Inductively Coupled Plasma
by Traci Bradford and M. Nicole Cook
Table of Contents
Inductively Coupled Plasma (ICP) is an analytical
technique used for the detection of trace metals in environmental samples.
The primary goal of ICP is to get elements to emit characteristic wavelength
specific light which can then be meausured. The technology for the ICP
method was first employed in the early 1960's with the intention of improving upon crystal growing techniques.
Photo of an ICP
Since then, ICP has been refined and used in
conjunction with other procedures for quantitative analysis. Following
is a cursory look at the driving forces behind this analytical tool, its
use in series with other analytical tools, and environmental applications
Workings of an ICP
ICP hardware is designed to generate plasma, which is a gas in which
atoms are present in an ionized state. The basic set up of an ICP consists
of three concentric tubes, most often made of silica. These tubes, termed
outer loop, intermediate loop, and inner loop, collectively make up the
torch of the ICP. The torch is situated within a water-cooled coil of a
radio frequency (r.f.) generator. As flowing gases are introduced into
the torch, the r.f field is activated and the gas in the coil region is
made electrically conductive. This sequence of events forms the plasma.
Schematic of ICP flame.
The formation of the plasma is dependent upon an adequate magnetic
field strength and the pattern of the gas streams follows a particular
rotationally symmetrically pattern. The plasma is maintained by inductive
heating of the flowing gases. The induction of a magnetic field generates
a high frequency annular electric current within the conductor. The conductor, in turn, is heated as the result of its ohmic resistance.
A Typical plasma torch.
In order to prevent possible short-circuiting as well as meltdown,
the plasma must be insulated from the rest of the instrument. Insulation
is achieved by the concurrent flow of gasses through the system. Three
gases flow through the system--the outer gas, intermediate gas, and inner
or carrier gas. The outer gas is typically Argon or Nitrogen. The outer
gas has been demonstrated to serve several purposes including maintaining
the plasma, stabilizing the position of the plasma, and thermally isolating
the plasma from the outer tube. Argon is commonly used for both the intermediate gas and inner or carrier gas. The purpose of the carrier gas is to convey the sample to the plasma.
An ICP typically includes the following components:
- sample introduction system (nebulizer)
- ICP torch
- High frequency generator
- Transfer optics and spectrometer
- Computer interface
Schematic of an ICP system
An ICP requires that the elements which are to be analyzed be in solution.
An aqueous solution is preferred over an organic solution, as organic solutions require special manipulation prior to injection into the ICP. Solid samples are also discouraged, as clogging of the instrumentation can occur.
The nebulizer transforms the aqueous solution into an aerosol. The
light emitted by the atoms of an element in the ICP must be converted to
an electrical signal that can be measured quantitatively. This is accomplished by resolving the light into its component radiation (nearly always by means of a diffraction grating) and then measuring the light intensity with a photomultiplier tube at the specific wavelength for each element line. The light emitted by the atoms or ions in the ICP is converted to electrical signals by the photomultiplier in the spectrometer. The intensity of the electron signal is compared to previous measured intensities of known concentration of the element and a concentration is computed.
Each element will have many specific wavelengths in the spectrum which
could be used for analysis. Thus, the selection of the best line the analytical application in hand requires considerable experience of ICP wavelengths.
Advantages and Disadvantages
Advantages of using an ICP include its ability to identify
and quantify all elements with the exception of Argon; since many wavelengths of varied sensitivity are available for determination of any one element, the ICP is suitable for all concentrations from ultratrace levels to major components; detection limits are generally low for most elements with a typical range of 1 - 100 g / L. Probably the largest advantage of employing an ICP when performing quantitative analysis is the fact that multielemental analysis can be accomplished, and quite rapidly. A complete multielement analysis can be undertaken in a period as short as 30 seconds, consuming only 0.5 ml of sample solution. Although in theory, all elements except Argon can be determined using and ICP, certain unstable elements require special facilities for handling the radioactive fume of the plasma. Also, an ICP has difficulty handling halogens--special optics for the transmission of the very short wavelengths become necessary.
An ICP can be used in the quantitative analysis in the
natural materials such as rocks, minerals, soil, sediment
air, water, and plant and animal tissue;pure and applied geochemistry,mineralogy,agriculture, forestry, animal husbandry,chemical ecology, and environmental sciences food industry, including purificationand distribution of water the analysis of elements not readily identified by AAS such as Sulfur, Boron, Phosphorus, Titanium, and Zirconium
Combining ICP with Atomic Emission Spectroscopy
Often, ICP is used in conjunction with other analytical
instruments, such as the Atomic Emission Spectroscopy (AES) and the Mass
Spectroscopy (MS). This is an advantageous practice, as both the AES and
MS require that sample to be in an aerosol or gaseous form prior to injection into the instrument. Thus, using an ICP in conjunction with either of these instruments eliminates any sample preparation time which would be required in the absence of an ICP.
Combining ICP with Mass Spectrometry
The efficiency of the Inductively Coupled Plasma in producing
singly-charged positive ions for most elements makes it an effective ionization source for mass spectrometry. Inductively coupled plasma-mass spectrometry is unique among the flame and plasma spectroscopy techniques in the ability to discriminate between the mass of the various isotopes of an element where more than one stable isotope occurs. Isotope dilution, in which the change in isotope ratio for two selected isotopes of an element of interest is measured in a solution after the addition of a known quantity of a spike that contains enrichment of one of the isotopes, permits calculation of the concentration of the element. Isotope dilution is the most reliable
method of accurate determination of elemental concentration. The conventional method of sample introduction for inductively coupled plasma-mass spectroscopy is by aspiration, via a nebulizer, into a spray chamber. A small fraction of the resulting aerosol is swept by argon into the torch. Approximately 1 mL of sample is required per analytical run, about 99% of which is wasted.
