Soil and Environmental Analysis: Modern Instrumental Techniques - Chapter 2 docx

2
Inductively Coupled Plasma
Spectrometry
Stephen J. Hill and Andrew Fisher
The University of Plymouth, Plymouth, England
Mark Cave
British Geological Survey, Nottingham, England
I. INTRODUCTION
Environmental samples are often complex mixtures of organic and
inorganic components that vary widely in their makeup depending on many
factors, which include geological controls, geographical location, depth,
climate, and anthropogenic inputs. Consequently, the chemical composition
of environmental samples is very varied. For example, a major chemical
component of one soil type can be a trace component of others. To provide
a comprehensive elemental analysis of environmental samples it is therefore
necessary to have an analytical method capable of determining most
chemical elements over concentration ranges varying from percent to
ultratrace.
The inductively coupled plasma (ICP) source used in conjunction with
either atomic emission spectroscopy (AES) or mass spectrometric (MS)
detection has a number of properties that make it ideally suited to the
analysis of soils and other environmental materials.
1. In principle, around 75 of the chemi cal elements can be determined.
2. By judicious choice of emission lines, mass-to-charge range, and
detectors, elements can be determined over 5 orders of magnitude in
one sample solution.
3. With care, both the precision and the accuracy of the measurement is high.
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4. ICP spectrometric methods can provide fast multielement measurement
of many elements in one sample. This is ideally suited to soil survey work,

where a large number of samples must be analyzed.
A. Development of ICP as an Atomic Emission Source
The ICP source, as we know it today, was introduced by Reed (1961),
who described an atmospheric argon (Ar) ICP sustained in a three-tube
quartz torch using an RF induction coil. In this first application, the
high temperature of the ICP was used for growing refractory crystals.
Greenfield et al. (1964) were the first to realize the potential of the ICP as a
source for multielement atomic emission. Almost simultaneously, but
independently, in the U.S., Wendt and Fassel (1965) were also experiment-
ing with an Ar ICP as an atomic emission source. From these early studies,
during the late 1960s and the early 1970s, further work by Greenfield, Fassel,
and other research groups established ICP-AES as a viable method for trace
element analysis, producing published articles on a variety of analytical
applications.
In 1975 Greenfield et al. (1975a, 1975b) described the analytical system
that they had been using for routine analytical work for several years. This
system had an ICP source coupled to a 30-channel direct reading
spectrometer, allowing simultaneous multielement analysis with automated
control of sample input and data readout. At a similar time Scott et al.
(1974) described a ‘‘compact’’ design ICP torch in a system with a
pneumatic nebulizer. The two systems generated much discussion on the
most suitable torch design and operating conditions for ICP. Greenfield’s
group advocated the use of a robust ICP with a relatively large torch (29 mm
o.d.) run at high powers of several kW, whereas Fassel’s group used a
smaller torch (20 mm o.d.) and lower power (1–2 kW). Despite these contro-
versies, the early published work on ICP-AES was enough to convince the
instrument manufacturers that the ICP was a marketable product, and the
first commercial instruments were introduced in the mid-1970s.
During the 1970s and the 1980s a wide variety of commercial
instruments were produced. In general these instruments fell into two

categories: systems with polychromator spectrometers that were able to
make simultaneous measurements of many emission lines and systems
with monochromator spectrometers that measured each emission line
sequentially. The former instruments had high sample throughput but were
usually more costly, whereas the latter were less expensive but had more
moving parts and a lower sample throughput. However, monochromator
spectrometers also had the advantage of being able to address many
more analytical lines than the fixed channel polychromator systems.
54 Hill et al.
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Some manufacturers opted to use a combination of both of these spectrom-
eters in a single system. Although there were a number of spectrometer
designs, by the end of the 1980s most commercial systems reflected the
consensus of opinion over the best ICP operating conditions for routine
analytical work. Torch dimensions and low power as originally advocated
by Fassel’s group (Scott et al., 1974) had become universally adopted, along
with the use of generators operating at 27.12 MHz, although it was known
that plasmas operating at 40–50 MHz provide a higher signal-to-back-
ground ratio (SBR) in the emission spectra (Capelle et al., 1982).
All of the early commercial systems relied on the photomultiplier
tube in the spectrometer as a means of converting light intensity into
an electrical signal that could be used to quantify concentration. Although
very sensitive, the photomultiplier tube is quite bulky (a minimum packing
cross sectional area of ca. 0.75–5 cm
2
), which in many instances makes it
difficult to fit the required number of lines necessary for a particular
application into a polychromator system. At the end of the 1980s, however,
developments in solid-state array optical detectors heralded a significant

change in ICP-AES instrument design. These new detectors allowed many
hundreds or thousands of detectors (referred to as pixels), each equivalent
to a single photomultiplier tube, to be packed into the area occupied by a
single photomultiplier tube. In one of the first applications of these new
detectors to ICP, Pilon et al. (1990) described an ICP system that combined
a charge injection device (CID) array detector with an echelle spectrometer.
This system allowed simultaneous analysis with continuous spectral
coverage from 185 to 511 nm, combining the advantages of the older
polychromator and monochromator systems in a single, more compact
instrument.
In conjunction with the advances in detector technology, the instru-
ment manufacturers, under pressure to obtain very low detection limits to
meet the needs of environmental legislation and keep pace with the advances
in ICP-MS, have revisited the use of axial viewing of the ICP, originally
described by Abdallah et al. (1976). This approach provided improvements in
detection limits for some elements by factors varying from 2 to 20 (Brenner
et al., 2000). Nearly all the major instrument manufacturers now offer solid-
state detector instruments with axial ICP viewing. The implications of these
current instrument design trends to environmental analysis will be discussed
in more detail in later sections.
Despite some recent changes in instrument design since the early days,
ICP-AES has been in everyday use for at least 25 years and can now be
considered a mature analytical technique. In recent reviews of atomic
spectrometry in environmental analysis (Cave et al., 2000; Cave et al., 2001;
Hill et al., 2002), it has been concluded that multielement analyses of, for
Inductively Coupled Plasma Spectrometry 55
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example, plant and soil digests by ICP-AES are now so routine in many
laboratories around the world that few reports of novel work, other than

