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Editor: JOSEPH SNEDDON
Department of Chemistry
McNeese State University
Lake Charles, Louisiana
VOLUME3 9 1997
Copyright 91997 by JAI PRESSINC.
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LIST OF CONTRIBUTORS
PLASMA SOURCE MASS SPECTROSCOPY
Andrew S. Fisherand Les Ebdon
MULTIELEMENT GRAPHITE FURNACEAND FLAME
ATOMIC ABSORPTION SPECTROMETRY
Joseph Sneddonand Kimberly S. Farah
DIRECT CURRENTARCSAND PLASMAJETS
Rudi Avni and IsaacB. Brenner
DIRECT AND NEAR REAL-TIME DETERMINATION OF
METALS IN AIR BY IMPACTION-GRAPHITE FURNACE
ATOMIC ABSORPTION SPECTROMETRY
LIST OF CONTRIBUTORS
Isaac B. Brenner
Kimberly S. Farah
Andrew S. Fisher
Geological Surveyof Israel
Department of Environmental Sciences
University of Plymouth
Department of Science
Department of Environmental Sciences
University of Plymouth
Department of Chemistry
Volume 3 of Advances in Atomic Spectroscopy continues to present cutting edge
reviews and articles in atomic spectroscopy as did the previous two volumes in this
Chapter 1 of this volume is devoted to plasma source mass spectroscopy, in
particular inductively coupled plasma mass spectrometry. This was proposed in the
early 1980s and has been commercially available since the mid-1980s. It has been
suggested that it will be the dominant force for trace and ultatrace metal determina-
tion in the coming years. This chapter describes the basic theory, instrumentation,
sample introduction techniques, and selected applications.
Chapter 2 covers simultaneous multielement atomic absorption spectrometry,
mostly with graphite furnace atomization but, where appropriate, with flame
atomization. Atomic absorption spectrometry has been around since the early to
mid- 1950s and is a well-established and accepted technique for trace and ultratrace
determination of elements. However, it is primarily regarded as a single-element
technique. The need to perform simultaneous multielement analyses became a need
and a desire in the early 1970s (the resurgence of atomic emission spectrometry
with the inductively coupled plasma at this time). Atomic absorption spectrometry
was slow to respond to the challenge of multielement analyses, with most work
from the early 1970s to the late 1980s using laboratory-constructed or modified
systems. However, since the late 1980s through to the present time, simultaneous
multielement atomic absorption spectrometry has attracted interest. This chapter
describes instrumentation and applications of simultaneous multielement atomic
Chapter 3 describes the direct current arc and plasma jets. Direct current arc and
plasma spectrometry has been around for a number of years but is still an integral
and indispensable method for determining metals in solids and liquids in many
laboratories. This chapter describes the arc and plasma, and the physical and
chemical interferences of the sample and its trace elemental constituents in the
direct current discharge and their correlation with spectral line intensities of each
trace element. The authors describe their experiences in the determination of trace
elements in refractory-type samples such as uranium, thorium, and plutonium
oxides, rare earth oxides, rock phosphates, silicate rocks, aluminum and titanium
oxides, and molybdenum and tungsten oxides.
Chapter 4 describes basic principles, design, instrumentation, evaluation, char-
acterization, and selected applications of the use of a single-stage impactor com-
bined with a graphite furnace for the direct collection ofmetals in air and subsequent
determination by atomic spectroscopic methods, primarily atomic absorption spec-
trometry. The advantage of this type of system is the ability to determine low
concentrations of metals, in the ng/m3range, within a few minutes.
PLASMA SOURCE MASS
Andrew S. Fisher and Les Ebdon
Abstract ...................................... 2
I. Introduction and Basic Theory .......................... 2
A. Interferences .................................. 4
B. Isotope Ratio Analysis ............................. 7
II. Instrumentation .................................. 8
A. Sample Introduction Systems ........................ 8
B. The Plasma .................................. 11
C. The Interface Region ............................. 12
D. The Ion Lenses ................................ 14
E. Mass Analyzer ................................ 15
F. Electron Multiplier .............................. 15
G. Vacuum Pumps ................................ 16
III. Sample Introduction Techniques ......................... 16
A. Laser Ablation ................................ 17
B. Electrothermal Vaporization ......................... 18
C. Slurry Nebulization .............................. 19
D. Flow Injection ................................ 20
E. Chromatography ............................... 22
F. Hydride Generation .............................. 23
Advances in Atomic Spectroscopy
Volume 3, pages 1-31
Copyright 9 1997 by JAI Press Inc.
All rights of reproduction in any form reserved.
2 ANDREW S. FISHERand LESEBDON
IV. Applications ................... ................ 25
V. Conclusions .................................... 26
References .................................... 27
The use, the instrumentation, and some of the applications of inductively coupled
plasma mass spectrometry (ICP-MS) is described. A brief description of some of the
basic theory is followed by an outline of the different components and their functions
and a more substantial description of the different methods of sample introduction
and their inherent advantages and disadvantages. The use of survey analysis and
isotope dilution analysis has also been described. A description of numerous appli-
cations involving different methods of increasing sensitivity or reducing interference
effects has also been included. Conclusions and a prediction of possible future trends
have also been made.
!. INTRODUCTION AND BASIC THEORY
The concept of plasma source mass spectrometry was first proposed by Gray and
in collaboration with Fassel's research group the inductively coupled plasma was
identified as the most suitable source. Inductively coupled plasma mass spectrome-
try (ICP-MS) was first demonstrated in Fassel's laboratory in the mid- 1970s (Houk
et al., 1980; Date and Gray, 1981). It is a marriage between two already successful
techniques (ICP and MS). The main problem encountered during this coupling was
the interface between the two. The ICP part is at atmospheric pressure, whereas the
MS is under high vacuum. This interface region has been developed and improved
over several years and this will be discussed in a later section (Section II.C).
There are several readable summaries of ICP-MS and ICP in general. Books by
Boumans (1987) and by Montaser and Golightly (1992) give excellent accounts of
the theory behind ICP, and a book by Date and Gray (1989) summarizes the early
developments of ICP-MS. This latter reference also contains a very large number
of applications. Other publications that are of use include the books by Thompson
and Walsh (1989), the handbook of ICP-MS edited by Jarvis, Gray, and Houk
(1992) and the Royal Society of Chemistry monograph by Evans et al. (1995). In
addition to this, there are journal articles that explain in simple terms the use of
such instruments (Ebdon and Evans 1988).
There are several types of instruments available commercially. The standard
instrument has a quadrupole-based mass spectrometer and costs in the region of
$150,000 to $200,000. These are low resolution spectrometers (approximately 0.5
daltons), but are sufficient for the vast majority of applications. For some applica-
tions, a more highly resolving spectrometer is required. Such instruments with
magnetic sector mass spectrometers are available but at much higher cost (approxi-
Plasma SourceMassSpectroscopy 3
mately $400,000). Much of the most recently introduced instrumentation is either
of the high resolution type or reduced size, i.e. benchtop versions of quadrupole
An ICP-MS instrument may be used to obtain concentration data for a large
number of analytes (approximately 70) in a very short period of time. It has several
advantages over other trace element techniques, and may be used in several different
ways. For the survey (semi-quantitative) method of analysis, the concentration of
approximately 70 analytes may be determined simultaneously to within a factor of
3 of the true concentration. Although this method is not particularly accurate, it
does serve to identify contaminants in a previously unknown sample. Once identi-
fied, these contaminants may be determined more accurately using fully quantita-
tive software with calibration standards in the normal way. In addition, most
instruments have single-ion monitoring and time-resolved analysis facilities. These
are software packages that monitor the signal at one isotope for single-ion moni-
toring or at several isotopes quasi simultaneously for time-resolved analysis. They
are most useful with transient signals such as those obtained with laser ablation,
flow injection, electrothermal vaporization, or when chromatography is being
coupled with ICP-MS, although there are other applications. Another advantage of
ICP-MS is ultratrace sensitivity. Modern ICP-MS instruments have limits of
detection superior for most analytes than even electrothermal atomic absorption
spectrometry. As well as having ultratrace detectability (limits of detection < 0.1
ng m1-1 for many analytes), it is also multielement and has a large linear working
range (manufacturers claim 8-10 orders of magnitude). Another advantage is that
it can supply isotopic information. This can be especially useful for analytes such
as lead whose isotopic ratio varies according to geological origin. The technique
also has some disadvantages, the most obvious of these being the high purchasing
and operational costs of the instruments.
The very basic principles of an ICP-MS instrument are that the sample enters a
plasma and is ionized. The ions are then taken from atmospheric pressure through
nickel cones into the interface region which is under partial vacuum. From this
region, the ions are directed using ion lenses under a stronger vacuum to a mass
spectrometer which is under still stronger vacuum. The mass spectrometer sorts the
ions into the mass-to-charge ratio and the ions are then detected by an electron
multiplier. A more detailed description of the method of work of each of the
components will be described later in Section II.