Recently, low cost, low uptake rate, high efficiency nebulizers have been
employed to combat this problem. The high efficiency nebulizer operates
more efficiently at 10-200 L/min. The detection limits and precision obtained with the high efficiency nebulizer are superior to conventional nebulizers.
Schematic of an ICP-MS system
The schematic diagram depicted above detail a two- or
three- stage differentially pumped interface used to extract ions from
the atmospheric pressure plasma into the low pressure mass spectrometer.
Ions pass through a cold sampling cone (typically Ni) with an orifice approximately 1mm in diameter. The gas expands behind the first orifice, and a portion passes through a second orifice in the skimmer cone. A series of ion lenses, maintained at appropriate voltages, are used to direct the ions into the quadrupole mass analyzer. The ions are transmitted through the quadrupole on the basis of their mass to charge ratios and then detected by an electron multiplier.
The use of a quadrupole mass analyzer gives better than
unit mass resolution over a mass range up to m/z=300. The inductively coupled plasma-mass spectrometry system is considered a sequential multielement analyzer that has scan times less than 20 ms for one sweep. The signal intensity is a function of the number of analyte ions in the plasma and the mass-dependent transport through the mass spectrometer.
The most important advantages of ICP-MS include multi-element
capability, high sensitivity, and the possibility to obtain isotopic information on the elements determined. Disadvantages inherent to the ICP-MS system include the isobaric interferences produced by polyatomic species arising from the plasma gas and the atmosphere. The isotopes of argon, oxygen, nitrogen, and hydrogen can combine with themselves or with other elements to produce isobaric interferences. ICP-MS is not useful in the detection of nonmetals.
Applications in Environmental Analysis
Environmental matrices, which may contain low concentrations
and contain interfering elements, have historically presented difficulties
in determining sample analysis. ICP-MS was developed in the 1980's and
has been used increasingly in the environmental field due to its high sensitivity and multi-element capabilities. ICP-MS offers the possibility of simple and direct determination of some of the elements in soils, such as boron, phosphorus, and molybdenum, at levels not accessible by other methods.
ICP-AES has been widely used since the 1970's for the
simultaneous multi-element analysis of environmental and biological samples
after dissolution. The excellent sensitivity and wide working range for
many elements- together with the low level of interferences, make ICP-AES
a nearly ideal method so long as sample throughput is high enough to justify
the initial capital outlay. Laser sampling, in conjunction with ICP is
a way to avoid dissolution procedures of solid samples prior to the determination of the elements.
ICP-AES has been approved for the determination of metals
by the EPA under Method 6010. Method 6010 describes the simultaneous, or
sequential, multielemental determination of elements by ICP-AES. This method
is approved for a large number of metals and wastes. All matrices, including
ground water, aqueous samples, EP extracts, industrial wastes, soils, sludges, sediments, and other solid wastes, require digestion prior to analysis. The following table lists the elements for which Method 6010 is applicable. Detection limits, sensitivity, and optimum ranges of the metals will vary with the matrices and model of spectrometer. The data shown in the following table provide concentration ranges for clean aqueous samples. Use of this method is restricted to spectroscopists who are knowledgeable in the correction of spectral, chemical, and physical interferences.
Table 1. Recommended wavelengths and estimated instrumental detection limits
||Wavelength (nm) ||Estimated |
||See note c |
The wavelengths listed are recommended because of their
sensitivity and overall acceptance. Other wavelengths may be substituted
if they can provide the needed sensitivity and are treated with the same
corrective techniques for spectral interference. In time, other elements
may be added as more information becomes available and as required. The
estimated instrumental detection limits shown are given as a guide for
an instrumental limit. The actual method detection limits are sample dependent and may vary as the sample matrix varies.
c Highly dependent on operating conditions
and plasma position.
Hear Steve Heckendorn, Manager, Virgina Tech Soils Testing Laboratory, speak on ICP use.
- Vela, N.P., Olson, L.K., and Caruso, J.A. Elemental speciation
with plasma mass spectrometry. Analytical Chemistry 65 (13)
- Alcock, N.W. Flame, flameless, and plasma spectroscopy.
Analytical Chemistry 67 (12) 503R-506R (1995).
- Liu, H. and Montaser, A. Evaluation of a low sample
consumption, high efficiency nebulizer for elemental analysis of biological
samples using ICP-MS. Journal of Analytical Spectrometry 11
(4) 307-311 (1996).
- Boonen, S., Vanhaecke, F., Moens, L., and Dams, R.
Direct determination of Se and As in solid certified reference materials
using electrothermal vaporization ICP-MS. Spectrochimica Acta 51(2) 271-278 (1996).
- Boumans, P.W.J.M. Inductively coupled plasma-emission
spectroscopy-Part 1. John Wiley & Sons. New York. 584 pp.
- Hoffman, E., Ludke, C., and Stephanowitz, H. Application
of laser ICP-MS in environmental analysis. Fresenius Journal of Analytical Chemistry355: 900-903 (1996).
- Inductively Coupled Plasma. ICP newsletter published since 1975.
- EPA Method 6010. Revision date: September 1986.
Send comments or suggestions to:
Student Authors: Traci Bradford and M. Nicole Cook
Faculty Advisor: Andrea Dietrich, email@example.com
Copyright © 1997 Daniel Gallagher
Last Modified: 12-22-1997