Neutral-loss scan

The most common modes of operation for ms/ms systems include daughter scan, parent ion scan, neutral loss scan, and selected reaction monitoring. The mode chosen depends on the information required. Stmctural identification is generally obtained using daughter or parent ion scan. The mass analyzers commonly used in tandem systems include quadmpole, magnetic-sector, electric-sector, time-of-flight, and ion cyclotron resonance. Some instmments add a third analyzer such as the triple quadmpole ms (27). 

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.

Figure 5.27 Selective detection of lactolated peptides from a tryptic digest of / -lacto-globulins by LC-electrospray-MS-MS, showing (a) the total-ion-cnrrent trace in full-scan mode, and (b) the total-ion-current trace in neutral-loss-scanning mode. Figure from Selective detection of lactolated peptides in hydrolysates by liquid chromatography/ electrospray tandem mass spectrometry , by Molle, D., Morgan, F., BouhaUab, S. and Leonil, J., in Analytical Biochemistry, Volume 259, 152-161, Copyright 1998, Elsevier Science (USA), reproduced with permission from the publisher.

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

Neutral-loss Scanning m/z = x Scanning m/z = x — a MSI and MS2 are scanning at a fixed m/z difference to monitor compounds that lose a common neutral species (screening)... 

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. 

S. Kazuno, M. Yanagida, N. Shindo and K. Murayama, Mass spectrometric identification and quantification of glycosyl flavonoids, including dihydrochalcones with neutral loss scan mode, Anal. Biochem., 347, 182 192 (2005). 

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). 

Phosphorylation NL = neutral loss scanning. R-OHR-0-P03H +79 Precursor m/z 63/-Precursor m/z 79/-... 

Figure 14.1. Tandem mass spectra of amino acids. Top left panel shows the product ion spectra of the buylester of phenylalanine. Fragmentation that explains the neutral loss of 102 Da is shown in the top left panel of the figure. The bottom panel is a neutral loss scan from m/z 125 to 270 which includes many common amino acids. The profile is obtained by an analysis of a blood spot from a patient with PKU.

In addition to neutral loss scans, mass spectrometers can be used to detect other compounds in a different manner. Acylcamitines are fatty acid esters of carnitine. The masses of acylcamitines differ by the size of the fatty acid attached to it. The tandem mass spectrometer can detect these selectively as well because they all produce a similar product, in this case an ion rather than a molecule. Because it is an ion, it can be detected by the second mass separation device. The ion has a mass of 85 Da and is common to all acylcamitines. Performing a precursor ion scan of 85 Da (essentially a scan of only molecules that produce the 85 ion) reveals a selective analysis of acylcar-nitines, as shown in Fig. 14.2. Additional scans have been added to more selectively detect basic amino acids, free carnitine, short chain acylcamitines and a hormone, thyroxin (T4) which has amino acid components. 

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

MS-MS is a general analytical method for the analysis of unknown compounds. This analytical technique must be applied if soft ionisation techniques are performed and fragment information for identification is needed. With the different types of data obtained by MS-MS application, e.g. product ion scan, parent ion scan, neutral loss scan,... 

These results obtained from the analyses of industrial blends proved that the identification of the constituents of the different surfactant blends in the FIA-MS and MS-MS mode can be performed successfully in a time-saving manner only using the product ion scan carried out in mixture analysis mode. The applicability of positive ionisation either using FIA-MS for screening and MS-MS for the identification of these surfactants was evaluated after the blends examined before were mixed resulting in a complex surfactant mixture (cf. Fig. 2.5.7(a)). Identification of selected mixture constituents known to belong to the different blends used for mixture composition was performed by applying the whole spectrum of analytical techniques provided by MS-MS such as product ion, parent ion and/or neutral loss scans. 

From quantification performed in the FIA mode, applying mixture analysis by FIA-MS-MS, reliable results can be achieved if product-, parent- or neutral loss scans (MRM mode), each of them of quite outstanding specificity, are recorded. Compared with FIA—MS, the time for recording has to be expanded marginally compared with column separation techniques. 

The standard addition procedure is another method for recognising and overcoming potential matrix effects in quantification. Both alternatives, FIA—MS or FIA—MS—MS, can be performed using this procedure. Despite the increased expenditure because of a multiplication in analyses, the FIA approach combined with standard addition remains the faster technique even with the application of specific analytical MS—MS techniques such as product-, parent- or neutral loss scans applying selected reaction monitoring (SRM). The greatest drawback of this technique is that the compounds to be quantified must... 

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