Tandem MS with the Quadrupole Ion Trap

Ion traps are tandem-in-time instruments, i.e., they perform the steps of precursor ion selection, ion activation, and acquisition of fragment ion spectra in the very same place. This advantageous property allows the multiple use of a single QIT to perform not only MS but also MS and higher order MS experiments - indeed a very economic concept. Depending on the abundance of the initial precursor ion. 

Unfortunately, there is one major disadvantage of QFTs for fragment ion analysis in that they cannot simultaneously store ions over the full m/z range. It is a commonly accepted property of QITs to loose ions below an m/z value of about one third of the precursor ion. This phenomenon is known as low-mass cutoff (LMCO) [113]. 

Example I The positive-ion electtospray tandem mass spectrum of the [M+2ir ion of the P-casein tryptic phosphopeptide FQpSEEQQQTEDELQDK was obtained on a Bruker Esquire 3000 quadrupole ion trap (Fig. 9.21) [98]. Sequential amino acid residue losses from lx)th terminal ends of the peptide are labeled according to the b- and y-ion nomenclature. Due to the phosphoserine, the C- and N-terminal peptide ions (bs- bis and y - y ) exhibit an increase of 80 u in mass as compared to the unphosphorylated form. In addition, most ions are accompanied by the corresponding fragments from H3PO4 loss (98 u). 

Example II MS on a QTT was used for the identification of beauverolides, cychc peptides from the fermentation broth of Beauveria bassiana, a pathogenic fungus of insects [114]. All MS (ESI-CID-QIT) experiments started from singly charged [M+Hj precursor ions (Fig. 9.22). 

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