Mass analyser

The role of the mass analyser is to separate ions emerging from the ion source according to their mass (w/z ratio). The separating capability or mass resolution, R, of the analyser is measured in terms of its ability to resolve ions to less than a 10% valley between peaks  

There are two main groups of analysers, non-magnetic analysers typified by quadrupoles and ion trap analysers and magnetic analysers which use permanent or electromagnets to separate the ions under the influence of a magnetic field. 

Once the molecular ions are produced they are usually accelerated and focused, using high-voltage electric fields and suitable electrodes, prior to injection into the mass analyser. A number of different mass analysis methods are now in use, and new techniques are continually being developed. However, all the methods described here share one thing in common they all separate ions on the basis of the mass/charge ratio mjz). 

A charged particle (ze) moving in a uniform magnetic field will tend to follow a circular trajectory in which the (inward) magnetic force (zevB) is balanced by the (outward) centrifugal force (mv jr).  

The velocity (v) is determined by the voltage on the electrodes used to accelerate the ions. The kinetic energy gained by the particle passing through an electrostatic potential (F) is equal to zeV  

The time-of-flight of the particle along a fixed path will depend on the inverse of its velocity  

Post ionisation and desolvation, analyte ions are separated according to their m/z ratio by a mass analyser. The analyte ions are usually separated using static or dynamic electric/magnetic fields. An exception to this would be time-of-flight which does not utilise a magnetic/electric field, excepf if a reflection is used, which employs a static electric held. Although many mass analysers and their variants have been developed, only the principles of a quadrupole and time-of-flight mass analysers will be described in this section in accordance with the techniques used in later chapters. 

The fundamental principles of a quadrupole mass analyser were described in 1953 by Paul and Steinwegen [112]. The device itself consists of four rods in a parallel arrangement, either of circular or hyperbolic cross-sections, and utilises the principle of stable ion trajectories within an oscillating electric field to separate tn/z ratios. For example a positive ion entering a quadrupole will be attracted to a negative rod, however if the polarity of the rod switches to positive before the ion has chance to discharge on it, then the ion will change its path direction. 

The motion of an ion in a quadrupole field along the x and y axis can be described by second order Mathieu equations  

In order to obtain a stable ion trajectory along the x and y axis, 0 for both equations. When ro = 0, the ion will hit the electrode and discharge. Therefore 

Considering the aforementioned variables of the Mathieu equation, it is possible to deduce that  

In mass spectrometers, ions are analysed according to the ml7. (mass-to-charge) value and not to the mass. While there are many possible combinations of technologies associated with a mass-spectrometry experiment, relatively few forms of mass analysis predominate. They include linear multipoles, such as the quadrupole mass filter, time-of-flight mass spectrometry, ion trapping forms of mass spectrometry, including the quadrupole ion trap and Fourier-transform ion-cyclotron resonance, and sector mass spectrometry. Hybrid instruments intend to combine the strengths of the component analysers. 

Mass discriminator0 Measured quantity Kinetic energy 

MS capability In tandem In tandem Very flexible MS/MS Very flexible 

After McLuckey [166]. Reprinted from S.A. McLuckey, in Advances in Mass Spectrometry, Vol. 14 (KJ. Karjalainen et al., eds), pp. 153-196, Copyright (1998), with permission from Elsevier. 

Each of the analyser types has a unique set of figures of merit that makes it optimally suited for particular applications (Table 6.27). The main ionisation modes in relation to various mass spectrometers are summarised in Table 6.28. 

---

A) MASS-ANALYSED ION KINETIC ENERGY SPECTROMETRY (MIKES)... 

Quadruple Mass Analyser Gloquad Manufecturer name... 

Q-ToF-LC-MS-MS quadrupole time-of-flight mass analyser in combination with (high performance) liquid chromatography and tandem mass spectrometry... 

When a pulsed laser is used, ions are only produced for the duration of the pulse, i.e. they are not produced continuously and the mass spectrometer used must be capable of producing a mass spectrum from these pulses of ions. As discussed below in Section 3.3.4, the time-of-flight (ToF) mass analyser is the most appropriate for this purpose and has the added advantage of being able to measure very high m/z ratios. Indeed, the recent dramatic developments in the performance of the ToF mass analyser have largely been occasioned by the requirement to produce useful spectra from MALDI. 

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. 

Figure 3.5 Schematic of a (quadrupole) ion-trap mass analyser. From applications literature published by Thermofinnigan, Hemel Hempstead, UK, and reproduced with permission.

Figure 3.7 Schematic of a time-of-flight mass analyser, involving the use of a reflectron .

The operation of this type of device is fundamentally different to those described previously in which ions of one m/z ratio at a time enter the mass analyser. By varying the conditions in the mass analyser, e.g. magnetic field, quadrupole field, etc., ions of different m/z values are brought to the detector and a corresponding mass spectrum obtained. 

In this instrnment, the final stage of the triple quadrnpole is replaced by an orthogonal time-of-fiight (ToF) mass analyser, as shown in Fignre 3.10. The con-fignration is typical of the latest generation of ToF instrnments in which a nnmber of reflectrons, in this case two, are used to increase the flight path of the ions and thns increase the resolution that may be achieved. 

The difference between this and other MS-MS instrnments is the way in which the MS2 nnit operates, as discussed in Section 3.3.4 above. To reiterate, in contrast to other mass analysers which are scanned seqnentially through the... 

---