Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry which is capable of detecting metals and several non-metals at concentrations as low as one part in 1012 (part per trillion). This is achieved by ionizing the sample with inductively coupled plasma and then using a mass spectrometer to separate and quantify those ions. An inductively coupled plasma is a plasma that is energized by inductively heating the argon gas with a electrical coil, and contains a sufficient concentration of ions and electrons to make the gas electrically conductive. A number of different ICP-MS designs are commercially available today and share similar components such as the nebulizer, spray chamber, plasma torch, interface and detector.
The liquid sample is pumped into a nebulizer where it is converted into a fine aerosol with argon gas. The fine droplets (1-2%) are separated from larger droplets using a spray chamber. The fine aerosol then emerges from the exit tube of the spray chamber and is transported into the plasma torch through a sample injector. The plasma is produced by the interaction of an intense magnetic field produce by radio frequency (RF) passing through a copper coil on a tangential flow of argon gas flowing through concentric quartz tube known as torch. This ionizes the argon gas. The plasma torch is used to generate positively charged ions. Once the ions are produced in the plasma, they are directed into the mass spectrometer via the interface region, which is maintained at vacuum of 10-2 torr with a mechanical roughing pump. This interface region consists of two or three metallic cone usually made of nickel and platinum, called the sampler and a skimmer cones. Each cone has a small (0.6 - 1.2 mm) orifice to allow the ions through to the ion optics where they are guided into the mass separation device.
Once the ions have been extracted from the interface region they are directed into the ion optics by a series of electrostatic lenses. So, main function of ions optics is to electrostatically focus the ions beam towards the mass separation device. The ion beam contain all the analyte and matrix ions exit the ion optics and pass into the mass separation device- mass spectrometer (vacuum about 10-6 torr). Most common types of mass spectrometer use quadrupole type mass spectrometer. The main function is to allow analyte ions of a particular mass-to-charge ratio through to the detector and to filter out all the non-analyte, interfering and matrix ions.
In the final process, an ion detector converts ions into an electric signal. The most common design used today is a discrete dynode detector. This contains a series of metal dynodes along the lengths of a detector where the ions emerge from the mass filter; they impinge on the first dynode and are converted into electrons. As the electrons are attracted to the next dynode, electrons multiplication takes place, resulting in a very high stream of electrons emerging from the final dynode. This electronic signal is then processed by the data handling system and converted into analyte concentration.
In recent years, industrial and biological monitoring has presented major need for metal analysis by ICP-MS and other uses is in the medical and forensic field, specifically, toxicology and heavy metal poisoning. ICP-MS is undoubtedly the fastest growing trace element technique available today. It allows determination of elements with atomic mass ranges 7 to 250. It is able to detect the elements upto part per trillion levels and this ability to carry out rapid multi-elements determination at the ultra-trace level have made it very popular in diverse range of applications areas including environment, geochemical, semiconductor, metallurgical, nuclear, chemical, climatic and biotechnology.
About Author / Additional Info:
Dr. Suresh Kaushik
Molecular Biology and Biotechnology
A Biotechnological Professional from India