Dimethyl carbonate as a green organic modifier in supercritical fluid extraction of pesticide residue analysis in apple samples

Significance of the topic

Replacing hazardous organic solvents with greener alternatives is a central goal of Green Analytical Chemistry. Supercritical CO2-based extraction already reduces solvent hazards, but efficient co-solvents are required to broaden the polarity range of extractable analytes. Introducing dimethyl carbonate (DMC) as a novel co-solvent for supercritical fluid extraction (SFE) targets improved sustainability, reduced matrix co-extraction, and compatible performance for multiresidue pesticide analysis in food matrices such as apples.

Objectives and study overview

The study evaluated DMC as an organic modifier in off-line SFE for extraction of 64 pesticides from apple puree and compared its performance to acetonitrile (ACN) and bio-ethanol (EtOH). Key aims were to optimize SFE parameters for each modifier, assess extraction yields across a wide range of pesticide physicochemical properties (log Kow −0.9 to 7.7; MW 141–532), quantify matrix co-extraction and matrix effects, and evaluate method greenness and practical applicability using low‑pressure GC–triple quadrupole MS (LP‑GC‑QqQMS).

Methodology and sample preparation

Summary of the experimental approach:

• Apple samples (six market samples) were homogenized, spiked (primary tests at 15 mg kg−1, additional tests at 0.05 mg kg−1), dehydrated with anhydrous agent (sample:agent 4:1), and loaded into 0.2 mL stainless steel SFE vessels (136 mg apple per vessel).
• SFE optimization variables: co-solvent type and percentage (10–50%), extraction temperature (30–70 °C), static/dynamic extraction times. Final optimized SFE conditions for all modifiers: 30% co-solvent, 150 bar, 70 °C, total dynamic/static arrangement yielding ~21 min total extraction time (1 min static + 20 min dynamic), total solvent flow 1 mL min−1 (make-up flow adjusted to maintain 1 mL min−1).
• Extracts were collected, diluted to defined volumetric levels (10 or 20 mL), and analyzed directly by LP‑GC‑QqQMS; for low-level tests (0.05 mg kg−1) extracts were concentrated under N2.

Instrumentation used

Key instrumentation and configuration:

• SFE system: Shimadzu Nexera‑UC off‑line SFE with 48‑vessel rack, LC‑30AD SF CO2 pump, dual‑plunger co‑solvent pump, back pressure regulator, fraction collector; static+dynamic extraction sequence; post‑BPR make‑up flow and gas–liquid separation to collect liquid fraction.
• Chromatography and detection: GC‑2010 Plus coupled to TQ8040 triple quadrupole MS (Shimadzu). Injection by autosampler in splitless mode (3 µL) using high‑pressure pulse. A two‑column arrangement with an uncoated precolumn and a 5 m × 0.53 mm Equity‑5 separation column prevented sub‑ambient pressure effects. Oven ramp: 40 °C (2 min) to 320 °C at 30 °C min−1. Electron ionization at 70 eV; MRM acquisition (two transitions per analyte) for targeted quantification; SCAN used for comparing extract cleanliness.

Main results and discussion

Extraction yield and selectivity:

• Panel: 64 pesticides spanning diverse chemical classes. Optimized methods produced average recoveries: ACN ~95.1%, EtOH ~106.4%, DMC ~85.0% (individual recoveries with DMC ranged 45.3–111.1%).
• DMC delivered substantially cleaner extracts: measured mass of co‑extracted matrix at optimized conditions was 5 mg (DMC) vs 28 mg (ACN) and 52 mg (EtOH) per extraction. For a 3 µL GC injection, co‑injected matrix mass was ~0.75 µg for DMC compared to ~7.8 µg for EtOH, reducing instrument contamination and maintenance frequency.
• Matrix effect (ME) assessment: average %ME values were 90.5% (ACN), 140.1% (EtOH), and 137.4% (DMC). DMC showed predominantly signal enhancement for many analytes (similar to EtOH) but overall cleaner chromatographic background in SCAN traces.
• Compound exceptions and limitations: 15 pesticides exhibited recoveries <70% with DMC (examples: flonicamid, dimethoate, pyridaben, bifenthrin). Precision for these low‑recovery cases remained acceptable (RSD ≤ 12%), consistent with SANTE guidelines that permit values outside 70–120% when precision is satisfactory. Low recoveries were not strictly correlated with log Kow; solubility differences or limited affinity for DMC likely contribute.
• Analytical sensitivity: using LP‑GC‑QqQMS and a concentration step, average S/N for spiked extracts at 0.05 mg kg−1 was ~199 (range 17–882), with two pesticides below LOD under the tested concentration.
• Real samples: six market apple samples analyzed—pesticide concentrations detected were <0.05 mg kg−1 for all quantified residues.

Benefits and practical applications

Practical advantages of DMC in SFE:

• Greener profile: DMC is biodegradable, lower toxicity, and can be produced from CO2—aligns with carbon capture and utilization concepts and GAC principles.
• Cleaner extracts reduce matrix loading onto GC‑MS systems, extending maintenance intervals and improving instrumental robustness.
• Low sample mass requirement (136 mg) enables high throughput and minimal sample consumption compared with typical QuEChERS workflows (commonly 10 g samples and 10–20 mL ACN).
• Compatibility with LP‑GC‑QqQMS yields rapid analysis (total chromatographic run ~10 min) and supports high sample throughput.

Future trends and opportunities

Potential development directions:

• Broaden application: evaluate DMC‑modified SFE on other food matrices (high fat, high pigment) and environmental samples to define the scope and limits across matrix types.
• Optimize solvent consumption: reduce total DMC usage (current protocol uses ~23 mL including washes) via online trapping or miniaturized collection to improve greenness scores further.
• Hybrid workflows: integrate SFE online with supercritical/SFC separation or automated cleanup (SPE trapping) to minimize manual handling and concentrate analytes selectively for low‑level residue monitoring.
• Method refinement for poor‑recovering analytes: investigate solvent blends or polarity tuning (DMC + small fraction of polar cosolvent), temperature/pressure windows, or use of modifiers that improve solubility for problematic pesticides while preserving extract cleanliness.
• Green metric improvements: continue using tools such as AGREEprep to guide trade‑offs between solvent selection, energy consumption, and instrument complexity; pursue CO2/DMC sourced from renewable feedstocks.

Conclusion

Introducing dimethyl carbonate as an SFE co‑solvent delivers a meaningful sustainability gain while maintaining broadly acceptable analytical performance for multiresidue pesticide analysis in apples. DMC produced significantly cleaner extracts than ACN or EtOH and achieved an average recovery of 85% across 64 pesticides, with satisfactory precision for low‑recovering analytes. Remaining challenges include poorer extraction of a subset of compounds and the need to manage solvent consumption and extraction time. Overall, DMC is a promising green alternative for SFE that can reduce laboratory hazards, improve instrument longevity, and align pesticide residue workflows with Green Analytical Chemistry objectives.

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