Reaction Mechanisms with Highly Reactive Gases and Discrimination by Selective Bandpass Mass Filtering

Another way of rejecting polyatomic interfering ions and the products of secondary reactions/colli- sions 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 quad- rupole (instead of a hexapole or octapole) inside the reaction/collision cell and use it as a selective bandpass (mass) filter. There are a number of commercial designs using this approach, so let’s take a look at them in greater detail in order to better understand how they work and how they differ.

Dynamic Reaction Cell

The first commercial instrument to use this approach was called dynamic reaction cell (DRC) technology,15 Similar in appearance to the hexapole and octapole CRCs, 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 them 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 analyzer quadrupole for conventional mass separation.

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 and not just as an ion-focusing guide. 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. It means that every time an analyte and interfering ions enter the DRC, the bandpass of the quadrupole can be optimized for that specific problem and then changed on the fly for the next one. This is shown schematically in Figure 13.5, where an analyte ion 56Fe+ and an isobaric interference 40Arl6O+ enter the DRC. As can be seen, the reaction gas NH, picks up a positive charge from the 40Arl6O+ ion to form atomic oxygen, argon, and a positive NH, ion (this is known as a “charge transfer reaction”). There is no reaction between the 56Fe+ and the NH„ as predicted by thermodynamic reaction kinetics. The quadrupole’s electrical field is then set to allow the transmission of the analyte ion 56Fe+ to the analyzer quadrupole, free of the problematic isobaric interference, 40Arl6O+. In addition, the NH,+ is prevented from reacting further to produce a new interfering ion.

The practical benefit of using highly reactive gases is that they increase the number of ion- molecule reactions taking place inside the cell, which results in a faster, more efficient removal of the interfering species. Of course, they will also generate more side reactions which, if not prevented, will lead to new polyatomic ions being formed and could possibly interfere with other analyte masses. However, the quadrupole reaction cell is well characterized by well-defined stability boundaries. So, by careful selection of bandpass parameters, ions outside the mass/charge (m/z,) stability boundaries are efficiently and rapidly ejected from the cell. It means that additional reaction chemistries, which could potentially lead to new interferences, are successfully interrupted. In addition, the bandpass of the reaction cell quadrupole can be swept in concert with the bandpass of the quadrupole mass analyzer. This allows a dynamic bandpass to be defined for the reaction cell

Elimination of the Ar’O interference with a DRC. (Copyright 2013, all rights reserved

FIGURE 13.5 Elimination of the 40Arl<’O+ interference with a DRC. (Copyright 2013, all rights reserved,

PerkinElmer Inc.) so that the analyte ion can be efficiently transferred to the analyzer quadrupole. The overall benefit is that within the reaction cell, the most efficient thermodynamic reaction chemistries can be used to minimize the formation of plasma- and matrix-based polyatomic interferences, in addition to simultaneously suppressing the formation of further reaction by-product ions.

The process described can be exemplified by the elimination of 40Ar+ by NH, gas in the determination of 40Ca+. The reaction between NH, gas and the 40Ar+ interference, which is predominantly charge transfer/exchange, occurs because the ionization potential of NH, (10.2eV) is low compared to that of Ar (15.8eV). This makes the reaction extremely exothermic and fast. However, as the ionization potential of Ca (6.1 eV) is significantly less than that of NH,, the reaction, which is endothermic, is not allowed to proceed.15 This can be seen in greater detail in Figure 13.6.

Of course, other secondary reactions are probably taking place, which you would suspect with such a reactive gas as ammonia, but by careful selection of the cell quadrupole electrical fields, the optimum bandpass only allows the analyte ion to be transported to the analyzer quadrupole, free of the interfering species. This highly efficient reaction mechanism and selection process translates into a dramatic reduction of the spectral background at mass 40, which is shown graphically in Figure 13.7. It can be seen that at the optimum NH, flow, a reduction in the ‘‘“Ar* background signal of about eight orders of magnitude is achieved, resulting in a detection limit of approximately 0.1 ppt for 40Ca+.

One final thing to point out is that when highly reactive gases are used, the purity of the gas is not so critical because the impurity is almost insignificant in determining the ion-molecule reaction mechanism. On the other hand, with collision and low-reactivity gases that contain impurities, such as carbon dioxide, hydrocarbons, or water vapor, the impurity could be the dominant reaction pathway as opposed to the predicted collision/reaction with the bulk gas. In addition, the formation of unexpected by-product ions or other interfering species, which have the potential to interfere with other analyte ions in a KED-based collision cell, is not such a serious problem with the DRC system because of its ability to intercept and stop these side reactions using the bandpass mass-filtering discrimination process.

These observations were, in fact, made by Hattendorf and Gunter, who attempted to quantify the differences between KED and bandpass tuning with regard to suppression of interferences

A reduction of eight orders of magnitude in the Ar background signal is achievable with the DRC, resulting in a 0.1 ppt detection limit for Ca'. (Copyright 2013, all rights reserved, PerkinElmer Inc.)

FIGURE 13.7 A reduction of eight orders of magnitude in the 40Ar+ background signal is achievable with the DRC, resulting in a 0.1 ppt detection limit for 40Ca't. (Copyright 2013, all rights reserved, PerkinElmer Inc.)

generated in a CRC for a group of mainly monoisotopic element (Sc, Y, La, Th) oxides.16 They observed that when the collision/reaction gas contains impurities such as water vapor or ammonia, a broad range of additional interferences are produced in the cell. Depending on the relative mass of the precursor ions (ions that are formed in the plasma) compared to the by-product ions formed in the cell, there will be significant differences in the way these interferences are suppressed. They concluded that unless the mass (or energy) differences between the precursor and by-product ions are large, there will be significant overlap of kinetic energy distribution, making it very difficult to separate them, which limits the effectiveness of KED to suppress the cell-generated ions. On the other hand, bandpass tuning can tolerate a much smaller difference in mass between the precursor and by-product interfering ions because of its ability to set the optimum mass/charge cut-off at the point where these interfering ions are rejected. In addition, they found that the bandpass tuning method can use a heavier or denser collision gas if desired, without suffering a loss of sensitivity due to scattering observed with the KED method. The overall conclusion of their study was that “under optimized conditions, the bandpass tuning approach provides superior analytical performance because it retains a significantly higher elemental sensitivity and provides more efficient suppression of cell-generated oxide ions, when compared to kinetic energy discrimination.”

 
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