2 Inductively Coupled Plasma Spectrometry Stephen J. Hill and Andrew Fisher The University of Plymouth, Plymouth, England Mark Cave British Geological Survey, Nottingham, England I. INTRODUCTION Environmental samples are often complex mixtures of organic and inorganic components that vary widely in their makeup depending on many factors, which include geological controls, geographical location, depth, climate, and anthropogenic inputs. Consequently, the chemical composition of environmental samples is very varied. For example, a major chemical component of one soil type can be a trace component of others. To provide a comprehensive elemental analysis of environmental samples it is therefore necessary to have an analytical method capable of determining most chemical elements over concentration ranges varying from percent to ultratrace. The inductively coupled plasma (ICP) source used in conjunction with either atomic emission spectroscopy (AES) or mass spectrometric (MS) detection has a number of properties that make it ideally suited to the analysis of soils and other environmental materials. 1. In principle, around 75 of the chemi cal elements can be determined. 2. By judicious choice of emission lines, mass-to-charge range, and detectors, elements can be determined over 5 orders of magnitude in one sample solution. 3. With care, both the precision and the accuracy of the measurement is high. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. 4. ICP spectrometric methods can provide fast multielement measurement of many elements in one sample. This is ideally suited to soil survey work, where a large number of samples must be analyzed. A. Development of ICP as an Atomic Emission Source The ICP source, as we know it today, was introduced by Reed (1961), who described an atmospheric argon (Ar) ICP sustained in a three-tube quartz torch using an RF induction coil. In this first application, the high temperature of the ICP was used for growing refractory crystals. Greenfield et al. (1964) were the first to realize the potential of the ICP as a source for multielement atomic emission. Almost simultaneously, but independently, in the U.S., Wendt and Fassel (1965) were also experiment- ing with an Ar ICP as an atomic emission source. From these early studies, during the late 1960s and the early 1970s, further work by Greenfield, Fassel, and other research groups established ICP-AES as a viable method for trace element analysis, producing published articles on a variety of analytical applications. In 1975 Greenfield et al. (1975a, 1975b) described the analytical system that they had been using for routine analytical work for several years. This system had an ICP source coupled to a 30-channel direct reading spectrometer, allowing simultaneous multielement analysis with automated control of sample input and data readout. At a similar time Scott et al. (1974) described a ‘‘compact’’ design ICP torch in a system with a pneumatic nebulizer. The two systems generated much discussion on the most suitable torch design and operating conditions for ICP. Greenfield’s group advocated the use of a robust ICP with a relatively large torch (29 mm o.d.) run at high powers of several kW, whereas Fassel’s group used a smaller torch (20 mm o.d.) and lower power (1–2 kW). Despite these contro- versies, the early published work on ICP-AES was enough to convince the instrument manufacturers that the ICP was a marketable product, and the first commercial instruments were introduced in the mid-1970s. During the 1970s and the 1980s a wide variety of commercial instruments were produced. In general these instruments fell into two categories: systems with polychromator spectrometers that were able to make simultaneous measurements of many emission lines and systems with monochromator spectrometers that measured each emission line sequentially. The former instruments had high sample throughput but were usually more costly, whereas the latter were less expensive but had more moving parts and a lower sample throughput. However, monochromator spectrometers also had the advantage of being able to address many more analytical lines than the fixed channel polychromator systems. 