Constant neutral loss scan

These rearrangement reactions may also occur in MS-MS instruments and the constant-neutral-loss scan enables the analyst to observe all of the ions in the mass spectrum that fragment with a particular mass loss and therefore contain a specific structural feature. This knowledge can be of great value when attempting to interpret the mass spectrum of an unknown material. 

A reported method for the screening for transformation products of a number of pesticides [16] provides an elegant example of the complementary nature of the product-ion, precursor-ion and constant-neutral-loss scans (see Section 3.4.2 above). 

The TIC trace from the LC-MS analysis of an extracted river water sample, spiked with 3 p.g dm of atrazine and three of its degradation products, is shown in Figure 3.30. The presence of significant levels of background makes confirmation of the presence of any materials related to atrazine very difficult. The TIC traces from the constant-neutral-loss scan for 42 Da and the precursor-ion scan for m/z 68 are shown in Figure 3.31 and allow the signals from the target compounds to be located readily. 

Figure 3.31 TIC traces for (a) a constant-neutral-loss scan of 42 Da, and (b) a pre-cursor-ion m/z 68 scan, obtained from the LC-MS analysis of a mixture of atrazine and its degradation products. Reprinted from J. Chromatogr., A, 915, Steen, R. J. C. A., Bobeldijk, I. and Brinkman, U. A. Th., Screening for transformation products of pesticides using tandem mass spectrometric scan modes , 129-137, Copyright (2001), with permission from Elsevier Science.

Constant-neutral-loss scan An MS-MS scan in which ions containing a particular structural feature may be identified. 

Heller, D. N. Murphy, C. M. Cotter, R. I Fenselau, C. Uy, O. M. Constant neutral loss scanning for the characterization of bacterial phospholipids desorbed by fast atom bombardment. Anal. Chem. 1988,60,2787-2791. 

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

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. 

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. 

MS/MS A process in which mass (m/z) selection or analysis is typically performed in two distinct serial steps. Operational examples include selected reaction monitoring or constant neutral loss scanning (see below). 

Constant Neutral Loss Scan (MS/MS) Determination of precursor/product ion combinations that exhibit a specific, characteristic loss of a portion of a molecular ion. Particularly useful when the characteristic species (loss) is neutral and cannot be detected directly by the mass spectrometer. Analysis of glutathione conjugates via neutral loss of 129 is an example. For the purposes of this book, NLS is used to describe these types of MS/MS experiments. 

Constant Neutral Loss Scan Precursor Fragment and... 

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). 

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.

Figure 3.21. Constant-neutral-loss scan (top panels) triggered product ion spectrum (bottom panel) of haloperidol glucuronide.

Comparison of MDF with Precursor Ion Scan and Constant-Neutral-Loss Scan 238... 

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