However, the availability of only [M+nH]n+ ion peaks in ESI-MS1 mass spectra is not generally sufficient for identification at its con- firmation level (this evidence is the equivalent of only two identi- fication points instead of 3–4 ones required for certainly true results [10, p.123]). So combination of both library and HRMS approaches should provide reliable recognition of such molecules, particularly, in samples of unknown origin. For this purpose, the HRMS version of our library which could be easily built on the base of subset of mass spectra acquired on high resolution mass spec- trometers would be especially effective.
On the other hand, relatively rare peptides with only two library spectra, i.e. a pair of one test spectrum and the only reference one, involved in test searches for a unique compound, did not find out very reliable identification. Here, TPR is not higher than 50%. The use of HRMS cannot also be a panacea for recognition of many rare cyclopeptides because of their isomerism. For example, there are not less than four known cyclic isomers of demethyl microcystin- LR, two of them occurring with comparable probability (Table 4). Those are not differentiated by HRMS1 and are partly distinguished by tandem mass spectra [40]. Also, there may be linear isomers of those cyclopeptides (Table 4). Obviously, in order to be granted a chance to reliably identify isomer structures the library should be far enlarged with their replicate mass spectra.