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API Publication 4658-1997: A Benchmark Study for Petroleum Hydrocarbon Analysis in Soil
Interlaboratory Validation of Three Analytical Methods for Total Petroleum Hydrocarbons and Polycyclic Aromatic Hydrocarbons in Environmental Media
Scope and Purpose
API Publication 4658-1997 (commonly referred to as API 4658) presents the results of a comprehensive interlaboratory study designed to evaluate the performance of three widely used analytical methods for the determination of petroleum hydrocarbons in soil. This publication, developed under the auspices of the American Petroleum Institute, addresses the critical need for reliable and comparable data when assessing total petroleum hydrocarbons (TPH) and polycyclic aromatic hydrocarbons (PAHs) in soil matrices. The study specifically targets methods employed in regulatory compliance and risk-based corrective action (RBCA) programs, offering a statistical basis for understanding method bias, precision, and detection limits. The scope of API 4658 extends to laboratories conducting environmental analysis for upstream and downstream petroleum operations, as well as consultants and remediation specialists engaged in site characterization and monitoring.
Analytical Methods Under Study
API Publication 4658 evaluates three core methods that were, at the time of publication, the most prevalent in environmental laboratories for quantifying petroleum hydrocarbons in soil:
- Method A – EPA SW-846 418.1 (Total Recoverable Petroleum Hydrocarbons by Infrared Spectrophotometry)
- Method B – EPA SW-846 8015B (Nonhalogenated Organics by GC/FID – Modified for TPH)
- Method C – Modified EPA SW-846 8270C (Semivolatile Organic Compounds by GC/MS with extended quantitation for hydrocarbon ranges)
The study included 16 participating laboratories that analyzed a set of characterized soil samples representing various hydrocarbon contamination profiles, from light fuels (e.g., gasoline) to heavier products (e.g., diesel and crude oil). Each laboratory performed analyses according to detailed standard operating procedures provided in the study protocol.
| Method | Analyte | Instrumentation | Relative Detection Limit (mg/kg) | Precision RSD (%) |
|---|---|---|---|---|
| EPA 418.1 | Total Recoverable Petroleum Hydrocarbons | Infrared (IR) spectrophotometer | 10 | 30–50 |
| EPA 8015B | TPH (C6–C44) and range fractions | GC with flame ionization detector | 1 | 15–35 |
| Modified EPA 8270C | PAHs and hydrocarbon ranges | GC with mass spectrometer (full/selective ion monitoring) | 0.5 | 10–25 |
Attention: The IR method (EPA 418.1) showed the highest variability and limited speciation capability. Its use is now discouraged for risk-based corrective action (RBCA) applications unless coupled with fractionation or corroborated by GC methods.
Key Findings and Implementation Highlights
Performance Metrics and Statistical Analysis
The study used robust statistics (including Horwitz ratio and Youden’s plots) to evaluate between-laboratory and within-laboratory reproducibility. The GC/MS-based method (modified EPA 8270C) provided the best overall performance for both TPH and PAH quantification, achieving recoveries of 80–120% for most analytes. However, the study noted that the modified EPA 8270C method required extensive quality control measures and higher operational expertise.
The GC/FID method (EPA 8015B) offered a practical balance between cost, throughput, and accuracy when TPH fractionation was required. The publication recommends that laboratories adopt GC/FID for routine TPH screening and use GC/MS only when detailed PAH speciation is necessary for risk assessment.
Best Practice: For sites transitioning to risk-based closures, select the method consistent with the target risk fractions (e.g., aliphatic vs. aromatic). The study supports a tiered approach: start with GC/FID for TPH and confirm with GC/MS for PAH-targeted projects.
Quality Assurance / Quality Control Requirements
API 4658 provides detailed guidance on QA/QC protocols necessary for reliable hydrocarbon analysis. Laboratories are instructed to:
- Use matrix-specific calibration standards (e.g., weathered diesel for aged contamination).
- Include surrogate spikes (such as o-terphenyl and decachlorobiphenyl) for GC-based methods.
- Regularly analyze control and blank samples to monitor background interferences.
- Report detection limits based on method detection limit (MDL) studies, not instrument detection limits.
