A Beginner’s Guide to ICP-MS

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S P E C T R O S C O P Y
T U T O R I A L
T U T O R I A L

A Beginner’s Guide to ICP-MS
Part III: The Plasma Source

R O B E R T T H O M A S

Part III of Robert Thomas’ series on induc-
tively coupled plasma–mass spectroscopy
( ICP-MS) looks at the area where the ions
are generated — the plasma discharge. He
gives a brief historical perspective of some
of the common analytical plasmas used
over the years and discusses the compo-
nents that are used to create the ICP. He
finishes by explaining the fundamental
principles of formation of a plasma dis-
charge and how it is used to convert the
sample aerosol into a stream of positively
charged ions.

Inductively coupled plasmas are by far

the most common type of plasma
sources used in today’s commercial
ICP–optical emission spectrometry
(OES) and ICP-MS instrumentation.
However, it wasn’t always that way. In the
early days, when researchers were at-
tempting to find the ideal plasma source
to use for spectrometric studies, it was
unclear which approach would prove to
be the most successful. In addition to
ICPs, some of the other novel plasma
sources developed were direct current
plasmas (DCP) and microwave-induced
plasmas (MIP). A DCP is formed when a
gas (usually argon) is introduced into a
high current flowing between two or
three electrodes. Ionization of the gas
produces an inverted Y-shaped plasma.
Unfortunately, early DCP instrumenta-
tion was prone to interference effects and
also had some usability and reliability
problems. For these reasons, the tech-
nique never became widely accepted by
the analytical community (1). However,
its one major benefit was that it could as-
pirate high levels of dissolved or sus-
pended solids, because there was no re-
strictive sample injector for the solid
material to block. This feature alone
made it attractive for some laboratories,
and once the initial limitations of DCPs

Ion
detector

Mass separation
device

MS
interface

Ion optics

ICP torch

Spray
chamber

Nebulizer

Radio frequency
power supply

Turbomolecular
pump

Turbomolecular
pump

Mechanical
pump

Figure 1. Schematic of an ICP-MS system showing the location of the plasma torch and radio
frequency (RF) power supply.

were better understood, the technique
became more accepted. In fact, for those
who want a DCP excitation source cou-
pled with an optical emission instrument
today, an Echelle-based grating using a
solid-state detector is commercially
available (2).

Limitations in the DCP approach led to
the development of electrodeless plasma,
of which the MIP was the simplest form.
In this system, microwave energy (typi-
cally 100–200 W) is supplied to the plasma
gas from an excitation cavity around a
glass or quartz tube. The plasma dis-
charge in the form of a ring is generated
inside the tube. Unfortunately, even
though the discharge achieves a very
high power density, the high excitation
temperatures exist only along a central fil-
ament. The bulk of the MIP never gets
hotter than 2000–3000 K, which means it
is prone to very severe matrix effects. In
addition, they are easily extinguished dur-

ing aspiration of liquid samples. For these
reasons, they have had limited success as
an emission source, because they are not
considered robust enough for the analysis
of real-world, solution-based samples.
However, they have gained acceptance as
an ion source for mass spectrometry (3)
and also as emission-based detectors for
gas chromatography.

Because of the limitations of the DCP
and MIP approaches, ICPs became the
dominant focus of research for both opti-
cal emission and mass spectrometric
studies. As early as 1964, Greenfield and
co-workers reported that an atmospheric-
pressure ICP coupled with OES could be
used for elemental analysis (4). Although
crude by today’s standards, the system
showed the enormous possibilities of the
ICP as an excitation source and most defi-
nitely opened the door in the early 1980s
to the even more exciting potential of us-
ing the ICP to generate ions (5).

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S P E C T R O S C O P Y T U T O R I A L

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Plasma
gas

Auxiliary
gas

Interface Outer
tube

Plasma

Middle
tube

Quartz
torch

(a)

(b)

(c)

Nebulizer gas
Sample injector

RF power

RF
coil

Figure 2. Detailed view of a plasma torch
and RF coil relative to the ICP-MS interface.

Figure 3. (right) Schematic of an ICP torch
and load coil showing how the inductively
coupled plasma is formed. (a) A tangential
flow of argon gas is passed between the
outer and middle tube of the quartz torch.
(b) RF power is applied to the load coil,
producing an intense electromagnetic field.
(c) A high-voltage spark produces free elec-
trons. (d) Free electrons are accelerated by
the RF field, causing collisions and ioniza-
tion of the argon gas. (e) The ICP is formed
at the open end of the quartz torch. The
sample is introduced into the plasma via
the sample injector.

