How to Analyze High-Matrix Samples with ICP-OES Techniques
Mon 7 Dec, 2020
What is a high-matrix sample?
A high-matrix sample usually has a high percentage of dissolved solids. These are referred to as "totally dissolved solids" (TDS) and are present in high-matrix samples in a concentration of 3 to 30 percent. Volatile organic compound (VOC) solutions, such as petrochemicals or fuels, also count as high matrix samples. These solids and organic solutions can cause considerable measurement interference and lead to a variety of other challenges in the analysis.
High-matrix samples are for example solutions containing highly concentrated salts, acids, bases or other chemicals. Solutions with a high minerals or metal content after sample preparation and contaminated wastewater are also included in this type of sample.
Challenges of analysis with ICP-OES techniques
High-matrix samples cause several challenges during the analysis process. Especially the effects on the quality of results and the high demands on the analytical instrument must be emphasized. For the user this means a lot of additional maintenance and cleaning.
The most common difficulties concern the long-term measurement stability and the amount of effort for the user:
- Signal fluctuation/drift due to dirty atomizers and injectors
- short service life of the glass components due to local matrix deposits
- high cleaning and maintenance effort and high costs for consumables
The efficient and reliable analysis of high-matrix samples requires the appropriate technology. However, many older ICP-OES on the market have their difficulties with these samples, as they are not primarily optimized for this type of sample. One example is the alignment of the plasma flare. This plays a key role in preventing matrix deposition. Many older ICP systems use a horizontal orientation of the plasma flare, which is very susceptible to deposits at the injector tip. These deposits cause increased background noise and falsify the results. Modern ICP-OES systems such as the PlasmaQuant 9100 therefore use a vertically oriented flare, which significantly minimizes the risk of deposits and ensures longer lifespan, less maintenance and more efficient operation of the spectrometer.
The analytical challenges of high matrix samples are
- loss of sensitivity due to plasma-based interference
- insufficient detection limits to detect trace elements
- drag effects and increased background noise
- poor signal to noise ratio
- increased risk of spectral interference
- plasma collapse
Diluting the samples solves only a few of these issues. Although matrix deposition can be minimized by sample dilution, the ability of the ICP-OES spectrometer to detect the analytes (low LODs) is reduced at high dilution ratios. For best analytical performance, as little dilution as possible is required.
The plasma performance of the ICP-OES is crucial for the detection of trace elements in the sample. High-matrix samples significantly influence the stability and performance characteristics of the plasma. This can have significant effects on the measurement results.
- change in electron density due to high levels of easily ionizable elements (alkaline, alkaline earth)
- intensity shift from ionic to atomic lines
- cooling of the plasma
- significant loss of sensitivity and thus loss of detection strength
- sudden change in matrix load can cause plasma to collapse
High-matrix samples have complex spectra that are the cause of a large number of measurement interferences of the spectrometer. In ores or metals, for example, rare earths, iron, and refractory metals can cause distortions. Often there is a direct overlap of two spectral lines. When analyzing chemical products such as urea, fertilizers or etching solutions, it is also possible for several lines to overlap simultaneously. Unspecific background lines are another phenomenon that can affect the measurement quality. For example, the composition of many petrochemical products, such as naphtha, causes strong carbon-based background lines. In addition, any unfavorable analyte combination leads to spectral distortions. One of the best-known examples is the frequently occurring cadmium-arsenic interference. The example of arsenic-cadmium is also illustrated in the graph below. Here it becomes clear that only a high resolution of the spectrometer can prevent possible interferences. Low-resolution spectrometers display the two elements as one line and make it impossible to quantify individual elements separately.
The solution for the analysis of high-matrix samples
With the PlasmaQuant 9100 ICP-OES, Analytik Jena is specifically addressing many of these analytical challenges. The plasma performance and the properties of the detection optics play a decisive role in the efficient detection of bulk and trace elements in high-matrix samples. The instruments of the PlasmaQuant 9100 series have been optimized accordingly.
An overview of the of the PlasmaQuant 9100 optimizations:
- Freely operating high-frequency generator for immediate power adjustment at high plasma loads
- High-load induction coil for higher power transmission and sensitivity
- wider plasma for improved signal quality (advantages for RSD and trace sensitivity)
- Adjustable output power from 700 to 1,700 W
- Minimal sample dilution for improved LODs
- Increased sensitivity due to long analysis zone
- Uniform excitation of different matrix types leads to lowest sensitivity losses in high-matrix samples
Spectral interference can be easily bypassed thanks to the Dual View Plus function. Thanks to this feature, it is possible to select different observation modes depending on the composition of the sample. The PlasmaQuant 9100 ICP-OES also features an Echelle spectrometer with a double monochromator. The spectral resolution is 2 pm at 200 nm and the wavelength range is 160 to 900 nm. With the help of the Ne-correction the accuracy is less than 0.4 pm. Thus, even individual trace elements in interference-prone sample matrices can be clearly and easily identified.
A solid state CDD detector allows simultaneous reading of peak and background signals. Integrated routines ensure automatic baseline adjustment. In addition, the detector automatically corrects possible interferences. The detector also allows simultaneous multi-element detection.
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