Characteristics of Mass Spectrometers

The duty cycle is an essential temporal characteristic of mass spectrometers [1]. It is defined as the ratio of the time spent on the collection of ions for analysis to the operation time of the ion source within a complete scan. It is typically expressed as a percentage. This parameter... 

Principles and Characteristics A mass spectrometer consists of various components which are necessary for the formation of ions from molecules, and for their separation and detection (Fig. 6.1). Miniaturisation of MS represents a strategic technology. 

In addition, several approaches to ion activation that are not well-suited to conventional types of mass spectrometers, are very well-suited for use with FTMS. These include photodissociation and ion-electron collisions [46-49]. In this paper, we would like to present some applications of ion-electron collisions to MS/MS, and show that the method may be a suitable alternative to collisional activation for some applications. We will also discuss some of the desirable characteristics of the dual cell geometry, as applied to MS/MS experiments. 

Besides the triple quadrupol instruments, other types of mass spectrometers might be used as well. Examples for these types of instruments are ion traps, time of flight mass spectrometers and also single quadrupol mass analyzers. Due to the characteristic and specific advantages and disadvantages of different instrument types, the overall assay performance (e.g. sensitivity, dynamic range and selectivity) may vary quite a bit from one instrument type to the other. 

The investigations by Stuckey and Kiser employed a rather unique type of mass spectrometer which, as will be seen, is important in the production of the doubly-charged ne tive ions. We shall review briefly the experimental approaches, observations, and initial interpretations of the results. To do so, an examination of the design and characteristics of the omegatron is germane. 

The general description mass spectrometer covers a variety of instruments that vary in size, cost, versatility, performance and operational complexity, but all find specific applications appropriate to their design. The common use of mass spectrometers as chromatographic detectors has resulted in a significant market for low cost instruments with modest performance characteristics that are easy to operate, robust, and require little bench space. Instruments in this category will be the focus of the following sections. 

A characteristic feature of mass spectrometers, which is not shared by optical instruments, is the requirement of an elaborate vticuum system to create low pressures (10 to 10 torr) in all of the instrument components except the signal processor and readout. The need for a high vacuum arises because such conditions lead to infrequent collisions with atmospheric components and allow the production and manipulation of free electrons and ions. 

One more feature is its potential for measuring stereo-, positional and conformational isomers as well as isotopomers in gas-phase species, as was illustrated in Chapter 6. The combination of ion-trap mass spectrometry and ion mobility spectrometry with MMW spectrometry to quantify isomer ratios is surely a proposition worth investigation. The vacuum system in the mass spectrometer is already in place and the ions can be held in position for an extended period in ion traps. There may be time enough to make multiple scans and obtain the spectra of the different isomeric forms or their fragments even at the low abundance characteristic of mass spectrometry systems. 

Fourier Transform Mass Spectrometer (FTMS) Fourier transformation (FT) of time-dependent image from the detector to m/z intensity is utilized for two types of mass spectrometers ion cyclotron resonance (ICR) and Orbitrap. FTICR mass spectrometers operate based on the ion cyclotron resonance principle ions in a magnetic field (B) move in circular orbits at frequencies (ft>c) characteristic of their m/z values as shown below (Marshall et al., 1998,2002) ... 

Parent ion scanning in the positive mode for phosphotyrosines Taking advantage of the greater resolving power of mass spectrometer that can distinguish the characteristic reporter ion, the pTyr immonium ion at m/z = 216.04 from the amino acid doublets and triplet series (e.g. m/z = 216.09 of 62 series of NT and QS doublets). 

Tandem MS (MS/MS) capability, small size, and lower cost are other desirable characteristics of a mass analyzer. Miniaturization of mass spectrometers is a growing area of interest for field applications. Tandem MS is helpful in the analysis of complex mixtures. 

Before detailing the characteristics of mass analyzers, it is worth noting that electric fields as well as collisions between ions with gas molecules affect ion motion. To ensure the highest possible sensitivity, modern mass spectrometers operate under a vacuum ranging... 

Table 2.1 compares the characteristics of the three types of mass spectrometers used in commercially available ICP-MS instrumentation. 

The mass analyzer is the heart of the mass spectrometer. In the mass analyzer, some aspect of ion response to electric or magnetic fields is exploited so that ions of different masses can be differentiated. Salient characteristics of mass analyzers are mass range, mass resolution, ion transmission, and as discussed in a previous section, scan times. 

The marketed spectra databases are sold by institutes, constructors of mass spectrometers, editors on CD-ROM, or by download. Publicity put aside, the two most well known are, by far, the NIST database (from the American National Institute of Standard and Technology) and the WILEY database (marketed by the famous scientific editor). These are databases that index mass spectra recorded in electron ionization at 70 eV. Generally speaking, the few marketed databases devoted to chemical ionization are not very reliable. With this soft ionization mode, the characteristics of spectra depend greatly on conditions of tanperature and pressure withiu the source. For that reason, spectral reproducibility in Cl from one mass spectrometer to another is generally mediocre. 

In GC-MS effluent from the column is introduced directly into the mass spectrometer s ionization chamber in a manner that eliminates the majority of the carrier gas. In the ionization chamber all molecules (remaining carrier gas, solvent, and solutes) are ionized, and the ions are separated by their mass-to-charge ratio. Because each solute undergoes a characteristic fragmentation into smaller ions, its mass spectrum of ion intensity as a function of mass-to-charge ratio provides qualitative information that can be used to identify the solute. 

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. 

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