Poor sample washout or carryover effects can be the cause of inaccurate results when using ICP analysis techniques. Ideally the instrument signal should return to background levels between analyzing samples and standards. Samples and standards at high concentrations may present poor washout conditions resulting in a high bias in the instrument results.
This checklist will cover key points about what elements may be of a higher risk for carryover effects, how to determine if your results are suffering from poor washout, and how to best correct your analysis procedure to prevent bias in the analysis.
Blanks
- To determine if your analysis is suffering from carryover or poor washout it is important to have a reference point for what the background instrument signal should be.
- Along with a standard calibration blank, make sure to run a reagent blank and method blank. More information on the types of blanks can be found in our Technical Glossary.
- Rerun the calibration blank after running your highest calibration standard. Compare the raw instrument signal of the blank before running standards and after. Elevated counts in the blank ran after the standards indicate that a longer method rinse time may be needed.
- Considering running a method blank after noticeably “dirty” samples.
Elements of Concern
- Si and B – If a glass sample introduction system is installed and HF is present in the sample or standard matrix, then expect to see Si and B carryover due to the HF attacking the glass components. If HF is present in sample or standard matrices then use a HF resistant sample introduction system.
- B, Br, I, Bi, Hg, and Os – These elements can become “sticky” in matrices that are mostly HNO3. Consider using an HCl method rinse to reduce carryover.
- Si, Sn, Sb, Te, Mo, Hf, Ta, and W – These elements require HF in the sample and standard matrix to remain stable. If measuring high concentrations of these elements consider adding a trace amount of HF to the method rinse acid.
- Ag – Analysts should be cautious of analyzing Ag when HCl may be present in sample and/or standard matrices. Ag can precipitate with HCl causing possible washout issues or clogging of the sample tubing.
- Se and Th – These elements appear to have a strong affinity to PVC peristaltic pump tubing. Consider replacing the pump tubing after running high concentrations of Se and Th.
Sample Introduction System
General Maintenance:
- Following a suitable maintenance schedule for your instrument and sample types is critical to preventing poor washout. See an example maintenance schedule here.
- Make note of specific sample introduction part numbers when performing maintenance. Tracking parts based on serial numbers can help indicate when specific components should be retired or taken out of service.
- Run an instrument performance check standard after performing instrument maintenance. Track your results over time to ensure that your instrument is performing as expected and your signal background is within specification.
Sample Delivery:
- Inspect pump tubing for elasticity and replace it if worn or stretched. Keep a supply of spare tubing on hand.
- Replace PVC peristaltic pump tubing regularly, ideally after 12-24 hours of use. "Sticky” elements have a strong affinity to PVC pump tubing and washout issues can be corrected easily by replacing old pump tubing.
- Release pressure on the pump tubing when the instrument is not in use.
- Don't overtighten the pump tubing; ensure a smooth and even sample flow.
- If using a switching valve, ensure maintenance is being performed regularly and any consumables are replaced as recommended by the manufacturer. Poor washout can occur from the valve if consumables are worn and should be replaced.
Nebulizer:
- Use the proper nebulizer for the samples being analyzed. Glass nebulizers are not suitable for samples and standards with HF present in the matrix. The concentration of total dissolved solids may also dictate the type of nebulizer that should be used. See Part 5 of our ICP Operations Guide for more information on nebulizer types.
- Optimize the sample and nebulizer gas flow rates to ensure ideal performance.
- Ensure the nebulizer is securely seated in the spray chamber. Check all fittings/O-rings for damage and replace as needed.
- Regularly inspect and clean the nebulizer to prevent blockages. Some ICP instrument software can help troubleshoot blockages by monitoring nebulizer back pressure. Nebulizers should be cleaned via back flushing. Many commercial cleaning devices are on the market to make nebulizer maintenance quick, easy, and effective.
Spray Chamber:
- The type of spray chamber used can greatly impact the instrument washout performance. Cyclonic spray chambers will perform better in regard to washout compared to Scott style spray chambers.
- Ensure the drain line is correctly installed. Monitor the drain tubing during instrument start up and analysis. Segmented flow is expected and indicates the spray chamber is properly draining. No flow indicates the spray chamber is filling up and will result in a plasma shutdown. Improper drainage can result in poor washout.
- Check the O-ring or ball joint between the spray chamber and torch. Ensure fittings are tight.
- Clean the spray chamber regularly. We have found that rinsing the instrument after analysis batches with a 2.5% v/v solution of RBS™-25, followed by rinses of deionized water helps to keep spray chambers performing as expected. This approach can help extend the time needed between performing full maintenance.
Rinse Solutions
Method Rinse Solutions:
- Common method rinse, ran between samples during analysis, solutions include:
- Dilute HNO3/HCl
- Deionized water
- Dilute acid with trace HF can be used as a method rinse but requires the use of an HF resistant sample introduction system.
- Ensure that the method rinse time is appropriate by analyzing calibration blanks during method development. Ideally there should be no difference in blanks ran before and after the highest concentration calibration standard.
- Periodically clean the method rinse station and reservoir. This rinse station is typically part of the ICP autosampler. The tubing that feeds the rinse station should be replaced occasionally.
End of Analysis or Batch:
- Consider running separate rinse solutions after your analysis or batch is complete. An extended rinse protocol can be used to thoroughly clean the instrument and even restore instrument performance after running highly concentrated or “dirty” samples.
- Specialty rinse solutions can be run to address carryover concerns for specific elements:
- NH4OH – 1-5% v/v solutions can be effective to rinse out B, Br, I, and Hg.
- HCl/Thiourea - This mixture is effective for particularly “sticky” elements such as Hg, Os, and Bi. Inorganic Ventures offers this solution as ICP-TRUE-RINSE, a ready to use rinse solution. Learn more about the benefits of ICP-TRUE-RINSE here!
- RBS™-25 – We would recommend rinsing the instrument after analysis batches with a 2.5% v/v solution of RBS™-25, followed by rinses of deionized water.
Following this checklist can significantly reduce washout and carryover concerns, leading to more accurate, precise, and reliable results.
Have more questions? Check out our ICP Operations Guide and Trace Analysis Guide for more tips and tricks to achieve an accurate ICP analysis. Our team is ready to assist in all your analysis needs! Please send us an email or schedule a meeting to talk directly with a Technical Support Chemist.