The inductively coupled plasma

The ICP is almost in local thermal equilibrium. Indeed, the excitation temperatures (from atomic line intensity ratios) are about 6000 K [430] and the rotation temperatures (from the rotation lines in the OH bands) are 4000-6000 K (see Refs. [431, 432]). From the broadening of the Hjj-lme, an electron number density of 10 cm is obtained, whereas from the intensity ratio of an ion and an atom line of the same element the electron number density found is 10 cm . It has also been reported that measured line intensity ratios of ion to atom lines are higher by a factor of 100 than those calculated for a temperature of 6000 K and the electron number density found is 10 cm . This indicates the existence of over-ionization. This can be understood from the excitation processes taking place. They include the following. 

ICP generators with frequencies ranging from 27 to 100 MHz have been used and in general 27.12 and 40 MHz generators are now available. As a result of detailed investigations by Mermet and co-workers only slight differences in the per- 

In pneumatic nebulization for ICP-OES, continuous sample feeding requires a sample aspiration time of about 30 s so as to attain a stationary signal, a measurement time of around 5-10 s, and a rinsing time again of 30 s at minimum. However, discrete sampling is also possible with injection systems known from flame AAS [140, 141] and by flow injection. Work with sample aliquots of down to 10 xL then becomes possible, which is particularly useful, for example, in work with microsamples [156] or for the analysis of solutions containing high salt contents [443]. 

A torch of the type according to Fassel has a diameter of ca. 18 mm and can be operated at 0.6-2 kW and with 10-20 L/min of argon. In so-called mini-torches gas consumption can be down to about 6 L/min, through the use of a special gas inlet 

Accordingly, an over-population of the argon metastable levels would explain both the over-ionization as well as the high electron number density in the ICP. Indeed, it could be accepted that argon metastables act both as ionizing species and at the same time are easily ionized [385]. This could explain the fairly low interferences caused by easily ionized elements and the fact that ion lines are excited very effi- 

For ICP-AES both sequential and simultaneous as well as combined instruments are used. In sequential spectrometers special attention is given to the speed of the wavelength access and in simultaneous spectrometers to the provision of background correction facilities. In combined instruments a number of frequently used channels are fixed and with a moving detector or an integrated monochromator 

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The inductively coupled plasma [19] is excited by an electric field which is generated by an RF current in an inductor. The changing magnetic field of this inductor induces an electric field in which tire plasma electrons are accelerated. The helicon discharge [20] is a special type of inductively coupled RF discharge. 

The inductively coupled plasma and the torch used in ICPMS are similar to that used in ICP-OES. In ICPMS, the torch is aimed horizontally at the mass spectrometer, rather than vertically, as in ICP-OES. In ICPMS the ions must be transported physically into the mass spectrometer for analysis, while in ICP-OES light is trans-... 

The Inductively Coupled Plasma (ICP) has become the most popular source for multielement analysis via optical spectroscopy since the introduction of the first commercial instruments in 1974. About 6000 ICP-Optical Emission Spectrometry (ICP-OES) instruments are in operation throughout the world. 

The inductively coupled plasma source (Fig. 20.11) comprises three concentric silica quartz tubes, each of which is open at the top. The argon stream that carries the sample, in the form of an aerosol, passes through the central tube. The excitation is provided by two or three turns of a metal induction tube through which flows a radio-frequency current (frequency 27 MHz). The second gas flow of argon of rate between 10 and 15 L min-1 maintains the plasma. It is this gas stream that is excited by the radio-frequency power. The plasma gas flows in a helical pattern which provides stability and helps to isolate thermally the outside quartz tube. 

The authors wish to thank Mr. Edmund Huff of the Chemical Technology Division for performing the inductively couple plasma atomic emission spectrographic analyses. Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U. S. Department of Energy under contract number W-31-109-ENG-38. 

The accuracy of the inductively-coupled plasma procedure was assessed by analysing waters of known sulfate composition, and by comparing measured sulfate values for a wide range of samples with those obtained for the same waters by an automated spectrophotometric procedure. Good agreement is obtained between the derived sulfate measurements and the normal values for International Standard Sea Water and an EPA Quality Control Standard. 

