Mass filters

A single magnetic sector can be used as a mass filter for other apparatus. However, much more infonnation of the simple mass spectrum of a species can be obtained using the tandem mass spectrometer. 

B1.7.4 QUADRUPOLE MASS FILTERS, QUADRUPOLE ION TRAPS AND THEIR APPLICATIONS... 

Another approach to mass analysis is based on stable ion trajectories in quadnipole fields. The two most prominent members of this family of mass spectrometers are the quadnipole mass filter and the quadnipole ion trap. Quadnipole mass filters are one of the most connnon mass spectrometers, being extensively used as detectors in analytical instnunents, especially gas clnomatographs. The quadnipole ion trap (which also goes by the name quadnipole ion store, QUISTOR , Paul trap, or just ion trap) is fairly new to the physical chemistry laboratory. Its early development was due to its use as an inexpensive alternative to tandem magnetic sector and quadnipole filter instnunents for analytical analysis. It has, however, staned to be used more in die chemical physics and physical chemistry domains, and so it will be described in some detail in this section. 

A schematic diagram of a quadnipole mass filter is shown in figure Bl.7.8. In an ideal, three-dimensional, quadnipole field, the potential ( ) at any point (x,y, z) within the field is described by equation (Bl.7.5) ... 

Figure Bl.7.8. Quadnipole mass filter consisting of four cylindrical rods, spaced by a radius r (reproduced with pennission of Professor R March, Trent University, Peterborough, ON, Canada).

Ideally, the rods in a quadnipole mass filter should have a hyperbolic geometry, but more conunon is a set of four cylindrical rods separated by a distance 2r, where r I. I6r[((figure Bl.7.8). This anangement provides... 

The values of a and b that satisfy this relationship are a = 1 and h = -1, so the potential inside the quadnipole mass filter takes on the fonn ... 

Figure Bl.7.9. (a) Stability diagram for ions near the central axis of a quadnipole mass filter. Stable trajectories occur only if the and values lie beneath tire curve, (b) Stability diagram (now as a fiinction of U and F) for six ions with different masses. The straight line miming tlirough the apex of each set of curves is the operating line, and conesponds to values of UIVthat will produce mass resolution (reproduced with pennission of Professor R March, Trent University, Peterborough, ON, Canada).

To be effective, it is necessary for the ions traversing the instniment to experience several RF cycles. Thus, unlike magnetic sector instmments, the ions fonned in the ion source of a quadnipole mass filter apparatus are accelerated to only a few eV kinetic energy (typically 5-10 eV). The timescale of the experiment is therefore... 

Aside from the smgle mass filter, the most connnon configuration for quadnipole mass spectrometers is the triple-quadnipole instrument. This is the simplest tandem mass spectrometer using quadnipole mass filters. Typically, the... 

A sehematie diagram of a SIFT apparatus is shown in figure Bl.7.12. The instrument eonsists of five basie regions, the ion soiiree, initial quadnipole mass filter, flow tube, seeond mass filter and finally the deteetor. The heart of the instrument is the flow tube, whieh is a steel tube approximately 1 m long and 10 em in diameter. The pressure in the flow tube is kept of the order of 0.5 Torr, resulting in earrier gas flow rates of... 

Another instrument used in physical chemistry research that employs quadnipole mass filters is the guided ion beam mass spectrometer [31]. A schematic diagram of an example of this type of instrument is shown in figure B 1.7.13. A... 

The Mathieu equation for the quadnipole ion trap again has stable, bounded solutions conesponding to stable, bounded trajectories inside the trap. The stability diagram for the ion trap is quite complex, but a subsection of the diagram, correspondmg to stable trajectories near the physical centre of the trap, is shown in figure Bl.7.15. The interpretation of the diagram is similar to that for tire quadnipole mass filter. 

There are otlier teclmiques for mass separation such as tire quadmpole mass filter and Wien filter. Anotlier mass spectrometry teclmique is based on ion chromatography, which is also capable of measuring tire shapes of clusters [30, 31]. In tills metliod, cluster ions of a given mass are injected into a drift tube witli well-defined entrance and exit slits and filled witli an inert gas. The clusters drift tlirough tills tube under a weak electric potential. Since the... 