For the majority of applications, the sample is introduced to the plasma via the
sample introduction system consisting of a peristaltic pump, a nebulizer, a spray
chamber, and a torch. In the plasma the sample is rapidly desolvated, atomized, and
ionized. The extent of ionization depends on the ionization energy of the respective
analytes. The ionization energy for argon (the most commonly used plasma gas) is
15.76 eV. Any analyte with an ionization energy below this value will be at least
partially ionized and hence available to be detected. For most metallic analytes the
ionization energy lies in the range 5-10 eV. For analytes with a low ionization
4 ANDREW S. FISHERanti LES EBDON
energy (e.g., caesium), the ionization is close to 100%; however arsenic which has
a much higher ionization energy may only be 30-40% ionized. The extent of
ionization obviously has some effect on the sensitivity of the analyte. Other
prospective analytes such as the halogens (ionization energies 11.8-17.4 eV) are
either very insensitive or completely undetectable. The extent to which the various
species within the plasma are ionized may conveniently be described by the Saha
equation shown below; although it must be stated that this equation assumes the
plasma is in local thermal equilibrium, which most plasmas are not,
NONe _ (2xmekT) 3/2 2zij e-eHkr
Naj h3 Zaj
where: NU= ion concentration of species j; Naj = atom concentration of species j;
N e = free electron concentration; m e = electron mass; k = Boltzman's constant; k =
Planck's constant; zij = partition function of ions of speciesj; Zaj = partition function
of atoms of species j; ej = ionization energy of species j; and T = ionization
Although, when first produced, these instruments were regarded as being inter-
ference-free, this has proved not to be the case. A number of interferences and types
of interference exist. There are basically four types of interference encountered
using ICP-MS. These are termed isobaric, polyatomic, doubly charged, and inter-
ferences observed arising from signal drift or decreased/increased nebulization
efficiency. There have been several reviews on the different types of interference
and methods of overcoming them (Vanhoe et al., 1994a; Evans and Giglio, 1993;
Sah, 1995). Examples of each of these types of interference and how they are
overcome are given below.
These are interferences that occur when more than one analyte has the same
nominal mass. Examples of this include 4~ on 4~ and l13In on ll3Cd. Such
interferences may be overcome using alternative isotopes, such as 44Ca, but this is
a far less abundant isotope and therefore the sensitivity is limited. Another method
of overcoming this problem is to use a high resolution spectrometer. As explained
earlier, this is an expensive solution.
This is by far the most troublesome type of interference. There are a large number
of polyatomic ions that interfere with numerous analytes. The majority of interfer-
ences occur below mass 80, although some oxide based ones do exist at higher
mass, e.g. for the rare earth elements. Some of the more common interferences are
Plasma SourceMassSpectroscopy 5
shown in Table 1. The interfering ions tend to come as part of the matrix, part of
the solvent, or are entrained from the air. There are several possible ways to
overcome the interferences. The use of an alternative isotope is the obvious one,
but some analytes (e.g. arsenic) are monoisotopic. Judicious tuning of the ion lenses
and optimization of torch position, forward powers, and injector (nebulizer) gas
flow lead to a decrease in the interferences, but do not always eliminate them
completely. The use of alternative gases bled into the nebulizer flow has had great
success in removing interferences arising from chloride (Branch et al., 199 la; Hill
et al., 1992a), oxides (Hill et al., 1992b; Ebdon et al., 1993a) sulfates and phosphates
(Ebdon et al., 1994b). Some authors have also bled nitrogen into the coolant gas
flow to overcome interferences (Lam, 1990).
Desolvation of the sample will also decrease interference effects derived from
the solvent, e.g. oxides, nitrogen-containing interferants, and hydrides. Cryogenic
desolvation has been used by Alves et al. (1992) with great effect. By cooling the
sample to -80 ~ oxide ratios for many analytes were decreased to 0.02-0.05%
while ArO§ ArC1+, and C10§ were also decreased. Desolvation has been achieved
in a variety of other ways. The most common method is to use a cooling jacket on
the spray chamber, but other methods include the use of Peltier coolers (Hartley et
al., 1993), gas permeable membranes (Branch et al., 1991b; Botto and Zhu, 1994;
Tao and Miyazaki, 1995) and a mixture of all of these. At least one instrument
manufacturer produces a nebulizer that specializes in desolvating organic solvents.
Table 1. A List of Some of the More Common Polyatomic Ion Interferences
M/Z Element Abundance (%) Interfering Ions
47 Ti 7.72 POt
48 Ti 73.48 32SO§ POH§
51 V 99.76 35C10+, 34SOH+
52 Cr 83.76 4~ 35C1OH+
53 Cr 9.55 37C10§
54 Fe/Cr 5.92/2.38 4~ +, 37C1OH+
55 Mn 100 4~
56 Fe 91.52 4~ +
63 Cu 69.09 4~ §
64 Zn 48.89 HPO~, 32SO~,32S~,63Cull+
65 Cu 30.91 H32SO~
69 Ga 60.16 37C10~
75 As 100 4~
76 Se 9.12 36Ar4~ +
77 Se 7.58 4~
78 Se 23.61 4~ +
79 Br 50.54 38Ar4~ +
80 Se 49.96 4~176 +
6 ANDREW S. FISHERand I.ESEBDON
Membrane separators also have the effect of removing interferences. Tao and
Miyazald found that the levels of ArO§ and C10§ were decreased by one and two
orders of magnitude, respectively, while the CeO+:Ce§ ratio was decreased to
1.1 x 10 -3. The Ba2+:Ba+ratio was also decreased to a larger extent than that found
using a cooled spray chamber. Desolvation devices made inhouse have also been
used. Tittes et al. (1994) have described such a device and found that interferences
arising from chloride (e.g., C10§ C1OH§ ArC1§ CI~, and C10~) were largely
overcome. The detection limit for V in 0.4 M HC1 was found to be improved by a
factor of 25.
This occurs only when an element which has a very low second ionization energy
is present in the sample. The most problematic element for this is barium. This
element produces Ba2+ions which, when in high concentration, can interfere when
determinations of gallium are being made at m/z 69. This can be circumvented by
determining Ga at its less abundant isotope at rn/z 71. Alternatively, if barium is the
analyte, erroneously low results can be obtained if the double ionization is not
prevented. This can be achieved by optimization of power etc.
The signal may drift over a long-term period if the temperature of the room
changes. This is the reason that instruments should always be placed in a tempera-
ture-controlled room. Short-term signal drift occurs when the dissolved/suspended
solids content of the sample changes. This would lead to a change in the nebuliza-
tion efficiency and hence to the amount of sample reaching the plasma. Signal drift
can be accounted for by the addition of an internal standard to all samples and
standards. The prime requisites for an internal standard are that it should not be
present naturally in any measurable amount in the sample, it should preferably have
an ionization energy close to the analyte(s) of interest, and should be reasonably
close in mass to prevent any mass discrimination effects. Some of the more common
internal standards include Co, In, and Rh. The use of an internal standard for normal
analyses is now routine, but care must be taken if chromatography is being coupled
with the ICP-MS since an internal standard is not always readily available. Simi-
larly, for slurry introduction, an internal standard will not necessarily correct for
the transport efficiency of the slurry particles to the plasma if these particles are too
large. An added disadvantage of samples containing high dissolved solids or an
organic solvent is that gradual blocking of the nebulizer, torch, or cones may occur.
This will inevitably lead to a decrease in signal.
Mass Discrimination Effects
In addition to the types of interference listed above, mass discrimination effects
may also occur. This happens especially when there is a high concentration of an
easily ionized heavy element. The most badly affected analytes are the lighter
Plasma SourceMassSpectroscopy 7
elements that have a high ionization potential. Careful optimization of the plasma
operating conditions may overcome these problems to a large extent. It must be
noted that the effect is dependent on the absolute amount of the heavy element rather
than the molar ratio to the analyte. Therefore, if sensitivity permits, the effects can
also be partially overcome by diluting the sample.
B. IsotopeRatio Analysis
Applications of isotope ratio measurements fall into one of two classes. Unspiked
analyses are used primarily in the geological sciences for the determination of the
ages of samples, measuring sedimentation rates and determining paleo- and mag-
matic-temperatures, or in the nuclear industry where applications include the
monitoring of isotopic composition during the production of enriched uranium and
monitoring for environmental contamination. Spiked or isotope dilution analysis is
used to determine the concentration of an analyte by adding a known concentration
to the sample. In ICP-MS, stable isotopes are normally used. In a way it is the ideal
form of internal standardization because any matrix-induced interference will affect
all the isotopes equally. For this type of analysis it is necessary to know the natural
abundance of each of the isotopes in the sample to ensure that the spike is as different
from it as practical. The advantage of this technique is that once the sample has
been spiked, it is not necessary to analyze it quantitatively. Another advantage is
that it can yield results with exceptionally good precision (typically < 1%). This
method also compensates for any losses of analyte during the sample preparation
procedures. If the amount of spike isotope added is known and by measuring the
isotope ratio in the spiked sample, the analyte concentration can be calculated using
the formula below (Thompson and Walsh, 1989),
MsK(A s - BsR )
W(BR - A)
where: C is the analyte concentration (~tgg-l); M~ is the amount of isotope spike
(~tg);A is the natural abundance of the reference isotope; B is the natural abundance
of the spike isotope; A~is the abundance of the reference isotope in the spike; B~is
the abundance of the spike isotope in the spike; K is the ratio of natural and spike
atomic weights; Wis the sample weight (g); and R is the measured reference/spike
isotope ratio after spike addition.