54 Hill et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. Some manufacturers opted to use a combination of both of these spectrom- eters in a single system. Although there were a number of spectrometer designs, by the end of the 1980s most commercial systems reflected the consensus of opinion over the best ICP operating conditions for routine analytical work. Torch dimensions and low power as originally advocated by Fassel’s group (Scott et al., 1974) had become universally adopted, along with the use of generators operating at 27.12 MHz, although it was known that plasmas operating at 40–50 MHz provide a higher signal-to-back- ground ratio (SBR) in the emission spectra (Capelle et al., 1982). All of the early commercial systems relied on the photomultiplier tube in the spectrometer as a means of converting light intensity into an electrical signal that could be used to quantify concentration. Although very sensitive, the photomultiplier tube is quite bulky (a minimum packing cross sectional area of ca. 0.75–5 cm 2 ), which in many instances makes it difficult to fit the required number of lines necessary for a particular application into a polychromator system. At the end of the 1980s, however, developments in solid-state array optical detectors heralded a significant change in ICP-AES instrument design. These new detectors allowed many hundreds or thousands of detectors (referred to as pixels), each equivalent to a single photomultiplier tube, to be packed into the area occupied by a single photomultiplier tube. In one of the first applications of these new detectors to ICP, Pilon et al. (1990) described an ICP system that combined a charge injection device (CID) array detector with an echelle spectrometer. This system allowed simultaneous analysis with continuous spectral coverage from 185 to 511 nm, combining the advantages of the older polychromator and monochromator systems in a single, more compact instrument. In conjunction with the advances in detector technology, the instru- ment manufacturers, under pressure to obtain very low detection limits to meet the needs of environmental legislation and keep pace with the advances in ICP-MS, have revisited the use of axial viewing of the ICP, originally described by Abdallah et al. (1976). This approach provided improvements in detection limits for some elements by factors varying from 2 to 20 (Brenner et al., 2000). Nearly all the major instrument manufacturers now offer solid- state detector instruments with axial ICP viewing. The implications of these current instrument design trends to environmental analysis will be discussed in more detail in later sections. Despite some recent changes in instrument design since the early days, ICP-AES has been in everyday use for at least 25 years and can now be considered a mature analytical technique. In recent reviews of atomic spectrometry in environmental analysis (Cave et al., 2000; Cave et al., 2001; Hill et al., 2002), it has been concluded that multielement analyses of, for Inductively Coupled Plasma Spectrometry 55 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. example, plant and soil digests by ICP-AES are now so routine in many laboratories around the world that few reports of novel work, other than unusual applications, are to be expected. II. FUNDAMENTAL PROPERTIES OF ICP-AES The basic process of using emission spectroscopy for chemical analysis consists of introducing the sample to be analyzed, in an appropriate form, into an excitation source, where it is dissociated into atoms and ions by thermal decomposition. The atoms and ions are further excited from their ground state energy to an energized state from where they spontaneously revert to a lower energy state, accompanied by the emission of a photon of light. The energy of the photon (expressed as its wavelength) is specific to the element being excited, and the number of photons, or light intensity, is proportional to the concentration of the excited atoms or ions. The instrumentation required for ICP-AES is shown in Fig. 1 and comprises three basic units: the source, a spectrometer, and a computer for control and data analysis. The ICP source is ideally suited as an emission source because of two features. 1. The very high temperature of the source allows analyte material to be vola- tilized easily, and excitation of ions and atoms of most elements can occur. Figure 1 Schematic diagram of an ICP-AES instrument. 56 Hill et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. 2. The unique geometry of the ICP allows the emission to be viewed in an ‘‘optically thin’’ region of the plasma. Here, degradation of the proportionality of the emission energy to the concentration of the determinant is not affected significantly by self-absorption of the photons by atoms or ions of the same element. A. Plasma Formation A three-turn water-cooled copper coil is used to couple the RF energy into a stream of flowing Ar within the plasma torch. When a RF current is applied to the coil, it sets up an oscillating magnetic field within the quartz tube. The argon flow through the torch is seeded with electrons from a spark discharge (usually produced by a Tesla coil). The electrons are accelerated by the oscillating magnetic field and collide with atoms of the gas, causing further ionization that leads to the formation of the hot ionized plasma. Rapidly, equilibrium is reached in which the rate of electron production is balanced by losses due to