Load
coil

(d)

Tangential flow
of argon gas

Electromagnetic
field

High voltage
spark

(e)

Sample introduced
through sample injector

Collision-induced
ionization of argon

Formation of inductively
coupled plasma

THE PLASMA TORCH
Before we take a look at the fundamental
principles behind the creation of an in-
ductively coupled plasma used in ICP-
MS, let us take a look at the basic compo-

nents that are used to generate the
source: a plasma torch, a radio frequency
(RF) coil, and RF power supply. Figure 1
shows their proximity to the rest of the
instrument; Figure 2 is a more detailed
view of the plasma torch and RF coil rela-
tive to the MS interface.

The plasma torch consists of three con-

centric tubes, which are usually made
from quartz. In Figure 2, these are shown
as the outer tube, middle tube, and sam-
ple injector. The torch can either be one-
piece with all three tubes connected, or it
can be a demountable design in which
the tubes and the sample injector are sep-
arate. The gas (usually argon) used to
form the plasma (plasma gas) is passed
between the outer and middle tubes at a
flow rate of ;12–17 L/min. A second gas
flow, the auxiliary gas, passes between
the middle tube and the sample injector
at ;1 L/min and is used to change the
position of the base of the plasma relative
to the tube and the injector. A third gas
flow, the nebulizer gas, also flowing at
;1 L/min carries the sample, in the form
of a fine-droplet aerosol, from the sample
introduction system (for details, see Part
II of this series: Spectroscopy 16[5],
56–60 [2001]) and physically punches a
channel through the center of the plasma.
The sample injector is often made from
materials other than quartz, such as alu-
mina, platinum, and sapphire, if highly
corrosive materials need to be analyzed.
It is worth mentioning that although ar-
gon is the most suitable gas to use for all
three flows, there are analytical benefits
in using other gas mixtures, especially in
the nebulizer flow (6). The plasma torch

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S P E C T R O S C O P Y T U T O R I A L

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6500 K

7500 K 8000 K

Preheating
zone

6000 K

Normal
analytical
zone

10,000 K

Initial
radiation
zone

Droplet (Desolvation) Solid (Vaporization) Gas (Atomization) Atom (Ionization) Ion

M(H2O)1 X2

(MX)n

MX

M

M1

From sample injector

To mass spectrometer

Figure 4. Different temperature zones in the plasma.

Figure 5. Mechanism of conversion of a droplet to a positive ion in the ICP.

is mounted horizontally and positioned
centrally in the RF coil, approximately
10–20 mm from the interface. It must be
emphasized that the coil used in an ICP-
MS plasma is slightly different from the
one used in ICP-OES. In all plasmas,
there is a potential difference of a few
hundred volts produced by capacitive
coupling between the RF coil and the
plasma. In an ICP mass spectrometer,
this would result in a secondary dis-
charge between the plasma and the inter-
face cone, which could negatively affect
the performance of the instrument. To
compensate for this, the coil must be
grounded to keep the interface region as
close to zero potential as possible. I will
discuss the full implications of this in
greater detail in Part IV of this series.

FORMATION OF AN ICP DISCHARGE
Let us now discuss the mechanism of for-
mation of the plasma discharge. First, a
tangential (spiral) flow of argon gas is di-
rected between the outer and middle tube
of a quartz torch. A load coil, usually cop-
per, surrounds the top end of the torch
and is connected to a radio frequency
generator. When RF power (typically
750–1500 W, depending on the sample) is
applied to the load coil, an alternating
current oscillates within the coil at a rate
corresponding to the frequency of the
generator. In most ICP generators this
frequency is either 27 or 40 MHz. This
RF oscillation of the current in the coil
causes an intense electromagnetic field to
be created in the area at the top of the
torch. With argon gas flowing through
the torch, a high-voltage spark is applied
to the gas, which causes some electrons
to be stripped from their argon atoms.
These electrons, which are caught up and
accelerated in the magnetic field, then
collide with other argon atoms, stripping
off still more electrons. This collision-

induced ionization of the argon continues
in a chain reaction, breaking down the
gas into argon atoms, argon ions, and
electrons, forming what is known as an
inductively coupled plasma discharge.
The ICP discharge is then sustained
within the torch and load coil as RF en-
ergy is continually transferred to it
through the inductive coupling process.
The sample aerosol is then introduced
into the plasma through a third tube
called the sample injector. This whole
process is conceptionally shown in
Figure 3.

THE FUNCTION OF THE RF GENERATOR
Although the principles of an RF power
supply have not changed since the work
of Greenfield (4), the components have
become significantly smaller. Some of the
early generators that used nitrogen or air
required 5–10 kW of power to sustain the
plasma discharge — and literally took up
half the room. Most of today’s generators
use solid-state electronic components,
which means that vacuum power ampli-
fier tubes are no longer required. This
makes modern instruments significantly
smaller and, because vacuum tubes were
notoriously unreliable and unstable, far
more suitable for routine operation.

