Tandem-MS

Mass spectrometry/mass spectrometry (MS/MS) or tandem MS allows the examination of individual ions in a mixture of ions [39]. The ions of interest serve as surrogates for neutral molecules. They are mass-selected and are dissociated upon collisions, and the resulting product ions are analyzed by a second mass measurement. Such experiments generate structurally informative fragment ions for structural characterization of complex molecules. 

In a typical MS/MS experiment, two mass analyzers (or two mass-analysis events) are utilized to measure the parent and product ions. The first mass analyzer is usually set to select the ion of interest (i.e., the parent ion), which then 

The tandem-in-time instruments are mostly ion-trapping devices, including ion trap and FT-ICR. They operate in a time sequence in the scan function to yield MS/MS data, mostly product ion spectra. No additional mass analyzer is required. In the case of an ion trap, the scan function begins with the isolation of ions of interest with ejection of all other ions from the ion trap, followed by (a) translational excitation of ions by applying a supplementary RF voltage to the trap and (b) mass analysis of the product ions using resonant ejection. 

The entire process can be repeated to multiple-stage mass analysis (MS ). Similar to an ion trap, an FT-ICR is capable of performing MS experiments in a time sequence with high sensitivity (simultaneous detection of all the ions) and super-high resolution. Both of these two instruments are excellent tools for structural characterization of complex molecules. 

A third type of MS/MS instruments is a hybrid of tandem-in-space and tandem-in-time devices, including the Q-trap (QQ-2D-linear trap) [45] and the ion trap-FT-ICR (2D-linear ion trap-FT-ICR) [46]. The Q-trap takes the configuration of triple quadrupole, with the third quadrupole replaced by a 2D-linear ion trap. The uniqueness of this design is that the 2D-linear ion trap component can be used to perform either (a) a normal quadrupole scan function in the RF/DC mode or (b) a trap scan function by applying the RF potential to the quadrupole. It is well-suited for both qualitative and quantitative studies. In the case of ion Trap-FT-ICR, it combines ion accumulation and MS features of a 2D-linear ion trap with excellent mass analysis capability (mass resolution, mass accuracy, and sensitivity) of FT-ICR. 

Liu and Hop [19] have reviewed various LC-tandem MS strategies, with or without chemical modification or derivatization, which have been successfully applied in the identification and the rational, but tentative, structural determination of drug metabolites. These techniques are equally applicable to the analysis of known plant secondary metabolites or to the de novo structural identification of new compounds. 

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Because there is little fragmentation on FD, it is necessary to activate the molecular or quasi-molecular ions if molecular structural information is needed. This can be done by any of the methods used in tandem MS as, for example, collisional activation (see Chapters 20 through 23 for more information on tandem MS and collisional activation). 

An enzyme-linked immunosorbent assay (eflsa) has been developed for the detection of residues on hands. As Httle as 50 pg of TNT can be detected (126). Liquid chromatography/thermospray negative-ion tandem ms has been successfully used to detect picogram levels of explosives in post-blast debris (127). 

FIGURE 5.24 Tandem mass spectrometry, (a) Configuration used in tandem MS. 

Qnadroni, M., et al., 1996. Analy.sis of global re.spon.ses by protein and peptide fingerprinting of protein.s i.solated by two-dimensional electrophore-.sis. Application to snlfate-starvation re.sponse of Escherichia coli. European Journal of Biochemistry 239 773-781. This paper de.scribes the n.se of tandem MS in the analysis of protein.s in cell extracts. 

Immunoaffinity extraction combined on-line with LC in conjunction with MS (108 -110) or tandem MS (111, 112) has also been demonstrated for the determination of analytes in biological fluids. Obviously, such systems offer a very high... 

The combination of an element-selective detector and MS can provide excellent information for determining the presence of an analyte or, just as important, eliminating the chance that a pesticide is present. The added selectivity gained in the use of tandem MS (MS") can also weigh heavily in confirmation of pesticide identity despite the possible lack of three ions of defined ratios. 

Chemical ionization can be used at ambient pressures. Chemical ionization was used in tandem MS, using a triple quadrupole instrument, to characterize the antipsychotic agent 2-amino-N(4-(4-(l,2-benzisothiazol-3-yl)-l-piperazinyl)butyl)benzamide from human plasma.5... 

