High-matrix samples mean more complexity in analytes and contaminants. They also mean less accurate analytical results if not corrected. High-matrix components produce physical, chemical, and spectral interferences (view webinar) when measuring elemental impurities using ICP-OES and ICP-MS. Unlike drinking water, environmental samples, such as wastewater and soil samples, often contain a large number of analytes and contaminants in a wide range of concentrations. In part I of the blog, I will summarize the 4 tips for ICP-OES analysis. The tips for ICP-MS will follow in part II of this blog.
Tip 1. Using Internal Standards
Internal standards are used for correcting physical and chemical interferences that affect the accuracy of your results. During analysis, ratios between the signals for the internal standard and the analyte of interest are used to correct for inaccuracies that might result from phenomena such as instrument drift and differences in aerosol generation and transport efficiency. To utilize internal standardization with minimal error, the internal standard must be present in every solution at the exact same concentration and should not be naturally occurring in any of your unknown samples. When measuring your internal standard, choose the emission wavelength to ensure that it is free from spectral interference and carefully select background correction points. If analytes are being measured in both axial and radial view modes, make sure an internal standard element is chosen for both plasma views. For constant and accurate internal standard addition, an on-line internal standard mixing kit should be used.
Tip 2. Calibrating with Matrix-Matched Standards
For ICP-OES, standards with different concentrations are used to generate a standard curve for calculating the concentrations of the analytes. Two standard calibration methods are used for different calibration purposes. The simple external standardization method is used for calibration when the standard and the sample have similar matrices. The method of standard additions is used when standards must be matrix-matched to the samples with very different matrices and the matrices are difficult to duplicate in the lab. The standard addition method minimizes the matrix effect from both physical and chemical interference and is a good choice if limited matrices of samples are tested. By spiking each analyte of known concentration to the samples and measuring them against the blank, a standard curve is established to calculate the unspiked sample.
Tip 3. Adding Ionization Buffers
The group I and II elements in the periodic table are easily ionized elements (EIE) and can cause chemical interference to other elements when the low concentrations of analytes are measured in axial view for higher sensitivity. Because they are easily ionized, they can easily produce electrons that shift the ionization equilibrium of other elements, including the analytes, more towards the atom form, away from ion form, leading to inaccurate results. By using ionization buffers, such as 500 to 1000 ppm lithium chloride, the production of electrons is kept constant, which minimizes EIE effects. The ionization buffer should be added on-line to ensure precision, as with adding internal standards. However, if the concentrations of the analytes are sufficiently high to be measured by radial mode instead, no ionization buffer should be used.
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Tip 4. Remove Spectral Interference
High-matrix samples may also generate inaccurate results due to spectral interference. The multiple spectrum overlap of the analytes with contaminants must be corrected to get the most accurate analysis. Three different ways to remove spectral interference include:
1) Choose a different spectral line that is free of interference
2) Move or turn off the background points to cancel out the interference
3) Use an interfering element correction (IEC) to correct the interference
Stay tuned for Part II of this blog for tips for ICP-MS when dealing with high-matrix samples.
Visit our metal analysis page to learn more, including other webinars for sample preparation, cost of ownership, precision improvement tips and tricks, and speciation.
If you work on environmental sample analysis using ICP-OES or ICP-MS, and would like to share your thoughts, I would love to hear from you.