Tandem experiments

A significant recent experimental advance is the introduction of tandem mass spectrometers for studying ion-molecule reactions. Examining various isotope effects as a function of translational energy can provide detailed information about reaction mechanisms. Tandem experiments can also observe many of the possible reaction channels for a given collision complex. Such information provides valuable clues to the chemical and physical nature of the intermediates in ion-neutral interactions. 

Obviously, experiments designed to measure cross-sections as a function of energy are needed. At present, tandem experiments are not capable of high precision at low energies because one must assume details of collision mechanics and because it is difficult to estimate collection efficiencies in forward scattering geometry (15). The extension of all known techniques to lower energy (64, 65) and the further development of pulse methods (58) offer the possibility for advances in this area. 

Fragmentation of ions in tandem experiments requires an input of energy to break internal covalent bonds. This is most commonly achieved by converting the kinetic energy of a collision, between the selected ion and an... 

The results of the obvious tandem experiment to study the reaction of C2H2 with C2H2 are reported in Table IV. The ions C4H2 and 4113 together account for some 98% of the total products. Results are reported at two collision chamber pressures, 5 and 50 //m. The experiments at 50 ]um are significant in that the ratio of mass 76 to 77 is so dramatically different from that of 50 to 51, indicating that the simple sequential analysis... 

The primary ions present in vinyl chloride in substantial abundance are C2H2, C2H3 , and the molecular ion C2H3C1. The reaction of the acetylene ion as determined by a straightforward tandem experiment is merely charge transfer, while the vinyl ion undergoes two competing... 

Considerable information is available on electronic states of NO and 2 and their lifetimes. Because tandem experiments have an intrinsic time delay of tens of microseconds between ion formation and reaction, excited singlet states will undergo radiative decay to the ground state, either directly or via cascade processes, before the reactant ions reach the... 

Tandem mass spectrometry or ms/ms was first introduced in the 1970s and gained rapid acceptance in the analytical community. The technique has been used for stmcture elucidation of unknowns (26) and has the abiUty to provide sensitive and selective analysis of complex mixtures with minimal sample clean-up (27). Developments in the mid-1980s advancing the popularity of ms/ms included the availabiUty of powerhil data systems capable of controlling the ms/ms experiment and the viabiUty of soft ionisation techniques which essentially yield only molecular ion species. 

Meyers has demonstrated that chiral oxazolines derived from valine or rert-leucine are also effective auxiliaries for asymmetric additions to naphthalene. These chiral oxazolines (39 and 40) are more readily available than the methoxymethyl substituted compounds (3) described above but provide comparable yields and stereoselectivities in the tandem alkylation reactions. For example, addition of -butyllithium to naphthyl oxazoline 39 followed by treatment of the resulting anion with iodomethane afforded 41 in 99% yield as a 99 1 mixture of diastereomers. The identical transformation of valine derived substrate 40 led to a 97% yield of 42 with 94% de. As described above, sequential treatment of the oxazoline products 41 and 42 with MeOTf, NaBKi and aqueous oxalic acid afforded aldehydes 43 in > 98% ee and 90% ee, respectively. These experiments demonstrate that a chelating (methoxymethyl) group is not necessary for reactions to proceed with high asymmetric induction. 

Tandem quadrupole and magnetic-sector mass spectrometers as well as FT-ICR and ion trap instruments have been employed in MS/MS experiments involving precursor/product/neutral relationships. Fragmentation can be the result of a metastable decomposition or collision-induced dissociation (CID). The purpose of this type of instrumentation is to identify, qualitatively or quantitatively, specific compounds contained in complex mixtures. This method provides high sensitivity and high specificity. The instrumentation commonly applied in GC/MS is discussed under the MS/MS Instrumentation heading, which appears earlier in this chapter. 

Tandem mass spectrometry (MS-MS) is a term which covers a number of techniqnes in which one stage of mass spectrometry, not necessarily the first, is used to isolate an ion of interest and a second stage is then nsed to probe the relationship of this ion with others from which it may have been generated or which it may generate on decomposition. The two stages of mass spectrometry are related in specific ways in order to provide the desired analytical information. There are a large nnmber of different MS-MS experiments that can be carried ont [9, 10] bnt the fonr most widely nsed are (i) the prodnct-ion scan, (ii) the precnrsor-ion scan, (iii) the constant-nentral-loss scan, and (iv) selected decomposition monitoring. 

LSIMS is a more suitable ionisation technique than FD for analysis of mixtures by means of tandem mass spectrometry, because of the higher ion currents generated from polymer additives using LSIMS. LSIMS/MS experiments may be used in conjunction with FD-MS as a screen to determine class, molecular weight and structure of mixtures of organic polymer additives. 

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

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