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

Various tandem MS instrument configurations have been developed, e.g. sector instruments, such as CBCE, CBCECB or CECBCE, and hybrid instruments, e.g. BCECQQ (B = magnetic sector analyser, E = electrostatic analyser, C = collision cell, Q = quadrupole mass spectrometer), all with specific performance. Sector mass spectrometers have been reviewed [168],... 

Despite the fact that the ion trap via tandem MS offers more extensive possibilities for identification than QMS, in practice it is not often applied, because routine interpretation of the spectra is often complex and time-consuming. 

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

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

Tandem MS (DFS equipped with EI/F1/FD source) in conjunction with off-line direct inlet HPLC-UV was used for separation and quantification of isomeric antioxidants, C22H30O2S (MW 358 Scheme 6.3), as antioxidants in THF extracts of surgeons gloves [232]. Collision activation MS enabled differentiation between the three isomeric structures (Fig. 6.20). Quantification was achieved by chromatographic analysis of the isomeric species, which are not distinguishable by MS. On-line LC-MS facilitates this kind of analysis. 

However, for nontargeted analysis, where the class of compound remains unknown, a two-step analysis involving direct analysis by tandem MS followed by HPLC is an advantageous strategy. 

Using tandem MS (DFS with EI/FPFD source), electron impact and collision activation mass spectra of a THF extract of an orthopaedic polymer bandage identified IV-isopropyl-A/ -phcnyl-p-phenylenediamine (IPPD, m/z 226) as a cause for contact dermatitis [232]. Fl-MS of the extract of surgeons gloves indicated thio-bis (t-butylcresol) (m/z 358 343, after CID). 

Direct polymer compound analysis by soft ionisation, tandem MS/MS and high-resolution (AC-MS) mass spectrometry, has been reviewed [236]. 

The potential of MS/MS is well documented. Tandem MS has been reviewed [238] and various books deal with the topic [203,237,239]. 

Direct solid-state polymer/additive mass analysis has involved various ionisation modes El (Section 6.2.1), Cl (Section 6.2.2), DCI (Section 6.2.2.1), FAB (Section 6.2.4), FI (Section 6.2.5), FD (Section 6.2.6) and LD. Survey mass spectra obtained with soft ionisation methods (FI-MS, CI-MS) provide diagnostic overviews of chemical composition. The supplemental tandem (MS/MS) and atomic composition (AC-MS) techniques are used to make specific identifications of various organic ingredients. Direct analysis of polymer systems for more than a few thousand daltons has only just begun. Ionisation methods employed are FD, ESI and MALDI. Solid-probe ToF-MS (or DI-HRMS) is a breakthrough [188]. 

LC-PB-MS is especially suited to NPLC systems. RPLC-PB-MS is limited to low-MW (<500 Da) additives. For higher masses, LC-API-MS (combined with tandem MS and the development of a specific mass library) is necessary. Coupling of LC via the particle-beam interface to QMS, QITMS and magnetic-sector instruments has been reported. In spite of the compatibility of PB-MS with conventional-size LC, microbore column (i.d. 1-2 mm) LC-PB-MS has also been developed. A well-optimised PB interface can provide a detection limit in the ng range for a full scan mode, and may be improved to pg for SIM analyses. 

HPLC-APCI-MS data are found to be most useful when the chemical identity of additives in a polymer is already known (verification) otherwise tandem MS... 

Soft ionisation modes, such as API, which leave the (pseudo)molecular ion intact without much fragmentation, offer more sensitivity, and are ideal for quantitative work at low levels (e.g. breakdown products). With the use of soft ionisation techniques in LC-MS, tandem MS... 

LC-tandem MS was recently used for polymer/additive characterisation. In cases of soft ionisation processes (e.g. ESI, APCI, etc.), MS/MS is often necessary to confirm the ionic species. QITMS has the potential to improve the detection limits for organotin analysis compared to QMS. HPLC-UV and LC-API-MS/MS have been employed for the characterisation of the products of photodegradation of benzotriazole-based UV absorbers (Tinuvin P/328/900) under mild conditions [642]. Among the photoproducts identified... 

As for sample preparation, SPE-GC has become more popular than NPLC-GC. Aqueous samples are not compatible with NPLC-GC, while RPLC-GC has never become a success. SPE-GC-(tandem)MS and SPE-GC-AED systems have demonstrated excellent performance. SPME is an equilibrium technique while SPE affords exhaustive extraction of the analytes. Laser desorption injection in LD-GC-MS can uniquely provide an additional dimension of spatial information for 2D surface chemical mapping [221]. 

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