Mass Difference Scans

Linked Scanning and Metastable Ions in Quadrupole Mass Spectrometry 

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. 

For a process, mi+— m2 + iio, in which the difference in mass (m, - mj) is a set value (Am), the quadrupoles can be set to discriminate only that difference. Ql scans the spectrum of ions from the source and Q3 scans the same masses but offset by Am. For example, with a set mass difference 

A constant-mass-difference scan. Source ions (m, f,. .., f,) are passed successively by Q1 into Q2, where coliisionaliy induced dissociation occurs. Q3 is set to pass only those ions produced in Q2 that have a predetermined mass difference (Am) between the ions passed by Ql. In this example, they are mj - f, (= Am) and fj - f,(= Am), so, although all ions pass into Q2, only f, f, have a mass difference (Am) equal to that selected for Q3. 

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 stmctures of unknown compounds, as in peptide and protein sequencing (see Chapter 40). 

Metastable and coliisionaliy induced fragment ions can be detected efficiently by a triple quadrupole instrument. By linking the scanning regions of the first and third quadrupoles, important information about molecular structure is easily obtained. 

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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. 

Note Mass accuracy is highly dependent on many parameters such as resolving power, scan rate, scanning method, signal-to-noise ratio of the peaks, peak shapes, overlap of isotopic peaks at same nominal mass, mass difference between adjacent reference peaks etc. An error of 5 mmu for routine applications is a conservative estimate and thus the experimental accurate mass should lie within this error range independent of the ionization method and the instrument used. [37] There is no reason that the correct (expected) composition has to be the composition with the smallest error. 

As shown in Figure 6.13b, Q1 and Q3 are both scanning from low to high mass but with a fixed mass difference that corresponds to the mass of the neutral molecule lost during CID. This can be especially useful when analysing the same class of compounds or for group-specific detection. 

Figure 13.4. Different scan lines in a quadrupole mass filter.

Instruments that incorporate two or three mass analysers in a series have been developed to study ion fragmentation. Several of the same type of mass analyser can constitute a tandem mass spectrometer, or they can be constructed using different mass analysers (hybrids). Hybrid spectrometers include the combination of magnetic sector followed by quadrupole, multiple quadrupole, quadrupole TOF, etc. In these instruments, a collision cell is placed between each analyser (Fig. 16.23). Tandem instruments have different scanning modes. 

Figure 3.6. Schematic of constant neutral loss scan (NLS). Q1 and Q3 scan through a defined mass range, but Q3 is set to scan a fixed mass difference below Q1. Only compounds that generate the specified neutral will hit the detector and register a signal.

Mazzarino M et al (2010) Mass spectrometric characterization of tamoxifene metabolites in human urine utilizing different scan parameters on liquid chromatography/tandem mass spectrometry. Rapid Commun Mass Spectrom 24 749-760... 

The scan mode determines the extent and quality of mass spectrometric data and has thus to be chosen with respect to the analytical requirements. Detection of unknown compounds, identification of unknown structures and confirmation of known molecules as well as quantification of distinct target analytes require different scan modes for reliable optimum analytical acuity. 

For MS work, the electron impact (El) mode with automatic gain control (AGC) was used. The electron multiplier voltage for MS/MS was 1450 V, AGC target was 10,000 counts, and filament emission current was 60 pA with the axial modulation amplitude at 4.0 V. The ion trap was held at 200°C and the transfer line at 250°C. The manifold temperature was set at 60°C and the mass spectral scan time across 50-450 m/z was 1.0 s (using 3 microscans). Nonresonant, collision-induced dissociation (CID) was used for MS/MS. The associated parameters for this method were optimized for each individual compound (Table 7.3). The method was divided into ten acquisition time segments so that different ion preparation files could be used to optimize the conditions for the TMS derivatives of the chemically distinct internal standard, phenolic acids, and DIMBOA. Standard samples of both p-coumaric and ferulic acids consisted of trans and cis isomers so that four segments were required to characterize these two acids. The first time segment was a 9 min solvent delay used to protect the electron multiplier from the solvent peak. 

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