This technique has found applications in several different fields, including
geological, environmental and health sciences, and the nuclear industry. One of the
problems associated with the technique is that it relies on an analyte having several
isotopes that are stable. When isotopic abundance can vary because of radioactive
decay, as is the case with lead and uranium, then problems may occur unless a
"double spike" technique is used, whereby two accurately mixed spikes are added
8 ANDREW S. FISHERand LES EBDON
to a portion of the sample and then the spiked portion and the unspiked portion are
analyzed separately and the data is normalized to the ratio of the spikes.
There have been numerous applications of isotope dilution published. A few of
them are outlined below. A study of the systematic and random errors for the
determination of fission products and actinides has been made by Garcia'Alonso
(1995). Systematic errors arising from mass discrimination effects, detector non-
linearity, and isobaric interferences were corrected.
Blood lead has been determined by Paschal et al. (1995). These authors spiked
blood with NIST 983 radiogenic lead isotopic standard enriched with 2~ to
92.15%. After acid digestion of the blood in a microwave oven, the isotope ratio of
2~176 was measured. The authors claimed that exceptionally accurate results
were obtained and up to ten whole blood samples could be analyzed in a day.
Another application of isotope dilution has been demonstrated by Enzweiler et al.
(1995). These authors determined Ir, Pd, Pt, and Ru in sodium peroxide fusions of
geological materials. Analysis of several certified reference materials (WGB-1,
TDB-1, UMT-1, WPR-1, WMG-1, and SARM-7) yielded results in excellent
agreement with certified values.
Elemental speciation with liquid chromatography ID-ICP-MS has been ad-
dressed by Heumann et al. (1994). The isotope spike was applied in one of two
ways. When the chromatographic system was well-characterized and the species
were well-defined, a spike was added to the sample before the separation stage. If
the species were not well-defined, a continuous on-line introduction of a spiked
solution was used. For the species specific spikes, the method was applied to the
determination of iodide and iodate in mineral water. The range found was 0.5-20
ng ml-l, and the precision of the analysis was 2%.
The instrumentation may be conveniently split into several sections, starting from
the sample introduction systems and ending with the electron multiplier.
There are a number of different ways that samples can be introduced to an
ICP-MS instrument. The most common method is by aspiration of aqueous-based
samples, but other methods such as laser ablation (LA), electrothermal vaporization
(ETV), flow injection (FI), hydride generation (HG), chromatography, and slurry
nebulization also exist. These sample introduction techniques will be discussed in
more detail in Section III.
The sample introduction system for simple aqueous samples consists of a
peristaltic pump, a nebulizer, a spray chamber, and a torch. In general the nebulizer
produces a fine mist of droplets that enter the spray chamber which acts as a
Plasma Source Mass Spectroscopy 9
dampener for pump noise and droplet size filter. The smallest droplets (typically
1-2% of the total solution) reach the torch and enter the plasma.
There are several different types of nebulizers available commercially. These
include: concentric glass pneumatic nebulizers such as the Meinhard; V-Groove
nebulizers such as the De Galan, Babington, and Ebdon (Ebdon and Cave, 1982);
direct injection devices; and ultrasonic nebulizers (Bear and Fassel, 1986). Various
frit (Layman and Lichte, 1982) and grid (Brotherton et al., 1987) style nebulizers
have also been developed.
The type of nebulizer used will depend on the application. The Meinhard
nebulizer is effective for aqueous applications, but may become blockecl if the
solution has a high dissolved/suspended solids content. The Meinhard operates with
low noise levels since the solution need not necessarily be pumped to it by a
peristaltic pump. The frit and grid style nebulizers have greater nebulization
efficiency (and hence a higher percentage of the sample reaches the plasma leading
to reduced wastage of sample), but are also prone to blocking if samples with high
solids are aspirated. The V-Groove nebulizers are far more robust. They are often
made of a polymer, thus are less fragile than the glass Meinhard. For example, the
Ebdon nebulizer is made of corrosion-resistant Kel-F and has proved to be virtually
unblockable, therefore it is used when the sample is corrosive or if it contains a high
dissolved/suspended solids. The disadvantage with V-Groove nebulizers is that they
rely on a peristaltic pump to transport the sample. This means that they are more
liable to pump noise, but less prone to variations in uptake rate due to sample
Direct insertion (or direct injection) devices lead to far higher sensitivity because
close to 100% of the sample reaches the torch (i.e. there is no spray chamber). Since
there is no spray chamber, the dead volume is very much decreased and the time of
analysis is decreased because there does not have to be a wash-out period between
samples. This has advantages for chromatographic applications, but the sample flow
rate has to be low (typically < 100 ~tl min-1) to prevent plasma extinction. In
addition, the DIN can be easily blocked if a sample or a chromatographic mobile
phase has a high dissolved solids content. There have been several papers published
using this type of nebulizer. A DIN has been evaluated in a paper published by
Zoorob et al. (1995). Other authors to have used DIN's include Shum et al. (1992),
Shum and Houk (1993), and Powell et al. (1995). It should be remembered that one
of the roles of a spray chamber is to dampen noise and fluctuations in sample uptake,
therefore nebulizers with no, or reduced, volume spray chambers may result in
Other types of nebulizers such as the thermospray (Vanhoe et al., 1994a) have
also been developed. The advantages of this device are that it improves the
sensitivity by an order of magnitude and improves the M+:MO+and M+:M2§ratios
by a factor of 2.5 when compared with a pneumatic nebulizer used in conjunction
10 ANDREW S. FISHERand LES EBDON
with a spray chamber. Vanhoe et al. have also produced other papers on this topic
(Vanhoe et al., 1994b and Vanhoe et al., 1995). Other authors have also reported
the use of a thermospray device (Arpino, 1992; Koropchak and Veber, 1992).
The hydraulic high pressure nebulizer (HHPN) has been described (Jakubowski
et al., 1992). The same research group has also used this type of nebulizer for
speciation of chromium (Jakubowski et al., 1994). Once combined with an effective
method of desolvation, the HHPN increases the sensitivity for most analytes when
compared with a conventional pneumatic nebulizer. The results for the speciation
yielded detection limits of down to 1 ng ml-~ for the different chromium species.
Although ultrasonic nebulizers were developed in the 1920s, they have been used
by several authors to improve the detection limits in atomic spectrometry (Woller
et al., 1995). Detection limits for many analytes are improved by factors of 5 to 50
when compared with a nondesolvated pneumatic nebulizer, or 3 to 8 times when
desolvation is used. Desolvation is normally used for ultrasonic nebulization,
otherwise the increased solvent loading of the plasma leads to plasma cooling and
possibly extinction. The increased solvent loading of the plasma would arise
because of the smaller droplet size generated by this type of nebulizer (Tarr et al.,
1992). Some commercial USN's consist of a temperature-controlled heated cell
followed by a water-cooled condenser to obtain dry aerosol particles. Ultrasonic
nebulizers are also prone to blockage by samples with high dissolved solids and
may suffer from troublesome memory effects. Yang and Jiang (1995b) have also
reported on the use of a USN recently.
High efficiency nebulizers have also been developed (Sang-Ho Nam et al., 1994).
This nebulizer operates at a very low solution uptake rate (10-100 ~tl min-1) and
its analytical performance in terms of detection limits (ng I-l), precision (0.7-4%),
and the M+:MO§ and doubly charged species ratios compared favorably with a
An excellent review containing 209 references on the theory, mechanism of
operation, and operating characteristics of pneumatic nebulizers was produced by
Sharp (1988a). In a more recent review, the noise characteristics produced by the
aerosols of different ICP nebulizers has been published (Luan et al., 1992).
The basic function of the spray chamber is to ensure that only the smallest
droplets reach the plasma. A review (140 references) of the fundamental processes
occurring within spray chambers has been produced by Sharp (1988b). This review
also covers references that compare nebulizer and spray chamber types. Droplets
in excess of 5-8 ~tm are effectively removed from the system in the spray chamber
and drawn or are pumped to waste. There are several types of spray chambers that
may be used, but the large majority of applications use a double-pass Scott-style
spray chamber. These are frequently made of glass but variants exist for when
corrosive materials such as hydrofluoric acid are being analyzed. These variants are
usually made from PTFE. The drawback with a double-pass spray chamber is that
it has a high internal surface area and a large volume. For chromatographic
applications this can lead to peak broadening. A single pass spray chamber has less
volume and a smaller internal surface area, so is often used for coupling high
performance liquid chromatography (HPLC) or flow injection (FI) with ICP-MS.
