Investigations into the use of helium collision mode and aerosol dilution for ultra-trace analysis of metals in mineral reference materials

Importance of the topic

The mining and geological survey industries face extreme analytical demands, requiring fast, cost-effective measurement of dozens of elements across concentration ranges from sub-ppb to thousands of ppm. Traditional workflows split high-level major elements (ICP-OES) from trace elements (ICP-MS), or rely on time-consuming mathematical interference corrections. A unified, single-run method that delivers accurate, ultra-trace multi-element data in complex, high-matrix rock digests can greatly improve laboratory throughput, reduce per-sample cost, and enhance data confidence.

Aims and overview of the study

This work evaluates a combined approach on the Agilent 7700x ICP-MS—integrated discrete sampling (ISIS-DS), high matrix aerosol dilution (HMI), and helium collision mode—to analyze 44 analytes plus three internal standards in six mineral certified reference materials (CRMs) in a single automated run. The goal was to extend dynamic range for high‐mass elements, remove polyatomic interferences without mathematical corrections, and maintain stability over hundreds of samples.

Methodology and instrumentation

Samples: Six milled geological CRMs were digested in quadruplicate with aquaregia (75 % HCl/50 % HNO3), diluted to 50 g, and analyzed directly without filtration. A single calibration covered Hg, Ca, Ti, Fe, and 38 additional elements over five concentration levels. A continuing calibration verification (CCV) and blank check (CCB) were run every 20 samples over 365 analyses.
Instrumentation used

• Agilent 7700x ICP-MS with nickel sample/skimmer cones and MicroMist nebulizer
• Integrated Sample Introduction System (ISIS-DS) for discrete sampling
• High Matrix Introduction (HMI) for aerosol dilution and robust plasma operation
• Helium collision mode at 4 mL/min for polyatomic interference removal and dynamic range extension

Internal standards 13C, 103Rh, and 193Ir were introduced online with 1 % butanol to improve washout and boost As/Se response.

Main results and discussion

The combined system achieved an average analysis time of 87 s per sample (365 samples in 8 h 49 min) with continuous stability over nine hours. Calibration linearity exceeded r=0.9999 across all elements. Method detection limits in digested rock ranged from 0.5 to 1000 µg/kg, with He mode lowering background equivalent concentrations and detection limits by up to an order of magnitude for interfered isotopes (e.g., Ti, V, Cr, Ni, Cu, As, Se). CRM recoveries were within 90–120 % of certified values, with precision (RSD) typically below 5 % for trace elements and below 3 % for major elements. Skimmer cone inspection after nine hours revealed minimal deposition, and washout tests with 20 mM thiourea rinse kept carryover below 0.1 % for sticky elements (Au, Ag, Hg, Tl).

Benefits and practical applications

• Single-run multi-element analysis across ultra-trace to major element concentrations
• Elimination of mathematical interference corrections via universal He collision mode
• Extended dynamic range for low-mass, high-concentration elements, removing the need for separate ICP-OES assays
• High throughput with minimal drift and low carryover reduces labor and maintenance

Future trends and possibilities

Further automation of sample prep and integration with laboratory information management systems (LIMS) will streamline workflows. Expanded use of optimized collision/reaction gases could target specific molecular interferences. Advances in high matrix introduction and discrete sampling may be applied to other challenging matrices such as environmental and industrial slurries.

Conclusion

By combining ISIS-DS discrete sampling, HMI aerosol dilution, and helium collision mode on the Agilent 7700x ICP-MS, laboratories can achieve a robust, single-run method for ultra-trace and major element analysis in complex geological digests. This protocol delivers high accuracy, excellent precision, and significant improvements in throughput and cost efficiency.

Reference

Yamanaka K., Wilbur S. Investigations into the use of helium collision mode and aerosol dilution for ultra-trace analysis of metals in mineral reference materials. Agilent Technologies White Paper, 2013.