Common Methods for Measuring Oil Content in Water

Accurately determining the concentration of oil and grease in water is crucial for environmental monitoring, regulatory compliance, and industrial process control. Several standardized methods are commonly employed:

Gravimetric Methods (e.g., EPA Method 1664):

Principle: Oil is extracted from the water sample using a solvent (like n-hexane). The solvent is then evaporated, and the residual oil is weighed.

Pros: Considered the reference method; measures total oil and grease (non-polar material); relatively straightforward conceptually.

Cons: Time-consuming; requires skilled technicians; susceptible to loss of volatile compounds during evaporation; uses large volumes of solvent.

Infrared (IR) Spectrophotometry (e.g., EPA Method 413.2, ISO 9377-2):

Principle: Oil is extracted using a solvent (typically tetrachloroethylene or S-316). The concentration of oil in the extract is then determined by measuring the absorbance of infrared light at specific wavelengths (around 2930 cm?1) characteristic of C-H bonds.

Pros: Faster than gravimetry; good sensitivity; less volatile compound loss; widely accepted standard.

Cons: Requires solvent extraction; solvent choice impacts results; some compounds (e.g., surfactants, organic acids) can interfere.

Gas Chromatography (GC) (e.g., EPA Method 8015):

Principle: Extracted oil components are separated within a chromatographic column based on their volatility and affinity. Individual components are then detected (often with a Flame Ionization Detector - FID) and quantified.

Pros: Highly specific; identifies and quantifies individual hydrocarbons; excellent sensitivity and precision.

Cons: Most complex and expensive method; requires significant expertise; lengthy analysis time; still requires extraction/pre-concentration.

Fluorometry:

Principle: Many oil compounds fluoresce when exposed to specific wavelengths of light. The intensity of the emitted fluorescence is proportional to the oil concentration.

Pros: Very sensitive to low concentrations; potential for rapid, real-time, or in-situ monitoring (field instruments); minimal sample preparation often needed.

Cons: Response varies significantly between different oil types; not a measure of total oil (only fluorescent components); susceptible to quenching and interference from other fluorescing substances; requires calibration for specific oil types.

Method Selection Factors:

Choosing the appropriate method depends on:

Required Accuracy & Precision: Regulatory standards often specify methods.

Target Compounds: Total oil vs. specific hydrocarbons.

Detection Limits Needed: Sensitivity requirements.

Sample Matrix: Potential interferences.

Turnaround Time & Cost: Lab resources and urgency.

Field vs. Lab Analysis: Portability needs.

Gravimetric and IR methods remain the most common standards for total oil and grease, while GC offers detailed speciation. Fluorometry is valuable for rapid screening and continuous monitoring, especially at low concentrations. Understanding the strengths and limitations of each technique is key to obtaining reliable water quality data.