Mass filtering discrimination Collision/reaction cells

Let us now go on to discuss the other major way of interference rejection in a collision/reaction cell nsing highly reactive gases and mass (bandpass) filtering discrimination. 

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. 

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... 

---