Wastewater discharged from industrial facilities or municipalities, as well as produced water from fracking activities, often contains complex matrix components and is challenging to analyze for toxic metals using ICP-OES and ICP-MS. In my last blog, I discussed four tips for analyzing high-matrix environmental samples by ICP-OES. These tips and tricks were discussed by Matthew Cassap in his recent webinar. In the same webinar, Marcus Manecki talked about four high-matrix sample dilution techniques for ICP-MS analysis.
One parameter to characterize the amount of matrix contained in a sample is the total amount of dissolved solids (TDS). Whereas ICP-OES can tolerate high amounts of TDS (up to 30%) ICP-MS can typically analyze only samples with <0.2% TDS without special precautions. The reason for this is that the matrix may gradually clot the small aperture on the sampling and skimmer cones between the ICP ion source and the mass spectrometer, and cause signal drift. High-matrix samples must therefore be diluted before analysis. To achieve high precision and accuracy, internal standards must also be added to all the solutions.
Manual dilution is the simplest and cheapest dilution approach. The downside to this approach is that it is labor-intensive and prone to error and contamination. This dilution method is recommended only when a small number of samples need to be analyzed.
Fixed and Constant Online Liquid Dilution
This dilution method uses a mixing tee and the instrument’s peristaltic pump to merge each sample with a diluent before pumping it to the nebulizer. Compared to manual dilution, this method reduces sample preparation time, and reduces the risk of introducing errors or contaminants. However, this approach prevents the use of varying dilution factors to accommodate different samples. Samples requiring larger dilution factors must be diluted off-line (typically using manual dilution). Samples containing analytes at concentrations outside the instrument’s calibrated range must be flagged and diluted off-line as well.
Argon Gas Dilution
Argon Gas Dilution (AGD) is a technique that uses a fixed flow of argon gas to dilute the sample as it passes from the spray chamber to the torch. It provides fixed, online dilution of each sample; however, dilution takes place using a constant stream of argon as opposed to using a liquid diluent. Automated AGD makes high-matrix sample analysis possible, as shown in technical note TN43202, where seawater (~3.5% TDS) has been successfully analyzed by ICP-MS without performing liquid dilution prior to analysis. In addition to a fixed dilution factor, AGD has other disadvantages for high-matrix sample analysis, all of which are addressed in the technical note.
Automated, Intelligent, Online Liquid Dilution
The most flexible way of diluting samples is the use of an automated and intelligent dilution system such as the prepFAST. This system is available as a fully integrated solution (both hardware and software) for both the iCAP 7000 Series ICP-OES and the iCAP Q Series ICP-MS. The prepFAST system can perform prescriptive dilution of samples using dilution factors up to 400x before they are introduced to the plasma for analysis. Therefore, a tailored dilution factor can be applied for different sample matrices in the same run. If a prescribed dilution factor is found to be insufficient, a second automated dilution (intelligent dilution) is automatically triggered to make sure samples are sufficiently diluted for analysis. In addition, if results for samples or QC standards fall outside the calibration range, those solutions are automatically re-analyzed using an appropriate dilution factor to bring them within the calibrated range. Samples are diluted and rerun as needed during sample analysis. There is no need to rerun them after the whole set of samples is complete. This system saves sample processing time and offers an efficient solution for high throughput laboratories. In addition, the operation of the prepFAST system can be controlled using Qtegra ISDS software, which controls the ICP-OES and ICP-MS. You can find out the details from technical note TN43208.
In summary, different dilution approaches can be used for high-matrix sample analysis using ICP-MS. Manual dilution is recommended for laboratories with limited budgets and a few samples to run. Fixed online dilution is a good choice for laboratories with low to medium sample throughput requirements that are looking to reduce errors and contamination. Argon gas dilution is an alternative to fixed, online liquid dilution, but it has drawbacks when used for high matrix applications. High throughput labs may want to consider using intelligent automatic dilution because it will increase data accuracy and precision, reduce errors and contamination, minimize hands-on procedures, and save time and overhead costs.
Visit our Metal Analysis page for more information on ICP-MS environmental applications.