1
An Overview

ICP emission spectrometry (ICP-OES) is one of the most important techniques of instrumental elemental analysis. It can be used for the determination of approximately 70 elements in a variety of matrices. Thanks to its versatility and productivity it is used in many different applications, and nowadays it carries the basic workload in many routine laboratories.

This book gives an introduction to the basic principles of ICP emission spectrometry and provides some background information as well as practical hints to the user. This knowledge should enable the reader to appreciate the possibilities and limitations of this analytical technique in order to use it in an optimal way.

Throughout the text, you will find complementary information, which is indicated by a frame around the text. Symbols indicate the type of information given:

• Practical tips
• Additional information
• Complementary theory

1.1 Features of ICP-OES

The heart of an ICP emission spectrometer is the plasma, an extremely hot "gas" with a temperature of several thousand Kelvin. It is so hot that atoms and mainly ions are formed from the sample to be analyzed. The very high temperature in the plasma destroys the sample completely, so the analytical result is usually not influenced by the nature of the chemical bond of the element to be determined (absence of chemical interference). In the plasma, atoms and ions are excited to emit electromagnetic radiation (light). The emitted light is spectrally resolved with the aid of diffractive optics, and the emitted quantity of light (its intensity) is measured with a detector. In ICP-OES, the wavelengths are used for the identification of the elements while the intensities serve for the determination of their concentrations.

Since all elements are excited to emit light in the plasma simultaneously, they can be determined simultaneously or very rapidly one after another. Consequently, the analytical results for a sample can be obtained after a short analysis time. The time needed for the determination depends on the instrument used and is of the order of a few minutes. The fact that all the elemental concentrations are determined in one analytical sequence and not by measuring one series of samples for one element, another series for another element, and so on usually makes the technique attractive with respect to speed.

Samples analyzed are normally liquids, occasionally solids, and (quite rarely) gases. For the determination of an element, no specific equipment (such as the lamp used in atomic absorption spectrometry) is needed. As a rule, one only needs a calibration solution of the element to be analyzed and a little time for method development. Hence, an existing analytical method can easily be extended to include another element. This makes ICP emission spectrometry very flexible.

ICP-OES has a very large working range, typically up to six orders of magnitude. Depending on the element and the analytical line, concentrations in the range from less than µg/L up to g/L can be determined. Time-consuming dilution steps are therefore rarely needed, which considerably increases the analysis throughput.

Particularly in environmental analysis, the working ranges for many elements correspond to the concentrations normally found in the samples, and this is one of the reasons why this technique is widely used in environmental applications; about half of all users of ICP-OES use it in these or related areas.

Because of the widespread use of this technique in environmental applications, there are a number of standards and regulations that apply. The most important of these are ISO 11885 [1] and EPA Method 200.7 [2]. Moreover, ICP-OES is used in a variety of other applications, such as metallurgy and the elemental analysis of organic substances.

Plasma was first described as an excitation source for atomic spectroscopy in the mid-1960s [3-6], and the first instrument appeared in research laboratories a decade later. After a further 10 years the technique was commercialized [7-9]. At first slowly, but then at an increasing rate, ICP emission spectrometers were introduced into routine laboratories. During the same period, the instruments were refined to make them more user friendly [10]. Since the early 1990s, ICP-OES has become the "workhorse" in the modern analytical laboratory [11, 12]. These years also brought a number of significant improvements, most importantly the use of solid-state detectors [13].

1.2 Inductively Coupled Plasma Optical Emission Spectrometry - the Name Describes the Technique

As a rule, the technique is referred to as ICP or ICP-OES. The latter is the abbreviation for inductively coupled plasma optical emission spectrometry. The complete name describes or implies the analytic features of this technique: "Plasma" describes an ionized gas at very high temperatures. The energy necessary to sustain the plasma is transferred electromagnetically via an induction coil. This method of energy transfer is found in the first part of the name of the technique: "Inductively coupled plasma."

The sample to be analyzed is introduced into this hot gas. As a rule, all chemical bonds are dissociated at the temperature of the plasma, so that the analysis is independent of the chemical composition of the sample. The atoms and ions are excited in the plasma to emit electromagnetic radiation ("light"), which mainly appears in the ultraviolet and visible spectral range. The emission of light occurs as discrete lines, which are separated according to their wavelength by diffractive optics and utilized for identification and quantification.

Spectrometry is a technique for quantification that uses the emission or absorption of light from a sample. Its goal is the determination of concentrations and differs from qualitative analysis by spectra, which is commonly referred to as spectroscopy [14].

As a rule, in ICP emission spectrometry there is a linear relationship between intensity and concentration over more than 4-6 decades. This intensity concentration function depends on a number of parameters, some of which are unknown. Hence, there is a need for empirical proportionality factors. Consequently, in ICP emission spectrometry, these factors have to be determined before the analysis (calibration). One assumes that the slope of the calibration functions does not change between standards and the samples. It is an important prerequisite to ensure good accuracy of the analytical results to prove that this is actually the case. Instrument performances as well as method development have a large influence on this, which can be challenging at times.

Since all atoms and ions emit light simultaneously, ICP-OES is a typical representative of a sample-orientated multielement technique. This means that the results for the elements in one sample are measured in one step, unlike the element-orientated mode of operation where all samples are examined for one element. After all the samples have been analyzed for the first element, they are then measured in a new series for the next element. A typical representative of the element-orientated mode of operation is classical atomic absorption spectrometry. The advantages of the sample-orientated mode of operation for routine analysis are obvious, since the sample is characterized very quickly.

ICP, ICP-OES, ICP-AES, ICP/AES, ICP emission spectrometry, ICP ES: What is the correct name for this technique?

The variety of names for this technique reminds one of the tower of Babel. Which version should one follow? We will try to throw some light on this while attempting to trace the origin of the terms used:

Let us start with "ICP," the abbreviation for "inductively coupled plasma." This is widely accepted. Everyone agrees to use this abbreviation, at least in written communications.

However, the abbreviation "ICP" alone is no longer sufficient as a clear identification of the technique since a similar technique, ICP-MS (ICP mass spectrometry), exists. To distinguish these from each other, it is recommended to add "OES" or "MS" to the abbreviation "ICP" in order to clearly specify which technique is meant. "ICP" should rather be understood as a generic term for both techniques.

The abbreviation "OES" is the short form for "optical emission spectrometry" and has been around for many decades. Originally, it was used in connection with excitation by spark or glow discharge long before inductively coupled plasma was used analytically. Since plasma only represents another excitation source, it makes sense to stay with the abbreviation "OES."

Sometimes one finds the name "atomic emission spectrometry" or "AES." Typically, this version is used by users and manufacturers who in many cases have worked with "atomic absorption spectrometry" (AAS) before. The use of the term "atomic" is plausible to some extent since ICP-OES as well as AAS and ICP-MS are categorized under the group name "atomic spectrometry." However, the reference to "atoms" is misleading in a way since most particles in the plasma are ions (Table...