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

Significance of the Topic

The elimination of polyatomic interferences in inductively coupled plasma mass spectrometry (ICP-MS) is critical for achieving trace-level detection of elements such as selenium, sulfur, phosphorus and iron. Helium collision mode with kinetic energy discrimination (KED) has become the standard approach for removing unknown and variable interferences in complex matrices. Improving this mode extends reliable ppt-level quantitation to elements that were previously inaccessible without reactive cell gases, simplifying multi-element workflows and boosting laboratory productivity.

Objectives and Article Overview

This study evaluates the performance gains achieved by upgrading from the ORS2 collision cell to the new ORS3 design in an Agilent 7700 Series ICP-MS when operating in He collision mode. The main goals were to:

• Assess the ability of ORS3 to remove Ar2 interferences on selenium isotopes at mass 78 while maintaining analyte sensitivity.
• Compare residual energy distributions and quantify collision-induced dissociation (CID) effects in ORS2 and ORS3.
• Determine background equivalent concentrations (BEC) and detection limits (DL) for Se, P and S under optimized conditions.
• Demonstrate practical improvements for difficult matrices and other elements prone to polyatomic overlaps.

Methodology and Used Instrumentation

All measurements were performed on an Agilent 7700 Series ICP-MS equipped with the Octopole Reaction System (ORS) operated in helium collision mode with kinetic energy discrimination. Key aspects include:

• Comparison of two ORS cell configurations: ORS2 (standard) vs ORS3 (enhanced design).
• Use of pure helium as collision gas, adjusting flow rate and bias voltage to optimize separation of analyte and polyatomic ions.
• Analysis of a mixed acid matrix (5% HNO3 + 5% HCl + 1% H2SO4 + 1% IPAM) in no-gas mode and He mode.
• Evaluation of collision-induced dissociation (CID) of Ar2 (bond energy 1.33 eV) under varying collision energies.
• Calibration and spike recovery experiments at 10 ppb levels and blank corrections for BEC and DL determination.

Key Results and Discussion

The major findings illustrate the superiority of ORS3 in helium mode:

1. Spectrum Comparison
   In no-gas mode the mixed matrix shows a complex background of polyatomic peaks. Switching to He mode with ORS3 removes virtually all interferences while preserving high analyte signals, as demonstrated by a clean 10 ppb spike spectrum.
2. Residual Ion Energy Profiles
   Energy distributions after the collision cell reveal significant overlap between Se and Ar2 ions in ORS2. ORS3’s higher collision energy (4.88 eV vs 0.98 eV) and increased He flow shift the distributions apart, enabling a bias voltage that excludes Ar2 with less than 10% loss of Se ions.
3. Collision-Induced Dissociation
   ORS3 conditions exceed the Ar2 bond energy, promoting efficient CID. This leads to a sharp drop in Ar2 signal with increasing He flow, as shown in cell gas optimization data.
4. Detection Limits and BEC
   Under optimized ORS3 helium mode, the BEC for 78Se falls below 5 ppt and the DL reaches approximately 4.5 ppt. Phosphorus (31P) and sulfur (34S) also benefit, with BECs of 291 ppt and 154 ppb and DLs of 170 ppt and 18.2 ppb, respectively.
5. Extension to Other Elements
   Elements such as Fe exhibit similar improvements, confirming that the enhanced ORS3 cell design broadens the applicability of He mode to analytes formerly requiring reactive gases.

Benefits and Practical Applications

These advancements offer multiple advantages for routine and research laboratories:

• Unified single-gas approach for multi-element analysis, eliminating the need to switch reactive cell gases.
• Improved detection limits down to single-ppt for selenium and enhanced sensitivity for other challenging elements.
• Reliable removal of unknown interferences in complex or variable sample matrices.
• Greater throughput and ease of use, particularly for transient signals and discrete sampling.
• Compatibility with hyphenated techniques such as HPLC-ICP-MS for speciation studies.

Future Trends and Potential Applications

Building on the ORS3 helium mode improvements, future developments may include:

• Expanded collision/reaction cell designs tailored for ultra-trace analysis in environmental, clinical and food safety applications.
• Integration with advanced front-end separations to resolve chemical species at even lower concentration levels.
• Automation of gas flow and energy settings based on real-time interference feedback.
• Combining KED with emerging reaction gases in hybrid cells for targeted interference removal without compromising multi-element capability.

Conclusion

The introduction of the ORS3 collision cell in Agilent 7700 Series ICP-MS has dramatically enhanced the effectiveness of helium mode with KED. By increasing collision energy and gas flow to enable collision-induced dissociation of Ar2, single-ppt detection limits for selenium are now achievable alongside significant gains for phosphorus, sulfur and iron. This unified single-gas method simplifies workflows, boosts sensitivity and reliability, and extends the scope of conventional ICP-MS to more demanding analytical challenges.

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. 2011 European Winter Conference on Plasma Spectrochemistry, Zaragoza, Spain.