Direct Analysis of Undiluted Soil Digests Using the Agilent High Matrix Introduction Accessory with the 7500cx ICP-MS

Significance of the Topic

Inductively coupled plasma mass spectrometry (ICP-MS) is widely regarded as the gold standard for trace metal analysis due to its exceptional sensitivity and multi-element capabilities. However, high total dissolved solids (TDS) in samples—common in environmental matrices such as soil digests—pose substantial challenges including interface clogging, signal drift, and instrument corrosion. Traditional approaches rely on extensive sample dilution to mitigate matrix effects, but this increases analysis time, introduces dilution errors, and generates additional waste. The development of techniques that enable direct measurement of high-matrix samples by ICP-MS is therefore of critical practical importance for laboratories seeking higher throughput, reduced costs, and improved data quality.

Objectives and Study Overview

This study, conducted in partnership between Agilent Technologies and Eurofins Analytico (Analytico Milieu, the Netherlands), evaluates the performance of the Agilent High Matrix Introduction (HMI) accessory fitted to the 7500cx ICP-MS with Octopole Reaction System (ORS). The primary goal was to determine whether undiluted aqua regia soil digests with TDS > 1% could be measured directly while meeting the stringent detection limits and quality criteria stipulated by Dutch regulation AS3000 for contaminated soil analysis. Key performance metrics included method detection limits (MDLs), accuracy and precision on certified reference materials (CRMs), long-term signal stability, and calibration verification over an extended sample sequence.

Methodology

Operational parameters were optimized on an Agilent 7500cx ICP-MS equipped with the second peristaltic pump and the HMI accessory. A Burgener MiraMist nebulizer delivered the sample aerosol; the plasma was run in ultra-robust mode with an effective aerosol dilution factor of 1/12, equivalent to a 12× conventional dilution. Helium collision mode was used for all analytes except selenium, which required hydrogen reaction mode to achieve sub-µg L⁻¹ detection.

Sample preparation followed a standard aqua regia microwave digestion: 1 g of soil plus 8 mL aqua regia, diluted to 50 mL with ultrapure water (final matrix ~4% HNO₃, 12% HCl). Method detection limits were calculated as three times the standard deviation of ten replicates at or near the target MDL, both within a single day and across ten days. Accuracy and precision were assessed via replicate analyses (n = 10) of two CRMs (FeNeLab River Clay and BCR-144R Sewage Sludge) and spiked samples at low and high concentration levels over a 30-day period. A 23-hour sequence of 235 unique soil digest samples, interspersed with continuing calibration verification (CCV) standards every 12 samples, was run to evaluate stability and productivity.

Instrumentation

• ICP-MS: Agilent 7500cx with Octopole Reaction System
• Sample Introduction: Agilent High Matrix Introduction (HMI) accessory
• Nebulizer: Burgener MiraMist
• Collision/Reaction Gases: Helium (4.0 mL min⁻¹) for most elements; Hydrogen (4.0 mL min⁻¹) for selenium
• Plasma Conditions: RF power 1600 W, carrier gas 0.28 L min⁻¹, HMI gas 0.67 L min⁻¹, sample uptake 0.17 mL min⁻¹

Main Results and Discussion

• Method Detection Limits: All calculated MDLs (based on 1 g soil to 50 mL) were at least an order of magnitude lower than Dutch AS3000 requirements, confirming that HMI’s effective 12× dilution does not compromise sensitivity.
• Accuracy and Precision: CRM recoveries ranged from 87% to 108% across 10 replicates of two reference soils. Spike recoveries for low (near MDL) and high levels were 93–106% with relative standard deviations below 7%.
• Long-Term Stability: Over a 23-hour, 235-sample run, internal standard drift was ~20%, fully compensated by correction factors. CCV recoveries for all analytes remained within ±10% throughout, eliminating the need for unscheduled recalibration.
• Productivity: Average analysis time per sample (including dual gas modes) was 5.9 minutes. Direct measurement without manual or online dilution reduced total throughput time and minimized waste generation.

Benefits and Practical Applications

• Elimination of Conventional Dilution: Simplifies workflow, reduces dilution errors, and lowers reagent and waste disposal costs.
• Low Maintenance: No moving parts or additional tubing as in autodiluters, reducing downtime.
• Enhanced Flexibility: Quick switching between standard and HMI modes without hardware changes.
• Instrument Consolidation: A single 7500cx/HMI system can replace multiple instruments (ICP-MS, ICP-OES, mercury analyzer) for contaminated soil and sludge analysis.

Future Trends and Potential Applications

Looking forward, the HMI approach may be extended to other challenging matrices such as wastewater, industrial effluents, and high-salt biological samples. Further developments could include optimized reaction cell chemistries for additional interference removal, higher aerosol dilution factors for extreme matrices, and integration with automated sample handlers for fully unattended operation. The technique also holds promise for rapid screening in environmental remediation projects, food safety testing, and pharmaceutical impurity analysis.

Conclusion

The Agilent High Matrix Introduction accessory effectively overcomes the long-standing matrix limitations of ICP-MS, enabling direct measurement of high-TDS soil digests while meeting stringent regulatory requirements. The combination of robust sensitivity, excellent accuracy and precision, and streamlined operation delivers significant gains in productivity, cost savings, and data quality. Laboratories performing high-throughput environmental analyses can thus consolidate instrumentation and workflows, enhancing overall efficiency and reliability.

References

• Wilbur S., Proper W., Yamada N., Sugiyama N. Performance Characteristics of the Agilent High Matrix Introduction Accessory for 7500 Series ICP-MS. Agilent Technologies Product Overview, 5989-7737EN, 2008.
• Dutch Regulation AS3000 for Soil Contamination Analysis.