A Beginner’s Guide to ICP-MS – Michigan State University

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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 I

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

Amazingly, 18 years after the com-

mercialization of inductively cou-
pled plasma mass spectrometry
(ICP-MS), less than 4000 systems
have been installed worldwide. If
you compare this number with another
rapid multielement technique, inductively
coupled plasma optical emission spec-
trometry (ICP-OES), first commercial-
ized in 1974, the difference is quite signif-
icant. In 1992, 18 years after ICP-OES was
introduced, more than 9000 units had
been sold, and if you compare it with the
same time period that ICP-MS has been
available, the difference is even more dra-
matic. From 1983 to the present day,
more than 17,000 ICP-OES systems have
been installed — more than four times
the number of ICP-MS systems. If the
comparison is made with all atomic spec-
troscopy instrumentation (ICP-MS, ICP-
OES, graphite furnace atomic absorption
[GFAA] and flame atomic absorption
[FAA]), the annual turnover for ICP-MS
is less than 7% of the total atomic spec-
troscopy market — 400 units compared
to approximately 6000 atomic spec-
troscopy systems. It’s even more surpris-
ing when you consider that ICP-MS of-
fers so much more than the other
techniques, including two of its most at-
tractive features — the rapid multiele-
ment capabilities of ICP-OES, combined
with the superb detection limits of GFAA.

ICP-MS — ROUTINE OR RESEARCH?
Clearly, one of the reasons is price — an
ICP-MS system typically costs twice as
much as an ICP-OES system and three
times more than a GFAA system. But in a
competitive world, the “street price” of an
ICP-MS system is much closer to a top-of-
the-line ICP-OES system fitted with sam-
pling accessories or a GFAA system that
has all the bells and whistles on it. So if
ICP-MS is not significantly more expen-

38 SPECTROSCOPY 16(4) APRIL 2001

sive than ICP-OES and GFAA, why hasn’t
it been more widely accepted by the ana-
lytical community? I firmly believe that
the major reason why ICP-MS has not
gained the popularity of the other trace
element techniques is that it is still con-
sidered a complicated research tech-
nique, requiring a very skilled person to
operate it. Manufacturers of ICP-MS
equipment are constantly striving to
make the systems easier to operate, the
software easier to use, and the hardware
easier to maintain, but even after 18 years
it is still not perceived as a mature, rou-
tine tool like flame AA or ICP-OES. This
might be partially true because of the rel-
ative complexity of the instrumentation;
however, in my opinion, the dominant
reason for this misconception is that
there has not been good literature avail-
able explaining the basic principles and
benefits of ICP-MS in a way that is com-
pelling and easy to understand for some-
one with very little knowledge of the
technique. Some excellent textbooks (1,
2) and numerous journal papers (3–5)
are available that describe the fundamen-
tals, but they tend to be far too heavy for

a novice reader. There is no question in
my mind that the technique needs to be
presented in a more user-friendly way to
make routine analytical laboratories more
comfortable with it. Unfortunately, the
publishers of the “for Dummies” series of
books have not yet found a mass (excuse
the pun) market for writing one on ICP-
MS. So until that time, we will be present-
ing a number of short tutorials on the
technique, as a follow-up to the poster
that was included in the February 2001
issue of Spectroscopy.

During the next few months, we will be
discussing the following topics in greater
depth:
• principles of ion formation
• sample introduction
• plasma torch/radio frequency genera-

tor

• interface region
• ion focusing
• mass separation
• ion detection
• sampling accessories
• applications.

We hope that by the end of this series,
we will have demystified ICP-MS, made it

Figure 1. Generation of positively charged ions in the plasma.

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

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

Figure 2. Simplified schematic of a chromium ground-state atom
(Cr0).

Figure 3. Conversion of a chromium ground-state atom (Cr0) to an
ion (Cr1).

a little more compelling to purchase, and
ultimately opened up its potential as a
routine tool to the vast majority of the
trace element community that has not yet
realized the full benefits of its capabilities.

GENERATION OF IONS IN THE PLASMA
We’ll start this series off with a brief de-
scription of the fundamental principle
used in ICP-MS — the use of a high-
temperature plasma discharge to gener-
ate positively charged ions. The sample,

typically in liquid form, is pumped into
the sample introduction system, which is
made up of a spray chamber and nebu-
lizer. It emerges as an aerosol and eventu-
ally finds its way — by way of a sample in-
jector — into the base of the plasma. As it
travels through the different heating
zones of the plasma torch it is dried, va-
porized, atomized, and ionized. During
this time, the sample is transformed from
a liquid aerosol to solid particles, then
into a gas. When it finally arrives at the
analytical zone of the plasma, at approxi-
mately 6000–7000 K, it exists as excited
atoms and ions, representing the elemen-
tal composition of the sample.
The excitation of the outer electron of
a ground-state atom, to produce
wavelength-specific photons of light, is
the fundamental basis of atomic emission.
However, there is also enough energy in
the plasma to remove an electron from its
orbital to generate an ion. It is the genera-
tion, transportation, and detection of sig-
nificant numbers of these positively
charged ions that give ICP-MS its charac-
teristic ultratrace detection capabilities.
It is also important to mention that,
although ICP-MS is predominantly used
for the detection of positive ions, negative
ions (such as halogens) are also pro-
duced in the plasma. However, because
the extraction and transportation of nega-
tive ions is different from that of positive
ions, most commercial instruments are
not designed to measure them. The
process of the generation of positively
charged ions in the plasma is shown con-
ceptually in greater detail in Figure 1.

