Isomeric/isobaric compounds, identifying

In complex VOC mixtures, some isomeric/isobaric compounds may be present, and there will be some fragment ions that have the same or close ionic masses as other product ions. Under such circumstances, it is almost impossible to identify and quantify every component present using the methods mentioned above. To analyze a complex mixture, one feasible solution is to combine a conventional separation technique GC with PTR-MS. GC-PTR-MS is able to separate and unambiguously identify isomeric/isobaric compounds. 

From the FIA—MS overview spectrum, speculation that there can be more than just one structurally defined molecule type behind an observable signal i.e. the presence of isobaric compounds, cannot be excluded whenever one signal defined by the m/z-ratio is examined in FIA-MS spectra. Consequently, the information obtained by FIA-MS is quite limited whenever we deal with complex mixtures of environmental pollutants rather than the analysis of pure products or formulations with a known range of ingredients. LC separation is inevitable when mixtures of isomeric compounds should be identified with MS-MS. Therefore, in FIA-MS-MS special attention has to be paid to avoid the generation of mixed product ion spectra from isomeric parent compounds. This would block identification by library search and may lead to misinterpretations of product ion spectra because of the fragmentation behaviour observed. 

In a traditional PTR-MS, the detector is a quadrupole and its mass resolution is rather limited, the ratios of mass to charge (tn/z) of the product ions cannot be served for a definite indicator of the identity of trace gases because numerous isomeric or isobaric compounds have the same or close molecular weight. In addition, mass overlap from probable fragmentation and cluster ions may be in operation. In particular, when a mixture containing unknown VOC components is investigated, it is inevitable to meet with a question how to identify the compounds. 

A number of low-molecular-weight A-acyl amino acid lipids of biological significance were identified based on values of t and miz values provided by IMS-MS. Both data acquisition and visnalization of the separated isomeric lipids is rapidly achieved. For example, A-arachidonoyl isoleucine, and A-arachidonoyl leucine exhibit t fingerprints that are sufficiently distinct to delineate isomeric composition. It is often difficult to obtain meaningful MS/MS fragment ions from these low abundant lipids for structural characterization. The mass window for parent ion selection for most MS instruments is insufficient to cleanly select isobaric compounds, and the fragmentation pattern of isomers is commonly very similar. 

Unfortunately, the monitored m/z in the SIM technique could represent the combination of isobaric/isomeric species leaving us without unique identifiers. This is typical for the SIM analysis of a very complicated sample such as a serum or plasma sample where many compounds have the same m/z including multiply charged m/z of the species or stable isotopologues. Therefore, the technique is a kind of trading specificity for sensitivity. This technique is seldom used in modem lipidomics analysis due to its limitations. 

---