Environmentally friendly oil in water analysis by FTIR spectroscopy, based on ASTM D7678-11

Significance of the topic

Accurate measurement of oil concentrations in water is essential for regulatory compliance, environmental monitoring and industrial process control. The use of Fourier transform infrared spectroscopy enables rapid, sensitive and onsite determination of total petroleum hydrocarbons and oil and grease in aqueous samples. The method described combines a cyclohexane extraction with portable mid-IR instruments to minimize environmental impact and operational costs.

Objectives and overview

This study outlines the implementation of ASTM D7678-11 for oil in water analysis using FTIR. It aims to replace halogenated solvents with cyclohexane, establish a robust calibration, validate precision and accuracy at low and high concentrations, and compare performance to existing IR and chromatographic techniques.

Methodology and instrumentation

A five-step liquid-liquid extraction protocol isolates hydrocarbons from 900 mL water samples using 20 mL cyclohexane. The cleaned extract is dried with sodium sulfate, adsorbent-treated with Florisil, filtered and measured in a 1000 µm pathlength liquid cell (DialPath or TumblIR). Quantification targets the methyl bending vibration at 1378 cm⁻¹, with partial least squares regression applied over 1370–1380 cm⁻¹.

Použitá instrumentace

• Agilent 4500 Series FTIR spectrometer
• Agilent 5500 Series FTIR spectrometer
• DialPath and TumblIR liquid cells (1000 µm pathlength)
• MicroLab FTIR software with PLS algorithms

Main results and discussion

• Calibration established over 11 non-zero points correlates mineral oil concentration in cyclohexane (0–1465 mg/L) to water concentrations (0–32.55 mg/L) with R²=0.9993.
• Validation at 9.3 mg/L (oil) and 1.4 mg/L (oil) demonstrated RSDs of 2.7% and 5.7% and recoveries of 102% and 103%, matching or exceeding ASTM D7678 targets.
• Use of cyclohexane, which lacks absorbance at 1378 cm⁻¹, enables clear measurement of hydrocarbon methyl absorption without halogenated solvent permits.
• The MicroLab software automates data processing and guides users through quantitative analysis.

Benefits and practical applications

• Onsite, rapid analysis suitable for refineries, offshore platforms and remediation sites.
• Elimination of ozone-depleting and restricted solvents, reducing regulatory burden.
• Lower cost per sample (~$1.40) compared to methods using S-316 solvent (~$30).
• Broad dynamic range from 0.25 mg/L detection to 1000 mg/L quantification.

Future trends and possibilities of use

Continued miniaturization of FTIR platforms, integration with automated sampling systems and cloud-based data management will enhance real-time monitoring. Advances in chemometric models may extend selectivity to complex matrices. Exploration of greener solvents and coupling FTIR with complementary techniques could further improve sensitivity and application scope.

Conclusion

The FTIR method based on ASTM D7678-11 offers a validated, environmentally benign and cost-effective solution for oil in water analysis. It provides equivalent or superior performance to existing standards, simplifies sample preparation and supports routine onsite monitoring with portable FTIR instruments.