Principles of Octopole Reaction System (ORS)
 The 7700x and 7700s ICP-MS instruments use a third generation Octopole Reaction System (ORS3) to eliminate interferences arising from the plasma and sample matrix.
The 7700x uses helium collision mode, while the 7700s utilizes helium and hydrogen gas modes as standard. Alternative gases can be introduced into the ORS3, depending on the application, e.g. xenon and ammonia.
Advantages of the ORS3
- Ease of use - the ORS3 employs helium collision mode so side reactions that would create unpredictable, new interferences are eliminated.
- Reliability - all measurements are made directly on the analyte mass giving more reliable results.
- Effectiveness – polyatomic interferences are reduced in the ORS3 under one set of cell conditions irrespective of the composition, source, or reactivity of the interfering species
- Fast gas switching - the small size of the octopole allows the cell to have a small internal volume, which allows fast switching between gas modes (where multiple gas modes are used).
The Cell at Work
The ORS3 operates effectively without using highly reactive gases. This is the result of efficient energy discrimination (ED) and the small internal volume, which means a high cell pressure can be used to increase the reactivity of the cell gas. The following two video clips show the immediate effect on the presence of the interference when hydrogen or helium is added to the cell.
- Video 1 – Helium mode: A solution of sodium chloride (1.1g/L) is being aspirated into the 7500. The video shows the signal for sodium argide that interferes with copper at mass 63. The other masses being monitored are copper at mass 65 (copper being present as a contaminant), and argon chloride at mass 75, causing an interference on the only isotope of arsenic at the same mass. Note the sharp reduction in the signals for the polyatomic species under one set of cell conditions.
- Video 2 – Hydrogen mode: The video shows the signal for the argon dimer at mass 78 and mass 80, and yttrium at mass 89 (89Y). Note the immediate dramatic drop in signal for both Ar2 species when hydrogen is introduced to the ORS3 at a flow rate of 4.5 mL/min. The counts per second for Y increase slightly due to collisional focusing, at the same time as the attenuation of the interferences. When the hydrogen is turned off, the cell vents rapidly (due to its small internal volume), leading to an immediate reversal of the process.
Interference Removal Processes [1,2]
The octopole ion guide is contained within the ORS3 cell, which can be pressurized with a gas, typically helium (hydrogen for semiconductor applications). The ORS3 is positioned between the ion lens assembly and the quadrupole mass filter. As ions from the sample enter the cell, they interact with the gas, resulting in the reduction of the molecular interference by the processes described below.
- Helium Mode
- Collisional Induced Dissociation (CID) - the polyatomic interference bond energies are lower than the collision energy between the He atom and interference. CID is limited to polyatomic species with low bond energies such as ArNa+, ArO+.
- Kinetic Energy Discrimination (KED) - the larger polyatomic species (greater ionic radii) collide more frequently with the cell gas, they lose more energy than the smaller analyte species, and can be prevented from entering the quadrupole by a positive potential step at the cell exit – effectively the cell acts as a molecular filter by resolving low energy (polyatomic) and higher energy (analyte) ions from each other in the ion beam
- The Agilent ShieldTorch System produces ions with a narrow energy spread, which makes interference removal by KED very efficient. This allows for effective operation in He collision mode, unlike all other cell ICP-MS which can only operate with a reactive gas.
- Hydrogen Mode
- Charge Transfer -- an electron is transferred from neutral hydrogen to the interfering ion, resulting in a charged H ion and a neutral interfering species, which is system not focused through the analyzer and so disappears from the mass spectrum.
- Proton Transfer -- a proton is transferred to or from the interfering species, removing it from the analyte mass. Typically the new species has low energy and is lost through ED processes (see ED for more information).
Hydrogen reaction processes are quite specific and mostly target argon-based polyatomic species, such as Ar+, ArAr+, and ArO+, but are also effective in removing backgrounds due to other species, such as N2. </UL
Hydrogen reaction processes are quite specific and mostly target argon-based polyatomic species, such as Ar+, ArAr+, and ArO+, but are also effective in removing backgrounds due to other species, such as N2.
References
1. "The Effects of Cell-gas Impurities and Kinetic Energy Discrimination in an Octopole Collision Cell ICP-MS under Non-thermalized Conditions," Yamada N., Takahashi J. and Sakata K., JAAS 2002, 17, 1213-1222
2. "Operating Principles of the Agilent Octopole Reaction System (ORS)," Yamada T., and Yamada N., The ICP-MS Journal, August 2002, Issue 13, Agilent publication number 5988-7502EN
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