LABTips: Handling 'Glitches' in the Environmental Matrix

Sewage, salt water, soil, sediment – all are examples of some of the complex, high-matrix samples encountered in the environmental lab. When performing trace elemental analysis using inductively coupled plasma-optical emission spectrometry (ICP-OES) or mass spectrometry (ICP-MS) on high-matrix samples, there are many things that could potentially go wrong, including physical, chemical and spectral interferences from components of the matrix itself.¹ Fortunately, there are a number of different techniques you can use to minimize or correct for these matrix effects, enabling accurate, reliable results and ensuring your method meets regulatory standards. Join us on a journey into the environmental matrix to learn tips for overcoming analytical challenges and to improve your ICP-OES and ICP-MS experiments. 

1. Choose Wisely When Selecting Internal Standards

Internal standards (ISs) are excellent tools for correcting physical interferences in sample transport or nebulization, which can result from matrix properties such as viscosity, surface tension and density of dissolved solids. However, ISs are only helpful when used correctly, and can even be harmful to your analysis if the wrong element is used. For example, the rare earth elements yttrium (Y) and scandium (Sc) are two commonly used ISs that have the advantage of not being present in typical environmental samples, but can easily precipitate out of matrices that contain fluoride and thus need to be avoided in certain situations.²

In addition to ensuring that your IS will not be natively present in your samples, and will remain chemically stable when added, it is essential that you select an element that will behave similarly to your target analyte. For ICP-OES, this means selecting an IS with ionic lines for analytes that have ionic lines, and ISs with atomic lines for analytes that have atomic lines. The analyte and IS emission wavelengths should also be similar (i.e. both visible or both UV). For ICP-MS, the analyte and IS should also have a similar mass and similar ionization properties to each other. For example, germanium (Ge) is a popular IS element used when analyzing copper (Cu) due to their similarity in atomic mass and ionization potential,³ as well as the fact that Ge is absent from typical sample matrices. 

Proper, error-free utilization of ISs is also key for an accurate analysis – ISs should be added at highly consistent concentrations to samples, standards and blanks, which can be aided through automation, and one should take care to ensure the IS remains free from impurities and contaminants.

2. Upgrade Your Dilution Methods

The total dissolved solids (TDS) in a sample can have a detrimental effect on analysis, especially for ICP-MS, which typically requires TDS levels less than 0.2%.⁴ Dilution is used to decrease the TDS concentration and further minimize matrix effects in conjunction with internal standards. However, not all dilution methods are created equal with regard to efficiency and consistency, and more dilution is not always better, as excessive dilution can make your analytes difficult to detect. ICP-OES instruments can tolerate TDS concentrations up to 30%, and because this method is less sensitive, dilution should only be used as needed to avoid drowning out your analytes beyond the detection limits. 

Manual dilution is the simplest, but least efficient, method, and is more prone to error and contamination than automatic methods. Manual dilution may be suitable when only a low number of samples need to be processed, but upgrading to an automatic, or even an intelligent, method can increase both throughput and consistency while reducing the labor-intensive aspects of manual dilution. A basic fixed online dilution scheme using a liquid diluent simply requires a mixing tee to combine the diluent and sample flowing from different channels in the instrument’s peristaltic pump prior to nebulization, making it a relatively easy-to-apply upgrade from manual dilution in terms of efficiency and error rate.⁵ 

Another method, which can enable ICP-MS analysis in matrices containing up to 25% TDS with no predilution, is online argon gas dilution (AGD), which involves using a lower nebulizer gas flow and an additional constant flow of diluent argon to reduce the amount of sample aerosol that reaches the plasma.⁶ AGD eliminates the need for a liquid diluent, reduces sample prep time and lowers contamination risk while allowing high-matrix sample matrices such as seawater with up to 25% sodium chloride (NaCL) content to be analyzed through ICP-MS. Additionally, this method increases plasma robustness and reduces the formation of metal oxide.⁷ A potential pitfall to be aware of with this method, however, is the decreased recovery of elements with higher 1st ionization potentials such as zinc (Ze), selenium (Se), cadmium (Cd) and tellurium (Te). Using an argon humidifier along with the addition of isopropanol (IPA) to the internal standard solution can help bring recovery of these elements back into the 80-120% range through “re-wetting” of the plasma and introduction of carbon via IPA. 

Fully automated dilution, which can maximize throughput and provided added flexibility through software-mediated results monitoring and dilution factor modification of individual samples, requires a greater initial investment cost but can be an efficient solution for mitigating matrix effects without manual intervention for high-volume labs handling a variety of complex samples. With all of these options in mind, consider whether a dilution method upgrade could become a valuable tool in your kit as you tackle the challenge of matrix effects.

3. Keep an Eye Out for EIEs

Easily ionizable elements (EIEs), such as alkali metals, are common in many environmental samples, making them another common source of matrix interferences you might see in your trace elemental analyses. For example, sodium (Na) and potassium (K) are abundant in soil matrices - Na is also an obvious concern for seawater analysis, and K can be present in plant digests as well. The problem with EIEs for ICP-OES is that when these elements become ionized, electron density increases and shifts the ionization equilibrium to enhance the expression of atomic lines of other elements (i.e. false positive results). This is primarily a problem in the axial view where sensitivity is higher.⁸ 

The use of an ionization buffer is an easy-to-implement solution for suppression of EIE effects from environmental matrices. As with ISs, ionization buffer elements are selected when they are not present in the sample matrix, and are in this case similar to native EIEs in terms of having a low ionization potential. Cesium (Cs) is one of the most commonly used and effective ionization suppressors due to its very low first ionization potential and absence from most environmental samples.

