Collision / reaction cells multipoles

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. 

A multipole cell at pressures around 1 to 15 mtorr, placed between the sampler-skimmer interface and the mass spectrometer, can serve two functions reduce the kinetic energy of the ions to nearly thermal energies (<0.5 eV) and carry out reactions with analyte or background ions. Of particular interest for ICP-MS are reactions that would dramatically reduce spectral overlaps due to elemental or polyatomic ions. Two potentially undesirable processes must be considered for successful use of a collision-reaction cell. Scattering losses can be severe if the mass of the collision or reaction gas is high compared to that of the analyte ion... 

A multipole collision/reaction cell consists of a multipole (consisting of 2n -F 2 parallel metallic rods) in an enclosed cell that can be pressurized with a gas [76-81], It is located between the interface and the mass analyzer (Figure 2.20). 

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. 

The first commercial instrument to use this approach was called dynamic reaction cell (DRC) technology Similar in appearance to the hexapole and octapole collision/reaction cells, the DRC is a pressurized multipole positioned prior to the analyzer quadrupole. However, this is where the similarity ends. In DRC technology, a quadrupole is used instead of a hexapole or octapole. A highly reactive gas such as ammonia, oxygen, or methane is bled into the cell, which is a catalyst for ion-molecule chemistry to take place. By a number of different reaction mechanisms, the gaseous molecules react with the interfering ions to convert than into either an innocuous species different from the analyte mass or a harmless neutral species. The analyte mass then emerges from the DRC free of its interference and is steered into the analyza quadrupole for conventional mass separation. 

Based on a quadrupole ICP-MS, another technique, termed collision/reaction cell, has frequently been used to reduce polyatomic ion interferences. " These techniques provide simple, efficient, and low-cost methods in the face of many difficult interference problems. In these methods, ions to be analyzed first enter a radio-frequency-only multipole (e.g. a quadrupole, hexapole, or octapole), in which the analytes react with the collision/reaction gas, which is usually oxygen, ammonia, xenon, or methane, " to remove polyatomic interference or generate a new analyte ion of mjz showing less interference. The RP-only multipole does not separate ions like a traditional quadrupole, but it has profound influence on collisional focusing of ions, both of the energy and spatial distributions. An example of removing the polyatomic interference is shown below, which uses ammonia gas to reduce any "" Ar" " interference in the measurement of " Ca ... 

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. " " ... 

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... 

The theoretical aspects of collision- and direct-reaction cells, including the use of multipole devices, have been treated in detail by Tanner et al. [619]. Ion-molecule reactions are of importance for decreasing the occurrence of polyatomic species in elemental mass spectrometry in two ways ... 

Since its commercial introduction in 1997, collision cell and reaction cell ICP-MS has come a long way and is now well accepted. A recent tutorial review by Tanner et al. covered the fundamentals of the operation of such devices, and listed the applications published up until September 2001. This section examines implementation and use of the ion-molecule processes enabled in the linear radio-frequency driven (r.f.) collision cells and reaction cells used in ICP-MS, partially drawn from the Tanner et al. review. Special attention is paid to ion dynamics, since the outcome of the ion-molecule processes strongly depends on it. Exart5)les of the principle applications are given along with a discussion of the fundamental processes occurring in pressurized r.f. multipole devices. 

The major differences between the two approaches are how the gaseous molecules interact with the interfering species and what type of multipole is used in the cell. These dictate whether it is an ion-molecule collision or reaction mechanism taking place. Let us take a closer look at each process because there are distinct differences in the way the interference is rejected and separated from the analyte ion. 

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