Ions, precursor

A single instrument — a hybrid of a quadrupole and a TOF analyzer — can measure a full mass spectrum of ions produced in an ion source. If these are molecular ions, their relative molecular mass is obtained. Alternatively, precursor ions can be selected for MS/MS to give a fragment-ion spectrum characteristic of the precursor ions chosen, which gives structural information about the original molecule. 

Figure 27.4 is a diagrammatic representation of a typical MS/MS experiment in which a main ion beam selected for ions (precursor ions) of mass m and having kinetic energy zeV has been directed... 

Perhaps it is worth emphasizing that the actual metastable ions are those that decompose, and the so-called metastable ions that are recorded are actually the products of decomposition, not the metastable ions themselves. It is more accurate to describe the recorded metastable ions as ions resulting from decomposition of metastable ions. The apparent m/z value of a metastable ion provides the link between an ion that fragments (the parent or precursor ion) and the ion that is formed by the fragmentation (the daughter or product ion). 

Precursor ions are selected by Ql and passed into the collision cell (Q2 orq2 of Figure 33.5). Here, collision with an inert gas (argon or helium) causes dissociation to occur, and the resulting fragment (product) ions are detected by scanning Q3 (Figure 33.6). 

A precursor ion scan. Source ions f,. . .., f, ) are all passed successively by Q1 into the collision cell, Q2, where a selected fragment (i ) is produced and detected by Q3. Only the ions (m, f,. fj) give f, fragment ions in this example. 

Scanning for metastable ions requires adjustment of electric and magnetic fields which can be adjusted individually or in conjunction with each other. If two fields are automatically adjusted at the same time, it is known as linked scanning. It is important to remember that metastable ions are not the product ions m2 but are the ions undergoing fragmentation (precursor ions mj). 

In an EW- of a B/E-linked scan using an electric/magnetic-sector instrument, a precursor ion is selected. In this case it is m, which might be a molecular ion but equally could be any fragment ion. All product ions (mj, m3, m4) from decomposition of m, in the first field-free region between the ion source and the ion collector are found, thereby giving connections mpm, m -m3, m -m4. 

In a B /E-linked scan, a product ion (fragment ion) is selected. In this case it is m4, which can be any fragment ion but not a molecular ion (connects with itself). All precursor ions (m, mj, m3), which decompose to give the product ion m4, are found, giving connections m -m4, m2-m4, m3-m4. Any one of the precursor ions could be a molecular ion. 

It sometimes happens that instead of finding which precursor ion gives which product ion (or vice versa), the object is to identify all pairs of precursor/product ions that show the loss of one particular mass. For example, it may be that a series of compounds contains some methyl esters (31 mass units). By looking for the loss of 31 mass units, viz., m, - m2 = 31, methyl esters can be distinguished from other compounds (Figure 34.5). 

In a (B/E)(l - E) -scanning mode, a mass difference is seiected. For exampie, in this case a precursor ion m, is chosen (it is shown as being made up of two parts of mass mj, n,). After fragmentation, the product ion is mj accompanied by a neutral particle of mass n,. The mass difference (n, = m, - mj) can be specified so only pairs of ions connected by this difference are found. 

Daughter ion. An electrically charged product of a reaction of a particular parent ion. In general, such ions have a direct relationship to a particular precursor ion and, indeed, may relate to a... 

A newer hybrid system available commercially is the magnetic sector—TOF hybrid (38). The precursor ions can be selected with better than unit—mass resolution by msl and the product ion ions detected at high sensitivity by the TOF ms2 (39). 

Prior to the introduction of ESI, ms /ms studies of peptides were generally limited to molecules mol wt < 3500 (33). This limitation was a consequence of the rapid drop in precursor ion intensity from Isims ion sources with increasing mass, and the inefficiency of coUisional activation. Good... 

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. 

Parent ion/precursor ion An ion that undergoes fragmentation to produce a daughter ion and a neutral product. 

Tandem mass spectrometry (MS/MS) is a method for obtaining sequence and structural information by measurement of the mass-to-charge ratios of ionized molecules before and after dissociation reactions within a mass spectrometer which consists essentially of two mass spectrometers in tandem. In the first step, precursor ions are selected for further fragmentation by energy impact and interaction with a collision gas. The generated product ions can be analyzed by a second scan step. MS/MS measurements of peptides can be performed using electrospray or matrix-assisted laser desorption/ionization in combination with triple quadruple, ion trap, quadrupole-TOF (time-of-flight), TOF-TOF or ion cyclotron resonance MS. Tandem... 

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