unusual applications, are to be expected.
II. FUNDAMENTAL PROPERTIES OF ICP-AES
The basic process of using emission spectroscopy for chemical analysis
consists of introducing the sample to be analyzed, in an appropriate form,
into an excitation source, where it is dissociated into atoms and ions by
thermal decomposition. The atoms and ions are further excited from their
ground state energy to an energized state from where they spontaneously
revert to a lower energy state, accompanied by the emission of a photon of
light. The energy of the photon (expressed as its wavelength) is specific to
the element being excited, and the number of photons, or light intensity, is
proportional to the concentration of the excited atoms or ions.
The instrumentation required for ICP-AES is shown in Fig. 1 and
comprises three basic units: the source, a spectrometer, and a computer for
control and data analysis.
The ICP source is ideally suited as an emission source because of two
features.
1. The very high temperature of the source allows analyte material to be vola-
tilized easily, and excitation of ions and atoms of most elements can occur.
Figure 1 Schematic diagram of an ICP-AES instrument.
56 Hill et al.
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2. The unique geometry of the ICP allows the emission to be viewed in
an ‘‘optically thin’’ region of the plasma. Here, degradation of the
proportionality of the emission energy to the concentration of the
determinant is not affected significantly by self-absorption of the photons
by atoms or ions of the same element.
A. Plasma Formation
A three-turn water-cooled copper coil is used to couple the RF energy into a
stream of flowing Ar within the plasma torch. When a RF current is applied

to the coil, it sets up an oscillating magnetic field within the quartz tube. The
argon flow through the torch is seeded with electrons from a spark discharge
(usually produced by a Tesla coil). The electrons are accelerated by the
oscillating magnetic field and collide with atoms of the gas, causing further
ionization that leads to the formation of the hot ionized plasma. Rapidly,
equilibrium is reached in which the rate of electron production is balanced
by losses due to recombination and diffusion, and a stable plasma is formed.
The plasma is effectively a conductor and is heated by the flow of current
induced by the RF field. Electrically, the coil and plasma form a transformer
with the plasma acting as a one-turn secondary coil of finite resistance.
Once formed, the ICP is constrained in a quartz torch made from three
concentric tubes, as shown in Fig. 2. The coolant argon flow (typically in the
range 10–20 L min
À1
) is introduced tangentially through the outer annulus
and performs a dual function of keeping the plasma from melting the outer
quartz tube while providing the argon to sustain the plasma. The
intermediate flow (typically 0–1 L min
À1
) allows the plasma to be moved
up or down in the torch and can be used to help prevent the buildup of salt
on the injector tip. The injector flow (typically 0.5–1.5 L min
À1
) punches
a hole through the center of the plasma and is used to carry the sample
(usually in the form of an aerosol) into the plasma for volatilization,
atomization, ionization, and excitation.
B. Spectrochemical Emission Properties of an ICP
The temperature profile and the four main regions in a typical annular ICP
that are important to the analyst are shown in Fig. 3. The preheating zone

(PHZ) occurs at the base of the plasma just before the analyte reaches the
central channel where desolvation of the sample aerosol takes place. The
initial radiation zone (IRZ) is where the sample undergoes volatilization and
atomization/ionization and excitation. Finally, there is the normal analytical
zone (NAZ), which is a low background region just above the bright plasma
fireball where the atomic emission measurements are made. There are two
Inductively Coupled Plasma Spectrometry 57
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important parameters that control the relative positions of these zones in the
plasma:
1. The injector flow rate, when increased, increases the diameter of the hole
through the center of the plasma and shifts the IRZ and NAZ higher in
the central channel. At higher injector flow rates the analyte residence
time within the central channel is decreased, experiencing less heating
from the plasma.
2. The RF power, when increased, tends to constrict the central channel for
a given injector flow rate and push the IRZ and NAZ lower in the
plasma. This increases the residence time of the analyte within the plasma
channel.
Figure 2 Schematic view of an inductively coupled plasma (ICP).
58 Hill et al.
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The third parameter that is dependent on both the power and the injector
flow rate is the viewing height at which the atomic emission is measured
(when the ICP is viewed radially). This is measured as the distance above
the load coil and is normally between 5 and 16 mm. As the relat ive
positions of the IRZ and NAZ are moved through changes in RF power or
injector flow rate, the relative viewing height within the NAZ also changes.

These three parameters can therefore be varied to obtain optimum
performance for any given emission line. Common optimization criteria
are signal-to-background ratio and signal-to-noise ratio. In most instances,
however, ICP-AES is used as a multielement tool, and therefore compro-
mise operating conditions, usually set by the manufacturer, are supplied
with commercial instrumentation.
In many instances, compromise operating conditions work extremely
well for a wide range of sample types. Nevertheless, there may be instances
where an unusual determinant or matrix requires some changes in operating
conditions. In these instances it is useful to have a broad understanding of
how plasma conditions affect particular line types. In general, emission lines
can be divided into ‘‘hard’’ (excitation potential > 4.5 eV) and ‘‘soft’’ lines
(excitation potential < 4.5 eV) as proposed by Boumans (1978). The
behavior of the two types of line when changing ICP operating parameters
can be summarized as follows.
Figure 3 Axial channel emission zone of an ICP. PHZ: preheating zone; IRZ:
initial radiation zone; NAZ: normal analytical zone. (Reproduced with permission
from Sharp, B. in Soil Analysis—Modern Instrumental Techniques, 2d ed. [Smith,
K.A., ed.]. New York: Marcel Dekker, 1991.)
Inductively Coupled Plasma Spectrometry 59
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Soft Lines
1. An increase in applied power enhances the emission and shifts the peak
emission signal lower in the plasma.
2. An increase in sample carrier flow rate reduces the emission intensity and
shifts the peak emission signal higher in the plasma.
3. The presence of an increasing concentration of an easily ionizable
element (EIE) enhances the emission and shifts the peak emis sion signal
lower in the plasma. Higher up in the vicinity of the NAZ, the