40 SPECTROSCOPY 16(4) APRIL 2001

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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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Table I. Breakdown of the atomic structure of copper isotopes.

Number of protons (p1)
Number of electrons (e2)
Number of neutrons (n)
Atomic mass (p1 1 n)
Atomic number (p1)
Natural abundance
Nominal atomic weight

63Cu

29
29
34
63
29
69.17%

65Cu

29
29
36
65
29
30.83%

63.55*

* Calculated using the formulae 0.6917n 1 0.3083n 1 p1 (referenced to the
atomic weight of carbon)

Figure 4. Mass spectra of the two copper isotopes — 63Cu1 and
65Cu1.

Figure 5. Relative abundance of the naturally occurring isotopes of all the elements (6). Reproduced with the permission of PerkinElmer
Instruments (Norwalk, CT).

APRIL 2001 16(4) SPECTROSCOPY 41

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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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ION FORMATION
Figures 2 and 3 show the actual process
of conversion of a neutral ground-state
atom to a positively charged ion. Figure 2
shows a very simplistic view of the
chromium atom Cr0, consisting of a nu-
cleus with 24 protons (p1) and 28 neu-
trons (n), surrounded by 24 orbiting elec-
trons (e2) (It must be emphasized that
this is not meant to be an accurate repre-
sentation of the electrons’ shells and sub-
shells, but simply a conceptual explana-
tion for the purpose of clarity). From this
we can say that the atomic number of
chromium is 24 (number of protons), and
its atomic mass is 52 (number of protons
1 neutrons).

If energy is then applied to the

chromium ground-state atom in the form
of heat from a plasma discharge, one of
the orbiting electrons will be stripped off
the outer shell. This will result in only 23
electrons left orbiting the nucleus. Be-
cause the atom has lost a negative charge
(e2) but still has 24 protons (p1) in the
nucleus, it is converted into an ion with a
net positive charge. It still has an atomic
mass of 52 and an atomic number of 24,
but is now a positively charged ion and
not a neutral ground-state atom. This
process is shown in Figure 3.

NATURAL ISOTOPES
This is a very basic look at the process,
because most elements occur in more
than one form (isotope). In fact,
chromium has four naturally occurring
isotopes, which means that the
chromium atom exists in four different
forms, all with the same atomic number
of 24 (number of protons), but with differ-
ent atomic masses (numbers of neu-
trons).

To make this a little easier to under-
stand, let’s take a closer look at an ele-
ment like copper, which has only two dif-
ferent isotopes — one with an atomic
mass of 63 (63Cu) and the other with an
atomic mass of 65 (65Cu). They both have
the same number of protons and elec-
trons, but differ in the number of neu-
trons in the nucleus. The natural abun-
dances of 63Cu and 65Cu are 69.1% and
30.9%, respectively, which gives copper a
nominal atomic mass of 63.55 — the
value you see for copper in atomic weight
reference tables. Details of the atomic
structure of the two copper isotopes are
shown in Table I.

When a sample containing naturally oc-

curring copper is introduced into the

plasma, two different ions of copper,
63Cu1 and 65Cu1, are produced, which
generate different mass spectra — one at
mass 63 and another at mass 65. This can
be seen in Figure 4, which is an actual
ICP-MS spectral scan of a sample contain-
ing copper. It shows a peak for the 63Cu1
ion on the left, which is 69.17% abundant,
and a peak for 65Cu1 at 30.83% abun-
dance, on the right. You can also see
small peaks for two Zn isotopes at mass
64 (64Zn) and mass 66 (66Zn) (Zn has a to-
tal of five isotopes at masses 64, 66, 67,
68, and 70). In fact, most elements have
at least two or three isotopes and many
elements, including zinc and lead, have
four or more isotopes. Figure 5 is a chart
that shows the relative abundance of the
naturally occurring isotopes of all the
elements.

During the next few months, we will

systematically take you on a journey
through the hardware of an ICP mass
spectrometer, explaining how each major
component works, and finishing the se-
ries with an overview of how the tech-
nique is being used to solve real-world ap-
plication problems. Our goal is to present
both the basic principles and benefits of
the technique in a way that is clear, con-
cise, and very easy to understand. We
hope that by the end of the series, you
and your managers will be in a better po-
sition to realize the enormous benefits
that ICP-MS can bring to your laboratory.

REFERENCES
(1) A. Montasser, Inductively Coupled Plasma
Mass Spectrometry (Wiley-VCH, Berlin,
1998).

(2) F. Adams, R. Gijbels, and R. Van Grieken,
Inorganic Mass Spectrometry (John Wiley
and Sons, New York, 1988.).

(3) R.S. Houk, V. A. Fassel, and H.J. Svec, Dy-
namic Mass Spectrom. 6, 234 (1981).
(4) A.R. Date and A.L. Gray, Analyst 106,

1255 (1981).

(5) D.J. Douglas and J.B. French, Anal.

Chem. 53, 37 (1982).

(6) Isotopic Composition of the Elements: Pure
Applied Chemistry 63(7), 991–1002
(1991).

Robert Thomas is the principal of his own
freelance writing and scientific consulting
company, Scientific Solutions, based in
Gaithersburg, MD. He can be contacted by
email at thomasrj@bellatlantic.net or via
his web site at www.scientificsolutions1.
com.u

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