How does it work?
In ICP-MS, elemental abundances or isotopic ratios are determined by the mass spectrometry (MS) of ions generated in an inductively coupled Ar plasma (ICP).
As shown on the schematic above, the ICP-Mass Spectrometer consists of several integrated components:
- sample injection system,
- inductively coupled plasma,
- plasma sampling interface,
- mass analyzer,
- detector,
- computer.
Using a stream of Ar carrier gas, liquid or solid sample from sample injection systems is introduced into hot plasma which serves as an efficient source of positively charged analyte ions. The Ar plasma is generated and maintained at the end of the glass torch located inside the loops of an water cooled copper load coil. A radio frequency (RF) potential applied to the coil produces an electromagnetic field in the part of the torch located within its loops. A short electric discharge from a wire inside the torch provides the electrons to ignite the plasma. In the electromagnetic field of the load coil these electrons are accelerated and collide with Ar atoms in the Ar gas flowing through the torch producing Ar+ ions and free electrons. Further collisions cause an increasing number of Ar atoms to be ionized and result in the formation of plasma. The plasma-forming process rapidly becomes self-sustaining and may be maintained as long as Ar gas continues to flow through the torch.
The development of the interface region was crucial in the development of ICP-MS instruments. The main problem lies in the fact that the ions have to be extracted from the high temperature (~ 6000K or more), atmospheric pressure (760 torr) environment of often chemically corrosive Ar plasma into a mass spectrometer operating in a high vacuum (10-5 torr) at room temperature. The interface region contains two successive Ni cones with millimeter-sized orifices through which the ions in the center of the plasma may be sampled. The ions are first extracted trough the orifice of the sample cone into the region between two cones held at a pressure of about 1-3 torr by a large capacity rotary vacuum pump. At this stage, most of the Ar atoms are removed by a vacuum pump. The ion beam is further extracted through the orifice of the skimmer cone into the front section of the mass spectrometer chamber that is maintained at a pressure of about 10-3 - 10-4 torr by a large turbo molecular vacuum pump.
Focused by an ion lens system, analyte isotopes are separated according to their mass/charge ratio by either a quadrupole or magnetic sector mass analyzer. Ions with a specific m/z ratios are transmitted sequentially to the ion detection system. Ions with a lower or higher mass/charge ratios have different trajectories and are lost ("filtered").
The ion detection and counting system consists of an electron multiplier used in a dual-gain pulse counting mode or low gain analog mode depending on the ion-beam intensity.. The most commonly used type of detector in ICP-MS is an electron multiplier. The signal intensity is measured simultaneously at two different points in the detector. In the upper, analog stage of the detector, the signal is measured as a current which is subsequently converted into a count per second (cps) equivalent. This stage is responsible for measuring high intensity signals. The lower, pulse stage of the detector measures low intensity signals as cps. To prevent detector damage and prolong its life span, signals of exceedingly high intensities are prevented from entering the detector.
The ICP-MS instrument is computer controlled. The computer controls various hardware components as well as acquisition, storage, display and processing of the data.
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