Tandem constant-neutral-loss scan

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.

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. 

Another technological development in tandem mass spectrometry is the combination of two TOP mass spectrometers, TOF/TOK These instruments have excellent sensitivity and throughput for MALDI-MS/MS and are especially suited for proteomics research. However, these instruments are unable to perform true precursor ion scans or constant neutral loss scans. The Bruker Daltonics ultra-flex and Applied Biosystem 4700 proteomics analyzer are examples of this type of instrument, Such mstruments have yet to make an impact in clinical chemistry. 

So-called hybrid mass spectrometers include a combination of two different types of mass spectrometers in a tandem arrangement. The combination of a magnetic sector mass spectrometer with a quadrupole mass spectrometer was an early instrument of this type. More popular is the combination of a quadrupole for MSI and a TOF for MS2, As with TOF/TOF, these instruments are presently used mainly for proteomics research but could eventually find applications in the clinical lab. These mstruments are unable to perform true precursor ion scans or constant neutral loss scans. Commercial examples of this type of instrument include the qTOF by Waters Micromass and the QSTAR by Apphed Biosystems/MDS Sciex. 

MS, especially in combination with advanced separation techniques, is one of the most powerful and versatile techniques for the structural analysis of bacterial glycomes. Modern mass spectral ionization techniques such as electrospray (ESI) and matrix-assisted laser desorption/ionization (MALDI) provide detection limits in the high atto- to low femto-mole range for the identification of peptides and complex carbohydrates. Structural characterization of these trace level components can be achieved using tandem MS. This provides a number of specific scanning functions such as product, precursor ion, and constant neutral loss scanning to... 

In addition to MRM, the other scan modes available on a QqQ have occasionally been used for residue analysis as well. A precursor ion scan can be used to identify precursor ions from a product ion, and therefore to identify analytes and metabolites or impurities, which generate the same product ion, in complex matrices. For example, erythromycin B was identified in yogurt using this function. In this application, Q3 was held constant to measure a fragment ion at m/z 158, which is a typical product ion of compounds or impurities related to erythromycin A with a desosamine residue. Q1 was then scanned over an appropriate range, from which a precursor ion at m/z 718 was detected. The latter was identified as erythromycin B, which was an impurity in the erythromycin fermentation product. Constant neutral loss scan, which has rare applications for antibiotic analysis, records spectra that show all the precursor ions that have fragmented by the loss of a specific neutral mass. In this instance, both Q1 and Q3 scan together with a constant mass offset between the two quadrupoles. Both precursor ion and constant neutral loss scans can be performed only with ion beam tandem in-space mass spectrometers. 

Another tandem screening method is known as constant neutral loss scanning. Here, both mass spectrometers are scanned simultaneously but are offset corresponding to the difference between precursor and product ion masses. A signal only appears when a precursor ion yields a product ion with the mass difference selected. This technique can be used to screen for compounds that contain a specific structural feature that yields a common fragmentation process. 

Note In contrast to tandem-in-space instruments, tandem-in-time instruments neither support precursor ion nor constant neutral loss scanning. While product ion scans just need a precursor to be isolated prior to its fragmentation, both precursor ion and constant neutral loss scan rely on the simultaneous application of selection plus scanning or double scanning, respectively. Fulfilling two criteria at the same time requires two distinct analyzers at work. 

Fig. 11.16. Representation of three tandem mass spectrometry (MS/MS) scan modes illustrated for a triple quadrupole instrument configuration. The top panel shows the attributes of the popular and prevalent product ion CID experiment. The first mass filter is held at a constant m/z value transmitting only ions of a single mlz value into the collision region. Conversion of a portion of translational energy into internal energy in the collision event results in excitation of the mass-selected ions, followed by unimolecular dissociation. The spectrum of product ions is recorded by scanning the second mass filter (commonly referred to as Q3 ). The center panel illustrates the precursor ion CID experiment. Ions of all mlz values are transmitted sequentially into the collision region as the first analyzer (Ql) is scanned. Only dissociation processes that generate product ions of a specific mlz ratio are transmitted by Q3 to the detector. The lower panel shows the constant neutral loss CID experiment. Both mass analyzers are scanned simultaneously, at the same rate, and at a constant mlz offset. The mlz offset is selected on the basis of known neutral elimination products (e.g., H20, NH3, CH3COOH, etc.) that may be particularly diagnostic of one or more compound classes that may be present in a sample mixture. The utility of the two compound class-specific scans (precursor ion and neutral loss) is illustrated in Fig. 11.17.

Fig. 11.17. Simulated mass chromatograms resulting from precursor ion and constant neutral loss tandem mass spectra (middle and bottom traces), illustrating the selectivity that those MS/MS scan modes can bring to chromatographic analyses. The top trace in the figure represents a total ion chromatogram obtained using a conventional single stage of mass analysis.

Ion traps are tandem-in-time instruments, i.e., they perform the steps of precursor ion selection, ion activation and acquisition of fragment ion spectra in the very same place. This advantageous property allows the multiple use of a single QTT to perform not only MS but also MS and higher order MS experiments - indeed a very economic concept. Depending on the abundance of the initial precursor ion, its fragmentation behavior - and of course, on the performance of the QIT - MS experiments are possible. [138] However, in contrast to tandem-in-space instruments, tandem-in-time instruments do not support constant neutral loss and precursor ion scans. 

Figure 6 Acquisition modes in tandem MS with two mass anaiyzers. Muitipie arrows indicate a fuii scan, a singie arrow a singie ion scan (mass fiiter fixed on a singie m/z). in the neutral loss scan, a full scan with a constant mass difference is performed with both anaiyzers, selecting only ions which show a neutral loss. (Graphic adapted from Ref. 23.)...

A scan in a tandem mass spectrometer with two or more mtz analysers e.g a triple quadrupole or a sector mass spectrometer that incorporates at least one magnetic sector and one electric sector. Two or more of the analysers are scanned simultaneously so as to preserve a predetermined relationship between scan parameters to produce a product ion, precursor ion or constant neutral loss or gain spectrum. 

Fig. 8 Schematic of a tandem quadrupole MS/MS instrument. A tandem quadrupole MS/MS instrument consists of two quad-rupole MS filters, MSI and MS2, separated by a collision cell. Each quadrupole MS filter consists of four cylindrical or hyperbolic shaped rods. A unique combination of direct current (dc) potential and radiofrequency (rf) potential is applied to each pair of rods (one pair 180° out of phase with the other). A mass spectrum results by varying the voltages at a constant rf/dc ratio. A variety of scan modes (e.g., full scan, product ion, precursor ion, neutral loss) provide unique capabilities for quantitative and qualitative structure analysis. (Courtesy of Micromass, Manchester, UK.)...

A scan procedure for a tandem mass spectrometer designed to monitor a selected neutral loss mass difference from precursor ions by detection of the corresponding product ions produced by metastable ion fragmentation or collision-induced dissociation. Synonymous terms are constant neutral mass loss scan and fixed neutral fragment... 

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