Recommendations and good practices for dissolved organic carbon (DOC) analyses at low concentrations

Area of Science:

• Environmental Chemistry
• Analytical Chemistry

Background:

• Existing ISO protocols for dissolved organic carbon (DOC) are certified for concentrations ≥ 1 mg L⁻¹, but many studies report lower values.
• Current ISO protocols lack guidance on vial cleaning, filtration materials, and cap/septum selection for low DOC analysis.

Purpose of the Study:

• To evaluate and optimize protocols for accurate measurement of low dissolved organic carbon (DOC) concentrations (≤ 1 mg L⁻¹).
• To assess the impact of sample containers, filtration, acid type, and matrix effects on DOC measurements.

Main Methods:

• Evaluation of decontaminated glass vials (450-500 °C).
• Testing of 0.45 µm hydrophilic polytetrafluoroethylene (PTFE) membranes with ultra-pure water rinses.
• Assessment of HCl acidification for sample storage.
• Analysis of matrix effects from sulfides, sodium, and calcium.
• Investigation of contamination from PTFE septa at different storage temperatures (4 °C and -18 °C).

Main Results:

• Decontaminated glass vials, rinsed PTFE filters, and HCl acidification yielded the lowest DOC contamination.
• Sulfides, sodium, and calcium showed minimal matrix effects (≤ 10%) on DOC analysis.
• Pierced PTFE septa caused slight DOC contamination at 4 °C and significant contamination at -18 °C for acidified samples.

Conclusions:

• Optimized protocols using specific vials, filters, and acidification are crucial for reliable low DOC quantification.
• Careful selection of septa and storage conditions is vital to prevent contamination in low DOC water samples.