Dimethylbutadiene and 1 4-naphthoquinone. 2 3-Di-methylanthraquinone. In a small round-bottomed flask, fitted with a reflux condenser, place a solution of 8 g. of freshly-distUled 2 3-dimethyl-butadiene (Section 111,147) and 8 g. of 1 4-naphthoquinone (Section IV,149) in 30 ml. of ethanol, and reflux for 5 hours. Keep the resulting solution in a refrigerator for 12 hours break up the crystaUine mass, filter, and wash with 5 ml. of alcohol. The yield of crude adduct, m.p. 147-149°, is 11-5 g. recrystaUisation from methanol raises the m.p. to 150°. 

To decompose the acetone semicarbazone, warm 58 g. with 50 ml. of concentrated hydrochloric acid until it just dissolves. Cool in ice the semicarbazide hydrochloride separates as a thick crystaUine mass. Filter at the pump through a sintered glass funnel, and wash with a small quantity of alcohol and then with ether dry in the air. The yield of pure semicarbazide hydrochloride, m.p. 173° (decomp.), is 35 g. A further quantity of product may be obtained either by saturating the mother liquor with hydrogen chloride or by treating it with twice its volume of alcohol and then with ether. 

Filter s final mass - filter s initial mass = g suspended solid... 

The hexapole cannot act as a mass filter by applying a DC field and is used only in its all-RF mode, in which it allows all ions in a beam to pass through, whatever their m/z values. In doing so, the ion beam is constrained, so it leaves the hexapole as a narrow beam. This constraint is important because the ion beam from the inlet system tends to spread due to mutual ion repulsion and collision with residual air and solvent molecules. By injecting this divergent beam into a hexapole unit, it can be refocused. At the same time, vacuum pumps reduce the background pressure to about 10 mbar (Figure 22.1). The pressure needed in the TOF analyzer is about 10 ... 

Quadrupole mass spectrometers (mass filters) allow ions at each m/z value to pass through sequentially for example, ions at m/z 100, 101, 102 will pass one after the other through the quadrupole assembly so that first m/z 100 is transmitted, then m/z 101, then m/z 102 (or vice versa), and so on. Therefore, the ion collector (or detector) at the end of the quadrupole unit needs to cover only one point or focus in space (Figure 29.1a), and a complete mass spectrum is recorded over a period of time. The ions arrive at the collector sequentially, and ions are detected in a time domain, not in a spatial domain. 

Ions of different m/z values pass sequentially in time through the quadrupole mass filter to reach an in-line, single-point ion collector. 

In a sector instrument, which acts as a combined mass/velocity filter, this difference in forward velocity is used to effect a separation of normal and metastable mj" ions (see Chapter 24, Ion Optics of Magnetic/Electric-Sector Mass Spectrometers ). However, as discussed above, the velocity difference is of no consequence to the quadmpole instrument, which acts only as a mass filter, so the normal and metastable mj ions formed in the first field-free region (Figure 33.1) are not differentiated. 

Quadrupole analyzer. A mass filter that, creates a quadrupole field with a DC component and an RF (radio frequency) component in such a manner as to allow transmission only of ions having a selected mass-to-charge (m/z) ratio. 

Quadrupoles. The quadmpole mass filter (16,17), which became available commercially in 1967, consists of four cylindrical rods having circular or, more recentiy, hyperboHc cross sections, arranged with their centers on the perimeter of a circle of radius Tq. Figure 2 is a schematic diagram of a... 

Each type of mass spectrometer has its associated advantages and disadvantages. Quadrupole-based systems offer a fairly simple ion optics design that provides a certain degree of flexibility with respect to instrument configuration. For example, quadrupole mass filters are often found in hybrid systems, that is, coupled with another surface analytical method, such as electron spectroscopy for chemical analysis or scanning Auger spectroscopy. 

To minimize surface damage, static SIMS mass spectrometers should be as efficient as possible for detecting the total yield of secondary ions from a surface. Also, to be able to separate elemental from molecular species, and molecular species from each other, the mass resolution usually given as the mass m divided by the separable mass Am, should be very high. With this in mind, two types of mass spectrometer have been used - in early work mainly quadrupole mass filters and, more recently, time-of-flight mass spectrometers. 

Quadrupole analyzer A mass filter that creates a quadrupole field with dc and rf components so that only ions of a selected mass-to-charge are transmitted to the detector. 

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