Automated Analysis of Foods by ICP-QQQ with Discrete Sampling and Autodilution

Importance of the topic

Modern food safety regulation and industry quality programs increasingly demand lower limits for toxic elements (As, Cd, Hg, Pb) and robust assurance that measurement results are accurate and reproducible. High-throughput, interference-resistant elemental analysis is therefore critical for compliance testing, product safety, and regulatory monitoring. Methods that reduce manual preparation, increase automation, and provide reliable interference control improve laboratory productivity and data quality in routine food testing workflows.

Objectives and study overview

The application note demonstrates an automated, AOAC 2015.01–compliant workflow for multi-element analysis of food matrices using the Agilent 9500 triple quadrupole ICP-MS (ICP-QQQ) combined with discrete sampling (AVS MS) and the ADS 2 autodilutor. The study validated method performance with multiple NIST food SRMs (oyster tissue, tomato leaves, mussel tissue), optimized for throughput and interference control, and assessed limits of detection/quantification, accuracy (SRM recoveries), and stability across a routine analytical sequence.

Methodology and experimental workflow

Sample preparation:

• SRMs used: NIST 1566b Oyster Tissue, NIST 1573a Tomato Leaves, NIST 2976 Mussel Tissue (trace elements and methylmercury).
• Digestion: microwave closed-vessel digestion (MARS 6) of ~0.25 g sample in PFA-lined vessels with 4 mL HNO3 + 1 mL H2O2. A 50 mg/L Au/Lu pre-digestion spike (0.1 mL) was added to stabilize Hg (Au) and provide a Lu spike for volatile recovery checks.
• Digests were brought to 20 mL (gravimetric) and then pre-diluted ~4×, yielding an overall minimum dilution factor of ~320× before analysis.

Calibration, standards and QC:

• Multi-element calibration prepared from Agilent Environmental Calibration standard; single-element stocks of Hg, Lu, and Au used to prepare Hg/Lu calibrants and the pre-digestion spike.
• ADS 2 autodilutor produced calibration points by automated serial dilution (400×, 250×, 25×, 2.5×, 1×). CCV solutions were automatically prepared and a QC block (two CCVs) was run every eight samples.
• Internal standards: Agilent ISTD mix (Sc, Ge, Rh, Bi) diluted to 100 µg/L in 5% HNO3/20% acetic acid and introduced online via the AVS MS discrete sampling accessory.

ICP-QQQ operation and interference control:

• Instrument: Agilent 9500 ICP-QQQ with Dual-Cell System (DCS) collision/reaction cell operated in two complementary gas modes: Advanced Helium Mode (AHM) and Air cell mode.
• AHM: an intelligent helium-based mode combining KED + CID to improve low-mass sensitivity and remove polyatomic interferences; used for most elements.
• Air cell mode: uses ambient air as a reactive cell gas to suppress doubly charged rare-earth element (REE2+) interferences (e.g., Nd2+, Sm2+, Gd2+) on As and Se without requiring an O2 gas supply.
• Sample introduction: MicroMist nebulizer, temperature-controlled quartz spray chamber, quartz torch (2.5 mm i.d. injector), nickel-plated copper sampler cone and nickel skimmer cone. Instrument tuning used the vendor’s Food/Clinical preset and autotune routine.

Used instrumentation

• Agilent 9500 Triple Quadrupole ICP-MS with Dual-Cell System (DCS)
• Agilent AVS MS discrete sampling accessory (integrated discrete sampling)
• Agilent ADS 2 autodilutor (2.5 mL sample loops)
• Agilent SPS 4 autosampler
• MARS 6 closed-vessel microwave digestion system (CEM Corporation)
• MicroMist glass concentric nebulizer, temperature-controlled quartz spray chamber, quartz torch, nickel sampler and skimmer cones
• Reagents/standards: Agilent Environmental Calibration standard, single-element Hg/Lu/Au stocks, Agilent ISTD mix (Sc, Ge, Rh, Bi)

Main results and discussion

Detection capability and throughput:

• Limits: Calculated DLs and AOAC LOQs showed the 9500 ICP-QQQ performance well below AOAC 2015.01 LOQ requirements for regulated elements (As, Cd, Pb, Hg). LOQs were derived as 6× the standard deviation of 10 method blanks, per AOAC requirements.
• Calibration ranges: typical calibration windows were 0.1–25 µg/L for most trace elements, 10–2500 µg/L for Fe, 0.01–1 µg/L for Hg, and 1–100 µg/L for Lu, providing broad dynamic range suitable for diverse food matrices.
• Throughput: average sample-to-sample time ≈ 2 minutes 45 seconds using the single-tune DCS method, about 30 seconds faster per sample than a comparable multi-tune single-quadrupole ICP-MS approach.

