Precursor scan techniques

Advanced mass spectrometry enables the detection of higher-molecular-weight compounds that can be expected to retain more specific structure information contained in the original complex materials. The application of MS/MS using various scan modes will further extend the capabilities for identification of compounds in complex mixtures. Precursor scan techniques improve insight into the origin of ions in complex pyrolysates... 

Another scanning technique, the defocusing voltage scan, allows the determination of precursor ions from a given fragment. 

Figure 26. Various linked scan techniques for determination of neutral loss, precursor and daughter ions.

Selective detection of phosphopeptides in a protein digest is usually performed via the negative-ion precursor-ion scanning technique. Two alternative positive-ion precursor-ion scanning methods were also discussed in this chapter. Discuss the concepts of these methods. 

For optimal selectivity, particularly for quantitation with ion trap or triple quadrupole analyzers, MS/MS scanning techniques can be utilized in GCMS. With these techniques, instruments are operated to perform one of three basic experiments including product ion analysis, precursor ion analysis or neutral loss analysis. Ion traps are normally limited to product ion scans in which a particular ion of interest is isolated in the ion trap, subjected to collisional activation and its fragments are detected. Triple quadrupoles can additionally be operated to detect all precursors that generate a common fragment or the analyzer can be set to detect a specific neutral loss characteristic of an analyte functional group. All these methods add specificity to mass spectral detection. 

In neutral loss scan, all precursor ions, which lose a particular neutral particle (that otherwise cannot be detected in MS), are detected. Both mass analysers scan, but with a constant selected mass difference, which corresponds to the mass of the neutral particle lost. This analysis technique is particularly meaningful if molecules contain the same functional groups (e.g., metabolites as acids, glucuronides or sulfates). In this way, it is possible to identify the starting ions which are characterized by the loss of a common structural element. Both MS/MS scan techniques can be used for substance-class-specific detection in triple quadrupole systems. Ion trap systems allow the mapping of these processes by linking the scans between separate stages of MS in time. 

Fixed-product ion scans (sector instruments). High-voltage scan or linked scan at constant B /E. Both techniques give a spectrum of all precursor (parent) ions that fragment to yield a preselected product (daughter) ion. 

In this study, we report on the GaN nanorod growth by HOMVPE technique with or without using a new precursor, tris(N,N-dimethyldithiocarbamato)gallium(III) (Ga(mDTC)3). The structural and optical properties of GaN nanorods were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL). 

Tandem mass spectrometry (MS-MS) is a term which covers a number of techniques 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 used 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 number of different MS-MS experiments that can be carried out [9, 10] but the four most widely used are (i) the product-ion scan, (ii) the precursor-ion scan, (iii) the constant-neutral-loss scan, and (iv) selected decomposition monitoring. 

Thus, scanning of the electric field E scan) yields an energy spectrum which allows for the determination of the kinetic energy release (KER) from the peak width (Chap. 2.8.2). The MIKE technique provides good precursor ion resolution, but poor product ion resolution according to the influence of KER on peak shapes. 

Example Tandem mass spectrometric experiments in quadmpole ion traps are performed by combining the techniques of resonant ejection, and forward and reverse scanning to achieve an optimum in precursor ion selection, ion activation, and fragment ion scanning (Fig. 4.45). [156]... 

Precursor ion and neutral loss scan functions are unique to a triple quadrupole mass spectrometer. These are powerful techniques for targeted detection of compounds and their related impurities that produces a characteristic neutral loss or a fragment ion corresponding to a unique structural feature. Both neutral loss and... 

---