A large number of custom-made spray chambers have also been produced. Wu and
Hieftje have developed a cyclone style spray chamber that has a substantially higher
transport efficiency than many other spray chambers.
A water jacket is incorporated into most commercial spray chambers. This is to
cool the aerosol and hence lower the solvent loading of the plasma. This can be
important when the solvent is water since large amounts of oxygen entering the
plasma can lead to deleterious effects and interferences. Decreasing the solvent
loading can be even more important when organic solvents are being used, because
many solvents cause quenching of the plasma and increased interferences. A
detailed study on the effects of organic solvents on plasmas has been made by Boorn
and Browner (1982). Although this work was performed on an emission spectrome-
ter, the overall conclusions hold true for MS instruments.
There are many types of torches available, but most are basically similar in
design. The Fassel-based torch (18 mm in diameter) is used far more than the larger
Greenfield type. The internal diameter of the injector (typically 2 mm) may differ
depending on the sample type. For high solids, a wider bore may be necessary (3
mm), whereas for organic solvents a narrow bore (1 mm) will reduce solvent
loading in the plasma. Demountable torches have the advantage that a variety of
injector bore sizes may be used in an attempt to optimize the system. Low-flow
torches in which the gas flow may only be half that of conventional torches have
also found some use (Evans and Ebdon, 1991).
Other Methods of SampleIntroduction
Many manufacturers produce "bolt-on" devices that may be bought as accesso-
ries and are easily interchangeable. Such devices exist for LA, ETV, FI, and HG,
although many laboratories use equipment that they already possess. Certainly for
FI and HG applications it is common for simple manifolds to be prepared in-house
rather than opting for the more expensive manufacturers products. For LA and ETV
applications, modification to already existing devices is common, but it is more
usual to invest in the manufacturers product for software compatibility reasons.
B. The Plasma
Torch boxes and RF generators differ between manufacturers. Some manufac-
turers use a standard 27.12-MHz generator, whereas others make use of the extra
stability and tolerance of 40.68-MHz generators. Virtually all generators are solid
state, but older instrumentation made use of the Henry generator. Most generators
12 ANDREW S. FISHERand LES EBDON
supply power up to 2000 W, but for most applications a power of 1200-1500 W is
sufficient. Exceptions include the low power plasma work developed by Evans and
Caruso (1993) where powers of less than 100 W may be used. This will be discussed
in more detail later.
The tolerance of generators to organic solvents depends on the design of the
instrumentation. Most modem ICP-MS instruments can withstand very high per-
centages of solvents such as methanol, ethanol, and acetonitrile without plasma
extinction, although for many, a bleed of oxygen into the nebulizer gas flow is
necessary to prevent the buildup of carbon on the cones, which will lead to excessive
signal drift and ultimately to complete blockage.
The basic principles of the ICP have been given in several texts (e.g., Boumans,
1987). Basically, a flow of argon gas is seeded with free electrons via a Tesla coil.
This produces a potential that overcomes the dielectric resistance of the gas. The
load coil produces a fluctuating magnetic and electric field which sustains the
plasma. These fields couple energy into the plasma by accelerating free electrons
into a region within the load coil. These electrons then transfer energy to other
plasma species by collision. This produces further breakdown and an avalanche
effect is produced. The argon then continues collisional energy exchange and a
fireball (plasma) is produced. The temperature reached in the plasma ranges
between 10,000 K at the hottest part to 6000 K in the sampling part.
C. The Interface Region
The plasma acts as an ion source, i.e. the constituents of the sample are dried,
atomized, and then ionized. The ionized analytes then pass from the atmospheric
pressure plasma to the interface region (also called the expansion chamber). A
complete diagram of the plasma and interface region is shown in Figure 1. A
comprehensive description of ion sampling in plasma mass spectrometry has been
given by Douglas and French (1988). Douglas has also given an account in a book
Figure 1. A diagram of the plasma and interface region.
Plasma Source Mass Spectroscopy 13
edited by Montaser and Golightly (1992). The sampling and skimmer cones are
usually made of nickel because it is inexpensive, relatively easy to machine, and is
durable, but any material with high conductivity will suffice. Other materials that
have been used include aluminum, copper, and platinum. Platinum tipped cones are
still used if the sample is likely to be corrosive to the nickel ones, e.g. if it contains
large amounts of phosphate or sulfate.
Behind the sampling cone a rotary pump produces a partial vacuum. The ionized
sample in the form of a gas passes through the aperture in the sampling cone and
on entering this region of lower pressure it accelerates until it exceeds the speed of
sound. The temperature also drops dramatically. Under these conditions, the kinetic
energy of the sample is converted into a directed flow along this axis. In effect, a
free jet is formed that is bounded by a shock wave known as "barrel shock". Barrel
shock helps prevent the gas jet from mixing with any surrounding gas and hence
helps prevent the formation of molecular species. A second shock wave exists across
this axis. This is formed when the expansion is halted by the background gas
pressure. This second shock wave is called the Mach disc. TheMach disc's position
is dependent on the diameter of the aperture in the sampling cone and on the
pressure. Typically, the Mach disc is approximately 10 mm behind the aperture.
Behind the Mach disc the ion beam becomes subsonic again and may mix with any
surrounding gas. To prevent this as much as possible a second nickel cone (the
skimmer) is placed at a distance of just over 6 mm away from the aperture on the
sampling cone. This then allows the gas jet to pass through to the next stage of the
spectrometer. The condition of the aperture on the skimmer cone is also vital. If it
is misshaped then further shock waves will be caused and this will attenuate the
transport of the gas jet through the orifice. Once passed the skimmer, the gas jet
becomes random and requires focusing onto the detector by a set of electrostatic
Cleaning of the cones has been shown to assist in the prevention of the formation
of some polyatomic interferences. Secondary discharge has also been shown to
cause interference effects that have a severely detrimental effect on the performance
of the instrumentation. Secondary discharge is caused by an excessively high
plasma potential causing discharge to occur between the plasma and the sampling
orifice. A bad discharge causes crackling of the plasma and is characterized by
bright white emission from the gas flowing into the orifice. The overall result is
that the sampling orifice becomes ablated (i.e. it becomes much larger), leading to
decreased vacuum in the expansion region which therefore leads to increased
interference effects such as more doubly charged ions and more polyatomic ions.
There have been several methods used to overcome this problem. Among these are
careful optimization of the sampling depth (i.e. from what part of the plasma the
ions are sampled) using a low nebulizer gas flow (<11 min-l) and reducing the
solvent loading of the plasma.
The account given above is a very simplified version of the events going on in
the interface region. Intense research into the positioning and dimensions of the
14 ANDREW S. FISHERand LESEBDON
cones is continuing. Since the inception of the technique the sensitivity of the
analysis has been improved over 10-fold by refining the engineering of the interface
region. Much fuller accounts of molecular beams and the sampling of them and of
the way in which polyatomic interferences may be formed in this region are given
in the literature (Campargue, 1984; Olivares and Houk, 1985; Douglas and French,
1988, Vaughan and Horlick, 1990). Another readable account is given in a mono-
graph written by Evans et al. (1995).
D. The Ion Lenses
The function of the ion lenses is to focus as many ions as possible from the cloud
formed behind the skimmer cone through a differential pumping aperture into an
axial beam of circular cross section at the entrance to the quadrupole mass analyzer.
The ion lenses are basically metal rings with electric potentials applied to them.
They are housed in the intermediate region of the instrument. There are several
different systems in use, but all have a photon stop present on the axis to prevent
photons from the plasma reaching the detector and adding to the background signal.
The diameter of the photon stop is another factor that determines the geometry of
the ion lenses. The geometry of the lens system used by one manufacturer is shown
in Figure 2. The negative voltage on the extraction lens attracts the positive ions in
the sample cloud and accelerates them towards the lens stack. Negative ions are
repelled and the neutral atoms pass to waste in the vacuum pump. The ion beam is
then collected on the "collector electrode" prior to being focused through the
differential pumping aperture by lens 1 and 2. Lens 3 and 4 then refocus the ion
beam into the entrance aperture of the quadrupole mass analyzer.
It must be noted that the region between the skimmer cone and the collector has
a very high "space charge" and a high background pressure of neutral species. This
makes the calculation of ion trajectories difficult. The optimization of the ion
trajectories obviously enables greater sensitivity to be achieved, therefore instru-
'L4"~L3"~ ~~~~'~~ -r". "~"skimmercone
Figure2. A diagram of the interface region and the ion lensarrangement.
Plasma SourceMassSpectroscopy 15
ment manufacturers have performed intense research into this area of the design.
The effects of space charges etc. have been described in a series of papers by Tanner
(Tanner, 1992; Tanner et al., 1994).
