The Inductively Coupled Plasma Ion Source

Self-evidently, collisions between the ions and gas molecules disturb the ion beam and have to be avoided to the lar st possible extent. As a result, in all MS instrumentation, the mass spectrometer and detection system are brought under 

The argon inductively coupled plasma (ICP) is a plasma ion source [2, 3]. A plasma is defined as a gas mixture at high temperature, containing ions and electrons in addition to neutral particles. The presence of these charged particles allows energy transfer into the plasma via induction. 

When sample aerosol (see below) is introduced into an ICP, the droplets are desolvated, molecules are broken down into the constituting atoms and these are ionized via electron impact and Penning ionization (ionization as a result of energy transfer from an excited Ar atom). Although an ICP is not in thermal equilibrium, its ionization efficiency can be decently estimated on the basis of the Saha equation [5]  

The degree of ionization a can be deduced from the ionization constant  

---

The inductively coupled plasma ion source was developed to accomplish exactly the opposite of the two soft ionization methods just described. Molecules are reduced to their atomic (i.e. elemental) components through the application of intense energy. Masses corresponding to elements of interest (e.g. 30.974 for phosphorus) are then specifically monitored. 

Guilhaus M (2000) Essential elements of time-of-flight mass spectrometry in combination with the inductively coupled plasma ion source. Spectrochimica Acta 55 1511-1525. 

The ion source is an essential component of all mass spectrometers where the ionization of a gaseous, liquid or solid sample takes place. In inorganic mass spectrometry, several ion sources, based on different evaporation and ionization processes, such as spark ion source, glow discharge ion source, laser ion source (non-resonant and resonant), secondary ion source, sputtered neutral ion source and inductively coupled plasma ion source, have been employed for a multitude of quite different application fields (see Chapter 9). 

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. 

One of the more recent branches of atomic spectrometry, although perhaps the most exciting one, is atomic mass spectrometry, which has had a very important impact on science and technology. At present, atomic mass spectrometry is ordinarily performed using inductively coupled plasma ion sources and either a quadrupole or a scanning sector-field mass spectrometer as an analyser. The remarkable attributes of such a combination, being an indispensable tool for elemental analysis, include ... 

The inductively coupled plasma (ICP) source is used largely for specific element identification and evolved from the ICP atomic emission spectrometer it is probably more commonly employed in LC/MS than GC/MS. In GC/ MS, the ICP ion source is used in the assay of... 

ICP-MS is advantageous over RIMS and AMS due to fast and relatively inexpensive multielement determination, precise and accurate isotopic analysis at the trace, and ultratrace concentration levels in any material. Another advantage of ICP-MS compared to other atomic mass spectrometric techniques is the simple sample preparation and introduction of aqueous solutions into the normal-pressure inductively coupled plasma ion source. 

Improvements in instrumentation with inductively coupled plasma ion sources, especially with the sensitivity available with plasma-source double-focusing magnetic sector inductively coupled plasma mass spectrometer (MS-ICP-MS) instrumentation, have enabled similar accuracy as that which can be obtained using... 

The corabination of an inductively coupled plasma ion source and a magnetic sector-based mass spectrometer equipped with a multi-collector (MC) array [multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS)] offers precise and reliable isotope ratio data for many solid elements. In fact, MC-ICP-MS provides data, the trueness (accuracy) and precision of which is similar to, or, in some cases, even superior to, that achieved by thermal ionization mass spectrometry (TIMS), considered the benchmark technique for isotope ratio measurements of most solid elements [1], The basic strength of ICP-MS lies in the ion source, which achieves extremely high ionization efficiency for almost all elements [2, 3]. Consequently, MC-ICP-MS is likely to become the method of choice for many geochemists, because it is a versatile, user-friendly, and efficient method for the isotopic analysis of trace elements [4-8], The ICP ion source also accepts dry sample aerosols generated by laser ablation [9-16], The combination of laser ablation (LA) with ICP-MS is now widely accepted as a sensitive analytical tool for the elemental and isotopic analysis of solid samples. 

O Connor, G., Ebdon, L., and Evans, E. H. (1997). Low pressure inductively coupled plasma ion source for molecular and atomic mass spectrometry The effect of reagent gases. J. Anal. At. Spectrom. 12(11), 1263. 

The inductively coupled plasma (ICP) technique described in Chapter 9 (Section 9.5) has been coupled with mass spectrometry. The ICP source provides the mass spectrometer with a source of charged monatomic ions such that the electron beam is not needed. The exhaust of the ICP source is fed into the mass spectrometer for analysis by the mass analyzer portion of the mass spectrometer (Figure 10.20). 

Figure 5. The inductively-coupled plasma source (inspired by Niu and Houk 1996). The original figure has been modified to show the electrical potentials, the vacuum cascade (top), and the distribution of ions and neutral (bottom) in an MC-ICP-MS similar to the VG Plasma 54. The zone with incipient voltage acceleration right behind the skimmer show maximum space-charge effect with the lighter ions being most efficiently driven off by the strong axial current of positive ions.

Mass analysis is a relatively simple technique, with the number of ions detected being directly proportional to the number of ions introduced into the mass spectrometer from the ion source. In atomic mass spectrometry the ion source produces atomic ions (rather than the molecular ions formed for qualitative organic analysis) which are proportional to the concentration of the element in the original sample. It was Gray who first recognized that the inductively coupled plasma would make an ideal ion source for atomic mass spectrometry and, in parallel with Fassel and Honk, and Douglas and French developed the ion sampling interface necessary to couple an atmospheric pressure plasma with a mass spectrometer under vacuum. 

Inductively coupled plasma mass spectrometry (ICP-MS) is the marriage of two well established techniques, namely the inductively coupled plasma and mass spectrometry. The ICP has been described as an ideal ion source for inorganic mass spectrometry. The high temperature of the ICP ensures almost complete decomposition of the sample into its constituent atoms, and the ionization conditions within the ICP result in highly efficient ionization of most elements in the Periodic Table and, importantly, these ions are almost exclusively singly charged. 

The main common parts of an ICP mass spectrometer as discussed above are the sample introduction system, the inductively coupled plasma (ICP) ion source for desolvation, atomization and ion formation of introduced sample material, and the mass spectrometer including the mass analyzer system for separation of extracted ion beams and a fast ion detection system to register separated ion beams as illustrated in Figure 5.1. 

In this method the soil sample is dried overnight at 85 °C and ground into an homogeneous mixture. A 1 g soil sample is placed into a beaker and 10 ml of concentrated nitric acid added. The solution is heated to dryness and 5 ml of concentrated nitric acid is added. The uranium is redissolved in 5 ml of 8 N nitric acid and diluted to 25 ml with distilled water. The inductively coupled plasma mass spectrometry system used was an ELAN Model 250. The ion source consists of a modified plasma Thermal Model 2500 control box. The forward power was set at 1200 W with the plasma flow, auxiliary flow and nebuliser pressure set at 131/min, 1.0l/min and 0.27 MPa, respectively. The focusing lenses B, El, P and S2 are set at +5.3 V, -12.5 V, -18.0 V and -7.6 V, respectively. The m/z238 ion was monitored for two sec-... 

---