enhancement is much less (Blades and Horlick, 1981).
Hard Lines
1. An increase in the applied power produces an increase in the emission
intensity, but the position of the peak emission intensity signal changes
very little.
2. An increase in sample carrier gas flow produces a small but significant
upward shift of the peak emission intensity and a reduction in intensity.
3. An increasing concentration of an EIE causes depression in emission
intensity in the vicinity of the peak emission intensity, but an
enhancement lower in the plasma. This results in a crossover region
where the effect of the interfering elements is minimized.
It is generally agreed that the excitation mechanism of soft lines is essentially
thermal in nature, but for hard lines the excitation mechanism is nonthermal
and involves interactions with metastable Ar ions (Blades, 1987).
The equation used to express the linear relationship between the
spectral radiance B and the concentration of free atoms in the plasma, as
used for calibration in analytical use, may be expressed as
B ¼
1
4
h
0
NL
ZðtÞ
g
k
A
ki
exp
ÀE

k
kT

where
h ¼Planck’s constant

0
¼the frequency of the emitted photons
N ¼ the number of atoms per unit volume
Z(t) ¼the partition function
g
k
¼the statistical weight of the kth state
A
ki
¼the Einstein transition probability for spontaneous emission
L ¼the optical depth of the source
E
k
¼the excitation energy of the kth state
k ¼the Boltzmann constant
60 Hill et al.
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C. Axial/Radial Viewing of ICP
Atomic emission spectrometers may either be used to view the ICP in the
radial position, i.e., the torch is vertical and the light emitted by the
determinants is detected through the side of the plasma, or may be viewed in
the axial position, in which case the torch is horizontal and the detected
determinants’ emissions have to pass through the tail flame (Fig. 4). For use

Figure 4 Typical configuration for ICP-AES instruments: (a) side-on radial
viewing; (b) axial viewing, of the ICP.
Inductively Coupled Plasma Spectrometry 61
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in ICP-AES, the most common orientation to date has been the radial
configuration (although in ICP-MS, the plasma is universally mounted
horizontally). A comprehensive review of many aspects of axial viewing,
including both instrumentation and analytical performance, has been
published by Brenner and Zander (2000).
The first practical point regarding the use of axial viewing for
ICP-AES is that the optical system is ‘‘looking’’ down the end of the plasma
and therefore needs protection from the hot gases in the plasma tail flame
and from the possibility of salt build up on the optical interface. There are
two approaches to this: a shear gas is directed in a near-perpendicular
stream at the tail flame of the plasma directing the tail flame away from the
optics; the optical interface has a counter-current of purge gas flowing out
from its input aperture directly against the plasma tail flame. The shear or
purge gas can be air or an inert gas, the latter being a better choice if low UV
wavelengths are to be measured. The gas has a dual function of protecting
the optics and removing the cooler end of the tail flame, which could cause
self-absorption or other interference effects.
By viewing the ICP end-on, an integrated emission from the whole
length of the sample channel is obtained. This removes the spatial variable
of viewing height, which is important in radial viewing. For axial viewing,
therefore, there are only two important parameters governing the analytical
properties of the plasma, RF power and injector gas flow rate. It is believed
that both signal and background are increased when moving from radial to
axial mode owing to the longer path length being viewed, but because the
NAZ does not have to be viewed through the side of the high background

plasma (as found in radial viewing), the signal increases more than the
background, producing superior signal-to-background ratios (SBR).
While it is acknowledged that axial viewing improves detection limits,
there is some debate as to whether there is reduction in the linear range an d
increase in interferences compared with viewing perpendicular to the central
channel. In their review Brenner and Zander (2000) conclude that there are
conflicting results with regard to linear dynamic range, but Bridger and
Knowles (2000) suggest that curvature may be due to ionization suppression
effects that can be alleviated by on-line addition of a CsCl buffer. It has also
been suggested (Brenner and Zander, 2000) that when run under robust
conditions (see Sec. II.D), axial viewing is as interference-free as radial
viewing (Table 1).
D. Robust Operating Conditions for ICP-AES
In Sec. II.B, the use of compromise operating conditions for multielement
analysis was discussed. One way of arriving at a set of operating conditions
62 Hill et al.
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that are practical for the relatively complex matrix obtained from
environmental samples is to use ‘‘robust’’ operating conditions. Mermet
(1989, 1991, 1998) used this term collectively to express energy transfer,
residence time, and response of the plasma to changes in atomization and
Table 1 Instrumental Detection Limits for ICP-AES and ICP-MS Using the Most
Sensitive Lines and Most Abundant Isotopes
Determinant
ICP-AES
(ng mL
À1
)
ICP-MS

(ng mL
À1
) Determinant
ICP-AES
(ng mL
À1
)
ICP-MS
(ng mL
À1
)
Ag 3 0.005 Nb 4 0.005
Al 2 0.05 Nd 2 0.001
As 12 0.005 Ni 6 0.005
Au 6 0.005 Os 5 0.005
B 2 0.05 P 18 0.5
Ba 0.1 0.001 Pb 14 0.001
Be 0.2 0.001 Pd 7 0.005
Bi 12 0.001 Pr 1 0.001
Ca 0.03 0.5 Pt 20 0.005
Cd 2 0.005 Rb 3 0.001
Ce 8 0.001 Re 11 0.005
Co 5 0.001 Rh 5 0.005
Cr 4 0.005 Ru 6 0.005
Cu 2 0.005 S 20 10
Dy 0.3 0.01 Sb 20 0.005
Er 1 0.001 Sc 0.4 0.05
Eu 0.3 0.001 Se 40 0.05
Fe 2 0.05 Si 5 0.5
Ga 7 0.001 Sm 7 0.001

