determination by electron impact (El) MS is not recommended. Desorption-ionization MS techniques such as chemical ionization, secondary ion MS, fast-atom bombardment (FAB), field, plasma- and matrix-assisted laser desorption have been very effective for molecular ion detection in the characterization of tetrapyrroles. These techniques do not require sample vaporization prior to ionization and they are effective tools for allomerization studies. 

Detection of (pseudo-) molecular ion Detection of molecular ion minus (e.g. CH3 or HF)... 

El (top), methane Cl (middle) and isobutane Cl (bottom) mass spectra of butyl methacrylate. The ionization techniques (El vs Cl) and the reagent gases (methane vs isobutane) influence the amount of fragmentation and the prominence of the protonated molecular ions detected at 143 Th. 

Comparison of El (top) and FI (bottom) spectra of methyl stearate. Field ionization yields simple spectrum that shows intense molecular ion detected at m/z 298 without fragmentation. Reproduced, with permission from Micromass documentation. 

In all types of degradation assays the use of LC-MS detection is desirable since it allows for selective detection and quantitation and sometimes allows for structural elucidation of the degradation products. In some cases, tauto-merization or intramolecular rearrangements could lead to the formation of degradation products with the same molecular weight. These molecules are usually indistinguishable from the parent compound using MS with molecular ion detection. The employment of LC-NMR technique may be needed to further elucidate the structures. 

FIGURE 6.4 Detection and characterization of the GSH adduct of acetaminophen using the API 4000 Q Trap instrument. A mixture of labeled and nonlabeled GSH was used to trap the reactive metabolite of acetaminophen (lOOmM) incubated with rat liver microsomes. (a) The ion chromatogram obtained from the NL experiment, (b) The precursor protonated molecular ions detected for the peak at 9.2min. (c) An MS/MS spectrum of the precursor ion at m/z 457. (From Mutlib, A. et al., Rapid Commun. Mass Spectrom., 19, 3482, 2005. With permission.)... 

Structural Assignments of Molecular Ions Detected in the DPMS of PST... 

Molecular Ions Detected by FAB-MS Analysis of the Aminolyzed Residue Obtained After Heating PC at 400 C for 1 h... 

This chapter will explore the qualitative and quantitative investigations of MALDl-MS for molecules with a mass less than 1500 Da. As an exhaustive exploration of studies involving any molecular ions detected at 1500 Da or less could yield thousands of articles, attention has been focused on highhghting relatively recent relevant studies that concentrate on LMM compounds. More detailed information on LMM biomolecules and polymers is available in Chapters 3-8. In addition to exploring alternative sample preparation approaches, the couphng of MALDI to various separation techniques will be examined as mechanisms to strengthen the utility of MALDI in the low-mass range. 

Molecular ion detected by electro spray ionization-mass spectrometry (ESI-MS) in CH3CN solution. 

In addition, time-of-flight analyzers are predominantly used for molecular ion detection as the entire mass range is collected simultaneously. 

The intensity of the molecular ion from primary and secondary alcohols is normally quite low, and there usually is no molecular ion detectable for tertiary alcohols. One of the most common fragmentation patterns for alcohols is loss of a molecule of water to give a peak corresponding to the molecular ion minus 18 (M - 18). Another common pattern is loss of an allyl group from the carbon bearing the —OH group to form a delocalization stabilized oxonium ion and an allyl radical. The oxonium ion is particularly stable because of delocalization of charge. 

Microwave spectra (giving pure rotational spectra) are especially usefiil for the detection of interstellar molecular ions (in some cases the microwave spectrum has first been observed in interstellar spectra ). 

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

Dimeric ion. An ion formed either when a chemical species exists in the vapor phase as a dimer and can be detected as such, or when a molecular ion can attach to a neutral molecule within the ion source to form an ion such as (2M)+, where M represents the molecule. 

High mass resolution techniques are used to separate peaks at the same nominal mass by the very small mass differences between them. As an example, a combination of Si and H to form the molecular ion Si H , severely degrades the detection limit of phosphorous ( P) in a silicon sample. The exact mass of phosphorous ( P) is 31.9738 amu while the real masses of the interfering Si H and Si H2 molecules are 31.9816 amu and 31.9921 amu, respectively. Figure 8 shows a mass... 

Molecular ion mass interferences are not as prevalent for the simpler matrices, as is clear from the mass spectrum obtained for the Pechiney 11630 A1 standard sample by electron-gas SNMSd (Figure 4). For metals like high-purity Al, the use of the quadrupole mass spectrometer can be quite satisfiictory. The dopant elements are present in this standard at the level of several tens of ppm and are quite evident in the mass spectrum. While the detection limit on the order of one ppm is comparable to that obtained from optical techniques, the elemental coverage by SNMS is much more comprehensive. 

The nitrogen rule of mass spectrometry says that a compound with an odd number of nitrogen atoms has an odd-numbered molecular weight. Thus, the presence of nitrogen in a molecule is detected simply by observing its mass spectrum. An odd-numbered molecular ion usually means that the unknown... 

Molecular ions are easily detected for most cycloalkylamines and have a characteristic m/z of 30. For methylcyclopentylyamine, mJz 30 is the base peak in the spectrum, whereas for cyclohexy-lamine a rearrangement fragment ion is the base peak. 

Molecular ion The molecular ion intensity decreases with increasing molecular weight, but is still detectable through CUl although the ion abundance is low (0.1%). 

Not all of these isomers are completely separated by the conditions given here, but all arc readily detected by plotting the masses of the molecular ions. Pesticides can interfere with polychlorinated biphenyl (PCB) analysis. 

The molecular ions decrease in intensity with increasing chain length but are still detectable at C40. In contrast to branched alkanes, the loss of a methyl group is not favored for n-alkanes. Usually the... 

Molecular ion The presence of sulfur can be detected by the 34S isotope (4.4%) and the large mass defect of sulfur in accurate mass measurements. In primary aliphatic thiols, the molecular ion intensities range from 5-100% of the base peak. 

The presence of chlorine and/or bromine is easily detected by their characteristic isotopic patterns (see Appendix 11). As in many aliphatic compounds, the abundance of the molecular ion decreases as the size of the R group increases. For example, in the El mass spectra of methyl chloride and ethyl chloride, the molecular ion intensities are high, whereas in compounds with larger R groups such as butyl chloride, the molecular ion peak is relatively small or nonexistent. 

Alkylbenzenes have molecular ions at the following m/z values 92, 106, 120, 148, and so forth. The molecular ion intensity decreases with increasing alkyl chain length, but can be detected up to at least Cifi. Characteristic fragment ions are m/z 39, 50, 51, 52, 63, 65, 76, 77, and 91. 

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