recombination and diffusion, and a stable plasma is formed. The plasma is effectively a conductor and is heated by the flow of current induced by the RF field. Electrically, the coil and plasma form a transformer with the plasma acting as a one-turn secondary coil of finite resistance. Once formed, the ICP is constrained in a quartz torch made from three concentric tubes, as shown in Fig. 2. The coolant argon flow (typically in the range 10–20 L min À1 ) is introduced tangentially through the outer annulus and performs a dual function of keeping the plasma from melting the outer quartz tube while providing the argon to sustain the plasma. The intermediate flow (typically 0–1 L min À1 ) allows the plasma to be moved up or down in the torch and can be used to help prevent the buildup of salt on the injector tip. The injector flow (typically 0.5–1.5 L min À1 ) punches a hole through the center of the plasma and is used to carry the sample (usually in the form of an aerosol) into the plasma for volatilization, atomization, ionization, and excitation. B. Spectrochemical Emission Properties of an ICP The temperature profile and the four main regions in a typical annular ICP that are important to the analyst are shown in Fig. 3. The preheating zone (PHZ) occurs at the base of the plasma just before the analyte reaches the central channel where desolvation of the sample aerosol takes place. The initial radiation zone (IRZ) is where the sample undergoes volatilization and atomization/ionization and excitation. Finally, there is the normal analytical zone (NAZ), which is a low background region just above the bright plasma fireball where the atomic emission measurements are made. There are two Inductively Coupled Plasma Spectrometry 57 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. important parameters that control the relative positions of these zones in the plasma: 1. The injector flow rate, when increased, increases the diameter of the hole through the center of the plasma and shifts the IRZ and NAZ higher in the central channel. At higher injector flow rates the analyte residence time within the central channel is decreased, experiencing less heating from the plasma. 2. The RF power, when increased, tends to constrict the central channel for a given injector flow rate and push the IRZ and NAZ lower in the plasma. This increases the residence time of the analyte within the plasma channel. Figure 2 Schematic view of an inductively coupled plasma (ICP). 58 Hill et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. The third parameter that is dependent on both the power and the injector flow rate is the viewing height at which the atomic emission is measured (when the ICP is viewed radially). This is measured as the distance above the load coil and is normally between 5 and 16 mm. As the relat ive positions of the IRZ and NAZ are moved through changes in RF power or injector flow rate, the relative viewing height within the NAZ also changes. These three parameters can therefore be varied to obtain optimum performance for any given emission line. Common optimization criteria are signal-to-background ratio and signal-to-noise ratio. In most instances, however, ICP-AES is used as a multielement tool, and therefore compro- mise operating conditions, usually set by the manufacturer, are supplied with commercial instrumentation. In many instances, compromise operating conditions work extremely well for a wide range of sample types. Nevertheless, there may be instances where an unusual determinant or matrix requires some changes in operating conditions. In these instances it is useful to have a broad understanding of how plasma conditions affect particular line types. In general, emission lines can be divided into ‘‘hard’’ (excitation potential > 4.5 eV) and ‘‘soft’’ lines (excitation potential < 4.5 eV) as proposed by Boumans (1978). The behavior of the two types of line when changing ICP operating parameters can be summarized as follows. Figure 3 Axial channel emission zone of an ICP. PHZ: preheating zone; IRZ: initial radiation zone; NAZ: normal analytical zone. (Reproduced with permission from Sharp, B. in Soil Analysis—Modern Instrumental Techniques, 2d ed. [Smith, K.A., ed.]. New York: Marcel Dekker, 1991.) Inductively Coupled Plasma Spectrometry 59 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. Soft Lines 1. An increase in applied power enhances the emission and shifts the peak emission signal lower in the plasma. 2. An increase in sample carrier flow rate reduces the emission intensity and shifts the peak emission signal higher in the plasma. 3. The presence of an increasing concentration of an easily ionizable element (EIE) enhances the emission and shifts the peak emis sion signal lower in the plasma. Higher up in the vicinity of the NAZ, the enhancement is much less (Blades and Horlick, 1981). Hard Lines 1. An increase in the applied power produces an increase in the emission intensity, but the position of the peak emission intensity signal changes very little. 