Whole molecule mass spectrometry

2 Mass Spectrometry and Isotope Ratio Mass Spectrometry 7.2.1 Whole Molecule Mass Spectrometry 

The most widely used method for ionization is electron impact (El). In an El source the sample is placed in the path of an electron beam. Although many newer kinds of ion sources have been developed, El is the method commonly used in classical isotope-ratio mass spectrometers (IRMS), i.e. mass spectrometers designed for precise isotopic analysis. In this type of spectrometer the ions, once formed, are electrostatically accelerated, and then ejected through a slit into a magnetic field held perpendicular to the ion trajectory. In the magnetic sector part of the instrument the particles are deflected in an arc described by  

B is the magnetic field intensity, r is the radius of the ion path, and V is the accelerating potential. In analytical mass spectrometers either B or V is varied systematically so that ions of different m/z are sequentially focused on the collector and the spectrum recorded. Such whole molecule mass spectrometers have been employed occasionally for isotope ratio measurements but their use is restricted to samples 

Apart from the need for isotopic enrichment and synthesis there are other problems in applying whole molecule mass spectrometry to measure isotope ratios. Assume, for example, that we want to determine isotopic composition of chlorine from the spectrum of chlorobenzene presented in Fig. 7.7. The peaks at 114 and 

Element Isotope Natural abundance (%) or half-life Standard isotope ratio or energy 

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A second problem in whole molecule mass spectrometry is that fluctuations in ion current may introduce substantial errors. Recall that ions of different m/z are not measured simultaneously in whole molecule mass spectrometry. If the ion current is not stable (and it commonly fluctuates in El sources), then after first peak (say m/z = 112 in our example) is measured, and instrumental parameters are changed in order to focus the next peak (m/z = 114) on the collector, the ion current of this second peak may no longer correspond to that existing at the time the first peak was measured. One can try to switch the detector from peak to peak more rapidly but that shortens the collection time for each peak, fewer ions will be counted, and errors in counting statistics will increase. Normally this problem is dealt with by statistical... 

To avoid the kind of problems which trouble whole-molecule mass spectrometry it is better to use instrumentation especially designed for high precision measurements of isotope ratios isotope-ratio mass spectrometry (IRMS). 

In this chapter, we present details of how the compilation of ion-manipulation methodologies in QIT mass spectrometers can eliminate many of the restrictions that limit currently large molecule mass spectrometry. Specifically, we highlight three MS" interrogation schanes that show genuine promise to enable whole protein analysis (i) ETD coupled with CDD (ii) CID and ETD coupled with Proton Transfer and (iii) lA coupled with CID. 

The reaction was second order in acid and first order in substrate, so both rearrangements and the disproportionation reaction proceed via the doubly-protonated hydrazobenzene intermediate formed in a rapid pre-equilibrium step. The nitrogen and carbon-13 kinetic isotope effects were measured to learn whether the slow step of each reaction was concerted or stepwise. The nitrogen and carbon-13 kinetic isotope effects were measured using whole-molecule isotope ratio mass spectrometry of the trifluoroacetyl derivatives of the amine products and by isotope ratio mass spectrometry on the nitrogen and carbon dioxide gases produced from the products. The carbon-12/carbon-14 isotope... 

The nitrogen and carbon-13 kinetic isotope effects found using the N- N, die 1,1/-13C2 and die 4,4/-13C2 substrates were measured by whole-molecule isotope ratio mass spectrometry on die bis-(triduoroacetyl) derivative. 

Mass spectrometers use the difference in mass-to-charge ratio (m/z) of ionized atoms, molecular fragments, or whole molecules to differentiate between them. Mass spectrometry is therefore useful for quantitation of atoms or molecules and also for determining chemical and structural information about them [329, 531-533]. Molecules have distinctive fragmentation patterns which provide information to identify structural components. The general operation of a mass spectrometer is to (1) create gas-phase ions, (2) separate the ions in space or time based on their mass-to-charge ratio, and (3) measure the quantity of ions of each mass-to-charge ratio. The ion separation power of a mass spectrometer is described by the resolution, which is defined as ... 

Mass spectrometry is not a true spectroscopic technique that involves absorption of energy at particular frequencies. Rather one excites the molecule as a whole... 

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