Gd 3 0.001 Sn 15 0.005
Ge 13 0.05 Sr 0.02 0.001
Hf 4 0.005 Ta 10 0.005
Hg 10 0.001 Tb 5 0.001
Ho 0.5 0.001 Te 30 0.05
In 18 0.001 Th 20 0.001
Ir 4 0.005 Ti 1 0.05
K 10 0.5 Tl 16 0.001
La 0.02 0.05 Tm 2 0.001
Li 1 0.001 U 20 0.001
Lu 0.05 0.001 V 2 0.05
Mg 0.1 0.05 W 20 0.005
Mn 0.3 0.0004 Y 0.2 0.001
Mo 4 0.005 Yb 0.3 0.001
Na 1 0.05 Zn 1 0.005
Inductively Coupled Plasma Spectrometry 63
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excitation conditions and chemical composition of the aspirated solution.
These workers determined that conditions that maximized the Mg II
280.270 nm/Mg I 285.213 nm emission line ratio should provide conditions
that give minimum interference in routine analysis. A number of studies
showed that such conditions were obtained when the internal diameter of the
torch injector exceeds 2 mm i.d., when the injector flow rate is approximately
0.5–0.7 L min
À1
, when the viewing height (in the case of radial viewing) is
low (4–6 mm above the load coil), and when the forward power is high
(typically > 1.4 kW). It is interesting to speculate that Greenfield et al.
(1975a) were perhaps right when they suggested that operating at high

powers had analytical advantages. These conditions are the same for both
radial and axial viewing. Brenner and Zander (2000) have tabulated Mg
ion/atom ratios for both radial and axial viewing and show that axial
viewing ratios are usually lower. This may be additional evidence that axial
viewing is less robust, but again, at present, the evidence is inconclusive.
However, it is generally agreed that as long as the Mg atom-to-ion ratio is
> 8, whatever the viewing geometry, robust conditions have been achieved.
III. INSTRUMENTATION
As stated above, a basic ICP instrument (Fig. 1) comprises an RF generator
to supply the power to the plasma torch, a gas manifold and controller
system, a sample introduction system, a detector, and a data readout system
(Mermet, 1998).
A. Generators
A radio-frequency (RF) generator is the device used to provide the power
for an ICP. It produces an alternating current at a desired frequency. There
are two basic types of RF generator, free running and crystal controlled.
The free running type is the more common in recent instruments. There are
several versions of free running generators, including the Armstrong, the
Hartley, the Colpitts and the tuned anode, tuned grid (TATG), but all
have the same basic principles. The frequency of oscillation is fixed by the
electrical components in the circuit. In the crystal controlled generators, the
main component is a crystal oscillator that consists of a crystal (quartz or
Rochelle salts) sandwiched between two metal plates. When a voltage is
applied to the plates, the crystal expands and contracts with changing
polarity. The frequency of expansion of the crystal is related to its thickness,
but in most instruments this is constant at 13.56 MHz. Other components
64 Hill et al.
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include a frequency multiplier, a buffer circuit, a directional coupler, an

impedance matching network, and a load coil.
Most generators in commercial instruments operate at either 27.12
MHz (a frequency doubler is used) or 40.68 MHz (where a frequency tripler
is used), since manufacturers have to comply with strict regulations over
emission of radiation and the generator should therefore not interfere with
other equipment. Either generator is usually capable of producing a power
of up to 2 kW, although operating powers are normally in the range of 900–
1500 W. Generators and their associated matching networks are of varying
quality. Some can accept the introduction of high solvent loading whereas
others cannot. This capability is dependent upon the instrument manu-
facturer. A more detailed description of generators and their associated load
coils and matching networks is given in the literature (Fisher and Hill, 1999).
B. Torch Design
Several variations in torch design have been produced over the years.
Originally, the Greenfield style torch was used, but although this was very
robust and tolerant of the introduction of other gases, it was much larger
than the alternative Fassel type and required much higher gas flows. The
running costs of the Greenfield torches were therefore higher, and so the
Fassel style torch has become the norm in most modern instruments. There
are, however, several different styles now available based on the basic Fassel
torch. In general, the torches for ICP-AES and ICP-MS instrumentation are
similar. In one ICP-AES torch design, there is a slot cut into the coolant
tube so that the light emitted from the analyte can be transferred more
efficiently, i.e., without diffraction, to the collection optics. As stated in Sec.
II.A, the torch allows the flow of three gases through it, and it is these gases
that sustain the plasma. All three gas flows are usually argon, but at very
different flow rates. The coolant (also called plasma gas or outer gas) flow
rate is typically 11–15 L min
À1
, the auxiliary (also called the intermediate) is

at approximately 1 L min
À1
, and the nebulizer (also called the injector) gas
flow is at between 0.7 and 1.2 L min
À1
, depending on the nebulizer type and
the determinant. It is the latter flow that forms the aerosol from the
nebulizer and transports this through the spray chamber into the torch. The
coolant and intermediate gases are introduced at right angles so that the gas
flows out of the torch in a spiral fashion. It is this spiralling gas flow that
keeps the walls of the torch cool and helps prevent it from melting.
A low-flow torch (also known as the minitorch) is also available that
operates at about one-third of the gas consumption of the standard Fassel
type torch using a forward power of 0.65 kW (Evans and Ebdon, 1991). The
problem associated with the low-flow torch is that it is prone to blocking if
Inductively Coupled Plasma Spectrometry 65
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samples containing in excess of 1% m/v solids are aspirated. Thus althoug h
offering financial advantages in terms of gas consumption, these torches are
not as widely used as the larger design. Demountable or semidemountable
torches offer several advantages over one-piece torches, since various parts of
the torch can be removed and replaced separately. In most designs, this
includes the injector tube and the outer tubes, which erode more quickly than
other parts of the torch, so that they can be replaced separately without
having to replace the whole torch assembly. In some cases a different style or
size of injector may be used for different sample types (Fig. 5). If a sample
contains a relatively large concentration of dissolved or suspended solids,
these may collect within the injector. After a while, the collected solid material
accumulates until the injector becomes blocked. This will lead to signal drift

