CE Update
Received 08.06.04 | Revisions Received 10.15.04 | Accepted 10.18.04
A Primer for ICP-Mass Spectrometry
DOI: 10.1309/Q6NBT2B15MJVGN2N
ICP-MS is an inorganic analytical
technique used primarily for the
quantitation of trace elements (metals) in
a variety of sample media.
It employs many of the same principles and
processes as atomic emission spectrometry
but has several improvements that give it
greater analytical capabilities.
After reading this article, the reader should be able to define ICP-MS
(Inductively coupled plasma mass spectrometry), briefly explain the 4
processes of the ICP-MS analytical sequence, and understand the
background behind its development and popularity.
The need for multi-element trace analysis has grown rapidly
since the 1980s as federal and state agencies such as OSHA demand more and better analytical services to support workplace
monitoring in their quest to protect employee health and defend
against possible litigation.
To help meet this need, inductively coupled plasma combined with mass spectrometry (ICP-MS) was introduced in the
early 1980s as an improvement over the earlier atomic emission
spectrometers. By using a high-temperature argon plasma,
greater sample decomposition and greater ionization of
elements occurs resulting in greater sensitivity. The ICP-MS
technology is a very powerful tool for trace and ultra-trace
analysis. It is also unique in that both aqueous and solid samples can be analyzed.1 For the scope of this article, the author
will consider the trace region to be ppm (parts per million) and
ppb (parts per billion), and the ultra-trace region to be ppt
(parts per trillion) and lower.
NOTE: Concentration abbreviations are defined as follows:
parts per million (ppm), 1 microgram analyte per milliliter; parts
per billion (ppb), 1 nanogram analyte per milliliter; parts per
trillion (ppt), 1 picogram analyte per milliliter.
Basic Theory Of ICP Technology
Inductively coupled plasma technology employs the
same principles as atomic emission spectrometry. Samples are
decomposed to neutral elements in a high-temperature argon
plasma and analyzed based on their mass to charge ratios.
The ICP analytical sequence can be considered as 4 main
processes as shown in Figure 1: sample introduction and
aerosol generation, ionization by an argon plasma source,
mass discrimination and detection of ions by the detector,
and finally, data analysis.
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ICP-MS is used widely in the fields of
toxicology, medicine, and industrial hygiene.
Chemistry exam 20405 questions and corresponding answer form are
located after the “CE Update” secton on p. 749.
Mass Discriminator
and Detector
Sample Introduction
and Aerosol
Generation
Ionization by
Argon Plasma
Data Analysis
Figure 1_Schematic of ICP-MS main processes. Figure by Steve
Kvech, used with permission.
Sample Introduction And Aerosol Generation
Samples which are aqueous after decomposition and diluted
to a suitable volume are introduced into the ICP through a nebulizer which aspirates the sample using high-velocity argon and
creates an aerosol. This sample mist then enters a spray chamber
where the larger droplets are removed via a drain. This process is
necessary to produce sample droplets small enough to be vaporized in the plasma torch. Solid samples can be introduced by
purchasing a laser ablation system as an accessory.1 In laser ablation, a soil or other finely-pulverized solid sample is subjected to
short-UV laser irradiation (ablation) and then introduced into
the nebulizer (Figure 2) as a solid mist.
Plasma Generation and Sample Ionization
At this point, an explanation of the plasma and its generation is appropriate. A plasma is defined as a gas consisting of
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Ben M. West, MT(ASCP)SC
(Navy Environmental Preventive Medicine Unit 5, San Diego, CA)
CE Update
Detector
Quadrupole Rods
Sample
Vacuum
Chamber
Argon gas
Figure 3_Quadrupole mass filter. Figure by Steve Kvech, used with
permission.
ions, electrons, and neutral particles. In ICP-MS, the plasma is
formed when argon is subjected to an intense magnetic field
provided by a radio frequency (RF) signal fed into a tightly
wound, water-cooled coil. Argon passes into a glass or quartz
torch, which is surrounded by the coil to which the RF signal is
applied. With the argon flowing, the RF signal is applied creating a magnetic field which ionizes the argon. A spark from a
Tesla unit (much like a car spark plug) causes this highly ionized
and now energized mixture to “light” causing the bright white
plasma flame. The plasma is maintained as long as the magnetic
field and argon flow continue uninterrupted. The temperature
of the plasma is quite high, reaching about 7,000 degrees
Kelvin.2
Now, let us look at what happens to a sample when it is
subjected to the plasma. Once the sample passes through the
nebulizer and partial solvent removal (desolvation) has occurred,
the sample moves into the plasma torch where it is mixed with
the constantly flowing argon. The plasma removes any remaining solvent and causes atomization of the sample followed by
ionization. In addition to being ionized, these sample ions are
excited in the hot plasma, the same phenomenon used in ICP
atomic emission spectrometry.1
for) are allowed to reach the detector. As shown in Figure 3, a
quadrupole mass filter is made of 4 metal rods aligned in a parallel diamond pattern. A combined DC and AC electrical potential is applied to the rods with opposite rods having a net
negative or positive potential. Ions enter into the path between
all of the rods. When the DC and AC voltages are set to certain
values, only 1 particular ion is able to continue on the path between the rods and others are forced out of the path. This 1 ion
will have a specific m/z ratio. Various combinations of voltages
are chosen which allows an array of different m/z ratio ions to be
detected. While the quadrupole arrangement is most commonly
used, other types of mass filters are available such as double-focusing magnetic-sector, time of flight, and collision/reactor cell.
