Other Mass Spectrometers

TOF mass spectrometers are very robust and usable with a wide variety of ion sources and inlet systems. Having only simple electrostatic and no magnetic fields, their construction, maintenance, and calibration are usually straightforward. There is no upper theoretical mass limitation all ions can be made to proceed from source to detector. In practice, there is a mass limitation in that it becomes increasingly difficult to discriminate between times of arrival at the detector as the m/z value becomes large. This effect, coupled with the spread in arrival times for any one m/z value, means that discrimination between unit masses becomes difficult at about m/z 3000. At m/z 50,000, overlap of 50 mass units is more typical i.e., mass accuracy is no better than about 50-100 mass 

On the other hand, there are some ionization techniques that are very useful, particularly at very high mass, but produce ions only in pulses. For these sources, the ion extraction field can be left on continuously. Two prominent examples are Californium radionuclide and laser desorption ionization. In the former, nuclear disintegration occurs within a very short time frame to give a 

In (a), a pulse of ions is formed but, for illustration purposes, all with the same m/z value. In (b), the ions have been accelerated but, because they were not all formed in the same space, they are separated in time and velocity, with some ions having more kinetic energy than others. In (c), the ions approach the ion mirror or reflectron, which they then penetrate to different depths, depending on their kinetic energies (d). The ones with greater kinetic energy penetrate furthest. In (e), the ions leave the reflectron and travel on to the detector (f), which they all reach at the same time. The path taken by the ions is indicated by the dotted line in (f). 

---

A connnon approach has been to measure the equilibrium constant, K, for these reactions as a fiinction of temperature with the use of a variable temperature high pressure ion source (see section (Bl.7.2)1. The ion concentrations are approximated by their abundance in the mass spectrum, while the neutral concentrations are known from the sample mlet pressure. A van t Hoff plot of In K versus /T should yield a straight Ime with slope equal to the reaction enthalpy (figure B1.7.11). Combining the PA with a value for basicityG at one temperature yields a value for A.S for the half-reaction involving addition of a proton to a species. While quadnipoles have been tire instruments of choice for many of these studies, other mass spectrometers can act as suitable detectors [19, 20]. 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

The ability to selectively excite a particular ion (or group of ions) by irradiating the cell with the appropriate radiofrequencies provides a level of flexibility unparalleled in any other mass spectrometer. The amplitude and duration of the applied RF pulse determine the ultimate radius of the ion trajectories. Thus, by simply turning on the appropriate radiofrequency, ions of a single m/z may be ejected from the cyclotron. In this way, a gas-phase separation of analyte from matrix is achieved. At a fixed radius of the ion trajectories the signal is proportional to the number of orbiting ions. Quantitation therefore requires precise RF control. 

The method for ion detection in an FTICR instrument is different from the majority of other mass spectrometers, where the ions hit a detector and are lost in the process. 

The instrumental design is similar to that of other mass spectrometers described above, with the end of the GC column directly inserted into the ion... 

When operated as a specific detector the ion-trap detector is more sensitive still but not to the extent that would be expected from the performance of other mass spectrometers operated in this mode in view of the large number of ions monitored in full scan mode there is little more sensitivity to be gained by spending a little extra time scanning a narrow mass range, and the detection limit in this mode is in the region of l-2pg. 

Time of flight ion probes (TOF SIMS) have unique capabilities not found in other mass spectrometers. A pulsed ion beam, typically either cesium or gallium, ejects atoms and molecules from the sample. Ionized species are accelerated down the flight tube and the arrival time in the detector is recorded, giving the mass of the species (see discussion of time-of-flight mass analyzers above). TOF SIMS instruments used in cosmochemistry have spatial resolutions of <1 pm. They are used to determine elemental abundances in IDPs and Stardust samples. The spatial distribution of elements within a small sample can also be determined. TOF SIMS instruments can obtain good data with very little consumption of sample. 

The quadrupole MS detector was the first, and is still the most common, detector used for LC/MS, but a number of other mass spectrometers have been adapted to this application. Both three-dimensional spherical (ITD) and linear (LIT) ion trap detectors offer tremendous potential for general, inexpensive LC/MS systems. They both offer the ability to be used as either a mass spectral detector or as a MS/MS detector. The 3D ITD (Fig. 15.5) allows ions to be trapped in the ion trap where they can be fragmented by heavy gas collision and the fragments released by scanning the dc/RF frequency of the trap. 

The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9) ultrahigh mass resolution (>1,000,000 at m/z. < 200) with accurate mass measurement even 1n gas chromatography/mass spectrometry experiments sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers versatile Ion sources (electron impact (El), self-chemical ionization (self-Cl), laser desorption (LD), secondary ionization (e.g., Cs+-bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission) trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. 

Ion transport from a continuous ion source to the extraction/acceleration region of the TOF-MS presents many of the challenges also associated with other mass spectrometers. Specifically, a means of efficiently focusing incoming ions into a beam of well-defined energy and spatial characteristics is required in order to achieve optimal performance for any mass analyzer. 

