Array detection mass spectrometer

Frassanito, R., Cantonati, M., Tardio, M., Mancini, I., and Guella, G. 2005. On-line identification of secondary metabolites in fresh water microalgae and cyanobacteria by combined liquid chromatography-photodiode array detection-mass spectromet-ric techniques. J. Chromatogr. A, 1082,33-42. 

These three examples illustrate technology developments over time (dual-channel detector, diode array detector, mass spectrometer). Note that while the overall methodology is very similar (methanolic extracts, methanol-based, acidified solvents used for HPLC, detection of eluted compounds), the exact conditions for successful separation need to be defined for each system. 

There are two common occasions when rapid measurement is preferable. The first is with ionization sources using laser desorption or radionuclides. A pulse of ions is produced in a very short interval of time, often of the order of a few nanoseconds. If the mass spectrometer takes 1 sec to attempt to scan the range of ions produced, then clearly there will be no ions left by the time the scan has completed more than a few nanoseconds (ion traps excluded). If a point ion detector were to be used for this type of pulsed ionization, then after the beginning of the scan no more ions would reach the collector because there would not be any left The array collector overcomes this difficulty by detecting the ions produced all at the same instant. 

A second use of arrays arises in the detection of traces of material introduced into a mass spectrometer. For such very small quantities, it may well be that the tiny amount of substance will have disappeared by the time a scan has been has been completed by a mass spectrometer equipped... 

Other types of mass spectrometer can use point, array, or both types of ion detection. Ion trap mass spectrometers can detect ions sequentially or simultaneously and in some cases, as with ion cyclotron resonance (ICR), may not use a formal electron multiplier type of ion collector at all the ions can be detected by their different electric field frequencies in flight. 

A second use of arrays arises in the detection of trace components of material introduced into a mass spectrometer. For such very small quantities, it may well be that, by the time a scan has been carried out by a mass spectrometer with a point ion collector, the tiny amount of substance may have disappeared before the scan has been completed. An array collector overcomes this problem. Often, the problem of detecting trace amounts of a substance using a point ion collector is overcome by measuring not the whole mass spectrum but only one characteristic m/z value (single ion monitoring or single ion detection). However, unlike array detection, this single-ion detection method does not provide the whole spectrum, and an identification based on only one m/z value may well be open to misinterpretation and error. 

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector can be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a TOP analyzer, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. Multipoint detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of any substance are available. For such fleeting availability of ions, only multipoint collectors can measure a whole spectrum or part of a spectrum satisfactorily in the short time available. 

In a mass spectrometer, ions can arrive at a multipoint collector as a spatially dispersed beam. This means that all ions of different m/z values arrive simultaneously but separated in space according to each m/z value. Each element of the array, depending on its position in space, detects one particular m/z value (see Chapter 29, Array Collectors ). 

Alternatively, the ions in a mass spectrometer can also arrive at a multipoint collector as a temporally dispersed beam. Therefore, at any point in time, all ions of the same m/z value arrive simultaneously, and different m/z values arrive at other times. Ail elements of this collector detect the arrival of ions of one m/z value at any one instant of time. This type of detector, which is also an array, is called a microchannel plate collector of ions. 

After the analyzer of a mass spectrometer has dispersed a beam of ions in space or in time according to their various m/z values, they can be collected by a planar assembly of small electron multipliers. There are two types of multipoint planar collectors an array is used in the case of spatial separation, and a microchannel plate is used in the case of temporal separation. With both multipoint assemblies, all ions over a specified mass range are detected at the same time, or apparently at the same time, giving these assemblies distinct advantages over the single-point collector in the analysis of very small quantities of a substance or where ions are produced intermittently during short time intervals. 

FTIR in multiply hyphenated systems may be either off-line (with on-line collection of peaks) [666,667] or directly on-line [668,669]. Off-line techniques may be essential for minor components in a mixture, where long analysis times are required for FT-based techniques (NMR, IR), or where careful optimisation of the response is needed. In an early study a prototype configuration comprised SEC, a triple quadrupole mass spectrometer, off-line evaporative FTIR with splitting after UV detection see Scheme 7.12c [667]. Off-line IR spectroscopy (LC Transform ) provides good-quality spectra with no interferences from the mobile phase and the potential for very high sensitivity. Advanced approaches consist of an HPLC system incorporating a UV diode array, FTIR (using an ATR flow-cell to obtain on-flow IR spectra), NMR and ToF-MS. 

