Neutral loss tandem mass spectrum

DBT, 2-DBT (2-methyldibenzo[3]thiophene), and 4,6-DBT (4,6-dimethyldibenzo[3]thiophene) in ESI tandem mass spectra have a a consistent 32 Da neutral loss, which is believed to be sulfur. Based on the tandem mass spectrum of the PASH compounds, the mechanism of fragmentation is considered to be a charge site-initiated reaction followed by a radical site-initiated fragmentation. Taking the example of DBT, Scheme 4 shows a possible mechanism. 

Other less common fragmentation pathways, e.g., resulting in internal fragments or neutral loss ions, are not discussed here. As an example we can look at the neuropeptide leucine enkephalin, which has a sequence of yggfl and a protonated monoisotopic mass of mh 556.28. The fragmentation of this ion in a collision cell through CAD might produce a tandem mass spectrum similar to the one in Eig. 3. 

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. 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.

Figure 3 Tandem electrospray mass spectrum (positive mode) of lactosylceramide, which shows the neutral loss of two hexose residues.

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.)...

In practical application scanning can be manipulated on-the-fly within a chromatographic separation to obtain maximum information. In metabolism studies or as a chemical dosimeter, the structural feature of the parent compound and its unique neutral loss occurring on collisional activation, marks the metabolite. The mass of the metabolite is then obtained from the TSP mass spectrum at Q1 and the product ion spectrum of the metabolite molecule ion is obtained by product ion scanning. Recent publications have discussed additional applications of both tandem mass spectrometry (26. 271 and thermospray tandem mass spectrometry (28) in metabolite structure elucidation. 

Figure 4.6 An example demonstrating the effects of collision energy on profiling phosphocholine-containing molecular species in the neutral-loss mode. A full ESI mass spectrum in the positive-ion mode was acquired directly from a diluted mouse hepatic lipid extract in the presence of a small amount of lithium hydroxide. Tandem MS profiling of PC and SM molecular species was performed through neutral-loss scanning of 183.1 amu (i.e., neutral loss of phosphocholine from the lithium adducts of PC species) under the aforementioned lipid solution condition with variation of collision energies (as indicated). IS denotes internal standard. All mass spectral scans are displayed after normalization to the base peak in each individual spectrum.

All four types of scan laws discussed in Section 4.2 can be implemented with a triple-quadrupole instrument. For example, to acquire a product-ion spectrum, Qi is set to transmit ions of a specified miz value into Q2, where they undergo a CID process. Q3 is scanned to mass-analyze the products formed in Q2. A precursor-ion spectrum is acquired by reversing this procedure that is, Q3 is set to transmit just the m/z value of a desired product ion, and Qi is scanned to transmit all precursors of this chosen product ion. As compared to the magnetic sector-based tandem instruments, a simple scan law is used in the triple-quadrupole instruments to monitor the loss of a neutral. The fields of Qi and Q3 are both scanned in tandem, but with an offset value related to the mass of the neutral. 

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