E. Mass Analyzer
The function of the quadrupole is to produce an electric field that selectively
allows a stable trajectory for ions that have a narrow mass-to-charge ratio. Although
it does not have as good a resolution as a magnetic sector instrument, it does have
an acceptable sensitivity/resolution trade-off. Typically, it is comprised of four
electrically conducting rods (12 mm diameter x 230 mm long) made of molybde-
num that are arranged to produce an oscillating electric field between them. As the
rods are arranged in a square, the electric field they produce between them is a good
approximation of the ideal hyperbolic quadrupole field. Before the main mass
analyzer there is a series of pre-rods that are only about 20-25 mm long. These rods
are used to improve the transmission of the lighter ions and to prevent contamination
of the main analyzer. Similarly at the end of the main analyzer, a set of rods are
present to improve the extraction of the ions. The entire assembly is under high
vacuum to ensure that there is no residual gas that can disrupt the ion trajectories
by scattering, and hence causing decreased sensitivity. The basic principles of a
quadrupole mass analyzer are beyond the scope of this text, but accounts of how
they work are given elsewhere (Dawson, 1986). It is sufficient to say that the ions
of the selected mass units "corkscrew" their way through the filter while the ions
of unrequired mass units are deflected off this trajectory and are lost. Quadrupole
analyzers have the advantages of being capable of scanning mass units very rapidly
(up to 3000 mass units per second), and they are substantially cheaper and can
withstand higher operating pressures when compared with magnetic sector spec-
As discussed earlier, high resolution spectrometers that utilize a magnetic sector
instead of a quadrupole mass selective filter also exist. These spectrometers are
more highly resolving than the quadrupole-based instruments (they have a resolving
power of about 5000). It must be noted though that sensitivity and resolution are
not always compatible. Although the magnetic sector instruments are far more
resolving, they are frequently unable to compete with the detection limits obtainable
using quarupole-based instrumentation. There are however some applications that
require high resolution to obtain accurate results, including the semiconductor
industry, some geological applications, the nuclear industry, and laboratories that
analyze certified reference materials.
F. Electron Multiplier
The ions transmitted from the quadrupole mass analyzer are detected by an
electron multiplier. Again, the detailed description of how electron multipliers work
is not appropriate in this text, but basically when a positive ion strikes the funnel
16 ANDREW S. FISHERand LES EBDON
of the multiplier one or more secondary electrons are ejected from the surface and
are accelerated down the tube. During the passage down the tube they collide with
the walls dislodging further electrons and thus an avalanche effect quickly builds
up. The curved nature of the tube prevents ionic feedback, i.e. the traveling of
electrons to the beginning of the funnel to restart another avalanche effect. At the
bottom of the tube the cloud of electrons leaves the base of the channel and is
attracted by a collector electrode. The signal is therefore measured as an electrical
Recently, another type of detector has been developed. The use of this "active
film multiplier" detector has become fairly widespread in the U.S. and it is
envisaged that its use will spread further. It is a new type of discrete dynode
multiplier that is coated with a material that is reputedly both resistant to chemical
attack and which is stable in air. Other advantages this detector possesses include
an increased total surface area (1100 mm2compared with 160 mm2in conventional
electron multipliers), which should enhance its lifetime substantially and offer a
large linear dynamic range.
The Faraday cup is also used by some manufacturers. This device is especially
useful when high concentrations of analytes are being determined.
As mentioned earlier, the mass spectrometer from the interface region to the
detector is under vacuum. The interface region is evacuated to typically 2 mbar by
a single-stage rotary pump. Often two rotary pumps in series are used i.e. the output
from one is the input for the second. This improves the vacuum further. The
intermediate (the ion lenses) and analyzer (quadrupole and detector)regions are
under much stronger vacuum. In older style instruments, diffusion pumps were used
but more recently the large majority of ICP-MS instruments use turbomolecular
pumps. This has several advantages. After cleaning the lens stack the turbo pumps
may evacuate the inner chambers within only a few minutes, whereas the diffusion
pumps would take several hours. This has the obvious advantage of decreasing the
downtime of the instrument. Although cleaning the lens stack is normally a 3 to 6
monthly task, if samples containing organic solvents are in regular use. the proce-
dure may have to be repeated every 2 to 3 weeks.
!!!. SAMPLEINTRODUCTION TECHNIQUES
As well as the conventional aspiration of aqueous samples, there is an array of other
methods of introducing samples to the instrument. These include the direct analysis
of solids by laser ablation, ETV or slurry nebulization; or the use of chromatogra-
phy, hydride generation, and flow injection. The relative merits of each of these will
be discussed in this section. An annual review of the advances of methods of sample
introduction to ICP-MS instruments is provided in theAtomicSpectrometryUpdate
Plasma Source Mass Spectroscopy 17
produced in each October issue of the Journal of Analytical Atomic Spectrometry.
This review also describes the fundamental studies, theory, and, to some extent, the
applications of ICP-MS.
Most manufacturers produce a laser accessory that simply bolts on to the front
end of their instrument. A laser (often a Nd:YAG laser) is focused onto (or close to)
the surface of a solid sample causing it to vaporize. The sample vapor is then swept by
a flow of carder gas to the plasma for ionization and detection. The sample does not
have to be conducting, so it has advantages over arc/spark technology. Laser
ablation (LA) has enjoyed great popularity with geologists since it enables them to
analyze rocks without resorting to traditional methods of sample destruction such
as fusion, or acid digestion using hydrofluoric acid. Other advantages it has include
the ability to analyze small samples and depth profiling (i.e. the ability to determine
elemental constituents at different distances from the surface of the sample). The
production of a dry vapor of the sample also leads to decreased interference effects
that normally arise from the solvent (i.e. the production of oxides, hydrides etc.).
There are, however, problems associated with the technique. As the laser only
vaporizes very small areas of the sample, spurious results will be obtained if the
sample is not homogeneous. Another very important disadvantage is that of
standardization. The calibration standards have to be very closely matrix-matched
with the samples and this can be very problematic. The use of certified reference
materials as standards is common for LA-ICP-MS. Because of the difficulty of
standardization, LA is used most frequently for qualitative analysis of samples.
As described earlier, the technique has had many applications in the geological
field. A review of the applications for geological exploration has been published by
Hall (1992). In a very comprehensive review (1019 references) the interaction of
laser radiation with solid materials and its significance to analytical spectrometry
has been discussed (Darke and Tyson, 1993). This review gives a simple explanation
of the ablation process, an introduction to laser microprobes, and a large number
of applications. Because this is such a comprehensive review, only a few of the more
recent applications will be discussed here so that a feel of what is possible is
Laser ablation has been used to determine numerous analytes in several different
geological samples. These include the rare earth elements in geological materials
(Jarvis and Williams, 1993); chalcophile elements in rocks, soils, and sediments
(Guo and Lichte, 1995); sulfide minerals (Watling et al., 1995); rocks (Lichte,
1995); garnet (Fedorowich et al., 1995); alkaline and rare earth elements in marine
ferromanganese deposits (DeCarlo and Pruszkowski, 1995): and weathered marble
(Ulens et al., 1994). As well as geological applications, LA has been applied to
numerous other samples. These include the analysis of steel (Yasuhara et al., 1992),
biological tissue (Wang et al., 1994), tree rings (Hoffmann et al., 1994), glass (Stix
18 ANDREW S. FISHERand LES EBDON
et al., 1995), and teeth (Outfidge and Evans, 1995). Other applications that have
been performed include the analysis of individual fluid inclusions by Shepherd and
Chenery (1995), the analysis of powders (Raith and Hutton, 1994), and the laser
vaporization of small volumes of solutions (Prabhu et al., 1993). Laser ablation has
been used to obtain semiquantitative results in a paper by Cromwell and Arrows-
mith (1995). A paper that reports the direct determination of metals in silver and
gold without matrix-matched standards has been published by Kogan et al. (1994).
This is a novel application, because as described earlier, the standards used for
calibration normally have to be matrix-matched.
Some papers have also been published on hardware developments. Cousin et al.
(1995) have developed an autofocus system that enables reproducible focusing of
Laser ablation using a microprobe has been used in several applications (Chenery
and Cook, 1993; Fryer et al., 1995; Gunther et al., 1995). A microprobe has the
advantage of permitting ablation from a much smaller site (10% of the size required
by conventional laser ablation). This greatly facilitates the analysis of small
individual grains, crystals, and powders. In another application a laser microprobe
was used to find the three-dimensional distribution of precious metals in sulfide
Isotope ratio measurements of copper have been made by Allen et al. (1995).