until, when blocking is complete, no signal is observed. It therefore helps if a
slightly wider injector is used (e.g., with a bore of 2 mm rather than 1.5 mm) or
if a gently tapering injector is used. Operating with an injector tip of at least
2 mm in diameter also ensures that robust operating conditions can be
achieved (see Sec. II.D). The capacity to change the injector is also of use
when samples dissolved in HF are to be analyzed. Such samples would
dissolve a standard quartz injector, and so it is useful to be able to change it to
one made from alumina. The problem with demountable torches is that
precision engineering is required to ensure perfect concentricity. If concen-
tricity is not achieved, then the plasma will not be stable, leading to poor
performance, and the torch may become quickly damaged.
Figure 5 Injector tubes used in torches for ICP-AES. (a) Turbulent constricted
injector tip; (b) intermediate laminar/turbulent capillary injector tip; (c) fully laminar
capillary injector; (d) streamlined capillary injector for demountable torch.
66 Hill et al.
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C. Sample Introduction
Sample introduction plays a key part in any successful ICP-AES analysis.
The ICP is very flexible in that it can accept samples in the form of solids,
liquids, or gases. From the earliest work until the present day, the area of
sample introduction has been a very fertile one for research. In a recent
review of developments in atomic emission spectroscopy (Hill et al., 2000),
the authors note the substantial literature concerning sample introduction
that appears each year. A comprehensive review of this area has been
produced by Montaser et al. (1998). However, despite this ongoing research,
there are relatively few tried and tested sample introduction systems suitable
for routine environmental analysis. These will be discussed in more detail in
this section.
1. Nebulization

The vast majority of analyses are carried out using pneumatic
nebulization. The three principal types used by commercial instruments,
the concentric, the cross-flow, and the Babington V-groove, are shown in
Fig. 6. The concentric and cross-flow nebulizers are self-priming, but the
Babington nebulizer requires a pump to deliver the solution. The glass
concentric nebulizer is probably still the most widely used, giving high
stability and sensitivity. Its main drawbacks are that the sample solution
has to pass through a narrow capillary (i.d. ca. 0.3 mm), which can
become blocked by particulates, and that the narrow annular gas orifice
(ca. 0.02 mm wide) can become clogged by the accumulation of salt
crystals.
The instrumentation used for the sample introduction process can be
identical for both ICP-AES and ICP-MS. Most samples are in liquid form,
e.g., solutions of acid digests or leachates of soil, sediment, or rock samples.
This liquid needs to be transported to the plasma, and this is normally
achieved using a nebulizer to transform the liquid into an aerosol. There
are numerous types of nebulizer available commercially. These include
Meinhard, cross-flow, Ebdon, Burgener, Hildebrand grid, and assorted
other pneumatic nebulizer types. The function of each is to shatter a stream
of liquid into a cloud of droplets, which may be transported to the plasma in
a stream of gas.
Depending on the application and the sample type, different nebulizers
can be optimal. A standard Meinhard nebulizer is a good choice for filtered
fresh waters, but if the sample contains a substantial amount of dissolved
solids (e.g., saline waters) or suspended solids, it is very prone to blocking.
Unblocking such nebulizers can be problematic, since they are made of glass
and are very fragile. Immersion in a strong acid or ultrasonication may
Inductively Coupled Plasma Spectrometry 67
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unblock the nebulizers, but attempting to use thin wire usually leads to
irreparable damage. Modified Meinhard style nebulizers have been produced
that are more capable of aspirating more awkward samples. Other nebulizer
types are much more tolerant, e.g., the Burgener, which can cope with a high
concentration of dissolved salts, but which may still suffer from the problem
of blockage when suspended solids are introduced. Various other types of
nebulizer, e.g., Veespray and Sea Spray, are also available and are reputedly
tolerant of both suspended and dissolved solids. The Ebdon V-groove
nebulizer is also regarded as being generally unblockable and is useful with
slurry samples.
Figure 6 Common nebulizer designs: (a) pneumatic (Meinhard); (b) V-groove;
(c) cross-flow.
68 Hill et al.
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Most of the nebulizers described above produce an aerosol, which then
enters a spray chamber. The function of the spray chamber is to separate the
larger droplets, which are passed to waste, from the smaller droplets, which
are transported by the nebulizer gas to the plasma. A wide range of spray
chamber designs is available. Scott double-pass chambers are common and
can be made of glass or plastic (Fig. 7). Single-pass, cyclone, and impact-bead
spray chambers are also available. The dimensions, surface area, and regions
of dead volume of each are different, and so the washout period between
samples is very variable. In general, the smaller the internal volume and
surface area, the fewer the regions of dead volume, and hence the shorter the
washout period. Unfortunately, a second function of the spray chamber is to
act as a pulse dampener, i.e., to diminish noise originating from the peristaltic
pump used to transport the sample to the nebulizer. The smaller spray
chambers are often less efficient at reducing such pulses when compared with
the larger Scott style designs, and consequently ‘‘noisier’’ signals may result.

Many spray chambers have a jacket of cooling liquid (e.g., water at
5–10

C) surrounding them so that a constant temperature within the spray
chamber is obtained. This has the effect of keeping the solvent loading of
the plasma at a constant level and hence makes the signals more stable. If
an organic liquid, e.g., methanol, is introduced, then the spray chamber
temperature should be decreased, ideally to À10

