Mass separation devices

There are a number of different mass separation devices - analysers - used in mass spectrometry and each has its own advantages and disadvantages. Those most likely to be encountered by users of LC-MS are described briefly below, while more detailed descriptions may be found elsewhere [2-4]. One property that is important in defining the analytical capabihties of a mass analyser is the resolution which it may achieve. 

In some respects, the time-of-tlight (ToF) analyser is the simplest of the mass separation devices. This system relies on the fact that if all of the ions produced... 

This is probably the most widely used MS-MS instrument. The hardware, as the name snggests, consists of three sets of quadrupole rods in series (Figure 3.8). The second set of rods is not used as a mass separation device but as a collision cell, where fragmentation of ions transmitted by the first set of quadrupole rods is carried out, and as a device for focussing any product ions into the third set of quadrupole rods. Both sets of rods may be controlled to allow the transmission of ions of a single mjz ratio or a range of mjz values to give the desired analytical information. 

In addition to neutral loss scans, mass spectrometers can be used to detect other compounds in a different manner. Acylcamitines are fatty acid esters of carnitine. The masses of acylcamitines differ by the size of the fatty acid attached to it. The tandem mass spectrometer can detect these selectively as well because they all produce a similar product, in this case an ion rather than a molecule. Because it is an ion, it can be detected by the second mass separation device. The ion has a mass of 85 Da and is common to all acylcamitines. Performing a precursor ion scan of 85 Da (essentially a scan of only molecules that produce the 85 ion) reveals a selective analysis of acylcar-nitines, as shown in Fig. 14.2. Additional scans have been added to more selectively detect basic amino acids, free carnitine, short chain acylcamitines and a hormone, thyroxin (T4) which has amino acid components. 

Fig. 8.1.1 Simple illustrations of a various mass spectrometers, a The triple-quadrupole tandem mass spectrometer (top panel). The middle set of quadrupoles are part of the collision cell (CC) and do not perform mass separation. MSI and MS2 indicate the first and second quadrupole mass separation devices, respectively. The bold arrow shows the path of ions, b Ion-trap mass spectrometer (middle left). The charged sections of the ion trap are not elliptical as drawn, but rather hyperbolic. The diagram is also two-dimensional, whereas the ion trap is three-dimensional. The ion path is such that ions enter the device and are trapped until a specific voltage ejects these ions, c Time of Flight mass spectrometer with a Reflectron (middle left). Ions are separated by the time it takes to pass through the instrument. The Reflectron improves/focuses the ions, d Hybrid Tandem mass spectrometer (bottom). The diagram shows that a quadrupole instrument can be combined with a different type of mass spectrometer, forming a tandem hybrid instrument...

Ionization is very important in that you have to have ions to detect and measure mass. Today, electrospray ionization is used this is a process where a streaming liquid is ionized and this imparts ionization of the molecules dissolved within it. However, large amounts of liquid or uncharged molecules are not desirable in a mass spectrometer operating in a vacuum. Hence, electrospray ionization is optimized such that nearly all of the solvent/spray is removed and only charged molecules enter the mass-separation devices. You may ask how I ensure that only the molecules I am interested in make it to the mass spectrometer rather than all of the other molecules that may be present in the mix. This is an important concept, since... 

R. Thomas, A beginner s guide to ICP-MS Part VII Mass separation devices D double-focusing magnetic-sector technology, Spectroscopy, 16 (2001), 22D27. 

The nucleus of ICP-MS is the mass separation device, generally called mass analyzer, is positioned between the ion optics and the detector. The task of this device is to separate the... 

Once the ions are produced in the plasma, they are directed into the mass spectrometer via the interface region, which is maintained at a vacuum of 1-2 torr with a mechanical roughing pump. This interface region consists of two metallic cones (usually nickel), called the sampler and a skimmer cone, each with a small orifice (0.6-1.2 mm) to allow the ions to pass through to the ion optics, where they are guided into the mass separation device. 

Once the ions have been successfully extracted from the interface region, they are directed into the main vacuum chamber by a series of electrostatic lens, called ion optics. The operating vacuum in this region is maintained at about 10" torr with a turbomolecular pump. There are many different designs of the ion optic region, but they serve the same function, which is to electrostatically focus the ion beam toward the mass separation device, while stopping photons, particulates, and neutral species from reaching the detector. 

The ion optics, shown in Figure 6.1, are positioned between the skimmer cone and mass separation device and consist of one or more electrostatically controlled lens components, maintained at a vacuum of approximately 10 torr with a turbo-molecular pump. They are not traditional optics that we associate with ICP emission or atomic absorption, but are made up of a series of metallic plates, barrels, or cylinders, which have a voltage placed on them. The function of the ion optic system is to take ions from the hostile environment of the plasma at atmospheric pressure via the interface cones and steer them into the mass analyzer, which is under high vacuum. The nonionic species such as particulates, neutral species, and photons are prevented from reaching the detector by using some kind of physical barrier, positioning the mass analyzer off axis relative to the ion beam, or electrostatically bending the ions by 90° into the mass analyzer. 

Although ICP-MS was commercialized in 1983, the first 10 years of its development utilized a traditional quadrupole mass analyzer to separate the ions of interest. These worked exceptionally well for most applications, but proved to have limitations when determining difficult elements or dealing with more complex sample matrices. This led to the development of alternative mass separation devices that allowed ICP-MS to be used for applications that were previously beyond the capabilities of quadrupole-based technology. Before we discuss these different mass spectrometers in greater detail, let us take a look at the proximity of the mass analyzer in relation to the ion optics and detector. Figure 7.1 shows this in greater detail. 

FIGU RE 7.1 The mass separation device is positioned between the ion optics and the detector. 

The simultaneous nature of sampling ions in TOP offers distinct advantages over traditional scanning (sequential) quadrupole technology for ICP-MS applications in which large amounts of data need to be captured in a short span of time. To understand the benefits of this mass separation device, let us first take a look at its fundamental principles. All TOP mass spectrometers are based on the same principle the kinetic energy (KE) of an ion is directly proportional to its mass (m) and velocity (VO. This can be represented by the following equation ... 

The ion beam containing all the analyte and matrix ions exits the ion optics and now passes into the heart of the mass spectrometer—the mass separation device, which is kept at an operating vacuum of approximately 10" torr with a second turbo-molecular pump. There are many different mass separation devices, all with their strengths and weaknesses. Three of the most common types are discussed in this... 

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