Using an ionization buffer is typically not necessary in radial view analyses, but should be considered for axial view analyses of samples with matrices likely to contain interfering alkali metals and/or alkali earth metals. A concentration of about 500 ppm Cs can be sufficient to suppress the ionization effects of many coexisting matrix elements on various analyte elements.⁹ Like internal standards and diluents, ionization buffers can be introduced online with the use of a multichannel peristaltic pump. Also be sure to add consistent concentrations of buffer to both the sample and calibrations standards. 

4. Clean Your Cones!

Regularly and appropriately maintaining your instrumentation is always of the utmost importance for any analytical method, but perhaps even more so for extremely sensitive methods like ICP-MS, especially when components frequently come in contact with high-matrix sample components. While thorough filtering, digestion, dilution etc. can help prevent clogs and damage to the ICP-MS interface cones, build up of matrix materials from complex environmental samples is inevitable and can become a source of interference and overall poor performance if not dealt with in an appropriate, timely manner.

The interface components of your instrument should be inspected frequently for visible buildup and blockages, but the precise schedule for cleaning the cones should be based how often you are running samples and the TDS of your samples, as well as how widely your samples vary.¹⁰ For example, if you are running a high volume of high-matrix samples continuously, cleaning may be needed daily, whereas running a relatively low number of samples daily or low proportion of high matrix samples could warrant weekly or even monthly cleaning. If your analyte concentrations vary widely from trace amounts to high concentrations, you may need to clean between runs to avoid cross-contamination, and of course, any blockage or excessive buildup that is apparent upon inspection will warrant a cleaning before any further use. However, keep in mind that cleaning your cones more frequently than needed can reduce their lifespan - as with dilution, more does not always mean better. Additionally, the sampling cone will typically require more frequent cleaning than the skimmer cone due to its closer proximity to the plasma. 

Cleaning procedures should be in accordance with manufacturers’ instructions, but if your cones are frequently in contact with high TDS matrices, routine maintenance will be more intensive than what would suffice when dealing with cleaner samples. This may include soaking and sonication in an acid cleaner, which should be performed carefully so as not to damage the cone tip via contact with ultrasonic bath hardware or other tools. After cleaning, tips should be allowed to dry completely before they are reinstalled. Again, follow the manufacturers’ guidance or instruction to understand the compatible cleaners, cleaning methods and drying methods that are acceptable for a specific product. As a final note, ensuring a long lifetime for your interface cone relies on both proper maintenance and proper selection of cone type for your high-matrix samples. In general, platinum tips will be more robust and have a higher matrix tolerance than nickel tips, and cones with copper components may suffer shorter lifespans from more aggressive cleaning or overcleaning. If you’ve followed all of the previous tips on this list and more, and continue to see problems such as a high background signal and loss of sensitivity, that may be a sign your cones are overdue for a cleaning, or a replacement!

References

1. "ICP-OES Data Analysis," Thermo Fisher Scientific. https://www.thermofisher.com/us/en/home/industrial/spectroscopy-elemental-isotope-analysis/spectroscopy-elemental-isotope-analysis-learning-center/trace-elemental-analysis-tea-information/icp-oes-information/icp-oes-data-analysis.html 

2. Evans Norris SJ. Improve your ICP-OES Performance by using an Internal Standard, ARMI | MBH CRM Solutions Blog Post. 2017. https://www.armi.com/blog/how-an-internal-standard-can-improve-your-icp-oes-performance 

3. Wilschefski SC, Baxter MR. Inductively Coupled Plasma Mass Spectrometry: Introduction to Analytical Aspects. Clin Biochem Rev. 2019;40(3):115-133. doi:10.33176/AACB-19-00024

4. Liu W. High-Matrix Environmental Samples Part II: Dilution Methods for ICP-MS. Thermo Fisher Scientific, AnalyteGuru.com Blog Post. https://www.analyteguru.com/t5/Blog/High-Matrix-Environmental-Samples-Part-II-Dilution-Methods-for/ba-p/3068 

5. Cassap M, Manecki M. Part II: Dealing with High Matrix Samples. Webinar hosted by Thermo Fisher Scientific. 2015. https://www.analyteguru.com/t5/Webinar-Library/Dealing-with-High-Matrix-Environmental-Samples-Part-2/ta-p/6218 

6. McCurdy E. Improving ICP-MS Analysis of Samples Containing High Levels of Total Dissolved Solids. Spectroscopy Supplements, 2014;29(11). https://www.spectroscopyonline.com/view/improving-icp-ms-analysis-samples-containing-high-levels-total-dissolved-solids 

7. Kutscher D, Wills JD, McSheehy Ducos S. Analysis of High Matrix Samples using Argon Gas Dilution with the Thermo Scientific iCAP Q ICP-MS. Thermo Fisher Scientific Technical Note 43202. http://tools.thermofisher.com/content/sfs/brochures/TN-43202-ICP-MS-Argon-Gas-Dilution-Seawater-TN43202-EN.pdf 

8. Schulz O. How to Overcome Analytical Challenges with Environmental Samples Using ICP-OES. Webinar hosted by American Laboratory and sponsored by SPECTRO. https://www.americanlaboratory.com/623-Videos/342921-How-to-Overcome-Analytical-Challenges-with-Environmental-Samples-Using-ICP-OES/ 

9. Morishige Y, Kimura, A. Ionization interference in inductively coupled plasma-optical emission spectroscopy. SEI Technical Review, 2008;106-111. https://global-sei.com/technology/tr/bn66/pdf/66-14.pdf 

10. Brennan R, Masone J. ICP-MS Cones: Why, When and How to Maintain. Glass Expansion Application Note. https://www.geicp.com/site/images/flyers/ICP-MS-Cones_appnote.pdf