On-mass analysis, as the name suggests, is based on transmitting the isotope of interest through the first quadrupole (Q1) while rejecting either the majority of, or all of, the other isotopes in the ion beam (more on this later). The selected mass plus its associated interference then enters the collision cell (Q2) which is filled with a gas such as O2. In this case, the analyte ion (e.g. Cd+) does not react with O2, while the interference (e.g. ZrO+ / ZrOH+) reacts rapidly to form higher oxides (such as ZrO2+ / ZrO2H+). The unreacted target mass is then transmitted, into the third quadrupole (Q3), where it is now effectively and completely separated from the interference, and detected.
In contrast, with mass-shift analysis, after mass filtering the isotopes of interest with Q1 (in the same way as with on-mass analysis), it is the target analyte that is reacted with the cell gas in Q2 while the associated interference doesn’t react. In the cases of CoO+ and Nd2+ interference on As, and Gd2+ interference on Se (which are problems that arise during analysis of, for example, nutraceuticals containing Vitamin B and environmental samples such as soil / sewage digests, respectively), As and Se are converted by collision with O2 into AsO+ and SeO+ while CoO+, Nd2+ and Gd2+ do not react in the same way. The AsO+ and SeO+ product ions are then detected, interference free, after mass selection by Q3.
As well as these two triple quadrupole modes, instruments such as the iCAP TQ ICP-MS can be operated in single quadrupole mode with no cell gas and with He/H2 cell gases for analyses that don’t require triple quadrupole selectivity. With all these different possible operating modes, you might be thinking that these instruments are very complicated to use. After all, if you are not so experienced with ICP-MS, how do you know which mode is best for all the elements you need to measure? The Thermo Scientific Qtegra Intelligent Scientific Data Solution (ISDS) software that operates the iCAP TQ ICP-MS solves this complexity using a software-based method assistant called Reaction Finder, which automatically selects the optimum measurement mode, Q1 analyte, reaction gas and Q3 mass to be detected for you. If you want to compare the performance of different modes for a particular analysis within a single run, you can simply duplicate the isotope of interest and select a different mode from a drop-down list of available modes.
On-mass and mass-shift operation with Reaction Finder mode selection is shown schematically in Figure 3.
In addition to the examples given above, the extended flexibility and capability of triple quadrupole ICP-MS also enables highly challenging analyses, including:
- Ti detection in clinical research specimens, which requires resolution of Ti+ from PO+, Ca+ etc., using NH3 cell gas to convert Ti to TiNH(NH3)3+ at mass 114;
- Separation of 87Sr+ from 87Rb+ for Sr isotope ratio studies, using O2 to convert Sr+ to SrO+ while leaving Rb+ unreacted;
- Ultra-trace detection of Se in Ni alloys, by converting Se+ to SeO+ using O2, to resolve it from NiO+, NiOH+ and BrH+ interferences; and
- Cd quantitation in Zr alloy nuclear fuel rod cladding material, using O2 to remove the ZrO+ and ZrOH+ interferences on Cd+.
As mentioned above, Q1 can select the isotope of interest and reject either the majority of, or all of, the other isotopes. In most applications, using Q1 to transmit only the isotope of interest is not essential and only serves to potentially reduce sensitivity and degrade detection limits compared to operating it with a wider mass range. The iCAP TQ ICP-MS uses the advantage of controlling the mass transmission through Q1 to optimise sensitivity and detection limits for analyses that do not require single mass Q1 transmission, using an optimised software protocol to control Q1 called Intelligent Mass Selection (iMS). For those few analyses that do benefit from single mass transmission through Q1, the iCAP TQ ICP-MS software can be set by the user to a ‘higher’ mass resolution mode to selectively transmit only the target isotope through Q1 at a peak resolution of less than 1 atomic mass unit (amu). All of the above-listed interference problems addressed by triple quadrupole ICP-MS can be solved using iMS — none of them require single mass transmission through Q1, making analysis more effective, simpler and faster for both routine and research applications.
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