Precision was found to be 0.85%. This paper also utilized a novel twin quadrupole
mass spectrometer. Having two spectrometers enabled the authors to analyze two
Electrothermal vaporization can be used to analyze liquid or solid samples. Some
manufacturers produce an accessory, but many laboratories simply choose to
modify existing hardware (Lamoureux et al., 1994; Wang et al., 1994). A review of
ETV into ICP-MS has been produced by Carey and Caruso (1992). The overall
benefits offered by this technique have been summarized in a paper produced by
Beres et al. (1994). This paper also discussed the importance of vaporizer design
and illustrated its conclusions with several applications. For the majority of ETV
applications the sample is dispensed on a rod or platform made of graphite or other
suitable material (tungsten or some other refractory metal), dried, ashed, and then
vaporized. The vapor is then swept to the plasma by a flow of carrier gas. This
technique has the advantage of separating the analytes from the matrix, which is
removed during the ash phase. This obviously decreases the prospective interfer-
ences arising from phosphates, halides, sulfates etc. Since the sample is dry, the
determinations are also free from interferences arising from solvents (e.g., oxides
and hydrides). A study of the interferences in ETV-ICP-MS has been made by
Shibata et al. (1993). These authors concentrated on the formation of oxides. The
attenuation of oxide formation in ETV-ICP-MS has also been reported by Clemons
Plasma Source Mass Spectroscopy 19
et al. (1995). These authors used a graphite torch injector to decrease the amount
of oxide formation. Many applications have been produced recently. Vaporization
from a metal platform has been achieved by Marawi et al. (1995); the vaporization
of boron has been assisted by the addition of mannitol as a matrix modifier (Wei et
al., 1995); rare earth elements, uranium, and thorium have been determined (Gre-
goire et al., 1995); and As, Cd, Pb, and Zn have been determined in soils and
sediments (Zaray and Kantor, 1995).
The analysis of solids in this way leads to greatly enhanced sensitivity as no
sample preparation or dilution is required. An alternative approach is to place the
sample on the end of a graphite rod and then insert the rod into the base of the
plasma. This technique is not so common, although there has been a move recently
towards the "electrically heated wire loop in torch vaporization" (Karanassios et
al., 1995). Similarly, an assessment of direct solid sample analysis by graphite pellet
ETV-ICP-MS has been made by Ren et al. (1995).
As well as being an analytical tool, ETV-ICP-MS has been used to elucidate
vaporization mechanisms in electrothermal atomic absorption spectrometry. The
mechanism of chloride interference has been discussed by Byme et al. (1993) and
the mechanism of vaporization of uranium in a graphite tube has also been reported
(Goltz et al., 1995). In the former paper, the use of ICP-MS allowed direct
observation of the signals for manganese along with the matrix components during
the ash (pyrolysis) and vaporization steps. The loss of Mn was reported to be due
to vaporization of manganese chloride.
Exotic combinations of techniques have also been published recently. An ETV
method of introducing slurries to an ICP-MS instrument has been reported by
Gregoire et al. (1994). This technique also used a patented ultrasonic slurry agitator
to ensure homogeneity of the sample. Electrothermal vaporization has also been
used for isotope dilution analysis (Bowins and McNutt, 1994). This procedure
enabled the determination of lead in blood to below the ng m1-1 level.
Although the technique of ETV-ICP-MS has proved very successful and sensi-
tive, there are a few drawbacks. These include the very slow sample throughput,
and the possibility of some refractory analytes not being vaporized quantitatively
from the graphite rod. This has led to the use of matrix modifiers as in electrothermal
C. Slurry Nebulization
Slurry nebulization has had wide acceptance as a means of sample introduction
into plasma emission spectrometry, and its use has spread into plasma mass
spectrometry. Slurry nebulization requires no extra instrumentation, although the
use of a high solids nebulizer is obligatory. Slurry nebulization has several advan-
tages. It eliminates lengthy sample preparation procedures, decreases the number
of sample preparation steps, decreases the use of hazardous acids, and may decrease
the amount of contamination. A major advantage is that slurries may be analyzed
20 ANDREW S. FISHERand LES EBDON
using aqueous solutions as calibrants. A comparative study of aerosol sample
introduction for solutions and slurries in atomic spectrometry has been made by
Ebdon et al. (1989). There are several methods of making slurries, but basically,
sample is powdered and then ground in the presence of an aqueous dispersant using
either a ball mill or the "bottle and bead" method. The dispersant used will depend
on the sample properties, but Aerosol OT and Triton X-100 are frequently used for
carboniferous samples, whereas sodium pyrophosphate or sodium hexametaphos-
phate are used for samples that are more inorganic in nature. The most important
parameter for slurry nebulization is the particle size. Particles larger than 2 ktm are
not transported to the plasma as efficiently as water droplets, hence low recoveries
will be obtained if calibration is against aqueous solutions. The particle size may
be decreased by lengthening the grinding time. The use of an internal standard may
not compensate for the poor transport efficiency since the standard is normally in
the aqueous phase.
There have been several applications of slurry nebulization ICP-MS. These
include geological samples (Jarvis, 1992; Halicz et al., 1993; Totland et al., 1993)
and environmental samples, e.g. sediments (Zaray and Kantor, 1995). A wide range
of analytes may be determined in this way. Zaray and Kantor determined As, Cd,
Pb, and Zn, whereas Totland et al. determined platinum group metals and gold. For
some analyses, slurries have been analyzed using ETV-ICP-MS (Gregoire et al.,
1994; Fonseca and Miller-Ihli, 1995; Zaray and Kantor, 1995).
Some studies have concentrated on the theoretical aspects of slurry introduction.
A paper by Goodall et al. (1993) gave a good theoretical basis for slurry nebulization
into plasmas. Fonseca and Miller-Ihli (1995) concentrated on transport studies of
slurries in ETV-ICP-MS and Hartley et al. (1993) found that desolvation improved
the transport and atomization efficiencies of slurries.
D. Flow Injection
Flow injection has been used by numerous workers to obtain a variety of effects.
It has proved to be a very popular and successful method of sample introduction
because it is so easily coupled with ICP-MS and is so versatile. The first authors to
couple flow injection with ICP-MS were Dean et al. (1988). Basic flow injection
has been used to introduce samples with a very high solids content (Stroh et al.,
1992; Richner, 1993), or to introduce organic solvents (Hill et al., 1992c,d). The
use of flow injection for these samples prevents a continual loading of the plasma
with high solids, thereby preventing torch/nebulizer/cone blockage; or, in the case
of the organic solvents, prevents continually high-reflected powers. Richner com-
pared the limits of detection for samples with increasing solids loading. He found
that LOD's at the ng g-1 level were obtained when samples containing 3% m/m Ni
were analyzed. This was attributed to the very low dilution factor used. Precision
was 2% on 13 replicate 200-~d injections. Organic solvents have been introduced
into ICP-MS by flow injection to maintain plasma stability (Hill et al., 1992c).
Plasma Source Mass Spectroscopy 21
Trimethylgallium and methyllithium were dissolved in diethyl ether and aliquots
of this solution (10-25 ~tl) were introduced to a 2% nitric acid stream. Limits of
detection in the ng ml-I range were obtained for several analytes including A1, Cu,
In, Pb, and Zn. Detection limits were improved further by the use of a desolvation
membrane. The authors concluded that this approach improved detection limits,
increased sample throughput, decreased memory time, avoided sample pretreat-
ment, and therefore improved the safety aspects. Flow injection is also useful when
only a limited amount of sample is available.
If microcolumns of exchange media are used, analytes may be preconcentrated
and the matrix can be eliminated and hence interferences removed. There have been
a large number of applications papers that have used this approach. Examples
include the determination of trace analytes in seawater (Orians and Boyle, 1993;
Bloxham et al., 1994), concentrated brines (Ebdon et al., 1993a), biological samples
(Ebdon et al., 1993b; Ebdon et al., 1994; Huang et al., 1995), steels (Coedo and
Dorado, 1994; Coedo and Lopez, 1994), iron (Coedo et al., 1995), waters (Dadfar-
mia and McLeod, 1994; Fairman and Sanz-Medel, 1995; Gomez and McLeod,
1995), high purity zinc (Sayama et al., 1995), soils (Hollenbach et al., 1994), and
geological materials (Eaton et al., 1992). Many of these papers use ion exchange
resins to retain the analytes while the bulk matrix is eluted to waste. This obviously
has the advantage that when the analytes are eluted to detection, the large majority
of potentially interfering species have already been removed. Large preconcentra-
tion factors are also obtainable using this methodology. If 5 ml of sample is passed
through the column and then the analytes eluted to detection in 250 ~tl of eluent, a
theoretical preconcentration factor of 20 is obtained. Often the preconcentration
factor will be substantially higher than this; e.g. Gomez and McLeod achieved a
factor of 160 when they determined gold in natural waters. Many exchange or
adsorption media have been used. Examples include chelating resins (especially for
transition metals, Cd, Pb, and Zn), acidic alumina (for As, Se, V, and Cr), ion
exchange resins and sulfydryl cotton (for Hg and Au). Work has also been per-
formed in which flow injection of samples into a gaseous carrier (air) was achieved
Some workers have again been coupling several well known techniques together
to improve the overall detection limit or precision. Lu et al. (1993) have coupled
flow injection with isotope dilution ICP-MS to determine Cd, Cu, and Pb in
biological and environmental samples; Colodner et al. (1993) have used the same
technique to determine Re and Pt in waters and Ir in sediments. Stroh and Vollkopf
(1993) have described a method of flow injection-hydride generation ICP-MS for
the determination of As, Hg, and Sb in water and seawater. Their results were
validated by the analysis of several certified reference materials. Quijano et al.