C, to prevent excess solvent
entering the plasma and causing perturbation or possible extinction. The
majority of combinations of nebulizer and spray chamber have an efficiency
of 1–2%, i.e., of every 100 mL of sample that is introduced, only 1–2 mL
reaches the plasma. Although this seems extremely inefficient, this represents
the optimal sample loading for the plasma before severe perturbation or
possible extinction occurs. The theory behind nebulization and spray
Figure 7 Scott double-pass spray chamber. Shaded areas represent ‘‘dead space’’
that may give rise to memory effects.
Inductively Coupled Plasma Spectrometry 69
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chamber design has been fully discussed in two papers by Sharp (1988a,b),
and the theory of aerosol generation and sample transport in plasma
spectrometry has been covered in detail elsewhere (Browner, 1999).
There are other types of nebulizer that do not require a spray chamber.
One such device is the ultrasonic nebulizer (USN), where the sample is
deposited onto a piezoelectric transducer that vibrates at ultrasonic speeds,
thus shattering the liquid stream to form an aerosol (nebular). The aerosol is
then transported via a desolvation device to the plasma. The desolvation
device is required because the USN has a much higher sample transport

efficiency, and hence excess liquid must be removed to prevent plasma
perturbation. In general, the overall transport efficiency of the analyte is
increased by an order of magnitude, and hence improved limit s of detection
are obtained. Since this type of nebulizer has the desolvation device as an
integral part of its design, an external spray chamber is not required. Several
examples of applications of this type of nebulizer have been published
(Pandey et al., 1998; Poitrasson and Dundas, 1999).
The direct injection nebulizer (DIN) is capable of operating at
exceptionally low sample flow rates (10–50 mLmin
À1
) and hence can be used
if only a very limited sample volume is available. Since the flow rate is so
small, the DIN may be plugged into the base of the torch and the sample
nebulized directly into the plasma. Although 100% of the sample reaches
the plasma, the absolute amount of material is approximately the same as
for a typical pneumatic nebulizer (i.e., 100% at 20 mLmin
À1
is the same as
2% of 1 mL min
À1
), and hence there is no perturbation of the plasma. An
example of the use of a direct injection nebulizer has been published
elsewhere (Acon et al., 2001). Electrospray and thermospray sample
introduction devices have also found use for environmental matrices
(Zhang and Koropchak, 1999; Zheng et al., 2001), but these high-efficiency
sample introduction devices have been used mainly in research laboratories,
and few routine applications have been reported to date.
2. Alternative Sample Introduction Techniques
Although sample introduction is normally achieved via a nebulizer and
spray chamber, several alternative methods are available, some of which

facilitate the analysis of solid samples. These have an advantage in that the
sample needs less manipulation, which reduces the possibility of contam-
ination or determinant loss (e.g., loss by volatilization if an elevated
temperature is used).
Laser ablation is one such technique that has become relatively
popular in recent years, especially for geological samples. The analysis of
rocks normally requires a complete acid digestion, usually using HF,
70 Hill et al.
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which is hazardous to handle and requires special fume extraction
facilities. The use of laser ablation in which a laser is focused at or near
the surface of the rock (or any solid sample) avoids the need for such
sample preparation. The laser is used to volatilize the sample so that the
ablated material can be transported by a flow of inert gas (usually argon)
directly to the plasma. The disadvantage of laser ablation is that it is
often difficult to find appropriate solid standards, since for accurate, fully
quantitative results to be obtained, a standard of very closely matched
matrix must be used. This is frequently difficult to achieve unless certified
reference materials (CRMs) are used. The application of CRMs for
environmental analysis will be discussed in more detail in Sec. IV.
However, in general terms, if we assume that the CRM or other reference
material is closely matrix-matched with the sample, then if the results
obtained are in good agreement with the certified value, we can have
some confidence in the results derived for the real sample. This approach
will not correct for poor sample homogeneity, which can be problematic
using laser ablation, since only a small area of sample is volatilized into
the plasma. Clearly, if a sample is not homogenous, a representative
portion of that sample may not be obtained.
Laser ablation is, however, a useful technique when there is only a very

small mass of sample, because the laser can be focused onto a small area.
One such application of laser ablation to environmental samples is the
determination of platinum group metals in airborne particulate matter
collected on filters (Rauch et al., 2001). Scanning laser ablation provides the
resolution required to analyze individual particles. If a larger sample is
available, then the laser can be used to ‘‘map’’ the sample, i.e., the laser can
be used to ablate spots at regular intervals across the sample surface,
building up a lattice of data points. This goes some way to overcoming
problems associated with poor homogeneity. Depth profiling is another
technique that can be achieved using laser ablation. If the laser is focused
and fired onto the same spot repeatedly, the elemental composition at the
surface, and then at successively lower levels, can be determined.
There are numerous types of laser available commercially, but the
Nd-Y-Al garnet (Nd : YAG) operated at 1064 nm, at 532 nm (if frequency
doubled), or at 266 nm (if frequency quadrupled) is the most commonly used
type for laser ablation. Such a laser operated at a power of 1–1.3 mJ will
produce a crater of approximately 15 mm in width. Other laser systems, e.g.,
infrared (IR) or ultraviolet (UV), have also found common usage.
Slurries are suspensions of solid materials in liquid matrices. They
sometimes provide a convenient way of introducing solid samples, since
their use may allow the sample to be aspirated through a nebulizer/spray
chamber assembly as with liquid samples. Slurries also benefit from the
Inductively Coupled Plasma Spectrometry 71
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advantages of minimal sample manipulation and of not requiring strong
acids for sample destruction. Another advantage is that slurry samples
may frequently be analyzed against a calibration curve prepared using
aqueous standards. There are, however, a few primary rules for slurry
introduction. The most important of these is that the particle size of the

sample must be extremely small. Grinding the sample so that the majority
of particles are in the region of 2 mm in diameter is mandatory if reliable
results are to be produced. The presence of larger particles will yield
low recoveries, because such particles will be transported through the
nebulizer/spray chamber assembly and into the plasma with lower
efficiency than true solutions. Whether low recoveries are obtained will
depend on the sample type. If the sample is composed of relatively
volatile material, e.g., a powdered plant, then the particle in the plasma
will become vaporized and atomized very rapidly, enabling all the atoms
present to be detected. Other samples, e.g., a soil that has a relatively
nonvolatile aluminosilicate matrix, may be only partially decomposed in
the plasma, leaving smaller particles that will contain some determinant
atoms that will not be available for detection. The theory behind using
slurries for ICP analysis has been described in detail by Goodall et al.
(1993).
Sample particle size may be reduced in several ways. The ‘‘bottle and
bead method’’ may be used, in which sample is weighed into a plastic
bottle to which ZrO
2
beads (2 mm diameter, typically 10 g) are added.
A dispersant (about 5 mL) is added, and the bottle is sealed and placed on a
mechanical shaker for several hours (depending on sample type). Several
dispersants are available that are ideal for this purpose, including Aerosol
OT and Triton X-100 for biological samples and sodium hexametaphos-
phate or sodium pyrophosphate for more refractory samples.
Applications papers on slurry introduction are numerous for
environmental samples, including soils (Lu and Jiang, 2001), vegetation
(Carrion et al., 2001), and dust particulates (Coedo et al., 2000). For some
sample types, e.g., vegetation, where the levels of determinants are likely to
be low, a preconcentration method can be used. The sample (5 g) is placed in

