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ICP-MS — Comparison of Techniques: High-Resolution
A typical commercially available high-resolution ICP-MS is able to operate at a resolution of 10,000 or more. This is enough to fully resolve most isotopes from any molecular species that would otherwise interfere with their analysis. High-resolution systems use a powerful electromagnet with a varying magnetic field rather than an RF/DC field used in a quadrupole.

Advantages of High-Resolution ICP-MS include:
  • Variable Resolution: The VG Axiom can be set up at any resolving power from 300 to 10,000 by means of two swinging gate slits. By varying the resolution of the system, it is possible to tailor an analysis in such a way that each element is analyzed at a resolution that will enable it to be fully resolved from any interferences, without over-resolving it. 56Fe theoretically can be resolved from its interference (40Ar16O) at a resolution of 2500. In practice the ArO peak is so big that it tails into the Fe peak, making it necessary to use a much higher setting.
  • High Sensitivity: High-resolution systems are known for their extremely high ion transport efficiency. This efficiency gives rise to very high levels of sensitivity. The VG Axiom at MURR usually returns a count rate of greater than 1,000,000 counts per second for a solution of 1ppb (1µg/l) of 115In.
  • Low Noise: Due to the design of the high-resolution mass analyzer, there is virtually no chance of any stray photon making its way all the way through the instrument. This results in extremely low background noise, typically less than 0.2 counts per second. Such low noise levels, along with very high sensitivity, give the high-resolution system unparalleled Limits of Detection (LOD). For some elements it is possible to reach LODs of less than 1ppq (1 fg/g).
Disadvantages of High-Resolution ICP-MS include:
  • Slow Analysis Speed: Because the high-resolution mass analyzer uses an electromagnet to separate the masses, it must change the magnetic field strength between one mass and the next. This takes significantly longer than changing an electrical field.
  • Mass Drift: At higher resolutions, peaks are extremely narrow, so it becomes much more difficult for the magnet to move to exactly the same mass each time an element is measured. For this reason, high-resolution peaks are always measured by scanning across the entire peak. While this ensures that no counts are missed, peak scanning takes longer than the "peak hopping" used in a quadrupole instrument.
High-resolution scan showing the clear separation of 56Fe from the interference peak 40Ar16O high resolution scan of Fe-56

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Last updated August 2008