Types of ICP analysis

ICP-MS now comes in various guises. The primary differences are in the way in which the sample is introduced into the plasma (either as a solution, or by laser ablation of a solid sample), and in the type of magnetic separation applied to the ions. Whilst the type of magnetic separation is a fixed property of the instrument and cannot therefore be changed, most instruments can accommodate either solution or laser ablation sample presentation. 

However, each of these requires a different hardware configuration, and therefore specific tuning of the instrument, before use. It is not possible to switch between these modes during the same experimental run. 

The digestion of solid samples to produce a solution is discussed in Section 13.2. For solution-based ICP MS analysis, the liquid is taken up through a thin tube via a peristaltic pump. This feeds directly into the instrument nebulizer, where argon gas is introduced into the liquid and a fine mist of droplets is expelled from the tip of the nebulizer. This sample aerosol is sprayed into the condenser to reduce the size of the droplets, ensuring an even sample loading and preventing cooling of the plasma. About 1% of the sample solution uptake is transported to the plasma torch, and any unused solution is drained away and may be recycled. 

The plasma is maintained at a temperature of 10 000° C by an external radio frequency current, as described in Section 3.3. At this temperature, many molecular species are broken down, and approximately 50% of the atoms are ionized. So far this is identical to ICP-OES, but for ICP-MS we are not interested in the emission of electromagnetic radiation, but rather in the creation of positive ions. To transfer a representative sample of this plasma ion population to the mass spectrometer, there is a special interface between the plasma and the mass spectrometer. This consists of two sequential cones 

The sample is usually ablated using a Nd-YAG ultraviolet laser (k = 266 nm the theory of laser light is given in Section 4.1). Earlier systems used an infrared laser, but chemical fractionation was found to be a problem (Jeffries et al. 1996). A movable stage and integral optical microscope allow 

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