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Chapter 2. Advances in ICP Emission and ICP Mass Spectrometry
PAFWIZ N. SOLTANPOUR ETAL.
C. Mass Discrimination
D. Unwanted Ions
E. Methods of Correction for Interferences(ICP-MS)
VII. Practical Applications
A. Grinding Soil Samples
B. Obtaining Soil Extracts
C. Digestion of Organic Matter and Dissolution of Silicates for Total
D. Analysis of Soil Extracts and Digests
E. Determination of Trace Levels of As, Se, and Hg Using the HydrideMercury Vapor Generator
VIII. Quality Control Methods
The application of inductively coupled plasma-atomic emission spectrometry
(ICP-AES) to the analysis of soil was reviewed in 1982 and again in 1996 with inclusion of ICP-mass spectrometry (ICP-MS) (Soltanpour et al., 1982, 1996). In
this review we treat ICP-MS more comprehensively and include a table for isotopes of elements (see Section 1V.B.) and an example for Ca, Fe, Ni, Zn, and Pb
isotope selection for plant-tissue analysis (Appendix 1).
New developments in ICP-AES include suspension-nebulization analysis of
clays (Laird et al., 1991); interfacing ICP spectrometers with flow-injection analyzers for automatic dilution, calibration, separation, concentration, standard additions, and other operations (Greenfield, 1983; LaFerniere et al., 1985); interfacing ICP-AES with liquid chromatographs for concentration and speciation of
elements (Roychowdhury and Koropchack, 1990); using high-salt nebulizers to
prevent clogging of nebulizers (Legere and Burgener, 1985): successfully using
concentration and reduction of spectral interference techniques such as chelation-solvent extraction (Huang and Wai, 1986; Bradford and Bakhtar, 1991); using computer programs such as orthogonal polynomials (Hassan and Loux, 1990),
simplex optimization (Belchamber et al., 1986), and that recommended by Taylor
and Schutyser (1986) to optimize spectrometer operating conditions and automatic correction for spectral interferences; and compiling ICP emission lines still in
progress (McLaren and Berman, 1985; Boumans, 1984; Parsons et al., 1980).
The ICP-MS method of analysis has been developed over the last 15 years.
Houk et al. (198 1) showed suprathermal ionization in an ICP Ar plasma. Within
the last 10 years the method has been applied to routine analytical concentration
determinations. Several review articles document the ICP-MS developmental
milestones (Beauchemin, 1989; Hieftje and Vickers, 1989; Douglas, 1989; Houk
ADVANCES IN ICP EMISSION AND ICP MASS SPECTROMETRY 29
and Thompson, 1988; Houk, 1986; Gray, 1985; Douglas and Houk, 1985). Between 1986 and 1988, ICP-MS enjoyed a surge of popularity. According to Cresser et al. (1988), the late A. R. Date attributed the success of ICP-MS to spectral
simplicity, very high sensitivity, and isotope ratio capability; Date considered ICPMS “the greatest thing to happen to atomic spectroscopy since chopped light”
(Date, 1986). Each year since 1986, papers published in the environmental area of
atomic analysis, including ICP-AES and ICP-MS, have been reviewed by Malcolm S. Cresser and co-workers (Cresser et al., 1986; Ebdon et al., 1987; Cresser
et al., 1988, 1989, 1990, 1991, 1992). Soil and biological material analysis is included in their scope. Another source of current literature on using ICP-MS to analyze geological and inorganic materials is the biennial review publication that appears in Analytical Chemistry (Jackson et al., 1989, 1991). Each January the ICP
Information Newsletter publishes an annual bibliography of the ICP field (Barns,
1992) and, like the Cresser review, abstracts papers on ICP-MS presented at national and international conferences. A review concerned with inorganic mass
spectrometry and X-ray fluorescence spectrometry with a section emphasizing developments in the ICP-MS field has been published yearly since 1988 (Ure er al.,
1988; Bacon et al., 1989, 1990, 1991). Date and Gray (1989) edited a volume on
applications of ICP-MS, and Holland and Eaton (1991) edited a volume containing 21 selected papers from the 2nd International Conference on Plasma Source
Mass Spectrometry held at Durham University in September 1990.
