Mass spectrum features

Abundant Cis isoprenoid ketone, Z/E pristenal, and Z/E phytenal as well as phytol biomarker compounds were detected in the sinking particulate materials from the Yongshu Reef Lagoon and the continental shelf of the East China Sea (Figs. 5.28 and 5.29). This appears to be the first report of these compounds in China marine environment. Detection of these compounds in the studied regions is of some importance to understand the evolutional processes of acyclic isoprenoid compounds in the seawater column and their formative pathways in marine sediments. Their identifications were based on chromatographic retention time, mass spectrum features (Figs. 5.28 and 5.29), and comparison with those reported previously. The mass spectrum of phytol trimethylsilyl ether exhibits a base peak at mlz 143 and a molecular ion at mjz 368. Cig isoprenoid ketone has a base peak at mjz 58, a molecular... 

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. 

The great advantage of the mass spectrometer is its abihty to use mass, more accurately the mass-to-charge ratio, as a discriminating feature. In contrast to, for example, the UV detector, which gives rise to broad signals with little selectivity, the ions in the mass spectrum of a particular analyte are often characteristic of that analyte. Under these conditions, discrete signals, which may be measured accurately and precisely, may be obtained from each analyte when they are only partially resolved or even completely umesolved from the other compounds present. 

These rearrangement reactions may also occur in MS-MS instruments and the constant-neutral-loss scan enables the analyst to observe all of the ions in the mass spectrum that fragment with a particular mass loss and therefore contain a specific structural feature. This knowledge can be of great value when attempting to interpret the mass spectrum of an unknown material. 

The general procednre is to nse reconstrncted ion chromatograms at appropriate m/z values in an attempt to locate componnds of interest and then look at the mass spectrum of the unknown to determine its molecnlar weight. MS-MS can then be employed to obtain spectra from this and related compounds to find ions that are common to both and which may therefore contain common stmctmal features. Having the same m/z value does not necessarily mean the ions are identical and further MS-MS data or the elemental composition may be required. If these data do not allow unequivocal structure identification, then further MS" information may be required. 

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). 

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

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

Sulfosuccinates, as presented with their general structural formula in Fig. 2.11.33, are applied as surfactants for personal hygiene because of their hypoallergenic features. The sodium salt of the sulfosuccinate blend with the formula ROOC-CH-(SC>3 )-CH2-COOR Na+ (R = CsH-iy) was examined by APCI-FIA-MS in the positive and negative modes. The addition of an excess of ammonium acetate under FIA-APCI-MS(+) conditions resulted in [M — NH4]+ ions with mlz 440 while [M — H] ions with mlz 421, however, were observed in the negative APCI-FIA-MS mode (Fig. 2.11.34(a)). Only one type of ion could be observed in this industrial blend by FIA-MS. This purity could also be confirmed by APCI-LC-MS(-), as shown in the total ion current trace (cf. Fig. 2.11.34(b)), which is presented in combination with the averaged mass spectrum under the signal in the inset of Fig. 2.11.34(b) [22],... 

Let IM be the intensity of a peak at mass M, normalized to the highest peak in the spectrum (the base peak with intensity 100%). A spectral feature Xj is a function of one or several peak intensities and is scaled to the range 0-100. Spectral features can be used—eventually together with original peak intensities—to characterize a mass spectrum of a compound. The typical parameters given here are for electron impact mass spectra. 

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

Problems 1 - 277 are all of the basic "structures from spectra" type, are generally relatively simple and are arranged roughly in order of increasing complexity. No solutions to the problems are given. It is important to assign NMR spectra as completely as possible and rationalise all numbered peaks in the mass spectrum and account for all significant features of the UV and IR spectra. 

---