Popular mass analyzers

The most popular mass analyzers for ICP-MS have been quadrapole, magnetic-sector, and double-focusing analyzers, although time-of-flight analyzers are also used. These analyzers vary in resolution, throughput, and scanning time. The resolution of a mass analyzer is defined as ... 

The quadrupole mass filter is the most popular mass analyzer for LC-MS because of its low cost, compactness, and ruggedness. In recent years, time-of-flight analyzers have been developed for interfacing with electrospray ionizers. 

Table 1 Characteristics of the most popular mass analyzers.

FIGURE 7.7 The two most popular mass analyzers. The magnetic sector instrument is normally configured in tandem with an electrostatic analyzer that narrows the kinetic energy spread of the ion beam. For this reason, it is called a double-focusing spectrometer. 

There are now several different types of machines that are all capable of microanalysis. All have advantages and disadvantages, but the choice of which to use is often governed by expense and availability to a particular institution. Electron probe microanalysis is by far the most popular, but here particle-induced X-ray emission (PIXE), the laser microprobe mass analyzer (LAMMA), electron energy loss spectroscopy (EELS), and secondary ion mass spectrometry (SIMS) are also considered. 

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.

In the past decade, as systems have become simpler to operate, mass spectrometry (MS) has become increasingly popular as a detector for GC. Of all detectors for GC, mass spectrometry, often termed mass selective detector (MSD) in bench-top systems, offers the most versatile combination of sensitivity and selectivity. The fundamentals of MS are discussed elsewhere in this text. Quadrupole (and ion trap, which is a variant of quadrupole) mass analyzers, with electron impact ionization are by far (over 95%) the most commonly used with GC. They offer the benefits of simplicity, small size, rapid scanning of the entire mass range and sensitivity that make an ideal detector for GC. 

Quadrupole or radio-frequency mass analyzers have only recently become available with the mass range and resolution 42) to be of general use in inorganic and organometallie mass spectrometry, although they have been popular as small mass spectrometers built into specific systems as reaction monitors. They do have the advantage of essentially linear... 

The quadrupole mass analyzer is a popular economical choice when known compounds are being analyzed and the filter can be set to a limited number of ions. 

Different types of instrumentation have been developed to introduce Hquid samples into the MS. Since Fenn has shown that molecular ions can be formed from liquids sprayed at atmospheric pressure in high electric fields, electrospray ionization (ESI) MS has gained increasing popularity for the analysis of biological samples [56]. In an electrospray inlet, the liquid sample is usually emitted as a spray from a capillary at a high potential compared to the mass analyzer into the electric field in front of the mass analyzer (Fig. 8). 

The first commercial ion trap mass spectrometers were introduced in the early 1980s. Since the introduction, ion traps have become one of the most popular types of mass analyzers due to sensitivity, MS" capabilities, relatively inexpensive price, and compact size. The details of ion trap theory are covered in a number of references (Nourse and Cooks, 1990 March, 1997, 1998, 2000b) and only the general details will be given here. 

Much of the work in the early development of the preceding techniques incorporated pulsed electron-impact ionization sources or any of several types of laser ionization techniques. In almost all of these cases the ions were created in a pulsed fashion in vacuum and formed in or sent into the acceleration region of the mass spectrometer, where a static acceleration field present there injected them into the mass spectrometer. Such ion sources use the TOF-MS very efficiently because the repetition rate of the spectrometer is limited by the frequency of the ionization event itself. This arrangement allows the TOF-MS to mass analyze of all of the ions formed completely. However, many of the most popular ionization techniques being used in inorganic analysis today are continuous in nature. 

A quadrupole is small and relatively inexpensive. It serves as an excellent collision cell for collision activations of ions and ion/molecule reactions. It can also be used as a broadband ion transmission device. A quadrupole is readily coupled with other mass analyzers for MS/MS experiments. One of the most popular configurations is a triple quadrupole mass spectrometer that has found wide applications in LC/MS and LC/MS/MS (see section on Tandem MS). 

The MALDI source has been coupled to other mass analyzers, but these combinations have gained little popularity,... 

Table 8.2 compares the main parameters of three mass analyzers. From Table 8.2 we can understand the reasons that the ToF analyzer has become so popular for static SIMS it provides high resolution, high transmission and high sensitivity. The major shortcoming of the ToF analyzer is its use of pulse primary ions. The ratio of primary beam on- to off-time is only about 10-4. Thus, it is not efficient for analysis such as depth profiling of chemical elements. 

The most popular MS instrument to perform SELDI-TOF MS analysis is the PBS-II, manufactured by Ciphergen Biosystems Inc. [16], The SELDI-TOF MS instrument is composed of three major components the ProteinChip arrays, the mass analyzer, and the data-analysis software. 

Commercial LITs were introduced in 2002 as either a stand-alone mass spectrometer (LTQ) [318] or as part of a triple quadrupole (Q-Trap) [319] or in 2005 as part of hybrid tandem mass spectrometers (LTQ-Orbitrap and LTQ-FTICR) [88,90], Application of LTQ-FTICR for metabolism studies has been reviewed by Shipkova et al. [90], In comparison to other mass analyzer types, FTICR-based mass spectrometers are not very popular for metabolite identification studies due to availability of less expensive and more user-friendly LTQ-Orbitrap and Q-TOF-based systems. Another limitation associated with the FTICR-based hybrid mass spectrometers is the TOF effect, which results in efficient trapping of only the high-mass ions [90],... 

MS has always been seen as one of the most conclusive techniques for positive identification of organic compounds. The availability, since the beginning of the 1980s, of benchtop GC-MS systems based on quadrupole mass analyzers (GC-Q-MS) made such an analytical tool extremely popular also for routine applications. However, when GC-Q-MS is operated in the full scan mode, limits of detection (LODs) are too high, especially in trace analysis, and its use is seldom restricted to a confirmation technique.When the selected ion monitoring mode (SIM) is employed, the sensitivity is dramatically enhanced. On the other hand, SIM implies the detection of specific analytes with the consequent loss of all other information. 

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