After ion separation occurs, the chosen ions pass through a small
opening to the detector.1
The ICP-MS Interface
Once the ions are produced in the plasma, they are directed
into the mass spectrometer via the interface region which is
maintained at a vacuum of about 1 to 2 torr by a mechanical
pump. The interface region consists of 2 cones (usually nickel)
called the sample and the skimmer. Each cone has a small (0.6
to 1.2 mm) orifice to allow the ions to pass through to the ion
optics (electrostatic lenses) where they are guided into the mass
separation device—the heart of the mass spectrometer. It is the
job of the ion optics to focus the ion beam formed in the cones
and containing both analyte and matrix ions into the mass separation device. The ion optics also serve the purpose of preventing stray photons, particulates, and neutral species from
eventually reaching the detector.3
The Mass Spectrometer (MS)
Once the ions have entered the mass spectrometer, they are
separated according to their mass-to-charge (m/z) ratio in the
quadrupole mass filter and only those ions selected (analyzed
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The Detector
In the detector, the ions are converted into electrons
thereby creating an electrical signal, which can be recorded and
converted into a concentration reading using a calibration graph.
The most common type of detector used in an ICP-MS system
is the channeltron electron multiplier. This is a cone or hornshaped tube which has a high voltage applied to it and is opposite to the charge of the ions being detected. As the ions leave
the quadrupole mass filter, they are attracted to the interior cone
surface. As the ions strike the surface, additional secondary electrons are emitted which move farther into the tube emitting additional secondary electrons. As this process continues, even
more electrons are formed, resulting in as many as 108 electrons
at the other end of the cone.1
Data Analysis
Data generated by the ICP-MS can be either quantitative
or semi-quantitative. In most federal and state government laboratories, methodologies for industrial hygiene and environmental
analysis such as NIOSH and OSHA require quantitative data
based on at least 3 calibration points plus a verification of the
resulting calibration curve by an outside standard purchased
from a commercial source. The final reported results are calculated in parts per million (ppm) or parts per billion (ppb). Semiquantitative data, which is often based on a 1-point calibration
curve, is very useful in screening unknown samples suspected of
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Figure 2_Generation of aerosol by nebulizer. Figure by Jenna Worley,
used with permission.
CE Update
having high concentrations of the analyte of interest prior to
dilution for quantitative analysis.
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1. Worley J, Kvech S. ICP-MS Web page, graduate project, Department of Civil
Engineering, Virginia Polytechnic Institute. Blacksburg, Virginia.
2. Glascock MD, Speakman J. ICP-MS at MURR (University of MissouriColumbia) Archaeometry Laboratory. Available at: www.missouri.edu.
Accessed October 18, 2004.
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Summary
ICP-Mass Spectrometry is a highly popular choice for
multi-element trace analysis in fields such as toxicology, environmental chemistry, and industrial hygiene chemistry. Additionally,
laboratories that offer environmental and industrial hygiene testing, which use federally-mandated methods such as OSHA and
NIOSH, are required to provide ever-lower limits of quantitation for environmental contaminants such as cadmium and lead
and to operate in an increasingly litigious environment.
As shown in the article, an ICP-MS instrument has many
of the same components and uses the same physical processes
as the earlier mass spectrometers. However, by adding a hightemperature plasma and adapting the sample intake system to
accommodate both liquid and solid samples give the ICP-MS
greater sensitivity and versatility.
Finally, while ICP technology may seem complicated and
intimidating to a new user, its theory and operation are well
within the technical training and education of most generalist
medical technologists and certainly those with an interest in
chemistry and toxicology. LM
3. A guide to inductively coupled plasma mass spectrometry. Spectroscopy
Magazine, Eugene, OR. Special Project Supplement. Poster by Scientific
Solutions.
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