As mentioned previously, two frequen-
cies have typically been used for ICP RF
generators: 27 and 40 MHz. These fre-
quencies have been set aside specifically
for RF applications of this kind, so they
will not interfere with other communica-
tion-based frequencies. The early RF gen-
erators used 27 MHz, while the more re-
cent designs favor 40 MHz. There
appears to be no significant analytical ad-
vantage of one type over the other. How-
ever, it is worth mentioning that the 40-
MHz design typically runs at lower power
levels, which produces lower signal inten-
sity and reduced background levels. Be-

cause it uses slightly lower power, this
might be considered advantageous
when it comes to long-term use of the
generator.

The more important consideration is
the coupling efficiency of the RF genera-
tor to the coil. The majority of modern
solid-state RF generators are on the order
of 70–75% efficient, meaning that 70–75%
of the delivered power actually makes it
into the plasma. This wasn’t always the
case, and some of the older vacuum
tube–designed generators were notori-
ously inefficient; some of them experi-
enced more than a 50% power loss. An-
other important criterion to consider is
the way the matching network compen-
sates for changes in impedance (a mater-
ial’s resistance to the flow of an electric
current) produced by the sample’s matrix
components or differences in solvent
volatility. In older crystal-controlled gen-
erators, this was usually done with servo-
driven capacitors. They worked very well
with most sample types, but because they
were mechanical devices, they struggled
to compensate for very rapid impedance
changes produced by some samples. As a
result, the plasma was easily extin-
guished, particularly during aspiration of
volatile organic solvents.

These problems were partially over-
come by the use of free-running RF gen-
erators, in which the matching network
was based on electronic tuning of small
changes in frequency brought about by
the sample solvent or matrix components.
The major benefit of this approach was
that compensation for impedance
changes was virtually instantaneous be-
cause there were no moving parts. This
allowed for the successful analysis of
many sample types that would probably
have extinguished the plasma of a
crystal-controlled generator.

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S P E C T R O S C O P Y T U T O R I A L

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IONIZATION OF THE SAMPLE
To better understand what happens to the
sample on its journey through the plasma
source, it is important to understand the
different heating zones within the dis-
charge. Figure 4 shows a cross-sectional
representation of the discharge along
with the approximate temperatures for
different regions of the plasma.

As mentioned previously, the sample
aerosol enters the injector via the spray
chamber. When it exits the sample injec-
tor, it is moving at such a velocity that it
physically punches a hole through the
center of the plasma discharge. It then
goes through a number of physical
changes, starting at the preheating zone
and continuing through the radiation

zone before it eventually becomes a posi-
tively charged ion in the analytical zone.
To explain this in a very simplistic way,
let’s assume that the element exists as a
trace metal salt in solution. The first step
that takes place is desolvation of the
droplet. With the water molecules
stripped away, it then becomes a very
small solid particle. As the sample moves
further into the plasma, the solid particle
changes first into a gaseous form and
then into a ground-state atom. The final
process of conversion of an atom to an
ion is achieved mainly by collisions of en-
ergetic argon electrons (and to a lesser
extent by argon ions) with the ground-
state atom (7). The ion then emerges
from the plasma and is directed into the
interface of the mass spectrometer (for
details on the mechanisms of ion genera-
tion, please refer to Part I of this series:
Spectroscopy 16[4], 38–42 [2001]). This
process of conversion of droplets into
ions is represented in Figure 5.

The next installment of this series will
focus on probably the most crucial area
of an ICP mass spectrometer — the inter-
face region — where the ions generated
in the atmospheric plasma have to be
sampled with consistency and electrical
integrity by the mass spectrometer,
which is under extremely high vacuum.

REFERENCES
(1) A.L. Gray, Analyst 100, 289–299 (1975).
(2) G.N. Coleman, D.E. Miller, and R.W.

Stark, Am. Lab. 30(4), 33R (1998).
(3) D.J. Douglas and J.B. French, Anal.

Chem. 53, 37-41 (1981).

(4) S. Greenfield, I.L. Jones, and C.T. Berry,

Analyst 89, 713–720 (1964).

(6)

(5) R.S. Houk, V. A. Fassel, and H.J. Svec,
Dyn. Mass Spectrom. 6, 234 (1981).
J.W. Lam and J.W. McLaren, J. Anal.
Atom. Spectom. 5, 419–424 (1990).
(7) T. Hasegawa and H. Haraguchi, ICPs in
Analytical Atomic Spectrometry, A.
Montaser and D.W. Golightly, Eds., 2d ed.
(VCH, New York, 1992).

Robert Thomas is the principal of his own
freelance writing and scientific marketing
consulting company, Scientific Solutions,
based in Gaithersburg, MD. He specializes
in trace-element analysis and can be con-
tacted by e-mail at thomasrj@bellatlantic.
net or via his web site at www.
scientificsolutions1.com. u

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