Inductively Coupled Plasma Mass Spectrometry ICP-MS
The ICP-MS is an analytical technique that determines the elemental content of samples. It is accomplished by counting the number of ions at a certain mass of the element. Most samples analyzed by ICP-MS are liquid. Solid samples can be analyzed but they must be vaporized using e.g. lasers or heat cells. Gas samples can also be measured by introducing them directly into the instrument.
The ICP-MS instrument measures most of the elements in the periodic table. The elements can be analyzed with detection limits at or below the part per trillion (ppt). The ICP-MS detects only elemental ions and can determine the individual isotopes of each element.
Fig. 1. Elements determined by ICP-MS and approximate detection capability (PerkinElmer)
An ICP-MS consists of the following components:
- sample introduction system – consist of the peristaltic pump, nebulizer, and spray chamber that introduces sample to the instrument,
- ICP torch – generates the plasma which serves as the ion source of the ICP-MS, converting the atoms to be analysed to ions,
- interface – the sample ions are extracted from the central plasma channel and separated from the bulk ions by cooled conical aperture plates with apertureopenings of 1/0.8 mm in the vacuum interface (vacuum <2mbar),
- vacuum system – provides high vacuum for ion optics, quadrupole and detector,
- quadrupole – the high frequency quadrupole acts as a mass filter to sort ions by their mass-to-charge ratio (m/e). The mass resolution with constant peakwidths from 0.5 to 1 amu at 10% peak height can be set in three steps,
- detector – after passing mass filter the ions are either detected through direct current measurements on the ion collector or the ions generate secondary electrons that are propagated in the multiplier. Together, both methods can cover an intensity range from a few ions/s to 1012 ions/s.
- data handling and system controller.
Fig. 2. Typical detection limit ranges for the major atomic spectroscopy techniques (Perkin Elmer)
Fig. 3. General selection guide for atomic spectroscopy instrumentation based on sample throughput and concentration range (Perkin Elmer)
The Main Unique Aspects Of The Machine
Speed: The quadrupole mass analyzer is able to scan the mass spectrum from 3-250amu 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: easily able to detect trace levels of many elements at levels well below a ppb (ng/g).
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.
Inability to resolve target isotopes 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 40Ar 16O 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.