The use of an Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) is a popular technique used in analytical laboratories.
This paper describes the theory behind ICP-OES instrumentation, followed by a specific discussion on ICP-OES methodology and its applications. The article will touch upon some of the most critical factors controlling sample preparation, instrumental considerations during testing, and typical data obtained from ICP-OES analysis. Some of the common interferences that may affect accurate results are also discussed.
If you are wondering what is ICP MS, is an inductively coupled plasma (ICP) has an ionization mechanism different from photometric techniques since it uses radiofrequency heating to generate electrons required to produce the plasma. This is done using a coil with an alternating current to cause an electromagnetic field around the plasma torch. Depending on the design, semi-concentric rings are used for gas distribution, or multiple dielectric spacers are used to distribute gases along flow lines. Various inductively coupled plasma instruments have different electrode geometry and power supply designs, but all operate under ordinary principles. A schematic representation of an ICP instrument is shown here.
How it works:
An ICP instrument uses three electrodes – cathode (which emits electrons), auxiliary (provides power for operation), and concentric skimmer(removes ions near the edge of the plasma). The high voltage generator provides high-frequency electric oscillations to heat the plasma column. The plasma column is maintained by a radio frequency generator that supplies power to the electrodes and receives feedback from an electromagnetic radiation detector. There are two sensors available on ICP instruments: optical emission spectrometer (OES) or wavelength dispersive X-ray fluorescence (WDXRF).
Instruments can be broadly classified as “open path” and “closed path.” In open-path instruments, the torch is exposed to air, while closed path torches use a quartz outer wall filled with Argon gas to maintain purity. One advantage of open-path instruments is that they do not need argon cylinders, while a closed system requires this expensive gas. On the other hand, open systems are more susceptible to contamination due to exposure to atmospheric air.
Schematic of an ICP-OES instrument is shown here:
The ICP torch consists of a vessel where the plasma is created and maintained, an auxiliary electrode, and a skimmer. There are two types of torches used in ICP instruments – side-on view (as described above) or axial flow configuration. The former gives less stable plasma compared to them later. Auxiliary power for ICP instruments can be provided either by a transformer or rectifier power supply system. The output signal from the plasma is passed through various components before detecting it with an OES detector. PFA tubing lines filled with argon gas form a path for sample vapors that enter into the plasma.
Why do we use ICP? The main reason for using ICP is its ability to determine the concentrations of multiple elements in a sample (10-200) with high accuracy and precision simultaneously, at low detection limits. Inductively coupled plasma provides various types of interferences that are well documented. However, following an approved method by an experienced analyst can help overcome many problems that may arise during analysis. To determine significant constituents (elements) in samples, quantitative spectroscopy is used along with internal standards-certified reference materials (CRMs). CRMs are commercially available standard reference materials containing known amounts of certain elements; these are added to unknown samples before analysis to assess the reliability and accuracy of results obtained.