2. An increase in sample carrier gas flow produces a small but significant upward shift of the peak emission intensity and a reduction in intensity. 3. An increasing concentration of an EIE causes depression in emission intensity in the vicinity of the peak emission intensity, but an enhancement lower in the plasma. This results in a crossover region where the effect of the interfering elements is minimized. It is generally agreed that the excitation mechanism of soft lines is essentially thermal in nature, but for hard lines the excitation mechanism is nonthermal and involves interactions with metastable Ar ions (Blades, 1987). The equation used to express the linear relationship between the spectral radiance B and the concentration of free atoms in the plasma, as used for calibration in analytical use, may be expressed as B ¼ 1 4 h 0 NL ZðtÞ g k A ki exp ÀE k kT  where h ¼Planck’s constant  0 ¼the frequency of the emitted photons N ¼ the number of atoms per unit volume Z(t) ¼the partition function g k ¼the statistical weight of the kth state A ki ¼the Einstein transition probability for spontaneous emission L ¼the optical depth of the source E k ¼the excitation energy of the kth state k ¼the Boltzmann constant 60 Hill et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. C. Axial/Radial Viewing of ICP Atomic emission spectrometers may either be used to view the ICP in the radial position, i.e., the torch is vertical and the light emitted by the determinants is detected through the side of the plasma, or may be viewed in the axial position, in which case the torch is horizontal and the detected determinants’ emissions have to pass through the tail flame (Fig. 4). For use Figure 4 Typical configuration for ICP-AES instruments: (a) side-on radial viewing; (b) axial viewing, of the ICP. Inductively Coupled Plasma Spectrometry 61 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. in ICP-AES, the most common orientation to date has been the radial configuration (although in ICP-MS, the plasma is universally mounted horizontally). A comprehensive review of many aspects of axial viewing, including both instrumentation and analytical performance, has been published by Brenner and Zander (2000). The first practical point regarding the use of axial viewing for ICP-AES is that the optical system is ‘‘looking’’ down the end of the plasma and therefore needs protection from the hot gases in the plasma tail flame and from the possibility of salt build up on the optical interface. There are two approaches to this: a shear gas is directed in a near-perpendicular stream at the tail flame of the plasma directing the tail flame away from the optics; the optical interface has a counter-current of purge gas flowing out from its input aperture directly against the plasma tail flame. The shear or purge gas can be air or an inert gas, the latter being a better choice if low UV wavelengths are to be measured. The gas has a dual function of protecting the optics and removing the cooler end of the tail flame, which could cause self-absorption or other interference effects. By viewing the ICP end-on, an integrated emission from the whole length of the sample channel is obtained. This removes the spatial variable of viewing height, which is important in radial viewing. For axial viewing, therefore, there are only two important parameters governing the analytical properties of the plasma, RF power and injector gas flow rate. It is believed that both signal and background are increased when moving from radial to axial mode owing to the longer path length being viewed, but because the NAZ does not have to be viewed through the side of the high background plasma (as found in radial viewing), the signal increases more than the background, producing superior signal-to-background ratios (SBR). While it is acknowledged that axial viewing improves detection limits, there is some debate as to whether there is reduction in the linear range an d increase in interferences compared with viewing perpendicular to the central channel. In their review Brenner and Zander (2000) conclude that there are conflicting results with regard to linear dynamic range, but Bridger and Knowles (2000) suggest that curvature may be due to ionization suppression effects that can be alleviated by on-line addition of a CsCl buffer. It has also been suggested (Brenner and Zander, 2000) that when run under robust conditions (see Sec. II.D), axial viewing is as interference-free as radial viewing (Table 1). D. Robust Operating Conditions for ICP-AES In Sec. II.B, the use of compromise operating conditions for multielement analysis was discussed. One way of arriving at a set of operating conditions 62 Hill et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. [...]