Impurity peak mass spectrum

A SSIMS spectrum, like any other mass spectrum, consists of a series of peaks of dif ferent intensity (i. e. ion current) occurring at certain mass numbers. The masses can be allocated on the basis of atomic or molecular mass-to-charge ratio. Many of the more prominent secondary ions from metal and semiconductor surfaces are singly charged atomic ions, which makes allocation of mass numbers slightly easier. Masses can be identified as arising either from the substrate material itself from deliberately introduced molecular or other species on the surface, or from contaminations and impurities on the surface. Complications in allocation often arise from isotopic effects. Although some elements have only one principal isotope, for many others the natural isotopic abundance can make identification difficult. 

Figure 12.9 MALDI-TOF mass spectrum of chicken egg-white lysozyme. The peak at 14,307.7578 daltons (amu) is due to the monoprotonated protein, M+H+, and that at 28,614.2188 daltons is due to an impurity formed by dimerization of the protein. Other peaks are various protonated species, M+H rH ...

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

Check which ionization method was used and examine the general appearance of the mass spectrum. Is the molecular ion peak intensive (as with aromatic, heterocyclic, polycyclic compounds) or weak (as with aliphatic and multifunctional compounds) Are there typical impurities (solvent, grease, plasticizers) or background signals (residual air, column bleed in GC-MS) ... 

FIGURE 15 ESI/MS/MS daughter ion mass spectrum of the impurity peak at RT = 3.8 min.The impurity is a weak acidic compound, and LC/ESI MS was operated at acidic condition. 

FIGURE 16 (a) UV spectrum, (b) parent ion and (c) daughter ion mass spectra of an impurity peak can be built into an LC/MS database. 

The mass spectrum of SOAz is shown Fig. 38 and its pattern is quite different from that of MYKO 63 in fact we no longer observe the fall of the Az leaves which characterizes any mass spectrum within the MYKO 63 series. The base peak is at m/z 320 and there are very few other secondary peaks till m/z 50. No chlorinated impurity could be detected either by mass spectrometry or by neutron activation. 

There is one major additional peak in the sample which runs after propranolol (impurity 2). The mass spectrum for propranolol is shown in Figure 9.28. 

An example of how ES-MS can be used to determine minor impurities in a recombinant protein is shown in Figure 9.33, where some small additional ions in the mass spectrum of recombinant insulin-like growth factor (IGF) can be seen. The major ions in the spectrum are due to IGF itself bearing varying charge but the minor impurities also give rise to peaks and these can be interpreted as shown in Table 9.4. 

The mass spectrum of the bromosuccinic acid (K K Laboratories, Inc.), a snow-white powder which melted smoothly in the range of 160°-165°C, showed no peak corresponding to the parent compound. No impurities could be identified in particular, there were no peaks corresponding to fragments containing two bromine atoms. The mass spectrum for bromosuccinic acid was not found in the literature, but that of the prepared acid was analogous to the one for succinic acid, e.g., no parent peak (25). 

Even the relative homogeneity of a peptide alkaloid may be ascertained from its mass spectrum. Impurities are often similar peptide alkaloids, which, when they differ in the end amino acid, are detectable in traces by their intense base peaks. 

Using this optimized method shown in Figure 8-49 that starts at 25v/v% acetonitrile, LC-MS studies were performed to determine the [M -i- H]+ ion of the impurity that has been resolved from the main peak. The mass spectrum of Product M was taken and was shown to be spectrally homogeneous. The mass spectrum of the impurity (RRT 1.04) that has now been resolved from the main peak was also taken. The UV and the total ion chromatograms are shown in Figure 8-50. This impurity, RRT 1.04, has the same [M -i- H]+ ion that was co-eluting with the main component in the initial separation on the C8... 

The dominant peak at c 28 minutes is PBO itself, whilst the other peaks are assumed to be impurities present from the manufacturing process. Many of these other peaks have been identified by the mass spectroinetric techniques employed in these studies, but the data are not considered relevant to the current discussion, ["he El mass spectrum of PBO [Pig- 6.2) is similar to the published library spectrum. The fragmentation pattern of PBO, and hence many of its degradation products, is unusual. Whilst the El spectrum shows a clear molecular ion rrs/z 338), the spectrum is dominated by the fragment ion at m/z 176, which has been attributed by previous workers Williams and Williamson, 1991) to the rearrange merit ion ... 

It is often difficult to determine the degree to which the chemistry proceeded on the entire library population and whether peaks in a mass spectrum are due to the product, side reactions, reagents, solvents, or impurities. Diversity Sciences developed mass-spectral methods to distinguish all components that are cleaved from a solid support and implemented the method into the analytical construct. While early studies demonstrated promising results for fragmentation methods with tandem mass spectrometry (MS/MS), stable isotopes were routinely implemented as signature peaks for the identification of compounds that are produced from solid-phase reactions [27]. 

The isotopic competitive method, whether the analyses are made mass spectrometrically or by radioactivity measurements, is very sensitive to impurities. In the use of radioactive isotopes, the sample to be counted must be decontaminated of other chemical species containing the same radioactive nuclide and it must be chemically pure to avoid dilution. Both of these effects must be reduced to the order of 0.1 per cent, which is a very exacting restriction. In the mass spectrometric method one must avoid impurities which give ion peaks (mjq), either from the parent or from a fragment, at the same mfe as the product analyzed. A scan of the mass spectrum usually enables one to detect such impurities and provides the basis for further purification processes. The latter must all be isotopically nonfractionating. 

The final 19-hydroxy derivative isolated was originally observed as an impurity in the mass spectrum of 131. The 19-hydroxy side chain could be deduced from a base peak at m/e 184, loss of 89 mu from the molecular ion at m/e 504, and from the C-19 proton at 3.98 ppm. Eight aromatic protons were observed but no olefinic protons. The compound was concluded to be -dihydrocinnamoyl-19-hydroxycylindrocarine (136), and catalytic reduction of 133 confirmed this structure assignment. 

Now consider the mass spectrum in Fig. 10.11 and compare it to the carbon dioxide spectmm. The spectrum in Fig. 10.11 is one that every mass spectrometrist should know. It is the spectmm of air, a mixture of components. (Why should a mass spectrometrist recognize this spectmm ) The peak at m/z = 44 is from carbon dioxide, while the other peaks are due to water (18), nitrogen (28), and oxygen (32). The peak at m/z 28 in the CO2 spectmm is due to a CO ion, whUe in the air spectmm, it is due to nitrogen plus any carbon monoxide that is in the sample. At this resolution, it is not possible to distinguish between N2 and CO. It is important to remember that not all mass spectra are of a single pure component. In the real world, mixtures and impure samples are much more commonly encountered. 

Poly(2,2,4-trimethyl-l,2-dihydroquinoline), an oligomeric antioxidant for rubber, is a typical example for this ionization method. Figure 6.3 shows the FD mass spectrum of a poly-TMDQ sample. Molecular ions for "normal" oligomers (M = 173n) are observed, along with minor peaks due to impurities (with differing end-groups) from the synthesis. 

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