Isotopes of 71 naturally occurring elements can be monitored using conventional positive ion, solution nebulization ICP-MS. Accuracies of the concentrations estimated using these measurements at the Division of Agriculture and Natural Resources (DANR) Analytical Lab at the University of California, Davis,
corrected for internal standard, are typically within 2.5% of the true concentrations
in favorable cases. For about 70% of these elements, more than one stable isotope
occurs in nature. Thus, they can be analyzed using isotope ratios and/or isotope dilution. Isotope ratios show precision of 0 . 1 4 3 % (Gregoire, 1989). Concentrations calculated using isotope dilution (Fassett and Paulsen, 1989) are generally
within 1% of their true concentrations-an accuracy and precision rate higher than
ICP-MS analyses done without the use of stable isotope addition (Viczian et al.,
1990; Van Heuzen et al., 1989; Garbarino and Taylor, 1987; McLaren et al., 1987;
Dolan et al., 1990). Concentrations for 13 other nuclei that are not naturally occurring can also be estimated using the ICP-MS, as indicated in P. G. Brown et al.
(1988, tab. 2); Igarashi et al. (1990); and Kim et al. (1989a, 1989b, 1991).
While elemental coverage and detection limits under relatively ideal conditions
are excellent, there are some problem areas in ICP-MS that must be investigated
before deciding whether or not the ICP-MS technique will work for you (Hiefje,
1992). Although most of the following problems have been overcome or circumvented to meet analytical needs in selected instances, the statements that follow
are generally valid for a generic, normal resolution (i.e., peak widths between 0.5
and 1.O dalton), normal aqueous aerosol generation ICP-MS:
PARVIZ N. SOLTANPOUR ETAL.
1. Although ICP-MS has been found to be satisfactory for soil and biological
tissue work, ICP-MS data are approximately three times less accurate and precise
than ICP-MS data. However, for concentrations determined from isotope dilution-ratio measurements, precision and accuracy are somewhat better than concentrations determined by ICP-AES (Gregoire, 1989; Dolan et al., 1990).
2. Isobaric overlaps (spectral interferences) occur with some regularity for elements between approximately 28 and 80 daltons and do occur throughout the mass
range. They are a result of ( 1 ) a common unit mass shared by more than one element; (2) doubly charged ions overlapping a singly charged isotope with half the
unit mass of the doubly charged species (Vaughan and Horlick, 1986); (3) elemental oxide, elemental hydride, and/or elemental hydroxide ions overlapping isotopes of other elements (Vaughan and Horlick, 1986; Munro ef al., 1986; Date et
al., 1987; Gray, 1986); and (4) background spectral problems (Vaughan and Horlick, 1986; Gray, 1986; Tan and Horlick, 1986). The isobaric interferences involving oxygen can be eliminated using techniques such as electrothermal vaporization (ETV), atomization, or laser-ablation sample aerosol production (Gregoire,
3. Ion response is significantly suppressed by concomitant concentrations. The
threshold concomitant values are low compared to emission suppressions noted
for ICP-AES. Nonspectroscopic interferences result from excessive dissolved
solids in the test solutions. For a number of reasons, the analyte ion amval rate at
the detector (i.e., analyte response) is suppressed under these circumstances
(Beauchemin et al., 1987; Olivares and Houk, 1986; Douglas and Ken; 1988; Gregoire, 1987a,b; Hieftje, 1992). Although at the DANR Analytical Lab the onset of
suppression is usually observed in the neighborhood of 100-500 mg liter-', Gregoire indicates somewhat higher levels using the same instrument model-manufacturer (Perkin-Elmer SCIEX 250, Gregoire, 1989).
4. The ICP, generated in Ar with normal aqueous solution nebulization, may be
unable to produce measurable amounts of positive ions for some analytes that
could be of interest, e.g., F, C1, and/or S . However, the halogens can be determined
in the negative ion mode (Hieftje et al., 1988; Chisum, 1992), whereas sulfur can
be detected if the water is removed from the sample prior to nebulization. Water
vapor can be removed from the sample aerosol using a cooled spray chamber (Hutton and Eaton, 1987). Water can be completely separated from the sulfur using an
electrothermal atomizer (Gregoire, 1989) or partially removed using nebulizationdesolvation equipment (Veillon and Margoshes, 1968).
