ICP-MS - Comparison of Techniques: Quadrupole
A commercially available quadrupole ICP-MS is typically able to work at an effective mass resolution of 3-400. This is enough to result in a peak width of a little under 1 mass unit. The quadrupole mass analyzer therefore is able to differentiate easily between the different isotopes of a single element or between one element and another of differing mass.
Advantages of Quadrupole ICP-MS include:
- Speed: The quadrupole mass analyzer is able to scan the mass spectrum from 3-250 very quickly. A mass spectrum of usable data can be acquired in just a few seconds depending on exact instrument settings.
- Mass Stability: As there are no magnetic fields in the quadrupole ICP-MS, it is able to move from mass to mass with a superb degree of precision. This enables the analysis technique known as "peak hopping" in which only a single point of data is acquired at the very top of the peak at each element during an analysis.
- Sensitivity: Modern quadrupoles such as the PlasmaQuad III at MURR are easily able to detect trace levels of many elements at levels well below a ppb (ng/g).
- Reliability: Quadrupole ICP-MS systems are rapidly becoming the workhorse of many analytical laboratories throughout the world. They reliably turn out accurate data day in and day out. Many are even left to run overnight unattended.
- Cold Plasma Capability: Cold or cool plasma is a technique whereby the temperature of the plasma is reduced by lowering the RF power. This partially prevents the formation of Ar-based molecular interferences by reducing the number of Ar+ ions in the plasma. While a little awkward to use, this technique allows for the analysis of elements with large molecular interferences such as potassium and iron.
Disadvantages of Quadrupole ICP-MS include:
- Inability to resolve target isotope easily from molecular interferences: Commercially available quadrupole ICP-MS systems are able to resolve a mass spectrum only to unit resolution. This means that while the mass analyzer can easily tell the difference between 56Fe and 57Fe, they cannot resolve 56Fe (mass 55.9349) from the 40Ar16O molecular species (mass 55.9573), which is very easily formed in an Argon plasma. To accurately determine the concentration of some difficult elements, it is necessary to compromise sensitivity with the use of techniques such as "cold plasma."
- High Background Noise: The ion optics of quadrupole mass analyzers make them susceptible to background noise on the detector, particularly when coupled to an ICP source. A few stray high-energy photons from the plasma source always seem to make it through to the detector, sending false pulses into the counting electronics. Because the ultimate limit of detection (LOD) of any system is directly proportional to variations in the background noise, higher noise levels obviously will result in compromised LODs.