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Using internal standards is a common technique to correct for variations in sample matrices and the affect this has on analyte intensities. There are several basic criteria to be considered when using internal standards: selection of appropriate internal standards, the concentration added to the solutions analyzed, setting up in the correct view (axial vs. radial), how to introduce the internal standard to the solutions to be analyzed, and evaluating the resulting data. Each of these topics are considered and suggestions presented.
In the analysis by plasma techniques, the percentage of each solution analyzed that reaches the plasma can vary due to differences in sample matrices, particularly when compared to simple standards. When this occurs, sample results can be incorrect. Compensating for this problem can be done by matrix matching by preparing calibration solutions in the same type of matrix as the samples; by using the method of standard additions for calibration; or by using internal standards as part of the method. Note that matrix matching and method of additions may not always be practical.
Internal standards are elements that are added to all analytical solutions. Their intensities are monitored during analysis. As intensities fluctuate in each solution, most instrument software can correct the readings of the analytes to be determined and report percent recoveries of the internal standards. The use of internal standards has become an integral part of most methods for analysis by plasma techniques for obtaining accurate data. While this has become a common correction technique, the analyst must be aware of several key points to consider when using internal standards.
When selecting elements to be used as internal standards, there are a few basic rules. First, internal standards are elements that don’t need to be determined. Second, they have to be elements that won’t be found in any measurable concentration in any of the samples. Third, be sure the internal standards won’t spectrally interfere with the elements to be determined, or that the other constituents of the samples won’t spectrally interfere with the internal standards. Fourth, internal standards should never be elements that are common environmental contaminants, even if these elements are not in any of your samples. Both yttrium (Y) and scandium (Sc) are chosen for many applications following the above criteria, though they may not be the correct choices for all types of analyses. While choosing internal standards for most types of samples is generally not difficult, for the analysis of some types of samples, selection can be challenging, and may require more rigorous method development.
The actual concentration of the internal standard should be based on the intensity produced by the internal standard. There should be enough intensity from the internal standard that precision of the replicates is optimum (better than 2% relative standard deviation [RSD] in calibration solutions). Poor precision should not play a major role in the correction of the analyte readings. The correction itself is a linear function, so the maximum intensity of the internal standard must be in the linear range for the wavelength chosen.
Since many inductively coupled plasma–optical emission spectroscopy (ICP–OES) devices have dual viewing capabilities, it is important to note that the internal standards must be in the same “view” as the analytes to be determined. If the analyte is viewed axially (down the plasma), the internal standard must be in that view also. If the analytes are viewed radially (across the plasma), so must the internal standard be viewed. This might require the use of multiple internal standards if the method has some elements in the axial view and some in the radial view.
How the internal standard will be added to each solution is of key importance. Concentrations of the internal standards must be the same in all analytical solutions in order for the correction to be accurate. Addition can be made manually by the analyst or automated using either another channel on a peristaltic pump or a valve system to introduce the internal standard to the analytical solutions. Both addition techniques have advantages and disadvantages, as shown in Table I. Whichever way is chosen, evaluation of the resulting data is essential.
There are three areas to consider when evaluating data:
Using internal standards for analysis by ICP-OES is an essential part of good method development for most types of samples, but the analyst must be aware of several points in order to insure the reporting of accurate data.
(1) Mermet, J. M. Use of Magnesium as a Test Element for Inductively Coupled Plasma Atomic Emission Spectrometry Diagnostics. Anal. Chim. Acta 1991, 250, 85–94. DOI: 10.1016/0003-2670(91)85064-Y
(2) Bradshaw, D. K. Identification and Correction of Interferences in Practical ICP-OES. Short course, Winter Plasma Conference 2022.
Deborah Bradshaw is an analytical chemist who has been working the field of atomic spectroscopy for over 35 years. For the past 20 years, she has been working as a consultant in the field of atomic spectroscopy, conducting training classes and giving technical support for atomic absorption (AA) spectroscopy, inductively coupled plasma–optical emission spectroscopy (ICP–OES), and inductively coupled plasma–mass spectrometry (ICP–MS). Direct correspondence to: email@example.com ●