API MPMS 19.3C 1998 (2002): Measurement of Oil in Water by Fluorescence – Technical Overview and Compliance

Scope of API MPMS 19.3C

API MPMS Chapter 19.3C, originally published in 1998 and reaffirmed in 2002, establishes a standard procedure for the determination of oil and petroleum hydrocarbons in water using fluorescence spectroscopy. This method is part of the broader API Manual of Petroleum Measurement Standards (MPMS) and addresses the need for a sensitive, quantitative technique to monitor low concentrations of oil in aqueous matrices such as produced water, refinery effluents, and environmental waters.

The fluorescence method is based on the natural UV-induced fluorescence of aromatic hydrocarbons present in petroleum. Under ultraviolet excitation, these compounds emit characteristic fluorescent signals that can be correlated to total petroleum hydrocarbon (TPH) concentrations when calibrated against a reference oil. The standard is widely used in the upstream, midstream, and downstream petroleum sectors for regulatory compliance, process control, and environmental surveillance.

Tip: API MPMS 19.3C is particularly valuable for screening large numbers of samples due to its speed (analysis time typically under 30 minutes per sample) and low detection limits, often reaching sub-ppm levels for light aromatic fractions.

Technical Requirements of the Fluorescence Method

Principle of Measurement

The method employs a fluorescence spectrometer or dedicated fluorometer to measure the intensity of fluorescent light emitted by aromatic hydrocarbons in a sample. Excitation is typically provided at a wavelength in the 250–300 nm range, and emission is detected at 350–400 nm. The specific excitation and emission optima depend on the nature of the oil and must be determined during method calibration.

Sample Preparation and Handling

Proper sample collection and preparation are critical. The standard prescribes that water samples be collected in glass containers with minimal headspace, preserved by acidification to pH < 2 using hydrochloric or sulfuric acid, and stored at 4 °C in the dark. Before analysis, the oil fraction must be extracted from the water using a suitable solvent (commonly n-hexane or cyclohexane). The extract is then cleaned of potential interferences (e.g., through a silica gel column for polar compounds) and subjected to fluorescence measurement.

Calibration Procedures

Calibration is performed using a reference oil that is representative of the source or target hydrocarbons. A series of calibration standards containing known concentrations of the reference oil in the extraction solvent is prepared. The fluorescence intensity at the selected emission wavelength is plotted against concentration to generate a linear or quadratic curve. The standard requires that the correlation coefficient (R²) must be ≥ 0.995 for acceptance. Blank and check standards are analyzed at regular intervals to monitor baseline drift and validate calibration stability.

Warning: The choice of reference oil significantly influences results. Using an oil with an aromatic composition different from that in the sample can introduce systematic errors. Always correlate reference oil composition with the expected hydrocarbon profile or use a mixed standard that covers the range of petroleum fractions present.

Instrument Specifications

Table 1 summarizes the typical instrument parameters recommended in API MPMS 19.3C.

Parameter	Recommended Range / Value	Remarks
Excitation Wavelength	250–300 nm	Optimize for maximum signal from target aromatics
Emission Wavelength	350–400 nm	Select based on fluorescence spectrum of the oil
Spectral Bandpass	10–20 nm	Balances sensitivity and selectivity
Detection Limit	0.1–1.0 mg/L TPH in water	Depends on extraction efficiency and instrument quality
Calibration Range	0.5–50 mg/L in extract	Equivalent to 0.1–10 mg/L in water (for 100 mL extract)
R² Minimum	≥ 0.995	Ensures linearity of calibration curve

Success: When implemented correctly, the fluorescence method under API MPMS 19.3C yields precise and reproducible results that are widely accepted by regulators and industry partners for produced water monitoring and oil spill assessment.

Implementation Highlights

Sample Throughput and Automation

The fluorescence method lends itself to automation. Many modern laboratories use flow-through or robotic systems that automatically inject samples, collect readings, and calculate concentrations. A typical operator can process 20–40 samples per hour including extraction steps if automated liquid handling is used. For manual batch methods, throughput is lower (8–15 samples per day).

