Both ICP – OES and ICP – MS are considered the most advanced level and rapid analysis techniques available for estimation of elements at trace and ultra trace levels. Both the techniques are based on atomisation and excitation of the sample in the plasma source. An understanding of the plasma excitation source is essential for appreciating the two techniques.
Plasma Excitation Source
Plasma also termed as the fourth state of matter is the most abundant form of matter in the universe because majority of stars comprise of matter in the plasma state. It contains free ionised atoms and electrons. It is possible to confine it into a small volume in the laboratory and use it as a source for atomisation and excitation of samples introduced into it as a liquid aerosol.
The ICP source comprises of three concentric tubes made of quartz for argon and sample aerosol introduction. Argon gas spirals tangentially upwards and the gas from the nebuliser with sample moves inside the inner sample tube. Plasma is generated above the inlet jet by means of radio frequency energy applied to the coil. The plasma attains a temperature of 6000° – 8000° K which is sufficient to excite the sample elements. The detection of elements beyond the source is based on measurement of light emission by elements (ICP – OES) or by separation of ions on the basis of mass to charge (m/z) ratio (ICP – MS)
ICP – OES
ICP – OES also referred to as ICP – Atomic Emission Spectroscopy (ICP – AES) isolates the emitted light by the source into discrete component wavelengths using a diffraction grating.
The sensitivity of the technique is in direct proportion to the light intensity emitted at each wavelength and simultaneous analysis systems afford analysis of up to 60 elements in a single run. Large linear dynamic range permits detection levels between sub ppm to about 1000 ppm levels in less than a minute. It is a robust technique affording analysis in presence of up to 20% dissolved solids.
ICP – MS
ICP – MS takes advantage of excitation leading to ionization of elements as well as separation of ionised species by quadrupole mass separation and detection by electron multiplier system. In addition to estimation of the elements it is also capable of providing isotope ratio studies.
ICP – MS can achieve higher linear dynamic range up to which permits determination at concentration levels ranging from sub-ppt to ppm levels. ICP – MS cannot be used for samples having total dissolved solids as these effect nebuliser and efficiency. Capabilities of ICP- MS have been further exploited by coupling with LC to conduct speciation studies on metals bound to proteins and other organic molecules.
ICP – MS systems are more expensive than ICP – OES systems and operational cost is also higher due to higher consumption of argon gas but the cost factor is offset by the distinct advantages offered by the technique.
A subsequent article will provide guidance on selection of elemental analysis techniques as per your analysis requirements.