Agilent 9500 ICP-MS with Dual-Cell System (DCS)
Introduction
The Agilent 9500 builds upon the expertise gained from four generations of Collision Reaction Cell (CRC) development and two generations of ICP-QQQ (or ICP-MS/MS) from the Agilent 8800 and 8900 ICP-QQQ systems.
The 9500 is not a replacement for the 8900. It is an additional instrument with features and developments suitable for labs requiring high-speed analysis and ICP-QQQ performance, but not necessarily all reaction gas modes. As standard, the 9500 is configured with two cell gas mass flow controllers, one for helium and the other for air (pure O2 can also be used). This configuration is intended to cover most applications. The 9500 can be used as a direct replacement for a single quadrupole ICP-MS (SQ-ICP-MS), Figure 1, as it requires only argon and helium gas supplies.
Helium collision mode has been significantly enhanced with a completely new CRC design featuring two independently controlled ion guides. These ion guides enable the DCS to deliver superior sensitivity and interference suppression, even surpassing the performance of the ORS4. This new mode of operation is called Advanced Helium Mode (AHM).
When operating in air reaction mode (Air Cell), the instrument draws ambient air through a built-in filter. The filtered air is then passed into the DCS to perform oxygen reactions on targeted interferences.
For most labs, this configuration will be sufficient. However, there is an additional option for highly advanced applications, such as for research and high-purity chemical testing. This option adds two additional gas controllers for the DCS, typically one for H2 and the other for ammonia (as a 10% blend in He). The controllers can be specified at the time of purchase or added at any time as an upgrade. The gases can be substituted for any supported gas*.
*Check with your Agilent Product Specialist for the currently recommended alternative gases.
Advanced Helium Mode
AHM combines the sensitivity of no gas mode for low-mass elements (for example, Li, Be, B), helium Kinetic Energy Discrimination (KED) for most interferences, and Collision Induced Dissociation (CID) for intense polyatomic interferences (for example, ArAr, ArO, ArCl, NaAr, and more) into a single advanced helium mode. Interference removal is further improved because the DCS features KED inside the cell as well as an external barrier to remove any unwanted cell-derived species, and both KED and CID mechanisms are permanently employed during AHM operation.
With no need to change modes or gas flow rates, a single acquisition can be used for all elements across the majority of sample types. AHM not only simplifies method development but also improves analysis speed. Due to the higher sensitivity of ICP-QQQ, shorter integration times can be used while retaining or exceeding current DLs and BECs compared to SQ-ICP-MS, further improving throughput.
How the DCS enables AHM
The DCS consists of two sets of rods that can be independently controlled. They are aligned as front and rear ion guides (called Front Cell and Rear Cell). When operated in a “locked” configuration, both ion guides have the same electrical properties and act as a single ion guide. This mode of operation would be used for no-gas or non-AHM helium modes, as well as reaction modes. Figure 2 shows the design of the DCS, including the front and rear cells.
When operating in AHM, the front and rear cells operate with different parameters. The front cell accelerates the ions to induce CID (similar to High Energy Helium mode) and also performs KED. The rear cell acts as the first KED barrier (it has a positive potential) and then “thermalizes” the ions. Thermalization is a process in which ions are slowed, and the energy difference between them is significantly reduced, thereby enabling greater focusing efficiency and sensitivity while ensuring good abundance sensitivity despite high-energy collisions in the front cell. A final KED barrier is applied after the rear cell by applying a potential bias voltage to Q2, before the detector (Figure 3).
When measuring low-mass ions (< 23 u), the cell rapidly switches its operating mode to significantly increase ion transmission efficiency and sensitivity for these ions, without altering the helium gas flow rate. Sensitivity for low-mass elements in AHM is 20x higher than when operating in helium KED; see Figure 4. This allows the full mass spectrum to be measured in a single, optimized acquisition; the operator does not need to select which mode, only the analyte, and the software works together with the 9500, providing the fastest, most optimal acquisition.
How the DCS enables Air Cell mode
When adding air to a CRC, care must be taken to control the reactions properly, as only oxygen reactions are desired. Any water vapor or other reactive contaminant in the laboratory air (for example, solvent vapors) would cause unpredictable side reactions, rendering the feature unusable. For ambient air use as a cell gas, the 9500 has Agilent proprietary Air cell technology featuring a built-in air filter (Figure 5) with a molecular sieve and activated charcoal to remove undesired reactive species from the air. This way, only nitrogen and oxygen are the primary species in the cell (with traces of argon and other noble gases, which have a negligible effect).
Because nitrogen is essentially inert in the CRC, the primary processes in Air Cell mode are oxygen reactions. The benefits of oxygen reaction mode are well known1 and can help address several problematic interferences that He KED mode cannot resolve. Table 1 lists potential interferences on key elements that oxygen reactions can mitigate. Note that these interferences may not be present in all samples, so solutions can often be measured by AHM only.
Table 1. Potential interferences where Air Cell mode (or O2) may provide an additional benefit in interference removal over helium KED.
Operating in Air Cell mode automatically switches to MS/MS operation and locks the front and rear cell guides so they act as a single large ion guide. The central ion guides scan their voltages and frequencies based on Q1 to produce optimal ion transmission for the selected ion.
Oxygen is the reactive species in the cell, so oxygen reactions predominate the interference-removal process. The inert nitrogen acts as a “buffer” gas, which can thermalize the ion beam and increase ion residence time within the cell. Some KED processes may also occur and are desirable for certain interferences; they can affect some product ion formation compared to pure oxygen. However, the difference is not significant compared with the interference-removal performance, and Air Cell mode still yields equivalent DL and BEC across most matrices.
Air Cell mode brings the convenience of oxygen reaction chemistry to all labs, regardless of whether a pure oxygen supply is available or not.
How does the DCS maintain sensitivity in Air Cell mode?
When operating in Air Cell mode, the majority of the gas is nitrogen (78%), which can “get in the way” of the analyte ions and reduce sensitivity due to collisional losses (i.e., N2 molecules collide with the analyte ions, potentially causing signal loss). The DCS features axial acceleration in each of the front and rear ion guides (Figure 6). This creates a “funnel” shape, producing a potential gradient between the entrance and exit of each part of the cell. When combined with optimized scanning of the ion guides, this design ensures maximum ion transmission and minimal sensitivity losses.
What applications can I use the DCS, AHM, and Air Cell for?
The 9500 and DCS are well-suited to routine analysis; AHM is designed as a single high-speed operating mode for most applications. For samples with high levels of Rare Earth Elements (REE) or very high levels of Mo or W, Air Cell mode can be employed. The 9500 with DCS, AHM, and Air Cell offers great flexibility without the complexity.
For real examples of 9500 with AHM and Air Cell performance, please check the following Application Notes:
For advanced applications (including additional cell gas controllers):
Reference:
- Handbook of ICP-QQQ Applications using the Agilent 8800 and 8900
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