Tip: When submitting soil samples for TPH analysis under a regulatory program, always request the laboratory to report the specific method version and the associated MDL. This facilitates comparison with historical data and ensures defensibility.
Compliance and Regulatory Implications
API Publication 4658 serves as a foundational reference for regulatory agencies and environmental consultants who require validated methods for petroleum hydrocarbon analysis. Many state and federal voluntary cleanup programs (e.g., ASTM E1739 for RBCA) cite this publication to support method selection and data quality objectives.
Acceptance Criteria
Regulatory acceptance of analytical data often hinges on demonstrating that the methods used meet the statistical performance criteria outlined in API 4658:
- Accuracy (recovery) must fall within ±40% of the true value for TPH methods.
- Precision (relative standard deviation) across replicate analyses must be less than 30% for the same matrix.
- Method detection limits must be below the applicable target risk concentrations derived from public health or ecological benchmarks.
Since the publication of API 4658 in 1997, advances in instrumentation and data processing have further refined these methods. However, the 1997 study remains the primary interlaboratory validation dataset for TPH analysis methods and continues to be referenced in forensic hydrocarbon fingerprinting and litigation support.
Important: The methods evaluated in API 4658 are subject to updates in the EPA SW-846 compilation. Users must verify that their laboratory follows the most current method revision to maintain compliance with local regulations. For example, EPA Method 8270 has been updated multiple times since 1997.
Frequently Asked Questions
Q: What is the primary difference between EPA Method 418.1 and EPA Method 8015B as assessed in API 4658?
A: Method 418.1 uses infrared absorption to measure total petroleum hydrocarbons without chromatographic separation, resulting in a single “TPH” value. Method 8015B uses GC/FID to separate hydrocarbons by boiling point, allowing quantification of carbon range fractions (e.g., C6–C12, C12–C28) that are more relevant for risk-based decisions.
A: Method 418.1 uses infrared absorption to measure total petroleum hydrocarbons without chromatographic separation, resulting in a single “TPH” value. Method 8015B uses GC/FID to separate hydrocarbons by boiling point, allowing quantification of carbon range fractions (e.g., C6–C12, C12–C28) that are more relevant for risk-based decisions.
Q: Are the data from API Publicitcation 4658 still valid for modern risk assessments?
A: Yes, the statistical conclusions regarding method precision and bias remain relevant. However, many regulatory programs now require additional quality metrics (e.g., data completeness, surrogate recoveries) that go beyond the 1997 study. The publication should be used as a baseline reference, supplemented with current method performance data from the laboratory.
A: Yes, the statistical conclusions regarding method precision and bias remain relevant. However, many regulatory programs now require additional quality metrics (e.g., data completeness, surrogate recoveries) that go beyond the 1997 study. The publication should be used as a baseline reference, supplemented with current method performance data from the laboratory.
Q: Did API 4658 evaluate all PAHs or only a subset?
A: The study included 16 priority parent PAHs and several alkylated homologues (e.g., C1–C4 naphthalenes, dibenzothiophenes). This range provides sufficient coverage for petroleum source identification and toxicity assessment.
A: The study included 16 priority parent PAHs and several alkylated homologues (e.g., C1–C4 naphthalenes, dibenzothiophenes). This range provides sufficient coverage for petroleum source identification and toxicity assessment.
Q: Can the methods in API 4658 be used for water or sediment samples?
A: The interlaboratory study was limited to soil samples. While the methods can be adapted to water or sediment, the performance characteristics (detection limits, recoveries) should be verified separately. Refer to API Publication 4662 (1998) for analogous studies on aqueous matrices.
A: The interlaboratory study was limited to soil samples. While the methods can be adapted to water or sediment, the performance characteristics (detection limits, recoveries) should be verified separately. Refer to API Publication 4662 (1998) for analogous studies on aqueous matrices.
Reference: American Petroleum Institute. (1997). Interlaboratory Study of Three Methods for Analyzing Petroleum Hydrocarbons in Soil. API Publication 4658. Washington, DC: API.
Updated Reference Standards: Users are advised to consult the latest editions of EPA SW-846 (Update VI) for current method approval status.
This article is prepared for general informational purposes and does not constitute legal or regulatory advice. Practitioners should confirm applicability with the relevant jurisdiction.