The most commonly used method for the identification of carotenoids is high-pressure liquid chromatography (HPFC) combined with the UV-Vis absorption detection. The introduction of diode array detection enabled parallel collection of pigment spectra, which greatly aids the quantification and localization of unknown compounds. Coupling HPFC with the mass-spectrometer significantly... 

Multiple mass analyzers exist that can perform tandem mass spectrometry. Some use a tandem-in-space configuration, such as the triple quadrupole mass analyzers illustrated (Fig.3.9). Others use a tandem-in-time configuration and include instruments such as ion-traps (ITMS) and Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS or FTMS). A triple quadrupole mass spectrometer can only perform the tandem process once for an isolated precursor ion (e.g., MS/MS), but trapping or tandem-in-time instruments can perform repetitive tandem mass spectrometry (MS ), thus adding n 1 degrees of structural characterization and elucidation. When an ion-trap is combined with HPLC and photodiode array detection, the net result is a profiling tool that is a powerful tool for both metabolite profiling and metabolite identification. 

Sr). Over the past 30 years, lead and strontium isotope ratios have been measured with thermal ionization mass spectrometry (TIMS). Elemental salts are deposited on a filament heated to produce ionized particles, which are then sent into a mass spectrometer where they are detected by multiple Faraday cups arrayed such that ions of several masses are collected simultaneously. TIMS is capable of high precision isotope discrimination, but the instruments tend to be large and expensive, and extensive sample preparation is required prior to sample introduction. Newer ICP-MS-based technologies like multi-collector ICP-MS (especially laser ablation) circumvent some of the sample preparation issues while exploiting the precision of simultaneous mass discrimination, but they are still limited by the number and configuration of ion collectors. 

Detector The detector is the last major portion of the mass spectrometer, and it detects the presence, and preferably abundance, of ions after they have exited the mass analyzer. Examples include the electron multiplier, common on quadrupole instruments, and the microchannel plate (an array of electron multipliers), which have been common on TOF instruments. For most users, the actual detector is a relatively invisible portion of the instrument that needs little or no regular attention. 

Martens-Lobenhoffer et al. [119] used chiral HPLC-atmospheric pressure photoionization tandem mass-spectrometric method for the enantio-selective quantification of omeprazole and its main metabolites in human serum. The method features solid-phase separation, normal phase chiral HPLC separation, and atmospheric pressure photoionization tandem mass spectrometry. The internal standards serve stable isotope labeled omeprazole and 5-hydroxy omeprazole. The HPLC part consists of Agilent 1100 system comprising a binary pump, an autosampler, a thermo-stated column component, and a diode array UV-VIS detector. The enantioselective chromatographic separation took place on a ReproSil Chiral-CA 5 ym 25 cm x 2 mm column, protected by a security guard system, equipped with a 4 mm x 2-mm silica filter insert. The analytes were detected by a Thermo Scientific TSQ Discovery Max triple quadrupole mass spectrometer, equipped with an APPI ion source with a... 

Micro structured wells (2 mm x 2 mm x 0.2 mm) on the catalyst quartz wafer were manufactured by sandblasting with alumina powder through steel masks [7]. Each well was filled with mg catalyst. This 16 x 16 array of micro reactors was supplied with reagents by a micro fabricated gas distribution wafer, which also acted as a pressure restriction. The products were trapped on an absorbent plate by chemical reaction, condensation or absorption. The absorbent array was removed from the reactor and sprayed with dye solution to obtain a color reaction, which was then used for the detection of active catalysts by a CCD camera. Alternatively, the analysis was also carried out with a scanning mass spectrometer. The above-described reactor configuration was used for the primary screening of the oxidative dehydrogenation of ethane to ethylene, the selective oxidation of ethane to acetic acid, and the selective ammonoxidation of propane to acrylonitrile. 

Elemental mass spectrometry has undergone a major expansion in the past 15-20 years. Many new a, elopments in sample introduction systems, ionization sources, and mass analyzers have been realized. A vast array of hybrid combinations of these has resulted from specific analytical needs such as improved detection limits, precision, accuracy, elemental coverage, ease of use, throughput, and sample size. As can be seen from most of the other chapters in this volume, however, the mass analyzers used to date have primarily been magnetic sector and quadrupole mass spectrometers. Ion trapping devices, be they quadrupole ion (Paul) [1] traps or Fourier transform ion cyclotron resonance (Penning) traps, have been used quite sparingly and most work to date has concentrated on proof of principal experiments rather that actual applications. 

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