5. The costs of instrumentation, operation, and maintenance for ICP-MS are
generally higher than those for ICP-AES, leading to a higher cost-per-analyte-concentration determination. We calculate that the cost-per-analyte-concentration determination for an off-the-shelf ICP-MS is about 2.5 times that of a state-of-theart automated sequential scanning ICP-AES instrument using the same
depreciation schedule for each instrument. Gregoire ( 1989) points out, however,
ADVANCES IN ICP EMISSION AND ICP MASS SPECTROMETRY 3 1
that the cost of analysis using ICP-MS is low relative to other methods capable of
producing data on individual isotopes. Similarly, the sample throughput is greater
by about a factor of five for ICP-MS than for other isotope methods.
6. Finally, while multielement capability exists for the ICP-MS, true simultaneous multielement analysis does not (Hieftje, 1992). For an ICP-AES simultaneous multielement system, adding more analytes does not require longer measurement times per sample to preserve detection limits. For the ICP-MS, however,
adding additional analytical isotopes requires longer analysis time per sample to
avoid detection limit and/or precision degradation.
JI. ICP-AES AND ICP-MS INSTRUMENTATION
A. ICP GENERATION
The ICP is produced by initially passing ionized Ar gas through a quartz torch
located inside a Cu coil connected to a radio frequency (RF)generator. The RF
generator provides up to 3 kW forward power (in most commercial units) at a frequency of 27.1 MHz. The high-frequency currents flowing in the Cu coil generate
oscillating magnetic fields whose lines of force are axially oriented inside the
quartz tube and follow elliptical closed paths outside the coil, as shown schematically in Fig. 1 (Fassel, 1977; Fassel and Kniseley, 1974). Electrons and ions passing through the oscillating electromagnetic field flow at high acceleration rates in
closed annular paths inside the quartz tube space. The direction and strength of the
induced magnetic fields vary with time, resulting in electron acceleration on each
half cycle. Collisions between accelerated electrons and ions, and ensuing unionized Ar gas, cause further ionization.
The collisions cause ohmic heating and, when measured spectroscopically, give
thermal temperatures ranging from 6000 to 10,000 K (Fig. 2) (Fassel, 1977). However, with the advent of the ICP-MS, it is evident that the true thermal temperature
of the plasma is much lower than this. For example, the Perkin Elmer SCIEX 500
that has been in the DANR Analytical Lab for over a year has run for hours with
the “6000 K ’ region of the plasma shown in Fig. 2 striking the copper interface
plate with no melting or etching of the copper metal surface. In addition, several
ICP-MS laboratories use copper as the sampler cone metal (Hieftje and Vickers,
1989; Houk, 1986). Copper appears to give satisfactory results in this role unless
sulfuric acid is present in the test solutions and the sampler cone aperture is relatively small (i.e., -0.4 mm); in which case, rapid erosion has been observed
(Munro et al., 1986). Copper metal melts at 1356 K and boils at 2840 K (Weast
and Astle, 1979).
The quartz torch has three concentric channels. The outer channel conducts Ar
PARVIZ N. SOLTANPOUR ETAL.
Figure 1 Magnetic fields (H) and eddy currents (shaded) generated by high-frequency currents (I)
flowing through coil. (Adapted from Fassel and Kniseley, 1974.)
gas at about 15-17 liters min-’ to the plasma to sustain the plasma and to isolate
the quartz tube from high temperatures. The innermost channel is for introducing
sample into the plasma. The middle channel conducts the auxiliary Ar gas at about
1 liter min-l and is used in ICP-AES only when starting the plasma or for organic samples; it is routinely used for all types of samples for ICP-MS (Fig. 3). The
ICP has an annular, or donut, shape when it is viewed from above. The hole has a
lower temperature than the donut body and offers less resistance to the sample injection. The sample is injected into the plasma by using Ar carrier gas at a rate of
about 1 liter min-’ for ICP-AES work. For ICP-MS work, the aerosol flow is approximately 1.5 liter min- l .
The ICP generater has unique physical properties that make it an excellent
source for vaporization, atomization, ionization, and excitation of elements. For
ICP-AES, the aerosol droplets containing the analyte are desolvated, the analyte
salts-oxides are vaporized, and the analyte is atomized at the high temperature region of the plasma in the vicinity of the Cu coil (Fig. 2). An initial radiation zone
(IRZ) has been defined by Koirtyohann et al. (1980) as the zone that begins in the
sample aerosol channel inside the load coil for ICP-AES (Fig. 4). The IRZ extends
upward to 1-2 mm above the load coil, taking on the appearance of an amber “bullet” during nebulization of many sample types related to agriculture. This is due