Direct Analysis of Seawater Using ICP-QQQ and Integrated Advanced Valve System

Significance of the topic

This application note addresses the analytical challenge of directly determining trace elements in seawater, a matrix with high salinity and abundant matrix components (Na, Mg, Cl, S) that generate ionization suppression and numerous polyatomic interferences in ICP-MS analysis. Accurate, interference-free quantification of trace metals and metalloids at sub-ng L-1 levels is essential for marine biogeochemistry, environmental monitoring, pollution assessment and regulatory compliance (EPA Method 200.8). The described workflow demonstrates how modern ICP-QQQ hardware and online dilution strategies can streamline sample handling, maintain sensitivity, and satisfy QC criteria for routine, high-throughput seawater analysis.

Objectives and overview of the study

The study evaluated an analytical workflow for direct seawater analysis using an Agilent 9500 triple-quadrupole ICP-MS (ICP-QQQ) equipped with an integrated Advanced Valve System for ICP-MS (AVS MS). Key aims were to: verify accuracy for 26 elements in certified reference materials (NMIJ CRM 7204-a and NMI MX014) and real seawater; demonstrate ppt-level method detection limits (MDLs); validate long-run stability during repeated injections of undiluted seawater; and implement an automated online reverse-dilution approach to reduce manual preparation and increase throughput.

Methodology

Samples and standards:

• Two seawater CRMs (NMIJ 7204-a, NMI MX014) and a Singapore seawater sample (acidified with concentrated HNO3) were analyzed.
• Calibration used Agilent multi- and single-element standards prepared in 2% HNO3 / 0.5% HCl; calibration ranges covered low-ppb to tens of ppb (detailed ranges per element).
• Internal standards (6Li, Sc, Ge, Y, In, Tb, Bi) were diluted to 5 ppb and introduced online.

Analytical approach:

• Direct analysis with online reverse dilution (~15× dilution achieved by tubing selection and a 0.25 mL sample loop) minimized manual sample prep and CRM consumption.
• ICP-QQQ MS/MS modes: Advanced Helium Mode (AHM, default He flow ~14 mL/min) used for most elements to exploit Kinetic Energy Discrimination (KED) and collision-induced dissociation (CID); Air cell mode (air ~0.4 mL/min) used for elements requiring on‑mass or mass-shift reactions (e.g., As, Se, V, Cu, Zn).
• Autosampler: Agilent SPS 4; sample introduction: u-lens, MicroMist glass concentric nebulizer and quartz spray chamber; Ni-plated sampler cone and Ni skimmer cone; General Purpose plasma conditions were applied. UHMI was not used to conserve CRM material.

Quality control and runtime:

• Internal standards monitored per run; continuing calibration verification (CCV) standards measured every 10 samples (2 ppb, 0.2 ppb for Hg).
• Method performance evaluated against EPA Method 200.8 criteria.

Used instrumentation

Key hardware and consumables summarized from the study:

• Agilent 9500 ICP-QQQ with Dual-Cell System (DCS) and AHM/Air cell capabilities.
• Integrated Advanced Valve System for ICP-MS (AVS MS) with 0.25 mL sample loop for online reverse dilution and associated preconfigured tubing kits.
• Agilent SPS 4 autosampler.
• Sample introduction: MicroMist glass concentric nebulizer, quartz spray chamber, quartz torch, u-lens assembly, nickel-plated sampler cone and nickel skimmer cone.
• Peristaltic tubing configuration to achieve reverse dilution: narrow tubing on sample line and wider tubing on ISTD line.
• Calibration and ISTD standards from Agilent (multi-element mixes and single-element standards).

Main results and discussion

Analytical sensitivity and detection limits:

• ppt-level MDLs were achieved across the examined element list. For elements measured in AHM, many MDLs were below 1 part-per-trillion (sub-1 ppt), demonstrating maintained sensitivity across low and high mass elements (including 9Be and 238U).

Accuracy and matrix effects:

• Analysis of two seawater CRMs and spiked real seawater showed element recoveries between 90–110% for certified and spiked values, confirming accurate interference removal and quantitation in high-salinity matrices.
• Both AHM and Air cell modes were essential: AHM handled broad polyatomic suppression via KED/CID, while Air mode enabled targeted mass-shift reactions for elements prone to Ar/Cl/S-based interferences (e.g., As, Se, V, Cu, Zn).

Stability and throughput:

• The method demonstrated robust stability over extended sequences: 100 continuous injections of undiluted seawater (over ~7 hours) produced internal standard recoveries within 80–120% and CCV recoveries within ±10%, meeting EPA 200.8 QC limits.
• Automated online reverse dilution and rapid gas-mode switching enabled high throughput, with a typical sample analysis cycle of approximately 140 seconds per sample.
• The integrated AVS MS and optimized rinse protocols limited salt buildup impacts, reducing the need for frequent cleaning and increasing run productivity.

Benefits and practical applications

This workflow offers multiple advantages for laboratories engaged in marine and environmental trace metal monitoring:

• Direct analysis of undiluted seawater reduces sample handling, contamination risks, and turnaround time compared with offline dilution or matrix-separation procedures.
• ICP-QQQ MS/MS with DCS (AHM + Air modes) delivers robust interference management across a wide element range, improving accuracy in saline matrices.
• Online reverse dilution via AVS MS streamlines high‑volume campaigns and conserves CRM resources by minimizing sample consumption.
• Consistent ISTD recovery and CCV performance support regulatory QA/QC requirements (EPA 200.8).

Future trends and possibilities for use

Anticipated directions and potential applications include:

• Wider adoption of ICP-QQQ with automated dilution for routine coastal and oceanographic monitoring, enabling denser spatial and temporal sampling campaigns.
• Integration with onboard or autonomous sampling platforms where online dilution and compact, robust ICP-QQQ systems can reduce shore‑based processing.
• Expanded multi-element and isotopic workflows combining MS/MS modes with chromatographic separation for speciation or source apportionment studies.
• Continued refinement of cell gases and reaction chemistries to further lower MDLs for problematic analytes (e.g., Hg species) and to extend compatibility with ultra-trace applications.

Conclusion

The presented method demonstrates that the Agilent 9500 ICP-QQQ combined with the AVS MS online reverse-dilution approach can perform fast, accurate, and stable direct analyses of seawater for 26 trace elements. The workflow achieved ppt-level MDLs, recoveries within 90–110% for CRMs and spiked samples, and sustained instrument stability for long batches of undiluted seawater, meeting EPA Method 200.8 QC criteria. This approach reduces sample preparation burden, increases throughput (≈140 s per sample), and is well suited to environmental monitoring and marine research laboratories requiring reliable trace element data from saline matrices.

References

1. Tagliabue A., Weber T. Novel Insights into Ocean Trace Element Cycling from Biogeochemical Models. Oceanography. 2024;37:131–141.
2. Jeandel C., Chase Z., Hatje V. Marine Biogeochemistry of Trace Elements and Their Isotopes. Elements. 2018;14(6).
3. U.S. EPA. Method 200.8: Determination of Trace Elements in Waters and Wastes by Inductively Coupled Plasma–Mass Spectrometry. Revision 5.4. 1994.
4. Agilent Technologies. Dual-Cell System (DCS) and Advanced Helium Mode (AHM). Publication 5994-8985EN.
5. Agilent Technologies. Air Cell Mode of the Agilent 9500 ICP-QQQ with Dual-Cell System. Publication 5994-8987EN.