Tandem MS experiments

Not all tandem MS experiments are possible (no neutral loss experiments)... 

In other words, in tandem MS experiments two (MS/MS or MS2) or more (MS") sequential stages of mass analysis (which can be separated spatially or temporally) are used in order to examine selectively the decomposition of given ions, occurring out of the ion source, in a mixture of ions. Type and performances of the analyzers determine the kind of MS experiments that can be carried out. There are two main kinds of reactions that cause decomposition of ions occurring out of the source ... 

No tandem MS experiment can be successful if the precursor ions fail to fragment (at the right time and place). The ion activation step is crucial to the experiment and ultimately defines what types of products result. Hence, the ion activation method that is appropriate for a specific application depends on the MS instrument configuration as well as on the analyzed compounds and the structural information that is wanted. Various, more or less complementary, ion activation methods have been developed during the history of tandem MS. Below we give brief descriptions of several of these approaches. A more detailed description of peptide fragmentation mles and nomenclature is provided in Chapter 2. An excellent review of ion activation methods for tandem mass spectrometry is written by Sleno and Volmer, see Reference 12, and for a more detailed review on slow heating methods in tandem MS, see Reference 13. 

Figure 6.3. Real-life example of a tandem MS experiment in an electrospray ion trap instrument. Top panel a complex peptide mixture. Middle panel ion at 1318.9 m/z was isolated from other sample components. Note the lack of any other peaks and a very low background. Bottom panel fragmentation spectrum of the selected parent ion (1318.9 m/z), note the different scale of the m/z axis. All peaks seen in this mass spectrum are product ions that were formed due to the controlled fragmentation of the parent ion. The main peak at 1300.8 m/z corresponds to the loss of water molecule, a lower intensity parent ion at 1318.9 m/z is also seen.

More recently, the catalytic activities of a large pool of transition-metal carbene complexes have been screened by means of ion-molecule reactions in tandem-MS experiments. [156-158] Different from the concepts and methods discussed so far, the latter experiments are not designed to study the fundamentals of mass spectrometry. Instead, sophisticated methods of modem mass spectrometry are now employed to reveal the secrets of other complex chemical systems. 

Quadrupole Energy and spatial distribution of ions produced in the ion source is not critical Low cost and easy to couple to LC Tandem MS experiments available in triple quadrupole or Q-TOF systems for sub-structure information and/or quantitative analysis Vacuum system demands are minimum Low resolution and low accuracy in mass measurement except in Q-TOF systems Mass range limited to approximately 4000... 

The third method which provides evidence for a knotted structure is mass spectrometry.42 With electrospray ionization (ESI) it is possible to ionize the knot and other similar molecules by protonation and to transfer them into the highly diluted gas phase of a mass spectrometer. In a so-called tandem-MS experiment, the parent ion, i.e. the protonated knot, is isolated and subjected to collisions with a stationary gas inside the... 

Fig. 11. The three common tandem MS experiments for carbohydrate analysis.

NM R experiments (ID and 2D, homonuclear and heteronuclear) are the preeminent techniques for the determination of molecular structures. However, careful application and analysis of mass spectral data can provide sufficient information to postulate tentative structures. In this respect, the application of tandem MS experiments, sometimes in conjunction with selective derivatization of the unknown compound, can be very informative about the stmcture. The high-resolution mass spectral data are critical to the support of NMR-deduced stmctures by providing molecular formulae for unknowns. 

HPLC. The structures were determined mainly by NMR investigations but a common APCI-MS fragment (m/z 226, Figure 13.7) was observed that could readily be used in tandem MS experiments to screen extracts for structurally related HPLC eluants. 

The unique ability of FTMS to provide an exact-mass measurement for each of the ions produced in multiple-stage tandem MS (MS") assists greatly in the structure determination of degradation products [97]. Winger et al. [96] carried out LC FTMS and tandem MS experiments for the detection and identification of various degradants from drug candidates. This approach drastically reduced the time required for isolation and purification of substantial quantities of material and expedited the identification process. 

Instrumentation. HPLC isolations were performed on a Beckman 421A system using a Vydac column (C-18, 4.6 x 250 mm). Liquid secondary ion mass spectra (LSIMS) were recorded in the positive ion mode on a Kratos (Manchester, UK) MS-50S mass spectrometer equipped with a 23 kG magnet and post-acceleration detector. The LSIMS ion source has been described elsewhere (28). A Cs+ ion beam of energy 10 keV was used as the primary beam (21). Spectra were recorded (300 sec per decade) with a Gould ES-1000 electrostatic recorder. Tandem MS experiments were performed on a Kratos Concept IIHH (Manchester, UK) four sector instrument of EBEB... 

The specific site of modification was established by high performance tandem MS experiments. In this technique the 12C isotope peak for the molecular ion in the first mass spectrometer is selectively introduced into an inert gas collision cell where fragmentation is induced. The fragments are separated and detected in the second mass spectrometer (22). Collisionally induced dissociation (CID) provides abundant structural information from which the amino acid sequence for the peptide (22) or, in this case, modified peptide can be deduced. 

Table 3 Single-Stage and Tandem MS Experiments of the Quadrupole Mass Analyzer Summarizing the Qualitative and Quantitative Applications of Each Configuration... 

Beyond Analytical Characterization Tandem MS Experiments for the Examination of the Cas-phase Chemistry of Supramolecules... 

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