Winge et al. [730] have investigated the determination of twenty or more trace elements in saline waters by the inductively coupled plasma technique. They give details of experimental procedures, detection limits, and precision and accuracy data. The technique when applied directly to the sample is not sufficiently sensitive for the determination of many of the elements at the low concentrations at which they occur in seawater, and for these samples preconcentration techniques are required. However, it has the advantages of being amenable to automation and capable of analyzing several elements simultaneously. 

Unlike halogenated solvents, it does not produce noxious substances in the inductively coupled plasma, has a very low aqueous solubility, and yields hundredfold concentration in one step. Detection limits ranged from 0.02 jtg/l (cadmium) to 0.6 pg/1 (lead). The results indicate that the proposed procedure should be useful for the precise determination of metals in oceanic water, although a higher sensitivity would be necessary for lead and cadmium. 

Brief mention has been made, particularly in connection with the inductively coupled plasma atomic absorption spectrometric technique, of the need to preconcentrate seawater samples prior to the determination of metals, in order to achieve adequate detection limits. 

Atomic absorption spectrometric methods and, more recently, the inductively coupled plasma atomic emission method, are, of course, mandatory if determination of elements is required (arsenic, selenium, boron, phosphorus and silicon). 

Detailed operating conditions for the inductively coupled plasma emission spectrometer have been described by Brzezinska et al. [126]. 

The inductively coupled plasma atomic emission spectrometric procedure [28] described in section 12.10.3.4 for the determination of arsenic in saline sediments has been applied to the determination of down to 0.5g L 1... 

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.

In the inductively coupled plasma atomic emission spectroscopy (ICPAES) method (ASTM DD 5600), a sample of petroleum coke is ashed at 700°C (1292°F) and the ash is fused with lithium borate. The melt is dissolved in dilute hydrochloric acid, and the resulting solution is analyzed by inductively coupled plasma atomic emission spectroscopy using aqueous calibration standards. Because of the need to fuse the ash with lithium borate or other suitable salt, the fusibility of ash may need attention (ASTM D1857). 

CONTENTS Preface, Joseph Sneddon. Analyte Excitation Mechanisms in the Inductively Coupled Plasma, Kuang-Pang Li and J.D. Winefordner. Laser-Induced Ionization Spectrometry, Robert B. Green and Michael D. Seltzer. Sample Introduction in Atomic Spectroscopy, Joseph Sneddon. Background Correction Techniques in Atomic Absorption Spectrometry, G. Delude. Flow Injection Techniques for Atomic Spectrometry, Julian F. Tyson. 

Different methods have different detection limits. For example, the flame atomic absorption spectrophotometry (AAS) method for aluminum has a detection limit of 30 parts per million, while the inductively coupled plasma... 

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. 

For resonance lines, self-absorption broadening may be very important, because it is applied to the sum of all the factors described above. As the maximum absorption occurs at the centre of the line, proportionally more intensity is lost on self-absorption here than at the wings. Thus, as the concentration of atoms in the atom cell increases, not only the intensity of the line but also its profile changes (Fig. 4.2b) High levels of self-absorption can actually result in self-reversal, i.e. a minimum at the centre of the line. This can be very significant for emission lines in flames but is far less pronounced in sources such as the inductively coupled plasma, which is a major advantage of this source. 

In a flame, as the concentration of atoms increases, the possibility increases that photons emitted by excited atoms in the hot region in the centre will collide with atoms in the cooler outer region of the flame, and thus be absorbed. This self-absorption effect contributes to the characteristic curvature of atomic emission calibration curves towards the concentration axis, as illustrated in Fig. 4.4. The inductively coupled plasma (ICP) tends to be hotter in the outer regions compared with the centre—a property known as optical thinness—so very little self-absorption occurs, even at high atom concentrations. For this reason, curvature of the calibration curve does not occur until very high atom concentrations are reached, which results in a much greater linear dynamic range. 

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 temperature of the inductively coupled plasma varies with the distance from the load coil and according to the setting of the ICP rf power and nebulizer gas flow rate. A typical profile of the plasma gas temperature along the torch axis as a function of distance from the load coil is shown in Figure 2.4. With increasing distance from the load coil and with a reduction of ICP rf power the gas plasma temperature decreases. 

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. 

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