Collision / reaction cells octapole

Collision-Reaction Cells. Perhaps the most exciting development regarding molecular ion removal in ICP-MS is the use of collision or reaction cells, introduced in Sec. 3.1.5. These rf-only quadrupole, hexapole, or octapole cells, typically operated at pressures around 10 mtorr, can provide two benefits The collision cell can be used to reduce the ion kinetic energy and to focus ions toward... 

Typical Detection Limits in Parts per Trillion (ppt) of a Dynamic Reaction Cell (DRC), an Octapole-Based Collision/Reaction Cell (CRC), and a Collision/Reaction Interface System (CRI)... 

More recently, Yamanaka and Fryer showed the use of collision/reaction cell technology ICP-MS to determine elements at the trace and macro levels in various plant materials. They demonstrated that by using both helium and hydrogen gases in an octapole-based collision cell, they could determine ppm levels of Cr, Cu, and Fe... 

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... 

Both types of process are a function of the collision frequencies and consequently of the pressure. Both can also be enhanced by increasing the residence times of the polyatomic species in the cell, denoted as the collision or reaction cell, and accordingly by introducing the gases into the quadrupole, octapole or hexapole cell, through which the ion trajectories and hence the collision probabilities are increased. 

A transmission hexapole, octapole or square quadrupole colhsion ceU to which an osciUating radio frequency potential is apphed that is filled with a collision gas at low pressure and used to generate collision induced dissociation (CID) of ions to form a product ion spectrum. The coUision cell has no mass separating capabihties. Axial fields accelerate product ions to leave the colhsion cell for a fast switch of SRM reactions. 

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