a muffle furnace (typically 400–450

C) until it is thoroughly carbonized, and
then the ash, which constitutes 5–10% of the sample mass, is slurried. This
method is obviously not suitable if very volatile determinants, e.g., Hg, Cd,
or Pb, are to be determined. Some workers have also used a combined acid
leach/slurry technique (Persaud et al., 1999).
A micronizer is also a suitable method for preparing slurries. Once
again, the sample is weighed into the container containing the grinding rods,
and a small volume of dispersant is added. The method of choice for slurry
preparation will often be governed by the nature of the determinants.
72 Hill et al.
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For example, if Hf and Zr are to be determined, then the bottle and bead
method is inappropriate. If, however, the determinants of interest include
Mg, the agate grinding rods of a micronizer will yield an enormous blank
value. The hardness of the sample must also be taken into account. If the
sample is harder than the grinding medium, then clearly the sample will not
be ground, while damage will be done to the beads or rods. For rock
samples, it is usual for a pregrinding to be undertaken in a tungsten carbide
Tema mill. This exceptionally hard material breaks down the rocks into a
powder that is then in a form ready to be ground further using ZrO
2
beads.
Electrothermal vaporization (ETV) is a method that has been adopted
from atomic absorption spectrometry. An aliquot of the sample (typically
15–50 mL), usually in a liquid form, is placed on a rod or in a tube that is
heated electrically in a stepwise mann er. The temperatures used will depend
on the sample type. Usually a drying step is required, in which the solvent is

driven off at a temperature sufficient to vaporize the sample evenly, without
frothing of the sample and potential loss due to excessive heating. Once
dried, the sample may be charred or ashed. This step is required for organic
samples to volatilize the matrix, leaving the determinant atoms on the
heated surface. The temperature of this step will depend on the nature of the
determinants and on the sample type, but it typically lies in the region of
300–1300

C. The overall effect of this process is to separate the potentially
interfering matrix from the determinants, hopefully leading to fewer
interference problems during the measurement step. The third step is to
volatilize the determinants into a stream of inert gas so that they may be
transported to the plasma for detection. Again, the temperature at which
volatilization occurs will depend on the determinant, but it will typically be
in the region of 1200–3000

C. The final step is usually a cleaning step to
prevent determinant carryover to the next sample. This is usually achieved
by heating at a temperature close to 3000

C.
It is possible to analyze solid materials directly using this technique,
but weighing only a few mg of sample accurately onto the heating element
can be problematical. The introduction of slurries into ETV is also possible
(Li et al., 1998). Several workers have used chemical additives to modify
the behavior of the determinant elements. Many elements, for example,
have a very volatile fluoride and hence determinants that under normal
circumstances are very refractory, e.g., Hf, Ti, W, or Zr may be volatilized
as the fluoride into the plasma, which dissociates the compounds into atoms
or ions. In cases such as this, the determinant may be volatilized in

preference to the matrix. Alternatively, a silica-based material may be
volatilized leaving the determinants on the atomizer. In any event, the
matrix and determinant(s) are separated, facilitating interference-free
determination.
Inductively Coupled Plasma Spectrometry 73
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Several fluorinating chemicals have been used, but the most common
has been PTFE (Peng et al., 2000). Numerous other matrix modifiers have
been reported in the literature. Further informat ion on this area can be
found in a review on ETV-ICP-MS prepared by Sturgeon and Lam (1999)
or in a review of the relative merits of laser ablation, slurry nebulization, and
electrothermal vaporization by Darke and Tyson (1994).
Some elements form compounds that are vapors at room temperature.
Examples include the metalloids Sb, As, Se, and Te. If acidified samples are
reacted with sodium tetrahydroborate, the gaseous hydrides of these
elements will be formed. These hydrides may then be separated from the
liquid matrix in a gas/liquid separator and transported as the gaseous form
in a stream of inert gas to the plasma. The optimum concentrations of the
acid and of the tetrahydroborate differ for each determinant, although if
several of these hydride-forming elements are to be determined simulta-
neously, compromise conditions may be used. The advantage of this
technique is that the determinants are transferred to the plasma at an
efficiency close to 100%, hence increasing the sensitivity and improving the
limits of detection significantly.
One potential disadvantage of the hydride technique is the risk of
interference effects from the matrix. Transition metals such as Cu or Fe, or
the Pt group metals, may alter the efficiency of formation of the hydrides.
When calibrating against aqueous standards, very low recoveries may be
obtained unless the chemistry of the method is optimized, e.g., by adding

a sequestering or complexing agent to bind with the potential interfering
species (Nakahara and Wasa, 1994). Another potential drawback is that not
all species of the determinant elements form a hydride, and some species
may form a hydride at a different rate or efficiency than others. For
example, Se(IV) readily forms a volatile hydride, but Se(VI) does not form a
hydride at all. In such a case, the sample is usually boiled with HCl,
to reduce the Se(VI) to Se(IV) (Pitts et al., 1995). Another example is As,
where both oxidation states, As(III) and As(V), form hydrides, but at
different rates, whereas other As species, e.g., arsenobetaine (the main As
species in many marine biological samples), do not form a hydride. In this
case, either a powerful oxidant is required to destroy the arsenobetaine,
e.g., alkaline persulfate (Cabon and Cabon, 2000) or photolytic decom-
position may be used (Rubio et al., 1995). For certain other elements,
other vapor-forming chemicals have been used. An example is sodium
tetraethylborate, which has been used to form volatile ethyl compounds
of Cd (Mota et al., 1999).
A modification of the above approach is used for Hg determinations.
If the sample is mixed with a reducing agent, e.g., tetrahydroborate, or more
commonly tin (II) chloride, elemental Hg vapor is formed. This may then be
74 Hill et al.
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transported directly to the plasma by a flow of inert gas. A more detailed
coverage of the history, theory, and application of the extensive subject of
vapor generation has been presented by Dedina and Tsalev (1995).
D. Line Isolation Devices and Detectors (ICP-AES)
The spectral lines of interest emitted by the determinant species in the
plasma need to be separated from all the other lines emitted, using some
form of line isolation device. Nearly all of the useful analytical lines lie in the
region 160–860 nm, although the majority occur below 450 nm. The ideal

