Molecular ion species

Tandem mass spectrometry or ms/ms was first introduced in the 1970s and gained rapid acceptance in the analytical community. The technique has been used for stmcture elucidation of unknowns (26) and has the abiUty to provide sensitive and selective analysis of complex mixtures with minimal sample clean-up (27). Developments in the mid-1980s advancing the popularity of ms/ms included the availabiUty of powerhil data systems capable of controlling the ms/ms experiment and the viabiUty of soft ionisation techniques which essentially yield only molecular ion species. 

Similarly, methane Cl spectrum of 18a was found to be dominated by the (C6H5C= CC6H5 + H)+ ion. A distinct molecular ion species at m/e value corresponding to (M + H)+ was observed in the methane mass spectra of this thiirene oxide (26% 2 40). Furthermore, the relative intensity of the (M +H)+ peak of 18a was shown to increase substantially in the isobutane and dimethyl amine Cl mass spectra91. 

Molecular ions obtained from thianthrenes are normally the base peak in their mass spectra. The principal fragmentation involves loss of sulfur (87PS377), and this is interpreted as formation of a dibenzothiophen radical cation (16). Further loss of sulfur then occurs. CSH is lost from both the dibenzothiophen fragment ion and from the molecular ion species such as 17, from the parent ion, are proposed (74JHC287). The mass spectroscopic fragmentation pattern of fluorothianthrenes is comparable (720MS373). 

Because extensive fragmentation is typical of El and Cl mass spectra, molecular ions or protonated molecules might not be observed. In order to confirm the molecular weight of a carotenoid, desorption El or desorption Cl (also known as in-beam El and Cl) can be utilized to increase the abundance of the molecular ion species. If the molecular weight of the carotenoid remains uncertain, then softer ionization techniques should be investigated, such as FAB-MS, ESI, MALDI, or APCI. 

Figure 1. Effect of pH on the molecular ion species of solutions of TAPS [tris(hydroxymethyl)methylaminopropanesulfonic acid] in 50% glycerol/water. HA represents the protonated amine, a zwitt-erion of molecular weight 243, and A- the conjugate base formed from dissociation of a proton from the acid. Reproduced from Ref. 2. Copyright 1983, American Chemical Society.

The example in Figure 8.68 shows the analysis of a mixture containing 11 bile acids. [311] The FAB spectrum containing the molecular ion species of all of the bile acids present in the mixture (more intense than the background noise) is not sufficient to determine the composition of the mixture. Only the various specific scans allow the determination of the nature of all the bile acids present in the mixture, without however determining their stereochemistry. 

The identification of all other PCDD/PCDF isomers is based on their retention times falling within their respective PCDD/PCDF retention time windows as established by a window defining mix. Confirmation of all PCDDs/PCDFs is based on a comparison of the ratio of the integrated ion abundance of the molecular ion species to the theoretical ion abundance ratio. 

The CI(NH3)-MS data supported the assignments given in Figure 1 (10). With NH3 reagent gas (13) all four materials gave rather Intense molecular ion species shifted by I8 mass units. The fraction. Mild acid hydrolysis released over 50 of the insoluble "I C as neutral products. Silica and RP-TLC suggested that most of the products are oxygenated and have lost EO units. 

While glucuronide and sulfate molecular ion species are not seen in the PB mass spectra, the SAX-LC separation groups the compounds by conjugate class. Phenols elute first, followed by glucuronides, and then sulfates. Thus, tentative identification of unknown metabolites is possible based upon identification of the aglycone and the retention time window. 

Features common to the spectra of glucuronic acid conjugates analysed by FAB, laser and field desorption were summarized several years ago (15). These appear to hold as well as for plasma desorption and thermospray spectra more recently examined. The situation with thermospray is somewhat more complicated as will be discussed later. Generally speaking, positive ion spectra contain protonated, natri ted or analogous molecular ions species, and usually (M+H-176) ions formed by the elimination of neutral dehydroglucuronic acid. 

Figure 80. Main decompositions occurring in the second FFR from molecular ion species formed in field ionization.

Figure 9.5 Schematic of Matrix Assisted Laser Desorption Ionisation (MALDI). The laser is fired at the sample analyte of interest admixed with a crystalline matrix that readily absorbs laser energy and allows the sputtering of analyte into the vapour phase in association with matrix molecules of solvation. Desolvation and proton transfer leads to naked molecular ion species, ready for mass analysis.

Figure 9.7 Schematic of Electrospray ionization. A "spray" of droplets is formed that evaporate until the destabilizing electrostatic forces cause the droplets to "explode" releasing multi-charged molecular ion species for mass analysis.

Figure 9.14 Comparison of protein mass analysis data from ESI and MALDI mass spectrometry, (a) ESI produces a family of multi-charged molecular ions, whereas (b) MALDI typically generates single molecular ion species (Reproduced from Glish and Vachet, 2003, Fig. 2).

---