Inductively Coupled Plasma Atomic Emission Spectroscopy

Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is an analytical technique for elemental determinations in the concentration range of major to trace, based on the principles of atomic spectroscopy. In theory, the technique applies to all elements except argon, and samples can be introduced as liquids, gases, or solids. In practice, favorable analytical results are obtained for approximately 70 elements, with detection limits usually attainable at the parts per billion level, and most samples are introduced in liquid form as aqueous solutions. The technique has found widespread application in the metallurgical, geological, environmental, agricultural, industrial, and biological fields and is an important technique in the modern analytical laboratory.

The ICP is an excitation source for atomic emission spectroscopy. It is an argon plasma operated at atmospheric pressure and sustained by inductive coupling to a radio frequency (RF) electromagnetic field. Argon gas flows axially through a quartz tube surrounded by three or four turns of an induction or work coil connected to an RF generator. The standard frequencies of operation are 27.12 MHz or, less commonly, 40.68 MHz, the frequencies allowed by the Federal Communications Commission for scientific and medical instrumentation. Power output is generally 1 to 5 kW.

The high-frequency current of up to 100 A flows in the water-cooled copper induction coils, generating oscillating magnetic fields whose lines of force are oriented axially inside the quartz tube and follow closed elliptical paths outside the tubes. If free electrons are present inside the tube, the induced magnetic fields cause the electrons in the gas to flow in oscillating closed annular paths inside the quartz tube space. This electron flow is termed the eddy current, and the electrons are accelerated by the time-varying magnetic field, causing collisions that result in further ionization of the argon gas and resistive heating. These electrical and magnetic fields responsible for the plasma are represented in Fig. 6.

Fig. 6 Electric and magnetic fields of the inductively coupled plasma

The energy transmission in the plasma is similar to an electrical transformer in which the induction coils are the primary winding and the ionized gas is the secondary. Because the argon gas is initially neutral and nonconducting, the plasma must be initiated by seed electrons. usually generated by a brief tesla discharge. With RF power applied, the plasma ignites instantaneously, then is self-sustaining. The resulting plasma is a highly ionized gas with temperatures in the proximity of 10,000 K.

Analytical Characteristics. The ICP as an analytical technique provides the capability of performing simultaneous multielement analysis for as many as 60 elements within 1 to 2 min. It is applicable to most elements in the periodic table and has a large linear dynamic range (calibration curves that are linear over three to six orders of concentration magnitude). This enables determination of trace, minor, and major components in a single analysis.

Detection limits are in the parts per billion (ng/mL) range for most elements; precision and accuracy on the order of 1%; and relative freedom from chemical interferences. Detection limits for the ICP are determined by first establishing a calibration curve (plot of signal intensity at a given wavelength versus concentration for a series of standard solutions). The detection limit is calculated as the concentration that would correspond to an analytical signal equal to two (or three, according to choice of definition) times the standard deviation (noise) of repeated measurements of a blank at that wavelength. This concentration is the lowest value measurable with any certainty. These detection limit values should be considered extreme limits, because they are determined under ideal situations. Practical detection limits will be somewhat higher. Some sample preparation procedures, such as dissolution and dilution, necessarily degrade the achievable detection limits of the elements in the original sample material.

Precision and Accuracy. The precision of the ICP technique is usually determined by making several consecutive measurements, then calculating the standard deviation of the replicates as a percentage of the mean value. Major causes of signal fluctuations in the ICP are small variations in the RF power applied to the plasma and changes in the nebulization process. Precision can be increased to less than 1% by close regulation of the RF power (most new instrumentation accomplishes this to ±0.1%), by improved nebulization techniques, or by use of an internal standard. Nebulization is stabilized by using a mass flow controller to regulate the nebulizer gas flow and by use of improved nebulizer designs, such as the high-pressure crossflow nebulizer. The accuracy of the ICP technique is essentially limited by the precision and by systematic errors, such as interference effects, but is usually shown to be comparable to the precision.

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