Features of a Mass Spectrum

What features of a mass spectrum indicate that a soft ionization technique has been employed (few if any fragment ions). 

After observation of a molecular or pseudomolecular ion, the most important feature of a mass spectrum is the fragmentation behavior, i.e., the differences between the observed ion mass-to-charge ratios and their relative abundances. The rules for structural elucidations for electron ionization (El) are well known and accessible. The reader is referred to McLafferty and Turacek (1993) either as an introduction or as a refresher.30... 

A striking feature of a mass spectrum is the existence of satellite peaks. Those peaks are isotopically shifted lines that appear at masses one or more units higher than the main peak M the mass of M is calculated using the atomic masses of the most abundant isotopic species (i.e., the primary isotope). The satellite peaks, designated as M +1, M + 2, and so on, reflect the differences in the natural abundances of the isotopes. From the elemental composition of a molecular ion or fragment ion, the abundances of its satellite peaks (i.e., the isotopic pattern) can be predicted (see Example 6.1). Consider a compound of the general formula CxHj,NjO the abundance of the [M - -1] ion relative to [M] = 100% is given by... 

Features of a Mass Spectrum Interpreting Mass Spectra... 

Oxostephasunoline (4) was isolated from the roots of Stephania japonica(4). The UV spectrum of oxostephasunoline (4) showed an absorption maximum at 286 nm, and the IR spectrum depicted bands at 3550,3500, and 1670 cm, indicating the presence of a hydroxyl group and a y-lactam. The mass spectrum (Table VI) exhibited the most abundant ion peak at m/z 258, and the H-NMR spectrum (Table II) revealed the presence of three methoxyl and one N-methyl group. The downfield shift (53.06) of the JV-methyl resonance indicated that oxostephasunoline (4) was a y-lactam, which was further supported by the IR band at 1670 cm 1, significant features of the mass spectrum (Table VI), and the 13C-NMR spectrum (Table III). On exhaustive H-NMR analysis similar to the case of stephasunoline (17), the structure of oxostephasunoline (4) including the stereochemistry was practically proved (4). 

In 1970, Clampiti and Jefferies [106] reported their molecular beam experiments for various ion clusters. For Ne Li", they found the largest peak for m = 1 followed by nearly identical signals for m = 2, 3, 4, 5, 6. The yield of ion clusters decreases abruptly beyond HcgLi". The same result was found for hydrogen clusters (H2) Li. A remarkable feature of the mass spectrum was the relative insignificance of clusters with m > 6. It was concluded [106] that six molecules around an ion probably constitute a complete shell . This result is important in comparison with theoretical studies of Ng Be " ions. 

Mass resolving power The ratio of the mass of an ion to the full-width-at-half-maximum (FWHM) of its spectral feature in a mass spectrum, or m/Am. Notably, the definitions of resolving power and resolution were reversed for historical reasons. Since in modern MS, one usually reports only m/Am, in this book we use the terms mass resolving power and resolution to refer to the same properties of mass spectra. For details please refer to Section 5.3.1. 

Dynamic range describes the ratio of the highest to lowest intensity feature in a mass spectrum. The lowest intensity feature needs to provide meaningful information on an ion, such as with the S/N ratio > 3. Dynamic range is an important factor when mass spectrometers are used to analyze samples with large concentration differences for example, when one of the sample molecules has an abundance several thousand times lower than others. Dynamic range is distinct from detection limit of a mass spectrometer because detection limit concerns the smallest detectable sample quantity without considering the interference of other species. 

Another prevalent feature of the mass spectrum, which is often more of a problem than elemental isobaric overlap, is the occurrence of peaks attributed to ionized molecular species. The formation of molecular ion species is discussed in Chapter 2. As discussed there, usually only diatomic homogeneous molecular species, such as, and diatomic heterogeneous molecular species, such as ArO", are observed in the mass spectrum from the ICP, although some triatomic species, such as ArOH" ", are observed when the components that form these polyatomic molecules are present at high concentrations. When molecules are present, only singly charged ions are observed. The principal m/z value observed in the mass spectrum for the molecular species is determined by the sum of the most abundant iso-... 

Multivariate data analysis usually starts with generating a set of spectra and the corresponding chemical structures as a result of a spectrum similarity search in a spectrum database. The peak data are transformed into a set of spectral features and the chemical structures are encoded into molecular descriptors [80]. A spectral feature is a property that can be automatically computed from a mass spectrum. Typical spectral features are the peak intensity at a particular mass/charge value, or logarithmic intensity ratios. The goal of transformation of peak data into spectral features is to obtain descriptors of spectral properties that are more suitable than the original peak list data. 

Similarly, a common feature in the mass spectrum of thiirene oxides is the high abundance of the substituted acetylene ion (e.g. [PhC CPh]7) formed by elimination of sulfur monoxide. In fact, this ion constitutes the base peak in the spectrum of 18a whereas the molecular ion has a rather insignificant intensity (0.25% I of M+)91. 

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