(1995) have used a similar approach to determine Se in water and serum. Using this
method, the authors claimed that the sensitivity was improved by two orders of
magnitude when compared with pneumatic nebulization (35 ng 1-l compared with
3 I.tg 1-1).
22 ANDREW S. FISHERand LES EBDON
Some manufacturers produce accessories for achieving flow injection/matrix
elimination/preconcentration, but basically all that is required is an injection valve,
a peristaltic pump, a glass tube, and some fittings to make the column. The vast
majority of laboratories make their own manifolds since it is substantially cheaper
and is often more robust.
While ICP-MS gives information on total elemental concentration, it is often the
form of the element, so-called "speciation", which is important. To obtain specia-
tion information it is necessary to couple a separation technique such as HPLC or
GC with ICP-MS. A review of coupling chromatography with plasma spectrometric
detection has been made by Hill et al. (1993), while a review concentrating on
coupling chromatography with ICP-MS has been produced by Seubert (1994).
There have been numerous papers appearing in the literature describing the cou-
pling of high performance liquid chromatography (HPLC) with ICP-MS. Chroma-
tography is frequently used for speciation studies and there have been numerous
papers published speciating several different analytes in many different sample
types. Examples include gold from anti-arthritis drugs (Zhao et al., 1992), alumi-
num in tea infusions by size exclusion chromatography (Owen et al., 1992);
platinum from anticancer drugs (Zhao et al., 1993); arsenic in water (Thomas and
Sniatecki, 1995), in urine (Larsen et al., 1993a), in fish (Branch et al., 1994), and
in chicken (Dean et al., 1994); organotin (Kumar et al., 1993 and Rivas et al., 1995);
and cadmium in pig kidney (Dean et al., 1987; Crews et al., 1989). Coupling HPLC
with ICP-MS is in theory very simple. Normally a PTFE tube from the end of the
column to the nebulizer is sufficient, but occasionally, when the mobile phase
contains organic solvents, it may be necessary to desolvate the nebular in some way.
There have been several methods published in the literature on this subject. A
summary of methods available for desolvation is given in Section I.
Some authors are again becoming more adventurous and in attempts to improve
the sensitivity or the precision of their analyses they have coupled together several
techniques. An example of this is the use of chromatography to separate Sb III and
Sbv, followed by hydride generation into ICP-MS detection (Smichowski et al.,
1995). In this paper the detection limits for a 100 ~tl sample were 0.04 and 0.008
ng of SblII and Sbv, respectively, which was an improvement of over an order of
magnitude compared with HPLC-ICP-MS. In a similar approach, lead has been
speciated by HPLC-HG-ICP-MS (Yang and Jiang, 1995a). Again, the limits of
detection for the HPLC-HG-ICP-MS were comparable to or better than conven-
tional pneumatic nebulization. The limits of detection were reported to be 0.6-6 ng
1-1 depending on the species. Lead has also been speciated using HPLC - isotope
dilution -ICP-MS (Brown et al., 1994).
Gas chromatography has been coupled with ICP-MS on far fewer occasions. This
is partially because of the difficulty in coupling the two together. A heated transfer
Plasma SourceMassSpectroscopy 23
line from the end of the chromatograph to the torch is required, but ensuring that
there are no cool points where analytes can condense often proves to be very
problematic. Another problem is ensuring that the transfer line reaches as far into
the torch as possible while ensuring that it does not act as an aerial for RF power
which, if directed back to the chromatograph, could lead to severe instrumental
failure and possible personal danger to the operator. That said, a few workers have
succeeded in making the coupling (Kim et al., 1992a,b; Peters and Beauchemin,
1993; Pretorius et al., 1993; Ebdon et al., 1994b). All of these papers describe the
transfer line or the interface between the chromatograph and the ICP torch. The
majority of papers in this field analyze organometallic compounds such as al-
kylleads in fuel (Kim et al., 1992b) and metalloporphyrins (Pretorius et al., 1993;
Ebdon et al., 1994b). A novel interface that can double for both GC and normal
sample nebulization has been described by Peters and Beauchemin. This interface
reportedly yielded detection limits that were favorable compared with those ob-
tained from a conventional nebulizer and spray chamber. Other workers that have
attempted to couple GC with ICP-MS include Prange and Jantzen (1995) and
Hintelmann et al. (1995). This latter paper described the determination of mercury
and organomercury species in sediments.
Supercritical fluid chromatography is one of the latest types of chromatography
to be developed, and this too has been coupled successfully with ICP-MS. Appli-
cations of this have been produced by Vela and Caruso (tin compounds)(1992) and
Blake et al. (various organometallic compounds)(1995). An overview of the method
has also been published (Carey and Caruso, 1992). The basic instrumentation
including the transfer line is similar to that for gas chromatography.
A few papers coupling capillary electrophoresis with ICP-MS have also been
published. One of the drawbacks with coupling these two techniques is the different
flow rates of the sample. Capillary electrophoresis uses a flow rate at the ~tl min-1
range or below, while ICP-MS typically uses 1 ml min-l. Lu et al. (1995) have
developed a special interface to overcome this problem. A speciation application
has also been produced by Liu et al. (1995).
F. Hydride Generation
Hydride generation (HG) is a technique that is used for several of the metalloid
elements. Arsenic and selenium are particularly prone to interference from chloride
ions. The use of hydride generation is an extremely useful way of separating the
analyte from the matrix. This means that even for difficult samples such as seawater,
As and Se may be determined. The other advantage of HG is that the transport
efficiency of the analyte to the ion source is far higher than for aqueous nebulization
(close to 100%). This inevitably leads to a substantial increase in sensitivity.
Hydride generation has a particular advantage for arsenic determinations in that by
separating As from the matrix the troublesome interference from ArC1§ at mass 75
may be avoided. At very low levels of As, chloride in the aerosol carried over from
24 ANDREW S. FISHERand LES EBDON
a conventional U-tube gas-liquid separator may be problematic. The use of a
membrane separator eliminates this very fine aerosol and has therefore particular
advantage in ICP-MS (Branch et al., 1991).
Other analytes have also been determined using this technique. Besides As and
Se, Ge, Pb, Sn, and Te may be determined by their hydrides. Other analytes such
as Hg may be determined by reducing any mercury present in a sample to the
elemental form using stannous chloride and then sweeping the Hg to the plasma in
a flow of argon. Cadmium has also been determined by ethylating it into a volatile
The only difficulty with this procedure is that the different species of an analyte
may form a hydride with different efficiency. Examples include SeTM which is
hydride forming and SevI which is not, and inorganic arsenic (AsnIand Asv) which
form hydrides (albeit at different rates) and arsenobetaine (the main arsenic com-
pound in many marine samples) which does not form a hydride (Dean et al., 1990).
There are procedures to overcome this problem, e.g. reducing the selenium and
arsenic species to SeTMand AsIu, respectively, using a reagent such as hydrochloric
acid or L-cysteine. This however does not work for some species, e.g. arsenobetaine
and selenomethionine, which are extremely stable. To destroy these compounds,
either very harsh acidic conditions such as perchloric acid are required, which can
on occasions be hazardous for the analyst; or photolysis may be used. There have
been numerous papers that have been published recently that describe the use of
HG-ICP-MS. Antimony, arsenic, and selenium have been determined in waters
(Haraldsson et al., 1992); As, Hg, and Sb have been determined in seawater (Stroh
and Vollkopf, 1993); and total and leachable As and Se have been determined in
soils (Anderson et al., 1994). Other solid materials have been analyzed. Arsenic and
selenium have been determined in marine CRM's with a detection limit of 0.3 and
2.5 pg ml-l respectively (Tao et al., 1993).
Other researchers have coupled different types of chromatography with HG-ICP-
MS to obtain speciation data. However, one of the major problems with this
technique is obtaining full recovery of all the species present without altering the
speciation. This obviously prohibits the use of conventional acid digestion of solid
samples. Arsenic speciation has been achieved as reported in papers by Le et al.
(1994) and Rubio et al. (1993). This latter paper is interesting because it involved
on-line photooxidation to convert the nonreducible species into ones that can form
hydrides. Arsenic has also been determined in seawater by HPLC-HG-ICP-MS
(Hwang and Jiang, 1994) and water (Thomas and Sniatecki, 1995). This work
utilized an interesting in-situ nebulizer hydride generator. Other speciation studies
that have been performed include the separation of SbIu and Sbv (Smichowski et
al., 1995), lead compounds (Yang and Jiang, 1995a), mercury compounds in biota
(Brown et al., 1995), and organotin compounds in mussels (Rivaro et al., 1995).
Other types of chromatography that have been used include micellar liquid chro-
matography for the speciation of tin (Inoue et al., 1995) and vesicle-mediated HPLC
for the speciation of Hg (Aizpun et al., 1995) and for As (Liu et al., 1993). The
Plasma SourceMassSpectroscopy 25
vesicle-mediated HPLC methods have used a Cl8-reversed phase column and a
mobile phase consisting of didodecyldimethylammonium bromide vesicles in a
phosphate buffer containing 0.5% methanol. Under these conditions, the four toxic
forms of As were separated in 10 minutes with sub ng ml-l detection limits. As an
application, As was speciated in urine and tap water.