spectrometer should therefore be capable of detecting all lines within this
region. At lower wavelengths, e.g., below 200 nm, the presence of molecular
gases such as O
2
and N
2
in the atmosphere will severely limit the sensitivity
because they will absorb much of the emitted light. For determinants such as
As, which has analytical wavelengths of 188.979 and 193.696 nm, and Al,
that has a wavelength of 167.017 nm, steps must be taken to ensure that
molecular gases are excluded from the light path. This may involve flushing
the spectrometer with an atomic inert gas, e.g., Ar, or using a vacuum pump
to evacuate the potential light-absorbing species.
Several different types of instrument are available, but in general they
may be split into two overall classes, sequential and simultaneous
spectrometers. As the name implies, a sequential spectrometer may only
interrogate one analytical line at a time. If a number of determinants are to
be measured, the spectrometer must change wavelength for each element
sequentially. In a modern instrument, this can be a relatively rapid process,
but overall the measurement process is fundamentally slower than when
using a simultaneous spectrometer.
In all sequential spectrometers, the line isolation device is a mono-
chromator. Schematic diagrams of two types of line isolation device are
shown in Fig. 8, although the Czerny–Turner mounting is the most widely
used. The diffraction grating is the component that separates the light
entering the monochromator into its constituent parts. There are several
different types, e.g., ruled, holographic, and echelle gratings, but all have
equidistant parallel grooves cut at an angle on the surface of a mirror.
The density of these grooves on such ‘‘blazed’’ gratings differs between
spectrometers but usually lies in the region of 600 to 4200 per mm. It is the

angle and the density of the blaze that determines the separation of the
wavelengths of the light. As the light, which is composed of multiple
wavelengths, strikes the grating, it is diffracted at an angle that is dependent
upon the wavelength. Light with longer wavelength will have a higher angle
of diffraction. The angle of diffraction also increases as the number of
grooves per unit area increases. The mirror focuses the light coming from
Inductively Coupled Plasma Spectrometry 75
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the plasma through the entrance slit onto the grating, and then the diffracted
light is focused onto the exit slit and detector. If more than one analytical
line is to be interrogated, then the grating can be rotated so that a second
wavelength is focused to the detector. This process may be repeated until all
the wavelengths of interest have been examined.
Polychromators are line isolation devices that allow several analytical
lines to be examined simultaneously. This not only saves time during an
analysis but also offers the opportunity to obtain simultaneous background
correction. In Paschen–Runge type polychromators, the grating is static,
Figure 8 Line isolation devices used for ICP-AES. (a) Single-channel scanning
spectrometer based on the Czerny–Turner optical configuration; (b) echelle
polychromator.
76 Hill et al.
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i.e., it does not rotate. This means that the instrument is relatively inflexible,
because only a certain number of exit slits and detectors are available to
the user. Most polychromators are programmed for between 20 and 30
analytical lines. Since changing to obtain another wavelength of interest is
time-consuming, considerable thought should go into choosing the original
set of wavelengths.

In both monochromator and polychromator spectrometers, the
detector used has traditionally been a photomultiplier tube (PMT).
Although it is not necessary here to describe in detail how these devices
work, a brief overview may be useful. A PMT is basically a quartz vacuum
tube containing a photosensitive cathode. When a photon exiting the
spectrometer hits the cathode it emits electrons. These electrons are then
accelerated down a series of between 9 and 16 ever more positive dynodes,
emitting further electrons every time a dynode is hit. An avalanche effect is
therefore created so that a 9-dynode PMT may yield 10
6
electrons per
photon strike. The electrons are then collected by the anode and the current
measured is proportional to the number of photons hitting the PMT, i.e., it
is proportional to the amount of light emitted from the determinant, which
in turn is proportional to the concentration of this element in the sample.
Numerous types of PMT exist, and each has a slightly different wavelength
operating range determined by the photosensitive material used to coat the
cathode. Gallium arsenide is a common material for this purpose, since it
has a fairly uniform response over a relatively wide wavelength range.
A more versatile version of the basic polychromator, the echelle-type
polychromator (Fig. 8b), is now incorporated into many new instrument
designs. This device uses both a prism and a grating to separate the
polychromatic radiation. Individually, the prism and the grating produce
poor resolution, with multiple overlapping wavelengths, but used in
combination very high resolution may be achieved. A more co mprehensive
description of how the echelle-type polychromator works may be found
elsewhere (Boss and Fredeen, 1997). One important feature of this
dispersion system is that it makes possible the use of a completely different
type of detector, the multichannel solid-state detector, which is now fitted to
many new ICP-AES instruments. Each of these detectors contains many

thousands of individual cells (pixels), usually arranged in a two-dimensional
rectangle (Fig. 9). The number of pixels varies but can be between 512 Â512
and 4096 Â4096. There are several different types of such so-called charge-
transfer devices (CTD), including charge-coupled (CCD), segmented
charge-coupled (SCD), and charge-injection devices (CID). The SCD is
slightly different from the others in that it contains individual collections
of small subarrays (over 200 in total) of 20–80 pixels each, rather than a
complete CCD, which contains hundreds of thousands of contiguous pixels.
Inductively Coupled Plasma Spectrometry 77
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