Tandem MS with Magnetic Sector Instruments

There are multiple ways of detecting metastable and collision-induced dissociations with magnetic sector instmments [12]. In fact, the whole phenomenon of so-called diffuse peaks in mass spectra was discovered with this particular type of mass analyzer (Chap. 4.3.2). In the mid-1940s, these broad signals were correctly interpreted as ions decomposing in transit [54,55]. Metastable ion spectra still represent one of the indispensable tools for studying the mechanism and thermochemistry of ion dissociations [1,2,13,14]. 

Due to conservation of velocity, i.e., vi = V2 = v, the momentum of a fragment ion m2 formed in a FFR preceding the magnetic sector is different from that of such a fragment ion arising from the ion source. The ion formed by metastable ion dissociation thus passes the magnet as if it had the virtual mass m  

This explains diffuse peaks at fractional m/z values in the B scan spectra of B and EB instruments as a result of metastable ion dissoedations [54,55]. In turn, the mass spectra obtained from BE instruments do not show any metastable ion peaks in normal operation. 

Example The metastable decay of the o-nitrophenol molecular ion, m/z 139, by loss of NO to yield the [M-NO] ion, m/z 109, has been studied on a single-focusing magnetic sector instrument (Fig. 9.13) [56]. The mass spectrum shows a flat-topped peak of low intensity expanding over three mass units. Some minor and narrow regular peaks corresponding to fragment ions formed within the ion 

There are multiple ways of detecting metastable and collision-induced dissociations with magnetic sector instruments. [82] In fact, the phenomenon was discovered with this particular type of mass analyzer (Chap. 2.7.1). [83,84] 

Dissociation in a field-free region (FFR) not only causes partitioning of ion kinetic energy (Eq. 4.10), but also goes with partitioning of momentum p 

Mass-analyzed ion kinetic energy spectra (MIKES) [85,86] can be measured on BE geometry instruments only. The precursor ion is selected by the magnet and the fragments from dissociations of in the 2.FFR are analyzed by the ESA due to their kinetic energy. This is possible, because the kinetic energy of the precursor is distributed among the product ion and the neutral. Derived from Eq. 4.10 we have 

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Tandem MS with magnetic sector instruments example ifE = constant... 

LC-PB-MS is especially suited to NPLC systems. RPLC-PB-MS is limited to low-MW (<500 Da) additives. For higher masses, LC-API-MS (combined with tandem MS and the development of a specific mass library) is necessary. Coupling of LC via the particle-beam interface to QMS, QITMS and magnetic-sector instruments has been reported. In spite of the compatibility of PB-MS with conventional-size LC, microbore column (i.d. 1-2 mm) LC-PB-MS has also been developed. A well-optimised PB interface can provide a detection limit in the ng range for a full scan mode, and may be improved to pg for SIM analyses. 

The most common types of MS/MS instruments available to researchers in food chemistry include triple quadrupole mass spectrometers and ion traps. Less common but commercially produced tandem mass spectrometers include magnetic sector instruments, Fourier transform ion cyclotron resonance (FTICR) mass spectrometers, and quadrupole time-of-flight (QTOF) hybrid instruments (Table A.3A.1). Beginning in 2001, TOF-TOF tandem mass spectrometers became available from instrument manufacturers. These instruments have the potential to deliver high-resolution tandem mass spectra with high speed and should be compatible with the chip-based chromatography systems now under development. 

The introduction of the open-tubular columns eliminated the need for concentrating devices as the mass spectrometer pumping system could cope with the entire column eluent. Consequently, the column eluent could be passed directly into the mass spectrometer and the total sample can enter the ionization source. The first mass spectrometer used in a GC-MS tandem system was a rapid-scanning magnetic sector instrument that easily provided a resolution of one mass unit. Contemporary mass spectrometers have vastly improved resolution and the most advanced system (involving the triple quadrupole mass spectrometer) gives high in-line sensitivity, selectivity, and resolution. 

Various tandem MS instrument configurations have been developed, e.g. sector instruments, such as CBCE, CBCECB or CECBCE, and hybrid instruments, e.g. BCECQQ (B = magnetic sector analyser, E = electrostatic analyser, C = collision cell, Q = quadrupole mass spectrometer), all with specific performance. Sector mass spectrometers have been reviewed [168],... 

This basically means that two instruments have been linked together. The first analyser can replace the traditional chromatographic separation step and is used to produce ions of chosen m/z values. Each of the selected ions is then fragmented by collision with a gas, and mass analysis of these product ions effected in the second analyser. The resulting mass spectrum is used for their identification. The potential combinations of the various magnetic sector and quadrupole instruments to form such coupled systems is considerable. Ion traps may also be operated in a tandem MS mode. 

ToF mass spectrometers as dynamic instruments gained popularity with the introduction of matrix assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) as effective pulsed ion sources for the soft ionization of large biomolecules (up to 10s dalton) due to their high ion transmission.38 ToF mass spectrometers, quadrupole analyzers and/or magnetic sector fields can be combined in tandem mass spectrometers (MS/MS) for the analysis of organic compounds. 

Instrumentation. HPLC isolations were performed on a Beckman 421A system using a Vydac column (C-18, 4.6 x 250 mm). Liquid secondary ion mass spectra (LSIMS) were recorded in the positive ion mode on a Kratos (Manchester, UK) MS-50S mass spectrometer equipped with a 23 kG magnet and post-acceleration detector. The LSIMS ion source has been described elsewhere (28). A Cs+ ion beam of energy 10 keV was used as the primary beam (21). Spectra were recorded (300 sec per decade) with a Gould ES-1000 electrostatic recorder. Tandem MS experiments were performed on a Kratos Concept IIHH (Manchester, UK) four sector instrument of EBEB... 

The ESI and APCI are both soft ionization techniques, hence the spectra obtained using these methods are usually very simple and consist predominantly of the pseudomolecular ion. When more information is required, tandem mass spectrometry is a popular option. Several different types of tandem LC/MS systems can be found, with triple quadrupole and ion trap being the most popular choices. Hybrid systems including magnetic sector-quadmpole, magnetic sector-TOF, quadrupole-TOF (Q-TOF) and ion trap-TOF have been described. The most successful among these hybrids is the Q-TOF instrument. 

Magnetic analyzers are often used in various combinations with electrostatic sectors. Sector instruments are characterized by good ion transmission. Depending on the order of these analyzers, very high resolution and wide mass ranges can be achieved. Sector instruments suffer from a relatively low scan rate, which makes their application in GC-MS and LC-MS studies somewhat limited. At the same time, some types of tandem mass spectrometry experiments with mass-selected ions can be performed only on this type of equipment. 

The first hyphenated approach to be considered is the on-line combination of MS and MS, i.e. tandem mass spectrometry (MS-MS). A variety of combinations of different mass analysers have been described, including quadrupole and magnetic-sector analysers as MS], and quadrupole, magnetic-sector, ion-trap and time-of-flight analysers as MS2. Instruments like triple-quadrupoles are widely used for MS-MS, either as stand-alone systems with sample introduction via a solids insertion probe or flow-injection analysis, or in on-line combination with GC or LC. The work of Yost and co-workers and of Hunt and colleagues exemplify these methods. 

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