In inductively coupled plasma (ICP) spectroscopy, the concentration of an element within a sample can be measured by either ICP mass spectrometry (ICP-MS) or ICP optical emission spectrometry (ICP-OES). In ICP-MS, the measurement of concentration is correlated to the mass of the element, and in ICP-OES, the measurement of concentration is correlated to the emission spectra of the element.
The accurate measurement of the concentration of an element is dependent on the choice of ICP method and on the calibration technique used.
ICP Systems
A typical ICP-MS system consists of a spray chamber, a nebulizer, an ICP torch, an interface region, a vacuum region, a lens system, a mass spectrometer, a detector, and a data handling system. A spray chamber, nebulizer, ICP torch, interface region, optical chamber, emission spectrometer, detector, and data handling system comprise a typical ICP-OES system.
To analyze a liquid sample or a dissolved solid sample, the sample is pumped into the nebulizer to produce an aerosol. The aerosol is dispersed in the spray chamber so that only the smallest aerosol droplets are introduced into the ICP torch. The plasma is generated in the ICP torch by applying energy to argon passing through the torch using a radio frequency coil. In the high-temperature plasma, the sample aerosol is dried, atomized, and ionized.
In the ICP-MS system, the ions generated in the plasma exit the sample introduction system into the interface region, the vacuum region, and the lens system. The lens system focuses the ions into the mass spectrometer, which separates the ions on the basis of mass. The specific elements corresponding to the ions are analyzed based on mass and measured by the detector and the data handling system.
In the ICP-OES system, atoms and ions are generated within the plasma. As electrons in the atoms and ions return to a lower energy state, photons are emitted into and are collected within the optical chamber. In the optical chamber the photons are separated into different wavelengths called emission lines, and are measured by the optical emission spectrometer. Elements are detected and measured based on the wavelength and intensity of specific emission lines.
ICP-OES
When developing an ICP-OES method, the following should be considered.
Sensitivity
The sensitivity of the procedure depends on the ability of the emission line to be accurately and quantitatively measured at the lower limits of detection without spectral correction.
Spectral interferences
These interferences include direct spectral overlap, wing overlap, and near wavelength neighbors that can cause background correction problems. In any measurement, these interferences must be accounted for by proper instrument calibration and the use of standards with well-defined trace metals impurity data.
Matrix effects
These effects, generated from the elemental matrix components, influence the plasma temperature and the nebulization efficiency, which leads to bias in the signal intensity and inaccurate measurement. The calibration techniques below, especially internal standardization, can be used to correct for matrix effects.
ICP-MS
When developing an ICP-MS procedure, the following should be considered.
Resolution
Tailing from a larger peak into a smaller peak can occur, giving false high results for the smaller peak.
Interferences
There are several types of major interferences.
- Isobaric interferences result from isotopes of different elements with indistinguishable masses in the sample solution. They can be resolved by using another isotope of the element to be measured, or by measuring the intensity of another isotope of the interfering element and applying the appropriate correction factor.
- Polyatomic interferences are due to the recombination of sample and matrix ions with argon or other matrix components. They can be avoided by eliminating specific matrix elements, using certain plasma techniques, and/or reducing the sample argon gas flow rate.
- Doubly charged ion interferences are due to doubly charged element isotopes with twice the mass of the analyte isotope. They frequently occur in matrices containing high levels of mid- to heavy mass element isotopes, and can be minimized by reducing the plasma temperature.
Matrix effects
These effects are generated from the sample matrix components. They suppress or quench the analyte signal, and influence the plasma temperature and nebulization efficiency. This leads to bias in the signal intensity and inaccurate measurement. The calibration techniques below can be used to correct for matrix effects.
Calibration techniques
The calibration techniques that can be used with ICP-OES and ICP-MS include the calibration curve technique and the standard additions techniques. With the calibration curve technique, matrix matching and internal standardization can be used, while with the standard additions technique a standard of the element being measured is added directly to the sample, thereby minimizing matrix effects.
Calibration curve
In this technique, calibration standards with known analyte concentrations are used to construct a curve relating analyte concentration to instrument response. The measurement of the sample analyte concentrations is accomplished by comparing the instrument response for the sample to the calibration curve.
Matrix matching
The measurement signal in both ICP procedures is affected by matrix effects. Therefore, it is preferable that the matrices of the calibration standards and samples be matched.
Matrix effects can be caused by the acid content of the matrix. Matching the type and concentration of acid between the calibration standards and samples is a typical approach to matrix matching.
Matrix effects can also be caused by matrix components, particularly those present in the samples. Therefore, matching the elemental matrix components of the calibration standards with the samples may also be necessary to obtain effective matrix matching.
Internal standardization
Internal standardization can be used with the calibration curve technique to correct for plasma-related effects. In this technique, an internal standard element is selected that behaves similarly to the analyte while providing a signal that can be distinguished from that of the analyte. The internal standard must be compatible with the matrix and does not introduce spectral interferences or trace impurities to the analyte signal, while giving a good signal-to-noise ratio. The internal standard element and the analyte must be affected to the same extent by their respective matrix effects.
Standard additions
The standard additions technique can be used when the matrix is variable, when dealing with unknown samples matrices, and to get around matrix interference through plasma-related effects. This technique can also be used to confirm the effectiveness of an internal standards calibration curve technique, or to correct for plasma-related effects.
In this technique, a standard is added directly to one aliquot of the analytical sample solution, which is compared to a separate aliquot of the same analytical sample solution that does not contain the standard. The measured difference between the two solutions allows the quantification of the sample analytes. Before an analysis is made, a semiquantitative analysis of the unknown solution must be performed to estimate the analyte levels in order to add the appropriate level of the analyte of interest to the unknown solution.
Isotope dilution
With ICP-MS, isotope dilution can be used with the standard additions technique to alleviate matrix effects. In this technique, a known amount of an enriched isotope of the element of interest is used as the standard and added to a sample with a known isotopic composition. The resulting isotopic ratio measured in the standard and sample mixture can then be used to calculate the element concentration in the sample.
Conclusion
For best performance of an ICP system, it is essential to select the right calibration technique and chemical standards. For more information, visit http://www.inorganicventures.com.
Lina Genovesi, Ph.D., JD, is a technical, regulatory, and business writer based in Princeton, NJ, U.S.A.; e-mail: [email protected]; www.linagenovesi.com