Normal and Metastable Ions

In a normal quadrupole instrument, the field-free regions are very short. Ions formed in region 1 will be transmitted by the quadrupole as normal ions. In region 2 there is no differentiation between metaatable and normal ions. 

Formation of normal ions in an ion source. In this example, some initially formed ions (m, ) dissociate (fragment) to give smaller ions (m2 ) and a neutral particle (no)- Unchanged ions (m, ) and the fragment ions (mj ) are drawn out of the source as beams moving with velocities v, Vj, respectively. 

The kinetic or translational energy of the ions is equal to the work done on moving the charged species through the potential, V, i.e., l/2mjVi = zV and l/2m2V2 = zV, where z is the charge on the ions and V2 are their final velocities. From this, we obtain Equations 33.1 and 33,2.------------------------- 

Ions (m,+) of velocity (v,) that dissociate outside the ion source give ions (mj ) of the same velocity (v,). 

Normal ions (m, or mj ) formed in the ion source will pass (filter) through the first, second, and third quadrupoles (( , 2, 3) if these are set correctly. If Q1 is set to pass only mi ions, then normal m2+ ions cannot reach the detector, and if Q3 is set to pass only mj ions, then m, ions cannot reach the detector. Any mj ions that reach the detector must have been formed (metastable or induced by collision) by dissociation of m, ions in Q2. 

Now consider the same fragmentation process, but this time the ion (mi+) dissociates outside the ion source. Initially, as for m2, mjVi = 2zV, and V, =. When fragmentation occurs, the total 

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Some mild methods of ionization (e.g., chemical ionization. Cl fast-atom bombardment, FAB electrospray, ES) provide molecular or quasi-molecular ions with so little excess of energy that little or no fragmentation takes place. Thus, there are few, if any, normal fragment ions, and metastable ions are virtually nonexistent. Although these mild ionization techniques are ideal for yielding molecular mass information, they are almost useless for providing details of molecular structure, a decided disadvantage. 

In a sector instrument, which acts as a combined mass/velocity filter, this difference in forward velocity is used to effect a separation of normal and metastable mj" ions (see Chapter 24, Ion Optics of Magnetic/Electric-Sector Mass Spectrometers ). However, as discussed above, the velocity difference is of no consequence to the quadmpole instrument, which acts only as a mass filter, so the normal and metastable mj ions formed in the first field-free region (Figure 33.1) are not differentiated. 

It seems reasonable to suppose that some excess energy will be required to drive a reaction with a rate constant of 10 sec. or greater, but the magnitude of the excess energy required is in considerable doubt. The difference between the appearance potentials of normal ions and metastable ions is taken as an indication of the order of excess energy involved in the kinetic shift (see also Cooks et al., 1969). 

The steps (reactions) by which normal ions fragment are important pieces of information that are lacking in a normal mass spectrum. These fragmentation reactions can be deduced by observations on metastable ions to obtain important data on molecular structure, the complexities of mixtures, and the presence of trace impurities. 

By introducing a collision gas into Q2, collision-induced dissociation (CID) can be used to cause more ions to fragment (Figure 33.4). For example, with a pressure of argon in Q2, normal ions (mj ) collide with gas molecules and dissociate to give mj ions. CID increases the yield of fragments compared with natural formation of metastable ions without induced decomposition. 

The importance of linked scanning of metastable ions or of ions formed by induced decomposition is discussed in this chapter and in Chapter 34. Briefly, linked scanning provides information on which ions give which others in a normal mass spectrum. With this sort of information, it becomes possible to examine a complex mixture of substances without prior separation of its components. It is possible to look highly specifically for trace components in mixtures under circumstances in which other techniques could not succeed. Finally, it is possible to gain information on the molecular structures of unknown compounds, as in peptide and protein sequencing (see Chapter 40). 

By measuring a mass spectrum of normal ions and then finding the links between ions from the metastable ions, it becomes easier to deduce the molecular structure of the substance that was ionized originally. 

Knowledge of which normal ions in a mass spectrum fragment to which others is important and can be obtained from observations on metastable ions. 

Again, as for metastable ions, linked scanning of the first and third quadrupoles reveals important information on fragmentation processes, viz., which normal ions fragment to give which product ions. 

Once the mass spectral information has been acquired, various software programs can be employed to print out a complete or partial spectrum, a raw or normalized spectrum, a total ion current (TIC) chromatogram, a mass chromatogram, accurate mass data, and metastable or MS/MS spectra. 

In magnetic-sector instruments, metastable ions are normally observed as small broad peaks. However, in GC/MS the analyst looks only at centrioded (processed) data thus, metastable peaks are not obvious and generally appear as part of the background. Metastable ions, when observed, can be used to link specific product and precursor ions. 

Note This explains the occurence of diffuse peaks due to metastable ion dissociations at fractional m/z values in the B scan spectra of B and EB instruments (Chap. 2.7.1). [83,84] In turn, the mass spectra obtained from BE instruments do not show any metastable ion peaks in normal operation. 

Normal metastable ions are formed between the source and magnet of single-focusing sector instruments. In double-focusing... 

Results of a low-resolution MIKES study of CeF6Si(CHs)j 280) are shown in Fig. 7, for transitions from the parent ion, the base peak, CsFsSifCHsla, and from the rearrangement ion (CH3)2SiF+. Only the more intense peaks are also observed as normal metastable ions (m = 69.4 for the transition C6F5Si(CH3)2+ -> C0F2CH3+ + CH3SiF3). 

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