Types of Plasma Sources

There are two popular types of plasma sources l) the direct current plasma (DCP), and 2) the inductively coupled plasma (ICP). In the commercial version of the former plasma source (marketed by Spectrometries, Inc.), the sample is aspirated with argon through a small orifice into a chamber where the large droplets settle out and the fine mist is conveyed by the argon stream through a chimney to the vertex of a plasma which is in the form of... 

An inductively coupled plasma (ICP) is a type of plasma source in which the energy is supplied by electrical currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields. See plasma. 

The analytical plasmas are classified according to the method of power transmission to the working gas. There are three dominant types of plasma source in use today (i) Inductively coupled plasmas, ICPs (ii) Direct current plasmas, DCPs (current carrying DC plasmas and current-free DC plasmas) (iii) Microwave plasmas (microwave induced plasmas, MIPs, and capacitively coupled microwave plasmas, CMPs). 

ICPs are by far the most common type of plasma sources used in today s commercial ICP optical emission (ICP-OES) and ICP mass spectrometric (ICP-MS) instrumentation. However, it was not always that way. In the early days, when researchers were attempting to find the ideal plasma source to use for spectrometric studies, it was not clear which approach would prove to be the most successful. In addition to ICPs, some of the other novel plasma sources developed were direct current plasmas (DCPs) and microwave-induced plasmas (MIPs). Before I go on to describe the ICP, let us first take a closer look at these other two excitation sources. 

When mass spectrometry was first used as a routine analytical tool, El was the only commercial ion source. As needs have increased, more ionization methods have appeared. Many different types of ionization source have been described, and several of these have been produced commercially. The present situation is such that there is now only a limited range of ion sources. For vacuum ion sources, El is still widely used, frequently in conjunction with Cl. For atmospheric pressure ion sources, the most frequently used are ES, APCI, MALDI (lasers), and plasma torches. 

Different types of ion sources used in mass spectrometry are compared in Table 2.2. Several plasma ion sources applied in analytical atomic spectrometry were described by Broekaert.33... 

Fourier transform ICR mass spectrometers together with any type of ion source, such as nanoESI, MALDI (or also an inductively coupled plasma ion source) permit mass spectrometric measurements to be performed at ultrahigh mass resolution (R = m/hm = 105—106) with a very low detection limit and the highest possible mass accuracy (Am = 10 3—10 4 Da). In addition, a high mass range is possible and FTICR-MS can be applied for MS/MS experiments.48 A comparison of different separation systems used in inorganic mass spectrometry is presented in Table 3.1. 

There are several types of ionization sources [MALDI, ESI, FAB (fast atom bombardment), PD (Cf-252 plasma desorption), El (electron ionization), Cl (chemical ionization) etc.], different types of mass analyzers [combinations of magnetic and electric sectors, quadrupolar filters (Q) and ion traps (IT), time-of-flight (TOF) and FT-ICR] and different detectors, each with its own advantages and drawbacks. We describe herein only the systems that presently have widespread use for the study of biomolecules ESI coupled to a quadrupole (or triple quadrupole, QqQ) mass analyzer or an ion trap, the MALDI source with the linear or reflectron TOF analyzer, and the FT-ICR system which can be equipped with both ESI and MALDI sources. 

With the so-called current-free or transferred plasma, the observation zone is situated outside the current-carrying zone. A source such as this can e.g. be realized by the use of a supplementary gas flow directed perpendicular to the direction of the arc current and by the observation zone being in the tail-flame. In this observation zone no current is flowing. This type of plasma reacts significantly on cooling as no power can be delivered to compensate for temperature drops. Therefore, it is fairly insensitive to the addition of easily ionized elements. They do not cause a temperature drop but only shift the ionization equilibrium and give rise to ambipolar diffusion, as discussed previously. 

Plasma sources are capable of producing intense emission from the elements. Types of plasma used in chromatographic detection are microwave induced plasmas (MIP) and inductively coupled plasma (ICP). An argon plasma is sustained in a microwave cavity which focuses into a capillary discharge cell. The most widely used cavities are cyhndrical resonance cavities and surfatron that operates by surface microwave propagation along a plasma column. Atmospheric pressure cavities are very simple to interface with capillary GC columns. 

Spark ablation. Spark ablation solid sampling uses the same type of spark source already described. The function of the spark in this case is to vaporize the solid sample the ICP plasma can atomize any nonatomic vapor reaching it. Spark ablation is limited to the analysis of solids that conduct electricity. It is very useful for metals and alloys because it eliminates time-consuming sample dissolution and costly high-purity acids. 

The original use of MS was for the detection and determination of elements. The elements have different masses, so MS provided a method of determining atomic weights. The various elements as well as their isotopes could be separated from each other with this technique. This made it possible to obtain the isotope distribution of pure elements (Appendix 10.1). The application of MS to the determination of atomic weights and isotope distribution was crucial in the development of atomic chemistry and physics. While there are several types of ionization sources for atomic MS, such as the glow discharge (GD) and spark source (described in Chapters 7 and 9), used for atomic mass spectrometric analysis of solids, it is the development of the inductively coupled plasma... 

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