by Traci Bradford and M. Nicole Cook
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 of 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 following areas:
- 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
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
|Potassium||766.491||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.
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