Plasma atomisers

Catalyst characterization - Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomisation (ICP-AES) on a CE Instraments Sorptomatic 1990. NH3-TPD was nsed for the characterization of acid site distribntion. SZ (0.3 g) was heated up to 600°C using He (30 ml min ) to remove adsorbed components. Then, the sample was cooled at room temperatnre and satnrated for 2 h with 100 ml min of 8200 ppm NH3 in He as carrier gas. Snbseqnently, the system was flashed with He at a flowrate of 30 ml min for 2 h. The temperatnre was ramped np to 600°C at a rate of 10°C min. A TCD was used to measure the NH3 desorption profile. Textural properties were established from the N2 adsorption isotherm. Snrface area was calcnlated nsing the BET equation and the pore size was calcnlated nsing the BJH method. The resnlts given in Table 33.4 are in good agreement with varions literature data. 

Commercial plasma atomisers became available in the mid 1970s and offer several advantages over flame atomisers. A plasma is a conducting gaseous mixture containing a significant concentration of cations and electrons. 

Atomic Emission Spectrometry with microwave-induced plasma atomisation (MIP-AES) (TNO-design), or inductively-coupled plasma atomisation (ICP-AES)(Kontron ASS 80), depending on the element and its eoncentration. 

For the Instrumental Neutron Aetivation Analysis (INAA), samples of about 0.2 g were irradiated with thermal neutrons and coimted with a Ge-detector eoupled to a 3072 channel analyser. The gamma-ray spectra were analysed using a single eomparator (Zn) method. At the BF samples of about 0.3 g were analysed after wet deeomposition (PTFE vessels, pressure bombs) (9). Solutions were analysed with the help of eleetro-thermal Zeeman AAS (Z 3030, Perkin-Elmer) and mass Spectrometry with inductively coupled plasma atomisation (ICP-MS) (PlasmaQuad, VG). 

This chapter describes the basic principles and practice of emission spectroscopy using non-flame atomisation sources. [Details on flame emission spectroscopy (FES) are to be found in Chapter 21.] The first part of this chapter (Sections 20.2-20.6) is devoted to emission spectroscopy based on electric arc and electric spark sources and is often described as emission spectrography. The final part of the chapter (Sections 20.7-20.11) deals with emission spectroscopy based on plasma sources. 

The use of a plasma as an atomisation source for emission spectroscopy has been developed largely in the last 20 years. As a result, the scope of atomic emission spectroscopy has been considerably enhanced by the application of plasma techniques. 

GC-AAS has found late acceptance because of the relatively low sensitivity of the flame graphite furnaces have also been proposed as detectors. The quartz tube atomiser (QTA) [186], in particular the version heated with a hydrogen-oxygen flame (QF), is particularly effective [187] and is used nowadays almost exclusively for GC-AAS. The major problem associated with coupling of GC with AAS is the limited volume of measurement solution that can be injected on to the column (about 100 xL). Virtually no GC-AAS applications have been reported. As for GC-plasma source techniques for element-selective detection, GC-ICP-MS and GC-MIP-AES dominate for organometallic analysis and are complementary to PDA, FTIR and MS analysis for structural elucidation of unknowns. Only a few industrial laboratories are active in this field for the purpose of polymer/additive analysis. GC-AES is generally the most helpful for the identification of additives on the basis of elemental detection, but applications are limited mainly to tin compounds as PVC stabilisers. 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

Both emission and absorption spectra are affected in a complex way by variations in atomisation temperature. The means of excitation contributes to the complexity of the spectra. Thermal excitation by flames (1500-3000 K) only results in a limited number of lines and simple spectra. Higher temperatures increase the total atom population of the flame, and thus the sensitivity. With certain elements, however, the increase in atom population is more than offset by the loss of atoms as a result of ionisation. Temperature also determines the relative number of excited and unexcited atoms in a source. The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 103 to 1010 or more. At higher temperatures (up to 10 000 K), in plasmas and electrical discharges, more complex spectra result, owing to the excitation to more and higher levels, and contributions of ionised species. On the other hand, atomic absorption and atomic fluorescence spectrometry, which require excitation by absorption of UV/VIS radiation, mainly involve resonance transitions, and result in very simple spectra. 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

In AFS, the analyte is introduced into an atomiser (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (HCL, EDL, or tuned laser). Subsequently, the fluorescence radiation is measured. In the past, AFS has been used for elemental analysis. It has better sensitivity than many atomic absorption techniques, and offers a substantially longer linear range. However, despite these advantages, it has not gained the widespread usage of atomic absorption or emission techniques. The problem in AFS has been to obtain a... 

