WCPS: Enhancing Helium Mode Performance to Provide Improved Detection Limits for Difficult Elements Including S, P, Fe, and Se

Importance of Helium Collision Mode with Kinetic Energy Discrimination

ICP-MS analysis of elements in complex matrices often suffers from polyatomic interferences that compromise sensitivity and detection limits. Helium collision mode combined with kinetic energy discrimination (KED) provides a nonreactive approach to reduce multiple unknown interferences across diverse sample types. Recent advances in collision cell design have further expanded its applicability to elements previously requiring reactive gases, notably selenium, sulfur, phosphorus and iron.

Study Objectives and Overview

This work evaluates the performance of the new ORS3 collision/reaction cell on the Agilent 7700 Series ICP-MS in Helium mode with KED. Key goals include:

• Assessing removal of Ar2-mediated interferences on selenium and other challenging elements
• Comparing sensitivity and detection limits between ORS2 and ORS3 configurations
• Demonstrating improvements in background equivalent concentrations (BEC) and detection limits (DL) for Se, P, S and Fe

Methodology and Instrumentation

The Agilent 7700 Series ICP-MS equipped with the updated ORS3 collision cell was operated in Helium mode under optimized cell gas flows and collision energies. The mechanism of collision induced dissociation (CID) was exploited by increasing center-of-mass collision energy above the bond energy of interfering polyatomic ions (for Ar2 this is 1.33 eV, reached at 4.88 eV in ORS3). Both no-gas and Helium mode spectra were recorded for mixed acid matrices spiked with target analytes.

Main Results and Discussion

• The ORS2 configuration reduced Ar2 interference on 78Se but retained low residual sensitivity and a BEC of ~150 ppt. Detection limits were compromised at around 150 ppt.
• ORS3 enhancement in collision energy and gas flow led to clear separation of residual ion energy profiles between Se and Ar2, allowing Ar2 rejection with less than 10% loss of Se signal.
• Collision induced dissociation in ORS3 produced a rapid decline in Ar2 signal as cell gas flow increased, illustrated by optimization curves.
• Calibration data showed a BEC of 2.7 ppt and DL of 4.5 ppt for 78Se in ORS3 Helium mode.
• Additional elements benefited similarly: phosphorus (BEC 291 ppt, DL 170 ppt) and sulfur (BEC 154 ppb, DL 18.2 ppb), with iron measurement also improved.

Practical Benefits and Applications

Helium collision mode with the ORS3 cell enables single-ppt quantification of selenium and significantly lowers detection limits for other elements without switching to reactive gases. This simplifies multi-element analyses in variable and complex matrices, enhances throughput for transient signals and discrete sampling, and offers reliable interference removal for routine QA/QC, environmental, clinical and industrial applications.

Future Trends and Potential Uses

• Broader adoption of high-energy Helium collision mode for speciation studies coupling HPLC-ICP-MS, enabling trace-level detection of individual chemical forms.
• Further optimization of collision cell designs to extend CID-based removal to heavier polyatomic interferences.
• Integration with automated methods for high-throughput screening in environmental and food safety laboratories.
• Expansion to on-line and portable ICP-MS platforms benefiting from universal gas-mode operation.

Conclusion

The enhanced ORS3 collision/reaction cell significantly improves Helium mode performance in ICP-MS, achieving ppt-level detection limits for difficult analytes such as selenium, phosphorus and sulfur. The combination of higher collision energy, increased gas flow and collision induced dissociation delivers robust interference removal without sensitivity loss, streamlining multi-element analysis in challenging sample matrices.

References

• Ed McCurdy, Naoki Sugiyama and Steve Wilbur. Enhancing Helium Mode Performance to Provide Improved Detection Limits for Difficult Elements Including S, P, Fe, and Se. Agilent Technologies poster at 2011 European Winter Conference on Plasma Spectrochemistry, Zaragoza, Spain.