Precursor ion scan mode

The precursor ion scan mode scheme is scan-disso-ciate-select (o-> in Kondrat symbols, and the opposite of the product ion scan mode). This mode is used in instruments containing two or more analyzers in series, but is unavailable in ion storage devices. The first analyzer scans the ion stream over a particular mass range and ions of increasing miz value are passed to the collision cell and fragmented. The second analyzer is fixed to select product ions of a specific miz ratio. Therefore, ions passing through... 

In addition to the product-ion scan, most MS-MS instruments allow other scan modes, e.g. neutral-loss and precursor-ion scan modes. The modes are not only useful in the elucidation of the fragmentation pattern of a particular compound, but also in the screening for a series of structurally-related compounds in complex samples. In the product-ion scan mode, the first mass analyser selects a particular precursor ion, while the product ions obtained by CID of this precursor are analysed in the second mass analyser. In the precursor-ion scan mode, this process is virtually reversed the first mass analyser transmits all ions in a preset mIz window to the collision cell, while the second analyser selects only the ions of one particular miz, e.g. a particular structure informative fragment for a series of ions or compounds. An example of the use of the precursor-ion scan mode is the monitoring of phthalate plasticisers by means of the common fragment ion at miz 149 due to protonated phthalic anhydride. In the neutral-loss scan modes, both mass analysers are... 

Orbital trapping mass spectrometers achieve resolutions of up to 105 and would be the next choice after ToF mass spectrometers if resolving powers above 104 are required. In addition to mass resolution, the selectivity of an MS can be critical to distinguish between co-eluting and not mass-resolved compounds. For example, typical triple-quad mass spectrometers usually cannot achieve better than unit-mass resolution. However, special operation modes like neutral loss scans and precursor ion scans can filter out compounds of interest even if neither LC separations nor MS scans would be sufficient to resolve these compounds (note that this is a filtering step). 

What are the three most common tandem mass spectrometry (MS/MS) scan modes (product ion scan, precursor ion scan, constant neutral loss scan). 

Fig. 9. Precursor ion scan on an electrospray triple quadrupole mass spectrometer. From all the peptides present of the digested protein only those that are phosphorylated are detected in a precursor ion scan for the phosphate ion (P03, mass 79 Da) in negative ion mode. From the TPX protein three phosphorylated peptides could be detected Ml, AQLTM PSTPTVLK M2, LSETSVNTEQNSK and M3, VQPVQTTPSKDDVSNSATHVC DVK. M, Oxidized methionine C, carbamidomethylated cysteine.

Whereas TOF instruments solely allow for the detection of product ions of a selected precursor, sector instruments offer additional modes of operation i) to exclusively identify product ions of a particular precursor ion, so-called precursor ion scans, [92,93] or ii) to detect only ions formed by loss of a specific neutral mass, so-called constant neutral loss (CNL) scan. [94] This can be achieved by some technically more demanding linked scans (Table 4.2). [95-98]... 

Skutlarek D, Faerber H, lippert F, Ulbrich A, Wawrzun A, Buening-Pfaue H (2004) Determination of glucosinolate profiles in Chinese vegetables by precursor ion scan and multiple reaction monitoring scan mode (LC-MS/MS). Eur Food Res Technol 219 643-649... 

A product ion scan can obtain stmctural information of a given precursor ion while a precursor ion scan is more suited to find stmctural homologues in a complex mixture. Bosentan (Mr = 551, Fig. 1.19) has two metabolites corresponding to the tert-butyl hydroxylation product (Mr = 567) and the dealkylation of the me-thoxy group to form the phenol (Mr = 537). Bosentan (Tracker, Actelion Phrama-ceuticals) is an oral duel endothelin receptor antagonist approved for the use in arterial hypertension [56]. Selection of the fragment at m/z 280 can fish out precursor ions corresponding only to bosentan and these two metabolites (Fig. 1.19C). A similar result is obtained with the constant-neutral loss scan mode (Fig. 1.19D) which is based on neutral loss of 44 units. 

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

A Q-TOF spectrometer is similar to a triple quadrupole but Q3 is replaced by an orthogonal TOF mass spectrometer. Using a Q-TOF instrument only the product ion scan mode can be collected, but because of its high resolving power, accurate masses for both the precursor ion and product ions can be obtained. (See the section below on accurate mass measurements.)... 

Different MS MS experiments of product ion scan, precursor ion scan, and neutral loss scan modes of selected flavonoids can be carried out in order to confirm the structure of flavonoids previously detected by the full-scan mode. In the product ion scan experiments, MS MS product ions can be produced by CID of selected precursor ions in the collision cell of the triple-quadrupole mass spectrometer (Q2) and mass analyzed using the second analyzer of the instrument (Q3). However, in the precursor ion scan experiments, Q1 scans over all possible precursors of the selected ion in Q3 of the triple quadrupole. Finally, in neutral loss... 

Several scan modes are unique to the triple-quadrupole instrument, and most of these modes are superior in duty cycle versus an ion trap, Fourier transform (FT), or time-of-flight (TOF) mass spectrometers. Different elements of the triple-quadrupole perform different operations for each scan mode. These scan modes, each of which will be described in detail, are single-reaction monitoring (SRM) or multiple-reaction monitoring (MRM), precursor ion scanning (PIS), and constant-neutral-loss scanning (NLS). These scan modes and applications for structural elucidation have been described in detail (Yost and Enke, 1978, 1979). 

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