For these techniques, a dissolved sample is usually employed in the analysis to form a liquid spray which is delivered to an atomiser e.g. a flame or electrically generated plasma). Concerning optical spectrometry, techniques based on photon absorption, photon emission and fluorescence will be described (Section 1.2), while for mass spectrometry (MS) particular attention will be paid to the use of an inductively coupled plasma (TCP) as the atomisation/ionisation source (Section 1.3). The use of on-line coupled systems to the above liquid analysis techniques such as flow injection manifolds and chromatographic systems will be dealt with in Section 1.4 because they have become commonplace in most laboratories, opening up new opportunities for sample handling and pretreatment and also to obtain element-specific molecular information. 

The flames commonly used as atomisers have temperatures in the range 2000-3000 K allowing for the analysis of elements such as Na, K and Cs by OES. The flame temperatures are not high enough to excite many other elements, so other atomisers such as spectroscopic plasmas have to be used. 

Based on the configurations in Figure 1.5, many analytical techniques have been developed employing different atomisation/excitation sources. For example, two powerful AAS techniques are widespread one uses the flame as atomiser (FAAS) whereas the other is based on electrothermal atomisation (ETAAS) in a graphite furnace. Although the flame has limited application in OES, many other analytical emission techniques have evolved in recent decades based on dilTerent atomisation/excitation plasma sources. 

Concerning AFS, the atomiser can be a flame, plasma, electrothermal device or a special-purpose atomiser e.g. a heated quartz cell). Nowadays, commercially available equipment in AFS is simple and compact, specifically configured for particular applications e.g. determination of mercury, arsenic, selenium, tellurium, antimony and bismuth). Therefore, particular details about the components of the instrumentation used in AFS will not be given in this chapter. 

Flames and plasmas can be used as atomisation/excitation sources in OES. Electrically generated plasmas produce flame-like atomisers with significantly higher temperatures and less reactive chemical environments compared with flames. The plasmas are energised with high-frequency electromagnetic fields (radiofrequency or microwave energy) or with direct current. By far the most common plasma used in combination with OES for analytical purposes is the inductively coupled plasma (ICP). 

Furnace atomisation plasma emission spectrometry (FAPES) this consists of an atmospheric pressure source combining a capacitively coupled radiofrequency helium plasma formed inside a graphite tube which contains an axial powered electrode. This miniplasma has rarely been used in analytical atomic spectrometry, probably because of the small number of users and a lack of information about its applications and capabilities [7]. 

R. E. Sturgeon, Furnace atomisation plasma emission/ionisation review of an underutilized source for atomic and molecular spectrometry. Can. J. Anal. Sci. Spectrosc., 49, 2004, 385-397. 

It is possible to extend the principle of photometric emission (FPD) by replacing the flame with a microwave plasma that has a temperature high enough to induce any element to radiate light. This is equivalent to atomic emission where each solute is atomised and gives rise to specific emission bands. 

Figure 14.16—Elements determined by AAS or FES. Most elements can be determined by atomic-absorption or flame emission using one of the available atomisation modes (burner, graphite furnace or hydride formation). Sensitivity varies enormously from one element to another. The representation above shows the elements in their periodic classification in order to show the wide use of these methods. Some of the lighter elements, C, N, O, F, etc. in the figure can be determined using a high temperature thermal source a plasma torch, in association with a spcctropholometric device (ICP-AbS) or a mass spectrometer (1CP-MS).

Besides plasmas, which are at the forefront of thermal atomisation devices, other excitation processes can be used. These methods rely on sparks or electrical arcs. They are less sensitive and take longer to use than methods that operate with samples in solution. These excitation techniques, with low throughputs, are mostly used in semi-quantitative analysis in industry (Fig. 15.2). Compared to the plasma torch, thermal homogeneity in these techniques is more difficult to master. 

Argon plasmas are used in optical emission spectrometry to atomise and ionise elements leading to the emission of characteristic spectral lines. Hence, a plasma torch (7-8 000 K) can be used for ionisation in mass spectrometry. Ions produced in the plasma are introduced into the mass analyser through a small orifice (called a skimmer) placed in the axial direction. Because the mass spectrometer is operated under a vacuum, the ions are sucked into the mass analyser through the skimmer. An aqueous solution of the sample can be aspirated into the plasma or, alternatively, the plasma can be placed at the exit of a gas chromatograph (e.g. speciation of organo-metallic compounds by GC/ICP-MS). Since all chemical bonds are broken in the plasma, the only accessible information is that concerning the concentration of monoatomic ions (Fig. 16.19). 

Ramendik [16] pointed to the possibilities of the creation and development of theoretical foundations based on mathematical modelling in elemental mass spectrometry after the creation of a plasma. For laser plasma mass spectrometry of geological RMs and a quasi-equilibrium approach based on atomisation and ionisation temperatures without relying on reference RMs materials, he claims to be able to arrive at average uncertainties for 40 elements totalling 20% [17]. This may not be ideal but it is a suitable accuracy for solving many practical analytical problems. 

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