Multipole filter

In general, electromagnetic resonators are of common use for material characterization at microwave frequencies. In addition, a resonator represents a basic element of a multipole filter or an oscillator circuit. Any type of electromagnetic resonator is characterized by the resonant frequency /o and the unloaded quality factor Qo of the selected resonant mode and its spectrum of spurious modes. In order to measure the resonator properties or to use a resonator as part of a filter structure, the resonator needs to be equipped with one or two... 

Some of these interferences may be removed by the use of reaction cell technology where a gas such as helium is added so that, by ion-molecule reactions, interfering ions may be converted into noninterfering species or removed by a multipole filter device. 

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. 

The quadrupole mass filter is the most abundant mass analyser today and RF-only multipoles are used as transmission devices/collision regions in various instrumental configurations. The mass filter is used extensively as a stand-alone mass analyser and as an analyser in multistage mass spectrometers. 

The Stark effect and the Zeeman effect of molecules in an inhomogeneous external field serve to select molecules due to their different rotational quantum states and are thus used to produce molecules with well defined preferential orientations in a beam molecules with different orientation are differently deflected in these fields. With regard to state selectors one can distinguish between simple deflection devices like Stern-Gerlach magnets (and their electrical analogues) and multipole fields where molecules with a well-defined Stark or Zeeman-effect are focused into the detector. In both cases the state selector works as a filter enhancing the relative number of molecules in a certain quantum state. 

Beeause an electrospray source transfers ions continuously to the gas phase, it is most easily coupled to a continuous mass analyzer. From the time of the first successful applieations of this technique, linear quadrupoles proved to be of great praetical use. In triple-quadrupole mass spectrometers, a linear multipole guides the ions to a first quadrupole mass filter, followed by a second ion guide leading to a seeond quadrupole mass filter. Finally, an ion multiplier deteets the ions. Differential pumping reduces the pressure from atmospheric pressure (API region, ESI source) to 10 mbar in the mass analyzer. 

Many of the low m/z ions are filtered out by the RF-only multipole ion guides. That said, in best case comparisons [82, 83] the TOF instruments have about an order of magnitude lower limit of detection than FTICR-MS instruments, although in both cases it is clear that the limiting factor is sample handling, and not instrument performance. 

The advantage of using a quadrupole in the reaction cell is that the stability regions are much better defined than higher-order multipoles, so it is relatively straightforward to operate the quadrupole inside the reaction cell as a mass or bandpass filter... 

However, the use of highly reactive gases such as ammonia and methane can lead to more side reactions and potentially more interferences unless the by-prodncts from these side reactions are rejected. The way around this problem is to ntilize a lower-order multipole, such as a quadrupole, inside the reaction/collision cell and nse it as a mass discrimination device. The advantages of using a quadrupole are that the stability boundaries are much better defined than a hexapole or an octapole, so it is relatively straightforward to operate the quadrupole inside the reaction cell as a mass or bandpass filter. Therefore, by careful optimization of the quadrupole electrical fields, unwanted reactions between the gas and the sample matrix or solvent, which could potentially lead to new interferences, are prevented. This means that every time an analyte and interfering ions enter the reaction cell, the bandpass of the... 

Another way of rejecting polyatomic interfering ions and the products of secondary collisions/reactions is to discriminate them by mass. As mentioned previously, higher-order multipoles cannot be used for efficient mass discrimination because the stability boundaries are diffuse and sequential secondary reactions cannot be easily intercepted. The only way this can be done is to utilize a quadrupole (instead of a hexapole or octapole) inside the collision/reaction cell and use it as a selective bandpass (mass) filter. There are a number of commercial designs using this approach, so let us take a look at them in greater detail in order to better understand how they work and how they differ. 

It should be noted that up until now, only single multipole-based cells have realized commercial success, but a recent development has placed an additional quadrupole prior to the collision/reaction cell multipole and the analyzer quadrupole. This first quadrupole acts as a simple mass filter to allow only the analyte masses to enter the cell, while rejecting all other masses. With all nonanalyte, plasma, and sample matrix ions excluded from the cell, sensitivity and interference removal efficiency is significantly improved compared to traditional collision/reaction cell technology coupled with a single quadrupole mass analyzer. 

Because of the disparity of the reaction rates of the two neutralization reactions, the analyte can be efficiently determined after the introduction of ammonia as a reactive gas into the multipole. There are many excellent reviews about the development and applications of collision/reaction cell in ICP-MS. " In order to eliminate the new isobaric interferences produced by secondary reactions, two methods are commonly used in the commercial instrument the discrimination of kinetic energy or mass filtering. " The former mainly utilizes the post-cell kinetic energy discrimination (KED) to suppress transport of the produces of the side reactions to the analyte in the hexapole and octapole cell instruments. Whereas in the latter, the quadrupole cell has a capability to reduce the formation of the unwanted side product ions by selecting an appropriate mass bandpass. The details of the KED and bandpass approaches can refer to many excellent books and reviews. " " ... 

---