Quadrupole mass separator

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...

Figure 21-11 shows a transmission quadrupole mass spectrometer, which is the most common mass separator in use today. It is connected to a gas chromatography column at the left to record the spectrum of each component as it is eluted. Compounds exiting the column pass through a heated connector into the ionization chamber where they are converted into ions and accelerated by 15 V before entering the quadrupole mass separator. 

As with the quadrupole, mass separation in the ion trap is based on stability of the ions in an oscillating electrical field. The ion trap consists of a ring electrode and two endcap electrodes on which AC and DC voltages are applied (Figure 3.37). Ions enter the ion trap through a hole in one of the endcap electrodes, are cooled down by helium present at low pressures, and are trapped in the electrical field. 

Tandem quadrupole mass separators Up to 20000 for multiple pass quadrupoles... 

Note that in mass spectrometry/mass spectrometry (MS/MS) applications, quadrupole and magnetic sectors can be used together advantageously. It is also worth noting that the quadrapole can be operated without the DC voltages. In this RF-only mode, no mass separation occurs, and these quadrapoles are used as ion transmission guides, described in Chapter 49. 

It is perhaps worth noting here that, if a quadrupole assembly is used in this all-RF mode, there is no significant mass separation as ions of different mass move through the guide. However, if a DC potential is applied to one pair of rods, the guiding potential changes to that shown in Equation 49.5, in which F is the applied DC potential. 

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

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. 

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. 

The resolution obtainable with a UTI-100C quadrupole mass analyser is m/Am 2m (Jjt)). These three peaks are also separated to baseline resolution In Figure 7a however, they appear as one peak due to the wide mass range which is displayed. 

As with all spectroscopic methods discussed previously, this method is best suited to measurement and elucidation of the characteristics of pure compounds. For this reason, MS is often used as a detector for gas chromatographs. The GC separates the mixture into pure compounds and the MS then analyzes each pure chemical as it exits the column. The most common MS for this application is the quadrupole mass spectrometer. For this reason, it is discussed in Chapters 14 and 15. 

---