Interference control and accuracy (SRM recoveries):

• AHM effectively reduced common polyatomic interferences (e.g., NaAr on Cu) while enhancing sensitivity at low masses.
• Air cell mode provided superior suppression of REE2+ doubly charged interferences on As and Se in samples containing elevated rare-earth elements (demonstrated using NIST 1573a Tomato Leaves), yielding recoveries nearer 100% compared with AHM where REE2+ caused biased signal responses.
• SRM recoveries: triplicate digests measured in triplicate gave recoveries within the AOAC ±25% acceptance for all certified analytes across the three SRMs. Examples: Cu and Zn in oyster tissue, Mn and Zn in tomato leaves, and Zn in mussel tissue were measured; where concentrations exceeded calibration ranges, the ADS 2 automatically performed reactive dilutions and remeasured.
• Lu spike recovery (volatile element check) exceeded 98% for all SRMs, confirming minimal loss of volatile elements during digestion and handling.

Stability and QC performance:

• Continuing calibration verification (CCV) samples run throughout a 5.5-hour sequence all recovered within the AOAC ±15% limit.
• ISTD recoveries ranged from 87–118% over the 5.5-hour run, comfortably inside the AOAC acceptance window of 60–125%.

Benefits and practical applications

• AOAC-compliant quantification of regulated toxic elements in complex food matrices with robust interference control for both polyatomic and doubly charged REE interferences.
• Broad dynamic calibration range and low detection limits suitable for trace-level regulatory monitoring and survey testing.
• Automation (ADS 2, AVS MS, SPS 4) reduces manual preparation, supports automated calibration and reactive dilution, and increases laboratory throughput and reproducibility.
• Air cell mode eliminates the need for a dedicated O2 gas supply to address REE2+ interferences, simplifying installation and operation in routine laboratories.

Future trends and potential uses

• Broader adoption of MS/MS ICP-QQQ platforms in regulatory and contract food-testing laboratories to manage increasingly stringent regulatory limits and complex matrices.
• Further integration of intelligent cell gas modes and automated sample-handling (autodilution, discrete sampling) to enable walk-away workflows and reduce analyst intervention.
• Expansion of multi-element workflows to include speciation (e.g., methylmercury) or coupling with chromatographic separations for targeted compound measurement while maintaining AOAC-compliant trace element quantitation.
• Development of standardised preset methods and software-driven autotune procedures to shorten method development time and enable non-expert operation at scale.

Conclusion

The Agilent 9500 ICP-QQQ combined with AVS MS discrete sampling and ADS 2 autodilution produced a high-throughput, AOAC 2015.01–compatible workflow for multi-element food analysis. Dual-Cell System gas modes (AHM and Air cell) provided complementary interference control—AHM for polyatomic interferences and Air cell mode for suppression of REE2+ doubly charged interferences on As and Se—resulting in accurate SRM recoveries, excellent volatile-element preservation (Lu >98%), low detection limits, and stable operation over extended sequences. Automation and prescriptive dilution reduced hands-on time and improved sample throughput, making the configuration suitable for routine regulatory and industrial food-testing laboratories requiring speed, reliability, and trace-level performance.

Reference

1. Association of Official Analytical Chemists (AOAC). Determination of Heavy Metals in Food by Inductively Coupled Plasma–Mass Spectrometry: First Action 2015.01. J AOAC Int. 2015;98(4):1113–1120.
2. Agilent Technologies. Dual-Cell System (DCS) and Advanced Helium Mode (AHM). Publication 5994-8985EN.
3. Agilent Technologies. Air Cell Mode of the Agilent 9500 ICP-QQQ with Dual-Cell System. Publication 5994-8987EN.
4. Agilent Technologies. Capabilities and Operation of the Advanced Dilution System 2. Publication 5994-7211EN.