Interferences and Mitigation

Common interferences include:

Natural organic matter (NOM) – Humic acids and other fluorescent organics can cause positive bias. The standard recommends pre‑extraction or use of a clean‑up step using silica gel.
Surfactants and dispersants – May affect extraction efficiency or contribute to fluorescence. Solvent selection and extraction pH are critical.
Instrument stray light – Should be minimized through proper monochromator or filter selection and routine maintenance.

Quality control measures include analysis of field blanks, matrix spikes, and duplicate samples at a frequency of at least 10% of total samples.

Danger: Failing to account for interference from background fluorescence can lead to gross overestimation of oil concentrations, potentially causing false positive violations of discharge permits. Always validate the absence of interfering substances in the sample matrix before routine application.

Compliance Notes and Best Practices

Regulatory Context

API MPMS 19.3C is referenced by many environmental regulatory agencies (e.g., EPA, OSPAR, national oil spill response plans) as an acceptable method for measuring oil in water. The method meets the detection limits required under most produced water discharge limits (e.g., 29 mg/L oil and grease, 15 mg/L for enhanced recovery).

Quality Assurance / Quality Control (QA/QC)

The standard requires a documented QA/QC plan that includes:

Initial and continuing calibration verification
Analysis of laboratory control samples (LCS)
Matrix spike / matrix spike duplicate (MS/MSD) recovery limits of 70–130%
Blank contamination less than method detection limit

Documentation and Reporting

Results should be reported as “total petroleum hydrocarbons (TPH) by fluorescence, as equivalents”. The report must include the method ID (API MPMS 19.3C 1998/2002), extraction solvent, reference oil type, and any deviations from the standard procedure. Instrument calibration curves and QC data should be retained for at least five years.

Tip: To simplify audits, maintain a standard operating procedure (SOP) that mirrors the API MPMS 19.3C requirements step by step. Include instrument maintenance logs, sample receipt records, and corrective action reports as part of your quality system.

Frequently Asked Questions

Q: What types of petroleum hydrocarbons does the fluorescence method detect?
A: The method primarily detects mono‑ and polycyclic aromatic hydrocarbons (PAHs) that fluoresce under UV light. Saturated hydrocarbons (such as alkanes) and highly polar compounds (such as asphaltenes) respond weakly or not at all. Therefore, the measured TPH concentration is biased toward the aromatic fraction. This is addressed by calibrating with a reference oil that matches the aromatic profile of the source oil.

Q: How does API MPMS 19.3C compare to infrared (IR) or gravimetric methods?
A: Fluorescence is generally 10–100 times more sensitive than IR or gravimetric methods for aromatic-rich oils. It also requires smaller sample volumes and shorter analysis times. However, it is more prone to matrix interferences and is less suitable for heavy end Bunker C oils with low aromatic content. The selection among MPMS 19.3A (IR), 19.3B (gravimetric), and 19.3C should be based on the expected hydrocarbon composition and regulatory requirements.

Q: Is the 2002 reaffirmation different from the 1998 original?
A: The 2002 reaffirmation did not introduce technical changes; it confirmed the continued validity of the 1998 edition. Users should always reference the latest reaffirmed version for compliance. Check with API or your jurisdiction for any amendments or updates beyond 2002, as newer editions of MPMS Chapter 19.3C may have been issued in subsequent years.

Q: Can the method be used for on‑site, real‑time monitoring?
A: Yes, portable fluorometers exist that follow the same operational principles. However, they must be validated against the full laboratory method. For regulatory reporting, results from field instruments often require confirmation by a laboratory that adheres strictly to API MPMS 19.3C. Always calibrate field units with site‑specific water and ensure that extraction efficiencies are comparable.

Published in accordance with API MPMS 19.3C 1998 (2002). Note that standards are subject to periodic revision; always confirm that you hold the latest version. This article provides general technical guidance and should not replace the original standard text for compliance decisions.

📥 Standard Documents Download

🔒

Please wait 10 seconds, the download links will appear after the ad loads