As the ions repel each other in the trap, their trajectories expand as a function of the time. To avoid ion losses by this expansion, care has to be taken to reduce the trajectory. This is accomplished by maintaining in the trap a pressure of helium gas which removes excess energy from the ions by collision. This pressure hovers around 10-3 Torr (0.13 Pa). A single high-vacuum pump with a flow of about 401 s-1 is sufficient to maintain such a vacuum compared with the 2501 s-1 needed for other mass spectrometers. The instrument is very simple and relatively inexpensive. 

I In an ion trap, in contrast to other mass spectrometers, D ionization and analysis occur in the same location but y at different times. A chromatographic sample is intro- duced directly into the space between the electrodes, I where ionization takes place. An appropriate voltage g across the electrodes causes appearance of an electri-% cal field within the trap. This field keeps the ions pro-... 

For the determination of isotope ratios, the precision of TOF-ICP-MS has been studied in a preliminary comparison with other mass spectrometer systems [521]. Typical isotope ratio precisions of 0.05% were obtained, thus overtaking sector field mass spectrometry with sequential detection, for which values of 0.1-0.3% for 63Cu/65Cu in Antarctic snow samples have been reported [522], Similar results were obtained by Becker et al. [523] for Mg and Ca in biological samples (0.4-0.5%). In principle, the features of TOF-ICP-MS may be superior to those of sequential sector field or quadmpole mass spectrometry, however, true parallel detection of the signals as is possible with multicollector systems may be the defini-... 

Other mass spectrometers are equipped with three-dimensional ion traps of which the geometry is much different to the quadrupoles previously described. In an ion-trap, the ions are confined between three electrodes (one toroidal and two end-caps), whose particular shape appears to result from a sort of anamorphosis of the four-bar set-up of a classic quadrupole. As in the previous category they operate under the effect of a variable electric field (with or without a superimposed fixed field). Although they are, in appearance, physically simple devices, the fundamental principle of ion trap is complex. These ion trap detectors are sensitive, less costly than quadrupoles and compatible with different ionization techniques. The volume defined by the electrodes, named superior, inferior and annular, is simultaneously the ion source and the mass filter (Figure 16.11). These analysers are almost exclusively linked with a separative technique (GC/MS). 

Another area of recent interest is the interfacing of API techniques with TOF mass spectrometers. TOF/MS instruments combine ease of operation, relatively low cost, excellent ion transmission, and virtually unlimited mass range. The only significant disadvantage with respect to other mass spectrometers is the limited mass resolution. A tremendous effort in development and performance opti-... 

AMS consists of a high-energy particle accelerator and an ion detector. Typical atom ratios for AMS isotope measurements are in the range of 10 10 to 10 15. These ratios fall well below the 1 to 10-9 range measured with other mass spectrometers, except for laser-based systems with resonance ionization techniques, which can match the range of the AMS. 

In operation, the major and rare isotopes are measured sequentially by peak switching from one isotope to the other, as with other mass spectrometers, to determine atom ratios. The major isotope may produce a current in the microampere range, whereas the rare isotope produces a signal as low as a few counts per minute. The raw isotope ratio consists of counted pulses divided by integrated current. Calibration by measurement standards with known isotope ratios provides the factor to convert the raw ratio into an atom ratio. 

The third situation is when the objectives (usually specific and limited in scope) require the highest resolution (>1,000,000) that is available only with FT-ICRMS systems. Because the timescales of data acquisition for these instruments are not compatible with LC, the current approach is to bypass the chromatographic step and use flow injection to introduce samples, and then use an LIT (often included) in conjunction with the resolving power of the FT-ICR to tease apart highly complex mixtures (e.g., top-down proteomics or crude oils). It is likely that the organization contemplating such an instrument already has other mass spectrometers along with experienced operators/researchers. 

The RF quadrupole trap and the Penning trap can be used not only to trap ions but also to perform very precise measurements of their masses [1219]. Unlike other mass spectrometers, the signal is not obtained from mass-selected particles impinging on the detector, but from an induction voltage picked up by external electrodes from the ion motion. Fourier analysis of this signal gives the frequencies of the three components. Since the cyclotron resonance frequency... 

In principle however, this method should be portable to other mass spectrometer systems capable of chemical ionization and tandem mass spectrometry. An [M+54] ion was reported using atmospheric pressure chemical ionization (APCI) with a predominantly acetonitrile solvent while analyzing extremely long-chain polyunsaturated fatty acids (16). Such an observation is promising for the use of this method for the analysis of low- or nonvolatile lipids, such as triglycerides and phospholipids. 

In addition to some other mass spectrometers, FTICRMS devices are also used. The latter, in addition to very high acquisition and operating costs (e.g., helium), has the disadvantage of low data acquisition rate (same problem as with the Orbitrap), so the coupling with a fast analysis, such as UHPLC, cannot be realized. However, they are unbeaten in resolution and an extremely useful tool in metabolomic research. 

---