Unlike simpler flame and furnace spectrometers, inductively coupled plasma optical emission spectroscopy (ICP) uses plasma to excite atoms in a sample into giving off light. The light can be analyzed by the spectrometer in order to identify and quantify the elements present. The plasma, almost universally made from argon gas, is created by electromagnetic induction. A radio-frequency electrical signal induces rapidly fluctuating electromagnetic fields, causing collisions and heating within the argon gas passing through the coil. The ICP torch can reach temperatures as high as 10,000 K, converting the argon gas into a charged plasma.

The sample, typically an aerosolized liquid or solution, is introduced directly into the plasma, which ionizes the sample causing it to emit light frequencies associated with the elements in the sample. The configuration of ICP instruments is much the same, but there are variations depending on the geometry and detector type. For sequential ICP, the light from the torch is directed to a diffraction grating that separates it into the different wavelengths. By rotating the diffraction grating, different wavelengths can be directed in turn to the detector, which is generally a photomultiplier tube (PMT). In other words, the PMT scans sequentially through the entire spectrum, one wavelength at a time. In contrast, simultaneous ICP either involves multiple PMTs or, as is more common nowadays, a solid-state area detector that can capture segments of the spectrum all at once.

One advantage of sequential ICP is that it allows the PMT-based ICP instruments to achieve lower detection limits than instruments with solid-state detectors. Sequential ICP also tends to be slightly less expensive than otherwise comparable ICP products. As with the general ICP market, sequential ICP has strong demand in environmental applications, such as water and soil testing. Similarly, evaluation of complex ores, minerals and metals can be achieved with the technique, including the assaying of gold and other precious metals. When only a few elements are of interest, atomic absorbance may be preferable to sequential ICP. When many elements are of interest, simultaneous ICP may be the preferable technique. Sequential ICP neatly fills the performance gap between the two.

Over the past five years, major ICP suppliers like Varian and Thermo Fisher Scientific have abandoned sequential systems in favor of solid-state detector systems. Three strong competitors remain in sequential ICP, though all three now also offer solid-state systems. Horiba Jobin Yvon specializes in high-end optical systems with excellent detection levels. Shimadzu and SII Nanotechnology compete primarily in the Japanese market. Other vendors of sequential ICP include Teledyne Leeman Labs, GBC Scientific and Beijing Beifenruili Analytical Instruments. The total market for sequential ICP, including aftermarket and service, was about $40 million in 2008.

Sequential ICP at a Glance:

Leading Suppliers

• Horiba Jobin Yvon

• SII Nanotechnology

• Shimadzu

Largest Markets

• Environmental

• Utilities

• Metals and Mining

Instrument Cost

• $45,000–$95,000