Several papers have reported on the coupling of flow injection with HG-ICP-MS
(Dean et al., 1990). In one study Se was determined in water and serum (Quijano
et al., 1995) and in another the effects of organic solvents on the hydride generation
of selenium was evaluated (Olivas et al., 1995). In this latter paper, it was found
that the presence of the organic solvents enhanced the Se signal. Under optimum
conditions (sodium borohydride 0.2%, pH 1, methanol load 6%) an LOD of 1 pg
for a 200 ~t1 sample was obtained.
Isotope measurements of antimony have been made in seawater by Kumamaru
et al. (1994): A method was developed to overcome the well-known chloride
interference on As determinations (Story et al., 1992). Other more exotic method-
ologies include the preconcentration of As, Bi, and Te hydrides in a palladium-
coated graphite tube, followed by ETV into the ICP-MS (Marawi et al., 1994). This
technique has the advantage of separating the excess hydrogen produced during
reduction, enabling a more stable plasma to be obtained. The linearity was limited
to the sub ng ml-~ range, but the procedure was validated by the analysis of CRM's.
The LOD for As was 0.002 ng m1-1.
There have been a very large number of applications involving determination by
ICP-MS over the last 10 years. Many of these applications have already been
detailed in the individual sections describing the different types of sample intro-
duction. Extremely useful sources of information include the AtomicSpectrometry
Updates (ASU) published in the Journal of Analytical Atomic Spectrometry.
Fundamental papers involving the theory, hardware, modifications, etc. of ICP-MS
are published annually in the October issue of the journal. Environmental applica-
tions including water, geological materials, air/airborne particulates, soils, etc.
appear in the February issue; clinical, biological materials, food, and beverages
appear in the April issue, and industrial analysis incorporating metals, chemicals,
and advanced materials appear in the December issue. These updates give informa-
tion on papers published in all majorjournals during the previous year. The majority
of the ASU's tabulate the data so that the samples being analyzed, the sample
preparation procedures, the techniques used, the analytes determined, etc. may be
seen at a glance.
Several reviews have also been published in recent years. Gray, (1994) has
discussed the current status of ICP-MS and considered the major problems. Other
reviews concerning clinical applications have also been published. These include
ones by Barnes (1993)(138 references) who summarized the advances in the
26 ANDREW S. FISHERand LES EBDON
application of ICP-MS to human nutrition and toxicology; Vanhoe (1993) who
reviewed the capabilities of ICP-MS for trace element analysis in body fluids and
tissues; and McKay (1993) who reviewed (with 17 references) the use of ICP-MS
for the study of pharmacokinetics of platinum drugs. Carey (1993) has discussed
the relative merits of conventional sample nebulization, ETV, HG, GC, LC, and
SFC (120 references). Uden (1995) has discussed the relative merits of HPLC- and
GC-ICP-MS. In a similar paper, Hill et al. (1993) also reviewed the coupling of
chromatography with ICP-MS. In another invaluable review, McLaren (1993)
provided an ICP-MS applications bibliography update. This review was designed
to include all of the most important ICP-MS applications papers. A review of
speciation in aquatic and biological environments has been produced by Lespes et
al. (1992). Reviews have also been produced for solid sample analysis by ICP-MS
(Baumann, 1992; Voellkopf et al., 1992; Darke and Tyson, 1994). Darke and Tyson
concentrated on slurry introduction, ETV, and laser ablation.
Recently, the use of mixed gas or alternative gas plasmas has increased substan-
tially. Sheppard and Caruso (1994) have produced a review concerning the use of
mixed gas plasmas to overcome polyatomic ion interferences (64 references). The
use of helium as an alternative gas has been used successfully to overcome the
interferences arising from argon, e.g. ArO§ ArC1§ and ArAr§ Similarly, mixed gas
plasmas that use ethene (Ebdon et al., 1994c), nitrogen (Hill et al., 1992a), hydrogen
(Ebdon et al., 1993a), and methane (Hill et al., 1992b) to reduce polyatomic ion
interferences have been reported. The use of mixed gas plasmasmcertainly nitrogen
to remove chloride interferencesmis now routine in many laboratories.
The use of the low-powered plasma developed by Evans and Caruso will probably
gain popularity as it is a halfway house between conventional ICP-MS and other
types of mass spectrometry (Evans and Caruso, 1993; Castillano et al., 1994).
Depending on the operating conditions, it may be used to break down molecules
completely in a similar manner to ICP-MS, but it may also be used to obtain a
fragmentation pattern in a similar manner to typical mass spectrometry techniques
(Evans et al., 1994). This will therefore enable a more unequivocal characterization
of a sample. Although some modification to the hardware of the interface region of
the instrument is required, this can be machined and may easily be interchangeable
with normal hardware.
Inductively coupled plasma mass spectrometry has now come of age. A review by
Horlick (1994) containing 36 references compared the current status of ICP-MS in
the context of Shakespeare's seven ages of man, and there has been an overview by
Gray (1994). Both concluded that the method had reached an encouraging degree
of maturity. Many of the problems associated with ICP-MS have been overcome,
such as the removal of interferences by either matrix separation using flow injec-
tion, hydride generation, or ETV; or by the use of mixed gases.
As the quest for more knowledge on the speciation of analytes progresses, it
would seem obvious that the number of papers that couple chromatography with
ICP-MS will increase. As more authors require greater sensitivity with fewer
interferences, the coupling of other techniques such as ETV, HG, LA, etc. will
increase. Similarly, as a better understanding of space charge effects, ion optics,
and the events in the expansion chamber is obtained, instrumentation with better
sensitivity will be developed. As work continues into the use of low-powered
plasmas, it is envisaged that this too will become more popular.
High resolution spectrometers are becoming more widespread as the need for
interference-free determinations increases. It is therefore envisaged that their use
will continue to increase in the future.
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MULTIELEMENT GRAPHITE FURNACE
AND FLAME ATOMIC ABSORPTION
JosephSneddon and KimberlyS. Farah
Abstract ...................................... 34
Introduction .................................... 34
Instrumentation .................................. 35
A. Multielement Line Sources .......................... 35
B. Continuum Source Multielement AAS .................... 36
C. Multielement Systems ............................ 38
D. Multielement Systems Using Lasers ..................... 46
Flame Atomization Applications ......................... 47
A. Multichannel Systems ............................ 47
B. SIMACC ................................... 48
C. Hitachi .................................... 49
D. Thermo Jarrell Ash-Baird .......................... 49
Graphite Furnace Applications .......................... 49
A. FREMS .................................... 51
B. Hitachi .................................... 51
C. Dual-Channel ................................. 52
Advances in Atomic Spectroscopy
Volume 3, pages 33-61
Copyright 9 1997 by JAI Press Inc.
All rights of reproduction in any form reserved.
34 JOSEPH SNEDDON and KIMBERLYS. FARAH
D. SIMAAC .................................. 53
E. Time-Divided, Single-Channel ....................... 54
F. Thermo Jarrell Ash-Baird .......................... 54
G. Fast Fourier Transform (FFT) ........................ 55
H. Perkin-Elmer SIMAA ............................ 57
V. Multielement System Summary ......................... 58
Acknowledgments ................................ 59
References .................................... 59
In recent years there has been a renewed interest in the use and application of
simultaneous multielement atomic absorption spectrometry (AAS). This chapter
describes the historical development of simultaneous multielement AAS, tracing its
use in laboratory-constructed or modified systems through to commercially available
systems. Several approaches have been proposed and used and are described, includ-
ing multielement line sources, continuum source, multichannel systems, and laser-
based systems. Selected results of the applications of simultaneous AAS are
presented, primarily on graphite furnace atomization, but where appropriate flame
Atomic absorption spectrometry (AAS) is widely used for the determination of
trace elements in numerous and complex matrices. It offers excellent and low
detection limits, high precision, high sensitivity and selectivity, and acceptable
accuracy. One drawback of AAS has been its lack of ability to simultaneously
determine several elements (multielement determinations). If knowledge of several
elements in a sample is desired, an analysis must be run for each sample separately.
This leads to an increase in the time required to complete the analysis. It also
requires that a separate sample be used for every analysis. This can prove to be
difficult when the available sample is small in quantity or is difficult to obtain.
Since the late 1960s, researchers have used a variety of techniques in performing
multielement AAS analysis. Both continuous and line sources have been used. Early
problems with these systems included a decrease in sensitivity, lack of linearity,
and spectral interferences. Not until the early 1980s did systems become available
that eliminated some of these problems.
In the past few years several commercial systems have become available. Newer
systems, some using a laser light source and solid state electronics for detection,
are being studied. Commercial multielement AAS systems are available from
Hitachi (Danbury, Connecticut) since 1988, Thermo Jarrell Ash-Baird (Franklin,
Massachusetts) since 1990, Leeman Labs (Lowell, Massachusetts) since 1993, and