Radiofrequency quadrupoles

Dawson PH and Whetten NR (1969) Radiofrequency quadrupole mass spectroscopy. Electronics and Electron Physics 27 58-158. 

Lunney, M. D. Moore, R. B. Cooling of mass-separated beams using a radiofrequency quadrupole ion guide. Int. J. Mass Spectrom. 1999, 190/191, 153-160. 

Note Standard quadrupole analyzers have rods of 10-20 mm in diameter and 15-25 cm in length. The radiofrequency is in the order of 1-4 MHz, and the DC and RF voltages are in the range of some 10 -10 V. Ions of about 10 eV kinetic energy undergo approximately 100 oscillation during their passage. 

Following ionization and fragmentation, the ions produced are separated based on their miz ratio in the mass filter (analyzer). There are several types of analyzers used in GC/MS, including time-of-flight (TOE), magnetic sector, and radiofrequency, which include both quadrupole and ion trap. 

A quadrupole mass analyzer is made of four hyperbolic or circular rods placed in parallel with identical diagonal distances from each other. The rods are electrically connected in diagonal. In addition to an alternating radiofrequency (RE) potential (V), a positive direct current (DC) potential (U) is applied on one pair of rods while a negative potential is applied to the other pair (Fig. 1.17). The ion trajectory is affected in x and y directions by the total electric field composed by a quadrupolar alternating field and a constant field. Because there is only a two-dimensional quadrupole field the ions, accelerated after ionization, maintain their velocity along the z axis. 

Quadrupoles are comprised of four metal rods, ideally of hyperbolic cross section, arranged as shown in Fig. 5.4. A combination of radiofrequency (RF) and direct current (DC) voltages are applied to each pair of rods, which creates an electric field within the region bounded by the rods. Depending on the RF/DC ratio, the electric field between the rods will allow ions in a narrow m/z range to pass, typically 0.8 m/z —just how narrow will depend on a number of factors which influence the resolution. Hence, by changing the RF/DC ratio in a controlled manner, the quadrupole can be... 

There are two ways to operate a quadrupole filter the voltages V and Frf are maintained constant while v, the radiofrequency, is scanned, or alternatively, v is maintained constant and both voltages are scanned together keeping their ratio at a fixed value. It is thus possible to sequentially obtain the mass spectrum of the compound. 

The maximum resolution is given by equation (16.16) where e represents a unit charge, Vz the acceleration potential, L the length of the quadrupole mass filter, r0 the radius of the filter and l/max the maximum voltage of the radiofrequency. 

Figure 16.23—Triple quadrupote MS MS instrument. In the triple qnadrupole arrangement, the middle quadrupole is used as a collision chamber. It is operated in the radiofrequency voltage mode only, where it will transmit all masses. A gas pressure introduced in the second quadrupole is responsible for collision activation. Triple quadrupole instruments can conduct all three types of MS — MS analysis described above.

In contrast to triple quadrupole instruments, where MS-MS experiments can be conducted in space in separate regions of the instrument, ion traps enable MS-MS sequentially in the same physical space, and thus, occur tandem in time. After the ions have been formed an trapped, a parent ion is selected by resonance ejection of all ions except those of the selected m/z ratio. This is done by applying a resonance ejection radiofrequency voltage to the end-cap electrodes which stimulates motion of the ions in the axial direction. The next step in the MS-MS sequence is to effect collisionally... 

The ion is trapped in a quadrupole radiofrequency trap that is a geometrical variant of the original Paul trap and basically consists of only a ring electrode... 

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