... 55.9349 62. 929 6 63. 929 1 63. 929 1 74. 921 6 79.9165 28 IV Interfering ion Nominal m/z 12 C2 N2 12 16 C O 14 16 1 N OH 15 16 N O 16 O2 12 16 C O2 32 16 S O 35 16 Cl O 35 16 1 Cl O H 40 Ar12C 40 Ar14N 40 Ar16O 40 Ar23Na 32 16 S O2 32 S2 40 Ar35Cl 40 Ar2 14 Accurate m/z Resolution required 24 .0000 28 .0060 27 .9949 31.0057 30.9950 31.9898 43.9898 47.9670 50.9637 51.9715 51.9 623 53.9653 55.95 72 62. 9 521 63.9619... 51.9 623 53.9653 55.95 72 62. 9 521 63.9619 63.94 42 74.9311 79. 924 6 1599 9 62 1555 968 1455 1807 128 2 25 11 25 86 1676 23 83 20 99 25 09 27 97 1950 423 4 7887 9867 SAMPLE PREPARATION AND APPLICATIONS ICP-AES and ICP-MS are now used routinely for the analysis of environmental samples in many laboratories worldwide Despite this widespread use, there continues to be a lack of standardization in the methodologies used to...Inductively Coupled Plasma Spectrometry 63 Table 1 Instrumental Detection Limits for ICP-AES and ICP-MS Using the Most Sensitive Lines and Most Abundant Isotopes Determinant ICP-AES (ng mLÀ1) ICP-MS (ng mLÀ1) 3 2 12 6 2 0.1 0 .2 12 0.03 2 8 5 4 2 0.3 1 0.3 2 7 3 13 4 10 0.5 18 4 10 0. 02 1 0.05 0.1 0.3 4 1 0.005 0.05 0.005 0.005 0.05 0.001 0.001 0.001 0.5 0.005 0.001 0.001... Yb Zn ICP-AES (ng mLÀ1) ICP-MS (ng mLÀ1) 4 2 6 5 18 14 7 1 20 3 11 5 6 20 20 0.4 40 5 7 15 0. 02 10 5 30 20 1 16 2 20 2 20 0 .2 0.3 1 0.005 0.001 0.005 0.005 0.5 0.001 0.005 0.001 0.005 0.001 0.005 0.005 0.005 10 0.005 0.05 0.05 0.5 0.001 0.005 0.001 0.005 0.001 0.05 0.001 0.05 0.001 0.001 0.001 0.05 0.005 0.001 0.001 0.005 that are practical for the relatively complex matrix obtained from environmental. .. ground to < 125 mm before it is weighed into the tube 2 Microwave Digestion Since the 1980s, microwave-assisted sample digestion techniques have become increasingly popular and are now widely used (Quevauviller et al., 1993; Smith and Arsenault, 1996; Levine et al., 1999; Betinelli et al., 20 00; Falciani et al., 20 00; Chen and Ma, 20 01; Ivanova et al., 20 02; Sun et al., 20 01) Microwave-assisted aqua... Rights Reserved  82 Hill et al Table 2 Examples of Polyatomic Ion Interferences When Using ICP-MS and the Mass Analyzer Resolution Necessary to Resolve the Interferences from the Determinant of Interest Analyte ion Nominal m/z Accurate m/z 24 Mg Si 28 Si 31 P 31 P 32 S 44 Ca 48 Ti 51 V 52 Cr 52 Cr 54 Fe 56 Fe 63 Cu 64 Zn 64 Zn 75 As 80 Se 23 .9850 27 .9769 27 .9769 30.9737 30.9737 31.9 721 43.9555 47.9479... al., 20 00) On return to the laboratory, the species were eluted by 5% thiourea in 0.5% HCl (500 mL) Speciation was then achieved by liquid chromatography and ICP-MS detection of the inorganic and methyl mercury Between-column precision was 12% at the 0 .2 ng mLÀ1 level, and the limit of detection was 5 .2 ng LÀ1 Stability of As, Sb, Se, and Te species in water, urine, fish, and soil extracts using HPLC-ICP-MS... many species was less than 20 % Organolead speciation in rainwater may be accomplished using a variety of detection methods including GC-ICP-TOF-MS (Baena et al., 20 01) The GC-ICP-TOF-MS method yielded a limit of detection of 15 pg (as Pb) Speciation of vanadium(V) and vanadium(IV) has been achieved using flow injection (FI)-ICP-AES, using an ultrasonic nebulizer (Wuilloud et al., 20 01) The flow injection... (Reproduced with permission from Sharp, B., in Soil Analysis Modern Instrumental Techniques, 2d ed [Smith, K.A., ed.] Marcel Dekker.) TM Copyright n 20 04 by Marcel Dekker, Inc All Rights Reserved 80 Figure 11 Hill et al ICP-MS interface determined, then the mass spectrometer must be capable of separating the 63 Cuþ from the 65Cuþ as well as the 20 6Pb, 20 7Pb, and 20 8Pb isotopes from each other In addition,... 1996) However, when ICP-AES is used for analysis, adding boric acid can create a matrix effect and was reported to result in a 20 % decrease in sensitivity for Mo, Ni, Pb, Sb, Se, and Sn, and TM Copyright n 20 04 by Marcel Dekker, Inc All Rights Reserved Inductively Coupled Plasma Spectrometry 85 50% and 70% decrease in sensitivity for P and S, respectively (Paudyn and Smith, 19 92) This procedure has . atomization and Table 1 Instrumental Detection Limits for ICP-AES and ICP-MS Using the Most Sensitive Lines and Most Abundant Isotopes Determinant ICP-AES (ng mL À1 ) ICP-MS (ng mL À1 ) Determinant ICP-AES (ng. optimization criteria are signal-to-background ratio and signal-to-noise ratio. In most instances, however, ICP-AES is used as a multielement tool, and therefore compro- mise operating conditions,. the analysis of soils and other environmental materials. 1. In principle, around 75 of the chemi cal elements can be determined. 2. By judicious choice of emission lines, mass-to-charge range, and detectors,