Many people are familiar with Inductively Coupled Plasmas (ICPs) as atom sources in optical emission spectroscopy. Similar devices are used in mass spectrometry - ICP-MS. Despite the superficial similarity, in ICP-MS, the ICP is an ion not an atom source. The ICP and sampling system must therefore be optimised to produce the maximum population of positively charged ions in mass spectrometry.
The population of ions in a plasma is a function of the element's ionisation potential and can be calculated theoretically using the Saha equation. Several parameters must be estimated, however the equation gives a good approximation of the actual values.
As well as offering the benefits of good matrix dissociation and low molecular interferences, higher temperature plasmas also allow the analysis of difficult to ionise elements such as As, Se and Hg.
Using the Saha equation, the degree of ionisation was calculated for three assumed plasma temperatures, 6000K, 7000K and 8000K. The graph highlights how the population of As ions changes with plasma temperature. Small changes in plasma temperature can have big changes on the degree of ionisation for elements with high ionisation potential, so it is important to ensure that the sampling/ionisation system of an ICP-MS is properly optimised.
Several factors combine to deliver robust sampling/ionisation and all should be considered when designing an ICP-MS:
- Low sample uptake (300-400 µL/min), with no sensitivity compromise, so that the plasma is not overwhelmed by excess sample.
- Cooling the spray chamber (preferably with an efficient thermoelectric device) reduces the amount of solvent vapour (usually water), from reaching the plasma and absorbing energy, so causing a 'cooling effect'.
- Wide diameter injector tubes in the torch allow the sample to be better dispersed and so extend the residence time in the plasma. This has the effect of ensuring more complete sample dissociation and ionisation.
- 27.12 MHz RF generators produce a hotter more efficient plasma than 40.68 MHz devices (that are more typically used in ICP-OES).
- Solid state crystal controlled RF electronics produce 75% coupling efficiencies (compared to 50-55% for older power tube designs), as well as offering longer lifetime and long-term better stability.
- Motorised 3-axis torch positioning that allows users to easily optimise the sampling position for maximum sensitivity and minimum interfering polyatomic species.
Element: oxygen bonds are some of the most stable and require a large amount of energy to break. The 'oxide' test in ICP-MS is a useful measure of the efficiency of the ICP. This test usually involves running a solution containing an element, and monitoring the ratio of the signal from the ion (M+) and the signal 16 mass units higher, the oxide peak (MO+). Cerium is the element most often used for this test because of the strength of the Ce:O bond. The specification is usually presented as the percentage MO+ as a function of M (MO+/M+%).
Measuring Trace Mercury In Wastewater
Samples with high dissolved solids absorb energy from the plasma, lowering the temperature and efficiency. Small changes in plasma temperature have a large effect on sensitivity, particularly of elements with high ionisation potential. A true test of plasma robustness is the analysis of a difficult to ionise element in a matrix of high dissolved solids.
The instruments in the Agilent 7500 series have been designed with an efficient sampling/ionisation system. By optimising the ICP for maximum efficiency, the source is powerful enough to ensure that elements with high ionisation potential can be easily ionised, even in matrices with high dissolved solid content.
