Neutral loss filter

All the previously described, unique triple-quadmpole scans—SRM/MRM, precursor ion, and neutral loss—filter out a significant amount of background noise, allowing for detection of species present at very low levels. 

FIGURE 15.5 Work flow of metabolic soft spot determination using EMS-EPI or MIM-EPI EIC, extracted ion chromatography PIF, product ion filter NLF, neutral loss filter. 

Product ion filter (PIF) and neutral loss filter (NLF) [174,175] Predicted fragmentation Sensitive detection of unexpected metabolites Not suited for metabolites that do not generate significant predictable fragmentation... 

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). 

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.

Zhu, M., Zhang, H., Yao, M., Zhang, D., Ray, K., and Skiles, G. L. (2004). Detection of metabolites in plasma and urine using a high resolution LC/MS-based mass defect filter approach Comparison with precursor ion and neutral loss scan analyses. Drug Metab. Rev. 36 (Suppl. 1) 43. 

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.)...

Examples of neutral loss SRM are shown in Figure 3. As described above the aryl sulfates fragment almost exclusively by neutral loss of S03. Thus, simultaneous control of the Q, and Q3 mass filters to pass ions m/z X and m/z (X - 80), respectively, allows sulfate conjugate- selective detection. Accordingly, phenol sulfate (m/z 173 > m/z 93), 4-nitrophenol sulfate (m/z 218 > m/z 138), and naphthol sulfate (m/z 223 --> m/z 143) are monitored (Figure 3). At this time we have not applied this technique to sufficient numbers of biological samples, and cannot yet refer to the technique as sulfate conjugate specific . 

A triple-quadrupole linear ion trap (QqLIT), which is the most widely used hybrid linear ion trap, is based on the ion path of a triple-quadrupole mass spectrometer with Q3 operated as either a conventional RF/DC quadrupole mass filter or a linear ion trap mass spectrometer. " A QqLIT combines the advantages of a QqQ and a QIT within the same platform without compromising the performance of either mass spectrometer. It retains classical QqQ functions such as MRM, product ion scan, precursor ion scan, and constant neutral loss scan for quantitative and qualitative analysis, and possesses MS" ion accumulation... 

Several MS acquisition and data processing strategies are used for detection and structure elucidation of metabolites. The common metabolite detection strategies are summarized in Section 9.2.1, which include full MS scan, constant neutral loss, parent ion scan, multiple reactions monitoring, and mass defect filtering. The structure elucidation strategies feature product ion scan, multistage scan, and accurate mass measurement, which are reviewed in Section 9.2.2. 

FIGURE 11.4 Metabolite profiles of clozapine in rat liver microsome incubation in the presence of glutathione, (a) UV profile of clozapine that displays four drug-related peaks, P (parent drug), P-CH2 (demethylated metabolite), P-hO (N-oxide metabolite) and P+GSH-2H (a GSH adduct), (h) TIC of full-scan MS analysis by LTQ-Obitrap mass spectrometry, (c) MDF processed profiling using the parent drug as a filter template, (d) TIC of data-dependent MS/MS scan, (e) Processed MS/MS data with neutral loss of 129 Da. 

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