The ICP as an ion source

The ICP has many desirable features as an ion source for elemental analysis. The ionization efficiency approaches 100% for elements with first ionization energies below about 8 ey which includes most metals. Naturally, nonmetals are not as efficiently ionized, but useful numbers of positive ions can be observed even from As (ionization energy = 9.8 eV), Se (9.75 eV), and Br (11.8 eV). Multiply charged ions are not very 

These characteristics are illustrated in the mass spectrum shown in Fig. 5.3. Lanthanum at 1 mg 1 yields nearly 10 counts s , which is near the upper end of the linear range of the detector. Peaks are observed for La and LaO+, but they are small relative to La . The low abundance of LaO shows that even refractory rare earth oxides are dissociated efficiently (but not H completely). Most elements yield much lower levels of M or MO than shown for La in Fig. 5.3. The two isotope peaks for Eu illustrate the isotopic capability of the technique. Finally, the total signal for the two isotopes of Eu is comparable to the signal for La because these two elements are ionized to nearly the same extent in the ICP and because they are extracted and detected with similar efficiency. 

---

As in inductively coupled plasma optical emission (ICP-OES) spectra, in addition to atomic lines, intense ionic lines are also observed, the use of an ICP as an ion source for MS seemed logical, but overcoming the difference in pressure between the ICP (generated at atmospheric pressure) and the mass spectrometer (10 —10 mbar) proved difficult and had to be accomplished via the use of a two-cone interface. Despite the advantages that double-focusing sector field mass spectrometers (higher mass resolution) and TOP analyzers (high data acquisition speed) can offer, approximately 90% of the ICP-MS units used worldwide are equipped with a quadrupole filter for mass analysis. 

Because of the limitations of the DCP and MIP approaches, ICPs became the dominant area of research for both optical emission and mass spectrometric studies. As early as 1964, Greenfield and coworkers reported that an atmospheric pressure ICP coupled with OES could be used for elemental analysis. Although crude by today s standards, it showed the enormous possibilities of the ICP as an excitation source and most definitely opened the door in the early 1980s to the even more exciting potential of using the ICP to generate ions. ... 

Inductively coupled plasma-mass spectrometry (ICP-MS) is a powerful technique that uses an inductively coupled plasma as an ion source and a mass spectrometer as an ion analyzer. It can measure the presence of more than 75 elements in a single scan, and can achieve detection limits down to parts per trillion (ppt) levels for many elements—levels that are two or three orders of magnitude lower than those obtained by ICP-AES (Keeler 1991). It is more expensive than ICP-AES and requires more highly skilled technical operation. Aluminum levels in urine and saliva were detected down to 0.02 g/mL and in blood serum to 0.001 g/mL using ICP-MS (Ward 1989). Speciation studies have employed ICP-MS as a detector for aluminum in tissue fractions separated by size-exclusion chromatography (SEC) with detection limits of 0.04 g/g in femur, kidney and brain (Owen et al. 1994). 

ICP-MS uses an inductively coupled plasma as an ion source for a mass spectrometer. The basic units of an ICP-MS system, in the order used, are the sample introduction... 

An inductively-coupled plasma (ICP) is an effective spectroscopic excitation source, which in combination with atomic emission spectrometry (AES) is important in inorganic elemental analysis. ICP was also considered as an ion source for MS. An ICP-MS system is a special type of atmospheric-pressure ion source, where the liquid is nebulized into an atmospheric-pressure spray chamber. The larger droplets are separated from the smaller droplets and drained to waste. The aerosol of small droplets is transported by means of argon to the torch, where the ICP is generated and sustained. The analytes are atomized, and ionization of the elements takes place. Ions are sampled through an orifice into an atmospheric-pressure-vacuum interface, similar to an atmospheric-pressure ionization system for LC-MS. LC-ICP-MS is extensively reviewed, e.g., [12]. 

Research developing environmental methods might consider the use of AA detection. Perhaps some of the more interesting are detectors that use inductively coupled plasma (ICP) as an energy source and either atomic emission (AE) or mass spectrometry (MS) as the detector. ICP-AE and ICP-MS are well-developed analytical tools. Once of the major advantages of these techniques is that mixture of metals can be analyzed without the need for separation. Thus, workers who use these instruments normally do not think about their use as detectors. However, ICP-AE and ICP-MS cannot determine the oxidation or chemical state of a particular metal ion. Some samples are quite important from a toxicological and environmental standpoint since the... 

The use of an ICP as an excitation source makes it possible to correct interference due to light scatter by means of several different techniques. The double-beam technique is possible because ion and resonance lines can be obtained by the ICP which do not appear in cooler atomizers. In this technique, non-resonance lines near the resonance line of the analyte are employed. 

The best-known technique based on a combination of methods is ICP-MS. Here, the excited atoms are introduced upon their return to a lower energy level, through an interface into the ion source of a quadru-pole of a mass spectrometer. The ICP thus acts as an ion source and the mass spectrometer as the ion detector. The latest development in atomic spectrometry is the electrothermal evaporation-ICP-MS technique, where a graphite furnace is coupled to an ICP-MS. In this case, use is made of the most remarkable property of a graphite furnace (elimination of matrix interferences) by a graphite tube atomizer and subsequent transport of the atomic phase into the plasma and quadrupole. 

Most interesting is the use of the Okamoto cavity as an ion source for mass spectrometry [736]. Here, a torch similar to the ICP torch is positioned inside a micro-wave resonator and a plasma is operated mostly in nitrogen at a power in excess of 1 kW (Fig. 139) [737]. It was found that aerosols produced by pneumatic nebulization of liquids could easily be introduced into such a system. They penetrate... 

---